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UDC 541.13
ELECTROCHEMICAL DEPOSITION OF THIN FILMS OF CADMIUM
CHALCOGENIDES
A.Sh.Aliyev, V.A.Majidzade, Sh.O.Eminov*, J.A.Guliyev*, D.M.Babanly, Mahmoud El-Ruby
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan Institute of Physics NAS of Azerbaijan
Received 16.03.2016
This paper presents the results of the works in the field of electrochemical deposition of thin films of cadmium chalcogenides from fluorboric, sulphite, thiosulphate and selenium acid electrolytes, including elaboration of compositions of electrolytes and finding out electrolysis conditions for obtaining thin films of CdTe, CdSe and CdS.
Keywords: electrodeposition, thin films, semiconductors, fluorboric electrolyte.
In resent years increasing attention has been drawn to the possibilities of applications of metal chalcogenides in various areas of technology, especially in the development of solid and liquid solar panels to convert solar energy into electric energy. Particularly, the problem of solar energy is becoming increasingly important due to the gradual depletion of fossil fuel resources. Therefore, the use of metal chalcogenides as photoanodes to convert solar energy not into electric energy, but into chemical one in electrochemical cells seems very promising as the band gap of CdTe (Eg = 1.45 eV), CdSe (Eg = 1.75 eV), CdS (Eg = 2.4 eV), MoS2 (Eg = 1.7 eV) allows to work successfully in the visible part of the sunlight.
Conversion of solar energy into electric or chemical energy in elements made of semiconductor materials with direct optical transitions takes plan on the surface layer of a depth of not more than 10 mkm, and the the majority of radiation is absorbed in a layer only 2-3 mkm in depth. Generally, as initial material for creation of such elements used basic samples with a thickness of 250-300 ^m, which isn't justified neither from physical, nor from economic standpoint and is explained only due to great technological difficulties and high percentage of defects that occur in production aimed to reduce thickness of brittle base plates from monocrystals or massive hemicrystalline semiconductors. Therefore it is logical that many users desire to obtain thin films of semiconductor materials with direct optical transitions to create on their basis the efficient and cheap solar cells.
Thin semiconductor films are produced by various methods: thermal evaporation in vacuum, pyrolytic decomposition chemical deposition and electrochemical method. The method of vacuum thermal evaporation is the well-developed and widely used method at present.
However, this and other methods either involve certain difficulties or even unacceptable, since some metal chalcogenides dissociate at elevated temperatures, which typically leads to nonuniform composition and thickness of the films. On the other hand, this method is labor-consuming, requires sophisticated equipment, and it is difficult to control the structure films during deposition.
The above-mentioned research and development of new methods for obtaining thin layers of metal chalcogenides from various electrolytes on different substrates is very important direction. In this regard, special attention is drawn to the electrochemical method, which (due to its electrical nature) allows precise process control and direct control of the film thickness.
Moreover, the electrochemical method opens wide possibilities of obtaining films with different structure and properties that significantly expands the possibilities of their use in engineering. Due to the simplicity of technical implementation, the method is very promising also in obtaining of films with high surface area and different configurations, especially at their mass production and offers great possibilities to automate the process, allows to obtain multi-
component films, the most promising for the technical application.
However, all advantages of the electrochemical deposition method of alloys do not exempt us from some of the difficulties. Addressing these issues is complicated by the lack of a developed theory of electrochemical deposition of semiconductor alloys with high electrical resistance. Method of electrochemical deposition of alloys in the form of films, in particular is developed to a considerable extent empirically.
In the case of deposition of thin semiconductor films there are some additional difficulties associated with a low concentration of charge carriers on the semiconductor surface because of the processes occur in transient conditions, and according to its description it isn't always possible to use those general regularities, which are installed in the electrolytic deposition of metal alloys. Therefore, studies of specific conditions for the formation of semiconductor films can provide valuable information for identifying electrodeposition mechanism, kinetics of electrode processes in their initial, more difficult stage.
In view of the above-mentioned the development and improvement of obtaining thin semiconductor materials suitable for use in the photoelectron and photoelectrochemical plants to convert solar energy into chemical and electrical ones are undoubtedly relevant.
1. Electrodeposition of thin films of CdTe from fluoboric electrolyte
Cathodic electrodeposition of thin films of Cd-Te alloys from aqueous solutions containing TeO2, CdSO4 and H2SO4, was carried out in the numeraus works of foreign authors [1-16] a review of which is presented in [17].
In all the above-mentioned works for the preparation of thin films of Cd-Te simple sulphuric acid electrolytes were used. Tellurium ions have the form of HTeO 2, and cadmium is in the form of simple cation Cd 2 in these electrolytes. Codeposition of tellurium with cadmium from such electrolytes is significantly more difficult since standard potentials of tellurium
(E 0Te =0.551 V) and cadmium (E Td = - 0.42 V)
considerably differ from each other. The difference of the standard potentials is 0.971 V. At such difference of potentials the codeposition of cadmium with tellurium can be carried out only at the maximum current of precious metal, i.e. at maximum current of tellurium. At the same time, as shown in [18], on the cathode it is not always possible to obtain high-quality cathodic deposits.
Furthermore, sulphuric acid solutions have some disadvantages as is mentioned below.
