Electrodeposition of Layered Manganese Oxide Nanocomposites

On the contrary, the layered PAH/MnOx film showed a good electrochemical response due to the redox reaction of Mn3+/Mn4+ couple with no change in the ...
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Langmuir 2006, 22, 3864-3869

Electrodeposition of Layered Manganese Oxide Nanocomposites Intercalated with Strong and Weak Polyelectrolytes Masaharu Nakayama* and Hiroki Tagashira Department of Applied Chemistry, Yamaguchi UniVersity, 2-16-1 Tokiwadai, Ube 755-8611, Japan ReceiVed NoVember 15, 2005. In Final Form: February 24, 2006 Multilayered manganese oxide nanocomposites intercalated with strong (poly(diallyldimethylammonium) chloride, PDDA) and weak (poly(allylamine hydrochloride), PAH) polyelectrolytes can be produced on polycrystalline platinum electrode in a thin film form by a simple, one-step electrochemical route. The process involves a potentiostatic oxidation of aqueous Mn2+ ions at around +1.0 V (vs Ag/AgCl) in the presence of polyelectrolytes. Fully charged PDDA polycations are accommodated tightly in the interlayer space by electrostatic interaction with negative charges on the manganese oxide layers, leading to an interlayer distance of 0.97 nm. The layered film prepared with PAH has a larger polymer content (PAH/Mn molar ratio of 0.98) than that (PDDA/Mn molar ratio of 0.43) made with PDDA because of the smaller charging degree of PAH, exhibiting a larger interlayer distance (1.19 nm). The interlayer PAH contains neutral (-NH2) and positively charged (-NH3+) amine groups, and the -NH3+ groups are associated with Cl- (to generate -NH3+Cl- ion pairs) as well as the negatively charged manganese oxide layers. Both polyelectrolytes once incorporated were not ion exchanged with small cations in solution. The layered structure of PDDA/MnOx was collapsed during the reduction process in a KCl electrolyte solution, accompanying an expansion of the interlayer as a result of incorporation of K+ ions for charge neutrality. On the contrary, the layered PAH/MnOx film showed a good electrochemical response due to the redox reaction of Mn3+/Mn4+ couple with no change in the structure. X-ray photoelectron spectroscopy revealed that, in this case, excess negative charges generated on the manganese oxide layers upon reduction can be balanced by the protons being released from the -NH3+Cl- sites in the interlayer PAH; the Cl- anions becoming unnecessary are inevitably excluded from the interlayer, and vice versa upon oxidation.

Introduction Nanocomposites sometimes offer improved mechanical, catalytic, electronic, and optical properties that are distinct from those of the pure component phases and those of the related macro- and microcomposites. Layered nanocomposites are usually composed of an organic polymer included between layers of an inorganic host having a nanoscale repeated unit. Great effort has been made to develop synthetic routes for these materials, which include the following: (1) in situ polymerization of intercalated monomers, (2) exfoliation of a layered host and subsequent adsorption of polymer and reaggregation, (3) template synthesis of host structures in polymer-containing solutions, and (4) direct melt intercalation of polymers into hosts.1-5 Synthesis of new porous manganese oxides is the focus of intensive research due to their promising properties as adsorbents,6 oxidation catalysts,7,8 and electrodes for battery applications.9,10 These materials are most commonly prepared in the form of powder using precipitation and/or hydrothermal techniques.11-13 * To whom correspondence should be addressed. Tel: +81-836-85-9223, Fax: +81-836-85-9201. E-mail: [email protected]. (1) Lerner, M. M.; Oriakhi, C. O. Polymers in Ordered Nanocomposites. In Handbook of Nanophase Materials; Goldstein, A., Ed.; Marcel Dekker: New York, 1997; pp 199-219. (2) Giannelis, E. P. AdV. Mater. 1996, 8, 29. (3) Oriakhi, C. J. Chem. Educ. 2000, 77, 1138. (4) Carrado, K. A.; Xu, L.; Seifert, S.; Csencsits, R.; Bloomquist, C. A. A. Polymer-Clay Nanocomposites Derived from Polymer-Silicate Gels. In PolymerClay Nanocomposites; Pinnavaia, T. J., Beall, G. W., Eds.; Wiley & Sons: New York, 2000; pp 47-63. (5) Carrado, K. A. Polymer-Clay Nanocomposites. In AdVanced Polymeric Materials: Structure Properly Relationships; Advani, S. G., Shonaike, G. O., Eds.; CRC Press: Boca Raton, FL, 2003; pp 349-396. (6) Shen, Y. F.; Zerger, R. P.; DeGuzman, R. N.; Suib, S. L.; McCurdy, L.; Potter, D. I.; O’Young, C. L. Science 1993, 260, 511. (7) Cao, H.; Suib, S. L. J. Am. Chem. Soc. 1994, 116, 5334. (8) Skordilis, C. S.; Pomonis, P. J. Stud. Surf. Sci. Catal. 1995, 91, 513. (9) Bach, S.; Pereira-Ramos, J.; Cachet, C.; Bode, M.; Yu, L. T. Electrochim. Acta 1995, 40, 785. (10) Bach, S.; Pereira-Ramos, J.; Baffier, N. J. Electrochem. Soc. 1996, 143, 3429.

