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Electrosynthesis of cadmium selenide films from sodium citrate–selenosulphite bath
Materials Chemistry and Physics 91 (2005) 399–403
Electrosynthesis of cadmium selenide films from sodium citrate–selenosulphite bath C.D. Lokhande, Eun-Ho Lee, Kwang-Deog Jung, Oh-Shim Joo∗ Eco-Nano Research Center, Korea Institute of Science and Technology, P.O. Box 131, CheongRyang, Seoul 136 650, Korea Received 7 July 2004; accepted 26 November 2004
Abstract Electrosynthesis of cadmium selenide (CdSe) film has been carried out from deposition bath containing sodium selenosulphite, along with cadmium complexed with sodium citrate under potentiostatic deposition condition on titanium substrates. The pH of deposition bath was weakly basic (< 9.0). The CdSe films up to 3.0 m were deposited. The X-ray diffraction (XRD) studies revealed that the CdSe films are microcrystalline with increased grain size after annealing. The scanning electron microscopy showed that the films are porous with cauliflower-like morphology. The photelectrochemical characterization showed that the CdSe films are photoactive. © 2004 Elsevier B.V. All rights reserved. Keywords: Electrosynthesis; Cadmium selenide; Thin films; Structural properties
1. Introduction Cadmium selenide, in the form of thin polycrystalline films, exhibits very interesting properties, such as direct transition, proper band gap width, short penetration length of light, etc. These properties make CdSe particularly suitable for various technical applications. In particular, its band gap value (1.7 eV) makes it useful for the conversion of solar energy in photovoltaic or photoelectrochemical (PEC) devices as well as catalysis for the photo-assisted decomposition of water. Using cadmium chalcogenide single-crystal electrodes, conversion of solar light into electrical energy has been achieved with efficiencies as high as 14% [1]. On the other hand, Hodes et al., obtained solar conversion efficiencies up to 8%, using CdSe0.65 Te0.35 polycrystalline layers in PEC cell [2]. Various chemical methods, e.g., electrosynthesis, chemical deposition, slurry painting etc. have been used to deposit polycrystalline CdSe thin films for the production of large area electrodes for the use in PEC cells. Amongst the various deposition methods, the electrosynthesis from ∗
Corresponding author. Tel.: +82 2 958 5215; fax: +82 2 958 5219. E-mail address: [email protected] (O.-S. Joo).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.11.046
aqueous electrolytes appears to be a very simple and low cost preparation method. This method has already been employed for the deposition of CdSe films. Thin films of CdSe have been deposited from aqueous and nonaqueous and acidic and alkaline baths [3]. Use of selenious acid as selenide ion source in acidic and alkaline has been reported [4–9]. The cadmium ions have been complexed using citric acid, ethylene-diamine-tetra-acetic acid (EDTA), ammonia, sodium cyanide, nitrilotriacetic acid (NTA), etc. In acidic medium, the pH of bath is about 2–3. The difficulty in depositing films from the solutions containing selenious acid is that a substantial excess of selenium is detected in the films. To remove an excess portion of selenium, the films must be heated at temperatures as high as 700 ◦ C. The mechanism of electrodeposition of CdSe from a solution containing Cd2+ and SeO2 has shown that the Se(IV+ ) is reduced to Se(II− ), i.e. H2 Se, which then reacts with Cd2+ present at the cathode surface to form CdSe [4]. It is thought that in the acidic medium, the selenium incorporation occurs according to the reaction
2CdSe + H2 SeO3 + 4H+ → 2Cd2+ + 3Se + 3H2 O
(1)
400
C.D. Lokhande et al. / Materials Chemistry and Physics 91 (2005) 399–403
It is important to avoid this reaction in the bath. In an alternate approach, in alkaline medium (pH > 10), cadmium ions are complexed with NTA or EDTA and selenocynide ions are incorporated in the solution, which are reduced at the cathode to produce selenide ions, which react with the cadmium ions to electrodeposit CdSe on the cathode [3,6]. In order to get device quality CdSe film, the deposition was carried out from elevated temperature (>333 K) baths. In the present paper, we report on the room temperature electrosynthesis of CdSe films from weakly alkaline (pH < 9) aqueous solution containing the selenosulphite ion and a cadmium ion complexed with sodium citrate. The CdSe film of the thickness up to 3.0 m was deposited on titanium substrate. The structural characterization of CdSe film was carried out using XRD and SEM techniques. The photovoltaic activity of CdSe film was tested by forming photoelectrochemical cell with polysulphide electrolyte.
