Fe2+ mole ratio in the solution

Electrochimica Acta 49 (2004) 2155–2165

Electrodeposition of Mackinawite films on Ti: effects of the S2 O32−/Fe2+ mole ratio in the solution A. Gomes, M.I. da Silva Pereira∗,1 , M.H. Mendonça, F.M. Costa C.C.M.M., Departamento de Qu´ımica e Bioqu´ımica, Faculdade de Ciˆencias da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal Received 6 June 2003; received in revised form 30 September 2003; accepted 26 December 2003

Abstract Mackinawite films have been deposited on Ti supports from aqueous solutions containing ferrous and thiosulphate ions, using a potentiostatic double pulse technique. Studies on the influence of the electrolyte concentration ratio [S2 O3 2− ]/[Fe2+ ] on the film properties were performed. Cyclic voltammetry was used as a diagnostic technique for the electrodeposition process. In situ characterisation of the deposits was performed by anodic stripping analysis. The structure and morphology of the films were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The experimental data provide evidence that Mackinawite deposits have been obtained from the entire electrodeposition baths although its purity is conditioned by the bath composition. Namely, when [S2 O3 2− ] = [Fe2+ ], oxidation products such as ␣-S and ␥-FeOOH were detected. © 2004 Elsevier Ltd. All rights reserved. Keywords: Pulsed electrolysis; Mackinawite; Iron sulphide oxidation; Anodic stripping

1. Introduction Iron sulfides constitute a wide class of materials with technological importance, mainly on corrosion science [1,2], batteries [3] and solar energy conversion [4]. Among them, pyrite exhibit a value of optical band gap (Egap = 0.9 eV) coupled with a high absorption coefficient which makes it an interesting and promising material in the field of semiconductors and energy conversion [5]. Therefore, some attention has been dedicated to the preparation of pyrite coatings and different techniques were used [6–9]. Although a lot of research work has been done on pyrite synthesis, in what concerns the electrochemical synthesis a clear understanding of the reaction mechanism is not yet reached. There is evidence in the literature that iron monosulphides, namely Mackinawite, are possible precursor to pyrite formation in aqueous solution, and the reaction of the iron monosulphides with dissolved sulphur species could lead to pyrite formation [10–12]. Benning et al. [13] assume that the conversion is a multi-step reaction pro∗

Corresponding author. Tel.: +351-21-750-0709; fax: +351-21-750-0088. E-mail address: [email protected] (M.I. da Silva Pereira). 1 ISE member. 0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.12.042

cess involving changes in aqueous sulphur species causing solid state transformation of Mackinawite to pyrite via the intermediate monosulphide greigite. In a previous work, we were able to electrodeposit Mackinawite on both Ebonex® and Ti substrates in acidic medium [14]. The choice of electrodeposition conditions allowed the incorporation of sulphur in the deposits. Annealing of the electrodeposits on Ti, under nitrogen flux, was performed and by controlling the temperature and duration of the heat treatment, different phases were obtained, namely pyrite [15]. In this work, Mackinawite films were prepared on Ti supports by pulse electrolysis. The influence of the bath concentration was investigated. The electrodeposits were characterised in situ by anodic stripping analysis and ex situ by scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM/EDS) and X-ray diffraction (XRD).

2. Experimental 2.1. Electrochemical studies The electroplating bath was a mixture of (NH4 )2 Fe(SO4 )2 (Merck p.a.) and Na2 S2 O3 (Riedel de Haën p.a.) aqueous

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Table 1 Electrodeposition bath composition

2.2. Ex situ characterisation

S2 O3 2− /Fe2+

Bath composition

1 100 0.01

0.25 M (NH4 )2 Fe (SO4 )2 + 0.25 M Na2 S2 O3 2.5 × 10−3 M (NH4 )2 Fe (SO4 )2 + 0.25 M Na2 S2 O3 0.25 M (NH4 )2 Fe (SO4 )2 + 2.5 × 10−3 M Na2 S2 O3

