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Effect of the substrate on the electrodeposition of iron sulphides
Solid State Sciences 4 (2002) 1083–1088 www.elsevier.com/locate/ssscie
Effect of the substrate on the electrodeposition of iron sulphides A. Gomes, M.I. da Silva Pereira ∗ , M.H. Mendonça, F.M. Costa CCMM, Departamento de Química e Bioquímica, Faculdade Ciências de Lisboa, 1749-016 Lisboa, Portugal Received 22 January 2002; received in revised form 14 May 2002; accepted 23 May 2002
Abstract The electrodeposition of iron sulphides films on titanium and Ebonex® , in aqueous solutions containing iron(II) ions and colloidal sulphur, has been assessed at 333 K, using periodic pulse electrolysis. Mackinawite was the only crystalline iron sulphide phase identified on the deposit. The structural and morphological characterization of the sulphide films obtained on both substrates was accomplished by X-ray powder diffraction (XRD) and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM/EDS). The results show that the film structure and morphology are sensitive to the substrate material. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Deposition process; Iron sulphides; Structural properties; Surface morphology
1. Introduction Iron sulphides are very interesting materials, presenting non-stoichiometry, and with promising applications on several fields such as solar energy conversion (iron disulphidepyrite) [1], lithium batteries (iron monosulphide) [2–4], or as electrocatalyst for hydrogen evolution reaction (FeSx ) [5]. Several studies on the electrodeposition of iron sulphides onto Pt, Au and Ti, under potentiostatic and potentiodynamic conditions have been reported [6–8]. The formation of non-stoichiometric films and/or amorphous films has been claimed, in general, but no discussion on the possible influence of the substrate on the properties of the electrodeposit has been brought about. However, it is well-known that the nature and surface characteristics of the substrate play a very important role on the initial formation of any electrodeposit, with consequences on their electronic and optical properties [9]. In this paper we report a comparative study on the characteristics of the electrodeposited iron sulphide (Fex Sy ) films onto, Ebonex® and titanium. This work is part of a more extensive project, where a systematic study on the
* Correspondence and reprints.
E-mail address: [email protected] (M.I. da Silva Pereira).
effect of the preparation conditions on the iron sulphide films properties is performed. The choice of these substrates are due to their properties, namely in the case of titanium, a typical support for activated anodes [10]. Concerning Ebonex®, a conducting ceramic, composed of titanium oxides of general formula Tix O2x−1 (4 x 10), it is referred in the literature as an electrode material, presenting interesting properties, such as good adhesion for metal electrodeposition and high overpotential for hydrogen evolution in acidic and basic aqueous solutions [11,12]. Our previous studies on the voltammetric behaviour of the Fe–S–Ebonex® system, at room temperature, show that the deposition of iron sulphide films occurs during electroreduction of sulphur in the presence of iron(II) ions [13]. Potentiostatic studies have been also performed, but it was not possible to get the experimental conditions in order to obtain an adherent deposit. To overcome this problem, periodic pulse electrolysis, has been used to obtain the Fex Sy electrodeposits on Ebonex®, at room temperature, from aqueous solutions containing iron(II) ions and colloidal sulphur [14]. This technique has been used with success for the deposition of other sulphides as it is referred in the literature [15,16]. In order to improve the film quality the electrodeposition has been performed at 333 K and another substrate, titanium, was tested.
1293-2558/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 2 ) 0 1 3 5 5 - 9
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2. Experimental 2.1. Substrates The working electrodes were Ebonex® plates and Ti discs with geometric area of approximately 2 and 1 cm2 , respectively. In order to guarantee good adhesion of the electrodeposits on the substrate, special attention was paid to their treatment [17]. The two substrates were mechanical polished with abrasive paper with different grades, followed by ultrasonic cleaning in water in the case of Ebonex®. Regarding the Ti electrodes the polishing was followed by a decrease with acetone and a chemical etching with a mixture of H2 O, HNO3 and H2 SO4 . These treatments were performed just before the substrate immersion in the solution, in order to avoid exposure to atmospheric oxygen. 2.2. Electroplating bath and deposition conditions The electroplating bath was a mixture of aqueous solutions 0.01 mol dm−3 (NH4 )2 Fe(SO4 )2 (Merck) and 0.35 mol dm−3 Na2 S2 O3 (Riedel de Haën) + 0.75 mol dm−3 Na2 SO4 (Merck) added at the proportion 1 : 3, without further purification. The pH was adjusted to 3, by adding H2 SO4 . The solutions were made daily and the mixing carried out inside the electrochemical cell, followed by deaeration with nitrogen during 15 min, just before the application of the pulse potential. For the applied pulse sequence the values used were Ea = 0.05, Ec = −1.00 V and ta = tc = 1 s. All potentials are referred to SCE. The deposition was performed under magnetic stirrer at ≈ 333 K for 3 hours. During the experiment, a black deposit was formed on the electrodes. When the deposition was finished the electrode was removed from the cell, rinsed with pure water and dried under nitrogen atmosphere.
by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) analysis performed with a Jeol (JSM 35C)/NORAN (VOYAGER) system.
