Chapter 1

Introduction 1.1 Motivation

“Yes, my friends, I believe that water will one day be employed as fuel,

that hydrogen and oxygen which constitute it, used singly or together,

will furnish an inexhaustible source of heat and light, of an intensity

of which coal is not capable. I believe, then, that when the deposits

of coal are exhausted we shall heat and warm ourselves with water.

Water will be the coal of the future.”

— Jules Verne’s novel The Mysterious Island

French novelist Jules Verne may be one of the earliest people with the thoughts of

hydrogen fuel. As early as 1874, he articulated in his novel “The mysterious Island”

the idea of splitting water to generate hydrogen and oxygen and recognized that

hydrogen is a valuable fuel which can be used to satisfy the energy needs of the

society. Jules Verne would be pleased, though not surprised to see that more than a

century and a quarter later, the idea of using hydrogen as the “coal of the future” is

beginning to move from the pages of scientific fiction into the experiments in

laboratories and lexicon of political and business leaders. In the context of

hydrogen economy, hydrogen, the most abundant element in the universe, is an

energy carrier, not an energy source. It is believed that, out of several available

energy resources, hydrogen keeps the potential to fulfill the world energy demands

if it can be harnessed from renewable energy resources [Dunn 2002].

Energy from the sun can easily provide enough power for all of our energy needs if

it can be efficiently harvested. While there already exists a number of devices that

can capture and convert electromagnetic energy, the most common: a photovoltaic

Introduction

2 Poonam Sharma

cell that produces electricity, which must be used immediately or stored in a

secondary device such as a battery or a flywheel [Sivula et al, 2011]. A more

elegant, practical, and potentially more efficient route of storing solar power is to

convert the electromagnetic energy directly into chemical energy in the form of

molecular bonds, analogous to the photosynthesis process exploited by nature.

Biologic photosynthesis effectively rearranges electrons in H2O and CO2 to store

solar energy in the form of carbohydrates. However, the extremely low overall

efficiency that natural photosynthesis exhibits implies the requirement of vast

amounts of land and farming resources to meet our energy demands [Blankenship et

al,2011]. Because of this, artificial photosynthetic routes including

photoelectrochemical (PEC) and photocatalytic (PC) solar energy conversion have

been intensely investigated over the last four decades. Given the abundance of H2O

on earth, the water splitting reaction, H2O →1/2O2+H2 (E0=1.23 V), is the most

appealing pathway for artificial photosynthesis [Bouroushian, 2010]. Indeed solar

water splitting would form the basis for a sustainable hydrogen based energy

economy [Artero et al, 2011].

A distinction can be made between the different approaches to artificial

photosynthesis: PCwater splitting systems use a dispersed material in water and

accordingly produce hydrogen and oxygen homogeneously throughout the solution.

This approach is under examination with both inorganic colloid materials and

molecular complexes [Sivulaet al, 2011]. In contrast, PEC systems employ

photoactive materials as electrodes. As in conventional water electrolysis, oxidation

(O2 evolution) occurs at the anode, reduction (H2 evolution) occurs at the cathode,

and an aqueous electrolyte completes the current loop between the electrodes and

an external circuit. One or both of the electrodes can be a photoactive

semiconductor, in which a space charge (depletion) layer is formed at the

semiconductor/liquid junction (SCLJ). The advantage of the PEC route is that it

allows the spatially separate production, and therefore collection, of H2 and O2.

Despite the difference between PC and PEC water splitting, the requirements of the

materials employed are essentially the same[Boddy, 1968]. Ever, since the seminal

demonstration of PEC water splitting with TiO2, [Fujishima and Honda, 1972]

researchers have sustained a vigorous search for a material combining the essential

requirements: a small semiconductor bandgap for ample solar light absorption,

Introduction

3 Poonam Sharma

conduction and valence band energies that straddle the water oxidization and

reduction potentials, high conversion efficiency of photogenerated carriers to the

water splitting products, durability in aqueous environments, and low cost.

However, to date no single semiconducting material has been found to meet all of

these requirements [Gratzel, 2001]. Transition metal oxides such as TiO2 possess

adequate stability but only absorb a small fraction of solar illumination due to their

large band gap (Eg=3.2 eV for anatase TiO2). Many other semiconductors have

smaller bandgaps and appropriate energy levels, such as InP (Eg=1.3 eV) and Fe2O3

(Eg=2.2 eV) however, they exhibit poor stability in aqueous environments. While

much work continues to focus on identifying an ideal material using combinatorial

approaches [Szklarczyk and Bockris, 1984] and by relaxing the constraint that only

one material (whose bandgap energy levels straddle the water redox potentials) can

be used, combinations of complementary semiconductors can be employed

[Woodhouse and Parkinson, 2009]. Systems delivering a record solar-to-hydrogen

(STH) efficiency over 12% have been demonstrated using III–V semiconductor

materials in a tandem cell using this approach, but their cost and stability remain

major disadvantages [Khaselev and Turner, 1998]. Alternatively, inexpensive and

stable oxide photoanodes following combination of a small and wide bandgap in a

single photoelectrode can be used to afford reasonable solar water splitting

efficiency. Semiconductor heterojunctions can absorb different region of the solar

spectrum. The advantage of composite structures is that each semiconductor needs

to satisfy one energetic requirement: matching the conduction band minimum

(CBM) or (VBM) with either the H2 reduction or O2 oxidation potential. Single

semiconductor materials typically cannot satisfy the requirements of suitable

bandgap energies for efficient solar absorption and meantime with band-edges

aligned with both the H2 and O2 redox potential of water [(Hwang et al, 2009), ]. It

has been reported that n/n semiconductor heterojunction thin films have different

band bending properties near the junction and have shown more promising

performance in PEC cell [Hwang et al, 2009]. However, very few reported the

photoelectrochemical response using multiple band gap thin film structures of

Fe2O3/TiO2, most of the work in this direction is focused on photocatalyts [(Zhang

and Lei, 2008), (Yang et al, 2009), (Mora et al, 2007), (Zhang and Lei,

2007),(Khedr et al, 2007)]. The research work presented in this Ph.D. thesis focuses

on the development of nanostructured bilayered thin films of Fe2O3/TiO2 consisted

Introduction

4 Poonam Sharma

of Fe2O3 and TiO2 that exhibit excellent physical properties, strong oxidizing

capability and long term stability in PEC cell for efficient production of solar

hydrogen. Bilayered Fe2O3/TiO2 thin film preparation is a two-step process

involving Fe2O3deposition by spray pyrolysis and TiO2 by sol-gel method

respectively. At the same time doping was also performed to reduce the band gap of

TiO2 and to improve the conductivity of Fe2O3 thin film photoelectrode [(Singh et

al, 2008), (Kumari et al, 2006)]. The effect of swift heavy ion irradiation on doped

and undoped bilayered thin films was performed and all samples were investigated

for best photoelectrochemical performance in PEC cell. An exhaustive study on

layered structures was further enhanced by modifying the porous iron oxide film

obtained from commercially available nano-powder. These porous structures were

further modified with carbon coatings.

