Passive film formation as a function of applied potential on tin in citrate buffer solution was studied by X-ray photoelectron spectroscopy (XPS) and cyclic ...
JO~F~NAL OF
ELSEVIER
Journal of Electroanalytical Chemistry 407 (1996) 83-89
Electrochemical and X-ray photoelectron spectroscopy studies of passive film on tin in citrate buffer solution M. Seruga a, M. Metiko]-Hukovi6 a,*, T. Valla b, M. Milun b, H. Hoffschultz c, K. Wandelt c a Institute of Electrochemistry, Faculty of Chemical Engineering and Technology, University ofZagreb, PO Box 177, 10000 Zagreb, Croatia b Institute of Physics of the University Zagreb, PO Box 304, 10000 Zagreb, Croatia c Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstrasse 12, 53115 Bonn, Germany Received 14 June 1994; in revised form 7 December 1995
Abstract
Passive film formation as a function of applied potential on tin in citrate buffer solution was studied by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. In order to avoid air oxidation and to minimise contamination of the samples, an electrochemical preparation and transfer system attached to the ultrahigh vacuum system was used. Quantitative evaluation of the electrochemical and XPS data showed the characteristic changes of the spectra in the prepassive and in the passive potential range. The prepassivation correlated with the clear presence of Sn°, Sn 2+ and Sn 4+ species, while true passivation correlated with the presence of Sn4÷ species only, as expected on thermodynamic grounds. Three different oxygen-containing species were found in both potential ranges: oxide, hydroxide and water. The hydroxyl oxygen peaks (oxygen atoms from tin-hydroxide species) were dominant in the O ls spectra due to electrochemical conditions used (first scan) to produce samples for XPS examination. Keywords: Tin; Passivity; Polarization; X-ray photoelectron spectroscopy
1. Introduction Tin is a moderately corrosion-resistant metal, widely applied, especially in the tin-plate and electronic industry. The corrosion resistance of tin in humid atmospheres and in aqueous solutions is attributed to the presence of a passive oxide/hydroxide film on the metal surface. Therefore, studies of its passivarion and the composition of passive films are of considerable interest. Numerous studies of tin have been focused on passivation in strongly and slightly alkaline solutions [1-8], less with neutral solutions [9,10] and strong acid solutions [11-13], while passivarion in weak acid solutions (e.g. organic acids) has been only rarely investigated [14-16]. According to these studies, passivation and properties of the passive film grown on tin strongly depend on experimental conditions, especially the pH of a solution [16]. Reports also show that various properties of the passive films are closely related to their composition.
• Corresponding author. 0022-0728/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 4 5 0 2 - 3
However, there is disagreement in the literature data on the composition of the electrochemically formed passive films on tin [2,6,9,11,12,17,18]. The studies in very alkaline solutions reported by Hampson and Spencer [1], employing electron diffraction, suggested a duplex structure of the passive film, consisting of a thick, poorly adherent and generally amorphous layer of 5SnO. 2 H 2 0 overlaying a thin, strongly adherent crystalline film of SnO 2. However, the X-ray photoemission spectroscopy (XPS) studies reported by Ansell et al. [2] showed that the passive film consisted of SnO 2 and S n ( O H ) 4 , w i t h no Sn(II) species. Varsanyi et al. [6] used M/Sssbauer spectroscopy and found that the film formed on tin in slightly alkaline solution at low potentials was duplex, consisting of highly amorphous Sn(OH) 2 or hydrated SnO and SnO 2 or Sn(OH) 4, while at higher potenrials the passive layer consisted only of Sn(IV) hydroxide or oxide. The studies of Do Duc and Tissot [9] on the composition of the passive film formed on tin in neutral solution (phosphate buffer), applying XPS and electron microprobe analysis, indicated that the film formed at higher potentials consisted of a mixture of tin(IV)-hydroxide and oxide,
84
M. Seruga et aL /Journal of Electroanalytical Chemistry 407 (1996) 83-89
