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Cadmium cathodic deposition on polycrystalline p-selenium: Dark and photoelectrochemical processes
Electrochimica Acta 56 (2011) 3562–3566
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Cadmium cathodic deposition on polycrystalline p-selenium: Dark and photoelectrochemical processes G.A. Ragoisha a,∗ , E.A. Streltsov b , S.M. Rabchynski b , D.K. Ivanou b a b
Research Institute for Physical Chemical Problems, Belarusian State University, Leningradskaya 14, Minsk 220030, Belarus Chemistry Department, Belarusian State University, Minsk 220030, Belarus
a r t i c l e
i n f o
Article history: Received 30 June 2010 Received in revised form 8 December 2010 Accepted 10 December 2010 Available online 21 December 2010 Keywords: Cadmium Selenium CdSe Underpotential deposition Potentiodynamic electrochemical impedance spectroscopy
a b s t r a c t Cathodic reduction of Cd2+ on p-Se proceeds at low overpotential in the dark and results in bulk Cd, while the underpotential deposition is kinetically inhibited. Cadmium adlayer is photoelectrochemically deposited on illuminated electrode 0.7 V above E(Cd2+ /Cd). The adlayer cathodic deposition under illumination proceeds with simultaneous formation of CdSe nanoparticles. Potentiodynamic electrochemical impedance spectroscopy has discriminated the two products of the photoelectrochemical reaction both by their potentials of anodic oxidation and by characteristic dependences of impedance on potential. Anodic oxidation of CdSe nanoparticles gives a sharp peak of real impedance in low frequencies close to the corresponding anodic current peak in cyclic voltammogram. The impedance peak appears below a threshold frequency ft . The latter separates two modes of diffusion in anodic dissolution of CdSe nanoparticles. The diffusion proceeds independently at different particles above ft and turns to cooperative mode below the threshold frequency. Due to this effect, information on spatial distribution of growing nuclei on electrode surface in early stages of electrodeposition can be obtained from potentiodynamic impedance spectra. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Cadmium and selenium codeposition has been widely used in electrosynthesis of CdSe [1–19]. The latter semiconductor is of special interest due to photovoltaic and optoelectronic applications [20–27]. CdSe electrodeposition is usually conducted from aqueous solution of cadmium salt (sulphate, chloride, nitrate, etc.) and selenium precursor such as H2 SeO3 , Na2 SeO3 , or Na2 SeSO3 . Cadmium and selenium codeposition can proceed both at underpotential [10–12] and at overpotential [18] relative to E(Cd2+ /Cd). Cadmium underpotential deposition (upd) on selenium has been applied as constituent process of electrochemical atomic layer epitaxy [28–34], with alternating deposition of Cd and Se atomic layers from different electrolyte solutions. Despite the use of cadmium and selenium codeposition and alternating deposition of Cd and Se atomic layers in CdSe electrosynthesis, cadmium adlayer growth mechanisms and kinetics on bulk selenium have not been investigated. Cd upd could be expected to proceed on selenium and other Se containing electrodes (CdSe, ZnSe and PbSe), due to strong Cd–Se atomic bonding and negative Gibbs energy of CdSe formation (f G ◦ 298 CdSe (solid) = − 143 kJ mol−1 [35]). In fact, adlayers of metals deposit on
∗ Corresponding author. E-mail address: [email protected] (G.A. Ragoisha). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.12.042
Se atomic layer and manifest irreversibility [36,37] in accord with the thermodynamic expectation. However, bulk selenium is p-type semiconductor with wide band gap (Eg = 1.9 eV for trigonal modification and Eg = 2.48 eV for monoclinic Se at 5 K [38]). The barrier attributed to the semiconductor surface space charge layer can prevent and control interfacial electron transfer in cathodic reactions on bulk Se [39,40], in particular Pb2+ upd is inhibited in the dark but Pb atomic layer can be cathodically deposited on bulk Se under illumination [40]. Though the strong Cd–Se atomic bonding favours the selenium affinity of accepting cadmium adlayer, the potential range of Cd upd is expected to be below the flat band potential of p-Se (Efb = +0.38 V, as evaluated in [40]), and this is unfavourable for electron transfer from p-Se to Cd2+ . On the other hand, the semiconductor photoexitation and a possible local inversion from p- to n-conductivity type can probably facilitate electron transfer from selenium to Cd2+ in specific potential ranges. So, one could expect different routes of Cd2+ cathodic reduction on selenium atomic layer (which is known to form selenium–metal bilayers by underpotential metal deposition [31,34,37]) and on bulk p-Se. Underpotential deposition and anodic oxidation of atomic layers and multilayers on metallic electrodes have been recently examined with potentiodynamic electrochemical impedance spectroscopy (PDEIS) [36,37,41,42]. PDEIS enables characterisation of the interfacial structure impedance constituents as functions of electrode potential, thus disclosing some interlayer interactions normally hidden in electrochemical examination. Due to the elec-
G.A. Ragoisha et al. / Electrochimica Acta 56 (2011) 3562–3566
troactive character of the system formed during Cd2+ cathodic reduction on selenium, the frequency response in alternating current acquired with PDEIS spectrometer as functions of variable electrode potential and time could be of interest for characterising the early stages of cadmium deposition. The goal of this work was to fill a gap in the knowledge of cadmium cathodic deposition on bulk p-Se concerning the atomic layer deposition in the dark and under illumination, using a combination of classical electrochemical techniques with PDEIS. The complementary goal of PDEIS application for the composite cathodic photodeposit characterisation was the development of a new approach to discriminate 2D and 3D nuclei by their different frequency responses in the anodic oxidation in the potentiodynamic mode. 2. Experimental Selenium film electrodes were prepared by cathodic deposition of Se on Au foil. The foil was polished using 0.7 m diamond paste, treated with concentrated H2 SO4 , washed with bidistilled water and