In this thesis, the advantageous properties of bilayered photoelectrodes as well as

the challenges it presents to photoelectrochemical water splitting have been

presented. The most recent efforts at controlling water oxidation at the SCLJ

(semiconductor liquid junction), improving photon harvesting by layering, and

increasing the understanding of this promising material for solar energy conversion

have been critically examined.

1.2 SolarHydrogen-A viable Alternative to Fossil Fuels

Solar hydrogen is considered as a nonpolluting and inexhaustible energy carrier for

the future due to its unique physical and chemical properties. In the context of

hydrogen economy, hydrogen, the most abundant element in the universe, is an

energy carrier, not an energy source [Styring, 2012].

Hydrogen is the lightest substance known, much lighter than air that it rises fast and

is quickly ejected from the atmosphere. This is why hydrogen as a gas (H2) is not

found by itself on earth and is available in combined form with oxygen as water and

combined with carbon and other elements in fossil fuels and innumerable

hydrocarbon compounds. Hydrogen has the highest energy content of any common

fuel byweight (about three times more than gasoline), but the lowest energy content

by volume (about four times less than gasoline). Thus, remarkable properties of

hydrogen boost its potential as energy source [Hoffmann and Dorgan, 2012].

Unlike fossil fuel reactions no carbon dioxide, carbon monoxide, sulfur dioxide or

Introduction

5 Poonam Sharma

particulates are produced, water is the only combustion byproduct of

hydrogen[Sivula, 2011]. Hence the replacement of fossil fuels with hydrogen will

result in the reduction of greenhouse gas emissions and improved air quality, which

in turn will greatly improve the health of the environment and the population.

Despite its attractive properties, hydrogen is not readily available for use as a fuel.

It exists in bound form with other elements (e.g. water, hydrocarbons). In general,

hydrogen can be prepared in different energy pathways as follows: electrolysis,

plasmolysis, magnetolysis, thermal approach (direct, catalytic and cyclic

decomposition of water, as well as magmalysis), use of solar energy

(photosensitized decomposition using dyes, plasma induced photolysis,

photoelectrolysis, photo-aided electrolysis, the indirect path towards hydrogen by

photoelectrolysis: the photoelectrochemical production of H2 and photovoltaic

electrolysis), biocatalytic decomposition of water, radiolysis and other approaches

[Bockriset al, 2002]. Each of these pathways has its own advantage and

disadvantage that should be considered in terms of cost, emissions, feasibility,

scale, and logistics. But, in order to make sustainable use of hydrogen energy, the

production of hydrogen should use renewable forms of energies and must be

environment friendly[Bockris et al, 2002].

The electrolysis of water to produce hydrogen is considered as one of the promising

energy pathways as it can be easily achieved using an electrochemical cell. If a

voltage is applied between two electrodes submerged in an acidic or basic aqueous

solution, watersplits (electrolyzed) into hydrogen and oxygen gases. The water

splitting reaction will only take place when external energy is applied in this case in

the form of electricity. If the energy required for the splitting of water

electrochemically is supplied by a renewable source, like solar energy, then

hydrogen produced in this way is termed as ‘solar hydrogen’ and is renewable.

A photoelectrochemical system based on semiconductor materials is an attractive

way of transforming solar energy directly into chemical energy. This system

sometimes is referred to as “Artificial trees” or “Artificial leafs” (Grätzel) [Sivula et

al, 2011], since the net effect of these systems is the production of hydrogen fuel

using sunlight and water as input.

Introduction

6 Poonam Sharma

1.3 Solar Hydrogen Production byPhotoelectrochemical Splitting

of Water

Photoelectrochemical decomposition of water using solar energy has been a goal of

scientists and engineers since 1972, when Fujishima and Honda reported the

generation of hydrogen in a PEC cell with TiO2 electrode illuminated with near

ultraviolet light [Fujishima and Honda, 1972]. In a PEC cell, a light-sensitive

semiconductor photoelectrode is immersed in an aqueous solution, with electrical

wiring connected to a metallic counter-electrode. With exposure to sunlight,

photons create, photogenerated electron–hole pairs in the semiconductor, interact

electrochemically with ionic species in solution at the solid/liquid interfaces.

Photoexcited holes drive the oxygen evolution reaction at the anode surface, while

photoexcited electrons drive the hydrogen evolution reaction at the cathode surface

[(Sivula et al, 2011), (Bak et al, 2002)]. Fig. 1.1 depicts aphotoanode system where

holes are injected into solution at the semiconductor surface for evolving oxygen,

while photoexcited electrons are shuttled to the counter-electrode where hydrogen

is evolved. Conversely, in photocathode systems, electrons are injected into

solution and hydrogen is evolved at the semiconductor surface, while oxygen is

evolved atthe counter electrode.

Figure 1.1:Photoelectrochemical based solar hydrogen production system

PEC cells have the advantage of combining the photovoltaic cell and electrolyzer

into one system, without wires. Further, the advantage of PEC cells over

photovoltaic cells is that, in PEC cells semiconductor-semiconductor junction is not

Electrolyte

WE CE

RE

hν

AA

VWR

A

e-

H+

H2O + h+ 2H+ + ½ O2 2 H+ + 2e- H2

Introduction

7 Poonam Sharma

required. The junction is formed spontaneously at the semiconductor-electrolyte

interface.

1.4 Relevant Theories for PEC Splitting of Water Using Metal Oxides

This section provides some important theoretical concepts that underpin the

development of photocatalyst materials for water splitting relating to

semiconducting materials and their interaction with the electrolyte when used as

photoelectrodes in a photoelectrochemical (PEC) cell.

1.4.1 Semiconductor Electrolyte Interface in PEC Splitting of Water

The key components of a PEC cell are, undoubtedly, the semiconductor electrode

and the electrolyte. The semiconductor-electrolyte junction is, thus, the main

functional unit and to understand the mechanism of the processes occurring in a

PEC cell, a clear idea of the chemistry and physics of semiconductor in contact with

the electrolyte solution, particularly under illumination, is desired.

Semiconductors are class of materials, having conductivity intermediate between a

conductor and an insulator. In semiconductor, highest and lowest energy levels of a

band are referred as the band edges. As with molecular orbitals, the highest

occupied orbitals, called the valence band (VB) and the lowest unoccupied orbitals

called the conduction band (CB). The bandgap (Eg) is the energy gap between these

bands i.e. the difference in energy between the upper edge of the valence band (Ev)

and the lower edge of the conduction band (Ec) that determines the properties of the

material [Zollner et al, 2000]. The energy bandgap (Eg) of a semiconductor is

usually determined by means of optical absorption. The bandgap of semiconductors

ordinarily varies from 0.2 to 2.5 eV.

