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Electrochemical Synthesis of Layered Manganese Oxides Intercalated with Tetraalkylammonium Ions Masaharu Nakayama,* Sayaka Konishi, Hiroki Tagashira, and Kotaro Ogura Department of Applied Chemistry, Yamaguchi University, Tokiwadai 2-16-1, Ube 755-8611, Japan Received July 21, 2004. In Final Form: October 14, 2004 Thin films of birnessite-type layered manganese oxides with various interlayer spacings have been prepared on a platinum electrode by a one-step electrochemical procedure. The process involves a potentiostatic oxidation of aqueous Mn2+ ions at around +1.0 V (Ag/AgCl) in the presence of tetraalkylammonium cations with different alkyl chain lengths. X-ray diffraction indicates that the films deposited with tetrabutylammonium (TBA), tetrapropylammonium (TPA), and tetraethylammonium (TEA) ions are composed of a single phase where unhydrated tetraalkylammonium ions are accommodated as a monolayer between manganese oxide layers. The interlayer spacing of the products increases in an order of TEA < TPA < TBA. The film deposited with tetramethylammonium (TMA) is a mixture of two phases relating to hydrated and unhydrated guest cations, the former being predominant probably as a result of less hydrophobic property of TMA compared to that of other tetraalkylammonium ions. The TBA+-intercalated Mn oxide film-coated electrode exhibits a good charge/discharge property in a KCl solution between 0 and +0.8 V. In this case, TBA+ ions between the Mn oxide layers are rapidly replaced with K+ in solution by ion exchange, accompanying a shrinkage of the interlayer. The incorporated K+ ions as well as protons play an important role in the electrochemical conversion between Mn4+ and Mn3+ in the oxide layer. In the TBACl solution, the interlayer TBA+ ions can be excluded electrochemically during the positive-going scan, concomitant with the oxidation of Mn3+ sites. This causes an anodic current and the accompanying shrinkage of the interlayer. On the reverse scan, however, the compressed interlayer does not allow the incorporation of bulky TBA+ ions from the electrolyte, with virtually no cathodic current observed. Such an obvious difference in electrochemical behavior between the two electrolytes can be recognized by considering that most of the Mn oxide surface is present inside the layered structure, not on the external surface. This indicates that the layered structure is formed over the entire film.
Introduction Manganese oxides with various valence states and crystalline structures including spinel, layered, and onedimensional tunnel have been extensively studied on their synthesis, structures, and physicochemical properties. As with other electroactive transition metal oxides, manganese oxide stores electrical charge by simultaneous injection of electrons and charge-compensating cations into the solid and are, therefore, potentially useful for charge storage applications such as cathodes in secondary lithium batteries, electrochromic devices, and recently electrochemical supercapacitors in aqueous electrolytes.1-5 Charge storage properties of transition metal oxides are closely related to electrical conductivity in the solid phase and ionic transport within the pores.6,7 In this regard, layered manganese oxides possessing bicontinuous networks of solid and pore are attractive candidates for application as active electrode materials. Birnessite has a two-dimensional layered structure that consists of edge* Author to whom correspondence should be addressed. Tel.: +81-836-85-9223. Fax: +818-836-85-9201. E-mail: nkymm@ yamaguchi-u.ac.jp. (1) Lee, H. Y.; Goodenough, J. B. J. Solid State Chem. 1999, 144, 220. (2) Lee, H. Y.; Kim, S. W.; Lee, H. Y. Electrochem. Solid-State Lett. 2001, 4, A19. (3) Xu, J. J.; Yang, J.; Jain, G. Electrochem. Solid-State Lett. 2002, 5, A223. (4) Toupin, M.; Brousse, T.; Be´langer, D. Chem. Mater. 2002, 14, 3946. (5) Kim, H.; Popov, B. N. J. Electrochem. Soc. 2003, 150, D56. (6) Owens, B. B.; Passerini, S.; Smyrl, W. H. Electrochim. Acta 1999, 30, 215. (7) Long, J. W.; Stroud, R. M.; Rolison, D. R. J. Non-Cryst. Solids 2001, 285, 288.
