- Email: [email protected]
Structural and photoelectrochemical studies of In2O3-TiO2 and WSe2 photoelectrodes for photoelectrochemical production of hydrogen
Int. J. Hydrogen Energy, Vol. 14, No. 8, pp. 53%544, 1989.
0360-3199/89 $3.00 + 0.00 Maxwell Pergamon Macmillan plc. International Association for Hydrogen Energy.
Printed in Great Britain.
STRUCTURAL AND PHOTOELECTROCHEMICAL STUDIES OF In203-TiO2 AND WSe2 PHOTOELECTRODES FOR PHOTOELECTROCHEMICAL PRODUCTION OF HYDROGEN G . PRASAD,
K. S. CHANDRABABU* and O. N. SRIVASTAVA
Physics Department, and *Department of Chemistry, Banaras Hindu University, Varanasi-221005, India
( Received for publication 16 November 1988) Abstract--The present paper describes a new photoelectrode system for photoelectrolysis of water based on TiO2. This comprises of a system with In203 islands overlaid on a TiO2 base. The TiO2 is synthesized through anodic oxidation of Ti sheets under glow discharge conditions. The In203 islands are formed by first depositing a thin In film on TiO2 by electrodeposition followed by subsequent oxidation. The ln2OwTiO2 photoelectrodes have been characterized through XRD, scanning electron microscopy and photoelectrochemical techniques. It has been found that as compared to TiO2, where the typical photocurrent under illumination was 4 mA cm 2, the In203 overlaid T i O 2 exhibits a much higher photocurrent of 14 mA c m -2. The hydrogen gas evolution under photo-assisted electrolysis employing a TiO2 photoanode was found to be 3 x 102/t[ over a cm2of electrode, on the other hand the I n 2 0 3 modified TiO2 exhibited a higher hydrogen gas evolution rate of 8 x 10 ttl over a c m 2 of electrode. Evidence and arguments have been put forward to show that the presence of ln203 over TiO 2 makes the system possess the advantages of a colloidal photochemical system, and the better performance of the new photoelectrode is thought to be due to improved spectral response and catalytic activity of 1n203 in regard to the gas evolution kinetics. Besides TiO2 the other photoanode studied in the present investigation corresponds to the M X 2 type layered material, WSe2 (WS2). The WSe2crystals were grown by the chemical vapour transport technique with SeCI4as the transporter. The pfiotoelectrochemical efficiency (light to electricity) conversion efficiency of the as grown n-WSe2 crystals has been raised to ~ 17% through a photoetching treatment imparted to WSe2crystal surfaces. The high PEC efficiency of these photoanodes may be utilized for electrolysis in the photovoltaic mode or for providing external bias to the TiO: based photoelectrolysis cells.
1. I N T R O D U C T I O N In recent times a great deal of attention has been paid to photoelectrochemical (PEC) methods of conversion of solar energy to electrical power or a storable energy from e.g. hydrogen. The photoelectrolysis of water using semiconductor electrode(s) has gained significant attention in the past decade. Fujishima and Honda [1] described the photosensitized electrolysis of water on n-type TiO2 single crystal electrodes and proposed this reaction to be used for solar energy conversion and storage in the form of "hydrogen". For efficient PEC decomposition of water, the n-type photoanode is required to have manageable band gap - 2 to - 2 . 5 eV) suitable negative flat band potential, good stability and high quantum efficiency. In initial and subsequent studies, n-TiO2 has been shown to be exceptionally stable with high quantum efficiency (-- 100%). However, its optical band gap (Eg - 3 eV) is a negative factor in regard to its application as a photoelectrode in solar powered photoelectrolysis systems. Most of the small band gap oxide and non-oxide semiconductors being investigated were found to exhibit either photocorrosion or unsuitable flat band potential. It seems that after an extensive search for suitable semiconductors during 1972-1985, the interest is returning to TiO2 [2]. Several different approaches are being pursued to circumvent the poor solar spectral response of TiO~
caused by its large band gap. In one of these, TiO2 is doped with certain cations like A1, Cr, Y, Cu, Mo, Nb, V etc. [3-5] for their improved visible response. In another approach, the band gap of TiO2 is reduced by alloying with VO2, sacrificing the major advantage of TiO2 [6]. Similarly, to improve the limited solar response of TiO:, mixed oxide alloys like TiO2-SiO2 [7, 8], TiO2-Fe203, TiO2-Co304 [9], TiO2-NbO2 [10], etc. have been studied with the known fact that the binary oxide catalysts show higher activity than their components. In yet another attempt to improve response through an improvement in gas evolution kinetics, loading of photoelectrodes with catalysts have been tried with TiO2. As, for example, improved gas evolution (oxygen and hydrogen) was found when TiO 2 was loaded with RuO~ [11, 12]. Similarly, SrTiO3 enhanced gas evolution with loading of the NiO catalyst [13]. In our present study, the influence of 1n203 admixtures over the TiO2 on its photoactivity and gas evolution kinetics has been studied. An indium oxide single crystal was photoelectrochemically tested by McCann and Bockris [14] and was reported to have an indirect optical absorption edge at 2.3-2.5 eV. With n-type conductivity polycrystalline In203 exhibited an indirect band gap at 2.8 eV and direct band gap at 3.6 eV [15]. The quantum efficiency of this photoanode was dependent on the preparative conditions and the highest value of 100% corresponds to the one prepared by thermal 537
538
G. PRASAD, K. S. CHANDRA BABU AND O. N. SRIVASTAVA
oxidation of liquid In metal. In the present study, thin film admixtures of In203 with TiO 2 have been synthesized and this has been employed as a photoelectrode. It may be mentioned that the thin films have good transport properties and reduced carrier recombination at the interface. A thin layer of TiO2 is prepared on Ti by anodicaily oxidizing a Ti sheet under conditions of high current density (50 m A cm -2) and glow discharge. Over this TiO2 surface, In is electrochemically deposited from 0.5 M In2(SO4) 3 solution. Later, the TiOE-In electrode is homogenized in an oxygen atmosphere at 500°C for 1 h (presumably this treatment transforms TiO2-In to TiO2-In203). The surface morphology, structural and photoelectrochemical characteristics of the base TiO 2 as well as the TiO2 overlaid with In203 thin film admixtures have been investigated in relation to hydrogen evolution through photoelectrolysis. Besides the oxide semiconductors, the interest in regard to developing viable semiconducting photoelectrodes centres also on the special MX 2 type layered semiconductors with d-d phototransition characteristics. These materials are typified by WSe 2. Such materials possessing optimal band gaps ( - 1.2 to - 2.0 eV) have high optical absorption coefficients ( - 105 cm-J). Above all, the optical transitions in these materials mostly involve lower and upper metal bands, as a result the metal--chalcogen bonds remain nearly unaffected during phototransition and hence photocorrosion in reasonably perfect crystals would be minimum [16, 17]. Keeping this fact in view, we have investigated the growth, synthesis characterization of WSe2 (WS2) crystals. We have shown that by improving the quality of the as-grown crystals through the use of SeC14 transporter and by improving on the crystal surface through photoetching, it is possible to achieve PEC (solar~electrical) conversion efficiency of about ~ 17%. These high efficiency cells could be useful in solar hydrogen production either as a source of bias for a TiO2 based cell or for direct production through photoelectrolysis.