finally heated at 600 ◦ C for 20 min in air. Selenium film was deposited from 1 M SeO2 + 9 M H2 SO4 aqueous solution (95 ◦ C) at 1 mA cm−2 . Se film thickness estimated by SEM was about 1 m. X-ray diffraction analysis showed that the Se film was polycrystalline (␥-Se phase). Current densities were calculated using geometric surface area. Electrochemical and photoelectrochemical measurements were performed in a three-electrode cell equipped with an optical quality window, a platinum counter-electrode and Ag/AgCl/KCl(sat.) reference electrode. Analytical-grade reagents and double distilled water were used in the electrolyte preparation. Most of the electrochemical measurements were carried out in 0.05 M H2 SO4 + 1 M Na2 SO4 and in 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 solutions which were deaerated with argon. Polychromatic light from a 50 W halogen lamp (5 mW cm−2 ) was used for the photoelectrochemical measurements. In the electrochemical characterization of cathodic deposit obtained under illumination, electrode was stirred shortly with Ar bubbling before anodic scan to remove the photoelectrochemically generated hydrogen selenide which could otherwise contribute to anodic current [40]. Electrochemical measurements in direct current were carried out with Gamry G300 potentiostat. A home-built impedance spectrometer specially designed for nonstationary measurements [41–45] was used for acquisition of potentiodynamic electrochemical impedance spectra (PDEIS) [43] and impedance spectra dependences on time. The 3D plots of variable frequency response obtained with these impedance techniques were presented as impedance dependences on potential and time in different frequencies. 3. Results and discussion Fig. 1 shows typical potentiodynamic curves of polycrystalline p-Se in acidified sulphate electrolyte solution with Cd2+ and without Cd2+ . Selenium behaves as nearly ideally polarisable electrode in the wide range of potential from −0.6 V to +0.8 V (curve 1). The cathodic current below −0.6 V results from molecular hydrogen evolution and selenium reduction to hydrogen selenide [39,40,46], the upper limit of the ideally polarisable range is due to selenium anodic dissolution [40,46], so the range of low current corresponds to the range of Se corrosion stability. Selenium behaves as typical p-type semiconductor under illumination in the intrinsic absorption range (curve 2). The cathodic photocurrent rises below the flat band potential (Efb = +0.38 V, as evaluated earlier [40]). The cathodic photocurrent is due to photogenerated charges separation in space-
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Fig. 1. Potentiodynamic voltammograms of Se in (1, 2) 0.05 M H2 SO4 + 1 M Na2 SO4 , (3) 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 acquired at 20 mV s−1 (1, 3) in the dark, (2) under chopped illumination.
charge region and the phoelectrons consumption for reduction of hydroxonium ions and selenium. Cd2+ addition to the electrolyte solution causes a strong decrease in the dark cathodic current below −0.6 V (curve 3 in Fig. 1). The passivity of selenium in the presence of Cd2+ is somewhat similar to Cd2+ inhibition in hydrogen evolution on iron [47] but the passivation is definitely not due to Cd2+ cathodic reduction. Cadmium reduction manifests itself as a reversible deposition of bulk phase with typical cathodic current loop close to the equilibrium potential E(Cd2+ /Cd) below −0.7 V followed by cadmium anodic oxidation peak in the anodic scan (curve 3). Our analysis of voltammograms obtained above E(Cd2+ /Cd) has shown no Cd upd on Se in the dark. A low and wide peak at 0.6 V in curve 3 was observed only on Se subjected to overpotential Cd deposition and the subsequent anodic oxidation of bulk Cd. This peak corresponds to CdSe which remains in very small quantity after anodic dissolution of Cd particles. The prevention of cadmium upd on selenium is evidently due to the space-charge region hindrance to the electron electron transfer from Se electrode to Cd2+ . We will show further that Cd atomic layer oxidation starts on Se at a little more negative potential to the flat band potential of selenium, so the range of the thermodynamic stability of cadmium adlayer is covered by the range of the cathodic current blocking by the semiconductor space-charge region. On the contrary, E(Cd2+ /Cd) is above 1 V more negative than Efb of selenium, so the electron energy that corresponds to the potential drop in the space-charge region turns to be approximately equal to half of the semiconductor band gap at E(Cd2+ /Cd), and this can be sufficient to cause local inversion of the conductivity type in the space-charge layer on Se surface. This correlations of energies and potentials may be the reason of bulk Cd phase unimpeded formation at low overpotential. Also, particles of bulk metal typically grow at sites of local energy minima on inhomogeneous semiconductor surface, which can give additional favour to electron tunneling through the space-charge layer in the cathodic reaction that produces metal particles. On the contrary, Cd upd would require electron tunneling through the space charge layer at any place of the surface. As this condition does not apply, the adlayer is not formed above E(Cd2+ /Cd). Cd2+ addition to the electrolyte causes also a strong decrease in the cathodic photocurrent (Fig. 2). This is a joint effect of electron–hole recombination, the surface limited character of Cd adlayer deposition considered below and also concurrent cathodic reactions suppression. The cathodic photocurrent (Fig. 3a) and impedance (Fig. 3b) are strongly nonstationary in the presence of Cd2+ , due to the surface limited character of Cd adlayer photodeposition. The anodic voltammogram of the product shows clearly two components with peak potentials A1 and A2 slightly variable and dependent on the potential and time of photodeposition (Fig. 4). Cd2+ photoreduction starts close to 0 V resulting predominantly in the product that gives A1 peak in the anodic voltammogram (Fig. 4a). The amount
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Fig. 2. Potentiodynamic voltammograms (cathodic scan, dE/dt = 20 mV s−1 ) of Se electrode under illumination in (1) 0.05 M H2 SO4 + 1 M Na2 SO4 ; (2) 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 .
