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Photoelectrochemical studies on galvanostatically formed multiple band gap materials based on CdSe and ZnSe
Solar Energy Materials & Solar Cells 71 (2002) 115–129
Photoelectrochemical studies on galvanostatically formed multiple band gap materials based on CdSe and ZnSe Kehar Singh*, Sameer S.D. Mishra Chemistry Department, D.D.U. Gorakhpur University, Gorakhpur, U.P. 273 009, India Received 27 November 2000; accepted 22 February 2001
Abstract The preparation of some multiple band gap semiconductor films based on CdSe and ZnSe has been carried out using galvanostatic electrochemical codeposition technique to investigate their photoelectrochemical characteristics on the basis of photoelectroconvertibility and 3 4 photoaction spectral studies using I2/I redox couples. These 3 and [Fe (CN) 6] /[Fe (CN) 6] composite systems show substantially improved photoelectrochemical properties compared to the constituent CdSe and ZnSe films prepared under comparable experimental conditions. These multiple band gap films were also found to exhibit enhanced resistance towards electrochemical corrosion. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Galvanostatic electrochemical codeposition; Multiple band gap semiconductor; Photoelectroconvertibility; Photoaction spectrum; Bilayered semiconductor films; Mixed semiconductor films; Corrosion rates; Tafel plots
1. Introduction Variously formed semiconductor electrodes have been extensively studied for the development of photoelectrochemical cells for the sustained and efficient capture and conversion of solar energy [1–6]. Cadmium selenide, in particular, is considered to be a potential candidate for photoelectrochemical conversion because of the compatibility of its band gap (1.7 eV) with the solar spectrum [7–9]. Photoelectrodes based on this material have, however, been found to be susceptible to electrochemical corrosion. Zinc selenide, on the other hand, exhibits acceptable resistance towards corrosion but its photoexcitability is confined only to high frequencies because of its *Corresponding author. 0927-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 1 ) 0 0 0 4 8 - 4
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relatively wide band gap (2.7 eV) [10–12]. Coupling of cadmium selenide and zinc selenide is expected to provide a multiple band gap semiconductor which, in principle, may be suitable for accomplishing the twin tasks of increased absorption of solar spectrum and enhanced resistance towards electrochemical corrosion [13–17]. In the present studies we propse to explore the possibilities of preparation of some multiple band gap semiconductor film electrodes based on CdSe and ZnSe using the galvanostatic electrochemical co-deposition technique. The following two configurations have been attempted for the preparation of the composite systems: 1. A CdSe layer was deposited using titanium foil. This was followed by the electrodeposition of ZnSe using different, constant current–densities for various time intervals. 2. Titanium supported mixed metal selenide (CdZn)Se films were prepared using an electroplating solution containing CdSO4, ZnSO4 and SeO2 using different constant current–densities. These preparations were tested for their photoresponsiveness and characterized on the basis of photoaction spectral studies. Current voltage measurements were also carried out to construct Tafel plots to determine their resistance towards electrochemical corrosion.
2. Experimental The electrochemical deposition of bilayered (Ti-CdSe/ZnSe) films and mixed selenide Ti-(CdZn)Se films was carried out galvanostatically using the usual three electrodes system [18]. In all cases, the passage of constant current was ensured with the help of a constant current power supply (Lake Shore, Model 120, USA). Titanium foils (M/s Titanium Equipment and Anode Manufacturing Co. Ltd., Chennai) were used as working and counter-electrodes. The titanium electrodes were cleaned with diamond paste using Metses Fluid (Madras Metallurgical Services Pvt. Ltd., Chennai) and washed successively with acetone and distilled water. CdSO4 and ZnSO4 (both CDH, India) and selenium dioxide (Fluka Chemika, Switzerland) were used as such without further purification. The preparations were tested using I2/I 3 and [Fe (CN)6]3/[Fe (CN) 6]4 redox couples. For photoelectroactivity studies a 1 kW Tungsten lamp, housed in an illumination chamber fitted with an exhaust fan to obviate heating of the system, was used. Photoaction spectral studies were carried out using f/3.4. Monochromator (Applied Photophysics, London). Corrosion characteristics of these electrodeposited multiple band gap materials were investigated on the basis of current–voltage studies using Tafel method.