2. Experimental Room temperature electrodeposition of CdSe thin film on titanium substrate has been carried out using following procedure. In brief, freshly prepared solutions of 0.02 M CdSO4 and 1.0 M sodium citrate were mixed into 2:1 ratio and pH of the resultant solution was made at 8.6. A solution of 0.02 M sodium selenosulphite (Na2 SeSO3 ) was added to it. The Na2 SeSO3 solution was prepared by dissolving 0.120 g of elemental selenium powder (0.05 M) in an aqueous solution containing 3.0 g of sodium sulphite (0.75 M) at pH 9.0. A platinum sheet with 1.5 cm × 1.5 cm area was used as a counter electrode. All potentials were measured with respect to Ag/AgCl as a reference electrode. A scanning galvanostat/potentiostat (E G and G model 273A) was used in potentiostatic mode. The deposition was carried out at room temperature (298 K). After deposition, film was cleaned with triple distilled water and dried under argon flow. Before inserting the titanium substrate in the deposition bath, it was polished with smooth polish paper and cleaned with triple distilled water. In order to remove the oily substances from surface, cleaned substrate was etched in 3% dil. HCl for 10 min and then ultrasonically cleaned with triple distilled water. After deposition, CdSe film was annealed at 773 K in air for 60 min. The CdSe film was kept in furnace at room temperature and then furnace temperature was raised at a heating rate of 5 K min−1 . The CdSe film was etched in 3% dil. HCl for 3 s and dried under argon flow. Thin film of CdSe was characterized using following techniques. The XRD pattern was obtained by using X ray diffractrometer (RINT/PMAX 2500, Rigaku, Japan). Micro structural study was carried out with scanning electron micrographs, obtained with FE-SEM, (SM-6340F, JEOL, Japan). Photoelectrochemical properties were studied by forming a cell with CdSe as a photoelectrode, platinum spiral wire as a counter electrode. The electrolyte used was 1 M poly-
sulphide (NaOH–Na2 S–S) aqueous solution. A Xe lamp (450 W, Thermo Oriel Co., USA), with illumination intensity of 100 mW cm−2 at 300 nm wavelength was used as a light source.
3. Results and discussion The mechanism of cathodic electrodeposition of CdSe from Cd and Se precursors in solution is still a matter of some contention. It may involve the co-deposition of Cd and Se, or the initial formation of Se2− followed by CdSe precipitation onto the electrode surface [4]. Furthermore, separate Cd and Se phases, which are in physical contact, may react during subsequent annealing. It is not unlikely that all these mechanisms are operating, but their contributions depend on the electrodeposition conditions. In case of CdSe deposition, Se being a noble metal, the surface concentration of Se ions has to be small as possible to allow the complete achievement of the low crystalline process of CdSe compound. It is therefore; necessary to reduce the concentrations of the total amount of Se ions dissolved in the bath and monitor the electrodeposition parameters in such a way that the current is totally limited by the mass transport of Se ions. The cathodic deposition potential depends upon bath temperature, nature of substrate, metal ion concentration, complexing agent and its concentration etc. [3]. The deposition potential for CdSe from the above bath was decided from polarization curve. Fig. 1(a and b) shows the polarization curves for the reduction of Cd from cadmium sulphate–sodium citrate and CdSe from cadmium sulphate–sodium citrate–sodium selenosulphite solutions, on titanium substrate. The sudden increase (curve a) in current above −800 mV versus Ag/AgCl corresponds to the reduc-
Fig. 1. Polarisation curves for the reduction of (a) cadmium from the solution containing 0.02 M cadmium sulphate–1.0 M sodium citrate and (b) cadmium selenide from the solution containing 0.02 M cadmium sulphate–1.0 M sodium citrate–0.02 M sodium selenosulphite, on titanium substrate. The scanning rate was 10 mV s−1 .
