Hydrogen Generation at Irradiated Oxide Semiconductor

ABSTRACT

Current energy system round the globe is based upon the consumption of fossil fuels and our
90% energy needs are fulfilled by fossil fuels. Unfortunately, finite fossil fuel reserves are
depleted largely, and their consumption produces carbon-based greenhouse gases which are
continuously harassing the environment. Development of an alternative, clean and sustainable
energy system (hydrogen-based energy system) is a key challenge for the scientists. Sun is the
natural mega source of clean and safe energy which can be used to produce hydrogen by water
splitting on semi-conductor catalyst via photocatalytic reaction. The development in the field of
hydrogen production via photocatalysis has been initiated by Fuji Shema and Honda since 1972
and till now, considerable work has been carried out but hydrogen production via photocatalysis
still faces many challenges. The review discusses the principles, strategies and research progress
adapted to generate hydrogen from water using semi-conductor catalyst and sun light. It also
describes challenges to overcome low photoactivity of catalysts and strategies to develop
efficient visible light catalysts.
Introduction

Environmental pollution is one of the most dangerous and destructive now a day. Major cause of
environmental pollution is ejection of greenhouse gases such as CO 2, CH4, N2O, and CFCs etc. to
the atmosphere. Among the various sources the decisive source of greenhouse gases is
consumption of fossil fuels for energy production. The content of greenhouse gases increases day
by day in the atmosphere because of the increasingly consumption of fossil fuels over the past
decades. As the population increases the rate of consumption of fossil fuels such as gasoline
increases rapidly, and content of greenhouse gases rises in atmosphere which contributes to
pollute the environment. Consequently, depletion in the fossil fuel reserves occurs. Scientists are
trying to find and set an alternative fuel instead of fossil fuels. In this regard hydrogen emerges
as potential candidate to meet the world’s energy demand because it is worst energy carrier.
Hydrogen has utmost energy yield per unit mass i.e. 122 KJ/g than that of gasoline which is 40
KJ/g (Acar et al., 2014).

Hydrogen is the most abundant and naturally occurring element in the universe, both in Free
State and in combine state. Hydrogen production via photochemical splitting of water attracts
attention of researchers after Pioneer work of Honda and Fujishima on photo electrochemical
cell in 1972. Water is easily available and abundant source of hydrogen and by the use of
suitable semi-conductor photo catalyst water can be split into hydrogen and oxygen in the
presence of sun light. Hydrogen based energy systems purposed as long-term solution to
environmental problems and also a sustainable solution to future energy demand.

Hydrogen can be produced by number of ways. Primary sources of hydrogen belong to

hydrocarbon class of organic compounds include naphtha, methanol, heavy oil, natural gas, coal
and biomass. Hydrogen can be produced via gasification and reforming of hydrocarbons (SMR)
given by equation 1 and equation 2.

CH4 + H2O  CO + 3H2O (1)

CO + H2O  CO2 + H2 (2)

Disadvantage of steam reforming technique is the production of CO 2, which is harmful to
environment and a major cause of global warming round the globe (Navarro Yerga et al., 2009).
There are three major processes in which research work has been carried out to produce
hydrogen and oxygen from water. (i) Thermochemical water splitting, (ii) Photo biological water
splitting, and (iii) Photo catalytic water splitting,

Thermochemical water splitting requires high temperature to split the water chemically into its
constituents but the splitting efficiency is low as compared to photo catalytic water splitting. This
method has advantage of low or no greenhouse gas emission. High temperature used here is
difficult to handle and large-scale production of hydrogen is hindered by the instrument’s huge
cost.

Alternatively, hydrogen can be produced by the photobiological water splitting. It uses special
type of microorganisms and enzymes to convert the water into H 2 and O2 in the presence of sun
light. This process produces carbon dioxide which is at least not desirable and low production of
hydrogen limits the process efficiency (Acar et al., 2014).

Photochemical water splitting has advantage over above described methods that it can produce
hydrogen without any by-product and expensive instruments. This process has higher efficiency
than anyone described earlier because it can produce hydrogen on large scale. Figure 1 shows the
basic principle of overall water splitting mechanism which consists of few successive steps (i)
light absorption, (ii) excitation of electrons from ground state to excited state, (iii) transfer of
charges from bulk of photocatalyst to the surface of photocatalyst and (iv) surface reaction
between photogenerated charges and water.

Figure 1. Basic principle of water splitting on photo-catalyst surface.

Here solar energy is used to split water in the presence of suitable semiconductor photocatalyst
(eq. 3)

2H2O Photocatalyst + sunlight 2H2 + O2 (3)

→

Honda and Fujishima initiated the research work in this field as they used the TiO 2 as photo
catalyst to split the water in the presence of sunlight. It is observed that photo catalyst used in
splitting is responsible for conversion efficiency of water to hydrogen. A considerable work has
been made to develop the efficient semiconductor photo catalyst which in turn to maximize the
efficiency.

The following section will review the generous work on hydrogen production. It also reviews the
basic concepts, and the progress made to develop visible light active material over the past
decades. It covers the different strategies adapted to develop the visible light active photocatalyst
and describes the use of different materials such as co-catalysts and sacrificial reagents to
develop visible light response in a UV-active catalyst for hydrogen production by splitting of
water (Navarro Yerga et al., 2009; Acar et al., 2014).

