Electrochemical Deposition and Characterization of Mixed-Valent

Sep 14, 2007 - Neil R. Murphy , Regina C. Gallagher , Lirong Sun , John G. Jones , John T. Grant. Optical Materials ... Benjamin P. Hahn , Keith J. St...
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Langmuir 2007, 23, 10837-10845

10837

Electrochemical Deposition and Characterization of Mixed-Valent Rhenium Oxide Films Prepared from a Perrhenate Solution Benjamin P. Hahn, R. Alan May, and Keith J. Stevenson* Department of Chemistry and Biochemistry, Center for Nano- and Molecular Science and Technology, Texas Materials Institute, UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed May 23, 2007. In Final Form: July 23, 2007 Cathodic electrodeposition of mixed-valent rhenium oxides at indium tin oxide, gold, rhenium, and glassy carbon electrodes from acidic perrhenate solutions (pH ) 1.5 ( 0.1) prepared from hydrogen peroxide and zerovalent rhenium metal is described. Cyclic voltammetry, variable angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS), UV-vis spectroelectrochemistry, and electrochemical quartz crystal microbalance (EQCM) data indicate that the chemical nature of the electrodeposited rhenium species depends mainly upon the potential and supporting electrolyte. The presence of SO42- as a supporting electrolyte inhibits the adsorption of perrhenate, ReO4-, at non-hydrogen adsorbing electrode materials. However, in acidic perrhenate solutions containing only protons and ReO4- anions, strong adsorption of ReO4- at potentials preceding hydrogen evolution occurs. This leads to the formation of an unstable ReIII2O3 intermediate which catalytically disproportionates to form mixed-valent rhenium films consisting of 72% ReIVO2 and 28% Re0. During the hydrogen evolution reaction (HER), hydrogen polarization causes the principle deposit to be more reduced, consisting of roughly 64% ReIVO2 and 36% Re0. Conclusively, metallic rhenium can be deposited at potentials preceding the HER at non-hydrogen adsorbing electrode materials, especially in the absence of SO42- anions.

Introduction Rhenium and its derivatives are interesting candidate materials for potential uses as catalysts,1 phosphors,2 structure directing templates,3 and antibiofouling coatings.4 Rhenium has also been explored for use as a barrier layer5 and to enhance the interfacial properties of microelectronic semiconductors.6 Several studies have explored the electrodeposition of rhenium species from acidic solutions containing perrhenate ions, ReVIIO4-, on various electrode materials.7 Although the morphology and microstructure of the rhenium deposits can be directly controlled by the preparation conditions (e.g., applied potential, solution composition, pH, etc.), a detailed mechanism has not been fully elucidated. Characterization of the composition of the electrodeposited species using X-ray diffraction and Raman spectroscopy has been challenging, because finely divided rhenium and low-valent rhenium oxides can rapidly oxidize, decompose, or disproportionate when exposed to ambient conditions and moderate temperatures. As summarized recently by Martins and co-workers,7 the simplest proposed electrodeposition mechanism involves the direct reduction of perrhenate ions, ReVIIO4-, to ReIVO2‚2H2O (i.e., ReVIIO4- + 4H+ + 3e- f ReIVO2‚2H2O). However at Pt and Rh electrodes, perrhenate ions strongly adsorb prior to reduction.7 As a result, strongly adsorbed ReVIIO4- is reduced * Author to whom all correspondence should be addressed. Phone: (512) 232-9160. Fax: (512) 471-8696. E-mail: [email protected]. (1) Blom, R. H.; Kollonitsch, V.; Kline, C. H. Ind. Eng. Chem. 1962, 54, 16-22. (2) Gong, X.; Ng, P. K.; Chan, W. K. AdV. Mater. 1998, 10, 1337-1340. (3) Selby, H. D.; Roland, B. K.; Zheng, Z. Acc. Chem. Res. 2003, 36, 933944. (4) Ha¨feli, U. O.; Warburton, M. C.; Landau, U. Biomaterials 1998, 19, 925933. (5) Petrovich, V.; Haurylau, M.; Volchek, S. Sens. Actuators A, Phys. 2002, 99, 45-48. (6) Giaddui, T.; Earwaker, L. G.; Forcey, K. S.; Aylett, B. J.; Harding, I. S. Nucl. Instrum. Methods Phys. Res. B 1996, 113, 201-204. (7) Me´ndez, E.; Cerda´, M. F.; Castro Luna, A. M.; Zinola, C. F.; Kremer, C.; Martins, M. E. J. Colloid Interface Sci. 2003, 263, 119-132.