for 1 h. Room temperature conductivities of TiO2 films before and after In203 modification are measured. These electrodes are subjected to structural and photoelectrochemical characterization. Simultaneously, In is thermally evaporated on to a vitreous quartz slide in a vacuum of 10 -5 torr and is annealed at 500°C for 1 h in oxygen flow. The absorbance of this thin film is measured in the range of 200-800 nm with a Hitachi 320 U.V.-Visible Absorption Spectrometer. Structural characterization of In203 deposited and base TiO, is performed by X-ray diffractometry using Philips PW-1710 X-ray Diffractometer equipped with a graphite monochromator. Surface morphology of these films is studied under scanning electron microscope (PhilipsCM-12). Photoelectrochmeical characterization of the electrodes is performed by measuring the currentpotential characteristics. The electrodes are connected to a pure copper wire with silver epoxy and sealed with epoxy resin leaving an exposed area of 1 cm 2. The sealant has been observed to be stable in 1 M NaOH. The measurement is done in a Pyrex cell with a quartz window employing conventional three electrode assembly. A ( - 2 cm 2) Pt was used as counter electrode and SCE with luggin capillary as reference. The I - V curves of the electrodes under dark and illumination were recorded with a Princeton Applied Research (PAR) Model 173 Potentiostat/Galvanostat, a P A R Model 175 Universal Programmer and a Houston Model 20(10 X - Y recorder. The illumination source being 1000 W X e - H g lamp (Oriel Corporation, U.S.A.) with a water filter to remove heating effects. The I - V characteristics at room temperature were measured in nitrogen purged 1 M NaOH. All the reagents are analytical grade and prepared with doubly distilled water. The evolved gases are measured by fixing an inverted burette.
3. RESULTS A N D DISCUSSION 3.1 Structural characteristics
2. E X P E R I M E N T A L Thin Ti sheets (Goodfellow Metals, U.K.) are decreased with methanol and ultrasonically cleaned in acetone for 30 rain. Before anodization every electrode is etched with HzO2:HF (1:1) for 10 s. These Ti sheets are anodically oxidized in 2 M Na2SO 4 solution witha Pt cathode, under high current density and glow discharge. The voltage applied between the electrodes (Ti and Pt) is increased gradually with the current never exceeding 50 m A c m -~. Sparking started around 85 V, and the anodization was continued at 120 V for 5 min. The uniform, compact, mouse-colored TiO2 films thus obtained exhibit n-type conductivity even without reduction. Over this anodized Ti, In is electrodeposited by cathodically polarizing the electrode in 0.5 M In2(SO4) 3 solution at 5 V for 10 rain. Later this electrode is desealed and annealed in an oxygen atmosphere at - 500°C
The X-ray diffractograms of anodized Ti exhibit a dominant rutile type of TiO2 with Ti (Fig. 1). After depositing In on TiO2, the XRD analyses confirm the presence of a rutile phase of TiO2 and In with (In-Ti) alloy phase and Ti (Fig. 2). The XRD pattern of oxidized In on TiO2 was found to contain (Ti-In) alloy phase and In in addition to TiO2 and In203 phases (Fig. 3). However, no mixed oxide alloy could be indexed. Thus, it can be concluded that In203 has formed a thin film over the surface of TiOz, without forming any compound oxide system of the type (Ti-In)xO v. The scanning electron micrographs o ( a n o d i z e d Ti reveal a porous surface with a film thickness of around 1000 ~, (Fig. 4a). T.he TiO2 films with In203 are found to exhibit a cratered surface. It is also observed that the In203 layer itself is not uniform but is in the form of clusters of 1000 • thickness (Fig. 4b). The absorbance characteristics of the In203 formed on vitreous quartz slide is shown in Fig. 5. This exhibits commencement of
In203-TiO2 AND WSe2 PHOTOELECTRODES
539
O
o
o 0
I
4
/
L
70
i I
60
I
50
I 30
40 ",
,
i 2o
2
Fig. 1. X-Ray diffraction pattern of TiO2 prepared by anodic oxidation of Ti under extraordinary conditions, o--TiO2; A--Ti.
O
o O 1
I
._
I
60
O'
50
40
3O
2O
20
Fig. 2. X-Ray diffractogram of TiO2 with electrodeposited indium metal film. o--TiOz; e--(In-Ti) alloy; A--In; &--Ti.
c,
t~
70'
60I
°
~t
~
~ 00~
50I
41 -
0
3/
A° ~,
20
Fig. 3. X-Ray diffractogram of TiO2 with indium oxide on surface. *--1n203; o--TiO2; A - - I n ; e--(In-Ti) alloy.
540
G. PRASAD, K. S. CHANDRA BABU AND O. N. SRIVASTAVA
Fig. 4. Scanning electron micrographs of TiO~ film (a) before and (b) after In20~ modification.
absorption around 460 nm indicating an optical transition commencing around 2.5 eV. The room temperature conductivity of the anodized Ti has been found to be 10- ~-f2 t cm -~, while that of the TiO, surface deposited with In~O3 is l02 f2 i cm i. From this, it is clear that the conductivity of the photoanode gets enhanced with In203 modification. 3.2 Photoelectrochemical studies
The photoelectrochemical behaviour of TiO2 overlaid with In_~O3 and base TiO2 is elucidated by studying the I - V characteristics of the two electrodes individually. From the I - V curves, a clear enhancement in the
photocurrent from 4 m A cm 2 to 14 m A cm -~is evident with In203 modification (Fig. 6). The photo-onset potential has been shifted to the negative side and the onset is less sluggish in nature, indicating reduced surface recombination. The enhancement in the photocurrent with In203 modification is thought to be due to the improved spectral response of photoanode due to the presence of surface In203. As is clear from Fig. 5 that [n203 shows c o m m e n c e m e n t of absorption around 2.5 eV and continues up to 3.6 eV, thereby producing carriers which evidently enhance the photoactivity of the anode. As the carrier recombination at the thin film interface is known to be low added by high light absorption [15 I, the thin In203 islands on TiO~ surface
14--
4
5 ,o
"-" 3
8
82
/
~6
/j
\[n203-TiO2
f
~._. .~. - - -: a---,C
"'"""""
""--"-TIO00
mV
;2 200
I
300
I
I
r
400 500 600 WaveLength (nm)
I
700
I
800
Fig. 5. Absorbancc characteristics of indium oxide fihns formed on to vitreous quartz slide by oxidizing indium metal film.
0
I 7 0 0 600
r 0
l 200
.9-°~k--
Potentiot (mV) vs SCE Fig. (/. Currcm-potential characteristics of TiO_~ (broken lines) and ln~O; modified Ti02 (solid lines) under dark and illumination in 1 M NaOH.
