Analysis of tellurium thin films electrodeposition from acidic citric bath

Analysis of tellurium thin films electrodeposition from acidic citric bath

G Model ARTICLE IN PRESS APSUSC-32893; No. of Pages 8 Applied Surface Science xxx (2016) xxx–xxx Contents lists available at ScienceDirect Applie...

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ARTICLE IN PRESS

APSUSC-32893; No. of Pages 8

Applied Surface Science xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Analysis of tellurium thin films electrodeposition from acidic citric bath a ˙ ´ nski Remigiusz Kowalik a,∗ , Dawid Kutyła a , Krzysztof Mech b , Piotr Zabi a b

AGH University of Science and Technology, Faculty of Non-Ferrous Metals, al. A. Mickiewicza 30, 30-059 Krakow, Poland AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, al. A. Mickiewicza 30, Krakow, Poland

a r t i c l e

i n f o

Article history: Received 11 January 2016 Received in revised form 15 March 2016 Accepted 16 March 2016 Available online xxx

a b s t r a c t This work presents the description of the electrochemical process of formation thin tellurium layers from citrate acidic solution. The suggested methodology consists in the preparation of stable acidic baths with high content of tellurium, and with the addition of citrate acid. In order to analyse the mechanism of the process of tellurium deposition, the electroanalytical tests were conducted. The tests of cyclic voltammetry and hydrodynamic ones were performed with the use of polycrystalline gold disk electrode. The range of potentials in which deposition of tellurium in direct four-electron process is possible was determined as well as the reduction of deposited Te◦ to Te2− and its re-deposition as a result of the comproportionation reaction. On the basis of the obtained results, the deposition of tellurium was conducted by the potentiostatic method. The influence of a deposition potential and a concentration of TeO2 in the solution on the rate of tellurium coatings deposition was examined. The presence of tellurium was confirmed by X-ray spectrofluorometry and electron probe microanalysis. In order to determine the phase composition and the morphology, the obtained coatings were analysed with the use of x-ray diffraction and scanning electron microscopy. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Tellurium is a particularly interesting element which exhibits intermediate properties of metals and non-metals featuring energy gap of 0.34 eV. Semiconducting properties of tellurium allow for a number of its applications in many industry branches. Physicochemical properties of thin tellurium layers are more and more frequently exploited in infrared photoconductive detectors, holographic recording materials, sensors, thermoelectric, piezoelectric and optoelectronic devices [1,2]. Recently, high interest has been focused on gas sensors constructed on the basis of thin tellurium layers whose sensitivity and selectivity significantly depend on the thickness of semiconducting coatings. They are able to detect small amounts of harmful gases, such as SO2 , CO2 , CO, NO2 and NH3 . Moreover, this element, when combined with d-block metals, allows for obtaining advanced optical materials used in constructing solar cells, diodes and radiation emitters of different wave length. The leading methods of thin tellurium coatings production are physical and chemical vapour deposition methods, which enable production of coatings with different thickness. However,

∗ Corresponding author. ˙ ´ E-mail address: [email protected] (P. Zabi nski).

one of the alternative ways to obtain tellurium coatings can be the electrochemical method allowing for a very precise control of the process and subsequently obtained layers of strictly defined properties. It is known that electrochemical technique allows to obtain various materials from aqueous solutions which are utilized within the area of contemporary thin film materials technologies [3]. However, the number of parameters of the electrolysis that should be adjusted requires intensive tests in order to achieve required results. Traditional electroanalytical techniques, such as cyclic voltammetry or the hydrodynamic method enable identification of regimes in which electrochemical reactions responsible for obtaining the desired material take place [4,5]. These techniques are widely applied in the analysis of the kinetics and mechanism of electrode reactions. The detailed analysis of the phenomena taking place during the process of electrodeposition by the electroanalytical techniques enables to produce metallic [6–10] and alloy coatings [11–16] as well as semiconductor thin films [17–22]. The process of electrochemical tellurium deposition can be performed both from acidic and alkaline aqueous solutions [23–40]. However, it is difficult to achieve it due to electrochemical properties of tellurium and its very low solubility in aqueous solutions [38]. The electrochemical properties of tellurium in aqueous solutions are described in the literature [25–29,35,37,41–47]. Because

http://dx.doi.org/10.1016/j.apsusc.2016.03.127 0169-4332/© 2016 Elsevier B.V. All rights reserved.

