Chapter 1
Introduction
Growing energy demand, depletion of fossil fuels and environmental concerns led
to the search for new, sustainable sources of energy, which can meet all our energy
needs in future. Most energy sources that are currently being scrutinized originate
from solar energy [1]. However, direct use of solar energy by e.g. solar thermal
power or photo-voltaic solar cells is still limited, and only 0.04% of the total energy
is generated by photo-voltaic [2]. On the other hand large-scale use of solar energy
requires an efficient energy storage solution. Hydrogen is one of the most
promising future energy carriers and has great potential for its use as fuel in
transport sectors [3]. Use of hydrogen is environment friendly. Conversion of solar
energy into hydrogen would provide long-term storage of the world’s most
abundant but intermittent source of energy. Hence, world is looking forward to the
development of clean and sustainable methods of its production from renewable
energy sources.
Production of hydrogen using solar energy by photo-oxidation of water in
photoelectrochemical cell is a very straightforward and attractive method, as it is
clean, sustainable and renewable [4]. Efficiency of hydrogen production in such
systems is mainly decided by the properties of the semiconductor used as
photoelectrode in PEC cell. Selection of material for efficient water splitting by the
direct solar energy is very crucial. First, the materials must be stable in water and
they must be stable (upon illumination) against photo corrosion. Second bandgap
must be small enough to absorb visible light and finally, their band edges must be
positioned below and above the redox potential of hydrogen and oxygen,
respectively [5]. 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. After the
pioneering work of Fujishima and Honda in 1972, showing the possibility of
electrolyzing water into hydrogen using solar illuminated TiO2 semiconductor
electrode, various metal oxides WO3, ZnO, SrTiO3, α-Fe2O3 etc have been tested as
a photoelectrode for PEC application [6].
Introduction
By inspiring the devotion of metal oxides in PEC application the present thesis is
motivated to overview the nanostructured hematite thin films for solar generation of
hydrogen. Hematite (α-Fe2O3) is a promising photo-electrode material due to its
significant light absorption, chemical stability in aqueous environments, and ample
abundance [7]. However, its performance as a water-oxidizing photoanode has been
crucially limited by poor optoelectronic properties that lead to both low light
harvesting efficiencies and a large requisite over potential for photoassisted water
oxidation [8, 9]. To overcome these problems and to improve its photoresponse for
water splitting, the development of the nanostructured hematite electrode can be
viewed as three bodies of present thesis. Firstly, the development and
characterization of nanostructured hematite thin films by simple and economical
techniques: 1) spray pyrolysis, 2) electrodeposition. Secondly, the investigation of
different metal ion doping e.g. Zr, Si, Al at various doping concentrations with
nanostructured hematite thin films prepared by both techniques and thirdly, the
effect of swift heavy ion irradiation on the various properties of doped/undoped
nanostructured hematite thin films prepared by both techniques. Morphological,
optical, electrical, semiconducting and photoelectrochemical properties of various
samples have been investigated and analyzed with respected to solar hydrogen
production.
Subsequent sections describe the greater details of basic principle involved in the
photoelectrochemical splitting of water for generation of hydrogen. Various
material related challenges involved in the process have also been discussed, which
is actually the motivation behind the work undertaken in this thesis.
1.1 Hydrogen: Alternative Source of Energy
What is hydrogen fuel?
Hydrogen is the simplest and most common element in the universe. It is a
colorless, odorless, and tasteless gas that has the highest energy content per unit of
weight of any known fuel. Hydrogen is very chemically active and rarely stands
alone as an element. It usually exists in combination with other elements, such as
oxygen in water, carbon in methane, and in trace elements as organic compounds.
Hydrogen, therefore, must be broken from its bonds with other elements in order to
be used as a fuel. Hydrogen gas was first isolated by Henry Cavendish in 1766 and
later recognized as a constituent of water by Lavoisier in 1783. The production of
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Introduction
hydrogen and oxygen by the electrolytic decomposition of water has been practiced
since the year 1800, when the process was first discovered by Nicholson and
Carlisle. Since then, the idea of society using hydrogen as a primary energy carrier
has been explored and refined. Jules Verne appears to be one of the earliest people
to recognize, or at least articulate, the idea of splitting water to produce hydrogen
(H2) and oxygen (O2) in order to satisfy the energy requirements of society. As
early as 1874 in The Mysterious Island, Jules Verne alluded to clean hydrogen
fuels, writing:
"Yes, my friends, I believe that water will someday be employed as fuel, that
hydrogen and oxygen, which constitute it, used singly or together, will furnish an
inexhaustible source of heat and light….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."
By consuming hydrogen, energy can be produced without emitting local air
pollutants or carbon dioxide, the gas that many scientists believe will be most
responsible for potential climate change in the 21st century.
Hydrogen as valuable energy carrier
Hydrogen could have the potential to reshape the entire energy industry.
Environmental benefits are obvious. Whether hydrogen is combusted or consumed
in a fuel cell, it directly produces almost no local air pollutants or greenhouse gas
emissions. Emissions of volatile organic compounds (VOCs, the precursors of
ozone), SOx, NOx, carbon monoxide, and particulate matter could be dramatically
reduced if all vehicles were fuelled by hydrogen [10].
However, when carbon-based fuels are reformed or gasified to produce hydrogen, a
stream of nearly pure carbon dioxide is easily produced as a byproduct.
Technologies are emerging to isolate this carbon dioxide from the atmosphere by
sequestering it in the ocean or in geological formations; however, the long-term
effects of carbon sequestration and storage security are not entirely known. If
sequestration proves to be a viable and economical technique, fossil fuels could be
converted to hydrogen and consumed to produce energy with small greenhouse gas
emissions. Electrolysis produces no carbon dioxide directly since there is no carbon
involved in splitting water into hydrogen and oxygen. However, the entire process
is only carbon-free if a non-carbon source of electricity, such as wind, solar, or
nuclear power is used.
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Introduction
Hydrogen may change the electric power industry as well. If electrolysis costs reach
a certain point, it could be used as a technique to store electricity. This could make
it economical for power companies to convert off-peak power into hydrogen, which
could be converted back into electricity during peak periods. This would require
less electric power capacity for the same amount of peak power. For example a 100
MW plant could serve an area which has a peak power demand of 110 MW if there
was lower demand during off-peak periods. The ability of hydrogen to store
electricity could also help stabilize and promote renewable electricity generation
from intermittent sources such as wind or solar power [11].
Following are some significant points which prove solar hydrogen an ideal energy
carrier:
1) Its raw material for production is water.
2) It is renewable fuel.
3)
It can be stored in gaseous, liquid or metal hydride form.
4)
H2 stores three times better energy per unit mass as gasoline and seven times
the energy per unit mass as coal.
5)
It can convert into other forms of energy more efficiently than other fuel.