shared MnO6 octahedra with water molecules and alkali metal cations or protons occupying the interlayer space. The interlayer spacing is typically about 0.7 nm.8-10 A number of procedures have been developed for the synthesis of birnessite, such as oxidation of Mn2+,8 reduction of MnO4-,9 and redox reactions between these two species.10 Furthermore, birnessite-type Mn oxides with larger interlayer space have been produced by using organic ammonium ions, where organic cations were intercalated between the layers of synthetic birnessite by an ion-exchange mechanism. For example, the intercalation of (C4H9)4N+ and (C12H25)NMe3+ ions led to the formation of layered oxides with interlayer distances of 1.28 and 2.41 nm, respectively.11 Several previous reports have pointed out that the interlayer spacing of birnessite with alkylammonium ions varies depending not only on the type of guest cations but also on the synthetic routes and drying conditions because they markedly influence the distribution of water molecules and cations.12-15 On the other hand, birnessite has a large discharge capacity as electrode materials in rechargeable lithium bat(8) Golden, D. C.; Chen, C. C.; Dixon, J. B. Clays Clay Miner. 1987, 35, 271. (9) Ching, S.; Petrovay, D. J.; Jorgensen, M. L.; Suib, S. L. Inorg. Chem. 1997, 36, 883. (10) Shen, Y. F.; Suib, S. L.; O’Young, C. L. J. Am. Chem. Soc. 1994, 116, 11020. (11) Luo, J.; Suib, S. L. Chem. Commun. 1997, 1031. (12) Brock, S. L.; Sanabria, M.; Suib, S. L.; Urban, V.; Thiyagarajan, P.; Potter, D. I. J. Phys. Chem. B 1999, 103, 7416. (13) Liu, Z.; Ooi, K.; Kanoh, H.; Tang, W.; Tomida, T. Langmuir 2000, 16, 4154. (14) Gao, Q.; Giraldo, O.; Tong, W.; Suib, S. L. Chem. Mater. 2001, 13, 778. (15) Liu, Z.; Wang, Z.; Yang, X.; Ooi, K. Langmuir 2002, 18, 4926.
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teries.16-18 In such cases, birnessite powder was mixed with acetylene black as a conductive additive and poly(tetrafluoroethylene) as a binder, and then the mixture was kneaded to form an electrode pellet. Thin films are the desirable form of metal oxides for use in electronic, optical, magnetic, or molecular recognition applications.19 Long et al. have found that sol-gelderived thin films of Na+-birnessite (birnessite with Na+ ions in the interlayer) cast on a transparent glass electrode showed high electrochromic efficiency as a result of the facile mobility of the interlayer cations.20,21 Among various techniques for the fabrication of thin metal oxide films, electrochemical deposition is one of the most promising approaches for obtaining thin and uniform films on substrates of complex shape with a high degree of reproducibility.22 Thin conducting films adhered well to a substrate need no binders and conductive additives in their electrochemical use. In addition, one can precisely control the film thickness by controlling the delivered electrical charge. However, no studies had been published on the electrochemical formation of layered manganese oxides until we first reported it in a short communication.23 On the basis of X-ray diffraction (XRD) analysis, the products were identified as birnessite-type layered Mn oxides intercalated with potassium, tetraethylammonium (TEA), and tetrabutylammonium (TBA) ions. In this paper, we describe in detail the electrochemical synthesis of layered manganese oxides using a series of quaternary ammonium salts with different methylene chain lengths. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) as well as XRD were used to characterize the products. Ion-exchange and electrochemical properties of the film intercalated with TBA ions were also investigated. Experimental Section Materials. All chemicals were of reagent grade and were used without further purification. Tetrabutylammonium chloride (TBACl, 99%) was obtained from Fluka. Tetrapropylammonium chloride (TPACl, 98%) was obtained from Wako Pure Chemicals. Tetraethylammonium chloride (TEACl, 99%) was obtained from Tokyo Kasei. Tetramethylammonium chloride (TMACl, 98%) was obtained from Aldrich. All solutions were prepared with doubly distilled water and deoxygenated by bubbling purified nitrogen gas just prior to use. Preparation of Manganese Oxide Films. All electrochemical experiments were carried out in a standard three-electrode glass cell. A Hokuto Denko HA-301 potentiostat and a Hokuto Denko HB-104 function generator were used to control the electrode potential. Electrodeposition of manganese oxide films was performed potentiostatically in aqueous MnSO4 (2 mM) solutions containing various ammonium chlorides at a concentration of 50 mM. A polycrystalline platinum foil (1.0 × 1.0 cm; thickness 0.5 mm; Niraco) was used as the working electrode to fabricate the films on it. Before electrodeposion, the electrode surface was ultrasonically cleaned in diluted HCl solution for 5 min and then rinsed thoroughly with distilled water. A platinum sheet (1.5 × 1.5 × 0.05 cm) and a Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The distance (16) Bach, S.; Pereira-Ramos, J.; Cachet, C.; Bode, M.; Yu, L. T. Electrochim. Acta 1995, 40, 785. (17) Bach, S.; Pereira-Ramos, J. P.; Baffier, N. J. Electrochem. Soc. 1996, 143, 3429. (18) Tsuda, M.; Arai, H.; Sakurai, Y. J. Power Sources 2002, 110, 52. (19) Giraldo, O.; Brock, S. L.; Willis, W. S.; Marquez, M.; Suib, S. L. J. Am. Chem. Soc. 2000, 122, 9330. (20) Long, J. W.; Qadir, L. R.; Stroud, R. M.; Rolison, D. R. J. Phys. Chem. B 2001, 105, 8712. (21) Long, J. W.; Young, A. L.; Rolison, D. R. J. Electrochem. Soc. 2003, 150, A1161. (22) Therese, G. H. A.; Kamath, P. V. Chem. Mater. 2000, 12, 1195. (23) Nakayama, M.; Konishi, S.; Tanaka, A.; Ogura, K. Chem. Lett. 2004, 33, 670.
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Figure 1. XRD patterns of Mn oxide films deposited on a Pt electrode from 2 mM MnSO4 solutions containing (a) NH4Cl, (b) TMACl, (c) TEACl, (d) TPACl, and (e) TBACl at a concentration of 50 mM. The films were prepared by applying a constant potential of +1.0 V for 30 min. between the working electrode and the counter electrode was fixed at 2 cm. A constant potential (+0.9 to +1.1 V) was applied to the working electrode for a certain period of time (10-120 min). Most experiments were carried out by applying +1.0 V for 30 min. After electrodeposition, the resulting electrodes coated with manganese oxide films were rinsed immediately with copious amounts of water, dried under a vacuum for at least 2 h, and then subjected to XRD, XPS, field-emission scanning electron microscopy (FE-SEM), and cyclic voltammetry. Characterization. XRD patterns were recorded using a Shimadzu XD-D1 diffractometer with Cu KR radiation (λ ) 0.154 05 nm). The data were collected in the 2θ range from 5 to 60° with a scan rate of 1° min-1. The beam voltage was 30 kV, and the beam current was 30 mA. 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). Wideand narrow-range 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: Mn(2p3/2) ) 9.17, N(1s) ) 1.80, and K(2p3/2) ) 2.62. FE-SEM images were obtained at an accelerating voltage of 20 kV with a Hitachi S-4700Y scanning electron microscope. Samples were studied without overcoating with an electronic conductor. Cyclic voltammograms (CVs) were acquired in 0.1 M KCl and 0.1 M TBACl solutions between 0 and 0.8 V at a scan rate of 20 mV s-1.