3. Results and discussion 3.1. Voltammetric studies The purpose of the voltammetric study was to define, for each substrate, the potential region in which the various reactions occur, and from that, select the potential of the anodic and cathodic pulses for the iron sulphide deposition. Fig. 1(a) and (b) shows the voltammetric curves (1st cycle) obtained for titanium and Ebonex® substrates by sweeping the potential from the positive limit towards the cathodic direction. It can be said that the two profiles are quite similar, having in mind the differences on the substrates, namely the porosity of the Ebonex®. Under the experimental conditions used, acidic medium, the thiosulfate ion is unstable and decomposes on to colloidal sulphur and HSO− 3 according to the reaction scheme [19–21]: + − S2 O2− 3(aq) + H(aq) ⇔ S(colloidal) + HSO3(aq)
(1)
The sulphur formation was evident due to the turbidity of the solution what favours the electrodeposition process [16]. On he cathodic sweep, sulphur is reduced giving rise, in both substrates, to a wave at approximately −0.80 V vs. SCE attributed to the reaction [22–24] S + 2 H+ + 2e− → H2 S
(2)
During the reduction of sulphur in the presence of the iron(II) ions, the iron sulphide film is formed according to reaction (3), once the solubility product is reached [25–27] Fe2+ + (1 − x) H2 S → FeS1−x + 2 H+
(3)
and/or 2.3. Electrochemical cell and apparatus A three electrode glass cell was used with a Pt mesh as counter electrode and a commercial SCE as reference. The electrochemical measurements were carried out using a EG&G potentiostat, model PAR 263 and a Philips 8271 x-y-t recorder. 2.4. Characterization of the electrodeposited films The structural characterization of the as-deposited films was done by X-ray powder diffraction (XRD) with a Philips X-ray diffractometer PW 1730 (Cu Kα , λ = 0.15406 nm) and automatic data acquisition (A PD Philips v. 35 B software). The indexation and refinements of the lattice parameters values were done using the L SUCRE software for Xray data [18]. The film characterization was supplemented
Fe2+ + (1 − x) S + 2e− → FeS1−x
(4)
Two current crossovers are observed on the cathodic scan. The one observed at less negative potential has been previously examined in this group and attributed to the nucleation of the iron sulphide film [17]. Concerning the other one, recorded at more negative potential, it can be due, either to the nucleation of another phase or to changes on the support/film interface. Supplementary studies point to the first hypothesis. Indeed SEM/EDS analysis has detected the presence of zinc on the electrodeposits, and similar experiments performed in the absence of the supporting electrolyte, showed only one crossover. Consequently the nucleation of a zinc containing sulphide phase can occur, what is in accord with the observation by Kashyout et al. [28] of a current cross-over, on the same potential region, for the electrodeposition of ZnFeS films on tin oxide substrates.
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were chosen on the basis that iron ions have to coexist with reduced sulphur species. Consequently, the cathodic pulse should be more negative than the peak potential assigned to the sulphur reduction and in a region where the iron(II) exists so that reactions (3) and (4) take place. The selected potential was −1.00 V vs. SCE. Previous voltammetric studies of the influence of the negative limit on the electrodeposition of iron sulphide films on titanium and Ebonex® show that at this potential the reduction of iron(II) ions to metallic iron is negligible [13,17]. Concerning the positive limit, it was set in the anodic peak region, where the sulphur is deposited. As previously reported the amount of black deposit formed on the electrodes surface was very sensitive to the negative potential of the cycle and practically independent on the positive value in the range of potential where the sulphur is formed [13,17]. In Fig. 1(c) the applied pulse sequence is presented.
(a)
3.2. Morphological characterization
(b)
(c) Fig. 1. Typical cyclic voltammograms at 333 K for: (a) Ti/Fe + S system, (b) Ebonex® /Fe + S. Scan rate 10 mV s−1 . Electrodes geometric area: Ti ≈ 1 and Ebonex® ≈ 0.3 cm2 . (c) Periodic pulse waveform applied to electrodeposit iron sulphides on both substrates.