1. The solubility of TeO2 in H2SO4 is rather low.
2. The acidity of the solution during the deposition of the cadmium is maintained at a certain level. In case of insufficient acidity of the solution at the cathode falls and accumulates cadmium hydroxide, due to hydrolysis of cadmium salts, which causes the formation on the cathode dark, more rough, sometimes loose deposits [19].
3. More concentrated solutions of cadmium (4N CdSO4) is required. Highly diluted solutions of cadmium (0.25N CdSO4 and below) allow to work only at very low current densities, otherwise on the cathode are allocated sponge deposits.
4. To improve the performance the electrolyte should be stirred or the temperature of the electrolyte (should be increase), which is undesirable, especially in the deposition of alloys, one component of which has more positive deposition potential. At the same time the cathode deposits consists of precious metal, in this case of tellurium.
As is known [18], the necessary condition for the codischarge of two or more ions at the cathode is the equality of their reduction potentials when the standard potentials of the deposited metals are slightly different from each other. In the case where the potentials of co-precipitated elements are significantly different from each other, an effective method of potential approaching is complexation. The successful use of complexing agents for co-deposition of the two metals is due to the fact that the equilibrium potentials of elements in the complex
electrolytes may be greatly shifted towards negative potentials. Thus, the shift of potential of more electropositive metal is very important. Therefore, in some works of foreign authors for co-deposition of tellurium with cadmium were used electrolytes containing tellurium [2022] complexes.
In the present work for co-deposition of tellurium with cadmium was used fluoboric electrolyte. Fluoboric electrolyte is significantly different from sulfate and other simple electrolytes for high performance and considerable capacity. Furthermore, in fluoboric electrolyte cadmium is in the form of simple Cd(BF4)2 salt, whereas according [23], tellurium in fluoboric electrolyte forms a complex type Te(OH)2F and deposition of tellurium from this electrolyte is accompanied by high polarization.
By preliminary experiences it has been established that, unlike simple sulfate electrolytes, the fluoboric electrolyte is stable and the films deposited from them are uniform, compact and have good adhesion with the cathode surface. The discharge of cadmium ions is accompanied by a slight polarization, which in turn creates a favorable condition for the convergence of tellurium and cadmium potentials.
The influence of various factors on the polarization of both tellurium and cadmium [24, 25] were for a better understanding of the codeposition mechanism of tellurium with cadmium from fluoboric electrolyte in this paper considered.
To study the cathode process in co-precipitation of tellurium with cadmium they have been measured common polarization curves on the electrodes of Cd, Te, CdTe and Pt as a potentiodynamic method, and the method of cyclic voltammetry. The data presented in the above stated studies [26, 27] indicate that the stationary potential of the cadmium and tellurium in the fluoboric electrolyte depending on the composition of the electrolyte are within 0.28-0.3V for tellurium and -0.62-(-0.63)] V for cadmium. As can be seen, the stationary potential of cadmium on an average of 0.91V more negatively charged compared with the potential of tellurium. With such a potential difference, as
shown in [18], co-deposition of cadmium with tellurium can be carried out at the maximum current of tellurium. However, polarization measurements performed by us have shown that the recovery of tellurium ions from this electrolyte goes through discharge step of anion of complex, and the discharge precedes the chemical dissociation reaction and as a result of this reaction tellurium deposition is accompanied by high polarization, whereas cadmium deposition takes place from the neutral complex and accompanied by low polarization. This in turn contributes to the convergence of tellurium and cadmium deposition potentials and creates favorable conditions for co-deposition.
To study co-deposition of tellurium with cadmium, general polarization curves of Cd-Te alloy, Te and Cd discharge were taken, respectively on the electrodes cadmium-tellurium, tellurium and cadmium (Figure 1).
ic,A/dmJ'1
Fig. 1. Cathodic potentiometric polarization curves for the deposition of tellurium, cadmium and an alloy of Cd-Te from fluoboric electrolyte (M): 1 - 0.006 TeO2 + 0.68 HBF4 + 0.16 H3BO3 + 0.1 NH4F, 2 - 0.25 Cd(BF4)2 + 0.68 HBF4 + 0.16 H3BO3 + 0.1 NH4F, 3 -0.006 TeO2 + 0.25 Cd(BF4)2 + 0.68 HBF4 + 0.11 6H3BO3 + 0.1 NH4F. Scanrate - 8 mV/s, t = 200C.
As can be seen from Figure 1, the polarization curves of the Cd-Te alloy discharge (curve 3) are located in the more positive potential area than the discharge curves of tellurium (curve 1) and cadmium (curve 2). It is known [18], that the total polarization curves are arranged in a more positive potential area than the
discharge curves of the individual components in the case when the deposition process is accompanied by depolarization and depolarization extends to both components of an alloy. However, the depolarization is observed in cases when the codeposition of components at the cathode is accompanied by the formation on the cathode chemical compounds or solid solutions between components. This, in turn gives reason to believe that in case of the codeposition of tellurium with cadmium at the cathode is really obtained compound CdTe.
Subseqnently, cyclic voltammetric curves for Pt-electrode were measured to establish codeposition potential of tellurium with cadmium and to determine the range of potentials at which on the cathode thin layers of CdTe are formed. It gives the chance to establish the phase composition of deposition products on the cathode cycle, through measuring cycle of oxidation [28, 29].
