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ELECTROCHEMICAL DEPOSITION OF THIN SEMICONDUCTIVE Mo-S FILMS V.A.Majidzade, S.F.Cafarova, A.Sh.Aliyev, N.B.Farhatova, Y.A.Nuriyev, D.B.Tagiyev
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Received 17.12.2018
The electrolytic co-deposition of thin Mo-S semiconductive films from aqueous electrolytes has been studied. The potential region for electrolytic co-deposition of molybdenum with sulphur was determined with the technical drawing of cyclic polarization curves by the potentiodynamic method on Pt and Ni supports. Samples of electrolytic co-deposition of molybdenum with sulphur were also obtained using a galvanostatic method on Ni supports. X-ray phase analysis of the obtained samples indicated to the formation of chemical compound of MoS2 composition.
Keywords: MoS2, polarization, thin films, electrodeposition, semiconductors.
https://doi.org/10.32737/0005-2531-2019-1-6-13
In recent years, thin films of metal chal-cogenides [1-4] have attracted much attention due to their unusual properties in functional materials and nanodevices. Thin films are widely used in the electronics, semiconductive engineering and optoelectronics. The utilization of such thin-film materials reduces the material capacity of devices appointed for various purposes.
Transition metal sulphides have been widely applied as solid lubricants, catalysts and cathode materials in lithium batteries. Amorphous chalcogenides are of more interest compounds.
Molybdenum disulphide (MoS2) is one of the most promising, stable n-type semiconductor of layered structure [5-9]. In addition, due to the width of the band gap (1.7 eV), it can be used as a part for solar cells [10] and photoelec-trode materials for the production of hydrogen [11-13]. Recent studies have shown that MoS2 nanoparticles are promising and inexpensive alternatives to platinum using for electrochemical and photochemical hydrogen production from water [14-17].
There are various ways of synthesis of thin MoS2 films: chemical vapor deposition [18, 19], thermal evaporation [20, 21], hydrothermal synthesis [22], electrochemical method [23-25] etc.
In the world literature, there are a number of studies pertaining to the electrochemical deposition of thin semiconductor MoS2 films [23-28].
The authors of [23] electrochemically deposited MoS2 films from an aqueous electrolyte on a conductive glass, coated by tin oxide, and titanium substrates. X-ray diffraction studies show that films are naturally polycrystalline with a hexagonal structure. It was established that the film is an n-type semiconductor. The width of the band gap is 1.68 eV.
In [24], the electrodeposition of thin MoS2 films was studied at room temperature using ionic liquids as electrolyte. To find the optimum composition and deposition regime, the initial components Mo and S were added at different concentrations. Electrochemical deposition of MoS2 was confirmed by both Raman spectroscopy and X-ray photoelectron spectroscopy. The electrodeposited MoS2 films indicated characteristic fluorescent properties. These materials have optoelectronic properties for their application in photodetectors and light-emitting devices.
In work [25], molybdenum sulphide thin films were electrodeposited on glass carboniferous electrodes from aqueous electrolytes containing 10 mM (NH4)2MoS4 and 0.2 M KCl. Adhesion of the films was adequate only for electrodes pretreated with a potential cycle in 1.0 M HNO3 and 0.1 M NaF to enhance the surface roughness. The deposited films that close to stoichiometry were obtained by annealing in Ar at 6000C for 60 min.
The authors of the review article [26] showed that MoS2 films can be electrochemi-
cally deposited in different media: from molten salts, ionic liquids, non-aqueous and aqueous solutions. The value of the potential at which MoS2 is deposited depends on the medium from which the deposition process occurs. Deposits obtained within more negative potentials were rich in molybdenum. It has been established that the temperature of the electrolyte significantly influences the growth of deposits, since a higher temperature leads to faster growth rates of the crystals, as well as to adhesion of the films. At high pH values, the deposition rate is relatively low, while at lower pH values, the mixed phase films of MoS2 are obtained. It was found that the electrodeposition of MoS2 from high-temperature molten salts makes possible to obtain well-defined crystals. The photoelectro-chemical behavior of MoS2 photoanodes in neutral and alkaline electrolytes has shown that the MoS2 semiconductor is oxidized, which leads to its decomposition into metal ions and molecular sulphur.
