Highly-Stabilized Polynuclear Indium−Hexacyanoferrate Film

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Langmuir 1998, 14, 6274-6279

Highly-Stabilized Polynuclear Indium-Hexacyanoferrate Film Electrodes Modified by Ruthenium Species Giuseppe E. De Benedetto and Tommaso R. I. Cataldi*,† Dipartimento di Chimica, Universita` degli Studi della Basilicata Via N. Sauro, 85, 85100 Potenza, Italy Received April 27, 1998. In Final Form: July 20, 1998 We have discovered a novel procedure that yields extremely stable ruthenium-modified indiumhexacyanoferrate(II/III) film electrodes. The process includes electrochemical deposition of the indium(III)-hexacyanoferrate (InHCF) film and its subsequent voltammetric conditioning in 1 mM RuCl3. The resulting inorganic film was characterized by electrochemical, piezoelectric microgravimetric, and surface spectroscopic techniques. Frequency changes due to mass increase of the indium-hexacyanoferrate film onto the platinum-crystal working electrode were correlated with the incorporation of ruthenium, which imparts an extensive cross-linking between hydrate InHCF particles through dinuclear [Fe, Ru] oxo (-O-) and cyanide (-CN-) bridges. InHCF films prepared by this procedure and analyzed by XPS support the conclusions drawn from EQCM results. Upon modification, the inorganic film exhibits remarkable electrochemical stability, there being no observed decrease in peak current signal after at least 8600 cycles in the potential window from 0.0 to +0.9 V vs Ag|AgCl. Evidence is also provided for the incorporation of K+ along with one water molecule during the reduction process. The cation selectivity of the rutheniummodified InHCF film is K+ > Na+ > Li+, Cs+ . NH4+, Rb+, and the fact that the original voltammetric profile was restored when the Ru-modified InHCF film electrode was cycled in K+ further confirms its greater selectivity.

1. Introduction Stability of inorganic films is highly desirable in order to expand their use in numerous practical applications, such as electrochromism, electrocatalysis, corrosion inhibition, and photochromic conversion. Indium(III)hexacyanoferrate(II/III) (InHCF) is a Prussian Blue (PB) analogue that is very attractive for its application in electrochromic1 and electrocatalysis2,3 fields as a photoelectrochemical converting material4 and as stabilizing coating for photoelectrodes.5 Apparently, the deposition of this inorganic film is propitiated by the sparingly soluble indium(III)-ferrocyanide(II) complex, which is present mostly as In4III[FeII(CN)6]3. This is based on the observation that the initially deposited InHCF film does not contain potassium ions. In agreement with previous work,6,7 the conversion of the initially formed material during electrochemical cycling involves some loss of In3+ from the lattice and its eventual removal from the film and replacement by K+. Very recently, we have characterized such a film by X-ray photoelectron spectroscopy (XPS),8 and in agreement with that investigation the redox transition in potassium-containing electrolytes may be written as follows:

InIIIFeIII(CN)6 + xe- + xK+ ) Kx{InIII[FeII(CN)6]}x{InIII[FeIII(CN)6]} (1) where x was found to be equal to about 0.2 and represents the fraction of the film that is electroactive. Note that * Corresponding author. † E-mail: [email protected]. (1) Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Electrochromism: Fundamentals and Applications, VCH: Weinheim, Germany, 1995; Chapter 6. (2) Chen, S.-M. J. Electroanal. Chem. 1996, 417, 145. (3) Li, H.; Wang, E. Microchem. J. 1994, 49, 91. (4) Gomathi, H.; Upadhyay, D. N.; Prabhakara Rao, G. Solar Energy Mater. 1993, 30, 161. (5) Gruszecki, T.; Holmstro¨m, B. J. Appl. Electrochem. 1991, 21, 430.