There are three waves of oxidation in the reverse anode cycle of voltampere curve of the co-deposition Te(IV) and Cd(II). At 0.5 and -0.5 V potentials, oxidation of tellurium and cadmium occurs respectively. Wave observed at the potential of 0.05 V, can be attributed to the oxidation of CdTe by the reaction: CdTe - 2e = Cd2+ + Te.
The high value of depolarization in the allocation of cadmium in an alloy suggests that during the co-precipitation of tellurium with cadmium, the main role is most probably played by non-equilibrium part of depolarization, i.e., the overvoltage in the allocation of cadmium on tellurium, as stimulates the deposition of cadmium at more positive potentials. This is evidenced by the fact that as soon as the surface of the platinum electrode is covered with a tellurium and achieved a certain value of potential (in this case, for example -0.1 V), there is a co-precipitation of cadmium with tellurium (Figure 2).
This deposition mechanism also occurs in the preparation of thin films to potential -0.3 V. However, the tellurium content is high enough in the deposits obtained at the -0.3 V potential (the Table). This is apparently due to the fact that the surface of the electrode is covered with elementary tellurium before the co-deposition potential of cadmium with tellurium.
ic,A/dm2i
0,4
Fig. 2. Cyclic voltametric curves of the alloy Cd-Te on Pt-electrode in solution (M): 0.003 TeO2 + 0.25 Cd(BF4)2 + 0.68 HBF4 + 0.16 H3BO3 + 0.1 NH4F, V=8 mV/sec, t=200C
The composition and type of conductivity of films obtained by potentiostatic deposition from an electrolyte containing: 0.006 M TeO2 + 0.0252 M Cd(BF4)2 + 0.682 M HBF4 + 0.161 M H3BO3;The time of electrolysis is 60 min
The deposition potential, V Phase composition, (Te content, wt%) The type of conductivity
-0.15 CdTe, Te (75) P
-0.2 CdTe, Te (65) P
-0.25 CdTe, Te (60) P
-0.3 CdTe, Te (57) P
-0.35 CdTe, Te (54) P
-0.4 CdTe, Te (53.5) P
-0.5 CdTe, Te (53.2) P
-0.6 CdTe, Cd (52.1) n
-0.65 CdTe, Cd (48) n
As shown above, the co-deposition of cadmium with tellurium occurs at potentials when Te2- ions formed on the surface of the electrode and these ions play the main role in prepotentials deposition of cadmium. As a result of the interaction of Te2- with Cd2+, the surface of the electrode is covered with a layer of CdTe and diffusion of tellurium (-2) ions through this layer to the surface of the electrode is difficult and they remain in the lower layers. Most likely not only these atoms, also the whole deposited in the initial stages of the process, tellurium is consumed for the formation of CdTe. Probably the content of free phase of Te obtained at potentials more positive than -0.3 V increases as the content of tellurium in electrolyte increasis.
Elemental analysis of the films using a microanalyzer (by thickness) showed that indeed the lower layer of films is composed of elemental tellurium. However, this problem can be avoided in two ways - 1) before deposition of CdTe, it is necessary to cover previously a surface of a platinum electrode with a thin layer of cadmium; however, this is not enough rational way, because after deposition of films require heat treatment for the purpose of their homoge-nization is required; 2) conditions, when the quantity of tellurium, deposited on a platinum electrode, is sufficiently small are preferable; these conditions may favor the deposition on the cathode thin films of CdTe stoichiometric composition; to ensure this condition it is necessary to use electrolytes with low concentration of Te02 and high concentration of Cd(BF4)2; in such conditions a small amount of tellurium reacts with cadmium ions, can form CdTe, very similar in composition to stoichiometric.
Taking into account this fact, electrolytes containing relatively low concentrations of Te02 and high concentrations of Cd(BF4)2 have been used in this work to obtain thin films of CdTe. However, to avoid inclusion of free phase of tellurium in the composition of the CdTe is not possible because the tellurium as a separate phase is present in all films. Still, a sharp decrease of the relative content of tellurium phase was observed in solutions containing large concentrations of Cd(BF4)2. We have found that elemental cadmium in the composition of the cathodic deposits can be included even at -0.6 V, i.e. at the potential more positive than the deposition potential of the cadmium, if the electrolysis conducts in cumulative mode at -0.6 V for 10 minutes. This is likely due to prepotentials recovery of cadmium ions on CdTe due to the release of the Gibbs energy of formation of compound CdTe:
Cd2+ + 2e + CdTe = CdTe:Cd.
Based on the analysis of cyclic voltam-metric curves and the results of X-ray phase analysis of CdTe deposits obtained at different potentials there were chosen deposition conditions of CdTe compounds on the platinum electrode at the constant potential. This potential
should be more positive than - 0.3V and negative -0.6 V. At more negative potentials, possible deposition of free cadmium phases in the of dendritic form on the surface that degrades the quality of deposits. Based on the analysis of all experimental data selected optimum composition of the electrolyte. Thus, it is shown that the phase CdTe is formed in the potential range (-0.3)-(-0.6) V from electrolyte with composition: (0.002-0.006) M Te02 + (0.25-0.5) M Cd(BF4)2 + 0.68 M HBF4 + 0.16 M H3BO3 + 0.1 M NH4F. The obtained CdTe films have a thickness of from 5 to 10 ^m and contain free tellurium in their composition. On the base of the conducted researches to obtain high-quality electrolytic layer of alloys of the Cd with thickness to 10 ^m thick, containing various amounts of tellurium and cadmium, it is possible to recommend the following composition of the electrolyte (M):
Cd(BF4)2 - 0.25-1.0, Te02 - 0.001-0.01, HBF4 - 0.15-0.59, H3BO3 - 0.08-0.16, NH4F - 0.08-0.14.
