15-Inorganic Multilayer Films of Manganese Oxide Nanosheets and

Inorganic Multilayer Films of Manganese Oxide Nanosheets and Aluminum Polyoxocations:Fabrication,Structure,andElectrochemical BehaviorLianzhou Wang,Nobuyuki Sakai,Yasuo Ebina,Kazunori Takada,and Takayoshi Sasaki* Ad V anced Materials Laboratory,National Institute for Materials Science,1-1Namiki,Tsukuba,Ibaraki305-0044,JapanRecei V ed October22,2004.Re V ised Manuscript Recei V ed December22,2004Inorganic multilayer films of exfoliated MnO2nanosheets and aluminum Keggin ions have been fabricated onto various substrates via sequential adsorption.Ultraviolet-visible absorption spectra in the multilayer buildup process indicated that nearly equal amounts of nanosheets could be assembled for each deposition cycle.X-ray photoelectron spectroscopic quantitative analysis and structural consideration based on charge balance indicate that aluminum Keggin ions should be sparsely distributed in the nanosheet galleries,occupying only13%of the gallery area.This low density of Keggin ions can explain a Bragg peak observed with a relatively small periodicity of0.73nm,suggesting that a major gallery area accommodates H2O molecules.This unique structure is different from ordinary multilayer structures,in which two components alternate,contributing to the film thickness with their own sizes.The multilayer film showed a very stable cyclic voltammogram involving reversible redox reactions of Mn III/Mn IV, providing new potential electrodes for electrochemical applications.The observed electron transfer suggests that a predominant amount of MnO2nanosheets in multilayer films was electrochemically active.IntroductionThe sequential layer-by-layer(LBL)adsorption technique introduced by Decher is one of the most promising new methods for film fabrication.1Initially applied for the multilayer assemblies of polyelectrolytes,1-3it has been successfully extended to wide varieties of other charged materials including dyes,biological macromolecules,and inorganic nanoscale materials,4-9providing an elegant way of controlling composition,thickness,and architecture of the films on a nanometer scale.Inorganic nanomaterials are currently one of the hottest research topics due to their intriguing physicochemical properties in comparison with bulk materials.10,11The incorporation of inorganic nanoma-terials into multilayer systems is also a fashion because they may render a wide range of applications for the systems in optics,microelectronics,and magnetics.12,13Organic poly-electrolytes have been mostly employed as linkers to com-bine inorganic nanomaterials,whereas inorganic binders have been paid much less attention.14,15Actually,appro-priate inorganic binders may lead to novel architectures and new physiochemical properties for the multilayer systems, which are hardly achieved with organic binders.Very recently,we have demonstrated that multilayer ultrathin films and hollow shells could be obtained by LBL assembly of delaminated titania nanosheets and inorganic polyoxoca-tions of[AlO4Al12(OH)24(H2O)12]7+,or Al13Keggin ions.16 The resulting multilayer films had a unique pillared archi-tecture and nanoporous nature,showing excellent thermal stability.*Corresponding author.Fax:+81-29-854-9061.E-mail:sasaki.takayoshi@ nims.go.jp.(1)(a)Decher,G.Science1997,277,1232.(b)Decher,G.;Schlenoff,J.B.Multilayer Thin Films:Sequential Assembly of NanocompositeMaterials;Wiley-VCH:Weinheim,2003;p1.(2)Tripathy,S.K.;Kumar,J.;Nalwa,H.S.Handbook of Polyelectrolytesand Their Applications;American Scientific Publishers:Los Angeles, CA,2002.(3)(a)Dubas,S.T.;Schlenoff,J.B.Macromolecules1999,32,8153.