Nanostructured thin films of molybdenum disulphide were deposited on various substrates in an electrolyzer at a constant current, from an aqueous electrolyte containing ions of molybdate and sulphide [27]. Annealing of deposited thin films at higher temperatures in the range of 450-7000C transforms the deposited amorphous films into a nanocrystalline structure. UV and visible spectra confirmed the presence of Mo-Sx in the deposited films.
In [28, 29] authors produced thin films of molybdenum sulphide (MoSx) on indium-tin oxide polyethylene naphthalate (ITO/PEN) substrates by pulsed electrochemical deposition way. The results showed that the thin MoSx films are uniformly deposited onto the flexible ITO/PEN substrates and they are active catalysts for the reduction of the triiodide. MoSx thin films are produced in a simple and rapid manner at room temperature and atmospheric pressure. This is an important practical contribution to the production of flexible low-cost thin-film solar cells based on plastic substrates.
Analysis of literature leads to the conclusion that there is no optimum variant for obtaining thin semiconductor MoS2 films. This de-
pends on the type and properties of the desired film, on the choice of substrate etc.
Therefore, the aim of our work is to study the kinetics and mechanism of the process of electrodeposition of molybdenum with sulphur from an aqueous solution, the determination of the potential region in which the MoS2 compound is deposited. As known, to obtain thin films of semiconductor materials by co-deposition, the first action is to study the kinetics and mechanism of the electroreduction process of the components separately. The latter was investigated by us beforehand in works [30, 31].
Experimental part
The process of electrochemical deposition of molybdenum with sulphur from aqueous electrolytes was carried out as follows. Solutions containing molybdenum and sulphur are prepared separately. The solution of molybdenum is 1M Na2MoO42H2O, and of sulphur is 0.1 M Na2SO3.
Polarization curves were drawn with the IVIUMSTAT Electrochemical Interface poten-tiostat. At the same time an electrochemical three-electrode glass cell was used. The Pt electrode with an area of 0.4 cm2 and a Ni electrode with an area of 2 cm2 were used as working electrodes. The reference electrode was a silver chloride electrode, and an auxiliary electrode was a platinum plate with an area of 4 cm2. To regulate the temperature in the cell, a universal ultratermostat UTU-4 was applied.
The phase composition of the obtained thin layers was studied using X-ray diffraction analysis of D2 Phazer of Bruker Company (CuK, Ni-filter).
Morphology, relief and elemental composition of the samples were analyzed using a Scanning Electron Microscope (SEM) "Carel Zeiss Siqma" equipped with an EDAX unit.
At the beginning of the experiments, the Pt and Ni electrodes were purified in concentrated nitric acid, and then washed with distilled water. Then, the Ni-electrodes were electro-chemically polished in a mixed solution of sulphuric, phosphoric and citric acids under certain conditions (T = 293-303 K, i = 510-1 A/dm2, t = 180 s) and washed with distilled water.
Results and discussion
As indicated above, with the study of the electrochemical reduction of molybdenum [30] and sulphur [31], the potentials regions for the deposited components is separately determined.
The results of the experiments showed that the electrochemical reduction of molybdate ions occurs in the range of potentials 0.2-(- 0.7) V and the ion sulphite within 0.4-(-0.3)V, respectively, according to the reactions (1), (2):
Mo(VI) ^ Mo(III) ^ Mo0 , (1)
S(IV) ^ S0 ^ S2- . (2)
Taking into account the potentials of electrochemical reduction of molybdate and sulphite ions, cyclic polarization curves with a stationary potential of -0.7 V were drawn in 1 M Na2MoO4-2 H2O + 0.1 M Na2SOs electrolyte (Figure 1).
MA
20.
