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Formation of a thin MnIII oxide film on noble metal electrodes from a manganese solution
Journal of Electroanalytical Chemistry 536 (2002) 47 /53 www.elsevier.com/locate/jelechem
Formation of a thin MnIII oxide film on noble metal electrodes from a manganese solution M. Nakayama *, C. Matsushima, K. Ogura Department of Applied Chemistry, Faculty of Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan Received 25 March 2002; received in revised form 12 September 2002; accepted 14 September 2002
Abstract MnII /glycine complex in slightly alkaline solution is oxidized at a less positive potential (/0.25 to /0.6 V vs. Ag j AgCl) where free Mn2 ion is not oxidizable, leading to the formation of a thin uniform film containing MnIII oxide. The film prepared by continuous cyclic scans in the potential region between /0.2 and /0.6 V exhibited an electron spin resonance (ESR) peak attributed to MnII(H2O)6, but this resonance peak disappeared when the lowest potential was shifted to 0 V. From the electrochemical quartz II crystal microbalance (EQCM) measurements, the film deposited anodically can be represented as MnIII 2 O6 [Mn (H2 O)6 ]3 which II III III II corresponds to a structure where Mn in Mn5O6 (Mn2 Mn3 O6 ) is not bound to the oxygen atom in the Mn /O network but is coordinated with water molecules. The MnII(H2O)6 in the film was suggested to be released during the reduction of the oxide matrix and oxidized to form the MnIII /O bond at more positive potentials than /0.6 V. # 2002 Elsevier Science B.V. All rights reserved. Keywords: ESR; EQCM; Manganese oxide; Glycine
1. Introduction Metal oxide films are prepared by chemical vapor deposition, radiofrequency sputtering, sol /gel and spin coating, and recently by electrochemical deposition. Among these methods, the electrodeposition technique has been paid much attention since very thin and uniform oxide films can be obtained with a high reproducibility even on complicated substrates. Films grown electrochemically in aqueous media show a high degree of hydration, offering sufficient charge transfer kinetics owing to ease of ion transport. Manganese oxides are good candidates as materials for positive or negative electrodes of primary and secondary batteries. Although the redox process of manganese oxides, especially MnO2, has been extensively studied by means of various voltammetric and spectroscopic techniques [1 /3], only a few studies were made for elucidating the formation process. Recently, Messaoudi et al. have
* Corresponding author. Tel.: /81-836-85-9223; fax: /81-836-859201 E-mail address: [email protected] (M. Nakayama).
investigated manganese oxide films formed by the anodic polarization of the Mn electrode in NaOH [4]. They revealed from in situ Raman spectroscopy that the electrode surface is covered by Mn3O4, Mn2O3 and MnO2 as the potential is shifted towards more positive values. Chigane and Ishikawa have prepared thin MnOx films by electrolysis of the manganese /ammine complex at various potentials, and the structure of the films was examined using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) [5]. Their XRD results indicated that the electrolysis at potentials lower than /0.3 V versus Ag j AgCl gives films mainly composed of g-Mn2O3 and/or Mn3O4 (hausmannite) and that at higher potentials produces Mn7O13 ×/5H2O. We have previously reported that metal/amino acid complexes can be oxidized at less positive potentials compared to uncomplexed ions, resulting in the deposition of CuO [6] and Co3O4 [7] films. In this case, the adsorption of complexing ligands onto the electrode surface was suggested to lower the overpotential for the oxidation of metal ions. In the present study, anodic oxidation of Mn2 was carried out similarly in the presence of glycine, and thin films containing MnIII oxide generated at positive potentials less than /0.6 V
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 1 1 9 1 - 9
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were characterized by means of electrochemical quartz crystal microbalance (EQCM), electron spin resonance (ESR) and Fourier-transform infrared (FTIR) techniques. ESR is able to detect MnII and MnIV, whereas MnIII is not detected due to the large splitting of energy levels [8]. Also, ESR is effective only to paramagnetically isolated species. For instance, MnII oxide (MnO) provides no distinct ESR signal because of a strong magnetic interaction of the MnII sites [9], but MnII dispersed in a matrix shows well-resolved peaks at ordinary temperature [10]. Hence, the use of ESR spectroscopy will be of great help in studying the nature of manganese species in the deposited films.
2. Experimental The electrolytic solutions were prepared by dissolving an appropriate amount of manganese(II) sulfate and glycine in distilled water. The pH of the solutions was adjusted by adding concd. NaOH. All chemicals used were of reagent grade, and purchased from the Wako Chemical Company. All electrochemical experiments were carried out in a three-electrode system. The counter and reference electrodes were a Pt plate and an Ag j AgCl j sat. KCl electrode, respectively. The working electrode (WE) in voltammetric and FTIR measurements was a Pt plate (1 cm2). EQCM experiments were carried out with a 6 MHz AT-cut quartz crystal, which was supplied with a thin film of gold deposited on both sides. A one-sided Au-plated crystal was exposed to the electrolyte and served as the WE. The active electrochemical area of this electrode was 1.55 cm2. The oscillation circuit was controlled with an automatic polarization system (Hokuto Denko, HZ-3000) and an EQCM controller (Hokuto Denko, HQ-101B). The current passing through the EQCM WE and the frequency of oscillation of the quartz disk were measured simultaneously. The frequency shift observed is linearly related to the added mass per unit surface area according to the Sauerbrey equation [11]. Sample films for ESR measurements were deposited on a Pt rod (1 mm f /30 mm). The electrode obtained was ultrasonicated in a water bath for at least 10 min and rinsed with copious amounts of water to remove physically adsorbed species. Following this treatment, the electrode was placed in a quartz tube (3 mm inner diameter) connected with a vacuum line, evacuated to be dried, and then exposed to water vapor at 259/1 8C overnight without contact to air, unless otherwise noted. ESR spectra were recorded at room temperature with a JEOL JES-FE1X spectrometer working at X-band frequency region. Samples for FTIR spectroscopy were prepared by the following procedure. A thick film deposited on a Pt
plate was peeled off and dried under vacuum. Two milligram of the sample was mixed with 40 mg of KBr, and this mixture was used to prepare a compressed pellet. The data were recorded on a Shimadzu DR-8000 FTIR spectrometer.