The position of energy bandgap of the semiconductor electrode with counter

(metal) electrode dipped in an electrolyte, before contact in dark has been shown in

Figure 1.2A. When a semiconductor electrode is immersed into an aqueous

solution, interfacial charge transfer and formation of a double layer takes place in

the presence of an electroactive species. On connecting the semiconductor electrode

with metal counter electrode in dark, if the Fermi level (EF) of the semiconductor

lies above that in solution, electrons flow from the inside to the surface to adjust the

EFof semiconductor [Bard and Faulkner, 1991]. At the thermodynamic equilibrium,

Introduction

8 Poonam Sharma

the EFof the semiconductor is shifted to the position of the redox potential in the

solution. A space charge layer (depletion layer) is formed near the surface of the

bulk semiconductor. As the semiconductor band edges are fixed, there is a

difference in potential between the surface and the inside of the semiconductor.

This phenomenon is known as band bending (Fig. 1.2B).

Upon illumination with photons having energy greater than the bandgap energy of

the semiconductor (hυ ≥ Eg), the energy is absorbed by the semiconductor electrode

creating the photogenerated electron-hole pair at the semiconductor/electrolyte

interface, as shown in Fig.1.2C. Electrons drift towards the bulk semiconductor in

the direction consistent with the existing electric field while the holes move in

opposite direction to the semiconductor-electrolyte interface [Bard and Faulkner,

1991].Only holes are available for reaction at the semiconductor surface. The

thickness of the space charge layer is in the order of 1 to 103 nm [Hashimato et al,

2005].

Due to this electron transfer to the bulk of the semiconductor, the semiconductor

becomes positively charged and this excess charge does not reside at the surface, as

it would in a metal, but instead is distributed in a space charge region (depletion

layer). The charge in the space charge layer is located in energy levels of impurities

and trapped holes [Morrison, 1980]. Within a planar compact semiconductor film,

the potential drop over the space charge region is given by equation 1.1:

2

2d

scD

wkTq L

1.1

Where∆ sc is the potential drop within the layer, wdis the width of the space charge

layer, q is the elementary charge and LD is the Debye length, expressed by the

following equation

2/1

22

D

oD Nq

kTL 1.2

Where is the dielectric constant, o the permittivity of free space and k, T, q and

NDhas their usual meanings.

Introduction

9 Poonam Sharma

Figure 1.2: Band diagram of junction between an n-type semiconductor and an electrolyte containing a redox couple O/R (A) before contact in the dark (B) after contact in dark and (C) junction under illumination.

φSc

Eg

H+/H2

O2/H2O

Semiconductor Metal

EF(metal)

φelectrolyte

EV

EC

EF

φSc

Eg

H+/H2

O2/H2O

Anode CAthode

EF(metal)

φelectrolyte

EV

EFEC

R

φSc H+/H2

O2/H2O

Photoanode Cathode

EF(metal)

φelectrolyte

EV

EF

EC

R

e-

h+

hυeVpH

eVB

A

B

C

Introduction

10 Poonam Sharma

The resulting electric field in the space-charge region causes a potential difference

between the surface and the bulk of the semiconductor. The band energies become

more negative with increasing distance into the semiconductor and then remain flat

in the field free bulk. This effect is called band bending[Bard et al, 2001]. Due to

this electric field an excess electron (for instance photo generated) in the space

charge region move towards the bulk semiconductor in the direction consistent with

the existing electric field. An excess hole in the space charge region moves toward

the semiconductor electrolyte interface.The electronstransferred from the

semiconductor reduce oxidized species in the electrolyte solution giving rise to

compensating charged layers. The one closest to the electrode is called the

Helmholtz layer and contains solvent molecules and sometimes specifically

adsorbed ions or molecules. This layer, reach from the semiconductor and the

position of the nearest non-adsorbed solvated ions at the surface, the so called outer

Helmholtz plane(OHP)[Bard et al, 2001]. At the semiconductor surface the charge

is located in the surface states or at states generated by adsorbed ions, whereas at

OHP the charge arises from the accumulation of ions attracted by the charged

surface. When semiconductor is immersed in an aqueous solution, protons and

hydroxide ions, and possibly other ions get adsorbed [Morrison, 1980]. The surface

acts as an acid-base couple, having the possibility to accept or donate protons. In

aqueous solution the H+ and OH- are potential determining ions at the

semiconductor/electrolyte interface. Accordingly, the positions of the energy bands

and also the Fermi level of the semiconductor shifts with the pH of the solution,

which modifies the kinetics of reaction at the interface.

1.4.2 Material Related Issues

The materials used as photoelectrodes to split water in PEC cell should satisfy

several specific functional requirements with respect to semiconducting and

electrochemical properties. Although these properties have been identified, it is

difficult to have materials which satisfied all of these requirements simultaneously.

The purpose of the present section is to discuss these important property

requirements.

Introduction

11 Poonam Sharma

Bandgap

The bandgap of the photoelectrode has a critical impact on the energy conversion of

photons [Bak et al, 2002]. That is, only the photons of energy equal to or larger than

that of the bandgap may be absorbed and used for conversion.

Figure 1.3: Solar energy spectrum (AM of 1.5) in terms of number of photons vs. photon energy, showing different flux photon regimes corresponding to specific properties of photoelectrodes.

Figure 1.3 illustrates the solar energy spectrum, depicting segments defining

phonon fluxes corresponding to different energy ranges. Water splitting being an

endothermic reaction requires the energy of 1.23 eV. Thus, the bandgap of

semiconductor necessary for photoelectrolysis of water should be 1.23 eV. In

practice, the energy that may be used for conversion is larger than the theoretical

energy limit. The difference between the two is due to energy losses caused by the

following [Wenham et al, 1994].

recombination of the photo-excited electron–hole pairs;

resistance of the electrodes;

polarization within the PEC;

resistance of the electrical connections;

Voltage losses at the contacts.

The estimated value of these combined losses is ~ 0.8 eV. Therefore, the optimal

energy range in terms of the photons available for conversion is -2.0 eV more

Introduction

12 Poonam Sharma

suitable. However, the semiconductor with bandgap >2.5 eV are not desirable

because they will not be able to absorbed solar radiation falling in the visible

region, where the sun emits the maximum energy.

Position of Band Edges

For easy transfer of charge carriers at semiconductor/electrolyte interface and in

order to successfully split water upon irradiation, the valence band of the

semiconductor has to be located at a lower energy level than the chemical potential

of O2 evolution (H2O/O2) in solution and the conduction band has to be positioned

at a higher energy level than the chemical potential of H2 evolution (H2/H+). If the

position of energy levels of the valence and conduction band is not fulfilled, an

external bias (Ebias) has to be applied in order to induce the photo-oxidation process.

Position of energy bands with respect to redox level of water for some oxide

semiconductors have been shown in Figure 1.4.

