Chapter 1A
Introduction to Sun-light Driven Photocatalysis and
Nano-Photocatalytic Materials
This introductory chapter is an overview of artificial photocatalysis and basic
principles involved in photocatalytic reactions. The chapter also discusses the
importance and significance of semiconductors as a photocatalyst, material
requirements to be satisfied to be a photocatalyst and a brief description
about photocatalytic materials and the fundamental challenges in
photocatalysis.
Chapter 1A
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1A.1. INTRODUCTION:
Photocatalysis is one of the most promising technologies and it represents an
easy way to utilize the energy of natural sunlight for development of a sustainable
society, and the Sun can easily provide enough power for all our energy needs if it
can be efficiently harvested.1,2 Photocatalysis is emerging as one of the possible
means that can provide viable solutions for the development of both pollution free
technologies for environmental remediation and alternative clean energy supply. The
word “photocatalysis” is composed of two parts, ‘photo’ and ‘catalysis’. Catalysis is
the process where a substance participates in modifying the rate of a chemical
transformation of the reactants without being altered or consumed in the end. This
substance is known as the catalyst which increases the rate of a reaction by reducing
the activation energy.3 Photocatalysis is a reaction which uses light to activate a
substance to modify the rate of a chemical reaction without itself being involved
and photocatalyst is the substance which can modify the rate of chemical reaction
using light irradiation.
Biogenic photocatalytic phenomena, such as those occurring along the lines
of natural photosynthesis, have been known since prehistoric times.4,5 In natural
photosynthesis, plant photosynthetic organisms effectively rearrange electrons in
H2O and CO2 to store solar energy in the form of carbohydrates.6,7 Photosynthetic
organisms universally exploit antenna systems to absorb light and funnel the
excitation energy to the reaction centers, where the charge separation occurs. It
provides an overview to demonstrate the feasibility of efficient solar energy
conversion via photo induced charge separation.8,9 Understanding of natural
Chapter 1A
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photosynthesis at the molecular level has been assisted and inspired further, by the
creation of artificial photosynthetic model system referred as Dye sensitized solar
cells (DSSCs) that utilize analogous mechanism to harvest sun energy and convert it
into electrical energy.10,11 Artificial photosynthesis based on semiconductor
nanostructures replicates the natural photosynthesis in many ways. For example,
Chlorophyll of plants is a type of photocatalyst which captures sunlight to turn water
and carbon dioxide into oxygen and glucose.12 This in turn has led to efforts to
develop photoelectrochemical cells for the synthesis of solar fuel that are
constructed by combining DSSCs with multi electron catalysts. These multi electron
catalysts must be capable of storing multiple redox equivalents and driving fuel
forming reactions such as water oxidation and CO2 reduction.10,13
The photocatalyst in the photocatalysis process corresponds to the
chlorophyll in the photosynthesis process. Figure (1) illustrates the similarities and
differences between natural photocatalysis to artificial photosynthesis. The
difference between chlorophyll photocatalyst to artificial photocatalyst is, primarily
the chlorophyll which captures sunlight to turn water and carbon dioxide into
oxygen and glucose, while in artificial photosynthesis the photocatalyst creates
strong oxidizing agent and electronic holes to breakdown the organic matter to
carbon dioxide and water in the presence of the photocatalyst, light and water.14 This
has for a long time motivated extensive research activity in chemistry, biology,
physics, and materials science to understand and even to mimic such biological
energy-transfer processes using artificial materials and technologies to achieve water
Chapter 1A
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splitting, CO2 fixation, green organosynthesis, and environmental purification
through sunlight.15
Figure 1. Illustration of the similarities and difference between natural photocatalysis and
artificial photosynthesis 16
1A.2. PRINICIPLES OF SEMICONDUCTOR PHOTOCATALYSIS:
From the point of view of semiconductor photochemistry, the role of
photocatalysis is to initiate or accelerate specific reduction and oxidation (redox)
reactions in the presence of irradiated semiconductors.17 Light absorption and the
consequent photoexcitation of electron-hole pairs takes place when the energy of the
incident photons matches or exceeds the band gap. Illumination induces a transition
of electrons from the valance band (VB) to the conduction band (CB), leaving an
equal number of vacant sites (holes). The population of both charge carriers, that is,
electrons and holes, in an illuminated semiconductor are higher than at equilibrium.