such a low potassium content (i.e., x < 1.0) is common to other inorganic films, such as the well-known Prussian Blue.9,10 When the InHCF film is repetitively cycled between 0.0 and +0.9 V in the same electrolyte employed for the deposition, i.e., 0.5 M KCl + HCl at pH 2, the electrochemical activity deteriorates considerably. As a result these films could not be used for long in those experimental conditions in which the redox transition FeII/III is involved, and although there is a lot of interest in the use of InHCF in relevant technological fields, only some attempts there exist for improving its stability. Previous efforts include the use of overlayers made by different inorganic films but without reporting comparison of data.2 Besides, Li and Wang3 proposed the application of the InHCF film as an amperometric sensor in liquid chromatography with electrochemical detection by coating it with a layer of Nafion; a change of response of about 15% over 30 min was observed. Better results were not obtained by coating the inorganic film with additional layers of Nafion, and this represents in any case a hybrid approach. A major objective of this research was to find out a general route for the stabilization of inorganic films. It is well-known that not only InHCF film electrodes but also several PB analogues exhibit electrochemical stability as the other side of the coin, as substantiated by the limited number of papers devoted to practical applications. Previously, we have reported a novel and extremely simple approach for improving the electrochemical stability of ruthenium purple [iron(III)-hexacyanoruthenate(II)] obtained by cycling the inorganic film in a solution of RuCl3.11 As ruthenium purple and InHCF are structurally very (6) Dong, S.; Jin, Z. Electrochim. Acta 1989, 34, 963. (7) Jin, Z.; Dong, S. Electrochim. Acta 1990, 35, 1057. (8) Cataldi, T. R. I.; De Benedetto, G. E.; Bianchini, A. J. Electroanal. Chem. 1998, 448, 111. (9) Itaya, K.; Ataka, T.; Toshima, S. J. Am. Chem. Soc. 1982, 104, 4767. (10) Itaya, K.; Uchida, I. Inorg. Chem. 1986, 25, 389. (11) Cataldi, T. R. I.; De Benedetto, G. E.; Campa, C. J. Electroanal. Chem. 1997, 437, 93.

S0743-7463(98)00477-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/17/1998

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close, we may expect a similar behavior in the attainment of film stability. Thus, this paper examines the modifications occurred to InHCF films upon conditioning in Ru(III)-chloride. Simultaneous cyclic voltammetry and microgravimetry with an electrochemical quartz crystal microbalance (EQCM) were used to follow the modification through the incorporation of ruthenium species. The resulting ruthenium-modified InHCF film was characterized by cyclic voltammetry and X-ray photoelectron spectroscopy (XPS), while the stabilization process was investigated by EQCM. We will also report on the longterm stability of the Ru-modified InHCF films. 2. Experimental Section 2.1. Chemicals. Reagent grade K3Fe(CN)6 99+%, InCl3‚ 4H2O 97%, RuCl3 xH2O (x e 1), and potassium chloride were purchased from Sigma-Aldrich Chemical Co. (Steinheim, Germany). All other chemicals employed were of analytical grade and were used without further purification. Doubly distilled and deionized water (Cecchinato, Venice Mestre, Italy) was used throughout. 2.2. Electrochemical Instrumentation. Cyclic voltammetry was performed on an EG&G (Princeton Applied Research, Princeton, NJ) Model 263A potentiostat/galvanostat. Data acquisition and potentiostat control were accomplished with a 486/50 MHz IBM-compatible computer running the M270 electrochemical research software (EG&G) version 4.11. All experiments were carried out at room temperature in a standard three-electrode glass cell employing a Ag|AgCl|KCl(sat) reference electrode and platinum gauze as counter electrode. The glassy carbon working electrode (4-mm diameter) used in cyclic voltammetry was purchased from EG&G. During electrochemical deposition, no care was taken to degas the solutions. The details of the EQCM apparatus have been described previously.11 EQCM experiments were carried out with a potentiostat (EG&G Model 263A) interfaced to a Seiko EG&G QCA 917 electrochemical quartz crystal microbalance. The piezoelectric quartz crystals used were 9 MHz AT-cut (EG&G) coated with platinum; the geometrical area was about 0.2 cm2. The EQCM measurements were performed in a Teflon quartz crystal holder, Model QA-CL4 (EG&G), using a three-electrode configuration (Pt wire auxiliary and Ag|AgCl|KCl(sat) reference electrode). Microgravimetric data were calculated using the Sauerbrey equation12 assuming that the observed change in the fundamental frequency of the coated quartz crystal, ∆f, is due entirely to the weight (∆m) of the rigid material attached to it, with the proportionality constant, K, dependent on the characteristic of the piezoelectric crystal, equals to 9.1 × 108 Hz g-1. 2.3. Electrodes. Before modification, the glassy carbon surfaces were polished with 0.05 µm R-alumina slurry on a microcloth polishing pad, washed with water, and sonicated for a few minutes in doubly distilled water. The deposition of InHCF films was accomplished as described elsewhere8 by a procedure similar to that reported by Kulesza and Faszynska.13,14 Briefly, the potential was cycled 25 times from 0.0 to +0.9 V in freshly prepared 0.5 M KCl at pH 2 solutions containing 1 mM InCl3 and 1 mM K3Fe(CN)6. Coverage of the modified electrode was determined by measuring the charge under the voltammetric waves for the hexacyanoferrate(II/III) redox process. All experiments reported herein have been carried out with surface coverages in the (2-6) × 10-9 mol/cm2 range. For XPS experiments the working electrode was platinum 99.99%, 0.05 mm thick (Aldrich), or glassy carbon plates, Sigradur K, obtained from HTW Hochtemperatur-Werkstoffe GmbH (Meitingen, Germany). InHCF films were rinsed thoroughly with water before XPS analysis to avoid measuring ionic species adsorbed onto the deposit. (12) Buttry, D. A. Applications of the Crystal Microbalance to Electrochemistry. In Electroanalytical Chemistry; Bard, A. J., Ed.; 1991; Vol. 17, p 1. (13) Kulesza, P. J.; Faszynska, M. J. Electroanal. Chem. 1988, 252, 461. (14) Kulesza, P. J.; Faszynska, M. Electrochim. Acta 1989, 34, 1749.