3. Results and discussion The electrodeposition of CdSe using known constant current densities was carried out by electrolysing solutions containing 0.01 M CdSO4 and 0.001 M SeO2 for
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different time intervals. The deposits were washed with distilled water and immersed in a solution containing 0.01 M ZnSO4 and 0.001 M SeO2. The electrodeposition of ZnSe was then carried out using different constant current–densities for various time intervals. Photoactivity data obtained using these successively galvanostatically electrodeposited bilayered selenide films is shown in Fig. 1. The variation of photopotential with deposition current–density at which ZnSe films were deposited for 15 min in the preparation of Ti-CdSe/ZnSe is shown in this figure. It has earlier been shown [19,20] that the nature of the photoelectroactivity of CdSe depends on electrodeposition current–density. Current–densities upto 0.3 mA cm2 invariably yield cadmium selenide films endowed with p-type semiconductivity in I2/I 3 redox system. On the other hand, zinc selenide films deposited under galvanostatic control are always endowed with p-type semiconductivity in the I2/I 3 redox couple. Experimental conditions for successive electrodepositions were chosen so as to obtain films which could yield enhanced photoresponsiveness since the constituents have a similar semiconducting nature. The results given in Fig. 1 clearly show that the presence of zinc selenide in conjunction with electrodeposited CdSe films results in considerably enchanced photoactivity, especially when the electrodeposition of ZnSe under galvanostatic control is carried out using current–densities in the range of 0.3–1.0 mA cm2. This obviously indicates absorption and photoexcitation by electromagnetic radiations from within a broader range of the electromagnetic spectrum. To investigate the contribution of each layer to the photopotential, we studied the photoaction spectral behaviour of Ti-CdSe/ZnSe electrodes. The results given in Fig. 2 support the contention that the photoactivity of the composite system is a consequence of the photoexcitation of both CdSe and ZnSe layers. Photoaction
Fig. 1. Photoactivity of some bilayered CdSe/ZnSe electrodes. Deposition current densities for CdSe deposits: (FJF) 0.1 mA cm2; (FKF) 0.2 mA cm2; (FWF) 0.3 mA cm2.
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Fig. 2. Photoaction spectral data (FJF) CdSe/ZnSe; (FKF) CdSe; (FWF) ZnSe.
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spectral data for single layers of Ti-CdSe and Ti-ZnSe are also included in Fig. 2 for the sake of comparison. Threshold wavelength, for Ti-CdSe and Ti-ZnSe, 730 and 550 nm, respectively, correspond to band gap values of 1.7 and 2.4 eV [21,22]. In this composite system, the photoexcitation of ZnSe dominates till l=550 nm beyond which the photoexcitation of both ZnSe and CdSe seems to be involved. The identification of threshold wavelength in this case is not possible so that no definite band gap value can be assigned to this spectral region. Furthermore, photoaction spectral behaviour at wavelengths beyond 650 nm is somewhat intriguing and defies explanation. Attempts were also made to prepare mixed (CdZn)Se films. For this purpose, a mixture of electroplating solutions containing 0.01 M CdSO4, 0.002 M SeO2 and a variable concentration of ZnSO4 was electrolysed using constant current–densities. When the electrodeposition of titanium supported mixed (CdZn)Se films was carried out, the deposition potential initially varied somewhat rapidly with time, before attaining a time invariant magnitude as shown in Fig. 3. This alteration obviously arises on account of the cummulative effect of polarization and alteration in the electrochemical character of the substrate as a result of electrodeposition. The deposition potential of Ti-(CdZn)Se, however, declines less rapidly in comparison to those for Ti-CdSe and Ti-ZnSe electrodepositions under comparable experimental conditions. This clearly indicates reduced polarization during the electrochemical formation of mixed selenide films compared to that for CdSe and ZnSe separately. These results also show that Ti-(CdZn)Se electrodeposits are compositionally and structurally more homogeneous. Film thickness values for Ti-(CdZn)Se electrodeposits under different experimental conditions given in Table 1 were estimated using the relationship [23] FT ¼
itðEWÞ ; FdA
ð1Þ
where, i is the constant deposition current, t the deposition time, EW the equivalent weight of the electrodeposit, F the Faraday constant, A the area of the electrodeposited film, and d the density of the electrodeposit. Relevant photoactivity data are given in Table 2. The photoactivity of mixed 3 4 deposits was tested using I2/I redox couples. The 3 and [Fe(CN)6] /[Fe(CN)6] mixed deposits have both p- and n-type semiconductivity. At lower current–densities the electrodeposits are endowed with p-type semiconductivity while at higher current–densities they exhibit n-type semiconductivity. [Fe(CN)6]3/[Fe(CN)6]4 redox couple yields considerably enhanced photoactivity. With a view to exploring the possibilities of further improvement in the photoresponsiveness of this mixed system, electrodepositions were carried out using electroplating solutions containing 0.05 M CdSO4 0.001 M SeO2 and different concentrations of ZnSO4. The results included in Table 3 show a substantial improvement in photoresponsiveness in almost all cases. Photoaction spectral data included in Fig. 4 indicate an improvement in the photoexcitibility of the electrodeposits with an increase in the concentration of ZnSO4 in the electroplating solution. These data also indicate that the essential character of the electrodeposits
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Fig. 3. Variation of deposition potential with time. Deposition current–density=0.4 mA cm2: (FJF) CdSe; (FKF) ZnSe; (FWF) (CdZn)Se.