The formation of free charge carriers (electrons and holes) follows several de-
excitation pathways. Initially, the energy of the incident photons is stored in the
semiconductor by photoexcitation, which is then converted into chemical form by a
series of electronic processes and surface/interface reactions (Figure (2)). The
Chapter 1A
4
charge carriers, once spatially separated, may migrate to the surface of the
photocatalyst and eventually migrate to the adsorbed acceptor molecules, thereby
initiating the corresponding reduction or oxidation process.18,19 At the surface, the
semiconductor can donate electrons to acceptors (pathway (A)), where as holes can
migrate to the surface, where they can combine with electrons from donor species
(pathway B)). Simultaneously, a large proportion of the generated electron-hole pairs
recombine, dissipating the input energy in the form of heat or emitted light (pathway
(C)). The carriers can recombine with their counterparts of opposite charge trapped
on the surface pathway (D)). Both these recombination processes are detrimental to
the efficiency of the photocatalytic reaction.20 The rate of charge transfer and
recombination depends on the bandedge position or the band gap and the redox
potential of the adsorbate species, respectively. For desired or favorable electron
transfer reaction to occur, the potential of the electron acceptor species should be
located below the conduction band of the semiconductor (more positive than),
where as the potential of electron donor species should me located above the valance
band of the semiconductor (more negative than). The actual reaction sites may be
located either directly on the surface of the semiconductor within which the
photoexcitation takes place, or indirectly across the interface at the surface of another
semiconductor or metal nanoparticle.21 Interfacial charge transfer, i.e., transfer of
electrons to or from surface adsorbed species onto the light activated semiconductor
is probably the most critical step in photocatalytic processes. There are various
factors which determine the recombination rates, mobility and trapping of charge
Chapter 1A
5
carriers, defect density in the semiconductor lattice, and the presence of an interface
with a secondary material which acts as an electron or a hole sink.22
Figure 2. Photoinduced formation of an electron–hole pair in a semiconductor with possible
decay paths. A=electron acceptor, D=electron donor.23
Another critical factor determining photocatalysis efficiency is the separation
and transport of the photogenerated electron–hole pairs. Achievement of a high
photocatalytic activity demands the efficient separation of the electron–hole pairs as
well as the rapid charge transport to their suitable active sites for the desired redox
reactions. Coupling between different semiconductors in photocatalytic systems
allow to alleviate the charge carrier recombination in individual semiconductor. A
good match of their CB and VB levels can realize a vectorial transfer of
photogenerated charge carriers from one to the other,24 where the relative positions
of the energy bands of the two particles are shown in terms of energetic rather than
spatial levels. After coupling the energy gap between corresponding band levels
drives the charge carriers from one particle to its neighbor to form a spatial
separation between electrons and holes.
Chapter 1A
6
In the case of metal nanoparticles supported on the semiconductor, the
hetero-junctions formed between the semiconductor and the co-catalyst facilitates
separation of the electron-hole pairs effectively. The improved separation translates
into slower recombination rates and an increase in the efficiency of the
photocatalytic process. Upon contact, a Schottky barrier can be formed at the
semiconductor–metal interface that promotes the separation of the charge carriers by
accumulation of the electrons in the metal, while the holes remain in the
semiconductor.25,26 This effect shifts the Fermi level of the semiconductor–metal
composite upwards, so that its potential becomes more negative.21,27 Then, the
energetic difference at the semiconductor/metal interface drives the electrons from
the CB of the semiconductor into the metal nanoparticles. The Fermi level of the
metal is thereby also negatively shifted so that a secondary electron transfer can
occur between the metal and electron acceptors in the redox couples from the
surrounding electrolyte. 28
1A.3. IMPORTANCE OF SEMICONDUCTORS:
Semiconductor materials are particularly employed for photocatalytic
reactions because of their favorable combination of electronic structure, light
absorption properties, charge transport characteristics and long excitation life time.
The semiconductor acts as a photocatalyst for the light-induced photochemical
reactions because of its unique electronic structure characterized by a filled valence
band (VB) and an empty conduction band (CB). The primary role of the
semiconductor in photocatalysis is to absorb an incident photon, generate an
electron–hole pair, facilitate its separation and transport and the system that should
Chapter 1A
7
be followed by promoting both the oxidation and reduction reactions (redox)
simultaneously.29
The photocatalytic properties of semiconductors strongly depend on the
electronic band structure. The semiconductor is nonconductive in its undoped ground
state, because of its large band gap. The electron transport between VB and CB in
semiconductor must occur only when the appropriate amount of energy is supplied.
As an ideal photocatalyst, the top of the valence band (measure of the oxidizing
power) and bottom of the conduction band (measure of the reducing power) must be
separated by about 1.23 eV to promote redox reaction.30 In case of metals, the top of
the valence band and bottom of the conduction band are almost identical and hence
they cannot be expected to promote pair redox reactions. This situation is achievable
with semiconductors as well as insulators. However, insulators are not suitable due
to the large band gap which demands high energy photons to create the appropriate
excitons for promoting both the reactions. The available photon sources for this
energy gap are expensive and again require energy intensive methods. Hence
insulators are not the favored choice for the purpose of photocatalytic reactions. For
the above reasons semiconductors are the only suitable materials for the promotion
of photocatalytic redox reaction.
A large number of semiconductor materials including metal oxide and
chalcogenides have been investigated with respect to their photocatalytic properties.
In general wide band gap semiconducting materials such as TiO2, prove to be better
photocatalysts than low band gap materials such as CdS, mainly due to the low
chemical and photochemical stability of the later. However low band gap materials
Chapter 1A
8
are better adapted to the solar spectrum thereby offering significant advantages of
their potential utilization for continuous and readily available power fromthe sun.
The catalytic activity of semiconductor in turn strongly depends on top of the
valence band and bottom of the conduction band. Figure (3) shows the band gap
energies and the band edge positions of common semiconductor photocatalysts
Figure 3. Bandgaps and redox potentials, using the normal hydrogen electrode (NHE) as a
reference, for several semiconductors. 31
A great deal of interest is paid to nanocrystalline semiconductor materials due
to the fact that they exhibit quantum-size effects in their optical, magnetic, and
electrical properties, which accounts for their high catalytic activity. The thermal
stability and high mobility of electrons in semiconductors are the essential
properties, since these features provide the necessary charge transfer upon contact
with donor and acceptor species. Moreover semiconductors are chemically and
biologically inert, photostable, inexpensive, nontoxic and are able to absorb visible
and/or UV light.
