Synthesis and structural study on MnO2 nanosheet material by X-ray absorption spectroscopic technique

Article abstract of DOI:10.1021/jp053854k

MnO₂ nanosheet with acetylene black composite material has been synthesized from layered K_0.45MnO₂ powder. The electrochemical lithiation reaction of nanosheet composite material proceeds in a different manner from that of the parent material, layered K _0.45MnO₂ powder. To elucidate the origin of the changes in discharge profile, the electronic and local structures for the nanosheet composites and its parent and protonated material have been investigated by Mn K-edge and O K-edge X-ray absorption spectroscopy (XAS). The results showed that local and electronic structure around Mn ions does not vary during nanosheet formation, while significant changes in electronic structure around oxide ions were observed. Accordingly, it is suggested that the difference observed in discharge profile is due to the electronic structural change induced by nanosheet formation.

Full text of DOI:10.1021/jp053854k

174
J. Phys. Chem. B 2006, 110, 174-177
Synthesis and Structural Study on MnO2 Nanosheet Material by X-ray Absorption
Spectroscopic Technique
Yoshihiro Kadoma,† Yoshiharu Uchimoto,‡ and Masataka Wakihara*,†
Department of Applied Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku,
Tokyo 152-8552, Japan, and Department of Interdisciplinary EnVironment, Graduate School of Human and
EnVironmental Studies, Kyoto UniVersity, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan
ReceiVed: July 13, 2005; In Final Form: September 2, 2005
MnO2 nanosheet with acetylene black composite material has been synthesized from layered K0.45MnO2 powder.
The electrochemical lithiation reaction of nanosheet composite material proceeds in a different manner from
that of the parent material, layered K0.45MnO2 powder. To elucidate the origin of the changes in discharge
profile, the electronic and local structures for the nanosheet composites and its parent and protonated material
have been investigated by Mn K-edge and O K-edge X-ray absorption spectroscopy (XAS). The results showed
that local and electronic structure around Mn ions does not vary during nanosheet formation, while significant
changes in electronic structure around oxide ions were observed. Accordingly, it is suggested that the difference
observed in discharge profile is due to the electronic structural change induced by nanosheet formation.
Introduction
It is important to investigate the electrochemical reaction
mechanism at the electrode/electrolyte interface, not only for
the fundamental electrochemistry but also for the development
of electrochemical applications, such as lithium ion battery, fuel
Figure 1. Schematic figure of the exfoliation: (a) layered K0.45MnO2
cell, etc. Most of the interfacial electrochemical reaction studies
have been performed by single crystals of noble metals (such
as Pt and Au). On the other hand, 3d transition metal oxides
are especially attractive for the practical use of electrochemical
devices;1-6 therefore, the knowledge about detailed interface
reaction mechanism between electrode/electrolyte is strongly