Figure 1.4: Position of energy bands of metal oxides (and a few other common semiconductors) at pH 1 with respect to the electrochemical scale. The standard electrode potentials of hydrogen and oxygen are also shown for reference

Resistance

The major sources of energy losses derive from the ohmic resistances of the

external and internal circuits of the PEC, including (a) electrodes (b) electrolyte (c)

electrical connections and (d) measuring and control equipment. In order to

Introduction

13 Poonam Sharma

achievethe maximum conversion efficiency, the electrical resistances of all of these

items should be minimum.

(a) The electrical resistance of the semiconducting photoanode is several orders of

magnitude larger than that of the metallic cathode. The electrical conductivity of

the photoanode, which is determined by the concentration of the charge and

their mobilities, is described in equation 1.3.

n p i ien ep Z ei 1.3

wheren is the concentration of electrons, p the concentration of holes, i the

concentration of ions, µn the mobility of electrons, µp the mobility of holes, µi

the mobility of ions and Zi charge number of ions.

At room temperature, the ionic component of the electrical conductivity may be

ignored. The mobility terms do not change with concentration when interactions

between the charge carriers are absent. However, at higher concentrations, these

interactions result in a decrease in the mobilities. Therefore, the maximal σ is a

compromise between the effects of increasing the concentrations while

decreasing the mobilities. The optimal value of σmay be achieved through the

imposition of a defect disorder that is optimal for conduction [Baket al, 1997].

The defect disorder and electrical properties may be modified through the

incorporation of aliovalent cations (forming donors and acceptors) and the

imposition of controlled oxygen partial pressure during processing. Again, these

required electrical properties may be achieved through in situ monitoring of the

electrical conductivity, thermoelectric power and work function during

processing.

(b) Analogous to the situation concerning the semiconducting electrode, maximal

conduction of the electrolyte may be achieved by selection of optimal ions and

their concentrations, leading to maximal mobility. The ions with the highest

mobilities are H+ and OH- [Baket al, 1997]. However, their use in high

concentrations is problematic owing to their chemical aggressiveness. Alkaline

cations, such as K+ and Ba2+, and anions, such as Cl- and NO3-, are alternative

candidates owing to their relatively high mobilities. These ions assume minimal

resistance at concentrations between 3 and 4 M.

Introduction

14 Poonam Sharma

(c) Electrical connections, such as those between wires and electrodes, may be

sources of high resistance due to (i) contact potential difference (CPD), which

develops between solids of different work function and (ii) local connection

resulting in the formation of high resistance scales. Therefore, it is desirable to

minimize or, preferably, eliminate the number of interwire connections. Also,

the engineering of other types of connections, those between the leads and the

other circuit elements, is of considerable importance.

(d) The internal resistances of the measuring and control equipment are important

because it is essential to maintain these resistances at the appropriate levels.

That is, voltmeters should have resistance as high as possible and ammeters

should have resistance as low as possible.

Condition for Stability

A very important property desired in the photoelectrodes is its high resistance to

chemical corrosion and photocorrosion. Free electrons and free holes generated in

the semiconductor, can also cause a chemical reaction between the electrolyte and

the semiconductor itself, which may alter the chemical nature of the semiconductor

and may therefore, destroy its semiconducting properties. This is termed as

'photocorrosion'. Therefore, it is essential for the photoelectrode to be resistant to

these types of undesired reactivities. In addition to the electrolyte, illumination can

also prove damaging on semiconductor material. The holes created under

illumination are reactive and might oxidize the material itself. The stability of the

material can be enhanced by catalytic surface treatments, which increase the rate of

charge transfer from the semiconductor surface to the solution. If the charge carriers

can be transferred to the solution as quickly as possible, then possibility of their

participation in recombination or corrosion is minimized.

Photogenerated holes may cause the oxidation of the semiconductor surface,

producing a blocking layer or the dissolution (corrosion) of the electrode. The

stability and mode of operation of electrodes can be attained by suitable choice of

solution and redox couple, as well as by the electrode surface.

Thus, the key functional element in photoelectrochemical water decomposition

using solar energy is the semiconductor photoelectrode. The efficiency of

photoelectrochemical production of hydrogen is determined by a combination of

such factors as bulk and surface properties of the semiconductor photoelectrodes,

Introduction

15 Poonam Sharma

their bandgap, resistance to the corrosion in aqueous electrolytes, and ability to

drive the water splitting reaction. Therefore, research is developing today in two

directions: the first one is further investigation of known semiconductor, with the

aim of the optimization of their electro-physical and electro-chemical parameters

and the second one discovery and manufacture of new semiconductor

photoelectrode. Unfortunately, to date, there is no such material that can meet all

the requirements simultaneously.

Among the various candidates for the photoanode, semiconductor metal oxides are

relatively inexpensive and have a better photochemical stability. Metal Oxide

semiconductors are generally of much lower purity than traditional semiconductors

and often possess lattice defects (such as vacancies and interstitials) that act as

donors or acceptors [Thursfield et al, 2006]. Metal-oxide semiconductors cannot be

thought of as intrinsic semiconductors. However, as the intrinsic, carrier density is

highly temperature dependent, there exists a high temperature range where the

concentration of intrinsic charge carriers exceeds that of the extrinsic carriers, so

the electrical properties of the semiconductor are independent of the impurities

within the material and the semiconductor exhibits intrinsic behavior [Kittel, 1953].

At lower temperature, the conductivity is dominated by impurity conduction

mechanisms resulting from the largely temperature-independent extrinsic defects

[Thursfield et al, 2006].

Many metal oxides have been extensively studied and considerable progress has

been made in recent years [Li and Zhang, 2010]. For a PEC cell, the conduction

band of most metal oxides is less negative than the H2 evolution potential; thus, a

small external potential needs to be applied to facilitate the PEC reactions.

1.5 Promises and Challenges of Using Nanostructured Metal

Oxide Semiconductors in PEC Splitting of Water

In the past few decades, the concept of PEC water splitting for hydrogen generation

has been validated by successful demonstration using PEC cells based on

semiconductor photoelectrodes. Earlier work was mainly based on planar metal

oxide semiconductor films [Li and Zhang, 2010]. A target hydrogen production

efficiency of 10 % has been quoted for water splitting technology to be

Introduction

16 Poonam Sharma

commercially viable [Bolton, 1995]. The efficiency of PEC devices has been

limited by several key factors, such as the limited light absorption efficiency in the

desired visible region and recombination of photoexcited electrons and holes. The

recent development of nanomaterial opens up new opportunities in addressing these

fundamental scientific issues.

Nanomaterials have attracted significant attention in recent years. The primary

reason is their unique physical and chemical properties compared to bulk materials

as well as their potential applications in various technologies including energy

conversion. Photoelectrodes based on nanostructured semiconductor materials have

been explored for a number of systems, usually single component nanomaterials

such as WO3 [Enescaet al, 2007], CdS [Parida et al, 2010], ZnO [Chen et al, 2010],

Fe2O3[Sivula et al, 2011] and, more commonly, TiO2 [Khan et al, 2002].