Chapter 1A
9
1A.4. HISTORY OF PHOTOCATALYTIC MATERIALS:
Since the demonstration by Honda and Fujishima of the photoelectrolysis of
water using a TiO2 electrode, intensive research efforts have been devoted to the
development of photocatalytic materials, understanding the fundamental principles,
enhancing the photocatalytic efficiency, and expanding the scope of applications.19,32
In addition to hydrogen fuel production, many other potential uses of TiO2
photocatalysis have been identified, such as the detoxification of effluents,
disinfection, superhydrophilic self-cleaning property, the elimination of
inorganic/organic gaseous pollutants, and the synthesis of organic fuels with the aim
of utilizing solar energy and thus addressing the increasing global concerns of
environmental remediation and clean fuel production. Decades of efforts have
successfully produced a wide range of efficient semiconductor-based photocatalytic
materials including TiO2, SnO2, Fe2O3 and ZnO. Among these, TiO2 has drawn much
attention because its exceptional photactivity. Unfortunately, TiO2 is not the best
for all purposes and performs rather poorly in processes associated with solar
photocatalysis. In principle, TiO2 with a wide band gap (3–3.2 eV) in the UV range
can utilize not more than 5% of the total solar energy impinging on the surface of the
earth. . Hence to enhance the photoactivity, and utilize the material appripriately for
all its useful properties the energy levels need to be modified by introducing new
intermediate energy levels in the band gap between the valence and conduction
bands. The popular optical modification strategies including energy band modulation
by doping with elements such as N, C, and S,33 the construction of hetero-junctions
by combining TiO2 with metals such as Pt or Pd, or other semiconductors such as
Chapter 1A
10
NiO, RuO2, WO3 or CdS,34 and the addition of quantum dots or dyes on the TiO2
surface for better light sensitization35 are being adopted to manipulate and shift the
absorption edge into the visible region.
Simultaneously, the use of conventional semiconductors such as SrTiO3 and
WO3 in photocatalysis has been investigated in the search for possible alternatives to
TiO2.36 A number of metal oxide complexes including In3+ Ga3+, Sb5+, Bi5+ and Ag+
and sulfides (GaN, Bi2S3, CdS and ZnS), nitrides, oxynitrides have been
investigated to exploit their photocatalytic activity in the solar spectrum.37 These
novel semiconductors have proved to be among the most successful photocatalysts
for several reactions. In addition to classic semiconductors, polymeric C3N4 was
recently identified as a new photocatalyst for the production of hydrogen from water
under visible light.38
1A.4.1. Titanium Dioxide (TiO2):
TiO2 has emerged as one of the most fascinating materials in the modern era
for a wide range of applications from environment to health . It has succeeded in
capturing the attention of physical chemists, physicists, material scientists, and
engineers in exploring distinctive semiconducting and catalytic properties. Inertness
to chemical environment and long-term photostability have made TiO2 an important
component in many practical applications and there are several examples where this
material has found its way in commercial products. From drugs to doughnuts,
cosmetics to catalysts, paints to pharmaceuticals, and sunscreens to solar cells, TiO2
is used as a desiccant, brightener, or reactive mediator and most widely used
benchmark photocatalyst in the field of energy and environmental applications.39
Chapter 1A
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TiO2 is biologically and chemically inert, environmental friendly, low cost, nontoxic
and stable with respect to photocorrosion and chemical corrosion and this has led to
its widespread use in photocatalytic applications.
Figure 4. TiO6 polyhedra (right) for the TiO2 phases rutile (a), anatase (b) and brookite (c)
and their corresponding unit cell structure (left)44
The three well-known polymorphs of TiO2 are rutile (tetragonal), anatase
(tetragonal), and brookite (orthorhombic). Rutile is the thermodynamically most
stable form, where as both anatase and brookite are meta-stable. All the three
polymorphs of TiO2 are grown from TiO6 octahedra and phase formation differs only
due to the nature of sharing corners or edges (shown in figure 4).40 There are four
Chapter 1A
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shared edges for octahedron in anatase, three in brookite, and two in rutile. Brookite
has both shared edges and corners, a compromise between anatase and rutile in terms
of shared faces.41
The anatase and rutile form can be described by their tetragonal structure in
terms of three parameters: two cell edges a, c and one internal parameter d. In rutile,
each Ti atom is coordinated to six neighboring oxygen atoms via two apical (long)
and four equatorial (short) bonds. Each O atom is coordinated to three Ti atoms via
one long bond and two short bonds lying in the same plane. The anatase phase is 9 %
less dense than rutile and has a tetragonal unit cell. The coordination of Ti and O
atoms are same as in rutile, however the octahedra are significantly more distorted.