In203-TiO2
AND
541
WSe 2 PHOTOELECTRODES
enhance the photoactivity of the anode, even under low absorption coefficiency at the region of indirect transition around 2.5 eV. As is evident from XRD studies and scanning electron micrographs, the In203 does not react with the base but forms only islands over the surface of TiO2. These photoactive islands appear to act similarly to Pt islands on TiO2 or SrTiO3, enhancing the gas evolution kinetics without forming any mixed oxide material.
-
f..- ~-'----.~x........x_...__
-- /
~-
(b)
B 0
L =:L
L g
~ o
3.3 Water photoelectrolysis
I
The potential at which the onset of photocurrent starts (V,,.), outlines the position of energy bands of the semiconductor. In our present study, Vo, is used instead of Vvb (flat band potential) because of nonlinearity of Mott-Schottky plots for both TiO 2 and In203 electrodes. Generally, the difference of Vvb-Vo, diminishes with increasing photon flux. So, in our high flux illumination, the difference (Vvb-Vo,) can be negelcted. In the photoassisted electrolysis mode of operation of the cell, n-TiO2/1 M NaOH/Pt, under the illumination of 1000 W Xe-Hg lamp fitted with a water filter, oxygen gas started evolving on the anode (hydrogen at the cathode) at - 0 . 8 5 V. The cell with configuration n-In203-TiO2/1 M NaOH/Pt showed gas evolution at - 0 . 9 V with the same illuminating source. The values ( - 0 . 8 5 and - 0 . 9 ) correspond to an applied bias voltage of 0.45 and 0.4 V, respectively. Theoretically, the minium bias required for photo-oxygen evolution on n-type semiconductors in a Schottky type photoelectrolysis cell, Vn-min bias when the flat band potential is more positive than the 2H+/H2 redox potential is [14]: Vn min bias :
c3
Vn Fb - - V2 H+/H, -
Substituting a value of - 1 . 0 3 V for V2 H+-U~in 1 M NaOH, the Vo, for V,-vb' the Vn_mi n bias of 0]13 V is required for the cell with In203 modified anode. This value after accounting the iR drop and overvoltages at the electrodes, corresponds well with the experimentally observed bias of 0.4-0.45 V. From the gas evolution characteristics (Fig. 7), it is clear that the evolved gases obey the Faraday laws and they are in stoichiometric quantities. The gas evolution has attained saturation after 10 min under an applied bias of +0.5 V and illumination. The clear enhancement of gas evolution kinetics for 1n203 admixtures with TiO2 as compared to TiO2 alone from 3 x 102 ul over cm 2 of electrode to 8 x 1(12 lq over a cm 2 of electrode is evident. From the I-V characteristics (Fig. 6) it is clear that In203 modified TiO2 shows an enhanced photocurrent and photovoltage. In the absorption studies, In203 films exhibit commencement of optical absorption at around 2.5 eV. Even with a low value of absorption coefficience at this indirect transition due to the facilitated charge transfer and reduced recombination rate caused by the high conductivity (1(12 ~ i cm l) of the anode, the photocurrent of the anode is evidently enhanced with
(a)
/
I IO
I 20
I 30 Time
I 40
I 50
o 60
(rain)
Fig. 7. Volume of evolved gases as a function of irradiation time at an applied bias of 0.5 V (light source--l(X)0 W Xe-Hg lamp with water filter) (a) pure TiO2 (b) In203 modified TiO2.
In203 modification. The highly conducting In203 reduces the overvoltage at the anode and catalyses the gas evolution, thus, causing the enhanced rate of gas production at the photoanode. The porous TiO substrate also facilitates in reducing the overvoltage at the anode by increasing the effective surface area of the anode. As is clear from the scanning electron micrographs (Fig. 4), the islands of In203 formed over the surface of TiO2 are thought to catalyse the anode reaction, probably with the added advantages of colloidal photochemical systems. The increased photoactivity is also thought to be due to a lower concentration of defects and impurities. Even though the mechanism of this facilitated charge transfer at the oxide/electrolyte interface is presently unknown, the enhanced photoactivity added by increased gas evolution kinetics can be thought to be due to surface modification with thin In203 islands which are having high light absorption, conductivity and wider spectral response than TiO 2 alone. It may, therefore, be taken that TiO2 overlaid with In203 surface clusters represents a new improved photoelectrode for hydrogen production through photoelectrolysis.
4. STUDIES ON WSe2 P H O T O A N O D E S As outlined earlier, besides TiO2, the other material on which studies in regard to photoelectrochemical
542
G. PRASAD, K. S. CHANDRA BABU AND O. N. SRIVASTAVA 4O 32
E 24
~2
g
o
4
600
400
l ~0
200
400
E
I 200
CeLt vo(tege ( m Y )
Fig. 8. Power characteristics of the cell with as-grown W S c 2 crystal (a) and after photoctching in 0.1 M aquaregia (b). Crystal area is 0.0125 cm2.
(photoelectrolysis) behaviour were carried out corresponded to WSe2. Now these investigations will be discussed. The single crystals of WSe2 using SeCI4, which is a transporter having an ingredient (Se) native to WSe2, were grown by the chemical vapour transport technique [18]. The transporter was added to presynthesized tungsten diselenide powder in a concentration of 0.2 mg cm -3. A temperature gradient of 2°C cm-I was maintained for 170 h in the growth furnace. Large single crystals having dimensions up to 0.5 x 0.5 x 0.1 cm were obtained after growth. Crystals were attached to a platinum wire, which was fused in a narrow tube containing a mercury pool. The back contact of the crystals was provided by l n - G a alloy. The electrolyte used for electrochemical studies corresponded to 1 M KI + 0.05 M 12 (pH - 9). All experiments were carried out by using Princeton Applied Research (PAR) electrochemical equipment. A fresh crystal surface, obtained by rough cleaving with adhesive tape, was used for fabrication of PEC cells. Illumination was provided with a tungsten lamp with intensity of 60 mW cm -2 (after correction to solution absorption) for both photoetching and electrochemical measurements. Typical PEC efficiencies of the order of ~ 6 to 8% were observed with the as-grown crystals. Photoetching was carried out in 0.1 M aqua regia in forward bias conditions. Several different etchants like perchloric acid, orthophosphoric acid, chromic acid and aqua regia were used for photoetching. It was found that the best results in regard to enhancement of efficiency were obtained in aqua regia. Figures 8(a) and (b) show the power characteristics of the crystal before and after photoetching. The PEC conversion efficiency before photoetching is - 8.3% (Fig. 8a) and this changes to 17.1% (Fig. 8b) after photoetching. This efficiency is one of the highest reported efficiencies for WSe2 based PEC solar cells. It should be mentioned that the said efficiency ( - 17.1%) was obtained for specifically good crystals exhibiting rather significant changes in the surface micro-structure
on photoetching for the WSe 2. Conversion efficiencies of about ~ 17.1% could be obtained on some other crystals also. Efficiencies in the range of 13 to 15% were generally obtained. A possible factor which is likely to contribute to the enhancement in conversion efficiency may come about from the growth parameters adopted in the present study. Unlike many other previous WSe 2 growth experiments, where a TeCI4 transporter is used, we have employed SeCI4 as the transporter. Here Se atoms are involved which are native to WSe2 crystals. Thus, unlike the case with Te, incorporation of Se in the growing crystal would not result in the creation of additional recombination type energy states (gap states) in the band gap. Analytical explorations of these crystals through the E D A X technique in scanning electron microscope mode and resistivity measurements confirmed the improved quality of SeCI4 grown crystals. In order to explore the influence of the photoetching on the microstructural characteristics of the surface, the effective crystal surface was characterized by employing scanning mode of the electron microscope (Philips EM-CM12). The crystal surfaces of several crystals where PEC measurements were done, were explored. Figures 9(a) and (b) represent the surface characteristics of the crystal obtained in the secondary electron scanning mode corresponding to PEC characteristics shown in Figs 8(a) and (b) before and after photoetching. A noticeable feature brought out by this Fig. 9(b) is the disappearance of several surface steps (see Fig. 9a) consequent to photoetching. The surface steps are known to be detrimental in the PEC conversion process through the energy states (recombination levels) which they produce in the electronic band structure [19]. A reduction in the density of surface steps would produce enhancement in the PEC efficiency. This is what has actually been observed in the present investigation (Fig. 8b). It has been surmized in earlier works that photoetching may lead to increased PEC efficiency through a decrease of the surface reflectivity also [20, 21]. It is not
In203-TiO2 AND WSe2 PHOTOELECTRODES
543
Fig. 9. (a) Scanning electron micrograph (secondary electron image) of the as-grown crystal, The edges of the crystals are outlined as A, B and C. Notice the presence of steps on the effective surface in the as-grown crystals, some of these are marked by arrows. After photoetching several of these surface steps disappear (b).