1.2 Pathways of Hydrogen Production:
Hydrogen can be produced from a variety of sources, including fossil fuels;
renewable sources such as wind, biomass, nuclear or solar heat-powered thermo
chemical
reactions,
solar
photolysis
or
biological
methods
and
photoelectrochemical method which utilize solar energy & water [12] [Figure 1.1].
Fossil fuels and other carbon fuels can be decarbonize to produce hydrogen.
Decarbonizing the fuels to produce pure hydrogen greatly reduces local air
pollutants, separates the carbon dioxide (which could possibly be isolated from the
atmosphere), and also allows the energy to be used more efficiently, since the
chemical process inside a fuel cell is more efficient that combustion. Steam
Methane Reforming (SMR) and coal gasification are currently used to produce
more hydrogen for chemical industries than any other technique. However, natural
gas prices are volatile (and rising); efficiency of conversion to hydrogen is 80-90%.
Coal gasification is slightly more expensive than SMR, but it offers several
advantages. Coal prices have historically remained relatively stable (and lower than
natural gas), and there is a vast supply of coal in many regions of the world.
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Introduction
Carbon dioxide can be easily separated from the feedstock and local air pollutants
are low in both cases. Another approach is biomass gasification––heating organic
materials such as wood and crop wastes.
Hydrogen Production Methods
Diesel
Natural Gas
Biomass
Coal
Biomass
Solar energy
Nuclear Energy
Solar Energy
Nuclear Energy
Solar Energy
Hydro Energy
Wind Energy
Wave Energy
Geothermal Energy
Biofuels
Reforming
Gasification
Thermo-chemical splitting of
water
Electrical
energy
Electrolysis
Fig. 1.1: Methods of hydrogen Production
Biomass hydrogen production approach includes thermochemical and biological
methods [13, 14]. Thermochemical methods refer to those processes that involve
thermal heating of biomass, such as energy crops, agricultural residues and wastes,
forestry waste and residues, or industrial and municipal wastes, to produce
hydrogen. Unfortunately, carbon monoxide and carbon dioxide are common side
products in these processes, which are not desired environmentally. Biological
methods are often based on microbial processes for hydrogen production with or
without the presence of light.
One of the most promising approaches is a photobiological method that involves
microorganisms such as green algae and cyanobacteria. These microorganisms can
perform photosynthesis by using solar as the energy source and water as the
electron donor to produce hydrogen with the help of H2 producing enzymes, such as
hydrogenases or nitrogenases [15, 16]. Although these biophotolysis methods are
very attractive, they are still at the early stages of research. At present, the
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Introduction
underlying mechanism is not fully understood, and the light-to-hydrogen
conversion efficiency is still low. Thus, the overall cost is high, which limits the
feasibility of these methods.
Motivated by the idea of photosynthesis, hydrogen generation from water splitting
(artificial photosynthesis) has also attracted a lot of attention [17-21]. This is a clean
reaction involving water as the primary reactant, which is abundant on earth. Water
splitting is a thermodynamically uphill or endothermic process, H2O → H2 + 1/2O2
(∆G ∼ 237.2 kJ/mol; E0 = 1.23V vs. Normal Hydrogen Electrode, NHE), and a
minimum potential of 1.23V is needed for the reaction to proceed. Considering the
recombination of photo excited electron-hole pairs and imperfection of devices such
as contact and electrode resistances, the optimal energy required for water splitting
is around 2 eV. The energy required for water splitting can be obtained from
renewable or non-renewable sources. It is well known that hydrogen can be
generated from electrolysis using an electrochemical cell if enough potential is
applied. However, this is just a process that transforms electricity into chemical
energy in the form of hydrogen but does not generate hydrogen from renewable
resources. Furthermore, due to the imperfection of devices, electrolysis involves
energy loss with a cell efficiency limit around 80% [22]. A better solution will be
the combination of a photovoltaic (PV) cell with an electrochemical cell. In this
case, the light harvested by the PV cell can be used to supply the requested energy
for electrolysis. Nevertheless, the efficiency of this two-step process is expected to
be low, given the typical efficiency of commercially available PV devices of ∼10–
15% and the energy loss in electrolysis. In addition, the relatively high cost of a PV
cell is another major drawback of this approach. In this regard, a
Photoelectrochemical (PEC) cell consisting of semiconductor photoelectrodes that
can harvest light and use this energy directly for splitting water is a more promising
and cost-effective way for hydrogen generation.
1.3 Photoelectrochemical Splitting of Water
Photoelectrochemical (PEC) water splitting, using sunlight to break apart water
molecules into constituent hydrogen and oxygen gases, remains one of the “holy
grail” technology for clean and renewable hydrogen production. It has been a goal
of scientists and engineers since 1972, when Fujishima and Honda reported the
generation of hydrogen in a photoelectrochemical cell with titanium dioxide
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Introduction
electrode illuminated with near ultraviolet light [18]. A PEC system combines the
harnessing of solar energy and the electrolysis of water into a single semiconductorbased device. Sunlight plus water gives us clean hydrogen plus oxygen. It sounds
good, but it’s not all that easy. When a PEC semiconductor device is immersed in a
water-based solution, solar energy can be converted directly to electrochemical
energy for splitting the water. This will happen only however, if all key criteria are
met. The semiconductor material must efficiently absorb sunlight and generate
sufficient photovoltage to split water, while the semiconductor interface must be
favorable to sustaining the hydrogen and oxygen gas evolution reactions. In
addition, the PEC system needs to remain stable in solution, and must be cheap for
any large-scale deployment.
1.3.1 PEC Water-Splitting Reactions
PEC system for water splitting with simple two-electrode setup has been shown in
Figure 1.2. In this canonical model, a light-sensitive semiconductor photoelectrode
is immersed in an aqueous solution, with electrical wiring connected to a metallic
counter-electrode. With exposure to sunlight, 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
(OER) at the anode surface, while photoexcited electrons drive the hydrogenevolution reaction (HER) at the cathode surface. Figure 1.2 depicts a photoanode
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 at the counter electrode. Similar to solid-state pn-junction solar
cells, PEC photoelectrodes typically act as minority carrier devices [23, 24]. The
semiconductor/liquid junction, like the p-n junction, allows the flow of minority
carriers,
while
blocking
majority-carrier
flow.
For
this
reason
n-type
semiconductors allowing minority-carrier hole injection are better suited as
photoanode, while p-type semiconductors are used as photocathode.
For semiconductor material, the reaction is expressed as:
e-SC + þ+SC
SC + hυ
7
1.1
Praveen Kumar
Introduction
Fig.1.2: Standard two electrode setup for PEC water splitting, shown in the photoanode
configuration with a separated counter electrode.