The anodic sweep shows the usual peak close to 0.00 V vs. SCE, assigned to sulphur deposition, due to the oxidation of sulphur reduced species previously formed on the cathodic cycle according to the back reaction (2) [29]. In addition partial dissolution of the iron sulphide film can occur according to the back reaction (4) and/or FeS1−x → (1 − x) S + Fe3+ + 3e−
(5)
leading to the formation of sulphur rich layers. No evidence of the oxidation of metallic iron formed during the cathodic cycle has been observed. Considering that our goal was to prepare iron sulphide films using pulsed electrolysis, the square wave potentials
The surface morphology of the deposits prepared on Ti and Ebonex®, is presented in Fig. 2. As it can be seen the film obtained on Ebonex® (Fig. 2(b)) is more heterogeneous and less compact than the one obtained on the titanium electrode (Fig. 2(a)), where a cauliflower like morphology is observed. Differences are also noticed on the crystallite morphology (Fig. 2(c) and (d)), both present as a porous microstructure. For the deposit on the Ebonex®, the estimated pore diameter, varies between 4 and 1.5 µm, while for the deposit on titanium the values range from 2.5 to 0.7 µm. These results clearly show that the film morphology is sensitive to the support material. Indeed the Ebonex® surface shows a high degree of porosity as it can be seen in Fig. 2(e) what could explain the differences on the electrodeposit characteristics. In order to accomplish the films characterization, EDS spectra were obtained for deposits prepared under similar conditions, on Ti and Ebonex®. As expected, the two electrodeposits contain mainly iron and sulphur. Compositional analysis of the as-deposited films indicates S/Fe ratio values greater than 1, changing along the surface on both substrates. The evaluated values for the S/Fe atomic ratio obtained on different points of the electrodeposit surface are presented in Table 1. To our surprise the presence of Zn was also detected. In order to investigate the origin of this impurity, some studies were performed mainly, experiments in the absence of supporting electrolyte, what led to conclude that the Zn comes from the Na2 SO4 [30]. The presence of small quantities of Na and O was detected and attributed to the expected contamination from the electroplating bath.
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Fig. 2. Scanning electron micrographies for the deposits prepared on Ti (a), (c) and Ebonex® (b), (d). Micrography of a clean Ebonex® surface (e).
Table 1 S/Fe atomic ratios for the films deposited on a Ti and Ebonex® support Titanium support
Ebonex® support
1.69 1.66 1.32 1.14
1.62 1.28 1.18 1.46
3.3. Crystal structure deposits characterization XRD patterns of as-deposited films obtained under the same conditions on Ti and Ebonex® are shown in Figs. 3 and 4, respectively. Regarding the two figures, characteristic peaks of the deposit are clearly seen, which are consistent with mackinawite, FeS1−x structure type, according to the ICDD file [31]. Weak intensity lines of an impurity
ZnS cubic phase [32] are also observed, which is in accordance with the results of SEM/EDS analysis already mentioned. The X-ray pattern for the Ebonex® support is much more complex than the one observed for metallic Ti, and therefore the diffractograms for this substrate, with and without deposit were analyzed in more detail. Comparing the diffraction lines of the deposit on both supports, it is clear that the deposit presents the maximum peak of the FeS1−x phase on the same 2θ region although, on the Ebonex®, these lines are displaced towards higher 2θ values. In addition, for the deposit on titanium an inversion on the intensity of the 2 0 0 and 1 2 2 planes occurs, in relation to the stated in the ICDD data file [31]. This fact should be due to an excess of sulphur in the deposit, with a consequent sulphur enrichment of the mackinawite structure, namely the 2 0 0 plane. The
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Table 2 Crystallographic data for tetragonal FeS1−x film deposited on a Ti and Ebonex® support Titanium support
Fig. 3. X-ray diffractograms for (a) Ti substrate and (b) a film deposited on Ti.
Ebonex® support
hkl
dobs (nm)
dcalc (nm)
dobs (nm)
dcalc (nm)
001 101 110 200 122 211 113 200
0.5090 0.2997 0.2612 0.1844 0.1815 0.1566 0.1408 0.1301
0.5021 0.2968 0.2602 0.1840 0.1807 0.1564 0.1408 0.1301
0.491 0.2916 0.2595 0.1820
0.4903 0.2921 0.2572 0.1819
Cell parameters: a = b = 0.3680, c = 0.5021 nm for titanium support, a = b = 0.3638, c = 0.4903 nm for Ebonex® support. Table 3 Crystallographic data for Ti, Ebonex® and mackinawite phases [31,33] Material Ti Ebonex® (Ti4 O7 ) Mackinawite
Cell parameters a (nm)
b (nm)
c (nm)
0.2950 0.5600 0.3679
0.2950 0.7130 0.3679
0.4686 1.2460 0.5047
and the mackinawite. Table 3 presents the crystallographic data for the mackinawite, titanium and Ti4 O7 phases [31, 33], where the differences on the cell parameters are clearly seen. 3.4. Sulphide formation
Fig. 4. X-ray expanded diffractograms for Ebonex® substrate, a film deposited on Ebonex® and a film deposited on Ti.