The electrolysis mode: the current density is 0.2-1.4 A/dm2, temperature - 293 K, the anode - combination of tellurium and platinum with the ratio of the surface of the STe:SPt = 1:2, cathode - Pt and Ni. The proposed composition of the electrolyte is protected by the copyright certificate [30].
2. Electrodeposition of thin films of CdS from a sulphite electrolyte
To obtain thin films of the alloy of CdS of n-type are applied various aqueous and nonaqueous electrolytes [31-44] containing dimethyl sulphoxide, sodium thiosulphate, di-methyleneglycolic and propylenecarbonate solutions containing colloidal sulfur. The main disadvantage of these electrolytes is that the film obtained from them, in some cases, heterogeneous system in composition as a result of the inclusion in the composition of the films colloidal sulfur, in other cases, obtained films are crack. The use of different composition of electrolytes for obtaining of thin layers of CdS, as-
sociated with the deficiencies found in each case and focused on finding a new electrolyte composition, giving the possibility of obtaining high-quality cathode films. In this paper, to determine the optimum conditions for obtaining thin layers of CdS carried out a systematic study of the regularities of sulfur discharge [45], cadmium [46, 47] and CdS [48-50] deposition on a platinum electrode in solutions containing Na2SO3, CdSO4, H2SO4 and in the presence of (NH2)2CS. After ascertaining the conditions of deposition of sulphur and cadmium separately held their codeposition. By means of the preliminary experiments carried out with solutions containing sulfur and cadmium compounds, it was found that deposits of the Cd-S alloy obtained at room temperature, have a poor adhesion with the cathode surface and uniform in composition. Homogeneous deposits with good adhesion, yellow color, were obtained at temperatures of 40-600C.
To determine the range of potentials at which on the cathode occurs co- deposition of cadmium with sulfur and formed thin CdS layers, cyclic voltametric curves on a platinum electrode were measured at different scan speeds of potential, temperatures, different concentrations of components of the electrolyte and its acidity. Figure 3 presents cyclic voltammetric curves taken on a platinum electrode in a solution containing 0.02 M CdSO4, 0.1 M Na2SO3, 0.1 M (NH2)2CS at the temperature of 400C. As can be seen from Figure 3 (curve 1), on the cyclic voltammetric curve of co-deposition of sulfur with cadmium there were observed three waves of recovery - (A), (B) and (D) and one oxidation wave (C). In the area of wave (A) sulfite ion is reduced to sulfur, and wave (B) corresponds to the restoration of S to S2-. In the area of wave (B) there is a co-deposition of sulfur with cadmium as a result of the interaction S2- with ions Cd in the near-electrode layer, which leads to the formation of CdS by the reaction: S2- + Cd2+ = CdS.
In the area of wave (D) the surface of the electrode is fully covered with a thin layer of CdS having a high electric resistance, because of which the electrode reaction rate decreases. By the physical and chemical analysis it is es-
Fig. 3. Voltametric curves on platinum cathode in a solution containing: 2102 M CdSO4 + 0.1 M Na2SO3 + 0.1 M (NH2)2CS at a temperature of 400C, Ev = 8 mV/s, pH = 2.
tablished that CdS thin layers of stoichiometric composition obtained in the potential range of -0.6-(-0.7) (s. c. e.). Films obtaining in the region of more positive potentials, consist of two phases - CdS and S, and in the composition of the films obtained at more negative potentials area then the discharge potential of cadmium, detected free-phase of cadmium. Wave (E) observed in the reverse cycle, probably due to the cathode recovery of CdS by the reaction: CdS + 2e ^ Cd + S2-, so that the potential of this reaction are in good agreement with the potential
recovery of S2- to S. Wave (C) refers to the oxi-
• 2
dation of S - to S and at more positive potentials occurs oxidation of S to HSO 3 . At recycle (curve 2) wave of recovery (D) on the voltam-metric curve becomes negligible. In addition, at anodic recycle oxidation wave (C) increases and shifts to the negative direction, which indicates sufficiently strong bond between Cd and S. Carrying out cyclic recovery on non-upda-table electrode of CdS, it is possible to obtain films of sufficient thickness. It is established that with increasing of temperature the rate of the co-deposition process of cadmium with sulfur increases. The recovery wave of S to S2-, and the area of potentials in which is formed of CdS, is shifted toward higher current densities.
In addition, with the increase of the concentration of sulfite ions in the solution increases the deposition rate of the alloy and expands the range of potentials in which the co-elec-trodeposition of sulfur with cadmium. Increasing of the CdSO4 concentration in the electrolyte does not cause any significant changes in the ca-thodic scan of current-voltage curves, while the oxidation wave of cadmium in a reverse anodic cycle increases. With increasing of the sweep rate of potential the rate of cathodic reduction and anodic oxidation is increased and the current-voltage curves are shifted in the positive direction. The observed dependence of E12 -ip has a classic, straight character, which indicates a diffusive nature of polarization in case of codeposition of cadmium with sulfur [50].