to an unstable rhenium(III) oxide intermediate that catalytically disproportionates to form a mixed-valent film of ReIVO2 and Re0. This type of mechanism has been previously proposed for Sn,8 and a thermodynamically favorable route could exist for rhenium as well.7,9 One complicating factor is that the hydrogen evolution reaction (HER) occurs as a competing side reaction during the reduction of perrhenate.10,11 Strong hydrogen adsorbing electrode materials such as Pt and Rh have been reported to directly catalyze the reduction of perrhenate to metallic Re0 and ReIVO2 at reduction potentials more positive than the HER.7 Martins et al. proposed that with electrodes of this nature strongly adsorbed hydrogen, Had, acts as a reducing agent to facilitate the adsorption and reduction of ReVIIO4- prior to catalyzing the disproportionation of unstable ReIII oxide intermediates.7 In contrast, for non-hydrogen adsorbing electrodes such as Au, a different mechanism has been advocated where ReVIIO4- is weakly adsorbed and reduction proceeds mainly during the cogeneration of molecular hydrogen, H2, at potentials coinciding with the HER where H2 acts as a reducing agent to form predominately metallic Re0.7,10 In the majority of these studies, supporting electrolytes have been added to the deposition bath (i.e., SO42-, ClO4-) to increase solution conductivity and current density and reduce concentration polarization effects in addition to controlling the solution pH. However, the role of the supporting electrolyte has not been fully evaluated since supporting electrolytes containing SO42or ClO4- ions are typically considered to be weakly adsorbed and generally non-interfering. A few reports have discussed the influence of electrolyte anions on the electrodeposition of (8) Szabo´, S. J. Electroanal. Chem. 1984, 172, 359-366. (9) Szabo´, S. Int. ReV. Phys. Chem. 1991, 10, 207-248. (10) Schrebler, R.; Cury, P.; Orellana, M.; Go´mez, H.; Co´rdova, R.; Dalchiele, E. A. Electrochim. Acta 2001, 46, 4309-4318. (11) Schrebler, R.; Cury, P.; Sua´rez, C.; Mun˜oz, E.; Vera, F.; Co´rdova, R.; Go´mez, H.; Ramos-Barrado, J. R.; Leinen, D.; Dalchiele, E. A. Thin Solid Films 2005, 483, 50-59.

10.1021/la701504z CCC: $37.00 © 2007 American Chemical Society Published on Web 09/14/2007