known to what extent this contributes to enhancement in conversion efficiency. However, the photoetching treatment did not appear to produce any significant decrease in reflectivity of the WSe2 (0001) crystal surface in the present investigation. The high efficiency WSe2 based PEC cells can be useful in ?egard to solar hydrogen production in two different ways. The high efficiency cells can be arrayed and the water can be subjected to photovoltaic driven electrolysis. Another mode in which the high efficiency WSe2 cells can be useful would be their use for producing external bias needed for the operation of TiO2 based photoelectrolysis cells. Thus a TiO2 cell backed by a WSe2 cell would form an all solar combination system for hydrogen production through water splitting. In the d - d phototransition dichalcogenide system the compound crystals of WS2 with direct and indirect band gaps of 1.7 and 1.3 eV is thought to be a better candidate for hydrogen production in combination with suitable catalysts [22], Keeping this fact in view we have initiated work on growth, synthesis and PEC characterization of WS2 crystals.
5. CONCLUSION In conclusion, it can be said that the modification of TiO2 with In20 3 admixtures enhances the photoproduction of hydrogen. The islands of In203 formed over the TiO. surface appear to achieve the advantages of colloidal photochemical systems, thereby enhancing the photoactivity of the anode. The overlaid thin In203 having higher light absorption extending from 2.5 to 3.6 eV enhance the photoactivity of the anode, leading to generation of higher density of carriers. The overvoltage at the anode is evidently reduced with the deposition of highly conducting In203 layers over the base TiO2 films,
thereby increasing the gas evolution kinetics. Besides TiO2, the other material studied corresponds to WSe~. High PEC efficiencies up to about 17'7, have been achieved through good quality photoanode crystals obtained through SeCI4 transporter and photoetching of crystal surfaces. The high efficiency WSe2 based PEC cells can be employed for solar hydrogen production either through their use as PV driven electrolysis systems or by using them for providing electrical bias to the TiO 2 based PEC cell. Further work on (a) Photoelectrodes consisting of admixtures of oxides e.g. In203TiO2 and (b) development of WS2 photoelectrodes, in relation to their use for production of solar hydrogen is being carried out and results will be forthcoming.
Acknowledgements--The authors are grateful to Prof. M, V. C.
Sastri, Prof. A. R. Verma, Prof. B. Venkataraman, Prof. G. V. Subba Rao, Prof. A. A. Balchin for helpful discussions. One of the authors (KSCB) greatly acknowledges Dr Dharam Singh (Chem. Dept., B.H.U.) for academic assistance. The presenl work was carried out under a Dept. of Non-conventional Energy Sources (Govt of India, New Delhi) project.
REFERENCES 1. A. Fujishima and K. Honda, Nature 238 37 ( 19721. 2. K. J, Hartig and N. Getoff, Hydrogen Energy Progress VI, p. 546 (1986); M. S. Wringhton in 5th Int. Conj~ on Photochemical Conversion and Storage of" Solar Energy
Osaka, Japan (1984). 3. C. Stalder and J. Augustynski, J. Electrochem. Soc. 126, 2[X17(1979). 4. A. K. Ghosh and H. P. Maruska, J. Electrochem. Soc. 124, 1516 (1977). 5. J. F, Houlihan, D. B. Armitage, T. Hoovkr, D. Bona Quist, D. Madacsi and L. N. Mulay, Mater. Res. Bull. 13, 1205 (1978).
544
G. PRASAD, K. S. CHANDRA BABU AND O. N. SRIVASTAVA
6. T. C. Philips, K. Moorjani, J. C. Murphy and T. O. Poehler, J. Electrochem. Soc. 129, 1210 (1982). 7. Masakazu, Anpo, N. Hiroki. K. Sukeya, K. Yutaka, Kuzunaridomen and Taka Haruonishi, J. Phys. Chem. 90, 1633 (1980). 8. K. V. C. Rao, V. C. Kumar and V. R. Gowarikar, private communication (1987). 9. S. Zielinski, A. Sobczynski, Acta Chim. Hung. 120, 229 (1985). 111. J. Gautron, P. Lamasson, B. Poumellec and J. F. Marucco, Sol. Energy Mater. 9, 101 (1983). 11. T. Kawai and T. Dakata, Chem. Phys. Lett. 72, 87 (1980). 12. W. Gissler and A. J. McEvoy, J. Electroanal. ('hem. 142,375 (1982). 13. K. Domen, S. Naito, T. Onishi and K. Tamaru, (71era. Phys. Lett. 92,433 (1982). 14, J. F. McCann and J. O'M. Bockris, J. Electrochem. Soc. 128, 1719 (1981).
15. C. C. Schumacher, S. Mamiche-Afara and M. J. Digham, J. Electrochem. Soc. 133, 716 (1986). 16. H. Tributsch, Ber. Bunsenges, Phys. Chem. 81, 361 (1977). 17. R. Cochoorn, C. Haas and R. A. deGroot, Phys. Rev. B. 35, 621)3 (1987). 18. G. Prasad, N. N. Rao and O. N. Srivastava, Cryst. Res. Technol. 21, 1303 (1986). 19. H. J. Lewerenz, A. Heller and F. J. Disalvo, J. Am. Chem. Soc. 102, 1877 (1980), 20. R. Tenne, N. Muller, Y. Mirovsky and D. Lando, J. Electrochem. Soc. 130, 852 (1983). 21. F. Decker, M. F. Decker, J. R. Moro and P. Motisuki, Solar Cells 20, 852 (1987). 22. A. Sobczynski, A. Yildiz, A. J. Bard, A. Campion, M. A. Fox, T. Mallouk, S. E. Webber and J. M. White, J. Phys. (71era. 92,2311 (1988).