tin-phosphate, and contained adsorbed phosphate species. Ammar et al. [17] reported, using XPS and Auger electron spectroscopy (AES), that passive films formed on tin in neutral phosphate buffer and in 0.1 M KC1 and 0.1 M Na2SO 4 solutions consisted of Sn(IV) and Sn(II) oxides and hydroxides mixed with the species from the solutions. The composition of a passive film on tin in acidic solutions has been investigated relatively rarely. Thus Stirrup and Hampson [11] studied the composition of a film formed on tin in buffered phosphate electrolyte by X-ray diffraction. They obtained Sn3(PO4) 2 species in the passive film containing a mixture of SnO 2 and SnO. Laitinen et al. [12] analysed the passive film formed on tin in sulphuric acid solution using scanning electron microscopy (SEM), X-ray diffraction (XRD), XPS and secondary ion mass spectroscopy (SIMS) techniques. The analysis showed that the film was mainly SnO, mostly amorphous, containing traces of tin sulphate a n d / o r adsorbed sulphate/ bisulphate. Chen et al. [18] studied the composition of the passive film formed on tin in citric acid, by X-ray diffraction and AES, and suggested the presence of SnO 2 and 2SnO. H 2 0 in the film. Correlation of the above data is not simple due to various experimental conditions (e.g. solution composition, pH, polarisation range, polarisation scan rate, etc.) on the one hand and the very different nature of the experimental techniques used on the other hand. This is probably the main reason for disagreements between the results of studies on the composition of the passive films on tin. The aim of the present work was to study the composition of the passive film formed electrochemically, under potentiodynamic conditions, on tin in citrate buffer solution, pH 6, by XPS. To our best knowledge this is the first XPS measurement on electrochemically treated tin transferred within the vacuum system. The formation and ageing processes of passive film on tin were studied by means of cyclic voltammetry.
2. Experimental
Electrodes for cyclic voltammetry measurements were prepared from spectroscopically pure tin rods (JohnsonMatthey) and sealed into glass tubes with epoxy resin. The exposed faces of electrodes (A = 0.5 cm 2) were polished to a mirror finish using 0.05 /zm alumina powder. The electrochemical experiments were performed in a standard three-electrode cell; the working electrode was a tin electrode, the counter electrode a platinum electrode and the reference electrode a saturated calomel electrode (SCE), to which all potentials were referred. The experiments were carried out in 0.05 M citrate buffer solution, pH 6. The solution was prepared from "Analar" grade citric acid and sodium citrate and twicedistilled water. All experiments were performed at 298 K. Cyclic voltammetry measurements were performed us-
ing standard polarisation equipment: Wenking PCA 72H potentiostat, Wenking VSG 72 V scan generator, Wenking SSI 70 integrator and Servogor Goerz X-Y recorder. XPS measurements were carried out with the electrode prepared from spectroscopically pure tin foil (JohnsonMatthey). The electrochemical cell was attached directly to the XPS apparatus and the electrode was transferred from the cell into the main chamber of the spectrometer, without breaking the vacuum. A detailed description of the apparatus is given in Ref. [19]. Mg anode (Mg K a line) was the X-ray excitation source. The outgoing electrons were analysed by use of a SPEC AE10 + hemispherical analyser, operating with 12 eV pass energy.
3. Results and discussion
3.1. Cyclic voltammetry study of the passive film formation Cyclic voltammetry measurements on a tin electrode in citrate buffer solution, pH 6, were performed in the potential range between H 2 and 0 2 evolution and, within the limited potential ranges, at scan rates between 50 and 250 mV s - l . Prior to anodic polarisation, in order to start with a clean metal surface, any previously spontaneously formed oxide film was removed by polarising the electrode cathodically at - 1.8 V for 15 min. The electrode was then subjected to a positive potential scan. The study of the influence of scan rate on tin electrode behaviour was carried out within the short potential range ( - 1 . 8 to 0.4 V) where the initial film formation and reduction occurred (Fig. 1).
5.0
B.O
2.0
1.0
0.0:
0.5
10
-0.45
/ J
-0.48 -1.5
-ols
o'.o elvvs.ScE o,5
Fig. 1. Cyclic voltammograms of tin electrode in 0.05 M citrate buffer solution, pH 6, at different scan rates: v = 50, 70, 100, 150, 200 and 250 mV s - i. EL c = _ 1.8 V, ELa = 0.4 V. The inset shows the dependence of the anodic peak II potential on the square root of the potential scan rate.
