Nanostructured bilayered thin films in photoelectrochemical water splitting – A review

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Review

Nanostructured bilayered thin films in photoelectrochemical water splitting e A review Surbhi Choudhary a, Sumant Upadhyay a, Pushpendra Kumar a, Nirupama Singh a, Vibha R. Satsangi b, Rohit Shrivastav a, Sahab Dass a,* a b

Department of Chemistry, Dayalbagh Educational Institute, Agra-282110, India Department of Physics & Computer Sciences, Dayalbagh Educational Institute, Agra-282110, India

article info

abstract

Article history:

In the quest for achieving the desired efficiency, balanced economics and prolonged

Received 20 April 2012

durability of the photoelectrochemical (PEC) system for hydrogen generation, hetero-

Received in revised form

structures consisting of two or more semiconductors are being looked upon as favourite

5 October 2012

material alternatives. This communication describes the basic principles involved and

Accepted 8 October 2012

summarizes most of the work done in this domain. Band gap, electronic band edge

Available online 3 November 2012

alignment of the materials with each other and with the redox potential of water, lattice mismatch of the materials and optimization of thickness of each layer at the junction in

Keywords:

the PEC devices appear to be crucial for attaining enhanced photoresponse and efficiency.

Photoelectrochemical

Based on the studies reported in the literature and from our own studies, heterojunction

Heterostructures

systems are considered as effective tool towards extending the spectrum to the visible

Lattice mismatch

range and for effective separation of charge carriers leading to development of efficient

Heterojunction

solar hydrogen production system. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Non-renewable energy resources such as fossil fuels and their reserves take millions of years to form but are being depleted very rapidly. The production, transmission and use of fossil fuels also lead to environmental degradation. Combustion of carbon-based fossil fuels generates CO2, which act as air pollutants and also form a part of greenhouse gases e the culprit for global climate change. Hydrogen serves as an attractive energy carrier because the use of hydrogen as a fuel can achieve much higher energy conversion efficiencies than the conventional fossil fuels and offers environmentally benign and more sustainable energy system with lesser

emission [1,2]. It has the potential to meet the world’s energy requirements and can act as a substitute for conventional fuel as a clean non-fossil fuel in the future provided it can be produced by using the world’s most abundant energy source i.e. the sun. While there are various routes available for solar hydrogen generation, PEC splitting of water into hydrogen and oxygen by the direct use of sunlight is an ideal, low cost, environment friendly and renewable method of hydrogen production that integrates solar energy collection and water electrolysis in a single photocell [3e6]. This review attempts to highlight the basics of the working of bilayered (heterojunction) systems as well as summarizes

* Corresponding author. Tel.: 91 9219695960; fax: þ91 0562 2801226. E-mail address: [email protected] (S. Dass). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.028

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various studies on this aspect by various researchers and from our own group.

2.

Photoelectrochemical water splitting

PEC solar hydrogen production, a well established method of hydrogen generation by the splitting of water is considered to be a superior technology as hydrogen could be obtained directly from abundantly available renewables i.e. water and solar light [3e8]. Photoelectrochemical water splitting incorporates conversion of solar energy into electrical energy by using semiconductor/electrolyte junction. The efficient photoelectrochemical cell converts water into hydrogen and oxygen using sunlight in a two electrode system; semiconductor (possessing either p or n type conductivity) as working electrode and platinum (Pt) as counter electrode (as shown in Fig. 1). However in most measurements reference electrode is also employed to investigate half reactions in the PEC cell. Upon illumination with photons having energy equal or larger than the band gap (Eg) of the semiconductor, electron-hole pairs are generated. In case of n-type semiconductor electrode, the holes react with water molecules at semiconductor surface resulting into O2 formation whereas electrons travel through substrate and are transported to the counter electrode where they reduce Hþ into H2. On the contrary, p-type semiconductor electrode produces H2 and O2 at semiconductor electrode and at counter electrode respectively [5]. In other words, fast transfer of electrons towards the electrolyte than that of holes leads to flow of cathodic photocurrent ( p-type semiconductor), while fast hole transfer towards the electrolyte resulting into anodic photocurrent (n-type semiconductor) in photoelectrochemical water splitting [9]. Fermi energy level e an electronically conducting phase of a metal/semiconductor, measured with respect to reference energy level, defines that the probability of occupancy of an electronic energy level (free electrons) is one-half. The relative Fermi energy (Ef) position of the semiconductor depends on

Fig. 1 e Schematic diagram of a typical photoelectrochemical cell consisting of n-type semiconductor photoanode, reference (SCE) and metal cathode for water splitting.

the electron and hole concentration i.e. on n- or p-type conductivity of the semiconductor. For p-type semiconductor, the Fermi energy level lie above the valence band, while Fermi energy level of n-type semiconductor is located below the conduction band. When the semiconductor electrode is immersed in a redox electrolyte, charge carriers are transferred at semiconductor/electrolyte interface until equilibrium is attained i.e. the condition when Fermi energy of the semiconductor and the redox potential are at the same level on both sides of the interface. The charge transfer process results in generation of potential gradient (i.e. space charge region) across the interface that leads to bending of energy bands either upward (n-type semiconductor) or downward (ptype semiconductor). Upon illumination, a photovoltage is generated in photoelectrode due to the separation of photogenerated eehþ pairs in the band bending region. The charge separation persists until the bands are flattened, afterwards the photovoltage or the charge separation cannot be increased by further intense illumination. The presence of light leads to lowering of the Hþ/H2 potential while application of external bias results in elevation of Fermi energy level of cathode above the Hþ/H2 energy level, thereby facilitating electron transfer to Hþ ions of the electrolyte for generation of hydrogen. In PEC system the electrons are mainly responsible for generation of molecular hydrogen either on the counter (Pt) electrode in ntype semiconductor or on the semiconductor electrode in the p-type semiconductor [10]. The average distance travelled by electron/hole before trapping or recombination is referred as e/hþ diffusion length (L). It has been reported that the light absorbed within a distance of space charge width and e/hþ diffusion length contribute to the photo-response in water splitting process [11,12]. For efficient PEC systems, the diffusion length of charge carriers (L) should be comparable or larger than the thickness of film (d ) i.e. L w d or L > d, so that photogenerated e/hþ are collected before their recombination i.e. enhancement in the lifetime of charge carriers [12]. For efficient water splitting photogenerated charge carriers needs to be separated quickly and transferred at the counter electrode through external circuit without recombining with holes. The recombination losses in the bulk as well as at the surface reduce efficiency if the charge transfer kinetics within the electrolyte is slow [13]. The positions of the conduction and valence band edges of the semiconductor are important in determining the spontaneous water splitting and also the stability of material in the PEC cell. A semiconductor capable of spontaneous water splitting must have band gap of w2 eV with conduction band energy higher and valence band energy lower than that of reduction and oxidation potential of water respectively [14]. These conditions are energetically favourable for efficient transfer of charge carriers between semiconductor and electrolyte. Thermodynamically water splitting is not a spontaneous process (DG ¼ 237 kJ/mol) under normal temperature and pressure. Theoretically thermodynamic potential of the order of 1.23 eV is required for the generation of hydrogen and oxygen by water splitting but taking into account the energy losses (w0.8 eV) caused by the recombination of the photogenerated charge carriers, resistance of the electrodes and

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electrical connections and voltage losses at the contacts etc., a voltage of 1.8 eV is needed for spontaneous water splitting. Accordingly a semiconductor with optimum band gap is considered more suitable in PEC water splitting [15]. The performance of any photoelectrochemical cell is best described by measuring its efficiency. Various types of efficiency terms described in the literature [15e17] are summarized as: a) Solar-to-hydrogen conversion efficiency (STH):- Also termed as Benchmark efficiency as it describes the overall efficiency of a PEC water splitting device exposed to AM 1.5G illumination under unbiased conditions. STH efficiency is expressed as a fraction of produced chemical energy to that of input solar energy (i.e. power out/power in). The produced chemical energy is defined as the product of rate of hydrogen production (measured by gas chromatography or mass spectrometry in mmol H2/s) with the change in Gibbs free energy per mol of H2 (at room temperature) while input solar energy is measured as the product of incident illumination power density (Ptotal, mW/cm2) with that of illuminated electrode area (cm2).  
STH ¼ ½ðmmol H2 =sÞ  ð237 kJ=molÞ= Ptotal mW=cm2
 Area cm2 AM 1:5G

(1)

b) Applied bias photon-to-current efficiency (ABPE):- ABPE measurement is not an actual solar-to-hydrogen efficiency value as this efficiency is measured in the presence of external bias i.e. the energy not generated from sunlight. However dependence on external bias is not the sole criterion for determining ABPE of water splitting process. In addition the magnitude of photocurrent generated is also an important factor towards achieving higher efficiency in photoelectrochemical process as shown in equation (2).