of the photodeposit increases with time approaching the limit in the region of A1 peak. The maximum of surface limited deposition evaluated from anodic voltammograms is about 360 C cm−2 , which corresponds to a monolayer of cadmium. Using lower potential and longer time of the cathodic photodeposition results in the predominant growth of A2 peak (Fig. 4b) and the A1 peak envelopment of by A2 peak (Fig. 4c). The latter continues to grow after A1 peak reaches its limit. The open circuit potential of Se electrode before Cd adlayer cathodic photodeposition was close to 0.4 V, i.e. slightly higher than the potential of Cd adlayer anodic oxidation, in the dark and turned to be even more positive under illumination, thus there was no possibility for Cd2+ spontaneous reduction on selenium at the open circuit condition. The macroscopic product accumulated on illuminated selenium in the presence of soluble cadmium salt was identified earlier [46] as CdSe. Cadmium selenide normally accumulates due to the interaction of photochemically generated H2 Se with Cd2+ : Cd2+ + H2 Se = CdSe + 2H+ .
Fig. 3. (a) Chronoamperograms at different potentials, (b) imaginary Z(im) and real Z(re) impedance dependences on time at −0.1 V during cadmium adlayer cathodic photodeposition on selenium in 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 . Down and up arrows point the On and Off switching of the illumination. Each consecutive curve in the series shown on the dark background in (b) represents the ac response in one of 23 frequencies decreasing from 1750 Hz (the lowest curve) to 19 Hz (the highest curve).
Cadmium adlayer could be expected to contribute to CdSe production in early stages of the photodeposition, provided its interaction with selenium could be fast. However, our test of Cdad –CdSe system stability (Fig. 5) has disclosed fairly low reactivity of the adlayer. Cdad interaction with bulk selenium was not completed even after 15 h storing in the solution from which the adlayer had been photoelectrochemically deposited, and the adlayer showed good stability on the scale of few hours (the fractional charge of the anodic dissolution related to A1 peak showed considerable decrease just after 8 h of storing the deposit at open circuit condition, Fig. 5b). Fig. 6 represents schematically Cd2+ diffusion from the anodically dissolving Cdad –CdSe deposit at different frequencies of ac probing in impedance spectroscopy. The diffusion from cadmium adlayer should respond in alternating current as predominantly planar diffusion and show plain decrease of impedance with frequency like the one of Warburg element, while the diffusion from CdSe nuclei was expected to show specific effect at frequencies below a threshold frequency ft . The latter frequency corresponds to the shortest time of diffusion sufficient to cause interference of the diffusion fronts generated at adjacent CdSe nuclei (shown in the right part of Fig. 6). At high frequencies (fx > ft ) the perturbation applied on the diffusion by ac probing produces independent responses from different particles, while the frequency range below ft corresponds to the interfering events in diffusion. The counterrunning wave produces additional hindrance to diffusion, therefore the diffusion impedance should start to increase when the probing frequency decreases below ft and this effect is expected to become apparent in impedance data in the potential range of diffusion control of the anodic dissolution, i.e. on the right slope of the anodic peak in potentiodynamic voltammogram.
Fig. 4. Anodic voltammograms of the cathodic photodeposit obtained at different potentials and time on Se in 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 . dE/dt = 1.6 mV s−1 .
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Fig. 5. (a) Anodic voltammograms (dE/dt = 20 mV s−1 ) of the cathodic photodeposit obtained on Se in 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 (1 min at 0 V) and kept at open circuit condition for different times in the same solution, (b) variation of (1) total charge and fractional charges related to (2) A1 and (3) A2 peaks.
Fig. 6. Schematic illustration of diffusion in anodic dissolution of Cdad –CdSe deposit probed at ac frequencies fx and ft . The threshold frequency ft corresponds to the shortest period of oscillation at which the ac perturbations applied on mass transfer at adjacent CdSe particles start to interfere (shown in the right).
