Nanocatalysis Synthesis and Applications (Polshettiwar/Nanocatalysis) || Nanomaterial-Based...

13

NANOMATERIAL-BASEDPHOTOCATALYSTS

Biswajit Mishra and Deepa Khushalani

INTRODUCTION

Finding easier and more economical ways of activating a diverse set of nonspontaneousorganic reactions has been the mainstay of a variety of researchers over the years. Fromvery early on, it has been clear that the main reason for use of a catalyst in a particularreaction is that it lowers the activation energy, providing for a more economical reactionpathway (from an energy perspective). It should be noted that this is viable for bothendo- and exothermic reactions. As a result, ideally the reaction proceeds faster andhopefully to a purer product without the presence of unwanted additional phases and/orstoichiometries. This definition still holds for the term “photocatalyst.” As the nameintimates, it is basically a material that is able to increase the rate of a reaction, however,only when specifically activated by either ultraviolet (UV) or visible electromagneticradiation (at least thus far in literature, predominantly the range of ∼250–800 nm hasbeen effectively studied). To this aim, using UV or visible light as a source of viableenergy (especially when it is provided naturally, i.e., in the form of sunlight) to promotereactions has been, and is currently being, actively studied by researchers in the areasas disparate as organic chemistry, electrochemistry, colloidal science, surface science,materials chemistry, analytical chemistry, and semiconductor physics. The ultimate aimof these researchers is to make a catalyst that is viable in terms of both energy andmonetary units.

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.

469

470 NANOMATERIAL-BASED PHOTOCATALYSTS

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Spectr

al irra

dia

nce

(W

/m2/n

m)

400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Wavelength (nm)

Visible(14%)

UV(7%)

IR(47%)

Figure 13.1. Solar irradiance spectrum.

The solar spectrum, displayed in Figure 13.1, has been analyzed to contain approxi-mately 7%UV radiation, 46%visible radiation, and 47% infrared radiation. It is apparentthat harnessing this clean energy (range of 5–0.5 eV) could dramatically assist a vari-ety of reactions in a cost-effective way, and moreover, these processes, if optimized,have such lucrative long-term potential that science-funding organizations in all majorcountries have dictated harnessing solar power efficiently as one of the most importantprimary research targets for the foreseeable future.

Strictly speaking, two aspects underlay the central tenet in photocatalysis: thecatalyst must be able to absorb photons (preferably with a large range of energies) andalso reactants onto its surface. The rationale behind this is that, when a large band-gapsemiconductor absorbs photons, the system undergoes a transition, whereby electronsfrom a filled valence band are promoted to an empty conduction band. Although this isapplicable to a large variety of materials, the crux of the principle lays in the fact that(1) the carriers generated (electrons and holes) must not recombine, (2) the chemicalpotential of these carriers must be commensurate with the chemical reactions that are tobe eventually initiated, and (3) as such, the carriers must be prevented from recombining(usually a fast process), and instead be allowed to diffuse to the surface (usually aslower process) where subsequently crucial reactive radicals are formed. Although thedetails of the exact processes are detailed later, Figure 13.2 depicts a quick schematicrepresentation of the vital processes taking place.

It should be noted that due to popularity of this field of research, several excellentreviews have recently appeared in the literature.1–4 In order to avoid redundancy withthese aforementioned studies, we focus specifically on the following aspects in thischapter: (1) solid oxide-based photocatalyticmaterials have been studies, (2) the functionof catalysts has been evaluated using exclusively visible light, (3) sole degradation of

HISTORICAL PERSPECTIVES IN PHOTOCATALYSIS 471

Reduction reaction

Oxidation reaction

CB

VB

h+

e–

λν

Figure 13.2. Schematic representation of the generation of the charge carriers by irradiation.

organic pollutants (or representative model compounds) has been studied, and finally(4) methods of synthesis of the catalysts have been summarized.

HISTORICAL PERSPECTIVES IN PHOTOCATALYSIS

At this juncture, in order to appreciate the latest inventions in this field, it is importantto be aware of the journey that research in photocatalysis has taken from its inceptionto reach the current state. Although the term “photocatalysis” has been around for overa century, with some of the earliest works using the term “photocatalyst” appearing by1911,5 its relation to explicitly referring to reactions where photogenerated electronsand holes actively participate in chemical reactions remains under some debate. Thusfar, one of the earliest citations that specifically refers to using a solid oxide materialfor effectively being involved in the degradation of organic materials appeared in 1911,where Eibner discussed the influence of zinc oxide on decomposition of organic com-pounds.6 Subsequently, a variety of papers on ZnO appeared, and in 1929 the first citedpaper appeared where instead of ZnO, TiO2 was observed as a contender for catalysis byKeidel.7 He published a paper entitled “Influence of Titanium White on the Fastness ofLight to Coal-Tar Days.” Although the exact mechanismwas not understood at that time,it was nonetheless obvious to researchers that these solid oxides played a pivotal role,especially when they were irradiated with light of various wavelengths, in contributingto the degradation of any organic compounds present in the vicinity of that oxide.

Subsequently, consistent research in this area continued for many decades; however,it was not until the breakthrough pioneered by Fujishima and Honda in 19728 that adramatic focuswas brought to this field of research. These authors showed in their pivotalNature publication that the rutile phase of TiO2 could split water and thereby releasehydrogen at the cathode and oxygen at the anode. However, this was solely accomplishedwhen (1) the reaction was specifically irradiated with UV light and (2) there was a biaspresent (be it an anodic bias or a chemical bias, i.e., the pH of the electrolyte solutionfor the working electrode was maintained at elevated levels). The area of photocatalysisbasically exploded following this publication, which was primarily because the authorsunambiguously showed that when a semiconductor oxide was irradiated with light of


wavelengths shorter than its band gap (∼415 nm for 3 eV band gap), photocurrent couldbe generated that flowed from the counter electrode to the TiO2-irradiated electrodethrough an external circuit. Hence, the direction of the current revealed that the oxidationreaction could be initiated at the TiO2 electrode and the reduction reaction at the counterelectrode (Pt was used by Fujishma and Honda).

