Electrodeposition of Manganese and Molybdenum Mixed Oxide Thin

May 21, 2005 - Received January 14, 2005. In Final Form: April 19, 2005. Manganese and molybdenum mixed oxides in a thin film form were deposited ...
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Langmuir 2005, 21, 5907-5913

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Electrodeposition of Manganese and Molybdenum Mixed Oxide Thin Films and Their Charge Storage Properties Masaharu Nakayama,* Akihiro Tanaka, Yoshimine Sato, Tsuyoshi Tonosaki, and Kotaro Ogura Department of Applied Chemistry, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan Received January 14, 2005. In Final Form: April 19, 2005 Manganese and molybdenum mixed oxides in a thin film form were deposited anodically on a platinum substrate by cycling the electrode potential between 0 and +1.0 V vs Ag/AgCl in aqueous manganese(II) solutions containing molybdate anion (MoO42-). A possible mechanism for the film formation is as follows. First, electrooxidation of Mn2+ ions with H2O yields Mn oxide and protons. Then, the protons being accumulated near the electrode surface react with MoO42- to form polyoxomolybdate through a dehydrated condensation reaction (by protonation and dehydration). The condensed product coprecipitates with the Mn oxide. Cyclic voltammetry of the Mn/Mo oxide film-coated electrode in aqueous 0.5 M Na2SO4 solution exhibited a pseudocapacitive behavior with higher capacitance and better rate capability than that of the pure Mn oxide prepared similarly, most likely as a result of an increase in electrical conductivity of the film. Electrochemical quartz crystal microbalance and X-ray photoelectron spectroscopy clearly demonstrated that the observed pseudocapacitive behavior results from reversible extraction/insertion of hydrated protons to balance the charge upon oxidation/reduction of Mn3+/Mn4+ in the film.

Introduction Electrochemical capacitors are currently receiving great attention for their use as energy-storage devises in highpower applications. From the charge-storage mechanisms, electrochemical capacitors can be classified into two different types: (1) double-layer capacitors, which are based on the double-layer capacitance arising from the nonfaradaic charge separation at an electrode/electrolyte interface, and (2) supercapacitors, which are based on the pseudocapacitance arising from fast and reversible faradaic redox reactions of electroactive materials having several oxidation states. Activated carbons are used in the former, while conducting polymers and transition metal oxides are promising materials for use in supercapacitors. Amorphous hydrated ruthenium oxide prepared by the sol-gel process exhibits ideal pseudocapacitive properties, i.e., a very large specific capacitance (720 F/g) as a result of high electronic and protonic conductivity and excellent reversibility within a large potential window of 1.4 V.1 However, ruthenium is an expensive and toxic material and RuO2 capacitors require the use of strongly acidic electrolyte such as 5 M H2SO4, making this material inadequate for commercial application. In this regard, cheaper alternatives with acceptable pseudocapacitive properties must be probed. Manganese oxide is an interesting electrode material in various energy-storage technologies. As with other transition metal oxides, manganese oxide can store electrical charge by simultaneous injection of electrons and charge-compensating cations into the solid. Although a wide variety of studies have been made on the MnO2 compounds in battery applications, only very limited papers deal with their application in electrochemical * Author to whom correspondence should be addressed. Tel: +81-836-85-9223. Fax: +81-836-85-9201. E-mail: nkymm@ yamaguchi-u.ac.jp. (1) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699.