-1-1-1-1-1-1-1-1-1-1-1-1-1-1-r~
-D.5 0.0 0.5 1.0
Potential v
Fig. 1. The cyclic polarization curve of electrolytic codeposition of molybdenum with sulfur from aqueous electrolytes on the Pt electrode; Т = 338 К, Ev=0.03 V/s.
Figure 1 shows the cyclic polarization
curve of the electrochemical co-deposition of
molybdenum with sulphur. As can be seen, the
electrochemical deposition occurs in steps within
the potential range of 0.5 - (-0.70)V on the Pt
electrode due to following reaction: 1 о и ,
S(IV) S° S2" ■ (3)
Mo6+ + 2S2- + 2e- ^ MoS2. (4)
On the surface of the electrode, the elec-trodeposition of sulphur occurs up to -0.3 V (reaction 3) starting from 0.3V (Figure 2).
mA
Potential v
Fig. 2. Cyclic polarization curve of electrolytic co-deposition of molybdenum with sulphur from aqueous electrolytes on a Ni-electrode; electrolyte: 1M Na2MoO42H2O+0.1M Na2SO3, 7=338 К, EV=0.03 V/s.
After that, from -0.4V potential begins a deep recovery of S before S2- and at the same time occurs electrodeposition molybdenum with sulphur to -0.7 V (reaction 3, II and reaction 4). Alongside with this the surface of electrode is covered by a black lager. The continuation of the process was represented on the anode branch of the cyclic curve. The peak obtained at potential of -0.3 V refers to the dissolution of the obtained molybdenum compound with sulphur, whereas after 0.2 to 1.0 V - to the dissolution of sulphur. The study of the process of electrodeposition to -0.7 V is due to the fact that after this potential, hydrogen evolution is observed which degrades the quality of the obtained Mo-S films.
The electrolytic co-deposition of molybdenum with sulphur was also investigated on a Ni- electrode. As can be seen from Figure 2, in comparison with the Pt electrode, the electro-deposition on the Ni-electrode occurs at more cathodic potential - within the range of -0.55 -(-13)V.
After finding the potential for electrolytic co-deposition of molybdenum with sulphur, further studies have been devoted to the selection of the optimum electrolyte composition and to the electrolysis regime for obtaining the stoichiometric composition of the MoS2 compound. For this, the influence of certain factors on the process of electrolytic co-deposition has been studied.
Fig. 3. The effect of the concentration of molyb-date ions on the electrolytic co-deposition of molybdenum with sulfur from aqueous electrolytes on the Pt electrode. Electrolyte - Na2MoO42H2O, M: 1 - 0.1, 2 - 0.2, 3 - 0.4, 4 - 0.8, 5 - 1.0, 6 - 1.2 + 0.1 Na2SO3. T = 338 K, EV=0.03 V/s.
Fig. 4. The effect of the concentration of sulfite ions on the electrolytic co-deposition of molybdenum with sulfur from aqueous electrolytes on the Pt electrode. Electrolyte - Na2SO3 , M: 1 - 0.01, 2 - 0.02, 3 - 0.04, 4 -0.08, 5 - 0.1, 6 - 0.2 +1.0 Na2MoO42H2O; T = 338 K, EV=0.03 V/s.
First, the effect of the concentration of the initial components on the electrolytic codeposition of thin Mo-S films was investigated. This process has been studied in two steps: the effect of the concentration of molybdate and sulphite ions separately (Figure 3).
Figure 3 shows the effect of the concentration of molybdate ions (0.1-1.2) M on the codeposition of molybdenum with sulphur. With increasing the concentration of molybdate ions, the potential value of electrolytic co-deposition shipts to the positive side. The increase in concentration also affects the composition and quality of the obtained Mo-S thin films. Table 1 shows the results of the experiments. As can be seen from the table, black, smooth, crystalline, uniform coatings of stoichiometric composition are obtained at 1.0 M of Mo concentration. After this, the quality of the films deteriorates and the amount of molybdenum in the composition increases. Therefore, the optimum concentration of molybdenum was chosen (1.0-1.1)M.
