Local atomic arrangement and electronic structure of nanocrystalline transition metal oxides determined by X-ray absorption spectroscopy

Article abstract of DOI:10.1021/jp034049d

The local crystal structure and electronic configuration of transition metal in X-ray amorphous MnO₂ and CrO₂ nanocrystals have been examined by using X-ray absorption (XAS) spectroscopy at Mn K and Cr K-edges. The Mn K-edge XAS study reveals that tetravalent manganese ions are stabilized in ?±-MnO₂-type local atomic arrangement consisting of the intergrowth of edge- and corner-shared MnO₆ octahedra. On the other hand, it is found from Cr K-edge XAS results that nanocrystalline CrO₂ possesses two different kinds of local structures around chromium, that is, Cr₂O₃-type with octahedral site and CrO₃-type with tetrahedral site. The presence of Cr^+VI species on the surface would be helpful for Li grafting process, giving rise to excellent electrochemical performances. This work can be regarded as a strong evidence for the usefulness of XAS to study nanocrystalline electrode materials.

Full text of DOI:10.1021/jp034049d

J. Phys. Chem. B 2003, 107, 5791-5796
5791
Local Atomic Arrangement and Electronic Structure of Nanocrystalline Transition Metal
Oxides Determined by X-ray Absorption Spectroscopy
Seong-Ju Hwang*,† and Jin-Ho Choy‡
Department of Applied Chemistry, College of Natural Sciences, Konkuk UniVersity Chungju Campus,
Chungju, Chungbuk 380-701, Korea, and National Nanohybrid Materials Laboratory, School of Chemistry,
Seoul National UniVersity, Seoul 151-747, Korea
ReceiVed: January 9, 2003; In Final Form: March 24, 2003
The local crystal structure and electronic configuration of transition metal in X-ray amorphous MnO2 and
CrO2 nanocrystals have been examined by using X-ray absorption (XAS) spectroscopy at Mn K and Cr
K-edges. The Mn K-edge XAS study reveals that tetravalent manganese ions are stabilized in R-MnO2-type
local atomic arrangement consisting of the intergrowth of edge- and corner-shared MnO6 octahedra. On the
other hand, it is found from Cr K-edge XAS results that nanocrystalline CrO2 possesses two different kinds
of local structures around chromium, that is, Cr2O3-type with octahedral site and CrO3-type with tetrahedral
site. The presence of Cr+VI species on the surface would be helpful for Li grafting process, giving rise to
excellent electrochemical performances. This work can be regarded as a strong evidence for the usefulness
of XAS to study nanocrystalline electrode materials.
Introduction
of an absorber through extended X-ray absorption fine structure
(EXAFS) analysis.13
Recently intense research interest has been focused on the
nanocrystalline transition metal oxides as promising electrode
materials for lithium rechargeable batteries.1-5 One of the main
advantages of nanocrystalline materials is their high structural
stability for repeated Li insertion-disinsertion processes, leading
to an improved cyclability and a great discharge capacity. This
is surely contrasted with well-crystalline materials that frequently
suffer from severe phase transition upon the Li intercalation-
disintercalation process.6,7 Until now, several nanocrystalline
metal oxides have been explored as an alternative electrode
material.1-5,8,9 Among them, nanocrystalline 3d transition metal
oxides such as MnO2 and CrO2 have attracted special attention