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rhenium.7,12-17 HClO4 has been reported to diminish the electrodeposition efficiency because Re ad-layers can catalyze the reduction of ClO4- to Cl-.7,12-16 At Au electrodes, Martins et al. speculated that the effect of SO42- and ClO4- ions on the reduction of ReVIIO4- was dominated by a competitive surface reaction between ReIVO2 and Re0 and not by competitive anion adsorption since electrochemical quartz crystal microbalance experiments demonstrated that ReVIIO4- was weakly adsorbed at a low surface coverage (∼8%) at potentials preceding the HER.7,17 Therefore, it was concluded that the reduction of ReVIIO4- at non-hydrogen adsorbing electrodes involves weak adsorption of ReVIIO4- via an entropy driven process requiring the displacement of six water molecules from the surface.17 Adsorbed ReVIIO4- is then reduced to ReIVO2 and finally to metallic Re0 only at potentials coinciding with HER, when molecular hydrogen is produced.7 Herein we provide strong evidence that the presence of SO42in the deposition bath significantly inhibits the reduction of perrhenate at non-hydrogen adsorbing electrodes at potentials positive to the HER. In the absence of SO42-, ReVIIO4- is strongly adsorbed and is reduced to ReIVO2 and metallic Re0 at potentials preceding the HER, consistent with the electrodeposition mechanism proposed for strongly adsorbing hydrogen electrode materials such as Pt and Rh. Electrochemical methods along with spectroscopic techniques have allowed us to gather useful information to understand the subtleties of the cathodic electrodeposition of perrhenate at various electrode materials. Experimental Section Analogous to procedures for preparing peroxo-complexes with molybdenum and tungsten,18,19 perrhenate solutions (pH ) 1.5 ( 0.1) were synthesized using rhenium metal (1.86 g, Strem). An excess of 30% (v/v) aqueous hydrogen peroxide (Fisher) was slowly added to the powder in a 9 H2O2:1 Re0 mole ratio, resulting in a vigorous reaction. Excess hydrogen peroxide was catalytically decomposed using a platinum bar coated with platinum black according to a standard procedure.20 The perrhenate solution was diluted with nanopure H2O (18 MΩ cm) to 100 mL in an amber volumetric flask resulting in a perrhenate concentration of ∼0.09 M. Prior to experimentation, the perrhenate was allowed to equilibrate for several days, and residual solid byproducts were removed from the perrhenate solution with a 0.22 µm filter (Millex GP). Perrhenate solutions used in this study registered a pH of 1.5 ( 0.1. Electrodeposited films used for variable angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM) studies were prepared directly from the standard perrhenate solution, with no additives, by performing chronocoulometry on a CH 440 potentiostat/galvanostat (CH Instruments). Deposition was performed in a homemade electrochemical cell that fixed the indium tin oxide (Delta Technologies Ltd., 15 Ω/square) working electrode area at 0.45 cm2. All indium tin oxide (ITO) substrates were cleaned prior to deposition as previously described.18 A Ag/AgCl (KCl sat’d, World Precision Instruments) electrode and a platinum wire were employed as the reference and counter electrodes, respectively, for all electrochemical depositions. Thin film growth was terminated after 0.04 C (for depositions at -0.30 V vs Ag/AgCl), and 1.0 C (for depositions at (12) Bakos, I.; Hora´nyi, G.; Szabo´, S.; Rizmayer, E. M. J. Electroanal. Chem. 1993, 359, 241-252. (13) Hora´nyi, G.; Bakos, I. J. Electroanal. Chem. 1994, 378, 143-148. (14) Hora´nyi, G.; Bakos, I.; Szabo´, S.; Rizmayer, E. M. J. Electroanal. Chem. 1992, 337, 365-369. (15) Bakos, I.; Hora´nyi, G. J. Electroanal. Chem. 1994, 375, 387-390. (16) Hora´nyi, G.; Bakos, I. J. Electroanal. Chem. 1994, 370, 213-218. (17) Jusys, Z.; Bruckenstein, S. Electrochem. Commun. 2000, 2, 412-416. (18) McEvoy, T. M.; Stevenson, K. J. Anal. Chim. Acta 2003, 496, 39-51. (19) Kondrachova, L.; Hahn, B. P.; Vijayaraghavan, G.; Williams, R. D.; Stevenson, K. J. Langmuir 2006, 22, 10490-10498. (20) Hills, G. J.; Ives, D. J. G. J. Chem. Soc. 1951, 305-310.