Brookite has an orthorhombic crystalline structure composed of octahedra. The
octahedra share edges and corners with each other to such an extent as to give the
crystal the correct chemical composition. The octahedra are distorted and present the
oxygen atoms in two different positions and all the bond lengths between the
titanium and oxygen atoms are different. These differences in lattice structures cause
different mass densities and electronic band structures between the three forms of
TiO2.42 These structural features are likely to be responsible for the difference in the
mobility of the charge carriers upon light excitation.43
However, a serious limitations of TiO2 photocatalysts is the massive
recombination of photogenerated electon-hole pair and its large bandgap. Because of
the latter it can only absorb photons with light wavelengths shorter than about 400
nm in the UV or near-UV wavelength regime, which accounts for less than 5% of the
total solar energy irradiation.45 Although TiO2 based photocatalysts can function as
Chapter 1A
13
effective photocatalysts, they cannot be used for effective solar energy harvesting
and conversions. Therefore many modification methods, such as metal or non-metal
doping, surface sensitization, semiconductor coupling, precious metal deposition,
and increasing crystal defects have been carried out in order to expand the spectral
response range and improve the photocatalysis quantum efficiency of TiO2.46
1A.4.2. Zinc Oxide (ZnO):
ZnO is a semiconductor material with a direct wide band gap energy (3.37
eV) is expected to exhibit impressive photocatalytic activity and is recognized as a
suitable alternative to TiO2.47 Therefore, it has been comparatively studied TiO2 in
terms of its photocatalytic performance.48 ZnO is biocompatible, biodegradable, and
biosafe for medical and environmental applications.49 ZnO crystallizes in two main
forms, hexagonal wurtzite and cubic zinc blende. Under general conditions, ZnO
exhibits a hexagonal wurtzite structure. The crystalline nature of ZnO could be
indexed to known structures of hexagonal ZnO, with a = 0.32498 nm, b = 0.32498
nm, and c = 5.2066nm (JCPS card no. 36–1451).50
Figure 5. ZnO structure: (a) the wurtzite structure model; (b) the wurtzite unit cell56
Chapter 1A
14
The structure of ZnO could be described as a number of alternating planes composed
of tetrahedrally coordinated O2− and Zn2+ stacked alternately along the c-axis (shown
in figure 5). The O2− and Zn2+ form a tetrahedral unit, and the entire structure lacks
central symmetry. Due to their remarkable performance in electronics, optics, and
photonics, ZnO is an attractive candidate for many applications such as UV lasers51
light-emitting diodes52 solar cells53 gas sensors54 photodetectors.55
1A.4.3. Tin Oxide (SnO2):
SnO2 is an n-type semiconductor with a bandgap energy of 3.8 eV, which
corresponds to an optical absorption edge below 330 nm.57 The most important form
of naturally occurring tin oxide (SnO2) is cassiterite, with the tetragonal rutile-type
crystalline structure. The combination of chemical, electronic and optical properties
make it advantageous in various applications in catalysis,58 gas-sensing,59 anode
materials for lithium-ion batteries60 and DSSCs.61 In particular, due to its stability,
sensitivity, and low cost, more recently much attention has been focused on SnO2 for
potential use in environmental remediation.62 SnO2 may have an onther advantage
regarding their band structures that can coupled with other semiconductors with
suitable matching of band levels, which provide another way to serve in
photocatalytic systems.63
1A.4..4. Silver Orthophosphate (Ag3PO4):
Recently, Ag3PO4 has attracted enormous attention due to its great potential
in harvesting solar energy for environmental purification and fuel production.64 The
direct and indirect bandgap of Ag3PO4 is 2.43 and 2.36 eV, that can able to absorb
solar energy with a wavelength shorter than 530 nm. The photocatalytic properties of
Chapter 1A
15
Ag3PO4 exhibits extremely high photooxidative capabilities for the O2 evolution
from water and the decomposition of organic dyes under visible-light irradiation.
Ag3PO4 is a body-centred cubic structure type with space group P4-3n and a lattice
parameter of ∼6:004 ˚A. The structure consists of isolated, regular PO4 tetrahedra
(P–O distance of ∼1.539 ˚ A) forming a body-centred-cubic lattice. The six Ag+ ions
are distributed among twelve sites of twofold symmetry.65 This indicates that each
Ag atom at (0.25, 0,0.50) actually occupies one of the two sites at (푥, 0, 0.50) and
(0.5 − 푥, 0, 0.50) on the 2-fold axis.66 The unit-cell structure of cubic Ag3PO4 is
shown in Figure (6), in which the Ag atom experiences 4-fold coordination by four O
atoms. The P atoms have 4-fold coordination surrounded by four O atoms, while the
O atoms have 4-fold coordination surrounded by three Ag atoms and one P atom.67
Figure 6. Unit-cell structure of cubic Ag3PO4, showing (a) ball and stick and (b) polyhedron
configurations. Red, purple, and blue spheres represent O, P, and Ag atoms, respectively 68
The electrode potential of Ag/Ag3PO4 is between the reduction potential of
H+ and Ag/Ag3PO4, which would mean that it cannot split water to generate H2.
However Ag3PO4 possesses strong photooxidative capabilities in presence of
sacrificial reagents like silver nitrate.69 The photodegradation rate of organic dyes
Chapter 1A
16
over Ag3PO4 is dozens of times faster than the rate over commercial TiO2−푥N푥.
Moreover, what is most interesting is that this novel photocatalyst can achieve a
quantum efficiency of up to 90% at wavelengths greater than 420 nm, which is
significantly higher than the previous reported values.70
1A.4.5. Other Semiconductor Photocatalytic Materials:
A great number of new photocatalytic materials have been studied as
potential substitutes of TiO2 with the aim of effective utilization of solar spectrum.
WO3, Fe2O3 (hematite), CdS, and BiVO4, are well known low band gap visible-light-
driven photocatalysts. Among them WO3 gained much attention due to the upper
edge of the VB of WO3 that is close to that of TiO2, and which exceeds the H2O/O2
oxidation potential. Thus, the photogenerated holes in WO3 upon bandgap excitation
are capable of oxidizing a wide range of compounds. The advantage of WO3 as a
photocatalyst is that the bandgap is only 2.6 eV, which is 0.6 eV narrower than
TiO2.71 Therefore, more visible light can be harnessed by WO3 from the sunlight
spectrum. Another virtue of WO3 is its remarkable photostability in acidic aqueous
solutions making it a powerful photocatalyst. On the other hand -Fe2O3 is another
important semiconductor photocatalyst possessing a relatively low band gap of about
2.1 eV. The band structure of -Fe2O3 is quite similar to that of WO3, with the VB
edge exceeding the standard redox potential of H2O/O2 and the CB edge being lower
than the standard redox potential of H2/H2O.72 A bias potential of Fe2O3 can help to
achieve total H2O splitting under visible irradiation. In the presence of appropriate
scavengers, however, the holes can function as powerful oxidants, and the electrons
as moderately powerful reductants.73
Chapter 1A
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1A.5. CHALLENGE OF SEMICONDUCTOR PHOTOCATALYSIS:
Nanomaterials have emerged as pioneering photocatalysts and account for
most of the current research in this area. Nanomaterials can provide large surface
areas, abundant surface states, diverse morphologies, and easy device modeling, all
of which are properties beneficial to photocatalysis. The photocatalytic activity of
photocatalyst depends on its physicochemical properties including the primary
particle size, degree of aggregation, surface area, morphology and crystalline
structures. The first two properties decide the adsorption capability of photocatalysts
for substrate molecules, which has a significant effect on many photocatalytic
reactions to proceed efficiently. Nevertheless, in all photocatalytic applications high-
surface-area geometries have a strong influence in obtaining higher overall reaction
rate. Thus, it is crucial to maximise surface area with various geometries including
1D, 2D and 3D nanostructures to achieve maximum overall efficiency. In particular
one-dimensional (1D) nanostructures such as wires, belts and tubes have attracted
considerable attention for photocatalytic applications due to their distinct electronic,
optical and chemical properties, which differ from their bulk counterparts.74 These
properties are dependent on the size and morphology of the materials, leading to the
development of strategies to optimize the photocatalytic reactivity.