In Figure 4, the effect of the concentration of the sulphite ions on the electrolytic codeposition of molybdenum with sulphur is shown. The study was carried out within the (0.01-0.2)M interval.
According to Figure 4, with increasing the concentration of sulphite ions, the potential of electrolytic co-deposition shifts to the positive side (from 0.15 to 0.35 V).
An increase in the concentration of sulphite ions in the electrolyte significantly affects the composition and quality of the obtained thin Mo-S films.
Table 2 shows the results of the experiments. As can be seen, black, smooth, crystalline, uniform Mo-S coatings of stoichio-metric composition are obtained at 0.1 M concentration of sulphite ions.
Table 1. The effect of molybdate ions concentration on the composition and quality of electrodeposited thin Mo-S films
Composition of electrolyte, M Amount of Mo in the film, % Appearance of the coatings
Na2MoO42H2O Na2SOs
0.1 0.1 21.6 black, uneven, amorphous, loose
0.2 0.1 33.2 black, uneven, semicrystalline, loose
0.4 0.1 42.8 black, amorphous, uniform, smooth
0.8 0.1 54.3 black, uniform, amorphous, smooth
1.0 0.1 60.1 black, smooth, crystalline, uniform
1.2 0.1 65.3 black, crystalline, uneven, loose
Table 2. The influence of the concentration of sulphite ions on the composition and quality of electrodeposited thin Mo-S films
Composition of electrolyte, М Amount of S in the film, % Appearance of coatings
Na2SO3 Na2MoO42H2O
0.01 1 11.7 black, uniform, amorphous, loose
0.02 1 19.2 black, uniform, amorphous, smooth
0.04 1 26.4 black, amorphous,uniform, smooth
0.08 1 32.1 black, uneven, semicrystalline, loose
0.1 1 39.9 black, smooth, semicrystalline, loose, uniform
0.2 1 59.3 black, semicrystalline, uneven, loose
In addition, from Figure 4 it follows that with an increase in the concentration of sulphite ions, the rate of the electrochemical reaction decreases. This is due to the fact that at high cathode potentials with an increase in the concentration of sulphite ions, on the electrode surface, first sulphur deposits which cover the electrode surface and makes it difficult for current to flow. Therefore, Mo-S electrodeposition occurs in small cathode currents. As soon as, at shift the cathode potential begins reduction sulphur to sulphide ions, the rate of the electrochemical reaction is accelerated, and thin films Mo-S there are deposited.
After studying the influence of the concentration of the initial components, the effect of temperature on the process of electrolytic codeposition of thin Mo-S semiconductor films was also studied by the potentiodynamic method within the 298-348 K intervals.
With increasing of the electrolyte temperature, the potential of electrolytic codeposition of Mo-S shifts to more positive direction. In addition, the temperature also affects the quality of the deposited thin films and their adhesion to the surface of the electrode. There-
fore, to obtain black, smooth, crystalline, uniform Mo-S coatings with the stoichiometric composition, an optimum temperature interval 333-338 K was chosen.
The effect of the scanrate was studied at an interval of 0.005-0.2 V/s.
With increasing the scanrate, the potential of electrolytic co-deposition of molybdenum with sulphur shifts to the positive side and the cathode current increases (Fiure 5). When investigating the effect of the scanrate, black, smooth, crystalline, uniform Mo-S coatings with stoichiometric composition are obtained at 0.03-0.06 V/s.
Figure 6 shows the morphology, topography and of elements composition of Mo-S samples obtained by electrolytic co-deposition using a scanning electron microscope.
The results of a scanning electron microscope show that electrodeposition of both components is carried out on the Pt and Ni electrodes. And the X-ray phase analysis shown in Figure 7 proves the preparation of the chemical compound MoS2, which was deposited by the potentiostatic method on the Ni electrode.