Different methods to produce hydrogen

Hydrogen production has become an important research topic round the globe. In this regard
several methods have been employed to check feasibility of hydrogen production. Many
processes such as thermochemical processes, electrochemical processes, direct solar water
splitting and biological processes have been introduced with different efficiencies and limitations

(1) Thermochemical processes

It uses the heat for the chemical reactions occurring in fossil fuels and biomass to produce
hydrogen. Some important techniques in thermochemical processes are (i) steam methane
reforming (SMR) (ii) coal gasification and (iii) biomass gasification.

(i) Steam methane reforming (SMR).

In SMR technique, methane gas is treated with steam in the presence of suitable catalyst at high
temperature (100°C-1000°C). This process is endothermic because energy is required to start and
to complete the reaction (eq. 4)
 CH4 + H2O(steam)  CO + 3H2 (4)

Subsequently carbon monoxide reacts with water to produce more hydrogen which is also known
as water gas shift reaction (eq. 5)

CO + H2O  CO2 + H2 (5)

Only small amount of carbon dioxide is produced which contribute to augment the content of
greenhouse gases in atmosphere, but the CO 2 gas produced can be captured and stored using
various separating techniques. Although SMR technique produces the greenhouse gases one half
as compared to greenhouse production from fossil fuels (Holladay et al., 2009; Acar et al.,
2014).

(ii) Coal gasification

Coal can be used to produce hydrogen and many other hydrocarbons due to its complex structure
and variability in properties. Coal is treated with oxygen and steam under high temperature and
high pressure to produce hydrogen and carbon monoxide. CO further undergoes water gas shift
reaction to produce more hydrogen. Hydrogen is captured by using separating system and CO 2 is
captured and stored (Holladay et al., 2009; Salam et al., 2018).

(iii) Biomass gasification

Here hydrogen is produced using same principle involved in thermochemical processes as

biomass react with steam at high temperature and pressure to produce hydrogen and carbon
monoxide and carbon monoxide react with water to yield hydrogen

(2) Electrolytic Processes

This process uses electricity to produce hydrogen and oxygen from water-based electrolytes.
This process works according to the principle of electrolysis. Oxidation of O−2 ion takes place at
+¿¿
anode to release O2 and Reduction of 2 H takes place to release hydrogen. In this process
hydrogen is produced with zero percent greenhouse gases emission. But electricity is required to
drive the whole process and its cost and method of production limits the electrolytic process for
hydrogen production (Sapountzi et al., 2017; Ju et al., 2018).
(3) Direct solar water splitting

It involves two major processes

(i) Photo-electrochemical water splitting.

Photo-electrochemical (PEC) water splitting is the most feasible and affordable process to
produce hydrogen from renewable resources belongs to nature. This process uses solar energy in
form of light to break the water into hydrogen and oxygen in the presence of suitable
semiconductor photocatalyst. Here photocatalyst is immersed into water and the sunlight derives
the process to exhaust hydrogen. It offers high solar to hydrogen conversion efficiency. This is
long term, sustainable solution to future energy demand with almost zero greenhouse gases
emission. This technique is most feasible one and requires more research to increase the
efficiency of the process (Zhao et al., 2016; Saraswat et al., 2018).

(ii) Photo biological water splitting.

Here bacteria and microorganisms are used to convert the water into its constituents in the
presence of sunlight. Hydrogen production through biological process required more research in
the field because development in this field is in earlier stage. This process is limited because of
low efficiency (Weaver et al., 1980; McKinlay and Harwood, 2010; Acar et al., 2014).

(4) Microbial Biomass Conversion

In this process bacteria and algae use to convert and digest the biomass and to release H 2 on a
small scale with no environmental impact. Research in this process is also in early stages so
development of this process for hydrogen production needs more research (Holladay et al., 2009;
Salam et al., 2018).

Basic Principle of Water Splitting

From thermodynamic point of view, the process of water splitting by catalyst is an endothermic
process with a significant positive change in Gibbs-free energy, round about 237 kJ/ mol. Photo
catalytic water splitting uses semiconductor photocatalyst because semiconductor substances are
capable to absorb light photons which excite its electron from valance band (VB) to conduction
band (CB). As a result of which electron hole pairs generated which on reaction with water split
the water into its constituents.
A semiconductor is a substance that absorbs photon of energy equal or greater than the band gap
between VB and CB. When it absorbs photon, electrons from VB promoted to CB by creating
holes (h+) of positive charge in VB and an electrons promotion to CB result in negative charge.
This hole and electron pairs use to split the water into hydrogen and oxygen via oxidation and
reduction.

Semiconductor used should be smaller band gap in order to absorb visible light for higher
efficiency. If band gap is large so that UV light is absorbed by photocatalyst than efficiency is
limited by 3-5% because of solar radiations has UV content less than 10%.

In this regard two type of configurations are introduced to produce H2 and O2 from photocatalytic
water splitting (i) Photo-electrochemical cell, and (ii) Particulate photocatalytic system. Photo-
electrochemical cell consists of two electrodes of which one is semiconductor photocatalyst
dipped in aqueous or water-based electrolyte (Fig-2). As the system is irradiated, the
photocatalyst produces h+ and e- pairs where h+ is used to oxidized water and e - is used to reduce
the 2H+ ions into H2 gas. Figure 2 shows the processing occur during whole process.

Figure 2 showing hydrogen production using electrolytic process.

Particulate photocatalytic system involves tiny electrodes which are in form of small particles or
granules of semiconductor material immersed in water-based electrolyte. When the light strikes
the system than the subsequent processes occur. (i) absorption of light and generation of charge
carriers (ii) charge carriers move from bulk of catalyst to the catalyst surface (iii) chemical
reaction between water and photogenerated charge carriers. In particulate photocatalytic system
water is adsorbs on the surface of photocatalyst. After 2nd step when charge carriers come out on
surface, Redox chemical reaction takes place so that water breaks into oxygen and hydrogen (eqs
(6) and (7))

1
H2O + h+  2H+ + O2 (6)
2

2H+ + e-  H2 (7)

Overall reaction is

1
H2O  H2 + O2 (8)
2

Process is shown in figure 3.

