- Email: [email protected]
Photoelectrochemical study of ZnSe electrodeposition on Cu electrode
Journal of Electroanalytical Chemistry 674 (2012) 108–112
Contents lists available at SciVerse ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Short Communication
Photoelectrochemical study of ZnSe electrodeposition on Cu electrode _ ´ ski a Remigiusz Kowalik a,⇑, Konrad Szaciłowski a,b, Piotr Zabin a b
AGH University of Science and Technology, Faculty of Non-Ferrous Metals, Al. Mickiewicza 30, 30-059 Krakow, Poland Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland
a r t i c l e
i n f o
Article history: Received 20 January 2012 Received in revised form 1 March 2012 Accepted 3 March 2012 Available online 15 March 2012 Keywords: ZnSe Electrodeposition Cyclic voltammetry Photoelectrochemistry
a b s t r a c t The paper describes the photoelectrochemical studies and the electrochemical synthesis of zinc selenide thin films. Obtained results specify the range of potentials where semiconducting compound is deposited. Photocurrent profiles indicate the p-type conductivity of the electrodeposited semiconductor. The generated photocurrent indicates the anomalous absorption of the light below the band gap range of ZnSe and suggests a plasmon–exciton coupling effect. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Solar energy harvesting requires semiconducting layers with optical properties allowing the most efficient photon-to-electron conversion. Various approaches to extend the photosensibility window are applied, the most common include photosensitization (in dye-sensitized solar cells) [1,2] and band gap engineering in thin layer cells. Usually significant decrease in band gap energy requires application of highly toxic (e.g. CdSe) or expensive (e.g. CIGS) materials [3]. Furthermore, fabrication of high quality thin layers of these materials is not straightforward. Electrodeposition, however, is a promising method for low-cost production of semiconducting thin films [4–7]. In this communication we report photoelectrochemical properties of freshly deposited ZnSe thin layers on copper substrates. These layers exhibit unusual photosensitivity range and offer an alternative way to increase the efficiency of thin layer solar cells. This approach is based on surface plasmon resonance observed in coinage metals (Cu, Ag, Cu) [8]. While all metallic phases show plasmon phenomena, only in the case of metal mentioned above the resonant energy falls within the visible part of electromagnetic spectrum. Plasmonic excitations in gold nanoparticles are reported to induce photosensitization of wide band gap semiconductors due to damping process involving electron transfer from d-states of gold into conduction band of TiO2 [9–11]. In this report we propose a new approach towards increased efficiency of photoconversion. Appropriate selection of material
(both semiconductor and metal) allows efficient coupling between excitonic states of semiconductor with surface plasmons in metals [12,13] in the process known as Fano resonance [14]. This resonance results in intense sub-bandgap transitions which inject electrons into conduction band of semiconductors. 2. Experimental The three-electrode system was used for electrochemical measurements. The working electrode was copper sputtered on glass with the active surface of 1 cm2. Platinum foil was employed as the counter electrode and the Ag/AgCl electrode as a reference electrode. The electrochemical measurements were carried out using Autolab PGSTAT30 potentiostat and Modular LED Illuminator (Instytut Fotonowy) equipped with UNO Plus Array RGBA and QUATTRO Mini NIR (ENFIS) as a light source. The maximum energy of the diodes were: 1.96 eV, 2.38 eV, 2.67 eV, 2.08 eV and 1.43 eV. The nominal maximum power of the diodes were 12.5 W for the visible range and 200 W for NIR, the aperture was 1.15 cm2 (VIS) or 4 cm2 (NIR). The ca. 25% of maximum power of the diodes were used in each experiment and a light chopping sequence of 0.25 s on, 0.75 s off was used. The total reflectance spectra were measured on Lambda 950 (Perkin Elmer) spectrophotometer equipped with a 150 mm integrating sphere using both Spectralon and unmodified, freshly sputtered copper as reference samples. 3. Results and discussion
⇑ Corresponding author. Tel./fax: +48 12 617 36 11. E-mail address: [email protected] (R. Kowalik). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.03.002
The formation of the semiconducting deposit during electrochemical measurements can be directly detected due to
R. Kowalik et al. / Journal of Electroanalytical Chemistry 674 (2012) 108–112
109
Fig. 1. Cyclic photovoltammogram recorded for the copper substrate in the solution: (a) 0.001 M H2SeO3, and (b) 0.001 M H2SeO3, 0.1 M ZnSO4; scan rate 20 mV/s.