Hahn et al. -0.70 V vs Ag/AgCl) of charge was measured by coulometry. Cyclic voltammograms were carried out in the same homemade electrochemical cell (0.45 cm2 fixed area) in which pre-cleaned, ITO, rhenium foil (Alfa Aesar) and glassy carbon (Alfa Aesar) were employed as working electrodes. Cyclic voltammograms on a gold disk electrode of known area were also conducted. Na2SO4 (Mallinckrodt) electrolyte was only used where explicitly indicated, to demonstrate the effects of SO42- on perrhenate adsorption. Unless stated otherwise, all potentials in this report are referenced to the Ag/AgCl (KCl sat’d) reference electrode, and all experiments were conducted under ambient conditions at room temperature (25 ( 2 °C). Spectroelectrochemical experiments were performed on a CH 700 bipotentiostat (CH Instruments) interfaced to an Agilent Instruments 8453 UV-visible spectrometer with a photodiode array detector. A homemade electrochemical cell with a fixed working electrode area of 0.45 cm2 was placed in the spectrometer sample holder with a platinum wire counter electrode, Ag/AgCl (KCl sat’d) reference electrode (CH Instruments) and a transparent ITO working electrode. For this experiment, a cyclic voltammogram (CV) was taken between +0.20 and -0.70 V at a scan rate of 10 mV s-1. Optical spectra were collected concurrently with electrochemical measurements every 4.5 s, over an integration time of 0.4 s. EQCM measurements were collected on a CH 440 potentiostat/ galvanostat (CH Instruments) using a 9.995 MHz AT-cut polished quartz crystal with a Ti adhesion underlayer (∼10 nm) and a Au electrode overlayer (∼100 nm) made to specification by a commercial manufacturer, International Crystal Manufacturers (ICM), Inc. Through a private communication with ICM, the Au was deposited via a diffusion pumped vacuum chamber with a resistive filament source, and the thickness was monitored using a crystal controlled vacuum monitor. A Au-coated crystal (0.201 cm2 area) served as the working electrode, and it was used in conjunction with a platinum counter and a Ag/AgCl (KCl sat’d) reference (World Precision Instruments). Prior to experimentation, the Au-Ti working electrode was cleaned with 0.1 M HNO3, followed by copious rinsing in water (18 MΩ cm). Calibration of the EQCM crystal was adapted from an established procedure,21 where cyclic voltammograms of a 10-3 M AgNO3, 0.2 M H2SO4 solution were collected. Using the Sauerbrey equation22 (∆f ) Cf∆m) where ∆f is the change in frequency of the EQCM crystal and ∆m is the change in mass, the proportionality constant, Cf, was determined based on five Ag+ deposition/stripping cycles taken at 10 mV s-1. For the reported EQCM experiment, the proportionality constant was 0.778 ( 0.006 Hz/ng. Following calibration, perrhenate solution (ca. 0.09 M) was placed in the EQCM cell and analyzed by cyclic voltammetry at a scan rate of 10 mV s-1. All EQCM data was acquired in a home-built Faraday cage. Ellipsometry measurements were taken on a J.A. Woollam M-2000 spectroscopic ellipsometer over a wavelength range of 300-1000 nm at 62.5°, 65.0°, and 67.5°. Transmission data, taken on the same instrument, was combined with ellipsometry data and fitted simultaneously via minimization of the mean square error using the Levenberg-Marquardt algorithm. All of the derived film thicknesses were verified with atomic force microscopy using a Veeco Instruments Bioscope Nanoscope IV. The optical constants of Re0 and ReIVO2 were determined by direct inversion of data obtained from optically thick samples. Films deposited at -0.30 V were fitted with a four layer model consisting of a Corning 1737 glass substrate layer (optical constants provided by J. A. Woollam Co.), a graded ITO layer, a film layer, and a 5 nm surface roughness layer. The graded ITO layer was similar to that described by Synowicki,23 but the Lorentzian oscillator describing the band gap was replaced with a Tauc-Lorentz24 oscillator to better model the transmission data. The film layer was modeled using three Lorentzian oscillators to account for the broad absorbance observed. Finally the surface roughness layer consisted of a Bruggeman effective medium (21) Bruckenstein, S.; Swathirajan, S. Electrochim. Acta 1985, 30, 851-855. (22) Sauerbrey, G. Z. Phys. 1959, 155, 206-222. (23) Synowicki, R. A. Thin Solid Films 1998, 313-314, 394-397. (24) Jellison Jr., J. A.; Modine, F. A. Appl. Phys. Lett. 1996, 69, 371-373.

Deposition of Mixed-Valent Rhenium Oxide approximation (BEMA) with a 50% mix between the lower layer and void space. Films deposited at -0.70 V were fit with a similar model except that the surface roughness layer was replaced by a ∼35 nm layer described by two Lorentzian oscillators. Fit parameters for the ReIVO2 standard, the Re0 standard, and rhenium thin films deposited from a perrhenate solution at -0.30 and -0.70 V are provided in the Supporting Information. In order to quantify the composition of films deposited at -0.30 and -0.70 V, the derived film optical constants were fitted using a BEMA model consisting of distorted, spherical Re0 (depolarization factor ) 0.55) suspended in a matrix of ReIVO2. For films deposited at -0.70 V, a void space component was also added. The following assumptions were made: (1) the surface ( 0.99), whereas the imaginary refractive index k is well fit by an exponential dependence (R2 > 0.99). The derived refractive index of rhenium thin films deposited at -0.30 V is included at 28% Re0 and is in excellent agreement with the observed dependencies. The refractive index for films deposited at -0.70 V is shown at 42%