0360-3199/89 $3.00 + 0.00 Maxwell Pergamon Macmillan plc. International Association for Hydrogen Energy.
Printed in Great Britain.
STRUCTURAL AND PHOTOELECTROCHEMICAL STUDIES OF In203-TiO2 AND WSe2 PHOTOELECTRODES FOR PHOTOELECTROCHEMICAL PRODUCTION OF HYDROGEN G . PRASAD,
K. S. CHANDRABABU* and O. N. SRIVASTAVA
Physics Department, and *Department of Chemistry, Banaras Hindu University, Varanasi-221005, India
( Received for publication 16 November 1988) Abstract--The present paper describes a new photoelectrode system for photoelectrolysis of water based on TiO2. This comprises of a system with In203 islands overlaid on a TiO2 base. The TiO2 is synthesized through anodic oxidation of Ti sheets under glow discharge conditions. The In203 islands are formed by first depositing a thin In film on TiO2 by electrodeposition followed by subsequent oxidation. The ln2OwTiO2 photoelectrodes have been characterized through XRD, scanning electron microscopy and photoelectrochemical techniques. It has been found that as compared to TiO2, where the typical photocurrent under illumination was 4 mA cm 2, the In203 overlaid T i O 2 exhibits a much higher photocurrent of 14 mA c m -2. The hydrogen gas evolution under photo-assisted electrolysis employing a TiO2 photoanode was found to be 3 x 102/t[ over a cm2of electrode, on the other hand the I n 2 0 3 modified TiO2 exhibited a higher hydrogen gas evolution rate of 8 x 10 ttl over a c m 2 of electrode. Evidence and arguments have been put forward to show that the presence of ln203 over TiO 2 makes the system possess the advantages of a colloidal photochemical system, and the better performance of the new photoelectrode is thought to be due to improved spectral response and catalytic activity of 1n203 in regard to the gas evolution kinetics. Besides TiO2 the other photoanode studied in the present investigation corresponds to the M X 2 type layered material, WSe2 (WS2). The WSe2crystals were grown by the chemical vapour transport technique with SeCI4as the transporter. The pfiotoelectrochemical efficiency (light to electricity) conversion efficiency of the as grown n-WSe2 crystals has been raised to ~ 17% through a photoetching treatment imparted to WSe2crystal surfaces. The high PEC efficiency of these photoanodes may be utilized for electrolysis in the photovoltaic mode or for providing external bias to the TiO: based photoelectrolysis cells.
1. I N T R O D U C T I O N In recent times a great deal of attention has been paid to photoelectrochemical (PEC) methods of conversion of solar energy to electrical power or a storable energy from e.g. hydrogen. The photoelectrolysis of water using semiconductor electrode(s) has gained significant attention in the past decade. Fujishima and Honda [1] described the photosensitized electrolysis of water on n-type TiO2 single crystal electrodes and proposed this reaction to be used for solar energy conversion and storage in the form of "hydrogen". For efficient PEC decomposition of water, the n-type photoanode is required to have manageable band gap - 2 to - 2 . 5 eV) suitable negative flat band potential, good stability and high quantum efficiency. In initial and subsequent studies, n-TiO2 has been shown to be exceptionally stable with high quantum efficiency (-- 100%). However, its optical band gap (Eg - 3 eV) is a negative factor in regard to its application as a photoelectrode in solar powered photoelectrolysis systems. Most of the small band gap oxide and non-oxide semiconductors being investigated were found to exhibit either photocorrosion or unsuitable flat band potential. It seems that after an extensive search for suitable semiconductors during 1972-1985, the interest is returning to TiO2 [2]. Several different approaches are being pursued to circumvent the poor solar spectral response of TiO~
caused by its large band gap. In one of these, TiO2 is doped with certain cations like A1, Cr, Y, Cu, Mo, Nb, V etc. [3-5] for their improved visible response. In another approach, the band gap of TiO2 is reduced by alloying with VO2, sacrificing the major advantage of TiO2 [6]. Similarly, to improve the limited solar response of TiO:, mixed oxide alloys like TiO2-SiO2 [7, 8], TiO2-Fe203, TiO2-Co304 [9], TiO2-NbO2 [10], etc. have been studied with the known fact that the binary oxide catalysts show higher activity than their components. In yet another attempt to improve response through an improvement in gas evolution kinetics, loading of photoelectrodes with catalysts have been tried with TiO2. As, for example, improved gas evolution (oxygen and hydrogen) was found when TiO 2 was loaded with RuO~ [11, 12]. Similarly, SrTiO3 enhanced gas evolution with loading of the NiO catalyst [13]. In our present study, the influence of 1n203 admixtures over the TiO2 on its photoactivity and gas evolution kinetics has been studied. An indium oxide single crystal was photoelectrochemically tested by McCann and Bockris [14] and was reported to have an indirect optical absorption edge at 2.3-2.5 eV. With n-type conductivity polycrystalline In203 exhibited an indirect band gap at 2.8 eV and direct band gap at 3.6 eV [15]. The quantum efficiency of this photoanode was dependent on the preparative conditions and the highest value of 100% corresponds to the one prepared by thermal 537
538
G. PRASAD, K. S. CHANDRA BABU AND O. N. SRIVASTAVA
oxidation of liquid In metal. In the present study, thin film admixtures of In203 with TiO 2 have been synthesized and this has been employed as a photoelectrode. It may be mentioned that the thin films have good transport properties and reduced carrier recombination at the interface. A thin layer of TiO2 is prepared on Ti by anodicaily oxidizing a Ti sheet under conditions of high current density (50 m A cm -2) and glow discharge. Over this TiO2 surface, In is electrochemically deposited from 0.5 M In2(SO4) 3 solution. Later, the TiOE-In electrode is homogenized in an oxygen atmosphere at 500°C for 1 h (presumably this treatment transforms TiO2-In to TiO2-In203). The surface morphology, structural and photoelectrochemical characteristics of the base TiO 2 as well as the TiO2 overlaid with In203 thin film admixtures have been investigated in relation to hydrogen evolution through photoelectrolysis. Besides the oxide semiconductors, the interest in regard to developing viable semiconducting photoelectrodes centres also on the special MX 2 type layered semiconductors with d-d phototransition characteristics. These materials are typified by WSe 2. Such materials possessing optimal band gaps ( - 1.2 to - 2.0 eV) have high optical absorption coefficients ( - 105 cm-J). Above all, the optical transitions in these materials mostly involve lower and upper metal bands, as a result the metal--chalcogen bonds remain nearly unaffected during phototransition and hence photocorrosion in reasonably perfect crystals would be minimum [16, 17]. Keeping this fact in view, we have investigated the growth, synthesis characterization of WSe2 (WS2) crystals. We have shown that by improving the quality of the as-grown crystals through the use of SeC14 transporter and by improving on the crystal surface through photoetching, it is possible to achieve PEC (solar~electrical) conversion efficiency of about ~ 17%. These high efficiency cells could be useful in solar hydrogen production either as a source of bias for a TiO2 based cell or for direct production through photoelectrolysis.