The films structural characterisation was done by X-ray powder diffraction with a Philips X-ray diffractometer PW 1710 (Cu K␣, λ = 1.5406 Å) working at 30 mA and 40 KV and automatic data acquisition (APD Philips (v 3.5 B) software). The diffractograms were obtained in the 2θ range of 10–80◦ , using a 0.02◦ step and acquisition time of 2 s per step. The indexation and refinements of the lattice parameters values were done using the LSUCRE software for X-ray data [16]. The average grain size of the film phases were calculated from the XRD patterns using the Scherrer equation [17]. Scanning electron microscopy coupled with energy dispersive spectroscopy analysis was performed with a Jeol (JSM 35C)/NORAN (VOYAGER) system at an accelerating voltage of 20 KeV, in order to characterise the films surface.

solutions. Table 1 shows the composition of the different used baths. The pH was adjusted to 3, by adding H2 SO4 . The solutions were made daily without further purification 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. A three-electrode glass cell was used with a Pt mesh as counter electrode and a commercial SCE as reference. The working electrode was a Ti disc (Goodfellows, 99.6%) with a 10 mm diameter. The Ti disc was mechanical polished with abrasive paper with different grades, followed by a degreasing with acetone and a chemical etching with a mixture of H2 O, HNO3 and H2 SO4 (3:1:1). The treatment was performed just before the substrate immersion in the solution, in order to minimise exposure to atmospheric oxygen. The electrochemical measurements were carried out using a Voltalab 32 Radiometer apparatus connected to an IMT 102 interface, controlled by a personal computer through the VoltaMaster 2 software. Iron monosulphide films were prepared by electrodeposition, using a potentiostatic double pulse technique. The deposition was performed under magnetic stirring at ≈333 K for 1 h. When the deposition had finished the electrode was removed from the cell, rinsed with pure water and dried under nitrogen atmosphere.

3. Results and discussion 3.1. Cyclic voltammetric studies The Ti/0.25 mol dm−3 S2 O3 2− system was studied by cyclic voltammetry over the potential range +0.400 to −1.200 V versus SCE at pH = 3. In acidic media, the S2 O3 2− species are not stable and disproportionate, giving rise to colloidal sulphur and HSO3 − according to: − + S2 O3 2− (aq) + H(aq) → Scolloidal + HSO3 (aq)

The result of the first cycle, recorded between +0.400 and −0.950 V and starting at the positive limit is presented in Fig. 1. A cathodic wave (C*) is observed for E < −0.700 V,

0,02

i / mAcm-2

0,00

-0,02

-0,04

C* -0,06

-0,08 -1000

-800

-600

-400

(1)

-200

0

200

400

E / mV vs. SCE Fig. 1. Cyclic voltamograms obtained for a Ti electrode in 0.25 mol dm−3 S2 O3 2− , at pH = 3 and 333 K. Sweep rate 10 mV/s.

A. Gomes et al. / Electrochimica Acta 49 (2004) 2155–2165

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1,0

0,5

30

-0,5

20 10

i / µAcm-2

i / mAcm-2

0,0

-1,0

-1,5

0 -10 -20 -30 -1000

-800

-600

-2,0

-400

-200

0

200

400

E / mV vs. SCE

-1000

-800

-600

-400

-200

0

200

400

E / mV vs. SCE Fig. 2. Cyclic voltamograms obtained for a Ti electrode in 0.25 mol dm−3 Fe2+ at pH = 3 and 333 K. Sweep rate 10 mV/s. Ei = 400 and Ef = −1050 mV. Inset Ei = 400 and Ef = −950 mV.

in contrast with the featureless anodic profile. Considering that colloidal sulphur could be adsorbed on the electrode surface, its reduction should be associated with the cathodic wave following the reaction scheme (2), S + 2H+ + 2e− → H2 S(aq)

(2)

The reduction of HSO3 − species with the formation of sulphur could also occur during the cathodic sweep as well as the homogeneous redox reaction (3) [18] + 2H2 S(aq) + HSO3 − (aq) + H → 3H2 O + 3S

(3)

(b)

(a)

i / mA cm-2

The capacitive anodic response can be explained by an inhibition effect due to an incomplete reduction of sulphur during the cathodic sweep, in addition to the growth of the semiconducting TiO2 film at positive potentials. Identical voltammetric results obtained for the Cvitreous /S2 O3 2− system, in the same conditions, supports the inhibition effect of sulphur [19]. In what concerns the Ti/0.25 mol dm−3 Fe2+ system, the voltammogram presented in Fig. 2 and obtained between +0.400 and −1.050 V, shows the increase of cathodic current associated with the Fe deposition and the corresponding

2

0,1

1

0,0

0

-0,1

-1

-0,2

-2

-0,3

-3

-0,4

-1200

-800

-400

0

E / mV vs. SCE

400

-800

-400

0

400

E / mV vs. SCE

Fig. 3. Cyclic voltamograms (first cycle) for a Ti electrode in 0.25 mol dm−3 Fe2+ +2.5×10−3 mol dm−3 S2 O3 2− (a) and 0.25 mol dm−3 S2 O3 2− +2.5×10−3 Fe2+ at pH = 3 and 333 K (solid lines) Sweep rate 10 mV/s. The dashed lines correspond to the respective isolated system.