   ABPE ¼ Jph mA=cm2 ð1:23jVb jÞðVÞ Ptotal mW=cm2 AM 1:5G (2) where Jph is the photocurrent density obtained under an applied bias Vb. c) External quantum efficiency (EQE) or Incident photon-to-current efficiency (IPCE):- IPCE defines the collected photocurrent per incident photon flux as a function of illumination wavelength. IPCE or EQE term for a photoelectrochemical device explains its efficiency in terms of “electrons out per photons in”. IPCE is calculated as the ratio of photocurrent measured by chronoamperometry (potentiostatic) experiment in three electrode system vs the rate of incident photons from monochromatic light source at various wavelengths.
  IPCEðlÞ ¼EQEðlÞ ¼ Jph mA=cm2  1240ðV  nmÞ
   Pmono mV=cm2  lðnmÞ

(3)

d) Internal quantum efficiency (IQE) or Absorbed photon-to-current efficiency (APCE):- IQE or APCE defines efficiency of a PEC device in terms of collected photocurrent per absorbed photon. In other words, APCE is used to establish

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a relationship between maximum photon absorption with minimum effective charge carrier movement within the semiconductor material.
  APCEðlÞ ¼IQEðlÞ ¼ Jph mA=cm2  1240ðV  nmÞ 

  Pmono mV=cm2  lðnmÞ  1  10A

(4)

where, A is the absorbance of thin film sample, calculated as the logarithmic ratio of measured output light intensity (I ) vs input light intensity (I0) and determined by UVevis spectroscopy. III-V semiconductor technology based multijunction PEC devices have been demonstrated at National Renewable Energy Laboratory (NREL) with 16% solar-to-hydrogen (STH) conversion efficiency [18] having GaAs PV junction as the bottom cell and Ga-InP PEC junction as the PEC top cell incorporated in hybrid tandem device. But these devices suffer from long duration stability issues and are very expensive [19]. Low cost WO3 multijunction based PEC devices have also been reported with stable STH conversion efficiencies in the range of 3e5% [5,20]. Following benchmarks emerge to make PEC technology commercially viable as reported by Department of Energy (DOE), U.S. [15]:➢ ➢ ➢ ➢

3.

Conversion efficiency e 10% Current density (Jpc) e 10e15 mA/cm2 Material durability > 2000 h Economically feasible

Semiconductor photoelectrodes

The aim of PEC material fabrication is to design a photoelectrode that has the potential to satisfy most of the requirements viz. (a) sufficient visible light absorption i.e. band gap in the range of 1.8e2.2 eV, (b) efficient separation and fast transport of photo-generated electron-hole pairs to prevent recombination, (c) favourable conduction and valence band edge position with respect to redox potential of water, (d) non-corrosive and high chemical stability in the electrolyte, and (e) low cost [15,21,22]. Large band gap semiconducting oxides such as TiO2, WO3, SrTiO3, BaTiO3, SnO2, ZnO etc. are stable in aqueous electrolyte but absorb in UV region which is only about 4% of the solar spectrum, whereas small band gap semiconductors such as Si, GaAs, InP, CdTe, CdSe, CuO etc. and optimum band gap semiconductor viz. Cu2O have the potential to absorb visible part of solar spectra but corrode when dipped in electrolyte [23,24]. Intermediate band gap semiconductor like Fe2O3 absorbs in the visible region but suffer from poor semiconductor characteristics due to redox level mismatch, low mobilities of holes and trapping of electrons by oxygen-deficient iron sites. In order to improve the lifetime of PEC system, the semiconductor material must have adequate electrochemical stability so that the charge carriers reaching at its surface drive only the water splitting reactions without any side reactions (i.e. electrode corrosion). Semiconductor materials are found to be more resistant to reduction reactions than that of oxidation reactions, which makes p-type material more suitable than n-type material with

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respect to stability. Alternatively, the thermodynamic instability of photoelectrodes is due to ability of photo-generated holes to oxidise the semiconductor [25]. Though, if the charge transfer kinetics across the n-type semiconductor/ electrolyte interface is faster than the anodic oxidative reaction, photo-corrosion can be inhibited [11,26,27]. The stability of both (n and p type) semiconductor material may be improved by catalytic surface treatments which increases the charge transfer from semiconductor surface to the solution [28e30]. In this context, various attempts have been made to advance the design, fabrication and modifications of semiconductor nanostructured materials used in PEC cell. It has been reported that efficiency of water splitting reaction is greatly influenced by the electron transfer process, band gap energy and band structure of the semiconductors [31]. Thus, efficient photoelectrodes based on visible-light-responsive semiconductors can be modified using various strategies such as: doping, swift heavy ion irradiation (SHI), metal ion loading, composites of mixed oxides, bilayered systems and dye-sensitization. Out of the above mentioned systems, bilayered systems are the most exciting and recent strategy to achieve absorption of large part of the solar spectrum for increasing the efficiency of the process. Multilayers expressed by ½ðAÞt1 =ðBÞt2 n where t1 and t2 are the thicknesses of semiconductor layers deposited n times, are called as bilayers (n ¼ 1) or heterostructures. Generally bilayers encompass two main aspects: (i) the fine control of the interfaces by synthetic techniques, which may lead to unexpected properties [32e34], and (ii) bilayer of two materials may cover large portion of solar spectrum [35,36]. While there are number of excellent reviews available on semiconductor based composite materials [37,38], thin film solar cells [39] and photoelectrochemical cells [40], exhaustive discussion on the bilayered semiconductor systems used in the PEC water splitting is apparently not adequately discussed. This article attempts to review the scientific and technical issues involved in the bilayered systems and their use in PEC water splitting.

4. Mechanism of bilayered photoelectrodes in PEC water splitting Bilayered electrodes comprises of two semiconductors, one with a wide band gap while another with a small/mid band gap. Bilayers are the multifunctional materials which combine different properties of the individual semiconductor layers. The small band gap semiconductor is mainly responsible for sensitizing the large band gap semiconductor through electron or hole injection by visible light absorption. Efficient electron injection requires proper positioning of conduction and valence bands of large and small/mid band gap semiconductor [41e43]. The electrons transfer between the two semiconductors may enhance the charge separation and inhibit the recombination rate by forming a potential gradient at the interface. The energy layers in the bilayered devices can cover broad visible spectrum thereby offering synergistic effect. The overlying semiconducting layer absorbs photons of

high energy, while being transparent to photons of lower energy. Subsequent underlying layer absorb the lower energy photons [44]. Thus, synergistic effect of both layers facilitates enhanced photocurrent and improved photoconversion efficiency [35,36]. A bilayered/heterojunction system comprises of electronaccepting semiconductor, hole-accepting semiconductor (with well aligned band edges and high electron or hole conduction ability) and the interface formed at the junction due to difference in the direction of electron or hole movement across the heterojunction [45]. The existence of interface with varying charges may lead to formation of internal electric field at the heterojunction which may induce the separation of photogenerated eehþ pairs in both the semiconductors and at the interface, resulting into reduction in eehþ recombination. The mechanism of bilayered semiconductor photoanode (in case, when the conduction band of the material deposited on substrate is lower than respective band of outer material which is in contact with electrolyte i.e. nen junction as shown in Fig. 2(a) and (b)) in the PEC cell can be understood as upon irradiation bilayered film leads to the excitation of electrons in the valence band of both the layers to the conduction band, leaving holes in the valence band. The absorbed light photons generate excitons throughout the bilayered semiconductors. The photogenerated electrons in the outer layer are injected into the conduction band of underlying material (coated on substrate) due to the effect of heterojunction interface, and the electrons easily reach at the substrate surface, and are transferred to the cathode via back contact to produce hydrogen. Photogenerated holes on the other hand move into the valence band of outer material in the opposite direction i.e. driven by the built-in field of heterojunction, and finally holes are carried out by the electrolyte (H2O) to produce oxygen at photoanode in the PEC cell. Alternatively, the electron-hole transfer across the favourable band edge position of the photocathodes (i.e. pep junction) occur in the opposite directions compared to nen junction (Fig. 2(c)) producing H2 and O2 at semiconductor and counter electrode respectively. Furthermore, the favourable energetics in pen junction leads to electron-hole transfer across the junction as shown in Fig. 2(d). In bilayered semiconductors, the photogenerated charge carriers travel few nanometers before reaching the interface, which is proportionate with the diffusion length of charge carriers [46], which may lead to efficient charge separation in bilayered systems. Electron-hole pairs are effectively separated at the junction of wide and mid/small band gap semiconductors as shown in Fig. 2, which leads to the enhanced photoelectrochemical activity of bilayered thin films than single layered films. The synergistic effect results in enhancement in the characteristic photoelectrochemical response as discussed earlier [36,47e49]. However, proper alignment of the energy levels between the wide and mid/small layers is the fundamental requirement for efficient charge transfer in PEC water splitting. Optimization of thickness of the bilayered films is yet another important criterion towards facilitation of efficient separation of photogenerated charge carriers and their movement across the interface for photocurrent improvement [50]. To summarize, the efficiency of bilayered systems

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Fig. 2 e Favourable band edge alignment of wide/mid/narrow band gap semiconductors with redox potential of water and possible charge transfer across various heterojunctions under illumination.

for water splitting depends on their appropriate band gap, proper band edge alignment with each other, minimum lattice mismatch and high stability. Significant prevalence of bilayered films over single layered thin films for enhancement in photocurrent and photoconversion efficiency may be ascribed to the following reasons: ➢ Absorbance of large band gap material shifted towards visible region i.e. red shift. ➢ Improvement in charge separation of both the material that results in decrease of recombination of charge carriers. ➢ Increase in lifetime of charge carriers. The characterizations that can be used in assessing the above mentioned advantages of bilayered films over monolayered films are: i) UVevis spectroscopy to determine the extent of visible light absorption by the bilayered system [51], ii) Electrochemical impedance spectroscopy (EIS), a powerful

tool to examine the interaction of semiconductor/electrolyte interface [52], iii) Time-resolved spectroscopy where the photocurrent is monitored as a function of time, depicts the existence of defects that change the concentration of the charge carriers within the system [53], iv) Surface-restricted grating method to generate spatial pattern of eehþ pairs and free-carrier density near the interface [54], v) Potentialmodulation-induced microwave reflectance method to determine the density of free carriers at the semiconductor/ electrolyte interface [55], vi) Photoluminescence spectroscopy to analyze distribution of charge and potential at semiconductor/electrolyte junction [56].