It is shown that the increase in the content of thiourea in the electrolyte extends the range of potentials in which CdS is formed. This decreases the amount of free cadmium, included in the composition of cathode deposits. Based on recorded polarization curves and the results of X-ray phase analysis of the films of CdS, obtained at different potentials were determined deposition conditions of CdS thin films at constant potential. The electrolysis time is 0.5-1 hour depending on thickness of deposited films. To obtain the films of composition very close to CdS, the deposition potential should be more positive than -0.5 V (s. c. e. -silver-chlorine half-cell) and negative at -0.65 (s. c. e.) To find the optimum conditions for obtaining high-quality cathode films of cadmium-sulfur was studied the influence of the concentration of components of the electrolyte, current density and temperature on the composition and quality of cathode deposits in galvanostatic mode, as it allows conducting experiments in more comfortable conditions than when conducting electrolysis in potentiostatic mode [49]. High-quality film of yellow-lemon color, with the composition which is very similar to CdS are obtained at 50-600C temperatures and current densities of 3-4 A/dm2. Films obtained at low temperatures had a loose structure and poor adhesion to the surface of the cathode. The film thickness depends on the time of electrolysis
and current density. Under certain conditions on the cathode it is possible to obtain thin layers of cadmium-sulfur up to a thickness of 12 mcm. To obtain thin layers alloys Cd-S alloys containing 8-84 wt.% Cd, the electrolyte of the following composition in recommended (M):
Na2SO3 - (0.1-0.5), CdSO4 - (0.005-0.04), (NH2)2CS - (0.1-0.3).
The electrolysis mode: ic = 1-5 A/dm , temperature of the electrolyte 50-600C, the pH of = 1.5-2, cathode - platinum, titanium, nickel, anode - platinum.
3. The electrochemical deposition of cadmium sulphide on Pt- and MWCNT-electrodes
After knowing the electrochemical behavior of cadmium and thiosulfateat platinum and carbon nanotube paste electrode (MWCNT), the optimum conditions for elec-trodeposition of CdS can be easily detected [51-53]. Nickel sheet was chosen as a substrate for further analysis (SEM, XRD and EDX) of the electrodeposited CdS in this work.
Figure 4 shows the cyclic voltammo-grams of 0.01 M Cd2+ + 0.1 M S2O32" without and with 0.1 M thiourea (TU) at 313 K on Pt-electrode.
s 01 -
T5
E, V
Fig. 4. Cyclic voltamogram (C.V.) of 0.01-M Cd2+ + 0. 1 M S2O32- without TU (a) and with TU (b) at 0.02 V/s, pH = 2.5 on Pt- electrode at 313 K.
The presence of thiourea as an additive can cause grain refining and brightening effect
0.3-
0. 2-
0. 0-
0.0
0.3
extremely smooth deposits were obtained. The surface roughness as well as the mean grain size of the deposits also approached minimum values above this thiourea concentration.
In this work it was established that the optimum condition for the electrodeposition of CdS from thiosulfate acidic aqueous solution on Pt-electrode (confirmed by XRD analysis below) were described in Figure 5(b).
-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
Fig. 5. C.V. of 0.01M Cd2+ +0.1M S2O 2 + 0.1 M
(NH2)2CS at 0.02V/s-1, at pH = 2.5 on Pt-electrode at (a) at 295 K and (b) 313 K.
Figure 5 shows the cyclic voltammograms of the Pt electrode in 0.01 M Cd2+ + 0.1 M S2O 2 + 0.1 M thiourea (TU) at room temperature 220C (a) and at 400C (b). As is seen from the shown data, the increase of temperature leads to an increase of the height of the cathodic peak current density (rate of the electrodeposition was accelerated) of the electrodeposition process. But at temperatures higher than 400C, it is noted that the electrodeposited cadmium sulfide leaves out the electrode surface. This may be attributed to that higher temperatures lead to becoming of the electrodeposited CdS an amorphous material and the adhesion to the electrode decreases at these conditions.
It is observed from Figure 5.2 (b), the first cathodic peak (I) at cathodic potential of about —0.55 V attributed to the electroreduc-tion of the colloidal sulphur produced from the dissociation of thiosulphate under at these conditions (acidic). It is also noted that in Figure 5 (b), the second cathodic peak potential (II) appears at a cathodic potential of about — 0.75 V
attributed to the electrodeposition of CdS (confirmed by XRD). At this potential the electrore-duced adsorbed sulfur ions react with the cadmium ions electrochemically on the electrode surface giving CdS.The resulted CdS is adsorbed on the electrode surface of platinum. The resulted liberated HSO 3 ions from the dissociated thiosulfate in acidic media can be reo_
duced to sulfur then to S ions and water. The formed sulfur ions can be adsorbed on the electrode surface and react with cadmium to form CdS; S2-(adsorbed) + Cd2+ = CdS (adsorbed at the electrode surface) second peak, II.
Also it can be noted in Figure 5 (b), the third cathodic peak potential, III, appears at a potential of —0.93 V at which the free unre-acted Cd2+ ions are discharged into Cd(0). The fourth cathodic peak IV, in Figure 5 (b) appears at p otential of —1.15 V attributed to the elec-troreduction decomposition of the electrodepos-ited CdS on the electrode surface resulting Cd(0) and S2-, CdS + 2e = Cd + S2-. The wave potential (II/) relates to oxidation of S2- to S. At more positive potentials, sulfur (S) can be oxidized to HSO 3 in acidic media. The wave potential (III/) relates to the oxidation of the elec-trodeposited cadmium sulfide on the platinum electrode surface; CdS = Cd2+ + S + 2e. The separation of the anodic and cathodic waves AEp, depending on the rate of the applied potential scan rate, this indicates to that the process dealing with a quasi-reversible processes.