Figure 1. Plot of mass vs time of an InHCF film grown on platinum during repeated potential cycling from 0.0 to +0.9 V vs Ag|AgCl. Supporting electrolyte: 0.5 M KCl at pH 2. The inset shows the first, second, and last EQCM mass plots. The “zero” mass refers to the electrode mass before film formation. Scan rate: 50 mV/s. 2.4. Surface Characterization. X-ray photoelectron spectra were obtained with a Leybold LH1 spectrometer interfaced to a personal computer for data acquisition. Spectra were acquired using either the achromic Mg KR (1253.6 eV) or Al KR (1486.6 eV) radiation at a power of 260 W (13 KV, 20 mA), and were recorded in fixed analyzer transmission mode (FAT) to achieve maximum instrumental resolution. Pressure in the analysis chamber was typically less than 5 10-9 Torr. Under these conditions, the binding energy (BE) scale was calibrated with respect to the Cu 2p3/2 (932.7 eV, full width at half-maximum (fwhm) ) 1.75 eV) and Au 4f7/2 (84.0 eV, fwhm ) 1.20 eV) transitions using spectroscopically pure metals (Johnson Matthey). Each sample was analyzed at a 90° angle relative to the electron detector. Wide and high-resolution spectra were recorded in constant pass energy (50 eV) with channel widths of 1.0 and 0.1 eV, respectively. The high-resolution spectra were averaged for number of scans to increase the signal-to-noise ratio. All transfers to the XPS spectrometer were made under ambient conditions (i.e., air at room temperature). Binding energies were assigned by referring the energy scale to that of the N 1s peak relevant to the nitrogen of ferrocyanide moieties, which was set to 398.0 eV.15,16 Data acquisition and spectra analysis were accomplished with a data processing program.17 Calculated spectra were generated using a mixed Gaussian-Lorentzian sum function. A Shirley-type function was used to generate the background curves. Semiquantitative analysis was based on the use of empirically derived atomic sensitivity factors,18,19 since the Physical Electronics 550 and Leybold LHS10 XP-spectrometers have similar transmission functions. Due to the intrinsic uncertainty of sensitivity factor values, the absolute accuracy of quantitative determinations by XPS analysis is around 10%.

3. Results and Discussion 3.1. InHCF Film Dissolution. Insoluble InHCF films prepared by electrochemical deposition on a conducting substrate, such as glassy carbon or platinum, exhibit a well-defined redox couple associated with the surfaceconfined iron centers (FeII/III). As apparent from the plot of Figure 1, the mass of a freshly InHCF film electrode presents a progressive loss as the number of potential cycles increases, resulting in a poor stability of the modified (15) Vannerberg, N. G. Chem. Scr. 1976, 9, 122. (16) Yatsimirskii, K. B.; Nemoshalenko, V. V.; Nazarenko, Y. P.; Aleshin, V. G.; Zhilinskaya, V. V.; Tomashevsky, N. A. J. Electron Spectrosc. Relat. Phenom. 1977, 10, 239. (17) Desimoni, E.; Biader Ceipidor, U. J. Electron Spectrosc. Relat. Phenom. 1991, 56, 189. (18) Wagner, C. D. Anal. Chem. 1979, 51, 466. (19) Briggs, D.; Seah, M. P. Practical Surface Analysis, J. Wiley: Chichester, England, 1994; Vol. I, Appendix 6.