supercapacitors.2-17 Advantages of manganese oxide are the lower cost for raw materials and the fact that manganese-based capacitors are environmentally benign in the point of view that they can operate in neutral solution. Lee et al. first recognized that amorphous hydrated manganese oxide (a-MnO2‚nH2O) mixed with carbon as a conducting agent behaves as a capacitor in 2 M KCl aqueous solution within a potential region of -0.2 to +1.0 V (vs SCE).2 Addition of carbon improves the electrical conductivity, as well as the porosity of the electrode, because MnO2 is an electrically low-conductive material.4 Jiang and Kucernak reported a direct electrochemical method to prepare carbon-containing Mn oxide films with good kinetic reversibility from an acetonitrile solution containing a Mn halide complex.11 Moreover, incorporation of other transition metals into MnO2 compounds has also been under extensive investigation in recent years to enhance their charge-storage capability.8,15 For example, amorphous Mn/Pb and Mn/Ni (2) Lee, H. Y.; Goodenough, J. B. J. Solid State Chem. 1999, 144, 220. (3) Pang, S. C.; Anderson, M. A.; Chapman, T. W. J. Electrochem. Soc. 2000, 147, 444. (4) Lee, H. Y.; Kim, S. W.; Lee, H. Y. Electrochem. Solid-State Lett. 2001, 4, A19. (5) Hu, C. C.; Tsou, T. W. Electrochem. Comm. 2002, 4, 105. (6) Hu, C. C.; Tsou, T. W. Electrochim. Acta 2002, 47, 3523. (7) Broughton, J. N.; Brett, M. J. Electrochem. Solid-State Lett. 2002, 5, A279. (8) Xu, J. J.; Yang, J.; Jain, G. Electrochem. Solid-State Lett. 2002, 5, A223. (9) Chin, S. F.; Pang, S. C.; Anderson, M. A. J. Electrochem. Soc. 2002, 149, A379. (10) Jeong, Y. U.; Manthiram, A. J. Electrochem. Soc. 2002, 149, A1419. (11) Jiang, J.; Kucernak, A. Electrochim. Acta 2002, 47, 2381. (12) Toupin, M.; Brousse, T.; Be´langer, D. Chem. Mater. 2002, 14, 3946. (13) Chang, J. K.; Tsai, W. T. J. Electrochem. Soc. 2003, 150, A1333. (14) Hu, C. C.; Wang, C. C. J. Electrochem. Soc. 2003, 150, A1079. (15) Kim, H.; Popov, B. N. J. Electrochem. Soc. 2003, 150, D56. (16) Toupin, M.; Brousse, T.; Be´langer, D. Chem. Mater. 2004, 16, 3184. (17) Brousse, T.; Toupin, M.; Be´langer, D. J. Electrochem. Soc. 2004, 151, A614.

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mixed oxides synthesized by reduction of KMnO4 with Pb and Ni salts provide improved specific capacitances compared to that of pure MnO2.15 Hence, the development of a simple method for constructing Mn-based mixed oxides in a thin-film form will provide a breakthrough to fabricate electrode materials for supercapacitors. For use in electronic, optical, and magnetic studies or applications, thin films are the desirable form of metal oxides. Among various techniques for preparing thin films of metal oxides, electrochemical deposition is one of the most promising approaches, especially for obtaining uniform films on substrates of complex shape with a high degree of reproducibility. More importantly, this method allows precise control of film thickness by simply changing the delivered electrical charge. Most of the effort in MnO2-based capacitors over the past five years has been aimed at obtaining compounds with high capacitance and good cyclability, whereas experimental insight into their charge-storage mechanism is largely missing. It is generally now accepted that cations from the electrolyte are involved in the pseudocapacitive process of MnO2 in neutral electrolyte (KCl, NaCl, Na2SO4, etc.); there still is, however, no consensus on whether the participating cation is alkaline metal ion,2 proton,3 or both.16 In our previous report, we have described a new electrochemical route to construct manganese and vanadium mixed oxide films.18,19 The process includes anodic oxidation of aqueous Mn2+ ions in the presence of oxovanadate anions (VO3-). Herein, we report the preparation of molybdenum-containing Mn oxide films by the similar approach and the characterization of the products using several techniques, such as X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. In the following, the charge-storage properties of the mixed Mn/ Mo oxide as a pseudocapacitor electrode in neutral electrolyte were investigated by means of electrochemical quartz crystal microbalance (EQCM) gravimetry and XPS. EQCM is a highly sensitive in situ technique for monitoring mass changes on the electrode during electrochemical processes, and it has been proved to be very powerful in elucidating electrochemical reaction mechanisms.20 However, very few EQCM studies available on pseudocapacitive processes have so far been conducted on RuO2 21-23 and NiOx 24 electrodes in strong acid (H2SO4) and alkaline (KOH) solutions, respectively. To the best of our knowledge, this is the first application of the EQCM technique to the pseudocapacitive process of a Mn-based electrode material in neutral media. Experimental Section Materials. All chemicals were of reagent grade (Wako Pure Chemicals) and were used without further purification. All solutions were prepared with doubly distilled water and were deoxygenated by the bubbling of purified nitrogen gas for at least 20 min just prior to use. Electrochemical Equipment. All electrochemical experiments were carried out in a standard three-electrode glass cell, housing a platinum working electrode, a platinum gauze counter electrode, and a Ag/AgCl (in saturated KCl) reference electrode. (18) Nakayama, M.; Nishio, M.; Ogura, K. J. Mater. Res. 2003, 18, 2364. (19) Nakayama, M.; Tanaka, A.; Konishi, S.; Ogura, K. J. Mater. Res. 2004, 19, 1509. (20) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355. (21) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Kluwer Academic/ Plenum Press: New York, 1999; pp 259-297. (22) Vukovic´, M.; C ˇ ukman, D. J. Electroanal. Chem. 1999, 474, 167. (23) Kim, I. H.; Kim, K. B. J. Electrochem. Soc. 2004, 151, E7. (24) Nam, K. W.; Kim, K. B. J. Electrochem. Soc. 2002, 149, A346.