3. Results and discussion 3.1. Anodic behavior of Mn2 in aqueous glycine solution Fig. 1a shows the voltammogram of a Pt electrode in a 2 mM MnSO4/20 mM glycine solution of pH 8. An anodic current starts to appear at /0.25 V and increases again from /0.6 V. The former current cannot be observed in a similar solution of pH 5.7 (Fig. 1b) and in a MnSO4 solution of pH 8 (Fig. 1c). On the other hand, the latter current is seen in all the solutions containing Mn2, and is attributed to the oxidation of free Mn2 ion. The voltammetric measurements were made similarly in MnSO4 solutions of various pHs in the presence of 20 mM glycine. In Fig. 2a, the maximum current density at the former wave (denoted as ja1, see Fig. 1a) was plotted as a function of the solution pH. The pH dependence of ja1 up to pH 9 seems similar to that of the amount of glycine in the anionic form (pKa2 /9.60) (Fig. 2b). Since the anionic glycine forms a complex with manganese ion, the amount of MnII /glycine complex should increase with increasing solution pH. An abrupt decrease appearing above pH 9 is probably due to the loss of soluble Mn2 as a result of the precipitation of manganese hydroxide. On the other hand, Fig. 2c displays the variation of ja1 with respect to the glycine concentration in the 2 mM MnSO4 solution of pH 8. It is found that the current density increases roughly
Fig. 1. Voltammograms of a Pt electrode obtained in 2 mM MnSO4 solutions with (a, b) and without (c) 20 mM glycine. pHs of the solutions were 8.0 (a, c) and 5.7 (b). Scan rate, 10 mV s 1.
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Fig. 3. (a) CV and (b) mass change /potential curves obtained with an Au electrode in a 2 mM MnSO4/20 mM glycine solution of pH 8. Scan rate, 10 mV s 1.
Fig. 2. (a, c) ja1 in the voltammograms of a Pt electrode plotted vs. the solution pH (a) and the concentration of glycine (c). The voltammograms were obtained in 2 mM MnSO4 solutions of various pHs with 20 mM glycine (a) and of pH 8 with various concentrations of glycine (c). Scan rate, 10 mV s 1. (b) Distribution of glycine species as a function of pH in the glycine /water system.
linearly with the glycine concentration. This also indicates that the increase of MnII /glycine complex causes an increase in the anodic current because a certain portion of the added glycine always dissociates at pH 8. Thus, the anodic current from /0.25 V can be attributed to the oxidation of MnII /glycine complex. 3.2. Characterization of deposited films 3.2.1. EQCM studies EQCM measurements were conducted in a 2 mM MnSO4/20 mM glycine solution of pH 8 during continuous potential cycling between 0 and /0.6 V. The potential scan rate was 10 mV s 1. Fig. 3 shows current /potential (a) and mass change /potential (b)
curves recorded for initial five cycles. At the first scan, the anodic current from /0.25 V is found to accompany an increase in the electrode mass. This suggests that MnII /glycine complex is oxidized to form a deposited layer containing MnIII on the electrode. The mass increase continues at the reverse scan to attain a constant value around /0.3 V. Toward more negative potential, a reduction current starts to appear at /0.23 V together with a decrease in the electrode mass, which corresponds to a transport of matter from the anodically deposited species. The mass change during each potential cycle does not return to the original value, resulting in an increase in the total mass of the electrode with each scan. After cycling the potential 90 times, a brown film characteristic of manganese oxide appeared, and this film gave a FTIR spectrum demonstrating the formation of the Mn /O bond and the absence of glycine or SO2 4 ; as indicated later. It is therefore suggested from the above observations that the deposited film is composed of MnIII oxide. Furthermore, the current response is noted to become larger as the number of cycles is increased while the mass change associated with each cycle gradually decreases both for the positive and negative going scans. This tendency continued until a slight mass change was observed constantly, meaning that the mass transport related to the redox process of the grown film is much smaller than that taking place during the early stage of the film deposition. To determine the species involved in the first anodic and subsequent cathodic processes, the apparent molar mass, i.e. the mass change per mole of electrons, was
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estimated. The mass change (Dm ) and the electric charge passed (DQ ) at an interval of 20 mV (2 s) calculated from the data shown in Fig. 3 were used to estimate the molar mass according to the following equation. molar mass F Dm=DQ
(1)
where F stands for the Faraday constant (96 500 C mol 1). In Fig. 4, the molar mass data thus obtained for the anodic (a) and cathodic (b) processes are shown as a function of the electrode potential. The anodic process (Fig. 4a) yields an almost constant value of about 315 g per mol of electrons in the region between 0.33 and 0.55 V. This value is considerably larger compared to those expected for the formation of pure MnIII-containing oxides such as Mn2O3, Mn3O4 and Mn5O6 from MnII (79, 114 and 185 g per mol of electrons, respectively). For the cathodic process (Fig. 4b), two constant values of about /280 (0.08 V B/E ) and /170 g per mol of electrons (E B/0.08 V) can be seen, suggesting that different reactions take place depending on the potential. Such large decreases in the electrode mass cannot be ascribed to the ion transport for charge compensation during the reduction of the grown film, as already described, and it is probably due to the dissolution of a certain portion of the film deposited anodically, because the mass change detected by EQCM involves electrically non-charged species. The assignments of the species
Fig. 4. Molar mass plotted vs. the electrode potential for the first anodic (a) and subsequent cathodic processes (b) from the data shown in Fig. 3.
involved in both anodic and cathodic processes will be dicussed later.
