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ArticleRoom-Temperature Synthesis of Manganese Oxide Monosheets
Kazuya Kai, Yukihiro Yoshida, Hiroshi Kageyama, Gunzi
Saito, Tetsuo Ishigaki, Yu Furukawa, and Jun Kawamata
J. Am. Chem. Soc., 2008, 130 (47), 15938-15943 • Publication Date (Web): 01 November 2008
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Published on Web 11/01/2008

Room-Temperature Synthesis of Manganese Oxide
Monosheets
Kazuya Kai,† Yukihiro Yoshida,*,† Hiroshi Kageyama,† Gunzi Saito,*,†,‡
Tetsuo Ishigaki,§ Yu Furukawa,§ and Jun Kawamata§DiVision of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku,
Kyoto 606-8502, Japan, Research Institute, Meijo UniVersity, Shiogamaguchi 1-501 Tempaku,
Nagoya 468-8502, Japan, and Department of Chemistry, Faculty of Science, Yamaguchi
UniVersity, Yoshida 1677-1, Yamaguchi 753-8512, Japan
Received June 13, 2008; E-mail: yoshiday@kuchem.kyoto-u.ac.jp; saito@kuchem.kyoto-u.ac.jp

Abstract: Preparation of single-layer manganese oxide nanosheets (monosheets) comprised of edgeshared MnO6 octahedra has relied on multistep processing involving a high-temperature solid-state synthesisof bulk templates, and ion-exchange and exfoliation reactions in solutions, requiring high cost and longprocessing time. Here we demonstrate the first single-step approach to directly access the MnO2
monosheets, by the chemical oxidation of Mn2+ ions in the presence of tetramethylammonium cations in
an aqueous solution. Of importance is that this template-free reaction readily proceeds within a day atroom temperature. The ability of the MnO2 monosheets to self-assemble allows aggregation, to form layered
structures with potassium cations and cationic tetrathiafulvalene analogues as intercalants. Furthermore,
Langmuir-Blodgett (LB) films composed of the MnO2 monosheets were successfully fabricated by the LB
deposition method, in which about one layer of the monosheets was deposited for each process.

1. Introduction

Nanosized materials with different morphologies, such as
nanoparticles and clusters (zero-dimension, e.g., fullerenes), andnanotubes and nanowires (one-dimension, e.g., carbon nanotubes), have become increasingly important as potential systems
for both fundamental interests and industrial applications.1,2
Nanosheets are a new class of two-dimensional nanomaterials
that are characterized by a thickness on the order of nanometers
and lateral dimensions of submicro- to micrometers.3,4 So far,a wide variety of transition-metal oxide nanosheets such as
Ca2Nb3O10,5 Nb6O17,6 Ti1-δO2,7 and MnO2,8 to name only a few,
have been discovered and investigated extensively due to therich chemistry and physics of the bulk counterparts.3,4 The
utmost feature of the nanosheets is their exceptionally highspecific surface area, rendering them promising candidates fora variety of applications, especially in photocatalysis andelectrodes of electrochemical devices.3,4 Furthermore, the ability†

Kyoto University.
Meijo University.
§
Yamaguchi University.
(1) Handbook of Nanostructured Materials and Nanotechnology; Nalwa,
H. S., Ed.; Academic Press: New York, 2000.
(2) Nanomaterials Chemistry; Rao, C. N. R., Mueller, A., Cheetham, A. K.,
Eds.; Wiley-VCH Verlag: Weinheim, 2007.
(3) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2002, 14, 1455–1471.
(4) Sasaki, T. J. Ceram. Soc. Jpn. 2007, 115, 9–16.
(5) Fang, M.; Kim, C. H.; Saupe, G. B.; Kim, H.-N.; Waraksa, C. C.;
Miwa, T.; Fujishima, A.; Mallouk, T. E. Chem. Mater. 1999, 11, 1526–
1532.
(6) Abe, R.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen,
K. J. Mater. Res. 1998, 13, 861–865.
(7) Sasaki, T.; Watanabe, M.; Hashizume, H.; Yamada, H.; Nakazawa,
H. J. Am. Chem. Soc. 1996, 118, 8329–8335.
(8) Liu, Z.-h.; Ooi, K.; Kanoh, H.; Tang, W.-p.; Tomida, T. Langmuir
2000, 16, 4154–4164.
‡