Figure 3 shows systematic diagrame of hydrogen production from particulate photocatalytic system.
Particulate photocatalytic system has limitation that charge carrier separations difficult as
compared to separation in photo-electrochemical cell and there exists difficulties in separating
the produced gases to prevent the reverse reaction. But it is simple and easy to produce hydrogen
using particulate photo catalytic system. Some reducing agents such as alcohols, sulphides,
slphites and EDTA and oxidizing agents such as persulphates, Ag +, and Fe+3 are also used to
facilitate the splitting reaction in photocatalytic water splitting but the process than not remains
photocatalytic rather is becomes test for evolution of H 2 and O2 (Navarro Yerga et al., 2009;
Saraswat et al., 2018).

Photocatalyst Requirement
 To achieve maximum solar visible light water splitting efficiency, it is necessary to develop a
photo catalyst which possesses the desired properties regarding band gap energy and
electrochemical properties. It must be capable to absorb visible portion of light for achieving
maximum efficiency. In this regard several binary metal oxides, metal-non-metal composites,
and doped metal alloys are tried to check their capability of converting the visible light to energy
based upon their light absorption capacity, kinetics of charge transfer and electronic structure
used for water splitting.

The material used as photocatalyst in particulate photocatalytic process (discussed in earlier

section) to split the water into its constituents must possess the several useful properties for
example it should be capable to absorb sufficient solar visible radiation to give maximum
efficiency along with the bang edge potential should be appropriate for water splitting. It should
not allow the photogenerated charge carriers to recombine which result in energy losses. It
should be stable toward photochemical and photo corrosion. Kinetically it should facilitate the
transfer of photo generated electrons from bulk of photocatalyst to surface in order to oxidize the
water.

To carry out water splitting, the conduction band of photoactive material must be at higher
negative potential than that of reduction potential of proton as 2H +/H2 on normal hydrogen
electrode (NHE) scale. To oxidize the water which already adsorbs on photocatalyst surface, the
oxidation potential of valance band of photocatalyst must be at lower potential than oxidation
potential of water (v= +1.23eV). Therefore, to achieve water splitting 1.23eV is minimum
required energy. This energy in equivalence with photon energy having wavelength round about
1010nm. Therefore, almost 70% of all solar photons are available to carry out energy-based
processes such as water splitting. Table below shows the distribution of energy in solar spectrum,
their corresponding wavelength and contribution to total spectrum.

Table No. 1 showing distribution of energy in solar spectrum

Spectral regions Near uv Blue Green/yellow Red Near IR IR

Wavelength (nm) 315-400 400-510 510-610 610-700 700-920 920-1400
Energy (eV) 3.93- 3.09-2.42 2.42-2.03 2.03-1.77 1.77-1.34 1.34-0.88
3.09
Contribution to 2.9 14.6 16.0 13.8 23.5 29.4
total spectrum
(%)
However there are several unavoidable energy losses such as electron hole recombination. Band
gap of semiconductor is basically determined by its electronic structure. Band position for
several semiconductor photocatalysts are shown in table no (Xu and Schoonen, 2000).

Among many semiconductor photocatalysts band position of SiC, KTaO3, CdS, SrTiO3, ZnS, and
TiO2 meet the energy requirements necessary to split the water. Another problem associated with
variety of photocatalysts such as CdS and GaP is that these photocatalyst when absorbs light than
they oxidize themselves rather to oxidize the water which limits the whole processes. The
selected photocatalyst for water splitting must capable of preventing the chemical reaction at
photocatalyst water interface to prevent electrochemical corrosion, photo-corrosion and
dissolution. (Bolton, 1996; Navarro Yerga et al., 2009; Saraswat et al., 2018)

Different efforts to develop efficient photocatalysts.

Properties of photocatalyst such as small band gap, charge carriers production and preventing
recombination of charges are identified and already been discussed in previous section but it is
difficult to find a single material which possess all the described properties. Research work to
develop efficient photocatalyst are still a key challenge for researchers round the globe.
Efficiency of a photocatalyst is determined by the ratio of hydrogen production to absorbed solar
energy in form of radiation and till now maximum efficiency (Quantum yield) by using Rh 2-
Y CrYO3/(Ga1-XZnX)(N1-XZnX) photocatalyst to produce hydrogen is limited by 5.9% because of
limited properties of photocatalyst. This catalyst is applied to clean water under visible light
irradiation having wavelength 420-440 nm using 300 W xenon lamp with a cut off filter (Maeda
et al., 2008). The low efficiency of the process is still a major limitation which prevent the
process from boosting on large scale. It is a key challenge for the researcher over the globe to
develop a photocatalyst that can give maximum efficiency so that H 2 can be used on commercial
scale.

Time resolved spectroscopic technique reveals that charge carriers generated via irradiation of
photocatalyst get recombined at all, about 90% charge carriers are readily recombine as they
generate while only 10% charges appear at the surface to oxidize the water and reduce the
hydrogen ions to hydrogen (Colombo Jr et al., 1995; Skinner et al., 1995).