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Table 1 Summary of experimental conditions for mixed system. Electroplating solutions: 0.01 M CdSO4+0.002 M SeO2+various concentrations of ZnSO4. Electrode Area: 1.0 cm2 Deposition current–density (mA cm2)
Film thickness (FT) 104 (cm)
Deposition time (min)
0.1 0.2 0.3 0.4 0.8 1.0
15 15 15 15 15 15
ZnSO4 0.01 M
ZnSO4 0.05 M
ZnSO4 0.1 M
0.92 1.82 2.78 3.68 7.35 9.19
0.78 1.57 2.35 3.14 6.28 7.84
0.75 1.50 2.26 3.00 6.02 7.52
Table 2 Photoactivity data of the mixed (CdZn)Se systems. Electroplating solutions: 0.01 M CdSO4+0.002 M SeO2+Various concentrations of ZnSO4 Deposition Deposition Photopotentials (mV) current–density time (min) 4 3 I2/I redox-system 3 redox-system [Fe(CN)6] /Fe(CN)6] (mA cm2)
0.1 0.1 0.2 0.2 0.3 0.4 0.8 1.0 1.2
10 15 10 15 15 15 15 15 15
ZnSO4 0.01 M
ZnSO4 0.01 M
ZnSO4 0.05 M
ZnSO4 0.1 M
+190 +235 +175 +190 38 65 105 95 110
+180 +190 264 294 360 385 345 328 345
128 372 370 472 540 384 362 348 342
272 364 378 394 480 398 388 368 372
Table 3 Photoactivity data of the mixed (CdZn)Se systems. Electroplating solution: 0.05 M CdSO4+0.001 M SeO2 Various concentrations of ZnSO4. Testing solution: 0.1 M Na2SO4+0.01 M K4Fe(CN)6+0.01 M K3Fe(CN)6 Deposition current–density (mA cm2) 0.1 0.2 0.3 0.4 0.8 1.0
Deposition time (min)
10 10 10 10 10 10
Photopotentials (mV)
ZnSO4 0 M
ZnSO4 0.01 M
ZnSO4 0.03 M
ZnSO4 0.05 M
385 540 535 410 440 460
435 565 560 462 445 425
528 570 585 478 485 470
625 610 590 535 535 482
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Fig. 4. Photoaction spectral data of some (CdZn)Se films. Electroplating solution: 0.05 M CdSO4+ 0.001M SeO2 +different concentrations of ZnSO4: (FJF) 0 M ZnSO4; (FKF) 0.01 M ZnSO4; (FWF) 0.03 M ZnSO4; (FEF) 0.05 M ZnSO4. Deposition current–density=0.1 mA cm2, deposition time=10 min.
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Fig. 5. Ep2 vs. l plots for the estimation of band gap of different (CdZn) Se films: (FJF) 0 M ZnSO4; (FKF). 0.01 M ZnSO4; (FWF) 0.03 M ZnSO4; (FEF) 0.05 M ZnSO4.
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Fig. 6. Variation of band gap of (CdZn)Se with composition of the electroplating solution: (FJF) theoritical; (FKF) experimental.
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Fig. 7. Typical current–voltage curves (FJF) 0.01 M CdSO4+0.001 M SeO2; (FKF) 0.01 M ZnSO4+0.001 M SeO2.
Fig. 8. Some representative Tafel plots for the estimation of corrosion rates of CdSe/ZnSe films. Current– density for CdSe deposition=0.1 mA cm2. Current–density for ZnSe deposition: (FJF) 0.1 mA cm 2; (FKF) 0.2 mA cm2; (FWF) 0.3 mA m2. Deposition time=15 min.
remains unaltered in spite of the variation in the concentration of ZnSO4 in the electroplating solution. For the estimation of band gap, E2p vs. l plots were constructed (Fig. 5) using photoaction spectral data. The variation of band gap with the concentration of ZnSO4 in the electroplating solution depicted in Fig. 6 indicates the formation of solid solution of CdSe and ZnSe. Band gap values theoritically calculated on the basis of the assumption that the solid solution composition equals
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Experimental conditions for the deposition of CdSe films in the preparation of CdSe/ZnSe electrodes
Experimental conditions for the deposition of ZnSe films in the preparation of CdSe/ZnSe electrodes
Deposition current–density (mA cm2)
Deposition time (min)
Deposition current–density (mA cm2)
Deposition time (min)
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
15 15 10 15 15 15 15 15 15 15 15 15
0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.8 1.0
10 15 15 10 15 30 5 10 15 15 15 15
Corrosion current Ic 106 (A)
Corrosion rate Rc 1010 (g/s)
2.180 1.990 1.819 0.478 0.436 0.398 0.251 0.229 0.199 0.187 0.248 0.229
7.52 6.90 6.28 1.65 1.52 1.37 0.866 0.791 0.687 0.646 0.856 0.791
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Table 4 Corrosion characteristics of CdSe/ZnSe system. Testing solution: 0.5 M Cd(CH3COO)2+0.1 M KI+0.2 mM I2 solution