Birnessite has a two-dimensional layered structure that consists of edge-sharing MnO6 octahedra. Small cations such as potassium or protons are normally located in the interlayer space to balance negative charges on manganese oxide layers. Some organic ammonium ions can also be intercalated between the layers of birnessite by an ion-exchange mechanism, accompanied by an expansion of the interlayer.14-16 Layered manganese oxides, however, possess a high charge density in the interlayer, which makes it very difficult to obtain nanocomposites intercalated with bulky guest ions or polymer by a conventional ion-exchange procedure. In 2002, Liu et al. provided a breakthrough to overcome such a problem.17 They prepared a layered nanocomposite with poly(diallydimethylammonium) (PDDA) cations incorporated between manganese oxide layers by an exfoliation/reassembly process, where the slurry of exfoliated manganese oxides was soaked in a PDDA solution. On the other hand, alternate adsorption of oppositely charged macromolecules, that is, the layer-by-layer (LBL) self-assembly process, is a well-established approach to build multilayered nanocomposites.18 Sasaki et al. applied this technique to fabricate for the first time ordered multilayered films comprising manganese oxide nanosheets and PDDA ions.19 Later, Suib et al. found that the composite film consisting of PDDA and layered manganese oxide on carbon electrode made by LBL adsorption was active for electrochemical catalysis of styrene epoxidation.20 (11) Luo, J.; Zhang, Q.; Suib, S. L. Inorg. Chem. 2000, 39, 741. (12) Chen, R.; Zavilij, P.; Whittingham, M. S. Chem. Mater. 1996, 8, 1275. (13) Feng, A.; Kanoh, H.; Miyai, Y.; Ooi, K. Chem. Mater. 1995, 7, 1226. (14) Luo, J.; Suib, S. L. Chem. Commun. 1997, 1031. (15) Brock, S. L.; Sanabria, M.; Suib, S. L.; Urban, V.; Thiyagarajan, P.; Potter, D. I. J. Phys. Chem. B 1999, 103, 7416. (16) Gao, Q.; Giraldo, O.; Tong, W.; Suib, S. L. Chem. Mater. 2001, 13, 778. (17) Liu, Z.; Yang, X.; Makita, Y.; Ooi, K. Chem. Mater. 2002, 14, 4800. (18) Decher, G. Science 1997, 277, 1232. (19) Wang, L.; Omomo, Y.; Sakai, N.; Fukuda, K.; Nakai, I.; Ebina, Y.; Takada, K.; Watanabe, M.; Sasaki, T. Chem. Mater. 2003, 15, 2873. (20) Espinal, L.; Suib, S. L.; Rusling, J. F. J. Am. Chem. Soc. 2004, 126, 7676.

10.1021/la053072i CCC: $33.50 © 2006 American Chemical Society Published on Web 03/21/2006