3.2.2. ESR studies Fig. 5 shows the ESR spectra of the manganese oxide films which were obtained by cycling the potential 90 times between /0.2 (a) or 0 (b) and /0.6 V in a 2 mM MnSO4/20 mM glycine solution of pH 8. The spectrum of the film obtained in the former potential region presents the resonance peak with a hyperfine structure, whereas this feature is not seen in the latter film. The observed sextet signal with a g factor of 2.00 and a hyperfine coupling parameter, A , of 95 Gauss can be identified as the Ms //1/2 l/ /1/2 transition of MnII, and these values are almost the same as those reported for MnII(H2O)6 (g /2.009/0.002, A / 96.5 Gauss) [12]. It is evident that this signal is not associated with the MnII sites in the reduced manganese oxide because it clearly appears when the potential scan was stopped before the reduction of the film. Also, it is known that manganese oxides show no hyperfine structure owing to a strong magnetic interaction of the Mn sites [9]. Hence, the observed signal can be attributed to MnII(H2O)6 incorporated in the MnIII oxide matrix. The MnII(H2O)6 species is considered to interact electrically with the negative charge of unshared oxygen in the MnIII oxide. On the other hand, the disappearance of the signal in the negative potential region suggests that MnII(H2O)6 is released from the film during the reduction of MnIII oxide. This view coincides with the large decrease in the electrode mass during the cathodic process (Fig. 3b and Fig. 4b). As also described later, these observations can be recognized by considering that the reduction of MnIII
Fig. 5. ESR spectra of the manganese oxide films deposited on a Pt electrode by cycling the potential 90 times between /0.2 (a) or 0 (b) and /0.6 V in a 2 mM MnSO4/20 mM glycine solution of pH 8. Scan rate, 10 mV s 1.
M. Nakayama et al. / Journal of Electroanalytical Chemistry 536 (2002) 47 /53
oxide leads to the formation of MnII(OH)2 by incorporating protons, and the MnII(H2O)6 held by anionic oxygen of the oxide matrix is expelled from the film. Fig. 6 shows the ESR spectra of the films prepared by potentiostatic electrolysis at /0.4 (a), /0.8 (b) and / 1.2 (c) V in the same solution, where the electric charge for the film preparation was fixed. The film prepared at /0.4 V provides signal due to MnII(H2O)6 similar to that described above. The signal intensity decreases markedly in intensity at /0.8 V and diminishes almost completely at /1.2 V, suggesting that the MnII(H2O)6 species in the film is oxidized at such positive potentials. In Fig. 7, ESR spectra of the films in various environments are shown, where the sample films deposited at a constant potential of /0.4 V were evacuated once to dry them, (a) and then exposed to methanol (b) or iso -propanol (c) vapor at 259/1 8C overnight. In the case of the film treated with methanol vapor, the signal due to MnII(H2O)6 is almost the same as that of the hydrated sample (Fig. 6a), confirming that the coordinated water molecules remain in the film after evacuation. However, no absorption is seen for the films dried (a) and treated with iso -propanol (c). A similar phenomenon has been reported by Brouet et al. for MnIIimpregnated aluminophosphate molecular sieves (MnIIAlPO) [13], in which the ESR signal due to MnII in the hydrated MnIIAlPO became smaller when the sample was evacuated. The authors explained this change by an increase of the spin /spin interaction of the MnII sites as a consequence of which MnII ions were closer to each other in the dry state. Furthermore, as small molecules such as methanol, which can enter the AlPO channels, were adsorbed, the spectrum showed the
Fig. 6. ESR spectra of the manganese oxide films on a Pt electrode prepared potentiostatically at /0.4 (a), /0.8 (b) and /1.2 (c) V by applying a constant electric charge of 75.6 mC cm 2 in a 2 mM MnSO4/20 mM glycine solution of pH 8.
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Fig. 7. ESR spectra of the manganese oxide films which were evacuated (a) and then exposed to methanol (b) and iso -propanol (c) at vapor pressure at 259/1 8C overnight. The oxide film was prepared at /0.4 V on a Pt electrode by applying an electric charge of 75.6 mC cm 2 in a 2 mM MnSO4/20 mM glycine solution of pH 8.