mackinawite phase was indexed according to ICDD data file [31] and the results are presented in Table 2. For the deposit on the titanium support, the cell parameters are closer to the ones reported on the ICDD data file, a = 0.3679 and c = 0.5047 nm. In contrast, the cell contraction observed on the Ebonex® should reflect the influence of the morphology and cell misfit between the Ebonex® substrate, mainly the Ti4 O7 phase,
Mackinawite is a metastable iron sulphide [34] and due to this fact it is not referred in the usual E–pH diagrams for the S–Fe–H2 O system. Iron sulphide, FeS, without any reference to stoichiometry/structure is always considered [27,35–37] and even for this sulphide the pH stability range differs from diagram to diagram. A great number of studies report the electrochemical deposition of mackinawite in the pH range from 7 to 12 [38,39], however studies on the corrosion of iron in aqueous H2 S solutions state that mackinawite is formed as corrosion product between pH = 3 and pH = 7 [40]. Lennie [41] also affirms that the dissolution of mackinawite occurs for pH values below 3. In this study XRD analysis show that mackinawite is formed on both substrates in aqueous solutions with an initial pH = 3. It must be said that, during the deposition process an increase of pH was observed, reaching values around 5 in the end of each experiment, which may be responsible for the stability of these phase. According with the potential perturbation used, the deposition process is composed by individual steps. Moreover the initial deposition of the film differs from subsequent deposition, because it occurs on the substrate, instead of the deposit. The first step involved the adsorption of sulphur and its reduction during the subsequent cathodic semi-cycle, leading to the formation of iron sulphide film in accordance
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with reaction (3) and/or (4). Once the support is coated with the sulphide film, on the following anodic pulse, part of it is oxidatively stripped out and simultaneously fresh sulphur is formed according to reactions (5) and back reaction (4). Film formation proceeds on the subsequent cathodic semicycle. The formation of mackinawite films sulphur enriched is in agreement with the higher content in sulphur given by the SEM/EDS analysis and also by the results obtained when these films are annealed at higher temperatures [14]. The results clearly show that ZnS cubic phase is also deposited, although the Zn ions are present at a much lower concentration than the iron ions on the deposition bath. The formation of the ZnS can be explained by the adsorption and coprecipitation of Zn2+ with mackinawite [42].
4. Concluding remarks From the analysis of the experimental results, it can be concluded that the tetragonal iron sulphide, mackinawite, is electrodeposited on both substrates, Ti and Ebonex®. XRD and SEM data show that both, film crystalinity and morphology are sensitive to the substrate material. A lattice contraction is observed for the deposit on Ebonex® when compared with the deposit obtained on titanium. The present results indicate, that it should be possible to electrodeposit iron sulphide films with incorporation of small quantities of Zn that could be suitable for future applications such as photoelectrochemical devices, after an adequated heat treatment.
Acknowledgement A. Gomes acknowledges a BD/5223/95 grant from FCT Programa Praxis XXI.
References [1] [2] [3] [4] [5] [6]
A. Ennaui, H. Tributsch, Sol. Energy Mater. 14 (1986) 46. Y. Uetani, K. Yokoyama, O. Okamoto, J. Power Sources 5 (1980) 89. J. Lesinski, J. Izydorek, L. Weblan, J. Power Sources 27 (1989) 337. Y. Shao-Horn, Q.C. Horn, Electrochim. Acta 46 (2001) 2613. A. Nidola, R. Schira, J. Hydrogen Energy 11 (1986) 449. A.S. Aricó, V. Antonucci, P.L. Antonucci, D.L. Cocke, N. Giordano, Electrochim. Acta 36 (1991) 581. [7] N.R. Tacconi, O. Mrdvedko, K. Rajeshawar, J. Electroanal. Chem. 379 (1994) 545.