A. Gomes et al. / Electrochimica Acta 49 (2004) 2155–2165

FeS → Fe2+ + S + 2e−

(4 )

Comparing Fig. 3(a) and (b) it is interesting to note that when the S2 O3 2− species is in excess, the iron dissolution peak does not appear, indicating that the iron deposit is formed by the reduction of the Fe2+ species in solution. In order to investigate the influence of the ratio [S2 O3 2− ]/ [Fe2+ ] on the iron sulphide electrodeposition, a voltammetric study was performed in stagnant solutions for three different bath at 333 K. Fig. 4(a-c) present cyclic voltammograms, obtained in the same conditions (seventh cycle) for the three solutions under study. The scans were started from the positive potential and the negative limit (−0.650 V) was increased by 50 mV each cycle. The influence on the voltammetric profile, of the sulphur/iron mole ratio in the solution, is well exemplified in Fig. 4 and the most striking feature is the changes in the current range, being the maximum current intensity observed when S2 O3 2− /Fe2+ = 1, in the deposition bath. On these conditions, three oxidation peaks (A1, A3, A4) and two shoulders (A2, A5) appeared in the positive scan. When the iron concentration decreases (S2 O3 2− /Fe2+ = 100), A4 is the most pronounced peak, and the others almost vanish. In contrast peak A1 is the most pronounced when S2 O3 2− /Fe2+ = 0.01. Concerning the cathodic counterpart, the influence of the bath composition is also clearly seen, for S2 O3 2− /Fe2+ = 1. A broad cathodic peak (C) is observed between −0.500 V and −0.800 V versus SCE followed by an increase of current intensity at the negative potential end. On the same potential range two peaks (C1 and C2) appear when S2 O3 2− /Fe2+ takes the value of 100. For the S2 O3 2− /Fe2+ = 0.01 a single peak commences at ≈−0.900 V giving a reverse loop and a current crossover

20

i / mAcm-2

10

A4

(a)

A1 A2

A3

A5

0 -10 -20

C

-30 -1000 -800 1 (b)

i / mAcm-2

dissolution peak. The current crossing, characteristic of a nucleation process is also observed. The inset shows the voltammetric profile recorded on a narrower potential range where the Fe2+ /Fe3+ characteristic pair of peaks is observed. Changes in the voltammetric behaviour are observed when both precursors are present in solution as Fig. 3 reveals. Fig. 3(a) shows the changes in the voltammetric profile due to the addition of 2.5 × 10−3 mol dm−3 S2 O3 2− to the iron solution. It is clear that when only iron is present the reduction of Fe2+ to Fe0 occurs at a more negative potential. A similar effect have been observed by us with the Fe-S-Ebonex® system [20] and by Itabashi at the Hg electrode [21] and explained in terms of the formation of an iron sulphide film on the electrode surface that stimulates the reduction of Fe2+ to Fe0 . On the other hand, when 2.5 × 10−3 mol dm−3 Fe2+ are added to the sulphur containing solution a different voltammetric profile develops. Indeed on the cathodic cycle and just before the increase of current, the cathodic wave C* is observed. On the reverse sweep, a current cross over and a well-defined anodic peak appears. This result supports the cathodic formation of iron sulphide film and its subsequent oxidation on the anodic sweep according to:

-600

-400

-200

0

200

400

A4

0 -1

C2

-2 -3 -1000 4

i / mAcm-2

2158

-800

(c)

C1

-600

-400

-200

0

200

400

-400

-200

0

200

400

A1

2 0 -2 -4 -1000

-800

-600

E / mV vs. SCE Fig. 4. Cyclic voltamograms (7th cycle) obtained for a Ti electrode in unstirred deposition bath at 333 K with S2 O3 2− /Fe2+ = 1 (a), S2 O3 2− /Fe2+ = 100 (b) and S2 O3 2− /Fe2+ = 0.01 (c). Sweep rate 10 mV/s.