Electrodeposition of Manganese Oxide Nanocomposites

Thin films are the desirable form of metal oxides for use in electronic, optical, magnetic, and electrochemical applications. Among various techniques for fabrication of thin metal oxide films, electrochemical deposition is one of the most promising approaches, particularly for obtaining thin and uniform films on substrates of complex shape with a high degree of reproducibility.21 Thin conducting films adhering well to a substrate need no binders or conductive additives in their electrochemical use. More importantly, one can precisely control the film thickness as desired by simply changing the delivered electrical charge. Over the past few years, nanostructured metal oxide films, including NiO,22,23 ZnO,24,25 and Cu2O,26,27 have been prepared by electrodeposition. Their growth processes are exclusively based on the cathodic polymerization of transition metal precursors in the presence of structure-directing agents. In 2004, we presented a new strategy for constructing birnessitetype layered manganese oxide materials intercalated with alkaline metal and alkylammonium ions in a thin film form.28 The process consists of a potentiostatic oxidation of aqueous manganese(II) ions in the presence of the corresponding guest ions. This methodology is simple, environmentally benign in the point of view that no heating is required, and remarkably versatile because the inorganic host can adjust itself to accommodate guest molecules during electrodeposition.29 In fact, this anodic electrochemistry could be extended with the use of a strong polyelectrolyte, PDDA, to form a multilayered nanocomposite consisting of manganese oxide and PDDA, as has been described in our short communication.30 The present paper follows up our previous study and reports on comparative investigations of the electrodeposited manganese oxide films from solutions containing strong and weak polyelectrolytes. Characterization of the products was performed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). Interestingly, the multilayered film intercalated with a weak polyelectrolyte (poly(allylamine hydrochloride), PAH) exhibited a good electrochemical response, although the layered film with PDDA was electrochemically inactive. Experimental Section Materials. Poly(diallyldimethylammonium chloride) (PDDA; Aldrich, Mw ) 400000-500000) and poly(allylamine hydrochloride) (PAH; Alfa Aesar, Mw ) 50000-65000) were commercially available and used without any further purification. PDDA was obtained as a 20 wt % aqueous solution, while PAH was supplied in the form of a powder. The corresponding molecular structures are shown in Figure 1. All other chemicals were of reagent grade and used as received. 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. Electrodeposition. Electrochemical experiments were conducted using a conventional three-electrode system in an undivided glass cell. A platinum sheet and a Ag/AgCl electrode (in saturated KCl) (21) Therese, G. H. A.; Kamath, P. V. Chem. Mater. 2000, 12, 1195. (22) Nelson, P. A.; Elliott, J. M.; Attard, G. S.; Owen, J. R. Chem. Mater. 2002, 14, 524. (23) Tan, Y.; Srinivasan, S.; Choi, K. J. Am. Chem. Soc. 2005, 127, 3596. (24) Yoshida, T.; Terada, K.; Schlettwein, D.; Oekermann, T.; Sugiura, T.; Minoura, H. AdV. Mater. 2000, 12, 1214. (25) Yoshida, T.; Minoura, H. AdV. Mater. 2000, 12, 1219. (26) Choi, K.; Lichtenegger, H. C.; Stucky, G. D. J. Am. Chem. Soc. 2002, 124, 12402. (27) Luo, H.; Zhang, J.; Yan, Y. Chem. Mater. 2003, 15, 3769. (28) Nakayama, M.; Konishi, S.; Tanaka, A.; Ogura, K. Chem. Lett. 2004, 33, 670. (29) Nakayama, M.; Konishi, S.; Tagashira, H.; Ogura, K. Langmuir 2005, 21, 354. (30) Nakayama, M.; Tagashira, H.; Konishi, S.; Ogura, K. Inorg. Chem. 2004, 43, 8215.

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Figure 1. Chemical structures of the monomer units of poly(diallyldimethylammonium chloride) (PDDA) and poly(allylamine hydrochloride) (PAH). were used as the counter and reference electrodes, respectively. A Hokuto Denko HA-301 potentiostat connected to a Hokuto Denko HB-104 function generator was used to control the electrode potential. Polycrystalline platinum foil (Niraco, 99.98%) of a 1 cm2 surface area was used as the working electrode to fabricate the films on it. Prior to electrodeposition, the electrode surface was ultrasonically cleaned in diluted HCl solution for 10 min and then rinsed thoroughly with distilled water. The deposition baths used consisted of 2 mM MnSO4 aqueous solutions mixed with 1-56 mM PDDA or 1-400 mM PAH. The pH was in the ranges of 5.0-3.9 and 5.2-3.3 for PDDA- and PAH-containing solutions, respectively. No buffers were added to the deposition solution to adjust the pH value. A constant potential was applied to the working electrode for a certain period of time while monitoring the charge delivered. Most experiments were carried out by applying +1.0 V with the same charge (330 mC/cm2). After electrodeposition, the resulting film coated on a Pt 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. XRD patterns were recorded using a Shimadzu XD-D1 diffractometer with Cu KR radiation (λ ) 0.15405 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). Wide- and narrowrange spectra were collected with a pass energy of 20 eV and channel widths 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 Lorenzian functions, while background subtraction was done according to the Shirley method. Semiquantitative estimates of the relative atomic concentrations were obtained from the peak area ratios by considering the appropriate sensitivity factors: Mn 2p1/2 ) 4.74, N 1s ) 1.80, and Cl 2p3/2 )1.51. Electrochemical Characterization. The Pt electrode coated with the as-deposited manganese oxide film was rinsed with water and transferred to a 0.1 M KCl solution. Cyclic voltammetry was performed in a potential region between 0 and +0.8 V at a scan rate of 20 mV/s. In Situ UV-vis Spectroscopy. In situ UV-vis spectra were recorded on a Shimadzu UV2400PC spectrometer with a quartz cuvette (10-mm path length) as the electrochemical cell. The working electrode (indium tin oxide (ITO) coated with the manganese oxide film) was placed in the optical path. The reference and counter electrodes were Ag/AgCl and Pt, respectively.