Re0 for the lower portion of the film and 15% Re0 for the upper portion as derived from the three component BEMA model. Note these values differ from the theoretical curves, illustrating that the two-component BEMA model is insufficient to describe them. Interestingly, the real portion of the refractive index is much lower than that predicted by the two-component BEMA model. Since the HER greatly increases the surface roughness of films deposited at -0.70 V, the apparent n is lowered necessitating the inclusion of void space into the model. Compared to the ITO substrate, films deposited at -0.30 V on Au (Figure 4a) have very similar optical constants. The real portion of the refractive index varies by an average of less than 0.01 across the entire spectral range tested. The imaginary portion is more variant, appearing to be shifted up by ∼0.05 units of k on Au compared to ITO. This shift could be due to a slightly enhanced presence of Re0; however, the imaginary refractive index of gold is very large in this spectral range (∼1.9-6.8),39 and it is likely that some of the absorbance of gold is not being fully deconvoluted from the rhenium film optical constants. Note that the optical constants of rhenium films deposited at -0.70 V on Au could not be determined because those films require a more complex two-layer model than films deposited at -0.30 V. On ITO, this model could be resolved through the addition of transmission measurements and a fixed total thickness determined by AFM. Obviously Au is not transparent, and reflectance measurements were not an acceptable substitute for the transmission measurements due to the large amount of scattering from the very rough surface of the film. Moreover, AFM thickness measurements were complicated by the Au substrate. Qualitatively the model of films deposited at -0.70 V on ITO matched up well with the experimentally derived Ψ and ∆ values for films deposited at -0.70 V on Au. Results from our VASE experiments are consistent with previous XPS depth profiling and Ar+ sputtering studies that indicate both metallic Re0 and ReIVO2 are present in films deposited at similar potentials.7,11 As discussed previously, the presence of metallic Re0 and ReIVO2 products for films deposited (39) Lynch, D. W.; Hunter, W. R. In Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press: Orlando, FL, 1985; Vol. 1, pp 293-294.

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Figure 5. Cyclic voltammograms for (a) ITO, (b) Au, (c) Re, and (d) C electrodes immersed in 0.09 M ReVIIO4- and cycled between +0.20 and -0.70 V. Scan rate is 10 mV/s. The first CV (red line) and four subsequent CVs (black dash) are shown. Note the y axes for the ITO, Re and C electrodes (-5 to +25 mA/cm2) are scaled differently from the Au electrode (-5 to +50 mA/cm2).

at -0.30 V strongly supports a reduction mechanism involving disproportionation of an unstable ReIII species.7 Since hydrogen evolution occurs simultaneously with deposition at -0.70 V, it is likely that molecular hydrogen is participating as a reducing agent and promoting the reduction of rhenium to the zerovalent state, consistent with previously proposed mechanisms.7 Although the XPS and VASE data provide reasonable estimates of the resultant film composition, additional information is required to establish the electrodeposition mechanism for films deposited at the two different potentials. Historically, the electrodeposition of perrhenate has been carried out in the presence of supporting electrolyte containing sulfate or perchlorate anions. Although it has been previously suggested that sulfate and perchlorate do not strongly adsorb at electrode surfaces in the presence of perrhenate ions, their influence should not be ignored. As noted earlier, since our perrhenate solution is prepared via dissolution of Re0 metal in hydrogen peroxide, it contains only protons and ReVIIO4- ions. In comparison to previous reports,10,11 cyclic voltammetry of non-hydrogen adsorbing electrode materials with perrhenate solutions prepared from metallic Re0 and hydrogen peroxide demonstrated consistent evidence for strong perrhenate adsorption at potentials preceding the HER. Figure 5 shows cyclic voltammograms for ITO, Au, Re, and C electrodes immersed in 0.09 M ReVIIO4-. All CVs show similar qualitative responses with the potential onset of electrochemical processes differing slightly. On ITO during the first CV, a small increase in the cathodic current appears near -0.20 V, a much larger increase in current corresponding to the onset of the HER transpires at -0.46 V, and a small anodic peak occurs close to +0.10 V. As the electrode is cycled in perrhenate multiple times, the cathodic peak near -0.15 V grows larger. Similarly the anodic peak near +0.10 V grows with each successive cycle, suggesting that these two peaks are associated with the reversible adsorption and reduction of perrhenate. This voltammetric behavior is characteristic of that seen at strongly H-adsorbing electrodes such as Pt even though Au, ITO, Re, and C electrodes are considered non-hydrogen adsorbing materials. This result also suggests that rhenium deposition can occur at potentials preceding the HER and that strongly adsorbed hydrogen is not required as a co-reactant to form metallic Re0.

Hahn et al.

Figure 6. Cyclic voltammograms for ITO electrodes immersed in (a) 0.09 M ReVIIO4- and (b) 0.09 M ReVIIO4-, 0.09 M Na2SO4. Cyclic voltammograms are also shown for Au electrodes immersed in (c) 0.09 M ReVIIO4- and (d) 0.09 M ReVIIO4-, 0.09 M Na2SO4. Scan rate was 10 mV/s for all. Note the y axes for CVs in the absence of SO42- anions are scaled differently from CVs in the presence of SO42- anions. The first CV (red line) and four subsequent CVs (black dash) are shown.