15938 9 J. AM. CHEM. SOC. 2008, 130, 15938–15943

of the nanosheets to self-assemble allows formation of restacked
lamellar aggregates with a disordered and porous nature,9leading to enhanced properties as found in H4Nb6O1710 and
HTiNbO511 for photocatalysis, Li/MnO212 for electrodes oflithium-ion batteries, and H/RuO2.113 for double-layer capacitors.
However, several hurdles must be overcome before possible
future applications can be realized. The synthesis of metal oxide
nanosheets, until very recently, has required multistep processing, involving a high-temperature solid-state reaction (to yielda thermodynamically stable precursor phase, e.g., K/MnO2),
protonation of interlayer alkali metal ions (to yield, e.g.,
H/MnO2), and an acid-base reaction with an aqueous solutionof quaternary ammonium cations (to finally yield negatively
charged nanosheets, e.g., MnO2δ- in the form of a colloidal
suspension). This top-down approach is costly and timeconsuming. Moreover, it is fairly difficult to exfoliate theprotonated compounds completely into single-layer nanosheets(monosheets). In practice, previously obtained nanosheets
possess a wide thickness distribution. It is highly desirable toobtain monosheets as a main product, because they possess alarge specific surface area and also serve as primitive building
units to construct well-defined self-assemblies with functionalorganic and/or inorganic counterions. In this work, we have(9) Ebina, Y.; Sasaki, T.; Harada, M.; Watanabe, M. Chem. Mater. 2002,
14, 4390–4395.
(10) Domen, K.; Ebina, Y.; Ikeda, S.; Tanaka, A.; Kondo, J. N.; Maruya,
K. Catal. Today 1996, 28, 167–174.
(11) Takagaki, A.; Sugisawa, M.; Lu, D.; Kondo, J. N.; Hara, M.; Domen,
K.; Hayashi, S. J. Am. Chem. Soc. 2003, 125, 5479–5485.
(12) Wang, L.; Takada, K.; Kajiyama, A.; Onoda, M.; Michiue, Y.; Zhang,
L.; Watanabe, M.; Sasaki, T. Chem. Mater. 2003, 15, 4508–4514.
(13) Sugimoto, W.; Iwata, H.; Yasunaga, Y.; Murakami, Y.; Takasu, Y.
Angew. Chem., Int. Ed. 2003, 42, 4092–4096.
10.1021/ja804503f CCC: $40.75  2008 American Chemical Society

Room-Temperature Synthesis of Manganese Oxide Monosheets

Figure 1. Photographs of (a) pale pink MnCl2 aqueous solution, (b) darkbrown colloidal suspension of MnO2 monosheets obtained by adding amixed aqueous solution of TMA · OH and H2O2 to the MnCl2 solution, and(c) black potassium-type birnessite precipitated by adding a KCl aqueous
solution to the colloidal suspension.