(b)Baur,J.W.;Rubner,M.F.;Reynolds,J.R.;Kim,ngmuir1999,15,6460.(c)Xie,A.F.;Granick,S.J.Am.Chem.Soc.2001, 123,3175.(4)(a)Tedeschi,C.;Caruso,F.;Mo¨hwald,H.;Kirstein,S.J.Am.Chem.Soc.2000,122,5841.(b)Schneider,T.;Lavrentovich,ngmuir2000,16,5227.(c)Va´zquez,E.;Dewitt,D.M.;Hammond,P.T.;Lynn,D.M.J.Am.Chem.Soc.2002,124,13992.(5)(a)Zhang,J.;Chua,L.S.;Lynn,ngmuir2004,20,8015.(b)Yu,A.;Caruso,F.Anal.Chem.2003,75,3031.(c)Pei,R.;Cui,X.;Yang,X.;Wang,E.Biomacromolecules2001,2,463.(6)(a)Caruso,F.;Lichtenfeld,H.;Giersig,M.;Mo¨hwald,H.J.Am.Chem.Soc.1998,120,8523.(b)Caruso,F.;Mo¨hwald,ngmuir1999, 15,8276.(c)Lvov,Y.;Munge,B.;Giraldo,O.;Ichinose,I.;Suib,S.L.;Rusling,ngmuir2000,16,8850.(7)(a)Fang,M.-M.;Kaschak,D.M.;Sutorik,A.C.;Mallouk,T.E.J.Am.Chem.Soc.1997,119,12184.(b)Fendler,J.H.Chem.Mater.1996,8,1616.(c)Cassagneau,T.;Mallouk,T.E.;Fendler,J.H.J.Am.Chem.Soc.1998,120,7848.(8)(a)Sasaki,T.;Ebina,Y.;Watanabe,M.;Decher,mun.2000,2163.(b)Sasaki,T.;Ebina,Y.;Tanaka,T.;Harada,M.;Watanabe,M.;Decher,G.Chem.Mater.2001,13,4661.(c)Sasaki, T.;Ebina,Y.;Fukuda,K.;Tanaka,T.;Harada,M.;Watanabe,M.Chem.Mater.2002,14,3524.(9)(a)Ma,R.;Sasaki,T.;Bando,Y.J.Am.Chem.Soc.2004,126,10382.(b)Tokudome,H.;Miyauchi,mun.2004, 958.(10)(a)Wang,Z.L.Ad V.Mater.2003,15,1497.(b)Shchukin,D.G.;Sukhorukov,G.B.Ad V.Mater.2004,16,671.(11)Cushing,B.L.;Kolesnichenko,V.L.;O’Connor,C.J.Chem.Re V.2004,104,3893.(12)(a)Gao,M.;Lesser,C.;Kirstein,S.;Mo¨hwald,H.;Rogach,A.L.;Weller,H.J.Appl.Phys.2000,87,2297.(b)Newman,A.K.;Liu,J.M.J.Appl.Phys.1997,82,4637.(13)Mamedov,A.A.;Kotov,ngmuir2000,16,5530.(14)Ollivier,P.J.;Kovtyukhova,N.I.;Keller,S.W.;Mallouk,T.E.Chem.Commun.1998,1563.(15)Wang,Z.-S.;Sasaki,T.;Muramatsu,M.;Ebina,Y.;Tanaka,T.;Wang,L.Z.;Watanabe,M.Chem.Mater.2003,15,807.(16)Wang,L.Z.;Ebina,Y.;Takada,K.;Sasaki,T.J.Phys.Chem.B2004,108,4283.1352Chem.Mater.2005,17,1352-135710.1021/cm048146a CCC:$30.25©2005American Chemical SocietyPublished on Web02/17/2005Considerable efforts have been made on the fundamental research and commercial applications of manganese oxides as they are promising with versatile properties including magnetic,electric,electrochemical,ion-exchange,and cata-lytic performances.17-19Nanometer-scale manganese oxides with various morphologies and crystallographic forms such as nanoparticles,nanowires,nanoporous materials,and hol-low nanospheres have been reported to date.20-23In addition, Ooi et al.and Suib et al.have recently reported the inter-calation and swelling behavior of layered manganese oxides upon treating them with organic ammonium ions.24,25More recently,our group demonstrated the successful exfoliation of a layered compound of K0.45MnO2into colloidal nanosheets of MnO2.26The resulting nanosheet consists of a two-dimensional array of edge-shared MnO6octahedra with a crystallographic thickness of subnanometer and a lateral size of several hundred nanometers,which distinctly differs from nanoparticles and one-dimensional nanostructures in terms of morphology and anisotropy.Thanks to its surface charge, unilamellar,and colloidal nature,MnO2nanosheet is par-ticularly favorable for electrostatic LBL film deposition.In a previous study,we have reported on the fabrication of multilayer ultrathin films containing delaminated MnO2 nanosheets and polyelectrolytes via the LBL technique,27 which may be considered as one of the first molecularly organized film systems based on manganese oxides.The present work describes the LBL deposition of aluminum polyoxocations and exfoliated MnO2nanosheets on various substrates,yielding a new kind of inorganic ultrathin films with an uncommon multilayer architecture. As the electrochemical performance of newly developed