The cooperative diffusion was too complicated for the impedance spectra analysis with equivalent circuits, therefore we considered the impedance spectra variation on the qualitative level. The total impedance magnitude |Z| has been found to be not very informative in the respect of the diffusion impedance monitoring, because |Z| variation with the potential in this kind of anodically dissolving system almost reproduces the potential dependence of the dominating imaginary part of impedance Z(im) (Fig. 7a and b, imaginary impedance is shown inverted as −Z(im)). The imaginary impedance in the potential range of Cdad –CdSe anodic dissolution shows the pattern which resembles the voltammogram (Fig. 7b). The rapidly increasing absolute value of Z(im) above 0.55 V (at the right slope of the voltammetric anodic dissolution peak) portrays essentially the double layer capacitance decrease which results from CdSe dissolution.
Contrary to Z(im) which characterises mainly the electrode surface status during the anodic destruction of the Cdad –CdSe deposit, the real part of impedance Z(re) (Fig. 7c) bares an imprint of the diffusion effect that was discussed above. The potentiodynamic responses in real impedance show significantly different patterns below and above 50 Hz. The two patterns of the potentiodynamic response are separated in Fig. 7 by a narrow dark zone which substitutes the potentiodynamic curve in 50 Hz range (we omit the narrow frequency interval around 50 Hz in PDEIS to minimise induced effects of power supply and surrounding electric equipment in impedance measurement). The sharp Z(re) peak in low frequencies in the potential range of the right slope of the anodic voltammetric peak discloses the hindrance to diffusion which originates from the interference of diffusion flows emitted from adjacent dissolving CdSe nuclei. Cdad anodic dissolution shows a pattern below 0.45 V with a similar but strongly reduced feature. Edge effects in the diffusion from the dissolving fragments of cadmium adlayer (left part in Fig. 6) can only slightly contribute to the collective effect in diffusion impedance in low frequencies. Thus, the potentiodynamic curves obtained with PDEIS discriminate the two components of Cdad –CdSe deposit both by potential of anodic oxidation and by different patterns of real impedance. In view of complexity of equivalent circuit analysis in systems with collective effects in diffusion, the demonstrated possibility of geometric visualisation in raw impedance data of the structurally
Fig. 7. Orthogonal projections of typical PDEIS spectra that represent anodic dissolution of Cdad –CdSe deposit from Se surface in 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 as (a) impedance magnitude, (b) imaginary impedance (and anodic voltammogram overlaid), (c) real impedance variation in the anodic scan at 1.6 mV/s. Each consecutive curve in the series shown on the dark background represents the ac response in a different frequency decreasing from 702 Hz (the lowest curve) to 19 Hz (the highest curve). The deposit was obtained under illumination at potentiostatic condition (20 min at E = −0.1 V) in the same solution.
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dependent diffusion on electrode surface can be of interest for fast in situ characterisation of multicomponent deposits. The threshold frequency ft , which separates the patterns of individual and collective behaviour in diffusion, is an imprint of time required for the fronts of diffusion perturbations emitted from adjacent particles to meet and interfere. With further more detailed elaboration of the model this characteristic time can give interparticle distances and the sharpness of the potentiodynamic impedance pattern change with frequency can characterise uniformity of the particles distribution (the more uniform is the distribution the sharper is the transition to the collective mode in diffusion). This approach can be especially helpful in examination of unstable systems and nonequilibrium distributions of new phase nuclei on electrode surface when only in situ examination is allowed. 4. Conclusions This work has elucidated the specific role of semiconductor in Cd2+ cathodic reduction on p-Se, which prevents from Cd atomic layer deposition at underpotential conditions in the dark and allows a two-component Cdad –CdSe deposit growth under illumination. Despite the known affinity of selenium to form atomic bilayers and multilayers by metal upd in the electrochemical assembly started from Se atomic layer, the electron transfer control by semiconductor directs the reduction into the different routes on p-Se. Cathodic reduction of Cd2+ on p-Se proceeds at low overpotential in the dark and results in bulk Cd, while the underpotential deposition is kinetically inhibited, due to hindrance to electron transfer that results from space-charge layer in semiconductor. Under illumination, cadmium atomic layer is cathodically deposited 0.7 V above E(Cd2+ /Cd) and the adlayer deposition is accompanied by CdSe nucleation on Se surface. CdSe nuclei are formed as a result of Cd2+ interaction with photoelectrochemically generated hydrogen selenide. The anodic voltammogram of the photodeposit has shown two peaks. The peak close to 0.3 V shows the surface limited character and was attributed to cadmium adlayer, while the peak at 0.2 V higher potential was attributed to CdSe particles. Besides the strong difference (about 1 V) in the anodic oxidation potential from bulk cadmium, the cadmium adlayer shows a notable stability on selenium surface, it can be stored for few hours with very slow transformation into CdSe. Potentiodynamic electrochemical impedance spectroscopy has discriminated the two products of the photoelectrochemical reaction both by their potentials of anodic oxidation and by characteristic dependences of impedance on potential. Anodic oxidation of CdSe nanoparticles gives a sharp peak of real impedance in low frequencies close to the corresponding anodic current peak in cyclic voltammogram. The impedance peak appears below a threshold frequency (approximately 50 Hz) which separates two modes of diffusion in the anodic dissolution of CdSe nanoparticles. The diffusion from different particles proceeds independently above 50 Hz and turns to cooperative mode at lower frequency. The switching between the two modes of diffusion produces clear patterns in impedance data, when the latter are presented as functions of potential in different frequencies simultaneously. This new possibility of PDEIS is of interest for obtaining information on spatial distribution of growing nuclei on electrode surface in early stages of electrodeposition.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Cadmium cathodic deposition on polycrystalline p-selenium: Dark and photoelectrochemical processes G.A. Ragoisha a,∗ , E.A. Streltsov b , S.M. Rabchynski b , D.K. Ivanou b a b