same signal as the hydrated sample, while molecules larger than the channel entrance, such as o-xylene, yielded a spectrum similar to that of an evacuated sample. Hence, the spectral change observed here may reflect the effect of the adsorbed molecules on the magnetic interaction between the incorporated MnII(H2O)6 and the MnIII oxide matrix. That is, only water and methanol can permeate through the film and reduce their magnetic interaction, which implies that the MnIII oxide matrix and the incorporated MnII(H2O)6 form a quite dense film. Fig. 8 shows the FTIR spectra of the electrodeposited films in the region between 2000 and 450 cm 1, where the film deposition was carried out under potentiostatic conditions in a 2 mM MnSO4/20 mM glycine solution of pH 8. No absorption related to glycine is noticed for all the compounds examined. In the spectrum of the film prepared at /0.4 V (Fig. 8a), a broad band appears in the region below 750 cm 1. This absorption can be assigned to the metal/oxygen (MO) stretching mode [14,15], which is for the formation of the Mn /O bond. The small absorption around 1080 cm 1 is attributable to the bending vibration of H /O MO [14,16], suggesting the presence of surface hydroxide in the film. The peak observed at 1630 cm 1 is due to structural water. The film prepared at /1.2 V (Fig. 8b) provides a larger Mn/O absorption than that of the film at /0.4 V. Fig. 8c shows the spectrum of the film which was first deposited at /0.4 V and cycled between /0.2 and /0.8 V in a Na-borate solution. In this spectrum, the intensity of the Mn /O band relative to the absorptions at 1630 and 1080 cm 1 is larger than that of the as-deposited film (Fig. 8a). This is probably because the MnII(H2O)6
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MnII(H2O)6 is released from the film at a negative potential where the MnIII oxide is reduced. As described above, the release of such a cationic species during the reduction of the film cannot be explained by ion transport relating to the redox reaction of the film. Hence, it is reasonable to consider that MnIII oxide is reduced to MnII hydroxide by the injection of proton and electron in a way similar to usual manganese oxides [1], and this causes a release of MnII(H2O)6 and H2O. II MnIII 2 O6 [Mn (H2 O)6 ]3 8H 2e
0 2MnII (OH)2 2H2 O3MnII (H2 O)6
Fig. 8. FTIR spectra of the manganese oxide films in KBr. The films were deposited potentiostatically at /0.4 V (a, c) and /1.2 (b) V in a 2 mM MnSO4/20 mM glycine solution of pH 8, and the film prepared at /0.4 V was subjected to potential cycling between /0.2 and /0.8 V in a Na-borate solution at a scan rate of 10 mV s 1 (c).
incorporated in the film prepared at /0.4 V was oxidized to form the MnIII /O bond at the positive potential.
(3)
Based on this equation, the mass decrease per mole of electrons is calculated to be 262 g, being similar to the observed molar mass ( /280 g per mol of electrons) at less negative potentials than 0.08 V in the cathodic process (Fig. 4b). The decrease in apparent molar mass at potentials more negative than 0.08 V is probably related to the involvement of a small amount of MnII(H2O)6 in the film. That is, it is suggested that reaction (3) takes place in the outer part of the deposited film, while the inner part is not completely transformed to MnII(OH)2, because the diffusion of H , H2O or MnII(H2O)6 should be more difficult. The apparent molar mass ( /170 g per mol of electrons) at potentials more negative than 0.08 V is close to MnII(H2O)6 (M.W., 162) per mole of electrons, and thus the following reduction scheme can be given. II MnIII 2 O6 [Mn (H2 O)6 ]3 4H 2e
0 2MnII O(OH)×MnII (H2 O)6 2MnII (H2 O)6
(4)
3.3. Mechanism of the film deposition From the above results, it is revealed that the oxidation of MnII /glycine complex at a positive potential less than /0.6 V yields a thin film consisting of MnIII oxide and MnII(H2O)6. Taking into account the film being electrically neutral, the deposited substance can be represented in the simplified formula of II MnIII 2 O2x [Mn (H2 O)6 ]2x 3 and is considered to be formed according to the following reaction from MnII /glycine (Gly ) complex. (2x 1)MnII (Gly )n (14x 18)H2 O II 0 MnIII 2 O2x [Mn (H2 O)6 ]2x 3 4xH
(2x 1)nGly 2e II (/MnIII 2 O6 [Mn (H2 O)6 ]3 ;
(2)
when x is 3 MW, 694), the molar mass expected for this reaction is estimated to be 347 g per mol of electrons, and becomes close to the value observed in Fig. 4a (315 g per mol of electrons). This formula corresponds to a structure where MnII in II Mn5O6 (MnIII 2 Mn3 O6 ) is not covalently bound to oxygen atoms in the MnIII /O network but coordinated with water molecules.
4. Conclusions MnII /glycine complex was oxidized at a less positive potential than /0.6 V to form a thin film of a mixture of MnIII oxide and MnII(H2O)6, which can be represented II as MnIII 2 O6 [Mn (H2 O)6 ]3 : This formula corresponds to a II structure where MnII sites in Mn5O6 (MnIII 2 Mn3 O6 ) are coordinated with water molecules, and not bound to oxygen atoms in the MnIII /O network. MnII(H2O)6 within the initially deposited film is expelled from the film during the reduction of the MnIII oxide to MnII hydroxide. On the other hand, at more positive potentials than /0.6 V, the incorporated MnII(H2O)6 is suggested to be oxidized to form the MnIII /O bond.
References [1] L.M. Gassa, H.T. Mishima, B.A. Lopez de Mishima, J.R. Vilche, Electrochim. Acta 42 (1997) 1717.
M. Nakayama et al. / Journal of Electroanalytical Chemistry 536 (2002) 47 /53 [2] N.D. Ivanova, S.A. Kirillov, A.B. Mishchenko, Electrochim. Acta 38 (1993) 2305. [3] B.A. Lopez de Mishima, T. Ohtsuka, H. Konno, N. Sato, Electrochim. Acta 36 (1991) 1485. [4] B. Messaoudi, S. Joiret, M. Keddam, H. Takenouti, Electrochim. Acta 46 (2001) 2487. [5] M. Chigane, M. Ishikawa, J. Electrochem. Soc. 147 (2000) 2246. [6] K. Ogura, K. Nakaoka, M. Nakayama, S. Tanaka, J. Electroanal. Chem. 511 (2001) 122. [7] K. Nakaoka, M. Nakayama, K. Ogura, J. Electrochem. Soc. 149 (2002) C159.