Another key issue influencing the photocatalytic capability of a
semiconductor is the nature of its surface/interface chemistry. The surface energy
and chemisorption properties play crucial roles in the transfer of electrons and
energy between substances at the interface, in governing the selectivity, rate and over
potential of redox reactions on the photocatalyst surface.75 Considering that
Chapter 1A
18
photocatalytic reactions take place on the surfaces of semiconductors, the exposed
crystal facets play a critical role in determining the photocatalytic reactivity and
efficiency.76 In general, a higher surface energy crystal yields higher catalytic
activity due to its high adsorption capability towards donor and acceptor molecules.
Attempts to deliberately fabricate such materials are challenged by the
thermodynamic growth mechanisms of the crystals.77 The synthesis of single crystals
with exposed highly reactive facets represent a promising and efficient method for
the further improvement of photocatalytic performance.
The most important property relevant to the photocatalytic activity of a
semiconductor is its energy band configuration, which determines the absorption of
incident photons, the photoexcitation of electron-hole pairs, the migration of carriers,
and the redox capabilities of excited-state electrons and holes.78 Most photocatalysts
available to date can only function in the ultraviolet (UV) or near UV regime with
limited efficiency due to a number of intertwined limiting factors including a
mismatch between the semiconductor bandgap and the solar spectrum, inefficient
charge separation and transport. Therefore, energy band engineering is a
fundamental aspect of the design and fabrication of semiconductor photocatalysts.
One of the important research directions is to change the semiconductor energy band
structure by raising the position of valence band, lowering the position of
conduction band and continuous modulation of both valance and/or conduction
band.79
The most popular approach for tailoring the absorption edges of
photocatalysts is to dope the host material with foreign species.80 This in turn will
Chapter 1A
19
enable the materials to absorb longer wavelength photons, hence exhibiting
photocatalytic activity under visible light irradiation. Besides, narrowing the
bandgap, the orbital hybridization can render band structures with different
configurations, i.e., flat or abrupt. This can also influence the photocatalytic
performance of the materials, because the effective masses of the charge carriers, and
therefore their mobilities, are sensitive to the band configurations.81
1A.6. CONCLUSIONS:
In conclusion, this chapter discussed the fundamental principle of photocatalysis and
the important role of semiconductors in a photocatalysis reaction. The chapter also
gives brief description about the semiconductors that has to satisfied to be a suitable
photocatalyst in terms of energy levels and electronic structures. As a final point, a
short account on photocatalytic materials and the fundamental challenges in
photocatalysis was discussed.
Chapter 1A
20
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Chapter 1B
Environmentally Benign Photocatalyst for
Harmonious Applications
In this chapter, the knowledge gained from the research on
photocatalysis and understanding the mechanism is further extended to
develop new materials suitable for possible environmentally harmonious
technologies to harness the solar energy. The chapter covers the potential
application of photocatalysis as well as the detailed fundamental
mechanisms behind each application.
28
1B.1. INTRODUCTION:
Energy and environment are the two most critical issues that are of
serious concern to modern society. Photocatalysis is emerging as one of the
possible means that could provide viable solutions to challenges from these two
areas. The key application on the energy front utilizing the abundantly available
sun energy are production of hydrogen by splitting of water using a photocatalyst,
generating electricity using solar cells and photocatalytic production of methane
and hydrogen fuels. Removal of pollutants from water by advanced
photocatalytic oxidation, purification of air, self cleaning, anti fogging and anti-
bacterial applications are typical applications of photocatalysis towards
environmental protection. 1
1B.2. Photocatalytic Oxidation:
Photocatalytic oxidation is an effective and inexpensive tool for the
removal of organic and inorganic pollutants from water due to its ability to
oxidize organic and inorganic substrates. The initial interest in heterogeneous
photocatalysis was triggered with the discovery of photochemical splitting of
water into hydrogen and oxygen with TiO2 by Fujishima and Honda in 1972.2
Since then extensive work has been carried out to produce hydrogen from water
by this novel oxidation reduction reaction using a variety of semiconductors. In
recent years, the increasing awareness and concern for the environment, is
leading researchers to direct their research activities towards processes that would
make possible complete photocatalytic mineralization for a variety of toxic
organic compounds into harmless end products.3 The technique could also be
utilized for the decomposition of organic and inorganic compounds, and removal
of trace metals as well as destruction of viruses and bacteria from water. This
29
technique also finds application in the decomposition of natural organic matter,
which has severe detrimental environmental and industrial impact. The principal
drawback of this method is that it is slow compared to traditional methods, but it
has the most important advantage of not leaving any toxic by product or sludge
that would need to be disposed.