photocurrent generation [15]. The copper electrode was illuminated by chopped light of four diodes simultaneously. The chopped light induces the photo-effects easily noticeable at the I–E curve as characteristic spikes. The broad cathodic shoulder appeared just at the beginning of the scan when selenous acid is present in the solution (Fig. 1a). According to the previous studies [16–18] this cathodic effect originates from electrodeposition of selenium according to reaction (1):
H2 SeO3 þ 4Hþ þ 4e ¼ Se þ 3H2 O
ð1Þ
No photocurrent is observed at the voltammogram up to the potential 0.5 V, where the deposition of selenium proceeds according to reaction (1). When zinc ions are present in the solution the codeposition of zinc starts at the potential of 0.5 V and it can be observed as a second cathodic peak in Fig. 1b:
Se0 þ Zn2þ þ 2e ¼ ZnSe 2+
ð2Þ
The process of Zn reduction proceeds at a more positive potential than the equilibrium one due to interaction with selenium deposited on the copper electrode at more positive potentials. This
phenomenon is well known as induced co-deposition [19]. According to our previous experiments the bulk deposition of zinc starts below 1.1 V [16–18] and the scan was reversed at this potential in order to minimize the deposition of metallic zinc. The characteristic cathodic spikes appeared on illumination when the potential of the electrode is lower than 0.55 V. This effect indicates the co-deposition of zinc and synthesis of photoactive material, i.e. ZnSe. It can be observed that selenium layers deposited at higher potentials do not generate any photocurrent (Fig. 1a). The potential range where the photoresponse is observed perfectly coincides with the induced co-deposition of zinc at selenium and indicates the electrosynthesis of the ZnSe phase. The Schottky barrier of 1.1 eV height may be formed at the Cu–ZnSe interface [20]. Therefore the photocurrent polarity indicates p-type conductivity of the ZnSe layer. The intensity of the photocurrent increases when the scan is continued to the potential of 1.1 V. This increase is a consequence of continuous increase of the thickness of the deposit, which in turn results in increased absorbance of the film. Increasing band bending may also contribute to this effect. The photocurrent is present up to 0.3 V during scanning to the positive direction. The amplitude of the photocurrent spikes follows the Butler
110
R. Kowalik et al. / Journal of Electroanalytical Chemistry 674 (2012) 108–112
equation [21], therefore one can conclude that loss of photocurrent during anodic sweeps is due to decreased band bending at the solid–liquid interface. The disappearance of the photocurrent spikes on the voltammogram overlaps with the anodic peak which originates from the dissolution of the ZnSe phase. Illumination of the electrode during the synthesis of semiconductor layers enables to estimate the range of the potentials where the photoactive material is deposited. The copper electrode was illuminated with light of five different photon energies in order to identify the photosensitivity range of the deposit. Surprisingly, significant photocurrent generation was observed on illumination with visible light down to 1.96 eV (Fig. 2), while 1.43 eV illumination only generated very weak photocurrents (data not shown). Photocurrent generation was very efficient, the IPCE value reached 73% at 2.67 eV, but at 1.96 eV it was still significant. This indicates that not only excitation of fundamental transition can generate charge carriers in studied system. Normal semiconductors should generate net photocurrent only when incident photons have sufficient energy to promote electrons from the valence to the conduction band. Taking account the value of the zinc selenide band gap of 2.7 eV, the photocurrent generation should be possible only when the electrode is illuminated with light of higher energy. Surprisingly the photocurrent was recorded on each voltammogram independently on the incident light wavelength. Photoelectrochemical techniques can be also used to follow the electrodeposition kinetics. The copper electrode was illuminated with the blue diode during the potentiostatic deposition (Fig. 3). Applied potentials were chosen according to the voltammetry results. Three potentials were selected, where pure selenium is deposited at potential of 0.4 V, and codeposition of zinc and selenium is expected, namely 0.6 and 1.0 V. Similarly to the voltammetry experiments there was no photoresponse observed when the potential of 0.4 V was applied. This result indicates that the selenium deposit does not generate any net photocurrent upon excitation within the fundamental transition. On the contrary, when lower potentials, namely below 0.5 V are applied, the photoeffect is clearly observed. Obviously the photocurrent increases during the time of the deposition due to increasing of the thickness of the deposit. The photocurrent is higher when the potential 0.6 V is applied as compared to photocurrent recorded at
Fig. 2. (a) Linear sweep photovoltammogram recorded for the copper substrate illuminated with different light in the solution 0.001 M H2SeO3, 0.1 M ZnSO4; scan rate 20 mV/s; and (b) radiant power distribution and integrated output power of LEDs.
