Raman spectroscopic evidence of the bronze-like recharging behavior for conducting films deposited from isopolytungstates

Electrochimica Acta 50 (2005) 1693–1702

Raman spectroscopic evidence of the bronze-like recharging behavior for conducting films deposited from isopolytungstates Barbara Palysa , Marina I. Borzenkob,∗ , Galina A. Tsirlinab,1 , Krystyna Jackowskaa,1 , Elena V. Timofeevab , Oleg A. Petriib,1 a b

Department of Chemistry, University of Warsaw, Pasteur 1, 02-093 Warsaw, Poland Department of Electrochemistry, Moscow State University, 119992 Moscow, Russia

Abstract In situ Raman spectroscopy (spectral regions 200–1700 cm−1 and 2800–3800 cm−1 ) at various potentials is applied to study the oxidation states of tungsten in oxotungstate films electrodeposited on Pt support from metastable Na2 WO4 + 0.5 M H2 SO4 solution. Comparison of in situ and ex situ spectra confirms that the films under study consist of the hydrated tungsten bronze (Hx WO3 , or Hx Wvx WVI (1−x) O3 ) and undergo quasi-reversible dehydration in air. Cyclic voltammetry (CV) and scanning tunnelling microscopy (STM) are applied to demonstrate the difference of freshly deposited and aged films. The pronounced changes of electrochemical properties and morphology found by these means are not accompanied by the changes in potential-dependent spectral behavior. The most pronounced effects of potential on Raman intensities are observed in the region of 0.05–0.60 V (reversible hydrogen electrode, RHE) and correspond to the decrease in WV content (i.e. x decrease) with potential. The behavior of bands at 2900–3800 cm−1 confirms that this process is accompanied by deprotonation. There is no any WV in the films at E > 0.60 V, which region corresponds to the thermodynamically stable hydrated WO3 . The experimental coulometric data are considered in the framework of the regular solution model and are applied to quantitative determination of potential—x dependence. The latter corresponds to reduction of one tungsten unity in [W10 O32 ]4− isopoly anion, which predominates in the deposition solution and, according to polarographic data, gives the main contribution to tungsten bronze deposition. © 2004 Elsevier Ltd. All rights reserved. Keywords: Isopolytungstates; Electrodeposition; Tungsten bronzes; Raman spectroscopy; Scanning tunnelling microscopy

1. Introduction Oxide films of group VI transition metals have been extremely actively studied in the context of electrochromic applications [1] based on their property to change the color reversibly when the applied potential or current are changed. Oxotungstate films present the most attractive electrochromic material as compared with other inorganic electrochromic compounds due to their higher reversibility, color efficiency, and stability. Rechargeability and redox mediating properties of tungsten oxides makes them also very attractive for electrocatalysis of fuel cell reactions [2]. ∗ 1

Corresponding author. Fax: +7 95 9328846. E-mail address: [email protected] (M.I. Borzenko). ISE member.

0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.10.012

Oxotungstate thin films can be prepared by various methods, namely: vacuum evaporation [3]; electron-beam sputtering [4]; chemical vapor deposition [5,6]; sol–gel deposition [7–13]; anodization [14], etc. Deposition conditions affect the structure, stoichiometry, adhesion, and water content in the films [1]; correspondingly, the properties of certain film material depend strongly on fabrication technique [15]. Mild room-temperature techniques of films fabrication are evidently advantageous, but there are only few approaches of this sort based on deposition from solution. Its disadvantages are low adhesion and formation of disordered or even amorphous oxides. The latter usually fail to demonstrate fast reversible recharging and require heat treatment for crystallinity increase. Heat treatment, in its turn, induces the same problems as the application of beam-assisted or