macromolecule-like MnO2nanosheets is of great interest from the fundamental and practical viewpoints,we examined electrochemical properties of the present inorganic multilayer systems.The obtained films showed good thermal stability and promising electrochemical performance.Experimental SectionReagents.Chemicals such as K2CO3,(CH3)4NOH,(C4H9)4NOH (Wako Pure Chemical Co.,Japan),Mn2O3(High Purity Chemical Co.,Japan),and Al(NO3)3‚9H2O(Kanto Chemical Co.,Japan)were of99.9%purity or higher.All other chemicals and solvents were of analytical grade.Water was purified to a resistivity of18MΩcm by passing through a Milli-Q water system.Al13Keggin Ions.A solution containing Al13polyoxocations was prepared by dropwise addition of(CH3)4OH solution(200cm3, 0.2mol dm-3)to a Al(NO3)3solution(80cm3,0.2mol dm-3)under stirring.28The resulting solution was aged overnight at room temperature before use for film fabrication.The molar ratio of OH/ Al was2.5,and the approximate concentration of Al13Keggin ions was4.4×10-3mol dm-3.MnO2Nanosheets.A starting layered material of K0.45MnO2 was synthesized by calcination of a stoichiometric mixture of K2CO3and Mn2O3at750°C for30h under an O2gas flow.26A protonic form of H0.13MnO2‚0.7H2O was prepared by treating the compound(∼10g)with HCl aqueous solution(2dm3,1mol dm-3) under stirring for10days.The resulting protonic oxide(1.6g)was then shaken vigorously with400cm3of(C4H9)4NOH solution (0.052mol dm-3)at room temperature for10days.The colloidal suspension containing well-dispersed exfoliated MnO2nanosheets was obtained after separation of an unexfoliated residue by centrifugation at10000rpm.Multilayer Film Fabrication.Solid substrates such as Si wafer chips and quartz glass slides were cleaned according to a previously described procedure.8b Indium-tin oxide(ITO)-coated glass sub-strates were cleaned by sonication in acetone and washed with ethanol and ultrapure water.Multilayer films were fabricated by sequential deposition of Al13polyoxocations and MnO2nanosheets onto a substrate.A substrate was dipped first in an aqueous solution containing Al13polyoxocations(concentration:9×10-4mol dm-3; pH≈5.0)for20min and then rinsed thoroughly with water.The Al13-precoated substrate was subsequently immersed into the colloidal suspension containing MnO2nanosheets(0.08g dm-3, pH≈7)for20min,thoroughly washed with water,and dried with a nitrogen stream.These deposition steps were repeated n times to obtain thin films of(Al13/MnO2)n.Film Characterizations.A Hitachi U-4000spectrophotometer was employed to record ultraviolet-visible(UV-vis)absorption spectra of multilayer films fabricated on a quartz-glass substrate.A Seiko SPA400atomic force microscope(AFM)was used to examine the surface topographical feature of the films prepared on a silicon wafer chip.The observations were conducted in tapping mode with silicon-tip cantilevers(20N m-1).X-ray diffraction (XRD)data were collected using a Rigaku Rint2000powder diffractometer with monochromatized Cu K R radiation(λ) 0.15405nm).X-ray photoelectron spectra(XPS)were taken on a Physical Electronics XPS-5700spectrometer with Al K R X-ray line (1486.6eV).FT-IR spectra for the multilayer films on Si wafer substrate(6×5cm2)were measured using a Digilab S-45FT-IR spectrometer.A film sample was aligned at a Brewster’s angle of 75°with respect to the incident beam to obtain the data in transmission mode.Electrochemical Measurements.Cyclic voltammograms(CV) were recorded on a Solartron SI-1287potentiostat using a three-electrode electrochemical cell.A thin film of(Al13/MnO2)n deposited on ITO-glass substrate was used as a working electrode, which was dried at110°C under vacuum for3h prior to CV(17)(a)Shen,Y.F.;Zerger,R.P.;DeGuzman,R.N.;Suib,S.L.;MaCurdy,L.;Potter,D.I.;O’Young,C.L.Science1993,260,511.