Nakayama et al. An EG&G Princeton Applied Research model 263A potentio/ galvanostat was used to control the electrode potential. EQCM experiments were performed using a QCA917 quartz crystal analyzer (Seiko EG&G) and the potentio/galvanostat. An ATcut 9 MHz platinum-plated quartz crystal (Seiko EG&G model OA-A9M-PT) with an active area of 0.16 cm2 was used as the working electrode. The EQCM data in most cases are presented in terms of changes in frequency (∆f in Hz). The frequency change can be transformed to the change in mass per unit area in the electrode (∆m) according to the Sauerbrey equation25 (∆f ) -C∆m), where the integral sensitivity constant (C ) 1.97 × 108 cm2 s-1 g-1) was reproducibly obtained by calibration using galvanostatic silver deposition experiments.26 For XPS, IR, fieldemission scanning electron microscopy (FE-SEM), and DC resistivity measurements, sample films were prepared on a platinum foil (1.0 × 1.0 cm2; thickness 0.05 cm; nilaco) substrate. Electrodeposition. Prior to electrodeposition, the surface of the working electrodes was cleaned with nitric acid and pure water followed by blow-drying in a stream of nitrogen. The deposition baths used consisted of 2 mM MnSO4 aqueous solutions with 0-20 mM Na2MoO4 (20 mM in most cases), in which the total sodium concentration was adjusted to be 120 mM by adding Na2SO4. Mn/Mo mixed oxide films were prepared on the platinumplated quartz crystal or platinum foil by cycling the electrode potential repeatedly between 0 and +1.0 V vs Ag/AgCl at a scan rate of 20 mV/s. If not otherwise mentioned, the potential cycling was stopped at the anodic limit on the positive-going scan. Immediately after electrodeposion, the resulting film coated on a foil electrode was rinsed thoroughly with water, dried under vacuum in a desiccator for at least 2 h, and then submitted to spectroscopic measurements within 6 h. Structural Characterization. The films thus prepared were visually uniform and adhered well to the substrate. An amorphous phase was confirmed for all the films from X-ray diffraction patterns recorded on a Shimadzu XD-D1 diffractometer with Cu KR radiation (λ ) 0.15405 nm). FE-SEM images were measured at an accelerating voltage of 20 kV with a Hitachi S-4700Y scanning electron microscope. IR spectroscopy was performed on a Horiba FT-710 spectrometer. Reflection-absorption spectra of the deposited films on a Pt substrate were acquired at an angle of incidence of 80° using a reflectance accessory. Transmittance spectra of the reactant were obtained using pellets of the compounds with KBr (Nakarai Tesque; IR grade). All spectra were measured from 4000 to 400 cm-1. The resolution was 8 cm-1, and 50 scans were collected. X-ray photoelectron spectra were collected using a Fisons Escalab 210 spectrometer, with an Al KR (1486.6 eV) unmonochromatic source (15 kV, 20 mA). Wide- and narrowrange spectra were collected with a pass energy of 20 eV and a channel width of 0.1 and 0.01 eV, respectively. The binding energy (BE) scale was calibrated with respect to the C(1s) (284.5 eV) signal. Curve fitting was made by a mixture of Gaussian and Lorentzian functions on a Shirley-type background. Semiquantitative estimates of the relative atomic concentrations were obtained from the peak area ratios by considering the appropriate sensitivity factors:27,28 Mn(2p3/2) ) 9.17, Mo(3d3/2) ) 3.88, O(1s) ) 2.93, and Na(2s) ) 0.42. Electrical Resistivity Measurements. DC resistivities of films deposited on a Pt substrate were determined on the basis of the film thickness estimated by FE-SEM cross-section photographs and the resistance measurements using a four-point probe method. A constant current was passed through the outer pins while the corresponding voltage was measured across the inside pins. Five measurements were made at different locations on each sample. Electrochemical Characterization. The Pt-plated quartz crystal coated with the as-deposited oxide film was rinsed with water and transferred to a 0.5 M Na2SO4 solution. Cyclic voltammetry and electrogravimetry using the EQCM were (25) Sauerbrey, G. Z. Phys. 1959, 155, 206. (26) Gabrielli, C.; Keddam, M.; Torresi, R. J. Electrochem. Soc. 1991, 138, 2657. (27) Nakayama, M.; Tagashira, H.; Konishi, S.; Ogura, K. Inorg. Chem. 2004, 43, 8215. (28) Nakayama, M.; Konishi, S.; Tagashira, H.; Ogura, K. Langmuir 2005, 21, 354.