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chemical vapour deposition-like techniques (cracking, lack of adhesion, etc.). Polyoxometalates themselves are poor film formers. Therefore, a carrier matrix (usually an organic polymer or resin) is required to provide the film-forming properties. At the same time, isopolytungstates are able to form infinite network structures [16]. Recently, a simple electrochemical technique of the preparation of a new type of the rechargeable oxotungstate films was reported [17]. Electrochemical deposition of solid conductive oxotungstates appeared to be possible from the specially prepared long-living non-equilibrium solutions of isopolytungstates in 0.5 M sulphuric acid. The films of submicrometer thickness demonstrate the relatively fast charge transport in acid. According to preliminary rough estimate [17], if one presents the film composition by the formula for hydrogen tungsten bronze, Hx WO3 , the reversible capacity in terms of x changes corresponds to x ∼ 0.1. As the films obtained in [17] demonstrated good crystallinity immediately after deposition, the film formation process can be considered as electrocrystallization. It looks like cathodic analogue of the anodic electrocrystallization of oxides [18], which is possible if the solubility of tungsten oxocompound is lower for the lower oxidation states. The examples of solubility decrease in the course of formation of various tungsten blues [19] confirm this hypothesis. We can also assume that for reduced oxotungstates the ability to oversaturation in the acidic media appears to be less pronounced than for completely oxidized isopolyanions, which results in the nucleation of partly reduced solid oxotungstates on the electrode surface. Detailed elaboration of this electrocrystallization hypothesis requires further experimental verification. We should stress that deposition solutions proposed in [17] are H2 O2 -free (in contrast to solutions used, for example, in refs. [9–11]), and contain no colloidal tungstic acid (in contrast to deposition media reported in [12,13]). Both features are of great importance for controllable deposition with coulometric monitoring of the film thickness. Actually, the presence of H2 O2 induces parallel electrochemical reactions and probably also chemical reactions with reduced oxotungstate. In the presence of colloids, the deposition mechanism cannot be considered as electrocrystallization, being most probably of electrophoretic nature. This paper presents spectroscopic monitoring of the recharging processes for films fabricated according to [17]. Our main idea is to compare the potential-dependent oxidation state of novel films with the similar features observed earlier for more conventional materials, and to confirm the hypothesis [17] about bronze-like behavior of the films under study. It is the first step of detailed film characterisation, which is necessary for starting the basic study of the equilibrium potential–composition diagrams. Raman spectroscopy is a powerful tool for elucidation of structural and compositional features of thin films. It gives possibility to avoid morphology damage (in contrast to electron microscopy) and provides rather high sensitivity for thin

films (in contrast to X-ray diffraction). Therefore, Raman spectroscopy investigations are frequently chosen as a tool applied to various oxotungstate films [18–20].

2. Experimental Film deposition was organized like in [17]. A possibility to stabilize tungsten species in acid by using the similar procedure was also reported in [21,22]. The portion of Na2 WO4 necessary for obtaining 10−3 M solutions was initially dissolved in a portion of boiling water, and later the solution of 18 M H2 SO4 was rapidly added to provide 0.5 M H2 SO4 concentration in resulting solution. In contrast to [23,24] in which the study of the aggregation of tungstate ions in acidic medium and gel formation took place in ca. 30 min, the solutions prepared according to our technique demonstrated no colloid formation1 for at least 90 min, some solutions remained completely transparent for many hours. Usually, the oversaturation period was long enough to make at least 200 potential cycles and to obtain the film of ␮m-scale thickness. The films were visible and looked like yellow or blue homogeneous layers. The nature of film colour and the peculiarities of electrochromic behavior will be presented in a separate communication. The electrochemical experiments were carried out with the use of CH Instruments Electrochemical Analyzer (Austin, TX, USA) in a three-electrode configuration cell with a Pt gauze counter electrode. Potentials of Pt measured versus silver–silver chloride electrode were rescaled and are given below versus the reversible hydrogen electrode (RHE). In a number of experiments RHE was applied directly. The support was (1 cm × 2 cm) Pt foil pretreated by etching in aqua regia. Electrodeposition mode consisted in potential cycling (scan rate: 0.05 V s−1 ) over the potential region from 0.05 to 1.30 V. Besides, cyclic voltammetry (CV) was used to test rechargeability of deposited films in 0.5 M H2 SO4 solution. The freshly deposited tungstate films were aged during 1–30 days under normal atmospheric conditions. Some films for comparative study were deposited on Pt/Pt support with the roughness of ca. 200 [25]. Polarographic tests were carried out on a dropping mercury electrode with the mercury flow rate of 0.65 mg s−1 and open circuit drop life of 10.1 s, the saturated calomel electrode (SCE) served as a reference electrode. The Raman measurements were carried out with Jobin–Ivon spectrometer equipped with CCD detector and micro-setup. For excitation the 514 nm line of Ar+ laser was used. During in situ spectral measurements, the potential was controlled by Microautolab (Eco Chemie) and Raman signal was collected from 1 ␮m3 volume of solution in the vicinity of interface.

1 This conclusion follows from UV–vis spectroscopic data [17]. Colloid formation induces a rapid increase of absorbance in a wide spectral rang.

