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Electrodeposition of cadmium selenide
Materials Science in Semiconductor Processing 50 (2016) 43–48
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Electrodeposition of cadmium selenide Remigiusz Kowalik a,n, Honorata Kazimierczak b, Piotr Żabiński a a b
AGH University of Science and Technology, Faculty of Non-Ferrous Metals, al. A. Mickiewicza 30, 30-059 Krakow, Poland Institute of Metallurgy and Material Science, Polish Academy of Sciences, Reymonta 25, 30-059 Krakow, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 1 February 2016 Received in revised form 13 April 2016 Accepted 14 April 2016
In the present work the process of electrochemical synthesis of cadmium selenide from sulphate solutions has been examined. Electrode reactions taking place at co-deposition of cadmium and selenium were tested with the use of cyclic voltammetry. Two mechanisms of co-deposition of cadmium and selenium were suggested. Within the range of more positive potentials, the following two reactions take place: reduction of selenious acid to selenium, and then underpotential deposition of cadmium on selenium. The second mechanism proceeds within the potentials range below 0.4 V vs. SCE and relies on simultaneous reduction of ions of cadmium and selenious acid in a six-electron process. Next, the influence of selenious acid concentration and potential on the Cd–Se coatings deposition process on copper sheets was examined. The coatings were analysed with X-ray spectrofluorometry, electron scanning microscopy and X-ray diffraction. It was demonstrated that it is possible to obtain coatings stoichiometry corresponding to CdSe compound of hexagonal structure from sulphate electrolyte. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Electrodeposition Cadmium selenide Thin films Cyclic voltammetry Cadmium Selenium
1. Introduction Broadband semiconductors of II–VI types enjoy a lot of interest due to their application in electronics. They can be used as materials for constructing light-emitting diodes, semiconductor lasers or light detectors. Elements containing such semiconductors are used to produce devices for recording information, displays and flat television screens as well as heterojunction photovoltaic cells. So large possible applications of the materials result in a necessity to work out new methods of manufacturing semiconductor systems, and particularly, thin coatings of required parameters. Currently, many research centres are conducting tests to get to know the mechanisms and kinetics of semiconductor compounds synthesis with electrochemical methods, which should lead to elaborating a technology to obtain such coatings electrochemically. It should be emphasized that electrochemical methods of deposition of coatings are commonly used in case of metallic layers [1]. Their greatest strengths are: a possibility to cover large surfaces, also porous ones, high uniformity of the obtained coatings, a possibility to obtain coatings of desired thickness, high purity of the spread coatings, good adhesion of the coatings, high velocity of spreading the coatings, a possibility of precise control of the deposition process and, the most important, no need of considerable financial outlays. The wide possibilities of the electrochemical method are confirmed by several publications concerning synthesis of CdSe of different structures: 3D n
Corresponding author. E-mail address: [email protected] (R. Kowalik).
http://dx.doi.org/10.1016/j.mssp.2016.04.009 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
structures onto the silica arrays [2–4] or a polystyrene template [5], “Mulberry-like” Nanoclusters [6], self-assembled nanowires [7], nanowires in a polycarbonate template [8], micro and nanowires in alumina membrane [9,10], 2D CdSe nanopillar arrays [11], core–shell nanowire arrays [12], nanofiber film [13], tubular end-capped nanofibers [14], two-dimensional photonic crystals in a polymer template [15]. Modification of both the structure and surface of the obtained coatings allows a precise tuning in of optoelectronic properties of cadmium selenide. In case of CdSe electrodeposition the influence of different parameters of the electrolysis process on the possibility to obtain a stoichiometric compound was examined. The applied electrolytes were aqueous solutions, organic ones [16–18], ion liquids [19] and molten salts [20,21]. The substrate was gold [22–26], silver [26,27], nickel [28–30], titanium [28–31], InP [32], HOPG [33] or ITO [34]. In the present work cadmium selenide was obtained on a copper electrode. Sulphate solution was applied as the electrolyte. The influence of the electrolyte components concentration and its pH on a possibility of a stoichiometric compound synthesis were examined. Preliminary tests were performed on a gold electrode. The quality of the coatings was tested with the use of scanning electron microscopy. The elemental and phase composition got confirmed by X-ray spectrofluorometry and X-ray diffraction.