Research Institute for Physical Chemical Problems, Belarusian State University, Leningradskaya 14, Minsk 220030, Belarus Chemistry Department, Belarusian State University, Minsk 220030, Belarus
a r t i c l e
i n f o
Article history: Received 30 June 2010 Received in revised form 8 December 2010 Accepted 10 December 2010 Available online 21 December 2010 Keywords: Cadmium Selenium CdSe Underpotential deposition Potentiodynamic electrochemical impedance spectroscopy
a b s t r a c t Cathodic reduction of Cd2+ on p-Se proceeds at low overpotential in the dark and results in bulk Cd, while the underpotential deposition is kinetically inhibited. Cadmium adlayer is photoelectrochemically deposited on illuminated electrode 0.7 V above E(Cd2+ /Cd). The adlayer cathodic deposition under illumination proceeds with simultaneous formation of CdSe nanoparticles. Potentiodynamic electrochemical impedance spectroscopy has discriminated the two products of the photoelectrochemical reaction both by their potentials of anodic oxidation and by characteristic dependences of impedance on potential. Anodic oxidation of CdSe nanoparticles gives a sharp peak of real impedance in low frequencies close to the corresponding anodic current peak in cyclic voltammogram. The impedance peak appears below a threshold frequency ft . The latter separates two modes of diffusion in anodic dissolution of CdSe nanoparticles. The diffusion proceeds independently at different particles above ft and turns to cooperative mode below the threshold frequency. Due to this effect, information on spatial distribution of growing nuclei on electrode surface in early stages of electrodeposition can be obtained from potentiodynamic impedance spectra. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Cadmium and selenium codeposition has been widely used in electrosynthesis of CdSe [1–19]. The latter semiconductor is of special interest due to photovoltaic and optoelectronic applications [20–27]. CdSe electrodeposition is usually conducted from aqueous solution of cadmium salt (sulphate, chloride, nitrate, etc.) and selenium precursor such as H2 SeO3 , Na2 SeO3 , or Na2 SeSO3 . Cadmium and selenium codeposition can proceed both at underpotential [10–12] and at overpotential [18] relative to E(Cd2+ /Cd). Cadmium underpotential deposition (upd) on selenium has been applied as constituent process of electrochemical atomic layer epitaxy [28–34], with alternating deposition of Cd and Se atomic layers from different electrolyte solutions. Despite the use of cadmium and selenium codeposition and alternating deposition of Cd and Se atomic layers in CdSe electrosynthesis, cadmium adlayer growth mechanisms and kinetics on bulk selenium have not been investigated. Cd upd could be expected to proceed on selenium and other Se containing electrodes (CdSe, ZnSe and PbSe), due to strong Cd–Se atomic bonding and negative Gibbs energy of CdSe formation (f G ◦ 298 CdSe (solid) = − 143 kJ mol−1 [35]). In fact, adlayers of metals deposit on
∗ Corresponding author. E-mail address: [email protected] (G.A. Ragoisha). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.12.042
Se atomic layer and manifest irreversibility [36,37] in accord with the thermodynamic expectation. However, bulk selenium is p-type semiconductor with wide band gap (Eg = 1.9 eV for trigonal modification and Eg = 2.48 eV for monoclinic Se at 5 K [38]). The barrier attributed to the semiconductor surface space charge layer can prevent and control interfacial electron transfer in cathodic reactions on bulk Se [39,40], in particular Pb2+ upd is inhibited in the dark but Pb atomic layer can be cathodically deposited on bulk Se under illumination [40]. Though the strong Cd–Se atomic bonding favours the selenium affinity of accepting cadmium adlayer, the potential range of Cd upd is expected to be below the flat band potential of p-Se (Efb = +0.38 V, as evaluated in [40]), and this is unfavourable for electron transfer from p-Se to Cd2+ . On the other hand, the semiconductor photoexitation and a possible local inversion from p- to n-conductivity type can probably facilitate electron transfer from selenium to Cd2+ in specific potential ranges. So, one could expect different routes of Cd2+ cathodic reduction on selenium atomic layer (which is known to form selenium–metal bilayers by underpotential metal deposition [31,34,37]) and on bulk p-Se. Underpotential deposition and anodic oxidation of atomic layers and multilayers on metallic electrodes have been recently examined with potentiodynamic electrochemical impedance spectroscopy (PDEIS) [36,37,41,42]. PDEIS enables characterisation of the interfacial structure impedance constituents as functions of electrode potential, thus disclosing some interlayer interactions normally hidden in electrochemical examination. Due to the elec-
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troactive character of the system formed during Cd2+ cathodic reduction on selenium, the frequency response in alternating current acquired with PDEIS spectrometer as functions of variable electrode potential and time could be of interest for characterising the early stages of cadmium deposition. The goal of this work was to fill a gap in the knowledge of cadmium cathodic deposition on bulk p-Se concerning the atomic layer deposition in the dark and under illumination, using a combination of classical electrochemical techniques with PDEIS. The complementary goal of PDEIS application for the composite cathodic photodeposit characterisation was the development of a new approach to discriminate 2D and 3D nuclei by their different frequency responses in the anodic oxidation in the potentiodynamic mode. 2. Experimental Selenium film electrodes were prepared by cathodic deposition of Se on Au foil. The foil was polished using 0.7 m diamond paste, treated with