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Figure 1. Cyclic voltammograms and frequency changes of a Pt-plated quartz crystal electrode during the first 10 cycles in 2 mM MnSO4 solutions (a) with and (b) without 20 mM Na2MoO4. Scan rate, 20 mV/s. performed in a potential region between +0.1 and +0.9 V vs Ag/AgCl at various scan rates. Specific capacitance (in F/g) was calculated by integrating the CV curve to obtain the voltammetric charge (in C), and subsequently dividing it by the mass of the electroactive material (in g) and the potential window (in V). Here, the mass of the electroactive material was calculated by subtracting the mass decrease during the potential cycling in Na2SO4 electrolyte from the as-deposited mass.

Results and Discussion Film Deposition. Figure 1a shows a series of cyclic voltammograms (CVs) and the accompanying frequency changes during the first 10 potential cycles in a 2 mM MnSO4 solution with 20 mM Na2MoO4, along with those obtained without Na2MoO4 for comparison (Figure 1b). Clearly, both voltammetric and frequency responses are affected by adding MoO42-. In both solutions, an oxidation current starts to appear around +0.6 V at the first anodic scan, accompanying a decrease in resonant frequency (correspondingly an increase in mass). This can be ascribed to the oxidation of Mn2+ ions in solution which leads to the deposition of Mn oxide.6,18 A pure Na2MoO4 solution gave no CV current (Supporting Information I), confirming that MoO42- is electrochemically inactive in this potential region. In the first anodic scan, the Mn2+ oxidation should take place on the Pt electrode surface, while the reaction after the second cycle can occur on the deposit. In the absence of MoO42- (Figure 1b), the decrease in frequency during each voltammetric cycle gradually becomes smaller in magnitude upon continuous cycling, indicating the formation of a film which blocks electron transfer to the electrode surface. In the case of MoO42- present, however, the magnitude of the frequency decrease in each cycle does not diminish even at the later cycles. From these observations, we deduce that the film prepared with MoO42- has better conductivity than that without MoO42-. As the number of cycles is increased, the Mn2+ oxidation peaks are gradually superimposed by capacitive-like currents, and they will be discussed later. Figure 2a shows anodic polarization curves recorded at the first scan in MnSO4 solutions with different Na2MoO4 concentrations. With increasing MoO42- concentration, the voltammetric profile shifts cathodically. This can be recognized by considering that the electrooxidation of Mn2+

Figure 2. Voltammograms of a Pt electrode taken at a scan rate of 20 mV/s in 2 mM MnSO4 solutions containing (a) 0-20 mM Na2MoO4, in which total sodium concentration was adjusted to be 120 mM by adding Na2SO4, and (b) sodium salts of the indicated anions at the sodium concentration of 120 mM.

ion with water, which produces Mn oxide and protons (e.g., eq 1),29 is accelerated by a subsequent chemical reaction that should be associated with MoO42-.