2. Experimental Aqueous solutions containing proper salts or acids originating from elements being components of the coatings were applied as
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the electrolyte. Acids or bases of ions common with the basic solution were used in order to assess pH of the solutions. Distilled water was applied to make the electrolyte. Voltammetric tests and potentiostatic deposition were conducted applying a classical three – electrode electrochemical vessel. It included a reference electrode – saturated calomel electrode, while the counter electrode was a platinum sheet of 12 cm2 surface. Gold or copper got used as the working electrode. Gold disks placed in a Teflon holder were applied for voltammetric tests. The electrodes were ground mechanically with sand paper of 320 gradation and then polished with diamond paste of 9 mm and 3 mm. The finishing polish was performed with a suspension of SiO2 0.04 mm. Before the measurement, the electrodes were flushed with distilled water and acetone. The electrodes surface was 0.196 cm2. The deposition of coatings were carried out only on copper sheets placed in a Teflon casing. Each sheet was mechanically prepared as well as gold disks, and then chemically polished in a mixture of HNO3 (analytical grade POCh), CH3COOH (analytical grade POCh) and H3PO4 (analytical grade POCh) of components proportions 1:1:1. Concentrated acids were used to make the solution. Before the measurement, the electrodes were flushed with distilled water and acetone. The electrode surface was 2.8 cm2. The time of deposition was 2 h for all experiments. Electrochemical tests were conducted with the use of PAR273A, Autolab PGSTAT30 and EDAQ EA 161 potentiostat/galvanostat. Voltammetric tests and potentiostatic deposition were carried out on them. The elemental composition of the obtained layers was examined and observation of the surface was performed in the Laboratory of Scanning Microscopy with Field Emission and Microanalysis in the Institute of Geological Sciences of the Jagiellonian University (HITACHI S-4700 and NORAN Vantage) and in the Laboratory of Electron Scanning Microscopy in the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences (SEM – PHILIPS XL30). Additionally, the samples were analysed with the method of X-ray fluorescence using a fluorescent spectrometer Rigaku Primini WDXRF. The elemental analysis carried out for the deposited coatings were concentrated on Cd and Se content in the coatings. Only percentage of cadmium is shown on graph for clarity. The phase analysis was examined by X-ray diffraction applying Rigaku Miniflex II diffractometer with Cu Kα radiation.
3. Results and discussion
Fig. 1. Cyclic voltammograms on gold electrode in different solutions, pH ¼ 2 at 20 mV/s.
Next, when scanning is performed towards positive potentials, there occurs a characteristic anodic peak, connected with dissolution of previously deposited cadmium. When voltammetric tests were conducted in a solution containing only selenious acid (IV), slight cathodic current is registered already from potential 0.2 V. It is likely to be connected with underpotential deposition of selenium on gold. It should be noticed that in researches on voltammograms presented in literature there appear characteristic peaks, perfectly visible [36–43], connected with underpotential deposition of selenium on gold. In the voltammogram presented in Fig. 1 only a slight increase of cathodic current is seen. The above effect can be linked to the type of gold electrode used during the tests and the method of preparing the surface before the experiment. In case of the cited works, the tests were carried out on electrodes obtained by sputtering gold on sheets of different types. In the results presented here, the function of the electrode was fulfilled by a polycrystalline gold disk. It can be assumed that the sputtered gold electrode demonstrates higher reactivity in relation to selenium than a disk electrode that previously was only mechanically polished. That is why not all subtleties connected with surface phenomena taking place in the system Au–H2SeO3 are so intensive. When scanning is continued towards negative potentials, a cathodic peak occurs at potential 0.3 V. The peak corresponds to the process of overpotential deposition of selenium following the reaction [39]:
Preliminary tests concerning a possibility of CdSe synthesis with an electrochemical method were conducted on a polycrystalline gold electrode. Fig. 1 presents cyclic voltammograms obtained in solutions containing, respectively, 0.1 M CdSO4, 0.001 M H2SeO3 and 0.1 M CdSO4 þ 0.001 M H2SeO3. Additionally, for comparative purposes, examinations in the base solution (H2SO4 pH ¼2) were also performed. When the solution contained only ions of cadmium, cathodic current occurs already from potential 0.1 V. It is probably connected with reduction of oxygen or underpotential deposition of cadmium on gold[35]. Next, an intensive increase of cathodic current is observed from potential 0.7 V. At the same time, two processes can take place here. Processes of hydrogen evolution and overpotential cadmium deposition begin:
From the potential 0.5 V another increase of cathodic current is observed that can be connected with the reactions[40,41]:
2H þ þ2e ¼H2
(1)
H2SeO3 þ6H þ þ 6e -H2Seþ3H2O
(4)
Cd2 þ þ2e ¼ Cd0
(2)
Seþ 2H þ þ2e -H2Se
(5)
H2SeO3 þ4H þ þ 4e -Se þ3H2O
(3)
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Below potential 0.7 V a process of intensive evolution of hydrogen begins following reaction (1). When the solution contains both dissolved cadmium and selenium, the mechanism of electrode reactions gets altered. A typical effect is occurrence of cathodic peak already at potential 0.2 V (peak A), in comparison to a voltammogram obtained in a solution containing only selenious acid (IV). Both an increase of peak maximum and its displacement towards more positive potentials can indicate a beginning of the process of co-deposition of cadmium with selenium. The process is initiated by underpotential deposition of selenium on gold. Next, underpotential deposition of cadmium on selenium takes place, which can be described by the reaction: Se0 þ Cd2 þ þ 2e ¼Cd0
(6)