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Figure 2. Cyclic voltammograms of an InHCF film grown on glassy carbon in a blank supporting electrolyte before (A) and after (B) modification in acidic 0.5 M KCl solution (pH 2) containing 1 mM RuCl3. The potential was cycled for 75 times between 0.0 and +0.9 V: surface coverage, 5.4 ( 0.2 10-9 mol/ cm2; scan rate, 50 mV/s.

electrode. Such a behavior is attributable to dissolution of the film following its oxidation,14 that is upon formation of the relatively high soluble complexes InFe(CN)6 and Kx{InIII[FeII(CN)6]}x{InIII[FeIII(CN)6]}.8 As can be seen in the inset of Figure 1, the film dissolution is concomitant with the oxidation of the iron centers. Indeed, during potential cycling the decrease of mass occurs mainly after expulsion of potassium (vide infra) and ends when the film is reduced back. Conceivably, the corresponding peak heights lower in intensity as well (not shown). 3.2. Stability Enhancement of InHCF Films. It is recognized the ability of ruthenium to form very stable mixed-valent Ru-oxide inorganic films cross-linked by cyanide.20-24 Some attention has recently been paid to the ability of ruthenium(III) to impart a remarkable stability to the inorganic film known as ruthenium purple.11 In the present case, the same idea was applied. Thus, as anticipated in the Experimental section, freshly InHCF film electrodes were subjected to continuous cycling between 0.0 and +0.9 V in solutions of 0.5 M KCl at pH 2 containing 1 mM RuCl3. In Figure 2, both voltammograms before and after the modification step are reported; the apparent surface coverage, (5.4 ( 0.2) × 10-9 mol/cm2, was estimated from the charge under the FeII/III redox peak of the InHCF film electrode, at a scan rate of 10 mV/s. Although there are no substantial differences in the main redox peaks, a small anodic peak appears at about +0.53 V whereas the corresponding cathodic peak, probably being quite broad, is not clearly distinguishable. Since indium is not electroactive in the potential window from 0.0 to +0.9 V, we ascribe such a new peak either to Fe(II/III) with a slightly different chemical environment compared with that of the iron centers involved in the main redox process, or most likely to Ru(II/III) as the oxo- and/or oxy-ruthenium species incorporated in the InHCF deposit during the modification process. The last suggestion seems more plausible, as will be substantiated in some detail below. Kulesza21 postulated the involve(20) Cox, J. A.; Kulesza, P. J. Anal. Chem. 1984, 56, 1021. (21) Kulesza, P. J. J. Electroanal. Chem. 1987, 220, 295. (22) Gorski, W.; Cox, J. A. J. Electroanal. Chem. 1995, 389, 123. (23) Cataldi, T. R. I.; Centonze, D.; Guerrieri, A. Anal. Chem. 1995, 67, 101. (24) Cataldi, T. R. I.; Salvi, A. M.; Centonze, D.; Sabbatini, L. J. Electroanal. Chem. 1996, 406, 61.

De Benedetto and Cataldi

Figure 3. Plot of mass vs time of an InHCF film in contact with 1 mM RuCl3 solution during repeated potential cycling between 0.0 and +0.9 V. The inset illustrates the initial and final EQCM mass plots. Other conditions are as in Figure 2.