demanded. Nevertheless, few papers in this field have been
reported, mainly due to the difficulty of characterizing the
surface structure of transition metal oxides. Recently, Sasaki et
al.7 reported the preparation of MnO2 nanosheets that have many
characteristic features as a single crystal in two dimensions:
molecular size thickness, micro size width of face, large surface
area, and so forth (see Figure 1).8-10 Therefore, the plane surface
being perpendicular to the z-axis dominantly forms an interface
with electrolyte solutions, giving us the concrete image regarding
the electrochemical reaction with MnO2 nanosheet. Hence, a
detailed investigation of the nanosheet surface structure is
required.
powder and (b) MnO2 nanosheet.
described in ref 8. A stoichiometric mixture of K2CO3 (Wako
Pure Chemical Industries) and Mn2O3 was heated at 800 °C
for 30 h under an O2 gas flow. [Mn2O3 was prepared by the
calcination of MnCO3 (99.9%, Soekawa Chemicals) at 600 °C
for 48 h in air.]
Protonated layered K0.45MnO2 was prepared by soaking 5 g
of K0.45MnO2 in 1 dm3 of a 1 mol dm-3 HCl aqueous solution
under stirring for 10 days. HCl solution was refreshed every
day to promote complete protonation. The resulting material
was washed and air-dried at ambient temperature, yielding the
protonated material. Then, the colloidal suspension of MnO2
nanosheet was prepared by vigorously stirring 0.2 g of the
protonated material in 500 cm3 of tetra-n-butylammonium
hydroxide [(C4H9)4NOH] solution at room temperature for 10
days. Acetylene black (AB) was mixed with the colloidal
suspension (the ratio of AB and colloidal suspension was 9:1,
w/w) and thereafter was vigorously agitated at room temperature
for 6 h. The water was evaporated by agitating the solution at
80 °C. After the composite suspension was evaporated for 1
week, the composite material was obtained by vacuum-drying
at 100 °C for 1 day. The protonation and exfoliation process
was followed according to the previous report.8
This paper describes the synthesis procedure of the MnO2
nanosheet and changes in the electronic and local structures for
the nanosheet, which were investigated by using Mn K-edge
and O K-edge XAS.11-12 Electrochemical lithiation reaction was
also studied to reveal the difference in equilibrium state between
the nanosheet and bulk MnO2 structure.
Characterization. Crystalline phase identification was carried
out by powder X-ray diffraction (XRD) on a Rigaku RINT2500V
diffractometer, with Cu KR radiation (λ ) 0.154 05 nm),
equipped with a curved graphite monochromator, at room
temperature.
XAS experiments were performed at the synchrotron facility,
Photon Factory (PF; Tsukuba, Japan). Mn K-edge and O K-edge
X-ray absorption near-edge structure (XANES) and extended
Experimental Section
Sample Preparation. Layered-type K0.45MnO2 (see Figure
1) was synthesized by conventional solid-state reaction as
* Corresponding author: tel +81-3-5734-2145; fax +81-3-5734-2146;
e-mail mwakihar@o.cc.titech.ac.jp.
† Tokyo Institute of Technology.
‡ Kyoto University.
10.1021/jp053854k CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/13/2005

