Nanocatalysis Synthesis and Applications (Polshettiwar/Nanocatalysis) || Nanocatalysts for Water...

14

NANOCATALYSTS FORWATER SPLITTING

Xu Zong, Gaoqing Lu, and Lianzhou Wang

INTRODUCTION

Water splitting is the general term for a chemical reaction in which water is dissoci-ated into oxygen and hydrogen as shown in Equation 14.1. This reaction is an uphillreaction with the standard Gibbs free energy change (� G0) of 237 kJ/mol. Therefore,the reaction is thermodynamically unfavorable under ambient conditions. However, thethermodynamic equilibrium of the reaction can be shifted toward the desirable rightdirection under certain circumstances, leading to the splitting of water.

H2O → H2 + 12O2; � G = 237 kJ/mol (14.1)

Water-splitting reaction is an important reaction, as it can provide hydrogen, whichserves as a potential energy carrier to solve severe energy and environmental prob-lems. Up till now, several techniques including electrolysis of water with electric power,thermolysis of water, photobiological water splitting with algae, photocatalytic or pho-toelectrochemical water-splitting reactions have been used to split water into H2 andO2 with the aim of realizing promising hydrogen economy. However, water splitting ina sustainable, environment-benign, economical, and efficient manner should be a keyconsideration for hydrogen supply. If hydrogen as a green energy carrier is be obtainedby using the abundant solar energy resource to split water, it can be considered an idealway of powering this blue planet.1, 2

Nanocatalysis: Synthesis and Applications, First Edition. Edited by Vivek Polshettiwar and Tewodros Asefa.© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

495

496 NANOCATALYSTS FOR WATER SPLITTING

Photocatalytic water splitting with semiconductor nanocatalysts can convert pho-ton energy into chemical energy, which is very similar to photosynthesis and can beregarded as an artificial photosynthesis. It has the simplicity of using a powder photo-catalyst in solution and sunlight to produce H2 and O2 from water without producinggreenhouse gases or inducing adverse effects to the environment. Therefore, photocat-alytic water splitting using semiconductor nanocatalysts is an attractive and promisingway of depositing solar energy in the form of hydrogen energy. Ideally, a photocatalyticwater-splitting system is composed of only solar energy, water, and a semiconductorphotocatalyst. The ease of availability of solar energy and water makes the developmentof cost-effective photocatalyst materials, the key research component in the system.

The extensive research on photocatalytic water splitting using solar energy wasinitiated by the discovery of photoelectrochemical splitting of water on n-type TiO2electrodes in 1972.3 In the past decade, this research regained lots of attention as oiland other nonrenewable fuels became increasingly depleted and expensive. Up to now,more than 100 semiconductor nanocatalysts have been developed as photocatalystsfor water splitting, especially those which are active under visible light irradiation(� � 420 nm). In 2003, La-doped NaTaO3 loaded with NiO cocatalyst was reported tosplit pure water into H2 and O2 with a quantum yield of 56% at 270 nm.4 In 2008, GaN–ZnO solid solution photocatalysts loaded with Cr–Rh oxide cocatalyst were reported tosplit water into H2 and O2 with a quantum yield of 5.9% at 420 nm.5,6 In 2009, CdSphotocatalysts coloaded with Pt and PdS cocatalyst were reported to produce H2 fromwater in the presence of sacrificial reagents with a quantum yield of 93% at 420 nm.7 Andin 2010, Ag3PO4 photocatalysts were reported to produce O2 from water in the presenceof sacrificial reagents with a quantum yield of more than 90% at 420 nm.8 These fourrecently developed semiconductor nanocatalysts are themost activematerials reported sofar for pure water splitting or water splitting in the presence of sacrificial reagents undervisible light or ultraviolet (UV) light irradiation, indicating the fruitful achievements inthe past decades with the development of materials science and nanotechnology.

In this chapter, we start with a brief introduction to the knowledge of water splittingon semiconductor nanocatalysts. Then we overview the semiconductor nanocatalystsaccording to their light absorption property and element compositions. The chapterconcludes with a brief discussion of important criteria for the design and developmentof efficient and stable semiconductor nanocatalysts. Readers interested in solar energyutilization could refer to a series of excellent review paper and books for more broadinformation.9–33

KNOWLEDGE ON PHOTOCATALYTIC WATER SPLITTING

Principles of Photocatalytic Water Splitting

The research on photocatalytic water splitting was initiated by the pioneer work ofFujishima and Honda, when photoelectrochemical water splitting into H2 and O2 wasrealized using a TiO2 electrode (Figure 14.1 left).3 When the TiO2 electrode was irra-diated by UV light under applied potential, oxygen evolution occurred at the TiO2

KNOWLEDGE ON PHOTOCATALYTIC WATER SPLITTING 497

e– e–

H+

Bias

(a) (b)

Photocatalyst

Platinum black

H2O

TiO2 photoanode

O2

H2O

H2O

O2H2

H+

H2

Figure 14.1. Photoelectrochemical (a) and photocatalytic water splitting (b).

electrode and hydrogen evolution occurred at the platinum black electrode. This con-cept was later applied to the photocatalytic system where every semiconductor pho-tocatalyst in powdered form worked as microelectrodes separately for photocatalyticwater-splitting reactions (Figure 14.1b).34, 35

In principle, the photocatalytic water-splitting process can be divided into threestages. A schematic illustration of the process is depicted in Figure 14.2. The first stagecan be defined as the semiconductor’s “photo-excited” state. When the semiconductorabsorbs incident light with enough energy, the electrons in the valence band of thephotocatalyst are excited to the conduction band, while the holes are left in the valenceband, creating negative-electron (e−) and positive-hole (h+) pairs. The energy differencebetween the valence and conduction bands is known as the “band gap,” and the energyof the incident light must exceed the band gap of the semiconductors to allow for

Figure 14.2. Schematic steps for photocatalytic water splitting. (See color insert.)


V/NHE

–1.0

0

hv+1.0

H+/H2

H2

O2

O2/H2O

H2O

+2.0

+3.0

+4.0

H+Conduction band

e– e– e– e– e– e–

Valence band

h+ h+ h+ h+ h+ h+

Band gap

Figure 14.3. Band structures of semiconductors for photocatalytic water splitting.

the effective absorption of the light. The second stage is referred to as the “chargesmigration” state. The photogenerated electrons and holes migrate to the surface of thesemiconductor photocatalysts before recombination; and the last stage can be referredto as the “surface redox reaction” stage. The photogenerated electrons and holes actas reducing and oxidizing agents, respectively, and can induce redox reactions similarto electrolysis. During this reaction, water molecules are reduced by the electrons toform H2 and are oxidized by the holes to form O2 on different active sites, respectively.Only when these three stages can be accomplished simultaneously, photocatalytic watersplitting can proceed.

The first stage obeys the first law of photochemistry (the Grotthuss–Draper law),which means that light must be absorbed by the photocatalyst in order for a photochem-ical reaction to take place. Considering the standard Gibbs free energy change (� G0) of237 kJ/mol for water splitting, a theoretical minimum band-gap energy (Eg) of 1.23 eVis required, corresponding to the light with a wavelength of 1100 nm (Figure 14.3). Ifvisible light is used for water splitting, the band gap of the semiconductors should beless than 3.0 eV (� � 400 nm). Otherwise, incident photons cannot be absorbed bythe photocatalyst to initiate photocatalytic reactions. This stage determines the amountof available photogenerated charges for further reactions, and therefore is the basicrequirement for further photocatalytic reactions.

In the second stage, themigration of photogenerated chargeswill be accompanied bythe charge recombination processes in the bulk or on the surface of photocatalysts. Chargerecombination is a competitive process of charge migration, which reduces the excitedcharges by emitting light or generating photons. This process decreases the amount ofthe available active charges for water-splitting reactions and impairs the photocatalyticperformance of semiconductors severely. Crystal structure, crystallinity, and particlesize are important considerations for the charge migration process. Methods that couldpreparematerials with smaller particle size (less migration distance), higher crystallinity,and fewer defects (less trapping and recombination centers) are very paramount for


decreasing the recombination. Therefore, it is fundamentally important to facilitatethe charge transfer and decrease the recombination of photogenerated charges towardefficient photocatalysis.

In the third stage, the available photogenerated charges will react with watermolecules to produce H2 and O2. From the thermodynamic point, both the reduction andoxidation potentials of water should locate within the band gap of the photocatalyst, asshown in Figure 14.3. The bottom level of the conduction band has to be more negativethan the redox potential of H+/H2 (0 V vs. normal hydrogen electrode (NHE)), and thetop level of the valence band has to be more positive than the redox potential of O2/H2O(1.23 V). Therefore, the match between the conduction and valence band positions ofphotocatalysts with the redox potential of water splitting is compulsory if water splittingneeds to be realized on a single photocatalyst. However, although the band gaps ofsemiconductors are well known, accurate information for the potentials of valence bandand conduction band has not been obtained for most of the materials. In some cases,there is much difference between the literature values for the same semiconductor. But-ler and Ginley developed a useful method to calculate the band-edge energies using anempirical relationship based on the electronegativity of the constituting elements (Eqs14.2 and 14.3).36 In the equations, � represents the electronegativity of the elementswith respect to vacuum as the zero energy reference and Eg represents the band gap ofsemiconductors.

Ec = −� + 12 Eg (14.2)

Ev = −� − 12 Eg (14.3)

Based upon the equations, we can tentatively obtain the conduction band-edge andvalence band-edge positions of semiconductor materials. Figure 14.4 shows the calcu-lated band positions of some metal oxide and metal sulfide semiconductor materials.37

It is evident that the conduction band and valence band-edge positions of different semi-conductors vary a lot. The conduction and valence band-edge positions of SrTiO3 andCdS match well with the potentials required for water-splitting reactions, indicating thatthey can split water from the thermodynamic point of view. However, the conductionand valence band-edge positions of CoS and PbS locate within the redox potentials ofwater splitting, indicating their inability to split water even excited by incident light.In some cases, both the conduction band and valence band-edge positions are morenegative than the redox potential of H+/H2 and O2/H2O, respectively. While in somecases, both the conduction band and valence band-edge positions are more positive thanthe redox potential of H+/H2 and O2/H2O, respectively. These two kinds of materialsonly have enough potential to reduce or oxide water, which may be useful for H2 or O2production from water splitting in the presence of sacrificial reagents.

In the third stage, it should be noted that cocatalysts are usually loaded on the sur-face of photocatalysts to catalyze the evolution of H2 or O2. The loading of cocatalystscan decrease the overpotential for H2 and O2 evolution on the surface of photocatalysts.Moreover, the presence of proper cocatalysts can reduce the recombination of photogen-erated electrons and holes, which enhances the photocatalytic performance. Therefore,


–3

–4H2/H+

H2/H+

OH–/O2

OH–/O2

–5

–6

–7

–8Ene

rgy

with

res

pect

to V

acuu

m (

eV)

–9

TiO

2

SrT

iO3

BaT

iO3

MgT

iO3

MnT

iO3

FeT

iO3

CoT

iO3

NiT

iO3

CuT

iO3

ZnT

iO3

V2O

5

Cr 2

O3

Fe 2

O3

Fe 3

O4

Cu 2

O

MnO

FeO

CoO NiO

CuO

ZnO

MnO

2

–3

–4

–5

–6

–7

–8

Ene

rgy

with

res

pect

to A

VS

(eV

)

–9

MnS

MnS

2

ZnS

ZnS

2F

e 3S

4C

u 2S

Ag 2

SR

h 2S

3

As 2

S3

In2S

3La

2S3

Sb 2

S3

MoS

2R

uS2

WS

2O

sS2

PtS

2

HfS

2

FeS

FeS

2

CoS

CoS

2

CuS

CuS

2

NiS

NiS

2

TiS

SnS

SnS

2

PbS

CdS

HgS

Figure 14.4. Calculated conduction band and valence band-edge positions at pH 0 for sev-

eral metal oxide and metal sulfide semiconductors. The bottom of open squares represents

conduction band edges, and the top of solid squares represents valence band edges. The solid

lines indicate water stability limits.37


cocatalysts play a crucial role in photocatalytic water-splitting reactions. For example,TiO2 alone is not active for water-splitting reactions. However, after loading Pt on TiO2,the activity of Pt/TiO2 is improved a lot. Another important point that should be noted isthat the water-splitting reactions may be accompanied by unwanted “back-reaction,” inwhich the produced H2 and O2 will react again to form water on special reactive surfacesites of photocatalysts. This back-reaction should and could be avoided or decreasedthrough engineering the surface of photocatalysts.

By analyzing these steps of water splitting, two important thermodynamic require-ments for semiconductor materials must be satisfied to realize water splitting. First, theband gap of semiconductors should be more than 1.23 eV. Second, the band-edge posi-tions of the conduction and valence bands of semiconductors should correlate well withthe potentials of water reduction and oxidation reactions. However, the band structureis just a thermodynamic requirement and not a sufficient condition. In the real photocat-alytic water-splitting reactions, different aspects affecting the reaction dynamics such asstability of photocatalysts, overpotential, reaction media, and lifetime of photogeneratedcharges should be considered.

Types of Photocatalytic Water Splitting

Overall Water Splitting. By definition, photocatalytic water splitting means thedissociation of water into H2 and O2 stoichiometrically under solar irradiation, whichis called overall water splitting (Figure 14.3). Exploring semiconductors capable ofrealizing overall water splitting in a sustainable, cost-effective, and environment-benignmanner is the ultimate goal of this research. In the previous section, this type of watersplitting has been well introduced.

Sacrificial Water Splitting. The large, positive, standard Gibbs free energychange of overall water splitting makes it hard to proceed. To remedy this difficulty, sac-rificial reagents are often employed to overcome the large uphill barrier for the reactions.As shown in Table 14.1,25 the standard Gibbs free energy changes of the listed reactionsare much smaller than those of pure water-splitting reactions. Therefore, in principle, thewater-splitting reaction with the aid of sacrificial reagents is more favorable comparedto overall water-splitting reactions. There are two kinds of sacrificial reagents for thewater-splitting reactions. One kind is called electron donors (hole scavengers) and theother kind is called electron acceptors (electron scavengers).

They are used separately to consume the photogenerated electrons or holes duringthe reaction to investigate the water reduction or oxidation reactions (Figure 14.5). Asshown in Table 14.2,25 the oxidative potentials for the listed reactions in the presenceof different sacrificial reagents are quite low compared with those of water oxidation,demonstrating the lower energy barrier for the reaction. Therefore, photogenerated holeswill react preferentially with the sacrificial reagents instead of water. During the reaction,photogenerated holes will be consumed through the oxidation reaction with the electrondonors irreversibly. The remaining photogenerated electrons will then reduce protons toproduce H2. The successful accomplishment of this reaction indicates the appropriateband structures of semiconductors for water reduction reactions without considering


TABLE 14.1. Change of Gibbs free energy at 298 K for the reactions of hydrogen production25

Entry Chemical equation � G0298K/kJmol

−1

1 CH4 + 2H2O(g)→ CO2 + 4H2 1142 C + 2H2O(g)→ CO2 + 2H2 633 CH3OH(g)→ HCHO(g) + H2 594 CH3OH(g) + H2O(g)→ C02 + 3H2 −45 HCHO(g) + H2O(g)→ HCOOH(g) + H2 −206 CO(g) + H2O(g)→ CO2 + H2 −297 HCOOH(g)→ CO2 + H2 −438 HCHO(g) + H2O(g)→ CO2 + 2H2 −639 C2H6 + 4H2O(g)→ 2CO2 + 7H2 15810 C2H5OH(g) + 3H2O(g)→ 2CO2 + 6H2 6511 C2H5OH(g)→ CH3CHO(g)+H2 3612 CH3CHO(g) + 3H2O(g)→ 2CO2 + 5H2 3013 CH3CHO(g) + H2O(g)→ CH3COOH(g) + H2 −1314 CH3COOH(g)→ CO2 + CH4 −7115 CH3CH(OH)COOH(s) + 3H2O(g)→ 3CO2 + 6H2 2516 CH3CH(OH)CH3(g)→ CH3COCH3(g) + H2 2117 C3H8O3(l) + 3H2O(g)→ 3CO2 + 7H2 2018 C6H12O6(s) + 6H2O(g)→ 6CO2 + 12H2 −8519 H2O(g)→ H2 + 1/2O2 22920 H2O(l)→ H2 + 1/2O2 237

the band structures for water oxidation reactions. This reaction is quite attractive ifpollutants such as H2S from natural gas or abundant organic compounds present innature and industries are used as the electron donors for H2 production.38,39 In the wateroxidation half-reaction, electron acceptors such as Ag+ are generally used. Duringthe reaction, photogenerated electrons will be consumed by the electron acceptors.

Figure 14.5. Schematic principles of water reduction or oxidation in the presence of sacrificial

reagents.


TABLE 14.2. Electrochemical reactions with corresponding oxidative potential25

Entry Reaction Eoox/V vs. NHE (pH = 0)

1 CH3CHO + H2O + 2h+ → CH3COOH + 2H+ −0.122 CO + H2O + 2h+ → CO2 + 2H+ −0.123 HCHO + H2O + 4h+ → CO2 + 4H+ −0.074 C6H12O6 + 6H2O + 24h+ → 6CO2 + 24H+ −0.015 H2 + 2h+ → 2H+ 0.006 CH3OH + H2O + 6h+ → CO2 + 6H+ 0.037 C2H5OH + 2h+ → C2H6 + 2CO2 + 2H+ 0.088 2CH3COOH + 2h+ → C2H6 + 2CO2 + 2H+ 0.129 CH4 + 2H2O + 8h+ → CO2 + 8H+ 0.1710 C2H5OH + 2h+ → CH3CHO + 2H+ 0.1911 H2O + 2h+ → 1/2O2 + 2H+ 1.23

The remaining photogenerated holes will then oxidize water to produce O2. The wateroxidation reaction is quite challenging because it requires the removal of four protons andfour electrons and the formation of an oxygen–oxygen double bond. The investigationon the water oxidation half-reaction is quite meaningful for developing novel materialswith the capability of oxidizing water, especially for the development of materials forestablishing the Z-scheme water-splitting system, which is defined in more detail in thefollowing section.40 Inmany cases, even photocatalysts are capable of splittingwater intoH2 and O2 separately in the presence of different sacrificial reagents; they are not liablefor overall water splitting. This means even if the thermodynamic requirement for watersplitting can be satisfied, the reaction dynamics may not be met in real photocatalyticreactions. In this respect, the reduction and oxidation water-splitting reactions are quitedifferent from overall water-splitting reactions.

Overall Water Splitting with Z-Scheme System. Single-component photo-catalysts capable of realizing overall water splitting, especially those active under visiblelight, are quite limited. However, overall water splitting can be realized by constructinga Z-scheme system with suitable multicomponent semiconductors. Figure 14.6 showsthe schematic principles of overall water splitting in the Z-scheme system. There aretwo semiconductor photocatalysts that play different roles in the Z-scheme system, andboth are coupled by reversible redox mediators such as I−/IO3−. During photocatalyticreactions, both semiconductors are excited to produce electrons and holes. The holes ofone semiconductor (Figure 14.6a) react with the mediator with the reductive state andthe electrons react with protons to produce H2. The electrons of the other semiconductor(Figure 14.6b) react with the mediator with the oxidative state to restore the mediatorto the reductive state and the holes react with water to produce O2. Therefore, with theassistance of reversible redox mediators, two semiconductors are coupled and H2 and O2can evolve on the two semiconductors, respectively. For example, Abe et al. constructeda Z-scheme system composed of TiO2-anatase, TiO2-rutile, and I−/IO3−.41 Water reduc-tion to H2 and I− oxidation to IO3− occur on Pt-loaded TiO2-anatase. Water oxidation


Figure 14.6. Schematic principles of overall water splitting in the Z-scheme system.

to O2 and the restoration of IO3− to I− occur on TiO2-rutile. With the help of I−/IO3−,overall water splitting can be realized in the Z-scheme even though Pt/TiO2-anataseand TiO2-rutile alone cannot split pure water. Because semiconductors in the Z-schemesystem only need to produce H2 or O2, the requirements for the band structures of thesesemiconductors are greatly lessened. This is quite meaningful because many photocata-lysts that cannot split pure water alone can be utilized to construct the Z-scheme system,which substantially extends the photocatalyst gallery for overall water splitting. This isespecially true when the Z-scheme system is constructed with visible light-responsivesemiconductors.

Dye-Sensitized Water Splitting. In the semiconductor-based photocatalyticsystem, semiconductor materials usually work as the light-absorbing component. How-ever, UV-responsive semiconductor photocatalysts can only utilize UV light. Sensiti-zation of semiconductors with dye can induce visible light activity on UV-responsivesemiconductors. Figure 14.7 shows the schematic principles of water reduction throughsensitization. Upon the absorption of visible light, dye is excited. The excited state of dyethen injects electrons to the conduction band of semiconductors, where photocatalytic H2production reactions occur. The oxidized dye is finally regenerated with electron donors.In essence, it is very similar with photocatalytic H2 production in the presence of sacrifi-cial reagent. However, dye works as the light absorbance component and semiconductorworks as the active sites for photocatalytic reactions. It should be noted that noble metalshould be loaded on the surface of semiconductors to catalyze H2 evolution. In thisdye sensitization scheme, dye, semiconductors, and electron donors are three importantcomponents. Different noble metals (especially ruthenium) and transition metal-baseddyes, and metal-free dyes have been successfully used as sensitizers for photocatalyticH2 production. Photocatalysts such as Pt/TiO2 are usually used as the semiconductorsubstrate. The type of dye and semiconductor, and their interaction between dye and


Figure 14.7. Schematic principles of water splitting through dye sensitization.

semiconductor can greatly influence the charge transfer between them and therefore candetermine the photocatalytic performance of the dye-sensitized system.

Photocatalytic Performance Evaluation

There are three important considerations regarding the photocatalytic performance ofsemiconductors for water splitting: photocatalytic activity, long-term stability, and lightabsorption property.

Photocatalytic Activity. The photocatalytic activity of semiconductors for watersplitting can be easily determined by measuring the amounts of hydrogen and oxygengases evolved within a certain period during photocatalytic reactions, in which unitssuch as �mol·h−1 and �mol·h−1·g−1 are often used. However, it should be noted thatthe photocatalytic activity obtained with this method can be greatly influenced by a lotof factors such as the light source (Xe or Hg lamp with different intensity), reactioncell (different dimension, top-irradiation, inner-irradiation or side-irradiation), reactionmedia (water or different sacrificial reagents), and the amount of photocatalysts used.Therefore, readers should be carefulwhen comparing the reported results in the literature.

Quantum yield is another important indicator for the photocatalytic activity ofsemiconductors.42 The quantum yield is defined by Equation (14.4), indicating thepercentage of absorbed photons for designated reactions.

Quantum yield (%) = Number of reacted electrons

Number of absorbed photons× 100% (14.4)

However, it is difficult to determine the real amount of photons absorbed by aphotocatalyst in a dispersed system. To address this problem, all incident photons are


assumed to be absorbed by the photocatalyst and the corresponding quantum yield iscalled the apparent quantum yield (Eq. 14.5).

Apparent quantum yield (%) = Number of reacted electrons

Number of incident photons× 100%

= 2× Number of evolved H2 moleculesNumber of incident photons

× 100%

= 4× Number of evolved O2 moleculesNumber of incident photons

× 100%

(14.5)

For measuring apparent quantum yield, the number of reacted electrons can becalculated from the amounts of H2 or O2 molecules produced in photocatalytic reactionsand the number of incident photons can bemeasured using a thermopile or Si photodiode.Because the number of absorbed photons is usually smaller than that of incident light,the apparent quantum yield is estimated to be smaller than the real quantum yield. Inaddition to the quantum yield, solar energy conversion efficiency that is usually usedfor evaluation of solar cells can be used for evaluating the photocatalytic activity ofsemiconductors (Eq. 14.6).

Solar energy conversion efficiency (%) = Output energy of H2 evolved

Energy of incident solar light×100% (14.6)

However, this measurement unit is seldom used due to the extremely low valuesachieved on all the existing photocatalysts. Nevertheless, the solar energy conversionefficiency should be the most important standard if photocatalytic water splitting isanticipated to be applied in practical applications. It is estimated that at least 10% oftotal solar energy conversion efficiency should be achieved to satisfy the economicfeasibility of this technology.43

Photocatalytic Stability. The long-term stability is a very important indicatorregarding the photocatalytic performance of semiconductors besides their high photo-catalytic activity. Many semiconductors will undergo deactivation during photocatalyticreactions and may totally lose their activity after a long run. Photocorrosion is consid-ered to be the main reason causing the instability of photocatalysts, which is detrimentalto practical applications. To test the photocatalytic stability, a long-term time courseexperiment or a repeated experiment is usually conducted to find the possible reasonsand corresponding solutions to the degradation. As shown in Figure 14.8, RuO2/Ge3N4undergoes degradation with the reaction time.44 However, the Pt/TaON–Pt/WO3 photo-catalysts are very stable after repeated reactions for 25 h.45 From this aspect, Pt/TaON–Pt/WO3 is a better choice in the practical applications.

Light Absorption Property. Semiconductors can only be excited by incidentlight with energy higher than their band gaps. Therefore, the light absorption propertyof semiconductors determines their action range under solar light irradiation. In theory,


3200

evac.

H2O2N2

150

100

50

0

2.5

2

1.5

1

0.5

0

Reaction time (h) Time (h)A

mou

nt o

f gas

evo

lved

(m

mol

)

Am

ount

of g

ases

evo

lved

(m

mol

)

0

(a) (b)

5 10 15 20 250 5 10 15 20 25

Figure 14.8. Time courses of overall water splitting on (a) RuO2/Ge3N4 and (b) Pt/TaON–

Pt/WO3 photocatalysts.44,45

the smaller the band gap of the semiconductor is, the wider the solar spectrum can beabsorbed. From this aspect, semiconductors with small band gaps are good candidatesin the scheme of solar energy utilization.