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Frequency range was chosen on the basis of previously published data for oxotungstate films. Usually it covers the region of W O and W O vibrations (200–1100 cm−1 ). As we were interested not only in the understanding the degree of WVI/V transformation, but also in protonation of oxygen atoms (which most probably accompanied the redox process in the film), we extended this range up to 1700 cm−1 and also collected the spectra in the region of 2800–3800 cm−1 . STM images were obtained ex situ with the use of Litscan2 device [26] with bias voltage of 0.2–0.3 V and typical tunnelling currents up to 0.2–0.3 nA. Three or five microscopic regions of each sample of specified type were scanned to provide representative images. Images were independent of scan direction and sign of tunnelling current. Na2 WO4 ·2H2 O and H2 SO4 were for analysis grade quality (Merck). Solutions were prepared using Millipore MilliQ water.

3. Results and discussion 3.1. Electrochemical behavior of deposition solution Acidification of an aqueous solution of tungsten salt results in the formation of a complex pH-dependent ensemble of polyoxotungstates [27]: paratungstate ‘A’ ([W7 O24 ]6− ), tungstate ‘Y’ (or decatungstate) ([W10 O32 ]4− ) and paratungstate ‘Z’ ([H2 W12 O42 ]10− ) ions are generated in the 0.5 M H2 SO4 solution. For [H2 W12 O42 ]10− anion the transformation into metatungstate anions ([H2 W12 O40 ]6− ) was reported in [17,28]. Data on the presence of pseudometatungstate anions ([H3 W6 O21 ]3− )] in solution of low pH are contradictory [28,29]. Actually, it follows from the analysis UV-spectra of deposition solution [17] that [W7 O24 ]6− and [W10 O32 ]4− predominate in this solution, both formed in the course of non-equilibrium hydrolysis/polymerisation of tungstate anion. Literature data on the corresponding redox potentials are limited by the values for pH ≥ 4–5. Polyoxotungstates reduction on mercury electrode in aqueous solutions of pH ≈ 5.0 takes place at −0.80 to 0.90 V (SCE) depending on both the nature and concentration of species [30–32]. An example for metatungstate anion is presented in Fig. 1, curve 1. No limiting current is observed because of wave coupling with the wave of catalytic hydrogen evolution. By decreasing the solution pH the reduction wave shifts to more positive potentials. For 0.5 M H2 SO4 medium, the potential of −0.34 V was reported [33]. Thus, the potential shift averages ca. 120 mV/pH. In contrast to this behavior, the deposition solution demonstrates two waves (Fig. 1, curve 2). For the second wave, the half-wave potential E1/2 is in a good agreement with the value reported in [33]. At the same time, the first wave demonstrates anomalously positive electroreduction potential (E1/2 ) equals to −0.20 V (SCE). Two successive single electron waves were earlier observed in [34] for [W10 O32 ]4− reduction in aqueous solution of pH

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Fig. 1. Polarization curves at Hg dropping electrode in solutions: (1) 2 × 10−4 M Na6 [H2 W12 O40 ] + 0.5 M acetate buffer (pH 4.7); (2) 10−3 M Na2 WO4 + 0.5 M H2 SO4 .

2.5. We avoid detailed comparison with [34] because of an uncertainty of pH dependence. If to assume even a relatively weak dependence of ∼60 mV/pH, data [34] also correspond to anomalously positive potentials. As [W10 O32 ]4− anion is a major component of deposition solution [17] (like in the previously studied colloidal solution obtained by means of acidification of tungstate solution [35]), it is rather natural to consider these anions as predominating reagents for film formation. It cannot be excluded that in 0.5 M H2 SO4 solution, the first wave corresponds to a two-electron reduction of decatungstate anion, and the second one should be referred to electroreduction of other polytungstate components of deposition solution. Anyway, the first wave corresponds to positive potentials in RHE scale and finds itself in the region used in [17] for film deposition. So there is no necessity to assume any special catalytic role of Pt support. Polyoxotungstates demonstrate strong chemosorption forming complete monolayer on the positively charged mercury electrode [30,36–38]. These monolayers induce strong inhibition of reduction of a number of anions as compared to tungstate-free solutions [30,38]. This rather general feature can manifest itself also in electroreduction of isopolyanions in the region under discussion (complications by both self-inhibition and inhibition by other tungstate species coexisting in the deposition solution). Sharp current increase at −0.55 V (SCE) (Fig. 1, curve 2) coincides with desorption potential [38] and corresponds to reduction of isopolyanions on the free surface of the dropping mercury electrode. Adsorption phenomena can explain the high slope and low height of the first wave, which are, however beyond the framework of this paper. The main aim of polarographic test (Fig. 1) was to demonstrate that single-electron reduction with blues formation in the course of potential cycling of Pt electrode occurs in the potential region in which the reagent is also redox-active on mercury. This means that reduction process is possible even if there is no any specific interaction with Pt, and catalytic effects are absent.