To develop an efficient photocatalyst researchers have to control the interdependence of the

electronic, surface and microstructural properties of photocatalyst. In this regard many efforts are
made to develop such type of photocatalyst. Among many efforts, some of them are listed here
(i) finding new single-phase material, (ii) tuning of UV-active photocatalysts (iii) surface
modulation of photocatalyst by installing a co-catalyst in it to reduce the energy of activation for
hydrogen evolution (iv) sensitization and (v) Nano designed photocatalyst for control over the
morphology and defects present in photocatalyst. These processes are applied by researchers to
enhance the efficiency of photocatalysts from 5.6% to maximum via reduction in recombination
of photogenerated charges.

Band-Gap Engineering

It will be fruitful to convert a UV-active photocatalyst to visible light active photocatalyst. For
the purpose, in band-gap engineering two processes are frequent (i) Cation Doping or anion
Doping in semi-conductor lattice and (ii) use of semi-conductor alloys. Frequently, doping has
used to develop visible light activity in UV-active substance (photocatalyst). Because of large
band gap in photocatalyst, they absorb high energy radiations (UV-radiations) which contribute
less than 10% of the total solar light. Transition metals like antimony, tantalum, choromium, zinc
and carbon can also be used as dopant in semiconductors to enhance the photocatalyst response
towards visible light.

(i) Cation-Doped semiconductor photocatalyst

A change in composition occur by doping or replacing a cation of other element into

semiconductor photocatalyst. Doping results in contraction of band gap in semiconductor by
injecting energy levels as impurities between VB and CB of semiconductor which facilitates the
photogenerated charge carriers to move to surface to oxidize and reduce the water which makes
the semiconductor material to enhances its response toward visible light. Although activity
toward visible light increases but cation doping provides active centers or active sites to
photogenerated charge carriers to recombine instead to oxidize the water (Ni et al., 2007). The
energy levels injected by cation doping within the semiconductor band gap hindered the
movement of holes and electrons from going to surface of catalyst to perform redox reaction
(Blasse et al., 1981). Cation-doped semiconductor photocatalyst should be fine-tuned to split the
water to give maximum quantum yield. Many chemical techniques such as precipitation and
impregnation are used to dope different metals into the host structure. Figure 3 shows a cation
doped catalyst having a wide band gap structure but responsive to (Anpo, 2000; Yamashita et al.,
2002; Anpo and Takeuchi, 2003).

Figure 3. Cation doped wide band gap semi-conductor material

(ii) Anion-Doped Semiconductor Photocatalyst

An alternative method to develop visible light response in large band gap UV-active
photocatalytic materials. Numerous researchers are interested in anion-doping in
semiconductor’s structure because of its feasibility and anion doping does not provide any active
centers to photogenerated charge carriers for combining as they formed and hence quantum yield
increases. Different anions such as “N” (Asahi et al., 2001; Khan et al., 2002; Umebayashi et al.,
2002) can be doped in oxide semiconductors. Oxide semiconductor photocatalyst has large band
gap in which 2P orbital of Oxygen is on the top of VB. The anion doping results in mixing of the
2P atomic orbital of anion with 2P of “O” to decrease the band gap energy of wide-band gap
semiconductor as shown in figure. This process raised the VB near to CB and make the UV-
active semiconductor a visible-active photocatalyst. Doping often yields defects in oxidation
state of oxygen which can act as recombination centers for photogenerated charge carriers.
Figure 4 evaluate the anion doping effect on semi-conductor’s band gap.
 Figure 4. anion doped wide band gap semi-conductor with visible light activity

Semi-Conductor Alloys

Interestingly in this technique two semiconductors among which one is of wide bang gap and
other is of small band gap are mixed to give a solid solution in which lattice sites of both
semiconductor materials are interspersed to make the photocatalyst responsive to visible light as
depicted in figure 5. By varying the composition of solid solution, band gap of mixed
semiconductor photocatalyst can be changed according to desired value. Examples of such alloys
include GaN-ZnO (Matsumura et al., 1983), ZnS-CdS (Yu et al., 2012), ZnSInS2 (Inoue et al.,
1994), and CdS-SdSe (Kambe et al., 1984).

Figure 5. representing the band gap of material obtained via solid solution of wide and narrow band gap
materials.
Surface co-catalyst

A surface catalyzed photochemical reaction takes place between photogenerated charge carriers
and water to produce H2 as already discussed above. The hydrogen production via water splitting
and surface adsorption of charges can be enhanced by using Nobel metals or their oxides adsorbs
on the surface of photocatalyst as co-catalyst . e.g. Rh, Pt, RuO2, and NiO2. Co-catalyst enhances
the rate surface reactions by capturing the photogenerated holes and electrons (Maruthamuthu
and Ashokkumar, 1989) and prevent them from recombing. It makes easier for holes to oxidized
water and reduce the hydrogen ions by lowering the activation energy and thus enhance the
visible light activity of UV-active photocatalyst (Maruthamuthu and Ashokkumar, 1988).

Nanostructure

Efficiency of a photocatalyst is determined by photogenerated charges either they recombine or

go to the surface of photocatalyst for hydrogen production. The properties of photocatalyst used
is responsible for the hydrogen production and that photocatalyst determines the recombination
or transfer of the charges. Properties such as surface properties, crystal size, crystallinity,
structural imperfections and their nature responsible for transferring of photogenerated charges
to surface of photocatalyst or allow them to recombine.