due to their lightweight and high energy density.3-5,8 However,
there have been only a few studies dealing with their crystal
and electronic structures.10,11 This would be because this kind
of nanocrystalline material prepared by Chimie Douce reaction
at low temperature is often X-ray amorphous because of the
lack of long range crystal order. For this reason, X-ray
diffraction (XRD), which is a common tool for structure analysis
of inorganic solids, is not suitable for studying these nanocrys-
talline materials. It is therefore required to employ other
spectroscopic methods to investigate the local chemical environ-
ment and, at the same time, the chemical bonding nature of a
nanocrystalline electrode. Actually, we have demonstrated that
X-ray absorption spectroscopy (XAS) is very effective in
probing the local atomic structure of electrode materials with
poor crystallinity, as well as in examining the effect of the Li
insertion-disinsertion process on it.7,12 Moreover, XAS can
provide electronic structural information on a specific absorbing
atom from X-ray absorption near-edge structure (XANES) data,
as well as local structural information on the neighboring atoms
In this study, we have applied XAS to the study on the
chemical bonding nature of nanocrystalline MnO2 and CrO2
compounds prepared by reduction of KMnO4 and K2CrO4 at
room temperature. Even though there are many structure types
for MnO2, the present XANES/EXAFS results allow us to
identify the exact local structure of nanocrystalline MnO2. In
the case of CrO2 nanocrystal, however, two different local crystal
structures corresponding to Cr2O3- and CrO3-types could be
observed.
Experimental Section
The nanocrystalline MnO2 and CrO2 samples were prepared
by reacting the aqueous solution of KMnO4 or K2CrO4 with
KBH4 at room temperature, as previously reported.4a,8 Briefly,
MnO2 nanocrystals were obtained by dropwise addition of 50
mL of 0.1 M KBH4 solution to 50 mL of 0.1 M KMnO4 solution
at pH 1, and CrO2 sample was prepared by dropwise addition
of 50 mL of 0.1 M KBH4 solution to 50 mL of 0.25 M K2CrO4
solution at pH 2. During the titration process, concentrated HCl
was added to maintain constant pH condition because pH tends
to increase due to the formation of KOH and KBO2. Nanocrys-
tals were washed thoroughly with distilled water and dried at
110 °C under vacuum. The effect of heat treatment on the crystal
structures of nanocrystalline MnO2 and CrO2 compounds was
studied by XRD measurements using Ni-filtered Cu KR
radiation with a graphite diffracted beam monochromator. The
chemical composition of these nanocrystals was examined by
using energy dispersive spectrometric (EDS) analysis, which
confirmed the absence of potassium ion and the formation of
binary oxides. As a reference material for XAS measurements,
nanocrystalline K-Li-Mn-O-I was prepared by mixing the
aqueous solution of KMnO4 with 1.5 equiv of LiI at room
temperature,10 and the spinel LiMn1.8Cr0.2O4 was synthesized
by conventional solid-state reaction with the stoichiometric
mixture of LiCO3, Mn2O3, and Cr2O3 precursors.12c
* To whom correspondence should be addressed. Tel: +82-43-840-3569.
Fax: +82-43-851-4169. E-mail: hwangsju@kku.ac.kr.
† Konkuk University Chungju Campus.
‡ Seoul National University.
10.1021/jp034049d CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/24/2003