1.0 V, while the dark current is much higher in the latter case. These results are effect of competitive reactions occurring at the most negative potential. Except of the hydrogen evolution, the process of six-electron reduction of selenous acid and reduction of Se0 are also possible:
H2 SeO3 þ 6Hþ þ 6e ¼ H2 Se þ 3H2 O þ
Se þ 2H þ 2e ¼ H2 Se
ð3Þ ð4Þ
These reactions increase the dark current, but the photocurrent generated at lower potentials seems to be unaffected by competitive electrochemical processes. The advantage of the photovoltammetric results enable to estimate the most efficient range of potentials where the desired semiconducting phase is synthesised. The presented results are in very good agreement with our earlier studies about the electrodeposition of ZnSe under potentiostatic control, where phase analysis indicates the presence of the cubic ZnSe as the sole product of electrolysis [16,17]. In order to understand the anomalous photocurrent generation freshly deposited films were studied spectroscopically. First of all, all the films are dark blue colored, which is in contrary to the band gap of the material. Similar dark coloration was, however observed already for the Cu/Cu2O system obtained during slow oxidation of copper colloids [22]. Recorded UV–VIS–NIR reflectance spectra indicate significant light absorption within 200–1500 nm range (Fig. 4a). The spectrum shows very sharp and intense peak at 1.98 eV, which does not correlate neither with the fundamental transition of ZnSe nor with the surface plasmon peak of bulk copper (ca. 2.40 eV, Fig. 4b). Furthermore, the 1.98 eV peak has almost perfect Lorenzian envelope and is partially overlapped with lower intensity Gaussian component with maximum at 2.34 eV. The latter can be attributed to the excitonic transition of ZnSe. Therefore the 1.98 eV absorption should be associated with a resonant transition of two coupled oscillators, one of them representing the continuum of states while the other having a discrete energy spectrum located within the continuum of the former oscillator. Such a resonance, usually called the Fano resonance is observed in the case of electromagnetic wave scattering in nanostructured media [14]. In this particular case the unusual absorption features can be attributed to the efficient plasmon–exciton coupling, which affects not only the exciton lifetime, but also the spectral signature of the structure and its photoelectrochemical properties [13,23]. In this particular case the unusual absorption features can be attributed to the efficient plasmon–exciton coupling, which affects not only the exciton lifetime, but also the spectral signature of the structure and its photoelectrochemical properties [13,23]. In the case of ZnSe overlayer at Cu surface the excitonic levels are discrete, while the surface plasmon resonance represents the whole continuum of states (cf. Fig. 4b). The resonant absorption maybe further facilitated by image charge interactions and metal-induced gap states [24]. At the metal–semiconductor interface the Fermi levels of both materials have to equilibrate. At the interface the wavefunction of an electron in the semiconductor must match that of an electron in the metal. In the case of finite system the wavefunctions of electrons are altered and states that are forbidden within the bulk semiconductor gap are allowed at the surface [25–29]. These new states may be in turn involved in electronic transitions, which result in promotion of electrons to the conduction band. This resonant process allows electronic energy transfer between plasmonic excited states within metallic phase and excitonic excited states within semiconductor [30]. Similar processes have been observed for the Au/Cu2O [31] and Ag/CdSe [32] systems. Therefore the Fano resonance between coupled transitions at metal surface may explain anomalous light absorption and photocurrent in the long wavelength region and anomalous photocurrent generation on sub-band gap illumination. Thin (up to 100 nm
R. Kowalik et al. / Journal of Electroanalytical Chemistry 674 (2012) 108–112
111
Fig. 3. I–t transients for the copper substrate recorded in the solution containing 0.001 M H2SeO3 and 0.1 M ZnSO4; (d) mechanism of photocurrent generation at ZnSe@Cu photoelectrodes.
