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Photoelectrochemical formation of indium and cadmium selenide nanoparticles through Se electrode precursor
Electrochemistry Communications 6 (2004) 1051–1056 www.elsevier.com/locate/elecom
Photoelectrochemical formation of indium and cadmium selenide nanoparticles through Se electrode precursor Sergej M. Rabchynski, Dzmitry K. Ivanou, Eugene A. Streltsov
*
Chemistry Department, Institute of Physico-Chemical Problems, Belarusian State University, Leningradskaya st. 14, 220050 Minsk, Belarus Received 29 July 2004; received in revised form 29 July 2004; accepted 30 July 2004
Abstract The technique of photoelectrochemical In2Se3 and CdSe nanoparticles preparation with the use of Se electrode precursor has been developed. In2Se3 and CdSe particles were prepared by illumination of Se electrode at applied cathodic potential in acidic solutions of corresponding nitrates. The average size of the particles was 40–80 nm. In2Se3 and CdSe nanoparticles were synthesised both at the Se cathode surface and in the electrolyte bulk. The concentration of the metal ions in the electrolyte was found to be the main controlling parameter of the photoelectrochemical reaction influencing on the localisation of the metal chalcogenide nanoparticles formed. At metal cations concentration above 0.01 mol l1 In2Se3 and CdSe colloid particles attain positive charge due to Men+ adsorption. The positive charge did not allow them to leave the negatively charged electrode surface, which resulted in the formation of Se|MexSey heterostructures. In contrast, at concentration below 0.01 mol l1 the colloidal particles were negatively charged and repulsed from the electrode surface thus forming colloidal solutions. In2Se3 and CdSe particles as well as Se|MexSey heterostructures were characterised by the TEM, AFM, XRD, RBS, cyclic voltammetry and photocurrent spectroscopy. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Selenium; Metal selenides; Electrodeposition; Semiconductor chalcogenides; Photoelectrochemistry
1. Introduction In the past decade various methods for nanostructured materials preparation were developed. The electrochemical approach is energy-efficient, provides convenient process control by electrode potential or current and high ecological safety with relatively simple and inexpensive equipment. Electrochemical methods were applied for various nanostructures: quantum dots [1,2], superlattices [3,4], semiconductor nanocomposites [5], ultra thin films [6–10], supported nanoparticles on electrodes [11], photonic crystals [12], etc. In the present work, In2Se3 and CdSe nanoparticles were photoelectrochemically synthesised using Se elec*
Corresponding author. Tel.: +375 17 2095178; fax: +375 17 2264696. E-mail address: [email protected] (E.A. Streltsov). 1388-2481/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2004.07.019
trode that was illuminated under cathodic polarisation in solutions that contained indium or cadmium salts. The concentration of metal cations was shown to determine the selenides deposition onto the electrode or formation of colloid solutions.
2. Experimental Selenium film electrodes were prepared by the electrochemical deposition of Se onto an Au foil. Before the deposition, the Au foil was polished using 0.7 lm diamond paste, treated with concentrated H2SO4, then washed with bidistilled water and finally heated at 700 °C for 20 min in air. Selenium film was galvanostatically deposited onto Au in an aqueous solution containing 5 M SeO2 + 9 M H2SO4 (T = 95 ± 2 °C) at a current density of 1 mA cm2. Its thickness estimated by SEM
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and Rutherford backscattering (RBS) was about 0.2 ± 0.05 lm. The working area of Se electrodes was 1 cm2. XRD showed that the Se film was polycrystalline (c-Se phase). In2Se3 and CdSe particles were prepared by illumination of Se electrode at applied cathodic potential (E = 0.2 V vs Ag|AgCl|KCl(sat.)) in solutions of corresponding metal nitrates in 0.1 M HNO3 at 18 ± 2 °C. The polychromatic light from a 50 W halogen lamp (J = 7 mW cm2) equipped with an infrared filter was used. To avoid bulk In and Cd deposition the potentials applied were more positive than equilibrium ECd2þ =Cd0 and EIn3þ =In0 potential. Analytical-grade reagents and doubly distilled water were used in the electrolyte preparation. Electrochemical and photoelectrochemical measurements were carried out in a three-electrode two-compartment cell (20 cm3) with an optical quality window, a platinum counter-electrode and Ag|AgCl|KCl(sat.) reference electrode. Spectral dependences of the photocurrent were obtained using high intensity grating monochromator, a 1000 W xenon lamp and 0.3 Hz light chopper. The photocurrent spectra were corrected for the spectral intensity distribution at the monochromator output. Optical spectra were measured using a Specord M40 UV–Vis spectrometer (Carl Zeiss, Jena). AFM images were obtained with a FemtoScan-001 (Advanced Technologies Center, MSU, Russia) operated in the constant force mode (1.5–5 nN) under ambient conditions. AFM tips were standard oxidesharpened Si3N4 cantilevers with spring constants of 0.06 and 0.12 N/m. The scanning frequency ranged from 1 to 10 Hz. TEM images were obtained with EM–125 (Russia). Smooth electrodes for AFM measurements were prepared by Se film electrodeposition onto mica covered in vacuum with a mirror Au film. RBS with 2.0 MeV alpha particles was used for analysing of Se electrodes and products of the photoelectrochemical reaction. The incident beam, usually normal to the sample surface, was collimated up to about 1 mm diameter. Backscattered particles were detected at 140° in General geometry with silicon surface barrier detectors. The solid angles and integrated charge were determined with uncertainty of about 3%. The spectra were analysed using the RUMP computer program [13]. X-ray diffraction analysis of the films was performed with HZG-4A diffractometer (Cu Ka radiation, Ni filter).