5.

Various bilayered system studied

This overview provides a brief discussion on some recent research activities in the area of PEC water splitting based on nanostructured bilayered semiconductor materials. Summary

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of various bilayered systems so far studied in photoelectrochemical splitting of water has been presented in Table 1.

5.1.

CdS and CdSe based bilayered systems

The stability of the PEC performance of the semiconductor electrode (CdSe/CdS) in redox electrolyte was accounted by Martirosyan et al. in 2001 [57]. It was found that the presence of Cd in the redox electrolyte (NaOH:S:Na2S) protects the substrate (CdSe/CdS) against the attack of the electrolyte, which may eventually cause short-circuiting in the PEC process. Mahapatra et al. in 2010 [58] electrodeposited mixed CdSeCdSe on nickel substrate and used thymol blue as a sensitizer to the redox electrolyte and reported 5% conversion efficiency of CdSeCdSe electrode in PEC cell against 4.42% without the addition of dye. The improved performance of PEC cell comprises of CdSeCdSe semiconductor electrode attributed to reduction in band gap, higher value of flatband potential and smaller electron affinity of the semiconductor electrodes. Addition of small amount of suitable dye to the redox electrolyte could enhance the efficiency, mainly due to increase in the density of charge carriers but the enhancement in efficiency values obtained after the addition of dye was not so significant, which may be due to the existence of surface states, acting as recombination centres for electron-hole pairs. In 2010, Lee et al. [59] introduced a cascade structure of TiO2/ CdS/CdSe in which band edges of the three materials aligned in stepwise manner which was beneficial in the transport of excited charge carriers across the composite electrode. Current response of electrodes at externally applied positive bias increased in the order: TiO2/CdSe z TiO2/CdSe/CdS < TiO2/ CdS < TiO2/CdS/CdSe, indicating that TiO2/CdS/CdSe electrode has a band edge structure with superior ability for charge transfer. The stepwise band-edge structure built in the TiO2/ CdS/CdSe electrode and also the electric field in space-charge region, was favourable to the electron injection and hole recovery of the system, responsible for the high photocurrent in the TiO2/CdS/CdSe electrode. While, for the inverse (TiO2/CdSe/ CdS) structure, the band edges of intermediate CdSe were higher than those of CdS, resulting in energy barriers for injected excited electrons from the outer CdS layer and transferring hole from the inner CdSe layer. Thus, significant recombination of electron and hole was expected leading to low photocurrent. A similar work on CdS and CdSe nanoclusters co-sensitized TiO2 NTWs (nanotubes with nanowires) arrayed films prepared by SILAR (successive ionic layer adsorption and reaction) deposition method has also been reported by Cheng et al. in 2012 [60]. The stepwise band-edge structure in the TiO2/CdS/CdSe/ZnS heterojunction electrode is suggested to create efficient charge transfer channel and towards triggering a high resistance to transport excited electrons back to the electrolyte. The enhanced photoelectrochemical response of heterojunction may also be attributed to the morphology of TiO2 NTWs, formation of fine heterojunction between CdS and TiO2 and broader spectral response of CdSe than CdS in the solar region.

5.2.

CdSe based bilayered systems

Photoelectrochemical behaviour of sequentially deposited SnO2 and CdSe on an optically transparent electrode (OTE) has

been reported by Nasr et al. in 1997 [61]. The favourable energetic positioning of the band edges of SnO2 and CdSe leads to improved performance of OTE/SnO2/CdSe over OTE/ CdSe in terms of enhanced efficiency and stability. In OTE/ SnO2/CdSe electrode, upon illumination electron-hole pairs were generated in visible light responsive CdSe. The photogenerated electrons in CdSe readily migrated to the lowerlying conduction band of SnO2 and migrated through the film to the back contact OTE and contribute to the photocurrent, while the holes were scavenged by the electrolyte. Consequently, the photogenerated electrons escape from recombination with photogenerated holes in CdSe and are collected in higher concentration at the back contact OTE producing enhanced photocurrent. Aloney et al. in 2009 [62] reported 10 mA/cm2 current density under illumination of 1950 lux light intensity by using CdSe/ZnSe/1 M NaOHeNa2SeS/C (graphite) based PEC cells. The PEC cell performance has been reported to improve with increased deposition time however dissolution of film was also observed in case of deposition time duration greater than 60/60 min, which is attributed to the increase in rate of dissolution than the rate of deposition after attaining the maximum thickness. The performance of these cells also improved by increasing the deposition current density (JD) during preparation of photoelectrodes, but at JD > 10 mA/cm2 the films did not adhere properly to the substrate and got dissolved in the electrolyte. The annealing effect on the photoelectrochemical performance of CdS/CdSe-sensitized TiO2 photoelectrode has been discussed by Chi et al. in 2010 [63]. They concluded that higher annealing temperature (400  C) may activate oxidation and decomposition of the sensitizers which may be harmful to the photoelectrodes but optimal annealing (w300  C) can increase the crystallinity of the CdS and CdSe and also enhance the charge transport characteristic of the photoelectrode, leading to better performance of the TiO2/CdS and TiO2/CdSe electrodes, though with little effect on inhibition of photocorrosion.

5.3.

CdS based bilayered systems

CdS photoelectrodes coated with titania nanosheets were studied by Yamada et al. in 2005 [64] towards protection of surface from photocorrosion and to enhance the catalytic activity for water oxidation. Titania coated CdS electrodes containing Cu(II), Ni(II), and In(III) ions exhibited suppressed dissolution but no photocurrent was recorded in Na2SO4 solution at the applied potential of 0.20 V (vs Ag/AgCl), probably because these metal ions act as the recombination centre for photoinduced electron-hole pairs in CdS. The corrosion of CdS/titania electrodes prepared with Zn(II) and Cd(II) ions, was blocked apparently due to the stabilization of surface S radicals by the formation of metal complexes. Under dark condition, metal ions were blocked for anodic corrosion, while the surface of CdS/titania electrodes corroded under irradiation similarly as in bare CdS indicating that Zn(II) and Cd(II) ions between titania sheets are not stable for the oxidation with the holes of CdS. For bare CdS, photocurrent increased initially because of the dissolution of CdS surface. When the surface was coated with one layer of titania nanosheets (TiOx) with cationic polymer PEI (polyethyleneimine), the dissolution

Table 1 e Summary of various bilayered semiconductor systems used in photoelectrochemical water splitting. Reference

Bilayered system

Fabrication method

Photocurrent

Voltage

OTE/ZnO/CdS OTE/TiO2/CdSe

Nasr et al., 1997 [61]

OTE/SnO2/CdSe

Martirosyan et al., 2001 [57]

CdSe/CdS

Chemical deposition Electrochemical deposition Electrochemical deposition Electrodeposition

Siripala et al., 2003 [35]

Cu2O/TiO2

Electrodeposition

0.7 mA/cm2

1 V vs Ag/AgCl

Liu et al., 2004 [87]

Mercurochrome-sensitized TiO2/SnO2 & ZnO/SnO2 composites Cu2O/ZnO/ITO peien heterojunction CdS/titania nanosheets

Sol-gel

0.757 mA/cm2 (TiO2/SnO2) 0.892 mA/cm2 (ZnO/SnO2) e

0V 0V

Yamada et al., 2005 [64]

Electrochemical deposition Chemical bath deposition (CBD) MW-CBD

Luo et al., 2007 [89]

Cu2Oeporous TiO2 heterostructure WO3/Fe2O3/FTO

Wang et al., 2007 [48]

SrTiO3/a-Fe2O3/FTO

Sol-gel Spin-coating

Yin et al., 2007 [72]

ZnFe2O4/TiO2/ITO

Dip coating

Yin et al., 2007 [65]

CdS/TiO2 nanotube arrays

Electrodeposition

Chi et al., 2008 [66]

Composite ITO/TiO2/CdS

Seabold et al., 2008 [73]

CdTe/TiO2/FTO

Yamane et al., 2009 [95] Yu et al., 2009 [96]

n-Si/p-CuI/ITO/Si/ GaP/ITO/RuO2 Silicon NW (nanowire)/TiO2

TiO2 by Spin-coating and CdS-QDs by CBD DC sputtering and potentiostatic anodization Vacuum deposition

Lin et al., 2009 [82]

TiO2/TiSi2

Vigil et al., 2005 [70]

Sol-gel Spin-coating

TiO2 by chemical vapour deposition and SiNW arrays by chemical etching of Si wafers ALD

0 V/SCE e

0.027 mA/cm2

e 0 V/SCE

e

e

e

e

e

e

e

Light source 1000 W xenon lamp 1000 W tungsten halogen lamp 250 W xenon lamp (300 W/cm2) Halogen lamp (70 mW/cm2) Xenon lamp (700 W/cm2) 400 W xenon lamp (30 W/m2) e 50 W tungsten lamp (3 mW/cm2) 150 W halogen xenon lamp 500 W xenon lamp

2.5 mA/cm2 (Fe2O3) 6 mA/cm2 (WO3/Fe2O3) 9.80 mA/cm2 (SrTiO3) 17.0 mA/cm2(a-Fe2O3) 52.7 mA/cm2(SrTiO3/a-Fe2O3) 0.02 mA/cm2 (TiO2) 0.1 mA/cm2 (ZnFe2O4/TiO2) w5 mA/cm2