Following this publication, the use of TiO2 in photocatalytic studies exponentiallygrew, and moreover, disparate fields of science such as organic chemistry, colloidalscience, materials science, and semiconductor physics started actively pursuing themechanistic details of this type of catalysis.

CURRENT STATE OF PHOTOCATALYSIS (FROM 2000 TO NOW)

In order to fully appreciate this field of research, it is imperative for the reader torealize the worldwide interest that has slowly gained momentum specifically over thelast decade. Figure 13.3a shows the numbers of publications that were released withthe keyword “photocatalys∗” as provided by ISI Web of Science from the start of thetwenty-first century. It is clear that there has been an exponential increase in the numberof publications, andmore interestingly, geographically, the research has not been isolatedto any one particular region. In fact, all the major research institutes the world over aredevoting centers of excellence to this field of science. The reason for this can be mainlyattributed to the current energy crisis pervading the entire planet. Research is beingguided primarily due to the economics of the various governments. In order to alleviatethe large dependence on crude oil, finding alternative sources of fuel is being ferventlypursued, and to this aim, it is obvious (as Fujishima and Honda’s pioneering papershowed), photocatalysis could perhaps hold a vital key to the future quality of life onthis planet—in terms of not only providing financially viable production of hydrogengas (detailed in Chapter 15 in this book) but also dealing with the pollution problem.

MECHANISTIC DETAILS

As mentioned earlier, this review exclusively assesses the current state of photocatalysisas it pertains to using visible light for remediation of organic pollutants (air- and/or water-borne) in both gas and liquid phases. The mechanism by which catalysis is performedis depicted in Figure 13.4.

Primary steps in the mechanism of photocatalysis are formation of charge carriersby photon absorption. This involves illumination of the semiconductor with appropriateenergy, allowing in turn the promotion of electrons from the valence to the conductionband (e.g., for anatase phase of TiO2, UV light irradiation—� �390 nm—correspondingto its band gap of 3.2 eV). This process leaves an unoccupied state in the valenceband, which in itself can be described as a particle (hole with a positive charge). Asa result, negative and positive particles are generated. Most of these carriers actuallyend up recombining and the energy released is usually in the form of heat; however, asmall percentage of the carriers are able to migrate (i.e., diffuse) to the surface of the

MECHANISTIC DETAILS 473

2500

01980

(a)

(b)

1985 1990 1995 2000 2005 2010

Year

Number of publications usingkeyword: “photocatalys*”

Num

ber

of p

ublic

atio

ns

500

1000

1500

2000

1000

01990 1995 2000 2005 2010

Year

Number of publications using keyword: “visible light photocatalys*”

Num

bers

of p

ublic

atio

ns

200

400

600

800

Figure 13.3. The number of publication using keywords “photocatalys∗” and “visible light

photocatalys∗.” (Data obtained from ISI Web of Science.)

semiconductor particle where they are captured by adsorbed molecules, and a catalyticreaction is initiated. The initiation is that of an oxidative pathway by the valence bandhole and a reductive pathway by the conduction band electron. As a result, it naturallyfollows that the type reaction is intrinsically dependent on the chemical potential ofthe carriers. As such, each semiconductor oxide gives rise to oxidizing and reducingspecies with differing capabilities. Figure 13.5 shows the relative chemical potential ofthe electrons and holes generated from a variety of oxides. In the case of TiO2, it canbe seen that the top of the valence band lies at approximately 2.91 V and the bottomof the conduction band lies at approximately −0.29 V (for the band gap of 3.2 eV, this


CB

VB

h+

hν(UV light) heat, hν'

e–e–

–h+

S

S

O2O2

–H+

O2H2

12O

+S

–

–

S

S

HO2 + H O2

H O2 2

2 OHH O2 2

OH

HO2

HO2O2 H O2 2

+ OH + OH

hν+ OH O2+

Figure 13.4. Schematic representation of the main processes involved in photocatalysis.

(Adapted from9.)

corresponds to an energy of 308 kJ/mol, which is a large amount of energy capableof being used for the degradation of organic compounds). Hence, these respectiveenergies suggest that for TiO2 the holes are capable of oxidizing water and the electronsare capable of reducing water simultaneously. From the perspective of degradation oforganic pollutants, the electrons are capable of reducing a variety of active oxygenspecies and the holes predominantly participate in generating hydroxide radicals (OH•),which are very strong oxidizing agents.

Although the processes discussed earlier seem simple enough, it is imperative torealize that the processes have been outlined only using a thermodynamic perspective.In reality, the scenario is far more complex as the reaction processes depend on a myriadof factors. One such factor that needs to be highlighted is that the redox reactions atthe surface of the oxide particle will predominantly be initiated only if the organiccontaminants are adsorbed on the surface. As such, usually before any experimentalcatalytic reaction is performed, it is preceded by a “dark reaction” during which the

0

–1

–2

–3

1

2

3

4

–2

–3

–4

–5

–6

–7

–8

5.1

VB

CB

HfO

2

ZrO

2

Ta

O2

5

4.6

4.3

3.4 3.0

3.2

3.2 1.