3. Results and discussion 3.1. Photoelectrochemical behaviour of bare Se electrodes Fig. 1 shows typical voltammograms of bare Se electrodes measured in the dark and under chopped illumination in 0.1 M HNO3. The dark current is negligible in
Fig. 1. Voltammograms of Se electrode in 0.1 M HNO3 recorded under chopped polychromatic illumination (cathodic scan, solid line) and in the dark (anodic scan, dotted line): (a) in stationary electrolyte; (b) in the electrolyte that was stirred by Ar bubbles. Potential scan rate: 0.02 V s1.
the wide potential range from 0.6 to 0.8 V. Above 0.85 V, the anodic current associated with selenium oxidation and dissolution appears. Under illumination, the cathodic photocurrent shows up below 0.1 V in accordance with normal behaviour of a p-type semiconductor. The height of the photopolarisation wave was proportional to the intensity of illumination. It was previously shown that photoreduction (photocorrosion) of Se electrode and hydrogen evolution is responsible for the cathodic photocurrent on Se in nitric acid solution [11]: Se þ 2Hþ þ 2e ¼ H2 Se
ð1Þ
2Hþ þ 2e ¼ H2
ð2Þ
Reaction (1) consumes above 35% of the cathodic charge. Selenium reduction by photoelectrons proceeds below Se flat band potential (Efb = +0.6 V) [11]. The peak AH2 Se recorded in the anodic scan (Fig. 1(a)) results from the anodic oxidation of the product of reaction (1). The peak disappears when the solution is stirred (Fig. 1(b)). The evolution of H2Se was also reported by Gissler [14] who detected H2Se by mass-spectrometry as a photodecomposition product of Se electrodes.
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The colour changes correspond to the changes in absorption spectra shown in Fig. 2, which results from the following reactions:
Fig. 2. Absorption spectra of: (a) In2Se3; (b) CdSe colloidal solutions prepared by Se electrode illumination in (a) 2 mM In(NO3)3 + 0.1 M HNO3 and (b) 2 mM Cd(NO3)2 + 0.1 M HNO3 for 15 min (1), 30 min (2) and 60 min (3). The potential applied was 0.2 V. Curves 4 show absorption spectra for colloid-free electrolyte.
3.2. Photoelectrochemical processes on Se electrode in solutions containing In3+ and Cd2+ ions In acidic solutions of indium and cadmium salts at concentrations below 0.01 mol l1 the illumination of Se electrode for 5–7 min in the potential range of cathodic photocurrent resulted in the formation of turbid solutions. The colour of thus formed colloid solution was orange in the case of In(NO3)3 and white in the case of Cd(NO3)2 ones.