These electrons-holes
holes get separated and migrate to the surface of the semiconductor
(hole) and to the counter-electrode
counter
(electron) without recombining. At the
photoanode (semiconductor working electrode), oxidation of water molecule by
holes forming oxygen gas (equation 1.2).
.2). At the cathode (metal counter-electrode),
counter
the free electrons react with water molecules to reduce the H+ and produce
hydrogen gas (equation 1.3).
2h+ + H2O (liquid)
1/2O2 (gas) + 2H+
1.2
H2 (gas)
1.3
2H+ + 2e-
Accordingly, the overall reaction of the photoelectrochemical splitting of water may
be expressed in the form
2hυ +H2O (liquid)
1/2O2 (gas) +H2 (gas)
1.4
This water splitting reaction takes place when the energy of the photons absorbed
by the photo-anode is equal to or larger than Et, the threshold energy:
1.5
Where ∆GoH2O is the standard free enthalpy per mole of equation 1.4, which is
equal to 237.141 kJ/mol; NA is Avogadro’s number is equal to 6.022 × 1023 mol-1).
This yield
Et = hυ = 1.2289 eV
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1.6
Praveen Kumar
Introduction
Thus, the water splitting reaction requires energy (Gibb’s free energy at 298 K) of
237.141 kJ mol-1, or 2.4578 eV per H2 molecule produced. Therefore, the energy
required for water splitting is 1.2289 eV (usually rounded to 1.23 eV) per electron
as shown in Figure 1.3. A photon with energy of 1.23 eV has a wavelength around
1000 nm (near infra-red), so theoretically the entire ultraviolet and visible parts of
the solar spectrum are available for solar water splitting. However, in practice
around 1.8 eV (cut-off wavelength around 685 nm) is required. The extra energy is
to account for unavoidable loss mechanisms such as electrode over potentials.
Energy
Level
hυ ≥ Eg
H2
EH2/H2O
1.23 ev
e-
H2 O
EO2 /H2O
H2O
Ev
O2
Fig. 1.3: Mechanism of semiconductor photoelectrolysis for hydrogen
production
1.3.2 Fundamental Process Steps
Fundamental process steps have been summarized as follows:
Photon Absorption/Charge Generation (solid-state):
In single-junction absorbers, photons with energies below the semiconductor
bandgap cannot be absorbed or converted. Photons with energies exceeding the
bandgap are absorbed at rates dependent on the allowed transitions in the
semiconductor. Direct bandgap materials absorb more efficiently than indirect
bandgap materials. Photogenerated electron hole pairs rapidly thermalize (usually
within picoseconds) to band-edge energy levels, losing energy to heat. Highbandgap semiconductors generate little photocurrent due to poor absorption, while
low bandgap semiconductors can suffer from low conversion efficiency due to high
thermalization losses.
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Introduction
Charge Separation and Transport (solid-state/interface):
While at band-edge energy states, the electron hole pairs can often survive for
several microseconds before recombining. During this time, they must be separated
and transported to electrochemical interfaces. This separation is assisted by the
electric fields set up by charge distributions in the semiconductor and at the solid/
liquid interface. Defects in the bulk and at the interface can adversely affect the
separation fields, and also result in poor mobility for charge transport. If wide
absorption widths are needed (for example, in indirect semiconductors) the charge
transport losses can be severe.
Charge Extraction/Electrochemical Product Formation (interface):
Ideally, charge is extracted via the water splitting half-reaction at the solid/liquid
interface. The extraction process can be slowed or completely inhibited by poor
energetic alignment or poor surface kinetics at the photoelectrode or counterelectrode surfaces. Moreover, parasitic or corrosion reactions competing with the
water-splitting reactions can result in substantial loss. Surface treatments can be
employed to kinetically and/or energetically favor water splitting over the parasitic
processes, but such treatments could also block sunlight. Surface incorporation of
nanoparticle catalysts is one approach. Since PEC water splitting is a low-currentdensity process (typically operating below 20mAcm-2), non-precious-metal
catalysts can be used. Additionally, nanostructuring of electrode surfaces can
increase effective surface area for enhanced charge extraction, although this can
also lead to higher surface recombination loss. On the solution side, the electrolyte
is an important factor determining stability, efficiency of the charge-extracting
reactions, and the electrochemical byproducts. Splitting seawater, for example, is a
challenge, since it is difficult to electrochemically suppress the production of
chlorine gas from the Cl ions [25].
Electrochemical Product Management (solution):
During PEC water splitting, the evolved hydrogen or oxygen gas must be efficiently
removed from the photoelectrode surface to avoid mass-transport losses in the
surface reactions, and to minimize adverse optical effects. Surfactants added to the
electrolyte have been successful in promoting rapid bubble formation and
dissipation. In solution, ionic conductivity losses tend to be a bigger problem. High
electrolyte concentrations can be used to minimize this loss, but the tradeoff is in
higher corrosivity. Photoelectrode geometry and counter-electrode proximity are
10
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Introduction
critical parameters to the redistribution of ions. In some geometry, gas-separating
membranes are needed, introducing further ionic-transport loss.
1.4 Solar Splitting of Water- Material Related Issues:
Selection of materials for its use as photoelectrodes in PEC cell to split water
efficiently using solar energy is crucial and requires satisfying several specific
semiconducting and electrochemical properties. The semiconductor should have
bandgap energy ~ 2 eV, strong optical absorption for all wavelengths up to the
band-gap energy, conduction and valence band edges straddle with water redox
potentials, efficient charge transfer between the semiconductor and electrolyte and
stability in strong electrolytes. These important requirements for an ideal
semiconductor have been discussed in detail as follows:
1.4.1 Band Gap:
The ideal material for use as a photoelectrode for water splitting should have a
bandgap energy Eg around 2 eV. The band gap of the photo-electrode has a critical
impact on the energy conversion of photons [26, 27]. Theoretically, the lowest limit
for the band gap of a PEC’s photo-anode is determined by adding energy required
to split the water molecule and over potential losses at junction, is 1.8 eV, which is
needed to be supplied by solar energy photons. The band-gap energy of a
semiconductor determines the fraction of the solar spectrum that can be utilized by
the material; only photons with energy greater than or equal to the bandgap energy
will excite valence band electrons to the conduction band. Figure1.4 illustrates the
solar energy spectrum, depicting segments defining phonon fluxes corresponding to
different energy ranges. As shown in Figure 1.4, the photon flux within the part of
the spectrum, represented by the integral of J1 −J2 is not available for conversion
owing to the theoretical energy limit of 1.23 eV [26]. The estimated value of these
combined losses [thermodynamic losses (~0.4 eV) and the overpotentials that are
required at various points in the system to ensure sufficiently fast reaction kinetics
(~0.3–0.4 eV)] is ~ 0.8 eV. Therefore, the optimal energy range in terms of the
photons available for conversion is ~2 eV. This situation is represented in Figure
1.4 by the integral of J1−J3. In consequence, the energy corresponding to the photon
flux J3 is available for conversion. The availability of this energy is contingent upon
the use of a photoanode with band gap of 2 eV. The material that has been used
most frequently as a photoanode, due to its high corrosion resistance, is TiO2.