on the return sweep at about −0.750 V, typical of nucleation and growth of a new phase [22]. Comparing the voltammetric response of the three systems, it can be said that A1 is clearly seen in systems with S2 O3 2− /Fe2+ = 0.01 and A2 and A3 appear for S2 O3 2− /Fe2+ = 1. In what concerns peak A4, it appears on the three systems, with intensities depending strongly on the bath composition, and presenting a shoulder (A5) for S2 O3 2− /Fe2+ = 1. For the system with S2 O3 2− /Fe2+ = 1, and considering that we are analysing the seventh cycle starting on the positive limit, the initial stages of film deposition and stripping have already taken place. During the previous cycle, the formation of sulphur by the oxidation of iron sulphide as well as by the reaction (3) has occurred. Consequently peak C is associated with the reduction of sulphur (reaction 2) and other products resulting from the partial oxidation of the iron sulphide film, namely iron oxides and/or hydroxides that can be reduced on this potential range (see Table 2). The occurrence of different reduction processes on the same potential range explains the broadening of the peak. The cathodic currents at the negative potential limit correspond to the formation of iron sulphide (reactions 4 and 4a) Fe2+ + S + 2e− → FeS

(4)

A. Gomes et al. / Electrochimica Acta 49 (2004) 2155–2165

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Table 2 Assignment of the cathodic process involved in the cyclic voltammograms studied S2 O3 2− /Fe2+ mole ratio in the bath

Cathodic peaks/potential range (V vs. SCE)

Cathodic reactions

1:1

C/−0.500 to −0.800

FeOOH + S + 3H+ + 3e− = FeS + 2H2 O S + 2H+ + 2e− = H2 S

(6) (2)

−0.125 −0.257

100:1

C1/−0.650 ± 0.050 C2/−0.750 ± 0.050

S + 2H+ + 2e− = H2 S Fe2+ + S + 2e− = FeS

(2) (4)

−0.257 −0.272

1:100

C3/−0.950 ± 0.050

Fe2+ + 2e− = Fe

(9)

−0.738

and/or Fe2+ + H2 S → FeS + 2H+

(4a)

This is well justified, by the cyclic voltammograms recorded for two different negative potential limits E␭ = −0.800 and −0.850 V versus SCE and presented on Fig. 5. The size increase of peak A4 for E␭ = −0.850 V is related with the cathodic current appearing for E > −0.800 V and not visible on the cyclic voltammogram obtained at E␭ = −0.800 V versus SCE. Thus, the increase of A4 is due to the oxidation of higher amounts of iron sulphide that are formed on the potential range after peak C. The reduction of Fe2+ species in solution that have not reacted previously may also occur, giving rise to the deposition of metallic iron. At higher sulphur/iron concentration ratio, the lower current intensities are due to the inhibition effect of the sulphur, as it has been evident on the Ti/S2 O3 2− system. On these conditions, the amount of adsorbed sulphur resulting from reactions (1) and (3) increases and, small amounts can be incorporated in the film during the electrodeposition process. Nishino et al. [23] obtained similar results for the electrode10

i / mAcm-2

0

Equilibrium potential (V vs. SCE)

position of CdS films by pulsed electrolysis. Moreover, the development of peaks, C1 and C2, on the potential range of peak C, points to the influence of the amount of sulphur on the kinetics of the reduction process. Our previous work, has assigned peak C1 to the reduction of sulphur deposited on the iron sulphide film, which is formed and not re-dissolved during the previous sweep [15]. At lower sulphur/iron concentration ratio (Fig. 4(c) only a single cathodic peak appears at E ≈ −0.950 V versus SCE that was assigned to iron deposition. Table 2 summarises a proposal of cathodic peak assignment based on the information collected from the voltammetric data and the equilibrium potentials calculated for the processes [24]. Since the Mackinawite phase does not appear on the electrochemical equilibrium Eh–pH diagrams for the sulphur–iron–water system [25–28], values quoted for the FeS without specification of the phase were used. The anodic peaks A1 and A2 according to our previous work [15] were assigned to the anodic dissolution of iron and to iron sulphide anodic formation, respectively. In what concerns peak A3, it has not been observed before and it is not reported in the literature also [29,30], what could indicate its relation with the bath composition, more precisely with the S2 O3 2− /Fe2+ ratio. Once peak A4 accounts for the oxidative dissolution of the iron sulphide electrodeposits, peak A3 could be assigned to an intermediary oxidation step, possibly the formation of a non-equilibrium metal-deficient surface layer, in a similar way as it was proposed for the oxidation of pyrrhotite by Mikhlin [31]. This oxidation could lead to an S-rich phase (FeSx with 1 < x < 2) assigned to reaction (5 ). FeS → FeSx + Fe2+ + 2e−