After knowing the suitable deposition potential for cadmium sulfide from cyclic volt-ammetric measurements and the optimum conditions, chronoamperometric method can be used for the electrodeposition of CdS. Potential step chronoamperometry is an important and useful experimental techniqueachieved by stepping the potential (Ed = -0.8 V, in between peak II and peak III as in Figure 5 b) from one point on a voltammetric wave to another and monitoring the current response.
During the chronoamperometric deposition process of cadmium sulfide and by passing the time, the current decreases but not sharply, moves to more positive direction, this due to the formation of well adsorbed cadmium sulphide
0.3
0.0
E, V
layers on the platinum electrode surface, this blocks the electrode surface, hence its electrochemical activitydecreases. Furthermore the formed layers of cadmium sulfide (semiconductor material) are less conductive than the platinum itself.
The heterostructure composite of MWCNTs and a semiconductor becomes very important if MWCNTs are employed in optoelectronic devices. CdS has been synthesized electrochemically on multi walled carbon nano-tubes paste electrode (MWCNT) from aqueous
9+ 9
solutions it consists of Cd and S2O3 at 313 K, using potentiostatic technique in this work [54, 55]. Cyclic voltammetric response of thio-sulfate and Cd2+ together has been investigated. The optimum conditions of CdS electrodeposi-tion were determined on MWCNT electrode.
Figure 6 shows the cyclic voltammo-grams of the MWCNT-electrode in a solution consists of 0.01 M Cd2+ + 0.1 M S2O33 + 0.1 M
(NH2)2CS at pH = 2.5, temperature of 313 K and potential scan rate of 0.02 V/s. It is observed from the figure that, the cathodic peak
2+ 2
potential of Cd and S2O ^ in mixture is shifted to more negative potential compared with the cathodic potential peaks of cadmium ions and thiosulfate alone. This shift may be attributed to the formation of cadmium sulfide.
0.000
<
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
Fig. 6. Cyclic voltamogram of 0.01-M Cd
+ 0.1 S2O M + 0.1 M (NH2)2CS at 0.02
V/s-1, at pH = 2.5 on MWCNT-electrode at 31 3 K.
The first cathodic peak (I) (Figure 6) is appeared at a potential of about —0.55 V at-
tributed to the electroreduction of the colloidal sulphur which dissociated from thiosulphate under these conditions converting into sulphide ions adsorbed on the electrode surface of carbon nano tubes. Moreover the colloidal sulfur can be resulted from hydrogen sulfite (the second product of the dissociation of thiosulfate in acidic media); the second cathodic peak potential (II) is a slight peak and appeared at a potential of about —0.85 V. This peak is attributed to the electrodeposition of cadmium sulfide and was confirmed by XRD analysis:
S (colloidal) + 2 e ^ S - (adsorbed at the electrode surface, first peak, I), (a) S2-(adsorbed) + Cd2+ = CdS (adsorbed at the electrode surface, second peak, II). (b)
At the third cathodic peak potential III, was observed at potential of —0.93 V, a crossover takes place indicating to presence of a nu-cleation process. At this peak the free un-reacted Cd2+ ions are discharged into cadmium metal Cd(0) and adsorbed into the electrode surface of carbon nano tubes which have high ad-sorbability. The wave potential (I/) relates to oxidation of S2- to S. At more positive potentials, Sulfur (S) can be oxidized to HSO 3 in
acidic media. The oxidation wave potential (II/) relates to the oxidation and dissociation of the electrodeposited cadmium sulfide on the MWCNT paste electrode surface; CdS = Cd2+ + S + 2e.
The oxidation wave potential (III/) relates to oxidation of the electrodeposited adsorbed free unreacted cadmium metal to cadmium metal ion Cd2+.
From XRD data, it is also illustrated that when the deposition potential is fixed at a potential of -0.80 V (cathodic peak II, Figure 5 b), pure CdS is only deposited on the electrode surface. But when the deposition potential is fixed at a potential of -1.2 V (cathodic peak IV) cadmium sulfide and cadmium metal Cd(0) are electrodeposited as confirmed from the XRD data. Also, it was found that when the deposition potential is fixed at potential less than -0.6 V, the electrodeposited CdS and sulfur are obtained, this also was confirmed by EDX analysis.
0.050
0.025
E, V
EDX elemental analysis is used for studying the chemical composition and per cent of Cd and S in the electrodeposited CdS. The effect of electrodeposition time and electrodeposition potential on the percent ratio of cadmium and sulfur in the electrodeposited CdS films were studied [56]. The SEM images were taken for the electro-deposited chrono-amperometrically CdS from a solution consists of 0.01 M Cd2+ + 0.1 M S2O33
at pH = 2.5, temperature = 313 K on Ni substrate at fixed certain potential at different electrodepo-sition certain times (Figure 7). From the obtained results it can be noted that, the shape and the morphology of the images of the electrodeposi-tion of CdS depend on the time and the deposition potential values at these conditions.