Based upon the light absorption range, semiconductor photocatalysts can be gener-ally clarified into two groups: UV-responsive photocatalysts (wide band-gap semicon-ductors) and visible light-responsive photocatalysts (narrow band-gap semiconductors).UV-responsive photocatalysts are only active under light with wavelength shorter than400 nm, while visible light-responsive are active under light with wavelength higherthan 400 nm. With respect to the solar spectrum, only a small fraction (3–5%) of thelight energy lies in the UV region, whereas the visible light accounts for around 46% ofthe solar spectrum. Therefore, to utilize solar energy efficiently, it is highly desirable todevelop visible light-responsive photocatalysts. In the following part of this chapter, weintroduces semiconductor photocatalysts based upon their light absorption propertiestogether with their elemental compositions.

To investigate the light absorption range of semiconductors, ultraviolet-visible (UV-Vis) spectra of semiconductors are usually obtained. From Figure 14.9(a), it is evidentthat the TiO2 photocatalysts can only absorb light with wavelength shorter than 400 nm(UV-responsive) while TiNxOyFz can absorb light with wavelength more than 600 nm(visible light-responsive). Therefore, the light absorption property can be clearly demon-strated from the UV-Vis spectra. However, absorption of light does not simply justifythat photocatalytic reaction will occur. In many cases, the recombination of photogener-ated charges after light absorption will totally inhibit the reactions. Therefore, an actionspectrum is usually needed to study whether the absorption of light in a specified rangecan induce photocatalytic reaction. For example, in the action spectrum shown in Fig-ure 14.9(b), the TiNxOyFz photocatalyst is found to be active at wavelengths more than420 nm (visible light range) and the photocatalytic activity of the semiconductors isfound to correlate very well with the UV-Vis spectrum.46 This indicates that the reac-tion proceeds photocatalytically after light absorption. Therefore, action spectrum can


300

(a) (b)

(i)

(ii)(iii)

400 500

Wavelength (nm)Cut-off wavelength (nm)

Ku

bel

ka-M

un

k (a

.u.)

Ku

bel

ka-M

un

k (a

.u.)

Rat

e o

f g

as e

volu

tio

n (μm

ol h

–1)

600 700 400 450 500 550 600

50

40

30

20

10

0

Figure 14.9. (a) UV-Vis diffuse reflectance spectra for (i) anatase-TiO2, (ii) TiNxOy, and

(ii) TiNxOyFz and (b) dependence of rate of O2 evolution on TiNxOyFz photocatalyst with a

cut-off wavelength of incident light.46

provide useful information by combining the photocatalytic activity and light absorptionproperty of semiconductors.

General Synthesis Method of Semiconductor Photocatalysts

Solid-State Reaction. Solid-state reaction is themost widely usedmethod for thepreparation of semiconductors. By heating a precursor mixture of solid materials withsuitable compositions, solid materials will fuse together to form newmaterials at desiredtemperatures in a certain reaction time. The calcination temperature and time, and thephysical and chemical states of the precursor materials will affect the final states of theresulting materials. In most cases, the amounts of the precursor solid materials are in astoichiometric state based upon the compositions of the resulting materials. However, ifthe reaction elements are easily volatile at high temperatures, higher amounts of thesematerials are needed in the precursor. For example, a calcination temperature of morethan 1073 K is usually needed for the preparation of alkali titanates. This will lead to thevolatilization of alkali ions such as Na+, K+, Rb+, and Cs+ and less than stoichiometricamount of alkali ions in the final materials. Therefore, higher amount of alkali reactantswill be added to the precursor to compensate for this.

Solid-state reactions can be employed to prepare a variety of semiconductors. Whenpreparing oxides, the precursors are usually heated in an air environment. Nonoxidematerials such as sulfide and oxysulfide can be prepared by heating the precursor in aninert atmosphere or in a sealed reaction tube pumped to vacuum conditions. Nonoxidematerials can also be prepared with the following method. The oxide precursor is heatedin a reaction tube with flowing reactant gases such as ammonia (NH3) and hydrogensulfide (H2S). Oxygen atoms will be replaced with nitrogen or sulfur atoms under certainreaction conditions. This method is usually used to prepare novel nitride, oxynitride,


sulfide, and oxysulfide materials or to dope anions such as N and S in the lattice ofpristine oxides to introduce photocatalytic activity under visible light irradiation.

Solid-state reactions can prepare semiconductors with high crystallinity. However,the surface area of the as-prepared materials is quite low due to their sintering at hightemperatures. Moreover, due to the mixing problem, inhomogeneity will exist more orless in the resulting materials.

Polymerizable Complex Method. The polymerizable complex (PC) methodhas been widely used to prepare multicomponent oxides at relatively low temperatures.In essence, the PC method is the same as solid-state reactions. The only difference restswith a much better control on the reactant precursor in the PC method. The PC methodis based on the condensation polymerization reaction between ethylene glycol and citricacid (CA) in the presence of soluble metal–CA complexes. Complete polymerizationand loss of solvent leads to the transition from the liquid to a rigid solid gel phase. Thecomplexes are immobilized in a rigid polyester network homogeneously and allow for theformation of cationic species in the molecular level. By calcining the complex precursorin air, the different components will combine to form the desired materials. Comparedwith the conventional solid-state method, the PC method can result in powders withhigh crystallinity and surface area at relatively low calcination temperature and shortcalcination time. Moreover, inhomogeneity problems can be inhibited a lot with thismethod. Therefore, semiconductor photocatalysts prepared with PC methods usuallyshow better activity than those prepared with traditional solid-state reaction methods.

Hydrothermal and Solvothermal Methods. The hydrothermal method iswidely used for the preparation of semiconductor materials with small particle sizeand high crystallinity at low temperature. Hydrothermal synthesis of materials is nor-mally conducted in a steel autoclave with Teflon liners. The reactants are put into theTeflon liners containing water and then tightly sealed in the autoclave. By heating theautoclave in an oven or furnace, the internal pressure of the autoclave is increased andthe reactant will undergo chemical reactions under high pressures. For example, whenthe heating temperature is set to more than 373 K, the water will boil and the pressurein the autoclave will increase. The high pressure will facilitate the reactions that cannotbe realized under normal pressure at similar temperature. The reaction temperature, theamount of solution added to the autoclave, and the reactant will determine the inter-nal pressure produced. The physiochemical properties of the product can be modifiedby using-morphology controlling reagents, different reactants, and tuning the reactionconditions.

The solvothermal method is almost identical to the hydrothermal method exceptthat the solvent used here is the organic solvent such as ethanol and ethylene glycol.Different organic solvents have different boiling points. Therefore, when solvents withlow boiling points such as ethanol are used, higher pressure can be achieved at thesame temperature. Higher temperature synthesis can be realized with solvents with highboiling points such as ethylene glycol. Moreover, materials sensitive to water or thosecannot be synthesized in aqueous solution can also be prepared in an inert organicsolution such as benzene. Generally, the solvothermal method normally has a more


versatile control on the particle size, morphology, and the crystallinity of semiconductormaterials than the hydrothermal method. However, the use of organic compound in thesynthesis is expensive and may cause environmental problems.

Other Methods. The above three methods are most widely used for the prepara-tion of semiconductor materials for water-splitting applications. Several other methodssuch as sonochemical, microwave, electrodeposition, chemical vapor deposition, andphysical vapor deposition methods are also used for the synthesis of semiconductormaterials, either in powder or film form.

Methods of Loading Cocatalyst on Semiconductor Photocatalysts

Noble metals and some transition metals are widely loaded on semiconductors as cocat-alysts to catalyze H2 or O2 evolution from photocatalytic water splitting. The chemicaland physical states of cocatalysts such as particle size, dispersibility, contact with pho-tocatalysts, and valence states are greatly influenced by the loading method, which iscrucial for the catalytic performance of the cocatalysts. Two methods are generally usedto load cocatalysts on semiconductors. The first one is called in situ photodepositionmethod. In this method, semiconductors are dispersed in a precursor solution of cocata-lysts in the presence of sacrificial reagents. Under light irradiation, the photogeneratedholes are consumed by the sacrificial reagents, and the photogenerated electrons willreduce the cocatalyst precursor on the surface of semiconductors. For example, for theloading of Pt on TiO2, TiO2 is dispersed in H2PtCl6 solution in the presence of methanolaqueous solution. Under UV irradiation, methanol will react with photogenerated holesandH2PtCl6 will be reduced by the photogenerated electrons to Pt on the surface of TiO2.In some cases, this method can be used to prepare cocatalysts with core–shell structureswhen photoreduction steps are carried out sequentially in different precursor solutions.The second method can be divided into two steps: the impregnation and postcalcinationsteps. Semiconductors are first dispersed in a precursor solution of cocatalysts followedby the evaporation and drying process. The as-obtained solid materials are then cal-cined under different atmosphere such as air and H2 to decompose the precursor to thedesired state. The calcination temperature, time, atmosphere, and type of precursors areimportant factors for the final state of photocatalysts. Pt/TiO2 photocatalysts can alsobe prepared with this method. After the impregnation of TiO2 with H2PtCl6 solution,the H2PtCl6/TiO2 will be calcined in H2 to decompose H2PtCl6 to Pt on the surface ofTiO2. When cocatalysts were treated consecutively in different atmosphere, cocatalystswith core–shell structure could also be obtained. For example, the loading of NiOx onsemiconductors usually follows H2 reduction and subsequent O2 oxidation treatments.These treatments could form a NiO/Ni double-layer structure and facilitate the electrontransfer from a photocatalyst substrate to the Ni/NiO cocatalyst and lead to enhancedphotocatalytic activity.

After introducing the basic knowledge about the semiconductor catalysts for watersplitting, we give an overview on the semiconductor nanocatalysts in the following parts.We begin with a general classification of the elements constructing the semiconductormaterials. Then we review the nanocatalysts by dividing them into UV-responsive and

ELEMENTS CONSTRUCTING SEMICONDUCTOR PHOTOCATALYSTS 511

visible light-responsive photocatalysts based upon the light absorption property. In everycategory, semiconductor materials are reviewed in detail based upon their elementalcompositions. Moreover, cocatalysts are discussed in combination with photocatalystsas an inseparable part of the whole photocatalytic system. It should be noted thatphotocatalysts capable of splitting water with the different pathways shown in Section“Types of Photocatalytic Water Splitting” are discussed together in this chapter.

ELEMENTS CONSTRUCTING SEMICONDUCTORPHOTOCATALYSTS

A number of elements in the periodic table contribute to the formation of semiconductormaterials under investigation. Kudo and Miseki have given an excellent summary aboutthese elements. As shown in Figure 14.10, these elements can be well classified into fourgroups based on their functions related with the construction of crystal and/or energystructures in the semiconductors.28

The first group of elements contributes to the construction of crystal and energystructures of semiconductors directly. Metal cations with d0 and d10 configurations andsome anions are among this type. These elements constitute most of the metal oxide,oxysulfide, and oxynitride photocatalysts. For these photocatalysts, the conduction bandsare usually composed of d and sp orbitals of metal elements, while the valence bandsare composed of O 2p, S 3p, and N 2p orbitals, respectively. In some cases, Cu3d, Ag4d, Pb 6s, Bi 6s, and Sn 5s orbitals can also form valence bands in some metal oxideand sulfide photocatalysts.

H

Li Be

Na

K

Mg

Ca Sc

Rb Sr Y

Cs Ba La Hf

Ti V

Zr Nb

Mn Fe Co

Tc Ru

Re Os Ir Pt Au

Cu

Pd Ag

Zn Ga Ge

Cd In Sn

Hg Tl Pb

Br Kr

I Xe

At RnBi Po

As Se

Sb Te

Ta W

Mo

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Ce

: d0 ion: d10 ion

Pr

: Nonmetal

Nd

(i)

(ii)(iii)(iv)

Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

15 16 17 18

B C

Al Si

F Ne

He

Cl ArP S

N O

Cr Ni

Rh

to construct crystal structure and energy structure

to construct crystal structure but not energy structureto form impurity levels as dopantsto be used for cocatalysts

Figure 14.10. Elements constructing semiconductor photocatalysts.28


The second group of elements contributes to the construction of crystal structurebut not energy structure. Alkali, alkaline earth, and some lanthanide ions are amongthis type. These elements can greatly influence the crystal structures of the resultingsemiconductor materials, while they do not participate in the formation of the conductionor valence bands directly. However, these elements can influence the energy structuresof the resulting materials indirectly. For example, Sr2+ in SrTiO3 does not contribute tothe formation of the conduction band or valence bands. However after introducing Sr inTiO2, the conduction band of SrTiO3 is elevated to a more negative position comparedwith that of TiO2, improving the reduction ability of the photogenerated electrons.

The third group of elements contributes to the formation of impurity levels as dopantsin the lattice of semiconductors to modify the energy structures. Some transition metalcations such as Cr3+, Ni2+, and Rh3+ can substitute pristine cations in the crystal latticeto form impurity levels in band gaps. This kind of doping can decrease the band gap ofpristine semiconductors and is usually employed to modify the band structures of wideband-gap semiconductors toward visible light response.

The fourth group of elements is mainly used as cocatalysts for water-splittingreactions. Materials composed of these elements are simply loaded on the surface of thesemiconductors and do not contribute to the construction of band or energy structures.Some transition metals and oxides such as Pt, Ru, Rh, Pd, Au, RuO2, IrO2, and NiO areoften used as cocatalysts to catalyze H2 or O2 evolution. However, it should be notedthat Pt is also good catalysts for the backward reaction between H2 and O2. To avoid thebackward reaction in overall water-splitting reactions,modification of the cocatalysts andproper selection of reaction conditions are important. For example, backward reactionhardly occurs on NiO and RuO2, and the presence of high concentration of Na2CO3 inthe can greatly inhibits the backward reaction on Pt.

Up to now,more than 100 semiconductormaterials based on the above elements havebeen reported. Considering the variety and complexity of the photocatalyst gallery, onlytypical photocatalysts are reviewed in this chapter. Photocatalysts with quite complexcompositions and low photocatalytic activity are not mentioned. Readers could refer tothe several excellent review papers or books previously mentioned in this chapter formore detailed information.

UV-RESPONSIVE SEMICONDUCTOR NANOCATALYSTS FORWATER SPLITTING

Titanium (Ti)-Based Oxides

TiO2 is the most extensively investigated photocatalyst since its first report in photoelec-trochemical water splitting. TiO2 crystallizes in three structure forms—rutile, anatase,and brookite—with TiO6 octahedra as the building unit. Different form of TiO2 hasslightly different band gaps of around 3.2 eV due to the variation of the crystal struc-tures. The conduction band of TiO2 is only slightly higher than the reduction potential ofwater. Similar with most photocatalysts, TiO2 alone is not active for pure water splitting.When loaded with appropriate cocatalysts, TiO2 can produce H2 and/or O2 from purewater or aqueous solutions containing sacrificial reagents under UV irradiation.

UV-RESPONSIVE SEMICONDUCTOR NANOCATALYSTS FOR WATER SPLITTING 513

Pt is usually loaded on TiO2 to catalyze H2 evolution for water splitting. In 1980,Sato and White found that when Pt cocatalyst was loaded on TiO2 via photoreductionof hexachloroplatinate in an acetic acid solution, drastically enhanced water-splittingefficiency was obtained.47 When TiO2 is simultaneously loaded with Pt and RuO2nanoparticles, pure water can be split stoichiometrically into H2 and O2 with a quantumyield of 30± 10% under UV light irradiation.48 In this Pt–RuO2/TiO2 bifunctional pho-tocatalyst, Pt works as a water reduction cocatalyst and RuO2 works as a water oxidationcocatalyst. The presence of RuO2 can facilitate the transfer of the photogenerated holefrom the valence band of TiO2 to the reaction media and the subsequent water oxida-tion reactions. However, backward reaction between H2 and O2 can easily occur on Pt,especially at high partial pressure, which will greatly impair the photocatalytic activityof Pt/TiO2. The addition of high concentration of carbonate salts to pure water can dra-matically enhance the photocatalytic activity for water splitting. It is proposed that thebackward reaction on Pt is suppressed effectively in the presence of carbonate ions andthe intermediate surface-adsorbed peroxycarbonate species promote the desorption ofO2 from TiO2 surface.33 A similar effect was observed for different oxide photocatalystssuch as Ta2O5 and SrTiO3 in the presence of aqueous Na2CO3 solution. On the otherhand, NiO/TiO2 photocatalyst cannot split pure water; however, it can split water intoH2 and O2 in the presence of aqueous NaOH (3 M) solution. The presence of NaOH isproposed to make the surface of TiO2 more favorable for water-splitting reactions.49

In order to achieve high photocatalytic performance on TiO2, different methodswere used to modify the physical as well as chemical properties of TiO2. The dopingof Nb5+ in TiO2 was reported to achieve enhanced water-splitting efficiency on Pt–RuO2/TiO2 photocatalysts.50 The doping of Ni2+ in the framework of TiO2 was foundto enhance the photoactivity of the TiO2 for hydrogen production from an aqueousmethanol solution.51 Lanthanide ion doping in TiO2 was systematically investigated andTiO2 doped with 0.5 mol % of Gd oxide gave the best performance for H2 evolutionfrom an aqueous methanol solution. A single-phase B/Ti binary oxide was obtainedwith a sol–gel method, and the modification of TiO2 with boron doping was found tobe effective for photocatalytic decomposition of pure water.52 When TiO2 nanoclus-ters were highly dispersed in the framework of mesoporous materials, the as-preparedphotocatalysts showed higher photocatalytic activity for hydrogen evolution in aqueousmethanol solution under UV irradiation than bulk TiO2.53,54 When TiO2 was coupledwith appropriate semiconductors such as SrTiO3 and ZrO2, the as-obtained compos-ites with heterophase structures exhibited higher rates of H2 evolution in the presenceof different kinds of sacrificial reagents under UV irradiation. The coupling betweenTiO2 and semiconductors with appropriate band structures is supposed to facilitate theintercharge transfer between the two components and reduce the charge recombination,resulting in enhanced photocatalytic efficiency of the composite material.

In recent years, a great research effort has been devoted to control crystal and surface-phase structures of TiO2. Among the three crystal phases, anatase and rutile phases ofTiO2 are commonly used in photocatalysis. The single anatase phase of TiO2 is reportedto show an overall higher photocatalytic activity than the single rutile phase withoutconsidering the effect of surface area.However,much higher efficiency could be achievedwhen mixed anatase and rutile-TiO2 are used. Li’s group systematically investigated the


anatase–rutile mixed phase system for photocatalytic H2 production.55 They found thatthe photocatalytic activity of TiO2 nanoparticles was directly related to the surface-phasestructure. The phase junction formed between the surface anatase and rutile particlescould facilitate the charge transfer and greatly enhance the photocatalytic activity forphotocatalytic H2 production in methanol aqueous solution under UV light. Therefore,the deliberate control synthesis of the surface-phase junction between anatase and rutilephases is an important strategy for promoting the efficiency of TiO2. Facet engineeringof TiO2 is another important strategy for modifying the physical and chemical propertiesof TiO2. It is reported that the average surface energy of {001} facet of anatase-TiO2is the highest among all the investigated facets. Therefore, {001} facet of anatase-TiO2 is supposed to be the most active facet for photocatalysis, and the preparation ofshape-controlled anatase-TiO2 crystals with high percentage of {001} facet is highlydesirable. In 2008, Yang et al. prepared anatase crystals with 47% of {001} facetsusing hydrofluoric acid as a capping agent under hydrothermal conditions.56 Followingthis work, extensive efforts have been devoted to the preparation of TiO2 with highpercentage of reactive {001} facet.57, 58 Up to now, anatase-TiO2 with nearly 100% of{001} facets has been synthesized, and the thickness of the anatase crystals has beendecreased to less than 2 nm along the [001] direction. The as-obtained anatase-TiO2 withhigher percentage of reactive {001}was found to have higher photocatalytic activity forH2 production from water splitting in the presence of sacrificial reagents.

Titanate was commonly obtained through solid-state reactions by alloying TiO2with different metal ions, especially with alkali, alkaline earth, and lanthanide ions.Most of these titanates have large band gaps and are only active under UV light. Amongthese titanates, SrTiO3 has been widely investigated as a photocatalyst for water split-ting. SrTiO3 crystallizes in the perovskite structure type with a band gap of 3.2 eV. Theconduction band level of SrTiO3 is much higher than that of TiO2, reflecting the higherreduction ability of the photogenerated electrons formed in the conduction band. SrTiO3was initially investigated as a photoanode material in a water-splitting electrochemi-cal cell. When combined with p-type photocathodes, a photon-to-electron conversionefficiency of less than 1% could be achieved without applied bias.59 This is differentfrom the photoelectrochemical cell using the TiO2 photoanode, where applied bias isneeded. Domen and coworker systematically investigated the water-splitting propertiesof SrTiO3 loaded with NiO cocatalysts in aqueous solutions. SrTiO3 alone is not activefor pure water splitting. After loading NiO cocatalysts on SrTiO3, SrTiO3 can split purewater into H2 and O2 stoichiometrically in both liquid and vapor phases under UVirradiation.60,61 Similar to TiO2, the activity of SrTiO3 for overall water splitting canbe improved with the addition of NaOH to the reaction solution. The performance ofNiO/SrTiO3 can be further improved by a pretreatment of reduction by H2 and subse-quent reoxidation by O2.61 The H2 reduction and subsequent O2 oxidation treatmentcould form a NiO/Ni double-layer structure and facilitate the electron transfer from aphotocatalyst substrate the to Ni/NiO cocatalyst. The SrTiO3 powders with high crys-tallinity and surface area could be prepared with a polymerized complex method anddemonstrated higher activity than those prepared with a traditional solid-state reactionmethod. Doping with some metal cations of appropriate amounts could also improve thephotocatalytic performance of SrTiO3. Inoue et al. investigated the effect of different


cocatalysts on the water-splitting performance of SrTiO3. Overall water splitting can beachieved on M/SrTiO3 with the activity order of Rh�Ru�Re� Pt� Ir� Pd�Os�Co. When tuning the ratios of Sr to Ti, different derivatives of SrTiO3 such as Sr3Ti2O7could be obtained and were also found to be able to split pure water when loaded withNiO as cocatalysts. When Sr was replaced with other alkaline earth metals, differentphotocatalysts could be formed. Inoue systematically investigated the photocatalyticproperties of a series of barium titanate materials with different Ti-to-Ba ratios.62 Thestructures of these materials contain tunnels occupied by barium metal ions. Amongthese barium titanates, RuO2/BaTi4O9 was found to be able to split pure water stoi-chiometrically into H2 and O2 under UV irradiation, while other materials could not.BaTi4O9 prepared with a PC method demonstrates a photocatalytic activity five timeshigher than that of a sample prepared by a traditional solid-state reaction. The increaseof the surface area is found to be an important factor influencing the photocatalyticactivity. However, controlling the lattice defects through high-temperature calcinationsis also quite important. Mizoguchi et al. investigated the photocatalytic water-splittingproperty of CaTiO3.63 CaTiO3 has band-gap energy of 3.5 eV. Under UV irradiation,CaTiO3 alone only shows trace activity for O2 production. The loading of differentcocatalysts was found to promote the activity of CaTiO3 for pure water splitting with theorder of Pt�Ru� Ir�Ni�Au. After doping Zr4+ in CaTiO3, the activity of CaTiO3can be further enhanced.

When TiO2 was alloyed with alkali ions, several alkali-metal titanates with typi-cal layered structure could be obtained. These titanates consist of layers of edge- andcorner-shared TiO6 octahedra, which are separated by layers of alkali ions. The alkaliions can be exchanged with protons through ion-exchange reactions. Onishi et al. inves-tigated several layered titanates such as Na2Ti3O7, K2Ti2O5, K2Ti4O9, and K2Ti6O13and found that these materials were active for photocatalytic H2 evolution from aqueousmethanol solutions. When alkali ions were exchanged with protons, higher activity wasobtained on H+–K2Ti2O5 and H+–K2Ti6O13 materials, and a quantum yield of 10%was obtained on H+–K2Ti2O5 at 330–360 nm. Kudo et al. studied a series of cesiumtitanates with various Cs-to-Ti ratios. When loaded with Pt cocatalysts, Cs2Ti2O5 withthe five-coordinate structure showed higher photocatalytic activity for H2 evolution fromaqueous methanol solution than Cs2Ti5O11 and Cs2Ti6O13 with six-coordinate structuresunder UV irradiation. The unsaturated coordination state of the five-coordinate structureis supposed to be the active sites for photocatalytic reactions. Moreover, the interlayersof Cs2Ti5O11 are more easily hydrated than those of Cs2Ti6O13, which is the otherreason for the higher activity obtained on Cs2Ti5O11. Inoue et al. studied a series ofalkali-metal titanates with a chemical formula of M2Ti6O13 (M = Na, K, Rb, and Cs)and found that a close relationship exists between the ability of produced photoexcitedcharges and photocatalytic activity. RuO2/M2Ti6O13 (M = Na, K, and Rb) with rect-angular tunnel structures showed higher photocatalytic activity than RuO2/Cs2Ti6O13with a layered structure. The high dipole moment present in distorted TiO6 octahedra isfound to contribute to the higher photocatalytic activity.