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[8] J.C. Vedrine, in: F. Delannay (Ed.), Characterisation of Heterogeneous Catalysts, Dekker, New York, 1984, p. 161. [9] M.S. Seehra, G. Srinivasan, J. Appl. Phys. 53 (1982) 8345. [10] T.I. Barry, L.A. Lay, Nature 208 (1965) 1312. [11] G. Sauerbrey, Z. Phys. 155 (1959) 206. [12] B.M. Kozyrev, Disc. Faraday Soc. 19 (1955) 135. [13] G. Brouet, X. Chen, C.W. Lee, L. Kevan, J. Am. Chem. Soc. 114 (1992) 3720. [14] J. Fitzpatrick, L.A. Maclean, D.A.J. Swinkels, F.L. Tye, J. Appl. Electrochem. 27 (1997) 243. [15] S. Peulon, D. Lincot, J. Electrochem. Soc. 145 (1998) 864. [16] O.K. Srivastava, E.A. Secco, Can. J. Chem. 45 (1967) 585.
Formation of a thin MnIII oxide film on noble metal electrodes from a manganese solution M. Nakayama *, C. Matsushima, K. Ogura Department of Applied Chemistry, Faculty of Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan Received 25 March 2002; received in revised form 12 September 2002; accepted 14 September 2002
Abstract MnII /glycine complex in slightly alkaline solution is oxidized at a less positive potential (/0.25 to /0.6 V vs. Ag j AgCl) where free Mn2 ion is not oxidizable, leading to the formation of a thin uniform film containing MnIII oxide. The film prepared by continuous cyclic scans in the potential region between /0.2 and /0.6 V exhibited an electron spin resonance (ESR) peak attributed to MnII(H2O)6, but this resonance peak disappeared when the lowest potential was shifted to 0 V. From the electrochemical quartz II crystal microbalance (EQCM) measurements, the film deposited anodically can be represented as MnIII 2 O6 [Mn (H2 O)6 ]3 which II III III II corresponds to a structure where Mn in Mn5O6 (Mn2 Mn3 O6 ) is not bound to the oxygen atom in the Mn /O network but is coordinated with water molecules. The MnII(H2O)6 in the film was suggested to be released during the reduction of the oxide matrix and oxidized to form the MnIII /O bond at more positive potentials than /0.6 V. # 2002 Elsevier Science B.V. All rights reserved. Keywords: ESR; EQCM; Manganese oxide; Glycine
1. Introduction Metal oxide films are prepared by chemical vapor deposition, radiofrequency sputtering, sol /gel and spin coating, and recently by electrochemical deposition. Among these methods, the electrodeposition technique has been paid much attention since very thin and uniform oxide films can be obtained with a high reproducibility even on complicated substrates. Films grown electrochemically in aqueous media show a high degree of hydration, offering sufficient charge transfer kinetics owing to ease of ion transport. Manganese oxides are good candidates as materials for positive or negative electrodes of primary and secondary batteries. Although the redox process of manganese oxides, especially MnO2, has been extensively studied by means of various voltammetric and spectroscopic techniques [1 /3], only a few studies were made for elucidating the formation process. Recently, Messaoudi et al. have
* Corresponding author. Tel.: /81-836-85-9223; fax: /81-836-859201 E-mail address: [email protected] (M. Nakayama).
investigated manganese oxide films formed by the anodic polarization of the Mn electrode in NaOH [4]. They revealed from in situ Raman spectroscopy that the electrode surface is covered by Mn3O4, Mn2O3 and MnO2 as the potential is shifted towards more positive values. Chigane and Ishikawa have prepared thin MnOx films by electrolysis of the manganese /ammine complex at various potentials, and the structure of the films was examined using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) [5]. Their XRD results indicated that the electrolysis at potentials lower than /0.3 V versus Ag j AgCl gives films mainly composed of g-Mn2O3 and/or Mn3O4 (hausmannite) and that at higher potentials produces Mn7O13 ×/5H2O. We have previously reported that metal/amino acid complexes can be oxidized at less positive potentials compared to uncomplexed ions, resulting in the deposition of CuO [6] and Co3O4 [7] films. In this case, the adsorption of complexing ligands onto the electrode surface was suggested to lower the overpotential for the oxidation of metal ions. In the present study, anodic oxidation of Mn2 was carried out similarly in the presence of glycine, and thin films containing MnIII oxide generated at positive potentials less than /0.6 V
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 1 1 9 1 - 9
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were characterized by means of electrochemical quartz crystal microbalance (EQCM), electron spin resonance (ESR) and Fourier-transform infrared (FTIR) techniques. ESR is able to detect MnII and MnIV, whereas MnIII is not detected due to the large splitting of energy levels [8]. Also, ESR is effective only to paramagnetically isolated species. For instance, MnII oxide (MnO) provides no distinct ESR signal because of a strong magnetic interaction of the MnII sites [9], but MnII dispersed in a matrix shows well-resolved peaks at ordinary temperature [10]. Hence, the use of ESR spectroscopy will be of great help in studying the nature of manganese species in the deposited films.
2. Experimental The electrolytic solutions were prepared by dissolving an appropriate amount of manganese(II) sulfate and glycine in distilled water. The pH of the solutions was adjusted by adding concd. NaOH. All chemicals used were of reagent grade, and purchased from the Wako Chemical Company. All electrochemical experiments were carried out in a three-electrode system. The counter and reference electrodes were a Pt plate and an Ag j AgCl j sat. KCl electrode, respectively. The working electrode (WE) in voltammetric and FTIR measurements was a Pt plate (1 cm2). EQCM experiments were carried out with a 6 MHz AT-cut quartz crystal, which was supplied with a thin film of gold deposited on both sides. A one-sided Au-plated crystal was exposed to the electrolyte and served as the WE. The active electrochemical area of this electrode was 1.55 cm2. The oscillation circuit was controlled with an automatic polarization system (Hokuto Denko, HZ-3000) and an EQCM controller (Hokuto Denko, HQ-101B). The current passing through the EQCM WE and the frequency of oscillation of the quartz disk were measured simultaneously. The frequency shift observed is linearly related to the added mass per unit surface area according to the Sauerbrey equation [11]. Sample films for ESR measurements were deposited on a Pt rod (1 mm f /30 mm). The electrode obtained was ultrasonicated in a water bath for at least 10 min and rinsed with copious amounts of water to remove physically adsorbed species. Following this treatment, the electrode was placed in a quartz tube (3 mm inner diameter) connected with a vacuum line, evacuated to be dried, and then exposed to water vapor at 259/1 8C overnight without contact to air, unless otherwise noted. ESR spectra were recorded at room temperature with a JEOL JES-FE1X spectrometer working at X-band frequency region. Samples for FTIR spectroscopy were prepared by the following procedure. A thick film deposited on a Pt
plate was peeled off and dried under vacuum. Two milligram of the sample was mixed with 40 mg of KBr, and this mixture was used to prepare a compressed pellet. The data were recorded on a Shimadzu DR-8000 FTIR spectrometer.