established a novel single-step route to synthesize the MnO2
monosheets within a day in a high yield. During our study, asingle-step synthesis of Ti1-δO2 monosheets with uniform shapeand size was reported by Yoon and co-workers,14 usingtitanium(IV) tetraisopropoxide in tetramethylammonium (TMA)
hydroxide aqueous solution. While their processing requires theapplication of heat under reflux to access the Ti1-δO2 monosheets,
our present reaction readily proceeds at room temperature (RT).
In addition, MnO2 monosheets have received much attention
for their potential applications in electrochemistry and catalysis,
mainly due to the low cost and safety hazards of manganese.
Lamellar stacked Li/MnO2 and layer-by-layer assembled poly(ethyleneimide)/MnO2, both of which are prepared from MnO2
monosheets, have been employed as electrodes for lithium-ion
batteries12 and double-layer capacitors,15 respectively, andexhibited improved performances associated with their porous
structures. It is thus apparent that such rapid and facile RT
processing promotes the work toward practical use of MnO2
monosheets.
2. Results and Discussion
2.1. Formation of MnO2 Monosheets. Our approach follows
the preparation of lithium- and sodium-type birnessites by usinghydrogen peroxide (H2O2) as an oxidizing agent for Mn2+
ions.16-18 Here we used, in addition to H2O2, TMA · OH in an
aqueous solution of manganese(II) chloride (MnCl2, Figure 1a;
see Supporting Information, Figure S1), which readily gave adark brown suspension in open air at RT (Figure 1b). The
observation of Tyndall light scattering in the suspension
confirms the colloidal dispersion of the product, and no
precipitation was observed in 3 days. Such a suspension was
obtained even when trivalent manganese(III) acetylacetonate(Mn(acac)3) was used instead of divalent MnCl2, presumably
as a reflection of the hydrolysis of Mn(acac)3. Similarly, spinel
LiMn2O4 and orthorhombic LiMnO2 were formed by hydrolysis
of Mn(acac)3 in a LiOH aqueous solution.19
Because of the difficulties in confirming the presence of MnO2
monosheets in the colloidal suspension, we observed the dried(14) Tae, E. L.; Lee, K. E.; Jeong, J. S.; Yoon, K. B. J. Am. Chem. Soc.
2008, 130, 6534–6543.
(15) Zhang, X.; Yang, W.; Evans, D. G. J. Power Sources 2008, 184, 695–
700.
(16) Feng, Q.; Sun, E.; Yanagisawa, K.; Yamasaki, N. J. Ceram. Soc. Jpn.
1997, 105, 564–568.
(17) Feng, Q.; Higashimoto, Y.; Kajiyoshi, K.; Yanagisawa, K. J. Mater.
Sci. Lett. 2001, 20, 269–271.
(18) Cai, J.; Liu, J.; Suib, S. L. Chem. Mater. 2002, 14, 2071–2077.
(19) Barriga, C.; Calero, A.; Morales, C. J.; Tirado, J. L. React. Solids
1989, 7, 263–271.