for 1 h. Room temperature conductivities of TiO2 films before and after In203 modification are measured. These electrodes are subjected to structural and photoelectrochemical characterization. Simultaneously, In is thermally evaporated on to a vitreous quartz slide in a vacuum of 10 -5 torr and is annealed at 500°C for 1 h in oxygen flow. The absorbance of this thin film is measured in the range of 200-800 nm with a Hitachi 320 U.V.-Visible Absorption Spectrometer. Structural characterization of In203 deposited and base TiO, is performed by X-ray diffractometry using Philips PW-1710 X-ray Diffractometer equipped with a graphite monochromator. Surface morphology of these films is studied under scanning electron microscope (PhilipsCM-12). Photoelectrochmeical characterization of the electrodes is performed by measuring the currentpotential characteristics. The electrodes are connected to a pure copper wire with silver epoxy and sealed with epoxy resin leaving an exposed area of 1 cm 2. The sealant has been observed to be stable in 1 M NaOH. The measurement is done in a Pyrex cell with a quartz window employing conventional three electrode assembly. A ( - 2 cm 2) Pt was used as counter electrode and SCE with luggin capillary as reference. The I - V curves of the electrodes under dark and illumination were recorded with a Princeton Applied Research (PAR) Model 173 Potentiostat/Galvanostat, a P A R Model 175 Universal Programmer and a Houston Model 20(10 X - Y recorder. The illumination source being 1000 W X e - H g lamp (Oriel Corporation, U.S.A.) with a water filter to remove heating effects. The I - V characteristics at room temperature were measured in nitrogen purged 1 M NaOH. All the reagents are analytical grade and prepared with doubly distilled water. The evolved gases are measured by fixing an inverted burette.
3. RESULTS A N D DISCUSSION 3.1 Structural characteristics
2. E X P E R I M E N T A L Thin Ti sheets (Goodfellow Metals, U.K.) are decreased with methanol and ultrasonically cleaned in acetone for 30 rain. Before anodization every electrode is etched with HzO2:HF (1:1) for 10 s. These Ti sheets are anodically oxidized in 2 M Na2SO 4 solution witha Pt cathode, under high current density and glow discharge. The voltage applied between the electrodes (Ti and Pt) is increased gradually with the current never exceeding 50 m A c m -~. Sparking started around 85 V, and the anodization was continued at 120 V for 5 min. The uniform, compact, mouse-colored TiO2 films thus obtained exhibit n-type conductivity even without reduction. Over this anodized Ti, In is electrodeposited by cathodically polarizing the electrode in 0.5 M In2(SO4) 3 solution at 5 V for 10 rain. Later this electrode is desealed and annealed in an oxygen atmosphere at - 500°C
The X-ray diffractograms of anodized Ti exhibit a dominant rutile type of TiO2 with Ti (Fig. 1). After depositing In on TiO2, the XRD analyses confirm the presence of a rutile phase of TiO2 and In with (In-Ti) alloy phase and Ti (Fig. 2). The XRD pattern of oxidized In on TiO2 was found to contain (Ti-In) alloy phase and In in addition to TiO2 and In203 phases (Fig. 3). However, no mixed oxide alloy could be indexed. Thus, it can be concluded that In203 has formed a thin film over the surface of TiOz, without forming any compound oxide system of the type (Ti-In)xO v. The scanning electron micrographs o ( a n o d i z e d Ti reveal a porous surface with a film thickness of around 1000 ~, (Fig. 4a). T.he TiO2 films with In203 are found to exhibit a cratered surface. It is also observed that the In203 layer itself is not uniform but is in the form of clusters of 1000 • thickness (Fig. 4b). The absorbance characteristics of the In203 formed on vitreous quartz slide is shown in Fig. 5. This exhibits commencement of
In203-TiO2 AND WSe2 PHOTOELECTRODES
539
O
o
o 0
I
4
/
L
70
i I
60
I
50
I 30
40 ",
,
i 2o
2
Fig. 1. X-Ray diffraction pattern of TiO2 prepared by anodic oxidation of Ti under extraordinary conditions, o--TiO2; A--Ti.
O
o O 1
I
._
I
60
O'
50
40
3O
2O
20
Fig. 2. X-Ray diffractogram of TiO2 with electrodeposited indium metal film. o--TiOz; e--(In-Ti) alloy; A--In; &--Ti.
c,
t~
70'
60I
°
~t
~
~ 00~
50I
41 -
0
3/
A° ~,
20
Fig. 3. X-Ray diffractogram of TiO2 with indium oxide on surface. *--1n203; o--TiO2; A - - I n ; e--(In-Ti) alloy.
540
G. PRASAD, K. S. CHANDRA BABU AND O. N. SRIVASTAVA
Fig. 4. Scanning electron micrographs of TiO~ film (a) before and (b) after In20~ modification.
absorption around 460 nm indicating an optical transition commencing around 2.5 eV. The room temperature conductivity of the anodized Ti has been found to be 10- ~-f2 t cm -~, while that of the TiO, surface deposited with In~O3 is l02 f2 i cm i. From this, it is clear that the conductivity of the photoanode gets enhanced with In203 modification. 3.2 Photoelectrochemical studies
The photoelectrochemical behaviour of TiO2 overlaid with In_~O3 and base TiO2 is elucidated by studying the I - V characteristics of the two electrodes individually. From the I - V curves, a clear enhancement in the
photocurrent from 4 m A cm 2 to 14 m A cm -~is evident with In203 modification (Fig. 6). The photo-onset potential has been shifted to the negative side and the onset is less sluggish in nature, indicating reduced surface recombination. The enhancement in the photocurrent with In203 modification is thought to be due to the improved spectral response of photoanode due to the presence of surface In203. As is clear from Fig. 5 that [n203 shows c o m m e n c e m e n t of absorption around 2.5 eV and continues up to 3.6 eV, thereby producing carriers which evidently enhance the photoactivity of the anode. As the carrier recombination at the thin film interface is known to be low added by high light absorption [15 I, the thin In203 islands on TiO~ surface
14--
4
5 ,o
"-" 3
8
82
/
~6
/j
\[n203-TiO2
f
~._. .~. - - -: a---,C
"'"""""
""--"-TIO00
mV
;2 200
I
300
I
I
r
400 500 600 WaveLength (nm)
I
700
I
800
Fig. 5. Absorbancc characteristics of indium oxide fihns formed on to vitreous quartz slide by oxidizing indium metal film.
0
I 7 0 0 600
r 0
l 200
.9-°~k--
Potentiot (mV) vs SCE Fig. (/. Currcm-potential characteristics of TiO_~ (broken lines) and ln~O; modified Ti02 (solid lines) under dark and illumination in 1 M NaOH.
In203-TiO2
AND
541
WSe 2 PHOTOELECTRODES
enhance the photoactivity of the anode, even under low absorption coefficiency at the region of indirect transition around 2.5 eV. As is evident from XRD studies and scanning electron micrographs, the In203 does not react with the base but forms only islands over the surface of TiO2. These photoactive islands appear to act similarly to Pt islands on TiO2 or SrTiO3, enhancing the gas evolution kinetics without forming any mixed oxide material.