Mn2+ + 2H2O f MnO2 + 4H+ + 2e-

(1)

To gain more information on the reaction mechanism, similar experiments were carried out in MnSO4 solutions containing various electrolyte anions with different proton affinity (Figure 2b). All the anions were confirmed to be electrochemically inactive in the potential region examined, and pHs of the solutions were in the range of 5-8. Proton affinity of the anions increases in the order: Cl< NO3- , SO42- (pKa 1.99) < C6H5COO- (pKa 4.20) < CH3COO- (pKa 4.56). Clearly, Mn2+ ion can be oxidized easily in the presence of anion with larger proton affinity. This observation supports the assumption that the consumption of protons by MoO42- promotes the preceding electrooxidation of Mn2+. In other words, MoO42- ions accept protons in the film growth process, which can lead to the formation of polymolybdate, as will be described later. Figure 3 depicts field-emission scanning electron microscopy (FE-SEM) images of as-deposited films on a Pt substrate by applying 36 potential cycles in 2 mM MnSO4 solutions (a) with and (b) without 20 mM Na2MoO4. The film deposited from the Mo-containing solution presents a uniform and dense morphology, and none of the underlying Pt is exposed. On the other hand, the film made without MoO42- has a less dense and spherical morphology with grain sizes of 0.4-0.5 µm in diameter. XPS wide-scan spectra of these two films are shown in Figure 4. Both curves commonly present Mn(2p) (646 eV), Mn(3s) (84 eV), and O(1s) (530 eV) peaks arising from Mn (29) Voinov, M. Electrochim. Acta 1982, 27, 833.

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Figure 5. XPS spectra of the Mn/Mo mixed oxide film in the energy regions of (a) Mn(3s), (b) Mo(3d), (c) Na(2s), and (d) O(1s). Figure 3. FE-SEM images of films prepared from 2 mM MnSO4 solutions (a) with and (b) without 20 mM Na2MoO4 by cycling the electrode potential between 0 and +1.0 V for 1 h (36 cycles) at a scan rate of 20 mV/s.

Figure 4. XPS spectra of films prepared from 2 mM MnSO4 solutions (a) with and (b) without 20 mM Na2MoO4 by cycling the electrode potential between 0 and +1.0 V for 1 h (36 cycles) at a scan rate of 20 mV/s.

oxide. In the presence of MoO42- (Figure 4a), other peaks are observed at 496, 405, and 233 eV, which can be assigned to the 3s, 3p, and 3d energy levels of Mo, respectively. This clearly indicates the incorporation of Mo into the Mn oxide, i.e., the formation of Mn/Mo mixed oxide. Several peaks relating to the Pt substrate are visible around 320 eV (Pt(4d)) and 70 eV (Pt(4f)) in curve b and are negligible for the Mn/Mo oxide film. This may be a result from the porous morphology of the Mn oxide film (see Figure 3b). Core-level spectra of the Mn/Mo mixed oxide for Mn(3s), Mo(3d), Na(2s), and O(1s) energy regions are shown in Figure 5. In curve a, the Mn(3s) spectrum provides a doublet of peaks that results from a parallel spin coupling between electrons in the 3s and 3d orbits. The energy separation (∆E) of the peaks is sensitive to the oxidation state of Mn in the oxide, i.e., the ∆E value increases due to more electrons in the 3d energy level when the oxidation state decreases.12,30,31 The observed ∆E value (5.18 eV) corresponds to an average oxidation state of 3.3 according to a linear relationship between the

Mn oxidation state and the ∆E value presented in the literature.12 This oxidation level means that the film is composed largely of Mn3+ with a minor amount of Mn4+. The Mo(3d) spectrum (Figure 5b) presents a doublet at 235.0 and 231.8 eV corresponding to the 3d3/2 and 3d5/2 states (spin-orbit coupling in a 3d state).32,33 These peaks are symmetrical and their peak area ratio (3d5/2/3d3/2) was calculated to be about 3:2, as expected for stoichiometrically pure MoVIO3.34 Clearly, all molybdenum ions in the product are in the 6+ oxidation state, that is, the valence of Mo remains unchanged during electrodeposition. There is no peak in the Na(2s) region (Figure 5c), indicating that incorporated Mo ions are not present in the form of Na2MoO4 salt. In the O(1s) region (Figure 5d), three different contributions can be observed at 529.8, 531.3, and 532.5 eV, which are attributable to oxide (O2-), hydroxide (OH-), and residual structural water.35 By considering the appropriate sensitivity factors, a chemical formula of MnMo6+0.18O1.88(OH)0.59(H2O)0.25 can be obtained from the peak area ratios in Figure 5 (Supporting Information II). Here, the relative contents of the oxygen species involve experimental errors mainly due to exposure of the sample to ambient air. Standard deviations in all the oxygen contents for three measurements were estimated to be within 0.02. On the other hand, the errors in Mn and Mo contents were negligibly small. The above formula leads to a Mn valence of 3.3, which is in good agreement with that estimated from the energy splitting of the Mn(3s) doublet. This oxidation level is lower than what was observed for the pure Mn oxide prepared similarly (∆E ) 4.87 eV). Such an internal balance of charge between Mn and Mo sites implies that these are mixed on an atomic level. Figure 6 shows the reflection FT-IR spectra of (a) mixed Mn/Mo and (b) pure Mn oxide films on a Pt substrate, (30) Carver, J. C.; Schweitzer, G. K.; Carlson, T. A. J. Chem. Phys. 1972, 57, 973. (31) Chigane, M.; Ishikawa, M. J. Electrochem. Soc. 2000, 147, 2246. (32) Guerfi, A.; Paynter, R. W.; Dao, L. H. J. Electrochem. Soc. 1995, 142, 3457. (33) Nakayama, M.; Ii, T.; Komatsu H.; Ogura, K. Chem. Commun. 2004, 1098. (34) Briggs, D.; Rivire, J. C. Spectral Interpretation in Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, 2nd ed.; Briggs, D., Seah, M. P., Eds.; Wiley: New York, 1990; Vol. 1, pp 85-141. (35) Casella, I. G.; Gatta, M. Anal. Chem. 2000, 72, 2969.