ment of Ru2O63+ species in the formation of thin inorganic films of mixed-valent ruthenium-hexacyanoruthenate(II/III) along with its ability to form dinuclear oxo complexes. We believe that the presence of such a species is beneficial since it forms anchoring between InHCF particles through dinuclear Fe and Ru oxo and cyano bridges. It follows that some cyanide groups have been substituted, resulting in a more stable film probably because an extended network among iron centers is favored by oxoruthenium species. Currently, we are investigating the exact mechanism by which such an effect has been induced. Since it is difficult to judge the change occurred to InHCF from the voltammetric profile, the EQCM was exploited for gaining further hints about its modification in the Rucontaining solution. Figure 3 shows a series of consecutive redox cycles for an InHCF film electrode in contact with a 1-mM solution of RuCl3. As can be ascertained, the plot of film mass against potential cycling time increased up to 1500 s, and then became almost constant with an estimated mass increment corresponding to about 15%. It can be seen from Figure 3 that the stabilization process is almost completed after 2200-2700 s, corresponding to 60-75 redox cycles. The concentration of Ru(III) and the number of cycles were found to be important factors in enhancing the stability of the polynuclear inorganic film (vide infra). Moreover, there are evidences that the mass increase occurs both in the forward and reverse potential scans, thereby suggesting modification of the film not only as a result of the voltammetric cycling (see inset of Figure 3) but also as due to positive ion-exchanging from the electrolyte solution. Interestingly, a marked decrease in frequency (i.e., mass increase) was also noted simply by dipping the InHCF deposit in the RuCl3 solution. This effect declines with time, and after not more than a half an hour, no further mass changes of the inorganic film could be recorded. Again, the present data may be interpreted as being due to inclusion of oxo-ruthenium species into the lattice structure of the InHCF film. 3.3. Stability Evaluation of the Ru-Modified InHCF Film. Repetitive redox cycling experiments were done to determine the extent of stability relevant to Rumodified InHCF film electrodes in acidic 0.5 M KCl solutions. A series of films was investigated; the percentage of relative anodic peak current as a function of the number of potential cycles is shown in Figure 4. Each data point was the average of three to four different working electrodes prepared with the same procedure; vertical bars represent the difference between the highest and lowest values observed. As anticipated above, the anodic peak of InHCF films decreases rather dramatically

Indium-Hexacyanoferrate Film Electrodes

Figure 4. Electrochemical stability of InHCF film electrodes. Normalized anodic peak height as a function of the number of cycles in acidic 0.5 M KCl (pH 2) solutions for the following InHCF films: (a) unmodified; (b) cycled 25 times in 1 mM RuCl3; (c) 75 cycles in the same solution. Io is the peak height evaluated on the first cycle. The error bars represent the difference between the highest and lowest value observed.

during the initial 1000 redox cycles (curve a). By examination of curve c corresponding to Ru-modified InHCF films for 75 cycles in 1.0 mM Ru(III), instead, the loss of electrochemical activity was suppressed to an almost negligible level. The redox process was sustained without changes for at least 8600 redox cycles in the potential window from 0.0 to +0.9 V vs Ag|AgCl, even using different scan rates, such as 50, 100, and 500 mV/s. The test was terminated after this number of scans. It turns out that is possible to obtain devices with exceptional stability. This is impressive in light of the fact that the modified electrode seems to maintain the same lattice structure of the parent InHCF film (see next section). Note that there was evidence of gradual signal decrease when the InHCF film electrode was conditioned just for 25 cycles in 1.0 mM RuCl3 (see curve b, Figure 4). However, about 80% of the peak height persisted up to 4600 repetitive cycles, suggesting that the corresponding Ru-modified InHCF film upon an initial dissolution retains a remarkable stability. It was also found that increasing the number of cycles above 75 (50 mV/s as scan rate) did not show much further improvement in the film stability. It is recognized that InHCF is a PB analogues with the interstitial spaces and many lattice sites filled with water molecules and electrolyte ions exchanged from the solution. Hamnett and co-workers25,26 pointed out that PB films are better described as formed by an extended periodic arrays of hydrate particles embedded in a hydrogen-bonded water matrix. Likewise, the substantial water content of the InHCF deposit, revealed by EQCM,8,27 seems enough consistent with a film model composed of aqueous hydrate microcrystalline particles. Upon modification with Ru(III), the tenacity of InHCF films is significantly enhanced not only because isolated inorganic particles are held together by hydrogen bonding but especially because they are strongly stabilized by new formed cyanide and oxo bridges, such as Fe-CN-Ru and Fe-O-Ru. Thus, the reason the incorporation of ruthenium prevent the film dissolution is probably due to the formation of a layered network of bridging oxo species intra- and interparticles. Accordingly, it is also expected that the extrusion of In3+ ions is strongly minimized and the film stability greatly enhanced. (25) Christensen, P. A.; Hamnett, A.; Higgins, S. J. J. Chem. Soc., Dalton Trans. 1990, 2233. (26) Hamnett, A.; Christensen, P. A.; Higgins, S. J. Analyst, 1994, 119, 735. (27) Cziro´k, E.; Ba´cskai, J.; Kulesza, P. J.; Inzelt, G.; Wolkiewicz, A.; Miecznikowski, K.; Malik, M. A. J. Electroanal. Chem. 1996, 405, 205.