The process of underpotential deposition of cadmium on selenium is forced by a negative value of Gibbs free energy of formation the compound CdSe ΔG ¼147 kJ/mol [44]. The mechanism has been applied, among others, for synthesis of semiconductor compounds with the method ECALD [45–47]. When the potential is still changed towards more negative values, another peak occurs at potential 0.55 V (peak B) indicating a beginning of the process of co-deposition of selenium and cadmium by a different mechanism. Within the same range of potentials a reduction of selenium takes place following reaction (4). On the basis of studies carried out by Mishra[48] it can be assumed that the process of co-deposition of cadmium and selenium proceeds following the reaction: H2SeO3 þ Cd2 þ þ 4H þ þ6e -CdSe þ3H2O
(7)
The voltammetric experiment was repeated for copper electrode in the solution containing selenium and cadmium ions. The carrying out of the voltammogram on copper was impeded due to limited electrochemical window from the positive side because of intensive dissolution of copper in acidic solutions above potential 0.0 V vs. SCE. The cathodic current starts flow at the beginning of voltammetry scan at 0.0 V. It can be assumed that process of selenium deposition starts at 0.0 V according to the reaction (3). Following the process of cadmium underpotential deposition is started immediately after the selenium is deposited at potential 0.1 V according to the reaction (6). Both processes overlap each other and in effect it is manifested in convoluted shape of the cathodic peak at potential 0.15 V (C). The position of the peak C covers with the position of the peak A at voltammogram obtained on the gold electrode. Although the rate of the reduction reactions related with selenium and cadmium deposition according to reactions (3)–(6) is faster on copper then on gold electrode. It is related with the nature of the substrate. As it was mentioned by Stickney [36–38] process of selenium deposition on gold is very sluggish and, by extension, the underpotential deposition of cadmium on selenium (6) is slower too. In turn selenium deposited on copper is faster and therefore the total cathodic current related with the selenium and cadmium co-deposition according to the mechanism (3)–(6) is higher. When the scan is continuing to more negative potentials the peak D appeared and it is probably related with co-deposition of selenium and cadmium according to the mechanism (7). The position of the peak D is similarly to the peak B at the voltammogram obtained on the gold electrode and again intensity for “copper” peak is higher than for “gold” one. Moreover when cyclic voltammograms obtained on gold and copper electrode are compered it is clearly seen that hydrogen evolution starts at 0.75 V for gold electrode and it is moved to more negative potentials when copper electrode was used. Additionally an extra peak (E) is observed on the voltammogram obtained on copper electrode as compared with the
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voltammogram obtained on gold. This peak is related with the bulk deposition of cadmium according to reaction (2). As it has been mentioned earlier, the peak related with bulk deposition of cadmium is not observed on gold electrode due to intensive hydrogen evolution. On the basis of the conducted voltammetric tests the process of deposition of Cd–Se coatings was realised at different potentials from electrolytes of different concentration of selenious acid (IV). The process of deposition of semiconductor coatings was performed on copper sheets. Application of copper as the substrate seems to be a very promising solution due to high overpotential of hydrogen evolution that at the initial phase of Cd–Se coating creation can inhibit the process of hydrogen reduction. Too intensive hydrogen evolution can lower efficiency of the electrolysis process and additionally, it can favour reaction (5). It can result in unfavourable changes in composition of the obtained coatings as well as their morphology [3,4,49,50]. Moreover, according to the voltammetry tests the rate of the reduction reactions related with co-deposition of selenium and cadmium are faster than for gold electrode. Besides a possibility of creating a thin layer of Cu2Se can favour better adhesion of the deposited coatings and at the same time perform the function of the back layer in heterojunction solar cells, like Cu2Te joined with CdTe[51,52]. The coatings elemental analysis shows that within potentials from 0.4 to 0.7 V the composition of the obtained coatings was very close to CdSe stoichiometry when the process was conducted from a solution of 0.001 M H2SeO3 concentration (Fig. 2 Cyclic voltammograms on gold (black) and copper (red) electrode in 0.001 M H2SeO3, 0.1 CdSO4, pH ¼2 at 20 mV/s). Fig. 3. An increase of the applied potential favours deposition of a higher amount of selenium. Additionally, the deposition process is slower than for application of lower potentials (Fig. 4). On the basis of voltammetric tests it can be deduced that the process of co-deposition of selenium and cadmium follows the mechanism (6) to potential 0.4 V. Despite much higher concentration of cadmium ions in the solution, the process of selenium reduction is faster than the subsequent process of underpotential deposition of cadmium on selenium, therefore selenium surplus occurs in the coating. The process of electrolysis in solutions containing concentrations of selenious acid higher than 0.001 M was only performed from potential 0.4 V. Below the potential the process of co-deposition additionally begins to proceed by the mechanism (7) which starts to compete with the two-stage mechanism following reactions (3) and (6). It can be observed that an increase of selenious acid concentration in the electrolyte systematically
Fig. 2. Cyclic voltammograms on gold (black) and copper (red) electrode in 0.001 M H2SeO3, 0.1 CdSO4, pH¼2 at 20 mV/s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. The effect of applied potential and H2SeO3 concentration on cadmium content in the deposits. Plating bath: CdSO4 0.1 M, pH ¼ 2.
Fig. 4. The mass increase of the electrodes registered after deposition of coatings from solutions containing different concentrations of H2SeO3, pH¼ 2.