C.D. Lokhande et al. / Materials Chemistry and Physics 91 (2005) 399–403
Fig. 2. Variation of deposition current density with deposition time for deposition of CdSe film from the solution containing 0.02 M cadmium sulphate–1.0 M sodium citrate–0.02 M sodium selenosulphite (pH = 8.6) on titanium substrate. The deposition potential applied was −1000 mV versus Ag/AgCl.
tion of cadmium on titanium substrate. This is followed by hydrogen evolution above −900 mV versus Ag/AgCl. When the potential becomes more negative, current does not stay stationary because of the fast formation of fern-like cadmium dendrites. However, the nature of polarization curve (curve b) was changed by addition of sodium selenosulphite. It showed current peaks at around −900 and −970 mV versus Ag/AgCl, which correspond to the reduction of Cd and CdSe, respectively. Loizos et al. [10] have observed similar current behavior in polarization curves for CdSe deposition from acidic selenious bath. By considering the reduction potential obtained from polarization curve, the deposition of CdSe films was carried out by applying deposition potential of −1000 mV versus Ag/AgCl. The variation of current density with deposition time for deposition of CdSe on titanium substrate is shown in Fig. 2. The deposition current is found to be substrate dependent. When substrate is polycrystalline, its orientations are varied and material gets deposited so as to attain the closest symmetry with the surface morphology of the substrate. In addition, the affinity of substrate material is different for different depositing materials. In present case, the deposition current was about 2 mA cm−2 and was found to vary continuously with deposition time. This may be due to irregular expansion of the effective area of depositing CdSe film on titanium substrate [10]. It is well known that sodium selenosulphite solution hydrolyses in an alkaline medium to give selenide ions as Na2 SeSO3 + OH− → Na2 SO4 + HSe−
(2)
HSe− + OH− → H2 O + Se2−
(3)
The concentration of selenide ions at cathode is sufficiently high to become associated with cadmium ions as dissociated from the complexed pool of cadmium ions to form
401
Fig. 3. The XRD pattern of annealed CdSe film on titanium substrate.
cadmium selenide, which deposits on the cathode. The cadmium, complexed with sodium citrate as Cd(Na3 C6 H5 O7 )2+ which has stability constant of 109.8 , reduces the concentration of free cadmium ions. The pH of deposition bath is weakly basic (< 9), in which chemical equilibrium of complexing cadmium ions shifts towards the associated complexed cadmium ions. This minimizes the competitive reactions of cadmium ions with hydroxide ions in the bath and any selenide ions, which may be free in the bath. The weak alkaline pH of bath makes the hydroxyl ion concentration in such a way that the equilibrium involving the seleonsulphite and hydroxide favors the selenosulphite ion as compared to the production of selenide when the cadmium ions exist in the solution complexed by sodium citrate [11]. By means of this complex, the solution remains stable for more than 24 h. The CdSe film thickness in the range of 0.5–3.0 m was obtained, depending upon deposition time period. After about thickness of 3.0 m, CdSe film started detaching from the titanium substrate. The crystal quality of CdSe films can be improved by appropriate annealing treatments. It has been reported that annealing below 573 K does not affect the crystal structure. Above 673 K, most of the CdSe film adopts the normal hexagonal wurtzite structure [10]. Therefore, the CdSe film deposited on titanium was annealed at 773 K, for 60 min in air. Fig. 3 shows the XRD pattern of CdSe film deposited titanium coated substrate. From this, it is found that CdSe films are microcrystalline with planes (1 0 0), (2 0 0), (1 0 2), (1 1 0), and (1 1 2) corresponding to hexagonal phase [12]. For electrodeposited CdSe films, improvement in grain size, liberation of Se from surface region and chemical combining of Cd and Se has been reported after annealing [13]. Scanning electron microscopy is a convenient and versatile method to study microstructure of the film and to determine the grain size. Fig. 4(a and b) shows the micrographs of as deposited and annealed CdSe films on titanium substrate at two different magnifications (5000× and 20,000×).
402
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Fig. 4. The scanning electron micrographs of cadmium selenide films at two different magnifications: (a) as deposited, and (b) annealed at 773 K in air. (c) Cross-sectional view of titanium/CdSe.