Results and Discussion Characterization of As-Deposited Films. Figure 2 shows XRD patterns of the films obtained from MnSO4 solutions mixed with PDDA (a) and PAH (b) at different concentrations. The electric charge delivered during electrodeposition was always 330 mC cm-2. In Figure 2a, we can see a series of diffraction peaks at 2θ ) 9.1, 18.2, and 27.3°, which are evenly spaced by 9.1°. These can be indexed to the 00l reflections (l ) 1, 2, and 3) for a layered structure, as has commonly done for birnessitetype manganese oxides.31,32 According to Bragg’s equation, the obtained 2θ values indicate d-spacings of 0.97, 0.49, and 0.33 (31) Golden, D. C.; Chen, C. C.; Dixon, J. B. Clays Clay Miner. 1987, 35, 271. (32) Wang, S. T.; Cheng. S. Inorg. Chem. 1992, 31, 1165.

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Figure 3. XRD patterns of manganese oxide films on a Pt electrode prepared from a 2 mM MnSO4 and 200 mM PAH solution by applying a constant potential of +1.0 V for indicated periods of time.

Figure 2. XRD patterns of manganese oxide films on a Pt electrode prepared by applying a constant potential of +1.0 V in 2 mM MnSO4 solutions mixed with (a) PDDA and (b) PAH at indicated concentrations. The electric charge passed during electrolysis was always 330 mC cm-2.

nm, respectively. The d001 value (0.97 nm) corresponds to the interlayer distance and is close to that (0.92 nm) observed by Sasaki et al. for a multilayer ultrathin film consisting of MnO2 nanosheets and PDDA ions.19 They attributed this repeating periodicity to the accommodation of a monolayer of PDDA in parallel arrangement between manganese oxide layers, on the basis of the crystallographic thickness of a MnO2 nanosheet (0.45 nm)33 and that of a PDDA monolayer (0.5 nm).34 Therefore, PDDA in our product fits tightly into the interlayer space. The intensity of diffraction peaks increases with an increase in the starting PDDA concentration until reaching a maximum at 5.6 mM, while their positions remain unchanged. The intensity decrease at the higher concentration can be accounted for by an increase in hydrophobic interaction between the PDDA molecules, which relatively weakens the electrostatic force between the polycations and negative charges on the manganese oxide deposited. The quality of the PDDA-containing films was varied also depending on the electrode potential.30 In Figure 2b, the film obtained with PAH also provides a similar series of evenly spaced (in 2θ) diffraction peaks. Although no assignment could be made on the basis of previous literatures, these are indexable to the 001, 002, and 003 reflections, similarly to the PDDA-incorporated product. The largest pattern is observed at 200 mM, in which each peak is broader than that of the PDDA composite, suggesting a lower degree of ordering of the PAH molecules in the structure. The interlayer distance of this product was calculated to be 1.19 nm. Unlike the case of PDDA, the peak positions shift toward larger d-spacing with increasing the PAH concentration to become constant at 200 mM. (33) Post, J. E.; Veblem, D. R. Am. Mineral. 1990, 75, 477. (34) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 3038.