[8] S.N. Sahu, C. Sanchez, J. Mater. Sci. Lett. 11 (1992) 1540. [9] R.K. Pandey, S.N. Sahu, S. Chandra, in: Handbook of Semiconductor Electrodeposition, Marcel Dekker, New York, 1996, p. 89. [10] S. Trasatti, Electrochim. Acta 32 (1987) 369. [11] R.R. Miller-Folk, R.E. Noffle, D. Pletcher, J. Electroanal. Chem. 274 (1989) 257. [12] J.E. Graves, D. Pletcher, R.L. Clarke, F.C. Walsh, J. Appl. Electrochem. 21 (1991) 848. [13] A. Gomes, M.I. da Silva Pereira, M.H. Mendonça, F.M.A. Costa, J. Appl. Electrochem. 25 (1995) 1045. [14] A. Gomes, M.I. da Silva Pereira, M.H. Mendonça, F.M.A. Costa, Port. Electrochim. Acta 13 (1995) 429. [15] G.C. Morris, R. Vanderveen, Sol. Energy Mater. 27 (1992) 305. [16] E. Fatas, P. Herrasti, F. Arjona, G. Camarero, J. Electrochem. Soc. 134 (1987) 2799. [17] A. Gomes, M.H. Mendonça, M.I. da Silva Pereira, F.M.A. Costa, J. Solid State Electrochem. 4 (2000) 168. [18] D.E. Appleman, H.T. Evans, Indexing and least-squares refinement of powder diffraction data, Report USG-GD-73-003, 1973. [19] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon, Oxford, 1986. [20] R.J. Tykodi, J. Chem. Ed. 67 (1990) 146. [21] F. Johnston, L.Mc. Amish, J. Coll. Interf. Sci. 42 (1973) 112. [22] T. Hemmmingsen, Electrochim. Acta 37 (1992) 2775. [23] R.J. Biernat, R.G. Robins, Electrochim. Acta 14 (1969) 809. [24] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Nace, 1974. [25] R.J. Biernat, R.G. Robins, Electrochim. Acta 17 (1969) 1261. [26] E. Ahlberg, A.E. Broo, J. Electrochem. Soc. 144 (1997) 1281. [27] I.C. Hamilton, R. Woods, J. Electroanal. Chem. 118 (1981) 327. [28] A.B. Kashyout, A.S. Aricò, G. Monforte, F. Crea, V. Antonucci, N. Giordano, Sol. Energy Mater. 37 (1995) 43. [29] M.N. Lehmann, M. Stichmoth, D. Walton, S.I. Bailey, J. Electrochem. Soc. 147 (2000) 3263. [30] A. Gomes, Ph.D. Thesis, University of Lisbon, 2001. [31] P.A. ICDD, International Center for Diffraction Data Swarthmore, 1998, Powder diffraction file nr 24-73. [32] P.A. ICDD, International Center for Diffraction Data Swarthmore, 1998, Powder diffraction file nr 5-566. [33] P.A. ICDD, International Center for Diffraction Data Swarthmore, 1998, Powder diffraction files nr 5-682; 18-1402. [34] P. Taylor, Am. Mineral. 65 (1980) 1026. [35] R.J. Biernat, R.G. Robins, Electrochim. Acta 17 (1969) 126. [36] T. Hemmingsen, H. Lima, Electrochim. Acta 43 (1998) 35. [37] R.M. Garrels, C.L. Christ, Solutions, Minerals and Equilibria, Harper and Row, New York, 1965. [38] D.W. Shoesmith, M.G. Bailey, B. Ikeda, Electrochim. Acta 23 (1978) 1329. [39] P. Brezdicka, J.C. Grenier, L. Fournés, A. Wattiaux, P. Hagenmuller, Eur. J. Solid State Inorg. Chem. 26 (1989) 353. [40] D.W. Shoesmith, P. Taylor, M.G. Bailey, D.G. Owen, J. Electrochem. Soc. 127 (1980) 1007. [41] A.R. Lennie, K.E.R. England, D. Vaughan, Am. Mineral. 80 (1995) 960. [42] J.W. Morse, T. Arakaki, Geochim. Cosmochim. Acta 57 (1993) 3635.
Effect of the substrate on the electrodeposition of iron sulphides A. Gomes, M.I. da Silva Pereira ∗ , M.H. Mendonça, F.M. Costa CCMM, Departamento de Química e Bioquímica, Faculdade Ciências de Lisboa, 1749-016 Lisboa, Portugal Received 22 January 2002; received in revised form 14 May 2002; accepted 23 May 2002
Abstract The electrodeposition of iron sulphides films on titanium and Ebonex® , in aqueous solutions containing iron(II) ions and colloidal sulphur, has been assessed at 333 K, using periodic pulse electrolysis. Mackinawite was the only crystalline iron sulphide phase identified on the deposit. The structural and morphological characterization of the sulphide films obtained on both substrates was accomplished by X-ray powder diffraction (XRD) and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM/EDS). The results show that the film structure and morphology are sensitive to the substrate material. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Deposition process; Iron sulphides; Structural properties; Surface morphology
1. Introduction Iron sulphides are very interesting materials, presenting non-stoichiometry, and with promising applications on several fields such as solar energy conversion (iron disulphidepyrite) [1], lithium batteries (iron monosulphide) [2–4], or as electrocatalyst for hydrogen evolution reaction (FeSx ) [5]. Several studies on the electrodeposition of iron sulphides onto Pt, Au and Ti, under potentiostatic and potentiodynamic conditions have been reported [6–8]. The formation of non-stoichiometric films and/or amorphous films has been claimed, in general, but no discussion on the possible influence of the substrate on the properties of the electrodeposit has been brought about. However, it is well-known that the nature and surface characteristics of the substrate play a very important role on the initial formation of any electrodeposit, with consequences on their electronic and optical properties [9]. In this paper we report a comparative study on the characteristics of the electrodeposited iron sulphide (Fex Sy ) films onto, Ebonex® and titanium. This work is part of a more extensive project, where a systematic study on the
* Correspondence and reprints.