Nanomaterials including zero-dimensional (0D) nanocrystals and one-dimensional

(1D) nanorods and nanotubes offer some potential advantages over their bulk

counterparts for photoelectrodes in PEC [Li and Zhang, 2010].

Nanostructured photoelectrodes, used in PEC cells, are commonly referred as

porous electrode built up from interconnected semiconductor particles of nanometer

size [Hodes, 2001]. These electrodes distinguish themselves by their porosity and

high surface-to-volume ratio, where the effective surface area can be enhanced

by1000-fold [Hagfeldt, 1995]. It provides the large contact area between

semiconductor and electrolyte, therefore, better and faster is the process of transfer

of carriers at the interface in PEC cell.

Electronic properties of materials show profound changes, as the dimensions of

materials are made smaller. The band structure of semiconductors, their chemical

and physical reactivity is affected by the size of the particle. For bulk materials, the

energy levels are continuous; however, as the dimensions are made sufficiently

small, they become discreet, due to the confinement of the electron wavefunction

because of the physical dimensions of the particles.The average spacing between

successive energy levels is called Kubo gap and is given by:

43

fEn

1.4

Introduction

17 Poonam Sharma

Where,Efis the Fermi energy of bulk material and n is number of electrons in the

particle. Size of the particle in the material can be optimized with respect to

efficient PEC generation of hydrogen.

Mechanism of the charge transportin nanostructured electrodes is also very different

from the bulk material and affects the PEC properties. For very small

semiconductor particles of radii ro, the total potential drop within the semiconductor

(∆Φsc) is

2

0

6

Dsc L

rq

kT 1.5

The potential difference between the surface and the bulk of the semiconductor (the

band bending) has to be at least 50 mV (2kT/q) in order to support migration of the

photogenerated charge carriers [Hagfeldt and Gratzel, 1995]. The electric field in

nanostructured semiconductor particles is usually small, unless high dopant levels

are present. Besides, the porous structure of nanostructured semiconductor

electrodes enables the electrolyte to fully penetrate the electrode and any existing

electric field is effectively screened [Pichot and Gregg, 2000]. Therefore, the

concept of band bending will no longer be applicable and the driving force for

charge separation cannot be built-in-electric field. Instead, it is generally accepted

that the transport is mainly a diffusional random walk process as suggested by

Sodergen et al. [Sodergen and Hagfeldt, 1994]. Thus, the driving force for the

transport is the concentration gradient over the nanostructured film of photoinduced

electrons [Pichot and Gregg, 2000].

In the absence of depletion layer the initial charge separation of photogenerated

charge is dependent on fast interfacial kinetics [Pichot and Gregg, 2000], [Hodges

et al, 1992]. The small particle size and the large nanostructured

semiconductor/electrolyte interface (NSEI) may facilitate a fast transport of

photogenerated charges to the interface, which can compete with the recombination

rate. A typical feature of nanostructured electrodes is that the most efficient charge

separation takes place close to the back contact [Hagfeldt et al, 1992].

In conclusion it can be said that, use of nanostructures in PEC cell has been

definitely observed to deliver a better and efficient system for solar splitting

ofwater, by overcoming many of the problems. With these potential advantages,

Introduction

18 Poonam Sharma

nanostructured semiconductor photoelectrodes could fundamentally change the

design of PEC cells and improve the solar to hydrogen conversion efficiency.

Research activity in exploring nanomaterials for PEC applications has grown

substantially in the last few years.

1.6 Materials of Interest

Research work in this thesis utilizes nanostructured metal oxide semiconductors for

PEC splitting of water. Photoelectrodes based on nanostructured semiconductor

materials have been explored for a number of systems, usually single component

nanomaterials such as WO3 [Enesca et al, 2007], CdS [Parida et al, 2010], ZnO

[Chen et al, 2010], Fe2O3[Sivula et al, 2011] and, more commonly, TiO2 [Khan et

al, 2002].But, most of the materials investigated to date as photoelectrodes do not

achieve real water splitting solely by the use of solar energy and have been found to

be susceptible to the photocorrosion and/or possess unsuitable interfacial energetic.

The known light absorbers however are either too inefficient (1-2%) in sun light or

too unstable in the field for practical implementation.Wide survey of literature

revealed nanostructured TiO2(anatase) andFe2O3(hematite) as most promising

candidates for photoelectrode in the PEC cell on account of their unique set of

properties e.g. long term chemical stability in many solvent over a wide pH range

and low cost [(Sivula et al, 2011), (Khan et al, 2002),(Schrebler et al, 2007) and

(Cesar et al, 2006)].

1.6.1 Nanostructured Titanium dioxide as Photoelectrode

TiO2 is the most popular and useful semiconductor for photocatalysis. Anatase TiO2

at pH 0 has valence and conduction band potentials of +2.9 V and -0.3 V vs. NHE,

respectively [Burdett et al, 1987]. An important property of TiO2 which makes it

most suitable for solar splitting of water is its band edges matching with the redox

level of the water, which facilitate easy transfer of charge carriers at

semiconductor/electrolyte junction in PEC cell. Some of important properties of

TiO2 are listed as follows:

Exceptional, optical and electrical properties

Chemical and biological inertness

Stability in terms of photo-corrosion and chemical corrosion

Non-toxic and available at a low cost

Introduction

19 Poonam Sharma

Its valence band holes are strong oxidants and the conduction band electrons

are good reductants

But,photoresponse of TiO2 as a photoelectrode is limited due to its large bandgap

~3.2 eV capable of absorbing only ~4% of incoming solar radiation[Fahmi et al,

1993]. Nanomaterials including zerodimensional (0D) nanocrystals and one-

dimensional (1D) nanorods and nanotubes offer some potential advantages over

their bulk counterparts for their use as photoelectrode in PEC cell. Fitzmaurice and

coworkers reported for the first time practical feasibility of nanostructured TiO2

membrane sensitized with Ru complexes for water splitting on account of long-

lived charge separation. [Hoyle et al, 1997]. Later, Khan and Akikusa reported the

photoresponse of nanocrystalline n-TiO2 and n-TiO2/Mn2O3 thin-film electrodes

duringwater splitting reactions [Khan and Akikusa, 1999]. Besides 0D

nanostructures, 1D nanostructures, such as nanotubes, nanowires, and nanorods are

expected toexhibit much improved transport properties than nanoparticles. The

photoresponse of n-TiO2 thin-film and nanowire electrodes towards the water

splitting reaction has been compared. A more than twofold increase in maximum

photoconversion efficiency was observed in experiments when a single-layer thin

film of n-TiO2was replaced by nanowires [Lindgren et al, 2002]. It is clear that the

PEC performance is related to the electronic and structural properties of nanorod

arrays.