Deposited current–density mA cm2
Deposition time (min)
0.1 0.2 0.3 0.4 0.8 1.0
15 15 15 15 15 15
Corrosion current 106 (A)
Corrosion rate (Rc) 1011 (g/s)
ZnSO4 0.01 M
ZnSO4 0.05 M
ZnSO4 0.1 M
ZnSO4 0.01 M
ZnSO4 0.05 M
ZnSO4 0.1 M
0.0794 0.0724 0.0660 0.06918 0.06025 0.06309
0.0549 0.0575 0.05495 0.06165 0.05128 0.0542
0.05248 0.05888 0.05623 0.0588 0.05248 0.0549
3.59 3.27 2.98 3.13 2.72 2.85
2.07 2.17 2.07 2.32 1.93 2.04
1.89 2.12 2.02 2.11 1.89 1.97
Table 6 Corrosion characteristics of galvanostatically formed mixed (CdZn)Se systems. Electroplating solution composition: 0.05 M CdSO4+0.001 M SeO2+Various concentrations of ZnSO4. Testing solution: 0.1 M Na2SO4+0.01 M K4Fe(CN)6+0.01 M K3Fe(CN)6 Deposited current–density Deposition Corrosion current 106(A) Corrosion rate (Rc) 1011 (g/s) 2 (mA cm ) time (min) ZnSO4 0 M ZnSO4 0.01 M ZnSO4 0.03 M ZnSO4 0.05 M ZnSO4 0 M ZnSO4 0.01 M ZnSO4 0.03 M ZnSO4 0.05 M 0.1 0.2 0.3 0.4 0.8 1.0
10 10 10 10 10 10
2.82 2.40 2.00 1.66 1.51 1.48
0.580 0.631 0.525 0.447 0.501 0.417
0.210 0.202 0.224 0.188 0.174 0.158
0.060 0.050 0.049 0.038 0.0372 0.033
163 138 115 96.08 87.63 85.64
31.07 34.08 28.34 24.12 27.06 22.51
10.23 9.89 10.96 9.225 8.51 7.76
2.76 2.30 2.25 1.75 1.71 1.49
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Table 5 Corrosion characteristics of galvanostatically formed mixed (CdZn)Se systems. Electroplating solution composition: 0.01 M CdSO4+0.002 M SeO2+Various concentrations of ZnSO4. Testing solution: 0.01 M Na2SO4+0.01 M K4Fe(CN)6+0.01 M K3Fe(CN)6
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that of the electroplating solutions, are also included in the figure for the sake of comparison. These results clearly show that the electrodeposits are loaded in favour of CdSe because of the easier dischargeability of Cd2+ ions (deposition potential=0.403 V) in comparision to that of Zn2+ ions (deposition potential=0.763 V). Current–voltage data presented in Fig. 7 also support the above inference. The mixed systems, however, do exhibit substantially improved photoresponsiveness. Obviously, this is possible because of the improved absorbability of the electromagnetic spectrum, due to the presence of ZnSe along with CdSe in the mixed system. The resistance of these two photoelectroactive systems, i.e. bilayered films based on CdSe and ZnSe and mixed (CdZn)Se films, towards electrochemical corrosion was investigated on the basis of current–voltage studies using Tafel plots. Representative data are shown in Fig. 8. The Tafel plots were used for the estimation of corrosion current using the relationship [24] i Z ¼ b log ; ð2Þ icorr where Z is the overvoltage potential, i, the current at the applied voltage, icorr the corrosion current and b is the symmetry factor. The corrosion rates, Rcorr were estimated using the relationship [25] : Rcorr ¼ icorr EW F
ð3Þ
The corrosion rates estimated using these equations are summarized in Tables 4–6. Bilayered films exhibit reduced corrosion rates in comparison to those for ZnSe alone. Mixed selenide films, however, show substantially improved resistance towards electrochemical corrosion. The studies presented herein show that multiple band gap electrodeposited films, obtained under galvanostatic control, exhibit improved photoelectroconvertibility and are endowed with enhanced resistance towards electrochemical corrosion.
Acknowledgements The authors thank the Head of the Chemistry Department for permitting the use of laboratory facilities. One of us (S.S.D. Mishra) thanks Prof. N.B. Singh for the benefit of association and help. Financial help from CSIR, New Delhi is also gratefully acknowledged.
References [1] Shinji Yae, Isao Nakanishi, Y. Nakato, Naoki Toshima, Hirotoro Mori, J. Electrochem. Soc. 141 (1994) 3077. [2] T. Torimoto, R.J. Fox, M.A. Fox, J. Electrochem. Soc. 143 (1996) 3712. [3] N.R. de Tacconi, H. Wenren, K. Rajeshwar, J. Electrochem. Soc. 144 (1997) 3159. [4] G.J. Meyer, P.C. Searsen, The Electrochem. Soc. (INTERFACE) 2 (1993) 23–27.