2In3þ þ 3H2 Se ¼ In2 Se3 þ 6Hþ
ð3Þ
Cd2þ þ H2 Se ¼ CdSe þ 2Hþ
ð4Þ
The particles of In2Se3 were non-uniformly distributed in size (Fig. 3(a)) with major part of the particles having the sizes from 40 to 80 nm. Unlike In2Se3 particles that were mainly spherical in shape, CdSe formed complex structure from particles of different sizes (Fig. 3(b)). The colloids were stable for 12–18 h with no addition of surface-active reagents. At longer time sedimentation was observed. XRD has shown In2Se3 and CdSe particles were amorphous. The element ratio in the sediment from indium selenide colloidal solution, obtained from RBS spectroscopy, corresponded to In2Se3.4. The excess of Se was probably due to partial oxidation of H2Se to Se0 by HNO3 or by traces of O2. When the concentration of indium and cadmium salts were above 0.01 mol l1 the illumination resulted in precipitation of selenide particles on Se surface and colloidal solution formation was not observed. Fig. 4 explains the possible reason of metal salt concentration effect. At low Men+ concentration there is an excess of HSe anions formed because of H2Se dissociation:
Fig. 3. TEM image of the: (a) In2Se3; (b) CdSe particles formed by Se photoreduction at 0.2 V in (a) 2 mM In(NO3)3 + 0.1 M HNO3 and (b) 2 mM Cd(NO3)2 + 0.1 M HNO3.
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Fig. 4. Scheme of: (a) colloid solution formation; (b) selenides particles deposition onto the electrode surface.
H2 Se ¢ Hþ þ HSe HSe anions adsorb on colloid particles and the particles attain the negative charge. In this case the structure of the micelle obtained can be presented by the following formula: z
fðMex Sey Þn ; xHSe ; yMenþ g z=nMenþ At concentrations below 0.01 mol l1 the repulsion of the negatively charged colloid particles from the electrode surface favours the colloid formation (Fig. 4(a)). The negative charge of the colloidal particles thus obtained was confirmed by electrophoretic measurements. On the contrary, the excess of Men+ ions determines the positive charge of colloid particles. In the case of Men+ excess, the micelle structure can be represented by the following formula: zþ
fðMex Sey Þn ; xMenþ ; yHSe g zHSe The positively charged particles are attracted to the negatively charged electrode (Fig. 4(b)) and thus prevent colloidal solution formation at concentration above 0.01 mol l1. Fig. 5 shows the RBS spectra of Se electrode in In2Se3 photodeposition for the cases of low and high In(NO3)3 concentrations. At In(NO3)3 concentration below 0.01 M the shift of the spectrum edge from 1.80 to 1.83 MeV (Fig. 5(a)) manifest lower deceleration of alpha particles by Se film which results from Se film thinning in the photoelectrochemical reaction. In the case of higher In(NO3)3 concentration no considerable shift of the spectrum edge was observed as selenium did not leave the surface in the photoelectrochemical reaction. Small peak near the spectrum edge was due to indium selenide deposition onto the electrode. Even in the low indium concentration range some indium selenide deposition was observed at Se surface (Fig. 6). The ‘‘block’’ surface structure (Fig. 6(a)) attribute to Se film smoothes during the deposition and a lot of spherical particles are formed (Fig. 6(b)).
Fig. 5. RBS spectra from 150 nm Se film on Au substrate before illumination (dotted line) and after illumination for 15 min at 0.2 V (solid line) in: (a) 2 mM In(NO3)3 + 0.1 M HNO3; (b) 50 mM In(NO3)3 + 0.1 M HNO3. Vertical dotted lines show the scattering energies of Se, In and Au surface atoms.
The precipitation of In2Se3 and CdSe particles on Se electrode surface resulted in Se|In2Se3 and Se|CdSe heterostructures formation. The heterostructures were characterised by cyclic voltammetry (Fig. 7) and photocurrent quantum yield spectroscopy (Fig. 8). After cathodic photopolarization of Se electrode in electrolyte containing In3+ and Cd2+ ions two peaks A1 and A2 were observed in the anodic scan. These peaks were not due to H2Se oxidation, as they were observed in the conditions of the solution stirring by Ar bubbles. The peak A2 was observed in the potential range that corresponds to bulk metal selenides oxidation and thus was attributed to In2Se3 and CdSe oxidation. The other peak A1 observed at lower potential corresponded to 0.7 monolayer of indium and 0.8 monolayer of cadmium oxidation. Cadmium and indium adatoms were formed as a result of photoinduced underpotential deposition. The similar photoinduced underpotential deposition of lead atoms was reported previously [11]. Cadmium and indium adatoms formed the corresponding selenides by the following reactions: Cd0 þ Se0 ¼ CdSe
ð5Þ
2In0 þ 3Se0 ¼ In2 Se3
ð6Þ
Therefore, Se|In2Se3 and Se|CdSe heterostructures can be formed not only by the processes (3) and (4), but also by the reactions (5) and (6). Eqs. (5) and (6) represent the other path for indium and cadmium selenides
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Fig. 6. AFM image of the polycrystalline Se film: (a) before illumination; (b) after illumination at 0.2 V in 2 mM In(NO3)3 + 0.1 M HNO3 for 15 min.