Kim et al. studied a series of lanthanum titanates oxides with different Ti-to-Laratios.64 La2TiO5, La2TiO5, and La2Ti2O7 with perovskite structure can produceH2 fromwater splitting with the activity order of La2Ti2O7 � La2TiO5 � La2TiO5. Moreover,


these materials with typical layered structure were found to exhibit higher photocatalyticwater-splitting activity than LaTiO3 under UV irradiation. The doping of alkaline earthions such as Ba, Sr, and Ca in La2Ti2O7 was found to improve the photocatalytic activitymarkedly. Particularly, after loading suitable amount of NiO as cocatalysts, Ba-dopedLa2Ti2O7 was reported to produce H2 with a quantum yield of close to 50% in thepresence of high concentration of NaOH additive under UV irradiation. Pr2Ti2O7 andNd2Ti2O7 with similar layered structure were also reported to exhibit activity for watersplitting. Abe et al. prepared a series of M2Ti2O7 (M = Y, Eu–Lu) with polymerizedcomplex methods and investigated their photocatalytic water-splitting properties forthe first time. Different from La2Ti2O7, these lanthanide titanates are cubic pyrochlorestructure. Among Ln2Ti2O7 (Eu–Lu) with partly filled 4f orbitals, only NiO/Lu2Ti2O7shows comparable activity for pure water splitting with nonstoichiometric H2-to-O2ratios. Other NiO/Ln2Ti2O7 (Eu–Lu) exhibited extremely low activity for pure watersplitting. However, NiO/Y2Ti2O7 exhibited quite high activity for pure water splitting toH2 and O2 in a stoichiometric ratio under UV irradiation. When Y2Ti2O7 was preparedwith the addition of excess amount (5%) of Y during its synthesis, higher activitywas obtained. This is due to the inhibition of the formation of impurity TiO2-rutile onthe surface of catalysts by the excess Y. After investigating a series of photocatalystssuch as Ln3MO7 and Ln2Ti2O7 (R = Y, Gd, and La), Abe et al. found that materialscomposed of a network of corner-shared octahedral units of TiO6 were active for watersplitting, while those without such a network were inactive. Therefore, the octahedralnetwork is supposed to increase the mobility of electrons and holes, thereby enhancingphotocatalytic activity.

When TiO2 was alloyed together with alkali (or alkaline earth) and lanthanide, morecomplex Ti-based photocatalysts could be obtained. Domen et al. investigated a seriesof photocatalysts with chemical formula of M2La2Ti3O10 (M = K, Rb, and Cs).65,66

These compounds are ion-exchangeable layered perovskites with alkali ions intercalatedbetween the La2Ti3O10 layers. K2La2Ti3O10 exhibited a high activity for overall watersplitting when loaded with cocatalysts under UV irradiation. When the reaction wascarried out in aqueous KOH solution, a high efficiency was obtained. Moreover, theactivity could be doubled if K2La2Ti3O10 was synthesized with a polymerized complexmethod instead of the conventional solid-state reaction method. NiO/Rb2La2Ti3O10 andNiO/Cs2La2Ti3O10 exhibited even higher photocatalytic activity for water splitting thanNiO/K2La2Ti3O10. And the quantum yield for water splitting on NiO/Rb2La2Ti3O10was estimated to be approximately 5% at around 330 nm. When part of Ti4+ wasreplaced with Nb5+ in M2La2Ti3O10 (M = K, Rb, and Cs), the activities of the result-ing M2–xLa2Ti3–xNbxO10 decreased dramatically. Miseki et al. investigated a series ofphotocatalysts with chemical formula of MLa4Ti4O15 (M = Ca, Sr, and Ba) with a(111) plane-type layered perovskite structure.67 The band gaps of these compounds areestimated to be 3.7–4.1 eV. When loaded with NiO as cocatalysts, these compoundsdemonstrated high photocatalytic activities for water splitting under UV irradiation, andNiOx/BaLa4Ti4O15 showed the highest activity for water splitting with a quantum yieldof 15% at 270 nm. The photocatalytic activities of MLa4Ti4O15 (M = Ca, Sr, and Ba)were found to depend upon the type of the alkaline earth metal ions. It is supposed that


Interlayer I H2

H2

O2O2Interlayer II

: Ni cocatalysts

: NbO6 unitInterlayer I

Figure 14.11. Schematic structure and proposed reaction mechanism of H2O splitting on

NiO/K4Nb6O17 photocatalysts.28

the difference of the surface properties of MLa4Ti4O15 (M = Ca, Sr, and Ba) is one ofthe important factors influencing the photocatalytic activity.

Niobium (Nb)-Based Oxides

Besides Ti-based oxide materials, a plentiful of Nb-based materials can be obtained byalloyingNb2O5 with alkali, alkaline earth, and lanthanide ions,which forms another largegroup of water-splitting photocatalysts responsive to UV light. The band-gap energy ofNb2O5 is about 3.4 eV and inactive for pure water splitting under UV irradiation whenloaded with Ni as the cocatalyst. However, in the presence of methanol as the sacrificialreagent, Nb2O5 can efficiently produce H2 after loading it with Pt cocatalysts.

The simplest alkali niobates with a chemical formula of MNbO3 (M = Li, Na,and K) have been investigated for water-splitting reactions by various researchers.68–70

Under UV irradiation, LiNbO3 with a hexagonal structure is active for photocatalytic H2production using formic acid as the sacrificial reagent even without loading cocatalystson it. NaNbO3 alone demonstrated activity for O2 production. When loaded with Ptas cocatalysts, NaNbO3 can produce H2 efficiently in the presence of methanol as thesacrificial reagent. When loading RuO2 as cocatalysts, NaNbO3 can split pure waterinto H2 and O2. Moreover, the NaNbO3 sample prepared by a PC method exhibits thehighest photocatalytic activity compared with samples prepared with the hydrothermalmethod and solid-state reactions.69 When loaded with Pt as cocatalysts, KNbO3 witha perovskite structure exhibited high activity for photocatalytic H2 production in thepresence of methanol as the sacrificial reagent. Moreover, KNbO3 nanowires preparedwith a hydrothermal method demonstrated higher activity than those with nanocube andpowder morphologies. A variety of alkali niobates could be obtained by changing themolar ratios of alkali to Nb. One typical example is a potassium niobate photocatalystwith a chemical formula of K4Nb6O17.71–74 The band gap of K4Nb6O17 is about 3.3 eV.It is composed of niobium sheets, in which potassium ions are intercalated between theinterlayers of niobium sheets (Figure 14.11). There are two types of interlayers. Onecontainswatermolecules and potassium ions, and the other contains only potassium ions.The potassium ions between the niobium oxide layers can be exchanged with many othercations such as protons and transition metal ions. Unlike TiO2 and SrTiO3, K4Nb6O17


alone is highly active for H2 production in the presence of methanol as the sacrificialreagent. When pure water was used as the reaction media, only small amount of H2 wasobserved while no O2 was detected. After loading NiO as the cocatalyst on K4Nb6O17,water can be decomposed into H2 and O2 stoichiometrically with an activity of onemagnitude higher than that with bare K4Nb6O17. Moreover, the photocatalytic activityof NiO/K4Nb6O17 was doubled with a ball-milling treatment, which was ascribed tothe decreased particle size and increased surface area of the resulting photocatalyst.When KOH and NaOH were added in water, NiO/K4Nb6O17 could split pure waterinto H2 and O2 with a quantum yield of 5.3% (330 nm) under optimum conditions.The high activity of NiO/K4Nb6O17 is ascribed to its unique “two-dimensional” layeredstructure (Figure 14.11), in which H2 evolution proceeds in one interlayer, with a nickelcocatalyst, and O2 evolution occurs in another interlayer, leading to the separation ofthe H2 and O2 evolution sites. When potassium ions in the interlayer of K4Nb6O17were exchanged with ions such as H+, Cr3+, and Fe3+, the activity of the as-exchangedK4Nb6O17 was drastically enhanced. The H+-exchanged K4Nb6O17 demonstrated thehighest photocatalytic activity for H2 evolution from aqueous methanol solution with aquantum yield of 30% at 330 nm. Rb4Nb6O17 has similar structure and band gap withthose of K4Nb6O17. When loaded with NiO as cocatalysts, Rb4Nb6O17 exhibited highactivity for overall water splitting with a quantum yield of 10% at 330 nm.75 Cs2Nb4O11,which is composed of NbO6 octahedra and NbO4 tetrahedra, has a band gap of 3.7 eV.When loaded with NiO as cocatalysts, Cs2Nb4O11 could split pure water into H2 and O2with an activity similar with that of K4Nb6O17 under similar reaction conditions.76

Several alkaline earth niobates have been investigated as photocatalysts for photo-catalytic water splitting. Highly donor-doped Ca2Nb2O7 with layered perovskite struc-ture has a band gap of 4.3 eV.77 Under UV irradiation, Ca2Nb2O7 loaded with NiOcocatalysts can produce H2 from pure water with a quantum yield of 7% at 290 nm.Similarly, the Sr2Nb2O7 with isostructure can produce H2 from pure water splitting witha quantum yield of 23% at 300 nm. It is supposed that these highly donor-doped (110)perovskites could create a narrower depletion layer than undoped perovskites, allowingmore efficient charge separation and enhancing the efficiency of water splitting as aresult.77 SrNb2O6 nanorods prepared with a simple hydrothermal method were reportedto split pure water into H2 and O2 under UV irradiation when loaded with RuO2 ascocatalysts. The SrNb2O6 nanorods showed higher photocatalytic activity than thoseprepared with traditional solid-state reactions due to the higher surface area in the for-mer. Sr5Nb4O15 was also reported to exhibit high activity for pure water splitting underUV irradiation. Ba5Nb4O15 with distorted perovskite structure contains NiO6 octahedralayers separated by Ba2+ cations. The band gap of Ba5Nb4O15 is 3.9 eV. When NiOwas loaded as cocatalysts, Ba5Nb4O15 prepared with a solid-state reaction method couldsplit pure water stoichiometrically into H2 and O2 under UV irradiation. The activitywas further enhanced four times when Ba5Nb4O15 was prepared with a PC method,achieving a quantum yield of 8% at 270 nm. The appearance of polarization due to thedistorted perovskite structure was supposed to be one of reasons for the high activity ofBa5Nb4O15.78

Arakawa et al. investigated the photocatalytic water-splitting abilities of lanthanideniobates with a chemical formula of R3NbO7 (R = Y, Gd, and La). All the materials


were prepared with a PC method and had a similar band gap of 3.9 eV. La3NbO7with an orthorhombic weberite structure was active for overall water splitting underUV irradiation when loaded with NiO as cocatalysts, while other materials such asY3NbO7 with a fluorite cubic structure, and Gd3NbO7 and La3NbO7 with a cubicpyrochlore structure were inactive. The presence of corner-shared octahedral units ofNbO6 in La3NbO7 with an orthorhombic weberite structure was supposed to increase themobility of both photogenerated electrons and holes, as a result inducing photocatalyticwater-splitting activity in La3NbO7.

When niobium oxide was alloyed together with alkali and alkaline earth (or lan-thanide) oxides, Nb-based photocatalysts with more complex compositions could beobtained.Domen et al. investigated a series of ion-exchangeable niobateswith a chemicalformula ofA(Mn–1NbnO3n+1) (A=Na,K,Rb, andCs;M=Ca, Sr, andLa).79 ALaNb2O7and ACa2Nb3O10 photocatalysts alone were active for photocatalytic H2 productionunder UV irradiation in the presence of methanol as the sacrificial reagent.80 After load-ing Pt as the cocatalyst, the photocatalytic activities of ALaNb2O7 and ACa2Nb3O10(A= K, Rb, and Cs) were greatly enhanced. When the alkali ions were exchanged withprotons, the resulting H+-exchanged materials demonstrated greatly enhanced activ-ity either with or without Pt cocatalysts, and the photocatalytic activities increasedwith the degree of the exchanged alkali ions. The increased degree of the exchangedalkali ions can facilitate the migration of hydrated water molecules and methanol to theinterlayer spaces, which is responsible for the enhanced photocatalytic H2 evolution.The importance of the migration of water and reactants into the interlayer space wasdemonstrated by the fact that photocatalytic H2 evolution decreased in the presenceof aliphatic alcohols, sacrificial reagents with long chains. When the alkali precursorsuch as K2LaNb2O7 was prepared with a polymerized complex method, the as-obtainedproton-exchanged H2LaNb2O7 showed high activity for H2 evolution than that preparedfrom traditional solid-state reactions. ALaNb2O7 and ACa2Nb3O10 (A=K, Rb, and Cs)were also active for O2 evolution in the presence of AgNO3 as the sacrificial reagent.However, the activity is quite low compared with that of H2 evolution. KCa2Nb3O10 wastested for photocatalytic splitting of pure water. Only little amount of H2 was detectedand no O2 was observed on KCa2Nb3O10 alone. When KCa2Nb3O10 was modified withRuO2 by the traditional impregnation–calcination method, pure water could be split intoH2 and O2 with little activity. The water-splitting activity of KCa2Nb3O10 was signifi-cantly enhanced by depositing RuO2 cocatalysts on a restacking aggregate of exfoliatedCa2Nb3O10 nanosheets prepared through flocculation with NaOH or KOH aqueoussolutions. In this case, pure water can be split into H2 and O2 with medium efficiencystoichiometrically. Other materials such as KSr2Nb3O7 and HSr2Nb3O7 are reported tobe highly active for H2 production in the presence of methanol as the sacrificial reagent.

Tantalum (Ta)-Based Oxides

A plentiful of Ta-based materials similar with those of Nb-based materials can beobtained by alloying Ta2O5 with alkali, alkaline earth, and lanthanide ions, which formanother large group of water-splitting photocatalysts responsive to UV light. How-ever, the conduction band levels of Ta-based materials are more negative than their


LiTaO3

LiTaO3

NaTaO3

NaTaO3

KTaO3

KTaO3NiO

(a) (b)

ilmeniteTa-O-Ta angle

Distortion Large Middle Small

Energydelocalization

Low Middle High

Band gap 4.7 eV 4.0 eV 3.6 eV

4.7 eV 4.0 eV 3.6 eV 3.6 eV

Pot

entia

l / e

V v

s N

HE

143° 163° 180°perovskite perovskite

CB

ET H2 evolution site

VB

–1

0

1

2

3

O2/H2O

H+/H2

Figure 14.12. Crystal (a) and band (b) structures of MTaO3 (M = Li, Na, and K).28

corresponding Nb-based materials with similar crystal structures, leading to differentphotocatalytic performance when comparing the two materials. The band gap of Ta2O5is about 4.0 eV. Without loading cocatalysts on it, Ta2O5 can only produce trace amountof H2 and no O2 from pure water splitting under UV irradiation. However, after mod-ification with NiO, Ta2O5 shows high activity for pure water splitting.81 MesoporousTa2O5 with amorphous wall was found to be more active in overall water splitting thanits crystallized counterpart after loading NiO as a cocatalyst.82

Kudo et al. systematically investigated a series of alkali tantalates with a chemicalformula of MTaO3 (M = Li, Na, and K) for photocatalytic water-splitting reactions.All the MTaO3 (M = Li, Na, and K) consist of corner-sharing TaO6 octahedra withperovskite-like structure type (shown in Figure 14.12), and their band gaps are 4.7 eV(LiTaO3), 4.0 eV (NaTaO3), and 3.7 eV (KTaO3), respectively, as determined fromdiffuse-reflectance spectra. In this study, MTaO3 (M= Li, Na, and K) were prepared byconventional solid-state reactions. Excess (5–10%) alkali precursor was used during thesynthesis to compensate the volatilization of the alkali elements at high temperatures.Without loading cocatalysts, all the MTaO3 demonstrated high activity for pure watersplitting with the order of LiTaO3 � NaTaO3 � KTaO3, which corresponds well withthe band gap and conduction band level of MTaO3 (M = Li, Na, and K). The activitiescould be increased by one to two orders of magnitude when excess alkali reagents wereused during the synthesis. This enhanced photocatalytic activity was ascribed to theincrease of the crystal size, inhibition of the formation of grain boundaries, and alkalidefects (charge recombination centers) in the presence of excess of alkali. After loadingNiO as a cocatalyst, the photocatalytic activities of NiO/LiTaO3 and NiO/KTaO3 weredramatically decreased, while the activity of NiO/NaTaO3 was enhanced by one order ofmagnitude. The apparent quantum yield achieved on NiO/NaTaO3 was 20% at 270 nm.The highest activity obtained on NiO/NaTaO3 is due to its suitable conduction bandlevel for the transfer to photogenerated electrons to NiO cocatalysts and delocalizationof excited energy caused by the proper distortion of TaO6 connection in perovskitestructure, as shown in Figure 14.12. In the following work, Kato et al. modified thephotocatalytic water-splitting properties of NaTaO3 through lanthanide and alkalineearth metal ion doping.4 Among all the dopants, lanthanum was demonstrated to be


H+ H+

h+

h+

h+h+

e–

e–

e–

H2

H2

O2

O2

H2O

H2ONiO

La doping

3–15nm

NiO ultra fine particle

NiO/NaTaO32–3 μm

NiO/NaTaO3/La0.1–0.7 μm

Figure 14.13. Mechanism of highly efficient photocatalytic water splitting over

NiO/NaTaO3/La photocatalysts.4

the most effective in enhancing the photocatalytic activity of NaTaO3. The as-preparedNaTaO3/La can split pure water with a quantum yield of 56% at 270 nm when loadingNiO as cocatalyst on it. This is the highest quantum yield ever reported for overallwater splitting. It was found that the doping of La could reduce the particle size ofNaTaO3 and introduce formation of nanosteps on the particle surface (Figure 14.13).The grooves of the nanosteps were proposed to be the active sites for O2 production,while NiO nanoparticles highly dispersed at the edges of NaTaO3 worked as the H2evolution sites. The spatial separation of the O2 and H2 evolution sites can decrease therecombination of the photogenerated charges, resulting in the enhanced photocatalyticactivity dramatically. Moreover, the enhancement of the lifetime of photogeneratedelectrons in the conduction band or shallow trap level of NaTaO3 by La doping isanother reason for the enhanced activity. Mitsui investigated the photocatalytic water-splitting abilities of KaTO3 doped with group 4 elements. The partial substitution ofTa5+ for group 4 elements such as Hf4+, Ti4+, and Zr4+ did not change the bandgap of KTaO3, while decreased the electrical conductivity of KTaO3 dramatically. Thephotocatalytic water-splitting activity of KTaO3 was increased after doping Hf4+, Ti4+,and Zr4+, which is ascribed to the increased life time of photogenerated charges causedby a charge carrier annihilation.83

Various researchers investigated the photocatalytic water-splitting properties of thealkaline earth tantalates.81,84–87 MgTa2O6 with a band gap of 4.4 eV showed traceactivity for pure water splitting into H2 and O2 without cocatalysts. CaTa2O6 with anorthorhombic structure has a band gap of 4.0 eV. Under UV irradiation CaTa2O6 cansplit pure water into H2 and O2 stoichiometrically. After loading NiO catalysts, thephotocatalytic activity of CaTa2O6 can be increased by more than three times. Ca2Ta2O7with a pyrochlore structure can also split pure water into H2 and O2 stoichiometricallyin an aqueous NaOH solution after loading NiO cocatalysts on it, and the activity wasmuch higher than its niobate counterpart. The higher activity was mainly due to itshigh conduction band level consisting of Ta 5d orbitals. SrTa2O6 and BaTa2O6 withsimilar orthorhombic structure have band gaps of 4.4 and 4.1 eV, respectively. UnderUV irradiation, the materials are also active for pure water splitting with the activityorder of SrTa2O6 � BaTa2O6 � CaTa2O6. The difference in the photocatalytic activity


is supposed to be due to the conduction band level and the magnitude of transferringexcitation energy. Kudo et al. investigated a series of strontium tantalates with a chemicalformula of SrmTanO(m+5n/2). All the SrmTanO(m+5n/2) photocatalysts prepared with a PCmethod were active for pure water splitting into H2 and O2 when loaded with NiO asa cocatalyst. The activity increased in the order of Sr2Ta2O7 � Sr5Ta4O15 � SrTa2O6� Sr4Ta2O9, which was most likely due to the differences in their structures. Thequantum yield of NiO/Sr2Ta2O7 was estimated to be about 24% at 270 nm. BaTa2O6crystallizes in three phases: hexagonal (4.0 eV), tetragonal (3.8 eV), and orthorhombic(4.1 eV). All the phases can split pure water under UV light with the activity order oforthorhombic � hexagonal � tetragonal, which corresponds well with the width of theband gap. After loading NiO as cocatalysts, the photocatalytic activity of BaTa2O6 canbe greatly enhanced. Sakata et al. found that Ba5Ta4O15 prepared with a PC method wasactive for pure water splitting under UV light irradiation. When loading NiO cocatalystson it, the activity could be enhanced by more than 100 times. More interestingly, if25% more tantalum reagent was used during the synthesis, the as-obtained Ba5Ta4O15and Ba0.5TaO3 mixture demonstrated about 300 times higher activity than Ba5Ta4O15when loaded with NiO cocatalysts. In a similar manner, a series of transition metaltantalates have also been investigated. However, among all the transition metal tantalatesinvestigated, only few photocatalysts such as NiTa2O6 and ZnTa2O6 demonstrate littleactivity for pure water splitting.

Machida et al. investigated the photocatalytic properties of lanthanide tantalate witha chemical formula of LnTaO4 (Ln = La, Ce, Pr, Nd, and Sm).88 Under UV irradiation,LaTaO4 loaded with NiO cocatalysts demonstrated high activity for pure water splittingto H2 and O2, while other materials were almost inactive. It is found that the conductionband positions of LnTaO4 strongly depend upon the nature of the Ln ion and thephotocatalytic activity, and is governed by the energy level of Ln 4f levels in the bandstructure. The empty La 4f level is supposed to be higher than the conduction band edge.The filled 4f levels of the NdTaO4 and SmTaO4 lie in the valence band, and the occupied4f levels of CeTaO4 and PrTaO4 lie within the forbidden gap. The empty 4f levels close tothe conduction band edge would play as trapping centers for photoexcited electrons andthus decrease the photocatalytic activity for the H2 evolution. The interactions betweenthe Ln 4f levels and carriers are possible reasons for the Ln-dependent photocatalyticactivity. Abe et al. investigated the photocatalytic water-splitting properties of lanthanidetantalate with a chemical formula of Ln3TaO7 (Ln = Y, Yb, Gd, and La).89,90 Similarwith Ln3NbO7 (Ln = Y, Yb, Gd, and La), the photocatalytic activities of Ln3TaO7(Ln = Y, Yb, Gd, and La) strongly depend upon their crystal structures. La3TaO7(4.6 eV) with an orthorhombic weberite structure was quite active for overall watersplitting when loaded with NiO as a cocatalyst, while other materials such as Y3TaO7(4.5 eV) and Yb3TaO7 (4.3 eV) with a fluorite cubic structure, Gd3TaO7 (4.7 eV) witha pyrochlore cubic structure were almost inactive. The Weberite structure type containschains formed by corner-shared TaO6 octahedra, and the lack of TaO6 chains in the latterthree materials is supposed to be the reason for the inactivity.

When tantalumoxidewas alloyed togetherwith alkali (alkaline earth) and lanthanideoxides, Ta-based photocatalysts with more complex compositions could be obtained.Machida et al. investigated the photocatalytic properties of a series of layered alkali


lanthanide tantalates with a chemical formula of MLnTa2O7 (M = Rb, Cs, Na, andH; Ln = La, Pr, Nd, and Sm).91,92 The band gap of MLnTa2O7 was dependent on theLn ions, but negligibly affected by the interlayer M ions. However, the photocatalyticactivity of MLnTa2O7 was affected by both Ln and M ions. RbLnTa2O7 with layeredperovskite structure showed the highest activity for water splitting with the activityorder of RbNdTa2O7 � RbSmTa2O7 � RbLaTa2O7 � RbPrTa2O7 without loadingcocatalysts. The effect of Ln was expected to be similar with those of LnTaO4 (Ln= La,Ce, Pr, Nd, and Sm). NaLaTa2O7 and HLaTa2O7 obtained by the ion-exchanged reactionexhibited inferior activity compared with the pristine RbLaTa2O7. However, the activityof NaLaTa2O7 could be dramatically enhanced by loading NiO cocatalysts. Kudo et al.investigated the photocatalytic properties of a series of potassium lanthanide tantalateswith a chemical formula of K2LnTa5O15 (Ln= La, Pr, Nd, Sm, Gd, Tb, Dy, Tm, Ce, Eu,and Yb).93 NiO/K2LnTa5O15 (Ln = Ce, Eu, and Yb) is inactive for pure water splittingunder UV light. However, NiO/K2LnTa5O15 (Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, andTm) is highly active for pure water splitting with the sequence of Sm � Pr �Dy �Gd� La = Tb =Tm � Nd. Therefore, the photocatalytic activities of K2LnTa5O15 werefound to strongly depend on the lanthanide ions. Mitsuyama et al. also investigated thephotocatalytic properties of a series of layered alkaline earth lanthanide tantalates with achemical formula of MCa2Ta3O10 (M= Cs, Rb, K, Na, and Li).94 MCa2Ta3O10 is a typeof ion-exchangeable layered perovskite-type material. After loading NiO cocatalysts, allthe MCa2Ta3O10 photocatalysts can split pure water stoichiometrically into H2 and O2with the sequence of Li �Na �K � Rb � Cs. The hydration of interlayer is found tobe an efficient way to modify the structure of ion-exchangeable layered oxides in orderto enhance the photocatalytic activity.

Other Transition Metal-Based Oxides

In transition metal-based semiconductor materials—which are active for water splittingunder UV irradiation—Ti, Nb, and Ta-based materials constitute the most notable ones.Other transition metals including zirconium (Zr), molybdenum (Mo), tungsten (W), andcerium (Ce) form only a small portion.