(b)Brock, S.L.;Sanabria,M.;Nair,J.;Suib,S.L.;Ressler,T.J.Phys.Chem.B2001,105,5404.(18)(a)Armstrong,A.R.;Bruce,P.G.Nature1996,381,499.(b)Rao,C.N.R.;Cheetham,A.K.;Mahesh,R.Chem.Mater.1996,8,2421.(19)(a)Feng,Q.;Kanoh,H.;Ooi,K.J.Mater.Chem.1999,9,319.(b)Brock,S.L.;Duan,N.-G.;Tian,Z.-R.;Giraldo,O.;Zhou H.;Suib, S.L.Chem.Mater.1998,10,2619.(20)Brock,S.L.;Sanabria,M.;Suib,S.L.;Urban,V.;Thiyagarajan,P.;Potter,D.I.J.Phys.Chem.B1999,103,7416.(21)(a)Gao,Q.-M.;Suib,S.L.;Rusling,mun.2002,2254.(b)Wang,X.;Li,Y.D.J.Am.Chem.Soc.2002,124,2880.(c) Ma,R.;Bando,Y.;Zhang,L.-Q.;Sasaki,T.Ad V.Mater.2004,16, 918.(22)(a)Tian,Z.-R.;Tong,W.;Wang,J.-Y.;Duan,N.-G.;Krishnan,V.V.;Suib,S.L.Science1997,276,926.(b)Ma,Y.;Suib,S.L.;Ressler, T.;Wong,J.;Lovallo,M.;Tsapatsis,M.Chem.Mater.1999,11,3545.(c)Floros,N.;Michel,C.;Hervieu,M.;Raveau,B.Chem.Mater.2000,12,3197.(23)(a)Yuan,J.-K.;Laubernds,K.;Zhang Q.;Suib,S.L.J.Am.Chem.Soc.2003,125,4966.(b)Wang,L.Z.;Ebina,Y.;Takada,K.;Sasaki, mun.2004,1074.(24)Liu,Z.-H.;Ooi,K.;Kanoh,H.;Tang W.-P.;Tomida,ngmuir2000,16,4154.(25)Gao,Q.-M.;Giraldo,O.;Tong,W.;Suib,S.L.Chem.Mater.2001,13,778.(26)Omomo,Y.;Sasaki,T.;Wang,L.Z.;Watanabe,M.J.Am.Chem.Soc.2003,125,3568.(27)Wang,L.Z.;Omomo,Y.;Sakai,N.;Fukuda,K.;Nakai,I.;Ebina,Y.;Takada,K.;Watanabe,M.;Sasaki,T.Chem.Mater.2003,15, 2873.(28)(a)Kooli,F.;Sasaki,T.;Watanabe,mun.1999,211.(b)Kooli,F.;Sasaki,T.;Watanabe,M.Microporous MesoporousMater.1999,28,495.Inorganic Multilayer Films of MnO2Nanosheets Chem.Mater.,Vol.17,No.6,20051353measurements to remove H 2O molecules from the nanosheet galleries.Pt black wire and an Ag/Ag +/acetonitrile electrode (+0.49V vs NHE)served as a counter and a reference electrode,respectively.The working electrode potential was referred to this reference electrode in this paper.The CV measurements were performed at a scan rate of 5-100mV s -1ranging from -1.5to 1.0V.A propylene carbonate (PC)containing 0.1mol dm -3LiClO 4was used as a supporting electrolyte solution,which was degassed with N 2prior to the experiments.Results and DiscussionFilm Growth.To initiate the film fabrication,the substrate was first dipped into the solution of Al 13Keggin ions.AFM cross-sectional analysis revealed that the mean surface -height difference for the Si wafer after the treatment with Al 13ions was larger than that for bare Si wafer,suggesting the direct adsorption of Al 13onto the substrate surface.In fact,several groups including ours have reported the adsorp-tion of Al 13ions directly onto bare substrates.14,16,29Upon immersing the substrate precoated with Al 13in the colloidal suspension of MnO 2nanosheets,the negatively charged nanosheets were spontaneously adsorbed onto the surface due to the electrostatic interactions.AFM image (Figure 1)visualizes the surface topography of the first bilayer of Al 13/MnO 2nanosheets thus fabricated.The substrate surface was densely covered with unilamellar MnO 2nanosheets of submicrometers in lateral size.The total surface coverage was ∼82%,which comprises the monolayer (∼45%)and overlapped regions (∼37%).The MnO 2nanosheet exhibits characteristic absorption centered at 380nm,whereas Al 13polyoxocations deposited on quartz glass apparently had no optical feature in the wavelength range of 200-800nm.The multilayer buildup can thus be monitored by UV -vis spectra measured im-mediately after each deposition cycle.A nearly linear enhancement of absorbance at 380nm was observed (Figure 2),indicating that Al 13ions and MnO 2nanosheets are uniformly assembled by the LBL technique.The absorbance