Please cite this article in press as: R. Kowalik, et al., Analysis of tellurium thin films electrodeposition from acidic citric bath, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.03.127

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of the properties of tellurium, most of the values are calculated from the thermodynamic data, and only some of them were experimentally proved. In case of tests performed in acid solutions, due to small solubility of tellurium, experiments were limited to the maximal concentration of 0.01 M Te [26,29,35,39,40,42]. It results in conducting deposition of tellurium under conditions of diffusion control, which means that the rate of the coating growth is very low and it results in amorphous or fine-crystalline structure. Moreover, there is a risk of obtaining coatings of dendritic structure or even powders. Additionally, due to the mechanism of tellurium reduction from aqueous solutions, a competitive reaction leading to the reduction of deposited tellurium to Te2− may occur causing dissolution of the tellurium coating and a decrease of the electrolysis process efficiency. The aim of this work is to analyse the process of electrochemical deposition of tellurium coatings in the solution not used so far. The tests were performed in citric acid solution at pH = 2, which enable to obtain electrolyte with tellurium concentration 0.128 M. The cyclic voltammetry and rotating disk electrode were applied to identify reactions taking place on the electrode and to determine ranges of potentials in which its deposition occurs. Next, the results were confirmed by conducting electrolysis under selected potentiostatic conditions. The deposited coatings were analysed by XRF, SEM and XRD. The particular attention was paid to the mechanism of tellurium deposition and morphology of obtained coatings.

copper sheet (2.8 cm2 ). The copper sheets were chemically etched in a HNO3 :CH3 COOH:H3 PO3 1:1:1 mixture for 60 s at the temperature 60 ◦ C prior to the process of deposition. The Autolab PGSTAT30 and EDAQ EA163 potentiostats were used for the voltammetry experiments as well as a deposition process in the potentiostatic mode. XRD diagrams were registered with the Rigaku Miniflex II diffractometer with Cu K␣ radiation. The morphology of the product was studied with the use of Hitachi SU-70 by scanning electron microscopy (SEM) on a SU 70 instrument (Hitachi). The chemical composition of electrodeposits was characterized by the wavelength dispersive X-ray fluorescence method (WDXRF) by Rigaku PriMini.

3. Results and discussion In order to determine the range of potentials in which deposition of tellurium is possible, preliminary voltammetric tests were performed in a solution containing 0.008 M TeO2 (Fig. 1). Electrochemical window was changed systematically in order to define electrode reactions proceeding in the analysed system. All scans started at 1.0 V. Following the first vertex potential was switched at potentials −0.4, −0.5 and −0.6 V (Fig. 1a) and −0.7, −0.8 and −1.0 V (Fig. 1b). Finally the scans were over at 1.0 V. The first cathodic peak (A) occurs at potential −0.5 V and is connected with the reduction of tellurium ions due to the reaction [39]:

2. Methodology of the research

HTeO2 + + 3H+ + 4e− → Te + 2H2 O

All chemicals used in this work were of analytical grade. Concentrations of solutions varied from 0.008 to 0.128 M TeO2 . All solutions consisted of 0.6 M C6 H8 O7 and the pH was adjusted to 2.0 by sulfuric acid addition. A conventional three electrode system was employed with a platinum foil as a counter electrode (6 cm2 ) and a saturated calomel electrode (SCE) as the reference electrode. Polycrystalline gold disk was used for voltammetric and hydrodynamic experiments. The substrate for electrodeposition was a

At the same time, an anodic peak occurs on the voltammogram at potential 0.5 V when sweeping is conducted towards positive potentials. The anodic peak C is connected with a dissolution of the deposited tellurium. When the electrochemical window is extended towards more negative potentials, peak C grows. It clearly indicates that the process of tellurium deposition is continued also at lower potentials. At potential −0.8 V another cathodic peak (B) appears, which in turn leads to a decrease of anodic peak (C). It

(1)

Fig. 1. Cyclic voltammograms on gold electrode in the solution 0.008 M TeO2 , 0,6 M C6 H8 O7 , pH = 2, v = 0.04 V/s with different vertex potential E2 . Parameters of cyclic voltammograms: E1 = 1.0 V → E2 = x → E3 = 1.0 V.