E-mail address: [email protected] (M.I. da Silva Pereira).
effect of the preparation conditions on the iron sulphide films properties is performed. The choice of these substrates are due to their properties, namely in the case of titanium, a typical support for activated anodes [10]. Concerning Ebonex®, a conducting ceramic, composed of titanium oxides of general formula Tix O2x−1 (4 x 10), it is referred in the literature as an electrode material, presenting interesting properties, such as good adhesion for metal electrodeposition and high overpotential for hydrogen evolution in acidic and basic aqueous solutions [11,12]. Our previous studies on the voltammetric behaviour of the Fe–S–Ebonex® system, at room temperature, show that the deposition of iron sulphide films occurs during electroreduction of sulphur in the presence of iron(II) ions [13]. Potentiostatic studies have been also performed, but it was not possible to get the experimental conditions in order to obtain an adherent deposit. To overcome this problem, periodic pulse electrolysis, has been used to obtain the Fex Sy electrodeposits on Ebonex®, at room temperature, from aqueous solutions containing iron(II) ions and colloidal sulphur [14]. This technique has been used with success for the deposition of other sulphides as it is referred in the literature [15,16]. In order to improve the film quality the electrodeposition has been performed at 333 K and another substrate, titanium, was tested.
1293-2558/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 2 ) 0 1 3 5 5 - 9
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A. Gomes et al. / Solid State Sciences 4 (2002) 1083–1088
2. Experimental 2.1. Substrates The working electrodes were Ebonex® plates and Ti discs with geometric area of approximately 2 and 1 cm2 , respectively. In order to guarantee good adhesion of the electrodeposits on the substrate, special attention was paid to their treatment [17]. The two substrates were mechanical polished with abrasive paper with different grades, followed by ultrasonic cleaning in water in the case of Ebonex®. Regarding the Ti electrodes the polishing was followed by a decrease with acetone and a chemical etching with a mixture of H2 O, HNO3 and H2 SO4 . These treatments were performed just before the substrate immersion in the solution, in order to avoid exposure to atmospheric oxygen. 2.2. Electroplating bath and deposition conditions The electroplating bath was a mixture of aqueous solutions 0.01 mol dm−3 (NH4 )2 Fe(SO4 )2 (Merck) and 0.35 mol dm−3 Na2 S2 O3 (Riedel de Haën) + 0.75 mol dm−3 Na2 SO4 (Merck) added at the proportion 1 : 3, without further purification. The pH was adjusted to 3, by adding H2 SO4 . The solutions were made daily and the mixing carried out inside the electrochemical cell, followed by deaeration with nitrogen during 15 min, just before the application of the pulse potential. For the applied pulse sequence the values used were Ea = 0.05, Ec = −1.00 V and ta = tc = 1 s. All potentials are referred to SCE. The deposition was performed under magnetic stirrer at ≈ 333 K for 3 hours. During the experiment, a black deposit was formed on the electrodes. When the deposition was finished the electrode was removed from the cell, rinsed with pure water and dried under nitrogen atmosphere.
by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) analysis performed with a Jeol (JSM 35C)/NORAN (VOYAGER) system.
3. Results and discussion 3.1. Voltammetric studies The purpose of the voltammetric study was to define, for each substrate, the potential region in which the various reactions occur, and from that, select the potential of the anodic and cathodic pulses for the iron sulphide deposition. Fig. 1(a) and (b) shows the voltammetric curves (1st cycle) obtained for titanium and Ebonex® substrates by sweeping the potential from the positive limit towards the cathodic direction. It can be said that the two profiles are quite similar, having in mind the differences on the substrates, namely the porosity of the Ebonex®. Under the experimental conditions used, acidic medium, the thiosulfate ion is unstable and decomposes on to colloidal sulphur and HSO− 3 according to the reaction scheme [19–21]: + − S2 O2− 3(aq) + H(aq) ⇔ S(colloidal) + HSO3(aq)
(1)
The sulphur formation was evident due to the turbidity of the solution what favours the electrodeposition process [16]. On he cathodic sweep, sulphur is reduced giving rise, in both substrates, to a wave at approximately −0.80 V vs. SCE attributed to the reaction [22–24] S + 2 H+ + 2e− → H2 S
(2)
During the reduction of sulphur in the presence of the iron(II) ions, the iron sulphide film is formed according to reaction (3), once the solubility product is reached [25–27] Fe2+ + (1 − x) H2 S → FeS1−x + 2 H+
(3)
and/or 2.3. Electrochemical cell and apparatus A three electrode glass cell was used with a Pt mesh as counter electrode and a commercial SCE as reference. The electrochemical measurements were carried out using a EG&G potentiostat, model PAR 263 and a Philips 8271 x-y-t recorder. 2.4. Characterization of the electrodeposited films The structural characterization of the as-deposited films was done by X-ray powder diffraction (XRD) with a Philips X-ray diffractometer PW 1730 (Cu Kα , λ = 0.15406 nm) and automatic data acquisition (A PD Philips v. 35 B software). The indexation and refinements of the lattice parameters values were done using the L SUCRE software for Xray data [18]. The film characterization was supplemented
Fe2+ + (1 − x) S + 2e− → FeS1−x
(4)
Two current crossovers are observed on the cathodic scan. The one observed at less negative potential has been previously examined in this group and attributed to the nucleation of the iron sulphide film [17]. Concerning the other one, recorded at more negative potential, it can be due, either to the nucleation of another phase or to changes on the support/film interface. Supplementary studies point to the first hypothesis. Indeed SEM/EDS analysis has detected the presence of zinc on the electrodeposits, and similar experiments performed in the absence of the supporting electrolyte, showed only one crossover. Consequently the nucleation of a zinc containing sulphide phase can occur, what is in accord with the observation by Kashyout et al. [28] of a current cross-over, on the same potential region, for the electrodeposition of ZnFeS films on tin oxide substrates.