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Figure 5. Cyclic voltammogram and simultaneously obtained mass change as monitored by a quartz crystal microbalance at the Ru-modified InHCF film electrode. Other experimental conditions as in Figure 1.

3.4. Electrochemistry of the Ruthenium-Modified InHCF Film Electrode. As mentioned earlier, the highly stabilized Ru-modified InHCF film exhibits a well-defined redox process in 0.5 M KCl at pH 2 with the anodic and cathodic peaks at +0.745 and +0.730 V vs Ag|AgCl, respectively. Such a redox process is reversible at low scan rates as the ratio between the charge passed during forward and reverse scan approaches unity. Moreover, the peak current of both anodic and cathodic peaks is directly proportional to the potential scan rate up to about 50 mV/s. The above eq 1 implies insertion of hydrated K+ ions during ferricyanide reduction and its release upon oxidation together with concomitant structural reorganization. This can be clearly seen in Figure 5, where the voltammetric plot of the Ru-modified InHCF film on Pt (curve A) with simultaneous mass change (curve B) in the usual electrolyte are shown. In this manner, it is possible to make a direct comparison between charge (Q) and frequency changes (∆f) using the following relation, ∆f ) 106(MW)CfQ/nF, where MW is the apparent molar mass involved in the redox process, Cf is the resonant frequency of the crystal,12 and all other symbols have their usual meaning. A value of 57 ( 2 g/mol as the apparent molar mass of the exiting (entering) species upon oxidation (reduction) was obtained, suggesting that potassium ion is most likely transferred with at least one molecule of water. The observation of solvent transport in inorganic films is quite common. It should be considered as dependent on the changes in thermodynamic activities of water and other components within the film during the redox process, as pointed out by Bruckenstein and Hillman.28 3.5. XPS Analysis. The above interesting data on stability led us to investigate the Ru-modified InHCF film electrodes by XPS. As demonstrated previously, some modifications in the iron signals of InHCF specimens occur under X-ray irradiation.8 In the present case spectra were acquired for period no longer than 10 min, and even though this did not ensure the absence of sample damage by radiation allowed us to compare high-resolution spectra of the C 1s + Ru 3d envelope. Figure 6 shows such a photoelectron emission region with the relevant fitting into six peaks: two Ru3d doublets and two peaks of the C1s signal. The Ru3d5/2 peaks with binding energies at (28) Bruckenstein, S.; Hillman, A. R. J. Phys. Chem. 1988, 92, 4837.

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De Benedetto and Cataldi

Figure 6. XP spectrum of the C1s + Ru 3d regions of a Rumodified InHCF film grown on platinum. The inorganic film was deposited and treated as described in Figure 1. The baseline includes both the Shirley background and satellites contribution. Spectrum not corrected for sample charging. Table 1. XPS Elemental Ratios ((SD) of InHCF Films upon Potential Cycling in 0.5 M KCl + HCl at pH 2 Containing 1 mM RuCl3a no. of potential scans

Fe/In

0b 20 45 75

0.76 ( 0.06 0.80 ( 0.06 0.88 ( 0.09 0.86 ( 0.09

N/Fe

Ru/Fe

5.1 ( 0.3 5.0 ( 0.2 0.16 ( 0.04 4.8 ( 0.3 0.27 ( 0.02 4.6 ( 0.2 0.40 ( 0.04

O/Fe

O/Ru

1.3 ( 0.4 1.8 ( 0.5 3.6 ( 1.6 2.1 ( 0.5 3.0 ( 0.8 2.4 ( 0.7 2.8 ( 0.7

a Molar ratios estimated with the Fe 2p , In 3d , N 1s, and K 3/2 5/2 2p3/2 levels. Errors represent standard deviations (SD) of five equally b prepared films. These data refers to unmodified InHCF film electrodes.