Fig. 7. Cyclic voltammograms of Se electrode in: (a) 2 mM In(NO3)3 + 0.1 M HNO3, (b) 2 mM Cd(NO3)2 + 0.1 M HNO3 solution. Cathodic scans were recorded under illumination. The illumination was switched off in the points of scan reversal (A, B, C). Curve 4 anodic voltammograms of the bulk (a) In2Se3 and (b) CdSe electrodes. Potential scan rate: 0.02 V s1.
formation that allows some deposition even at low concentration of metal cations when the main part of selenide particles does not precipitate on Se electrode but forms colloid solution.
Fig. 8. Photocurrent quantum yield (Y) spectra of bare Se electrode (1) and Se electrode with In2Se3 particles (2,3) in acetic buffer solution (pH 7) at 0.2 V. The In2Se3 particles were formed under illumination of Se electrode for 30 s (2) and 90 s (3) at 0.2 V in 50 mM In(NO3)3 + 0.1 M HNO3 solution.
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In Fig. 8 the spectral dependences of photocurrent quantum yield (Y) for Se and Se|In2Se3 electrodes are presented. In2Se3 accumulation on selenium electrode surface results in the significant decrease of the photocurrent quantum yield in the wide spectral region up to 700 nm. The decrease is attributed to the formation of recombination centres and energy barrier for charge transfer through Se|In2Se3 interface. However, above 700 nm In2Se3 deposition increased the photocurrent quantum yield. The increase is probably due the formation of intra-band-gap surface states by indium atoms.
4. Conclusions Colloidal semiconductor In2Se3 and CdSe nanoparticles with average particle size from 40 to 80 nm were synthesised by the photoelectrochemical technique through Se electrode precursor. The colloid particles were formed as the result of Men+ ions interaction with H2Se generated photoelectrochemically. At metal concentration below 0.01 mol l1 metal selenide colloidal particles were negatively charged because of HSe anion adsorption and this favored the evacuation from the electrode surface into the solution. At indium and cadmiun cations concentration above 0.01 mol l1 selenide particles were positively charged because of Men+ cation adsorption and this resulted in attraction of the particles to the negatively charged electrode that resulted in the formation of selenium–selenide heterostructures. Thus the variation of the metal cations concentration provided the control of the photoelectrochemical reaction.
The similar approach may be of interest for various chalkogenides nanocolloids preparation. Acknowledgements We would like to thank Dr. G.K. Zhavnerko from the Institute for Chemistry of New Materials, NASB, Minsk, Belarus for AFM measurements, Dr. P.I. Gaiduk from the Belarusian State University for RBS and Dr. S.K. Poznyak from the Institute of Physico-Chemical Problems, Belarusian State University for photoelectrochemical measurements. References [1] B. Alperson, H. Demange, I. Rubinstein, H. Hodes, J. Phys. Chem. B 103 (1999) 4943. [2] Y. Golan, B. Alperson, J. Hutchison, H. Hodes, I. Rubinstein, Adv. Mater. 9 (1997) 236. [3] J.A. Switzer, M.J. Shane, R.J. Phillips, Science 247 (1990) 444. [4] T.L. Wade, R. Vaidyanathan, U. Happek, J.L. Stickney, J. Electroanal. Chem. 500 (2001) 322. [5] D.K. Ivanov, N.P. Osipovich, S.K. Poznyak, E.A. Streltsov, Surf. Sci. 532 (2003) 1092. [6] E. Herrero, L. Buller, H.D. Abruna, Chem. Rev. 101 (2001) 1897. [7] G.A. Ragoisha, A.S. Bondarenko, P. Osipovich, E.A. Streltsov, J. Electroanal. Chem. 565 (2004) 227. [8] B.W. Gregory, J.L. Stickney, J. Electroanal. Chem. 300 (1991) 543. [9] T. Oznuluer, U. Demir, J. Electroanal. Chem. 529 (2002) 34. [10] M. Froment, D. Lincot, Electrochim. Acta 40 (1995) 1293. [11] E.A. Streltsov, S.K. Poznyak, N.P. Osipovich, J. Electroanal. Chem. 518 (2002) 103. [12] A.L. Rogach, N.A. Kotov, D.S. Koktysh, W. Ostrander, G.A. Ragoisha, Chem. Mater. 12 (2000) 2721. [13] L.R. Doolitle, Nucl. Instrum. Meth. B 9 (1985) 344. [14] W. Gissler, J. Electrochem. Soc. 127 (1980) 1713.