increases the content of selenium in deposit if coatings obtained at the same potentials are compared. It is particularly seen when electrolysis was performed at potential 0.4 V in a solution of the highest concentration of selenious acid. Moreover, there is a danger that an increased amount of selenium will appear in the deposit in connection with a possibility of a synproportionation reaction: H2SeO3 þ2H2Se-3Se þ3H2O
(8)
However, it should be noticed that despite applying very low potentials in solutions of higher concentrations of selenious acid, the reaction (8) practically does not take place. It is proved by the fact that the difference in content of cadmium in the coatings does not exceed 5% at., when electrolytes with different concentrations of H2SeO3 from 0.001 to 0.008 M were applied. It should be mentioned that an increase of H2SeO3 concentration accelerates the process of Cd–Se coatings deposition, but only if potential 0.7 V is applied. The results can indicate that reaction (7) is controlled by diffusion of H2SeO3 molecules to the electrode surface. In case of more positive potentials, despite increased concentration of selenious acid in a solution up to 0.008 M the deposition process gets inhibited in relation to solutions of lower concentrations. It is clearly seen when the deposition process was conducted at potential 0.4 V where the process of deposition from a solution of 0.008 M concentration of H2SeO3 is slower even than in the case when the solution contained only 0.002 M of selenious acid. The above effect can be explained by the fact that
an increase of H2SeO3 concentration in the electrolyte is accompanied by move of the range of co-deposition of cadmium and selenium following the reactions (3) and (6) towards more negative potentials and thus it inhibits the reaction (7). An additional effect is lowering the content of cadmium in the deposit as the reaction (6) is slower than reaction (3) which can be observed in case of applying more positive potentials than 0.4 V and in a solution containing only 0.001 M H2SeO3. When potential 0.8 V was applied, the content of cadmium in deposit exceeded 70% at. In such low potentials the overpotential deposition of cadmium takes place following reaction (2). High content of cadmium in coatings results from much higher concentration of cadmium ions in relation to selenious acid. According to the mass increase the rate of deposition varies from 0.1 to 0.2 mg/cm2/h when the selenius acid concentration was 0.001 M in the electrolyte for samples with compositions close to stoichiometry of CdSe compound (potential of deposition from 0.4 to 0.7 V). The rate of deposition increases up to around 0.5 mg/cm2/h for sample obtained at potential 0.7 V from solution with 0.008 M H2SeO3. If it assumed that only CdSe phase is present in the deposit the thickness of the coating is around 1.7 mm after 2 h of deposition. The phase analysis of the samples clearly indicates presence of an intermetallic compound CdSe of hexagonal structure, although the analysis was difficult due to the use of copper substrate (Fig. 5). Peaks characteristic for CdSe phase are visible within angles range 2Θ ¼24.26°, 2Θ ¼25.77° and 2Θ ¼ 42.69° following the JCPDS 01075-5679 card. The obtained coatings were so thin that peaks originating from the copper sheet also appeared on the diffractogram at 43.30° and 50.43° (JCPDS 00-004-0836). In consequence the copper peaks overlap with peaks related with CdSe phase at 42.29° and 50.54° (JCPDS 01-075-5679) in particular. Additionally small peak is visible at 13° for sample obtained from solution with 0.004 H2SeO3 and it fits only to Cu2Se phase. It seems that during process of deposition the reaction between the deposited selenium and copper is possible. The peaks obtained in the range from 20° to 30° are very wide and overlap to each other. The shape of the peaks imply the small grain size or even an amorphous structure of the deposited CdSe. Morphology of the obtained coatings was examined with the use of a scanning electron microscope (Fig. 6). The surface of coatings obtained within potentials range from 0.4 to 0.7 V is very similar and does not depend on the concentration of selenious acid in the electrolyte. The coatings are grey and they feature metallic lustre. There is no dendrites or fringe patterns appeared on the surface of films. They are compact and homogeneous, well adhere to the copper substrate, and they do not crack or fall off the substrate. Whereas, coatings obtained at potential 0.8 V feature
Fig. 5. XRD patterns of the Cd–Se coatings deposited from the electrolyte 0.1 M CdSO4, pH¼2 at potential 0.7 V vs. SCE with different concentration of H2SeO3.
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process (3) takes place, then cadmium is deposited on previously deposited selenium during underpotential deposition (6). The second mechanism occurs within the range of more negative potentials and it relies on simultaneous reduction of selenium and cadmium following reaction (7). On the basis of the obtained voltammetric results, the process of deposition of Cd–Se coatings on copper sheets was conducted. Within the potentials range from 0.4 to 0.7 V the coatings are characterised by a composition close to stoichiometry of CdSe compound. In case of applying potentials above 0.4 V, the amount of selenium in deposit increases indicating that reaction (6) is lower than reaction (3), despite much higher concentration of cadmium ions than selenious acid in the electrolyte. Whereas, after exceeding potential 0.8, overpotential deposition of cadmium takes place and the content of metal in deposit increases. The coatings obtained within potentials range from 0.4 to 0.7 V were homogeneous and they well adhered to the substrate. The featured grey colour and metallic lustre. An analysis using X-ray diffraction confirmed presence of polycrystalline CdSe phase of hexagonal structure.
Acknowledgments This work was supported by the Polish National Centre of Science under Grant 2011/01/D/ST5/05743.
References
Fig. 6. SEM images of the coatings deposited from the electrolyte 0.1 M CdSO4, pH¼ 2 at different potentials: (a) 0.4, (b) 0.6, (c) 0.8 V vs. SCE.
dendritic structure and they are covered by numerous burls. The coatings were brittle and some dendritic burls fell off when the samples were being removed from the electrolyte. That explains significant differences in mass growth of the examined samples obtained at potential 0.8 V from solutions of different concentration of selenious acid (Fig. 4).