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aggregate using electron microscopy. Scanning electron microscopy (SEM) images of freeze-dried aggregates of the
product showed a flexible flaky form with lateral dimension of
micrometers, as shown in Figure 2a. The sheet-like structure
was also evident from a tapping-mode atomic force microscopy(AFM) image of the sample deposited by spin-coating of thecolloidal suspension onto a fluorinated mica substrate precoatedwith poly(ethyleneimine) (Figure 2c). The lateral dimension of
the sample lies in the range of 50-500 nm, which is much larger
than that of the Ti1-δO2 monosheets obtained by the singlestep approach described by Yoon et al. (<30 nm).14 The heightprofile scan demonstrates a fairly flat surface of the sheets with
an approximate thickness of 0.9 nm. The dimensions arecomparable to those of the MnO2 monosheets obtained by theconventional processing,20 suggesting a successful formationof the monosheets. Given the thickness of the MnO2 monosheet(0.52 nm), hydration on both sides of the monosheets canreasonably explain the observed thickness, as discussed in ref
21. Furthermore, the AFM image displays stepwise structurewith a step of ca. 0.7 nm, consistent with the thickness expectedfrom the MnO2 monosheet hydrated on the top. The mostremarkable observation in this study is that the majority of thecoverage area is composed of monosheets (Figure 2d). We notethat replacing the TMA cation with a bulkier tetrabutylammonium cation while keeping the other conditions the same
produces the similar results. When we used primary nbutylamine instead of quaternary tetraalkylammonium cations,on the other hand, the black precipitate of layered hybrid
n-butylammonium/MnO2 was readily obtained. This result will
be reported in a separate paper.22
Figure 3 shows the absorption spectra in the range of
250-800 nm as a function of concentration of the colloidal
suspension. An absorption band attributable to the d-d transitionof Mn ions in the MnO6 octahedra of the MnO2 monosheets19,23,24
was observed around 380 nm. This band follows Beer’s law inthe measured concentration range (inset of Figure 3), and themolar extinction coefficient at 380 nm is estimated to be 9.6 ×
103 M-1 cm-1, provided that the composition is (TMA)0.20MnO2 ·0.2H2O (Vide infra). The energy and intensity of the band arecomparable to those obtained by the conventional multistep
processing.20
A similar strategy to obtain metal oxide nanosheets withouta high-temperature treatment was recently implemented by Oaki
and Imai.25 They employed ethylenediaminetetraacetate (EDTA)
as chelating agent for Mn2+ ions so as to inhibit the rapid
precipitation of Mn(OH)2. The Mn2+ ions were oxidized slowly
by dissolved oxygen in the solution. However, the precipitateis far from monosheets but consists of multilayers with athickness of at least 10 nm (i.e., more than 10 layers), and thewhole processing requires 3-5 days. In our processing, on theother hand, H2O2 serves as an oxidizing agent to form MnO2
monosheets in alkaline medium, and TMA cations, previously(20) Omomo, Y.; Sasaki, T.; Wang, L.; Watanabe, M. J. Am. Chem. Soc.
2003, 125, 3568–3575.
(21) Sasaki, T.; Ebina, Y.; Kitami, Y.; Watanabe, M.; Oikawa, T. J. Phys.
Chem. B 2001, 105, 6116–6121.
(22) Kai, K.; Yoshida, Y.; Kageyama, H.; Saito, G.; et al., to be submitted.
(23) Gao, Q.; Giraldo, O.; Tong, W.; Suib, S. L. Chem. Mater. 2001, 13,
778–786.
(24) Wang, L.; Omomo, Y.; Sakai, N.; Fukuda, K.; Nakai, I.; Ebina, Y.;
Takada, K.; Watanabe, M.; Sasaki, T. Chem. Mater. 2003, 15, 2873–
2878.
(25) Oaki, Y.; Imai, H. Angew. Chem., Int. Ed. 2007, 46, 4951–4955.
J. AM. CHEM. SOC. 9 VOL. 130, NO. 47, 2008 15939

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Kai et al.

Figure 2. SEM images of (a) freeze-dried aggregate of MnO2 monosheets and (b) self-assembled K/MnO2. (c) Tapping-mode AFM image of MnO2 monosheets
and height profile along the red line in the image. The monosheets are deposited onto fluorinated mica substrate precoated with poly(ethyleneimine) by
spin-coating (1000 rpm for 15 s and 2000 rpm for 30 s) of 0.1 g L-1 colloidal suspension. (d) Histogram of thickness of the MnO2 sheets on the substrate.

used for the exfoliation of layered H/MnO2,8,26 would play anessential role during the course of two-dimensional growth ofthe monosheets; namely, the cations serve not only to inhibit
the flocculation of the monosheets but also to exfoliate thelocally aggregated monosheets in the suspension. Although we
do not have information about the mechanism of production of
the MnO2 monosheets, as the d-d absorption band appears andsaturates within the measurement limit of our time acquisition
measurement system (ca. 0.2 s), we believe that our rapid andfacile processing promotes the practical use of the MnO2
monosheets in lithium-ion batteries, catalysts, selective adsorbents, and so on, and also is generally applicable to thefabrication of other metal oxide monosheets.
2.2. Structural and Physical Properties of Dried Aggregate. In
order to obtain further evidence of the formation of MnO2
monosheets, powder X-ray diffraction (XRD) of a dried sampleof the colloidal suspension, which was separated by filtration,
washed with copious amounts of distilled water and methanol,and then air-dried at RT, was measured. The XRD pattern is
indexed as a hexagonal unit cell and has intense (00l) reflections
associated with the preferred orientation of the sample (Figure4a), which is similar to that of the dried aggregate TMA/MnO2
obtained by the conventional processing.26,27 The interlayer
spacing of 0.95 nm indicates the flocculation of the MnO2
monosheets together with TMA and water molecules,26,27 which(26) Yang, X.; Makita, Y.; Liu, Z.-h.; Sakane, K.; Ooi, K. Chem. Mater.
2004, 16, 5581–5588.