concentrated H2 SO4 , washed with bidistilled water and finally heated at 600 ◦ C for 20 min in air. Selenium film was deposited from 1 M SeO2 + 9 M H2 SO4 aqueous solution (95 ◦ C) at 1 mA cm−2 . Se film thickness estimated by SEM was about 1 m. X-ray diffraction analysis showed that the Se film was polycrystalline (␥-Se phase). Current densities were calculated using geometric surface area. Electrochemical and photoelectrochemical measurements were performed in a three-electrode cell equipped with an optical quality window, a platinum counter-electrode and Ag/AgCl/KCl(sat.) reference electrode. Analytical-grade reagents and double distilled water were used in the electrolyte preparation. Most of the electrochemical measurements were carried out in 0.05 M H2 SO4 + 1 M Na2 SO4 and in 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 solutions which were deaerated with argon. Polychromatic light from a 50 W halogen lamp (5 mW cm−2 ) was used for the photoelectrochemical measurements. In the electrochemical characterization of cathodic deposit obtained under illumination, electrode was stirred shortly with Ar bubbling before anodic scan to remove the photoelectrochemically generated hydrogen selenide which could otherwise contribute to anodic current [40]. Electrochemical measurements in direct current were carried out with Gamry G300 potentiostat. A home-built impedance spectrometer specially designed for nonstationary measurements [41–45] was used for acquisition of potentiodynamic electrochemical impedance spectra (PDEIS) [43] and impedance spectra dependences on time. The 3D plots of variable frequency response obtained with these impedance techniques were presented as impedance dependences on potential and time in different frequencies. 3. Results and discussion Fig. 1 shows typical potentiodynamic curves of polycrystalline p-Se in acidified sulphate electrolyte solution with Cd2+ and without Cd2+ . Selenium behaves as nearly ideally polarisable electrode in the wide range of potential from −0.6 V to +0.8 V (curve 1). The cathodic current below −0.6 V results from molecular hydrogen evolution and selenium reduction to hydrogen selenide [39,40,46], the upper limit of the ideally polarisable range is due to selenium anodic dissolution [40,46], so the range of low current corresponds to the range of Se corrosion stability. Selenium behaves as typical p-type semiconductor under illumination in the intrinsic absorption range (curve 2). The cathodic photocurrent rises below the flat band potential (Efb = +0.38 V, as evaluated earlier [40]). The cathodic photocurrent is due to photogenerated charges separation in space-
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Fig. 1. Potentiodynamic voltammograms of Se in (1, 2) 0.05 M H2 SO4 + 1 M Na2 SO4 , (3) 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 acquired at 20 mV s−1 (1, 3) in the dark, (2) under chopped illumination.
charge region and the phoelectrons consumption for reduction of hydroxonium ions and selenium. Cd2+ addition to the electrolyte solution causes a strong decrease in the dark cathodic current below −0.6 V (curve 3 in Fig. 1). The passivity of selenium in the presence of Cd2+ is somewhat similar to Cd2+ inhibition in hydrogen evolution on iron [47] but the passivation is definitely not due to Cd2+ cathodic reduction. Cadmium reduction manifests itself as a reversible deposition of bulk phase with typical cathodic current loop close to the equilibrium potential E(Cd2+ /Cd) below −0.7 V followed by cadmium anodic oxidation peak in the anodic scan (curve 3). Our analysis of voltammograms obtained above E(Cd2+ /Cd) has shown no Cd upd on Se in the dark. A low and wide peak at 0.6 V in curve 3 was observed only on Se subjected to overpotential Cd deposition and the subsequent anodic oxidation of bulk Cd. This peak corresponds to CdSe which remains in very small quantity after anodic dissolution of Cd particles. The prevention of cadmium upd on selenium is evidently due to the space-charge region hindrance to the electron electron transfer from Se electrode to Cd2+ . We will show further that Cd atomic layer oxidation starts on Se at a little more negative potential to the flat band potential of selenium, so the range of the thermodynamic stability of cadmium adlayer is covered by the range of the cathodic current blocking by the semiconductor space-charge region. On the contrary, E(Cd2+ /Cd) is above 1 V more negative than Efb of selenium, so the electron energy that corresponds to the potential drop in the space-charge region turns to be approximately equal to half of the semiconductor band gap at E(Cd2+ /Cd), and this can be sufficient to cause local inversion of the conductivity type in the space-charge layer on Se surface. This correlations of energies and potentials may be the reason of bulk Cd phase unimpeded formation at low overpotential. Also, particles of bulk metal typically grow at sites of local energy minima on inhomogeneous semiconductor surface, which can give additional favour to electron tunneling through the space-charge layer in the cathodic reaction that produces metal particles. On the contrary, Cd upd would require electron tunneling through the space charge layer at any place of the surface. As this condition does not apply, the adlayer is not formed above E(Cd2+ /Cd). Cd2+ addition to the electrolyte causes also a strong decrease in the cathodic photocurrent (Fig. 2). This is a joint effect of electron–hole recombination, the surface limited character of Cd adlayer deposition considered below and also concurrent cathodic reactions suppression. The cathodic photocurrent (Fig. 3a) and impedance (Fig. 3b) are strongly nonstationary in the presence of Cd2+ , due to the surface limited character of Cd adlayer photodeposition. The anodic voltammogram of the product shows clearly two components with peak potentials A1 and A2 slightly variable and dependent on the potential and time of photodeposition (Fig. 4). Cd2+ photoreduction starts close to 0 V resulting predominantly in the product that gives A1 peak in the anodic voltammogram (Fig. 4a). The amount
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Fig. 2. Potentiodynamic voltammograms (cathodic scan, dE/dt = 20 mV s−1 ) of Se electrode under illumination in (1) 0.05 M H2 SO4 + 1 M Na2 SO4 ; (2) 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 .