-10

(5 )

Peak A4 was assigned to the oxidative dissolution of the iron sulphide formed on both negative and positive scans and described by reactions (4 ) and: -20 -1000

-800

-600

-400

-200

0

200

400

E / mV vs. SCE

Fig. 5. Cyclic voltamograms obtained for a Ti electrode in unstirred deposition bath with S2 O3 2− /Fe2+ = 1, at 333 K at the negative potentials limits, −0.80 V (—) and −0.85 V (- - -) vs. SCE, respectively. Sweep rate 10 mV/s.

FeS + 2H2 O → FeOOH + S + 3H+ + 3e−

(6 )

In addition, the oxidation of Fe2+ to Fe3+ in the monosulphide structure possibly occurs and the shoulder (A5) could be assigned to: + − Fe2+ (solid) + 2H2 O → FeOOH + 3H + 1e

(7 )

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Table 3 Assignment of the anodic process involved in the cyclic voltammograms studied S2 O3 2− /Fe2+ mole ratio in the bath

Anodic peaks/potential range (V vs. SCE)

Anodic reactions

1:1

A1/−0.600 ± 0.050 A2/−0.450 ± 0.050 A3/−0.200 ± 0.050 A4/0.000 ± 0.050

Fe = Fe2+ + 2e− Fe + H2 S = FeS + 2H+ + 2e− FeS = FeSx + Fe2+ + 2e− H2 S = S + 2H+ + 2e− FeS = Fe2+ + S + 2e− FeS + 2H2 O = FeOOH + S + 3H+ + 3e− 3+ − Fe2+ (solid) = Fe(solid) + 1e 2+ Fe(solid) + 2H2 O = FeOOH + 3H+ + 1e−

(9 ) (8 ) (5 ) (2 ) (4 ) (6 )

A5

Equilibrium potencial (V vs. SCE) −0.738 −0.761 −0.257 −0.213 −0.125

(7 )

100:1

A4/0.000 ± 0.050

H2 S = S + 2H+ + 2e− FeS = Fe2+ + S + 2e− FeS + 2H2 O = FeOOH + S + 3H+ + 3e−

(2 ) (4 ) (6 )

−0.257 −0.213 −0.125

1:100

A1/−0.650 ± 0.050 A4/−0.050 ± 0.050

Fe = Fe2+ + 2e− H2 S = S + 2H+ + 2e− FeS = Fe2+ + S + 2e− FeS + 2H2 O = FeOOH + S + 3H+ + 3e−

(9 ) (2 ) (4 ) (6 )

−0.738 −0.198 −0.213 −0.125

Table 3 summarises the anodic peak assignments based on the information collected from the electrochemical experiments and the equilibrium potentials calculated for the processes [24].

in accordance with schemes (4–4a and 9). On the positive semi-cycle (E = 0.05 V versus SCE) the oxidation of metallic iron occurs with the consequent anodic formation of FeS and its partial oxidation in conjunction with the FeS deposited on the negative cycles. Other redox processes might occur on both half cycles depending on the S2 O3 2− /Fe2+ ratios in solution. Graphical integration of the anodic and cathodic current transients show that the charge involved on the redox processes depends on the bath composition. For all studied systems, the cathodic charge is higher than the anodic one indicating that the deposit is not completely stripped on the positive cycle, as might be expected for a cathodic

3.2. Electrodeposition process Iron sulphide electrodeposits were obtained by pulsed electrolysis. The characteristics of the square wave were selected from the voltammetric results in such a way that during the negative semi-cycle (E = −0.95 V versus SCE) the sulphur reduction occurs simultaneously with both the iron sulphide formation and the deposition of metallic iron,

50

30

a

q / mC

40

20

10

0 0

500

1000

1500

t

dep

2000

2500

3000

/s

Fig. 6. Variation of the anodic charges with deposition time for the three bath composition studied, S2 O3 2− /Fe2+ = 1 (䊏), S2 O3 2− /Fe2+ = 100 (䊐) and S2 O3 2− /Fe2+ = 0.01 (ρ).