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3.2. Films deposition and ageing Deposition under potentiodynamic mode was accompanied by systematic increase of charge in the region 0.05–0.8 V and slight decrease of charge in the O-upd region. The features of deposition voltammograms are considered in detail in [17] and explained by the interplay of redox WVI/V transformations of soluble tungstate species and growth of rechargeable film. To check the specific features of the latter process, we report below CV of already prepared films (after thorough washing) in 0.5 M H2 SO4 solution (Fig. 2). The shape of cyclic voltammograms both for fresh and aged tungstate films deposited on Pt support differs essentially from that one for Pt. For freshly deposited films the broadening of H-upd region is the most pronounced feature (Fig. 2a); the charge in H-upd region for tungstate film is approximately 3.5 times higher as compared to bare Pt. Charge values are independent of the scan rate which, however, affects two anodic peaks: the first one at E ≈ 0.08 V increases, when the second one at E ≈ 0.25 V decreases with the scan rate. Charge in the H-upd region approaches the values typical for smooth Pt when the tungstate film is aged (Fig. 2b). It seems doubtful to refer any CV features to soluble species

resulting from partial dissolution of the film in the course of cycling because there is no visible decrease of film thickness even after prolonged treatment. The rechargeable film with rather high rate of charge transport described in [17] was freshly deposited, and its ageing was not specially considered. From the first glance there is no evidence of more slow charge transport in the aged film, as no scan rate effects are found for H-upd region. However we were able to observe slow charge transport in the same potential range for the film deposited on Pt/Pt. Details of Pt/Pt modification by tungstate are given in [25]. Tungstate is unable to penetrate into the nanosize pores of Pt/Pt [39] and forms the porous film on its surface. Under these circumstances both H-upd and O-upd on the major (internal) part of the Pt/Pt surface are not suppressed. Correspondingly, the charge spent for related to redox transformations of external tungstate film gives minor contribution to the total charge of H-upd region, and the process in the film can be observed only in the vicinity of 0.40 V (in the double layer region of pure Pt/Pt). This feature at the fixed scan rate decreases with the film ageing (Fig. 3a). For fresh film (curve 2 in Fig. 3a), the charge under discussion does not depend on the scan rate in the region of

Fig. 2. Cyclic voltammograms (current, I normalized on scan rate, v) measured in solution of 0.5 M H2 SO4 at bare (1) and modified (2–6) Pt foil recorded at: 2–0.10, 3–0.05, 4–0.02, 5–0.01, 6–0.005 V s−1 for as deposited (a) and aged (b) tungstate film.

Fig. 3. (a) Cyclic voltammograms (current, I normalized on scan rate, v) recorded at 0.10 V s−1 for film degradation in solution of 0.5 M H2 SO4 at: bare (1) and modified (2–5) Pt/Pt: 2, as deposited film; 3–5, successive steps of film ageing. (b) Scan rate dependence of the anodic currents normalized on scan rate, v for aged tungstate film in solution of 0.5 M H2 SO4 at bare (1) and modified (2–7) Pt/Pt recorded at: 2–0.10, 3–0.050, 4–0.02, 5–0.01, 6–0.005, 7–0.002 V s−1 .