1.6.2 Nanostructured Hematite as Photoelectrode

Hematite (-Fe2O3) is the most abundant and photo-sensitive form of iron oxide. It

has received attention of researchers all over the world as solar energy harvester in

PEC cell for splitting of water, [Sivula et al, 2011] just after the first report on the

PEC splitting of water by Fujishima and Honda in 1972. Hematite (α-Fe2O3) is a

good PEC material due to its:

Good bandgap ~2.0-2.2 eV lying in the visible region of solar spectrum

where the sun emits the maximum energy.

It is capable of absorbing all UV light and most of the visible light of the

solar radiation from 295 nm to 565 nm, which allows utilization of 40% of

the incident solar radiation.

Introduction

20 Poonam Sharma

Excellent stability in both acidic and alkaline conditions, even under

illumination [Cesar et al, 2006].

Even though the bandgap of α-Fe2O3 is suitable for efficient absorption of incident

solar radiation [Turneret al, 1984], its photoresponse is reported quite low,

mainlydue to its high resistivity and high recombination rate of photogenerated

charge carriers at the semiconductor/electrolyte interface [(Cesar et al, 2006),

(Sivula et al, 2011) and (Lindgren et al, 2002)].Another problem with hematite is

that its energy bandedges do not match with the redox level of water, therefore

external voltage, in addition to solar energy is required to split water photo

electrochemically.However, hematite in its nanostructured form has shown better

performance in PEC cell than bulk hematite thin film photoelectrodes. This is due

to fact that in nanostructured hematite, sufficiently small diameter alleviate hole

transport limitations, while their large specific surface area helps addressing

intrinsically slow OER kinetics. Khan and Akikusa reported the photoresponse of

nanocrystalline n-Fe2O3 thin-film electrodes duringwater splitting reactions.

Significantly, the nanocrystallinen-Fe2O3 films showed higher photoresponse

compared tothose prepared by compression of n-Fe2O3 powder or bythermal

oxidation of metallic iron sheets. Since then,the PEC performance of different

nanocrystalline metaloxide films has been studied.The first demonstration of PEC

water splitting using1D nanostructures as photoelectrodes was reported byLindquist

and coworkers in 2000 [(Lindquist et al, 200), (Beermann et al, 2000)]. They

reportedthe photoelectrochemistry of hematite nanorod arrays forthe photoanode. It

is generally accepted that recombinationof electrons and holes, trapping of electrons

by oxygen deficiencysites, and low mobility of the holes cause the

lowphotoresponse for hematite films. In comparison to nanoparticles,nanorods

improve the transportation of carriers andthus reduce the recombination losses at

grain boundaries.

1.7 Strategies to Improve the PEC Response of Nanostructured

TiO2and Fe2O3 Thin Films The potential advantages of 0D and 1D metal oxide nanostructures as photoanodes

have been demonstrated in the abovementioned section. However, it should be

noted that PEC performance depends strongly on the band gap and band edge

Introduction

21 Poonam Sharma

positions of the photoanodes [Bak et al, 2002], and the large band gap of metal

oxides significantly limits the light harvesting in the visible region of the solar

spectrum. In the section, we will review the recent research strategies with the goal

to address this issue by developing more sophisticated nanostructures, including

hybrid semiconductor nanomaterials, dopants, metal ion implementation, swift

heavy ion irradiation, sensitized nanostructures, direct water splitting and

heterogeneous nanostructureswhich offer the possibility to manipulate the band

structure and thereby to enhance visible-light absorption and PEC performance.

Hybrid Semiconductor Nanostructures (HSNs)

Major efforts have been focused on attaining a broad photoresponse range and

achieving a better efficiency in water splitting using wide band gap semiconductors,

such as TiO2.Among the strategies, combining semiconductors with other elements

or compounds to form hybrid semiconductor nanomaterials (HSNs) is considered as

promising. In these cases, HSNs not only show higher efficiency in light absorption

but also suppress the recombination of the photogenerated electrons and holes, thus

demonstrating notably better performance in water splitting compared to pure

semiconductors. Doping and metal ion implantation in nanostructured

semiconductor thin films is most commonly referred to as HSNs.

Doping

One approach to alter the properties of semiconductor nanomaterials is to dope

them with metal ions such as iron and copper [(Morr et al, 2007), (Choi et al, 1994),

(Anpo et al, 1997), (Ghosh et al, 1977), (Ingler et al, 2005)]or non-metal species

such as silicon [(Yarahmadi et al, 2009), (Mohapatra et al 2007)], nitrogen [(Yan et

al, 2007), (Cui et al, 2008)], phosphorus [Lin et al, 2005], carbon [Xu et al, 2007,

Sakthivel and Kisch, 2003], sulfur [Ohno et al, 2003], boron [Zhao et al, 2004], and

halides [Luo et al, 2004]. For efficient doping to enhance the photoactivity of HSNs

for hydrogen generation, the following requirements should be achieved [Asahi et

al, 2001]: (i) doping should produce states in the band gap of semiconductors to

enhance visible-light absorption; (ii) ECB, including subsequent impurity states,

should be higher than the H2 evolution level to ensure its photoreduction activity.

Doping with metal ions in TiO2resulted in much higher efficiency in PEC cell [Choi

et al, 1994]. It has been reported that in the Fe3+doped anatase TiO2, Fe3+ has the

Introduction

22 Poonam Sharma

energy level for Fe3+/Fe2+ below the conduction band edge of TiO2 and the energy

level for Fe3+/Fe4+ above the valence band edge. Therefore, Fe3+ could act as traps

for electrons as well as holes and inhibit e−/h+ recombination, whereas no

noticeable change in band-gap energy of n-TiO2 was observed [Choi et al,

1994].Several dopants have been reported to improve the photoresponse of the iron

oxide. Silicon has been known as a good dopant for Fe2O3nanomaterials for PEC

water splitting [(Cesar et al, 2006) and (Mohapatra et al, 2007)]. The

nanocrystalline thin-film Si-doped Fe2O3photoelectrodes prepared by atmospheric

pressure chemical vapor depositionproduced significantly better photocurrent

thanundoped Fe2O3 electrodes. Also, Zn doped Fe2O3 nanostructured thin film long

term stability in PEC cell with enhanced photoelectrochemical response [Kumari et

al, 2006].

Ion implantation

Implantation of metal ions such as Ru3+ and Cr3+,in TiO2 caused effective

decrement in the band-gap energy with introduction of intra band-gap states

resulting in visible-light absorption [(Choi et al, 1994) and (Kato and Kudo, 2002)].

However, in most cases the metal ions mainly act as traps of photogenerated

carriers that help to suppress recombination of the photogenerated charge carriers

rather than to narrow the band gap [(Choi et al, 1994), (Litter et al, 1994)].