Please cite this article in press as: R. Kowalik, et al., Analysis of tellurium thin films electrodeposition from acidic citric bath, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.03.127

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Fig. 2. Cyclic voltammograms on gold electrode in solutions containing different concentrations of TeO2 , 0.6 M C6 H8 O7 , pH = 2, v = 0.04 V/s.

indicates that a reaction (2) takes place, which is responsible for a dissolution of previously deposited tellurium [34,49]: Te + 2H+ +2e− → H2 Te

(2)

Below potential −0.9 V another increase of cathodic current is connected with the evolution of hydrogen: 2H+ + 2e− → H2 ↑

(3)

Next, cyclic voltammetry measurements were performed within potentials range from 1.0 to −0.8 V in solutions of different concentration of TeO2 (Fig. 2). It is clearly seen that the increase of TeO2 concentration in the electrolyte is accompanied by a systematic increase of maximum of peak A (Fig. 3). However, it should be noticed that the peak maximum moves first towards more positive potentials, but at the concentration of 0.016 M TeO2 this trend changed and it moves towards more negative values (Fig. 4). This behaviour may indicate a change of the mechanism of tellurium deposition. It can be connected either with a transition of reaction (1) from the area of diffusion control to the kinetic one or because of a very high concentration of tellurium in the electrolyte with the change of its ionic form and the level of complexation. Thus, it can decide about the overpotential value needed to reduce a given ionic form of tellurium to Te◦ . Another argument supporting the change of the deposition of tellurium mechanism is the direction of the

Fig. 3. The dependence of the maximum of peak A from the concentration of TeO2 in solution 0.6 M C6 H8 O7 , pH = 2, v = 0.04 V/s.

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Fig. 4. The dependence of the peak A potential on the concentration of TeO2 in solution 0.6 M C6 H8 O7 , pH = 2, v = 0.04 V/s.

shift of cathodic peaks maxima. The increase of TeO2 concentration in the solution is accompanied by a systematic deviation of experimental points from the linear dependence. The above phenomenon can also be explained by a growth of tellurium coating on the surface of the gold electrode. Due to semiconducting properties of the deposited tellurium, and its lower conductivity in comparison with metals, the electrode reactions can be slowed down. Consequently, the type of control of reaction (1) was examined with the use of voltammetry with different sweeping rate, and rotating disk with different rotation velocity. At first, the voltammetric tests with different rate of scanning for solutions of different TeO2 concentration were performed (Fig. 5). Maximum of peak A was visible on voltammograms when solutions of concentration up to 0.064 M TeO2 were applied. Above the concentration of 0.064 M, the peak A overlapped with the peak B, therefore it was impossible to read it. Peak A maximum grows linearly from v0.5 indicating diffusion control for reaction (1) for concentrations range from 0.001 to 0.064 M TeO2 . Next, hydrodynamic tests were conducted that confirmed voltammetric results. Fig. 6 presents values of currents registered during voltammetric tests on the disk electrode depending on rotation velocity. The results obtained in solutions with concentrations 0.032 and 0.064 M are presented for comparison. Linear dependence typical for diffusion control is visible within the potentials

Fig. 5. The dependence of peak A current density i = f(v0.5 ) obtained in the solutions containing different concentrations of TeO2 , 0.6 M C6 H8 O7 , pH = 2.

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Fig. 7. The mass increase of the electrodes registered after deposition of coatings at different potentials in the solutions containing different concentrations of TeO2 , 0.6 M C6 H8 O7 , pH = 2.

Fig. 6. The dependence of current density i = f(v0.5 ) obtained at different potentials in the solutions containing (a) 0.32 M TeO2 , (b) 0.64 M TeO2 and 0.6 M C6 H8 O7 , pH = 2.

range from −0.5 to −0.8 V (Fig. 6a) for the solution of 0.032 M TeO2 . Whereas, for concentration of 0.064 M TeO2 , cathodic current does not grow with an increase of the electrode rotation velocity indicating transition into the area of kinetic control (Fig. 6b). Both voltammetric tests, with variable rate of sweeping and hydrodynamic one, clearly demonstrate that reaction (1) mechanism changes from diffusion control to the kinetic one depending on TeO2 concentration in the electrolyte. Moreover, even in case of a relatively intensive growth of thickness of the deposited tellurium, the process of its reduction was not slowed down during hydrodynamic tests. The dependence f(␻−1 ) = i−1 obtained from the results has a characteristic linear dependence for the solutions in which reaction (1) is under diffusion control. Thus, it can be assumed that neither semiconducting properties of the growing tellurium layer, nor a change in control of reaction (1) explained the change of peak A position depending on tellurium concentration (Fig. 3). Therefore, it can be suspected that the above effect is connected with either the change of ions form or the level of tellurium complexation in citrate solutions connected with its high concentration. Next, on the basis of the performed electroanalytical tests, the process of potentiostatic deposition of tellurium from solutions of different concentration of TeO2 was carried out. The results of the electrodes mass gain obtained after the deposition process correspond well with the suggested mechanism of tellurium coatings deposition. The growth of TeO2 concentration in the electrolyte is accompanied by an increase of the deposited mass. Moreover, application of low concentrations of tellurium within the range from 0.008 to 0.032 M allowed to obtain coatings of similar thickness, regardless of the potential applied. The results