after 10deposition cycles was approximate 0.9,which is comparable with that for a film with 10bilayers of poly-electrolytes/MnO 2nanosheets.26This indicates that Al 13Keggin ion is as efficient as organic binders in the LBL assembly.The visible color evolution of the films with increasing deposition cycles (Figure 3)also supports the consecutive growth of the films.Film Characterizations and Multilayer Nanostructure.XPS data provided evidence for the incorporation of Al 13polyoxocations and MnO 2nanosheets in multilayer films.An XPS survey scan of (Al 13/MnO 2)10film detected Mn,Al,O,and C in the films and/or substrate.Peaks assignable to Mn (2p 1/2)and Mn (2p 3/2)were detected at a binding energy of 653.6and 642.4eV (Figure 4a).30An additional peak attributable to Mn (3s)was also observed at 83.8eV (Figure 4b).Al (2p)and Al (2s)peaks were detected at 73.6and 118.4eV,respectively (Figure 4b).XPS quantitative analysis revealed that the atomic ratio of Mn to Al was 4.8for the(29)Keller,S.W.;Kim,H.-N.;Mallouk,T.E.J.Am.Chem.Soc .1994,116,8817.(30)Moulder,J.;Stickle,W.;Sobol,P.;Bomben,K.In Handbook of X-rayPhotoelectron Spectroscopy ;Chastain,J.,Ed;Perkin-Elmer Corp.:Eden Prairie,MN,1992.Figure 1.Tapping-mode AFM image of the first Al 13/MnO 2bilayer on a Si waferchip.Figure 2.(a)UV -vis absorption spectra for multilayer films of (Al 13/MnO 2)n on a quartz glass substrate.Nanosheet concentration,0.08g dm -3;pH value,7;Al 13polyoxocation concentration,0.9g dm -3;pH value,∼5.0;deposition time,20min each.(b)Dependence of observed absorbance at 380nm on the number of depositioncycles.Figure 3.Photograph of thin films of (Al 13/MnO 2)n deposited on quartz glass substrates:n )0,5,10for a,b,c,respectively.1354Chem.Mater.,Vol.17,No.6,2005Wang et al.film.A peak centered at 284.0eV is assignable as C (1s).This signal has been frequently observed in other thin films as contamination adsorbed at film surfaces.15,16Figure 5depicts XRD data for the multilayer films of (Al 13/MnO 2)n .The Bragg peak that appeared indicates a repeating periodicity of 0.73nm.Progressive enhancement in its intensity with an increase in the number of deposi-tion cycles strongly suggests the growth of nanostructured (Al 13/MnO 2)n assemblies.The multilayer spacing generally corresponds to the sum of a thickness of two counterpart layers in a layer-by-layer sandwich structure,as reported in the literature.8,16,27Considering the crystallographic thickness of 0.52nm for MnO 2nanosheet 26and the diameter of 0.86nm for the spherical Al 13polyoxocation,31the repeating distance of the (Al 13/MnO 2)bilayer should be 1.38nm ()0.52+0.86nm),which is much larger than the experi-mental value of 0.73nm.This drastic difference suggests that Al 13polyoxocations and MnO 2nanosheets are organized in a different architecture other than the layer-by-layer sandwich structure commonly formed for multilayer systems.What is a possible architecture of the multilayer films in the present study?The structural arrangement of film components should be discussed by considering various parameters,that is,charge balance,chemical composition,and architectural accommodation.As reported previously,26MnO 2nanosheet has a small negative charge of 0.13-per chemical formula,while one Al 13polyoxocation has a high positive charge of 7+.31Taking into account the fact that the two-dimensional unit cell of the nanosheet in hexagonal symmetry (a )0.284nm)contains one formula,54unit cells of MnO 2nanosheets are needed to balance the positive charge of one Al 13Keggin ion.On the other hand,XPS quantitative analysis revealed that the atomic ratio of Mn/Al was 4.8for the film.This indicates