The as deposited films are rough and show agglomeration of small grains. However, the morphology was changed after annealing the films. After annealing, the films showed porous structure with irregular grain growth of about one micron size. These grains were grouped on the surface by combination with each other to form cauliflower-like structure. This is clearly seen at (Fig. 4b) high magnification. Hodes et al. [2] have reported similar type of morphology for CdSe and Cd(Se, Te) films deposited from acidic and alkaline media on titanium substrate. They found no correlation between the surface morphology and the efficiency of CdSe PEC cells. Moreover, the large photocurrents obtained were ascribed to the ability of the electrolyte to make good physical contact with such an irregular surface. Fig. 4(c) shows the cross sectional view of titanium substrate/CdSe film. From this, the growth of grains and their merging into one another to form bigger grains is also seen. From this, CdSe film thickness was estimated to be about 2–3 m. In recent years, photoelectrochemical cells based on semiconductor–electrolyte junctions have been attracting a great deal of interest in solar and non-solar applications, as they have many advantages over conventional solid-state junctions. These PEC cells are easy to make and many processing steps of p–n junction are simplified or eliminated.
Since the junction with liquid is formed spontaneously upon contact, irregular shaped single crystal or thin films can be used. Therefore, PEC cells with semiconductor under investigation can be used as a simple and quick technique to test the quality of solar cell materials [14]. In the present case, photelectrochemical cell was formed with annealed CdSe film and illuminating the cell with light, its photovoltaic activity was tested. The short circuit current (Isc ) of the order of 1–2 mA cm−2 and open circuit voltage (Voc ) of the order of 450–500 mV were obtained under the illumination of 100 mW cm−2 . The typical current–voltage characteristic under chopped light illumination is shown in Fig. 5. The photoelectrochemical measurements, which were performed here only to test the quality of the CdSe films, confirms the photoactivity of CdSe films from weakly alkaline bath using selenosulphite as selenium source. By comparison, it is important to note that most of the as-deposited CdSe electrodeposits that were mentioned in the literature never showed any semiconducting behavior. Of course, the photocurrents obtained in the present study are not sufficient to be immediately useful for a practical application, but it is well known that conversion efficiency of such films can be considerably increased by thermal, chemical and (photo) electrochemical surface treatments [3,14].
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gram, funded by the Ministry of Science and Technology of Korea. One of the authors, (CDL), wishes to thank Korean Federation of Science and Technology Societies (KOFST), Korea for the award of Brain Pool Fellowship (2003–2004). He is grateful to Prof. M.G. Takawale, Vice Chancellor and Prof. V.M. Chavan, Pro. Vice Chancellor of Shivaji University, Kolhapur, India for their constant encouragement and sanction of leave.
References
Fig. 5. The current–voltage characteristics of CdSe based PEC cell under chopped light. The illumination intensity is 100 mW cm−2 .
4. Conclusions In conclusion, the room temperature electrosynthesis of cadmium selenide films on titanium substrate was carried out from weakly alkaline bath (pH < 9.0) consisting of cadmium sulphate–sodium citrate–sodium selenosulphite bath. After annealing at 773 K, CdSe films became microcrystalline and surface morphology changed to porous and cauliflowerlike. Formation of photelectrochemical cells with CdSe films showed that films are photoactive.