0.3 V 0.3 V 0.3 V vs Ag/AgCl 0 V/SCE 0 V/SCE 0.2 V vs Ag/AgCl

5.5 mA/cm2

0.5 V vs Ag/AgCl

0.05 mA/cm2 (TiO2) 0.23 mA/cm2 (CdTe/TiO2)

0V 0 V vs Ag/AgCl

1.88 mA/cm2

w0.9 V vs Ag/ AgCl/Sat. KCl 3.0 V vs SCE

Solar simulator AM 1.5G (100 mW/cm2) Xenon short arc lamp (100 mW/cm2)

0 V vs Ag/AgCl

150 W xenon lamp (100 mW/cm2)

0.50 mA/cm2 (n-Si/TiO2) 1.44 mA/cm2 (n-SiNW/TiO2)

0.6 mA/cm2

w0.7 V w0.7 V vs Ag/AgCl

400 W xenon lamp (240 mW/cm2) 200 W xenon lamp Xenon lamp (50 mW/cm2) 150 W xenon lamp (100 mW/cm2) 300 W xenon arc lamp (6.0 W/cm2)

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Hotchandani et al., 1992 [41] Liu et al., 1993 [69]

Zhang et al., 2004 [84]

0.06 mA/cm

2

(continued on next page)

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Reference

Bilayered system

Fabrication method

Cu2O/TiO2 nanotube heterojunction arrays

Kale et al., 2009 [44]

TiO2/In2S3/CdSe

Electrochemical anodization combined with photoreduction deposition Wet chemical

Aloney et al., 2009 [62]

CdSe/ZnSe

Electro-co-deposition

Sivula et al., 2009 [90]

FTO/WO3/Fe2O3

APCVD

Kuang et al., 2009 [91]

Fe2O3/TiO2

S-CBD

Sharma et al., 2010 [50]

ZneFe2O3/FeeTiO2/ITO

Mahapatra et al., 2010 [58]

CdS/CdSe

Sol gel for FeeTiO2 & spray deposition for ZneFe2O3 Electrochemically galvanostatic deposition

Wang et al., 2010 [85]

CdSeZnOeZnOeCdSe nanowire arrays ITO/ZnO/CdS

Dang et al., 2010 [49] Dang et al., 2010 [74] Ajuba et al., 2010 [86] Shin et al., 2010 [76]

ITO/TiO2/CdS nanocomposite ZnO/NiO CdS or CdSe decorated TiO2 nanotube arrays

Hydrothermal Vacuum deposition by thermal evaporation Vacuum deposition by thermal evaporation Chemical bath deposition Spray pyrolysis deposition

Wijesundera et al., 2010 [67]

Ti/p-CuO/n-Cu2O/Au

Electrodeposition

Lee et al., 2010 [59]

ITO/TiO2/CdS/CdSe

Spin coating for TiO2 & CBD for CdS and CdSe

Zhang et al., 2010 [77]

TiO2/SrTiO3 nanotube arrays WO3:Mo/WO3

Electrochemical anodization

Gaillard et al, 2010 [92] Hensel et al, 2010 [75]

CdSe (QDs)/TiO2:N

Reactive RF magnetron sputtering TiO2 nanoparticles by Sol-gel method and CdSe QDs by CBD

2

Voltage

Light source

0.04 mA/cm (TiO2 nanotube arrays) 0.24 mA/cm2 (Cu2O/TiO2 heterojunction arrays)

0 V vs SCE 0 V vs SCE

Simulated sunlight (33 mW/cm2)

0.05 mA/cm2 (ITO/TiO2/In2S3) 0.08 mA/cm2 (ITO/TiO2/In2S3/CdSe) 1.16 mA/cm2 (ITO/In2S3/CdSe) 0.64 mA/cm2 (CdSe) 0.28 mA/cm2 (ZnSe) 1.031 mA/cm2 (CdSe/ZnSe) 1.41 mA/cm2 (Fe2O3) 1.71 mA/cm2 (WO3/Fe2O3) w0.1 mA/cm2 (TiO2 NTs) w0.5 mA/cm2 (Fe2O3/TiO2 NTs) 0.072 mA/cm2 (ZneFe2O3) 0.7 mA/cm2 (ZneFe2O3/FeeTiO2) 2.9 mA/cm2 (CdSe) 3.3 mA/cm2 (CdS) 4.2 mA/cm2 (CdS/CdSe) w12 mA/cm2

0V 0V 0V 0V 0V 0V 1.43 V 1.43 V vs Ag/AgCl 0.1 V/SCE 0.1 V/SCE

80 mW/cm2

412 mA/cm2

0V

0.5 mA/cm2 (TiO2) 35 mA/cm2 (TiO2/CdS) e 0.1 mA/cm2 (TiO2 nanotubes) 5.0 mA/cm2 (CdS or CdSe modified nanotube arrays) 310 mA/cm2

0V 0V

Halogen lamp

e 1 V vs Ag/AgCl 1 V vs Ag/AgCl

e 150 W xenon lamp (100 mW/cm2)

0 V/SCE

4.9 mA/cm2 (TiO2/CdS) 5.1 mA/cm2 (TiO2/CdSe) 14.9 mA/cm2 (TiO2/CdS/CdSe) 1.0 mA/cm2 (TiO2) 1.75 mA/cm2 (TiO2/SrTiO3) 3.18 mA/cm2 (WO3) 1.91 mA/cm2 (WO3:Mo) 3.63 mA/cm2 (WO3:Mo/WO3) w25 mA/cm2 (TiO2 & TiO2:N) 0.15 mA/cm2 (CdSe/TiO2) w0.30 mA/cm2 (CdSe (QDs)/TiO2:N)

w0.7 V w0.7 V w0.7 V vs Ag/AgCl 0.2 V/SCE 0.2 V/SCE 1.6 V vs SCE 1.6 V vs SCE 1.6 V vs SCE 0 V vs Ag/AgCl 0 V vs Ag/AgCl 0 V vs Ag/AgCl

100 W tungsten lamp (90 mW/cm2) 300 W xenon lamp with AM 1.5G filter (100 mW/cm2) 450 W xenon lamp

0.95 V/SCE 0.95 V/SCE 0 V/SCE 0 V/SCE 0 V/SCE 0.4 V vs Ag/AgCl

200 W tungsten lamp (1950 lux) 450 W Xenon lamp (100 mW/cm2) Xe lamp (100 mW/cm2)

150 W xenon lamp (150 mW/cm2) 500 W tungsten lamp (40 mW/cm2) 1000 W xenon lamp (100 mW/cm2) Halogen Lamp

Solar simulator AM 1.5G 1000 W xenon arc lamp (100 mW/cm2)

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Hou et al., 2009 [71]

Photocurrent

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Table 1 e (continued )

Electrodeposition followed by ALD

7.6 mA/cm2

0 V vs Ag/AgCl

AM 1.5 (100 mW/cm2)

Wet impregnation

Yang et al., 2011 [79]

CuO and WO3 loaded TiO2 nanotube

0.8 V vs Ag/AgCl 0.8 V vs Ag/AgCl 0.75 V vs Ag/AgCl 0.75 V vs Ag/AgCl

300 W xenon lamp (6.7 mW/cm2) 300 W UV lamp

Park et al., 2011 [80]

BiOxeTiO2/Ti

Iodization method for TiO2 annotate & Wet impregnation method for CuO and WO3 loading Sequential coating

w22 mA/cm2 (TiO2 nanotube) w29 mA/cm2 (WO3/TiO2 nanotube) w0.55 mA/cm2 (TiO2 annotate) w0.85 mA/cm2 (CuO & WO3 loaded TiO2 annotate) 1.7e2.0 mA/cm2

1.3 V/SCE

Dai et al., 2011 [81]

BiOI/TiO2 nanotube arrays

Banerjee et al., 2011 [83]

TiSi2/TiO2 nanotubes

Anodization for TiO2 & impregnating hydroxylation for BiOI Sonoelectrochemical anodization

w0.009 mA/cm2 (TiO2 NTs) w0.022 mA/cm2 (BiOI) w0.045 mA/cm2 (BiOI/TiO2) 0.9 mA/cm2 (TiO2) 3.49 mA/cm2 (TiSi2/TiO2)

0.5 V 0.5 V 0.5 V vs Ag/AgCl 0.2 V 0.2 V vs Ag/AgCl

450 W HgeXe arc lamp 300 W xenon lamp (25 mW/cm2)

Su et al., 2011 [94]

FTO/WO3/BiVO4 heterojunction

Spin-coating & solvothermal technique

0.8 mA/cm2 (planar WO3/BiVO4) 1.6 mA/cm2 (nanorod WO3/BiVO4)

1 V/SCE 1 V/SCE

McDonald et al., 2011 [93]

FTO/Fe2O3/ZnFe2O4 composite

Electrodeposition

0.4 V 0.4 V vs Ag/AgCl

Sharma et al., 2012 [97]

ITO/FeeTiO2/ZneFe2O3

0.95 V/SCE 0.95 V/SCE

150 W xenon lamp (150 mW/cm2)

Cheng et al., 2012 [60]

CdS, CdSe, and ZnS QDs sensitized TiO2 NTWs Cu2O/CuO composite

Sol-gel spin-coating method for FeeTiO2 & spray pyrolysis for ZneFe2O3 SILAR

w0.1 mA/cm2 (Fe2O3/ZnFe2O4 before Al-treatment) w0.4 mA/cm2 (Fe2O3/ZnFe2O4 after Al-treatment) 0.262 mA/cm2 (FeeTiO2) 1.65 mA/cm2 (FeeTiO2/ZneFe2O3) 4.30 mA/cm2