Fig. 4. Total reflectance spectra of metallic copper and ZnSe@Cu (a) and corresponding Kubelka–Munk functions together with fitted Lorenzian resonance peak and Gaussian exciton peak (b).
according to quartz microbalance studies) films of zinc selenide studied in this paper can be regarded as quantum wells (under weak confinement regime), as both the electron and hole wave functions are confided in two dimensions, which in turn justifies observed plasmon–exciton coupling effect. Similar plasmonic effects in ZnSe were very recently reported by Dhanasekaran et al. [33]. This report confirms the importance of plasmonic phenomena at the conductor/semiconductor interface, and indicates that careful tuning of plasmon and exciton energies are necessary for efficient resonant process, which could not be observed on ITO surfaces due to energetic mismatch.
4. Concluding remarks The spectroscopic and photoelectrochemical studies presented in this paper supplement the previously reported data on the process of ZnSe electrosynthesis. Presented techniques facilitate the
analysis of the electrodeposition mechanism of semiconducting materials. New spectral features resulting from the Fano resonance between the surface plasmon within metal and exciton within semiconductor were observed. Interestingly, excitation of the metal–semiconductor systems within this resonance band result in generation of cathodic photocurrent. Acknowledgements The financial support from Polish Ministry of Education and Science under Contract Nos. DPN/N27/GDRE-GAMAS/2009, 694/NPOLONIUM/2010/0, 1609/B/H03/2009/36 and by AGH-UST under contract 11.11.180.509/11 is gratefully acknowledged. References [1] N. Robertson, Angew. Chem. Int. Ed. 45 (2006) 2338–2345. [2] N. Robertson, Angew. Chem. Int. Ed. 47 (2008) 1012–1014.
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[3] T. Markvart, L. Castañer, Practical Handbook of Photovoltaics, Elsevier, Oxford, 2003. [4] K. Rajeshwar, Adv. Mater. 4 (1992) 23–29. [5] R.K. Pandey, S.N. Sahu, S. Chandra, Handbook of Semiconductor Electrodeposition, Marcel Dekker, New York, 1996. [6] D. Lincot, J.F. Guillemoles, S. Taunier, D. Guimard, J. Sicx-Kurdi, A. Chaumont, O. Roussel, O. Ramdani, C. Hubert, J.P. Fauvarque, N. Bodereau, L. Parissi, P. Panheleux, P. Fanouillere, N. Naghavi, P.P. Grand, M. Benfarah, P. Mogensen, O. Kerrec, Solar Energy 77 (2004) 725–737. [7] I.M. Dharmadasa, J. Haigh, J. Electrochem. Soc. 153 (2005) G47–G52. [8] C. Sönnichsen, Plasmons in Metal Nanostructures, PhD dissertation, Munich, 2001. [9] A. Furube, L. Du, K. Hara, R. Katoh, M. Tachiya, J. Am. Chem. Soc. 129 (2007) 14852–14853. [10] N. Sakai, Y. Fujiwara, Y. Takahashi, T. Tatsuma, Chem. Phys. Chem. 10 (2009) 766–769. [11] Y. Nishijima, K. Ueno, Y. Yokota, K. Murakoshi, H. Misawa, J. Phys. Chem. Lett. 1 (2010) 2031–2036. [12] C. Symonds, J. Bellessa, J.C. Plenet, E. Cambril, A. Miard, L. Ferlazzo, A. Lemaître, Superlatt. Microstruct. 47 (2010) 50–54. [13] P. Vasa, R. Poraenke, S. Schwieger, Y.I. Mazur, V. Kunets, P. Srinivasan, E. Johnson, E. Runge, G. Salamo, C. Lienau, Phys. Stat. Sol. C 6 (2009) 466–469. [14] A.E. Miroshnichenko, S. Flach, Y.S. Kivshar, Rev. Mod. Phys. 82 (2010) 2257– 2298. [15] K.K. Mishra, K. Rajeshwar, J. Electroanal. Chem. Interfacial Electrochem. 273 (1989) 169–182.