1B.2.1. Operating Principle:
When the light of appropriate energy illuminates the semiconductor, an
electron from the valence band is promoted to the conduction band, leaving an
electron deficiency or hole (h+) in the valence band and an excess of negative
charge in the conduction band (e−) as illustrated in figure 1, which are the
equivalent oxidizing and reducing components respectively and can participate in
redox reactions. The electron and hole may migrate to the catalyst surface where
they participate in redox reactions with sorbed species. The positive hole oxidizes
either the pollutant directly or reacts with water to produce hydroxyl radical •OH,
whereas the electron in the conduction band reduces the oxygen adsorbed on the
photocatalyst to O2-•.4 These hydroxyl radicals (•OH) and superoxide radical
anions (O2-•) are the primary oxidizing species in the photocatalytic oxidation
processes that would result in the degradation of pollutants. Generation of these
active species by photo-irradiation of semiconductor (SC) can be represented by
the equations mentioned below.5
The pathway for the photocatalytic oxidation of organic pollutants can
proceed via two mechanisms: indirect oxidation and direct oxidation. In the
indirect oxidation mechanism, photogenerated valence holes react primarily with
physisorbed H2O and surface-bound hydroxyl groups (–OH) on semiconductor
particles to give OH radicals that then react with organic molecules. The highly
30
active OH radicals are capable of mineralizing most organic pollutant molecules.
Oxygen molecules dissolved in H2O, which usually serve as scavengers of
photogenerated electrons, also lead to the formation of OH radicals. Other
oxidizing routes have also been proposed, including direct oxidation by the
photogenerated holes, generation of oxidizing species from reactions involving
intermediates formed in the solution.6,7
-
•2
- •
2 2
2
2
( ) ( ) - - - - - - - - - - - - - - - (1)
( ) - - - - - - - -(2)
( ) - - - - - - - - - - - - - (3)
( ) - - - - - - - - - - - - - - - (4 )
- - - - - - - - - - - - - - - - - - - -(5)
hcb vb
vb
vb
vb
SC e sc h sc
SC h H O SC H O H
SC h O H SC H O
SC e O SC O
O H H O O
H O O e H O O H H
2 - - - - - -(6)O
Figure 1. Schematic diagram illustrate the photocatalytic oxidation mechanism8 .
The primary cause for the photocatalytic activity of TiO2 is believed to be
the formation of OH• radicals by rapid conversion of photogenerated holes upon
contact with the adsorbed H2O molecules on TiO2. The hydroxyl radical is an
extremely powerful oxidation agent and thefollowing table is a listing of common
chemical oxidants, placed in the order of their oxidizing strength.
31
Table 1. Comparison of oxidation potential of various oxidizing agents.9
Oxidizing Agents Oxidation Potential (V)
Fluorine 2.87
OH radical 2.80
Ozone 2.07
Hydrogen Peroxide 1.77
Manganese Peroxide 1.51
Hydrochlorous acid 1.50
Chlorine 1.36`
Oxygen 1.23
1B.3. Photocatalytic CO2 Reduction:
Greenhouse gases such as CO2, CH4, and CFCs are the primary causes of
global warming. The atmospheric concentration of CO2 has steadily increased
owing to human activity and this accelerates the greenhouse effect. In 1979
Fujishima et al. reported pioneering studies on photocatalytic CO2 reduction
using various inorganic semiconductor photocatalysts. Since then, various other
photocatalytic systems that employ semiconductors have been studied. Although
there are many problems such as low selectivity of the products and low quantum
yield, further development of semiconductor photocatalysts seems to be an
attractive proposition that could be attributed primarily to the durability of
inorganic semiconductors (typically metal oxides) and their efficient light
harvesting properties. In photosynthesis process, the energy obtained from
sunlight is ultimately used to convert CO2 into glucose that is stored in the form
of chemical energy. The process of artificial photosynthesis may be executed via
the photoreduction of CO2 to produce hydrocarbons. This would mean that solar
energy is directly transformed and stored as chemical energy. Consequently, the
32
photoreduction of CO2 to chemicals, such as methanol, is particularly interesting,
and achieving a high efficiency for this reaction is highly desirable. The research
into solar fuels is evolving rapidly owing to the long-term motivation to find
alternative transportation fuels that can be obtained photocatalytically from water
and CO2 using sunlight. In this context, CO2 can be converted into high value
added products such as CH3OH or CH4 that could be stored and handled more
conveniently than H2, as the energy density per volume or mass unit of the former
is much higher than that of H2.10 The photocatalytic reduction of CO2 requires
multiple electron transfers and can lead to the formation of many different
products depending on the specific reaction pathway followed and the number of
electrons transferred, which determines the final oxidation state of the carbon
atom. Carbon monoxide, formic acid, formaldehyde, methanol, methane, ethane,
and ethene have been observed in many experiments, but oxalic acid,
acetaldehyde, ethanol, higher alcohols, and higher hydrocarbons were also
detected in some systems.11
1B.3.1. Operating Principle:
Absorption of light by a semiconductor electrode or particles causes the
transition of an electron from the valence band (VB) to the conduction band
(CB). Both, an excited electron (e-) and a hole (h+) are generated concurrently in
the CB and VB, respectively. The photo-generated excited e- can potentially be
used for CO2 reduction. Because protons can also accept the excited electron,
hydrogen evolution often competes with CO2 reduction, and this remains as one
of the most serious problems to be solved in the field. On the other hand, the
photo-generated h+ is quenched by electron injection from a reductant such as
organic molecules or water. For the use of water as a reductant, the potential of
33
the VB must be more positive than the oxidation potential of water. Considerable
amounts of various organic molecules, such as acetic acid, can be adsorbed on the
surface of semiconductors that have not undergone any pre-treatment. Because
such organic adsorbates can work as reductants and/or carbon sources for the
products, removal of the organic contamination is essential prior to photocatalytic
CO2 reduction experiment. The efficient photoreduction of CO2 with H2O is one
of the most challenging tasks in photocatalysis because the efficiency of
photoconversion is very low. According to thermodynamics, transformation of 1
mole of CO2 into methanol requires 228 kJ (H) at 25 0C. The Gibbs free energy
of this reaction is 698.7 kJ (G) at 25 0C, indicating that the equilibrium is
highly unfavorable for the formation of the products, methanol, and oxygen.