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were chosen on the basis that iron ions have to coexist with reduced sulphur species. Consequently, the cathodic pulse should be more negative than the peak potential assigned to the sulphur reduction and in a region where the iron(II) exists so that reactions (3) and (4) take place. The selected potential was −1.00 V vs. SCE. Previous voltammetric studies of the influence of the negative limit on the electrodeposition of iron sulphide films on titanium and Ebonex® show that at this potential the reduction of iron(II) ions to metallic iron is negligible [13,17]. Concerning the positive limit, it was set in the anodic peak region, where the sulphur is deposited. As previously reported the amount of black deposit formed on the electrodes surface was very sensitive to the negative potential of the cycle and practically independent on the positive value in the range of potential where the sulphur is formed [13,17]. In Fig. 1(c) the applied pulse sequence is presented.
(a)
3.2. Morphological characterization
(b)
(c) Fig. 1. Typical cyclic voltammograms at 333 K for: (a) Ti/Fe + S system, (b) Ebonex® /Fe + S. Scan rate 10 mV s−1 . Electrodes geometric area: Ti ≈ 1 and Ebonex® ≈ 0.3 cm2 . (c) Periodic pulse waveform applied to electrodeposit iron sulphides on both substrates.
The anodic sweep shows the usual peak close to 0.00 V vs. SCE, assigned to sulphur deposition, due to the oxidation of sulphur reduced species previously formed on the cathodic cycle according to the back reaction (2) [29]. In addition partial dissolution of the iron sulphide film can occur according to the back reaction (4) and/or FeS1−x → (1 − x) S + Fe3+ + 3e−
(5)
leading to the formation of sulphur rich layers. No evidence of the oxidation of metallic iron formed during the cathodic cycle has been observed. Considering that our goal was to prepare iron sulphide films using pulsed electrolysis, the square wave potentials
The surface morphology of the deposits prepared on Ti and Ebonex®, is presented in Fig. 2. As it can be seen the film obtained on Ebonex® (Fig. 2(b)) is more heterogeneous and less compact than the one obtained on the titanium electrode (Fig. 2(a)), where a cauliflower like morphology is observed. Differences are also noticed on the crystallite morphology (Fig. 2(c) and (d)), both present as a porous microstructure. For the deposit on the Ebonex®, the estimated pore diameter, varies between 4 and 1.5 µm, while for the deposit on titanium the values range from 2.5 to 0.7 µm. These results clearly show that the film morphology is sensitive to the support material. Indeed the Ebonex® surface shows a high degree of porosity as it can be seen in Fig. 2(e) what could explain the differences on the electrodeposit characteristics. In order to accomplish the films characterization, EDS spectra were obtained for deposits prepared under similar conditions, on Ti and Ebonex®. As expected, the two electrodeposits contain mainly iron and sulphur. Compositional analysis of the as-deposited films indicates S/Fe ratio values greater than 1, changing along the surface on both substrates. The evaluated values for the S/Fe atomic ratio obtained on different points of the electrodeposit surface are presented in Table 1. To our surprise the presence of Zn was also detected. In order to investigate the origin of this impurity, some studies were performed mainly, experiments in the absence of supporting electrolyte, what led to conclude that the Zn comes from the Na2 SO4 [30]. The presence of small quantities of Na and O was detected and attributed to the expected contamination from the electroplating bath.
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Fig. 2. Scanning electron micrographies for the deposits prepared on Ti (a), (c) and Ebonex® (b), (d). Micrography of a clean Ebonex® surface (e).
Table 1 S/Fe atomic ratios for the films deposited on a Ti and Ebonex® support Titanium support
Ebonex® support
1.69 1.66 1.32 1.14
1.62 1.28 1.18 1.46
3.3. Crystal structure deposits characterization XRD patterns of as-deposited films obtained under the same conditions on Ti and Ebonex® are shown in Figs. 3 and 4, respectively. Regarding the two figures, characteristic peaks of the deposit are clearly seen, which are consistent with mackinawite, FeS1−x structure type, according to the ICDD file [31]. Weak intensity lines of an impurity
ZnS cubic phase [32] are also observed, which is in accordance with the results of SEM/EDS analysis already mentioned. The X-ray pattern for the Ebonex® support is much more complex than the one observed for metallic Ti, and therefore the diffractograms for this substrate, with and without deposit were analyzed in more detail. Comparing the diffraction lines of the deposit on both supports, it is clear that the deposit presents the maximum peak of the FeS1−x phase on the same 2θ region although, on the Ebonex®, these lines are displaced towards higher 2θ values. In addition, for the deposit on titanium an inversion on the intensity of the 2 0 0 and 1 2 2 planes occurs, in relation to the stated in the ICDD data file [31]. This fact should be due to an excess of sulphur in the deposit, with a consequent sulphur enrichment of the mackinawite structure, namely the 2 0 0 plane. The
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Table 2 Crystallographic data for tetragonal FeS1−x film deposited on a Ti and Ebonex® support Titanium support
Fig. 3. X-ray diffractograms for (a) Ti substrate and (b) a film deposited on Ti.