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0.005–0.10 V s−1 . On the contrary, for aged film the pronounced dependence on the scan rate is observed (Fig. 3b). This fact demonstrates that ageing of tungstate can result in the slower charge propagation. Most probably the time scales of charge transport differ for aged films on smooth and platinized Pt. This difference will be considered in a special communication. In spectroscopic experiments presented below, we apply potentiostatic modes and relatively high charging times in order to observe the equilibrium (or at least quasi-equilibrium) states of films on smooth Pt at various potentials. Unusual behavior of aged film in the O-upd region (Fig. 2b) hardly corresponds to the shift of charge-discharge region to the more positive potentials as compared to similar region for freshly deposited film (extremely high change of free Gibbs energy of solid tungstate in the course of ageing). To continue with this problem, we first need spectroscopic data on the ratio of WVI and WV at various potentials. 3.3. Assignment of the bands in Raman spectra Fig. 4a and b presents the in situ Raman spectra of freshly deposited and aged films recorded in 0.5 M H2 SO4 solution at various potentials 0.05, 0.20, 0.60 and 1.00 V. These spectra are more complex in comparison to the spectra of Keggintype heteropolyanions [40,41], most probably because of the presence of at least two isopolyoxotungstates of different composition. Another possible reason of complex spectra is the symmetry: Td point symmetry is typical for ␣-isomers of really Keggin heteropolytungstates, when C2v type is known for metatungstate-anion. The later is relative to the same type anions group as a result of the presence of protons in tetrahedral cavity [41]. Comparison of Fig. 4a and b demonstrates that the number of bands and the ratio of intensities of the most pronounced bands I (981)/I (957), I (1050)/I (981) are not affected by film ageing. Higher number of bands is found when comparing in situ Raman spectra with ex situ spectrum obtained for dry film. The latter spectrum exhibits only few peaks: a strong peak at 957 cm−1 , a strong but broad peak at about 670 cm−1 , and a weaker one at about 220 cm−1 . According to the literature [40–42], two former bands may result from stretching vibration of terminal bonds WVI O (957 cm−1 ) and antisymmetric stretching vibration of WVI O bonds (670 cm−1 ). The band at 220 cm−1 is rarely observed, some authors ascribed it to the presence of WIV states [43–45]. Taking into account that the film is formed with participation of at least two anions, metatungstate-, [W7 O24 ]6− , and [W10 O32 ]4− , which differ in W O bond strength, the broadening of 670 cm−1 band is not surprising. It should be noted that dry film exists in oxidised or partially oxidised state. The new bands observed exclusively when Raman spectra of deposited films are recorded in the solution (Fig. 4b) are as follows: broad and weak band at about 1630 cm−1 , strong bands at 1050 and 981cm−1 , weak broad bands at 890 and 440 cm−1 . Much more, the intensity of all these bands and

Fig. 4. In situ Raman spectra for fresh (a) and aged (b) tungstate films recorded in solution of 0.5 M H2 SO4 at (1) 0.05, (2) 0.2, (3) 0.6 and (4) 1.0 V.

the bands observed also for dry film (670, 957 cm−1 ) are potential dependent. Additionally, the band at about 670 cm−1 decreases and shifts to lower energies when the film is reduced. The band at 1628 cm−1 may be assigned to the in plane bending vibration of O H group. The bands at 1050 and 981 cm−1 may originate from sulphate ions (SO4 2− , HSO4 − ) present in solution. To evaluate the contribution of the supporting electrolyte, we recorded Raman spectra of the bare platinum electrode immersed in 0.5 M H2 SO4 under the same mode as for the film. We failed to find any bands characteristic for sulphate ions, probably because of too low surface area as compared to porous nanocrystalline film [17]. Due to high surface area, the latter accumulates higher quantity of sulphate as compared with bare support. However, at this stage there is no solid reason to assume that sulphate ions contribute to the film spectra as soon as these bands decrease with potential. The variety of polytungstates allows the appearance of W O bands still not described in the literature. Bands at 1030–1050 cm−1 are typical for heteropolytungstates [40,41] and are refered to stretching vibrations of heteroatom—O bonds. One cannot exclude the specific features of the cavity of central tetrahedron of metatungstate-polyanion and its