M. Seruga et al. / Journal of Electroanalytical Chemistry 407 (1996) 83-89
When anodic polarisation was forced from the initial potential of - 1 . 8 V, the current rose sharply and two anodic current peaks (I and II) followed the region of active tin dissolution. The returning scan exhibits one well expressed cathodic peak. Thermodynamic data were used to predict the possible oxidised species constituting a passive film on tin at a given potential. For the various oxides and hydroxides of tin the equilibrium potentials were taken from Pourbaix [20], calculated against SCE at pH 6 and shown in Table 1. Analysis of the effect of scan rate on the peak I showed only the peak I height dependence. The anodic peak I potential was not dependent on the scan rate, and the experimental value of - 0 . 6 9 5 V was in very good accordance with the reversible potential for the equilibrium (i) or (ii) (Table 1 and Fig. 1). To get more information about the mechanism of the initial passivity stage under potentiodynamic conditions, cyclic voltammograms in the small potential region of peak I were recorded as a function of the upper potential limit E l , a (not shown in Fig. 1). The appearance of the reactivation peak during the negative going scan could be attributed to the rupture of the very thin (one or less than one monolayer) initial Sn(II) film, a n d / o r active dissolution of tin in the presence of that non-protective film. It was found that the peak current (I) increased on stirring the solution and adding Sn 2+ ions to the citrate solution. The possibility of forming soluble species or complexes prior to oxide film formation during the anodic scan on Sn in citrate buffer could be demonstrated by comparing the anodic peak c.d. in a borate buffer (where the formation of soluble complexes and incorporation of anions are unlikely or negligible) and in a citrate buffer solution. The results clearly indicate that the current peak was always higher in a citrate buffer than in a borate buffer at the same pH value and scan rate [21]. In other words, the citrate buffer was characterised by higher disso-
Table 1 Equilibrium potentials o f possible oxidised tin species Equilibrium species a n d equilibrium potential
E / V vs. S C E (pH 6)
(i) S n / S n ( O H ) 2 E = - 0 . 3 3 3 to 0.0591 pH (ii) S n / S n O E = - 0 . 3 4 6 to 0.0591 pH (iii) S n / S n ( O H ) 4 E = - 0 . 2 5 0 to 0.0591 pH (iv) S n / S n O 2 E = - 0.348 to 0.0591 pH (v) S n O / S n O 2 E = - 0 . 3 5 0 to 0.0591 p H (vi) S n ( O H ) 2 / S n O 2 E = - 0.363 to 0.0591 pH (vii) S n O / S n ( O H ) 4 E = - 0 . 1 5 4 to 0.0591 pH (viii) S n ( O H ) 2 / S n ( O H ) 4 E = - 0 . 1 6 7 to 0.0591 pH
- 0.688 V (SCE) -0.701 V (SCE) - 0.605 V (SCE) - 0.703 V (SCE) - 0.705 V (SCE) -0.718 V (SCE) - 0.509 V (SCE) - 0.522 V (SCE)
85
lution rate prior to oxide formation as compared with the dissolution rate in borate buffer. According to the literature data [22-24], most of the Sn 2÷ ions produced by dissolution of tin in citrate solutions exist in the form of different soluble chelate Sn(II)-citrate complexed species, whereas only a small fraction of the total Sn(II) ions are in the form of free (aquated) Sn 2+ ions [22]. Different Sn(II)-citrate complexes and Sn 2÷ ions undergo hydrolysis [25], giving rise to the formation of a thin layer of either Sn(OH) 2 (Kps = 5 × 10 -26 [26]) a n d / o r SnO