(27) Brock, S. L.; Sanabria, M.; Suib, S. L.; Urban, V.; Thiyagarajan, P.;
Potter, D. I. J. Phys. Chem. B 1999, 103, 7416–7428.

Figure 3. UV-vis absorption spectra of the colloidal suspension of MnO2

monosheets as a function of the nanosheet concentration: from bottom totop, 1.03 × 10-5, 2.06 × 10-5, 4.12 × 10-5, 6.18 × 10-5, 8.24 × 10-5,
1.03 × 10-4, 1.24 × 10-4, 1.44 × 10-4, 1.65 × 10-4, 1.85 × 10-4, and2.06 × 10-4 M. The inset shows the plot of absorbance at 380 nm againstmonosheet concentration, and the solid line corresponds to the fit indicating
the molar extinction coefficient of 9.6 × 103 M-1 cm-1.

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Room-Temperature Synthesis of Manganese Oxide Monosheets

Figure 4. Powder X-ray diffraction patterns of (a) dried aggregate TMA/

MnO2, (b) self-assembled K/MnO2, (c) self-assembled TTF/MnO2, and (d)self-assembled BEDO-TTF/MnO2.
Scheme 1. Molecular Structures of Tetrathiafulvalene (TTF) andBis(ethylenedioxy)-Tetrathiafulvalene (BEDO-TTF)

should be driven mainly by electrostatic interactions between
negatively charged monosheets and TMA cations. Together withthis observation, the elemental analysis (Calcd: C, 9.12; H, 2.68;
N, 2.66. Found: C, 9.16; H, 2.67; N, 2.69.) leads us to conclude
that the composition of the dried aggregate is formulated as(TMA)0.20MnO2 · 0.2H2O, which yields the valence state on Mnof ca. +3.8.
Figure 5a displays the temperature dependence of DC
susceptibility (χ) for the dried aggregate in an applied magnetic
field of 1 kOe. Above 20 K, it follows the Curie-Weiss
expression, χ ) C(T - θ)-1, with Curie constant (C) of 1.44
emu K mol-1 and Weiss temperature (θ) of -36.4 K. As seenin the inset of Figure 5a, a susceptibility cusp was observed
around 10 K, below which there is a distinct difference between
the zero-field-cooled (ZFC) and field-cooled (FC) susceptibilities. The cusp was also observed in both in-phase (χ′) and outof-phase (χ′′) components of the AC susceptibility and shows
an increase of ca. 1 K with increasing frequency (ν) on goingfrom 1 Hz to 1 kHz (Figure 5b for χ′). The frequency shift inTf (peak temperature of χ′) is diagnostic for a glassy effect suchas spin-glass or superparamagnetism and is defined as γ ) ∆Tf/
[Tf(0)∆(log ν)], where Tf(0) is the peak temperature extrapolated
at log ν ) 0. The calculated small γ value (0.035) strongly
indicates that the behavior is not due to the superparamagnetism,
but arises from the spin-glass formation.28 This behavior should
be related to the geometrical frustration caused by the triangulararrangement of the mixed-valence Mn4+/3+ ions in the MnO2
layer. A similar trend was observed for K/MnO2, H/MnO2, andTMA/MnO2 obtained by the conventional processing (Supporting Information, Figures S2-S4).
2.3. Self-Assembly of MnO2 Monosheets. Since the MnO2
monosheets have a negative charge, as mentioned above, theelectrostatic interactions with organic and/or inorganic cations(28) Mydosh, J. A. Spin Glass; Taylor & Francis: London, 1993.