ZrO2 has a wide band gap of 5.0 eV. Compared with most semiconductors, theflat-band potential of ZrO2 is highly negative (Efb = −1.0 eV vs. NHE, pH = 0).Therefore, the photogenerated electrons in the conduction band of ZrO2 have quite highreduction potential for the water reduction reaction. In fact, Sayama and Arakawa foundthat ZrO2 could split pure water stoichiometrically into H2 and O2 with high efficiencyunder UV irradiation without loading any cocatalyst on it.95, 96 This is different frommost semiconductors, where cocatalysts such as Pt and NiO are necessary for achievingpure water splitting. The high conduction band level of ZrO2 is proposed to facilitate thegeneration of H2 without the assistance of cocatalysts. However, the high conductionband level of ZrO2 makes the electronic barrier of the semiconductor–cocatalyst junctionquite high, preventing the migration of electrons from ZrO2 to the cocatalyst. Therefore,when different cocatalysts such as RuO2, Au, and Cuwere loaded on the surface of ZrO2,some of the reaction sites on the ZrO2 were blocked, and the efficiency for water splittingon ZrO2 was decreased as a consequence. When Pt was loaded on ZrO2 as a cocatalyst,


only trace amount of H2 was observed. The backward reaction between H2 and O2 onPt is supposed to be another reason for the decreased activity. It was also found thatthe addition of carbonate, such as Na2CO3, NaHCO3, and KHCO3, led to a remarkableincrease in the activity and stability of the gas evolution rate on ZrO2 and ZrO2 loadedwith different cocatalysts. The carbonate species absorbed on ZrO2 is supposed to playan important role in desorbing O2 via the carbonate radical. Moreover, the presence ofcarbonate on Pt can prevent the backward reaction between H2 and O2 on Pt. Liu andWang prepared Zr–MCM-41 by incorporating Zr into the amorphous wall of MCM-41through a hydrothermal reaction. Photocatalytic H2 production from water splitting onZr–MCM-41 was found to be enhanced by more than 2.5 times compared with that onconventional bulk ZrO2. The enhancement may be due to the high dispersion of ZrO2.97

Yuan et al. investigated the photocatalytic water-splitting ability of BaZrO3 with a cubicperovskite structure.98 The flat-band level of BaZrO3 is highly negative. Therefore,similar with ZrO2, BaZrO3 alone can split water to produce H2 with a quantum yield of3.7% under UV irradiation. Density functional theory (DFT) calculations indicated thatthe top of the valence band of BaZrO3 was composed of O 2p orbitals and the bottom ofthe conduction band was composed of Zr 4d orbitals. The highly negative flat-band level,the largely dispersed conduction band, and the 180º Zr–O–Zr bond angle are likely tobe responsible for the high photocatalytic activity of BaZrO3 for water splitting. Inagakiet al. investigated the photocatalytic water-splitting abilities of a series of mesoporouszirconium-titanium phosphates (ZTPs) with different Zr-to-Ti ratios. When loading Ptcocatalysts on ZTP with an impregnation–calcination process, all the ZTPs showedphotocatalytic activity toward H2 production from water splitting. The highest activitywas obtained on ZTPwith a Ti-to-Zr molar ratio of 1 : 1, possibly as a result of synergeticeffect between zirconium and titanium in ZTPmesoporous materials. Like in many otheroxide-based photocatalysis, the addition of Na2CO3 was found to dramatically promotethe photocatalytic water-splitting performance of ZTP.

Kato et al. investigated the photocatalytic water-splitting properties of a series oftungstates and molybdates with a scheelite structure.99 PbMoO4 is active for photocat-alytic H2 and O2 production in the presence of sacrificial reagents under UV irradia-tion. The substituted compounds (NaBi)0.5MoO4 (3.1 eV), (AgBi)0.5MoO4 (3.0 eV), and(AgBi)0.5WO4 (3.2 eV) showed that activity for photocatalytic O2 production fromwatersplitting in the presence of AgNO3 sacrificial reagent and (NaBi)0.5WO4 (3.5 eV) couldproduce H2 from aqueousmethanol solutionwhen loadedwith Pt cocatalysts. Other pho-tocatalysts such as (AgLa)0.5MoO4 (3.5 eV), (AgCe)0.5MoO4 (2.3 eV), (AgEu)0.5MoO4(3.2 eV), and (AgYb)0.5MoO4 (3.1 eV) were inactive for H2 or O2 production in thepresence of sacrificial reagents. Na2W4O13 with a layered structure was also reported tobe active for photocatalytic hydrogen and oxygen evolution in the presence of methanoland AgNO3 as the sacrificial reagents, respectively.100 Kadowaki et al. investigated thephotocatalytic properties of PbWO4.101 PbWO4 consists of a WO4 tetrahedron. Whenloading RuO2 as a cocatalyst on it, PbWO4 could split pure water to H2 and O2 stoi-chiometrically under UV irradiation. However, CaWO4 with a similar crystal structuredid not show any activity. DFT calculations indicated that large dispersions existed inboth the valence and conduction bands of PbWO4. Therefore, photoexcited holes and


electrons with large mobility can be generated, which leads to high photocatalytic activ-ity of RuO2-loaded PbWO4.

CeO2 with a band gap of 2.9 eV was not active for H2 production under UVirradiation. However, CeO2 demonstrated relatively high activity for O2 evolution in thepresence of Fe3+ or Ce4+ as the sacrificial reagents. When CeO2 was doped with Sr, theas-obtained photocatalyst was quite active for overall water splitting when loading RuO2as the cocatalyst. However, the single-phase CeO2, SrCeO3, or Sr2CeO4 did not exhibitactivity for overall water splitting under UV irradiation. The doping of Sr is supposedto inhibit the formation of Ce3+ working as a recombination center for photoexcitedcharges, therefore enhancing the photocatalytic activity.102,103 BaCeO3 with a band gapof 3.2 eV was reported to produce H2 and O2 in the presence of CH3OH and AgNO3sacrificial reagents, respectively. It was also active for pure water splitting under UVirradiation when loaded with RuO2 cocatalysts.104

Main Group Metal Oxides

Sakata et al. investigated the photocatalytic water-splitting properties of Ga2O3.105,106

Ga2O3 has a band gap of 4.5 eV and can only produce little H2 from pure water splitting.After loading NiO as the cocatalyst on it, Ga2O3 can split pure water stoichiometricallyinto H2 andO2.WhenGa2O3 was further modified by doping different metal ions such asZn,Ca, Sr, Ba, Ta, andCr, the photocatalytic activity of the resultingGa2O3 was increaseddramatically. In particular, the Zn-modified Ga2O3 can split pure water with a quantumyield of about 20% when loading NiO as cocatalysts. When Ga2O3 was alloyed withIn2O3 at high temperature, Ga2–xInxO3 solid solution materials could be obtained. Withthe increase of content of Ga, the band gap of the Ga2–xInxO3 solid solution materialsincreased gradually and the flat-band potentials were shifted negatively. The Ga2–xInxO3solid solution materials showed photocatalytic activity for O2 production from aqueousAgNO3 solution or H2 evolution from aqueous methanol solution when loaded with Ptcocatalysts.107, the GaInO3 photocatalyst demonstrated the highest activity. ZnGa2O4is composed of an octahedrally coordinated Ga3+ ion and has a band gap of 4.3 eV. Itexhibited high activity for overall water splitting to H2 and O2 under UV irradiationwhen loading RuO2 as cocatalysts.108 DFT calculations indicated that there was alarge dispersion in the conduction band of ZnGa2O4 and therefore a large mobility ofphotoexcited electrons, which was considered to be responsible for high photocatalyticperformance.

Inoue et al. investigated the photocatalytic properties of a series of binary indateswith a chemical formula of Min2O4 (M = Li, Na, Ca, Sr, and Ba).109–112 All the indateswere composed of an octahedrally coordinated In3+metal ion. LiInO2 andBaIn2O4 alonewere not active for overall water splitting even when loaded with RuO2 as cocatalysts.NaInO2 exhibited little activity for overall water splitting under UV irradiation whenloaded with RuO2 as cocatalysts, and the activity decreased a lot when part of Na ionsin NaInO2 was exchanged with Li and K ions. CaIn2O4 and SrIn2O4 exhibited highactivity for overall water splitting when loaded with RuO2 as cocatalysts following theactivity order of CaIn2O4 � SrIn2O4 � NaInO2. The activities of the solid solution


materials CaxSr1–xIn2O4 increased monotonically with the increase of the content ofCa. It was found that the photocatalytically active indates possessed distorted InO6octahedra with dipole moments, leading to the formation of internal fields that promotedthe charge separation process. Moreover, the broad sp conduction bands with largedispersion permitted photoexcited electrons from indates to move to RuO2 cocatalysts.When In2O3 was alloyed with Y2O3 with cubic structure, YxIn2–xO3 (0.9 � x �1.5)solid solution materials could be obtained. With the increase of the Y content, the bandgap of the YxIn2–xO3 solid solution materials increased gradually and the conductionband positions shifted to a more negative position as well. All the YxIn2–xO3 solidsolution materials demonstrated activity for overall water splitting into H2 and O2, andtheY1.3In0.7O3 photocatalyst exhibited the highest activity. The photocatalytic activity ofYxIn2–xO3 was supposed to be influenced by the change in the geometric and electronicstructures with the contents of Y and In in the solid solution materials.113

Zn2GeO4 is composed of heavily distorted GeO4 tetrahedra.114 Different anti-monates such as CaSb2O6, NaSbO3, Ca2Sb2O7, and Sr2Sb2O7 are composed of distortedSbO6 octahedra.115 Similarly, stannates such as Sr2SnO4 are composed of distorted SbO6octahedra.110 All the above-mentioned photocatalysts showed relatively high activity foroverall water-splitting reactions when loaded with RuO2 cocatalysts. Similar with thephenomenon observed in gallates and indates, the distortion in these materials can alsogenerate a dipole moment, leading to the formation of internal electric fields that pro-mote the charge separation process. Moreover, a large dispersion in the conduction bandwas observed, indicating the large mobility of photoexcited electrons. These two factorsare considered to be responsible for the high activity of these photocatalysts.

Nonoxide Photocatalysts

Although oxide materials form the most common UV-responsive photocatalysts, metalnitrides and sulfides are two kinds of semiconductor photocatalysts that have also beeninvestigated for water splitting. Domen et al. systematically studied Ge3N4 and GaNphotocatalysts for overall water splitting.44,116–119 The two nitride photocatalysts wereobtained by calcining their corresponding oxide precursors in flowing NH3. By varyingthe preparation parameters such as calcination temperature, calcination time, and flowrate of NH3, the physiochemical properties of the resulting nitride photocatalysts canbe tuned to obtain the optimum performance. Ge3N4 consists of corner-shared GeN4tetrahedra and has a band gap of about 3.8 eV. Ge3N4 alone exhibits only little activityfor water splitting. After loading RuO2 as the cocatalyst, the performance of Ge3N4 wasdramatically enhanced and pure water could be split into H2 and O2 stoichiometricallyunder UV irradiation. The optimum activity of RuO2/Ge3N4 was obtained in a 1 MH2SO4 solution with a pH value of 0, and a pH dependence of the photocatalytic activitywas observed. This is quite different from the general character of transition metaloxide-based photocatalysts. Moreover, the valence band of Ge3N4 is composed of N 2porbitals, which is also different from the oxides whose valence band is usually composedof O 2p orbitals. The RuO2/Ge3N4 catalyst is not stable during photocatalytic reactions,and degradation of Ge3N4 was found to proceed due to the oxidation of Ge3N4 torelease N2. The stability and photocatalytic activity of Ge3N4 can be improved a lot by a

VISIBLE LIGHT-RESPONSIVE SEMICONDUCTOR NANOCATALYSTS FOR WATER SPLITTING 527

post-high-pressure ammonia treatment due to the inhibition of the anion defects in thebulk and surface.117 In the following work, GaN was also found to be inactive alone, butbecame active for overall water splitting under UV light when loaded with Rh2–xCrxO3cocatalysts. However, the loading of RuO2 did not bring appreciable effect on GaN,and only H2 was observed during the splitting of pure water. The doping of divalentmetal ions Zn2+ and Mg2+ in GaN was found to enhance the stability and activity ofGaN dramatically when loading RuO2 as the cocatalyst, which was possibly due to theincreases in the mobility and concentration of holes and formation of acceptor levels bythe divalent metal ion doping.

ZnS is a major metal sulfide investigated for photochemical water splitting. It isn-type semiconductor with a band gap of 3.80 eV for the hexagonal wurtzite phase and of3.66 eV for the cubic zinc blende phase. Reber andMeier investigated the photocatalyticwater-splitting ability of ZnS comprehensively.120 In the presence of different sacrificialreagents such as S2−, SO32−, S2O32−, and H2PO2−, ZnS can produce H2 efficientlyunder UV irradiation without loading any cocatalysts. The loading of cocatalysts suchas Pt does not improve the photocatalytic activity of ZnS significantly. The performanceof ZnS is greatly affected by the preparation conditions, and a quantum yield of 90%at 313 nm is obtained on the optimum ZnS sample. The wide band gap of ZnS allowsthe oxidation of less reducing reagents such as thiosulfate to sulfate, which cannot berealized in small band-gap semiconductors such as CdS.

VISIBLE LIGHT-RESPONSIVE SEMICONDUCTORNANOCATALYSTS FOR WATER SPLITTING

UV-responsive photocatalysts are active under irradiation with wavelength shorter than400 nm. This part of irradiation only accounts for around 3–5% of the total solar energy,in great contrast with visible light that accounts for 46%. In order to utilize solar energyefficiently, it is highly desirable to develop photocatalysts capable of splittingwater undervisible light. In the past decade, great effort has been devoted to the development ofnovel visible light-responsive photocatalysts or the band engineering of existent pristinematerials toward higher performance in the visible region, and great progress has beenmade. Generally, three strategies have been adopted to develop visible light-responsivephotocatalysts for water-splitting reactions (Figure 14.14). The first strategy (type A) isto find materials that have suitable valence band and/or conduction band levels and bandgaps for water splitting under visible light. For example, nitride, oxynitride, sulfide,and oxysulfide materials can form a valence band with more negative levels than thatformed in the corresponding oxides due to the less electronegativity of nitrogen andsulfur elements. Similarly, orbitals of Pb 6s in Pb2+, Bi 6s in Bi3+, Sn 5s in Sn2+, andAg 4d in Ag+ can also form valence bands above the valence band consisting of O 2porbitals in metal oxide photocatalysts, and therefore their participation in the formationof a semiconductor can lead to smaller band gaps. The second strategy (type B) is todope foreign elements into the crystal lattice of active photocatalysts to form donoror acceptor levels in the forbidden band. For example, cation doping, anion doping,and cation/anion codoping have been widely used to introduce visible light activity


V/NHE

A B C

–1.0

0 hv+1.0

H+/H2

H2

O2

O2/H2O

H2O

+2.0

+3.0

+4.0

H+

Conduction bande– e– e– e– e– e–

Valence bandh+ h+ h+ h+ h+ h+

Positive CB

Donor level

Acceptor level

Wide BGSolid solution

Narrow BG

Negative VB

Figure 14.14. Strategies for developing visible light-responsive photocatalysts: finding mate-

rials with suitable valence band (VB) or conduction band (CB) levels (type A), formation of

doping levels in the forbidden band through foreign elements doping (type B), and forma-

tion of solid solution materials between wide band-gap and small band-gap semiconductors

(type C).

to TiO2. Moreover, doping strategy can also be applied to modify the physiochemicalproperties of visible light-responsive photocatalysts toward better photocatalytic perfor-mance. Finally, the last strategy (type C) is to form solid solution materials betweenwide band-gap and narrow band-gap photocatalysts with suitable energy levels. Theresulting solid solution materials will have intermediate band gaps and band positionlevels and usually demonstrate higher performance than either the wide band-gap orthe narrow band-gap photocatalysts under visible light irradiation. In most cases onlyone strategy will be used, and in some cases more than one strategy will be combinedto achieve higher photocatalytic performance. We dwell on the visible light-responsivephotocatalysts according to the developed strategies and the element compositions.

Semiconductors with Suitable Energy Levels for WaterSplitting (Type A)

Oxide Materials. The band gap of n-type WO3 is 2.8 eV, and its conductionband level is too positive to allow for the H2 production under visible light. However,WO3 has been successfully used in photoelectrochemical cells for the splitting of waterunder applied bias.121 Moreover, WO3 has been found to be quite efficient for thephotocatalytic oxidation of water to produce O2.122 When WO3 was irradiated withlight with a wavelength less than 500 nm, O2 could be produced in the presence ofFe3+ as the sacrificial reagent. During the photocatalytic reactions, Fe3+ was reducedto Fe2+ and water was oxidized to O2. The accumulation of Fe2+ was found to greatlydecrease the photocatalytic activity of WO3 due to the backward reaction. The loadingof noble metals such as Pt, Ru, and Rh decreased the photocatalytic activity of WO3,while the loading of RuO2 could enhance the activity by two times. When the surface ofWO3 was modified with a thin layer of TiO2 nanoparticles, the photocatalytic activityof WO3 was improved in the presence of Fe2+ due to the suppression of the harmfuleffect of Fe2+ by TiO2.123 When AgNO3 was used instead of Fe3+ as the sacrificial


reagent, much higher amount of O2 could be produced, which was ascribed to theirreversible reduction of Ag+ to Ag.124 However, the deposition of Ag on WO3 canblock the absorption of incident light, leading to decreased photocatalytic activity withthe reaction. Nevertheless, WO3 has been successfully used as an important componentfor the fabrication of complex photocatalysts or photocatalytic systems. When n-typeWO3 was coupled with p-type PbBi2Nb1.9Ti0.1O9 through a W ohmic contact layer, H2and O2 could be produced efficiently in the presence of methanol and AgNO3 as thesacrificial reagent, respectively. The ohmic layer is supposed to be important for thephotocatalyst with p–n structure.125 When loaded with Pt cocatalysts, Pt/WO3 can bean efficient O2-evolution photocatalyst for the fabrication of a Z-scheme system. A lotof H2-evolution photocatalysts such as Pt/SrTiO3–Cr/Ta,126 Pt/TaON,45 and Pt/ATaO2N(A = Ca, Sr, and Ba)127 have been successfully coupled with WO3 through the I−/IO3−mediator in the Z-scheme system. The Pt/WO3 photocatalyst has good adsorption abilityfor IO3− instead of I−. During the reactions, Pt/WO3 will reduce IO3− to I− and oxidizewater to O2 simultaneously. TheH2-evolution catalyst will oxidize I− and reduce protonsto H2. Therefore, even though WO3 cannot produce H2 due to the positive conductionband level, WO3 can be an important part for the fabrication of an overall water-splitting system when coupled with appropriate photocatalysts. Bi2WO6 has a bandgap of 2.8 eV and can produce trace amount of O2 in aqueous AgNO3 solution undervisible light. It is also active for H2 production in aqueous methanol solution underUV light.128

Kudo et al. discovered BiVO4 photocatalysts for photocatalytic water split-ting.129–131 BiVO4 crystallizes with monoclinic scheelite structure and consists of VO4tetrahedra separated by Bi3+ cations. DFT calculations indicate that the conductionband of BiVO4 is composed of V 3d orbitals and the valence band is composed of Bi6s orbitals. The valence band level formed by Bi 6s orbitals is above that formed byO 2p orbitals in traditional oxide photocatalysts and therefore leads to much smallerband gaps. Although BiVO4 is inactive for H2 production under visible light due tothe low conduction band level, it shows quite high activity for O2 production undervisible light in the presence of aqueous AgNO3 solution. The photocatalytic activity ofBiVO4 prepared by the aqueous process is much higher than that of BiVO4 prepared bya conventional solid-state reaction due to the inhibition of the formation of the defects athigh temperatures. Recently, Xi and Ye found that monoclinic BiVO4 nanoplates withhigh percentage of {001} facets demonstrated much higher activity for photocatalyticO2 production from aqueous AgNO3 solution under visible light.132

Like Bi3+-containing oxides, the participation of Sn2+ with a 5s configuration canalso lead to the formation of small band-gap photocatalysts. SnNb2O6 has a band gapof 2.3 eV and can split water to produce H2 from aqueous methanol solution undervisible light irradiation. It can also produce O2 in the presence of AgNO3 solutions,and the activity for O2 evolution was remarkably increased when IrO2 was used as thecocatalyst.133

Ag+-based oxides form a large group of visible light-responsive oxide photocat-alysts. The valence band formed by the Ag 4d orbitals is above that formed by O 2porbitals and therefore leads to the formation of narrow band-gap photocatalysts. Kudoet al. developed a series of Ag-based oxides for photocatalytic water splitting.134,135


Perovskite-type AgNbO3 has a band gap of 2.8 eV. The valence band of AgNbO3 iscomposed of hybridized Ag 4d and O 2p orbitals, and the conduction band is composedof Nb 4d orbitals. Under visible light irradiation, AgNbO3 can produce H2 from aqueousmethanol solution when loaded with Pt cocatalyst. AgNbO3 can also produce O2 fromaqueous AgNO3 solution under visible light irradiation.135 In the following work, Kudoet al. investigated the photocatalytic properties of AgVO3, Ag4V2O7, and Ag3VO4 andfound that only Ag3VO4 showed activity for photocatalytic O2 production from aqueousAgNO3 solution under visible light. Ye and Zou et al. also developed several kinds ofAg-based materials for photocatalytic reactions.136–141 AgInW2O8 with a layered struc-ture demonstrated trace activity for O2 evolution from aqueous AgNO3 solution undervisible light irradiation.136 AgSbO3 has a band gap of 2.6 eV. It can oxidize water toproduce O2 in aqueous AgNO3 solution under visible light. The activity of AgSbO3 forO2 evolution was even higher than that ofWO3.140 Recently, Yi et al. developed Ag3PO4photocatalysts. It was found that Ag3PO4 could oxidize water to produce O2 in aqueousAgNO3 solution with a quantum efficiency of more than 90% at 420 nm. This is thehighest reported value for photocatalytic O2 from water splitting under visible light upto now.141

RbPb2Nb3O10 is an ion-exchangeable material crystallizing with perovskite struc-ture. It has a band gap of 2.5 eV and can produce trace amount of H2 in aqueousmethanolsolution under visible light when loaded with Pt cocatalysts. When the Rb+ ions areexchanged with protons, the as-obtained Pt/HPb2Nb3O10 exhibited much higher activityfor H2 evolution. When [Pt(NH3)4]Cl2 was used instead of H2PtCl6 as the platinizationsource, much higher H2 photocatalytic activity was observed due to the high dispersionof Pt particles in the internal channel of HPb2Nb3O10. RbPb2Nb3O10 can also producetrace amount of O2 in the presence of aqueous AgNO3 solution, while HPb2Nb3O10 isinactive for photocatalytic O2 production.142 PbBi2Nb2O9 crystallizes with perovskitestructure type. It has a band gap of 2.9 eV and can produce H2 and O2 from aqueousmethanol and AgNO3 solution, respectively, under visible light. Especially, the quantumyield of O2 evolution is estimated to be as high as 29%.143

Nitride and Oxynitride Materials. Nitride and oxynitride materials are quiteattractive photocatalytic materials because of their small band gaps that allow for theabsorption of visible light. Compared with the corresponding oxide materials, nitrideand oxynitride materials usually have smaller band gaps due to the elevated valenceband by the N 2p orbitals (Figure 14.14, type A, left). Domen et al. developed a seriesof nitride or oxynitride materials for photocatalytic water splitting.144–157

TaON and Ta3N5 are the first and among most investigated photocatalysts. They aretypically obtained by heating Ta2O5 powder in a NH3 flow at 1123 K. By controlling theflow rate of NH3 at 20 ml min−1 or allowing NH3 bubbling through water, TaON canbe obtained. When the high flow rate of NH3 was used, Ta2O5 could be fully nitridedto Ta3N5. Compared with Ta2O5, the band gaps of Ta3N5 and TaON were decreased to2.1 eV and 2.5 eV, respectively. Ta3N5 is highly active for O2 evolution in the presence ofAgNO3 as the sacrificial reagent, when La2O3 is added to maintain the pH at 8. However,the activity of Ta3N5 for H2 production in the presence of aqueous methanol solutionis quite low even though Pt is used as the cocatalyst. When Ta3N5 is posttreated with


high-pressure NH3, the rate of H2 evolution on Ta3N5 can be enhanced by five times.152

TaON has a band gap of 2.5 eV, and like Ta3N5, TaON is highly active for O2 evolutionin the presence of AgNO3 as the sacrificial reagent. Moreover, the activity of TaON forH2 production is only a little higher than that of Ta3N5 when Pt is used as the cocatalyst.However, the activity for H2 production on TaON can be greatly improved when Ru isused as the cocatalyst possibly due to the better interfacial electronic structure betweenRu and TaON. Moreover, when the surface of TaON is modified with ZrO2 particles, theprepared ZrO2/TaON nanocomposite material shows higher activity for H2 evolutiondue to the suppression of the reduced tantalum species that form during nitridation.156

Even though Ta3N5 and TaON are both active for H2 and O2 production in the presencemethanol and AgNO3 as the sacrificial reagents, respectively, they cannot split purewater under visible light. However, they have been successfully used for the splitting ofpure water in a Z-scheme system.151 When combined with WO3, Pt/TaON and can splitpure water into H2 in the presence of IO3−/I− redox mediator. Similarly, Pt/ZrO2/TaONcan split pure water into H2 and RuO2/TaON, and Ir/Ta3N5 can split water into O2 inthe presence of IO3−/I− redox mediator.157–159

Domen et al. developed a series of alkaline earth tantalum oxynitrides with achemical formula of MTaO2N (M = Ca, Sr, and Ba). All the oxynitrides crystallize inthe perovskite structure type, and the band gaps are 2.5, 2.1, and 2.0 eV for M= Ca, Sr,and Ba, respectively. MTaO2N can produce H2 from aqueous methanol solution undervisible light, but they cannot produce O2 in the presence of AgNO3 sacrificial reagentseven they have valence band levels sufficiently positive for water oxidation. Similar withTaON and Ta3N5, in the presence of IO3−/I− redox mediator, MTaO2N (M=Ca and Ba)has been successfully used as H2-evolution photocatalysts in a Z-scheme system whencombined with Pt/WO3 photocatalyst. SrTaO2N is not stable due to the self-oxidationin the presence of I− and therefore not suitable in a Z-scheme system.160

Liu et al. developed lanthanide tantalum oxynitrides Y2Ta2O5N2.161 Y2Ta2O5N2crystallizes in the pyrochlore structure type and has a band gap of 2.2 eV. Under visiblelight, Y2Ta2O5N2 can oxidize water to produce O2 in the presence of AgNO3 as thesacrificial reagent. When loading Pt or Ru as cocatalysts, Y2Ta2O5N2 can reduce waterto H2 in the presence of methanol solution. Interestingly, when Pt and Ru are coloadedon Y2Ta2O5N2, much enhanced activity for H2 could be obtained possibly due to thesynergy effect between Pt and Ru for catalyzing H2 evolution.