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effect of H O vibrations similar to the effect of heteroatom. It should be stressed that this band was not observed earlier neither in Raman nor in IR spectral investigations of metatungstate anion [41,46,47]. However, these studies covered only the restricted spectral region. Both bands at 981 and 957 cm−1 can be attributed to terminal WVI Ot bonds. Actually, up to three bands were found in [41] in the region under discussion. This fact can be explained by two reasons: (1) distortion of symmetry of metatungstate anion, and (2) contribution of octahedrallycoordinated polyanions [W7 O24 ]6− (957 cm−1 [46]) and/or [W10 O32 ]4− (984 [46] or 995 cm−1 [48]). Other bands, which appear at about 890 and 670 cm−1 can be assigned to the antisymmetric stretching vibrations of virtually linear corner- and edge-sharing bridging bonds WVI O WVI , respectively [41]. The ratio of intensities confirms this hypothesis. Band at 444 cm−1 can be attributed to vibration of WV O. Similar band at 450 cm−1 was observed in [45] for amorphous WO3 film, which was electrochemically reduced or treated in hydrogen gas. The presence of 439 cm−1 band in the spectra confirms that the film of tungsten bronze is formed under reductive conditions. It should be noted that pronounced band at about 805 cm−1 typical for spectra of dry crystalline WO3 films [20,49] is absent from our Raman spectra. However, the interplay of hydration and crystallinity effects should be taken into account, and the absence of the specified peaks (805 cm−1 ) cannot be considered as a solid evidence of amorphous state. In particular, we observed a shoulder at 710 and a band in the vicinity of 960 cm−1 in the ex situ Raman spectrum, which were earlier reported for the hydrated crystalline WO3 [50,51]. In situ Raman spectra were also recorded in 2800– 3800 cm−1 range. Results obtained for the 1600–1700 cm−1 and 3000– 3800 cm−1 region confirm that the film is strongly hydrated. The absence of these bands in our ex situ experiments demonstrates that protonation of oxotungstate film starts immediately when we put it into the solution. There is no any unified assignment of O H stretching bands in the 3000–3800 cm−1 region for tungstate compounds in the literature. Moreover, various numbers of bands (from one to four) over the O H stretching vibrations region has been stated. In accordance with [52], it is assumed that water presents in the form of physically adsorbed, chemisorbed and structurally involved molecules. Water, which survived the annealing procedure, is assumed to be ‘structural’, and three peaks at 3050, 3200 and 3530 cm−1 correspond to these kinds of water. The Raman spectrum recorded in 3000–3800 cm−1 range for dry film is rather broad and of low intensity, so we cannot exclude pronounced dehydration in air, involving the loss of strongly bonded water. Reproducibility of electrochemical and spectral features after 1–30 days drying and transfer to the solution confirms that dehydration is quasi-reversible.

Four peaks on the IR spectra over the range of the O H stretching frequencies for crystalline powder of WO3 ·2H2 O obtained by sol–gel technique were discussed in [53]. The peaks at 3240 and 3180 cm−1 were assigned to vibrations of coordinated water and those at 3530 and 3380 cm−1 to the interlayer cocrystallised water. At the same time for monohydrate, there is only one broad band in the range of O H stretching frequencies at 3424 cm−1 . Absorption peaks at 2991 and 3362 cm−1 (oxidized WO3 ) and 3104 and 3492 cm−1 (reduced WO3 ) have been attributed to the water incorporation within the WO3 matrix [54]. Single band was observed for a number of polyoxometalates in solution [55]. Therefore, very broad potential dependent duplicate band with maxima at about 3260 and 3400 cm−1 found in our spectra may be ascribed to the symmetric and antisymmetric O H stretching vibrations of water molecules in the film, which can also be adsorbed on internal surfaces in the form of hydroxyl groups. The most pronounced bands observed for deposited films and deposition solution and bands previously observed are listed in Table 1. 3.4. Potential-dependent behavior of Raman spectra There is no qualitative difference in the potentialdependent spectral behavior of freshly deposited and aged films. This fact gives no chance to explain the difference of CV in Fig. 2a and b by any shift of characteristic charge/discharge potential region, and we are forced to consider this difference as induced by charge transport peculiarities. The most pronounced potential dependence was observed in the region of 0.05–0.60 V for the bands in the region of 900–1050 cm−1 and at about 670 cm−1 (Figs. 4(a and b) and 5) both for freshly deposited and aged films. Slight changes induced by potential shift from 0.60 to 1.00 V look less systematic. Minor film dissolution cannot be avoided at 1.00 V, so we concentrate below on the potential region 0.05–0.60 V in which the effects are most pronounced. The decrease of the potential (film reduction) results in the increase of intensity of 981 and 1050 cm−1 bands and in the disappearance of 957 cm−1 band. It should be noted that the studies of the reduction process for various tungstate films demonstrated various types of spectral behavior. For example, graduate decrease and final disappearance of the Raman signal at 950 cm−1 with the amount of injected charge were reported [14,56]; no changes were obtained in the Raman spectra [57]; the increase of the Raman intensity at 950 cm−1 was observed in the course of the reduction for tungsten oxide films [58]. These results suggest that terminal W O bonds can either be created [58] or eliminated [14,56] depending remarkably on the film preparation conditions: intensity of terminal W = Ot vibrations is either decreasing in the course of reduction (for amorphous films) or is increasing for films with a higher degree of crystallinity [52]. Formally, curves for 957 cm−1 in

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Table 1 Survey of data related to the most pronounced spectral bands In situ