that 62()4.8×13)unit cells of MnO 2are related to one Al 13polyoxocation.Thus,the experimental data are in rough agreement with the calculated value based on electrostatic balance,indicating the success of alternate deposition of two counterparts via electrostatic interactions.Let us consider the film architecture from a viewpoint of structural arrangement of the two components.Because the two-dimensional unit-cell area of MnO 2nanosheet is 0.070nm 2()0.284nm ×0.284nm ×sin 120°),the area for 62units (XPS data)is around 4.3nm 2.On the other hand,one Al 13polyoxocation can occupy a projected surface area of 0.58nm 2()π×0.432)if we approximate the shape of Al 13ion as spherical.Consequently,Al 13polyoxocations occupy only 13%()0.58/4.3),as schematically shown in Scheme 1.Sparsely distributed Al 13polyoxocations can stabilize the multilayer assembly as the inorganic linker.The origin of repeating distance of 0.73nm is important to understand the film yered manganese oxides of birnessite and related compounds frequently show an interlayer distance of around 0.7nm,when hydrated mono-valent cations are accommodated in the interlayer space.24,26,32The flocculation of exfoliated MnO 2nanosheets with Li +ions produced a restacked layered structure with a repeating(31)Ohtsuka,K.Chem.Mater .1997,9,2039.(32)(a)Yang,X.-J.;Makita,Y.;Liu,Z.-H.;Ooi,K.Chem.Mater .2003,15,1228.(b)Ching,S.;Petrovay,D.J.;Jorgensen,M.L.;Suib,S.L.Inorg .Chem .1997,36,883.(c)Omomo,Y.;Sasaki,T.;Watanabe,M.Solid State Ionics 2002,151,243.Figure 4.High-resolution XPS spectra in (a)Mn 2p and (b)Al regions for a (Al 13/MnO 2)10thinfilm.Figure 5.XRD pattern of as-prepared multilayer thin films of (Al 13/MnO 2)n .Here,n is 1for (a),3for (b),5for (c),and 10for (d),respectively.Scheme 1.A Schematic Illustration for Surface OccupationAreas of 62Units of MnO 2Nanosheet and One Al 13Polyoxocation (Top)and Possible Multilayer Nanoarchitecture in the (Al 13/MnO 2)n Films(Bottom)Inorganic Multilayer Films of MnO 2Nanosheets Chem.Mater.,Vol.17,No.6,20051355spacing of0.72nm.33As discussed above,Al13polyoxo-cations are sparsely distributed in the nanosheet galleries, occupying13%of the total area.Because the nanosheets are very flexible reflecting their extremely small thickness, they can easily bend to cover or wrap these Al13islands and, in other major areas(∼87%),stack in parallel accommodat-ing H2O molecules.The presence of H2O molecules is confirmed by FT-IR data(Supporting Information I).On the average,six or seven oxide sheets stacked with H2O molecules are mixed with one Al13ion wrapped with the sheet,if we focus on the projected surface area where one polyoxocation occupies.A model in Scheme1represents such structural features,which can explain the observed multilayer spacing of0.73nm.Because of a disordered arrangement of Al13ions,they do not contribute to the diffraction pattern.The major structure order arises from thestacked area with H2O molecules between the MnO2 nanosheets.We have recently reported the fabrication of multilayer ultrathin films of(Al13/Ti0.91O2nanosheets)n.16The observed multilayer spacing of 1.5nm can be explained by the ordinary layer-by-layer sandwich structure,that is,as a sum of the nanosheet thickness(0.7nm)and the diameter of Al13 polyoxocation(0.86nm).A difference in structural feature between these two types of nanosheets may account for this apparently contrasting behavior:The higher negative charge of Ti0.91O2nanosheet requires the accommodation of alumi-num Keggin ions in higher packing density,which can prop open the nanosheet gallery.XRD patterns of the multilayer films of(Al13/MnO2)10 upon heating at different