Results and Discussion Characterization of Electrodeposited Films. Figure 1 shows XRD patterns of the manganese oxide films on a Pt electrode prepared using various ammonium salts. In the presence of NH4Cl (Figure 1a), two peaks are observed at 2θ ) 12.3 and 24.7°, along with that due to the Pt substrate. According to Bragg’s equation, these 2θ values correspond to d spacings of 0.72 and 0.36 nm, which can be indexed to (001) and (002) reflections of a layered structure, respectively. This pattern with an interlayer distance of 0.72 nm is consistent with that typically observed for bulk birnessite with hydrated small cations (Na+, K+, H+, etc.).8 The ammonium ionic radius (0.143
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Figure 2. Plots of the d001 spacings obtained in Figure 1 versus the dimensions of alkylammonium ions from the literature data.26
nm) is as small as that of potassium (0.133 nm),24 and the interlayer spacing is maintained by water molecules. All films except for that prepared with tetramethylammonium (TMA) show the patterns arising from a single phase consisting of the (00l) reflections. In contrast, the pattern of the film with TMA appears to be composed of two phases, one consisting of two peaks at 0.98 and 0.49 nm and the other at 0.75 and 0.37 nm. With the exception of the 0.98-nm phase in the TMA film, the set of diffraction lines is found to shift to lower angles and correspondingly larger d spacings with an increase in the size of the organic cations used. The patterns of the films with TEA and tetrapropylammonium (TPA) could not be indexed to any known structure and are, therefore, not easily assignable. On the other hand, the pattern observed for the TBA film agrees well with that of the TBA+-intercalated birnessite sample prepared by Omomo et al. through an ion-exchange reaction which involves the equilibration of synthesized proton birnessite in an aqueous TBAOH solution.25 They explained that the obtained interlayer spacing (1.25 nm) corresponds to the accommodation of one molecular layer of unhydrated TBA+ ions with the C2 axis normal to the plane of Mn oxide layers. In Figure 2, the interlayer spacings (d001) observed for the electrodeposited films were plotted against the dimensions of the corresponding alkylammonium ions from the literature data.26 In this figure, plots for the films prepared with TBA, TPA, and TEA and the 0.75-nm plot for the TMA sample can be fitted to a straight line. This enables us to assume that the unknown patterns can be assigned in the same manner as the TBA-incorporated product. The other plot for the TMA film (0.98 nm), which deviates from the straight line, can be ascribed to hydration of TMA+ ions probably due to the less hydrophobic property of TMA in comparison to other alkylammonium ions. Figure 3 displays XRD patterns of the films prepared in the presence of TBA at different applied potentials. Diffraction lines indicative of a layered structure appear in the potential region from +0.95 to +1.05 V. To gain more information on the crystalline growth, XPS analysis was made for the films prepared at +0.9, +1.0, and +1.1 V. Figure 4 shows XPS spectra for the Mn(2p), N(1s), and O(1s) core electronic transitions, where all spectra were normalized in intensity with respect to the Mn(2p) peaks to allow easier comparisons. The Mn(2p) region consists (24) Graf, E.; Kintzinger, J. P.; Lehn, J. M.; LeMoigne, J. J. Am. Chem. Soc. 1982, 104, 1672. (25) Omomo, Y.; Sasaki, T.; Wang, L.; Watanabe, M. J. Am. Chem. Soc. 2003, 125, 3568. (26) Nikam, P. S.; Nikumbh, A. B. J. Chem. Eng. Data 2002, 47, 400.
Figure 3. XRD patterns of Mn oxide films on a Pt electrode prepared from a 2 mM MnSO4 and 50 mM TBACl solution by applying constant potentials of (a) +0.9, (b) +0.95, (c) +1.0, (d) +1.05, and (e) +1.1 V for 30 min.
Figure 4. XPS spectra in the Mn(2p), N(1s), and O(1s) regions of Mn oxide films on a Pt electrode prepared from a 2 mM MnSO4 and 50 mM TBACl solution by applying constant potentials of (a) +0.9, (b) +1.0, and (c) +1.1 V for 30 min.