0 V/SCE

AM 1.5G (100/cm2)

1.54 mA/cm2

0 V vs Ag/AgCl

AM 1.5G (100 mW/cm2)

Wang et al., 2011 [78]

Zhang et al., 2012 [68]

Electrodeposition of Cu film on ITO followed by anodization

300 W solar simulator with AM 1.5 filter (87 mW/cm2) 150 W xenon lamp and AM 1.5 filter (100 mW/cm2) 300 W xenon lamp with AM 1.5 filter (100 mW/cm2)

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Highly active oxide FTO/Cu2O/AleZnO/TiO2/Pt nanoparticles WO3/TiO2 nanotube array

Paracchino et al., 2011 [88]

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was delayed but the photocurrent was decreased, and also significant decrease in photocurrent was observed upon doubly coating the TiOx. These observations confirmed that the photocurrent for CdS/PEI/TiOx/PEI/TiOx originates from the corrosion of CdS through an opening in the deposited nanosheets. Yin et al. in 2007 [65], proposed a unique fabrication route of core/sheath heterostructure CdS/TiO2 nanotube arrays by ac electrodeposition method. It was found that core/sheath CdS/TiO2 nanotube arrays electrode could potentially improve the efficiency of charge separation by increasing contact areas of CdS with electrolyte and CdS with TiO2 compared with sandwich electrodes. They reported that magnitude of the photocurrent density is related to the CdS deposition time, i.e. the CdS thickness. Thus, CdS nanotube arrays with thick walls have more band bending than a thin walled similar array and the rate of surface recombination decreases with the large band bending of the thicker walls, thereby increasing the photocurrent. Additionally, as the tube length of TiO2 nanotube columnar structure was increased, more photons were absorbed and consequently the photocurrent increased. Though the tube length was increased, the bulk and surface recombination rate of photoexcited electron-hole pairs were much less than the charge transfer rate at the interface. This may be due to the core/sheath heterostructure CdS/TiO2 architecture that resulted into a more effective surface area available to the electrolyte that enabled transport of holes to oxidizable species in the electrolyte. In 2008, Chi et al. [66] developed CdS-sensitized nanocrystalline TiO2 photoanode for water decomposition offering photoconversion efficiency of 3.67% under visible light illumination. The higher efficiency obtained in the CdS/TiO2 system implies that an applied potential helps in the effective separation of photoexcited electron-hole pairs which therefore, increases the energy conversion efficiency of the photoelectrode. Thicker TiO2 film resulted in a higher value of photocurrent (5.5 mA/cm2) which has been ascribed to the larger surface area available for incorporation of CdS and also the incorporation of CdS extends the optical absorption of TiO2 electrode to visible light, enhancing the visible-lightinduced photocurrent. In CdS/TiO2 photoelectrode, the photoexcited electrons in the CdS can be easily injected into the conduction band of TiO2, as the conduction band of CdS aligned higher than that of TiO2. The injected electrons were accumulated on conduction band of TiO2 and transferred to counter electrode where water was reduced to hydrogen. In order to prevent the photocorrosion by reducing the holes of CdS, S2 and SO2 3 were used as sacrificial reagents.

5.4.

Copper oxide based bilayered system

Wijesundera et al. [67] in 2010, electrodeposited single-phasic Cu2O on Ti/CuO electrodes in an aqueous solution containing 0.1 M sodium acetate and 0.01 M cupric acetate in the potential range of 250 to 550 mV/SCE. Well-covered photoactive ntype Cu2O thin films were electrodeposited on the Ti/p-CuO electrode at 550 mV/SCE in similar electrolytic conditions in which Cu2O was deposited on the Ti substrate. It was concluded that the thicknesses of the CuO and Cu2O semiconducting layers and annealing of the CuO/Cu2O

heterojunction plays a major role in enhancing the photoresponse in the PEC water splitting. Zhang et al. in 2012 [68] introduced a proficient two-step electrochemical strategy for the fabrication of highly efficient and stable copper oxide composite photocathode materials. The photocurrent density of copper oxide composite was found to improve more than 2 times than that for bare Cu2O electrode and the stability was also significantly enhanced from 30.1% to 74.4%. It was concluded that the CuO (top layer) in the Cu2O/CuO composite facilitates reduction of Cu2O photocorrosion and act as recombination inhibitor for the photogenerated electrons and holes from Cu2O, which leads to enhanced stability and PEC performance of Cu2O/CuO composite. The appropriate band energy structure of Cu2O/CuO composite permits the transfer of photogenerated electrons from Cu2O to the conduction band of CuO where the recombination of the electrons and holes considerably get reduced due to the involvement of phonon during transition in CuO. Moreover, Cu2O/CuO composite showed absorption of large portion of solar spectrum which is facilitated by narrow band gap of CuO, leading to a remarkable enhancement in PEC performance of Cu2O/ CuO composite.

5.5.

TiO2 based bilayered systems

A good amount of work has also been reported using TiO2 as one of the materials in bilayered photoelectrodes in PEC water splitting. Liu et al. (1993) [69] reported improved charge separation in the coupled TiO2/CdSe system offering advantageous effect in improving the photocurrent stability of semiconductor photoelectrode. The open-circuit voltage of these films was found to be independent of film thickness and remained constant around 650 mV exhibiting that the photoelectrochemical effect was initiated by the excitation of CdSe, and there was no direct contribution from the TiO2 film in initiating the photoelectrochemical effect. The mechanism of charge separation in thin CdSe films is reported to be governed by the different rates of electron and hole transfer at the semiconductor/electrolyte interface. The major issue in attaining photocurrent stability was increased recombination of charge carriers. By coupling CdSe thin film with TiO2 film, it was possible to inject photogenerated electrons into the conduction band of TiO2 which may have retarded the charge recombination within the CdSe film. Attempts have been made to address the corrosion limitation of Cu2O by electrodeposition of Cu2O/TiO2 heterojunction configuration by Siripala et al. [35] in 2003. They explained that the light absorbed by the Cu2O layer produces charge carriers where the excited electrons were driven to the TiO2/electrolyte interface through conduction band of TiO2 while holes were driven to the counter electrode by back contact. Thus, the charge separation basically occurred at the Cu2O/TiO2 interface. Similarly, sensitization of TiO2 with copper oxide was introduced by Vigil et al. in 2005 [70] using microwave-activated chemical bath deposition (MW-CBD) technique. Electrons were injected from copper oxide to TiO2 in Cu2Oeporous TiO2 heterostructure upon illumination, requiring no external bias for photocurrent generation in PEC, as confirmed by photocurrent spectra and its time behaviour. Hou et al. in 2009 [71] investigated the photoelectrochemical

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activity of Cu2O/TiO2 nanotube heterojunction arrays, synthesized by photoreduction of Cu2O nanoparticles on TiO2 nanotube arrays. On the basis of electrochemical impedance spectroscopy and photocurrent measurements, it was apparent that the interaction between p-type Cu2O and n-type TiO2 in Cu2O/TiO2 nanotube heterojunction array promotes the enhancement in charge separation efficiency and extended response of TiO2 into visible region. In 2007, Yin et al. [72] prepared ZnFe2O4/TiO2 coupled semiconductor system by dip-coating technique and observed a remarkable red-shift of the fundamental absorption edge with the increase in ZnFe2O4 thickness in the ZnFe2O4/TiO2 multilayered films. It was found that the photocurrent was mainly influenced by the thickness of ZnFe2O4. It was concluded that the difference in band-gap positions of ZnFe2O4 and TiO2 may lead to improved efficiency and enhanced concentration of photogenerated carriers in ZnFe2O4/TiO2 coupled semiconductor system, and thus greatly (five fold) enhance the photocurrent of ZnFe2O4/TiO2 as compared to TiO2 films. Seabold et al. in 2008 [73], fabricated CdTe/TiO2 bilayered photoelectrode by electrochemically filling tubes and tube-to-tube voids of TiO2 nanotube arrays with CdTe. It was found that thinner and smooth CdTe coating on the TiO2 tubes facilitates efficient electron transfer at the CdTe/TiO2 junction and hole transfer at the CdTe/ electrolyte junction, minimizing electron-hole recombination in the CdTe layer. Furthermore, the photocurrent generated by the CdTe layer alone was unstable, but CdTe/TiO2 electrode generated stable photocurrent, indicating that the efficient removal of photons, generation of electrons and holes from the CdTe layer were also beneficial for kinetically suppressing photocorrosion in the CdTe layer. CdTe/FTO exhibited p-type behaviour and generated cathodic photocurrent, while CdTe/ TiO2 electrodes generated anodic photocurrent, indicating that formation of TiO2/CdTe/electrolyte junction and the injection of photogenerated electrons from CdTe to TiO2 was more favoured than that from CdTe to the electrolyte. Kale et al. [44] in 2009 fabricated multi-layered photoelectrodes ITO/TiO2/In2S3/CdSe and analyzed the role of every descending band gap energy layer on the performance of PEC cell. It has been reported that photons having less energy than the band gap of TiO2, upon irradiating n-TiO2/n-In2S3/n-CdSe interfaces, generate electron-hole pairs which may be separated by the presence of electric field in the depletion region of TiO2/In2S3/CdSe. The presence of TiO2 thin film in multilayered electrode blocks electron-hole recombination due to its more negative conduction band level with respect to NHE. It was also observed that the electrodes with three successive descending band gap energies exhibited a good absorbance as photons of all energies could be absorbed, resulting in better photoconversion efficiency performance than the bilayers (ITO/TiO2/In2S3 and ITO/In2S3/CdSe). Dang et al. (2010) [74] reported significantly improved photoelectrochemical performance of TiO2/CdS composite film electrode as compared to ITO/TiO2 film, which opened the gateway for TiO2 microporous structures sensitized by CdS thin films to be used for fabricating high efficiency devices. When TiO2/CdS composite photoelectrode were irradiated by visible light, electrons generated in CdS were quickly transferred into the conduction band of the TiO2 and the holes