temperatures(Supporting Informa-tion II)provided the information of structural evolution of the films.The diffraction peak of d≈0.7nm remained up to200°C.In contrast,comparable multilayer films fabricated with MnO2nanosheets and organic binders lost their basal reflection above100°C.27This suggests that the present inorganic films have improved thermal stability in compari-son with those films containing organic linkers.The diffrac-tion peak of0.7nm disappeared at higher temperatures,and no crystalline phase was formed even at a temperature as high as800°C for the films of(Al13/MnO2)10.This is also contrasting to the behavior for the multilayer films of MnO2 nanosheets and organic binders,which showed the formation of Mn2O3phase at a much lower temperature of500°C.27 Al13polyoxocations in the films,even though sparsely distributed,play a role of an inhibitor of the crystallization at a temperature where manganese oxides such as Mn2O3 normally crystallize.Electrochemical Performances.It is of significant im-portance to clarify the electrochemical behavior of the present inorganic multilayer systems with unique nanostructure. Films fabricated on an ITO-coated glass substrate exhibited a XRD peak that was nearly identical to that for the sample on quartz glass slide in terms of position and line width, suggesting that a similar nanostructured film can be formed. UV-vis spectra also showed progressive enhancement of absorbance due to MnO2nanosheets in the multilayer. However,the peak absorbance was approximately one-half of that for the film on quartz glass.A rough surface of the ITO-coated glass substrate may be responsible for this lower loading.A similar tendency was apparently observed for various multilayer assemblies.34CV response of the multilayer films did not entirely develop at the initial scans,and some cycles were required to activate the whole multilayer film.Typically,a film of (Al13/MnO2)10required about20-30cycles to develop a stable CV response(Figure6),which is associated with the reduction/oxidation of MnO2nanosheets accompanying insertion/extraction of Li+ions into/from the nanosheet galleries,as formulated by:The slow activated response of CV signal may be due to the fact that the multilayer films should be very compact and electrolyte-free before the electrochemical measurements, consistent with the multilayer structure discussed above.The subsequent CV curves were very stable upon hundreds of cycling with only a slight redox peak drift.UV-vis spectra showed a negligible change for the films which experienced 100CV cycles.These data strongly suggest that the atomic architecture of MnO2nanosheets is fundamentally stable toward electrochemical measurements.Note that Al13Keggin ions contain no redoxable pairs under the present conditions and thus do not contribute to the electron transfer during CV measurements.However,Keggin ions play an important role in stabilizing the multilayer film structure upon elec-trochemical cycling.Figure7describes the CVs of thin films with different numbers of bilayers after100CV cycles.The peak current enhanced almost linearly with an increase in the number of bilayers,suggesting that approximately equal amounts of electrochemically active Mn species were incorporated in the films for each deposition cycle.One distinct reduction peak at approximately-0.60V vs Ag/Ag+and one oxidation peak at around-0.20V vs Ag/Ag+were detectable for(33)Wang,L.Z.;Takada,K.;Kajiyama,A.;Onoda,M.;Michiue,Y.;Zhang,L.-Q.;Watanabe,M.;Sasaki,T.Chem.Mater.2003,15,4508.