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would bring about self-assembly to yield lamellar aggregatesusing the present colloidal suspension. In the following, we
present the preparation of lamellar aggregates formed with alkali
and π-conjugated organic cations, and Langmuir-Blodgett (LB)
films deposited onto hydrophilic silica substrate.
2.3.1. Potassium-Type Birnessite. A self-assembled potassium-type birnessite was readily precipitated in nearly quantitative yield by adding an excess of KCl to the colloidal suspension
shown in Figure 1c. In comparison with the dried aggregate
TMA/MnO2, the XRD pattern (Figure 4b) and SEM image(Figure 2b) of the self-assembled K/MnO2 indicate that it hasa layered structure as well, but the interlayer spacing is greatly
reduced to 0.70 nm, which is comparable to that of thepotassium-type birnessite previously reported.26 The K/Mn ratio
was estimated to be 0.20 on the basis of energy-dispersive X-ray
spectroscopy (EDS; Supporting Information, Figure S5), andthe valence state on Mn of ca. +3.8 is in good agreement withthat of the dried aggregate.
2.3.2. Organic-Inorganic Layered Hybrids. Layered hybrids
composed of tetrathiafulvalene (TTF) and bis(ethylenedioxy)TTF (BEDO-TTF) (Scheme 1), which have afforded a numberof organic metals including superconductors,29,30 were immediately obtained by adding powdered (TTF)3(BF4)231 and(BEDO-TTF)2BF4 to the colloidal suspension. Taking themolecular lengths of TTF and BEDO-TTF as ca. 0.9 and 1.3
nm, respectively, the significantly expanded interlayer spacingsof 1.45 and 1.84 nm (Figure 4c,d) suggest the formation of thelayered hybrids with TTF analogues oriented almost perpendicular to the MnO2 layers. According to the S/Mn ratios
estimated from EDS (Supporting Information, Figures S6 andS7), the composition of the hybrids can be formulated as(TTF)0.15MnO2 · 1.0H2O and (BEDO-TTF)0.21MnO2 · 1.6H2O, inwhich the water contents were estimated from thermogravimetricanalyses. UV-vis absorption spectra of the hybrids dispersed
in a compressed KBr disk show a low-energy band around 5 ×
103 cm-1 (Supporting Information, Figures S8 and S9). Thisband can be assigned to the intermolecular transition between
partially charged TTF analogues, suggesting that these hybrids
can be potentially a metal,29,30 though the semiconducting
behavior observed was possibly due to the inhomogeneouspacking of TTF analogues.
2.3.3. Langmuir-Blodgett (LB) Films. Figure 6a shows thesurface pressure-area (π-A) isotherm of the diluted colloidal
suspension of MnO2 monosheets (0.01 g L-1). A clear solidcondensed phase was observed, as was the case for Ti1-δO2
monosheets.32 The LB films were fabricated on fused silica
substrate by the LB deposition method without any amphiphilic
additives at the air-water interface. UV-vis spectra of the films
for each deposition process display an absorption band around368 nm (Figure 6b), similar to those observed for the colloidal
suspensions, and the absorbance of the band increases linearlywith each deposition process (inset of Figure 6b). Since AFM
observation of the film fabricated by a single deposition process
reveals that the majority of the coverage area is composed of
MnO2 monosheets, it appears that about one layer of monosheets
was transferred for each deposition process. The XRD patternof the film with 30 layers, which displays a reflection at 2θ )(29) Saito, G.; Yoshida, Y. Bull. Chem. Soc. Jpn. 2007, 80, 1–137.
(30) Horiuchi, S.; Yamochi, H.; Saito, G.; Sakaguchi, K.; Kusunoki, M.
J. Am. Chem. Soc. 1996, 118, 8604–8622.
(31) Wudl, F. J. Am. Chem. Soc. 1975, 97, 1962–1963.
(32) Muramatsu, M.; Akatsuka, K.; Ebina, Y.; Wang, K.; Sasaki, T.; Ishida,T.; Miyake, K.; Haga, M. Langmuir 2005, 21, 6590–6595.
J. AM. CHEM. SOC. 9 VOL. 130, NO. 47, 2008 15941

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Kai et al.