Acknowledgement This research was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D pro-
[1] K.W. Frese Jr., Appl. Phys. Lett. 40 (1982) 275. [2] G. Hodes, J. Manassen, S. Neagu, D. Cahen, Y. Mirovski, Thin Solid Films 90 (1982) 433. [3] C.D. Lokhande, S.H. Pawar, Phys. Status Solidi A 111 (1989) 17, and references therein. [4] M. Skyllas-Kayacos, B. Miller, J. Electrochem. Soc. 127 (1980) 869. [5] S.J. Lade, M.D. Uplane, C.D. Lokhande, J. Mater. Sci. 9 (1998) 477. [6] M. Skyllas-Kayacos, J. Electroanal. Chem. 148 (1983) 233. [7] K. Singh, S.S.D. Mishra, Sol. Energy Mater. Sol. Cells 63 (2000) 275. [8] M. Bouroushian, Z. Loizos, N. Spyrellis, G. Maurin, Thin Solid Films 229 (1993) 101. [9] M.C.A. Fantini, J.R. Moro, F. Decker, Sol. Energy Mater. 17 (1988) 247. [10] Z. Loizos, N. Spyrellis, G. maurin, Thin Solid Films 204 (1991) 139. [11] M. Cocivera, US patent 4,536,260 (1985). [12] JCPDS data card No. 08-0459. [13] K.T.L. de Silva, D.J. Miller, D. Haneman, Sol. Energy Mater. 4 (1981) 233. [14] C.D. Lokhande, S.H. Pawar, Mater. Chem. Phys. 11 (1984) 201.
Electrosynthesis of cadmium selenide films from sodium citrate–selenosulphite bath C.D. Lokhande, Eun-Ho Lee, Kwang-Deog Jung, Oh-Shim Joo∗ Eco-Nano Research Center, Korea Institute of Science and Technology, P.O. Box 131, CheongRyang, Seoul 136 650, Korea Received 7 July 2004; accepted 26 November 2004
Abstract Electrosynthesis of cadmium selenide (CdSe) film has been carried out from deposition bath containing sodium selenosulphite, along with cadmium complexed with sodium citrate under potentiostatic deposition condition on titanium substrates. The pH of deposition bath was weakly basic (< 9.0). The CdSe films up to 3.0 m were deposited. The X-ray diffraction (XRD) studies revealed that the CdSe films are microcrystalline with increased grain size after annealing. The scanning electron microscopy showed that the films are porous with cauliflower-like morphology. The photelectrochemical characterization showed that the CdSe films are photoactive. © 2004 Elsevier B.V. All rights reserved. Keywords: Electrosynthesis; Cadmium selenide; Thin films; Structural properties
1. Introduction Cadmium selenide, in the form of thin polycrystalline films, exhibits very interesting properties, such as direct transition, proper band gap width, short penetration length of light, etc. These properties make CdSe particularly suitable for various technical applications. In particular, its band gap value (1.7 eV) makes it useful for the conversion of solar energy in photovoltaic or photoelectrochemical (PEC) devices as well as catalysis for the photo-assisted decomposition of water. Using cadmium chalcogenide single-crystal electrodes, conversion of solar light into electrical energy has been achieved with efficiencies as high as 14% [1]. On the other hand, Hodes et al., obtained solar conversion efficiencies up to 8%, using CdSe0.65 Te0.35 polycrystalline layers in PEC cell [2]. Various chemical methods, e.g., electrosynthesis, chemical deposition, slurry painting etc. have been used to deposit polycrystalline CdSe thin films for the production of large area electrodes for the use in PEC cells. Amongst the various deposition methods, the electrosynthesis from ∗
Corresponding author. Tel.: +82 2 958 5215; fax: +82 2 958 5219. E-mail address: [email protected] (O.-S. Joo).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.11.046
aqueous electrolytes appears to be a very simple and low cost preparation method. This method has already been employed for the deposition of CdSe films. Thin films of CdSe have been deposited from aqueous and nonaqueous and acidic and alkaline baths [3]. Use of selenious acid as selenide ion source in acidic and alkaline has been reported [4–9]. The cadmium ions have been complexed using citric acid, ethylene-diamine-tetra-acetic acid (EDTA), ammonia, sodium cyanide, nitrilotriacetic acid (NTA), etc. In acidic medium, the pH of bath is about 2–3. The difficulty in depositing films from the solutions containing selenious acid is that a substantial excess of selenium is detected in the films. To remove an excess portion of selenium, the films must be heated at temperatures as high as 700 ◦ C. The mechanism of electrodeposition of CdSe from a solution containing Cd2+ and SeO2 has shown that the Se(IV+ ) is reduced to Se(II− ), i.e. H2 Se, which then reacts with Cd2+ present at the cathode surface to form CdSe [4]. It is thought that in the acidic medium, the selenium incorporation occurs according to the reaction
2CdSe + H2 SeO3 + 4H+ → 2Cd2+ + 3Se + 3H2 O
(1)
400
C.D. Lokhande et al. / Materials Chemistry and Physics 91 (2005) 399–403
It is important to avoid this reaction in the bath. In an alternate approach, in alkaline medium (pH > 10), cadmium ions are complexed with NTA or EDTA and selenocynide ions are incorporated in the solution, which are reduced at the cathode to produce selenide ions, which react with the cadmium ions to electrodeposit CdSe on the cathode [3,6]. In order to get device quality CdSe film, the deposition was carried out from elevated temperature (>333 K) baths. In the present paper, we report on the room temperature electrosynthesis of CdSe films from weakly alkaline (pH < 9) aqueous solution containing the selenosulphite ion and a cadmium ion complexed with sodium citrate. The CdSe film of the thickness up to 3.0 m was deposited on titanium substrate. The structural characterization of CdSe film was carried out using XRD and SEM techniques. The photovoltaic activity of CdSe film was tested by forming photoelectrochemical cell with polysulphide electrolyte.