4. Conclusions The presented voltammetric tests show a possibility of co-deposition of selenium and cadmium in a sulphate solution of pH ¼ 2 following two mechanisms. Within the range of more positive potentials, at first a reduction of selenious acid in a four-electron
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Electrodeposition of cadmium selenide Remigiusz Kowalik a,n, Honorata Kazimierczak b, Piotr Żabiński a a b
AGH University of Science and Technology, Faculty of Non-Ferrous Metals, al. A. Mickiewicza 30, 30-059 Krakow, Poland Institute of Metallurgy and Material Science, Polish Academy of Sciences, Reymonta 25, 30-059 Krakow, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 1 February 2016 Received in revised form 13 April 2016 Accepted 14 April 2016
In the present work the process of electrochemical synthesis of cadmium selenide from sulphate solutions has been examined. Electrode reactions taking place at co-deposition of cadmium and selenium were tested with the use of cyclic voltammetry. Two mechanisms of co-deposition of cadmium and selenium were suggested. Within the range of more positive potentials, the following two reactions take place: reduction of selenious acid to selenium, and then underpotential deposition of cadmium on selenium. The second mechanism proceeds within the potentials range below 0.4 V vs. SCE and relies on simultaneous reduction of ions of cadmium and selenious acid in a six-electron process. Next, the influence of selenious acid concentration and potential on the Cd–Se coatings deposition process on copper sheets was examined. The coatings were analysed with X-ray spectrofluorometry, electron scanning microscopy and X-ray diffraction. It was demonstrated that it is possible to obtain coatings stoichiometry corresponding to CdSe compound of hexagonal structure from sulphate electrolyte. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Electrodeposition Cadmium selenide Thin films Cyclic voltammetry Cadmium Selenium
1. Introduction Broadband semiconductors of II–VI types enjoy a lot of interest due to their application in electronics. They can be used as materials for constructing light-emitting diodes, semiconductor lasers or light detectors. Elements containing such semiconductors are used to produce devices for recording information, displays and flat television screens as well as heterojunction photovoltaic cells. So large possible applications of the materials result in a necessity to work out new methods of manufacturing semiconductor systems, and particularly, thin coatings of required parameters. Currently, many research centres are conducting tests to get to know the mechanisms and kinetics of semiconductor compounds synthesis with electrochemical methods, which should lead to elaborating a technology to obtain such coatings electrochemically. It should be emphasized that electrochemical methods of deposition of coatings are commonly used in case of metallic layers [1]. Their greatest strengths are: a possibility to cover large surfaces, also porous ones, high uniformity of the obtained coatings, a possibility to obtain coatings of desired thickness, high purity of the spread coatings, good adhesion of the coatings, high velocity of spreading the coatings, a possibility of precise control of the deposition process and, the most important, no need of considerable financial outlays. The wide possibilities of the electrochemical method are confirmed by several publications concerning synthesis of CdSe of different structures: 3D n
Corresponding author. E-mail address: [email protected] (R. Kowalik).
http://dx.doi.org/10.1016/j.mssp.2016.04.009 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
structures onto the silica arrays [2–4] or a polystyrene template [5], “Mulberry-like” Nanoclusters [6], self-assembled nanowires [7], nanowires in a polycarbonate template [8], micro and nanowires in alumina membrane [9,10], 2D CdSe nanopillar arrays [11], core–shell nanowire arrays [12], nanofiber film [13], tubular end-capped nanofibers [14], two-dimensional photonic crystals in a polymer template [15]. Modification of both the structure and surface of the obtained coatings allows a precise tuning in of optoelectronic properties of cadmium selenide. In case of CdSe electrodeposition the influence of different parameters of the electrolysis process on the possibility to obtain a stoichiometric compound was examined. The applied electrolytes were aqueous solutions, organic ones [16–18], ion liquids [19] and molten salts [20,21]. The substrate was gold [22–26], silver [26,27], nickel [28–30], titanium [28–31], InP [32], HOPG [33] or ITO [34]. In the present work cadmium selenide was obtained on a copper electrode. Sulphate solution was applied as the electrolyte. The influence of the electrolyte components concentration and its pH on a possibility of a stoichiometric compound synthesis were examined. Preliminary tests were performed on a gold electrode. The quality of the coatings was tested with the use of scanning electron microscopy. The elemental and phase composition got confirmed by X-ray spectrofluorometry and X-ray diffraction.
2. Experimental Aqueous solutions containing proper salts or acids originating from elements being components of the coatings were applied as
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the electrolyte. Acids or bases of ions common with the basic solution were used in order to assess pH of the solutions. Distilled water was applied to make the electrolyte. Voltammetric tests and potentiostatic deposition were conducted applying a classical three – electrode electrochemical vessel. It included a reference electrode – saturated calomel electrode, while the counter electrode was a platinum sheet of 12 cm2 surface. Gold or copper got used as the working electrode. Gold disks placed in a Teflon holder were applied for voltammetric tests. The electrodes were ground mechanically with sand paper of 320 gradation and then polished with diamond paste of 9 mm and 3 mm. The finishing polish was performed with a suspension of SiO2 0.04 mm. Before the measurement, the electrodes were flushed with distilled water and acetone. The electrodes surface was 0.196 cm2. The deposition of coatings were carried out only on copper sheets placed in a Teflon casing. Each sheet was mechanically prepared as well as gold disks, and then chemically polished in a mixture of HNO3 (analytical grade POCh), CH3COOH (analytical grade POCh) and H3PO4 (analytical grade POCh) of components proportions 1:1:1. Concentrated acids were used to make the solution. Before the measurement, the electrodes were flushed with distilled water and acetone. The electrode surface was 2.8 cm2. The time of deposition was 2 h for all experiments. Electrochemical tests were conducted with the use of PAR273A, Autolab PGSTAT30 and EDAQ EA 161 potentiostat/galvanostat. Voltammetric tests and potentiostatic deposition were carried out on them. The elemental composition of the obtained layers was examined and observation of the surface was performed in the Laboratory of Scanning Microscopy with Field Emission and Microanalysis in the Institute of Geological Sciences of the Jagiellonian University (HITACHI S-4700 and NORAN Vantage) and in the Laboratory of Electron Scanning Microscopy in the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences (SEM – PHILIPS XL30). Additionally, the samples were analysed with the method of X-ray fluorescence using a fluorescent spectrometer Rigaku Primini WDXRF. The elemental analysis carried out for the deposited coatings were concentrated on Cd and Se content in the coatings. Only percentage of cadmium is shown on graph for clarity. The phase analysis was examined by X-ray diffraction applying Rigaku Miniflex II diffractometer with Cu Kα radiation.