Figure 5. Temperature dependence of magnetic susceptibility of the dried aggregate TMA/MnO2. (a) DC magnetic susceptibility (χ) under an appliedmagnetic field of 1 kOe. Above 20 K, the χ value follows the Curie-Weiss law with Curie constant of 1.44 emu K mol-1 and Weiss temperature of -36.4
K. The inset shows the low-temperature range of χ, showing the difference between the zero-field-cooled (ZFC) and field-cooled (FC) susceptibilities. (b)In-plane AC magnetic susceptibility (χ′) at different frequencies (1, 10, 100, and 1000 Hz).

Figure 6. (a) Surface pressure-area (π-A) isotherm of MnO2 monosheets in a diluted colloidal suspension (0.01 g L-1). The initial surface area of thesubphase was 130 cm2. The values in the horizontal axis of the isotherm indicate the relative surface area against the initial one. (b) UV-vis absorption
spectra of the LB films as a function of the number of layers (1-10). The inset shows the plot of absorbance at 368 nm against the number of layers.

12°, corresponding to an interlayer spacing of 0.73 nm (Supporting Information, Figure S10), confirms the regular stackingof the MnO2 monosheets, although the nature of the interlayer
cations is unclear at present.
3. Conclusions

In this study, we found a rapid and facile route to access acolloidal suspension of MnO2 monosheets in a high yield.
Although our single-step approach cuts the processing time toless than a day and eliminates the need for special equipment,
the structural, spectroscopic, and magnetic properties of the
products are comparable to those of MnO2 monosheets (or theiraggregate) obtained by the conventional multistep processing.
Also, our process makes it possible to fabricate high-purity
organic-inorganic layered hybrids and LB films composed of
the MnO2 monosheets. Further works, particularly on theapplications of our processing into the fabrication of other metaloxide monosheets, are in progress.
4. Experimental Section
4.1. Measurements. SEM and EDS experiments were conductedwith a JEOL JSM-5510LVN scanning electron microscope operated
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J. AM. CHEM. SOC. 9 VOL. 130, NO. 47, 2008at 20 kV. Samples are mounted on carbon tape. A Seiko SPA400
AFM instrument was employed to visualize the surface topographyof the nanosheets. Measurements were carried out in tapping modewith a silicon-tip cantilever having a force constant of 20 N m-1.
Samples were deposited onto fluorinated mica substrate (FluoroPhologopite, Topy Industry Ltd.) precoated with poly(ethyleneimine) by spin-coating (1000 rpm for 15 s and 2000 rpm for 30 s)of a 0.1 g L-1 colloidal suspension. Powder XRD measurements
were carried out with a MAC Science M18XHF diffractometerusing Cu KR radiation at a scanning rate of 0.01° s-1 in a 2θ rangeof 5-80°. UV-vis absorption spectra were taken in a quartz cellwith light path length of 1 cm on a Shimadzu UV-3100 spectrophotometer (250-800 nm). Time acquisition of the absorption
spectra was measured using an Ocean Optics HR-2000 spectrophotometer. FT-IR spectra were taken in dispersed KBr pellets on
a Perkin-Elmer 1000 series spectrophotometer (400-7800 cm-1).
A Quantum Design MPMS-XL superconducting quantum interference device (SQUID) magnetometer was used to collect DC
magnetic susceptibility data between 1.9 and 300 K. The AC
magnetic susceptibilities for 1.9-20 K were determined in the rangeof 1 Hz to 1 kHz using a Quantum Design MPMS-5S SQUID
magnetometer.
4.2. Synthesis. Solvents (water, methanol, acetonitrile, andchloroform) were distilled prior to use. MnCl2 · 4H2O (99.9%, Wako
Pure Chemical), TMA · OH (1.0 M in H2O, Aldrich), H2O2 (30 wt