3. Results and discussion 3.1. Anodic behavior of Mn2 in aqueous glycine solution Fig. 1a shows the voltammogram of a Pt electrode in a 2 mM MnSO4/20 mM glycine solution of pH 8. An anodic current starts to appear at /0.25 V and increases again from /0.6 V. The former current cannot be observed in a similar solution of pH 5.7 (Fig. 1b) and in a MnSO4 solution of pH 8 (Fig. 1c). On the other hand, the latter current is seen in all the solutions containing Mn2, and is attributed to the oxidation of free Mn2 ion. The voltammetric measurements were made similarly in MnSO4 solutions of various pHs in the presence of 20 mM glycine. In Fig. 2a, the maximum current density at the former wave (denoted as ja1, see Fig. 1a) was plotted as a function of the solution pH. The pH dependence of ja1 up to pH 9 seems similar to that of the amount of glycine in the anionic form (pKa2 /9.60) (Fig. 2b). Since the anionic glycine forms a complex with manganese ion, the amount of MnII /glycine complex should increase with increasing solution pH. An abrupt decrease appearing above pH 9 is probably due to the loss of soluble Mn2 as a result of the precipitation of manganese hydroxide. On the other hand, Fig. 2c displays the variation of ja1 with respect to the glycine concentration in the 2 mM MnSO4 solution of pH 8. It is found that the current density increases roughly
Fig. 1. Voltammograms of a Pt electrode obtained in 2 mM MnSO4 solutions with (a, b) and without (c) 20 mM glycine. pHs of the solutions were 8.0 (a, c) and 5.7 (b). Scan rate, 10 mV s 1.
M. Nakayama et al. / Journal of Electroanalytical Chemistry 536 (2002) 47 /53
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Fig. 3. (a) CV and (b) mass change /potential curves obtained with an Au electrode in a 2 mM MnSO4/20 mM glycine solution of pH 8. Scan rate, 10 mV s 1.
Fig. 2. (a, c) ja1 in the voltammograms of a Pt electrode plotted vs. the solution pH (a) and the concentration of glycine (c). The voltammograms were obtained in 2 mM MnSO4 solutions of various pHs with 20 mM glycine (a) and of pH 8 with various concentrations of glycine (c). Scan rate, 10 mV s 1. (b) Distribution of glycine species as a function of pH in the glycine /water system.
linearly with the glycine concentration. This also indicates that the increase of MnII /glycine complex causes an increase in the anodic current because a certain portion of the added glycine always dissociates at pH 8. Thus, the anodic current from /0.25 V can be attributed to the oxidation of MnII /glycine complex. 3.2. Characterization of deposited films 3.2.1. EQCM studies EQCM measurements were conducted in a 2 mM MnSO4/20 mM glycine solution of pH 8 during continuous potential cycling between 0 and /0.6 V. The potential scan rate was 10 mV s 1. Fig. 3 shows current /potential (a) and mass change /potential (b)
curves recorded for initial five cycles. At the first scan, the anodic current from /0.25 V is found to accompany an increase in the electrode mass. This suggests that MnII /glycine complex is oxidized to form a deposited layer containing MnIII on the electrode. The mass increase continues at the reverse scan to attain a constant value around /0.3 V. Toward more negative potential, a reduction current starts to appear at /0.23 V together with a decrease in the electrode mass, which corresponds to a transport of matter from the anodically deposited species. The mass change during each potential cycle does not return to the original value, resulting in an increase in the total mass of the electrode with each scan. After cycling the potential 90 times, a brown film characteristic of manganese oxide appeared, and this film gave a FTIR spectrum demonstrating the formation of the Mn /O bond and the absence of glycine or SO2 4 ; as indicated later. It is therefore suggested from the above observations that the deposited film is composed of MnIII oxide. Furthermore, the current response is noted to become larger as the number of cycles is increased while the mass change associated with each cycle gradually decreases both for the positive and negative going scans. This tendency continued until a slight mass change was observed constantly, meaning that the mass transport related to the redox process of the grown film is much smaller than that taking place during the early stage of the film deposition. To determine the species involved in the first anodic and subsequent cathodic processes, the apparent molar mass, i.e. the mass change per mole of electrons, was
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M. Nakayama et al. / Journal of Electroanalytical Chemistry 536 (2002) 47 /53
estimated. The mass change (Dm ) and the electric charge passed (DQ ) at an interval of 20 mV (2 s) calculated from the data shown in Fig. 3 were used to estimate the molar mass according to the following equation. molar mass F Dm=DQ
(1)
where F stands for the Faraday constant (96 500 C mol 1). In Fig. 4, the molar mass data thus obtained for the anodic (a) and cathodic (b) processes are shown as a function of the electrode potential. The anodic process (Fig. 4a) yields an almost constant value of about 315 g per mol of electrons in the region between 0.33 and 0.55 V. This value is considerably larger compared to those expected for the formation of pure MnIII-containing oxides such as Mn2O3, Mn3O4 and Mn5O6 from MnII (79, 114 and 185 g per mol of electrons, respectively). For the cathodic process (Fig. 4b), two constant values of about /280 (0.08 V B/E ) and /170 g per mol of electrons (E B/0.08 V) can be seen, suggesting that different reactions take place depending on the potential. Such large decreases in the electrode mass cannot be ascribed to the ion transport for charge compensation during the reduction of the grown film, as already described, and it is probably due to the dissolution of a certain portion of the film deposited anodically, because the mass change detected by EQCM involves electrically non-charged species. The assignments of the species
Fig. 4. Molar mass plotted vs. the electrode potential for the first anodic (a) and subsequent cathodic processes (b) from the data shown in Fig. 3.
involved in both anodic and cathodic processes will be dicussed later.