—I—,—,—,—,—I—,—,—,—,—
-0.5 0.0 0.5
Potential
Fig. 5. The effect of scanrate on the electrolytic co-deposition of molybdenum with sulfur from the aqueous electrolytes on the Pt electrode. T=338 K. Electrolyte: 1M Na2MoO4-2H2O + 0.1M Na2SO3, EV (V/s.): 1 - 0.005, 2 - 0.01, 3 -0.03, 4 - 0.06, 5 - 0.12, 6 - 0.2.
Fig. 6. SEM and EDX analyses of electrodeposited thin films of Mo-S. Electrolyte - 1M Na2MoO4-2H2O + 0.1 M Na2SO3; T = 338 K, EV=0.03 V/s.
Fig. 7. X-ray phase analysis of electrodeposited Mo-S thin films. Electrolyte: 1 M Na2MoO4-2H2O + 0.1M Na2SOs. T = 338 K, Ev=0-03 V/s.
Figure 7 shows 3 radiographs of the compounds obtained. Despite the fact that the samples were deposited in the same optimum mode, depending on the heat treatment temperature in an argon atmosphere, when heated to 623 K, there is formed compound MoS2, by heating to the 653 K the compound Mo3S4 and by heating to 923 K the compound NiMoO4 .
Conclusions
The electrodeposition process of Mo-S on Pt and Ni electrodes from aqueous electrolytes was studied by cyclic voltamperometry. It was determined that the electrolytic codeposition on the Pt electrode occurs within the
potential range of -0.5 - (-0.70) V, and on the Ni electrodes within 0.55 - (-1.3) V. The influence of various factors (concentration of components, temperature, and potential sweep) on the electrolytic co-deposition of molybdenum with sulphur has been also studied. The results of the experiments show that black, smooth, crystalline, uniform coatings are obtained at 1.0 M concentration of the molybdate ions, 0.1 M concentration of the sulphite ions, at 0.03 V/s scanrate, at 338 K temperature and 7 mA /cm2 current density.
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Mo-S NAZiK YARIMKEÇiRiCi TOBaQOLORlNiN ELEKTROKlMYOVi ÇÔKDÙRÙLMOSi
V.A.Maridzad3, S.F.Cafarova, A.§.Oliyev, N.B.Farhatova, Y.O.Nuriyev, D.B.Tagiyev
Sulu mahlullardan Mo-S nazik yarimkeçirici tabaqalarinin birga elektroçokdurulmasi tadqiq edilmiçdir. Potensiodinamik usulla Pt va Ni elektrodlari uzarinda tsiklik polyarizasiya ayrilarinin çakilmasila molibdenin kukurd ila birga çokma prosesinin potensial sahasi muayyan edilmiçdir. Hamçinin qalvanostatik usulla Ni elektrodlari uzarinda Mo-S nazik tabaqalarinin numunalari alinmiçdir. Alinan bu numunalarin rentgen-faza analizinin naticalari gostarir ki, tadqiqatlar naticasinda MoS2 kimyavi birlaçmasi alinmiçdir.
Açar sozter: MoS2, polyarizasiya, nazik t3b3q3hr, elektroçôkma, yarimkeçiricibr.
ЭЛЕКТРОХИМИЧЕСКОЕ ОСАЖДЕНИЕ ТОНКИХ ПОЛУПРОВОДНИКОВЫХ ПЛЕНОК Мо-8
В.А.Меджидзаде, С.Ф.Джафарова, А.Ш.Алиев, Н.Б.Фархатова, Я.А.Нуриев, Д.Б.Тагиев
Исследовано совместное электроосаждение тонких полупроводниковых пленок Мо-8 из водных электролитов. Снятием циклических поляризационных кривых потенциодинамическим методом на Р1 и N1 подложках определена область потенциалов совместного электроосаждения молибдена с серой. Гальваностатическим методом на N1 подложках получены образцы совместного электроосаждения молибдена с серой. Рентгенофазовый анализ полученных образцов указал на образование химического соединения Мо82.
Ключевые слова: МоБ2, поляризация, тонкие пленки, электроосаждение, полупроводники.