Synthesis and Structural Study on MnO2 Nanosheets
J. Phys. Chem. B, Vol. 110, No. 1, 2006 175
Figure 2. XRD pattern for the layered K0.45MnO2 powder as
synthesized.
X-ray absorption fine structure (EXAFS) spectra were measured
in order to clarify the change in electronic structure and local
symmetry. O K-edge XANES spectra were measured at BL 11A
beam lines at PF by the method of total electron yields at room
temperature. Mn K-edge EXAFS measurements were performed
at BL 9A and 12C beam lines at PF, at room temperature in
transmission mode with gas ionization detectors. The appropriate
amount of each sample powder for EXAFS measurement
(determined to obtain suitable X-ray absorption) was mixed with
200 mg of boron nitride as a binder. The absolute energy scales
were calibrated by using the value of Mn metal. Fourier
transformations were performed over the k range 3-12 Å-1 with
k3 weighting. The structural parameters were determined by
curve-fitting procedures with Rigaku REX2000 data analysis
software.13 Theoretical parameters of backscattering factors and
phase shifts used in the curve-fitting analysis were calculated
by FEFF8 (multiple-scattering XAS simulation code: see details
in refs 14 and15).
Figure 3. Charge-discharge profiles of (a) layered K0.45MnO2 powder
and (b) nanosheet composite material. Current density was 10 µA cm-2
cutoff potential was 2.0-4.3 V.
;
Electrochemical Lithiation. The electrochemical lithiation
was performed with a three-electrode-type cell. The ratio of the
composite electrode for this measurement of the parent material
was 70 wt % active materials of sample, 25 wt % acetylene
black, and 5 wt % poly(tetrafluoroethylene) (PTFE), while the
electrode of the nanosheet composite was composed of 90 wt
% active materials and 10 wt % PTFE. Lithium foil was used
as counterelectrode and reference electrode, with current col-
lectors of aluminum mesh. The electrolyte used was 1 mol dm-3
LiClO4 in 1:1 (v/v) ethylene carbonate (EC)/diethyl carbonate
(DEC) solution (Tomiyama Pure Chemical Industries Ltd). All
of the experiments were carried out galvanostatically at 10 µA
cm-2 in an Ar-filled glovebox at room temperature. The cutoff
potentials were set at 2.0 V (vs Li/Li+) and 4.3 V.
Figure 4. Mn K-edge XANES spectra of layered K0.45MnO2 powder
(O) protonated material (4), and nanosheet composite material (0),
with Mn2O3 (‚‚‚) and MnO2 (- - -) for reference.
difference in discharge profile of layered K0.45MnO2 and the
nanosheet composite would result from the difference between
the lithium ion insertion into bulk structure of MnO2 and
adsorption onto surface structure. Since the cell potential in the
discharge process was affected by the crystal and electronic
structure of host materials, the following section will describe
them by means of the XAS technique.
It is well-known that the X-ray absorption near-edge structure
(XANES) technique is sensitive to the electronic configuration
and local environment around X-ray absorbed atoms, such as
valence state, chemical bonding character, local coordination,
and so on. To clarify the changes in electronic structure and
local symmetry for the prepared sample, Mn K-edge and O
K-edge XANES spectra were measured. Figure 4 depicts the
Mn K-edge XANES spectra for the nanosheet composite
material, its parent material, and protonated material, together
with the data for reference compounds Mn2O3 and MnO2, where
the valence states of manganese ions are +3 and +4, respec-
tively. The absorption edge energy of the nanosheet composite
does not change from the parent and protonated materials,
indicating that the valence states of manganese ions in three
samples are almost the same.18 Chances are the cations of
ammonium hydroxide are replaced by H+ on the nanosheet
surface to maintain charge neutrality during the exfoliation
process. Considering the XANES spectra of the reference
materials, the valence states of manganese ions for the prepared
samples are between +3 and +4, which is in good agreement
with previous reports and also the expectation from the formula
Results and Discussion
The powder XRD pattern for the layered K0.45MnO2 powder
is showed in Figure 2, in good agreement with the previously
reported data.16 Two intense diffraction peaks at 12.5° and 25.1°
can be indexed as 001 and 002 in the hexagonal cell. Accord-
ingly, remarkable preferred orientation is observed.
The discharge profile of the layered K0.45MnO2 powder is
represented in Figure 3a. During the first discharge curve, the
layered K0.45MnO2 powder indicates a pseudoplateau near 3.1
V due to the redox reaction of the Mn3+/Mn4+ couple.17 Figure
3b shows the charge-discharge profiles of the nanosheet
composite material of MnO2 nanosheet mixed with acetylene
black. In the case of the nanosheet composite, the initial charge
capacity indicates the removal of tetra-n-butylammonium
hydroxide cations, which were used for exfoliation of layered
K
0.45MnO2. On the other hand, the first discharging of the
nanosheet composite illustrated continual decrease in its voltage
diagrams from 4.0 to 2.0 V with capacity of ca. 150 mA h g-1
which is comparable to that of layered K0.45MnO2. Such a
,