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Introduction
However, its band gap is 3 eV [18, 27-29] and, consequently, the part of the energy
spectrum available for conversion corresponds to photon flux J4. Thus, there is a
need to increase the amount of energy available for conversion from J4 to J3. This
can be done by processing a corrosion-resistant material, which is the challenge for
materials engineers.
Fig 1.4: 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.
1.4.2 Position of Band Edges in Semiconductors
The positions of the conduction band and valence band edges of the semiconductor
are important as they determine whether spontaneous water splitting will occur and
whether the material will be stable in a photoelectrochemical cell. Much higher
hydrogen production efficiency can be achieved if spontaneous water splitting
occurs, without the need to apply an electrical bias. A semiconductor capable of
spontaneous water splitting has a bandgap ≥ 2 eV with conduction band energy
higher than that of the H+/H2 redox potential, and valence band energy lower than
that of the O2/H2O redox potential as shown in Figure 1.5. Under these conditions
charge transfer between the semiconductor and the electrolyte is energetically
favorable. Most metal oxides have a valence band edge at a similar energy, well
below the O2/H2O potential [30]. Therefore, to have a conduction band edge above
the H+/H2 potential, the material will have a wide band-gap (over 3 eV). Narrower
bandgap materials with band edges that straddle the water splitting redox potentials
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Praveen Kumar
Introduction
(such as CdS and CdSe) are unstable. Maeda and Domen [30] suggest that
oxynitride and oxysulphide materials may be more suitable for spontaneous water
splitting than metal oxides, as their valence band edges are closer to the O2/H2O
potential, and those with a suitable band-gap around 2 eV are able to have band
edges that straddle the water splitting potentials.
Fig. 1.5: A semiconductor capable of spontaneous water splitting has a bandgap ≥ 2 eV with
conduction band energy Ec higher than that of the H+/H2 redox potential, and valence band
energy Ev lower than that of the O2/H2O redox potential.
1.4.3 Stability of the Electrodes:
A semiconductor suitable for use as a photoelectrode for water splitting must be
resistant to corrosion and photocorrosion in the presence of the electrolyte in the
photoelectrochemical cell. Therefore, it is essential for the photoelectrode to be
resistant to these types of undesired relativities. Corrosion can be described as an
increased reactivity of the surface atoms which increases the interaction with
reagents in the electrolyte, resulting in dissolution of the semiconductor [31].
Strong electrolytes are required to minimize resistive losses (provide good charge
conduction) between electrodes in the cell. A material will corrode if the free
energy (Fermi level) of electrons or holes exceeds the value where cathodic and
anodic processes would be in equilibrium. Under illumination, decomposition is
also possible due to photogenerated minority carriers. Figure 1.6 shows the stability
conditions for electrolytic decomposition of semiconductors [31], illustrating the
relationship between, the decomposition energy of the electrons (nEdecomp), the
decomposition energy of the holes (pEdecomp), the conduction band energy (Ec), and
13
Praveen Kumar
Introduction
the valence band energy (Ev), for materials that are (A) stable, (B) unstable, (C)
stable against cathodic decomposition, (D) stable against anodic decomposition.
Thus, 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.
Fig. 1.6: Stability conditions for electrolytic decomposition of semiconductors, reproduced
from Gerischer, 1985 (1). (A) stable, (B) unstable, (C) stable against cathodic decomposition,
(D) stable against anodic decomposition
1.4.4 Semiconductor – Electrolyte Interface:
When a semiconducting electrode comes into contact with electrolyte in a
photoelectrochemical cell a complex interface is formed. The properties of this
interface are critical to the cell performance and various photoelectrochemical
experiments can be performed to elucidate the nature of the interface. Several
review articles give detailed descriptions of the semiconductor-electrolyte interface
[32-36]. For n type semiconducting electrode(photoanode) submerged in an
electrolyte with a metal electrode acting as the counter-electrode (cathode) the
semiconductor-electrolyte interface are discussed by four electrochemical
conditions as shown in Figure 1.7: (A) initial condition before equilibrium is
reached, (B) equilibrium condition in the dark, (C) illuminated condition, and (D)
illuminated condition with a bias voltage applied.
In Figure 1.7 (A) it is assumed that the metal cathode is in equilibrium with the
electrolyte, therefore the Fermi levels of the electrolyte and metal are the same. At
equilibrium in the dark (Figure 1.7 (B)) the Fermi level of the semiconductor
(electrochemical potential of the electrons) equilibrates with that of the electrolyte
by flow of electrons from the semiconductor to the electrolyte, resulting in a region
depleted of electrons at the semiconductor surface known as the depletion layer or
14
Praveen Kumar
Introduction
space charge layer [36]. This positively charged region attracts negatively charged
ions in the electrolyte, which form a very thin (< 1 nm) Helmholtz layer. The
equilibration of the electrochemical potentials of the electrolyte and the
semiconductor leads to “band bending” of magnitude VB. When the
photoelectrochemical cell is illuminated, Figure 1.7 (C), charge carriers are
generated, which are separated by the electric field in the space charge layer. In the
case of an n-type semiconductor, the electrons move into the bulk and the holes
migrate to the electrolyte interface. A photovoltage is generated and the Fermi level
is moved upward toward the flat-band potential Vfb (the potential of the
semiconductor when at a condition of zero charge). The band bending is reduced as
a result of electron-hole pairs being generated by the absorbed photons. Under these
conditions no current is flowing. The over potential of an electrode is the difference
in potential of an electrode at equilibrium (with no current flowing) and when
current is flowing. It is a measure of the additional energy required to drive the
reaction. The value of the over potential will depend on the magnitude of the energy
barriers involved in the chemical reactions, arising from factors such as the
thermodynamics, kinetics, and charge carrier concentration differences between the
solution and the interface [35, 36]. The total over potential η is the sum of the over
potential across the depletion region ηd and that across the Helmholtz layer ηH. The
voltage drop in the electrolyte can be assumed to be small (in concentrated
solutions) and can be ignored in most cases. The interfacial activation energies for
electron and hole transfer are related to the over potentials. Under illuminated
conditions, Figure 1.7 (C), the maximum Fermi energy possible is the flat band
potential, which is still below the H+/H2 redox potential, so hydrogen generation is
not possible. When a bias voltage V bias is applied, Figure 1.7 (D), the Fermi
energy in the metal electrode is raised above the H+/H2 potential, allowing the water
splitting reaction to proceed. If a bias voltage needs to be applied the efficiency of
the water splitting is reduced. The properties of the semiconductor photoelectrode
are critical to determining the efficiency of the water splitting process.