on the metal surface according to the equilibrium reactions (i) and (ii) (Table 1). Accordingly, peak I could be regarded as a reversible formation of Sn(II) species with a deposition process involving an electron transfer mechanism and solution diffusion effects [27]. This initial Sn(II) film during the further positive going scan was oxidised to a more stable Sn(IV) species. The anodic peak II potential and cathodic peak potential, as well as the corresponding peak currents, changed with the scan rate. With increasing scan rate, the anodic peak II shifted to more positive potentials, whereas the cathodic peak shifted in the negative direction (Fig. 1). The difference between the anodic and cathodic peak potentials indicates the irreversible nature of the anodic processes. There is a linear relationship between the height of the anodic current peak II and the square root v ~/2 of the potential scan rate. A linear dependence was also recorded between the peak potential Ep(II) and u j/2 (Fig. 1, inset) which, when extrapolated to a zero scan rate, gave a spontaneous oxidation potential, i.e. the anodic nucleation potential Eo.a. Because of the absence of any polarisation effect this potential value was most suitable for comparison with thermodynamic data. It may be concluded that the experimental value Eo.a = - 0 . 4 9 0 V for peak II was in relatively good accordance with the equilibrium potentials (vii) and (viii) for the oxidation of SnO and Sn(OH) 2 to Sn(OH) 4 (see Table 1). The effect of u on the peak II corresponded to an irreversible process involving a dissolution-precipitation reaction under diffusion control [28,29]. The effect of u on the cathodic peak, as well as of the u 1/2 dependence found for the cathodic peak parameters, could be explained by a dissolution-precipitation process under ohmic resistance control [29-31]. The proportionality factor ( 6 E p / u i / 2 ) / ( 6 1 o / u L/2) has dimensions of resistance and a value of 100 J2. The reduction of passive films was a solid state decomposition process, leading to Sn-metal nucleation limited by resistance in the pores between the deposited metal sites [7]. The cathodic nucleation potential, i.e. spontaneous reduction potential obtained from the intercept of the straight line of cathodic peak potential Ep against u ~/2 at u = 0 showed the value of - 0 . 8 3 0 V vs. SCE. The cyclic voltammograms obtained on a tin electrode over a wide potential range, from H 2 to O 2 evolution, are shown in Fig. 2. After the initial passivation stage, charac-
M. Seruga et al./ Journal of Electroanalytical Chemistry 407 (1996) 83-89
86
0.4
of Sn(OH) 4 to SnO 2 involves a Gibbs energy change A r G = -- 38 kJ m o l - ~ and is therefore a highly irreversible process [20]. It could be assumed that dehydration of Sn(OH) 4 to SnO 2 • H 2 0 (or SnO 2 • x H 2 0 ) was also highly irreversible. The rate of this reaction is known to be slow, although it can be increased by heating [32], or by the applied field, as was the case in our experiments.
j
II
0.2
0.0
-0.2
?
3.2. XPS characterisation of the passive film Iv
-0.4
-0.6
-2.0
v I
I
I
I
I
~
-1.5
-I.O
-0,5
0.0
0.5
1.0
I
I
E/Vvs.SCE
Fig. 2. Cyclic voltammograms of tin electrode in 0.05 M citrate buffer solution, pH 6. ELc = - 1.8 V, E~,a = 2.0 V. The potential scan rate is 0.1 V s - ~. n = 1, freshly prepared electrode; n, stabilized electrode.