2. Experimental Room temperature electrodeposition of CdSe thin film on titanium substrate has been carried out using following procedure. In brief, freshly prepared solutions of 0.02 M CdSO4 and 1.0 M sodium citrate were mixed into 2:1 ratio and pH of the resultant solution was made at 8.6. A solution of 0.02 M sodium selenosulphite (Na2 SeSO3 ) was added to it. The Na2 SeSO3 solution was prepared by dissolving 0.120 g of elemental selenium powder (0.05 M) in an aqueous solution containing 3.0 g of sodium sulphite (0.75 M) at pH 9.0. A platinum sheet with 1.5 cm × 1.5 cm area was used as a counter electrode. All potentials were measured with respect to Ag/AgCl as a reference electrode. A scanning galvanostat/potentiostat (E G and G model 273A) was used in potentiostatic mode. The deposition was carried out at room temperature (298 K). After deposition, film was cleaned with triple distilled water and dried under argon flow. Before inserting the titanium substrate in the deposition bath, it was polished with smooth polish paper and cleaned with triple distilled water. In order to remove the oily substances from surface, cleaned substrate was etched in 3% dil. HCl for 10 min and then ultrasonically cleaned with triple distilled water. After deposition, CdSe film was annealed at 773 K in air for 60 min. The CdSe film was kept in furnace at room temperature and then furnace temperature was raised at a heating rate of 5 K min−1 . The CdSe film was etched in 3% dil. HCl for 3 s and dried under argon flow. Thin film of CdSe was characterized using following techniques. The XRD pattern was obtained by using X ray diffractrometer (RINT/PMAX 2500, Rigaku, Japan). Micro structural study was carried out with scanning electron micrographs, obtained with FE-SEM, (SM-6340F, JEOL, Japan). Photoelectrochemical properties were studied by forming a cell with CdSe as a photoelectrode, platinum spiral wire as a counter electrode. The electrolyte used was 1 M poly-
sulphide (NaOH–Na2 S–S) aqueous solution. A Xe lamp (450 W, Thermo Oriel Co., USA), with illumination intensity of 100 mW cm−2 at 300 nm wavelength was used as a light source.
3. Results and discussion The mechanism of cathodic electrodeposition of CdSe from Cd and Se precursors in solution is still a matter of some contention. It may involve the co-deposition of Cd and Se, or the initial formation of Se2− followed by CdSe precipitation onto the electrode surface [4]. Furthermore, separate Cd and Se phases, which are in physical contact, may react during subsequent annealing. It is not unlikely that all these mechanisms are operating, but their contributions depend on the electrodeposition conditions. In case of CdSe deposition, Se being a noble metal, the surface concentration of Se ions has to be small as possible to allow the complete achievement of the low crystalline process of CdSe compound. It is therefore; necessary to reduce the concentrations of the total amount of Se ions dissolved in the bath and monitor the electrodeposition parameters in such a way that the current is totally limited by the mass transport of Se ions. The cathodic deposition potential depends upon bath temperature, nature of substrate, metal ion concentration, complexing agent and its concentration etc. [3]. The deposition potential for CdSe from the above bath was decided from polarization curve. Fig. 1(a and b) shows the polarization curves for the reduction of Cd from cadmium sulphate–sodium citrate and CdSe from cadmium sulphate–sodium citrate–sodium selenosulphite solutions, on titanium substrate. The sudden increase (curve a) in current above −800 mV versus Ag/AgCl corresponds to the reduc-
Fig. 1. Polarisation curves for the reduction of (a) cadmium from the solution containing 0.02 M cadmium sulphate–1.0 M sodium citrate and (b) cadmium selenide from the solution containing 0.02 M cadmium sulphate–1.0 M sodium citrate–0.02 M sodium selenosulphite, on titanium substrate. The scanning rate was 10 mV s−1 .