Photoelectrochemical formation of indium and cadmium selenide nanoparticles through Se electrode precursor Sergej M. Rabchynski, Dzmitry K. Ivanou, Eugene A. Streltsov
*
Chemistry Department, Institute of Physico-Chemical Problems, Belarusian State University, Leningradskaya st. 14, 220050 Minsk, Belarus Received 29 July 2004; received in revised form 29 July 2004; accepted 30 July 2004
Abstract The technique of photoelectrochemical In2Se3 and CdSe nanoparticles preparation with the use of Se electrode precursor has been developed. In2Se3 and CdSe particles were prepared by illumination of Se electrode at applied cathodic potential in acidic solutions of corresponding nitrates. The average size of the particles was 40–80 nm. In2Se3 and CdSe nanoparticles were synthesised both at the Se cathode surface and in the electrolyte bulk. The concentration of the metal ions in the electrolyte was found to be the main controlling parameter of the photoelectrochemical reaction influencing on the localisation of the metal chalcogenide nanoparticles formed. At metal cations concentration above 0.01 mol l1 In2Se3 and CdSe colloid particles attain positive charge due to Men+ adsorption. The positive charge did not allow them to leave the negatively charged electrode surface, which resulted in the formation of Se|MexSey heterostructures. In contrast, at concentration below 0.01 mol l1 the colloidal particles were negatively charged and repulsed from the electrode surface thus forming colloidal solutions. In2Se3 and CdSe particles as well as Se|MexSey heterostructures were characterised by the TEM, AFM, XRD, RBS, cyclic voltammetry and photocurrent spectroscopy. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Selenium; Metal selenides; Electrodeposition; Semiconductor chalcogenides; Photoelectrochemistry
1. Introduction In the past decade various methods for nanostructured materials preparation were developed. The electrochemical approach is energy-efficient, provides convenient process control by electrode potential or current and high ecological safety with relatively simple and inexpensive equipment. Electrochemical methods were applied for various nanostructures: quantum dots [1,2], superlattices [3,4], semiconductor nanocomposites [5], ultra thin films [6–10], supported nanoparticles on electrodes [11], photonic crystals [12], etc. In the present work, In2Se3 and CdSe nanoparticles were photoelectrochemically synthesised using Se elec*
Corresponding author. Tel.: +375 17 2095178; fax: +375 17 2264696. E-mail address: [email protected] (E.A. Streltsov). 1388-2481/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2004.07.019
trode that was illuminated under cathodic polarisation in solutions that contained indium or cadmium salts. The concentration of metal cations was shown to determine the selenides deposition onto the electrode or formation of colloid solutions.
2. Experimental Selenium film electrodes were prepared by the electrochemical deposition of Se onto an Au foil. Before the deposition, the Au foil was polished using 0.7 lm diamond paste, treated with concentrated H2SO4, then washed with bidistilled water and finally heated at 700 °C for 20 min in air. Selenium film was galvanostatically deposited onto Au in an aqueous solution containing 5 M SeO2 + 9 M H2SO4 (T = 95 ± 2 °C) at a current density of 1 mA cm2. Its thickness estimated by SEM
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and Rutherford backscattering (RBS) was about 0.2 ± 0.05 lm. The working area of Se electrodes was 1 cm2. XRD showed that the Se film was polycrystalline (c-Se phase). In2Se3 and CdSe particles were prepared by illumination of Se electrode at applied cathodic potential (E = 0.2 V vs Ag|AgCl|KCl(sat.)) in solutions of corresponding metal nitrates in 0.1 M HNO3 at 18 ± 2 °C. The polychromatic light from a 50 W halogen lamp (J = 7 mW cm2) equipped with an infrared filter was used. To avoid bulk In and Cd deposition the potentials applied were more positive than equilibrium ECd2þ =Cd0 and EIn3þ =In0 potential. Analytical-grade reagents and doubly distilled water were used in the electrolyte preparation. Electrochemical and photoelectrochemical measurements were carried out in a three-electrode two-compartment cell (20 cm3) with an optical quality window, a platinum counter-electrode and Ag|AgCl|KCl(sat.) reference electrode. Spectral dependences of the photocurrent were obtained using high intensity grating monochromator, a 1000 W xenon lamp and 0.3 Hz light chopper. The photocurrent spectra were corrected for the spectral intensity distribution at the monochromator output. Optical spectra were measured using a Specord M40 UV–Vis spectrometer (Carl Zeiss, Jena). AFM images were obtained with a FemtoScan-001 (Advanced Technologies Center, MSU, Russia) operated in the constant force mode (1.5–5 nN) under ambient conditions. AFM tips were standard oxidesharpened Si3N4 cantilevers with spring constants of 0.06 and 0.12 N/m. The scanning frequency ranged from 1 to 10 Hz. TEM images were obtained with EM–125 (Russia). Smooth electrodes for AFM measurements were prepared by Se film electrodeposition onto mica covered in vacuum with a mirror Au film. RBS with 2.0 MeV alpha particles was used for analysing of Se electrodes and products of the photoelectrochemical reaction. The incident beam, usually normal to the sample surface, was collimated up to about 1 mm diameter. Backscattered particles were detected at 140° in General geometry with silicon surface barrier detectors. The solid angles and integrated charge were determined with uncertainty of about 3%. The spectra were analysed using the RUMP computer program [13]. X-ray diffraction analysis of the films was performed with HZG-4A diffractometer (Cu Ka radiation, Ni filter).