terised by two anodic peaks, a very good low-intensity stationary current plateau was formed. In the plateau region the current was constant, indicating thickening of the initial amorphous film of Sn(OH) 4 under potentiodynamic conditions by ionic conductivity under the influence of a strong electric field [8]. The charge measured in the current plateau region corresponded to a film thickness of about 2.5 nm, under the assumption that extremely insoluble Sn(OH) 4 film was formed (solubility product Kps = 1 × 10 -56 [26]). At a potential of about 1 V peak III was superimposed on the capacitance current region in the first positive scan (n = 1). In the stabilised electrode case (n > 10) a small kink appeared at the same potential region, probably due to some chemical and physical transformations of the passive film. Finally, at sufficiently positive potentials (around 1.8 to 2.0 V) the current rose abruptly (Fig. 2) due to oxygen evolution. This clearly indicates formation of an electronically conducting passive film. The reverse, negative going scan exhibited two cathodic reduction current peaks (peak IV at - 0 . 9 5 V and peak V at - 1 . 2 6 V). The presence of two cathodic peaks indicated that the final passive film contained Sn(IV) species of different electrochemical stability and activity (oxide, hydroxide), and it also supported the conclusion that the initial anodically formed species were transformed to a more stable state. Namely, the most stable tin species under anodic polarisation conditions was the hydrated oxide, SnO 2 • H 2 0 [4,5]. There was a good reason to assume that the initially formed amorphous Sn(OH) 4 film dehydrated with time and with increasing anodic polarisation potentials to a more thermodynamically stable state, crystalline SnO 2 • x H 2 0 film of different degrees of hydration [5], changing thus the film stoichiometry. Dehydration
Two samples (1 and 2) were prepared for XPS measurements under the same conditions as for cyclic voltammetry measurements. The electrode for sample 1 was subjected to potentiodynamic polarisation in the positive direction, from - 1 . 8 V to a final positive potential of 1.8 V at a scan rate of 0.1 V s- 1. Immediately after reaching the final potential, the electrode was transferred into the vacuum chamber of the spectrometer, as described in the Experimental section. Sample 2 was prepared in the same way, but the final potential was - 0 . 6 V, i.e. the electrode was removed from the cell at a potential between peaks I and II (Fig. 2). XPS spectra of Sn 3d levels of samples 1 and 2 are shown in Fig. 3(a). Before discussing the Sn 3d spectra obtained in this work, it should be mentioned that there was a discrepancy between the literature data on the Sn 3d peak positions of the Sn 2+ and Sn 4+ species. Some authors [17,33,34] suggested that there was no (or only a slight) difference between the Sn 3d peak positions of SnO 2 and SnO, while others [2,35] reported a significant difference of 0.5-0.7 eV. Recently, Themlin et al. [35] clearly showed that exposing a clean tin or SnO surface to air led to the immediate formation of a rather thick (around 30 ,~) SnO 2 film. This may explain some of the above experimental findings, that there was no chemical shift difference between the Sn 2+ and Sn 4+ core level spectra. The peak position difference of about 0.6 eV has been confirmed in several independent laboratories [36,37]. In this work, the binding energy scale was calibrated using the Au 4f signal from the sample-holder and then shifted by a constant amount to place the Sn 4+ 3d5/2 peak at 486.4 eV, the value found by several research groups (e.g. Refs. [33,36,37]). The Sn 3d spectra of both samples are shown in Fig. 3(a). In the following, for the sake of simplicity, the discussion will concentrate on the 3d5/2 levels. Sample 1 showed a very simple Sn 3d spectrum: the peak might be fitted with one Gaussian function centred at 486.4 eV and with full-width at half-maximum (FWHM) of 1.5 eV. However, a small contribution was necessary on both the low and high binding energy side of the peak in order to allow for the width of the peak at the baseline. Both contributions were small and highly dependent on the background subtraction procedure. The interpretation of the main peak as originating from the S n 4+ species was
M. Seruga et a l . / Journal o f Electroanalytical Chemistry 407 (1996) 8 3 - 8 9
S n 4+
~oo xes us~
a
~ : sn2 +
--
s, ~dja
'
6000
Sno
i__ 500
498
496
494
492
490
488
486
484
482
480
Bindingenergy/ e V
b
A
experimental
2OOO
0
......... h- ....... i......... i ......... i ......... I......... i ......... i......... 1......... | ......... #90 489 488 487 486 485 484 483 482 481 #80
Binding energy/eV F i g . 3. (a) X P S s p e c t r a o f S n 3 d 5 / 2
and 3d3/2
e l e c t r o n l e v e l s o f tin
e l e c t r o d e s a f t e r p o l a r i z a t i o n in 0.05 M citrate b u f f e r solution, p H 6. T h e s c a n rate is 0.1 V s - l .