Ex situ Deposition solution

Substances for which the certain bands were previously observed XW12 O40 n−b [41], H2 W12 O40 6− [41], a-WO3 [45] H2 W12 O40 6− [41], peroxopolytungstic acid (PPTA)[66], 5%WO3 /Al2 O3 [67] PPTA [24], WO3 a (nH2 O) [68] c-WO3 [50,51] PPTA [3], XW12 O40 n−b [40,41], H2 W12 O40 6− [41], WO3 [54] XW12 O40 n−b [40], H2 W12 O40 6− [41,70], PPTA [68,69], [W7 O24 ]6− [70], − WO3 /Nb2 O5 [67], WO3 /TiO2 [71] PPTA [24], XW12 O40 n−b [40,41], H2 W12 O40 6− [41,70], WO3 /TiO2 [69], [W10 O32 ]4− [70]

Freshly deposited

Aged

Freshly deposited

439m sha 600

439m sha 600m

442m 600m

– sha 594

Increase Decrease

673s sha 710 890w

673s sha 710 890w

670w

667w

885m

878w

Decrease Decrease Increase

957s

957s

946w

954s

Decrease

981m

981m

980m

sha 980w

Increase

1050s 1628m

1050s 1628m

1050s 1640m

1050s 1640w

Increase Increase

3243m –

3243s 3390s

Increase Increase

3260vs 3400vs

Aged

Potential dependency of peak intensity under reduction

Polytungstic acid[24], a-WO3 [10,54,72], PPTA [69], WO3 /SiO2 [72] WO3 a 2H2 O[53], PPTA [66] a-WO3 [10], WO3 a H2 O[53] PPTA [69], WO3 /SiO2 [72]

Peak intensity: w, weak; m, middle; s, strong; vs, very strong. a Shoulder. b X = P, Si, B, As and Ge.

Fig. 5 correspond to amorphous-type behavior. At the same time, we have direct X-ray diffraction evidence of film crystallinity. Most probably crystalline vibrational correlations reported earlier are rather approximate and not uniquely suitable for any oxotungstate. Intensity of a broad band at about 670 cm−1 decrease with potential decrease, and this peak disappears at 0.05 V. Simultaneously, the peak over 1600–1700 cm−1 region increases suggesting formation of W O H bonds. The 890 cm−1 band typical for O WVI O bond is more pronounced at lower potentials. All these facts confirm that the transformation of WVI =O and WVI O bonds followed by the decrease of intensity of corresponding spectral peak (957 and 670) leads to formation of W O H bonds and W O W bonds.

Fig. 5. Potential dependence of peak intensity for fresh (solid lines, filled symbols) and aged (dashed lines, open symbols) tungstate films at fixed wavenumbers.

In accordance with our aforementioned results, the film formation is one-electron process consisting in the reduction of a single tungsten ion for each isopoly anion. This electron introduced into the film is delocalized over all fragments of metatungstate [59,60], or over eight equatorial WO6 octahedral sites in the decatungstate-anion structure [61,62]. In the latter case, the reduction results in the stronger influence of added electron on some bonds and relatively weak effect on the others. Delocalization smoothes the consequence of electroreduction and leads to the redistribution of intensities of bands corresponding to terminal W O bonds. Thus two phenomena, namely small portion of reduced WV ions as compared to total number of WVI in isopolyanion and delocalization of introduced electron, result in the low intensity of band corresponding to WV . Two bands observed at 3000–3800 cm−1 region also demonstrate the potential dependence of intensity (Fig. 6).

Fig. 6. In situ Raman spectra for freshly deposited tungstate film measured in solution of 0.5 M H2 SO4 at (1) 0.05, (2) 0.2, (3) 0.6 and (4) 1.0 V.

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Both peaks increase in the course of electrochemical reduction of the film, indicating hydrated tungsten bronze formation and simultaneous adsorption or/and incorporation of H2 O molecules into the film. These protonation/hydration phenomena can take place even for water-free tungstate [63]. Nanoporous nature of the films under study [17] makes the initial hydration of oxidized film very fast. We always observed spectral features of hydration for both oxidized and reduced films in the solution, independently of ageing degree. 3.5. Thermodynamics

H-upd contribution. For the latter, we consider two limiting cases: (1) H-upd is completely suppressed, total charge corresponds to the film redox transformation; (2) H-upd is the same as on bare support, and the value for film charging can be obtained by subtraction of curve on bare Pt from that one for tungstate film. Curves 1 and 2 in Fig. 7 correspond to versions (1) and (2). Now we apply Eq. (2) to check for what values of A (parameter of the model of regular solution) the model dependence is close to the region of experimental values. It is easy to see that the model is highly sensitive to the choice of x in the region under study (0.05–0.60 V). If we assume that x changes from 0 to 0.25 (Fig. 7a), there