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N.S. Lewis, Electrochem. Soc. (INTERFACE) 5 (1996) 28. P. Carlsson, K. Uosaki, B. Holmstrom, Hideakikita, J. Electrochem. Soc. 136 (1989) 524. D. Ham, K.K. Mishra, K. Rajeshwar, J. Electrochem, Soc. 138 (1991) 100. Ph. Allongue, R. Tenne, J. Electrochem. Soc. 138 (1991) 261. A.J. Bard, A.B. Bocarsly, F.R.I. Tam, E.G. Waallon, M.S. Wrightor, J. Am. Chem. Soc. 102 (1980) 373. J.M. Dona, J. Herrero, J. Electrochem. Soc. 142 (1995) 764. G.H. Schoenmakers, E.P.A.M. Bakkers, J.J. Kelly, J. Electrochem. Soc. 144 (1997) 2329. K. Singh, R.K. Pathak, Electrochem. Acta. 39 (1994) 2693. S. Licht, et al., Electrochem. Solid State Lett. 1 (1998) 20. S. Licht, Electrochem. Soc. (Interface) 6 (1993) 37. Masutoshi Fujji, Tomasi Kawai, Shichio Kawai, J. Electrochem. Soc. 136 (1989) 649. H.C. Chou, A. Rohatgi, E.W. Thomas, S. Kamra, A.K. Bhat, J. Electrochem. Soc. 142 (1995) 254. Tomoyuki Yoshida, J. Electrochem. Soc. 142 (1995) 3232. K. Singh, J.P. Rai, Mater. Sci. Lett. 4 (1985) 1401. K. Singh, S.S.D. Mishra, Ind. J. Chem. Tech. 5 (1998) 95. K. Singh, S.S.D. Mishra, Sol. Eng. Mat. Sol. Cells. 63 (2000) 275. K. Singh, Md. Rashid Tanveer, Sol. Energy Mat. Sol. cells 36 (1995) 415. K. Singh, S.S.D. Mishra, Ind. J. Eng. Mat. 6 (1999) 48. K. Singh, R.K. Pathak, Ind. J. Chem. 30A (1991) 674. J. O’M. Bockris, A.K.N. Reddy, Modern Electrochemistry, Vol. 2, Plenum Press, New York, 1970, p. 893. EG & G. Princeton Applied Research Corporation, Application Note Corr., 1980.
Photoelectrochemical studies on galvanostatically formed multiple band gap materials based on CdSe and ZnSe Kehar Singh*, Sameer S.D. Mishra Chemistry Department, D.D.U. Gorakhpur University, Gorakhpur, U.P. 273 009, India Received 27 November 2000; accepted 22 February 2001
Abstract The preparation of some multiple band gap semiconductor films based on CdSe and ZnSe has been carried out using galvanostatic electrochemical codeposition technique to investigate their photoelectrochemical characteristics on the basis of photoelectroconvertibility and 3 4 photoaction spectral studies using I2/I redox couples. These 3 and [Fe (CN) 6] /[Fe (CN) 6] composite systems show substantially improved photoelectrochemical properties compared to the constituent CdSe and ZnSe films prepared under comparable experimental conditions. These multiple band gap films were also found to exhibit enhanced resistance towards electrochemical corrosion. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Galvanostatic electrochemical codeposition; Multiple band gap semiconductor; Photoelectroconvertibility; Photoaction spectrum; Bilayered semiconductor films; Mixed semiconductor films; Corrosion rates; Tafel plots
1. Introduction Variously formed semiconductor electrodes have been extensively studied for the development of photoelectrochemical cells for the sustained and efficient capture and conversion of solar energy [1–6]. Cadmium selenide, in particular, is considered to be a potential candidate for photoelectrochemical conversion because of the compatibility of its band gap (1.7 eV) with the solar spectrum [7–9]. Photoelectrodes based on this material have, however, been found to be susceptible to electrochemical corrosion. Zinc selenide, on the other hand, exhibits acceptable resistance towards corrosion but its photoexcitability is confined only to high frequencies because of its *Corresponding author. 0927-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 1 ) 0 0 0 4 8 - 4
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relatively wide band gap (2.7 eV) [10–12]. Coupling of cadmium selenide and zinc selenide is expected to provide a multiple band gap semiconductor which, in principle, may be suitable for accomplishing the twin tasks of increased absorption of solar spectrum and enhanced resistance towards electrochemical corrosion [13–17]. In the present studies we propse to explore the possibilities of preparation of some multiple band gap semiconductor film electrodes based on CdSe and ZnSe using the galvanostatic electrochemical co-deposition technique. The following two configurations have been attempted for the preparation of the composite systems: 1. A CdSe layer was deposited using titanium foil. This was followed by the electrodeposition of ZnSe using different, constant current–densities for various time intervals. 2. Titanium supported mixed metal selenide (CdZn)Se films were prepared using an electroplating solution containing CdSO4, ZnSO4 and SeO2 using different constant current–densities. These preparations were tested for their photoresponsiveness and characterized on the basis of photoaction spectral studies. Current voltage measurements were also carried out to construct Tafel plots to determine their resistance towards electrochemical corrosion.
2. Experimental The electrochemical deposition of bilayered (Ti-CdSe/ZnSe) films and mixed selenide Ti-(CdZn)Se films was carried out galvanostatically using the usual three electrodes system [18]. In all cases, the passage of constant current was ensured with the help of a constant current power supply (Lake Shore, Model 120, USA). Titanium foils (M/s Titanium Equipment and Anode Manufacturing Co. Ltd., Chennai) were used as working and counter-electrodes. The titanium electrodes were cleaned with diamond paste using Metses Fluid (Madras Metallurgical Services Pvt. Ltd., Chennai) and washed successively with acetone and distilled water. CdSO4 and ZnSO4 (both CDH, India) and selenium dioxide (Fluka Chemika, Switzerland) were used as such without further purification. The preparations were tested using I2/I 3 and [Fe (CN)6]3/[Fe (CN) 6]4 redox couples. For photoelectroactivity studies a 1 kW Tungsten lamp, housed in an illumination chamber fitted with an exhaust fan to obviate heating of the system, was used. Photoaction spectral studies were carried out using f/3.4. Monochromator (Applied Photophysics, London). Corrosion characteristics of these electrodeposited multiple band gap materials were investigated on the basis of current–voltage studies using Tafel method.
