Electrodeposition of the Sb 2 Se 3 thin films on various substrates from the tartaric electrolyte

The present contribution is devoted to the electrochemical deposition of Sb 2 Se 3 thin films from tartrate electrolyte. The study was conducted by potentiodynamic, potentiostatic and galvanostatic methods carried out under different conditions at Pt, Cu and Ni electrodes. The kinetics and mechanism of the electroreduction of antimony and selenite ions in the tartaric acid were studied separately for the electrochemical deposition. Comparison of the obtained polarization curves showed that co-deposition occurs between electroreduction potentials of antimony and selenium, indicating depolarization electrode effect for antimony ions. The influence of electrolyte composition, pH, current density, temperature, etc. has been studied. On the basis of cyclic polarization, X-ray phase and SEM-EDX analyses, it is found that Sb-Se thin films are deposited on Pt and Ni electrodes, but not on Cu electrode. Black, uniform, crystalline and shiny films of the stoichiometric composition of Sb 2 Se 3 compound are deposited on Pt and Ni electrodes within the 338-348 K temperature interval, pH 1.85, current density of 2.5-3.0 A/dm 2 , and annealing temperature of 703 K. Experiments were carried out using the optimal electrolyte composition containing 0.05 M SbOCl + 0.05 M H 2 SeO 3 + 0.007 M C 4 H 6 O 6 .

Antimony selenide (Sb 2 Se 3 ) is considered one of the most promising semiconductor materials of the type A 2 B 3 with a relevant bandgap of ~1.78 eV for solar cell applications [14].This gives a solid argument for the production of this type of semiconductor materials which complements the field of application in electrochemistry.Relatively lower prices of Sb and Se, as well as a low toxicity of Sb 2 Se 3 merit its exploration for photovoltaic application.
All above-mentioned methods, however, are time, finance and effort consuming.The electrochemical method is more accurate, time-saving, and efficient method for the synthesis of metal chalcogenides including Sb 2 Se 3 [7,8,28,29].Usually, this method is used to get thin layers of the required composition from electrolytes of different compositions in a short time and at relatively low temperatures.One of the advantages of this method is the possibility to alter the composition of the obtained layers by changing the composition of electrolyte and conditions of the electrolysis.The nucleation and the growth mechanisms of antimony selenide (Sb 2 Se 3 ) on indium-doped tin oxides (ITO) and fluorine tin oxide (FTO) substrates were achieved using the electrochemical method [7,30].
To obtain high-quality Sb 2 Se 3 thin films with an electrochemical method, more accurate experimental results in this field of research are still required.This work is a part of our systematic investigation in the area of electrodeposition of thin films of semiconductor chalcogenides [31][32][33][34][35][36][37].Therefore, the present contribution is devoted to exploring the optimal conditions for obtaining high-quality films of antimony selenide via studying the effect of the substrate material on the electrodeposition process.

Experimental part
The composition of an electrolyte solution used in our experiments was 0.01 -0.09 M SbOCl (supplied by 2A Pharmachem, USA), 0.01 -0.09 M H 2 SeO 3 (supplied by Reachim LTD, Russia) and 0.007 M C 4 H 6 O 6 (tartaric acid) (supplied by IndiaMART, India).The cyclic polarization curves were performed via "IVIUMSTAT Electrochemical Interface" potentiostat.During experiments, a three-electrode electrochemical cell with 100 ml volume, Pt wire of 210 -6 m 2 surface and Cu and Ni plates of 110 -4 m 2 surface area were used as working electrodes.The silver-silver chloride (Ag/AgCl/KCl) was used as a reference electrode and a Pt sheet as a counter electrode.The temperature was regulated by the UTU -4 universal thermostat.The phase composition of the prepared thin films was analyzed by "D2 Phazer" X-ray phase analyzer (CuK α ; Ni filter) of the "Bruker" company.Morphology, topography and chemical elemental composition of the deposited samples were measured by "Carel Zeiss Sigma" scanning electron microscope (SEM) provided with EDX unit.
At the beginning of the experiments, Pt, Ni, and Cu electrodes were purified in the concentrated nitric acid and then washed with bi-distilled water.Ni-electrodes were additionally subjected to electrochemical polishing in a mixed solution of sulfuric, phosphoric and citric acids at certain conditions (T = 293-303 K, j = 0.5 A/dm 2 , τ = 180 s) and washed with distilled water.