[16] R. Kowalik, K. Fitzner, Metall. Foundry Eng. 30 (2004) 129–141. _ ´ ski, K. Fitzner, Electrochim. Acta 53 (2008) 6184–6190. [17] R. Kowalik, P. Zabin [18] R. Kowalik, K. Fitzner, J. Electroanal. Chem. Interfacial Electrochem. 633 (2009) 78–84. [19] F.A. Kröger, J. Electrochem. Soc. 125 (1978) 2028–2034. [20] Y. Koide, T. Kawakami, M. Murakami, N. Teraguchi, Y. Tomomura, A. Suzuki, J. Electron. Mater. 27 (1998) 772–775. [21] M.A. Butler, D.S. Ginley, J. Electrochem. Soc. 125 (1978) 228–232. [22] K.P. Rice, E.J. Walker Jr., M.P. Stoykovich, A.E. Sauders, J. Phys. Chem. C 115 (2011) 1793–1799. [23] C. Tagenkamp, H. Pfnür, T. Langer, J. Baringhaus, W.H. Schumacher, J. Phys. Condens. Matter 23 (2011) 012001. [24] Q. Fu, T. Wagner, Surf. Sci. Rep. 62 (2007) 431–498. [25] W. Mönch, Rep. Prog. Phys. 53 (1990) 221–278. [26] J.O. McCaldin, T.C. McGill, Annu. Rev. Mater. Sci. 10 (1980) 65–83. [27] A. Zur, T.C. McGill, D.L. Smith, Phys. Rev. B 28 (1983) 2060–2067. [28] C. Noguera, G. Bordier, J. Phys. III France 4 (1994) 1851–1864. [29] V. Heine, Phys. Rev. 138 (1965) 1689–1696. [30] M. Achermann, J. Phys. Chem. Lett. 1 (2010) 2837–2843. [31] R. Bardhan, N.K. Grady, T. Ali, N.J. Halas, ACS Nano 4 (2010) 6169–6179. [32] D.E. Gómez, K.C. Vernon, P. Mulvaney, T.J. Davis, Nano Lett. 10 (2010) 274–278. [33] V. Dhanasekaran, T. Mahalingam, J.-K. Rhee, J.P. Chu, Opt. – Int. J. Light Electron. Opt. (2012), http://dx.doi.org/10.1016/j/ijleo.2011.11.063.
Contents lists available at SciVerse ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Short Communication
Photoelectrochemical study of ZnSe electrodeposition on Cu electrode _ ´ ski a Remigiusz Kowalik a,⇑, Konrad Szaciłowski a,b, Piotr Zabin a b
AGH University of Science and Technology, Faculty of Non-Ferrous Metals, Al. Mickiewicza 30, 30-059 Krakow, Poland Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland
a r t i c l e
i n f o
Article history: Received 20 January 2012 Received in revised form 1 March 2012 Accepted 3 March 2012 Available online 15 March 2012 Keywords: ZnSe Electrodeposition Cyclic voltammetry Photoelectrochemistry
a b s t r a c t The paper describes the photoelectrochemical studies and the electrochemical synthesis of zinc selenide thin films. Obtained results specify the range of potentials where semiconducting compound is deposited. Photocurrent profiles indicate the p-type conductivity of the electrodeposited semiconductor. The generated photocurrent indicates the anomalous absorption of the light below the band gap range of ZnSe and suggests a plasmon–exciton coupling effect. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Solar energy harvesting requires semiconducting layers with optical properties allowing the most efficient photon-to-electron conversion. Various approaches to extend the photosensibility window are applied, the most common include photosensitization (in dye-sensitized solar cells) [1,2] and band gap engineering in thin layer cells. Usually significant decrease in band gap energy requires application of highly toxic (e.g. CdSe) or expensive (e.g. CIGS) materials [3]. Furthermore, fabrication of high quality thin layers of these materials is not straightforward. Electrodeposition, however, is a promising method for low-cost production of semiconducting thin films [4–7]. In this communication we report photoelectrochemical properties of freshly deposited ZnSe thin layers on copper substrates. These layers exhibit unusual photosensitivity range and offer an alternative way to increase the efficiency of thin layer solar cells. This approach is based on surface plasmon resonance observed in coinage metals (Cu, Ag, Cu) [8]. While all metallic phases show plasmon phenomena, only in the case of metal mentioned above the resonant energy falls within the visible part of electromagnetic spectrum. Plasmonic excitations in gold nanoparticles are reported to induce photosensitization of wide band gap semiconductors due to damping process involving electron transfer from d-states of gold into conduction band of TiO2 [9–11]. In this report we propose a new approach towards increased efficiency of photoconversion. Appropriate selection of material