9

3.5 2

.7 1.3

1.7

1.7

U, V

E, eV

Nb

O5

2

Sb

O3

2

ZnO

TiO

2

α-F

eO

32

SnO

2S

nO

2

WO

3W

O3

PtO

PtO

Au

O3

2A

uO

32

CuO

Figure 13.5. Electronic structure of different metal oxides and the relative position of their

band edges. (Adapted from10.)

USE OF VISIBLE LIGHT 475

surface area and chemical functionality of the oxide (extent of hydrophilicity) play a vitalrole in allowing as much of the pollutant to be adsorbed. Moreover, parameters such asrecombination and trapping of charged carriers (in inaccessible sites), temperature of thereaction, types of reaction media used, thermal/temporal stability of the semiconductingoxides, morphology, and accessible surface area of the oxides are all crucial to evaluatingthe efficacy of any oxide photocatalyst synthesized.

USE OF VISIBLE LIGHT

As has been presented earlier, in photocatalysis, the redox reactions will only be initiatedif the energy of radiation is larger than the band gap of the semiconductor of interest.As such, in the interest of making them active under visible light, the energy of the bandgap must be in the range of 3.1–1.8 eV. It follows naturally therefore that modificationof the oxides, to make them active to visible light, has been pursued fervently over thelast decade (Figure 13.3b).

Furthermore, even though Figure 13.5 exhibits a large number of wide band-gapsemiconductors that are suited for photocatalytic reactions (albeit, in their native state,they are ideal for UV light absorption only), it is important to realize that actually thepredominant citations for photocatalysis happen to investigate modified TiO2. In fact,TiO2 appears to be the favorite choice because of its inexpensiveness, nontoxicity, andchemical stability in aqueous environment in addition to the perfect matching of the bandedges with the most important half-reactions to generate reactive oxygen species. Toillustrate the importance of TiO2 and its popularity in literature, a survey of the publishedworks, cited in Figure 13.3b, illustrates that over 80% of the citations are work on TiO2.Hence, its potential to provide for a viable visible light photocatalyst is undeniable.

Specifically, the electronic structure of TiO2 consists of the valence band comprisingof O 2p orbital and conduction band with Ti 3d. Position of conduction band minimum isat approximately−0.3 eV and valence bandmaximum at+2.91 eV; that is, oxidation andreduction powers of the photogenerated holes and electrons, generated upon absorptionof near-UV irradiation, are strong enough to perform the water splitting or to producethe common reactive oxygen species, which can further attack and mineralize organicpollutants, an aspect that has been detailed previously. Thus far there have been manymethodologies adopted to alter the electronic structure of the oxides tomake them visiblelight active. The approaches to do this are detailed here, and TiO2 is used as the modeloxide of choice in the following examples:

1. Doping the semi-conducting oxides to form states that can absorb visible light:Doping plays an invaluable role in modifying the electronic properties of theoxides. Although the details of the key issues involving doping (e.g., natureof chemical states, location of dopants, and localized vs. delocalized midgapstates) have still not been clearly elucidated, doping remains the most sought-after strategy for altering semiconducting oxides to be able to absorb visiblelight. Specifically doping involves inclusion or substitution of another atom intothe lattice of the oxide. This is done in such a way that it replaces either thecation or the anion into the framework, and as a result new electronic states


1.54 eV

2.18 eV3.3 eV

Conductionband

Valence

band

Black TiO2 White TiO2

Figure 13.6. Schematic representation of DOS of disorder-engineered “black TiO2” (formed

by hydrogen reduction of pristine TiO2) as compared to that of unmodified TiO2 nanocrystals.

(Adapted from12.)

are introduced and hence new transitions (optical) are seen that have not beenobserved in the pure oxide.11

There are specifically two types of doping:a) Self-doping: This is a process whereby vacancies are simply introduced intothe lattice and in fact no new atoms are added. As an ideal example, TiO2-xis a perfect representative of an oxygen-deficient (and hence n-doped) TiO2.These defects as a result create occupied gap states, which are normallythought to be located below the conduction band edge (states that predomi-nantly Ti 3d related). Hence, it is assumed a pseudoband gap is created that islower in energy than the conventional 3.2 eV for TiO2 (anatase phase). Theseoxygen vacancies create energy levels at 0.75–1.18 eV below the conductionband of TiO2. As a result photogenerated electrons in conduction bands areno longer powerful enough to reduce water to H2 or molecular oxygen tooxygen radical anion.

A slightly altered approach to create self-doped defects was recentlyshown by Chen and coworkers.12 The authors reported in their Science publi-cation a route to create broad, continuous defect states in the band-gap regionof TiO2 by simply creating a disordered TiO2 network through reducing crys-talline TiO2 in the presence of hydrogen over a prolonged period. The authorspurported that they developed an alternative approach to improving visibleand infrared optical absorption by engineering the disordered nanophaseof TiO2 with simultaneous dopant incorporation (i.e., formation of oxygenvacancies). The disorder-engineered nanophase TiO2 that was formed con-sisted of two phases: a crystalline TiO2 quantum dot or nanocrystal as acore, and a highly disordered surface layer where defects were introduced.Although an ensemble of nanocrystals retained the benefits of crystallineTiO2 quantum structures for photocatalytic processes, the introduction of dis-order and dopants at their surface was found to enhance visible and infrared

USE OF VISIBLE LIGHT 477

absorption, and a benefit of carrier trapping was also cited (thereby reduc-ing recombination rates). The density of states (DOS) of disorder-engineeredsemiconductor nanocrystals, as compared to those of unmodified nanocrys-tals, is shown schematically in Figure 13.6. The authors observed a shift in theonset of absorption in such disorder-engineered TiO2 nanocrystals, from theUV to near-infrared after hydrogenation, accompanied by a dramatic colorchange and substantial enhancement of solar-driven photocatalytic activity.