3. Results and discussion The electrodeposition of CdSe using known constant current densities was carried out by electrolysing solutions containing 0.01 M CdSO4 and 0.001 M SeO2 for
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different time intervals. The deposits were washed with distilled water and immersed in a solution containing 0.01 M ZnSO4 and 0.001 M SeO2. The electrodeposition of ZnSe was then carried out using different constant current–densities for various time intervals. Photoactivity data obtained using these successively galvanostatically electrodeposited bilayered selenide films is shown in Fig. 1. The variation of photopotential with deposition current–density at which ZnSe films were deposited for 15 min in the preparation of Ti-CdSe/ZnSe is shown in this figure. It has earlier been shown [19,20] that the nature of the photoelectroactivity of CdSe depends on electrodeposition current–density. Current–densities upto 0.3 mA cm2 invariably yield cadmium selenide films endowed with p-type semiconductivity in I2/I 3 redox system. On the other hand, zinc selenide films deposited under galvanostatic control are always endowed with p-type semiconductivity in the I2/I 3 redox couple. Experimental conditions for successive electrodepositions were chosen so as to obtain films which could yield enhanced photoresponsiveness since the constituents have a similar semiconducting nature. The results given in Fig. 1 clearly show that the presence of zinc selenide in conjunction with electrodeposited CdSe films results in considerably enchanced photoactivity, especially when the electrodeposition of ZnSe under galvanostatic control is carried out using current–densities in the range of 0.3–1.0 mA cm2. This obviously indicates absorption and photoexcitation by electromagnetic radiations from within a broader range of the electromagnetic spectrum. To investigate the contribution of each layer to the photopotential, we studied the photoaction spectral behaviour of Ti-CdSe/ZnSe electrodes. The results given in Fig. 2 support the contention that the photoactivity of the composite system is a consequence of the photoexcitation of both CdSe and ZnSe layers. Photoaction
Fig. 1. Photoactivity of some bilayered CdSe/ZnSe electrodes. Deposition current densities for CdSe deposits: (FJF) 0.1 mA cm2; (FKF) 0.2 mA cm2; (FWF) 0.3 mA cm2.
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Fig. 2. Photoaction spectral data (FJF) CdSe/ZnSe; (FKF) CdSe; (FWF) ZnSe.
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spectral data for single layers of Ti-CdSe and Ti-ZnSe are also included in Fig. 2 for the sake of comparison. Threshold wavelength, for Ti-CdSe and Ti-ZnSe, 730 and 550 nm, respectively, correspond to band gap values of 1.7 and 2.4 eV [21,22]. In this composite system, the photoexcitation of ZnSe dominates till l=550 nm beyond which the photoexcitation of both ZnSe and CdSe seems to be involved. The identification of threshold wavelength in this case is not possible so that no definite band gap value can be assigned to this spectral region. Furthermore, photoaction spectral behaviour at wavelengths beyond 650 nm is somewhat intriguing and defies explanation. Attempts were also made to prepare mixed (CdZn)Se films. For this purpose, a mixture of electroplating solutions containing 0.01 M CdSO4, 0.002 M SeO2 and a variable concentration of ZnSO4 was electrolysed using constant current–densities. When the electrodeposition of titanium supported mixed (CdZn)Se films was carried out, the deposition potential initially varied somewhat rapidly with time, before attaining a time invariant magnitude as shown in Fig. 3. This alteration obviously arises on account of the cummulative effect of polarization and alteration in the electrochemical character of the substrate as a result of electrodeposition. The deposition potential of Ti-(CdZn)Se, however, declines less rapidly in comparison to those for Ti-CdSe and Ti-ZnSe electrodepositions under comparable experimental conditions. This clearly indicates reduced polarization during the electrochemical formation of mixed selenide films compared to that for CdSe and ZnSe separately. These results also show that Ti-(CdZn)Se electrodeposits are compositionally and structurally more homogeneous. Film thickness values for Ti-(CdZn)Se electrodeposits under different experimental conditions given in Table 1 were estimated using the relationship [23] FT ¼
itðEWÞ ; FdA
ð1Þ
where, i is the constant deposition current, t the deposition time, EW the equivalent weight of the electrodeposit, F the Faraday constant, A the area of the electrodeposited film, and d the density of the electrodeposit. Relevant photoactivity data are given in Table 2. The photoactivity of mixed 3 4 deposits was tested using I2/I redox couples. The 3 and [Fe(CN)6] /[Fe(CN)6] mixed deposits have both p- and n-type semiconductivity. At lower current–densities the electrodeposits are endowed with p-type semiconductivity while at higher current–densities they exhibit n-type semiconductivity. [Fe(CN)6]3/[Fe(CN)6]4 redox couple yields considerably enhanced photoactivity. With a view to exploring the possibilities of further improvement in the photoresponsiveness of this mixed system, electrodepositions were carried out using electroplating solutions containing 0.05 M CdSO4 0.001 M SeO2 and different concentrations of ZnSO4. The results included in Table 3 show a substantial improvement in photoresponsiveness in almost all cases. Photoaction spectral data included in Fig. 4 indicate an improvement in the photoexcitibility of the electrodeposits with an increase in the concentration of ZnSO4 in the electroplating solution. These data also indicate that the essential character of the electrodeposits
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Fig. 3. Variation of deposition potential with time. Deposition current–density=0.4 mA cm2: (FJF) CdSe; (FKF) ZnSe; (FWF) (CdZn)Se.