(both semiconductor and metal) allows efficient coupling between excitonic states of semiconductor with surface plasmons in metals [12,13] in the process known as Fano resonance [14]. This resonance results in intense sub-bandgap transitions which inject electrons into conduction band of semiconductors. 2. Experimental The three-electrode system was used for electrochemical measurements. The working electrode was copper sputtered on glass with the active surface of 1 cm2. Platinum foil was employed as the counter electrode and the Ag/AgCl electrode as a reference electrode. The electrochemical measurements were carried out using Autolab PGSTAT30 potentiostat and Modular LED Illuminator (Instytut Fotonowy) equipped with UNO Plus Array RGBA and QUATTRO Mini NIR (ENFIS) as a light source. The maximum energy of the diodes were: 1.96 eV, 2.38 eV, 2.67 eV, 2.08 eV and 1.43 eV. The nominal maximum power of the diodes were 12.5 W for the visible range and 200 W for NIR, the aperture was 1.15 cm2 (VIS) or 4 cm2 (NIR). The ca. 25% of maximum power of the diodes were used in each experiment and a light chopping sequence of 0.25 s on, 0.75 s off was used. The total reflectance spectra were measured on Lambda 950 (Perkin Elmer) spectrophotometer equipped with a 150 mm integrating sphere using both Spectralon and unmodified, freshly sputtered copper as reference samples. 3. Results and discussion
⇑ Corresponding author. Tel./fax: +48 12 617 36 11. E-mail address: [email protected] (R. Kowalik). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.03.002
The formation of the semiconducting deposit during electrochemical measurements can be directly detected due to
R. Kowalik et al. / Journal of Electroanalytical Chemistry 674 (2012) 108–112
109
Fig. 1. Cyclic photovoltammogram recorded for the copper substrate in the solution: (a) 0.001 M H2SeO3, and (b) 0.001 M H2SeO3, 0.1 M ZnSO4; scan rate 20 mV/s.
photocurrent generation [15]. The copper electrode was illuminated by chopped light of four diodes simultaneously. The chopped light induces the photo-effects easily noticeable at the I–E curve as characteristic spikes. The broad cathodic shoulder appeared just at the beginning of the scan when selenous acid is present in the solution (Fig. 1a). According to the previous studies [16–18] this cathodic effect originates from electrodeposition of selenium according to reaction (1):
H2 SeO3 þ 4Hþ þ 4e ¼ Se þ 3H2 O
ð1Þ
No photocurrent is observed at the voltammogram up to the potential 0.5 V, where the deposition of selenium proceeds according to reaction (1). When zinc ions are present in the solution the codeposition of zinc starts at the potential of 0.5 V and it can be observed as a second cathodic peak in Fig. 1b:
Se0 þ Zn2þ þ 2e ¼ ZnSe 2+
ð2Þ
The process of Zn reduction proceeds at a more positive potential than the equilibrium one due to interaction with selenium deposited on the copper electrode at more positive potentials. This
phenomenon is well known as induced co-deposition [19]. According to our previous experiments the bulk deposition of zinc starts below 1.1 V [16–18] and the scan was reversed at this potential in order to minimize the deposition of metallic zinc. The characteristic cathodic spikes appeared on illumination when the potential of the electrode is lower than 0.55 V. This effect indicates the co-deposition of zinc and synthesis of photoactive material, i.e. ZnSe. It can be observed that selenium layers deposited at higher potentials do not generate any photocurrent (Fig. 1a). The potential range where the photoresponse is observed perfectly coincides with the induced co-deposition of zinc at selenium and indicates the electrosynthesis of the ZnSe phase. The Schottky barrier of 1.1 eV height may be formed at the Cu–ZnSe interface [20]. Therefore the photocurrent polarity indicates p-type conductivity of the ZnSe layer. The intensity of the photocurrent increases when the scan is continued to the potential of 1.1 V. This increase is a consequence of continuous increase of the thickness of the deposit, which in turn results in increased absorbance of the film. Increasing band bending may also contribute to this effect. The photocurrent is present up to 0.3 V during scanning to the positive direction. The amplitude of the photocurrent spikes follows the Butler
110
R. Kowalik et al. / Journal of Electroanalytical Chemistry 674 (2012) 108–112