-
f..- ~-'----.~x........x_...__
-- /
~-
(b)
B 0
L =:L
L g
~ o
3.3 Water photoelectrolysis
I
The potential at which the onset of photocurrent starts (V,,.), outlines the position of energy bands of the semiconductor. In our present study, Vo, is used instead of Vvb (flat band potential) because of nonlinearity of Mott-Schottky plots for both TiO 2 and In203 electrodes. Generally, the difference of Vvb-Vo, diminishes with increasing photon flux. So, in our high flux illumination, the difference (Vvb-Vo,) can be negelcted. In the photoassisted electrolysis mode of operation of the cell, n-TiO2/1 M NaOH/Pt, under the illumination of 1000 W Xe-Hg lamp fitted with a water filter, oxygen gas started evolving on the anode (hydrogen at the cathode) at - 0 . 8 5 V. The cell with configuration n-In203-TiO2/1 M NaOH/Pt showed gas evolution at - 0 . 9 V with the same illuminating source. The values ( - 0 . 8 5 and - 0 . 9 ) correspond to an applied bias voltage of 0.45 and 0.4 V, respectively. Theoretically, the minium bias required for photo-oxygen evolution on n-type semiconductors in a Schottky type photoelectrolysis cell, Vn-min bias when the flat band potential is more positive than the 2H+/H2 redox potential is [14]: Vn min bias :
c3
Vn Fb - - V2 H+/H, -
Substituting a value of - 1 . 0 3 V for V2 H+-U~in 1 M NaOH, the Vo, for V,-vb' the Vn_mi n bias of 0]13 V is required for the cell with In203 modified anode. This value after accounting the iR drop and overvoltages at the electrodes, corresponds well with the experimentally observed bias of 0.4-0.45 V. From the gas evolution characteristics (Fig. 7), it is clear that the evolved gases obey the Faraday laws and they are in stoichiometric quantities. The gas evolution has attained saturation after 10 min under an applied bias of +0.5 V and illumination. The clear enhancement of gas evolution kinetics for 1n203 admixtures with TiO2 as compared to TiO2 alone from 3 x 102 ul over cm 2 of electrode to 8 x 1(12 lq over a cm 2 of electrode is evident. From the I-V characteristics (Fig. 6) it is clear that In203 modified TiO2 shows an enhanced photocurrent and photovoltage. In the absorption studies, In203 films exhibit commencement of optical absorption at around 2.5 eV. Even with a low value of absorption coefficience at this indirect transition due to the facilitated charge transfer and reduced recombination rate caused by the high conductivity (1(12 ~ i cm l) of the anode, the photocurrent of the anode is evidently enhanced with
(a)
/
I IO
I 20
I 30 Time
I 40
I 50
o 60
(rain)
Fig. 7. Volume of evolved gases as a function of irradiation time at an applied bias of 0.5 V (light source--l(X)0 W Xe-Hg lamp with water filter) (a) pure TiO2 (b) In203 modified TiO2.
In203 modification. The highly conducting In203 reduces the overvoltage at the anode and catalyses the gas evolution, thus, causing the enhanced rate of gas production at the photoanode. The porous TiO substrate also facilitates in reducing the overvoltage at the anode by increasing the effective surface area of the anode. As is clear from the scanning electron micrographs (Fig. 4), the islands of In203 formed over the surface of TiO2 are thought to catalyse the anode reaction, probably with the added advantages of colloidal photochemical systems. The increased photoactivity is also thought to be due to a lower concentration of defects and impurities. Even though the mechanism of this facilitated charge transfer at the oxide/electrolyte interface is presently unknown, the enhanced photoactivity added by increased gas evolution kinetics can be thought to be due to surface modification with thin In203 islands which are having high light absorption, conductivity and wider spectral response than TiO 2 alone. It may, therefore, be taken that TiO2 overlaid with In203 surface clusters represents a new improved photoelectrode for hydrogen production through photoelectrolysis.
4. STUDIES ON WSe2 P H O T O A N O D E S As outlined earlier, besides TiO2, the other material on which studies in regard to photoelectrochemical
542
G. PRASAD, K. S. CHANDRA BABU AND O. N. SRIVASTAVA 4O 32
E 24
~2
g
o
4
600
400
l ~0
200
400
E
I 200
CeLt vo(tege ( m Y )
Fig. 8. Power characteristics of the cell with as-grown W S c 2 crystal (a) and after photoctching in 0.1 M aquaregia (b). Crystal area is 0.0125 cm2.
(photoelectrolysis) behaviour were carried out corresponded to WSe2. Now these investigations will be discussed. The single crystals of WSe2 using SeCI4, which is a transporter having an ingredient (Se) native to WSe2, were grown by the chemical vapour transport technique [18]. The transporter was added to presynthesized tungsten diselenide powder in a concentration of 0.2 mg cm -3. A temperature gradient of 2°C cm-I was maintained for 170 h in the growth furnace. Large single crystals having dimensions up to 0.5 x 0.5 x 0.1 cm were obtained after growth. Crystals were attached to a platinum wire, which was fused in a narrow tube containing a mercury pool. The back contact of the crystals was provided by l n - G a alloy. The electrolyte used for electrochemical studies corresponded to 1 M KI + 0.05 M 12 (pH - 9). All experiments were carried out by using Princeton Applied Research (PAR) electrochemical equipment. A fresh crystal surface, obtained by rough cleaving with adhesive tape, was used for fabrication of PEC cells. Illumination was provided with a tungsten lamp with intensity of 60 mW cm -2 (after correction to solution absorption) for both photoetching and electrochemical measurements. Typical PEC efficiencies of the order of ~ 6 to 8% were observed with the as-grown crystals. Photoetching was carried out in 0.1 M aqua regia in forward bias conditions. Several different etchants like perchloric acid, orthophosphoric acid, chromic acid and aqua regia were used for photoetching. It was found that the best results in regard to enhancement of efficiency were obtained in aqua regia. Figures 8(a) and (b) show the power characteristics of the crystal before and after photoetching. The PEC conversion efficiency before photoetching is - 8.3% (Fig. 8a) and this changes to 17.1% (Fig. 8b) after photoetching. This efficiency is one of the highest reported efficiencies for WSe2 based PEC solar cells. It should be mentioned that the said efficiency ( - 17.1%) was obtained for specifically good crystals exhibiting rather significant changes in the surface micro-structure
on photoetching for the WSe 2. Conversion efficiencies of about ~ 17.1% could be obtained on some other crystals also. Efficiencies in the range of 13 to 15% were generally obtained. A possible factor which is likely to contribute to the enhancement in conversion efficiency may come about from the growth parameters adopted in the present study. Unlike many other previous WSe 2 growth experiments, where a TeCI4 transporter is used, we have employed SeCI4 as the transporter. Here Se atoms are involved which are native to WSe2 crystals. Thus, unlike the case with Te, incorporation of Se in the growing crystal would not result in the creation of additional recombination type energy states (gap states) in the band gap. Analytical explorations of these crystals through the E D A X technique in scanning electron microscope mode and resistivity measurements confirmed the improved quality of SeCI4 grown crystals. In order to explore the influence of the photoetching on the microstructural characteristics of the surface, the effective crystal surface was characterized by employing scanning mode of the electron microscope (Philips EM-CM12). The crystal surfaces of several crystals where PEC measurements were done, were explored. Figures 9(a) and (b) represent the surface characteristics of the crystal obtained in the secondary electron scanning mode corresponding to PEC characteristics shown in Figs 8(a) and (b) before and after photoetching. A noticeable feature brought out by this Fig. 9(b) is the disappearance of several surface steps (see Fig. 9a) consequent to photoetching. The surface steps are known to be detrimental in the PEC conversion process through the energy states (recombination levels) which they produce in the electronic band structure [19]. A reduction in the density of surface steps would produce enhancement in the PEC efficiency. This is what has actually been observed in the present investigation (Fig. 8b). It has been surmized in earlier works that photoetching may lead to increased PEC efficiency through a decrease of the surface reflectivity also [20, 21]. It is not
In203-TiO2 AND WSe2 PHOTOELECTRODES
543
Fig. 9. (a) Scanning electron micrograph (secondary electron image) of the as-grown crystal, The edges of the crystals are outlined as A, B and C. Notice the presence of steps on the effective surface in the as-grown crystals, some of these are marked by arrows. After photoetching several of these surface steps disappear (b).