of the photodeposit increases with time approaching the limit in the region of A1 peak. The maximum of surface limited deposition evaluated from anodic voltammograms is about 360 C cm−2 , which corresponds to a monolayer of cadmium. Using lower potential and longer time of the cathodic photodeposition results in the predominant growth of A2 peak (Fig. 4b) and the A1 peak envelopment of by A2 peak (Fig. 4c). The latter continues to grow after A1 peak reaches its limit. The open circuit potential of Se electrode before Cd adlayer cathodic photodeposition was close to 0.4 V, i.e. slightly higher than the potential of Cd adlayer anodic oxidation, in the dark and turned to be even more positive under illumination, thus there was no possibility for Cd2+ spontaneous reduction on selenium at the open circuit condition. The macroscopic product accumulated on illuminated selenium in the presence of soluble cadmium salt was identified earlier [46] as CdSe. Cadmium selenide normally accumulates due to the interaction of photochemically generated H2 Se with Cd2+ : Cd2+ + H2 Se = CdSe + 2H+ .
Fig. 3. (a) Chronoamperograms at different potentials, (b) imaginary Z(im) and real Z(re) impedance dependences on time at −0.1 V during cadmium adlayer cathodic photodeposition on selenium in 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 . Down and up arrows point the On and Off switching of the illumination. Each consecutive curve in the series shown on the dark background in (b) represents the ac response in one of 23 frequencies decreasing from 1750 Hz (the lowest curve) to 19 Hz (the highest curve).
Cadmium adlayer could be expected to contribute to CdSe production in early stages of the photodeposition, provided its interaction with selenium could be fast. However, our test of Cdad –CdSe system stability (Fig. 5) has disclosed fairly low reactivity of the adlayer. Cdad interaction with bulk selenium was not completed even after 15 h storing in the solution from which the adlayer had been photoelectrochemically deposited, and the adlayer showed good stability on the scale of few hours (the fractional charge of the anodic dissolution related to A1 peak showed considerable decrease just after 8 h of storing the deposit at open circuit condition, Fig. 5b). Fig. 6 represents schematically Cd2+ diffusion from the anodically dissolving Cdad –CdSe deposit at different frequencies of ac probing in impedance spectroscopy. The diffusion from cadmium adlayer should respond in alternating current as predominantly planar diffusion and show plain decrease of impedance with frequency like the one of Warburg element, while the diffusion from CdSe nuclei was expected to show specific effect at frequencies below a threshold frequency ft . The latter frequency corresponds to the shortest time of diffusion sufficient to cause interference of the diffusion fronts generated at adjacent CdSe nuclei (shown in the right part of Fig. 6). At high frequencies (fx > ft ) the perturbation applied on the diffusion by ac probing produces independent responses from different particles, while the frequency range below ft corresponds to the interfering events in diffusion. The counterrunning wave produces additional hindrance to diffusion, therefore the diffusion impedance should start to increase when the probing frequency decreases below ft and this effect is expected to become apparent in impedance data in the potential range of diffusion control of the anodic dissolution, i.e. on the right slope of the anodic peak in potentiodynamic voltammogram.
Fig. 4. Anodic voltammograms of the cathodic photodeposit obtained at different potentials and time on Se in 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 . dE/dt = 1.6 mV s−1 .
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Fig. 5. (a) Anodic voltammograms (dE/dt = 20 mV s−1 ) of the cathodic photodeposit obtained on Se in 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 (1 min at 0 V) and kept at open circuit condition for different times in the same solution, (b) variation of (1) total charge and fractional charges related to (2) A1 and (3) A2 peaks.
Fig. 6. Schematic illustration of diffusion in anodic dissolution of Cdad –CdSe deposit probed at ac frequencies fx and ft . The threshold frequency ft corresponds to the shortest period of oscillation at which the ac perturbations applied on mass transfer at adjacent CdSe particles start to interfere (shown in the right).
The cooperative diffusion was too complicated for the impedance spectra analysis with equivalent circuits, therefore we considered the impedance spectra variation on the qualitative level. The total impedance magnitude |Z| has been found to be not very informative in the respect of the diffusion impedance monitoring, because |Z| variation with the potential in this kind of anodically dissolving system almost reproduces the potential dependence of the dominating imaginary part of impedance Z(im) (Fig. 7a and b, imaginary impedance is shown inverted as −Z(im)). The imaginary impedance in the potential range of Cdad –CdSe anodic dissolution shows the pattern which resembles the voltammogram (Fig. 7b). The rapidly increasing absolute value of Z(im) above 0.55 V (at the right slope of the voltammetric anodic dissolution peak) portrays essentially the double layer capacitance decrease which results from CdSe dissolution.