b) Cation and/or anion doping: The second form of doping (and definitely theone with more widespread popularity) is introduction of either n or p carriers.In the specific case of anion doping, Asahi and coworkers pioneered thisarea through their seminal publication in 2001.13 Here, they showed initiallythrough theoretical calculations that substitutional doping with N or S inplace of O could be most effective due to the mixing of N 2p with O 2p,resulting in band-gap narrowing. However, replacing O2− with S2− costslarge formation energy due to the larger ionic radius of S2−. On the otherhand, substitutional doping with C and P creates states but too deep in theband gap. When nitrogen was doped interstitially, it was found to be presentas molecular NO or N2, and this created bonding states below O 2p valenceband and antibonding states deep in the band gap, which were found to bewell screened. Moreover, they substantiated this theoretical prediction byexperimentally preparing N-doped TiO2. They sputtered a TiO2 target inN2/Ar gas mixture and showed that the visible light photocatalytic activityincreased with the increase of the area under the XPS (N 1S) peak at 396 eV,which corresponded to the �-N from Ti–N bonding; that is, N substitutionat the O position led to the visible light activity of the TiO2. Therefore,this publication experimentally showed the formation of TiO2-xNx, where thenitrogen was doped into substitutional sites and as a result, the ensuing oxidewas found to have a smaller band gap and also showed facile degradation ofmethylene blue and gaseous acetaldehyde (when compared to conventionalbulk TiO2). This proposition that the narrowed band gap formed because ofmixing O 2p states with the N 2p states of the substituted nitrogen atomsin the newly formed valence band was challenged later on when Irie andcoworkers in 2003 argued that it was the isolated N 2p states that formedabove the valence band maximum of TiO2. Moreover, it was these statesthat were the origin of the visible light absorption band of nitrogen-dopedTiO2.14 In addition, they found that increasing the amount of nitrogen loadingled to lower efficient catalysts, suggesting that these midgap states served asrecombination centres for the generated carriers. Since then, arguments onthe origin of this visible light absorption by nitrogen doping (and other anionssuch as S, P, and C) have continued, especially at the theoretical level wherethe chemical state, dopant concentration, and different methods of calculationhave all been thoroughly considered.15–25 Some of the latter arguments arebased on the formation of oxygen vacancies by doping and then subsequentadvent of color centres (e.g., F, F+, F++, and Ti3+) that absorb the visible


light radiation.26–28 However, a consistent conclusion as to whether the bandgap can be narrowed and what causes it has not yet been arrived at.

Cation doping, although not as favourable as N doping, has had a fewpositive results. Many groups have examined how cation doping shifts theTiO2 absorption properties into the visible light. A variety of metals on TiO2have been explored, including Fe, Cr, V, Ni, Nb, Mn, Cu, Al, Bi, and Co, andthe references provided are for the best representative examples.29–39 In mostof these cases, publications have shown that cation doping of TiO2 results inproducts that easily show visible light absorptivity; however, in general thisdoes translate into photoactivity. However, having stated this, there have beensome successful reports such as the one by Iino and coworkers where theyhave shown that cation doping (using Cr3+) of TiO2 results in both visiblelight absorptivity and photoactivity toward NO.40 Most importantly, the pho-tocatalysis activity was, however, observed only for samples prepared by ionimplantation and not for the samples whose surfaces were chemically dopedwith Cr3+. This suggests that the chemical and/or structural environment ofthe cation dopant has a strong influence on the nature and extent of pho-toactivity. Based on density functional theory calculations for substitutionalmetal doping, it has been stated that the metals do create midgap states, andvisible excitations can occur (a) between TiO2 valence band and the unoccu-pied metallic midgap states or (b) excitation of partially filled midgap statesinto the TiO2 conduction band. The authors have further proposed that theposition of the dopant’s t2g state in the oxide lattice’s octahedral field (forsubstitutional first row metal cation dopants) is key to whether optical tran-sitions result from the dopant into TiO2 states or from TiO2 states into thedopant. While both types of transitions may occur using visible light, they donot have the same potential for catalysis.

2. Improving visible light activity by forming heterostructures with the semi-conducting oxide: This process is based on the premise of incorporatinglight-absorbing species (e.g., active dyes, semiconducting quantum dots, andstrong electron conductors such as Ag ions or Ag nanoparticles) and couplingthem with the surface of the oxide (e.g., TiO2).41 This coupled system issupposed to benefit synergistically by (a) utilizing the light-absorbing capabilityof the additive and with (b) the catalytically active radical-generating capabilityof wide band-gap semiconductor. These hybrid systems possess significantadvantages because they promote the separation of electron–hole pairs (reducerecombination) and also keep the reduction and oxidation reactions at twodifferent reaction sites. In addition, they extend the light–response rangeby coupling suitable electronic structures onto the surface of the oxidicmaterials.