Kasahara et al. developed Ti-based oxynitride, LaTiO2N, for photocatalytic water-splitting reactions. LaTiO2N crystallizes in the perovskite structure and has a band gapof 2.1 eV.146 LaTiO2N demonstrated high efficiency for O2 production in the presence ofAgNO3 as the sacrificial reagent under visible light. When loaded with Pt as cocatalysts,LaTiO2N could produce H2 from aqueous methanol solution.

Mishima et al. developed Zr-based oxynitride, Zr2ON2, for photocatalytic water-splitting reactions.162 Zr2ON2 was prepared by nitridization of amorphous ZrO2 at highNH3 flow and temperature. Compared with ZrO2, the band gap of Zr2ON2 is reducedfrom 5.0 to 2.6 eV. DFT calculations indicate that the top of the valence band of Zr2ON2is composed of N 2p orbitals, leading to the drastic decrease of the band gap. Undervisible light irradiation, Zr2ON2 showed stable activity for H2 and O2 production in thepresence of methanol and AgNO3 aqueous solution, respectively.


Sulfide and Oxysulfide Materials. Sulfide and oxysulfide materials form alarge group of attractive visible light-responsive photocatalytic materials. Similar withnitride and oxynitride materials, sulfide and oxysulfide materials usually have smallerband gaps than the corresponding oxide materials due to the elevated valence band bythe S 3p orbitals (Figure 14.14, type A, left).

CdS is the most investigated and efficient sulfide-based photocatalyst.163–165 It cancrystallize with two structures: hexagonal wurtzite and zinc blende structures. CdS has aband gap of 2.4 eV and its conduction band level is sufficiently high to reduce water (flatpotential= −0.66 V vs. NHE, pH= 7) to produce H2, and its valence band level is ther-modynamically suitable to oxidize water to produce O2. CdS alone shows trace activityfor H2 evolution. However, when loaded with cocatalysts, CdS can efficiently split waterinto H2 in the presence of sacrificial reagents such as ethylenediaminetetraacetic acid(EDTA) and cysteine under visible light.163,164 Na2S was also used as the sacrificialreagent for the H2 production. However, the efficiency for H2 production was greatlyaffected due to the formation of S22− ions that blocked the absorption of light by thephotocatalysts.165 When S2− and SO32− were used together as the sacrificial reagents,H2 could be produced efficiently on Pt/CdS photocatalysts due to the suppression ofthe formation of S22−. The photocatalytic activity of CdS is greatly influenced by thecrystal phase and morphology of CdS and the cocatalysts. CdS with nanoporous struc-tures was prepared with a two-step aqueous route. When loaded with Pt as cocatalysts,it can produce H2 with a quantum yield of 60.34% at 420 nm in the presence of S2−

and SO32- ions as the sacrificial reagents.166,167 When CdS was loaded with Pt and PdSas cocatalysts, the as-prepared Pt–PdS/CdS could split water into H2 with a quantumyield of 93% at 420 nm.7 This is the highest value reported for sacrificial H2 productionunder visible light and almost approaches what has been obtained in natural photosyn-thetic systems. In this three-component photocatalyst, Pt is considered to catalyze theH2 evolution and PdS is proposed to catalyze the oxidation reaction. The separation ofthe reduction and oxidation sites decreases the recombination of photogenerated chargesand therefore improves the efficiency dramatically. Despite the excellent photocatalyticperformance achieved on CdS photocatalysts, photocorrosion and the inability for O2production remain the main problems for CdS-based photocatalysts.

Fu et al. discovered the In2S3 semiconductor for photocatalytic water splitting.168

In2S3 was prepared by the hydrothermal method. It has a band gap of 2.0 eV and can existin tetragonal and cubic crystal structure. The vacancy-ordered tetragonal In2S3 showedno activity for H2 production, whereas the vacancy-disordered cubic In2S3 exhibitedstable activity in aqueous Na2S/Na2SO3 solution under visible light. The photoactivity ofcubic In2S3 can be significantly improved by loading Pt as cocatalysts. Lei et al. preparedthe ZnIn2S4 photocatalyst with layered structure by a solvothermal method for the firsttime.169 The ZnIn2S4 photocatalyst has an intermediate band gap of 2.3 eV betweenthose of ZnS and In2S3. ZnIn2S4 alone can produce H2 from aqueous Na2S/Na2SO3solution under visible light irradiation. After loading Pt cocatalysts on it, the activityof ZnIn2S4 for H2 evolution can be further enhanced. The photocatalytic activity ofZnIn2S4 photocatalysts can be improved by choosing different synthesis methods andconditions. Similarly, CdIn2S4 and NaInS2 with layered structure demonstrated high


activity for photocatalytic H2 production from aqueous S2−/SO32− solution under visiblelight irradiation.170,171

Chen and Ye developed the AgIn5S8 photocatalyst for photocatalytic H2 produc-tion.172 The conduction band of AgIn5S8 was composed of In 5s5p orbitals, and thevalence band was composed of hybridized S 3p and Ag 4d orbitals. When loading thePt cocatalyst on the material, AgIn5S8 could produce H2 from aqueous S2− and SO32−

solution under visible light irradiation.Domen et al. investigated a series of Ti, Ga, and In-based oxysulfides for photo-

catalytic water splitting under visible light.173–178 Sm2Ti2S2O5 was the first reportedoxysulfide material used for water splitting. It was obtained by heating a mixture ofSm2S3, Sm2O3, and TiO2 in a sealed quartz tube under vacuum at 1273 K. Comparedwith Sm2Ti2O7, the band gap of Sm2Ti2S2O5 was reduced from 3.4 to 2.0 eV. DFT cal-culations indicate that the S3p orbitals constitute the upper part of the valence band ofSm2Ti2S2O5, leading to the small band gap. Under visible light irradiation, Sm2Ti2S2O5can split water to produce H2 or O2 from aqueous solutions containing a sacrificialelectron donor (Na2S–Na2SO3 or methanol) or acceptor (Ag+) when loading Pt or IrO2as cocatalysts on it. The modification of Sm2Ti2S2O5 with metal ions such as Ag+

and Mg2+ was found to increase the photocatalytic activity of Sm2Ti2S2O5 by seventimes.179 Compared with sulfide materials, oxysulfide material is quite attractive for theability to catalyze water oxidation. In the following work, Domen investigated a series ofoxysulfides with a chemical formula of Ln2Ti2S2O5 (Ln= Pr, Nd, Sm, Gd, Tb, Dy, Ho,and Er). Ln2Ti2S2O5 samples were synthesized by sulfurization of amorphous Ln2Ti2O7precursors under H2S flow. Compared with UV-responsive Ln2Ti2O7 photocatalysts, theband gaps of Ln2Ti2S2O5 were decreased to 1.9–2.2 eV. DFT calculations indicate thatthe top of the valence band of [Gd–Er]2Ti2S2O5 is composed of hybridized O 2p, S3p, and Ln 4f orbitals, while Lf 4f orbitals are localized in other [Pr–Sm]2Ti2S2O5.Moreover, the conduction band of [Gd–Er]2Ti2S2O5 is composed of S 3p+ Ln 4f and Ti3d orbitals, while that of [Pr–Sm]2Ti2S2O5 is composed of Ti 3d orbitals (Figure 14.15).

CB

VB VB

Ln = Gd, Tb, Dy, Ho, ErLn = Pr, Nd, Sm

CB

H+/H2

O2/H2OBgBg

Ti3d Ti3d

S3p + Ln4f

O2p + S3p O2p + S3p

O2p + S3p + Ln4f

Figure 14.15. Schematic band structures of Ln2Ti2S2O5.176


Ln2Ti2S2O5 can split water to produce H2 or O2 from aqueous solutions containing asacrificial electron donor (Na2S–Na2SO3 or methanol) or acceptor (Ag+) under visi-ble light irradiation, and Sm2Ti2S2O5 demonstrated the highest photocatalytic activity.Similarly, several La–Ga (La3GaS5O) and La–In (La5In3S9O3)-based oxysulfide mate-rials were also reported to be able to split water into H2 or O2 from aqueous solutionscontaining a sacrificial electron donor (Na2S–Na2SO3 or methanol) or acceptor (Ag+)under visible light irradiation.177,178

Other Materials. Besides the above types of photocatalysts widely investigated,a few types of other visible light-responsive photocatalysts have been studied for pho-tocatalytic water splitting.180

Titanium silicides with a chemical formula of TiSi2 can absorb light with a wave-length of 360–800 nm, and the absorption characteristic is different from traditionalsemiconductor photocatalysts. The Fermi level of TiSi2 is about −0.4 V at pH =7,sufficiently fulfilling the physical requirement for the reduction of protons to produceH2. Under light irradiation, TiSi2 can produce H2 from pure water without loading itwith any cocatalysts at ambient conditions. When the temperature of the reaction mediawas heated to 373 K, O2 could also be detected due to the release of O2 absorbed on thesurface of TiSi2. Therefore, TiSi2 is a very promising photocatalyst capable of separatingH2 and O2 from water splitting under light irradiation.180

Maeda and Wang et al. investigated the photocatalytic water-splitting property ofcarbon nitride, C3N4.181,182 Carbon nitride is prepared with a thermal condensationmethod, and the fully condensed graphitic carbon nitride has a band gap of 2.7 eV. DFTcalculation indicates that the valence band of C3N4 is composed of N pz orbital and theconduction band is composed of C pz orbital. When loaded with Pt cocatalysts, C3N4can reduce water to produce H2 in aqueous triethanolamine solution under visible light.When loaded with RuO2 cocatalysts, C3N4 can oxidize water to produce O2 in aqueousAgNO3 solution containing La2O3 buffer under visible light. The photocatalytic activityof C3N4 for H2 evolution can be improved by one order of magnitude by introduc-ing the right type of mesoporosity into polymeric C3N4.183 By copolymerization withbarbituric acid during the synthesis, the optical absorption of g-C3N4 was extendedinto the visible region of up to about 750 nm, and the as-prepared material demon-strated activity for H2 production in aqueous triethanolamine solution under visible lightirradiation.184

Doping Strategies (Type B)

Metal Ion Doping

Ti-Based Materials. Even thoughTiO2 has beenwidely investigated for its excel-lent characteristics in photocatalysis, one of the major problems for TiO2 is its largeband gap that restricts the utilization of visible light. Doping of TiO2 with suitable metalions is an effective strategy for introducing visible light-induced water-splitting abilityto TiO2. Kudo et al. investigated a series of metal ion-doped TiO2 photocatalysts forwater-splitting reactions.17,185–188 Different metal ions such as Cr3+, Ni2+, Rh3+, Ta5+,


Nb5+, and Sb5+ have similar radii with that of Ti4+, which can replace Ti4+ in TiO2during solid-state synthesis at high temperatures. However, only the doping of Cr3+,Ni2+, and Rh3+ ions can form donor levels in the forbidden band of TiO2 and introducevisible light activity while other metal ions such as Ta5+, Sb5+, and Nb5+ cannot. Butthese metal ions are indispensable to be used together with Cr3+ and Rh3+ to compen-sate the charge imbalance in TiO2. TiO2 doped with Cr3+ or Rh3+ is not active for H2or O2 production under visible light due to the presence of Cr5+ and Rh4+ that workas the recombination centers. However, TiO2 codoped with Cr3+/Sb5+ and Rh3+/Sb5+

can oxidize water to produce O2 in aqueous AgNO3 solution under visible light. Thecodoping of Sb5+ and Nb5+ can suppress the formation of Cr5+ and Rh4+ and prolongthe life time of photogenerated electrons. The visible light responses are ascribed to thetransitions from electron donor levels consisting of Cr3+ and Rh3+ to the conductionband of the TiO2 host. TiO2 doped with Ni2+ alone is active for O2 production undervisible light. The codoping of Rh3+/Sb5+ and Rh3+/Sb5+ in TiO2 can increase the O2production by two times. Even metal ion-doped TiO2 demonstrates good activity forO2 under visible light, but all of them are inactive or exhibit inferior activity for H2production possibly due to the material’s low conduction band positions.

SrTiO3 with a high conduction band level can be doped with metal ions toachieve visible light-inducedwater-splitting ability. Codoping of Cr3+/Ta5+, Cr3+/Nb5+,Cr3+/Sb5+, and Ni2+/Ta5+ and doping of Rh3+ are effective in the sensitization ofSrTiO3.185,186, 189, 190 All the cation-doped SrTiO3 photocatalysts are active for H2 pro-duction in the presence of methanol as the sacrificial reagent under visible light. Espe-cially, the Rh-doped SrTiO3 is one of the rare oxide photocatalysts that can efficientlyproduce H2 under visible irradiation. However, all the metal ion-doped SrTiO3 photo-catalysts are inactive or inefficient for O2 production in the presence of aqueous AgNO3solution under visible light, but this disadvantage can be partially remedied by utilizingthem in the fabrication of Z-scheme system. In fact, SrTiO3 codoped with Cr3+ andTa5+ has demonstrated to be an efficient H2-evolution photocatalyst in Z-scheme systemwhen using Pt as the cocatalyst, IO3−/I− as the redox mediator, and Pt/WO3 as theO2-evolution photocatalyst.191,192 Several other Z-scheme systems such as (Ru/SrTiO3–Rh)−(Fe3+/Fe2+)−(BiVO4), (Pt/SrTiO3:Rh)−(Fe3+/Fe2+)−(BiVO4), and (Pt/SrTiO3–Rh)−(Fe3+/Fe2+)−(WO3) have also demonstrated high efficiency for overall watersplitting under visible light.40, 193, 194 More interestingly, a direct Z-scheme system madeof the simple mixture of Ru/SrTiO3–Rh and BiVO4 can split pure water stoichiomet-rically into H2 and O2 under visible light without the aid of a redox mediator.195 It issupposed that the direct electron transfer between Ru/SrTiO3–Rh and BiVO4 will occurduring the reactions. Doping of Rh3+ in CaTiO3 and Ca3Ti2O7 and doping of Cr3+ andFe3+ in La2Ti2O7 photocatalysts can also introduce visible light-induced H2 productionin the presence of methanol as the sacrificial reagent.196–198

Besides the metal ions mentioned above, the doping of different metals such asV5+, Sn4+, and Mo6+ on Ti-based materials has been investigated. Even though thedoping of some of these metals can introduce visible light activity for the photocatalyticdegradation of pollutants, they are inactive or quite inefficient for the water-splittingreactions. Moreover, the doping of a lot of metal ions cannot form the doping level inthe forbidden band of pristine materials and therefore cannot decrease the band gap.


Other Metal Ion-Doped Oxide Materials. Ta and Nb form large groups ofhighly efficient UV-responsive photocatalysts for water splitting. The doping of propermetal ions in these host materials can lead to efficient visible light-responsive photo-catalysts. Iwase et al. investigated the codoping of Ir ions and alkaline earth metal ionsor lanthanum ions in NaMO3 (M = Ta and Nb) with perovskite structure.199 NaMO3(M = Ta and Nb) doped with Ir ions alone was inactive or quite inefficient for H2 andO2 production under visible irradiation. When NaMO3 (M = Ta and Nb) was codopedwith Ir and alkaline earth metal ions or lanthanum ions, the resulting materials showedphotocatalytic activities for H2 or O2 evolution from an aqueous solution containinga sacrificial reagent under visible light irradiation. Ir was found to substitute M at Bsites of perovskite structure and alkaline earth metal and lanthanum ions were found tosubstitute Na in NaMO3 (M= Ta and Nb). Similar with what has been observed in TiO2and SrTiO3, the codoping contributed to maintaining the charge balance and forming offine crystals of the NaMO3 powder. The electronic transition from the electron donorlevel formed with partially filled 5d orbitals of Ir3+ to the conduction band composedof Nb 4d or Ta 5d orbitals leads to the visible light response. In a similar manner, Yanget al. found that the NaTaO3 photocatalyst codoped with Cr3+ and La3+ was active forH2 production from aqueous methanol solution under visible light.200 Zou et al. alsoinvestigated the photocatalytic water-splitting abilities of InTaO4 doped with differentmetal ions such as Fe, Co, Ni, Cu, and Mn.201–203 The Ni-doped InTaO4 was reportedto split pure water into H2 and O2 stoichiometrically with a quantum yield of 0.66% at402 nm. The Ni 3d donor level formed in the forbidden band of InTaO4 was supposedto lead to the visible light response.

Shimodaira et al. investigated the photocatalytic water-splitting ability of PbMoO4doped with Cr (denoted as PbMo1–xCrxO4).204 After Cr6+ doping, the band gap ofPbMoO4 was decreased from 3.3 to 2.1–2.4 eV. The as-prepared PbMo1–xCrxO4 showedhigh photocatalytic activity for O2 evolution from an aqueous solution containingAg+ or Fe3+ as the sacrificial reagents under visible light irradiation. The band gapand photocatalytic activity of PbMo1–xCrxO4 depend upon the concentration of Cr6+

dopants. The optimum PbMo0.98Cr0.02O4 photocatalyst can produce O2 with a quantumyield of 6% at 420 nm. DFT calculations indicate that the top of the valence band ofPbMo1–xCrxO4 consisted of O 2p and Pb 6s orbitals and an accepter level composed ofCr 3d orbitals were formed in the forbidden band below the conduction band consistingof Mo 4d orbitals (Figure 14.16). The visible light response was due to the excitationfrom the Pb 6s + O 2p valence band to the Cr 3d accepter level. In contrast with theCr3+ doping in TiO2 where an electron–donor level was formed above the valence band,the present Cr6+ doping in PbMoO4 could form an electron–acceptor level below theconduction band, leading to a reduced band gap in the host photocatalyst (Figure 14.14,type B, right).

In2O3 and Ba2In2O5 can produce trace amounts of H2 and O2 in the presenceof methanol and AgNO3 as sacrificial reagents, separately, under visible light. Whenboth materials were doped with Cr and calcined together, the as-prepared Cr-In2O3/Cr-Ba2In2O5 nanocomposite demonstrated improved photocatalytic activity for H2 produc-tion in the presence of methanol as the sacrificial reagents.205


CaMoO4 PbMoO4 Pb(Mo1-xCrx)O4

Cr3d

Mo4dMo4dMo4d

BG:3.8 eV BG:3.3 eV EG:2.2 eV

1.1 eV

O2p+Pb6sO2p+Pb6sO2p

0.5 eV

Ene

rgy

−

Figure 14.16. Band structures of Cr6+-doped PbMoO4 and CaMoO4.204

Sulfide Materials. ZnS is an efficient photocatalyst for H2 production underUV light. The doping of appropriate metal ions in ZnS can extend the light absorptionof ZnS from the UV to the visible region. Kudo and Sekizawa investigated the dopingof metal ions such as Ni2+ and Cu2+ in ZnS.206,207 After doping Ni2+ and Cu2+ inZnS, the band gap of the host ZnS photocatalyst was reduced from 3.7 to 2.3 and2.5 eV, respectively. The visible light response was due to the formation of 3d donorlevels by the doped Ni2+ and Cu2+ in the forbidden band of the ZnS host photocatalyst.The doped ZnS photocatalysts showed high activities for H2 evolution from aqueoussolutions containing S2− and or SO32− as the sacrificial reagents under visible light. Theloading of cocatalyst did not promote the activity a lot due to the high conduction bandlevel of ZnS that allowed the easy reduction of protons to H2 on the surface of ZnS.

CdS is an excellent photocatalyst for H2 production under visible light when loadedwith cocatalysts. The doping of Mn was reported to improve the photocatalytic activityand stability of CdS for H2 production under visible light without cocatalysts on it.208

However, the activity of MnxCd1–xdS remains quite low. Similar with what has beenobserved in ZnS, the doping of Cu2+ and Ni2+ in ZnxCd1–xS solid solution materialscan also increase the photocatalytic activity of ZnxCd1–xS for H2 production.209–212 Thedoping of Cu2+ and Ni2+ can further decrease the band gap of ZnxCd1–xS by formingdonor levels in the forbidden band of ZnxCd1–xS. Cd0.1Cu0.01Zn0.89S and 0.1 wt% Ni2+-doped Cd0.1Zn0.9S can produce H2 in the presence of S2− and SO32− as the sacrificialreagents with a quantum yield of 9.6% and 15.9% at 420 nm, respectively. Certainamount of Cu2+ doping in ZnIn2S4 photocatalysts can also enhance the light absorptionand photocatalytic H2 production in the presence of S2− and SO32− as the sacrificialreagents in the visible region.213

Nonmetal Ion Doping. Nonmetal ion doping is also a very important strategyfor modifying photocatalyst toward visible light response. Asahi et al. first investigated


the doping of some nonmetal ions such as C, N, F, P, and S for O in TiO2 and found thatthe doping of N, C, S, P could narrow the band gap of TiO2. N-doped TiO2 photocatalystwas particularly the most efficient for dye degradation under visible light.214 Thus,extensive research has been focused on the sensitization of TiO2 and otherUV-responsivephotocatalysts by nonmetal ion-doping or codoping strategies as well.215 Despite theseefforts, most of the works focused on the photocatalytic mineralization of pollutants.Photocatalytic water-splitting property was rarely achieved except photoelectrochemicalwater splitting with the help of applied bias.

Domen et al. investigated the photocatalytic water-splitting property of non-metal-doped Ti-based materials. When TiO2 was codoped with and N and F, the band gap ofTiO2 was decreased from 3.3 to 2.2 eV. Instead of a tail-like absorption observed in N orF-doped TiO2, N/F-codoped TiO2 demonstrated a sharp absorption edge in the visibleregion, indicating a band-to-band excitation occurring upon light irradiation.46,216, 217

DFT calculations indicate that the doping level formed by N 2p orbitals in the forbiddenband above the valence band consisting of O 2p orbitals contributes to the visible lightresponse of N/F-codoped TiO2. The presence of F will help to maintain the chargebalance and stabilize the structure of TiO2. Under visible light irradiation, N/F-codopedTiO2 is quite active for photocatalytic O2 production in the presence of AgNO3 asthe sacrificial reagent. N/F-codoped TiO2 also demonstrated trace activity for H2 evo-lution under visible light in aqueous methanol solution when loading Pt cocatalysts.Wang et al. prepared N/F-codoped TiO2 plates with high percentage of {001} facetby nitriding TiOF2 cubic crystals in flowing ammonia. The as-prepared N/F-codopedTiO2 plates demonstrated high activity for photocatalytic O2 production.218 By nitridiza-tion of Zn2TiO4 precursors in flowing NH3, N was doped into Zn2TiO4 photocatalysts.Compared with the UV-responsive Zn2TiO4 photocatalyst, the as-prepared ZnxTiOyNx

photocatalyst had a band gap of 2.3 eV and demonstrated very high activity for thephotocatalytic O2 evolution in the presence of AgNO3 as the sacrificial reagent undervisible light. When loaded with Pt as cocatalysts, ZnxTiOyNx photocatalyst can alsoreduce water to produce H2 in the aqueous methanol solution under visible light, but theactivity is quite low.219

Hagiwara et al. investigated the photocatalytic water-splitting property of N-dopedKTa(Zr)O3.220 The absorption ofKTa(Zr)O3 was extended from theUV to visible regionsafter N doping. When loading the Pt cocatalyst on it, N-doped KTa(Zr)O3 was reportedto split pure water into H2 and O2 stoichiometrically under visible light irradiation. Jiet al. investigated the N-doped Sr2Nb2O7 for water-splitting reactions.221 The absorptionof Sr2Nb2O7 was extended from the UV to visible regions after N doping. The top of thevalence band of Sr2Nb2O7–xNx consists of N 2p orbitals, leading to the reduced band gap.Sr2Nb2O7–xNx was active for photocatalytic H2 production in aqueous methanol solutionunder visible light irradiation when loading Pt as the cocatalyst. The photocatalyticH2 production is supposed to be contributed by Sr2Nb2O7–xNx with both Sr2Nb2O7and SrNbO2N crystal phases. Baeg et al. reported that Nb2Zr6O17−xNx photocatalystsynthesized by thermal ammonolysis of Nb2Zr6O17 at 1073K demonstrated high activityfor photocatalytic H2 production from S2− aqueous solution under visible light whenloading Pt or RuO2 as cocatalysts.


Wang et al. investigated the photocatalytic water-splitting properties of several non-metal-doped ion-exchangeable materials.222–224 N-doped and N/S-codoped CsTaWO6can reduce water to produce H2 in aqueous ethanol solution under visible light. N-doped CsCa2Ta3O10 was reported to be able to catalyze water oxidation to produce O2in aqueous AgNO3 solution under visible light, and N-doped TiOx nanosheet showedactivity for electrochemical water splitting under visible light.225

Jang et al. investigated the photocatalytic water-splitting ability of C-doped ZnS.226

By calcining a ZnS(en)0.5 complex (en = ethylenediamine) at different temperatures, Cwill substitute some of the S atoms in ZnS and form Zn–C bonds. The contribution fromthe C 2p orbitals in the formation of the upper valence band should be responsible for thevisible light absorption of ZnS. C ZnS was reported to exhibit photocatalytic activityfor H2 production in aqueous S2− and SO32− solution under visible light irradiation.