equation [21], therefore one can conclude that loss of photocurrent during anodic sweeps is due to decreased band bending at the solid–liquid interface. The disappearance of the photocurrent spikes on the voltammogram overlaps with the anodic peak which originates from the dissolution of the ZnSe phase. Illumination of the electrode during the synthesis of semiconductor layers enables to estimate the range of the potentials where the photoactive material is deposited. The copper electrode was illuminated with light of five different photon energies in order to identify the photosensitivity range of the deposit. Surprisingly, significant photocurrent generation was observed on illumination with visible light down to 1.96 eV (Fig. 2), while 1.43 eV illumination only generated very weak photocurrents (data not shown). Photocurrent generation was very efficient, the IPCE value reached 73% at 2.67 eV, but at 1.96 eV it was still significant. This indicates that not only excitation of fundamental transition can generate charge carriers in studied system. Normal semiconductors should generate net photocurrent only when incident photons have sufficient energy to promote electrons from the valence to the conduction band. Taking account the value of the zinc selenide band gap of 2.7 eV, the photocurrent generation should be possible only when the electrode is illuminated with light of higher energy. Surprisingly the photocurrent was recorded on each voltammogram independently on the incident light wavelength. Photoelectrochemical techniques can be also used to follow the electrodeposition kinetics. The copper electrode was illuminated with the blue diode during the potentiostatic deposition (Fig. 3). Applied potentials were chosen according to the voltammetry results. Three potentials were selected, where pure selenium is deposited at potential of 0.4 V, and codeposition of zinc and selenium is expected, namely 0.6 and 1.0 V. Similarly to the voltammetry experiments there was no photoresponse observed when the potential of 0.4 V was applied. This result indicates that the selenium deposit does not generate any net photocurrent upon excitation within the fundamental transition. On the contrary, when lower potentials, namely below 0.5 V are applied, the photoeffect is clearly observed. Obviously the photocurrent increases during the time of the deposition due to increasing of the thickness of the deposit. The photocurrent is higher when the potential 0.6 V is applied as compared to photocurrent recorded at
Fig. 2. (a) Linear sweep photovoltammogram recorded for the copper substrate illuminated with different light in the solution 0.001 M H2SeO3, 0.1 M ZnSO4; scan rate 20 mV/s; and (b) radiant power distribution and integrated output power of LEDs.
1.0 V, while the dark current is much higher in the latter case. These results are effect of competitive reactions occurring at the most negative potential. Except of the hydrogen evolution, the process of six-electron reduction of selenous acid and reduction of Se0 are also possible:
H2 SeO3 þ 6Hþ þ 6e ¼ H2 Se þ 3H2 O þ
Se þ 2H þ 2e ¼ H2 Se
ð3Þ ð4Þ
These reactions increase the dark current, but the photocurrent generated at lower potentials seems to be unaffected by competitive electrochemical processes. The advantage of the photovoltammetric results enable to estimate the most efficient range of potentials where the desired semiconducting phase is synthesised. The presented results are in very good agreement with our earlier studies about the electrodeposition of ZnSe under potentiostatic control, where phase analysis indicates the presence of the cubic ZnSe as the sole product of electrolysis [16,17]. In order to understand the anomalous photocurrent generation freshly deposited films were studied spectroscopically. First of all, all the films are dark blue colored, which is in contrary to the band gap of the material. Similar dark coloration was, however observed already for the Cu/Cu2O system obtained during slow oxidation of copper colloids [22]. Recorded UV–VIS–NIR reflectance spectra indicate significant light absorption within 200–1500 nm range (Fig. 4a). The spectrum shows very sharp and intense peak at 1.98 eV, which does not correlate neither with the fundamental transition of ZnSe nor with the surface plasmon peak of bulk copper (ca. 2.40 eV, Fig. 4b). Furthermore, the 1.98 eV peak has almost perfect Lorenzian envelope and is partially overlapped with lower intensity Gaussian component with maximum at 2.34 eV. The latter can be attributed to the excitonic transition