One of the popular ways of forming these coupled systems was pioneeredby Spanhel and coworkers who effectively coupled fluorescent quantum dotsof CdS onto the surface of TiO2.42 As early as 1987, Spanhel and coworkerssystematically investigated and confirmed the efficient electron injection process

SYNTHETIC METHODS 479

from CdS particles to the conduction band of attached TiO2 particles illuminatedwith visible light. In the past few years, studies of this system have become morepopular, due mainly to the explosive development of a variety of quantum dotsand nanosized oxides with varied morphologies.43–48

Another effective additive is the use of a sensitizer. The sensitizer couldbe an organic molecule, or as it has lately been demonstrated, it could be ap or n semiconductor such as graphene oxide (GO). Chen and coworkers andothers have recently showed that by coupling GO (band gap �2.4 eV) on thesurface of TiO2 for example, the resultant composite could be used as the light-absorbing species, and the excited electrons could be transferred to TiO2 toeffectively mediate a host of redox reactions involving a variety of organicmolecules.49–52 However, one obvious drawback of these mechanisms is that theredox potential of transferred electrons/holes is greatly reduced by the release ofa portion of the potential energy during electron transfer at the interfaces. Hence,the efficiency of these kinds of heterostructures is still quite low. Nonetheless,a large number of successful results have been shown for use of a variety ofother structures such as palladium oxide-functionalized TiO2,53 plasmonic Aunanoparticle-functionalized TiO2,54–56 and even LaFeO3-functionalized TiO2.57

At this juncture, a word of caution needs to be noted. Despite the inordinatenumber of theoretical publications available on the doping effects on oxides,or even the experimental literature available where a variety of heterostructureshave been synthesized, the practical translation of this information into viablecatalysts is still difficult, simply due to the impediments encountered during thesynthesis of these catalysts. As has been stated earlier, location of the dopant,its concentration, its ionic radius and charge, and finally its interaction with theother elements all play a critical role in determining the overall electronic energylevels and the ensuing band gap. In the case of heterostructures, the interfacialinteraction at the surface of the oxide with the visible light absorber is the crucialmitigating factor. Hence, the manner in which these materials are synthesizedis the most often researched area and has a direct influence on the efficiency ofthe catalyst that is synthesized. As a result, therefore, the next section presents asummary of themost popular types of approaches that have been used to fabricatevisible-light-active photocatalysts. Their details have been summarized here.

SYNTHETIC METHODS

Sol–Gel

Themost easy and cost-effective procedure available to a synthetic chemist for formationof a variety of oxides (pure or doped) is that of sol–gel technique. A sol is basically ahomogeneous dispersion of solid particulates in colloidal form in fluid (liquid or solid).On the contrary, a gel is basically an interconnected three-dimensional continuous solidnetwork with submicrometer size pores filled with continuous liquid phase. Sol–gelprocess refers to the hydrolysis and polycondensation of the precursors for the metal or


metalloid oxides. Precursors used are mainly alkoxides and rarely inorganic (containingno carbon) salts of the metals or metalloids. Sols from the inorganic salts are less stablethan those from the alkoxides.58 Alkoxides fall in the class of metalorganic compoundsin which organic ligands coordinate to a metal or metalloid through oxygen atoms, notby the direct metal–carbon bonds (organometallics). The sol–gel process, irrespectiveof the nature of precursors, solely consists of the following six steps:

1. Formation of sol: Sol is basically formed by the hydrolysis of the precursor thatproduces hydroxides (monomers), and these subsequently condense to oligomersand form a stable solution that does not precipitate. As a sol is less viscous, itcan be cast in a mold (i.e., in a template, be it a microemulsion or a porousmembrane such as anodic alumina) or manipulated at this stage in a facilemanner. Usually, dopants are added at this stage because there exists a strongpossibility of creating strong covalent interactions between the dopant and thereactive monomeric species of the metal oxide (i.e., M(OH)nm+).

2. Formation of gel: Gel is formed by the polycondensation of the aforementionedmonomers/oligomers to polymers, and this is concomitantly observed with thesudden rise in viscosity of the solution. At this stage, it is also important torealize that the gel consists of large volumes of hydrogen-bonded solvent thatcan contribute to eventual porosity of the resulting oxide (and hence a largesurface area).

3. Aging: During the aging period, polycondensation continues until the gel solid-ifies and the porosity decreases. This can take up to several days (dependingon the type of solvent and metal precursor used) and can be done at elevatedtemperatures to improve the process.

4. Drying: Drying could be done in two ways. A xerogel can be formed by simplethermal evaporation of the liquid, which results in small (�20 nm) pores of thegel. Analogously, an aerogel can also be formed; however, it is crucial that theprocess of removal of the solvent is done using supercritical drying. This resultsin a low density and a highly porous gel network structure as the final product.

5. Dehydration: During this stage, surface-bound M-OH groups are removed byheating at higher temperature (800 ◦C) leading to further decrease in porosity,albeit with a stronger framework structure.

6. Densification: Densification of the gels at high temperatures is usually done toensure complete removal of any organic moiety, and this is usually accompaniedby further loss in porosity.

Figure 13.7 schematically details these steps using the general example of siliconalkoxide Si(OR)4, whereR=methyl, ethyl, or propylmoiety. It is important to appreciatethat the first two steps are crucial as they control the chemical nature of the final oxidethat results.

Both types of hydrolysis in Figure 13.7 are nucleophilic substitution (SN2) con-densation, which may occur in either of two ways—dealcoholation or dehydration—depending on the pH of the system:


ORSi

OR

OR

RO + :OH- Si

RO OR

OR

RO

++

OH- Si

OR

OR

OH + RORO -

OH

H+RO Si

OR

ORH

Si

RO OR

ORH

OH

H

++

OR + ROH + HSi

OR

OR

HO +O+ R O+

R(a)

(b)

Figure 13.7. Hydrolysis (Step 1) of alkoxides in (a) acidic (b) or basic solution.

Dealcoholation in acid solution:

ORSi

OR

OR

RO H+ +Fast..

O+

R

HSi

OR

OR

RO

Si

OR

OR

+RO O+

R

HSi

OR

OR

ROSlow

Si

RO OR

OR

O +

++

O+

R

HH

Si

OR

OR

ROOH

Si

OR

OR

RO Si

OR

OR

ORO + ROH H+ +

Dehydration in acid solution:

Si

OR

OR

RO H+ +Fast..