By increasing the electrodeposition time, the electrodeposited CdS arranged in a crystalline hexagonal shape and the alloying process is enhanced at this condition. Image (1), at which the electrodeposition potential is two hours, shows a layer of thin film of the electrodeposited material (gray color) surmounted white-colored crystalline materials. The corresponding EDX analysis for image (1) shows a better percent
between sulfur and cadmium of the electrode-posited CdS.
The best one as shown in the image (1) at potential -0.8 V at these conditions was observed. But when the potential is increased to be -0.9 V [image (2)] the cadmium sulfide is deposited with a high percentage of cadmium metal as confirmed with XRD and EDX analysis, also cadmium is appeared clearly in this image as a black particles.
The SEM images of the electrodeposited chrono-amperometrically CdS from a solution consists of 0.01 M Cd2+ + 0.1 M S2O33 at pH = 2.5, temperature=313 K on MWCNT paste electrode at electrodeposition time (for two hours) at fixed electrodeposition potential of -0.8 V (optimum condition), with and without the template (anodized aluminum oxide, AAO) were taken. It can be noted that, the electrodeposited particles of CdS are in the range of nanometer of average size 15-20 nm without using AAO template, depending on the substrate material (MWCNT) and formed with the shape of the substrate material. Image (4), after using the AAO as a template, shows the electrodeposited CdS in the form of
WD HV ZOOM DIM
5 mm 20 KV X25000 50 nm
Fig. 7. SEM images of the electrodeposited CdS on Ni (1, 2); (1) at E = -0.9 V, (2) at E = -0.8 V and MWCNT (3, 4), substrate (1) without AAO template (2) with AAO template (conditions as described in Figure 6).
nanowires in the range of about 200 nm in long and 15-18 nm in diameter.
4. Electrodeposition of cadmium -selenium alloys from selenite solutions
Electrochemical methods are often used in order to obtain thin semiconductor films of cadmium selenide [44, 57-68]. Recently electrochemical deposition of films of CdSe have shown considerable interest, since they can also be used when creating liquid solar batteries, as well as photoanodes for converting solar energy to chemical energy. However, the mechanism for CdSe deposition of alloys is poorly studied. In the works, there are mostly presented the data on the phase composition, morphology and properties of the obtained deposits. The authors of [63] believe that for the formation of cadmium selenide it is necessary to deposit elemental cadmium and selenium on the electrode but do not specify how to implement their interaction for direct obtaining of CdSe. The scheme of the deposition process of two structural forms of cadmium selenide on titan is offered without experimental confirmation [68].
In both cases, it is expected that for electrodeposition of CdSe it is necessary the deposition of Cd. Our experimental data cannot be explained from the point of view stated in [63, 68]. We obtained the cadmium selenide on titanium and platinum substrates by electrolysis in conditions, when the cathode codeposition of cadmium with selenium occurred at more positive potentials E d2+ than the equilibrium
potential £cd2+/cd and therefore, the discharge of
the cadmium at these potentials are excluded. Co-electrodeposition of cadmium with selenium from acidic electrolytes containing selenous acid and the Cd ions, is associated with considerable difficulties, since the standard potentials of cadmium (-0.42 V) and selenium (0.74) differ greatly and the achievement of discharge potential of cadmium in the presence of sele-nous acid is hardly possible. However, it has been noticed that in the electrolysis of solutions containing selenous acid, even on electrically
inert platinum and titanium electrodes form films of elemental selenium, blocking surface of the cathode. As a result, the potential of the cathode is shifted in the negative direction until the discharge potential of hydrogen and in certain conditions can significantly closer to to the discharge potential of cadmium.
In addition, polarization measurements performed on Pt and Ti electrodes showed that at more negative potentials until the discharge potential of hydrogen, the selenium deposited on the metal electrodes, undergoes to deep ca-thodic reduction with formation of the selenideion Se - which may contribute to co-deposition of cadmium with selenium at more positive potentials than the discharge capacity of cadmium, which was proved by recording the current-voltage curves on Se (Pt) and Se (Ti) electrodes [57, 59, 69]. In Figure 8 and 9 are presented the curves of the coelectrodeposition of cadmium and selenium from selenite solutions respectively on the platinum and titanium electrodes. It has been established that co-deposition of cadmium with selenium begins at potential of -0.4 V (s. c. e.) that corresponds to the potential formation of Se2- ions on the surface of the cathode. The rate of the process increases beginning from this potential and reaches its limiting value of -0.54 V (s. c. e.) (Figure 8, curve 1). As the surface of the electrode is covered with a CdSe film having higher electrical resistance, on voltage-current curve appears the area of the limiting current at the potential of (-0.5^0.6V) (s. c. e.), and electrode surface passivated up to the discharge potential of cadmium.
At potentials more negative than the discharge potential of cadmium, the rate of the process increases again due to the achievements of potential discharge of cadmium. On the reverse anodic cycle voltage-current curve of codeposition of selenium with cadmium observed wave (a), corresponding to the oxidation of cadmium, which isn't connected to the CdSe compound, and the small wave (c), connected, probably, with the oxidation of CdSe compound to elemental selenium by the reaction:
CdSe - 2e = Cd2+ + Se.
Fig. 8. Cyclic voltammetric curves of Pt-electrode in the electrolyte: a) 0.5 M CdSO4 + 1M H2SO4; b) 1 - 0.005M H2SeO3 + 0.5M CdSO4 + 0.5M H2SO4, 2 - 0.005M H2SeO3 + 0.5M CdSO4 + 1M H2SO4. Ev = 8 mV/s, t = 200C.