With an increase in time of electrolysis, the X-ray diffractions due to the PAH-intercalated phase increase in intensity (Figure 3). This confirms that the crystalline growth proceeds electrochemically and the bulk of the film is composed of a layered structure. The positions of diffraction peaks are practically independent of the amount of deposits (within experimental error of 0.1° in 2θ), meaning that the epitaxial condition is maintained in the same bath composition. The growth of the composites with PDDA and PAH can be achieved by the anodic formation of manganese oxide and the simultaneous assembly of polycations to negative charges on the deposited manganese oxide. This can be represented by eq 1, referring to the usual electrodeposition of MnO2 (Mn2+ + 2H2O f MnO2 + 4H+ + 2e-).35

Mn2+ + 2H2O + xA+ f A+xMn3+xMn4+1-xO2 + 4H+ + (2 - x)e- (1) where A+ corresponds to one cationic unit. Figure 4A displays XPS survey spectra of the manganese oxide films made with PDDA (a) and PAH (b) that exhibited the highest crystallinity in Figure 2, a and b, respectively. Mn 2p, O 1s, and Mn 3s signals arising from manganese oxide are observed in curves a and b. For the O 1s region in both spectra, the area contribution of O2- was much larger than that of OH(Supporting Information I), confirming that the films are mainly composed of oxide. The N 1s and C 1s peaks are visible at 400 and 285 eV. These peaks are ascribed to nitrogen and carbon atoms belonging to the incorporated polycations. XPS spectra of the N 1s core-level are given in Figure 4B. The film deposited with PDDA provides one component at 401.4 eV, while the spectrum of the PAH/MnOx film consists of two distinct chemical states that are centered at 400.9 and 398.9 eV. According to the literature, the higher BE component (around 401 eV) can be attributed to positively charged nitrogen, and the lower one to neutral nitrogen.36 From the ratio of the peak area of -NH3+ to that of total N 1s, the fraction of electrically charged nitrogen groups, [-NH3+]/[N], was estimated to be 0.68. Using the peak area ratios of N 1s and Mn 2p1/2 and their sensitivity factors, the N/Mn atomic ratios in the PDDA- and PAH-containing films were calculated to be 0.43 and 0.98, respectively. The higher composition of PAH over PDDA can be ascribed to a difference in the degree of ionization of the polymers in the film; i.e., PDDA is fully dissociated, while some amine groups in PAH are (35) Voinov, M. Electrochim. Acta 1982, 27, 833. (36) Zahr, A. S.; Villiers, M.; Pishko, M. V. Langmuir 2005, 21, 403.

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Figure 5. Cyclic voltammograms of the layered PDDA/MnOx film with a layered structure in a 0.1 M KCl solution at a scan rate of 20 mV/s. Twenty consecutive cycles are shown. The film was prepared on a Pt electrode prepared from a 2 mM MnSO4 and 5.6 mM PDDA solution at a constant potential of +1.0 V with an electric charge of 330 mC cm-2.

Figure 4. (A) XPS survey spectra of manganese oxide films prepared at +1.0 V in 2 mM MnSO4 solutions with (a) 5.6 mM PDDA and (b) 200 mM PAH, along with the core-level spectra in (B) N 1s and (C) Cl 2p regions of the same samples.

un-ionized. The assembly of polyelectrolyte molecules is driven by electrostatic attraction between cationic nitrogen groups of the polyelectrolyte and negative charges on the deposited manganese oxide. Consequently, many PAHs are needed to construct a layered structure, leading to a larger interlayer distance than the PDDA composite, as observed in the XRD measurements. For the Cl 2p region (Figure 4C), no feature is observed for the PDDA-containing film, confirming that the PDDA polymer is intercalated in a polycation form, but not in a chloride salt. On the other hand, the PAH/MnOx film exhibits a doublet that can be curve-fitted with two peaks assignable to the Cl 2p1/2 and 2p3/2 states, along with the Cl 2s peak at 268 eV. This finding enables us to realize that the -NH3+ groups in the interlayer PAH are associated with Cl- to generate -NH3+Cl- ionic pairs as well as with the negatively charged manganese oxide layers. The Cl to Mn atomic ratio was calculated to be 0.29 using the Cl peak area, which means that the fraction interacting with negative charges on manganese oxide is 0.38 (0.98 × 0.68 0.29). The estimated value is somewhat smaller than the PDDA/ Mn molar ratio (0.43) but is comparable to those observed for bulk birnessite. The structural arrangement of PAH should be affected by various factors such as charge balance, environmental pH, and hydrophobic interaction. The higher PAH concentration in the deposition bath can cause an increase in the fraction that does not contribute to balance the charges on manganese oxide, resulting in an expansion of the interlayer space and rather improved crystallinity (see Figure 2b). The effect of the electrolyte pH will be published elsewhere. The energy separation (∆E) in the doublet of peaks of Mn 3s core level was estimated to be ∼4.8 eV for both PDDA and PAH composites (not shown). These values correspond to an average oxidation states of about 3.8 according to a linear relationship between manganese oxidation state and ∆E value presented in the literature.37 This means that both films are composed largely of Mn4+ with a minor amount of Mn3+. (37) Toupin, M.; Brousse, T.; Be´langer, D. Chem. Mater. 2002, 14, 3946.