At nanoscale, crystalline structure of photocatalyst behaves differently as compared to bulk

scale. At nanoscale, crystal size, crystallinity, crystal imperfections and properties related to
surface have been a great effect on charge transportation. Reduction in size of photocatalyst
allow the photogenerated charges to move to the surface easily, as a result of which efficiency of
the process increases (Ashokkumar, 1998). Higher the crystallinity will be higher the rate of
charge transfer although it is necessary to control structural imperfections because they can make
the charges to recombine (Reber and Meier, 1984; Kominami et al., 1995; Ikeda et al., 1997).
New electronic states are introduced at nanoscale between bang gap of photocatalyst which act
as to prevent recombination of charges. Therefore, photocatalyst used should be at nanoscale
because it allows to control the band gap of material by making changes in particle size as the
band gap of crystalline material depends upon particle size (Brus, 1984; Brus, 1986; Rino and
Studart, 1999).

Efforts have been made to study particle size effect on light absorption, movement of
photogenerated charges and surface area which leads to synthesize the materials at nanoscale
with controlled particle size, structure defects and dimensions. Material can bring at nanoscale
by various methodologies. Nonconventional technologies such as sol-gel (Do Kim and Kim,
2002; Lee and Yang, 2005), hydrothermal (Cot et al., 1998; Chae et al., 2003) , micelles and
inverse micelles (Lin et al., 2002; Yu et al., 2002), chemical vapor deposition (CVD) (Ayllon et
al., 1999; Seifried et al., 2000) and sonochemical technique (Ho and Jimmy, 2006) have been
tried to synthesize nanoparticles with controlled dimensions and morphology which in turn a
control over photoactivity of photocatalyst. Problem of agglomeration is associated with these
generated Nano particles because of interparticle interactions. Therefore, a microporous
framework (zeolites (Fujishima et al., 2000; Hu et al., 2006) and activated carbon (Li et al.,
2007; Liu et al., 2013)) is introducing during the process which act as a host for Nanoparticles
and prevent the agglomeration. Three-dimensional structure of nanoparticles is also a reasonable
effect on the efficiency related problems. Wang et al. (Wang et al., 2003) reported the superior
activity of the colloidal TiO2 Nanoparticles photocatalyst when they form a three-dimensional
network along a given crystallographic plane. The author than proposed a so-called antenna
effect in which an antenna system is responsible for the transferring of charge carriers to surface
of photocatalyst. Recently powdered type photocatalyst used which carries strong electronic
coupling forces between particles enough to get agglomerate and result in production of particles
at microscale. At microscale antenna effect rises, so that rate of transfer of charge carriers
increase which in turn boost up the efficiency of the process. Apart from this, smart molecular
engineering can also be applied to synthesize the photocatalyst at nanoscale which have more
photoactivity than the present system.

Multiphoton water splitting

It is an interesting technique in which two semiconductor photocatalysts are used to split the
water into its major constituents. Here positive charge carriers from one photocatalyst used to
oxidize the water and negative charge carriers from second photocatalyst used to reduce the
hydrogen ions to hydrogen gas. Semiconductor photocatalysts applied to water should have
small band gap and water get simultaneously oxidized and reduced via two-step water splitting
reaction as depicted by figure 6 (Tennakone and Wickramanayake, 1986; Sayama et al., 1997;
Fujihara et al., 1998; Kudo et al., 1999; Abe et al., 2005).

Figure 6. representing dual photocatalyst system employing a redox shuttle

Oxygen is evolving at one semiconductor photocatalyst while hydrogen is evolving from other
semiconductor photocatalyst. However, it is very difficult to control the recombination reactions
between the generated charges here. Therefore, process is limited until a suitable solution to this
problem would proposed (Tennakone and Wickramanayake, 1986).

Recent advancement in developing visible light active catalysts for water splitting under
visible light

Advances in hydrogen production using visible light photocatalyst from water have been
achieved by combining photocatalyst material with sulphides, nitrides, nitrides, carbides,
phosphides (almost more than 130 materials). After combination a more efficient photocatalyst is
achieved which can produce hydrogen at higher rate. Following table contains photocatalysts and
sacrificial agents along with co-catalyst which on combination forms suitable electronic structure
for visible light activity (Navarro Yerga et al., 2009).
 Summary recently developed visible light active photocatalyst
Photocatalyst. Sacrificaial reagent. Co-catalyst. Activity (µmoleh -1g-1) Acivity (µmoleh-1g-1)
References O2 H2

(Kato and TiO2/Cr/Sb AgNo3 (0.05M) - 90 -

Kudo, 2002)

(Yu et al., (CuAgInx)Zn2(1-x)S2 S2--SO32- (0.35M/ Ru - 7666

2012) 0.25M)

(Kim et al., TiO2-N CH3OH Pt 0 -

2004)

(Yu et al., Zn2(1-x)S2(CuIn)x S2--SO32- (0.35M/ Ru - 4100

2012) 0.25M)

(Kim et al., TiO2-N AgNo3 (0.05 M) Pt 0 221

2004)

(Kudo et al., (AgIn)xZn2(1-x)S2 - Pt - 116

2002)

(Ishii et al., SrTiO3-Cr-Ta CH3OH (0.1M) Pt - 140

2004)

(Kudo et al., Zn2(1-x)S2(AgIn)x S2--SO32- (0.35M/ Pt - 3133

2002) 0.25M)

(Kato and SrTiO3-Cr-Sb CH3OH (0.1M) Pt - 156

Kudo, 2002)

(Tsuji and ZnS-Pb S2-/SO32- (0.1 M/ 0.04 - - 40

Kudo, 2003) M)

(Hwang et al., La2Ti2O2-Cr CH3OH (0.3M) Pt - 30

2005)

(Kudo and ZnS-Ni S2--SO32- (0.11M/ - - 450

Sekizawa, 0.04 M)
2000)

(Hwang et al., La2Ti2O2-Fe CH3OH (0.3M) Pt - 20

2005)