explicitly confirm that within the concentrations from 0.008 to 0.032 M, the electrode reactions responsible for the process of tellurium deposition are under diffusion control. Within the potentials range from −0.2 to −0.6 V the process of tellurium deposition takes place mainly following the reaction (1). The deposition process efficiency is maintained at a very high level, reaching even 100%. A slight decrease of the coatings deposition process velocity observed at potential −0.7 V is caused by reaction (2). However, it should be noticed that the rate of the coatings deposition changes insignificantly, indicating probably that the mechanism of tellurium deposition undergoes changes, and an additional reaction occurs in the system [36,46]: 2H2 Te + HTeO2 + → 3Te + 2H2 O + H+

(4)

The above reaction results in an incomplete dissolution of the obtained tellurium coatings, and continuation of the deposition process. However, a decrease of efficiency of the tellurium deposition process for potentials below −0.7 V (Fig. 8) is visible. It can be caused by an escape of H2 Te molecules from the electrode surface before they can react with HTeO2 + ions. Moreover, also reaction (3) can be responsible for the decrease of the process efficiency when low potentials are applied, particularly in acidic solutions. For the concentration 0.064 M TeO2 , a strong dependence of the electrode mass gain on the applied potential is visible, which may indicate that in the potentials range from −0.3 to −0.5 V the limiting current, typical for diffusion controlled processes, is not reached. These results are consistent with the results obtained during hydrodynamic tests (Fig. 6). Efficiency of the electrolysis process, likewise for solutions containing lower concentrations of TeO2 , was also maintained at a very high level reaching 100%. Application of potentials lower than −0.5 V resulted in the formation of very brittle coatings, which fell off the electrode surface already after a few minutes of the electrolysis process. That is why Figs. 7 and 8 do not include further results obtained during electrolysis performed at lower potentials. Similar effects were observed by applying solutions of higher concentrations than 0.064 M TeO2 . Brittleness of the coatings can be caused by a too fast growth of tellurium and generation of inner stresses in the coating making it to fall off the substrate. In order to avoid observed problems with the coatings brittleness, further studies including surface active agents [46] are planned. They can solve the existing problem and enable application of more concentrated solutions. The morphology of the obtained coatings was analysed depending of the applied potential and concentration of TeO2 . At first, the

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Fig. 8. The dependence of the current efficiency of the process of tellurium electrodeposition on different potentials in the solutions containing different concentrations of TeO2 , 0.6 M C6 H8 O7 , pH = 2.

coatings obtained in the wide potential range from −0.2 to −0.8 V in solutions of different concentration of TeO2 were analysed. As it is seen in Fig. 9, the coatings morphology is strongly dependent on both: applied potential and the electrolyte composition. Coatings deposited within potentials range from −0.2 to −0.4 V are homogeneous with a few holes when applying solution of concentration 0.008 M TeO2 . Whereas, the coating obtained at potential −0.5 V consists of tiny spherical burls (better visible at higher magnification – Fig. 10), which are additionally covered with tiny plates at the surface. Further decreasing of the potential results in the formation of spongy, porous coatings. The morphology transition can be linked to a change of the tellurium deposition mechanism. Within the range of more positive potentials, tellurium is deposited by reaction (1). The occurrence of plate burls or spongy deposits can be attributed to reaction (4), which appears at lower potentials. Similar observations were also made during selenide coatings electrodeposition, where the morphology of the coatings strongly depends on the mechanism of selenium deposition [48,49]. An increase of tellurium concentration in the solution to 0.032 M TeO2 shifts the potentials range in which coatings of different morphologies are obtained. Homogeneous and coherent coatings were obtained for potentials from −0.2 to −0.3 V. A characteristic feature visible in the photographs is the occurrence of tiny cracks. However, the obtained coatings well adhered to the substrate and did not fall off. The characteristic spherical burls starts to appear already from potential −0.4 V, and are very well visible when the deposition