Ebonex® support
hkl
dobs (nm)
dcalc (nm)
dobs (nm)
dcalc (nm)
001 101 110 200 122 211 113 200
0.5090 0.2997 0.2612 0.1844 0.1815 0.1566 0.1408 0.1301
0.5021 0.2968 0.2602 0.1840 0.1807 0.1564 0.1408 0.1301
0.491 0.2916 0.2595 0.1820
0.4903 0.2921 0.2572 0.1819
Cell parameters: a = b = 0.3680, c = 0.5021 nm for titanium support, a = b = 0.3638, c = 0.4903 nm for Ebonex® support. Table 3 Crystallographic data for Ti, Ebonex® and mackinawite phases [31,33] Material Ti Ebonex® (Ti4 O7 ) Mackinawite
Cell parameters a (nm)
b (nm)
c (nm)
0.2950 0.5600 0.3679
0.2950 0.7130 0.3679
0.4686 1.2460 0.5047
and the mackinawite. Table 3 presents the crystallographic data for the mackinawite, titanium and Ti4 O7 phases [31, 33], where the differences on the cell parameters are clearly seen. 3.4. Sulphide formation
Fig. 4. X-ray expanded diffractograms for Ebonex® substrate, a film deposited on Ebonex® and a film deposited on Ti.
mackinawite phase was indexed according to ICDD data file [31] and the results are presented in Table 2. For the deposit on the titanium support, the cell parameters are closer to the ones reported on the ICDD data file, a = 0.3679 and c = 0.5047 nm. In contrast, the cell contraction observed on the Ebonex® should reflect the influence of the morphology and cell misfit between the Ebonex® substrate, mainly the Ti4 O7 phase,
Mackinawite is a metastable iron sulphide [34] and due to this fact it is not referred in the usual E–pH diagrams for the S–Fe–H2 O system. Iron sulphide, FeS, without any reference to stoichiometry/structure is always considered [27,35–37] and even for this sulphide the pH stability range differs from diagram to diagram. A great number of studies report the electrochemical deposition of mackinawite in the pH range from 7 to 12 [38,39], however studies on the corrosion of iron in aqueous H2 S solutions state that mackinawite is formed as corrosion product between pH = 3 and pH = 7 [40]. Lennie [41] also affirms that the dissolution of mackinawite occurs for pH values below 3. In this study XRD analysis show that mackinawite is formed on both substrates in aqueous solutions with an initial pH = 3. It must be said that, during the deposition process an increase of pH was observed, reaching values around 5 in the end of each experiment, which may be responsible for the stability of these phase. According with the potential perturbation used, the deposition process is composed by individual steps. Moreover the initial deposition of the film differs from subsequent deposition, because it occurs on the substrate, instead of the deposit. The first step involved the adsorption of sulphur and its reduction during the subsequent cathodic semi-cycle, leading to the formation of iron sulphide film in accordance
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with reaction (3) and/or (4). Once the support is coated with the sulphide film, on the following anodic pulse, part of it is oxidatively stripped out and simultaneously fresh sulphur is formed according to reactions (5) and back reaction (4). Film formation proceeds on the subsequent cathodic semicycle. The formation of mackinawite films sulphur enriched is in agreement with the higher content in sulphur given by the SEM/EDS analysis and also by the results obtained when these films are annealed at higher temperatures [14]. The results clearly show that ZnS cubic phase is also deposited, although the Zn ions are present at a much lower concentration than the iron ions on the deposition bath. The formation of the ZnS can be explained by the adsorption and coprecipitation of Zn2+ with mackinawite [42].
4. Concluding remarks From the analysis of the experimental results, it can be concluded that the tetragonal iron sulphide, mackinawite, is electrodeposited on both substrates, Ti and Ebonex®. XRD and SEM data show that both, film crystalinity and morphology are sensitive to the substrate material. A lattice contraction is observed for the deposit on Ebonex® when compared with the deposit obtained on titanium. The present results indicate, that it should be possible to electrodeposit iron sulphide films with incorporation of small quantities of Zn that could be suitable for future applications such as photoelectrochemical devices, after an adequated heat treatment.
Acknowledgement A. Gomes acknowledges a BD/5223/95 grant from FCT Programa Praxis XXI.
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