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Figure 6. Reflection FTIR spectra of (a) mixed Mn/Mo and (b) pure Mn oxide films on a Pt substrate and a transmittance spectrum of (c) Na2MoO4 powder in a KBr pellet. The films were prepared the same way as that in Figure 3.

along with the transmittance spectrum of (c) Na2MoO4 powder in a KBr pellet. Curves a and b show a peak at 683 cm-1 due to the Mn-O stretching vibration.36 On the other hand, the Mo-O stretching mode,37 which is observed at 860 cm-1 for the reactant, is seen as a shoulder on the higher-wavenumber side of the ν(Mn-O) band. In curve a, we should note that a large monotonic absorption appears above 1500 cm-1, which is unnoticeable in the spectrum of pure Mn oxide. Similar findings have been reported for conducting ZnO and WO3 films and attributed to the absorption due to free carriers.38,39 Hence, the appearance of this feature strongly suggests improvement in electrical conductivity of the mixed Mn/Mo oxide film. In fact, electrical resistivities of the films were measured at room temperature using a four-point probe method. The resistivity of the mixed Mn/Mo oxide film with a thickness of ∼0.5 µm, made by applying 36 CV cycles, was calculated to be 3.0 × 101 Ω cm. The film thickness was estimated by FE-SEM cross-section images (Supporting Information III). The obtained resistivity value is much lower than that (∼106 Ω cm) of pure bulk MnO2 and is approximately the same order of magnitude as those reported for the composites with conductive carbon.40 In contrast, the Mn oxide film (∼0.5 µm in thickness, made by 50 cycles) exhibited a large resistivity of at least 5.0 × 104 Ω cm that is the maximum value measurable in our system. Although the resistivity values we obtained might be affected by the underlying Pt, it is certain that the mixed oxide film has better conductivity than the Mo-free film, which agrees with the CV and IR results. On the basis of the above results and discussion, a possible mechanism for the film formation of Mn/Mo mixed oxide is proposed as follows. Initially, the electrooxidation of aqueous Mn2+ ions yields Mn oxide and protons (see eq 1). Then, the protons being accumulated in the vicinity of the electrode surface react with MoO42- ions. Conversion of MoO42- into polymolybdate(VI), typically Mo7O246-, under acidic conditions is a prominent feature in the chemistry of molybdenum.41,42 The reaction proceeds (36) Allen, G. C.; Curtis, M. T.; Hopper, A. J.; Tucker, P. M. J. Chem. Soc., Dalton Trans. 1974, 14, 1525. (37) Mahadevan Pillai, V. P.; Pradeep, T.; Bushiri, M. J.; Jayasree, R. S.; Nayar, V. U. Spectrochim. Acta Part A 1997, 53, 867. (38) Peulon, S.; Lincot, D. Adv. Mater. 1996, 8, 166. (39) Opara Krasˇovec, U.; Sˇ urca Vuk, A.; Orel, B. Electrochim. Acta 2001, 46, 1921. (40) Frysz, C. A.; Shui, X.; Chung, D. D. L. J. Power Sources 1996, 53, 41. (41) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; John Willey and Sons: New York, 1980; pp 852-857.