of a spin-orbit doublet attributed to the 2p1/2 and 2p3/2 states around 654 and 642 eV. BE values of Mn(2p3/2) peaks are often used to estimate the oxidation state of manganese.27-29 According to the literature, the BE values of 642 eV are generally assigned to Mn4+, 641 eV to Mn3+, and those around 640 eV to Mn2+. All the Mn(2p3/2) peaks in Figure 4 are located at about 642 eV. Hence, all the films are likely to be composed largely of Mn4+, though the limited resolution of the XPS instrument prevents the precise determination of manganese oxidation states. A slight peak shift to the lower BE side observed at +0.9 V may account for a higher fraction of Mn3+. The N(1s) core-level spectra show a peak at 401.7 eV, which can be attributed to positively charged nitrogen arising from alkylammonium.30 The absence of the nitrogen peak at the lower potential (+0.9 V) indicates that the TBA+ ions are not incorporated into the film, resulting in the formation of an amorphous product. On (27) Hasemi, T.; Brinkman, A. W. J. Mater. Res. 1992, 7, 1278. (28) Aronson, B. J.; Blanford, C. F.; Stein, A. J. Phys. Chem. B 2000, 104, 449. (29) Nakayama, M.; Tanaka, A.; Konishi, S.; Ogura, K. J. Mater. Res. 2004, 19, 1509. (30) Lvov, Y.; Munge, B.; Giraldo, O.; Ichinose, I.; Suib, S. L.; Rusling, J. F. Langmuir 2000, 16, 8850.
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Figure 6. SEM images of a Mn oxide film on a Pt electrode prepared from a 2 mM MnSO4 and 50 mM TBACl solution by applying a constant potential of +1.0 V for 30 min.
Figure 5. (A) XRD patterns of Mn oxide films on a Pt electrode prepared from a 2 mM MnSO4 and 50 mM TBACl solution by applying a constant potential of +1.0 V for (a) 10, (b) 30, (c) 60, and (d) 120 min. (B) Plots of the intensity of the (001) peak and the charge density passed during deposition versus electrolysis time.
the other hand, the films deposited at +1.0 and +1.1 V demonstrate the incorporation of TBA. Wide-range XPS spectra of the films gave no peaks in the Cl(2p) (around 200 eV) region (not shown), confirming that the incorporated TBA exists in a cation form and not as a chloride salt. From the peak area ratios and the appropriate sensitivities, the TBA/Mn molar ratios were calculated to be 0.35 and 0.37 for the films prepared at +1.0 and +1.1 V, respectively. In contrast to the clear difference in their crystallinity (Figure 3c,e), there is no significant change in their TBA/Mn ratios. This means that the incorporation quantity of TBA+ ions is not the key factor causing the loss in crystallinity at +1.1 V. Application of more positive potentials should induce a faster deposition rate of Mn oxide, which may inhibit self-assembly to construct a layered structure. In the O(1s) region, three different constituents are observed at 529.6-529.9, 531.0-531.3, and 532.8-533.2 eV, which can be assigned to oxide (Mn-O-Mn), hydroxide (Mn-OH), and structural water, respectively.31 The area contributions of O2- and OH- for the layered product (curve b) are much larger and smaller, respectively, than those for the amorphous films (curves a and c). The surface of metal oxides is readily adsorbed by water in air, and the dissociative chemisorption of water leads to a high surface concentration of hydroxide.32,33 Thus, the high degree of O2- content in the crystalline product can be recognized (31) Chigane, M.; Ishikawa, M. J. Electrochem. Soc. 2000, 147, 2246.
by considering that most of the Mn oxide surface is associated with hydrophobic TBA+ cations. This indicates that the layered structure is formed over the entire film. Figure 5A shows diffraction patterns of the films prepared with TBA by applying a constant potential of +1.0 V for different periods of time. In Figure 5B, the intensity of the (001) peak and the charge passed during the electrodeposition process were plotted against the electrolysis time. It is apparent from both figures that the peak intensity increases with an increase in the delivered charge while maintaining the diffraction pattern. Furthermore, the peak due to the fourth-order reflection can be seen at a longer time. These results confirm that the bulk of the electrodeposited film is composed of manganese oxide layers and the crystalline growth proceeds electrochemically. The surface morphology of the manganese oxide film deposited with TBA was investigated by SEM and is shown in Figure 6. The film thickness was estimated to be ∼1.5 µm, which corresponds to a 30-min electrodeposition (174 mC cm-2). The electrode surface was entirely covered with a thin and uniform film. The higher magnification image shows a sheetlike morphology and no other kinds of crystallized phase particles, which may be regarded as a result of the growth of layer structure on the polycrystalline Pt substrate. All the above data support the electrochemical formation of birnessite-type Mn oxides in a thin film form, whose thickness can be regulated by controlling the applied electrical charge. Birnessite is denoted as a general formula of AxMnO2‚nH2O, in which A represents a cation. On the basis of an assumption that the electroformation of the birnessite-type oxide films takes place similarly to that of the electrolytic manganese dioxide, a form of γ-MnO2,34 the deposition process can be expressed as follows.