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accumulated in the valence band of the CdS, thereby reducing probability of electron decay and successful charge separation was achieved. Hensel et al. in 2010 [75] investigated the synergistic effect of CdSe quantum dots (QDs) sensitization and elemental (nitrogen) doping on enhanced photoelectrochemical performance of TiO2 nanostructured (nanoparticles and nanowires) photoanodes. Significant enhancement in charge transport and PEC performance in case of composites was observed than that for N-doped TiO2 or CdSe QD sensitized TiO2. It was explained that conduction band edge alignment of CdSe QDs and TiO2 nanoparticles permit efficient electron transfer from CdSe to TiO2 which leads to increased photocurrent. The photoelectrochemical performance of TiO2 nanotube arrays sensitized by low band-gap materials, viz. CdS and CdSe has also been investigated by Shin et al. in 2010 [76]. Remarkable increase in photocurrent from about 0.2 mA/cm2 (for 400 nm TiO2 nanotubes) to about 5.0 mA/cm2 (for the CdS or CdSe modified nanotube arrays) with 8.5% conversion efficiency has been reported. The photocurrent response of the CdS or CdSe modified TiO2 nanotube arrays has been reported to be 25 times higher than that of unmodified TiO2 nanotube arrays. CdSe decorated TiO2 nanotube arrays on the other hand with relatively short nanotubes (3 mm tube length) showed significant photocurrent value (10 mA/cm2), which confirmed that the charge-transfer processes between the well connected aggregated CdS or CdSe nanoparticles facilitates fast transfer of the photogenerated electrons from one CdS or CdSe to another and ultimately to the conduction band of TiO2, leading to reduced electron-hole recombination. In the same year Zhang et al. [77] accounted the photoelectrochemical performance of vertically aligned TiO2eSrTiO3 heterostructure array having controlled growth of SrTiO3 particles over TiO2 nanotube arrays and found that only welldispersed SrTiO3 nanocrystallites on TiO2 nanotube arrays, enhanced the overall PEC performance. They reported that TiO2/SrTiO3 composite heterostructure obtained with 1 h or less hydrothermal treatment exhibited the best PEC performance with nearly 100% increase in external quantum efficiency at 360 nm as compared to the untreated TiO2 nanotube electrode, which is attributed to enhanced crystallinity of SrTiO3, higher stability and a large negative flatband potential, indicative of large accumulation of electrons in the coupled heterostructure and decreased recombination of charge carriers. WO3/TiO2 nanotube array electrode have also been fabricated via wet impregnation method by Wang et al. in 2011 [78], which exhibited higher separation efficiency of photogenerated electron-hole pair and yielded higher steady-state photocurrent for oxidizing glucose as compared with the pure TiO2 nanotube array electrode. PEC studies were performed with electrolytes containing different concentrations of glucose. The photoelectrochemical oxidation of glucose with TiO2 and WO3/TiO2 nanotube array electrodes was investigated at constant potential bias (0.8 V) offering higher current in high concentration range of glucose (<2 mM) for WO3/TiO2 nanotube array electrode as compared with TiO2 nanotube array electrode. This suggests that the photoelectrocatalytic reaction of glucose on the surface of WO3/TiO2 nanotube array electrode is faster than that on the surface of

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TiO2 nanotube array electrode. Upon UV light illumination of WO3/TiO2 nanotube array electrode, the photogenerated electrons got transferred from the conduction band of TiO2 to that of WO3 as analyzed by EIS measurement and the holes were effectively scavenged by water or glucose, resulting in effective charge carrier separation and improved photoelectrochemical oxidation performance for glucose in WO3/ TiO2 nanotube array electrode than that of TiO2 nanotube array electrode. Yang et al. in 2011 [79] fabricated TiO2 annotate by loading CuO and WO3 on TiO2 by wet impregnation method. At 1 V, the photocurrent and hydrogen generation rate of CueWO3 loaded annotate was found to be higher than that of unloaded and pure WO3 loaded annotate and the photocurrent increases with the WO3 concentration. It was concluded that the improvement in efficiency of photoelectrocatalytic properties carried out by co-loading of CueWO3 on TiO2 annotates. Park et al. in 2011 [80] fabricated multi-layered BiOxeTiO2 electrodes with high doping of Bi (25 mol %) and observed that along with high dopant amount they retained photoelectrocatalytic and electrocatalytic activities. When exposed to light, the BiOxeTiO2 electrodes generated higher current than IrO2 and SnO2 electrodes due to the synergistic behaviour of BiOxeTiO2 electrocatalysts. These electrodes were studied for degradation of phenol and hydrogen production under direct UV irradiation. The anodic photocurrent for BiOxeTiO2 electrode increased by w50% under illumination on addition of phenol to the electrolyte, which was attributed to the charge carriers produced upon irradiation of BiOxeTiO2 anode and inhibition of recombination of charge carriers by the electron donor i.e. phenol. Dai et al. in 2011 [81], investigated photoelectrocatalytic activity for degradation of methyl orange (MO) solutions upon visible-light irradiation by BiOI/ TiO2 NTs (pen junction). They reported high photocurrent density in BiOI/TiO2 NTs as compared to the respective pure BiOI and TiO2 counterparts which was attributed to the pen junction, which can reduce the recombination of photogenerated electrons and holes by the internal electrostatic field in the junction region and an external electrostatic field which enhance the transfer and separation of photogenerated electrons and holes in BiOI/TiO2 NTs. Thus, the combined effect of the internal and external electric fields is reported to result the highest photocurrent density in such pen heterojunction. The enhanced PEC activity of BiOI/TiO2 NTs was attributed to the synergistic effect of strong visible-light absorption, pen junction structure and the applied external electrostatic field. Lin et al. in 2009 [82] studied combination of highly conductive TiSi2 nanonets (NNs) with photoactive TiO2 coating. Upon illumination, charge carriers were generated, out of which electrons were collected in the TiSi2 core and readily transported away whereas holes were transferred to the electrolyte for further chemical reaction. In TiO2/TiSi2 heterostructures, the TiO2/electrolyte junction area and the charge transport was increased by passing through more conductive TiSi2. A similar piece of work based on composite photocatalyst comprised of titania (TiO2) nanotubes (NTs) coupled with titanium disilicide (TiSi2) nanoparticles has also been reported by Banerjee et al. in 2011 [83]. A novel architecture, TiSi2 nanorods inside TiSi2 nanotubes, was prepared.

A combination of better solar light absorption (TiSi2) and high charge transport properties (one-dimensional TiO2 NTs) has been reported for the excellent photoactivity of this hybrid material. Upon UVeVis irradiation, electrons of both semiconductors (TiSi2 and TiO2) were excited and injected from TiSi2 to TiO2. Simultaneously, more photoelectrons were generated from TiO2 NTs by harvesting UV photons. Therefore, a high concentration of electrons was obtained in the conduction band of TiO2 compared to TiO2 alone. In addition to thermodynamically favourable energy bands, geometric architecture of the composite was also reported to be an important factor which controls the lifetime of charge carriers. One-dimensional TiO2 NTs collect the photoelectrons from TiSi2 and pass them to the back contact. The holes (from both TiSi2 and TiO2) generated in the process transferred to the solideliquid interface to generate protons from the solution.

5.6.

ZnO based bilayered systems

Hotchandani et al. in 1992 [41], developed coupled semiconductor films for PEC water splitting and elucidated the charge-transfer processes in CdSeZnO coupled semiconductor systems. Coupling the ZnO film with CdS particles lengthen its photoresponse in the visible region. The long lifetime of charge carriers in CdSeZnO system as analyzed by picosecond laser flash photolysis experiments confirmed the role of coupled semiconductor systems in retarding the recombination of trapped charge carriers. Better charge separation in the coupled semiconductor system lead to an enhancement in the efficiency of interfacial charge transfer to the adsorbed substrate. Upon optical excitation of CdS, the photogenerated electrons were readily transferred to ZnO while the holes accumulated at the CdS particle. At the interface, thin junction of ZnxCd1  xS was formed which was reported to be responsible in improving the process of charge separation by providing the necessary energy gradient for the flow of electrons towards ZnO film. Important observation of peien heterojunction and calculation based understanding of lattice mismatch between the two materials at the heterojunction was made by Zhang et al. [84] in 2004. They found that Cu2O/ZnO/ITO heterojunction exhibits a distinct property with smaller turn-on voltage, due to the tunnel recombination process that was based on the existence of the interface defect states in Cu2O and ZnO. This heterojunction structure offered an easy way for electron injection from n-type ITO to the p-type Cu2O and blocked the back hole injection from the p-to-n side under the forward applied voltage. In addition, the semi-insulated ZnO layer makes smoother energy band edge between Cu2O and ITO, which resulted in decrease in interfacial conduction and valence band discontinuities and an easy transition between bulk energy bands of Cu2O and ITO. Wang et al. (2010) [85] reported CdS and CdSe quantum dot co-sensitized with ZnO nanowire arrayed photoanode for PEC water splitting offering photocurrent density of w12 mA/cm2 at 0.4 V vs Ag/AgCl. This structure was analogous to tandem cell structure, in which ZnO nanowire arrays were deposited on ITO substrate followed by sensitization of CdS and CdSe quantum dots on each side. The photocurrent and IPCE were