of ZnSe. Therefore the 1.98 eV absorption should be associated with a resonant transition of two coupled oscillators, one of them representing the continuum of states while the other having a discrete energy spectrum located within the continuum of the former oscillator. Such a resonance, usually called the Fano resonance is observed in the case of electromagnetic wave scattering in nanostructured media [14]. In this particular case the unusual absorption features can be attributed to the efficient plasmon–exciton coupling, which affects not only the exciton lifetime, but also the spectral signature of the structure and its photoelectrochemical properties [13,23]. In this particular case the unusual absorption features can be attributed to the efficient plasmon–exciton coupling, which affects not only the exciton lifetime, but also the spectral signature of the structure and its photoelectrochemical properties [13,23]. In the case of ZnSe overlayer at Cu surface the excitonic levels are discrete, while the surface plasmon resonance represents the whole continuum of states (cf. Fig. 4b). The resonant absorption maybe further facilitated by image charge interactions and metal-induced gap states [24]. At the metal–semiconductor interface the Fermi levels of both materials have to equilibrate. At the interface the wavefunction of an electron in the semiconductor must match that of an electron in the metal. In the case of finite system the wavefunctions of electrons are altered and states that are forbidden within the bulk semiconductor gap are allowed at the surface [25–29]. These new states may be in turn involved in electronic transitions, which result in promotion of electrons to the conduction band. This resonant process allows electronic energy transfer between plasmonic excited states within metallic phase and excitonic excited states within semiconductor [30]. Similar processes have been observed for the Au/Cu2O [31] and Ag/CdSe [32] systems. Therefore the Fano resonance between coupled transitions at metal surface may explain anomalous light absorption and photocurrent in the long wavelength region and anomalous photocurrent generation on sub-band gap illumination. Thin (up to 100 nm
R. Kowalik et al. / Journal of Electroanalytical Chemistry 674 (2012) 108–112
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Fig. 3. I–t transients for the copper substrate recorded in the solution containing 0.001 M H2SeO3 and 0.1 M ZnSO4; (d) mechanism of photocurrent generation at ZnSe@Cu photoelectrodes.
Fig. 4. Total reflectance spectra of metallic copper and ZnSe@Cu (a) and corresponding Kubelka–Munk functions together with fitted Lorenzian resonance peak and Gaussian exciton peak (b).
according to quartz microbalance studies) films of zinc selenide studied in this paper can be regarded as quantum wells (under weak confinement regime), as both the electron and hole wave functions are confided in two dimensions, which in turn justifies observed plasmon–exciton coupling effect. Similar plasmonic effects in ZnSe were very recently reported by Dhanasekaran et al. [33]. This report confirms the importance of plasmonic phenomena at the conductor/semiconductor interface, and indicates that careful tuning of plasmon and exciton energies are necessary for efficient resonant process, which could not be observed on ITO surfaces due to energetic mismatch.
4. Concluding remarks The spectroscopic and photoelectrochemical studies presented in this paper supplement the previously reported data on the process of ZnSe electrosynthesis. Presented techniques facilitate the
analysis of the electrodeposition mechanism of semiconducting materials. New spectral features resulting from the Fano resonance between the surface plasmon within metal and exciton within semiconductor were observed. Interestingly, excitation of the metal–semiconductor systems within this resonance band result in generation of cathodic photocurrent. Acknowledgements The financial support from Polish Ministry of Education and Science under Contract Nos. DPN/N27/GDRE-GAMAS/2009, 694/NPOLONIUM/2010/0, 1609/B/H03/2009/36 and by AGH-UST under contract 11.11.180.509/11 is gratefully acknowledged. References [1] N. Robertson, Angew. Chem. Int. Ed. 45 (2006) 2338–2345. [2] N. Robertson, Angew. Chem. Int. Ed. 47 (2008) 1012–1014.
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