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Introduction
Fig. 1.7: Band diagrams of a two-electrode photoelectrochemical cell. (A) The system before
the semiconductor-electrolyte interface is formed. (B) The semiconductor in equilibrium with
the electrolyte. (C) The semiconductor is illuminated. (D) The semiconductor is illuminated
and a bias voltage is applied. Eg is the semiconductor band-gap, EF represents the Fermi
energies, Vfb is the flat-band potential, V bias is the bias voltage, VB is the band bending, V
Helmholtz is the potential of the Helmholtz layer, and V photo is the difference between the
Fermi energies of the semiconductor and the electrolyte when the semiconductor is
illuminated. Reproduced from Nozik and Memming [35]
1.5 Metal Oxides Suitable 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 metal oxide semiconductor
photoelectrodes. After the pioneering work of Fujishima and Honda in 1972,
16
Praveen Kumar
Introduction
showing the possibility of electrolyzing water into hydrogen using solar illuminated
TiO2 semiconductor electrode, attention of several investigators was directed to
other such metal oxides, namely WO3, ZnO, Cu2O, α-Fe2O3 etc. But so far, no
material has been found to exhibit efficient splitting of water because of either
having too large bandgap or poor semiconductor characteristics or being chemically
unstable in electrolyte. For example 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) [18]. Tungsten oxide (WO3) shares many of the same attributes with
TiO2 in terms of chemical inertness and exceptional photoelectrochemical and
chemical stability in aqueous media over a very wide pH range. However, its flatband potential (Vfb) lies positive of that of TiO2 (anatase) such that spontaneous
generation of H2 by the photogenerated electrons in WO3 is not possible. Also
because of its relatively large band gap (Eg: 2.6 to 3.0 eV) [37], it absorbs the solar
spectrum near the ultraviolet and blue regions. While ZnO has enjoyed extensive
popularity in the photochemistry community (even comparable to TiO2 in the early
days prior to ~ 1980), it is rather unstable under illumination and in the oxygen
evolution reaction (OER) and hydrogen evolution reaction (HER) regimes [38]. The
report of photocatalytic water splitting on Cu2O powder suspensions has been
greeted with skepticism by others who have also pointed out that the Cu2O bandedges are unlikely to bracket the H+/H2 and O2/H2O redox levels as required [39].
α-Fe2O3 is an attractive candidate for the photoelectrolysis of water due to low band
gap value, good photoelectrochemical stability and chemical inertness. However
it’s photoelectrochemical activity is limited by several key factors such as relatively
poor absorptivity very short excited-state lifetime (∼10-12 s), poor oxygen evolution
reaction kinetics, and a short hole diffusion length (2-4 nm) [40]. Hence the
resulting low efficiency of solar hydrogen production of metal oxides in existing
systems, are indeed the major impediments in the commercial viability of
photoelectrochemical splitting of water.
1.5.1 Nanostructured Metal Oxide for PEC Water Splitting
As mentioned in earlier section that efficiency of PEC devices is limited by
properties of semiconductor metal oxides such as limited light absorption efficiency
in the desired visible region and the recombination of photoexcited electrons and
holes and instability in electrolytic solution etc. Nanomaterials with their unique
physical, chemical, electrical and electronic properties have attracted significant
17
Praveen Kumar
Introduction
attention in recent years, due to their potential applications in various technologies
including energy conversion. Photoelectrodes based on nanostructured metal oxides
such as ZnO [41-43], WO3 [44-50], Fe2O3 [51-53], TiO2 [54–56]etc. including zero
dimensional (0D) nanocrystals and one dimensional (1D) nanorods and nanotubes
have been found to offer additional advantages over their bulk counterparts in PEC
splitting of water for hydrogen generation due to many reasons:
1. Nanostructured photoelectrodes in PEC cell provide extremely large surface
area for the redox reactions to take place, which could significantly increase
the efficiency [57].
2. Additionally solar energy absorption also increases with increased specific
area provided by nanomaterials [58].
3.
Electron-hole overlap factor and electron-hole exchange interaction
increase greatly due to quantum size confinement in nanomaterials, resulting
in increased bandgap energy as compared to bulk materials [59].
4.
Fundamental optical and electronic properties can be designed and
modified through controlled variation of nanomaterial structure. For
example, the band gap of semiconductor nanocrystals can be tuned by
varying their size to increase the light absorption in the solar spectrum.
5. Moreover, the separation of electrons and holes would be significantly
enhanced in nanorods if their diameters are comparable to the width of the
depletion layer [60].
6.
Unique bottom-up synthetic strategy allows the growth of single-crystal.
Nanomaterials on different substrates without the formation of dislocations
due to the lattice mismatch between substrate and semiconductor. These
high-quality nanostructured photoelectrodes with low density of defects
reduce trapping or recombination of electrons and holes [61].
7. In comparison to the conventional planar PEC electrode, as the coating of
nanomaterials on conducting substrates naturally forms an antireflection
layer, the energy loss due to light reflection can be reduced [62].
With these potential advantages, nanostructured semiconductor photoelectrodes
could fundamentally change the design of PEC cells and improve the solar to
hydrogen conversion efficiency.
18
Praveen Kumar
Introduction
1.6 Material of Interest: Hematite (α-Fe2O3):
Hematite is the material of investigation in present thesis as it fulfills most of the
requirements of a good photocatalysts material such as appropriate bandgap,
chemical and photoelectrochemical stability, low cost, and ease of fabrication [63].
However, solar to chemical conversion efficiencies reported for hematite are
relatively low [64] due to some drawback. General properties of hematite have been
specified below:
1.6.1 Crystalline Structure:
Iron is the fourth most common element in the earth’s crust (6.3% by weight) and
because iron is readily oxidized in air to the ferrous (+2) and ferric (+3) states, iron
oxide is ubiquitous. Since the ferrous and ferric forms of iron are separated by a
relatively small energy difference, many well-defined crystalline forms of iron
oxide and oxyhydroxide exist in nature.
Hematite is the most thermodynamically stable form of iron oxide under ambient
conditions and as such, it is also the most common form of crystalline iron oxide.
The iron and oxygen atoms in hematite arrange in the corundum structure, which is
trigonal-hexagonal scalenohedral with space group R-3c, lattice parameters
a=5.0356 Å, c= 13.7489 Å, and six formula units per unit cell [65]. It is easy to
understand hematite’s structure based on the packing of the anions O2- which are
arranged in a hexagonal closed-packed lattice along the [001] direction. The cations
(Fe3+) occupy the two-thirds of the octahedral interstices (regularly, with two filled
followed by one vacant) in the (001) basal planes, and the tetrahedral sites remain
unoccupied. The arrangement of cations can also be thought of producing pairs of
FeO6 octahedral that share edges with three neighboring octahedral in the same
plane and one face with an octahedron in an adjacent plane in the [001] direction
(Figure 1.8). Hematite is antiferromagnetic at temperatures below 260 K and a
weak (parasitic) ferromagnet at room temperature. The latter is due to the
ferromagnetic coupling of the spins within the (001) basal planes and
antiferromagnetic coupling between iron layers along the [001] direction [66].