Figure 6 shows the voltammetric response of ITO and Au electrodes immersed in a solution containing 0.09 M ReVIIO4with 0.09 M Na2SO4. The presence of sulfate anions significantly interferes with the adsorption of perrhenate at potentials preceding the HER. For example, Figure 6, panels b and d, exhibits ∼10100 times smaller current densities when SO42- is added as electrolyte compared to that seen when only perrhenate anions are present under the same conditions (Figure 6, panels a and c). On ITO, the presence of SO42- is most dramatic as the current density is lowered by 2 orders of magnitude (Figure 6b). The cathodic and anodic waves in Figure 6, panels b and d, show larger currents at potentials close to the reduction and oxidation of rhenium oxide, suggesting that, even though SO42- interferes with the adsorption of ReVIIO4-, there remain a small fraction of available sites on the electrode surface for ReVIIO4- to be adsorbed and reduced. This is consistent with the submonolayer oxide coverage observed by Schrebler and co-workers during the electrochemical deposition of rhenium in the presence of SO42- anions.11 A rise in the cathodic current between ca. -0.30 and -0.40 V in some of the voltammograms in Figure 6 is likely due to the applied potential approaching the onset of the HER. When SO42- ions are present in the perrhenate solution (Figure 6, panels b and d), electrolysis is shifted slightly positive compared to perrhenate solutions without SO42- (Figure 6, panels a and c). This observation is in agreement with previous reports of ReVIIO4- adsorption on Au10,17 that suggest ReVIIO4- adsorption causes the HER to occur at more negative potentials. To summarize, these experiments suggest that perrhenate must adsorb prior to deposition and that sulfate impedes the reduction of perrhenate at both ITO and Au. We have also observed similar diminishment of the current density response for the reduction of perrhenate at C and Re electrodes (data not shown). All of these electrode materials do not strongly adsorb hydrogen and are not known to strongly adsorb sulfate anions within this potential window. Experiments detailing the scan rate dependence of perrhenate reduction on ITO (Figure 7) suggest that at least one chemical step must exist within the reduction process before the HER. As the potential is cycled at faster scan rates, the cathodic and anodic waves broaden, indicative of competition between chemical step-

Deposition of Mixed-Valent Rhenium Oxide

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Figure 8. Spectroelectrochemical response of an ITO electrode immersed in 0.09 M ReVIIO4-. CV (line) is shown together with differential absorbance (dash) at 480 nm in (a). Change in absorbance during potential cycling is shown in (b). Scan rate was 10 mV/s.

Figure 7. Cyclic voltammograms were acquired at (a) 1, (b) 10, and (c) 100 mV/s between +0.20 and -0.40 V for ITO electrodes immersed in 0.09 M ReVIIO4-. The first CV (red line) and four subsequent CVs (black dash) are shown.

(s) and electrochemical step(s). The nucleation and growth behavior of rhenium deposits has been previously detailed and is very complex.7,10 However, based on a critical review of various studies for rhenium deposition, the data presented herein for the reduction of perrhenate at potentials positive of the HER at nonhydrogen adsorbing electrodes are consistent with the catalytic disproportionation of an unstable ReIII oxide intermediate involving both electrochemical and chemical steps (eq 2-5):

preadsorption step (2)

electrochemical step +

4Re O4

(ads)

chemical step ReVIIO4-(ads) + 2H2 f ReIVO2‚2H2O

(6)

electrochemical step

ReVIIO4- f ReVIIO4-(ads)

VII

known redox couples suggest that our proposed mechanism is feasible and is supported by further analysis of this topic by Martins and co-workers.7 The chemical step, eq 4, was taken directly from thermodynamic experiments on ReIII2O3 by Busey and co-workers.40 Due to our inability to identify ReIII2O3 solid with certainty, we cannot rule out the possibility that ReIII2O3 exists as a soluble intermediate. However, if Re3+ were being generated in solution, we should have observed additional absorbance in the visible region that could be attributed to the formation of the red, soluble rhenous ion, Re3+.41 Information on this topic is scant, but the literature7,11,30,40 suggests disproportionation is the likely pathway. Note that the ratio of ReIV to Re0 in the proposed net-half reaction (3:1) is close to the VASE determined percentages of ReIVO2 to Re0 for films deposited at -0.30 V (72%:28%). At -0.70 V deposition of rhenium during the HER forms more Re0. Therefore, at potentials coinciding with the HER it is likely hydrogen gas participates to form ReIVO2 and Re0 in the two-step deposition mechanism put forth by Martins and co-workers (eqs 6 and 7).7

-

+ 20H + 16e f

2ReIII2O3

+ 10H2O

(3)

chemical step 2ReIII2O3 + 6H2O f Re0 + 3(ReIVO2‚2H2O)