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process was carried out at potential −0.5 V. It can be thought that the dominating reaction within the potentials range from −0.2 to −0.5 V is reaction (1). For potential lower than −0.6 V the characteristic plates or spongy deposits connected with reaction (4) start to occur. The above observations well reflect the previously conducted electroanalytical tests. It should be noticed that, together with the growth of concentration of TeO2 in the electrolyte, the reduction process connected with reaction (1) starts at more and more positive potentials (Fig. 2). Additionally, peak A is getting wider. Thus, the potentials range in which it is possible to obtain homogeneous and coherent coatings is extended. Whereas, the reaction (2) and deposition of tellurium by reaction (4) move towards more negative potentials. That is why the increase of TeO2 concentration in the electrolyte is accompanied by the appearance of spongy, porous deposit at lower potentials. The growth of porous deposit can be explained in the following way. Deposited tellurium acts as a nucleation site for further formation of tellurium according to reaction (4). It seems that the edges are favourite areas for growth of the coating and as a result spongy structure is formed. Additionally, the samples obtained in the solution of TeO2 with concentration 0.064 M were examined with the use of a scanning electron microscope, and compared with coatings obtained in solutions of lower tellurium concentrations. Higher magnifications were applied for a better view of the obtained coatings morphology. The tests included coatings obtained at potential −0.4 and −0.5 V. By comparing the pictures, it can be noticed that an increase of TeO2 concentration in the electrolyte favours more homogeneous coatings with compact structure. Characteristic spherical burls are smaller and they adhere more tightly to each other than the ones that occur in coatings obtained from a solution having two times lower concentration. Analysing the change of the morphology of coatings obtained from electrolytes with different concentrations of TeO2 , it can be assumed that it is connected with the transition of reaction (1) from the range of diffusion control to the kinetic one. It should be emphasized that the improvement of coatings cohesion can positively influence properties of tellurium coatings [46], and particularly their conductivity. Finally, the process of tellurium deposition was conducted from solutions of different TeO2 concentration at potential −0.5 V at different durations (Fig. 11). The gain of the samples mass depends linearly on the time of the process. It can be concluded that despite of a decrease of conductivity connected with the growth of a semiconducting layer, its further growth was not inhibited. The coatings did not crack and they adhered very well to the substrate. The coatings morphology was homogeneous and coherent on the whole surface are. The grains tightly adhered to each other, and there were no cracks or pores in the coating. The phase analysis of the obtained coatings was difficult due to the use of copper substrate. The diffractograms present very

Fig. 9. The surface morphology of deposited coatings obtained after deposition at different potentials in the solutions containing different concentrations of TeO2 , 0.6 M C6 H8 O7 pH = 2, a–g) 0.008 M TeO2 , h–m) 0.032 M TeO2 .

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Fig. 10. The surface morphology of deposited coatings obtained after deposition at different potentials in the solutions containing different concentrations of TeO2 , 0.6 M C6 H8 O7 , pH = 2, a and b) 0.008 M TeO2 , c and d) 0.032 M TeO2 , e and f) 0.064 M TeO2 .

clear peaks connected with copper at 43.3◦ and 50.4◦ (JCPDS 00004-0836) and overlapping peaks related with tellurium at 43.1◦ and 50.2◦ (JCPDS 01-085-0563) (Fig. 12). However, it should be noticed that the height of peaks originating from pure Te systematically grows with the growth of the coating thickness. Whereas, peaks originating from the copper sheet get smaller. Moreover, an additional effect complicating a proper analysis of the results is a possibility of interfacial Cu-Te phases formation, which may occur due to the reaction between the deposited tellurium and copper [50,51]. Peaks positioned at 24.5◦ , 27.5◦ , 29.1◦ and 45.0 ◦ Can be attributed simultaneously to Cu2 Te (JCPDS 00-057-0477), Cu7 Te4 (JCPDS 00-057-0477) or Cu13 Te7 (JCPDS 00-057-0196) phases or other nonstoichiometric Cux Tey compounds [51]. In turn peaks at 24.5◦ and 29.0◦ fits very well to the tellurium pattern (JCPDS 01085-0563) and moreover one peak at 27.5 ◦ Can also correspond to tellurium according to the card JCPDS 01-085-0736.