A. Gomes et al. / Electrochimica Acta 49 (2004) 2155–2165

electrodeposition process. For long times, the two charges approach the same value. Fig. 6 shows the variation of the anodic charges with time for the three studied bath compositions. The higher anodic charges obtained for the deposition bath with equal amounts of ferrous and thiosulphate ions are in accord with the cyclic voltammetric data. Initially, the increase of the anodic charge is very sharp indicating a fast film stripping, probably due to the low thickness and no uniformities of the films going together with defects that may act as active sites during the oxidation. As the time growth increases, the film stripping becomes more difficult, as a result of the thickening, uniformity and restructuring phenomena. Some passivation may also occur due to sulphur and iron oxides deposits. Different plateau levels are observed depending of the [S2 O3 2− ] to [Fe2+ ] ratio. The appearance of these regions might be due to the dissolution of outer layers poorly attached to the

2161

film that were produced and reduced back during each cycle. These results indicate that there is a limit to the amount of film that can be grown and the beginning of the plateau corresponds to the optimum time growth for deposition. 3.3. Iron sulphide films characterisation 3.3.1. Anodic stripping analysis In order to characterise in situ the deposits, anodic stripping analysis was performed immediately after the pulsed deposition. The stripping curves are presented in Fig. 7. Although the general trend is comparable to voltammetric data, the number, definition and position of the peaks present some dissimilarity. For all the bath compositions, the peaks position indicates that the film is mainly composed by iron sulphide. For the bath with higher amounts of iron, peak A1 indicates that iron is anodically dissolved.

20

(a)

i / mAcm-2

10 0 -10 -20 -30

1

(b) i / mAcm-2

0

-1

-2

-3

2

(c)

i / mAcm-2

1 0 -1 -2 -3 -1000

-800

-600

-400

-200

0

200

400

E / mV vs. ESC Fig. 7. Stripping curves for the electrodeposits obtained at 333 K for the three bath compositions S2 O3 2− /Fe2+ = 1 (a), S2 O3 2− /Fe2+ = 100 (b) and S2 O3 2− /Fe2+ = 0.01 (c). Sweep rate 2.5 mV/s.

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Ti

3000

2500

1-x

*

FeS

FeS

* b)

Ti Ti

(220)

1-x

(211)

1-x

Ti

c)

FeS (200) 1-x FeS (112)

(111) FeS

1-x

(101) 1-x

FeS

1000

Ti

Ti

(001) 1-x

1500

FeS

I / a.u.

Ti

Ti

2000

500 a)

0 20

40

60

80

2θ / º Fig. 8. XDR patterns of the electrodeposits obtained on Ti at 333 K for the three bath compositions S2 O3 2− /Fe2+ = 1 (a), S2 O3 2− /Fe2+ = 100 (b) and S2 O3 2− /Fe2+ = 0.01 (c). *Reflexions due to the sample holder.

The films thickness was estimated from the integration of the anodic peak assigned to the iron sulphide dissolution. The value of 117.0 ␮m was found for the films obtained from the bath with [S2 O3 2− ]/[Fe2+ ] = 1. For the other two baths, a lower value, 4.6 ␮m, was found. This value compares well with 3.0 ± 0.5 ␮m obtained by SEM for the bath with [S2 O3 2− ]/[Fe2+ ] = 100 and reported in a precedent paper [32]. 3.3.2. Structural characterisation XRD measurements were performed to characterise structurally the electrodeposits obtained from the different baths. The results showed that all the as-electrodeposited films, independently of the bath composition, present patterns containing peaks consistent with tetragonal Mackinawite phase (FeS1−x ), indexed according to the ICDD file 24–73

[33], in addition to the lines of the titanium substrate and sample holder. The electrodeposits obtained from the bath with equal amounts of ferrous and thiosulphate ions contain other phases as Fig. 8(a) illustrates. By comparison with the suitable ICDD files [34], it was possible to identify, ␣-S and ␥-FeOOH as the crystalline phases that go with the Mackinawite as is shown in Table 4. The observed broadening of the diffraction lines indicates that the crystallinity of Mackinawite film is not high in contrast with the other samples. For the deposit obtained from the bath with the higher S2 O3 2− /Fe2+ ratio, although the pattern is very similar to the tetragonal Mackinawite phase, changes on the relative intensities of the (2 0 0) and (1 1 2) peaks are clearly seen, indicating a preference for the (2 0 0) plane of the FeS1−x , probably due to the incorporation of S2− in the structure.