19
Praveen Kumar
Introduction
Fig.1.8: The unit cell (left) of hematite shows the octahedral face-sharing Fe2O9 dimers
forming chains in the c direction. A detailed view (right) of one Fe2O9 dimer shows how the
electrostatic repulsion of the Fe3+ cations produce long (light grey) and short (dark grey) Fe-O
bonds.
1.6.2 Optical Characteristics:
The absorption of photons by hematite begins in the near-infrared spectral region
where weak absorption bands (with absorption coefficients, α, of the order 103
cm-1) are due to d–d transition states between electron energy levels of the Fe3+ ion
[67]. While photoexcitation of hematite at these wavelengths has been shown in one
case to increase its conductivity [68], sustained photocurrent is not observed in a
photoelectrochemical system upon irradiation below the bandgap energy, Eg
(which, depending on the method of preparation of hematite, is usually reported to
be between 1.9 and 2.2 eV corresponding to λ=650 to 560 nm) [69]. Hematite’s
strong absorption of yellow to ultraviolet photons in the visible region and
transmission of orange to infrared photons gives it a characteristic red color. Since
the electronic nature of the bandgap in hematite is of great interest to understand its
performance as a material for solar energy conversion, much work has focused on
this aspect. The Tauc analysis of the bandgap absorption onset, assumes that the
energy bands are parabolic with respect to the crystal momentum, most frequently
indicates an indirect (phonon-assisted) bandgap transition [70]. However, a few
recent reports of a direct bandgap in hematite have been attributed to quantum sizeeffects [71, 72]. The initial orbital assignments of the bandgap suggested it was due
to an indirect transition of Fe3+ d–d origin, [68, 73] and that a stronger direct
transition involving a charge transfer from an O2p orbital to Fe3d did not occur
20
Praveen Kumar
Introduction
until 3.2 eV [73]. This led to the hypothesis that two different types of p-type
charge carriers (holes) could be produced in hematite, depending on the excitation
mechanism,
and
was
responsible
for
the
observed
difference
in
photoelectrochemical (PEC) performance as a function of wavelength [74, 75].
1.6.3 Conductivity Mechanism in Hematite:
Hematite has very low electrical conductivities (ca. 10-14 Ω-1cm-1) with conduction
election concentrations of 1018 cm-3 at 1000 Kelvin, and electron mobility on the
order of 10-2 cm2 V-1 s-1 [68, 69]. These unusually small values obliged electrical
conduction to be explained by Fe3+/ Fe2+ valence alternation on spatially localized
3d orbital. Further studies of hematite single crystals identified an anisotropic
conductivity of hematite, which is up to 4 orders of magnitude higher within the
(001) basal plane (e.g., in [110] direction) than orthogonal to them[78-79]. As such,
conduction in the [001] direction could only involve the movement of holes in the
form of Fe3+
Fe4+ electron transfer. This process is significantly slower in
hematite [80]. Conduction mechanism and observed anisotropy are clearly
important for orientating hematite crystals in a photoelectrode, its intrinsic
conduction properties have been shown to be inadequate for PEC applications. Its
conductive properties must be significantly enhanced by adding impurities to act as
electronic dopants. Indeed, it is possible to increase conductivities and obtain both
p-type or n-type hematite by substitutional doping using atoms such as Mg2+, Cu2+
(p-type) or with Ti4+, Sn4+, Zr4+, Nb5+ (n-type)[80] By substituting at sufficient
levels, high carrier conductivities can be attained. For example, Zr4+ was doped into
single crystals to give donor densities on the order of 1019 cm-3, conductivities
around 0.1 Ω-1cm-1, and increased electron mobility (perhaps due to an increase in
dielectric constant) of 0.1 cm2V-1 s-1 [81]. Substitutional dopants used for n-type
iron oxide and their affect on electronic and photoelectrochemical properties has
been very well presented by Shinar and Kennedy in their review article [82].
1.6.4 Hematite as Photoelectrode
Very soon after the first report of water splitting using TiO2 [18], Hardee and Bard
[83] were first to study Fe2O3 as a material for water photolysis in 1976, seeking a
photoanode material that was both stable under anodic polarization and capable of
absorbing light with wavelengths longer than 400 nm. However, it was quickly
realized that hematite gave very low photoconversion efficiencies. This was
attributed to poor optical absorption [84], rapid electron-hole recombination
21
Praveen Kumar
Introduction
resulting in short diffusion lengths of charge carriers [84], slow surface reaction
kinetics, and unfavourable band-edge positions [85] (meaning an electrical bias is
required). Thus, with the hematite as photoelectrode in PEC cell, reported water
splitting efficiencies have not yet come close to the theoretical maximum efficiency
for this material of 12.9 % [64] and lot of work is being carried to overcome these
shortcomings by modifying properties of the material using various strategies [71,
88-93]. Some important ones have been described in the next section.
1.7 Modifications in Hematite Photoelectrode
In the direction of improving PEC response of hematite various strategies have been
studied in literature like tailoring the hematite structure in nanodimension [71], by
surface modification [94], layering of other metal oxides [95], modification by swift
heavy ions irradiation [93], and doping with heteroatom like Si, Pt, Ti etc.
[91,92,96,97].
1.7.1 Use of Nanostructures:
The ability to control the particle size and morphology of nanoparticle
semiconductor has crucial importance in PEC cells. These electrodes are commonly
referred as porous electrode and have high surface-to-volume ratio, where the
effective surface area can be enhanced by1000-fold [98]. It provides the large
contact area between semiconductor and electrolyte, therefore, better and faster
process of transfer of carriers obtained at the interface in PEC cell. Besides, the
porous structure, nanostructured semiconductor electrodes enables the electrolyte to
fully penetrate the electrode [99]. An obvious solution to the problem of majority
carrier transport in hematite films created from the colloidal approach is to use
nanometer-sized rod or wire arrays. An array of single-crystal nanorods with
diameters in the 10 nm range, attached and oriented orthogonally to a conducting
substrate would eliminate grain boundaries, and provide a direct path for electron
collection while still allowing photogenerated holes to efficiently reach the SCLJ.