Figure 7. (a) Cyclic voltammograms of the Mn/Mo mixed oxide film deposited on a Pt-plated quartz crystal when the electrode potential was cycled in a 0.5 M Na2SO4 solution at a scan rate of 20 mV/s and (b) the frequency change recorded concurrently during the potential cycling. The film was prepared the same way as that in Figure 3a.

through dehydrated condensation of MoO42- (by protonation and dehydration):

7MoO42- + 8H+ f Mo7O246- + 4H2O

(2)

It is, therefore, reasonable to think that polymolybdate(VI) thus formed coprecipitates with manganese oxide, although the MoxOy species could not be identified. Electrochemistry of the Mn/Mo Mixed Oxide Film Electrode. EQCM experiments were carried out with freshly prepared films on a Pt-plated quartz crystal electrode. Figure 7a shows voltammograms of the mixed Mn/Mo oxide film when cycled repeatedly in 0.5 M Na2SO4 electrolyte at a scan rate of 20 mV/s. With increasing the number of cycles, the voltammetric current decays gradually to become almost constant at 40 cycles. The steady-state voltammogram exhibits a rectangular image, corresponding to a typical pseudocapacitive behavior where the current flow is independent of the electrode potential. The frequency change recorded concurrently is displayed in Figure 7b as a function of time. During the first 4000 s (corresponding to 50 cycles), an increase in frequency on the anodic scan was not recovered completely by a decrease in frequency on the cathodic scan, resulting in an irreversible loss in the electrode mass. Then, the frequency change becomes reversible, similar to the CV response. The initial mass loss was calculated to be 6.6 µg cm-2, which is 6.8% of the as-deposited mass. On the other hand, from the XPS experiments described later (see Figure 13), the Mo/Mn atomic ratio was found to decrease to 0.02 from the initial value of 0.18 during the potential cycling, (42) Cruywagen, J. J.; Rohwer, E. F. C. H. Inorg. Chem. 1975, 14, 3136.

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Figure 8. FTIR spectra of the Mn/Mo mixed oxide film taken (a) before and (b) after CV cycling in a 0.5 M Na2SO4 solution. The film was prepared the same way as that in Figure 3a.

Figure 10. Frequency changes recorded concurrently with the CVs in Figure 9a. Scan rate: (a) 5, (b) 20, (c) 50, (d) 100, and (e) 200 mV/s. Table 1. Specific Capacitances of the Electrodeposited Mn/Mo and Mn Oxide Films specific capacitance (F/g)

Figure 9. Cyclic voltammograms of (a) Mn/Mo and (b) Mn oxide films taken at various scan rates in a 0.5 M Na2SO4 solution.

which was accompanied by an increase in the average oxidation state of Mn. These findings enable us to consider that the initial loss of the electrode mass is a result of leaching of Mo6+ ions, probably for compensating irreversible oxidation of Mn3+ within the film. As shown in Figure 8, the repeated potential cycling induces a decrease in intensity of the ν(Mo-O) absorption. However, the free carrier absorption still remains and indicates the maintenance of electrical conductivity of the film. Certainly, the resistivity value of the film remained low (9.6 × 101 Ω cm). Figure 9 compares the effect of scan rate on CV behaviors of (a) mixed Mn/Mo oxide versus (b) pure Mn oxide electrodes at the steady state, in which the mass of the electroactive material was deduced by the procedure described in the Experimental Section. The specific current of the Mn/Mo oxide film is significantly larger than that of the pure Mn oxide, and its rectangular shape seems to be maintained even at high potential scan rates. From the IR and electrical resistivity data, such improved

scan rate (mV/s)

Mn/Mo oxide

Mn oxide

5 20 50 100 200

190.9 175.5 162.8 150.2 132.0

82.6 65.6 55.4 47.9 38.2

charge-storage properties can be attributed to an increase in conductivity of the film electrode. In contrast, in the case of the pure Mn oxide film, the distortion from rectangularity becomes obvious near the potential limits at scan rates higher than 20 mV/s. Table 1 summarizes specific capacitances of the Mn/Mo and Mn oxide electrodes, which were calculated from the data in Figure 9. The specific capacitance values observed for the former electrode are similar to, or better than, those (150-200 F/g) reported previously for a-MnO2‚nH2O with conductive carbon.2,4,15 The corresponding frequency changes for the Mn/Mo oxide film at various scan rates are shown in Figure 10. Regardless of the rate, the positive-going scan causes an increase in frequency, followed by a decrease on the reverse scan, corresponding to the extraction of cations (Na+ ions and/or protons) upon oxidation to neutralize the charge in the film and vice versa upon reduction. At 5 and 20 mV/s, the mass of the electrode changes monotonically and almost linearly for the potential change. This means that the mass change rate is unaffected by the electrode potential, which is similar to the response observed for RuO2 capacitors in H2SO4.21-23 At all scan rates, the initial mass of the electrode is completely recovered at the end

Electrodeposition of Mixed Oxide Thin Films

Figure 11. Plot of the mass change versus the scan rate for the data of Figure 10.