However, HSNs with metal-ion implantation sometimes encountered problems such

as thermal instability, increase in carrier-recombination centers, and requirement of

an expensive ion-implantation facility [Anpoet al, 1997].

SHI irradiation

SHI is an important tool to tailor the material properties for best photoresponse by

altering its electrical, structural and surface morphological properties [Hwang et al,

2009]. Change in morphology, shape and size of pores in iron oxide thin films upon

irradiation were attributed to the defects and the strains. Nanostructured Fe2O3

[Singh et al, 2009], CuO [Chaudharyet al, 2006] and TiO2 [Singh et al, 2010] thin

films irradiated with swift heavy ions have shown several times better

photoresponse in comparison to their undoped counterpart. SHI is a useful way to

alter the material property but requirements of expensive ion beam accelerators

facility limits its application for commercial scale.

Introduction

23 Poonam Sharma

Sensitized Nanostructures

Sensitizing semiconductors by dyes to achieve a better use of solar light has been

intensely investigated in the fields of photovoltaics, photocatalysis, and

photoelectrochemistry [(Yang et al, 2009), (Ohtaniet al, 2003) and (Nazeeruddinet

al, 1993)]. For example, 1,1-binaphthalene-2,2-diol sensitized TiO2 showed

noticeable H2 evolution under visible light, whereas the bare TiO2 did not [Ohtaniet

al, 2003]. In severalstudies, dye sensitization has been used to improve PEC

reactions by enhancing visible-light absorption and photoconversion

[(Nazeeruddinet al, 1993), (Youngblood et al, 2009)]. A recent study found that

nanoparticulate anatase TiO2 sensitized with ruthenium polypyridyl dyes showed

visible light induced water splitting [Youngblood et al, 2009]. The sensitized TiO2

film has shown a better photoresponse than the unsensitized ones. However, the

photocurrent densities (typically 10–30 µA/cm2) of these dye-sensitized structures

are limited by the bleaching of dyes and the slow electron transfer from

nanoparticles to oxidized dyes.

SurfaceMorphology

This general concept of using 1D nanostructure for PEC photoelectrodes has been

further extended to nanotubes [(Lin et al, 2009), (Li and Zhang, 2010)]. In

comparison to nanorods or nanowires, nanotubes exhibit less material for light

absorption but higher surface area for redox reactions. The photosplitting of water

using highly ordered TiO2 nanotube arrays has been reported in 2005 [Moret al,

2005]. Grimes and coworkers demonstrated highest reported efficiency for

TiO2nanotube arrays. In addition, the coupling between 1D metal oxide

nanostructures with 0D narrow band gap semiconductor photosensitizers such as

CdS and CdSe nanoparticles or quantumdots (QDs) for water splitting has attracted

much attention lately [Chen et al, 2006]. Similar to dye sensitization, quantum dot

sensitized nanostructured photoanodes enable light absorption in the visible region

of interest and effective transportation of excited electrons. One added advantage

with QDs is that their absorption can often be controlled by varying the particle

size, which is usually not possible for dye molecules [Murray et al, 1993]. It has

been recently suggested that TiO2 nanorod arrays can be sensitized by different

sizes of CdSe nanoparticles to form rainbow solar structures [Kongkananandet al,

Introduction

24 Poonam Sharma

2008]. The band gap and band edge positions can be engineered by synthetically

tailoring the particle size of QDs. This multiple-junction quantum dot

heterostructures could maximize the light absorption efficiency in the same way

that has been demonstrated in tandem PV cells. In related studies, it has been shown

that increasing structural complexity of heterogeneous nanostructures could further

improve the efficiency and stability of PEC cells. For example, various core/shell

nanowire heterostructures, CdS/TiO2 and Si/TiO2, have been studied for PEC water

splitting [(Yin et al, 2007)]. These composite heterostructures combine the

properties of two semiconductors in unique architectures to help address the

shortcomings of individual semiconductors as photoanode, representing a new and

promising approach for PEC and other applications

Direct Water Splitting

One limitation with semiconductor metal oxide photoanodes is that they do not

have a sufficiently negative photocurrent onset potential to allow for direct water

splitting in the presence of a metal cathode due to the overpotential and ohmic drop

losses [Alexander et al, 2008]. To solve the problem, different approaches have

been explored. One strategy is to use PEC/PV tandem cells based on direct

integration of a PEC cell with a PV cell. The PV cell is intended to provide the bias

voltage required for PEC water splitting. An efficiency of 12.4% for the H2

generation was achieved in a tandem cell based on GaInP2/GaAs p/n multiple band

gap structures [Khaselev and Turner, 1998]. The energy required for spontaneous

water splitting could also be provided by a PEC cell consisting of a semiconductor

photoanode and a semiconductor photocathode [(Kochaet al, 1994), (Nozik, 1977)

and (Wang et al, 2008)]. The combination of two semiconductors relaxes the

criteria of the energetic overlap for photoelectrodes, which increases the number of

semiconductors that could be applied toward water splitting [(Kochaet al, 1994) and

(Nozik, 1977)]. However, the observed photocurrent density is low (∼20 µA/cm2)

even at 1W/cm2 light irradiation due to the poor photoresponse of the hematite or

WO3 photoanode.

Heterogeneous Nanostructures (Layered nanostructures)

The idea of sensitization or surface modification has been extended to the coupling

of two semiconductors possessing different energy levels for their corresponding

Introduction

25 Poonam Sharma

CB and VB. Various studies have shown improved visible-light photoactivity with

coupled semiconductors with different band gaps, e.g. TiO2/CdS [(Chen et al,

2006), (Lin et al, 2009), (Yin et al, 2007), (Park et al, 2008) and (Vogel et al,

1990)], TiO2/CdSe [Liu and Kamat, 1993], TiO2/Si [Park et al, 2006], ZnO/CdS

[Hotchandani and Kamat, 1992], ZnS/CdS [Innocentiet al, 2004], SnO2/TiO2 [Nasr

et al, 1998], SnO2/ZnO [Kumara et al, 2003], and SnO2/CdSe [Nasr et al, 2007].

These heterogeneous nanostructures are typically composed of two semiconductors,

one with a wide band gap and another with a smaller band gap. The small band gap

semiconductor is primarily responsible for visible light absorption and sensitizing

the large band gap semiconductor through electron and/or hole injection. Efficient

electron injection requires that the bottom of the CB of the small band gap

semiconductor be above the bottom of the CB of the large band gap semiconductor

[Kamat, 2007, Haoet al, 1999]. The electron transfer between the two

semiconductors could also enhance the charge separation and inhibit the

recombination rate by forming a potential gradient at the interface. These composite

heterostructures combine the properties of two semiconductors in unique

architecture to address the shortcomings of individual semiconductors as

photoanode, representing a new and promising approach for PEC and other

applications.