O+

H

HSi

OR

OR

RO

Si

OR

OR

RO + O+

H

HSi

OR

OR

ROSlow

Si

RO OR

OR

O+

++

O+

H

HH

Si

OR

OR

ROOH

Si

OR

OR

RO Si

OR

OR

ORO + H2O H+ +

OH


Dealcoholation in basic solution:

Si

OR

OR

RO + :OH –O R Si

OR

OR

ORO -ROH+

Dehydration in basic solution:

Si

OR

OR

RO + :OH –O H Si

OR

OR

ORO – + H2O

Si

OR

OR

ORO – Si

OR

OR

OHRO+ Si

RO OR

OR

O

++

OHSi

OR

OR

RO

Si

OR

OR

RO Si

OR

OR

ORO + OH –

The degree of hydrolysis (n in Si(OR)4-n(OH)n) could be controlled by varying theSi/H2O ratio.

Si(OR)4 + n H2O Si(OR)4-n(OH)n + n ROH

AsOH− is a better leaving group than OR, the gel network structures largely dependon the n:

Dimers:

Si

OR

OR

OHRO2

1=f

Si

OR

OR

RO Si

OR

OR

ORO

One-dimensional chains:

Si

OR

OR

OHHOn

2=f

Si

OR

OR

HO Si

OR

OR

OHO

n-1


Two-dimensional rings:

Si

O

O

RO

RO

Si

OR

OR

Si

OR

OR

O

Si

OR

OR

Si

OR

OR

O

Si

O

O

OR

OR

n-4

Si

OR

OR

OHHOn

2=f

Three-dimensional fractals:

Si OHHOn

4=f

OH

OH

Si OHO

OH

O

OSi

OH

O

Si OHO

OH

OSi

OH

OHSi

OH

O

OHSi

OHn-4

Between pH 2 and pH 6, the rate of condensation increases with [OH−]. This pHrange is above the isoelectric point of the silanol. Below pH 2, the rate of condensationis proportional [H+] and gel time is longer than in the former case.59

When the above pathway is considered for other elements, such as titanium, it isimportant to realize that Ti is more electropositive (+0.63) than Si (+0.32) as suchtitanium alkoxides are more susceptible to nucleophilic attack and hence have a fasterrate of hydrolysis. In terms of actual synthesis, this means that titania is more difficult(compared with silica) to follow any templating or doping paradigm. In addition, Ti hasa higher coordination number than Si, thereby it tends to more easily form oligomers andpolymers simultaneously during the hydrolysis process. This normally translates intothe fact that the products are highly heterogeneous in terms of structure and morphology.Moreover, in presence of acid, the rate of hydrolysis increases as the –OR groups areprotonated and become better leaving groups. However, at higher acid concentrations,the active hydroxyl groups for condensation become less nucleophilic as they becomeprotonated and hence the rate of condensation slows down. Interestingly, the contraryholds for basic solutions, where the active hydroxyl groups are deprotonated for thecondensation, hence are more nucleophilic and therefore an increase in the rate of con-densation is observed. In order to retard the rates of hydrolysis and condensation (so thatmore control is gained in terms of dopant incorporation and the ensuing morphology),chemical modifications of Ti(OR)4 with alcohols, chlorides, acids or bases, chelatingligands, and so on, by partial substitution (SN) or nucleophilic addition are usually


done.60–65 Stability of the modified ligand is expected to be higher if the modifying lig-and is more nucleophilic than the initial alkoxy group. For example, acetylacetone andother �-diketones are much stronger chelating agents, as such for this case precipitationdoes not occur even with the presence of a large excess of water.66

Thus far in literature, synthesis of doped semiconductor oxides, in particular TiO2,has been done predominantly by this method, albeit with somemoderate level of success.The sol–gel process does have limitations in terms of the amount of dopant that can beincorporated (usually�5% is normally feasible), but more importantly, there is usually alarge preponderance of the dopant species appearing as simple surface-adsorbed specieson the resulting oxide as opposed to being incorporated into the lattice framework. As aresult, normally the outcomes of sol–gel synthesis are doped oxides that do show visiblelight absorption but with little or no photoactivity.

Hydrothermal Synthesis

The term “hydrothermal” has a geological origin, where it implies a regime of hightemperatures and water pressure. If water is replaced by another solvent, then the methodis referred to as solvothermal synthesis.67 This synthesis is contingent on using high-temperature autoclaves. These are simply thick-walled steel cylinders with an airtightseal which must be able to withstand high temperatures and pressures for prolongedperiods. Inside the cylinders, inert liners are inserted (usually made of Teflon or otheranticorrosive material). These liners serve as the vessels inside which the reaction takesplace. The insert may have the same shape as that of the autoclave, and they fit in theinternal cavity (contact-type insert).

This technique is one of the most extensively used methods for crystal growth andformation of binary, ternary, and quaternary oxides. The principle relies on the fact thatthe reaction mixture inside the line/autoclave needs to be heated in order to create twotemperature zones. The chemical dissolves in the hotter zone, and the saturated aqueoussolution in the lower part is transported to the upper part by convective motion of thesolution. The cooler and denser solution in the upper part of the autoclave descends,while the counterflow of solution ascends. The solution becomes supersaturated in theupper part as the result of the reduction in temperature and crystallization sets in. Thistechnique has many advantages in that the products that are inevitably powders can beformed directly from solution and can be either crystalline or amorphous depending onthe requirement. Moreover, the process is able to control the particle size, shape, andchemical composition, stoichiometry in a relatively facile manner (the two parametersthat simply need to be varied are the temperature of the reaction mixture and thesolvent employed so as to obtain high pressures and hence supersaturation at lowertemperatures).