As a visible light-responsive metal-free photocatalyst, C3N4 doped with variousnonmetal ions has been investigated for photocatalytic water splitting.227 C3N4 homo-geneously doped with sulfur leads to an elevated conduction band level and an increasedvalence bandwidth. When loading with Pt as cocatalysts, the resulting C3N4–xSx demon-stratedmuch higher activity for H2 evolution than C3N4 under visible light irradiation.227

C3N4 doped with phosphor demonstrated a much better electric (dark) conductivity upto four orders of magnitude and an improvement in photocurrent generation by a factorof up to five.228 C3N4 doped with fluorine also demonstrated higher activity for photo-catalytic H2 production from aqueous triethanolamine solution under visible light dueto the modification of the electronic band structures.229

Metal/Nonmetal Ion Codoping. Metal ion and nonmetal ion-doping strate-gies could be combined to modify the physiochemical properties of photocatalyststoward high activity under visible light. Similar with nonmetal doping, most work onmetal/nonmetal ion codoping focused on photocatalytic mineralization of pollutant ortheoretical calculations, and only a few studies reported photocatalytic water splittingwith metal/nonmetal ion codoped photocatalyst.

Kudo reported Pb and halogen-codoped ZnS. Pb-doped ZnS can produce H2 fromaqueous K2SO3 solution under visible light irradiation without loading Pt cocatalystson the material. The formation of the donor level by the fully filled Pb 6s orbitals wassupposed to result in the visible light response. After codoping with different halogenions such as Cl−, Br−, and I− ions, the activity of Pb-doped ZnS was greatly enhanced.Pb and Cl−-codoped ZnS demonstrated an activity three times that of Pb-doped ZnS.The doped halogen ions were supposed to relax the distortion by the doping of largePb cations and suppress the formation of nonradiative recombination centers betweenphotogenerated electrons and holes.230

Lei et al. investigated the codoping of Zn2+ and S2− in In(OH)3 photocatalysts. Theband gap of In(OH)3 was found to be narrowed after S2− doping, which was ascribedto the contribution of S 3p orbitals to the formation of the top of the valence band.In(OH)xSy demonstrated photocatalytic activity for H2 production from aqueous Na2Sand Na2SO3 solution under visible light when loaded with Pt cocatalysts. After Zn2+

doping in In3+ sites, the activity for photocatalyticH2 evolution on In(OH)xSywas greatly


increased. The doping of Znwas supposed to increase the level of the conduction band byhybridizing the Zn 4s4p with In 5s5p orbitals, and therefore increase the thermodynamicdriving force for the reduction of water to produce H2.231

For all the doped photocatalysts mentioned above, the dopant level formed is usuallydiscrete and thus inconvenient for the migration of photogenerated charges. Moreover,the formation of recombination centers between photogenerated electrons and holes bythe dopant is inevitable, which decrease the photocatalytic activity dramatically. How-ever, doping of transition metals is a good strategy to develop visible light-responsivephotocatalysts.

Solid Solution Materials (Type C)

Oxide-Based Materials. Ye et al. developed a series of oxide-based solid solu-tion materials for photocatalytic water splitting and pollutant degradation under visiblelight.232–236 Na0.5Bi1.5VMoO8 can be regarded as a solid solution between narrow band-gap BiVO4 and wide band-gap (NaBi)0.5MoO4 photocatalysts.233 The band gap andconduction and valence band levels of Na0.5Bi1.5VMoO8 are between those of BiVO4and (NaBi)0.5MoO4. Under visible light irradiation, Na0.5Bi1.5VMoO8 can oxidize waterto produce O2 from aqueous AgNO3 solution, and the activity is higher than that ofBiVO4 prepared with the similar solid-state reaction. Figure 14.17 shows schematicband structures of (NaBi)0.5MoO4, Na0.5Bi1.5VMoO8, and BiVO4. The valence band ofNa0.5Bi1.5VMoO8 is composed of hybridized Bi 6s and O 2p orbitals, and the conductionband is composed of Mo 4d and V 3d orbitals. The high activity of Na0.5Bi1.5VMoO8for O2 evolution was attributed to its more positive potential of the valence band witha larger oxidation power of the photogenerated holes. Similarly, CaMoO4 and BiVO4could form Ca1–xBixVxMo1–xO4 solid solution with tetragonal crystal structures in awide range. Ca1–xBixVxMo1–xO4 demonstrated high activity for photocatalytic O2 pro-duction from aqueous AgNO3 solution under visible light, and Ca0.3Bi0.7V0.7Mo0.3O4

O 2p O 2p + Bi 6s

2.85 eV

0.46 eV

2.39

eV

2.5

eV

3.23

eV

Pot

entia

l

−0.13 eVCB

VB

+

−

Na0.5Bi0.5MoO4

Na0.5Bi1.5VMoO8BiVO4

O2/H2O

3.1 eV

V 3dMo 4d + V 3dMo 4d

(Bi 6s)O 2p

(Bi 6s)

Figure 14.17. Schematic band structures of (NaBi)0.5MoO4, Na0.5Bi1.5VMoO8, and BiVO4.233


with a band gap of 2.42 eV showed the highest activity.234 Yi and Ye also developedNa1−xLaxTa1−xCoxO3 (x = 0–0.25), which could be regarded as a solid solution betweenNaTaO3 and LaCoO3 with similar perovskite structure.232 It has strong absorption inthe visible region due to the hybridization of the Co 3d and O 2p orbitals in the bandstructures. When loaded with Pt cocatalysts, Na1−xLaxTa1−xCoxO3 (x= 0–0.25) showedactivity for photocatalytic H2 production from aqueous methanol solution under visi-ble light, with Na0.9La0.1Ta0.9Co0.1O3 photocatalyst specifically exhibiting the highestactivity. In the following work, Ye et al. developed a series of (AgNbO3)1–x(SrTiO3)x(0� x �1) solid solution between narrow band-gap AgNbO3 and large band-gap SrTiO3photocatalysts. (Ag1–xSrx)(Nb1–xTix)O3 crystallized in an orthorhombic (0 � x � 0.9)or a cubic (0.9 � x � 1) system. The conduction band of (AgNbO3)1–x(SrTiO3)x solidsolution is composed of hybridized Ti 3d and Nb 4d orbitals, and the valence band iscomposed by the hybridized O 2p + Ag 4d orbitals. Under visible light irradiation,(AgNbO3)1–x(SrTiO3)x could oxidize water to produce O2 in aqueous AgNO3 solutionand (AgNbO3)0.75(SrTiO3)0.25 demonstrated the highest activity for O2 evolution.235,236

Wang and Liu et al. developed a series of solid solution photocatalysts.237,238

BiYWO6 can be regarded as the solid solution between Y2WO6 and Bi2WO6. Bi 6sand Y 4d orbitals were supposed to contribute to a new valence band and a conduc-tion band, respectively, leading to visible light response and proper band structure forwater-splitting reactions. Under visible light irradiation, BiYWO6 can produce H2 or O2in aqueous Na2SO3 and AgNO3 solution, respectively. Moreover, when loading RuO2as cocatalysts on the material, BiYWO6 can split pure water into H2 and O2 stoichio-metrically under visible light irradiation. In the following work, the authors discoveredBi0.5Dy0.5VO4 solid solution composed of BiVO4 and DyVO4. Bi0.5Dy0.5VO4 has aband gap of 2.76 eV. Under visible light irradiation, Bi0.5Dy0.5VO4 solid solution canproduce H2 or O2 in aqueous Na2SO3 and AgNO3 solution, respectively. Moreover,when loading Pt–Cr2O3 as the cocatalyst, Bi0.5Dy0.5VO4 is shown to split pure waterinto H2 and O2 stoichiometrically under visible light irradiation.

Sulfide Materials. Kudo et al. developed several sulfide-based solid solutions,such as ZnS–AgInS2, ZnS–CuInS2, and ZnS–AgInS2–CuInS2, for photocatalytic H2production.239–243 Wide band-gap ZnS and narrow band-gap AgInS2 photocatalysts havevery similar hexagonal wurtzite structure and can form (AgIn)xZn2(1–x)S2 solid solutionsin a wide range.241 Figure 14.18 shows a schematic illustration of the band structures ofZnS, AgInS2, and (AgIn)xZn2(1–x)S2 solid solutions. It was found that the band gap andthe conduction and valence band levels of the (AgIn)xZn2(1–x)S2 solid solutions werebetween those of ZnS and AgInS2. By changing the compositions of ZnS and AgInS2,the energy structures of the resulting solid solution material can be systematically tuned.DFT calculations indicated that the changes in the band energy structures were caused bythe contribution of the Ag 4d and In 5s5p orbitals to the valence and conduction bands,respectively. In photocatalytic water-splitting reactions, AgInS2 and ZnS alone hardlyshowed any activity under visible light irradiation. However, (AgIn)xZn2(1–x)S2 solidsolutions alone showed high activity for photocatalytic H2 production from aqueousSO32− and S2− solution under visible light. The loading of Pt cocatalysts can furtherimprove the photocatalytic activity of the material. The (AgIn)0.22Zn1.56S2 solid solution


S3p+

Ag4d

Zn4s4p+

In5s5p

Zn4s4p

S3p+

Ag4d

In5s5p

1.80 eV3.55 eV

H+/H2

−

O2/H2O

Pot

entia

l (V

)

S3p

ZnS (Agln)xZn2(1-x)S2 AglnS2

Figure 14.18. Band structures of (AgIn)xZn2(1–x)S2 solid solutions, ZnS, and AgInS2.241

containing optimum amount of Pt can produce H2 from photocatalytic water splittingwith a quantum yield of 20% at 420 nm. Similarly, ZnS and CuInS2 can form solidsolutions in a wide range and the Cu 3d orbitals contribute to the formation of the valenceband in ZnS–CuInS2 solutions.240 The optimized (CuIn)0.09Zn1.82S2 solid solution canproduce H2 from aqueous SO32− and S2− solution with a quantum yield of 12.5% at420 nm. When ZnS, AgInS2, and CuInS2 were alloyed together, the as-obtained ZnS–AgInS2–CuInS2 solid solution exhibited a higher photocatalytic activity for H2 evolutionthan ZnS–AgInS2 and ZnS–CuInS2 solid solutions under the irradiation from a solarsimulator.239

Several groups investigated the photocatalytic water-splitting property of CdxZn1–xSsolid solution formed between narrow band-gap CdS and wide band-gap ZnS photo-catalysts.244–246 CdS and ZnS have similar crystal structure and can form CdxZn1–xS(0 � X � 1) solid solutions in a wide range. The valence band of CdxZn1–xS is com-posed of S 3p orbitals, and the conduction band is composed of Zn 4s4p and Cd 5s5porbitals. The changes in the band energy structures are mainly caused by the changeof the conduction band level due to the variation of the compositions. This is differ-ent from ZnS–AgInS2–CuInS2 solid solutions whose conduction and valence bandsare modified simultaneously. Under visible light irradiation, CdxZn1–xS prepared witha coprecipitation method demonstrated high photocatalytic activity for H2 productionfrom aqueous Na2S/Na2SO3 solution.244 CdxZn1–xS prepared by the thermal sulfidationof the corresponding mixed oxide precursors displayed a much higher photoactivity forH2 production than that prepared by the coprecipitation method.246

Jang et al. investigated the photocatalytic water-splitting property of AgGa1–xInxS2solid solutions.247 AgGaS2 and AgInS2 have a band gap of 2.7 eV and 1.9 eV, respec-tively. The band gap of AgGa1–xInxS2 solid solutions can be tuned between 2.7 eV and1.9 eV, depending on the compositions of AgGaS2 and AgInS2. Similar with CdxZn1–xSsolid solutions, the band structure of AgGa1–xInxS2 is mainly modified by the changeof the conduction band consisted of In 5s5p and Ga 4s4p orbitals. When loaded with Ptas cocatalysts, AgGaS2 showed activity for photocatalytic H2 production from aqueous


GaN(Ga1-xZnx)(N1-xOx)

with x = 0.05(Ga1-xZnx)(N1-xOx)

with x = 0.22

N2p+

Zn3d, O2p

N2p+

Zn3d, O2p

2.8 eV3.4 eV

Ga4s, 4pC.B.

V.B.

Ga4s, 4p Ga4s, 4p

2.6 eV

N2p

Figure 14.19. Schematic band structures of GaN and GaN–ZnO solid solutions.248

Na2S/Na2SO3 solution under visible light irradiation, while AgInS2 was inactive. TheAgGa1–xInxS2 solid solutions also exhibited activity for H2 production, and the optimumactivity was achieved on AgGa0.9In0.1S2 solid solution.

Oxynitride Materials. Domen et al. developed a series of oxynitride solid solu-tion photocatalysts, which were capable of splitting pure water into H2 and O2 stoichio-metrically under visible light.5, 6, 20, 46,248–259 GaN–ZnO was the first and most investi-gated oxynitride solid solution photocatalyst. It was obtained by heating the Ga2O3 andZnOmixture precursor in a flow of NH3. By controlling the heating temperature, heatingtime, and flow rate of NH3, the physiochemical properties of the solid solution can besystematically tuned. The GaN–ZnO solid solution has a wurtzite structure similar withthat of GaN and ZnO. Interestingly, the band gap of the solid solution (2.4 eV) is muchsmaller than that of the separate components, GaN (3.4 eV) and ZnO (3.2 eV). This is dif-ferent from what has been commonly observed in oxide photocatalysts, where the bandgap of solid solution is between those of the components. This unusual phenomenonis considered to be due to a raised valence band level resulting from p–d repulsion ofZn 3d and N 2p orbitals. Although GaN–ZnO alone is inactive for overall water split-ting under visible light (Figure 14.19), after loading it with RuO2 cocatalysts, it cansplit pure water stoichiometrically into H2 and O2 under visible light. When RuO2 wasreplaced with the CrxRh2–xO3-mixed oxide cocatalyst, the activity of GaN–ZnO could begreatly enhanced. The loading of noble metal/Cr2O3 core/shell nanoparticles and tran-sition metal–Cr2O3 composite particles can also dramatically increase the activity ofGaN–ZnO.250,253 Postcalcination of GaN–ZnO solid solution in static air at appropriatetemperature can improve the overall photocatalytic water-splitting ability under visiblelight, a quantum yield of 5.9% at 420–440. This enhancement in activity is ascribed to adecrease of zinc- and/or oxygen-related defects that function as recombination centers forphotogenerated electrons and holes.6 GaN–ZnO can also produce H2 and O2 in the pres-ence of methanol and AgNO3 as the sacrificial reagents. The rate-determining step foroverall water splitting using GaN–ZnO was supposed to be the H2-evolution process.260

ZnGeN2–ZnO solid solution photocatalyst is the other oxynitride solid solutioncapable of splitting pure water into H2 an O2 under visible light.255,256 It was obtained


by heating the GeO2 and ZnO mixture precursor in a flow of NH3. Similar with GaN–ZnO, the band gap of ZnGeN2–ZnO (2.7 eV) is smaller than that of both ZnO (3.2 eV)and ZnGeN2 (3.2 eV). When loaded with RuO2 as cocatalysts, ZnGeN2–ZnO couldsplit pure water into H2 and O2 stoichiometrically under visible light irradiation. WhenRhxCr2–xO3 was loaded as cocatalysts on the material, the activity on ZnGeN2–ZnOcould be further improved by eight times compared with that loaded with RuO2 ascocatalysts. Likewhatwas observed inGaN–ZnO solid solution, the activity of ZnGeN2–ZnO photocatalyst for overall water splitting under visible light could be drasticallyenhanced by a heat treatment in N2, achieving a quantum yield of 2.0% at 420–440 nm.The enhanced activity is due to a decrease of defects that act as recombination centersfor photogenerated electrons and holes.261

SUMMARY AND FUTURE PERSPECTIVE

Semiconductor-based nanocatalysts are the key components for photocatalytic watersplitting with utilization of solar energy. The development of cost-effective, highlystable, visible light-responsive, and environment benign photocatalysts is crucial forthe practical application of this technology. Up to now, more than 100 semiconductormaterials have been developed with the capability of catalyzing overall water splitting orwater splitting in the presence of sacrificial reagents underUVor visible light. Especially,a plentiful of highly efficient photocatalysts such as La–NiO/NaTaO3, GaN–ZnO solidsolution, Pt–PdS/CdS, and Ag3PO4 have been developed in the past decades as researchin material science and nanotechnology has rapidly progressed. These achievementsrepresent the great progress in this field with the combined effort of scientists frommultidisciplinary backgrounds. However, the fundamental mechanism underlying thephenomenon in this field is far from clear. Many aspects controlling the photocatalyticwater-splitting performance have not been elucidated in sufficient detail. Moreover, tobe practical for applications, the solar energy conversion efficiency for water splittinginto H2 and O2 should be higher than 5%, corresponding to a quantum yield of 30% at600 nm. The semiconductor photocatalysts developed up to now is far from this target.Furthermore, some highly efficient photocatalysts capable of splitting pure water intoH2 and O2, such as Rh2–xCrxO3/GaN–ZnO, Rh–SrTiO3–BiVO4, contain rare or toxicelements, which makes it impractical in real applications. To develop an “omnipotent”semiconductor photocatalyst, efforts in the following two aspects should be considered.First, the fundamental issue affecting the photocatalytic water-splitting process needs tobe clearly illustrated at a microcosmic level. This could be realized with the developmentof advanced characterization techniques such as time-revolved infrared and fluorescencespectroscopy. Second, the novel band-gap engineering strategy and synthesis methodsshould be developed to fine-tune the crystal structure, electronic structure, surface state,and morphology of photocatalysts. Theoretical calculations should also be combined toprovide useful guidance for the development of novel photocatalytic materials. After all,these efforts represent significant opportunities and challenges of this field—the HolyGrail of chemistry.

REFERENCES 545

REFERENCES

1. Muradov N. Z., Veziroglu T. N. “Green” path from fossil-based to hydrogen economy: Anoverview of carbon-neutral technologies. Int. J. Hydrogen Energ. 2008;33:6804.

2. Zuttel A., Borgschulte A., Schlapbach, L.Hydrogen as a Future Energy Carrier. Weinheim:Wiley-VCH Verlag GmbH; 2008.

3. Fujishima A., Honda K. Electrochemical photolysis of water at a semiconductor electrode.Nature 1972;238:37.

4. KatoH., AsakuraK., KudoA.Highly efficientwater splitting intoH2 andO2 over lanthanum-dopedNaTaO3 photocatalystswith high crystallinity and surface nanostructure. J. Am. Chem.Soc. 2003;125:3082.

5. Maeda K., Takata T., Hara M., Saito N., Inoue Y., Kobayashi H., Domen K. GaN:ZnO solidsolution as a photocatalyst for visible-light-driven overall water splitting. J. Am. Chem. Soc.2005;127:8286.

6. Maeda K., Teramura K., Domen K. Effect of post-calcination on photocatalytic activity of(Ga1-xZnx)(N1-xOx) solid solution for overall water splitting under visible light. J. Catal.2008;254:198.

7. Yan H. J., Yang J. H., Ma G. J., Wu G. P., Zong X., Lei Z. B., Shi J. Y., Li C. Visible-light-driven hydrogen production with extremely high quantum efficiency on Pt-PdS/CdSphotocatalyst. J. Catal. 2009;266:165.

8. Yi Z. G., Ye J. H., Kikugawa N., Kako T., Ouyang S. X., Stuart-Williams H., Yang H., CaoJ. Y., Luo W. J., Li Z. S., Liu Y., Withers R. L. An orthophosphate semiconductor withphotooxidation properties under visible-light irradiation. Nat. Mater. 2010;9:559.

9. Linsebigler A. L., Lu G. Q., Yates J. T. Photocatalysis on TiO2 surfaces: Principles, mechan-ims, and selected results. Chem. Rev. 1995;95:735.

10. Abe R. Recent progress on photocatalytic and photoelectrochemical water splitting undervisible light irradiation. J. Photochem. Photobiol. C 2010;11:179.

11. Bard A. J., Fox M. A. Artificial photosynthesis – solar splitting of water to hydrogen andoxygen. Acc. Chem. Res. 1995;28:141.

12. Hernandez-Alonso M. D., Fresno F., Suarez S., Coronado J. M. Development of alternativephotocatalysts to TiO2: Challenges and opportunities. Energ. Environ. Sci. 2009;2:1231.

13. Inoue Y. Photocatalytic water splitting by RuO2-loaded metal oxides and nitrides with d0-and d10-related electronic configurations. Energ. Environ. Sci. 2009;2:364.

14. Kudo A. Photocatalyst materials for water splitting. Catal. Surv. Asia 2003;7:31.

15. Kudo A. Development of photocatalyst materials for water splitting. Int. J. Hydrogen Energ.2006;31:197.

16. Kudo A. Photocatalysis and solar hydrogen production. Pure Appl. Chem. 2007;79:1917.

17. Kudo A., Kato H., Tsuji I. Strategies for the development of visible-light-driven photocata-lysts for water splitting. Chem. Lett. 2004;33:1534.

18. Liu G., Wang L. Z., Yang H. G., Cheng H. M., Lu G. Q. Titania-based photocatalysts-crystalgrowth, doping and heterostructuring. J. Mater. Chem. 2010;20:831.

19. Maeda K., Domen K. New non-oxide photocatalysts designed for overall water splittingunder visible light. J. Phys. Chem. C 2007;111:7851.

20. Maeda K., Teramura K., Domen K. Development of cocatalysts for photocatalytic overallwater splitting on (Ga1-xZnx)(N1-xOx) solid solution. Catal. Surv. Asia 2007;11:145.


21. Navarro R. M., Alvarez-Galvan M. C., de la Mano J. A. V., Al-Zahrani S. M., FierroJ. L. G. A framework for visible-light water splitting. Energ. Environ. Sci. 2010;3:1865.

22. Navarro R. M., Sanchez-Sanchez M. C., Alvarez-Galvan M. C., del Valle F., Fierro J. L. G.Hydrogen production from renewable sources: Biomass and photocatalytic opportunities.Energ. Environ. Sci. 2009;2:35.

23. Ni M., Leung M. K. H., Leung D. Y. C., Sumathy K. A review and recent developments inphotocatalytic water-splitting using TiO2 for hydrogen production. Renew. Sust. Energ. Rev.2007;11:401.

24. Nozik A. J. Photoeletrochemistry-applications to solar-energy conversion. Annu. Rev. Phys.Chem. 1978;29:189.

25. Shimura K., Yoshida H. Heterogeneous photocatalytic hydrogen production from water andbiomass derivatives. Energ. Environ. Sci. 2011;4:2467.

26. Walter M. G., Warren E. L., McKone J. R., Boettcher S. W., Mi Q. X., Santori E. A., LewisN. S. Solar water splitting cells. Chem. Rev. 2010;110:6446.

27. Zhu J. F., Zach M. Nanostructured materials for photocatalytic hydrogen production. Curr.Opin. Colloid Interface Sci. 2009;14:260.

28. Kudo A., Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc.Rev. 2009;38:253.

29. Chen X. B., Shen S. H., Guo L. J., Mao S. S. Semiconductor-based photocatalytic hydrogengeneration. Chem. Rev. 2010;110:6503.

30. Osterloh F. E. Inorganic materials as catalysts for photochemical splitting of water. Chem.Mater. 2008;20:35.

31. Gratzel M. Energy Resources through Photochemistry and Catalysis. New York: Academicpress; 1983.

32. Rajeshwar K., Mcconnell R., Licht, S. Solar Hydrogen Generation. New York: Springer;2008.

33. Serpone N., Pelizzetti E. Photocatalysis. New York: Academic press; 1989.

34. Bard A. J. Photoelectrochemistry and heterogeneous photocatalysis at semiconductors.J. Photochem. 1979;10:59.

35. Bard A. J. Photoelectrochemistry. Science 1980;207:139.

36. Butler M. A., Ginley D. S. Prediction of flatband potentials at semiconductor-electrolyteinterfaces from atomic electronegativities. J. Electrochem. Soc. 1978;125:228.

37. Xu Y., Schoonen M. A. A. The absolute energy positions of conduction and valence bandsof selected semiconducting minerals. Am. Miner. 2000;85:543.

38. Kawai T., Sakata T. Conversion of carbohydrate into hydrogen fuel by a photocatalyticprocess. Nature 1980;286:474.

39. Kawai T., Sakata T. Photocatalytic hydrogen production from liquid methanol and water.J. Chem. Soc. Chem. Commun. 1980;694.

40. Kato H., Hori M., Konta R., Shimodaira Y., Kudo A. Construction of Z-scheme typeheterogeneous photocatalysis systems for water splitting into H2 and O2 under visible lightirradiation. Chem. Lett. 2004;33:1348.

41. Abe R., Sayama K., Domen K., Arakawa H. A new type of water splitting system composedof two different TiO2 photocatalysts (anatase, rutile) and a IO3−/I− shuttle redox mediator.Chem. Phys. Lett. 2001;344:339.

REFERENCES 547

42. Serpone N., Sauve G., Koch R., Tahiri H., Pichat P., Piccinini P., Pelizzetti E., HidakaH. Standardization protocol of process efficiencies and activation parameters in heteroge-neous photocatalysis: Relative photonic efficiencies zeta(r). J. Photochem. Photobiol. A1996;94:191.

43. Nowotny J., Sorrell C. C., Bak T., Sheppard L. R. Solar-hydrogen: Unresolved problems insolid-state science. Sol. Energy 2005;78:593.

44. Sato J., Saito N., Yamada Y., Maeda K., Takata T., Kondo J. N., Hara M., Kobayashi H.,Domen K., Inoue Y. RuO2-loaded beta-Ge3N4 as a non-oxide photocatalyst for overall watersplitting. J. Am. Chem. Soc. 2005;127:4150.