3. Results and discussion
Fig. 1. Cyclic voltammograms on gold electrode in different solutions, pH ¼ 2 at 20 mV/s.
Next, when scanning is performed towards positive potentials, there occurs a characteristic anodic peak, connected with dissolution of previously deposited cadmium. When voltammetric tests were conducted in a solution containing only selenious acid (IV), slight cathodic current is registered already from potential 0.2 V. It is likely to be connected with underpotential deposition of selenium on gold. It should be noticed that in researches on voltammograms presented in literature there appear characteristic peaks, perfectly visible [36–43], connected with underpotential deposition of selenium on gold. In the voltammogram presented in Fig. 1 only a slight increase of cathodic current is seen. The above effect can be linked to the type of gold electrode used during the tests and the method of preparing the surface before the experiment. In case of the cited works, the tests were carried out on electrodes obtained by sputtering gold on sheets of different types. In the results presented here, the function of the electrode was fulfilled by a polycrystalline gold disk. It can be assumed that the sputtered gold electrode demonstrates higher reactivity in relation to selenium than a disk electrode that previously was only mechanically polished. That is why not all subtleties connected with surface phenomena taking place in the system Au–H2SeO3 are so intensive. When scanning is continued towards negative potentials, a cathodic peak occurs at potential 0.3 V. The peak corresponds to the process of overpotential deposition of selenium following the reaction [39]:
Preliminary tests concerning a possibility of CdSe synthesis with an electrochemical method were conducted on a polycrystalline gold electrode. Fig. 1 presents cyclic voltammograms obtained in solutions containing, respectively, 0.1 M CdSO4, 0.001 M H2SeO3 and 0.1 M CdSO4 þ 0.001 M H2SeO3. Additionally, for comparative purposes, examinations in the base solution (H2SO4 pH ¼2) were also performed. When the solution contained only ions of cadmium, cathodic current occurs already from potential 0.1 V. It is probably connected with reduction of oxygen or underpotential deposition of cadmium on gold[35]. Next, an intensive increase of cathodic current is observed from potential 0.7 V. At the same time, two processes can take place here. Processes of hydrogen evolution and overpotential cadmium deposition begin:
From the potential 0.5 V another increase of cathodic current is observed that can be connected with the reactions[40,41]:
2H þ þ2e ¼H2
(1)
H2SeO3 þ6H þ þ 6e -H2Seþ3H2O
(4)
Cd2 þ þ2e ¼ Cd0
(2)
Seþ 2H þ þ2e -H2Se
(5)
H2SeO3 þ4H þ þ 4e -Se þ3H2O
(3)
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Below potential 0.7 V a process of intensive evolution of hydrogen begins following reaction (1). When the solution contains both dissolved cadmium and selenium, the mechanism of electrode reactions gets altered. A typical effect is occurrence of cathodic peak already at potential 0.2 V (peak A), in comparison to a voltammogram obtained in a solution containing only selenious acid (IV). Both an increase of peak maximum and its displacement towards more positive potentials can indicate a beginning of the process of co-deposition of cadmium with selenium. The process is initiated by underpotential deposition of selenium on gold. Next, underpotential deposition of cadmium on selenium takes place, which can be described by the reaction: Se0 þ Cd2 þ þ 2e ¼Cd0
(6)
The process of underpotential deposition of cadmium on selenium is forced by a negative value of Gibbs free energy of formation the compound CdSe ΔG ¼147 kJ/mol [44]. The mechanism has been applied, among others, for synthesis of semiconductor compounds with the method ECALD [45–47]. When the potential is still changed towards more negative values, another peak occurs at potential 0.55 V (peak B) indicating a beginning of the process of co-deposition of selenium and cadmium by a different mechanism. Within the same range of potentials a reduction of selenium takes place following reaction (4). On the basis of studies carried out by Mishra[48] it can be assumed that the process of co-deposition of cadmium and selenium proceeds following the reaction: H2SeO3 þ Cd2 þ þ 4H þ þ6e -CdSe þ3H2O
(7)