(4)

net half-reaction 4ReVIIO4-(ads) + 20H+ + 16e- f Re0 + 3(ReIVO2‚2H2O) + 4H2O (5) ReIII2O3 is known to be unstable in the presence of water and air. Thermodynamic calculations of ReIII2O3 disproportionation to form Re0 and ReIVO2‚2H2O based on Nernst potentials of

ReIVO2‚2H2O + H2 + 2H+ + 2e- f Re0 + 4H2O (7) To follow the catalytic disproportionation of ReIII2O3, spectroelectrochemical and EQCM experiments were conducted on ITO and Au electrodes, respectively. Figure 8 shows a cyclic voltammogram of an ITO electrode immersed in 0.09 M ReVIIO4along with the absorbance measured at 480 nm. At the beginning of the cathodic sweep from +0.20 V, there is no significant spectroscopic or electrochemical change until ca. -0.22 V, where the current increases substantially. Interestingly, the absorbance at 480 nm does not change until approximately -0.29 V, 70 mV past the onset of the current, implying that ReVIIO4- adsorbs to the ITO surface prior to its reduction. The absorbance at 480 nm (40) Busey, R. H.; Sprague, E. D.; Bevan Jr., R. B. J. Phys. Chem. 1969, 73, 1039-1042. (41) de Zoubov, N.; Pourbaix, M. In Atlas of Electrochemical Equilibria in Aqueous Solutions; Pourbaix, M., Ed.; Pergamon Press Ltd.: Oxford, 1966; pp 300-306.

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is most likely associated with the 4Ag f 4B1g electronic transition42 and signifies the formation of ReIVO2. Consistent with the mechanistic steps outlined above, the increase of absorbance at 480 nm signals the onset of perrhenate reduction to the intermediate sesquioxide, ReIII2O3, which then quickly disproportionates into a mixed-valent film containing ReIVO2 and Re0. At more negative potentials coinciding with the HER (< -0.50 V), a large increase in the baseline absorbance is seen and is attributed to the deposition of Re0. The differential absorbance plot at 480 nm also shows there is a time delay between when the HER starts and when the baseline absorbance increases significantly (Figure 8), suggesting that a major quantity of molecular H2 must be generated before the deposition mechanism switches from one controlled via a catalytic disproportionation (eq 2-5) reaction to that consistent with a reaction proceeding predominantly via hydrogen polarization (eq 6 and 7). The anodic scan at ITO is complicated by sluggish electron transport that occurs at the electrode-solution interface. In Figure 8, the absorbance at 480 nm approaches a local minimum (ca. -0.39 V) in the reverse anodic scan as a result of HER termination. We speculate that the gradual oxidative current that appears ca. -0.29 V is representative of a kinetically slow process to form ReVIO3, as the presence of ReVI at this potential was confirmed by XPS. At +0.10 V, an oxidative peak appears simultaneously with a loss of absorbance at 480 nm suggesting the oxidation of ReIVO2‚ 2H2O to form ReVIIO4- (i.e., ReIVO2‚2H2O f ReVIIO4- + 4H+ + 3e-). The voltammetric response for the reduction of perrhenate at the rhenium electrode (Figure 5) suggests that it is difficult to oxidize Re0 back to perrhenate within our potential region of interest (+0.20 V to -0.70 V), indicated by the lack of a large anodic current. EQCM experiments were conducted at Au electrodes to confirm our proposed deposition mechanism. Using the EQCM, a molecular weight equivalent (MW/n) for our deposition product can be determined by rearranging Faraday’s law to MW/n ) (∆m/∆Q)F. Molecular weight equivalents can be determined from the slope of a mass (∆m) versus charge (∆Q) plot where F is the Faraday constant. Figure 9 shows an EQCM experiment acquired at a Au electrode immersed in 0.09 M ReVIIO4- solution. As the cathodic sweep moves in a negative direction from +0.20 V, the first process detected is the pre-adsorption of ReO4- anions at the gold surface, noticeable by the small increase in nonfaradaic current in the CV without simultaneous reduction of the frequency at the QCM crystal. At approximately -0.25 V (beginning of region I, Figure 9), the electrodeposition of a rhenium species can be easily distinguished by a large increase in the slope of the mass-charge plot, and as the potential becomes more cathodic, the electrochemical deposition of rhenium is masked by the HER starting at ca. -0.46 V (region II). Using the mass-charge plot in Figure 9, we estimate an experimental MW/n of 63.3 g mol-1 eq-1 for region I (-0.25 to -0.46 V) with R2 > 0.99. This value is very close to the theoretical MW/n value (63.8 g mol-1 eq-1) predicted for the net half-reaction, eq 5, where 4ReVIIO4-(ads) + 20H+ + 16e- f Re0 + 3(ReIVO2‚2H2O) + 4H2O. Note that our theoretical value of 63.8 g mol-1 eq-1 assumes that all of the water species in the product of eq 5 are associated with the rhenium deposition product. If we speculate that two H2O units are associated with each ReIVO2 unit, but the remaining H2O units are unassociated, then the theoretical MW/n value for eq 5 would be 59.3 g mol-1 eq-1. Although we do not have enough information to determine the extent of hydration exactly, all of our data and results presented in this report indicate that eq 5 (42) Edreva-Kardzhieva, R.; Andreev, A. A. Russ. J. Inorg. Chem. 1977, 22, 1089-1091.