45. Abe R., Takata T., Sugihara H., Domen K. Photocatalytic overall water splitting undervisible light by TaON and WO3 with an IO3−/I− shuttle redox mediator. Chem. Commun.2005;3829.

46. Maeda K., Shimodaira Y., Lee B., Teramura K., Lu D., Kobayashi H., Domen K. Studies onTiNxOyFz as a visible-light-responsive photocatalyst. J. Phys. Chem. C 2007;111:18264.

47. Sato S., White J. M. Photodecomposition of water over Pt/TiO2 catalysts. Chem. Phys. Lett.1980;72:83.

48. Duonghong D., Borgarello E., Gratzel M. Dynamics of light-induced water cleavage incolloidal systems. J. Am. Chem. Soc. 1981;103:4685.

49. Kudo A., Domen K., Maruya K., Onishi T. Photocatalytic activities of TiO2 loaded withNiO. Chem. Phys. Lett. 1987;133:517.

50. Borgarello E., Kiwi J., Pelizzetti E., Visca M., Gratzel M. Sustained water cleavage byvisible-lgiht. J. Am. Chem. Soc. 1981;103:6324.

51. Jing D., Zhang Y., Guo L. Study on the synthesis of Ni doped mesoporous TiO2 and itsphotocatalytic activity for hydrogen evolution in aqueous methanol solution. Chem. Phys.Lett. 2005;415:74.

52. Moon S.-C., Mametsuka H., Tabata S., Suzuki E. Photocatalytic production of hydrogenfrom water using TiO2 and B/TiO2. Catal. Today 2000;58:125.

53. Zhao D., Budhi S., Rodriguez A., Koodali R. T. Rapid and facile synthesis of Ti-MCM-48mesoporous material and the photocatalytic performance for hydrogen evolution. Int. J.Hydrogen Energ. 2010;35:5276.

54. Kiwi J., Borgarello E., Pelizzetti E., Visca M., Gratzel M. Cyclic water cleavage by visiblelight: Drastic improvement of yield of H2 and O2 with bifunctional redox catalysts. Angew.Chem. Int. Ed. 1980;19:646.

55. Zhang J., Xu Q., Feng Z., Li M., Li C. Importance of the relationship between surface phasesand photocatalytic activity of TiO2. Angew. Chem. Int. Ed. 2008;47:1766.

56. Yang H. G., Sun C. H., Qiao S. Z., Zou J., Liu G., Smith S. C., Cheng H. M., Lu G. Q.Anatase TiO2 single crystals with a large percentage of reactive facets.Nature 2008;453:638.

57. Han X. G., Kuang Q., Jin M. S., Xie Z. X., Zheng L. S. Synthesis of titania nanosheetswith a high percentage of exposed (001) facets and related photocatalytic properties. J. Am.Chem. Soc. 2009;131:3152.

58. Chen J. S., Tan Y. L., Li C. M., Cheah Y. L., Luan D. Y., Madhavi S., Boey F. Y. C.,Archer L. A., Lou X. W. Constructing hierarchical spheres from large ultrathin anatase TiO2nanosheets with nearly 100% exposed (001) facets for fast reversible lithium storage. J. Am.Chem. Soc. 2010;132:6124.

59. Ohashi K., McCann J., Bockris J. O. M. Stable photoelectrochemical cells for splitting ofwater. Nature 1977;266:610.


60. Domen K., Naito S., Soma M., Onishi T., Tamaru K. Photocatalytic decomposition of watervapour on an NiO-SrTiO3 catalyst. J. Chem. Soc. Chem. Commun. 1980;543.

61. Domen K., Naito S., Onishi T., Tamaru K. Photocatalytic decomposition of liquied wateron SrTiO3 catalyst. Chem. Phys. Lett. 1982;92:433.

62. Inoue Y., Niiyama T., Asai Y., Sato K. Stable photocatalytic activity of BaTi4O9 combinedwith ruthenium oxide for decomposition of water. J. Chem. Soc. Chem. Commun. 1992;579.

63. Mizoguchi H., Ueda K., Orita M., Moon S. C., Kajihara K., Hirano M., Hosono H. Decom-position of water by a CaTiO3 photocatalyst under UV light irradiation. Mater. Res. Bull.2002;37:2401.

64. Kim J., Hwang D. W., Kim H. G., Bae S. W., Lee J. S., Li W., Oh S. H. Highly efficientoverall water splitting through optimization of preparation and operation conditions oflayered perovskite photocatalysts. Top. Catal. 2005;35:295.

65. Takata T., Furumi Y., Shinohara K., Tanaka A., Hara M., Kondo J. N., Domen K. Photocat-alytic decomposition of water on spontaneously hydrated layered perovskites. Chem. Mater.1997;9:1063.

66. Ikeda S., HaraM., Kondo J. N., DomenK., Takahashi H., OkuboT., KakihanaM. PreparationofK2La2Ti3O10 by polymerized complexmethod and photocatalytic decomposition ofwater.Chem. Mater. 1998;10:72.

67. Miseki Y., Kato H., Kudo A. Water splitting into H2 and O2 over niobate and titanate photo-catalystswith (111) plane-type layered perovskite structure.Energ. Environ. Sci. 2009;2:306.

68. Ding Q. P., Yuan Y. P., Xiong X., Li R. P., Huang H. B., Li Z. S., Yu T., Zou Z. G., YangS. G. Enhanced photocatalytic water splitting properties of KNbO3 nanowires synthesizedthrough hydrothermal method. J. Phys. Chem. C 2008;112:18846.

69. Li G. Q., Kako T., Wang D. F., Zou Z. G., Ye J. H. Synthesis and enhanced photocatalyticactivity of NaNbO3 prepared by hydrothermal and polymerized complex methods. J. Phys.Chem. Solids 2008;69:2487.

70. Zielinska B., Borowiak-Palen E., Kalenzuk R. J. Preparation and characterization of lithiumniobate as a novel photocatalyst in hydrogen generation. J. Phys. Chem. Solids 2008;69:236.

71. Domen K., Kudo A., Shibata M., Tanaka A., Maruya K., Onishi T. Novel photocatalysts,ion-exchanged K4Nb6O17, with a layer structure. J. Chem. Soc. Chem. Commun. 1986;1706.

72. Domen K., Kudo A., Shinozaki A., Tanaka A., Maruya K., Onishi T. Photodecomposi-tion of water and hydrogen evolution from aqueous methanol solution over novel niobatephotocatalysts. J. Chem. Soc. Chem. Commun. 1986;356.

73. KudoA., TanakaA., DomenK.,MaruyaK., AikaK., Onishi T. Photocatalytic decompositionof water over NiO-K4Nb6O17 catalyst. J. Catal. 1988;111:67.

74. Kudo A., Sayama K., Tanaka A., Asakura K., Domen K., Maruya K., Onishi T. Nickel-loaded K4Nb6O17 photocatalyst in the decomposition of H2O into H2 and O2-structure andreaction mechanism. J. Catal. 1989;120:337.

75. Sayama K., Tanaka A., Domen K., Maruya K., Onishi T. Photocatalytic decomposition ofwater over a Ni-loaded Rb4Nb6O17 catalyst. J. Catal. 1990;124:541.

76. Miseki Y., Kato H., Kudo A. Water splitting into H2 and O2 over Cs2Nb4O11 photocatalyst.Chem. Lett. 2005;34:54.

77. Kim H. G., Hwang D. W., Kim J., Kim Y. G., Lee J. S. Highly donor-doped (110) layeredperovskite materials as novel photocatalysts for overall water splitting. Chem. Commun.1999;1077.

REFERENCES 549

78. Miseki Y., Kato H., Kudo A. Water splitting into H2 and O2 over Ba5Nb4O15 photocatalystswith layered perovskite structure prepared by polymerizable complex method. Chem. Lett.2006;35:1052.

79. Domen K., Kondo J. N., Hara M., Takata T. Photo- and mechano-catalytic overall watersplitting reactions to form hydrogen and oxygen on heterogeneous catalysts. Bull. Chem.Soc. Jpn. 2000;73:1307.

80. Domen K., Yoshimura J., Sekine T., Tanaka A., Onishi T. A novel series of photocatalystswith an ion-exchangeable layered structure of niobate. Catal. Lett. 1990;4:339.

81. Kato H., Kudo A. New tantalate photocatalysts for water decomposition into H2 and O2.Chem. Phys. Lett. 1998;295:487.

82. Takahara Y., Kondo J. N., Takata T., Lu D. L., Domen K. Mesoporous tantalum oxide.1. Characterization and photocatalytic activity for the overall water decomposition. Chem.Mater. 2001;13:1194.

83. Mitsui C., Nishiguchi H., Fukamachi K., Ishihara T., Takita Y. Photocatalytic decompositionof purewater overNiO supported onKTaMO3 (M=Ti4+,Hf4+,Zr4+) perovskite oxide.Chem.Lett. 1999;28:1327.

84. Kato H., Kudo A. Water splitting into H2 and O2 on alkali tantalate photocatalysts ATaO3(A = Li, Na, and K). J. Phys. Chem. B 2001;105:4285.

85. Kato H., Kudo A. Photocatalytic water splitting into H2 and O2 over various tantalatephotocatalysts. Catal. Today 2003;78:561.

86. Kato H., Kudo A. Photocatalytic decomposition of pure water into H2 and O2 over SrTa2O6prepared by a flux method. Chem. Lett. 1999;28:1207.

87. Yoshioka K., Petrykin V., Kakihana M., Kato H., Kudo A. The relationship between photo-catalytic activity and crystal structure in strontium tantalates. J. Catal. 2005;232:102.

88. Machida M., Murakami S., Kijima T., Matsushima S., Arai M. Photocatalytic property andelectronic structure of lanthanide tantalates, LnTaO4 (Ln= La, Ce, Pr, Nd, and Sm). J. Phys.Chem. B 2001;105:3289.

89. Abe R., Higashi M., Sayama K., Abe Y., Sugihara H. Photocatalytic Activity of R3MO7 andR2Ti2O7 (R = Y, Gd, La; M = Nb, Ta) for water splitting into H2 and O2. J. Phys. Chem. B2006;110:2219.

90. Abe R., Higashi M., Zou Z., Sayama K., Abe Y., Arakawa H. Photocatalytic water splittinginto H2 and O2 over R3TaO7 and R3NbO7 (R = Y, Yb, Gd, La): Effect of crystal structureon photocatalytic activity. J. Phys. Chem. B 2003;108:811.

91. Machida M., Miyazaki K., Matsushima S., Arai M. Photocatalytic properties of layeredperovskite tantalates, MLnTa2O7 (M = Cs, Rb, Na, and H; Ln = La, Pr, Nd, and Sm).J. Mater. Chem. 2003;13:1433.

92. Machida M., Yabunaka J.-I., Kijima T. Synthesis and photocatalytic property of layeredperovskite tantalates, RbLnTa2O7 (Ln = La, Pr, Nd, and Sm). Chem. Mater. 2000;12:812.

93. Kudo A., Okutomi H., Kato H. Photocatalytic water splitting into H2 and O2 overK2LnTa5O15 powder. Chem. Lett. 2000;29:1212.

94. Mitsuyama T., Tsutsumi A., Hata T., Ikeue K., Machida M. Enhanced photocatalytic watersplitting of hydrous LiCa2Ta3O10 prepared by hydrothermal treatment. Bull. Chem. Soc.Jpn. 2008;81:401.

95. SayamaK., Arakawa H. Photocatalytic decomposition of water and photocatalytic reductionof carbon dioxide over ZrO2 catalyst. J. Phys. Chem. 1993;97:531.


96. Sayama K., Arakawa H. Effect of carbonate addition on the photocatalytic decompositionof liquid water over a ZrO2 catalyst. J. Photochem. Photobiol. A 1996;94:67.

97. Liu S.H.,WangH. P. Photocatalytic generation of hydrogen onZr-MCM-41. Int. J. HydrogenEnerg. 2002;27:859.

98. Yuan Y. P., Zhang X. L., Liu L. F., Jiang X. J., Lv J., Li Z. S., Zou Z. G. Synthesis andphotocatalytic characterization of a new photocatalyst BaZrO3. Int. J. Hydrogen Energ.2008;33:5941.

99. Kato H., Matsudo N., Kudo A. Photophysical and photocatalytic properties of molybdatesand tungstates with a scheelite structure. Chem. Lett. 2004;33:1216.

100. Kudo A., Kato H. Photocatalytic activities of Na2W4O13 with layered structure. Chem. Lett.1997;26:421.

101. Kadowaki H., Saito N., Nishiyama H., Kobayashi H., Shimodaira Y., Inoue Y. Overallsplitting of water by RuO2-loaded PbWO4 photocatalyst with d10s2-d0 configuration. J. Phys.Chem. C 2006;111:439.

102. Bamwenda G. R., Uesigi T., Abe Y., Sayama K., Arakawa H. The photocatalytic oxidationof water to O2 over pure CeO2, WO3, and TiO2 using Fe3+ and Ce4+ as electron acceptors.Appl. Catal. A 2001;205:117.

103. Kadowaki H., Saito N., Nishiyama H., Inoue Y. RuO2-loaded Sr2+-doped CeO2 with d0

electronic configuration as a new photocatalyst for overall water splitting. Chem. Lett.2007;36:440.

104. Yuan Y. P., Zheng J., Zhang X. L., Li Z. S., Yu T., Ye J. H., Zou Z. G. BaCeO3 as anovel photocatalyst with 4f electronic configuration for water splitting. Solid State Ionics2008;178:1711.

105. Yanagida T., Sakata Y., Imamura H. Photocatalytic decomposition of H2O into H2 and O2over Ga2O3 loaded with NiO. Chem. Lett. 2004;33:726.

106. Sakata Y., Matsuda Y., Yanagida T., Hirata K., Imamura H., Teramura K. Effect of metalion addition in a Ni supported Ga2O3 photocatalyst on the photocatalytic overall splitting ofH2O. Catal. Lett. 2008;125:22.

107. Kudo A., Mikami I. Photocatalytic activities and photophysical properties of Ga2-xInxO3solid solution. J. Chem. Soc. Faraday Trans. 1998;94:2929.

108. Ikarashi K., Sato J., Kobayashi H., Saito N., Nishiyama H., Inoue Y. Photocatalysis forwater decomposition by RuO2-dispersed ZnGa2O4 with d10 configuration. J. Phys. Chem. B2002;106:9048.

109. Sato J., Kobayashi H., Saito N., Nishiyama H., Inoue Y. Photocatalytic activities for waterdecomposition of RuO2-loaded AInO2 (A= Li, Na) with d(10) configuration. J. Photochem.Photobiol. A 2003;158:139.

110. Sato J., Saito N., Nishiyama H., Inoue Y. New photocatalyst group for water decompositionof RuO2-loaded p-block metal (In, Sn, and Sb) oxides with d10 configuration. J. Phys. Chem.B 2001;105:6061.

111. Sato J., Kobayashi H., Inoue Y. Photocatalytic activity for water decomposition of indateswith octahedrally coordinated d10 configuration. II. Roles of geometric and electronic struc-tures. J. Phys. Chem. B 2003;107:7970.

112. Sato J., Saito N., Nishiyama H., Inoue Y. Photocatalytic activity for water decompositionof indates with octahedrally coordinated d10 configuration. I. Influences of preparationconditions on activity. J. Phys. Chem. B 2003;107:7965.

REFERENCES 551

113. Arai N., Saito N., Nishiyama H., Shimodaira Y., Kohayashi H., Inoue Y., Sato K. Photocat-alytic activity for overall water splitting of RuO2-loaded YxIn2−xO3 (x=0.9− 1.5). J. Phys.Chem. C 2008;112:5000.

114. Sato J., Kobayashi H., Ikarashi K., Saito N., Nishiyama H., Inoue Y. Photocatalytic activityfor water decomposition of RuO2-dispersed Zn2GeO4 with d10 configuration. J. Phys. Chem.B 2004;108:4369.

115. Sato J., Saito N., Nishiyama H., Inoue Y. Photocatalytic water decomposition by RuO2-loaded antimonates, M2Sb2O7 (M= Ca, Sr), CaSb2O6 and NaSbO3, with d10 configuration.J. Photochem. Photobiol. A 2002;148:85.

116. Maeda K., Saito N., Lu D. L., Inoue Y., Domen K. Photocatalytic properties of RuO2-loadedbeta-Ge3N4 for overall water splitting. J. Phys. Chem. C 2007;111:4749.

117. LeeY.G.,Watanabe T., Takata T., HaraM., YoshimuraM., DomenK. Effect of high-pressureammonia treatment on the activity of Ge3N4 photocatalyst for overall water splitting. J. Phys.Chem. B 2006;110:17563.

118. Arai N., Saito N., Nishiyama H., Inoue Y., Domen K., Sato K. Overall water splitting byRuO2-dispersed divalent-ion-doped GaN photocatalysts with d10 electronic configuration.Chem. Lett. 2006;35:796.

119. Maeda K., Teramura K., Saito N., Inoue Y., Domen K. Photocatalytic overall water splittingon gallium nitride powder. Bull. Chem. Soc. Jpn. 2007;80:1004.

120. Reber J. F., Meier K. Photochemical production of hydrogen with zinc sulfide suspensions.J. Phys. Chem. B 1984;88:5903.

121. BaerM.,Weinhardt L.,MarsenB., ColeB.,GaillardN.,Miller E., HeskeC.Mo incorporationin WO3 thin film photoanodes: Tailoring the electronic structure for photoelectrochemicalhydrogen production. Appl. Phys. Lett. 2010;96:032107.

122. Darwent J. R., Mills A. Photo-oxidation of water sensitized by WO3 powder. J. Chem. Soc.Faraday Trans. 1982;78:359.

123. Ohno T., Tanigawa F., Fujihara K., Izumi S., Matsumura M. Photocatalytic oxidation ofwater on TiO2-coatedWO3 particles by visible light using Iron(III) ions as electron acceptor.J. Photochem. Photobiol. A 1998;118:41.

124. Erbs W., Desilvestro J., Borgarello E., Graetzel M. Visible-light-induced oxygen generationfrom aqueous dispersions of tungsten(VI) oxide. J. Phys. Chem. 1984;88:4001.

125. Kim H. G., Jeong E. D., Borse P. H., Jeon S., Yong K. J., Lee J. S., Li W., Oh S. H.Photocatalytic Ohmic layered nanocomposite for efficient utilization of visible light photons.Appl. Phys. Lett. 2006;89:064103.

126. Sayama K., Mukasa K., Abe R., Abe Y., Arakawa H. Stoichiometric water splitting into Hand O using a mixture of two different photocatalysts and an IO3−/I− shuttle redox mediatorunder visible light irradiation. Chem. Commun. 2001;2416.

127. Higashi M., Abe R., Teramura K., Takata T., Ohtani B., Domen K. Two step water splittinginto H2 and O2 under visible light by ATaO2N (A = Ca, Sr, Ba) and WO3 with IO3−/I−

shuttle redox mediator. Chem. Phys. Lett. 2008;452:120.

128. Kudo A., Hijii S. H2 or O2 evolution from aqueous solutions on layered oxide photocat-alysts consisting of Bi3+ with 6s2 configuration and d0 transition metal ions. Chem. Lett.1999;28:1103.

129. Kudo A., Ueda K., Kato H., Mikami I. Photocatalytic O2 evolution under visible lightirradiation on BiVO4 in aqueous AgNO3 solution. Catal. Lett. 1998;53:229.


130. Kudo A., Omori K., Kato H. A novel aqueous process for preparation of crystal form-controlled and highly crystalline BiVO4 powder from layered vanadates at room temperatureand its photocatalytic and photophysical properties. J. Am. Chem. Soc. 1999;121:11459.

131. Tokunaga S., Kato H., Kudo A. Selective preparation of monoclinic and tetragonal BiVO4with scheelite structure and their photocatalytic properties. Chem. Mater. 2001;13:4624.

132. Xi G., Ye J. Synthesis of bismuth vanadate nanoplates with exposed {001} facets andenhanced visible-light photocatalytic properties. Chem. Commun. 2010;46:1893.

133. Hosogi Y., Shimodaira Y., Kato H., Kobayashi H., Kudo A. Role of Sn2+ in the bandstructure of SnM2O6 and Sn2M2O7 (M = Nb and Ta) and their photocatalytic properties.Chem. Mater. 2008;20:1299.

134. Konta R., Kato H., Kobayashi H., Kudo A. Photophysical properties and photocatalyticactivities under visible light irradiation of silver vanadates. Phys. Chem. Chem. Phys.2003;5:3061.

135. Kato H., Kobayashi H., Kudo A. Role of Ag+ in the band structures and photocatalyticproperties of AgMO3 (M: Ta and Nb) with the perovskite structure. J. Phys. Chem. B2002;106:12441.

136. Tang J. W., Zou Z. G., Ye J. H. Photophysical and photocatalytic properties of AgInW2O8.J. Phys. Chem. B 2003;107:14265.

137. Ouyang S., Li Z., Ouyang Z., Yu T., Ye J., Zou Z. Correlation of crystal structures, electronicstructures, and photocatalytic properties in a series of Ag-based oxides: AgAlO2, AgCrO2,and Ag2CrO4. J. Phys. Chem. C 2008;112:3134.

138. Ouyang S., Kikugawa N., Chen D., Zou Z., Ye J. A systematical study on photocatalyticproperties of AgMO2 (M=Al, Ga, In): Effects of chemical compositions, crystal structures,and electronic structures. J. Phys. Chem. C 2009;113:1560.

139. Ouyang S., Ye J. �-AgAl1−xGaxO2 solid-solution photocatalysts: Continuous modulation ofelectronic structure toward high-performance visible-light photoactivity. J. Am. Chem. Soc.2011;133:7757.

140. Kako T., Kikugawa N., Ye J. Photocatalytic activities of AgSbO3 under visible light irradi-ation. Catal. Today 2008;131:197.

141. Yi Z., Ye J., Kikugawa N., Kako T., Ouyang S., Stuart-Williams H., Yang H., Cao J., LuoW., Li Z., Liu Y., Withers R. L. An orthophosphate semiconductor with photooxidationproperties under visible-light irradiation. Nat. Mater. 2010;9:559.

142. Yoshimura J., Ebina Y., Kondo J., Domen K., Tanaka A. Visible light-induced photocatalyticbehavior of a layered perovskite-type rubidium lead niobate, RbPb2Nb3O10. J. Phys. Chem.1993;97:1970.

143. Kim H. G., Hwang D. W., Lee J. S. An undoped, single-phase oxide photocatalyst workingunder visible light. J. Am. Chem. Soc. 2004;126:8912.

144. Hitoki G., Ishikawa A., Takata T., Kondo J. N., Hara M., Domen K. Ta3N5 as anovel visible light-driven photocatalyst (lambda � 600 nm). Chem. Lett. 2002;31:736.

145. Hitoki G., Takata T., Kondo J. N., Hara M., Kobayashi H., Domen K. An oxynitride, TaON,as an efficient water oxidation photocatalyst under visible light irradiation (lambda ≤ 500nm). Chem. Commun. 2002;1698.

146. Kasahara A., Nukumizu K., Hitoki G., Takata T., Kondo J. N., Hara M., Kobayashi H.,Domen K. Photoreactions on LaTiO2N under visible light irradiation. J. Phys. Chem. A2002;106:6750.

REFERENCES 553

147. Hara M., Hitoki G., Takata T., Kondo J. N., Kobayashi H., Domen K. TaON and Ta3N5 asnew visible light driven photocatalysts. Catal. Today 2003;78:555.

148. Hara M., Nunoshige J., Takata T., Kondo J. N., Domen K. Unusual enhancement of H2evolution by Ru on TaON photocatalyst under visible light irradiation. Chem. Commun.2003;3000.

149. Hara M., Takata T., Kondo J. N., Domen K. Photocatalytic reduction of water by TaONunder visible light irradiation. Catal. Today 2004;90:313.

150. Lu D. L., Hitoki G., Katou E., Kondo J. N., Hara M., Domen K. Porous single-crystallineTaON and Ta3N5 particles. Chem. Mater. 2004;16:1603.

151. Abe R., Takata T., Sugihara H., Domen K. Photocatalytic overall water splitting undervisible light by TaON and WO3 with an IO3−/I− shuttle redox mediator. Chem. Commun.2005;3829.

152. Lee Y., Nukumizu K., Watanabe T., Takata T., Hara M., Yoshimura M., Domen K. Effect of10 MPa ammonia treatment on the activity of visible light responsive Ta3N5 photocatalyst.Chem. Lett. 2006;35:352.

153. YashimaM., Lee Y., DomenK. Crystal structure and electron density of tantalum oxynitride,a visible light responsive photocatalyst. Chem. Mater. 2007;19:588.

154. Higashi M., Abe R., Ishikawa A., Takata T., Ohtani B., Domen K. Z-scheme overall watersplitting on modified-TaON photocatalysts under visible light (lambda � 500 nm). Chem.Lett. 2008;37:138.

155. Higashi M., Abe R., Teramura K., Takata T., Ohtani B., Domen K. Two step water splittinginto H2 and O2 under visible light by ATaO2N (A = Ca, Sr, Ba) and WO3 with IO3−/I−

shuttle redox mediator. Chem. Phys. Lett. 2008;452:120.

156. MaedaK., TerashimaH., KaseK., HigashiM., TabataM., DomenK. Surfacemodification ofTaON with monoclinic ZrO2 to produce a composite photocatalyst with enhanced hydrogenevolution activity under visible light. Bull. Chem. Soc. Jpn. 2008;81:927.

157. Maeda K., Abe R., Domen K. Role and function of ruthenium species as promoters withTaON-based photocatalysts for oxygen evolution in two-step water splitting under visiblelight. J. Phys. Chem. 2011;115:3057.