In the case of photocatalysis, this technique has shown dramatic results, especiallyfor incorporating dopants into the oxides and also for predominantly forming highlycrystalline structures. The activity of semiconductor photocatalysts depends criticallyon formation of many carriers that can migrate to the surface to participate in redox reac-tions. As such it is imperative to avoid recombination, which for certain types of oxidespredominantly seems to occur at crystal lattice defects.68, 69 As an example, Amanao


and coworkers recently showed how two bismuth tungstate (Bi2WO6) samples of almostthe same specific surface areas containing only the amorphous or crystalline phase wereprepared by hydrothermal reaction of a precursor under the same synthesis conditions.In addition to successful synthesis of these tungstates using the hydrothermal prepara-tion, they were able to show that the crystalline phase of Bi2WO6 showed high quantumefficiency for complete oxidative decomposition of gaseous acetaldehyde (AcH) evenunder visible irradiation owing to the slow recombination while negligible photocat-alytic activity of amorphous Bi2WO6 was observed owing to the fast recombination ofelectron–hole pairs.70

Chemical Vapor Deposition and Atomic Layer Deposition

In chemical vapor deposition (CVD) normally, two reactants (A and B) react in the gasphase in one step over a heated substrate and the product is then subsequently depositedon that substrate. It has been used extensively as it is relatively easy to set up a CVDreactor and a variety of organic precursor molecules are commercially available foroxide/mixed oxide formation.71

Atomic layer deposition (ALD) however is a special modification of the CVD.However, in ALD each reactant is allowed to react sequentially with the surface of thesubstrate.72 ALD consists of four steps: (1) exposure of the first precursor (A), (2) purgeof the reaction chamber, (3) exposure of the second precursor (B), and (4) a further purgeof the reaction chamber. As this involves chemisorption of the reactants at the activesurface adsorption sites (which are finite in number) on the substrate, the growth processis strictly self-limiting, assuming the reactions are at completion at the end of each of thesteps 1 and 3. Typical thicknesses per cycle may be in between 0.1 A and 3 A dependingon the size of the precursors used; that is, for a fixed precursor size, the thickness ofthe film can be very easily controlled by just controlling the number of cycles. The timeallocated for the completion of the reaction step varies from 0.5 to 5 s. Hence, � G forthe reactions is desired to be negative in order for the reaction to go to the completionduring that time window. Moreover, ALD is quite adaptable as precursors could begases, volatile liquids, or even solids (introduced albeit at elevated temperatures). Allsolids and few liquids are needed to be heated to generate high enough vapor pressuresfor effective mass transport. Precursors need to be thermally stable enough at the growthtemperature. Otherwise advantages associated with the self-limiting growth cannot beobtained. Although recently ALD processes have been reported at temperatures as lowas room temperature especially for biological samples.73

Binary compounds such as oxides (TiO2, ZnO, and Al2O3) or metal nitrides (TiN,TaN, and W2N) have been deposited successfully by ALD. They have higher negative� G values associated with their deposition reactions because they occur spontaneouslywithout the need for use of plasma-generated radicals. Hence, in this case the processis simply referred to as thermal ALD.74 However, when using ALD to deposit singleelements (such as metal or metalloids), it is necessary that radicals are generated by theuse of plasma and hence the process is referred to as plasma or radical-enhanced ALD.For example, Ta and Si are usually deposited using H2 plasma.75 The energetics are suchthat they induce reactions that are not possible when only the thermal process is used.


Among the thin-film deposition techniques (such as CVD, physical vapor deposi-tion, or electrochemical deposition), ALD has two main advantages: (1) Film thicknesscan be controlled very precisely on an angstrom length scale, and (2) since the processrelies exclusively on chemisorption, it has been observed that once the substrate surfaceis saturated, there is no further deposition even in the presence of excess reactants. As aresult, excellent conformality can be maintained even with high-aspect ratio substrates(i.e., ALD can be done on even porous substrates in order to yield high surface areaoxides).76

Ion Beam Techniques

Ion Implantation. This is an engineering process by which ions of a materialare accelerated in an electrical field and impacted into another solid.77 The processinvolves using controlled amounts of chosen foreign species that are introduced intosurface regions of a material in the form of accelerated beam of ions. The energies usednormally lie between 10 keV and 500 keV, with corresponding penetrations rangingfrom 100 A to 1 �m. Note that if lower energies are employed, then the ions cansimply be either surface deposited or sputtered onto the target. However, for specificallyimplantation, the ions impinge on the substrate with kinetic energies 4–5 orders ofmagnitude, greater than the binding energy of the solid substrate, and form an alloy withthe surface upon impact. Virtually any element can be injected into the near-surfaceregion of any solid substrate. The injection of material into a target specimen in theform of an accelerated ion beam offers a valuable tool for altering its physical, chemical,structural, surface, and interface properties in a controlled manner and tailoring newmaterials. This is particularly the case for doping in semiconductors, where ion–solidinteraction physics provides a unique way for controlling the produced defects of thedesired type at a desired location. The ions can introduce both a chemical change in thetarget, in that they can introduce a different element than the target or induce a nucleartransmutation, and a structural change, in that the crystal structure of the target can bedamaged or even destroyed by the energetic collision cascades.

The advantage of the process over other surface modification techniques is that it isindependent of temperature and solubility limits, requires little or no postheat treatment,and is a controlled and universal process, while the major drawback is having access toaccelerator (high or low energy) facilities.