Figure 6. XRD patterns of the layered PDDA/MnOx film taken after scanning the electrode potential in a 0.1 M KCl solution and stopping at (a) +0.8 and (b) 0 V on the first cycle and at (c) +0.8 V on the second cycle. The film was prepared in the same way as that in Figure 5.

Electrochemistry of Nanocomposite Films. XRD and XPS data of the PDDA/MnOx film after being immersed in 0.1 M KCl solution for 12 h were the same as those of the as-deposited film. Obviously, PDDA polycations once incorporated are not ion exchanged with small cations in solution. Figure 5 shows initial 20 consecutive voltammograms performed on the layered PDDA/MnOx film in a 0.1 M KCl solution at a scan rate of 20 mV. In the potential region examined, the conversion between Mn4+ and Mn3+ can take place.21,38 The first cathodic scan provides a large reductive wave, whereas virtually no corresponding oxidation current is observed upon scan reversal. The response decreases with consecutive scans. This behavior was quite different from that observed for the similarly prepared amorphous counterpart which yielded a stable response associated with the oxidation/reduction process of Mn3+/ Mn4+ couple in the oxide film.30 This means that the amorphous surface of manganese oxide is accessible to charge compensating cations in solution. To interpret the above voltammetric behavior of the crystalline product, XRD measurements were conducted for the films that had been polarized in a 0.1 M KCl solution. The patterns presented in Figure 6 were taken after scanning the electrode potential and stopping at +0.8 (a) and 0 (b) V on the first cycle and at +0.8 V (c) on the second cycle. The pattern in Figure 6a is practically the same as that of the as-deposited sample, but it dramatically decreases in intensity at 0 V. A closer look revealed that this pattern comprises three evenly spaced peaks characteristic of a (38) Nakayama, M.; Tanaka, A.; Sato, Y.; Tonosaki, T.; Ogura, K. Langmuir 2005, 21, 5907.

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Figure 7. XPS spectra of the layered PDDA/MnOx film taken after scanning the electrode potential in a 0.1 M KCl solution and stopping at (a) +0.8 and (b) 0 V on the first cycle. The film was prepared in the same way as that in Figure 5.

Nakayama and Tagashira

Figure 9. XRD patterns of the layered PAH/MnOx film taken after being polarized at (a) +0.8 and (b) 0 V for 20 min following 40 CV cycles. The film was prepared in the same way as that in Figure 8a.

Figure 10. XPS core-level spectra in (a) K 2p, (b) N 1s, and (c) Cl 2p regions of the layered PAH/MnOx film taken after being polarized at +0.8 and 0 V for 20 min following 40 CV cycles. Figure 8. Voltammeric behaviors of (a) layered and (b) amorphous films of PAH/MnOx taken in a 0.1 M KCl solution at a scan rate of 20 mV/s. The films were prepared on a Pt electrode at 1.0 V from 2 mM MnSO4 solutions containing (a) 200 and (b) 1 mM PAH, respectively.

layered structure with d001 ) 1.38 nm. This corresponds to an expansion of the interlayer during the reduction process while lowering the crystalline nature. When the electrode is reoxidized, the pattern is recovered only in part. Besides, in situ UV-vis spectra were recorded while the PDDA/MnOx film coated on a transparent ITO electrode was kept at potentials between +0.45 (rest potential) and 0 V in a KCl solution (Supporting Information II). No significant change was observed in the absorption (400600 nm) due to charge-transfer transitions of manganese oxide layers39 during the cathodic polarization, removing the possibility of reductive dissolution of manganese species.40 Figure 7 displays XPS spectra of the sample films prepared in Figure 6, a and b. The Mn 2p region was not influenced by the polarized potential (Supporting Information III), confirming again that manganese remains in the film upon reduction. The N 1s spectra of both samples are the same as that of the asdeposited film, which indicates that the intercalated PDDA polycations are stable enough to remain in the interlayer. In the K 2p region, a doublet of peaks at 295.3 and 292.0 eV assignable to 2p1/2 and 2p3/2 states, which is absent in the oxidized film, appears in the reduced film. Considering the fact that PDDA cations remain fixed during reduction, injection of electrons must be balanced by incorporation of electrolyte cations. The incorporation of K+ into the tightly packed layer structure would bring about an expansion and collapse of the layered structure. (39) Giraldo, O.; Brock, S. L.; Willis, W. S.; Marquez, M.; Suib, S. L. J. Am. Chem. Soc. 2000, 122, 9330. (40) Bakardjieva, S.; Bezdie`ka, P.; Grygar, T. J. Solid State Electrochem. 2000, 4, 306.