Figure 2. Schematic representation for the photocatalytic reduction of CO2 with H2O on
TiO2.12
1B.4. Photocatalytic Water Splitting:
Water splitting by adopting the photocatalytic route is a challenging
reaction which would contribute to building an ultimate green sustainable society
by mitigating major energy and environmental issues. Photocatalytic water
splitting is one of the most-promising means for producing hydrogen because
34
water presents a virtually limitless source of hydrogen. Hydrogen is the simplest
element that can be considered as a viable alternative fuel and “energy carrier”
for the future. Hydrogen can be effortlessly used in fuel cells for the generation
of electricity without any CO2 emission. Since the early work of TiO2
photoelectrochemical hydrogen production was reported by Fujishima and
Honda, many researchers have taken up the challenge of this dream reaction, and
there have been numerous papers focused on photocatalytic water splitting.13,14
There are two basic methods to achieving solar hydrogen generation, one an
electrode system and the second a particle based system, as schematically shown
in Figure 3. The principle of operation for both systems are similar; the
semiconductor acts as the light absorber in which electron–hole pairs are
generated upon solar irradiation, while the metal acts as the electron trapper or a
co-catalyst.15,16
Figure 3. Schematic illustration of the photocatalytic hydrogen generation system. (A)
An electrode system and (B) a photocatalytic particle system.17
1B.4.1.Operating Principle:
Photo-generated electrons and holes are produced when the
semiconductor electrode absorbs sunlight and the electrons are excited from the
valence band to the conduction band. The excited electrons can be either
transported to the metal electrode connected by an external circuit in the case of
35
an electrode system, or directly transferred to the surface of the semiconductor
particle in the case of a particle system. The excited charges separate and migrate
to the surface of the photocatalyst and cause redox reactions. Overall water
splitting is achieved through the reduction of H+ ions to H2 by photogenerated
electrons and the oxidation of H2O to O2 by holes, respectively. In this step, the
surface sites and state of the photocatalyst play key roles in achieving overall
water splitting. Water splitting into H2 and O2 is an uphill reaction. It has a
standard Gibbs free energy change (G) of 237 kJ mol-1 (equivalent to 1.23 eV).
Therefore, the band gap of photocatalytic materials and the edges of the
conduction and valence bands must be suitable for decomposing water.
Meanwhile, the reverse reaction to form water from the reaction between evolved
H2 and O2 proceeds readily because it is a downhill reaction. The prerequisite for
an efficient photocatalyst is that the redox potential for the evolution of hydrogen
and oxygen from water and for the formation of reactive oxygenated species
should lie within the band gap of the semiconductor. The bottom level of the
conduction band has to be more negative than the redox potential of H+/H2 (0 V
vs. NHE), while the top level of the valence band would have to be more positive
than the redox potential of O2/H2O (1.23 V). Therefore, the theoretical minimum
band gap for water splitting is 1.23 eV that corresponds to light of wavelength
about 1100 nm.
1B.5. Super Hydrophilicity:
The degree of water repellence on the surface of a specific material can
be measured by the water contact angle. A hydrophobic surface has a water
contact angle that is higher than a hydrophilic surface. The fogging effect that can
be experienced on the bathroom mirrors and the windows of cars is caused by the
36
condensation of water that forms small droplets on the surfaces. Until now, the
main approach to prevent this type of fogging has been to create a hydrophobic
surface that repels the water molecules. This method still leads to creation of
water drops that have to be removed from the surfaces for example by blowing or
shaking. For glass and other inorganic materials, the water contact angle is
between 20° and 30°. Almost no surface is known with a water contact angle less
than 10°, compared to the water contact angle of the illuminated surface of
titanium dioxide being less than 1° (Figure 4).18
Figure 4. Change of the water contact angle with UV irradiation on TiO2 -Silicon
surface25
The superhydrophilic effect of TiO2 is formed when the surface is
exposed to UV light, and after a certain time of moderate illumination the water
contact angle approaches zero.19 When the illumination ceases, the
superhydrophilic effect disappears. If the surface is prepared with a water-
retaining material like silicon dioxide or silica gel, the superhydrophilic effect
can be maintained even after the light is turned off (Figure 4). On the surface of a
hydrophilic material, the condensation of water forms a uniform film, which
flattens out instead of fogging the surface with water drops. The applications of
super hydrophilicity with photocatalyst are very useful in various fields including
37
anti fogging, self-cleaning of all types of household appliances, vehicles and anti-
corrosive coatings.20-22
1B.5.1.Operating Principle:
Figure 5. Super hydrophilicity mechanism on TiO2 surface 25
When the surface of photocatalytic film is exposed to light, the contact
angle of the TiO2 photocatalyst surface with water is reduced gradually. After
enough exposure to light, the surface reaches super-hydrophilic point and the
water takes the form of a highly uniform thin film, which behaves optically like
a clear sheet of glass.23The superhydrophilic effect is also caused by the
production of holes because the electrons tend to reduce the Ti(IV)-cations to
Ti(III)-ions and the holes oxidize the O2− anions.