approximately 282.0 ( 0.2 and 283.4 ( 0.2 eV suggest the presence of two species of ruthenium with different oxidation states.24 The effect of number of scans on the film composition was investigated and the most relevant XPS data are collected in Table 1. No chloride ion was found in the spectra of either of the films. The elemental ratios were calculated from the integrated XPS highresolution peak, after cross-section correction. Several essential observations can be drawn from these data. First, the Fe/In elemental ratio in Ru-modified InHCF films is not significantly different from that one obtained for the parent InHCF film. Second, the loading of ruthenium species is proportional to the number of redox cycles in good agreement with the voltammetric and microgravimetric results mentioned above. Support for the view that cationic oxo-ruthenium species such as Ru2O63+ are involved in the stabilization process is provided by the fact that the intensity of O1s peak follows the same tendency. Supposing a constant amount of water within both InHCF and Ru-modified InHCF films, XPS analysis gave an oxygen-to-ruthenium ratio of 2.8 ( 0.7 in the case of InHCF films treated for 75 cycles in the Ru(III) solution (see Table 1). Such a finding is enough consistent with the entrance of Ru2O63+ species, thus providing concrete evidence for the formation of oxo-bridged dinuclear species, i.e., Fe-O-Ru, put forth earlier. Third, the N/Fe ratio (i.e., CN/Fe ratio) appears slightly lower in the rutheniummodified samples, suggesting that some terminal -CN ligands are probably lost during the conditioning process. As a result, the following interesting features of Rumodified InHCF films may be highlighted: the constancy of the Fe/In ratio, which suggests that the lattice structure of the indium hexacyanoferrate is not extensively affected by ruthenium entrance. The concomitant ruthenium and oxygen increases suggests also that such a metal ion goes into the film as oxo and/or oxy species. Moreover, the

Figure 7. Cyclic voltammetric behavior of the Ru-modified InHCF film electrode in the following supporting electrolytes (0.5 M) at pH 2: (A) KCl; (B) NaCl; (C) LiCl; (D) NH4Cl; (E) CsNO3; (F) RbNO3. Scan rate: 20 mV/s.

apparent lowering of the N/Fe elemental ratio confirms the possibility of terminal cyanide substituted by oxobridged, as already mentioned. That N-coordinated indium ions may undergo a substitution reaction is possible as well. In such a case, there will be formation of bridged dinuclear Fe-CN-Ru species. Examination of some InHCF samples simply modified by the dipping technique in a solution of RuCl3 reveals a similar C 1s + Ru 3d spectra as illustrated in Figure 6. In addition, in this case a good curve-fitting was obtained invoking the presence of two oxidation states of ruthenium incorporated in the InHCF film. Rather, the Ru/Fe ratio was 0.10 ( 0.03, a value slightly lower than that reported in Table 1 for electrochemically modified InHCF films. Hence, because of the ion balancing process the electrochemical cycling is more favorable to ruthenium entrance with a consequent more extensive substitution. 3.6. Electrolyte Effects. The effect of some metal ions on the voltammetric behavior and electrode stability for the Ru-modified InHCF film was also studied. As part of our investigations, we were interested in ascertaining whether the Ru-modified InHCF deposit would retain its zeolitic-like character because it is often found that small variation in the film composition gives rise to altered lattice parameters. Dong and Jin,6 and Kulesza and Faszynska14 reported an anomalous behavior about the ion selectivity of InHCF film electrodes with charge compensation during redox transitions not simple related to ion size. For instance, although the hydrated ionic radii of NH4+ and K+ are ca. the same (1.25 Å29,30) the electrochemical behavior is dramatically different. Figure 7 shows the voltammetric curves obtained with the same Ru-modified InHCF film electrode in different supporting electrolytes, containing K+ (A), Na+ (B), Li+ (C), NH4+ (D), Cs+ (E), and Rb+ (F) at a solution concentration of 0.5 M. Table 2 summarizes data on the anodic and cathodic peak potentials along with integrated current area observed with the modified electrode in each electrolyte; the ionic and hydration radii of the alkali cations are listed as (29) Itaya, K.; Uchida, I.; Neff, V. D. Acc. Chem. Res. 1986, 19, 162. (30) Itaya, K.; Ataka, T.; Toshima, S. J. Am. Chem. Soc. 1982, 104, 3751.