Similar problem related with the proper assignment of the peaks appeared when the structure analysis were carried out for samples obtained at different potentials (Fig. 13). The group of peaks between 20◦ and 30 ◦ C an be attributed simultaneously to tellurium or Cu-Te phases. However it can be observed that there are characteristic differences between analysed samples. Peaks registered at 43.1◦ (Te) are very well visible for coatings obtained at −0.4 and −0.5 V when compared with the related peaks for samples obtained at other potentials. There is no peak at 43.1◦ for samples deposited at −0.7 and −0.8 V and it is probably caused by a low crystallinity or even amorphous structure of the coatings. The structure of these coatings can be deduced from pictures obtained by scanning electron microscope (Fig. 9). It can be observed that the deposited tellurium at low potentials exhibits porous and spongy morphology which is characteristic for amorphous structure. Probably it is the reason why it does not give any reflexes. In turn, the peak at

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43.1◦ for a sample obtained at −0.3 V is very small despite of the homogenous and compact structure of the coating. But according to the mass increment measurements performed after deposition the coating is thinner than others (Fig. 7) what may have an effect on the intensity of this peak. Therefore, it can be deduced that the peak at 43.1◦ presented on the intensity diagrams originates mainly from pure tellurium, while the others are very hard to interpret. The crystallinity of the coatings can be improved by applying higher temperature of the electrolyte during process of electrodeposition, but the formation of Cu-Te compounds would be promoted. However, it should be mentioned that this reaction can be used, among others, for synthesis of copper tellurides which can function as back layer for CdTe thin film solar cells [52–54]. Besides, formation of copper tellurides as an intermediate layer between tellurium coating and the copper sheet can improve adhesion of the coating to the substrate.

Fig. 11. The mass increase of the electrodes registered after deposition of coatings at −0.5 V vs. SCE at different times in the solutions containing different concentrations of TeO2 , 0.6 M C6 H8 O7 , pH = 2.

4. Conclusions The conducted tests clearly indicate a possibility of obtaining tellurium coatings from citrate solutions with high concentration of TeO2 . The performed analyses of the elemental and phase composition confirm the presence of tellurium in the obtained coatings. The applied electrolyte exhibits significantly higher solubility of tellurium than other inorganic acids, and at the same time it enables application of higher pH. The voltammetric tests show that the tellurium deposition process can proceed following two mechanisms. Within the range of more positive potentials, between −0.2 and −0.6 V, the deposition of tellurium takes place according to the reaction: HTeO2 + + 3H+ + 4e− = Te + 2H2 O

(I)

Below the potential −0.6 V the deposited tellurium is then reduced to H2 Te which reacts with HTeO2 + ions present in the solution producing again Te◦ : Te + 2H+ +2e− → H2 Te

Fig. 12. XRD patterns (CuK␣) of the coatings obtained after different time of electrolysis from electrolyte: 0.032 M TeO2 , 0.6 M C6 H8 O7 , pH = 2.

(II)

2H2 Te + HTeO2 + → 3Te + 2H2 O + H+ It is clear that the type of the mechanism decides about the morphology of the obtained deposit. In case of the first mechanism, homogeneous and coherent coatings are obtained. In the second case, the deposit is porous and spongy. An increase of tellurium in the electrolyte results in widening the range of potentials in which deposition of tellurium proceeds following the mechanism (I) and it shifts the occurrence of tellurium reduction reaction (II) towards more negative potentials. An increased concentration of tellurium ions favours formation of more compact coatings without burls and dendrites, and with lower number of pores characteristic for more dilute solutions. The above effect is connected with the transition of reaction (I) from the range of diffusion control to the activation one. Moreover, it should be emphasized that an increase of tellurium ions concentration significantly accelerates the process of coatings deposition in comparison to solutions of lower concentrations. Acknowledgments The author is grateful to Dr. Tomasz Tokarski for SEM measurements. This work was supported by the Polish National Science Center under grant 2011/01/D/ST5/05743. References

Fig. 13. XRD patterns (CuK␣) of the coatings obtained at different potentials from electrolyte: 0.032 M TeO2 , 0.6 M C6 H8 O7 , pH = 2.

[1] F.A. Devillanova, W.-W. Du Mont, Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium, vol. 1, RSC Publishing, Cambridge, 2013.

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Please cite this article in press as: R. Kowalik, et al., Analysis of tellurium thin films electrodeposition from acidic citric bath, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.03.127