176 J. Phys. Chem. B, Vol. 110, No. 1, 2006
Kadoma et al.
TABLE 1: Mn K-Edge EXAFS Spectra Determined
Structural Parametersa for Layered K0.45MnO2 Powder,
Protonated Material, and Nanosheet Composite Material
Mn-O
Mn-Mn
residuec
(%)
CNb R/Å σ/Å CNb R/Å σ/Å
layered K0.45MnO2 powder
protonated K0.45MnO2
MnO2 nanosheet + AB
6
6
6
1.899 0.053
1.893 0.063
1.904 0.075
6
6
6
2.894 0.043 2.943
2.864 0.036 3.746
2.888 0.087 3.348
a CN, coordination number; R, bond length; σ, Debye-Waller factor.
b Fixed parameter for the curve-fitting procedure. c Residue )
100∑{k3øobs(k) - k3øcalc(k)}2/∑{k3øobs(k)}2.
Figure 5. Fourier transforms of Mn K-edge EXAFS spectra of layered
K0.45MnO2 powder (O), protonated material (4), and nanosheet
composite material (0). The range over which Fourier filtering has
been applied is shown by the arrows.
Figure 7. O K-edge XANES spectra of layered K0.45MnO2 powder
(O), protonated material (4), and nanosheet composite material (0).
(R < 4.0 %) and good accord between observed and simulated
EXAFS oscillation shown in Figure 6. As seen in Table 1, no
marked changes in interatomic distances were observed with
protonation and exfoliation reaction (the changes are less than
∼1.0 %), indicating the oxidation state and local coordination
environment around manganese ions are unchanged between
bulk and nanosheet materials.
Figure 7 shows the O K-edge XANES spectra for the
nanosheet composite material and the parent and protonated
materials. It is known that the broad peaks at more than about
535 eV correspond to the transition from O 1s to the mixing
bands of O 2p and Mn 4sp orbital, while the features around
530 eV are ascribed to the transition from O 1s to the mixing
bands of O 2p and Mn 3d orbital.19-20 The peak feature in the
O K-edge XANES for the layered K0.45MnO2 powder is almost
the same as the protonated material, implying that the electronic
structure of two samples is unchanged by protonation. Two
intensive absorption peaks at 529 eV (peak A in Figure 7) and
532 eV (peak B in Figure 7) were observed in the parent and
protonated materials. On the other hand, the peak at 529 eV
(peak A) disappears in the nanosheet composites, and large
spectral difference was also observed in O 2p-Mn 4sp
hybridized level. Therefore, it is revealed that the exfoliation
reaction caused electronic structural changes around oxide ions
but roughly no changes around manganese ions. The formation
of dangling bonds on the nanosheet surface, which is due to
the breakage of the oxygen bond during exfoliation, would cause
significant changes in O K-edge XANES, meaning electronic
structural changes around oxide ions.
Figure 6. Inverse Fourier transforms of Mn K-edge EXAFS spectra
of (a) layered K0.45MnO2 powder, (b) protonated material, and (c)
nanosheet composite material.
of K0.45MnO2 (the formal oxidation state of manganese ions
should be +3.55). In addition, it is noted that the difference in
the shape of the spectra for the protonation and exfoliation of
layered K0.45MnO2 shows no considerable change. Therefore,
the local environment with respect to the crystal and electronic
structure was kept unchanged during the reaction mentioned
above.
The local structure of manganese ion in the layered K0.45
-
MnO2 powder, protonated material, and nanosheet composite
material was investigated by the EXAFS technique. As shown
in Figure 5, the k3-weighted Mn K-edge EXAFS spectra for
the samples are Fourier-transformed (FT) in the k range 3.0-
12.00 Å-1. All of the FT spectra, which show pseudoradial
distribution functions, exhibit two intense peaks at 1.5 and 2.5
Å attributed to the Mn-O and Mn-Mn interactions, respec-
tively. To perform the nonlinear curve-fitting analysis, the FT
spectra are inversely Fourier-transformed to k space as shown
in Figure 6, and the two-shell model composed of Mn-O and
Mn-Mn with six coordinates was adopted for the fittings (the
range over which Fourier filtering has been applied is shown
by the arrows in Figure 5). The results of fitted structural
parameters are summarized in Table 1. The validity of the
present fitting results was supported by relatively small residues
Summing up, the difference in electronic structure between
nanosheet and bulky MnO2 is observed only around oxide ions,
while no marked change was observed around manganese ions.
Therefore, the electrochemical behaviors of lithiation into
layered material and adsorption onto nanosheet would strongly
depend on the difference in electronic structure because there
is no considerable change around manganese ions.
Acknowledgment. The present work was supported in part
through a Grant-in-Aid for Scientific Research on Priority Area,
Nanoionics (439) by the Ministry of Education, Culture, Sports,
and Technology and Science. Y. K. thanks the 21st-century COE

Synthesis and Structural Study on MnO2 Nanosheets
J. Phys. Chem. B, Vol. 110, No. 1, 2006 177
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