C.D. Lokhande et al. / Materials Chemistry and Physics 91 (2005) 399–403
Fig. 2. Variation of deposition current density with deposition time for deposition of CdSe film from the solution containing 0.02 M cadmium sulphate–1.0 M sodium citrate–0.02 M sodium selenosulphite (pH = 8.6) on titanium substrate. The deposition potential applied was −1000 mV versus Ag/AgCl.
tion of cadmium on titanium substrate. This is followed by hydrogen evolution above −900 mV versus Ag/AgCl. When the potential becomes more negative, current does not stay stationary because of the fast formation of fern-like cadmium dendrites. However, the nature of polarization curve (curve b) was changed by addition of sodium selenosulphite. It showed current peaks at around −900 and −970 mV versus Ag/AgCl, which correspond to the reduction of Cd and CdSe, respectively. Loizos et al. [10] have observed similar current behavior in polarization curves for CdSe deposition from acidic selenious bath. By considering the reduction potential obtained from polarization curve, the deposition of CdSe films was carried out by applying deposition potential of −1000 mV versus Ag/AgCl. The variation of current density with deposition time for deposition of CdSe on titanium substrate is shown in Fig. 2. The deposition current is found to be substrate dependent. When substrate is polycrystalline, its orientations are varied and material gets deposited so as to attain the closest symmetry with the surface morphology of the substrate. In addition, the affinity of substrate material is different for different depositing materials. In present case, the deposition current was about 2 mA cm−2 and was found to vary continuously with deposition time. This may be due to irregular expansion of the effective area of depositing CdSe film on titanium substrate [10]. It is well known that sodium selenosulphite solution hydrolyses in an alkaline medium to give selenide ions as Na2 SeSO3 + OH− → Na2 SO4 + HSe−
(2)
HSe− + OH− → H2 O + Se2−
(3)
The concentration of selenide ions at cathode is sufficiently high to become associated with cadmium ions as dissociated from the complexed pool of cadmium ions to form
401
Fig. 3. The XRD pattern of annealed CdSe film on titanium substrate.
cadmium selenide, which deposits on the cathode. The cadmium, complexed with sodium citrate as Cd(Na3 C6 H5 O7 )2+ which has stability constant of 109.8 , reduces the concentration of free cadmium ions. The pH of deposition bath is weakly basic (< 9), in which chemical equilibrium of complexing cadmium ions shifts towards the associated complexed cadmium ions. This minimizes the competitive reactions of cadmium ions with hydroxide ions in the bath and any selenide ions, which may be free in the bath. The weak alkaline pH of bath makes the hydroxyl ion concentration in such a way that the equilibrium involving the seleonsulphite and hydroxide favors the selenosulphite ion as compared to the production of selenide when the cadmium ions exist in the solution complexed by sodium citrate [11]. By means of this complex, the solution remains stable for more than 24 h. The CdSe film thickness in the range of 0.5–3.0 m was obtained, depending upon deposition time period. After about thickness of 3.0 m, CdSe film started detaching from the titanium substrate. The crystal quality of CdSe films can be improved by appropriate annealing treatments. It has been reported that annealing below 573 K does not affect the crystal structure. Above 673 K, most of the CdSe film adopts the normal hexagonal wurtzite structure [10]. Therefore, the CdSe film deposited on titanium was annealed at 773 K, for 60 min in air. Fig. 3 shows the XRD pattern of CdSe film deposited titanium coated substrate. From this, it is found that CdSe films are microcrystalline with planes (1 0 0), (2 0 0), (1 0 2), (1 1 0), and (1 1 2) corresponding to hexagonal phase [12]. For electrodeposited CdSe films, improvement in grain size, liberation of Se from surface region and chemical combining of Cd and Se has been reported after annealing [13]. Scanning electron microscopy is a convenient and versatile method to study microstructure of the film and to determine the grain size. Fig. 4(a and b) shows the micrographs of as deposited and annealed CdSe films on titanium substrate at two different magnifications (5000× and 20,000×).