(34)Sakai,N.;Ebina,Y.;Takada,K.;Sasaki,T.J.Am.Chem.Soc.2004,126,5851.Figure6.The initial20th CVs of a(Al13/MnO2)10film on ITO glass substrate in PC electrolyte containing0.1mol dm-3LiClO4(scan rate,10 mV s-1;potential range,-1.5-1.0V).The scan started from the rest potential of ca.-0.10V vs Ag/Ag+to a negative direction.The inserted arrows indicate the peak evolution directions.Mn IV O2+x(Li++e-)T LixMn IV(1-x)Mn IIIxO21356Chem.Mater.,Vol.17,No.6,2005Wang et al.multilayer films,which are attributable to the redox reaction of Mn III /Mn IV species.The electron-transfer rates were estimated to be ∼1.60×10-8and ∼1.55×10-8mol cm -2for cathodic and anodic reactions from the integrated peak areas of the 100th CV data on the (Al 13/MnO 2)10film.If a perfect monolayer film of MnO 2nanosheets is assumed,a content of Mn should be 2.37×10-9mol cm -2based on the structural considerations discussed above.27On the basis of the characterization data described above,the actual monolayer film on ITO-coated glass substrate contains MnO 2nanosheets in an amount approximately 60%of that in the hypothetical ideal film.That is to say,the amount of MnO 2in the working electrode of a 10-bilayer film should be approximately 1.42×10-8mol cm -2,which is nearly comparable to the observed electron-transfer rates.A predominant portion of the MnO 2nanosheets in the film is electroactive.The slightly higher charge transfer observed over the theoretical limit may beexplained by the charge/discharge of electric double layer developed on the electrode surface.The electrochemical redox reactions of manganese oxides are essential for their applications as electrodes in batteries,catalysis,and energy-storage devices.17-19,35The reversibility and electrochemical conversion efficiency have been the main targets for these materials.36Recently,Long reported the sol -gel synthesis of a mesoporous thin film of birnessite-type manganese oxide with a film thickness of sub-to micrometers.37They conducted electrochemical examinations under experimental conditions similar to ours and reported the insertion/extraction of Li ions to be 0.36per MnO 2unit.The multilayer films in the present study exhibited remark-ably higher electrochemical conversion capability.The unique nature of the MnO 2nanosheets in terms of high crystallinity and lamellar architecture may account for these higher electrochemical performances.ConclusionsWe demonstrated the fabrication of inorganic multilayer films via alternate self-assembly of MnO 2nanosheets and aluminum Keggin ions.Characterizations revealed that Al 13polyoxocations were sparsely distributed in the nanosheet galleries and H 2O molecules were accommodated in a remaining major area.MnO 2nanosheets in the multilayer films predominantly underwent electrochemical redox pro-cesses of Mn III /Mn IV ,giving very stable CV cycling.Acknowledgment.This work was supported by CREST of Japan Science and Technology Agency (JST).Supporting Information Available:FT-IR spectra for an as-prepared (Al 13/MnO 2)10multilayer film and XRD patterns of thin films of (Al 13/MnO 2)10heated at different temperatures (PDF).This material is available free of charge via the Internet at .CM048146A(35)Espinal,L.;Suib,S.L.;Rusling,J.F.J.Am.Chem.Soc .2004,126,7676.(36)(a)Toupin,M.;Brousse,T.;Be ´langer,D.Chem.Mater.2002,14,3946.(b)Long,J.W.;Young,A.L.;Rolison,D.R.J.Electrochem.Soc .2003,150,A1161.(37)Long,J.W.;Qadir,L.R.;Stroud,R.M.;Rolison,D.R.J.Phys.Chem.B 2001,105,8712.Figure 7.(a)The 100th cyclic voltammogram of (Al 13/MnO 2)n films.Here,1,2,3,5,7,and 10indicate the numbers of bilayers for the films,respectively.(b)The dependence of cathodic peak currents on the number of bilayers.Inorganic Multilayer Films of MnO 2Nanosheets Chem.Mater.,Vol.17,No.6,20051357。