3. Results and discussion 3.1. Photoelectrochemical behaviour of bare Se electrodes Fig. 1 shows typical voltammograms of bare Se electrodes measured in the dark and under chopped illumination in 0.1 M HNO3. The dark current is negligible in
Fig. 1. Voltammograms of Se electrode in 0.1 M HNO3 recorded under chopped polychromatic illumination (cathodic scan, solid line) and in the dark (anodic scan, dotted line): (a) in stationary electrolyte; (b) in the electrolyte that was stirred by Ar bubbles. Potential scan rate: 0.02 V s1.
the wide potential range from 0.6 to 0.8 V. Above 0.85 V, the anodic current associated with selenium oxidation and dissolution appears. Under illumination, the cathodic photocurrent shows up below 0.1 V in accordance with normal behaviour of a p-type semiconductor. The height of the photopolarisation wave was proportional to the intensity of illumination. It was previously shown that photoreduction (photocorrosion) of Se electrode and hydrogen evolution is responsible for the cathodic photocurrent on Se in nitric acid solution [11]: Se þ 2Hþ þ 2e ¼ H2 Se
ð1Þ
2Hþ þ 2e ¼ H2
ð2Þ
Reaction (1) consumes above 35% of the cathodic charge. Selenium reduction by photoelectrons proceeds below Se flat band potential (Efb = +0.6 V) [11]. The peak AH2 Se recorded in the anodic scan (Fig. 1(a)) results from the anodic oxidation of the product of reaction (1). The peak disappears when the solution is stirred (Fig. 1(b)). The evolution of H2Se was also reported by Gissler [14] who detected H2Se by mass-spectrometry as a photodecomposition product of Se electrodes.
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The colour changes correspond to the changes in absorption spectra shown in Fig. 2, which results from the following reactions:
Fig. 2. Absorption spectra of: (a) In2Se3; (b) CdSe colloidal solutions prepared by Se electrode illumination in (a) 2 mM In(NO3)3 + 0.1 M HNO3 and (b) 2 mM Cd(NO3)2 + 0.1 M HNO3 for 15 min (1), 30 min (2) and 60 min (3). The potential applied was 0.2 V. Curves 4 show absorption spectra for colloid-free electrolyte.
3.2. Photoelectrochemical processes on Se electrode in solutions containing In3+ and Cd2+ ions In acidic solutions of indium and cadmium salts at concentrations below 0.01 mol l1 the illumination of Se electrode for 5–7 min in the potential range of cathodic photocurrent resulted in the formation of turbid solutions. The colour of thus formed colloid solution was orange in the case of In(NO3)3 and white in the case of Cd(NO3)2 ones.
2In3þ þ 3H2 Se ¼ In2 Se3 þ 6Hþ
ð3Þ
Cd2þ þ H2 Se ¼ CdSe þ 2Hþ
ð4Þ
The particles of In2Se3 were non-uniformly distributed in size (Fig. 3(a)) with major part of the particles having the sizes from 40 to 80 nm. Unlike In2Se3 particles that were mainly spherical in shape, CdSe formed complex structure from particles of different sizes (Fig. 3(b)). The colloids were stable for 12–18 h with no addition of surface-active reagents. At longer time sedimentation was observed. XRD has shown In2Se3 and CdSe particles were amorphous. The element ratio in the sediment from indium selenide colloidal solution, obtained from RBS spectroscopy, corresponded to In2Se3.4. The excess of Se was probably due to partial oxidation of H2Se to Se0 by HNO3 or by traces of O2. When the concentration of indium and cadmium salts were above 0.01 mol l1 the illumination resulted in precipitation of selenide particles on Se surface and colloidal solution formation was not observed. Fig. 4 explains the possible reason of metal salt concentration effect. At low Men+ concentration there is an excess of HSe anions formed because of H2Se dissociation:
Fig. 3. TEM image of the: (a) In2Se3; (b) CdSe particles formed by Se photoreduction at 0.2 V in (a) 2 mM In(NO3)3 + 0.1 M HNO3 and (b) 2 mM Cd(NO3)2 + 0.1 M HNO3.