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Table 1 Summary of experimental conditions for mixed system. Electroplating solutions: 0.01 M CdSO4+0.002 M SeO2+various concentrations of ZnSO4. Electrode Area: 1.0 cm2 Deposition current–density (mA cm2)
Film thickness (FT) 104 (cm)
Deposition time (min)
0.1 0.2 0.3 0.4 0.8 1.0
15 15 15 15 15 15
ZnSO4 0.01 M
ZnSO4 0.05 M
ZnSO4 0.1 M
0.92 1.82 2.78 3.68 7.35 9.19
0.78 1.57 2.35 3.14 6.28 7.84
0.75 1.50 2.26 3.00 6.02 7.52
Table 2 Photoactivity data of the mixed (CdZn)Se systems. Electroplating solutions: 0.01 M CdSO4+0.002 M SeO2+Various concentrations of ZnSO4 Deposition Deposition Photopotentials (mV) current–density time (min) 4 3 I2/I redox-system 3 redox-system [Fe(CN)6] /Fe(CN)6] (mA cm2)
0.1 0.1 0.2 0.2 0.3 0.4 0.8 1.0 1.2
10 15 10 15 15 15 15 15 15
ZnSO4 0.01 M
ZnSO4 0.01 M
ZnSO4 0.05 M
ZnSO4 0.1 M
+190 +235 +175 +190 38 65 105 95 110
+180 +190 264 294 360 385 345 328 345
128 372 370 472 540 384 362 348 342
272 364 378 394 480 398 388 368 372
Table 3 Photoactivity data of the mixed (CdZn)Se systems. Electroplating solution: 0.05 M CdSO4+0.001 M SeO2 Various concentrations of ZnSO4. Testing solution: 0.1 M Na2SO4+0.01 M K4Fe(CN)6+0.01 M K3Fe(CN)6 Deposition current–density (mA cm2) 0.1 0.2 0.3 0.4 0.8 1.0
Deposition time (min)
10 10 10 10 10 10
Photopotentials (mV)
ZnSO4 0 M
ZnSO4 0.01 M
ZnSO4 0.03 M
ZnSO4 0.05 M
385 540 535 410 440 460
435 565 560 462 445 425
528 570 585 478 485 470
625 610 590 535 535 482
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Fig. 4. Photoaction spectral data of some (CdZn)Se films. Electroplating solution: 0.05 M CdSO4+ 0.001M SeO2 +different concentrations of ZnSO4: (FJF) 0 M ZnSO4; (FKF) 0.01 M ZnSO4; (FWF) 0.03 M ZnSO4; (FEF) 0.05 M ZnSO4. Deposition current–density=0.1 mA cm2, deposition time=10 min.
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Fig. 5. Ep2 vs. l plots for the estimation of band gap of different (CdZn) Se films: (FJF) 0 M ZnSO4; (FKF). 0.01 M ZnSO4; (FWF) 0.03 M ZnSO4; (FEF) 0.05 M ZnSO4.
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Fig. 6. Variation of band gap of (CdZn)Se with composition of the electroplating solution: (FJF) theoritical; (FKF) experimental.
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Fig. 7. Typical current–voltage curves (FJF) 0.01 M CdSO4+0.001 M SeO2; (FKF) 0.01 M ZnSO4+0.001 M SeO2.
Fig. 8. Some representative Tafel plots for the estimation of corrosion rates of CdSe/ZnSe films. Current– density for CdSe deposition=0.1 mA cm2. Current–density for ZnSe deposition: (FJF) 0.1 mA cm 2; (FKF) 0.2 mA cm2; (FWF) 0.3 mA m2. Deposition time=15 min.