EL e = _ 1.8 V. ELa = 1.8 V ( s a m p l e 1), - 0 . 6
V
( s a m p l e 2). (b) F i t t i n g o f the S n 3 d 5 / 2 p e a k o f s a m p l e 2.
straightforward and in full accordance with the XPS results of Ansell et al. [2] and Themlin et al. [35] (they reported positions of the Sn 4+ 3d5/2 peak at 486.3 eV), and also with the electrochemical data presented in Section 3.1. Sample 2 gave rise to a more complex spectrum, as shown in Fig. 3(a). Two peaks were clearly visible, the higher one being approximately at the position of the peak of sample t but exhibiting significantly higher FWHM value. This broadening is a consequence of strong asymmetry on the low binding energy side of the peak. The lower peak was centred at 484 eV and may safely be
87
attributed to metallic tin, i.e. Sn °. This assignation was based on measurements of metallic films of tin taken in this work as well as on the data of Themlin et al. [35], who placed the Sn ° 3d5/z peak 2.5 eV below the Sn 4+ value. That peak alone might not account for the asymmetry of the higher peak. Namely, fitting the complete Sn 3d5/2 spectrum of sample 2 with only two synthetic peaks (Lorentzian/Gaussian mixed functions), one of which was characterised by the values used for fitting the Sn 4+ peak of sample l, did not reproduce the experimental spectrum. The best fit was achieved using three Gaussian functions centred at 486.5, 485.6 and 484.0 eV. This fit is shown in Fig. 3(b). Based on available data on the position of the Sn 2+ 3d5/2 peak [2,35-37] and the fitting shown in Fig. 3(b) we ascribed the asymmetry of the high binding energy peak to the existence of the 3d5/2 peak originating from the Sn 2÷ species. The metallic signal shows that the thickness and composition of the surface films formed in citrate buffer solution, pH 6, strongly depend on the potential of formation. In the prepassive region (sample 2) the metal substrate is detectable (Fig. 3). According to the precipitation-dissolution model discussed above, a total layer thickness after peak I is less than one monolayer and, in addition, the Sn(II) hydroxide/oxide species are not strongly passivating. In the passive region (sample 1) a continuous and homogeneous layer thicker than one monolayer is found, and consequently no Sn ° derived 3d peak is present in the spectrum of sample 1 (Fig. 3). Quantitative evaluation of the electrochemical data gives a total thickness of 2.5 nm. In the experiments in which the upper potential limit was varied in the small prepassivation potential range between - 0 . 7 and - 0 . 5 V, the appearance of anodic currents during cathodic polarisation (negative scan) was attributed to rupture of the initial thin porous Sn(II) film and active dissolution of tin substrate. The XPS spectra confirm this statement. In the prepassive region the electrochemical conditions used to produce sample 2 (see Fig. 2) were such that neither the completion of the first layer nor the complete oxidation of Sn ° to Sn 4+ or Sn 2÷ to Sn 4÷ was possible. According to (iii) and (viii) of Table 1 both reactions are thermodynamically possible. It can be concluded that two distinct species are present in the prepassive region on the electrode surface. Furthermore, it could be expected for thermodynamic reasons that the layer nearest the electrode was that of the lowest oxidation state. More information may be obtained from the O Is spectra of both samples, as shown in Fig. 4. A rather broad peak in each case is clear evidence of several oxygen species present in the film. We fitted the O ls spectra under assumption of three distinct oxygen species; i.e. in terms of O ls signals of tin-oxides, tin-hydroxides and adsorbed water [2,38,39]. The spectra were fitted with three peaks keeping all their parameters free. Each peak was represented with four parameters: height, peak position, FWHM and ratio of the linear combination
88
M. Seruga et a l . / Journal of Electroanalytical Chemistry 407 (1996) 83-89 4oo0
hydroxide 0 ls - sample 1
o 1,
..,~ ][ ~ , ~ oxide
t "l I
Vittosample2
W" ..i\ ~'i
syntheti .... lope
0
536
.~34
532 530 Binding energy/eV
528
526
Fig. 4. XPS spectra of the O ls electron level of a tin electrode after polarization in 0.05 M citrate buffer solution, pH 6, for samples l and 2. Fitting is shown for the experimental spectrum of sample 2.