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Fig. 4. Scheme of: (a) colloid solution formation; (b) selenides particles deposition onto the electrode surface.
H2 Se ¢ Hþ þ HSe HSe anions adsorb on colloid particles and the particles attain the negative charge. In this case the structure of the micelle obtained can be presented by the following formula: z
fðMex Sey Þn ; xHSe ; yMenþ g z=nMenþ At concentrations below 0.01 mol l1 the repulsion of the negatively charged colloid particles from the electrode surface favours the colloid formation (Fig. 4(a)). The negative charge of the colloidal particles thus obtained was confirmed by electrophoretic measurements. On the contrary, the excess of Men+ ions determines the positive charge of colloid particles. In the case of Men+ excess, the micelle structure can be represented by the following formula: zþ
fðMex Sey Þn ; xMenþ ; yHSe g zHSe The positively charged particles are attracted to the negatively charged electrode (Fig. 4(b)) and thus prevent colloidal solution formation at concentration above 0.01 mol l1. Fig. 5 shows the RBS spectra of Se electrode in In2Se3 photodeposition for the cases of low and high In(NO3)3 concentrations. At In(NO3)3 concentration below 0.01 M the shift of the spectrum edge from 1.80 to 1.83 MeV (Fig. 5(a)) manifest lower deceleration of alpha particles by Se film which results from Se film thinning in the photoelectrochemical reaction. In the case of higher In(NO3)3 concentration no considerable shift of the spectrum edge was observed as selenium did not leave the surface in the photoelectrochemical reaction. Small peak near the spectrum edge was due to indium selenide deposition onto the electrode. Even in the low indium concentration range some indium selenide deposition was observed at Se surface (Fig. 6). The ‘‘block’’ surface structure (Fig. 6(a)) attribute to Se film smoothes during the deposition and a lot of spherical particles are formed (Fig. 6(b)).
Fig. 5. RBS spectra from 150 nm Se film on Au substrate before illumination (dotted line) and after illumination for 15 min at 0.2 V (solid line) in: (a) 2 mM In(NO3)3 + 0.1 M HNO3; (b) 50 mM In(NO3)3 + 0.1 M HNO3. Vertical dotted lines show the scattering energies of Se, In and Au surface atoms.
The precipitation of In2Se3 and CdSe particles on Se electrode surface resulted in Se|In2Se3 and Se|CdSe heterostructures formation. The heterostructures were characterised by cyclic voltammetry (Fig. 7) and photocurrent quantum yield spectroscopy (Fig. 8). After cathodic photopolarization of Se electrode in electrolyte containing In3+ and Cd2+ ions two peaks A1 and A2 were observed in the anodic scan. These peaks were not due to H2Se oxidation, as they were observed in the conditions of the solution stirring by Ar bubbles. The peak A2 was observed in the potential range that corresponds to bulk metal selenides oxidation and thus was attributed to In2Se3 and CdSe oxidation. The other peak A1 observed at lower potential corresponded to 0.7 monolayer of indium and 0.8 monolayer of cadmium oxidation. Cadmium and indium adatoms were formed as a result of photoinduced underpotential deposition. The similar photoinduced underpotential deposition of lead atoms was reported previously [11]. Cadmium and indium adatoms formed the corresponding selenides by the following reactions: Cd0 þ Se0 ¼ CdSe
ð5Þ
2In0 þ 3Se0 ¼ In2 Se3
ð6Þ
Therefore, Se|In2Se3 and Se|CdSe heterostructures can be formed not only by the processes (3) and (4), but also by the reactions (5) and (6). Eqs. (5) and (6) represent the other path for indium and cadmium selenides
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Fig. 6. AFM image of the polycrystalline Se film: (a) before illumination; (b) after illumination at 0.2 V in 2 mM In(NO3)3 + 0.1 M HNO3 for 15 min.