remains unaltered in spite of the variation in the concentration of ZnSO4 in the electroplating solution. For the estimation of band gap, E2p vs. l plots were constructed (Fig. 5) using photoaction spectral data. The variation of band gap with the concentration of ZnSO4 in the electroplating solution depicted in Fig. 6 indicates the formation of solid solution of CdSe and ZnSe. Band gap values theoritically calculated on the basis of the assumption that the solid solution composition equals
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Experimental conditions for the deposition of CdSe films in the preparation of CdSe/ZnSe electrodes
Experimental conditions for the deposition of ZnSe films in the preparation of CdSe/ZnSe electrodes
Deposition current–density (mA cm2)
Deposition time (min)
Deposition current–density (mA cm2)
Deposition time (min)
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
15 15 10 15 15 15 15 15 15 15 15 15
0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.8 1.0
10 15 15 10 15 30 5 10 15 15 15 15
Corrosion current Ic 106 (A)
Corrosion rate Rc 1010 (g/s)
2.180 1.990 1.819 0.478 0.436 0.398 0.251 0.229 0.199 0.187 0.248 0.229
7.52 6.90 6.28 1.65 1.52 1.37 0.866 0.791 0.687 0.646 0.856 0.791
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Table 4 Corrosion characteristics of CdSe/ZnSe system. Testing solution: 0.5 M Cd(CH3COO)2+0.1 M KI+0.2 mM I2 solution
Deposited current–density mA cm2
Deposition time (min)
0.1 0.2 0.3 0.4 0.8 1.0
15 15 15 15 15 15
Corrosion current 106 (A)
Corrosion rate (Rc) 1011 (g/s)
ZnSO4 0.01 M
ZnSO4 0.05 M
ZnSO4 0.1 M
ZnSO4 0.01 M
ZnSO4 0.05 M
ZnSO4 0.1 M
0.0794 0.0724 0.0660 0.06918 0.06025 0.06309
0.0549 0.0575 0.05495 0.06165 0.05128 0.0542
0.05248 0.05888 0.05623 0.0588 0.05248 0.0549
3.59 3.27 2.98 3.13 2.72 2.85
2.07 2.17 2.07 2.32 1.93 2.04
1.89 2.12 2.02 2.11 1.89 1.97
Table 6 Corrosion characteristics of galvanostatically formed mixed (CdZn)Se systems. Electroplating solution composition: 0.05 M CdSO4+0.001 M SeO2+Various concentrations of ZnSO4. Testing solution: 0.1 M Na2SO4+0.01 M K4Fe(CN)6+0.01 M K3Fe(CN)6 Deposited current–density Deposition Corrosion current 106(A) Corrosion rate (Rc) 1011 (g/s) 2 (mA cm ) time (min) ZnSO4 0 M ZnSO4 0.01 M ZnSO4 0.03 M ZnSO4 0.05 M ZnSO4 0 M ZnSO4 0.01 M ZnSO4 0.03 M ZnSO4 0.05 M 0.1 0.2 0.3 0.4 0.8 1.0
10 10 10 10 10 10
2.82 2.40 2.00 1.66 1.51 1.48
0.580 0.631 0.525 0.447 0.501 0.417
0.210 0.202 0.224 0.188 0.174 0.158
0.060 0.050 0.049 0.038 0.0372 0.033
163 138 115 96.08 87.63 85.64
31.07 34.08 28.34 24.12 27.06 22.51
10.23 9.89 10.96 9.225 8.51 7.76
2.76 2.30 2.25 1.75 1.71 1.49
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Table 5 Corrosion characteristics of galvanostatically formed mixed (CdZn)Se systems. Electroplating solution composition: 0.01 M CdSO4+0.002 M SeO2+Various concentrations of ZnSO4. Testing solution: 0.01 M Na2SO4+0.01 M K4Fe(CN)6+0.01 M K3Fe(CN)6
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that of the electroplating solutions, are also included in the figure for the sake of comparison. These results clearly show that the electrodeposits are loaded in favour of CdSe because of the easier dischargeability of Cd2+ ions (deposition potential=0.403 V) in comparision to that of Zn2+ ions (deposition potential=0.763 V). Current–voltage data presented in Fig. 7 also support the above inference. The mixed systems, however, do exhibit substantially improved photoresponsiveness. Obviously, this is possible because of the improved absorbability of the electromagnetic spectrum, due to the presence of ZnSe along with CdSe in the mixed system. The resistance of these two photoelectroactive systems, i.e. bilayered films based on CdSe and ZnSe and mixed (CdZn)Se films, towards electrochemical corrosion was investigated on the basis of current–voltage studies using Tafel plots. Representative data are shown in Fig. 8. The Tafel plots were used for the estimation of corrosion current using the relationship [24] i Z ¼ b log ; ð2Þ icorr where Z is the overvoltage potential, i, the current at the applied voltage, icorr the corrosion current and b is the symmetry factor. The corrosion rates, Rcorr were estimated using the relationship [25] : Rcorr ¼ icorr EW F
ð3Þ
The corrosion rates estimated using these equations are summarized in Tables 4–6. Bilayered films exhibit reduced corrosion rates in comparison to those for ZnSe alone. Mixed selenide films, however, show substantially improved resistance towards electrochemical corrosion. The studies presented herein show that multiple band gap electrodeposited films, obtained under galvanostatic control, exhibit improved photoelectroconvertibility and are endowed with enhanced resistance towards electrochemical corrosion.
Acknowledgements The authors thank the Head of the Chemistry Department for permitting the use of laboratory facilities. One of us (S.S.D. Mishra) thanks Prof. N.B. Singh for the benefit of association and help. Financial help from CSIR, New Delhi is also gratefully acknowledged.
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