Mn2+ + 2H2O + xA+ f AxMn3+xMn4+1-xO2 + 4H+ + (2 - x)e- (1) Taking into account the TBA/Mn ratio (0.35) and the slight contribution of hydroxide (OH-/O2- ) 0.05) from the XPS data, the composition of the layered product can be estimated to be TBA0.35Mn3+0.35Mn4+0.65O1.95(OH)0.05. This leads to an average Mn valence of 3.7, which is similar to that typically observed for bulk birnessite.9 In this reaction, the role of the guest cation is to compensate negative charges on the Mn oxide layers generated electrochemically. (32) McIntyre, N. S.; Zetaruk, D. G. Anal. Chem. 1977, 49, 1521. (33) Nakayama, M.; Ii, T.; Komatsu, H.; Ogura, K. Chem. Commun. 2004, 1098. (34) Voinov, M. Electrochim. Acta 1982, 27, 833.
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Figure 7. -intercalated Mn oxide film on a Pt electrode after immersing in a 0.001 M KCl solution for (a) 0, (b) 0.25, (c) 1, (d) 10, (e) 60, and (f) 720 min. The film was prepared the same way as that in Figure 6.
Ion-Exchange and Electrochemical Properties of the TBA+-Intercalated Birnessite Oxide Film. A TBA+-intercalated birnessite oxide film coated on a Pt electrode was immersed in a 0.001 M KCl solution for given periods of time, and the dried sample was subjected to XRD analysis. The patterns obtained at different immersion times are shown in Figure 7. A set of three peaks due to the TBA+-intercalated phase rapidly decreases in intensity with increasing time, disappearing at 10 min and being replaced with two peaks at 2θ ) 12.3 and 24.7°. This pattern with an interlayer spacing of 0.72 nm can be attributed to birnessite with hydrated potassium ions, indicating that the layered structure is compressed after replacement of the large TBA+ ions with K+ ions. Interestingly, we cannot find any mixture of the individual patterns from the TBA+- and K+-intercalated phases, which is likely to reflect the uniformity of the electrodeposited film. Electrochemical studies on the TBA+-birnessite filmcoated electrode were carried out. Figure 8 illustrates the comparison between voltammetric responses during the first 20 cycles in aqueous electrolytes of KCl and TBACl, where both CVs were taken immediately after the electrode being immersed. In a KCl solution (Figure 8a), the current response gradually increases during initial cycling. As expected from Figure 7, this increase suggests the incorporation of K+ ions from the electrolyte solution by ion exchange and the involvement of the interlayer K+ ions in the electrochemical reactions of the film. At the latter cycles, the voltammetric response is rather capacitive, with a very broad reversible peak around +0.45 V observed, and exhibits no defined reduction or oxidation peaks. This type of response involves continuous change of the oxidation state of manganese following the anodic and cathodic scanning.35 In the potential region examined, the conversion between Mn4+ and Mn3+ can take place. As shown in Figure 8b, the CV in the TBACl solution is quite different. An anodic current appears from +0.5 (35) Conway, B. E. J. Electrochem. Soc. 1991, 138, 1539.
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Figure 8. CVs of a TBA+-intercalated Mn oxide film-coated Pt electrode in 0.1 M (a) KCl and (b) TBACl solutions. Scan rate, 20 mV s-1. The film was prepared the same way as that in Figure 6.
Figure 9. (A) XRD and (B) XPS data of a TBA+-intercalated Mn oxide film on a Pt electrode taken after 20 potential cycles between 0 and +0.8 V in 0.1 M (a) KCl and (b) TBACl solutions. The potential cycling was stopped at +0.8 V. The film was prepared the same way as that in Figure 6.