402
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Fig. 4. The scanning electron micrographs of cadmium selenide films at two different magnifications: (a) as deposited, and (b) annealed at 773 K in air. (c) Cross-sectional view of titanium/CdSe.
The as deposited films are rough and show agglomeration of small grains. However, the morphology was changed after annealing the films. After annealing, the films showed porous structure with irregular grain growth of about one micron size. These grains were grouped on the surface by combination with each other to form cauliflower-like structure. This is clearly seen at (Fig. 4b) high magnification. Hodes et al. [2] have reported similar type of morphology for CdSe and Cd(Se, Te) films deposited from acidic and alkaline media on titanium substrate. They found no correlation between the surface morphology and the efficiency of CdSe PEC cells. Moreover, the large photocurrents obtained were ascribed to the ability of the electrolyte to make good physical contact with such an irregular surface. Fig. 4(c) shows the cross sectional view of titanium substrate/CdSe film. From this, the growth of grains and their merging into one another to form bigger grains is also seen. From this, CdSe film thickness was estimated to be about 2–3 m. In recent years, photoelectrochemical cells based on semiconductor–electrolyte junctions have been attracting a great deal of interest in solar and non-solar applications, as they have many advantages over conventional solid-state junctions. These PEC cells are easy to make and many processing steps of p–n junction are simplified or eliminated.
Since the junction with liquid is formed spontaneously upon contact, irregular shaped single crystal or thin films can be used. Therefore, PEC cells with semiconductor under investigation can be used as a simple and quick technique to test the quality of solar cell materials [14]. In the present case, photelectrochemical cell was formed with annealed CdSe film and illuminating the cell with light, its photovoltaic activity was tested. The short circuit current (Isc ) of the order of 1–2 mA cm−2 and open circuit voltage (Voc ) of the order of 450–500 mV were obtained under the illumination of 100 mW cm−2 . The typical current–voltage characteristic under chopped light illumination is shown in Fig. 5. The photoelectrochemical measurements, which were performed here only to test the quality of the CdSe films, confirms the photoactivity of CdSe films from weakly alkaline bath using selenosulphite as selenium source. By comparison, it is important to note that most of the as-deposited CdSe electrodeposits that were mentioned in the literature never showed any semiconducting behavior. Of course, the photocurrents obtained in the present study are not sufficient to be immediately useful for a practical application, but it is well known that conversion efficiency of such films can be considerably increased by thermal, chemical and (photo) electrochemical surface treatments [3,14].
C.D. Lokhande et al. / Materials Chemistry and Physics 91 (2005) 399–403
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gram, funded by the Ministry of Science and Technology of Korea. One of the authors, (CDL), wishes to thank Korean Federation of Science and Technology Societies (KOFST), Korea for the award of Brain Pool Fellowship (2003–2004). He is grateful to Prof. M.G. Takawale, Vice Chancellor and Prof. V.M. Chavan, Pro. Vice Chancellor of Shivaji University, Kolhapur, India for their constant encouragement and sanction of leave.
References
Fig. 5. The current–voltage characteristics of CdSe based PEC cell under chopped light. The illumination intensity is 100 mW cm−2 .
4. Conclusions In conclusion, the room temperature electrosynthesis of cadmium selenide films on titanium substrate was carried out from weakly alkaline bath (pH < 9.0) consisting of cadmium sulphate–sodium citrate–sodium selenosulphite bath. After annealing at 773 K, CdSe films became microcrystalline and surface morphology changed to porous and cauliflowerlike. Formation of photelectrochemical cells with CdSe films showed that films are photoactive.
Acknowledgement This research was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D pro-
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