(Navarro et al., CdS S2--SO32- (0.1M/ Pt - 350

2008) 0.04M)

(Ishikawa et al., Sm2Ti2S2O5 CH3OH (0.3M) Pt - 40

2002)

(Navarro et al., ZnO-CdS-CdO S2--SO32- (0.1M/ - - 11.9

2008) 0.04M)

(Hitoki et al., TaON CH3OH (0.06M) Pt - 50

2002)

(Curie, 2013) (Zn1+xGe)(N2Ox) - RuO2 466 930

(Hitoki et al., TaON AgNO3 (0.01 M) - 3300 -

2002)

(Maeda et al., (Ga1-xZnx)(N1-xOx) - Cr/Rh 466 930

2007)

(Yamasita et al., CaTaO2N CH3OH (0.06 M) Pt - 37

2004)

(Konta et al., Ag3VO4 AgNO3 (0.05 M) - 17 0

2004)

(Yamasita et al., CaTaO2N AgNO3 (0.01 M) 0 -

2004)

(Konta et al., Ag3VO4 CH3OH (0.1 M) - - -

2004)

(Yamasita et al., SrTaO2N CH3OH (0.06 M) Pt - 50

2004)
In this section recent development in the visible light photocatalyst via material formulation have
been reviewed. Material formulation via modern techniques customize the morphology of
photocatalyst, and crystallinity, morphology including structural defects have a great effect on
water splitting under visible light as already discussed in previous section.

Titanium oxide and titanite.

Titanium oxide is considered the first semiconductor to be used as a photocatalyst for water
splitting (Fujishima, 1972) but due to large band gap (more than 3 eV), it can only utilizes UV
radiations which is small portion of the solar spectrum. Later work proved to be very efficient for
TiO2 photocatalyst. Many changes have been made in its structure to make it visible light active
such as doping of metal ions (Fe 3+,V5+,Co2+,Cr3+,Ni2+) in TiO2 lattice (Konta et al., 2004; Hwang
et al., 2005). A little increase in efficiency achieved using this is not admirable at all. Kato and
Kudo (Kato and Kudo, 2002) reported the doping of the TiO2 photocatalyst at nanoscale with
Sb5+and Cr3+. This will result in efficient evolution of oxygen from an aqueous solution using
AgNo3 as a sacrificial reagent under visible light. Cr3+ doped in TiO2 lattice to introduce a new
electronic level between bang gap of TiO 2. This makes the semiconductor visible active due to
decrease in band gap. But problem arises due to unbalancing of charge, which is compensated
with the co-doping of Sb5+ which prevent the formation of Cr6+ ion and prevent the defects in
lattice due to oxygen (Navarro Yerga et al., 2009). Thin film of TiO2 shows visible light activity
after going under doping by recent ion-implantation technique with transition metal ions (Anpo,
2000; Yamashita et al., 2002; Anpo and Takeuchi, 2003). TiO2 thin films doped with ions such
as V+5 and Cr+3 shows photo activity under visible light by using CH 3OH as a sacrificial reagent
for water splitting to evaluate hydrogen having quantum yield of 1.25 (Matsuoka et al., 2007).
Ion-implantation technique of making a wide band gap material to narrow band gap material is
limited by its huge cost and low quantum yield (Navarro Yerga et al., 2009).

Visible light activity can also be achieved by doping TiO 2 with anions such as S (Umebayashi et
al., 2002), N (Asahi et al., 2001), C (Anpo and Takeuchi, 2003), because of p orbitals of these
anions get mixed with the 2p orbitals of oxygen present in TiO 2 lattice which give rise to VB
edge upward and band gap decreases (Navarro Yerga et al., 2009). Metal titanite such as SrTiO3,
LaTi2O7, Sm2Ti2O7 obtained when TiO2 react with SrO, Ln2O3 (Ln stands for lanthanides), BaO
but they have wide band gap and unable to absorb visible light. Doping of such titanite with Cr 3+-
Ta5+ or Cr3+-Sb5+ on rhodium make the photocatalyst visible light active (Kato and Kudo, 2002;
Ishii et al., 2004; Konta et al., 2004). Using Pt- co-catalyst in SrTiO3 not only make it responsive
to visible light but also produce hydrogen from aqueous methanol and La 2Ti2O7 when doped with
Cr3+-Fe3+ irradiation under visible light promote electron from 3d orbitals of Cr 3+ or from 3d
orbitals of Fe3+ to conduction band make it photoactive (Hwang et al., 2004; Hwang et al., 2005).
But it cannot split the pure water because it has very low redox potential. This can be made to
split the water if methane is used here as electron donor sacrificial reagent. Another technique
used to make Ln2Ti2O7 responsive to visible light up to 650 nm wavelength is the oxygen
substitution with Sulfur anions in the Ln2Ti2O7 such as Sm2Ti2S2O5 as shown in figure 7
(Ishikawa et al., 2002).

Figure 7. UV/visible diffuse reflectance spectra of Sm2Ti2O7 and Sm2Ti2S2O5

S and 3P valance orbitals of Sm 2Ti2S2O5 constitute to lower the band gap to 2.0 eV that of 3.5 eV
of Sm2Ti2O7. Under visible light Sm2Ti2S2O5 is stable photocatalyst and can reduce hydrogen ion
to hydrogen gas and can oxidize the water to oxygen gas by using electron donor sacrificial
reagent Na2S-Na2So3 or methanol or Ag+ acceptor ion (Navarro Yerga et al., 2009).