Oxidation of selenium (wave a) takes place at more positive potentials the rate of deposition slows down during second cycle. For obtain cathodic deposition having a crystalline structure, the electrolysis should be carried out at temperatures of 40-500C. Cyclic voltammet-ric curves on Ti electrode are presented in Figure 9. The analysis of cathodic deposits shows that the co-deposition of selenium with cadmium occurs at a potential of -0.4 V (s.c.e.), i.e. at Se2- formation potential, it was proved by the recording of the current-voltage curves on Ti (Se) electrode in an electrolyte containing only H2SO4. The rate of electrode reaction decreases as the electrode surface is covered with the sufficient thickness of cathode deposit. Upon reaching the potential of -0.57 V (s.c.e.) the rate of electrode reaction increases again, probably due to further dissolution of the free selenium atoms which are on an electrode surface.
2 2+
These Se - and Cd ions interact and the are of limiting current is detected on the voltammetric curve. By physico-chemical analysis of cathod-ic deposits it was established that, exactly at the
potentials the composition of the obtained thin films are very close to CdSe. At the anode cycle, recorded at potentials -0.65 (s. c. e.), the oxidation peak of the free phase of cadmium is observed on current-voltage (fl). The cathodic wave observed in the range of potentials -0.3-(-0.5) V (s. c. e.) when recorded the anode cycle probably due to cathodic reduction of CdSe by the reaction: CdSe + 2e = Cd + Se2-.
Fig. 9. Cyclic voltammetric curves on titanium electrodes in solution 0.5 M CdSO4+0.005 M H2SeO3 +1 M H2SO4. Ev = 10 mV/s, 1 - first cycle, 2 - second cycle. t = 500C.
As a result of this reaction on the volt-ammetric curves there is a new wave of oxidation of cadmium (E), which is a part of CdSe. At a recycle of volt-ampere curves (Figure 7, curve 2) the rate of electrode reaction decreases as a result of coating the surface by poorly conducting current film of the CdSe with sufficient thickness, as in the period of first anode cycle not all CdSe deposited in the cathode, is oxidized. This is evidenced by the rest current observed in the potential range of 0.1 and -0.1V in the (s.c.e.) Figure 9. By means of the microana-lyzer it is established that on the surface of electrode remains the passive layer consisting of CdSe and Se of noticeable thickness.
X-ray phase analysis and microanalyses show that in the composition of the thin layers of the Cd-Se alloy, obtained at positive than - 0.6 V (s.c.e.) potentials contain significant amount of free Se phase, however the thin films obtained at more negative than -0.65 (s.c.e.) of CdSe, CdS and Se. In this case, selenium is situated in the lower and Cd in the upper layers of the films. Films of the stoichiometric composition are obtained in a narrow potential range of -0.62^0.65 V (s.c.e.). It is established that the significant impact on the composition and quality of the ca-thodic deposits comes from the current density, the concentration of major components in the electrolyte and the electrolysis temperature [59].
The following composition of electrolyte (mol/l) is recommend based on the experimental investigation to obtain high-quality semiconductor layers of Cd-Se alloy: H2SeO3 -0.002-0.01; CdSO4 - 0.5-1.0; H2SO4 - 1.0-2.0. The electrolysis conditions: ik =-0.5-0.2 A/dm , temperature 20-600C, the cathode Pt and Ti, the anode - combined from selenium and platinum with the ratio of surfaceareas: SSe:SPt = 1: 2.
Acknowledgment: This work was supported by the Science Development Foundation under the President of the Republic of Azerbaijan, grant № EIF-2014-9(24) KETPL-14/04/4-M-13.
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KADMIUM XALKOGENiDLORlN NAZlK TOBOQOLORlNlN ELEKTROKlMYOVl YOLLA ALINMASI
A.§.0liyev, V.A.Maddzad3, §.O.Eminov, C.A.Quliyev, D.M.Babanli, Mahmud Ol-Rubi
Bu i§da elektrokimyavi yolla kadmium xalkogenidlarinin flüorborat, sulfit, tiosulfat va selenit elektrolitlarindan alinmasinin naticalari verilmi§dir. CdTe, CdSe va CdS-in nazik tabaqalarinin alinmasi ügun elektrolitin tarkibi va elektrolizin §araiti i§lanib hazirlanmi§dir.
Agar sözlari: elektrolitik gökms, nazik t3b3q3, yarimkegirici, flüorborat elektroliti.
ЭЛЕКТРОХИМИЧЕСКОЕ ПОЛУЧЕНИЕ ТОНКИХ ПЛЕНОК ХАЛЬКОГЕНИДОВ КАДМИЯ
А.Ш.Алиев, В.А.Меджидзаде, Ш.О.Эминов, Дж.А.Гулиев, Д.М.Бабанлы, Махмуд Эль-Руби
Представлены результаты работ в области электрохимического получения тонких пленок халькогенидов кадмия из борфтористоводородных, сернистокислых, тиосульфатных и селенистокислых электролитов, включая разработку составов электролитов и нахождение условий электролиза для получения тонких пленок CdTe, CdSe и CdS.
Ключевые слова: электроосаждение, тонкие пленки, полупроводники, борфтористоводородный электролит.