The resulting manganese oxide polymorph is probably less electrochemically active, which is the origin of the attenuated CV response in Figure 5. Electrochemistry of the PAH/MnOx film was quite different from that of the PDDA/MnOx film. Curve a in Figure 8 shows a typical cyclic voltammogram of the layered PAH/MnOx film in the same electrolyte solution taken when it reached a steady state, along with that of the amorphous film obtained from the manganese solution containing 1 mM PAH. Obviously, the CV of the layered product is much larger in size than that of the amorphous film. Moreover, XRD measurements were made for the films polarized at oxidizing (+0.8 V) and reducing (0 V) potentials for 20 min after the voltammetric steady state had been reached (Figure 9). Unlike the PDDA-containing film, the layered structure is maintained during the potential cycling and further polarization. These CV and XRD results strongly suggest that the electrochemical process including the redox of Mn3+/ Mn4+ takes place at the interior surface of the multilayered film. To gain more information on the redox process of the PAH/ MnOx film, XPS experiments were further conducted for the above polarized films and are presented in Figure 10. No feature can be seen in the K 2p region, removing the possibility of the participation of K+ ions in the redox process here. Note that, in the N 1s core-level spectra, the higher BE fraction (i.e., -NH3+) decreases at the reduced state. This phenomenon cannot be ascribed to a decrease in the proton content of the film because the motion of protons should be opposite to the direction of the charge compensation. As shown in Figure 10c, the Cl 2p doublet appearing at the oxidizing potential completely disappears in the reduced film. Since the net charge of manganese oxide is negative, the charge balancing mechanism cannot be explained only by the motion of Cl-. That is, the observed behaviors would not happen individually. Figure 11 schematically represents a scenario of the electrochemical process occurring in the PAH/MnOx film, which is consistent with all the above results. As already noted,

Electrodeposition of Manganese Oxide Nanocomposites

Figure 11. Schematic illustration of the redox process that takes place in the layered PAH/MnOx film.

the oxidized film involves neutral (-NH2) and positively charged (-NH3+) amine groups in the interlayer region, and some of the -NH3+ groups are associated with Cl- anions rather than negatively charged manganese oxide layers. Injection of electrons in the manganese oxide upon reduction can be balanced by the protons being released from -NH3+Cl- sites, giving rise to both an increase and decrease in the -NH2 and -NH3+ fractions, respectively. This situation is probably favored over the H+ incorporation from the bulk of electrolyte. At this time, the Clanions become unnecessary and are inevitably excluded from the interlayer to the solution.

Conclusions Multilayered manganese oxide nanocomposites intercalated with strong (PDDA) and weak (PAH) polyelectrolytes were

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produced in a thin film form through a direct electrochemical route. Fully charged PDDA cations are accommodated tightly in the interlayer space, while the PAH involves neutral (-NH2) and positively charged (-NH3+) amine groups, leading to a larger interlayer distance than that observed for the PDDA/MnOx film. The N/Mn atomic ratios in the PDDA- and PAH-containing films were estimated to be 0.43 and 0.98, respectively. The PAH/ MnOx film coated on a Pt electrode showed a much larger CV response than the PDDA/MnOx film, where the -NH3+Clspecies in PAH play an important role as anion-exchange site, as summarized in Figure 11. Acknowledgment. This research was supported in part by the Japan Society of the Promotion of Science (16750175). Supporting Information Available: O 1s core-level XPS spectra of the PDDA- and PAH-containing films with a layered structure, in situ UV-vis spectra of the layered PDDA/MnOx film upon reduction in a KCl electrolyte solution, and Mn 2p core-level XPS spectra of the layered PDDA/MnOx film taken after the potential scan. This material is available free of charge via the Internet at http://pubs.acs.org. LA053072I