Data presented above give a good basis for consideration of tungstate films under study as hydrated bronzes Hx WO3 . However, the values of x cannot be quantified from Raman data and are still under the question because of the uncertainty of film thickness, which prevents calculation of x from coulometric data. We attempted to clarify this problem by comparing the film composition calculated in the framework of regular solution model with the experimental coulometric data for fresh films under the assumption that total charge or its major part corresponds to film oxidation followed by the decrease of x value. We applied an approach proposed earlier in [64]. First, we considered Hx WO3 as hydrated oxide, which can be formed theoretically from stoichiometric oxides (hydration is assumed for any oxides): WO3 + xe− + xH+ → Hx WO3

(I)

WO2 + H2 O → Hx WO3 + (2 − x)e− + (2 − x)H+

(II)

Second, we applied the model of regular solution to express the free energy of non-stoichiometric oxide, ∆Gns : G◦ns = xG◦1 + (1 − x)G◦2 + RT [x ln(x) + (1 − x) ln(1 − x)] − Ax(1 − x), where subscripts 1 and 2 are given for WO2 and WO3 , respectively, G◦1 = −603 and G◦2 = −866 kJ mol−1 [65], and A is the model parameter of nonideality. The next step was to get potential–composition (x) relations. For any x, the redox potential for Hx WO3 /WO2 equilibria is given by E=

(G◦1 + G◦H2 O − G◦ns ) (2 − x)F

−

RT pH F

(2)

where G◦H2 O is the standard free Gibbs energy for H2 O (−307 kJ mol−1 [65]). On the basis of spectroscopic data, one can conclude that there is no any WV in the film at E > 0.60 V, and this region corresponds to thermodynamically stable hydrated WO3 . This means that the charge spent for the film redox transformation in the region of 0.05 ≤ E ≤ 0.60 corresponds to transfer of x electrons to each WVI unity, resulting in the V formation of Hx WV x W(1−x) O3 . This charge can be obtained from the curves in Fig. 2a within the accuracy of residual

Fig. 7. Approximation of experimental data for Hx WO3 (solid lines) with the model of regular solid solutions (dropped lines with model parameter A, presented on the graph). Model calculated under the assumption that (1) H-upd is completely suppressed, (2) H-upd is the same as on Pt support and x is (a) 0.25, (b) 0.10 and (c) 0.05 at 0.05 V.

B. Palys et al. / Electrochimica Acta 50 (2005) 1693–1702

is no agreement of model slopes (dashed lines) with experiment. The agreement becomes satisfactory if one assumes x of 0.05–0.10 in the region under consideration (Fig. 7b and c). Taking into account approximations of regular solution model we can conclude that the recharging degree is close to 0.10. It supports the assumption that reduction of [W10 O32 ]4− gives main contribution to tungsten bronze formation because of easier reducibility of this anion. Summarizing, it is safe to assume that reduction of deposited tungstate film possibly composed of [W10 O32 ]4− anion results in the formation of one WV for each decatungstate anion. 4. Conclusions The results of Raman measurements point out that during the reduction of isopolytungstate films the W OH fragments are created. The process involves the reduction of WVI species to WV with WV O bond formation and electron injection, accompanied by protonation and creation of W OH bonds. Such reactions are typical for tungstate bronzes. The electrochemical behavior of the films under study demonstrates satisfactory agreement with the model of regular solutions. There is no chance to explain the increase of charge in the O-upd region for aged films (Fig. 2b) by WVI /WV redox transition. Unusual spillover of oxygen requires special studies and is of interest in connection with methanol electrocatalysis [66]. According to analysis of experimental data presented above, the stoichiometric range of rechargeability at E > 0.05 V (RHE) is rather narrow (x ∼ 0.1). However, it should not be considered as disadvantage of our films as compared with other oxotungstate films described in [49] (x up to 0.1–0.23). Actually, for electrochromic applications and mediating of electron transfer the reversibility of WVI/V transition plays a crucial role, but not recharging capacity. The studies of electrochromic and electrocatalytic characteristics of films are in progress, for which interpretation the direct data on bronze stoichiometry (presented above) are obligatory.

Acknowledgements A mutual agreement on scientific cooperation between Lomonosov Moscow State University and University of Warsaw is appreciated. M.I.B., G.A.T., E.V.T. and O.A.P. were supported by RFBR, Russia (Project No. 02-03-33285a).

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