3.2.2. ESR studies Fig. 5 shows the ESR spectra of the manganese oxide films which were obtained by cycling the potential 90 times between /0.2 (a) or 0 (b) and /0.6 V in a 2 mM MnSO4/20 mM glycine solution of pH 8. The spectrum of the film obtained in the former potential region presents the resonance peak with a hyperfine structure, whereas this feature is not seen in the latter film. The observed sextet signal with a g factor of 2.00 and a hyperfine coupling parameter, A , of 95 Gauss can be identified as the Ms //1/2 l/ /1/2 transition of MnII, and these values are almost the same as those reported for MnII(H2O)6 (g /2.009/0.002, A / 96.5 Gauss) [12]. It is evident that this signal is not associated with the MnII sites in the reduced manganese oxide because it clearly appears when the potential scan was stopped before the reduction of the film. Also, it is known that manganese oxides show no hyperfine structure owing to a strong magnetic interaction of the Mn sites [9]. Hence, the observed signal can be attributed to MnII(H2O)6 incorporated in the MnIII oxide matrix. The MnII(H2O)6 species is considered to interact electrically with the negative charge of unshared oxygen in the MnIII oxide. On the other hand, the disappearance of the signal in the negative potential region suggests that MnII(H2O)6 is released from the film during the reduction of MnIII oxide. This view coincides with the large decrease in the electrode mass during the cathodic process (Fig. 3b and Fig. 4b). As also described later, these observations can be recognized by considering that the reduction of MnIII
Fig. 5. ESR spectra of the manganese oxide films deposited on a Pt electrode by cycling the potential 90 times between /0.2 (a) or 0 (b) and /0.6 V in a 2 mM MnSO4/20 mM glycine solution of pH 8. Scan rate, 10 mV s 1.
M. Nakayama et al. / Journal of Electroanalytical Chemistry 536 (2002) 47 /53
oxide leads to the formation of MnII(OH)2 by incorporating protons, and the MnII(H2O)6 held by anionic oxygen of the oxide matrix is expelled from the film. Fig. 6 shows the ESR spectra of the films prepared by potentiostatic electrolysis at /0.4 (a), /0.8 (b) and / 1.2 (c) V in the same solution, where the electric charge for the film preparation was fixed. The film prepared at /0.4 V provides signal due to MnII(H2O)6 similar to that described above. The signal intensity decreases markedly in intensity at /0.8 V and diminishes almost completely at /1.2 V, suggesting that the MnII(H2O)6 species in the film is oxidized at such positive potentials. In Fig. 7, ESR spectra of the films in various environments are shown, where the sample films deposited at a constant potential of /0.4 V were evacuated once to dry them, (a) and then exposed to methanol (b) or iso -propanol (c) vapor at 259/1 8C overnight. In the case of the film treated with methanol vapor, the signal due to MnII(H2O)6 is almost the same as that of the hydrated sample (Fig. 6a), confirming that the coordinated water molecules remain in the film after evacuation. However, no absorption is seen for the films dried (a) and treated with iso -propanol (c). A similar phenomenon has been reported by Brouet et al. for MnIIimpregnated aluminophosphate molecular sieves (MnIIAlPO) [13], in which the ESR signal due to MnII in the hydrated MnIIAlPO became smaller when the sample was evacuated. The authors explained this change by an increase of the spin /spin interaction of the MnII sites as a consequence of which MnII ions were closer to each other in the dry state. Furthermore, as small molecules such as methanol, which can enter the AlPO channels, were adsorbed, the spectrum showed the
Fig. 6. ESR spectra of the manganese oxide films on a Pt electrode prepared potentiostatically at /0.4 (a), /0.8 (b) and /1.2 (c) V by applying a constant electric charge of 75.6 mC cm 2 in a 2 mM MnSO4/20 mM glycine solution of pH 8.
51
Fig. 7. ESR spectra of the manganese oxide films which were evacuated (a) and then exposed to methanol (b) and iso -propanol (c) at vapor pressure at 259/1 8C overnight. The oxide film was prepared at /0.4 V on a Pt electrode by applying an electric charge of 75.6 mC cm 2 in a 2 mM MnSO4/20 mM glycine solution of pH 8.