Room-Temperature Synthesis of Manganese Oxide Monosheets% in H2O, Santoku Chemical), and HBF4 (48 wt % in H2O, Aldrich)were commercially available and used without purification. Commercially available TTF was purified by sublimation twice. BEDOTTF was synthesized according to the literature procedure33 andrecrystallized twice from cyclohexane. (TTF)3(BF4)2 was preparedusing the chemical oxidation of TTF by H2O2 in the presence of
HBF4 in acetonitrile, according to literature procedure.31 (BEDOTTF)2BF4 was prepared in the same manner as (TTF)3(BF4)2.
4.2.1. Preparation of Colloidal Birnessite Monosheets. Typically, 20 mL of a mixed aqueous solution of 0.6 M TMA · OH and3 wt % H2O2 was added to 10 mL of 0.3 M MnCl2 · 4H2O aqueous
solution within 15 s. The resulting dark brown suspension was
stirred vigorously overnight in the open air at RT, which isaccompanied by O2 gas generation. Dried aggregate was separated
by filtration (Millipore, type-JH, 0.45 µm pore size), washed withcopious amounts of distilled water and methanol, and then air-driedat RT. No trace of Cl was detected from EDS of the dried aggregate(Supporting Information, Figure S11).
4.2.2. Preparation of Self-Assembled Potassium-Type Birnessites. Self-assembly of MnO2 monosheets was induced by mixing
the colloidal suspension of the monosheets with a KCl aqueous
solution. Typically, 50 mL of 1 M KCl aqueous solution was added
dropwise within 1 h to 50 mL of the colloidal suspension of MnO2
monosheets (2 × 10-2 M) at 40 °C. Immediately after the addition,flocculation occurred in the mixed solution. After the solution wasleft to stand at RT for a week, the resulting black precipitate wasfiltered off, washed with copious amounts of distilled water andmethanol, and air-dried at RT (yield 94%). The K/Mn ratio wasestimated to be 0.20 on the basis of EDS (Figure S5).
4.2.3. Preparation of Self-Assembled Organic-Inorganic Layered Hybrid TTF/MnO2. Typically, 0.50 g (0.64 mmol) of
powdered (TTF)3(BF4)2 was added to 300 mL of the colloidal
suspension of MnO2 monosheets (3 × 10-3 M) during N2 bubblingat RT. After a few minutes of stirring, flocculation occurred in themixed solution. The resulting black precipitate was filtered off,
washed with acetonitrile, and air-dried at RT (yield 78%). The TTF/
Mn ratio was estimated to be 0.15 on the basis of EDS (Figure

ARTICLES

S6), and the IR absorption spectrum indicates no trace of BF4 anions(Supporting Information, Figure S12). Layered hybrid BEDO-TTF/
MnO2 was prepared by a procedure similar to that for TTF/MnO2
as described in the Supporting Information.
4.2.4. Fabrication of LB Films. Fused silica substrates fordepositing the films were cleaned with detergent followed by several
rinses with water, and sonicated in fresh water three times for 30
min each. Their surfaces were made hydrophilic by treating withpiranha solution (H2O2:H2SO4 ) 7:3) for 90 min, followed bysonication and rinsing in water. The π-A isotherm measurements
and film fabrications employed a USI Co. Ltd. model FDS-23 LB
trough equipped with a Wilhelmy plate for pressure sensing. A
diluted aqueous suspension of MnO2 monosheets (0.01 g L-1) wasused as a subphase. After the suspension was left to stand for 1 h,
the surface of the subphase was compressed at a rate of 5 cm2
min-1 by moving the barrier. The compressed film was deposited
onto hydrophilic silica substrate at a surface pressure of 10 mN
m-1 by the vertical deposition method. All experiments were carriedout at 20 °C, and high-purity water (<0.1 µS s-1) was used for all
operations.
Acknowledgment. This work was in part supported by the 21stCentury COE program on Kyoto University Alliance for Chemistry
and Grants-in-Aid for Scientific Research (Nos. 15205019, 17684018,
19052004, and 20360035) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. K.K. acknowledges support
by the Global COE program “Integrated Materials Science” (No.
B-024).
Supporting Information Available: Detailed characterizationof the products. This material is available free of charge via
the Internet at http://pubs.acs.org.
JA804503F(33) Suzuki, T.; Yamochi, H.; Srdanov, G.; Hinkelmann, K.; Wudl, F.
J. Am. Chem. Soc. 1989, 111, 3108–3109.

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