Table 4 Identification of crystalline phases, besides Mackinawite, present in electrodeposit obtained on Ti at 333 K with bath composition S2 O3 2− /Fe2+ = 1. ␥-FeOOH [29]

Electrodeposit

␣-S [29]

2θ obs (◦ )

dobs (nm)

Irel (%)

dtab (nm)

Irel (%)

hkl

14.33 23.19 24.74 25.85 27.16 27.96 36.47 47.05 59.10 60.85 64.98

0.6174 0.3832 0.3596 0.3444 0.3281 0.3189 0.2462 0.1930 0.1562 0.1521 0.1434

4.6 6.0 1.1 1.6 5.4 10.5 6.0 2.3 2.4 1.6 1.8

0.6260

100

020

0.3290

90

120

0.2470 0.1937 0.1566 0.1524 0.1433

80 70 20 40 20

031 051 080 231 103

dtab (nm)

Irel (%)

hkl

0.3850 0.3570 0.3440

100 8 40

222 133 026

0.3210

60

206

0.1930

2

444

A. Gomes et al. / Electrochimica Acta 49 (2004) 2155–2165

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Table 5 Cell parameters and crystallite size for Mackinawite prepared from the three bath composition studied S2 O3 2− /Fe2+ mole ratio in the bath 1:1 100:1 1:100

Cell parameters (nm3 )

a (nm)

c (nm)

V

0.3566 0.3664 0.3666

0.5355 0.5016 0.5017

6809 × 10−2 6734 × 10−2 6743 × 10−2

Crystallite size (nm) 5 8 8

In addition, the reflection at 2θ = 23◦ is a sign that small amounts of ␣-S are starting to form as a separate phase. For the electrodeposits obtained from the bath with lower S2 O3 2− /Fe2+ ratio, although other extra diffraction lines might be expected, namely those corresponding to metallic iron they are not detected. The lattice parameters and average crystallite size of Mackinawite films, prepared under the different bath conditions are presented in Table 5. The average crystallite size was obtained from the width at half maximum for the (0 0 1) diffraction line, which exhibits the maximum height. For the baths with different amounts of ferrous and thiosulphate ions, the values obtained for the lattice parameters are similar and in accordance with the values published by Muller et al. [35] and Lennie et al. [36] for Mackinawite samples prepared from aqueous sulphide solutions and metallic iron in acid medium. For the deposits obtained from the bath containing equal amounts of ferrous and thiosulphate ions the cell volume presents a higher value which can be influenced by the presence of other crystalline phases previously referred. The average value of the crystallite size was estimated as 8 nm for the electrodeposits obtained from the baths with different amounts of ferrous and thiosulphate ions, and lower for the other one. This could be related with different deposition rates. 3.3.3. Morphology and composition characterisation by SEM/EDS The observation of the samples by SEM shows that the surface morphology is dependent of the bath composition. Similar morphologies were observed for the electrodeposits obtained from the baths with higher amounts of either ferrous or thiosulphate ions. A different morphology was presented by the film obtained from the bath with equal amounts of ferrous and thiosulphate ions. Fig. 9 shows the micrographies of the films surfaces obtained from the bath with S2 O3 2− /Fe2+ ratios of 1/1 and 1/100, respectively (a) and (b), where a cluster-like surface morphology is seen. A more rough surface is observed for the samples obtained from the bath with S2 O3 2− /Fe2+ = 1 in comparison with the other samples. This could be associated with the higher total concentration of S2 O3 2− and Fe2+ species and consequently to a higher deposition rate. The EDS spectra are shown in Fig. 10 and reveal the presence of Fe, S and O. In addition, the presence of Ti from

Fig. 9. Scanning electron micrographies for the electrodeposits obtained on Ti at 333 K for the bath compositions S2 O3 2− /Fe2+ = 1 (a) and S2 O3 2− /Fe2+ = 0.01 (b).