of Gaussian and Lorentzian functions. The parameters obtained for the best fit were later used to fit the next O 1s spectrum, keeping only the peak intensity free. When a reasonable correspondence between the synthetic envelope and the experimental spectrum was achieved, the remaining parameters were also set free in order to reach the best fit; e.g. three synthetic peaks at binding energies of 532.7, 531.5 and 530.4 eV (shown in Fig. 4) represented the best fit procedure for sample 2. The fit for sample 1 (not shown) is very similar, the only difference being the small intensity increase of the oxide peak. The peak at 532.7 eV might reasonably be ascribed to water molecules adsorbed on the surface (water of hydration). This was in accordance with values of 532.3 eV reported by Ansell et al. [2] and 532.6 eV reported by Tarlov and Evans [38] for the adsorbed water molecules on tin oxidised surface. Water molecules might adsorb dissociatively on tin oxide surfaces to form hydroxyl groups, which could in turn act as hydrogen bonding sites for the molecular adsorption of water (tin oxide surface hydrated a n d / o r hydroxylated by exposure to atmosphere or aqueous solutions [38-43]). The O ls peak at 530.4 eV might be ascribed to tin-oxide oxygen, which is in full accordance with the results of previous works [2,17,33,34,38,44]. However, it is difficult to assign this peak unanimously to any particular oxide species (SnO 2 or SnO) only on the basis of comparison of the position of the experimental O 1s peak and literature data. Some authors [17,33,34] reported that there is no (or just a small) difference between O ls spectra in SnO2 and SnO; in contrast, Ansell et al. [2] reported a significant difference of 0.6 eV. However, the results of electrochemical measurements presented in Section 3.1 and the Sn 3d spectra shown in Fig. 3 clearly
suggest that in the case of sample 1 (i.e. in the full passive region) the oxygen oxide peak might reasonably be ascribed to the presence of Sn(IV) oxide, whereas in the case of sample 2 (i.e. in the prepassive region) the peak might be ascribed to the presence of both Sn(II) and Sn(IV)-oxide species. This assignation is in full accordance with the results of Ansell et al. [2]. The peak at 531.5 eV might be ascribed to hydroxyl oxygen (oxygen atoms from tin-hydroxide species), in accordance with the values of 531.6 eV reported by Tarlov and Evans [38] and 531.4 eV reported by Ansell et al. [2] for the OH- group (there is no literature data on differences between positions of hydroxyl oxygen peaks in Sn(II) and Sn(IV) oxides). That is a dominant peak in both O 1s spectra (Fig. 4), and according to the electrochemical measurements (Figs. 1 and 2) and thermodynamic data (Table 1) presented, should be ascribed to the presence of Sn(OH) 2 and Sn(OH) 4 species. The electrochemical measurements, thermodynamic calculations and Sn 3d spectra shown in this paper suggest the presence of Sn(OH) 4 species at higher oxidation potentials (sample 1, passive region), and the presence of Sn(OH) 2 and Sn(OH) 4 species in the prepassive region (sample 2). These assignations are in full accordance with data presented by Ansell et al. [2].
4. Conclusions
This study on the passivity of tin in citrate buffer solution, pH 6, shows that dissolution-precipitation kinetics, having a diffusion controlled rds, play an important role in the stepwise oxidation of Sn(0)-Sn(II) to Sn(IV) in the prepassive range. As a result, the initial prepassivating, purely protective film is formed. Its thickness is less than one monolayer. True passivation under dynamic conditions results in the formation of a thin Sn(OH) 4 film which is one to two layers thick. The thickness estimated by cathodic removal of the surface film formed up to an upper potential limit E l , a = 1.8 V is ca. 3 nm. Prepassivation correlates with the Sn °, Sn 2÷, and S n 4 + species as shown by XPS. True passivation correlates with Sn 4+ species only, as expected on thermodynamic grounds. Three different oxygen-containing species are found in both the prepassive and passive regions: tin-oxide (characterised by the O ls peak at 530.4 eV), tin-hydroxide species (oxygen peak at 531.5 eV), and adsorbed water (peak at 532.7 eV). The tin-hydroxide species are dominant film components of both samples. Electrochemical measurements clearly show that the potentiodynamically passivated and stabilised tin electrode (n > 10 scans) exhibits chemical and physical transformations. There are sufficient grounds for the assumption that the initially formed Sn(OH) 4 film dehydrates with time and increasing positive potentials to a more thermodynamically stable state. Dehydration of Sn(OH) 4 to SnO 2 involves a Gibbs energy change of - 3 8 kJ mo1-1.
M. Seruga et al./ Journal of Electroanalytical Chemistry 407 (1996) 83-89
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