Figure 12. Relationship between the mass change and the electrical charge passed for the anodic scan. Scan rate, 20 mV/ s.

of the cycle, indicating that redox reaction of this film is highly reversible. Frequency changes per cycle estimated from the data in Figure 10 were plotted in Figure 11 against scan rates. With increasing the scan rate, the amount of participating ions decreases linearly. At higher scan rates, there is not sufficient time for the electrolyte cations to diffuse into/from the bulk of the solution when the scan direction is changed. From the current- and frequency-potential curves at a scan rate of 20 mV/s, the mass change for the anodic scan segment was plotted against the charge passed (Figure 12). We can see a linear relationship having a slope of -0.49 µg/mC, corresponding to a loss of 47.3 g mol-1 of electrons. To further characterize the charge-storage mechanism, XPS experiments were conducted on the films after being held at oxidizing (+0.9 V) and reducing (+0.1 V) potentials for 20 min in Na2SO4 electrolyte following 100 potential cycles. The resulting spectra for the Mn(3s), Mo(3d), Na(2s), and O(1s) core electronic transitions are depicted in Figure 13. The ∆E values of the Mn(3s) doublets are 4.74 and 4.92 eV for the oxidized and reduced films, which correspond to the Mn valences of 4.0 and 3.6, respectively, on the basis of the literature relationship as mentioned above.12 This confirms that the charging and discharging of our product is a faradaic process. No significant difference in the Mo(3d) region indicates that the redox reaction of Mo sites are not involved in the process. As shown in Figure 13c, the signal due to Na+ ions that might be trapped in the film during the initial cycling is

Langmuir, Vol. 21, No. 13, 2005 5913

Figure 13. XPS spectra of the Mn/Mo oxide films after being held at (a) +0.1 and (b) +0.9 V for 20 min in a 0.5 M Na2SO4 solution following 100 potential cycles.

unaffected by the potential change. On the other hand, the area contribution of OH- species in the O(1s) spectrum for the reduced film (33%) is apparently larger than that (14%) for the oxidized one. Clearly, protons, not sodium ions, are taking part in the pseudocapacitive process of the Mn/Mo oxide film electrode in neutral electrolyte to compensate for the charge generated upon oxidation/ reduction of Mn3+/Mn4+. Accordingly, the molar mass observed above (47.3 g mol-1 of electrons) can be explained by the participation of a proton and 2-3 water molecules. The observed variation of the Mn valence between the oxidized and reduced states is somewhat small when compared to that in the work of Toupin et al. for a thin MnO2 film being composed of the small particles (i.e., between 2.9 and 4.0 for the potential change of 0 to +0.9 V vs Ag/AgCl). This can be associated with the fact that our product has a dense structure that does not allow for diffusion of Na+ cations, unlike the film made by them. Conclusions We demonstrated a novel electrochemical route to synthesize mixed Mn and Mo oxides in a thin film form. The process involves a potentiodynamic oxidation of aqueous Mn2+ ions in the presence of MoO42- ions. Cyclic voltammetry of the Mn/Mo mixed oxide film-coated electrode indicated a pseudocapacitive behavior with higher specific capacitance and better reversibility compared to that of pure Mn oxide, most likely due to an increase in electrical conductivity of the film by incorporation of Mo. EQCM and XPS data clearly demonstrated that this pseudocapacitive process arises as a consequence of monotonous and reversible extraction/insertion of protons with 2-3 water molecules to neutralize the charge in the film upon oxidation/reduction of Mn3+/Mn4+. Acknowledgment. This research was supported in part by The Mazda Foundation’s Research Grant. Supporting Information Available: Cyclic voltammogram taken in a pure Na2SO4 solution, XPS data for quantification, and FE-SEM images for estimation of film thickness. This material is available free of charge via the Internet at http://pubs.acs.org. LA050114U