Results and discussion
As is already known, the composition and quality of the obtained thin layers considerably depend on the composition of electrolyte and concentration of primary components during electrodeposition.In the present study, Sb-Se thin films were electrochemically deposited from tartrate electrolyte by galvanostatic and potentiostatic methods.We carried out the deposition process by changing certain conditions, such as current density, concentration of components, temperature, etc. Different substrate electrodes, i.e.Pt, Cu, and Ni have been used to obtain the stoichiometric compound Sb 2 Se 3 .As a result, the composition of the electrolyte and the condition of electrolysis were optimized to the necessary extent.
Initially, Pt electrode was used to study the kinetics and mechanism of the electrochemical codeposition process.Experiments were carried within 0.01 -0.1 M concentration range of SbOCl and H 2 SeO 3 acid in the presence of the tartaric acid, wherein the pH value varied in the interval of 1.65-2.1.The potentiodynamic method was used to record the cyclic polarization curves on the Pt electrode.After that, thin Sb-Se films were deposited on Ni and Cu electrodes under optimal experimental conditions.
As can be seen from Figure 1, in comparison with other processes, the electroreduction of selenite ions occurs in stages.According to the reference [38], the standard electrode potential of selenite ions is more positive than antimony ions.This affects co-electrodeposition and formation of the compound Sb 2 Se 3 on Pt and Ni electrodes, which occurs between the potentials of the electroreduction of the antimony ions (-0.46 V) [32] and selenite ions (0.4 V) [33].Co-deposition is indicated at -0.44 V, suggesting depolarization, i.e. decrease of the cathodic potential of electrode for antimony deposition.Starting from -0.44 V, the surfaces of Pt and Ni electrodes become completely covered with the black Sb-Se layer.As can be seen from the curve 6 in Figure 1, formation of Sb-Se compound on Cu electrode does not occur.
Further studies of the process of electrolysis on Ni-electrodes were carried out using both galvanostatic and potentiostatic methods.The films obtained by the galvanostatic method at current density of 2-6 A/dm 2 from an electrolyte of 0.05 МSbOCl + 0.05М H 2 SeO 3 + 0.007 МC 4 H 6 O 6 were thermally treated at T thermal = 673-773 K temperature range, in an argon atmosphere.The processes of electrodeposition and heat treatment lasted for 60 min each.The purpose of heat treatment was to obtain crystalline films of Sb 2 Se 3 .Before and after annealing, the obtained thin films were investigated using X-ray phase, SEM and elemental analyses.The results of SEM-EDX analysis presented in Figure 2 show that thin films obtained at a current density of 2-3 A/dm 2 are close to the stoichiometric composition.The content of antimony and selenium in the film is 61.91 at.% and 38.09 at.%, respectively.It was shown previously that the antimony content in the deposited film decreases with an increase of current density to 6 A/dm 2 [34].The co-deposition of Sb-Se layers on Ni electrode by the potentiostatic method was carried out at 298-358 K temperature range.Obtained results show that the stoichiometric Sb 2 Se 3 samples are formed starting from 338 K.This is evidenced by the X-ray phase results of samples (Figure 3).It was established that co-deposition of Sb-Se thin films at 298 K occurs at -0.6 V, while at 358 K, the potential of co-deposition is shifted to -0.36 V, suggesting that an increase in temperature has a positive effect on co-precipitation.During these experiments, the color of the samples varied from golden red to black.It was also observed that the composition of layers obtained within the temperature range 338 -348 K is closer to the stoichiometric one (Figure 2c).Adhesion of films obtained at T > 348 K, however, deteriorates and therefore falls from the surface of the electrode.
Sb-Se thin films were also deposited on Cu electrode.This process was carried out under the galvanostatic regime at 2-2.5 A/dm 2 current density in different concentrations of the electrolyte.The obtained samples were further annealed at 723 K in an argon environment.As shown in Figures 4 and  5, the results of X-ray phase analysis on Cu-electrode differ from the results obtained on Ni electrodes.
The duration of electrolysis and heat treatment on Cu-electrodes lasted for 60 min each.As can be seen from Figures 4 and 5, the electrodeposition of antimony does not occur on the surface of Cu electrode.In all studied concentration intervals of the initial components, the chemical compound Cu 2 Se is formed.In our opinion, this is because selenium is deposited at more positive potential as compared to Sb, and the electrode surface is completely covered with selenium.Therefore, the formation of Cu 2 Se compound occurs after heat treatment and the interaction of antimony with selenium does not occur.The main results of present experiments are summarized in Table 1, showing that the chemical composition of Sb-Se films is closer to the stoichiometric one when they are deposited on the surface of Pt and Ni electrodes.The compound Sb 2 Se 3 does not form on the Cu electrode.Thus, the obtained results have proved that the electrode support material has a considerable influence on the co-deposition process, and that Pt and Ni electrodes are more effective for co-deposition of Sb-Se thin films than Cu electrode.

Figure 1 .
Figure 1.Cyclic polarization curves (I vs. E) of electroreduction of selenite and antimony ions in tartaric acid and co-deposition of Sb and Se on different electrodes.Composition of the electrolyte (M):

Table 1 .
Chemical compositions of Sb 2 Se 3 thin films formed by galvanostatic deposition* from tartaric electrolyte containing different concentrations of Sb and Se on different substrate electrodes.