same signal as the hydrated sample, while molecules larger than the channel entrance, such as o-xylene, yielded a spectrum similar to that of an evacuated sample. Hence, the spectral change observed here may reflect the effect of the adsorbed molecules on the magnetic interaction between the incorporated MnII(H2O)6 and the MnIII oxide matrix. That is, only water and methanol can permeate through the film and reduce their magnetic interaction, which implies that the MnIII oxide matrix and the incorporated MnII(H2O)6 form a quite dense film. Fig. 8 shows the FTIR spectra of the electrodeposited films in the region between 2000 and 450 cm 1, where the film deposition was carried out under potentiostatic conditions in a 2 mM MnSO4/20 mM glycine solution of pH 8. No absorption related to glycine is noticed for all the compounds examined. In the spectrum of the film prepared at /0.4 V (Fig. 8a), a broad band appears in the region below 750 cm 1. This absorption can be assigned to the metal/oxygen (MO) stretching mode [14,15], which is for the formation of the Mn /O bond. The small absorption around 1080 cm 1 is attributable to the bending vibration of H /O MO [14,16], suggesting the presence of surface hydroxide in the film. The peak observed at 1630 cm 1 is due to structural water. The film prepared at /1.2 V (Fig. 8b) provides a larger Mn/O absorption than that of the film at /0.4 V. Fig. 8c shows the spectrum of the film which was first deposited at /0.4 V and cycled between /0.2 and /0.8 V in a Na-borate solution. In this spectrum, the intensity of the Mn /O band relative to the absorptions at 1630 and 1080 cm 1 is larger than that of the as-deposited film (Fig. 8a). This is probably because the MnII(H2O)6
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MnII(H2O)6 is released from the film at a negative potential where the MnIII oxide is reduced. As described above, the release of such a cationic species during the reduction of the film cannot be explained by ion transport relating to the redox reaction of the film. Hence, it is reasonable to consider that MnIII oxide is reduced to MnII hydroxide by the injection of proton and electron in a way similar to usual manganese oxides [1], and this causes a release of MnII(H2O)6 and H2O. II MnIII 2 O6 [Mn (H2 O)6 ]3 8H 2e
0 2MnII (OH)2 2H2 O3MnII (H2 O)6
Fig. 8. FTIR spectra of the manganese oxide films in KBr. The films were deposited potentiostatically at /0.4 V (a, c) and /1.2 (b) V in a 2 mM MnSO4/20 mM glycine solution of pH 8, and the film prepared at /0.4 V was subjected to potential cycling between /0.2 and /0.8 V in a Na-borate solution at a scan rate of 10 mV s 1 (c).
incorporated in the film prepared at /0.4 V was oxidized to form the MnIII /O bond at the positive potential.
(3)
Based on this equation, the mass decrease per mole of electrons is calculated to be 262 g, being similar to the observed molar mass ( /280 g per mol of electrons) at less negative potentials than 0.08 V in the cathodic process (Fig. 4b). The decrease in apparent molar mass at potentials more negative than 0.08 V is probably related to the involvement of a small amount of MnII(H2O)6 in the film. That is, it is suggested that reaction (3) takes place in the outer part of the deposited film, while the inner part is not completely transformed to MnII(OH)2, because the diffusion of H , H2O or MnII(H2O)6 should be more difficult. The apparent molar mass ( /170 g per mol of electrons) at potentials more negative than 0.08 V is close to MnII(H2O)6 (M.W., 162) per mole of electrons, and thus the following reduction scheme can be given. II MnIII 2 O6 [Mn (H2 O)6 ]3 4H 2e
0 2MnII O(OH)×MnII (H2 O)6 2MnII (H2 O)6
(4)
3.3. Mechanism of the film deposition From the above results, it is revealed that the oxidation of MnII /glycine complex at a positive potential less than /0.6 V yields a thin film consisting of MnIII oxide and MnII(H2O)6. Taking into account the film being electrically neutral, the deposited substance can be represented in the simplified formula of II MnIII 2 O2x [Mn (H2 O)6 ]2x 3 and is considered to be formed according to the following reaction from MnII /glycine (Gly ) complex. (2x 1)MnII (Gly )n (14x 18)H2 O II 0 MnIII 2 O2x [Mn (H2 O)6 ]2x 3 4xH
(2x 1)nGly 2e II (/MnIII 2 O6 [Mn (H2 O)6 ]3 ;
(2)
when x is 3 MW, 694), the molar mass expected for this reaction is estimated to be 347 g per mol of electrons, and becomes close to the value observed in Fig. 4a (315 g per mol of electrons). This formula corresponds to a structure where MnII in II Mn5O6 (MnIII 2 Mn3 O6 ) is not covalently bound to oxygen atoms in the MnIII /O network but coordinated with water molecules.
4. Conclusions MnII /glycine complex was oxidized at a less positive potential than /0.6 V to form a thin film of a mixture of MnIII oxide and MnII(H2O)6, which can be represented II as MnIII 2 O6 [Mn (H2 O)6 ]3 : This formula corresponds to a II structure where MnII sites in Mn5O6 (MnIII 2 Mn3 O6 ) are coordinated with water molecules, and not bound to oxygen atoms in the MnIII /O network. MnII(H2O)6 within the initially deposited film is expelled from the film during the reduction of the MnIII oxide to MnII hydroxide. On the other hand, at more positive potentials than /0.6 V, the incorporated MnII(H2O)6 is suggested to be oxidized to form the MnIII /O bond.
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[8] J.C. Vedrine, in: F. Delannay (Ed.), Characterisation of Heterogeneous Catalysts, Dekker, New York, 1984, p. 161. [9] M.S. Seehra, G. Srinivasan, J. Appl. Phys. 53 (1982) 8345. [10] T.I. Barry, L.A. Lay, Nature 208 (1965) 1312. [11] G. Sauerbrey, Z. Phys. 155 (1959) 206. [12] B.M. Kozyrev, Disc. Faraday Soc. 19 (1955) 135. [13] G. Brouet, X. Chen, C.W. Lee, L. Kevan, J. Am. Chem. Soc. 114 (1992) 3720. [14] J. Fitzpatrick, L.A. Maclean, D.A.J. Swinkels, F.L. Tye, J. Appl. Electrochem. 27 (1997) 243. [15] S. Peulon, D. Lincot, J. Electrochem. Soc. 145 (1998) 864. [16] O.K. Srivastava, E.A. Secco, Can. J. Chem. 45 (1967) 585.