Sputtering Deposition (RF or DC). This is a physical vapor method of deposit-ing thin films or clusters. Substrates are placed in a vacuum chamber, and are pumpeddown to their process pressure. Sputtering starts when a negative charge is applied to thetarget material (material to be deposited), causing a plasma or glow discharge. Positive-charged gas ions generated in the plasma region are attracted to the negative-biased targetplate at a very high speed. This collision creates a momentum transfer and ejects atomicsize particles in the form of neutral particles—either individual atoms, clusters of atoms,or molecules onto a substrate. Sputtered atoms ejected from the target have awide energydistribution, typically up to tens of electron volts (e.g., 100,000 K). The sputtering gasis often an inert gas such as argon. For efficient momentum transfer, the atomic weight

FUTURE DIRECTIONS 487

of the sputtering gas should be close to the atomic weight of the target, so for sputteringlight elements neon is preferable; while for heavy elements krypton or xenon is used.78

One of the main advantages of this technique for photocatalysis is the fact thatclusters can be deposited in facile and uniform manner. As a result, it is ideal forcreating heterostructures as has been recently demonstrated by Kao and coworkers whoused the low-cost sputtering technique (as opposed to the higher cost of ion implantationor ALD) to deposit Co on the surface of ZnO. Subsequently, they employed a simpleannealing process to diffuse the Co ions in the lattice of ZnO to form a catalyst that wasable to not only degrade organic pollutants but also prevent growth of Escherichia colibacteria.79

FUTURE DIRECTIONS

Despite the work that has been summarized previously, there have been a few recentpublications that are pioneering the photocatalysis in new directions. Specifically, therehave been some reports detailing redox reactions involving hazardous inorganic ionsand converting them into benign species using the principle of photocatalysis. Sun andcoworkers reported Cr(VI) reduction and organic chemical oxidation on the surface ofTiO2 with help of synergic effect of both the cation (Cr species) and an organic pollu-tant.80 Cr(VI), which is widely used in industries, is mutagenic and carcinogenic whereasCr(III) is relatively benign. Predominantly, either direct electric current or Fe(II)/Fe(III)redox couple has been used to convert Cr(VI) to Cr(III). Instead, Smirniotis used a veryeconomical way to reduce Cr(VI) and simultaneously oxidize another common pollutant4-chlorophenol using visible light and different types of TiO2 (Degussa P25, anataseor rutile). It should however be noted that currently the exact mechanism is not clearlyunderstood. Furthermore, Li and coworkers reported As(III) to As(V) oxidation on thesurface of nitrogen-doped titania/palladium oxide heterostructures.81 As(III) and As(V)are notorious species responsible for the groundwater contaminations. Although As(V)can be removed effectively by conventional techniques such as coagulation, precipita-tion, membrane process, or adsorption/ion exchange process, As(III) is arguably hardto remove due to its low affinity to adsorption since it is mainly present in the nonionicform, H3AsO3. As such different types of pretreatments are needed to oxidize As(III).Shang in his article described the use of TiON/PdO nanoparticles as the photocatalystto easily oxidize As(III) under the visible light.

Recently, Qin and coworkers detailed a very interesting aspect of using lanthanidesin photocatalysis.82 It is well-known that lanthanides have the ability for upconversiondue to the presence of a number of metastable energy levels. They can emit higher energyphotons by absorbing lower energy photons. Qin and his team have exploited this ideafor the first time to use upconverted photons to activate wide band-gap semiconductorsuch as TiO2 to generate electron–hole pairs. They have shown that TiO2 can effectivelydegrade methylene blue dye by absorbing near-infrared (980 nm) light, however onlywhen the oxide is synthesized and have a core–shell structure with the core beingYF3 : Yb3+ and Tm3+. The synthesis for this was done in a very controlled manner toget the nucleation of TiO2 precisely on the YF3 : Yb3+ and Tm3+ nanocrystals such


that the oxide shell of TiO2 was having a thickness of only approximately 10 nm. It isobserved that the nanocrystals (uncoated) have three UV-range and two visible-rangeemissions when they are irradiated with 980 nm (near-infrared energy). However, uponforming the core–shell structures, the three emission peaks below 365 nm were foundto either diminish or vanish, yet the visible range emissions (452 nm and 475 nm)were unperturbed. This observation was ascribed to the fact that upon irradiation of thecore–shell structure, resulting UV emissions from nanocrystals were able to be absorbedby the undoped TiO2 shell, and subsequently it was clearly shown that methylene blue(model organic pollutant) was able to be degraded by the irradiation of near-infraredenergy.

CONCLUDING REMARKS

We have summarized the ongoing efforts in effectively forming photocatalysts that areactive using visible light. Before terminating, we would like to add a note of caution.It is imperative to evaluate all the publications that are available with some level ofjudicious care. Currently, there are no standard procedures or protocols for evaluatingsuch catalysts. Hence, comparison of efficacy of photocatalysis from one publicationto another is currently a nontrivial task. The reader must realize that the parametersthat can dramatically influence the efficiency are mass of catalyst used, wavelength ofirradiation, distance between the reaction container and the light source, intensity (flux)of energy used, length of “dark reaction” (i.e., time the catalyst and pollutant spend whilenot being irradiated with light), temperature of the reaction while it is being monitored(ideally it should be kept under isothermal conditions), and finally most importantly,the model organic compound must be impervious to the irradiation that is being used toexcite the oxide. This last aspect is usually not considered vital by many authors sincea large variety of publications recently detailed are evaluating photocatalysts (usingvisible light) to monitor degradation of dyes (that also absorb in the visible range).As a result, extensive sensitization (electron transfer from dye to oxide) is known tooccur. Consequently, we believe, it is not prudent to exploit this paradigm to evaluate aphotocatalyst. The catalyst as a result will have little or no use in degrading other speciesthat do not absorb visible light.

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