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Table 2. Cyclic Voltammetric Data for the Ru-Modified InHCF Film in Some Supporting Electrolytesa

a

electrolyte (0.5 M as a chloride salt)

ionic radius (Å)b

hydration radius (Å)

anodic charge (µC)

Ep(A) (V)

cathodic charge (µC)

EpC) (V)

K+ Na+ Li+ NH4+ Cs+ c Rb+ c

1.38 1.02 0.76 0.99 1.67 1.52

1.25 1.83 2.37 1.25 1.19 1.28

34 31 33 1 13 2.5

732 855 610 730 590 740

36 34 34 18 15 1.3

726 770 550 610 480 713

See experimental conditions stated in Figure 7. b From ref 35. c Employed as nitrate.

well.31-35 In the solution of K+, Na+, Li+, and to some extent also in that containing Cs+, curves are well-defined, suggesting that each of these cations can penetrate the modified InHCF film freely. The voltammetric profiles are rather very poor in solution containing NH4+ or Rb+, especially with the last ion, probably because the structure of the film does not allow access to relatively large cations. Considering however the relevant voltammetric differences between Cs+ and Rb+, it appears that selective ion transport in the Ru-modified InHCF lattice cannot be simply explained in terms of the hydrated ionic radii and channel size. It is feasible that electrostatic factors, ionic polarizabilities and structural disorder of the microcrystalline particles may be responsible for the observed ion migration through the InHCF-based films.29 It is worth of note that the pH of all solutions was 2, thus the proton exchanges with whichever cation may also play a role, especially in those cases of large hydrated cationic radii. We wish to emphasize that the same film was used for establishing the effect of all cations; upon deposition in K+ and subsequent cycling of the modified inorganic film in the supporting electrolyte containing a determined monovalent ion, the initial voltammetric profile was obtained back in the KCl solution. Although few potential cycles were accomplished for getting the initial voltammetric profile for all cations, the extrusion of Rb+ and, especially of Cs+ was more problematic. For instance, the release of Cs+ from the ruthenium-modified InHCF film was completed only upon cycling it for about 48 h in 0.5 M KCl at pH 2 using a scan rate of 50 mV/s. It there(31) Lundgren, C. A.; Murray, R. W. Inorg. Chem. 1988, 27, 933. (32) Crumbliss, A. L.; Lugg, P. S.; Morosoff, N. Inorg. Chem. 1984, 23, 4701. (33) Sihna, S.; Humphrey, B. D.; Bocarsly, A. B. Inorg. Chem. 1984, 23, 203. (34) Schneemeyer, L. F.; Splengler, S. E.; Murphy, D. W. Inorg. Chem. 1985, 24, 3044. (35) Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J. Sen, D. Chem. Rev. 1985, 85, 271.

fore appears that the Ru-modified InHCF film retains the same sort of structure as postulated for the starting material. This would suggest that the inclusion of oxoruthenium species play an important role in stabilizing the system without altering the zeolitic-like structure of the film. 4. Conclusions Although dissolution/decomposition represents the most important drawback of several Prussian Blue analogues, in the present paper, it has been demonstrated that electrode stability of electrochemically grown InHCF films can be improved substantially by potential cycling them in a solution containing RuCl3. The attainment of such a remarkable stability makes the resulting Ru-modified InHCF film amenable to be used both in electrochemical and solid-state applications. Some evidences of interfacial structures containing bridging cyanometalates inferred by XPS have been presented. Spectroscopic data also support the view that oxo-ruthenium species are involved in the stabilization of InHCF films through the formation of bridging oxo species. Apparently, the inclusion of oxoruthenium species does not introduce significant lattice distortion within the inorganic film. Moreover, as ion selectivity of Ru-modified InHCF films is not simply related with the hydrated ionic radius of monovalent ions, further work is needed for a better understanding of such an effect. Acknowledgment. We wish to express our gratitude to Dr. A. Bianchini for his assistance during some experimental work and to A. Galasso for taking XP spectra. Regione Basilicata provided partial funding through “LaMI”. This work was also supported by the National Research Council of Italy (CNR) and Ministero dell’Universita` e della Ricerca Scientifica e Tecnologica (MURST). LA980477C