Titanium disilicate (TiSi2) emerges as a potential candidate recently to split the water into
hydrogen and oxygen by acting as proto type photocatalyst under visible light (Ritterskamp et
al., 2007; Navarro Yerga et al., 2009).

Tantalates and Niobates

Structural regularities present in the tantalates and niobates make them photoactive under UV
light because of high band gap (4.0-4.7 eV). Tentalates and niobates shared a corner in MO 6 unit
(M stands for Metals such as Ta, Nb). They provide a high transfer rate of photogenerated
charges and do not allow them to get combine again. Hence under UV irradiation this can split
the pure water into H2 and O2 (Takata et al., 1998; Kudo et al., 1999; Kato and Kudo, 2001; Kim
et al., 2005). Kato and Kudo reported that the MTaO 3 (metal tentalates) where M= Na, Li, K,
etc., are effective in photocatalytic water splitting (Kudo et al., 1999)and their band gap depends
upon cation (using Li= 4.71 eV, using Na=4.01 eV, Using K= 3.70 eV) (Kato and Kudo, 2001).
NaTaO3 in combination with NiO produce H2 and O2 from water under UV-irradiation to give
efficiency about 20-28% (Navarro Yerga et al., 2009). Teng et al. (Lin et al., 2006) reported that
NaTaO produce via sol-gel method has high efficiency than any other metal tantalates because of
higher surface area of product produced and monoclinic structure of product achieved using this
method instead of orthorhombic with a band gap of (4-4.2 eV)

Among many approaches to develop a visible light active photocatalyst, one approach is to
produce oxynitrides compounds using tentalates and niobates. Here in a metal oxide partial
substitution of oxygen atom with N-atom causing hybridization between 2P orbitals of oxygen
and 2P orbitals of Nitrogen which cause the VB to shift upward to higher potential which in turn
narrowing the band gap. Following the strategy MTa 2N (M=Sr, Ca, Ba), TaON andSr2Nb2O7-xNx
(X=1.49-2.81) photocatalyst have been evaluated and studied under visible light to split the
water (Hitoki et al., 2002; Yamasita et al., 2004; Ji et al., 2005). TaON, MTa2N (M=Sr, Ca, Ba)
show a band gap of 2.5 eV and 2.5-2.0 eV respectively which absorbs visible light up to 630 nm.
Figure 8 shows Uv/visible spectra of Tantalates and Niobates (Yamasita et al., 2004).
 Figure 8. Diffuse reflectance UV-visible spectra of different tantalates and Niobates

TaON photocatalyst produce H2 using methanol as sacrificial reagent and produce O 2 from
aqueous solution using electron donor AgNo3. But MTa2N cannot oxidize water because of lower
oxidation potential while it can produce H 2 using an electron donor sacrificial reagent methanol
under visible light irradiation (Navarro Yerga et al., 2009). Nitrogen based oxynitrides such as
Sr2Nb2O7 oxide can show photoactivity when irradiated with visible light because of lowering of
band gap by hybridization of 2P orbitals of N and with 2P orbitals of O. The higher the content
of nitrogen in the oxynitrides higher will be the photoactivity up till the original structure of
oxynitrides maintained. When he nitrogen content tends to exceed the limit than original
structure of layered Sr2Nb2O7 cannot maintained and efficiency decreases (Ji et al., 2005;
Navarro Yerga et al., 2009).

Other transition metal oxides

Photocatalytic water splitting reaction can be carried out using specific compounds of vanadium
and tungsten such as Ag3NO4 with structure same as that of CaTiO3(perovskite). BiVO4 with
structure like an ore of vanadium (sheltie) are photoactive to produce oxygen from an aqueous
solution AgNO3 under visible light irradiation (Kudo et al., 1999; Konta et al., 2004). These
oxides have lower band gap because hybridization of 2P orbitals of oxygen with valance orbitals
of BiVO4 and Ag3VO4. Thus, valance band potential rises near to CB to become visible light
active but conduction band electrons of BiVO 4 and Ag3VO4 have sufficient potential to reduce
2H+ ion to hydrogen gas. WO3 is also a candidate to evolve oxygen under visible light irradiation
only when it coupled with platinum which is the most expensive. Pt-WO 3 can be used in
presence of NaIO3 and Pt/SrTiO3. Overall water splitting reaction takes place using
monochromatic source of light of 420.7 nm (Sayama et al., 2001).

Metal nitrides and oxynitrides

Transition metal ions having d10 configuration such as Ga3+, Ge4+ react with nitrides and
oxynitrides to yield transition metal nitrides and oxynitrides e.g. ZnO and GaN react to give a
solution of composition (N1-xOx)(Ga1-xZnx) known as solid solution (Maeda et al., 2005) which
have band gap of 2.4-2.8 eV. ZnO and GaN separately have bang gap energies greater than 3 eV,
but they modified to give visible light response as in solid solution. Beause in solid solution of
both valance band contains electrons from the 3d orbitals of Zn and 2P orbitals of N which
which results in p-d repulsion (Navarro Yerga et al., 2009). Consequently, band gap decreases to
make it visible light active. However solid solution of composition (N 1-xOx) (Ga1-xZnx) is
modified using deposition of co-catalyst sacrificially to exhibit higher visible light activity
(Maeda et al., 2007; Maeda et al., 2008). Rh and Cr are proved to be efficient co-catalyst for
(Ga1-xZnx)(N1-xOx). This process does not utilize sacrificial reagents to produce H 2 and O2.
Quantum efficiency of solid solution after modification with Rh-Cr is 5.9% under visible light
(Maeda et al., 2008). Powder X-ray differaction and UV/visible spectra of GaN, GaN:ZnO,
GaN:ZnO, GaN:ZnO and ZnO is shown in figure 9

Figure 9. A) X-ray diffration peaks and B) Diffuse reflectance UV/visible spectra of a) GaN, b)
GaN:ZnO, c)GaN:ZnO, d)GaN:ZnO, and e) ZnO

Also solid solution of ZnO with ZnGeN 2 having composition (Zn1-xGe)(N2Ox) has been studied.
It is also photoactive under visible light with narrow band gap from parent molecules. Here also
p-d repulsion causes the bad gap to narrower. If RuO 2 used as co-catalyst than evolution of
hydrogen and oxygen takes place stoichiometrically from water (Navarro Yerga et al., 2009).