known to what extent this contributes to enhancement in conversion efficiency. However, the photoetching treatment did not appear to produce any significant decrease in reflectivity of the WSe2 (0001) crystal surface in the present investigation. The high efficiency WSe2 based PEC cells can be useful in ?egard to solar hydrogen production in two different ways. The high efficiency cells can be arrayed and the water can be subjected to photovoltaic driven electrolysis. Another mode in which the high efficiency WSe2 cells can be useful would be their use for producing external bias needed for the operation of TiO2 based photoelectrolysis cells. Thus a TiO2 cell backed by a WSe2 cell would form an all solar combination system for hydrogen production through water splitting. In the d - d phototransition dichalcogenide system the compound crystals of WS2 with direct and indirect band gaps of 1.7 and 1.3 eV is thought to be a better candidate for hydrogen production in combination with suitable catalysts [22], Keeping this fact in view we have initiated work on growth, synthesis and PEC characterization of WS2 crystals.
5. CONCLUSION In conclusion, it can be said that the modification of TiO2 with In20 3 admixtures enhances the photoproduction of hydrogen. The islands of In203 formed over the TiO. surface appear to achieve the advantages of colloidal photochemical systems, thereby enhancing the photoactivity of the anode. The overlaid thin In203 having higher light absorption extending from 2.5 to 3.6 eV enhance the photoactivity of the anode, leading to generation of higher density of carriers. The overvoltage at the anode is evidently reduced with the deposition of highly conducting In203 layers over the base TiO2 films,
thereby increasing the gas evolution kinetics. Besides TiO2, the other material studied corresponds to WSe~. High PEC efficiencies up to about 17'7, have been achieved through good quality photoanode crystals obtained through SeCI4 transporter and photoetching of crystal surfaces. The high efficiency WSe2 based PEC cells can be employed for solar hydrogen production either through their use as PV driven electrolysis systems or by using them for providing electrical bias to the TiO 2 based PEC cell. Further work on (a) Photoelectrodes consisting of admixtures of oxides e.g. In203TiO2 and (b) development of WS2 photoelectrodes, in relation to their use for production of solar hydrogen is being carried out and results will be forthcoming.
Acknowledgements--The authors are grateful to Prof. M, V. C.
Sastri, Prof. A. R. Verma, Prof. B. Venkataraman, Prof. G. V. Subba Rao, Prof. A. A. Balchin for helpful discussions. One of the authors (KSCB) greatly acknowledges Dr Dharam Singh (Chem. Dept., B.H.U.) for academic assistance. The presenl work was carried out under a Dept. of Non-conventional Energy Sources (Govt of India, New Delhi) project.
REFERENCES 1. A. Fujishima and K. Honda, Nature 238 37 ( 19721. 2. K. J, Hartig and N. Getoff, Hydrogen Energy Progress VI, p. 546 (1986); M. S. Wringhton in 5th Int. Conj~ on Photochemical Conversion and Storage of" Solar Energy
Osaka, Japan (1984). 3. C. Stalder and J. Augustynski, J. Electrochem. Soc. 126, 2[X17(1979). 4. A. K. Ghosh and H. P. Maruska, J. Electrochem. Soc. 124, 1516 (1977). 5. J. F, Houlihan, D. B. Armitage, T. Hoovkr, D. Bona Quist, D. Madacsi and L. N. Mulay, Mater. Res. Bull. 13, 1205 (1978).
544
G. PRASAD, K. S. CHANDRA BABU AND O. N. SRIVASTAVA
6. T. C. Philips, K. Moorjani, J. C. Murphy and T. O. Poehler, J. Electrochem. Soc. 129, 1210 (1982). 7. Masakazu, Anpo, N. Hiroki. K. Sukeya, K. Yutaka, Kuzunaridomen and Taka Haruonishi, J. Phys. Chem. 90, 1633 (1980). 8. K. V. C. Rao, V. C. Kumar and V. R. Gowarikar, private communication (1987). 9. S. Zielinski, A. Sobczynski, Acta Chim. Hung. 120, 229 (1985). 111. J. Gautron, P. Lamasson, B. Poumellec and J. F. Marucco, Sol. Energy Mater. 9, 101 (1983). 11. T. Kawai and T. Dakata, Chem. Phys. Lett. 72, 87 (1980). 12. W. Gissler and A. J. McEvoy, J. Electroanal. ('hem. 142,375 (1982). 13. K. Domen, S. Naito, T. Onishi and K. Tamaru, (71era. Phys. Lett. 92,433 (1982). 14, J. F. McCann and J. O'M. Bockris, J. Electrochem. Soc. 128, 1719 (1981).
15. C. C. Schumacher, S. Mamiche-Afara and M. J. Digham, J. Electrochem. Soc. 133, 716 (1986). 16. H. Tributsch, Ber. Bunsenges, Phys. Chem. 81, 361 (1977). 17. R. Cochoorn, C. Haas and R. A. deGroot, Phys. Rev. B. 35, 621)3 (1987). 18. G. Prasad, N. N. Rao and O. N. Srivastava, Cryst. Res. Technol. 21, 1303 (1986). 19. H. J. Lewerenz, A. Heller and F. J. Disalvo, J. Am. Chem. Soc. 102, 1877 (1980), 20. R. Tenne, N. Muller, Y. Mirovsky and D. Lando, J. Electrochem. Soc. 130, 852 (1983). 21. F. Decker, M. F. Decker, J. R. Moro and P. Motisuki, Solar Cells 20, 852 (1987). 22. A. Sobczynski, A. Yildiz, A. J. Bard, A. Campion, M. A. Fox, T. Mallouk, S. E. Webber and J. M. White, J. Phys. (71era. 92,2311 (1988).