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

1

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

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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.

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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

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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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