Hagfeldt et al first time reported a simple method to create hematite arrays on a
variety of substrates from the controlled precipitation of Fe3+ in aqueous solution
and investigated for water photoelectrolysis soon after [100]. A report [101]
examining the surface photovoltage on electrodes prepared in the same way,
suggests that bulk or surface defects are the major factors limiting the performance
of hematite prepared by this route. Another facile method to produce hematite
22
Praveen Kumar
Introduction
nanowires is the simple thermal oxidation of iron foils and has been reported by
many groups [102-105]. These nanowire arrays have large surface area, sufficient
light absorption and a direct path for the conduction of electrons to the substrate
(since basal planes are oriented perpendicular to the substrate [105], making them a
very attractive morphology for hematite.
The recent development of nanostructuring techniques using potentiostatic
anodization has provided another possible route to create structured hematite
photoelectrodes. Prakasam et al. first showed that iron foils could be nanostructured
using anodization in a glycerol-based electrolyte containing 1% NH4F+1%
HF+0.2% HNO3.[106]. Ordered nanopores were observed with pore size ranging
from 20 to 250 nm and depths up to 600 nm depending on the anodization voltage
and time. Under simulated solar illumination these photoanode produced a
photocurrent of 0.05 mAcm-2 at 0.4 VSCE in 1M NaOH (1.45 VRHE). Work by the
same group on anodized Ti-Fe-O electrodes is notable here due to the high
photocurrent densities reported (1.1 mAcm-2 at 1.4 VRHE in 1M NaOH) despite the
presence of both hematite and rutile in the photoanode prepared at the optimized
conditions [107]. In contrast, very well defined nanotube arrays of pure iron oxide
created from iron foils have been subsequently reported by a different research
group [108]. In this work, a single anodization step with 0.1M NH4F+3 vol% water
in ethylene glycol created nanotubes with walls less than 50 nm and lengths of
about 1.5 mm. After an optimized annealing treatment these electrodes were found
to be a mixture of both hematite and maghemite by XRD and had only small
photocurrents (160 mAcm-2, 1.23 VRHE under AM1.5 illumination compared to
120 mAcm-2 dark current).
Nanostructured
hematite
photoanode
also
have
been
prepared
by
the
electrodeposition of precursors from solution. Recently McFarland and co-workers
reported a method to deposit iron hydroxides from FeCl3 solutions under cathodic
polarization. The subsequent annealing at high temperature (700 oC) then resulted
in porous hematite films [96,109]. This method readily allows for the incorporation
of dopants which were found to have an effect on the morphology of the sintered
electrodes. In a subsequent report, an isovalent substitutional dopant Al3+ was
added to the hematite to modulate the lattice strain a factor predicted to benefit
polaron migration and offer a novel way to increase conductivity [109]. Another
example of electrodeposition has been recently reported by Spray and Choi [110]
23
Praveen Kumar
Introduction
using an anodic electrodeposition. They were able to demonstrate impressive
morphology control ranging from wires arrays to porous films by varying the
solution pH. The films were photoactive in an electrolyte containing iodide, but
water oxidation photocurrents were not reported suggesting surface recombination
issues. In general, while impressive morphologies can be obtained with
electrochemical deposition techniques, water splitting photocurrents have been
limited by the quality of the material produced.
1.7.2 Doping in Hematite:
In a metal oxides doping play distinct role, as the chemical composition of metal
oxides can be altered by doping. Doping of hematite has been investigated by many
workers in an attempt to yield a reasonable photoelectrochemical performance. The
introduction of group I A or IV B dopants such as Ge, Pb, Sn, Si and Ti produce ntype material while group II A, I B or VII B dopants such as Ca, Cu, Mg and Ni
produce p-type material [111]. Shinar and Kennedy [82] published a good review of
doped hematite for water splitting, and presented photoelectrochemical data for
sintered polycrystalline hematite doped with ZrO2 (IV B), HfO2 (IV B), CeO2, V2O5
(V B), Nb2O5 (V B), Ta2O5 (V B), WO3 (VI B) and Al2O3 (III A), as well as suboxide materials. Of the many dopants introduced into hematite, tetravalent like Zr4+,
Si4+, Ti4+, Pt4+, Sn4+ dopants promising for enhancing the photoelectrochemical
response[112,91,92,97,96,113]. They act as donors in hematite, thereby enhance the
conductivity of the material by substitutionally replacing Fe3+.
Si doping in
hematite has been reported to exhibit significantly enhanced photocurrent, which
was attributed to typical interesting nanostructured morphology [91, 92]. Si-doped
hematite nanocrystalline films prepared by APCVD produced a benchmark
photocurrent density of 2.7 mA/cm2 at 1.23 V/ RHE under a simulated solar light of
100 mW/cm2, due to the formation of hematite dendrites with largely increased
surface area, which reduced the hole diffusion length and thereby the electron hole
recombination.
1.7.3 Surface Modification:
Based on the flatband potential (Vfb) usually reported for hematite, an external bias
of only 0.3–0.4 V RHE should be necessary to initiate the water splitting reaction
[69]. Once the applied bias is greater than Vfb, the band bending drives
photogenerated holes to the semiconductor liquid junction (SCLJ). However, the
onset of water oxidation photocurrent is usually not observed until 0.8–1.0 V/RHE
24
Praveen Kumar
Introduction
even at a high pH of 13.6 (1M NaOH) and for single crystal electrodes. The
remaining over potential of ca. 0.5–0.6 V is a major drawback for the
implementation of hematite-based tandem cells [114] and has been attributed to two
distinct surface properties. Firstly there is evidence that mid-bandgap energy states
resulting from both oxygen vacancies [94] and crystalline disorder [92] can trap
holes at the surface. This can even result in Fermi level pining in some electrodes
[90]. Secondly, the oxygen evolution reaction (OER) kinetics is sluggish, as
compared to other oxides semiconductors [114]. This may be due to the increased
Fe3+ character of the valence band compared to other oxides [115].
To overcome the limitation of poor OER kinetics, various catalysts have been
attached to the surface of hematite photoanode. For example, water oxidation by
cobalt has been extensively studied and is known to be particularly rapid [116]. The
treatment of Fe2O3 photoanode (prepared by a CVD method) with a monolayer of
Co2+ resulted in a ca. 0.1 V reduction of the photocurrent onset potential [91]. Since
this treatment also increased the plateau photocurrent. It was good evidence that the
reaction rate was increased, and the Co2+ did not just fill surface traps. The
application of a recently-reported amorphous cobalt-phosphate (Co-Pi) based water
oxidation catalyst [117] on Fe2O3 gave a composite photoanode with a similar
photocurrent onset potential to that of the Co2+ treatment [118]. However, the
increased efficacy of the Co-Pi catalyst at more neutral pH afforded a noticeable
enhancement of the over potential reduction at pH 8 [119]. A drawback of this
approach is the unproductive light absorption by the Co-Pi catalyst, which allows
only a thin layer to be deposited.