Fig. 7. Cyclic voltammograms of Se electrode in: (a) 2 mM In(NO3)3 + 0.1 M HNO3, (b) 2 mM Cd(NO3)2 + 0.1 M HNO3 solution. Cathodic scans were recorded under illumination. The illumination was switched off in the points of scan reversal (A, B, C). Curve 4 anodic voltammograms of the bulk (a) In2Se3 and (b) CdSe electrodes. Potential scan rate: 0.02 V s1.
formation that allows some deposition even at low concentration of metal cations when the main part of selenide particles does not precipitate on Se electrode but forms colloid solution.
Fig. 8. Photocurrent quantum yield (Y) spectra of bare Se electrode (1) and Se electrode with In2Se3 particles (2,3) in acetic buffer solution (pH 7) at 0.2 V. The In2Se3 particles were formed under illumination of Se electrode for 30 s (2) and 90 s (3) at 0.2 V in 50 mM In(NO3)3 + 0.1 M HNO3 solution.
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In Fig. 8 the spectral dependences of photocurrent quantum yield (Y) for Se and Se|In2Se3 electrodes are presented. In2Se3 accumulation on selenium electrode surface results in the significant decrease of the photocurrent quantum yield in the wide spectral region up to 700 nm. The decrease is attributed to the formation of recombination centres and energy barrier for charge transfer through Se|In2Se3 interface. However, above 700 nm In2Se3 deposition increased the photocurrent quantum yield. The increase is probably due the formation of intra-band-gap surface states by indium atoms.
4. Conclusions Colloidal semiconductor In2Se3 and CdSe nanoparticles with average particle size from 40 to 80 nm were synthesised by the photoelectrochemical technique through Se electrode precursor. The colloid particles were formed as the result of Men+ ions interaction with H2Se generated photoelectrochemically. At metal concentration below 0.01 mol l1 metal selenide colloidal particles were negatively charged because of HSe anion adsorption and this favored the evacuation from the electrode surface into the solution. At indium and cadmiun cations concentration above 0.01 mol l1 selenide particles were positively charged because of Men+ cation adsorption and this resulted in attraction of the particles to the negatively charged electrode that resulted in the formation of selenium–selenide heterostructures. Thus the variation of the metal cations concentration provided the control of the photoelectrochemical reaction.
The similar approach may be of interest for various chalkogenides nanocolloids preparation. Acknowledgements We would like to thank Dr. G.K. Zhavnerko from the Institute for Chemistry of New Materials, NASB, Minsk, Belarus for AFM measurements, Dr. P.I. Gaiduk from the Belarusian State University for RBS and Dr. S.K. Poznyak from the Institute of Physico-Chemical Problems, Belarusian State University for photoelectrochemical measurements. References [1] B. Alperson, H. Demange, I. Rubinstein, H. Hodes, J. Phys. Chem. B 103 (1999) 4943. [2] Y. Golan, B. Alperson, J. Hutchison, H. Hodes, I. Rubinstein, Adv. Mater. 9 (1997) 236. [3] J.A. Switzer, M.J. Shane, R.J. Phillips, Science 247 (1990) 444. [4] T.L. Wade, R. Vaidyanathan, U. Happek, J.L. Stickney, J. Electroanal. Chem. 500 (2001) 322. [5] D.K. Ivanov, N.P. Osipovich, S.K. Poznyak, E.A. Streltsov, Surf. Sci. 532 (2003) 1092. [6] E. Herrero, L. Buller, H.D. Abruna, Chem. Rev. 101 (2001) 1897. [7] G.A. Ragoisha, A.S. Bondarenko, P. Osipovich, E.A. Streltsov, J. Electroanal. Chem. 565 (2004) 227. [8] B.W. Gregory, J.L. Stickney, J. Electroanal. Chem. 300 (1991) 543. [9] T. Oznuluer, U. Demir, J. Electroanal. Chem. 529 (2002) 34. [10] M. Froment, D. Lincot, Electrochim. Acta 40 (1995) 1293. [11] E.A. Streltsov, S.K. Poznyak, N.P. Osipovich, J. Electroanal. Chem. 518 (2002) 103. [12] A.L. Rogach, N.A. Kotov, D.S. Koktysh, W. Ostrander, G.A. Ragoisha, Chem. Mater. 12 (2000) 2721. [13] L.R. Doolitle, Nucl. Instrum. Meth. B 9 (1985) 344. [14] W. Gissler, J. Electrochem. Soc. 127 (1980) 1713.