4h+ + 4O2− →2O2
This process leads to expulsion of oxygen atoms and creation of oxygen
vacancies at the TiO2 surface. These vacancies are covered by water molecules
forming OH-groups that create the superhydrophilic effect. The super-
hydrophilicity would also be caused by the action of photo-catalyst. As the
38
photocatalyst decomposes hydrophobic molecules existing on the surface of
material, a very thin film of physisorbed water forms on the surface, and this thin
film of water is the origin of the super-hydrophilicity.24
1B.6. Dye Sensitized Solar Cells:
Photovoltaic cells are the devices that directly convert renewable energy
like solar into electrical energy. These photovoltaic cells also have the property of
providing electricity without emitting any carbon dioxide during operation.
Nowadays many different types of solar cells exist, but the most widely
manufactured solar cells are based on crystalline silicon.. However, their main
drawback is relatively expensive production costs due to the high consumption of
materials and energy, and the purity requirements that make it essential to
maintain stringent clean room facility conditions during the growth of the single
crystal silicon and fabrication of the solar cells. Other photovoltaic technologies
are thin film solar cells based on silicon, copper indium gallium selenide (CIGS)
or cadmium telluride (CdTe), and highly efficient concentrator solar cells in multi
junction configuration.26 In 1990s new solar cell technologies emerged such as
organic solar cells and dye-sensitized solar cells (DSSCs).27-29 The DSSC is
recognized as one of the world’s leading innovation in nanosciences and
photovoltaic technology. The development of DSSCs has been driven by their
many attractive features, e.g. low cost potential, high efficiency (up to 12%), and
short energy payback time.
Dye sensitized solar cells or ‘Grätzel Cells’ have shown immense
promise in recent years based on semiconducting oxides and suitable dye
molecules. It is based on photo-electrochemistry at the interface between the dye
adsorbed onto a mesoporous titanium dioxide layer and an electrolyte.28 As
39
sowed in figure 6, a DSSC consists of a photoactive electrode, a counter
electrode, and an electrolyte. The photoactive electrode is a transparent
conductive oxide (TCO) on glass or flexible substrate, coated with mesoporous
TiO2 sensitized with a monolayer of a dye, while the counter electrode is a TCO
on glass (or flexible substrate) coated with a thin catalytic layer. The gap between
the two electrodes is filled with an electrolyte containing a redox couple.
Figure 6. Cross-section of a Dye-Sensitized solar cell 30
In DSSCs, charge recombination in the dye/TiO2 interface and electron
transport at the photoanodes are the two important factors to be considered for
promoting the efficiency of charge collection of the device. A good photo anode
with high internal area to enable absorptivities for surface attached dye will
facilitate light harvesting, electron injection and electron collection from the dye
molecule. Depending on the physical state of the electrolyte , the DSSC can be
divided into one of three types: liquid electrolyte-, quasi-solid-state electrolyte
and solid-state electrolyte-based DSSC. Additionally, other types of DSSC exist,
where the electrolyte is replaced by a solid-state hole conductor.
1B.6.1. Operating Principle:
The heart of the system is a mesoporous oxide layer composed of
nanometer-sized particles which have been sintered together to allow for
40
facilitating electronic conduction . Photo excitation of dye attached to the surface
of mesoporous oxide layer, injects electrons to the CB of oxide layer. The
original state of the dye is subsequently restored by electron donation from the
electrolyte usually an organic solvent containing redox system, such as the
iodide/triiodide couple. The regeneration of the sensitizer by iodide intercepts the
recapture of the conduction band electron by the oxidized dye. The iodide is
regenerated in turn, by the reduction of the triiodide at the counter electrode and
the circuit being completed via electron migration through the external load. The
voltage generated under illumination corresponds to the difference between the
Fermi level of the electron in the solid and the redox potential of the electrolyte.
Overall the device generates electric power from light without suffering any
permanent chemical transformation.31
The most significant electrical loss mechanism in the DSSC is the
recombination of conduction band electrons in TiO2 with the I3- ions in the
electrolyte. Other recombination mechanisms are (i) an excited dye molecule may
directly relax into its ground state and (ii) electrons from the conduction band of
the TiO2 may recombine with the oxidized dye molecule, before the dye is
reduced by the redox couple in the electrolyte. In addition, a decrease in
efficiency of DSSCs is also affected by its internal resistance (RS), which consists
of a series of resistances, e.g. resistance at the interface between the electrolyte
and the platinum, resistance at the interface between the TCO and TiO2 layer, and
resistances of TCO and TiO2 layer. Optical losses come from total reflection at
the front side, absorption inactive layers and transmission through the cell in case
of semitransparent cell type.33
41
Figure 7. Energy level and device operation of DSCs; the sensitizing dye absorbs a
photon (energy hν), the electron is injected into the conduction band of the metal oxide
(titania) and travels to the front electrode (not shown). The oxidized dye is reduced by
the electrolyte, which is regenerated at the counter-electrode (not shown) to complete the
circuit. VOC is determined by the Fermi level (EF) of titania and the redox potential
(I3−/I−) of the electrolyte.32
1B.7. CONCLUSIONS:
This chapter deals with an insight into photocatalysis mechanism and how this
could be utilized to improve and develop new environmentally harmonious
technologies. The chapter includes the potential application of photocatalysis as
well as detailed fundamental mechanisms behind each applications.
42
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