the support was also detected. These results are in accordance with the structural analysis that was able to identify the FeS1−x , S and ␥-FeOOH phases. Taking into account the electrodeposition conditions the formation of amorphous iron sulphides cannot be discarded and consequently the signal of the Fe and S would be enhanced. Moreover, the oxygen signal is higher for the samples obtained from the bath with equal amounts of sulphur and iron and the S/Fe atomic ratio is lower than in the other two electrodeposits, what could be related with the formation of the ␥-FeOOH phase. Concerning the other deposits, the values are near the expected ones for typical Mackinawite phases whose composition varies between FeS and FeS0.93 [37], although higher average values for the S/Fe atomic ratio were obtained for the deposits prepared in the bath with higher amounts of sulphur, as the results on Table 6 show. Table 6 S/Fe atomic ratios for the electrodeposits prepared from the three bath composition studied. S2 O3 2− /Fe2+ mole ratio in the bath

S/Fe

1:1 100:1 1:100

0.76 ± 0.14 1.02 ± 0.20 0.99 ± 0.19

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cycles are completed, the inner film, mainly Mackinawite, is covered by the oxidation products that are partially reduced during the next cycle with the cathodic formation of Mackinawite. On the subsequent cycles the process is repeated, but on the negative half cycle the oxidised film is more difficult to reduce and a complete reduction is not attained due to the potential drop across the film/solution interface. Further oxidation of the inner layers is inhibited by the presence of incorporated sulphur that is not completely removed on the negative pulses. This inhibition is probably caused by the fact that there is little access of the electrolyte to the inner layers, although the porous nature of the coatings. For long times, where the charge is constant (Fig. 6), it seems that a stable film is obtained and probably an electropolishing and recrystallisation of the deposit takes place during the anodic and cathodic cycles, respectively, for all the deposition conditions. The voltammetric data shows higher currents for the films obtained from the bath with equal quantities of ferrous and thiosulphate ions, consequently the amount of deposit is higher and its reduction more difficult. This fact can explain the appearance of ␥-FeOOH and ␣-S on the films prepared on these conditions. Considering that XRD and energy dispersive X-ray analysis are ex situ techniques, the possible oxidation of the films by the air is under discussion [13,38]. Having in mind that all the films were prepared and analysed in the same conditions and that ␥-FeOOH was only detected in the films prepared from the bath with equal amounts of ferrous and thiosulphate ions it can be concluded that, on these conditions the ␥-FeOOH formation occurs during the deposition process. On the other conditions, ␥-FeOOH produced is negligible or inexistent since it could not be detected. Fig. 10. EDS spectra of the electrodeposits obtained on Ti at 333 K for the three bath compositions S2 O3 2− /Fe2+ = 1 (a), S2 O3 2− /Fe2+ = 100 (b) and S2 O3 2− /Fe2+ = 0.01 (c).

3.4. Electrochemical growth of Mackinawite films XRD analysis on the iron sulphide electrodeposits show that Mackinawite is the iron sulphide phase formed, for all the bath compositions, although different additional phases are detected, when the S2 O3 2− /Fe2+ ratio is equal to 1 and 100, respectively. In this work, during the first anodic half pulse, the titanium surface is modified and on the subsequent negative half pulse the initial stage of Mackinawite formation takes place (according to scheme 4 and 4a) for all the studied bath compositions. When the Fe amount on the bath is higher the reduction of Fe2+ to Fe0 may also happen. The second stage of the process, which occurs at the second positive half pulse, is the oxidation of the surface of the film deposited on the previous negative pulse that is in contact with the solution which becomes hydrated. In addition dissolution of metallic Fe and anodic formation of Mackinawite may take place. When the first three half

4. Conclusions Mackinawite films have been obtained for all the studied electrodeposition baths and additional phases were detected depending on the bath composition. From the studies presented, it can be conclude that in order to prepare uniform Mackinawite films with relatively high purity, electrodeposition baths with concentration ratios [S2 O3 2− ]/[Fe2+ ] = 1 should be used. Cyclic voltammetric studies makes possible clearing up the electrodeposition processes associated with the electrochemical oxidation of the iron sulphide film. ␥-FeOOH and ␣-S were detected by XRD in the film obtained from the bath with equal amounts of thiosulphate and ferrous ions. Acknowledgements A. Gomes acknowledges financial support of Fundação para a Ciˆencia e Tecnologia (Portugal) under a Ph.D. grant BD/5223/95.

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