The voltammetric experiment was repeated for copper electrode in the solution containing selenium and cadmium ions. The carrying out of the voltammogram on copper was impeded due to limited electrochemical window from the positive side because of intensive dissolution of copper in acidic solutions above potential 0.0 V vs. SCE. The cathodic current starts flow at the beginning of voltammetry scan at 0.0 V. It can be assumed that process of selenium deposition starts at 0.0 V according to the reaction (3). Following the process of cadmium underpotential deposition is started immediately after the selenium is deposited at potential 0.1 V according to the reaction (6). Both processes overlap each other and in effect it is manifested in convoluted shape of the cathodic peak at potential 0.15 V (C). The position of the peak C covers with the position of the peak A at voltammogram obtained on the gold electrode. Although the rate of the reduction reactions related with selenium and cadmium deposition according to reactions (3)–(6) is faster on copper then on gold electrode. It is related with the nature of the substrate. As it was mentioned by Stickney [36–38] process of selenium deposition on gold is very sluggish and, by extension, the underpotential deposition of cadmium on selenium (6) is slower too. In turn selenium deposited on copper is faster and therefore the total cathodic current related with the selenium and cadmium co-deposition according to the mechanism (3)–(6) is higher. When the scan is continuing to more negative potentials the peak D appeared and it is probably related with co-deposition of selenium and cadmium according to the mechanism (7). The position of the peak D is similarly to the peak B at the voltammogram obtained on the gold electrode and again intensity for “copper” peak is higher than for “gold” one. Moreover when cyclic voltammograms obtained on gold and copper electrode are compered it is clearly seen that hydrogen evolution starts at 0.75 V for gold electrode and it is moved to more negative potentials when copper electrode was used. Additionally an extra peak (E) is observed on the voltammogram obtained on copper electrode as compared with the
45
voltammogram obtained on gold. This peak is related with the bulk deposition of cadmium according to reaction (2). As it has been mentioned earlier, the peak related with bulk deposition of cadmium is not observed on gold electrode due to intensive hydrogen evolution. On the basis of the conducted voltammetric tests the process of deposition of Cd–Se coatings was realised at different potentials from electrolytes of different concentration of selenious acid (IV). The process of deposition of semiconductor coatings was performed on copper sheets. Application of copper as the substrate seems to be a very promising solution due to high overpotential of hydrogen evolution that at the initial phase of Cd–Se coating creation can inhibit the process of hydrogen reduction. Too intensive hydrogen evolution can lower efficiency of the electrolysis process and additionally, it can favour reaction (5). It can result in unfavourable changes in composition of the obtained coatings as well as their morphology [3,4,49,50]. Moreover, according to the voltammetry tests the rate of the reduction reactions related with co-deposition of selenium and cadmium are faster than for gold electrode. Besides a possibility of creating a thin layer of Cu2Se can favour better adhesion of the deposited coatings and at the same time perform the function of the back layer in heterojunction solar cells, like Cu2Te joined with CdTe[51,52]. The coatings elemental analysis shows that within potentials from 0.4 to 0.7 V the composition of the obtained coatings was very close to CdSe stoichiometry when the process was conducted from a solution of 0.001 M H2SeO3 concentration (Fig. 2 Cyclic voltammograms on gold (black) and copper (red) electrode in 0.001 M H2SeO3, 0.1 CdSO4, pH ¼2 at 20 mV/s). Fig. 3. An increase of the applied potential favours deposition of a higher amount of selenium. Additionally, the deposition process is slower than for application of lower potentials (Fig. 4). On the basis of voltammetric tests it can be deduced that the process of co-deposition of selenium and cadmium follows the mechanism (6) to potential 0.4 V. Despite much higher concentration of cadmium ions in the solution, the process of selenium reduction is faster than the subsequent process of underpotential deposition of cadmium on selenium, therefore selenium surplus occurs in the coating. The process of electrolysis in solutions containing concentrations of selenious acid higher than 0.001 M was only performed from potential 0.4 V. Below the potential the process of co-deposition additionally begins to proceed by the mechanism (7) which starts to compete with the two-stage mechanism following reactions (3) and (6). It can be observed that an increase of selenious acid concentration in the electrolyte systematically
Fig. 2. Cyclic voltammograms on gold (black) and copper (red) electrode in 0.001 M H2SeO3, 0.1 CdSO4, pH¼2 at 20 mV/s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. The effect of applied potential and H2SeO3 concentration on cadmium content in the deposits. Plating bath: CdSO4 0.1 M, pH ¼ 2.
Fig. 4. The mass increase of the electrodes registered after deposition of coatings from solutions containing different concentrations of H2SeO3, pH¼ 2.