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increased than that of quantum dot sensitized structures as a result of the proper band edge alignment of CdS and CdSe in electrolyte. Furthermore, the double-sided design exhibited better efficiency for charge collection than that of single-sided co-sensitized layered structures, which was attributed to direct contact between quantum dot and nanowire. It was concluded that the electron transfer in the CdSeeCdSeZnO was less efficient compared to electron transfer in CdSeZnOeZnOeCdSe, which is attributed to the proper alignment of fermi levels of CdS, CdSe and ZnO. Moreover, the conduction band edges of CdS and CdSe were close enough allowing electrons to delocalize between the conduction bands. Although the electrons created in CdSe can be transferred to ZnO through the CdS layer, the presence of intermediate layer in CdSeeCdSeZnO may increase the chance of electron-hole recombination and limits its electron collection efficiency. Dang et al. [49] in 2010 attempted ZnO/CdS bilayered film as a working electrode in PEC cell and found better PEC performance of bilayered film than ITO/ZnO film. They also observed inverse relation between the amount of Voc (465 mV) and Jsc (412 mA) with the thickness of CdS (70e180 nm), explained by Anderson’s model. In ZnO/CdS nanocomposite, CdS acted as a visible sensitizer while ZnO, being a wide band semiconductor, apparently responsible for charge separation, thereby suppressed the recombination process. Thus, ZnO/ CdS nanocomposite thin films can absorb significant portion of visible light and smoothly transfer the photoexcited electrons into the ZnO conduction band. The lifetime of electrons increased upon diffusion in the conduction band of ZnO, an area with non-availability of free holes under visible light excitation. The effect of annealing temperature on the optical properties and band-gap energy of ZnO/NiO multilayer thin films fabricated by CBD method has been studied by Ajuba et al. [86] in 2010. They found that the band gap energy decreases with increase in annealing temperature which may be either due to evaporation of water molecules off the films and/or reorganization of the films.

5.7.

TiO2 and ZnO based bilayered systems

Liu et al. [87] in 2004 fabricated mercurochrome-sensitized composite TiO2/SnO2 and ZnO/SnO2 PEC cells and examined the influence of mixed ratios of TiO2/SnO2 and ZnO/SnO2 on the performance of the composite semiconductor in the PEC cells. The energy barrier between different semiconductors contributed in the improvement of the Jsc by suppressing recombination of photo-induced electrons and holes in the oxidized dye resulting in the improvement of the IPCE of the composite TiO2/SnO2 and ZnO/SnO2 cells as compared to corresponding TiO2 and ZnO composite cells. Paracchino et al. [88] in 2011 reported a modified approach with multiple layer ALD (atomic layer deposition) protection strategy utilization to offer improved corrosion protection for the photoelectrodes used in PEC cell. Photocurrent 7.6 mA/cm2 at potential of 0 V for FTO/Cu2O/Al doped ZnO/TiO2/Pt nanoparticles and 100% Faradaic efficiency was reported. Aluminium doping in this case was considered to stabilize ZnO layer in the photoelectrode. The electrodes showed good

18725

photocurrents even after 1 h of testing but the photocurrent decreased with time without structural failure of the protecting layers or significant chemical degradation of Cu2O, which was probably due to electron accumulation in the TiO2 layer (by Ti3þ traps), whose Fermi level was not optimally positioned for water splitting. It was observed that the photoelectrons generated in Cu2O flowed without hindrance into Al:ZnO and to TiO2, but as the Fermi level of TiO2 in the dark was found to be very close to the water reduction potential, electrons readily moved into the electrolyte and accumulated in the protective layer as long-lived Ti3þ states.

5.8.

Fe2O3, WO3 and SrTiO3 based bilayered systems

Higher photocurrent and higher IPCE in case of SrTiO3/a-Fe2O3 heterojunction was found than that of the single SrTiO3 or aFe2O3 film as reported by Wang et al. [48] in 2007 which may be due to the electric field formed by the junction at the interface and the special band structures of SrTiO3 that favour the transfer of holes from a-Fe2O3 to SrTiO3, and the improved charge separation at the SrTiO3/a-Fe2O3 interface. Thus, it was concluded that the additional SrTiO3 layer facilitates increased separation of charge carriers along with the rapid transfer of holes to the electrolyte. A similar report on WO3/Fe2O3 bilayered semiconductor in PEC water splitting has also appeared [89] in 2007, which reported higher photocurrent density and higher IPCE values of WO3/Fe2O3 heterojunction than that for Fe2O3 and WO3 alone. In WO3/Fe2O3 electrodes, the conduction band of WO3 is higher than that of Fe2O3, which makes the transfer of photogenerated electrons easier and decreases the combination between the electrons and the holes. Thus, the WO3/Fe2O3 interface improved the photocurrent and IPCE of the composite structure. In 2009, Sivula et al. [90] introduced host/ guest (WO3/Fe2O3) architecture in which hematite has been reported to act as the guest absorber and a scaffold host material (WO3) was chosen on the basis of lower lying conduction band than that of hematite to allow efficient electron transport across the host/guest interface and larger band gap than hematite so that the scaffold host material did not compete with the light absorption. Increased photocurrent in PEC was observed, which was due to photon absorption by WO3 followed by hole transfer to the Fe2O3. Further, the quantum efficiency was improved by using the host/guest approach with thinner layer of iron oxide and scaffold layer with increased roughness and porosity, which allowed 20% increase in the photocurrent for hematite. Accordingly, increased quantum efficiency especially from wavelengths near the hematite absorption edge where photons have long penetration depths was also observed, as the greater fraction of photons may be absorbed near the hematite/electrolyte interface. Kuang et al. in 2009 [91], investigated improved photoelectrochemical behaviour of Fe2O3-modified TiO2 nanotube (NT) arrays which is attributed to the increased probability of charge carrier separation that extends the range of TiO2 photoresponse from ultraviolet (UV) to visible region due to the low band gap (2.2 eV) of Fe2O3. Under UVeVis illumination, electrons were excited from the valence band (VB) to the conduction band (CB) of anatase. In the absence of Fe2O3, most

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of the charge carriers quickly recombine but when Fe2O3 nanoparticles were attached to the surface of the TiO2 NTs, the relative position of the Fe2O3 CB edge permitted the transfer of electrons from the TiO2 surface allowing charge separation, stabilization and hindered recombination. With the increasing amount of Fe2O3 deposited on the TiO2 NT, the probability of the photo-generated eehþ recombination increases as all the photoelectrons on the nanoparticle cannot be scavenged immediately by the applied bias. Fe2O3 then becomes the recombination centre of eehþ induced by light, resulting in decrease of photoactivity with higher Fe content. An important finding showing the possible explanation for improved photoelectrochemical performance of WO3:Mo/ WO3 bilayered system compared to single layer i.e. WO3 or WO3:Mo photoelectrode was reported by Gaillard et al. in the year 2010 [92]. Electron transport property of pure WO3 was reported to be limited by crystallographic shear defect planes in the lattice. It was observed that Mo incorporation in WO3 resulted in reduction of PEC activity than that for pure WO3 which may be due to formation of defects by Mo incorporation, where photogenerated charge carriers were trapped. 20% enhancement in photocurrent density was observed for WO3:Mo/WO3 bilayered system and 100% improvement in photocurrent density of WO3:Mo based system was investigated than that for pure WO3 at 1.6 V vs SCE. The remarkable improvement in PEC performance of WO3:Mo/WO3 bilayered photoelectrode is attributed to modification in chemical and electronic surface properties of WO3 by overlying WO3:Mo thin film while maintaining its morphological and bulk properties and positive influence of high crystallinity of WO3 on coherent growth of overlying WO3:Mo thin film (for improved efficiency). Favourable energetic positions, efficient charge collection and built-in electric field at WO3/WO3:Mo junction has also been beneficial for the movement of holes from WO3 towards electrode surface and electrons towards back-side electrode. Significantly enhanced photoresponse of Fe2O3/ZnFe2O4 compared to the bare Fe2O3 electrode was investigated by McDonald et al. in 2011 [93]. ZnFe2O4 has band edges shifted to 200 mV in the negative direction from that of Fe2O3 which allowed the improved separation of charge carriers at the Fe2O3/ZnFe2O4 interface. Further improvement in photocurrent was observed through the Al3þ treatment of the composite electrodes, which may form thin solid solution coating layers (i.e. ZnFe2  xAlxO4 or Fe2  xAlxO3) and reduce surface states that may serve as the electron-hole recombination centres. Fe2O3 core with ZnFe2O4 shell structure may allow the photon generated holes in the Fe2O3 core to be transferred to the ZnFe2O4 layer and then consumed at the ZnFe2O4/electrolyte junction. This can effectively increase photon to photocurrent conversion ratio of Fe2O3 that has an extremely short diffusion length of hole. The photon generated electrons in the ZnFe2O4 layer may be transferred to the Fe2O3 core and move to the back contact. Su et al. in 2011 [94] reported WO3/BiVO4 nanorod-array heterojunction photoanode prepared by solvothermal deposition method for PEC water splitting with photocurrent density 1.6 mA/cm2 at 1.0 V/SCE, which is higher than that for the planar sample (0.8 mA/cm2 at 1.0 V). The WO3/BiVO4 heterojunction offered improved photoconversion efficiency

and photocorrosion stability. The nanorod-array films showed significantly improved photoelectrochemical properties than that of planar WO3/BiVO4 heterojunction films, probably due to the high surface area and improved separation of the electron-hole pairs at WO3/BiVO4 interface. More precisely, WO3/BiVO4 nanorod-array heterojunction was formed vertical to the substrate, along which light is absorbed. Thus, more carriers were generated close to the heterojunction where they were efficiently separated. In addition, the large surface area is offered by nanorod array structures which provide a direct path for the movement of electrons towards the substrate and a short path for holes to reach the aqueous electrolyte for water oxidation.