V and decreases in intensity with increasing the number of cycles. The reverse scan induces virtually no cathodic current. XRD and XPS analyses were carried out for the films obtained after the 20 potential cycles in KCl and TBACl solutions (Figure 9). In both films, we can see the same
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Figure 10. XPS spectra in (a) Mn(2p), (b) K(2p), and (c) O(1s) regions of the Mn oxide film-coated electrode taken after being poralized at +0.8 and 0 V for 20 min following the 20 potential cycles in a 0.1 M KCl solution. The film was prepared the same way as that in Figure 6.
diffraction pattern typical of birnessite with an interlayer spacing of 0.72 nm, corresponding to the interlayer shrinkage due to the removal of large TBA+ ions in both solutions while maintaining the layered structure upon potential cycling. As shown in Figure 9B(a), the N(1s) peak disappears completely with a concomitant occurrence of new K(2s) and (2p) signals, confirming that TBA+ cations are quantitatively replaced by K+. Hence, the appearance of the C(1s) peak is attributed to contaminated hydrocarbon, not arising from TBA. A very weak N(1s) signal in Figure 9B(b) indicates the loss of the intercalated TBA+ ions. Because no ion exchange occurs in a TBA solution, the decrease in the TBA content suggests that the interlayer TBA+ ions can be excluded electrochemically to maintain electroneutrality within the film upon oxidation. As seen from Figure 9A(b), the electrochemical TBA+ deintercalation is accompanied by the shrinkage of the layered structure and the incorporation of H2O. The compressed interlayer can no longer accommodate bulky TBA+ ions from the electrolyte during the negative-going scan. This is the reason no cathodic current was observed in Figure 8b. Such an obvious difference in electrochemical behavior between the two electrolytes suggests that most of the electroactive surface in our film is present inside the layered structure, not on the external surface. To better understand the CV response in KCl, further XPS experiments were conducted for the films polarized at oxidizing (+0.8 V) and reducing (0 V) potentials for 20 min following the 20 potential cycles. The resulting spectra are displayed in Figure 10. The oxidized film shows the Mn(2p3/2) peak which is almost the same as that of the as-deposited film (Figure 4). However, the reduced film exhibits a small but distinct shoulder around 241 eV, probably because of an increase in the Mn3+ content upon reduction. On the other hand, doublet peaks appearing at 295.3 and 292.5 eV are assigned to K(2p1/2) and (2p3/2) states, respectively, being larger for the reduced film (K/ Mn ) 0.38) than for the oxidized one (K/Mn ) 0.23). In
the O(1s) region (Figure 10c), it is noteworthy that the contribution of OH- species becomes larger in the reduced film. This indicates that a certain portion of oxides are associated with protons from solution to form hydroxides. In general, the redox reactions of conventional Mn oxides in neutral electrolytes have been ascribed to homogeneous insertion/deinsertion of protons.36 However, the above electrochemical and spectroscopic results clearly indicate that not only protons but also alkali metal cations are related to the electrochemical reactions of the electrodeposited birnessite-type Mn oxide. Similar behavior has been observed for the Pt electrode modified with chemically prepared birnessite in a KCl solution by Ooi et al.37 They described that the electrochemical reaction proceeds based on a mechanism consisting of insertion/deinsertion of H+ and ion exchange between K+ and H+. Such an alternate mechanism may be emphasized in the nanostructured forms of Mn oxide. Conclusions We demonstrated the new electrochemical approach for the synthesis of birnessite-type layered manganese oxides with various interlayer spacings in a thin film form. XRD revealed that the interlayer spacing can be regulated by the size of the electrolyte cations used, and the film thickness is controlled by the delivered electrical charge. According to the XPS and voltammetric data, most of the manganese oxide surface is present inside the layer structure and not on the external surface. This indicates that the layered structure is formed over the entire film. Acknowledgment. This research was supported in part by the Japan Society of the Promotion of Science (No. 16750175). LA048173F (36) Ruetschi, P. J. Electrochem. Soc. 1988, 135, 2657. (37) Kanoh, H.; Tang, W.; Makita, Y.; Ooi, K. Langmuir 1997, 13, 6845.