158. Tabata M., Maeda K., Higashi M., Lu D. L., Takata T., Abe R., Domen K. Modified Ta3N5powder as a photocatalyst for O2 evolution in a two-step water splitting system with aniodate/iodide shuttle redox mediator under visible light. Langmuir 2010;26:9161.

159. Maeda K., Higashi M., Lu D. L., Abe R., Domen K. Efficient nonsacrificial water splittingthrough two-step photoexcitation by visible light using a modified oxynitride as a hydrogenevolution photocatalyst. J. Am. Chem. Soc. 2010;132:5858.

160. Higashi M., Abe R., Takata T., Domen K. Photocatalytic overall water splitting under visiblelight using ATaO2N (A= Ca, Sr, Ba) and WO3 in a IO3−/I− shuttle redox mediated system.Chem. Mater. 2009;21:1543.

161. Liu M. Y., You W. S., Lei Z. B., Zhou G. H., Yang J. J., Wu G. P., Ma G. J., Luan G. Y.,Takata T., Hara M., Domen K., Can L. Water reduction and oxidation on Pt-Ru/Y2Ta2O5N2catalyst under visible light irradiation. Chem. Commun. 2004;2192.

162. Mishima T., Matsuda M., Miyake M. Visible-light photocatalytic properties and electronicstructure of Zr-based oxynitride, Zr2ON2, derived from nitridation of ZrO2. Appl. Catal. A2007;324:77.

163. Darwent J. R. H2 production photosensitized by aqueous semiconductor dispersions.J. Chem. Soc. Faraday Trans. 1981;77:1703.


164. Darwent J. R., Porter G. Photochemical hydrogen production using cadmium sulphidesuspensions in aerated water. J. Chem. Soc. Chem. Commun. 1981;145.

165. Buhler N., Meier K., Reber J. F. Photochemical hydrogen production with cadmium sulfidesuspensions. J. Phys. Chem. 1984;88:3261.

166. Bao N., Shen L., Takata T., Domen K., Gupta A., Yanagisawa K., Grimes C. A. Facilecd-thiourea complex thermolysis synthesis of phase-controlled CdS nanocrystals for photo-catalytic hydrogen production under visible light. J. Phys. Chem. C 2007;111:17527.

167. Bao N. Z., Shen L. M., Takata T., Domen K. Self-templated synthesis of nanoporous CdSnanostructures for highly efficient photocatalytic hydrogen production under visible. Chem.Mater. 2008;20:110.

168. Fu X. L., Wang X. X., Chen Z. X., Zhang Z. Z., Li Z. H., Leung D. Y. C., Wu L., Fu X. Z.Photocatalytic performance of tetragonal and cubic beta-In2S3 for the water splitting undervisible light irradiation. Appl. Catal. B 2010;95:393.

169. Lei Z. B., You W. S., Liu M. Y., Zhou G. H., Takata T., Hara M., Domen K., Li C.Photocatalytic water reduction under visible light on a novel ZnIn2S4 catalyst synthesizedby hydrothermal method. Chem. Commun. 2003;2142.

170. Kudo A., Nagane A., Tsuji I., Kato H. H2 evolution from aqueous potassium sulfite solutionsunder visible light irradiation over a novel sulfide photocatalyst NaInS2 with a layeredstructure. Chem. Lett. 2002;31:882.

171. Kale B. B., Baeg J. O., Lee S. M., Chang H. J., Moon S. J., Lee C.W. CdIn2S4 nanotubes and“marigold” nanostructures: A visible-like photocatalyst. Adv. Funct. Mater. 2006;16:1349.

172. Chen D., Ye J. Photocatalytic H2 evolution under visible light irradiation on AgIn5S8 pho-tocatalyst. J. Phys. Chem. Solids 2007;68:2317.

173. Ishikawa A., Takata T., Kondo J. N., Hara M., Kobayashi H., Domen K. OxysulfideSm2Ti2S2O5 as a stable photocatalyst for water oxidation and reduction under visible lightirradiation (lambda � or = 650 nm). J. Am. Chem. Soc. 2002;124:13547.

174. Ishikawa A., Yamada Y., Takata T., Kondo J. N., Hara M., Kobayashi H., Domen K. Novelsynthesis and photocatalytic activity of oxysulfide Sm2Ti2S2O5. Chem. Mater. 2003;15:4442.

175. Kasahara A., Nukumizu K., Takata T., Kondo J. N., Hara M., Kobayashi H., DomenK. LaTiO2N as a visible-light (� 600 nm)-driven photocatalyst (2). J. Phys. Chem. B2003;107:791.

176. Ishikawa A., Takata T., Matsumura T., Kondo J. N., Hara M., Kobayashi H., Domen K.Oxysulfides Ln2Ti2S2O5 as stable photocatalysts for water oxidation and reduction undervisible-light irradiation. J. Phys. Chem. B 2004;108:2637.

177. Ogisu K., Ishikawa A., Shimodaira Y., Takata T., Kobayashi H., Domen K. Electronic bandstructures and photochemical properties of La-Ga-based oxysulfides. J. Phys. Chem. C2008;112:11978.

178. Ogisu K., Ishikawa A., Teramura K., Toda K., Hara M., Domen K. Lanthanum-indiumoxysulfide as a visible light driven photocatalyst for water splitting. Chem. Lett. 2007;36:854.

179. Zhang F. X., Maeda K., Takata T., Domen K. Improvement of the photocatalytic hydro-gen evolution activity of Sm2Ti2S2O5 under visible light by metal ion additives. J. Catal.2011;280:1.

180. Ritterskamp P., Kuklya A., Wustkamp M. A., Kerpen K., Weidenthaler C., Demuth M. Atitanium disilicide derived semiconducting catalyst for water splitting under solar radiation-reversible storage of oxygen and hydrogen. Angew. Chem. Int. Ed. 2007;46:7770.

REFERENCES 555

181. Wang X., Maeda K., Thomas A., Takanabe K., Xin G., Carlsson J. M., Domen K., AntoniettiM. A metal-free polymeric photocatalyst for hydrogen production from water under visiblelight. Nat. Mater. 2009;8:76.

182. Maeda K., Wang X. C., Nishihara Y., Lu D. L., Antonietti M., Domen K. Photocatalyticactivities of graphitic carbon nitride powder for water reduction and oxidation under visiblelight. J. Phys. Chem. C 2009;113:4940.

183. Wang X. C., Maeda K., Chen X. F., Takanabe K., Domen K., Hou Y. D., Fu X. Z., AntoniettiM. Polymer semiconductors for artificial Photosynthesis: Hydrogen evolution by meso-porous graphitic carbon nitride with visible light. J. Am. Chem. Soc. 2009;131:1680.

184. Zhang J., Chen X., Takanabe K., Maeda K., Domen K., Epping J. D., Fu X., Antonietti M.,Wang X. Synthesis of a carbon nitride structure for visible-light catalysis by copolymeriza-tion. Angew. Chem. Int. Ed. 2010;49:441.

185. Kato H., Kudo A. Visible-light-response and photocatalytic activities of TiO2 and SrTiO3photocatalysts codoped with antimony and chromium. J. Phys. Chem. B 2002;106:5029.

186. Niishiro R., Kato H., Kudo A. Nickel and either tantalum or niobium-codoped TiO2 andSrTiO3 photocatalysts with visible-light response for H2 or O2 evolution from aqueoussolutions. Phys. Chem. Chem. Phys. 2005;7:2241.

187. Niishiro R., Konta R., Kato H., ChunW. J., Asakura K., KudoA. Photocatalytic O2 evolutionof rhodium and antimony-codoped rutile-type TiO2 under visible light irradiation. J. Phys.Chem. C 2007;111:17420.

188. Ikeda T., Nomoto T., Eda K., Mizutani Y., Kato H., Kudo A., Onishi H. Photoinduceddynamics of TiO2 doped with Cr and Sb. J. Phys. Chem. C 2008;112:1167.

189. Ishii T., Kato H., Kudo A. H2 evolution from an aqueous methanol solution on SrTiO3photocatalysts codoped with chromium and tantalum ions under visible light irradiation.J. Photochem. Photobiol. A 2004;163:181.

190. Konta R., Ishii T., Kato H., Kudo A. Photocatalytic activities of noble metal ion dopedSrTiO3 under visible light irradiation. J. Phy. Chem. B 2004;108:8992.

191. Sayama K., Mukasa K., Abe R., Abe Y., Arakawa H. Stoichiometric water splitting intoH2 and O2 using a mixture of two different photocatalysts and an IO3−/I− shuttle redoxmediator under visible light irradiation. Chem. Commun. 2001;2416.

192. Sayama K., Mukasa K., Abe R., Abe Y., Arakawa H. A new photocatalytic water splittingsystem under visible light irradiation mimicking a Z-scheme mechanism in photosynthesis.J. Photochem. Photobiol. A 2002;148:71.

193. Sasaki Y., Iwase A., Kato H., Kudo A. The effect of co-catalyst for Z-scheme photocatalysissystems with an Fe3+/Fe2+ electron mediator on overall water splitting under visible lightirradiation. J. Catal. 2008;259:133.

194. Abe R., Sayama K., Sugihara H. Development of new photocatalytic water splitting intoH2 and O2 using two different semiconductor photocatalysts and a shuttle redox mediatorIO3−/I−. J. Phys. Chem. B 2005;109:16052.

195. Sasaki Y., Nemoto H., Saito K., KudoA. Solar water splitting using powdered photocatalystsdriven by Z-schematic interparticle electron transfer without an electron mediator. J. Phys.Chem. C 2009;113:17536.

196. Nishimoto S., Matsuda M., Miyake M. Photocatalytic activities of rh-doped CaTiO3 undervisible light irradiation. Chem. Lett. 2006;35:308.

197. Okazaki Y., Mishima T., Nishimoto S., Matsuda M., Miyake M. Photocatalytic activityof Ca3Ti2O7 layered-perovskite doped with Rh under visible light irradiation. Mater. Lett.2008;62:3337.


198. Hwang D. W., Kirn H. G., Lee J. S., Kim J., Li W., Oh S. H. Photocatalytic hydrogenproduction from water over m-doped La2Ti2O7 (M = Cr, Fe) under visible light irradiation(lambda � 420 nm). J. Phys. Chem. B 2005;109:2093.

199. Iwase A., Saito K., Kudo A. Sensitization of NaMO3 (M: Nb and Ta) photocatalysts withwide band gaps to visible light by Ir doping. Bull. Chem. Soc. Jpn. 2009;82:514.

200. Yang M., Huang X. L., Yan S. C., Li Z. S., Yu T., Zou Z. G. Improved hydrogen evo-lution activities under visible light irradiation over NaTaO3 codoped with lanthanum andchromium. Mater. Chem. Phys. 2010;121:506.

201. Zou Z. G., Ye J. H., Sayama K., Arakawa H. Direct splitting of water under visible lightirradiation with an oxide semiconductor photocatalyst. Nature 2001;414:625.

202. Zou Z. G., Ye J. H., Arakawa H. Photocatalytic behavior of a new series of In0.8M0.2TaO4(M = Ni, Cu, Fe) photocatalysts in aqueous solutions. Catal. Lett. 2001;75:209.

203. Zou Z. G., Ye J. H., Arakawa H. Surface characterization of nanoparticles ofNiOx/In0.9Ni0.1TaO4: Effects on photocatalytic activity. J. Phys. Chem. B 2002;106:13098.

204. Shimodaira Y., Kato H., Kobayashi H., Kudo A. Investigations of electronic structuresand photocatalytic activities under visible light irradiation of lead molybdate replaced withchromium(VI). Bull. Chem. Soc. Jpn. 2007;80:885.

205. Wang D., Zou Z., Ye J. Photocatalytic water splitting with the Cr-doped Ba2In2O5/In2O3composite oxide semiconductors. Chem. Mater. 2005;17:3255.

206. Kudo A., Sekizawa M. Photocatalytic H2 evolution under visible light irradiation on Ni-doped ZnS photocatalyst. Chem. Commun. 2000;1371.

207. Kudo A., Sekizawa M. Photocatalytic H2 evolution under visible light irradiation onZn1−xCuxS solid solution. Catal. Lett. 1999;58:241.

208. Ikeue K., Shiiba S., Machida M. Novel visible-light-driven photocatalyst based onMn-Cd-Sfor efficient H2 evolution. Chem. Mater. 2009;22:743.

209. Liu G. J., Zhao L., Ma L. J., Guo L. J. Photocatalytic H2 evolution under visible lightirradiation on a novel CdxCuyZn1−x−yS catalyst. Catal. Commun. 2008;9:126.

210. ZhangW., Zhong Z., Wang Y., Xu R. Doped solid solution: (Zn0.95Cu0.05)1−xCdxS nanocrys-tals with high activity for H2 evolution from aqueous solutions under visible light. J. Phy.Chem. C 2008;112:17635.

211. Zhang W., Xu R. Surface engineered active photocatalysts without noble metals: CuS-ZnxCd1−xS nanospheres by one-step synthesis. Int. J. Hydrogen Energ. 2009;34:8495.

212. Zhang X. H., Jing D. W., Liu M. C., Guo L. J. Efficient photocatalytic H2 productionunder visible light irradiation over Ni doped Cd1−xZnxS microsphere photocatalysts. Catal.Commun. 2008;9:1720.

213. Shen S. H., Zhao L., Zhou Z. H., Guo L. J. Enhanced photocatalytic hydrogen evolutionover Cu-doped ZnIn2S4 under visible light irradiation. J. Phys. Chem. C 2008;112:16148.

214. Asahi R., Morikawa T., Ohwaki T., Aoki K., Taga Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001;293:269.

215. Khan S. U. M., Al-Shahry M., Ingler W. B. Efficient photochemical water splitting by achemically modified n-TiO2. Science 2002;297:2243.

216. Nukumizu K., Nunoshige J., Takata T., Kondo J. N., Hara M., Kobayashi H., Domen K.TiNxOyFz as a stable photocatalyst for water oxidation in visible light (�570 nm). Chem.Lett. 2003;32:196.

REFERENCES 557

217. Maeda K., Lee B. J., Lu D. L., Domen K. Physicochemical effects on photocatalyticwater oxidation by titanium fluorooxynitride powder under visible light. Chem. Mater.2009;21:2286.

218. Zong X., Xing Z., Yu H., Chen Z. G., Tang F. Q., Zou J., Lu G. Q., Wang L. Z. Photocatalyticwater oxidation on F, N co-doped TiO2 with dominant exposed {001} facets under visiblelight. Chem. Commun. 2011;11742.

219. Hisatomi T., Hasegawa K., Teramura K., Takata T., Hara M., Domen K. Zinc and titaniumspinel oxynitride (ZnxTiOyNz) as a d0-d10 complex photocatalyst with visible light activity.Chem. Lett. 2007;36:558.

220. Hagiwara H., Kumagae K., Ishihara T. Effects of nitrogen doping on photocatalytic water-splitting activity of Pt/KTa0.92Zr0.08O3 perovskite oxide catalyst. Chem. Lett. 2010;39:498.

221. Ji S. M., Borse P. H., Kim H. G., Hwang D.W., Jang J. S., Bae S. W., Lee J. S. Photocatalytichydrogen production from water-methanol mixtures using N-doped Sr2Nb2O7 under visiblelight irradiation: Effects of catalyst structure. Phys. Chem. Chem. Phys. 2005;7:1315.

222. Zong X., Sun C. H., Chen Z. G., Mukherji A., Wu H., Zou J., Smith S. C., Lu G. Q., WangL. Z. Nitrogen doping in ion-exchangeable layered tantalate towards visible-light inducedwater oxidation. Chem. Commun. 2011;6293.

223. Marschall R., Mukherji A., Tanksale A., Sun C. H., Smith S. C., Wang L. Z., Lu G. Q.Preparation of new sulfur-doped and sulfur/nitrogen co-doped CsTaWO6 photocatalysts forhydrogen production from water under visible light. J. Mater. Chem. 2011;21:8871.

224. Mukherji A., Marschall R., Tanksale A., Sun C. H., Smith S. C., Lu G. Q., Wang L. Z.N-Doped CsTaWO6 as a new photocatalyst for hydrogen production from water splittingunder solar irradiation. Adv. Funct. Mater. 2011;21:126.

225. Liu G., Wang L. Z., Sun C. H., Chen Z. G., Yan X. X., Cheng L., Cheng H. M., LuG. Q. Nitrogen-doped titania nanosheets towards visible light response. Chem. Commun.2009;1383.

226. Jang J. S., Yu C. J., Choi S. H., Ji S. M., Kim E. S., Lee J. S. Topotactic synthesis of meso-porous ZnS and ZnO nanoplates and their photocatalytic activity. J. Catal. 2008;254:144.

227. Liu G., Niu P., Sun C. H., Smith S. C., Chen Z. G., Lu G. Q., Cheng H. M. Unique electronicstructure induced high photoreactivity of sulfur-doped graphitic C3N4. J. Am. Chem. Soc.2010;132:11642.

228. Zhang Y. J., Mori T., Ye J. H., Antonietti M. Phosphorus-doped carbon nitridesolid: Enhanced electrical conductivity and photocurrent generation. J. Am. Chem. Soc.2010;132:6294.

229. Wang Y., Di Y., Antonietti M., Li H. R., Chen X. F., Wang X. C. Excellent visible-lightphotocatalysis of fluorinated polymeric carbon nitride solids. Chem. Mater. 2010;22:5119.

230. Tsuji I., Kudo A. H2 evolution from aqueous sulfite solutions under visible-light irradiationover Pb and halogen-codopedZnS photocatalysts. J. Photochem. Photobiol. A 2003;156:249.

231. Lei Z. B., Ma G. J., LiuM. Y., YouW. S., Yan H. J., Wu G. P., Takata T., HaraM., Domen K.,Li C. Sulfur-substituted and zinc-doped In(OH)3: A new class of catalyst for photocatalyticH2 production from water under visible light illumination. J. Catal. 2006;237:322.

232. Yi Z. G., Ye J. H. Band gap tuning of Na1−xLaxTa1−xCoxO3 solid solutions for visible lightphotocatalysis. Appl. Phys. Lett. 2007;91:254108.

233. YaoW., Ye J. A new efficient visible-light-driven photocatalyst Na0.5Bi1.5VMoO8 for oxygenevolution. Chem. Phys. Lett. 2008;450:370.


234. Yao W., Ye J. Photophysical and photocatalytic properties of Ca1−xBixVxMo1−xO4 solidsolutions. J. Phys. Chem. B 2006;110:11188.

235. Wang D., Kako T., Ye J. Efficient photocatalytic decomposition of acetaldehyde over asolid-solution perovskite (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3 under visible-light irradiation. J. Am.Chem. Soc. 2008;130:2724.

236. WangD., KakoT., Ye J. New series of solid-solution semiconductors (AgNbO3)1−x(SrTiO3)xwith modulated band structure and enhanced visible-light photocatalytic activity. J. Phys.Chem. C 2009;113:3785.

237. Wang Q., Liu H., Jiang L., Yuan J., Shangguan W. Visible-light-responding Bi0.5Dy0.5VO4solid solution for photocatalytic water splitting. Catal. Lett. 2009;131:160.

238. Liu H., Yuan J., ShangguanW., Teraoka Y. Visible-light-responding BiYWO6 solid solutionfor stoichiometric photocatalytic water splitting. J. Phy. Chem. C 2008;112:8521.

239. Tsuji I., Kato H., Kudo A. Visible-light-induced H2 evolution from an aqueous solution con-taining sulfide and sulfite over a ZnS-CuInS2-AgInS2 solid-solution photocatalyst. Angew.Chem. Int. Ed. 2005;44:3565.

240. Tsuji I., Kato H., Kobayashi H., Kudo A. Photocatalytic H2 evolution under visible-lightirradiation over band-structure-controlled (CuIn)xZn2(1−x)S2 solid solutions. J. Phys. Chem.B 2005;109:7323.

241. Tsuji I., Kato H., Kobayashi H., Kudo A. Photocatalytic H2 evolution reaction from aqueoussolutions over band structure-controlled (AgIn)xZn2(1−x)S2 solid solution photocatalysts withvisible-light response and their surface nanostructures. J. Am. Chem. Soc. 2004;126:13406.

242. Tsuji I., Kato H., Kudo A. Photocatalytic hydrogen evolution on ZnS-CuInS2-AgInS2solid solution photocatalysts with wide visible light absorption bands. Chem. Mater.2006;18:1969.

243. Kudo A., Tsuji I., Kato H. AgInZn7S9 solid solution photocatalyst for H2 evolution fromaqueous solutions under visible light irradiation. Chem. Commun. 2002;1958.

244. Reber J. F., Rusek M. Photochemcial hydrogen production with platinized suspensionsof cadmium sulfide and cadmium zinc sulfide modified by silver sulfide. J. Phys. Chem.1986;90:824.

245. Youn H. C., Baral S., Fendler J. H. Dihexadecyl phosphate, vesicle-stabilized and in situgenerated mixed cadmium sulfide and zinc sulfide semiconductor particles: Preparation andutilization for photosensitized charge separation and hydrogen generation. J. Phy. Chem.1988;92:6320.

246. Zhang K., Jing D. W., Xing C. J., Guo L. J. Significantly improved photocatalytic hydrogenproduction activity over Cd1−xZnxS photocatalysts prepared by a novel thermal sulfurationmethod. Int. J. Hydrogen Energ. 2007;32:4685.

247. Jang J. S., Borse P. H., Lee J. S., Choi S. H., Kim H. G. Indium induced band gap tailoringin AgGa1−xInxS2 chalcopyrite structure for visible light photocatalysis. J. Chem. Phys.2008;128:154717.

248. Maeda K., Teramura K., Takata T., Hara M., Saito N., Toda K., Inoue Y., Kobayashi H.,Domen K. Overall water splitting on (Ga1−xZnx)(N1−xOx) solid solution photocatalyst:Relationship between physical properties and photocatalytic activity. J. Phys. Chem. B2005;109:20504.

249. Yashima M., Maeda K., Teramura K., Takata T., Domen K. Crystal structure and opticalproperties of (Ga1−xZnx)(N1−xOx) oxynitride photocatalyst (x = 0.13). Chem. Phys. Lett.2005;416:225.

REFERENCES 559

250. Maeda K., Teramura K., Lu D. L., Saito N., Inoue Y., Domen K. Noble-metal/Cr2O3core/shell nanoparticles as a cocatalyst for photocatalytic overall water splitting. Angew.Chem. Int. Ed. 2006;45:7806.

251. Maeda K., Teramura K., Lu D. L., Takata T., Saito N., Inoue Y., Domen K. Photocatalystreleasing hydrogen fromwater-enhancing catalytic performance holds promise for hydrogenproduction by water splitting in sunlight. Nature 2006;440:295.

252. Maeda K., Teramura K., Masuda H., Takata T., Saito N., Inoue Y., Domen K. Efficientoverall water splitting under visible-light irradiation on (Ga1−xZnx)(N1−xOx) dispersed withRh-Cr mixed-oxide nanoparticles: Effect of reaction conditions on photocatalytic activity.J. Phys. Chem. B 2006;110:13107.

253. Maeda K., Teramura K., Saito N., Inoue Y., Domen K. Improvement of photocatalyticactivity of (Ga1−xZnx)(N1−xOx) solid solution for overall water splitting by co-loading Crand another transition metal. J. Catal. 2006;243:303.

254. Hirai T., Maeda K., Yoshida M., Kubota J., Ikeda S., Matsumura M., Domen K. Origin ofvisible light absorption in GaN-rich (Ga1-xZnx)(N1−xOx) photocatalysts. J. Phys. Chem. C2007;111:18853.

255. Lee Y., Terashima H., Shimodaira Y., Teramura K., Hara M., Kobayashi H., Domen K.,Yashima M. Zinc germanium oxynitride as a photocatalyst for overall water splitting undervisible light. J. Phys. Chem. C 2007;111:1042.

256. Lee Y. G., Teramura K., HaraM., Domen K.Modification of (Zn1+xGe)(N2Ox) solid solutionas a visible light driven photocatalyst for overall water splitting.Chem. Mater. 2007;19:2120.

257. Maeda K., Teramura K., Lu D. L., Saito N., Inoue Y., Domen K. Roles of Rh/Cr2O3(core/shell) nanoparticles photodeposited on visible-light-responsive (Ga1−xZnx)(N1−xOx)solid solutions in photocatalytic overall water splitting. J. Phys. Chem. C 2007;111:7554.

258. Sun X., Maeda K., Le Faucheur M., Teramura K., Domen K. Preparation of (Ga1−xZnx)(N1−xOx) solid-solution from ZnGa2O4 and ZnO as a photo-catalyst for overall water split-ting under visible light. Appl. Catal. A 2007;327:114.

259. Wang X. C., Maeda K., Lee Y., Domen K. Enhancement of photocatalytic activity of(Zn1−xGe)(N2Ox) for visible-light-driven overall water splitting by calcination under nitro-gen. Chem. Phys. Lett. 2008;457:134.

260. Maeda K., Hashiguchi H., Masuda H., Abe R., Domen K. Photocatalytic activity of(Ga1−xZnx)(N1−xOx) for visible-light-driven H2 and O2 evolution in the presence of sac-rificial reagents. J. Phys. Chem. C 2008;112:3447.

261. Wang X. C., Maeda K., Lee Y., Domen K. Enhancement of photocatalytic activity of(Zn1+xGe)(N2Ox) for visible-light-driven overall water splitting by calcination under nitro-gen. Chem. Phys. Lett. 2008;457:134.

Nanocatalysis Synthesis and Applications (Polshettiwar/Nanocatalysis) || Nanocatalysts for Water...

Documents

Transcript of Nanocatalysis Synthesis and Applications (Polshettiwar/Nanocatalysis) || Nanocatalysts for Water...