5.9.

Silicon based bilayered system

In order to achieve efficient water splitting, composite semiconductor electrode n-Si/p-CuI/ITO/neiep a-Si/nep GaP/ITO/ RuO2 has been fabricated by Yamane et al. (2009) [95]. PEC cell comprised of n-Si/p-CuI/ITO/neiep a-Si/nep GaP/ITO/RuO2 photoelectrode, generated 1.88 mA/cm2 photocurrent density under simulated solar illumination with 2.3% efficiency. The electrode showed a stable photoanodic current due to oxygen evolution with a large negative photo shift (Vp) of about 2.2 V from the corresponding anodic current at the RuO2 electrode. They found that the efficiency can be increased by using GaP with a well-regulated pen junction and the addition of neiep a-Si to nep GaP/ITO/RuO2 resulted into a shift in the onset potential of the anodic photocurrent towards more negative value (0.9 V). Yu et al. in 2009 [96] investigated the photoelectrochemical performance of nen and pen SiNW (nanowire) heterojunction with TiO2 fabricated by chemical etching of Si wafer. When SiNW/TiO2 heterojunction system was illuminated with solar light, the short wavelength (UV light) and long wavelength (visible light) portions were absorbed by TiO2 and SiNW respectively. TiO2 serves as window layer in the heterojunction array and allows visible light to permeate through it and absorbed by SiNW. The window effect of heterojunction photoelectrode leads to absorption of both visible and UV portion of the solar spectrum, resulting into enhanced photoelectrochemical performance of the heterojunction system. It was observed that the existence of window effect only in nen junction may lead to superior photoconversion ability of nSiNW/TiO2 rather than p-SiNW/TiO2 heterojunction system.

6. Summary of work on bilayered thin films in PEC water splitting by the authors Efforts have also been made by our group to prepare FeeTiO2/ ZneFe2O3, in which thickness of FeeTiO2 film deposited on ITO was varied while thickness of overlying ZneFe2O3 film was kept constant and the system was studied as photoelectrode in PEC cell for generation of hydrogen through water splitting [50]. A 10 fold enhancement in photocurrent density at 0.95 V/SCE was observed for FeeTiO2/ZneFe2O3 photoelectrodes than that of ZneFe2O3, which is attributed to efficient separation of photogenerated charge carriers at the

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interface, reduction in resistance and improved light absorption ability. In yet another system comprises of bilayered FeeTiO2/ ZneFe2O3 photoelectrode, PEC response was optimized with respect to thickness of the overlying layer of ZneFe2O3 [97]. Bilayered FeeTiO2/ZneFe2O3 photoelectrode was observed to possess enhanced efficiency for separation of charge carriers and could generate nine folds better photocurrent density than pure FeeTiO2. It was concluded that thickness of the ZneFe2O3 was found to be crucial in deciding photoelectrochemical properties of FeeTiO2. Improved absorption, mixed oxide formation and higher value of flatband potential in bilayered films were found to be responsible for better performance of modified photoelectrode. Efforts have also been made to investigate the effective role of CuO/SrTiO3 heterojunction in PEC water splitting for hydrogen generation by varying the thickness of CuO. Significantly higher photocurrent and efficiency of CuO/SrTiO3 bilayered photoelectrodes was observed than that for the CuO or SrTiO3 alone [Under Review].

7.

Concluding remarks

Based on the preceding review, it may be concluded that the prescribed modification technique, bilayered systems in PEC water splitting offer a promising approach, allowing absorption of large part of the solar spectrum, improved charge separation across the heterojunction and reduction in recombination losses. Further progress is expected to bridge the gap between the experimentally achieved photocurrent values and the theoretically expected values, and a detailed understanding of the interfaces in the bilayered structure and behaviour of photons and excited electrons in the PEC system. Reported research work on bilayered photoelectrodes with various preparation techniques, light source, applied voltage and achieved photocurrent values have been summarized in Table 1. In most of the reports an external bias voltage is used to achieve the photo-splitting of water, but the ultimate sustainable goal is to work without externally applied bias and with solar radiation under AM 1.5 conditions. Therefore, it seems difficult to pin point the best combination of bilayered photoelectrode out of the work reported here with respect to efficient PEC water splitting for generation of hydrogen. Significantly good photoresponse obtained by the various bilayered photoelectrodes as derived from the literature survey have been summarized below, although the light sources are different: a) Unassisted PEC water splitting (without external bias): A modified approach FTO/Cu2O/Al doped ZnO/TiO2/Pt nanoparticles with multiple layer ALD protection strategy utilization and improved corrosion protection for the photoelectrodes used in PEC cell offering 7.6 mA/cm2 photocurrent with 100% Faradaic efficiency under 450 W Xenon lamp irradiation [88].  CdS and CdSe nanoclusters co-sensitized TiO2 nanotubes with nanowires (NTWs) arrayed photoelectrodes exhibited 4.30 mA/cm2 photocurrent density and corresponding

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energy conversation efficiency of 2.408%, under AM 1.5G illumination [60].  CdS/CdSe photoelectrodes yielded 4.2 mA/cm2 current density under the illumination of 500 W Tungsten lamp [58]. b) Assisted PEC water splitting (with external bias): TiO2/CdS/CdSe electrode yielded 14.9 mA/cm2, photocurrent at w0.7 V vs Ag/AgCl under the illumination of UV cutoff AM 1.5 [59].  Double-sided CdS and CdSe quantum dot co-sensitized ZnO nanowire arrayed photoanode offered w12 mA/cm2 photocurrent density at 0.4 V vs Ag/AgCl using 1000 W Xenon arc lamp as white light source [85].  CdS-sensitized nanocrystalline TiO2 photoanode yielded 5.5 mA/cm2, photocurrent at 0.5 V vs Ag/AgCl with 3.67% photoconversion efficiency under the illumination of 150 W Xenon lamp [66].  Core/sheath heterostructure CdS/TiO2 nanotube arrays showed w5 mA/cm2 photocurrent density at 0.2 V vs Ag/ AgCl using Xenon lamp as light source [65].  Composite TiSi2/TiO2 NTs photoanode exhibited 3.49 mA/ cm2 photocurrent density at 0.2 V vs Ag/AgCl under illumination of 300 W Solar Simulator [83].  CdS or CdSe modified TiO2 nanotube arrays showed photocurrent enhancement from about 0.2 mA/cm2 to 5.0 mA/cm2 at 1 V vs Ag/AgCl with 8.5% conversion efficiency using 150 W Xenon lamp as light source [76]. The listed data clearly reveal that significant photocurrent with enhanced efficiency can be obtained by combination of nanostructured wide band gap metal oxide with either low band gap metal oxide or non-metal oxide. In the concept of bilayers both layers do not always contribute to the photocurrent generation, but behave differently in the process, like in the case of buffer layers, which are generally introduced for better electron or hole accepting/transporting/blocking properties. The bilayered heterojunction architecture clearly plays a crucial role in the performance of the semiconductor active component in PEC devices. In order to improve the efficiency of PEC water splitting to produce hydrogen, some factors such as band gap, electronic band edge alignments of the materials with each other and with the redox potential of water, lattice mismatch of the materials and optimization of thickness of each layer of the junction are found to be crucial. Choice of proper electrolyte, film thickness, annealing temperature of the films etc. could also improve the process. Attention should therefore be focused on the optimization of above said various experimental parameters in order to improve the performance of PEC cell. It is expected that by considering the above limiting factors and analysis carried out by theoretical simulations with suitably fabricated heterojunction architectures may lead to a stable, visible light responsive and an efficient unbiased PEC system. Bilayered semiconductor structures, thus, appear to be a promising direction in the development of efficient photoelectrode for PEC splitting of water. Precise control over the thickness of each semiconducting layer and the role of defects in kinetic stability of materials in water and energetics of semiconductor surfaces are critical issues that need to be addressed. Bilayered structures of wide band gap material (viz.

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TiO2, ZnO, SrTiO3) with mixed metal oxide/non-oxide materials, viz. BiVO4, ZnFe2O4, BiOI, TiSi2 etc. seems to be attractive option on account of higher stability offered by wide band gap material, absorption of visible part of solar spectrum obtainable by low band gap material and appropriate band edge straddling of both semiconductor materials resulting into facile transfer of charge carriers across the heterojunction followed by minimized recombination of charge carriers. Besides this, bilayering of semiconductor materials along with unique nano architectures such as nanosheet, nanotube arrays, nanowires and quantum dots as described in this article have also exhibited potential in advancement in efficient PEC system that needs exhaustive study.

Acknowledgements The authors gratefully acknowledge the financial support received from University Grants Commission, New Delhi, India to carry out this work vide project no. 39-838/2010 (SR). We acknowledge the valuable comments of reviewers.

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