increases the content of selenium in deposit if coatings obtained at the same potentials are compared. It is particularly seen when electrolysis was performed at potential 0.4 V in a solution of the highest concentration of selenious acid. Moreover, there is a danger that an increased amount of selenium will appear in the deposit in connection with a possibility of a synproportionation reaction: H2SeO3 þ2H2Se-3Se þ3H2O
(8)
However, it should be noticed that despite applying very low potentials in solutions of higher concentrations of selenious acid, the reaction (8) practically does not take place. It is proved by the fact that the difference in content of cadmium in the coatings does not exceed 5% at., when electrolytes with different concentrations of H2SeO3 from 0.001 to 0.008 M were applied. It should be mentioned that an increase of H2SeO3 concentration accelerates the process of Cd–Se coatings deposition, but only if potential 0.7 V is applied. The results can indicate that reaction (7) is controlled by diffusion of H2SeO3 molecules to the electrode surface. In case of more positive potentials, despite increased concentration of selenious acid in a solution up to 0.008 M the deposition process gets inhibited in relation to solutions of lower concentrations. It is clearly seen when the deposition process was conducted at potential 0.4 V where the process of deposition from a solution of 0.008 M concentration of H2SeO3 is slower even than in the case when the solution contained only 0.002 M of selenious acid. The above effect can be explained by the fact that
an increase of H2SeO3 concentration in the electrolyte is accompanied by move of the range of co-deposition of cadmium and selenium following the reactions (3) and (6) towards more negative potentials and thus it inhibits the reaction (7). An additional effect is lowering the content of cadmium in the deposit as the reaction (6) is slower than reaction (3) which can be observed in case of applying more positive potentials than 0.4 V and in a solution containing only 0.001 M H2SeO3. When potential 0.8 V was applied, the content of cadmium in deposit exceeded 70% at. In such low potentials the overpotential deposition of cadmium takes place following reaction (2). High content of cadmium in coatings results from much higher concentration of cadmium ions in relation to selenious acid. According to the mass increase the rate of deposition varies from 0.1 to 0.2 mg/cm2/h when the selenius acid concentration was 0.001 M in the electrolyte for samples with compositions close to stoichiometry of CdSe compound (potential of deposition from 0.4 to 0.7 V). The rate of deposition increases up to around 0.5 mg/cm2/h for sample obtained at potential 0.7 V from solution with 0.008 M H2SeO3. If it assumed that only CdSe phase is present in the deposit the thickness of the coating is around 1.7 mm after 2 h of deposition. The phase analysis of the samples clearly indicates presence of an intermetallic compound CdSe of hexagonal structure, although the analysis was difficult due to the use of copper substrate (Fig. 5). Peaks characteristic for CdSe phase are visible within angles range 2Θ ¼24.26°, 2Θ ¼25.77° and 2Θ ¼ 42.69° following the JCPDS 01075-5679 card. The obtained coatings were so thin that peaks originating from the copper sheet also appeared on the diffractogram at 43.30° and 50.43° (JCPDS 00-004-0836). In consequence the copper peaks overlap with peaks related with CdSe phase at 42.29° and 50.54° (JCPDS 01-075-5679) in particular. Additionally small peak is visible at 13° for sample obtained from solution with 0.004 H2SeO3 and it fits only to Cu2Se phase. It seems that during process of deposition the reaction between the deposited selenium and copper is possible. The peaks obtained in the range from 20° to 30° are very wide and overlap to each other. The shape of the peaks imply the small grain size or even an amorphous structure of the deposited CdSe. Morphology of the obtained coatings was examined with the use of a scanning electron microscope (Fig. 6). The surface of coatings obtained within potentials range from 0.4 to 0.7 V is very similar and does not depend on the concentration of selenious acid in the electrolyte. The coatings are grey and they feature metallic lustre. There is no dendrites or fringe patterns appeared on the surface of films. They are compact and homogeneous, well adhere to the copper substrate, and they do not crack or fall off the substrate. Whereas, coatings obtained at potential 0.8 V feature
Fig. 5. XRD patterns of the Cd–Se coatings deposited from the electrolyte 0.1 M CdSO4, pH¼2 at potential 0.7 V vs. SCE with different concentration of H2SeO3.
R. Kowalik et al. / Materials Science in Semiconductor Processing 50 (2016) 43–48
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process (3) takes place, then cadmium is deposited on previously deposited selenium during underpotential deposition (6). The second mechanism occurs within the range of more negative potentials and it relies on simultaneous reduction of selenium and cadmium following reaction (7). On the basis of the obtained voltammetric results, the process of deposition of Cd–Se coatings on copper sheets was conducted. Within the potentials range from 0.4 to 0.7 V the coatings are characterised by a composition close to stoichiometry of CdSe compound. In case of applying potentials above 0.4 V, the amount of selenium in deposit increases indicating that reaction (6) is lower than reaction (3), despite much higher concentration of cadmium ions than selenious acid in the electrolyte. Whereas, after exceeding potential 0.8, overpotential deposition of cadmium takes place and the content of metal in deposit increases. The coatings obtained within potentials range from 0.4 to 0.7 V were homogeneous and they well adhered to the substrate. The featured grey colour and metallic lustre. An analysis using X-ray diffraction confirmed presence of polycrystalline CdSe phase of hexagonal structure.
Acknowledgments This work was supported by the Polish National Centre of Science under Grant 2011/01/D/ST5/05743.
References
Fig. 6. SEM images of the coatings deposited from the electrolyte 0.1 M CdSO4, pH¼ 2 at different potentials: (a) 0.4, (b) 0.6, (c) 0.8 V vs. SCE.
dendritic structure and they are covered by numerous burls. The coatings were brittle and some dendritic burls fell off when the samples were being removed from the electrolyte. That explains significant differences in mass growth of the examined samples obtained at potential 0.8 V from solutions of different concentration of selenious acid (Fig. 4).
4. Conclusions The presented voltammetric tests show a possibility of co-deposition of selenium and cadmium in a sulphate solution of pH ¼ 2 following two mechanisms. Within the range of more positive potentials, at first a reduction of selenious acid in a four-electron
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