Metal sulphides

Metal sulfides are visible light active photocatalysts with small band gap however these are least
stable under water oxidation reaction in visible light. Perhaps the reason behind this is that the S 2-
anion is higher reducing agent than that of water. So S 2- oxidized by catalyst rather to oxidize the
water which causes the degradation of photocatalyst (Williams, 1960). By using electron donor
sacrificial reagent such as Na 2S/Na2SO3 salt, photocatalytic degradation of photocatalyst can be
prevented (Kamat, 2007). There are many metal sulfides available, among them wurtzite
structured CdS is the most effective photocatalyst (Kalyanasundaram et al., 1986; Meissner et
al., 1988; Ashokkumar, 1998) because if its narrow band gap of 2.4 eV which is visible light
active and can produce hydrogen and oxygen from water. Perhaps use of Pt as co-Catalyst can
enhance the photoactivity of catalyst which results in higher quantum yield about 25% (Buehler
et al., 1984). Composites of CdS with the semiconductor photocatalysts such as ZnO (Spanhel et
al., 1987; Navarro et al., 2008), TiO2 (Fujii et al., 1998), CdO (Navarro et al., 2008) have been
studied where electrons transfer from CdS to other photocatalyst while holes are remain in CdS
which facilitates the charge separation and prevent recombination. Thus, increase the efficiency
of the process (Navarro Yerga et al., 2009).

ZnS incorporated into CdS structure to make a solution is another best one approach to make
visible light active photocatalyst for water splitting. Both makes continuous sheet of solid
solution having composition Cd1-xZnxS in which crystal lattice is incorporated metal atoms
(Nayeem et al., 2001). Photochemical and photophysical properties of Cd 1-xZnxS solid solution is
studied by valle et al., (Del Valle et al., 2009) by varying Zinc concentrations (0.20<X<0.36).
Fig 16 indicates for a blue shift of solid solution. As the concentration of zinc rises from 0.2 to
0.3 the photocatalytic activity of solid solution (Cd1-x/ZnxS) increases as shown in figure 10.

Figure 10. Hydrogen producing effeciency from solution of Na 2S and Na2SO3 under visible light over
solutions of Cd1-xZnxS with varying Zn concentrations 0.2, 0.26, 0.30, and 0.39 in 0.15L reactant solution
of concentration of Na2S 0.1m and 0.04m concentration of Na2SO3
When the concentration of zinc gets changed the band gap modification takes place by there is a
positive change in electronic states occur between VB and CB in solid solution. That is why
evolution rate of H2 gas also varies as structural changes are responsible for band structure and
mobility of photogenerated charge carriers. It is useful to study structural changes in
photophysical properties of Cd1-xZnxS solid solution induced using thermal treatment. At high
temperature (8733-113 K) Cd1-xZnxS shows an increase in crystalline size and modification in
Cd1-xZnxS structure occurs because of substitution of Zinc increases into CdS structure. All these
changes lead the photocatalyst to higher crystallinity and high degree of substitution of Zn into
CdS structure results into higher activity of catalyst toward visible light (Navarro Yerga et al.,
2009).

Other sulphides such as ZnS have been studied. Band gap of ZnS does not allow it to absorbs
visible light but doping with various transition metals such as Ni 2+, Cu2+, Pb2+ make it responsive
to visible light. Due to doping band gap becomes narrower and photocatalyst become visible
active for hydrogen production from water based solutions using SO 32--S2- as electron donor
sacrificial reagent. Other sulphides such as In 3+ type and a type of transition metals such as Cd 2+,
Cu2+, Zn2+ ,Mn2+ have been studied and investigated, however, quantum yield for water splitting
using visible light is still limited to 3.7% at 420nm for the Na 14In17C43S35 photocatalyst which is
considered most active among In3+ type (Navarro Yerga et al., 2009)

Conclusion

Research in photocatalysis initiated by Fujishema and Honda to split the water has been proved
to very trending and till now more than 130 photocatalysts have been investigated. Use of
sacrificial reagents increases the research interest to oxidize and reduce the water which results
in hydrogen production. After understanding the dynamics of the process, it is revealed that the
efficiency of whole process is dependent on photocatalyst used. Due to many problems
associated with photocatalyst used and the mechanism inside photocatalyst is not fully explored
quantum yield is only 5.9%. Improvements requires are (i) understanding the detail mechanism
of oxidation reduction of water on the surface of photocatalyst (ii) whenever a co-catalyst
involves charge transfer mechanism from the surface of photocatalyst to co-catalyst (iii) surface
chemistry of photocatalysts and imperfections on surface of photocatalyst. Thus, all
photocatalysts need more effective research and understanding. Improvements in the areas of
research such as effect of impurities present on the surface area and other material effect can bse
helpful in discovering more and more about water splitting. Using synthetic methods both the
electronic structure and reactivity can be modified. Consequently, efficiency of photocatalyst
water splitting process increases in future using developed photocatalysts

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