Contrary to Z(im) which characterises mainly the electrode surface status during the anodic destruction of the Cdad –CdSe deposit, the real part of impedance Z(re) (Fig. 7c) bares an imprint of the diffusion effect that was discussed above. The potentiodynamic responses in real impedance show significantly different patterns below and above 50 Hz. The two patterns of the potentiodynamic response are separated in Fig. 7 by a narrow dark zone which substitutes the potentiodynamic curve in 50 Hz range (we omit the narrow frequency interval around 50 Hz in PDEIS to minimise induced effects of power supply and surrounding electric equipment in impedance measurement). The sharp Z(re) peak in low frequencies in the potential range of the right slope of the anodic voltammetric peak discloses the hindrance to diffusion which originates from the interference of diffusion flows emitted from adjacent dissolving CdSe nuclei. Cdad anodic dissolution shows a pattern below 0.45 V with a similar but strongly reduced feature. Edge effects in the diffusion from the dissolving fragments of cadmium adlayer (left part in Fig. 6) can only slightly contribute to the collective effect in diffusion impedance in low frequencies. Thus, the potentiodynamic curves obtained with PDEIS discriminate the two components of Cdad –CdSe deposit both by potential of anodic oxidation and by different patterns of real impedance. In view of complexity of equivalent circuit analysis in systems with collective effects in diffusion, the demonstrated possibility of geometric visualisation in raw impedance data of the structurally
Fig. 7. Orthogonal projections of typical PDEIS spectra that represent anodic dissolution of Cdad –CdSe deposit from Se surface in 0.05 M CdSO4 + 0.05 M H2 SO4 + 1 M Na2 SO4 as (a) impedance magnitude, (b) imaginary impedance (and anodic voltammogram overlaid), (c) real impedance variation in the anodic scan at 1.6 mV/s. Each consecutive curve in the series shown on the dark background represents the ac response in a different frequency decreasing from 702 Hz (the lowest curve) to 19 Hz (the highest curve). The deposit was obtained under illumination at potentiostatic condition (20 min at E = −0.1 V) in the same solution.
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dependent diffusion on electrode surface can be of interest for fast in situ characterisation of multicomponent deposits. The threshold frequency ft , which separates the patterns of individual and collective behaviour in diffusion, is an imprint of time required for the fronts of diffusion perturbations emitted from adjacent particles to meet and interfere. With further more detailed elaboration of the model this characteristic time can give interparticle distances and the sharpness of the potentiodynamic impedance pattern change with frequency can characterise uniformity of the particles distribution (the more uniform is the distribution the sharper is the transition to the collective mode in diffusion). This approach can be especially helpful in examination of unstable systems and nonequilibrium distributions of new phase nuclei on electrode surface when only in situ examination is allowed. 4. Conclusions This work has elucidated the specific role of semiconductor in Cd2+ cathodic reduction on p-Se, which prevents from Cd atomic layer deposition at underpotential conditions in the dark and allows a two-component Cdad –CdSe deposit growth under illumination. Despite the known affinity of selenium to form atomic bilayers and multilayers by metal upd in the electrochemical assembly started from Se atomic layer, the electron transfer control by semiconductor directs the reduction into the different routes on p-Se. Cathodic reduction of Cd2+ on p-Se proceeds at low overpotential in the dark and results in bulk Cd, while the underpotential deposition is kinetically inhibited, due to hindrance to electron transfer that results from space-charge layer in semiconductor. Under illumination, cadmium atomic layer is cathodically deposited 0.7 V above E(Cd2+ /Cd) and the adlayer deposition is accompanied by CdSe nucleation on Se surface. CdSe nuclei are formed as a result of Cd2+ interaction with photoelectrochemically generated hydrogen selenide. The anodic voltammogram of the photodeposit has shown two peaks. The peak close to 0.3 V shows the surface limited character and was attributed to cadmium adlayer, while the peak at 0.2 V higher potential was attributed to CdSe particles. Besides the strong difference (about 1 V) in the anodic oxidation potential from bulk cadmium, the cadmium adlayer shows a notable stability on selenium surface, it can be stored for few hours with very slow transformation into CdSe. Potentiodynamic electrochemical impedance spectroscopy has discriminated the two products of the photoelectrochemical reaction both by their potentials of anodic oxidation and by characteristic dependences of impedance on potential. Anodic oxidation of CdSe nanoparticles gives a sharp peak of real impedance in low frequencies close to the corresponding anodic current peak in cyclic voltammogram. The impedance peak appears below a threshold frequency (approximately 50 Hz) which separates two modes of diffusion in the anodic dissolution of CdSe nanoparticles. The diffusion from different particles proceeds independently above 50 Hz and turns to cooperative mode at lower frequency. The switching between the two modes of diffusion produces clear patterns in impedance data, when the latter are presented as functions of potential in different frequencies simultaneously. This new possibility of PDEIS is of interest for obtaining information on spatial distribution of growing nuclei on electrode surface in early stages of electrodeposition.
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