Hahn et al.

Figure 9. EQCM at a Au electrode immersed in 0.09 M ReVIIO4and cycled from +0.20 to -0.70 V at a scan rate of 10 mV/s. CV (top) and frequency response (middle) were collected simultaneously. Plot of charge versus mass change at bottom. MW/n for I and II is 63.3 and 1.84 g mol-1 eq-1, respectively.

is the most plausible explanation for rhenium electrodeposition from -0.25 to -0.46 V. The determination the deposition mechanism for region II (Figure 9) is more difficult, because hydrogen evolution occurs simultaneously with rhenium deposition at this potential. As a result of the HER, the charge measured by the potentiostat (QT) is the summation of charge passing across the working electrode due to hydrogen electrolysis (QH) plus the charge associated with the reduction of perrhenate (QRe) where QT ) QH + QRe.10 Therefore, our experimentally determined molecular weight equivalent for region II (MW/n ) 1.84 g mol-1 eq-1) is of little use in determining the half-reaction for perrhenate reduction without knowing the charge passed due to hydrogen evolution. We can hypothesize the reaction efficiency (QRe/QT) of eq 7 by estimating QRe using eq 8 where ∆m equals the mass measured by the EQCM, nRe is the number of electrons crossing the interface, and wRe represents the combined molecular weights of the products in eq 7.10

QRe ) ∆mFnRewRe-1

(8)

This analysis yields QRe/QT ) 0.014 for region II (Figure 9) which is consistent with efficiency values calculated during the HER at similar reduction potentials reported by Schrebler and co-workers that indicate QRe/QT in this potential window is close to zero.10

Deposition of Mixed-Valent Rhenium Oxide

Conclusion At potentials preceding the HER, perrhenate reduction proceeds through a catalytic disproportionation mechanism in which ReVIIO4- pre-adsorbs to the working electrode and gets reduced to ReIII2O3 prior to disproportionation to form a mixed-valent film of ReIVO2 and Re0. We also found that the addition of sulfate anions significantly blocks the adsorption of ReVIIO4- at non-hydrogen adsorbing electrode materials which effectively terminates the catalytic disproportionation mechanism. At deposition potentials coinciding with the HER, a hydrogen polarization mechanism is predominant where molecular hydrogen acts as reducing agent to form Re0. Surface analytical methods demonstrate freshly prepared films are covered with a native oxide layer (ReIVO2) that slowly oxidizes to form ReVIO3. Spectroscopic ellipsometry results indicate that the film composition deposited at -0.30 V, prior to the HER, contained 72% ReIVO2 and 28% Re0; however, films deposited at potentials after the onset of hydrogen evolution (-0.70 V) possessed 64% ReIVO2 and 36% Re0. Conclusively, electrode materials that

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strongly adsorb hydrogen at potentials positive of the HER are not required as a co-reactant to form metallic Re0. The same mechanistic behavior is also observed for non-hydrogen adsorbing electrode materials, especially when the acidic perrhenate deposition solution does not contain SO42- anions. Acknowledgment. The authors thank Fei Yang for preparing a rhenium solution used in the study, in addition to Ryan D. Williams and Lilia Kondrachova for performing SEM and AFM measurements, respectively. This work was generously supported by the Robert A. Welch Foundation (Grant F-1529), the National Science Foundation (CHE-0134884), and the Strategic Partnership for Research in Nanotechnology (SPRING). R.A.M. further acknowledges the NSF for an Integrative Graduate Education and Research Traineeship (DGE-054917). Supporting Information Available: Fit parameters for VASE analysis. This material is available free of charge via the Internet at http://pubs.acs.org. LA701504Z