The material often reported as the most effective catalyst for the OER is IrO2[120121].The application of IrO2 nanoparticle to the surface of hematite by
electrophoretic deposition resulted in an impressive shift of the photocurrent onset
by about 200 mV giving J=0.3 mAcm-2 at 0.9 V and 1.16 mAcm-2 at 1.0 V/RHE
[122].
1.7.4 Layering of Other Metal Oxides:
The layering of other metal oxide on hematite thin films to prepare composite
semiconductor thin films of different bandgap energies have gained considerable
interest on account of its modified optical and charge transportation properties . It is
well accepted that the wide band gap semiconductors generate a high photovoltage
but have low photocurrent. Smaller band gap semiconductors can utilize a larger
25
Praveen Kumar
Introduction
fraction of the incident photons but generate lower photovoltage [123]. Therefore, it
is believed that a device having multiple band gap energy layers can cover broad
range of solar spectrum. Very recently Sharma et al reported a combination of
α-Fe2O3 and TiO2 which provided a better and efficient PEC system for generation
of hydrogen as compared to single hematite photoelectrode [95].
1.7.5 Swift Heavy Ion Induced Modification of the Materials
Swift heavy ion (SHI) irradiation is an effective tool to modify materials, in which
ions having velocity comparable to or higher than the orbital velocity of the lattice
atoms dissipate their energy predominantly by electronic excitation and ionization
of the target atoms [124]. The rapid energy transfer during the electronic excitation
can result in a variety of effects in materials including defect creation, defect
annealing, crystallization, amorphization, etc [125]. Several models such as thermal
spike, Coulomb explosion and lattice relaxation have been proposed to explain the
transfer of the electronic energy to the lattice during ion beam irradiation [126-128].
SHI irradiation is now known to affect the photoelectrochemical properties of the
metal oxides through some of the modifications stated above. Effect of 170
MeVAu13+ ion irradiation on undoped and doped metal oxide thin films for
photoelectrochemical splitting of water was first time reported by Chaudhary et al
[129,130]. Significant improvement in photoresponse of hematite and TiO2 thin
films irradiated with 120 MeVAg9+ ions has also been reported by Singh et al
[93,131].
Finally through the review on modification strategies of hematite use as
photoelectrode in PEC cell for solar generation of hydrogen resulted in the
following important observations:
1. Electrodeposition technique is a simple technique, which has the potential in the
preparation of nanostructured doped/undoped thin films of hematite with
desired particle size and porous morphology and desired dopant by co
deposition of dopants [96,109]. This technique has not been well studied in PEC
splitting of water.
2. Hematite is a semiconductor with short hole diffusion length. Hence
nanostructuring in hematite is expected to improve photoresponse because small
sizes of nanomaterials reduce the distance for photogenerated holes to diffuse to
26
Praveen Kumar
Introduction
the
photoanode/electrolyte
interface
which
reduce
the
electron-hole
recombination [91, 92].
3. Spray pyrolysis economical technique for preparation of doped/undoped thin
films of hematite has been reported to exhibit reasonably good photoresponse as
compared to other methods [40, 52, 63].
4. Doping and SHI irradiation are known strong tools to modify properties of iron
oxides thin films and found to be effective in improving its photoresponse [91,
92, 112,113, 93, 129, 130].
5. Tetravalent ions doping like Zr4+, Si4+, Ti4+, Sn4+ Pt4+ etc has been found to be
effective in improving PEC response. They act as donors in hematite, thereby
enhancing the conductivity of the material [112,91,92, 97,113,96]
6. Si doping in Fe2O3 showed various attractive morphology, suitable in exhibiting
good photocurrent [91, 92].
7. Zr doped Fe2O3 although exhibited significant photocurrent in pellet (bulk) form
but, have not studied in nanostructured thin film form [81, 82].
8. Trivalent dopant like Al3+ form oxide, isostructural with hematite, thereby
directing the crystallinity of the thin films. Isovalent Al does not make a
significant contribution to the electronic structure around the band edges
because no change in Fe3+ is produced by Al3+ [109,110].
9. Irradiation of SHI on the thin films of nano hematite is effective in enhancing its
PEC response by modifying the morphology of the films [129-131].
With this motivation, present thesis has been focused on the development and
characterization of nanostructured hematite thin film for its successful use in
photoelectrochemical splitting of water using solar energy. The study utilizes two
simple and economical techniques electrodeposition and spray pyrolysis for
preparation of nanostructured photoelectrodes. Properties of hematite thin film
were modified by using three dopants Zr, Si and Al at different doping levels and
various parameters related to hydrogen production were investigated. Swift heavy
ion (SHI) has also been used as tool to induce modifications associated with any of
the crystallographic, optical, electrical and morphological properties of
nanostructured hematite thin films, which may improve the PEC behavior.
27
Praveen Kumar
Introduction
1.8 Present Study and Objectives
The purpose of the present work was to synthesize undoped/doped/modified
nanostructured hematite thin films to investigate the photoelectrochemical behavior
of materials synthesized, towards splitting of water. The main stages of the present
work are:
1.
Preparation of nanostructured undoped and doped (e.g. Zr, Si and Al)
hematite thin films.
2. 100 MeV Si8+ ions irradiation on undoped and Zr doped hematite thin films
prepared in step I
3.
Characterization of unmodified/ modified thin films prepared for
a) Phase formation and particle size
b) Surface morphology
c) Optical absorption and band edge energy
d) Elemental composition
4.
PEC studies of unmodified/ modified thin films prepared in step I and step II
a) Current-voltage characteristics under dark and illumination to determine
I.
II.
III.
Photocurrent density
Nature of charge carrier
Resistivity
b) Mott-Schottky plot (1/C2 vs. Vapp) for the determination of
I.
II.
5.
Flatband potential
Carrier density
The samples exhibiting significantly good PEC response were finally
employed in PEC cell for
a) Solar to hydrogen (STH) conversion efficiency calculation
b) Collection and quantification of hydrogen evolved
28
Praveen Kumar
Introduction
1.9 Overview of the Study
Present PhD thesis mainly presents the development of nanostructured hematite thin
films used as photoelectrode for a PEC cell for efficient production of solar
hydrogen. The thesis is divided into 4 Chapters. A brief introduction of hydrogen as
future fuel, various theories related to PEC generation of hydrogen including basics
of semiconductors and their interaction with the electrolyte and the motivations for
this research with aims and objectives have been presented in this chapter.
Chapter 2 describes earlier work carried out on hematite as photoelectrode in
photoelectrochemical water splitting for solar generation of hydrogen. Details of the
experimental methods used to synthesize, modify and characterize the prepared
material have been described in Chapter 3. Chapter 4 describes the various results
and
discussion
on
characterization
and
PEC
parameters
for
the
modified/unmodified hematite thin film prepared. Finally conclusion along with
summary of the research carried out has been presented in chapter 5.
29
Praveen Kumar
Introduction
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