1.8 Bilayered Metal Oxides: Potential Photoelectrodes for Solar

Hydrogen Generation While much work continues to focus on identifying an ideal material using

combinatorial approaches [(Lin et al, 2009), (Suet al, 2011)] and by relaxing the

constraint that only one material (whose bandgap energy levels straddle the water

redox potentials) cannot be used, combinations of complementary semiconductors

can be employed. Alternatively, inexpensive and stable oxide photoanodes

following combination of a small and wide bandgap in a single photoelectrode can

be used to afford reasonable solar water splitting efficiency. Advantages of using

bilayered thin films in PEC cell are listed as follows:-

Bilayered semiconductors can absorb in different region of the solar

spectrum.

Introduction

26 Poonam Sharma

These structures are capable of satisfying at least one energetic requirement:

matching the conduction band minimum (CBM) or (VBM) with either the

H2 reduction or O2 oxidation potential.

Bilayered structures formed by combining n/n type semiconductor thin films

have different band bending properties near the junction and have shown

more promising performance in PEC cell.

Literature survey revealed iron oxide and titanium dioxide are attractive PEC

material. But, still reported efficiency for both these oxide semiconductors is much

below the level of commercialization. TiO2 has been extensively studied as a

photoanode due to its high resistance to photocorrosion. However, its conversion

efficiency of solar energy to hydrogen is still low (less than 4 %) due to its large

bandgap (3.0 ~3.2 eV). TiO2 requires an external bias to reduce water for H2

production to overcome the chemical over potential. Fe2O3 (Eg = 2.2 eV), on the

other hand, can absorb sunlight efficiently. It is however challenging to use Fe2O3

for photoelectrolysis is its short diffusion length and corrosion in water. It has been

reported that n/n semiconductor heterojunction thin filmshave different band

bending properties near the junction and have shown more promising performance

in PEC cell. Keeping this idea of n/n semiconductor heterojunction, we prepared

TiO2-Fe2O3 bilayered thin films and studied their photoelectrochemical properties.

Several reports are available on the improved photocatalytic performance of

hetrojunction metal oxide thin films [Lin et al, 2009]. However, very few reported

the photoelectrochemical response using multiple band gap thin film structures of

Fe2O3/TiO2. It is expected that TiO2-Fe2O3thin films will show higher photocurrent

than the planar TiO2 and Fe2O3 thin films. Present thesis is aimed at design,

development of an ideal bilayered photoelectrode capable of efficiently split water

for hydrogen generation. Several strategies have been adopted to improve the

performance of bilayered structure in PEC cell. Aiming this, an exhaustive study

has been undertaken in this thesis focusing bilayered Fe2O3/TiO2 thin film

modification via doping each individual oxide material to tune its band edges at

interface. Irradiating thus modified bilayered structures with swift heavy ions.

Introduction

27 Poonam Sharma

1.8.1 Strategies to Overcome the Interfacial Charge Carrier Recombination

However multiple bandgap photoelectodes are stable in electrolyte and efficient of

extended absorption, but still the reported efficiency is low due to poor carrier

transfer across the interface due to misaligned band edge position. To overcome this

several strategies have been adopted and bilayered photoelectrode has been

designed accordingly.

Using Dopants

Since, problem in bilayered thin films is the position of their band edges, therefore

an attempt was made by doping each individual metal oxide to tune its bandedges in

appropriate way to favor carrier movement without recombination. In present

thesis, iron oxide thin films were doped with Zn and TiO2 with iron and thin films

were made in various combinations accordingly to optimize the

photoelectrochemical response.

SHI Irradiation of Bilayered Thin Films

Swift heavy ion irradiation is an important tool to tailor the material properties for

best photoresponse by altering its electrical, structural and surface morphological

properties. During swift heavy ion irradiation, the incident beam of ions imparts

energy into the target material through collisions with nuclei and electrons.

Collisions with nuclei are elastic and incident ions lose their energy to the target

lattice resulting in atomic displacement, producing interstitial and lattice defects.

SHI causes mixing at the interface in oxide bilayer thin films [Schattatet al, 2002].

Since, mixed oxides of TiO2 and Fe2O3 are good absorber of visible light [Thimsen

et,al, 2009]. Also, mixed oxide formation at the interface inhibits recombination of

carrier. Earlier reports on SHI irradiated metal oxide thin films of iron oxide and

titanium dioxide have shown significant photoelectrochemical response. Motivated

from these results, an attempt has been made to irradiate bilayered thin film with

SHI.

Introduction

28 Poonam Sharma

1.9 Overview of the Study

Important observations from the literature in relevance to present thesis work have

been listed as follows:

Nanostructured TiO2 and Fe2O3thin films as most stable, promising and

photoactive materials for its use in PEC splitting of water as bilayered thin

film[(Sivulaet al, 2011), (Khan et al, 2002), (Souza et al, 2009), (Schrebleret al,

2011) and (Cesar et al, 2006)].

Doped nanostructured thin films of TiO2 and Fe2O3performsbetter in PEC cell

than undoped thin films due to improved light absorption and

conductivity[(Singh et al, 2008), (Kumariet al, 2006)].

Swift heavy ion irradiation seems to be an effective way to improve the

photoresponse of the thin films by modifying its physico-chemical and

electrical properties.

Bilayered thin films not only inhibit the recombination of photogenerated

charge carriers but also enhance the absorption in comparison to the single

photolelecrode of same material [Lin et al, 2009].

So far very few reports are available on improved photoelectrochemical

performance of bilayered structure of Fe2O3/TiO2 thin films

Based on these studies, specific objectives of this thesis have been mentioned

below:

(1) To synthesize undoped/doped nanostructured bilayered thin films by spray-

pyrolysis and sol-gel methods, respectively.

(2) To modify bilayered thin films by swift heavy ion irradiation using different ion

species and energy for the better performance of the material in PEC cell.

(3) To characterize thin films for various physico-chemical properties.

(4) To explore the use of these layered structures as photoelectrodes in PEC cell for

measurement of photoresponse and efficiency with respect to hydrogen

generation.

(5) To obtain the Mott-Schottky plot (1/C2 vs. V/SCE) for determination of donor

density and flatband potential at semiconductor/electrolyte junction.

Introduction

29 Poonam Sharma

(6) Finally, collection and quantification of hydrogen with the samples exhibiting

good photoresponse.

This thesis is divided into 6 chapters. The motivations for this research with aims

and objectives along with various theories related to PEC generation of hydrogen

including basics of semiconductors and their interaction with the electrolyte have

been presented in this chapter. Chapter 2 examines the present status of research on

the use of layered structures in PEC cell for water splitting. Details of the

experimental methods used to synthesize, modify and characterize the prepared

material have been presented in chapter 3. Results and discussion on nanostructured

Fe2O3-TiO2bilayered thin films have been presented in Chapter 4. Chapter 5

presents the results of the characterization and discussion on nanostructured Fe2O3,

modified by carbon. Finally conclusions of the thesis and recommendations for

further work are presented in Chapter 6.