5792 J. Phys. Chem. B, Vol. 107, No. 24, 2003
Hwang and Choy
Figure 2. Mn K-edge XANES spectrum for the nanocrystalline MnO2
compound (s), in comparison with those for the references Mn2O3
(O), nanocrystalline K-Li-Mn-O-I (- - -), spinel LiMn1.8Cr0.2O4
(- ‚ -), â-MnO2 (‚‚‚), and KMnO4 (b). The inset presents correspond-
ing second derivative spectra for preedge region of 6535-6543 eV. In
the inset, data of KMnO4 are multiplied by 0.07 because of their great
amplitude.
Figure 1. Powder XRD patterns of (a) nanocrystalline MnO2 com-
pound and (b) its derivative after the heat treatment at 600 °C, together
with (c-d) the corresponding data of CrO2 compound. In panels b and
d, the indexed reflections originate from Mn2O3 and Cr2O3, respectively.
The XAS experiments on the nanocrystalline MnO2 and CrO2
compounds and some references were performed with the
EXAFS facility installed at the beam line 7C at the Photon
Factory in Tsukuba.14 The XAS data were collected at room
temperature in a transmission mode using gas-ionization detec-
tors. All of the present spectra were calibrated by measuring
the spectra of Mn and Cr metal foils. The data analysis for the
experimental spectra was carried out by the standard procedure
as reported previously.13 We have constructed a variety of
structure models by adopting the crystallographic structure data
of manganese oxides and chromium oxides, such as R-MnO2,
â-MnO2, γ-MnO2, CrO2, Cr2O3, and CrO3, for quantitative
EXAFS analysis.15-20
on the chemical analysis.21 In the preedge region, all of the
spectra presented here show small preedge peaks (denoted P
and P′), which are assigned as the transitions from the core 1s
level to unoccupied 3d states. Even though they are not allowed
by the electronic dipolar selection rule, ∆l ) (1, the preedge
peaks can be discerned either because of quadrupole-allowed
transitions or because of the mixing of 4p and 3d states or both.22
Therefore, this feature obtains significant spectral weight for
the case of noncentrosymmetric local geometry of an absorbing
atom. In fact, a very intense feature P is detected for the
reference KMnO4 with the tetrahedral Mn+VII ion. In contrast,
the weak intensity of the preedge peak for nanocrystalline MnO2
suggests that the manganese ions are stabilized in octahedral
site with an inversion center, as in Mn2O3, spinel LiMn1.8Cr0.2O4,
and â-MnO2. The position and shape of these preedge peaks
are closely related to the oxidation state of the absorbing ion
and the local arrangement of backscattering ions, respec-
tively.10,13 As can be seen clearly from the inset of Figure 2,
the relative energies of these peaks are consistent with the order
of the main edge positions. It is also evident that the spectral
shape of preedge features is strongly dependent upon Mn
oxidation state; one feature P at 6537 eV is observed for Mn2O3
containing trivalent manganese ions only, whereas two preedge
peaks P and P′ are discernible for other materials containing a
considerable amount of tetravalent manganese ions. A closer
inspection reveals that Mn oxidation state also has influence
on the intensity ratio of the two preedge peaks P and P′. That
is, while the peak P′ is more intense than the peak P for both
nanocrystalline and reference MnO2 compounds, a reversal of
relative intensity is detected for LiMn1.8Cr0.2O4 and K-Li-
Mn-O-I with mixed Mn+III/Mn+IV oxidation state. This
confirms that the average oxidation state of manganese ion in
nanocrystalline MnO2 is tetravalent as in â-MnO2. In the main-
edge region, there are some peaks (denoted A, B, and C) that
are assigned as the dipole-allowed transitions from the core 1s
level to unoccupied 4p states. Among these features, the peak
C should be carefully noted because it can provide useful
information on the crystal framework of manganese oxide.10
As shown in Figure 2, the nanocrystalline MnO2 and reference
â-MnO2 show a weak and broad peak C, whereas an intense
and sharp peak C appears for the spinel lithium manganate and
Results and Discussion
Figure 1 represents the powder XRD patterns of MnO2 and
CrO2 compounds before and after heat treatment at 600 °C.
Well-defined reflection was not observed for both samples,
indicating their nanocrystalline nature. Unlike CrO2 nanocrystal,
MnO2 shows several very broad and diffuse features at around
12°, 19°, and 38°, indicating that the structural coherence unit
is longer for MnO2 than for CrO2. No change was observed in
XRD patterns before and after heating at 110 °C under vacuum.
In contrast, the heat treatment at 600 °C in air gives rise to a
crystallization of amorphous phase to M2O3 (M ) Mn and Cr)
phase, together with a loss of 0.5 oxygen per transition metal.
As mentioned above, X-ray amorphous character of the as-
prepared nanocrystals makes XRD useless in the determination
of their crystal structures. In this regard, we have applied XAS
not only to identify the local crystal structure but also to probe
the electronic structure of these materials.
The Mn K-edge XANES spectrum of nanocrystalline MnO2
is presented in Figure 2, in comparison with the reference spectra
of nanocrystalline manganese oxyiodide K-Li-Mn-O-I,
cubic spinel LiMn1.8Cr0.2O4, Mn2O3 (bixbyite), â-MnO2 (pyro-
lusite), and KMnO4. The position of edge jump for the nano-
crystalline MnO2 is nearly the same as that for the reference
â-MnO2 but slightly higher than those for nanocrystalline
manganese oxyiodide and LiMn1.8Cr0.2O4. This indicates that
manganese ions in MnO2 nanocrystal have an average valence
state of ∼4, which is well consistent with the previous results

Local Atomic Arrangement and Electronic Structure
J. Phys. Chem. B, Vol. 107, No. 24, 2003 5793
Figure 3. Experimental Mn K-edge EXAFS spectra (O) in (a) R- and
(b) k-space, in comparison with the simulated spectra (s) based on
the structure of (i) R-MnO2, (ii) â-MnO2, and (iii) γ-MnO2.
Figure 4. Fourier (a) transformed and (b) filtered Mn K-edge EXAFS
spectra for (i) nanocrystalline MnO2, (ii) nanocrystalline K-Li-Mn-
O-I, and (iii) spinel LiMn1.8Cr0.2O4. The range over which the Fourier
filtering has been made is shown by the arrows. The solid lines and
empty circles represent the fitted and experimental data, respectively.
nanocrystalline manganese oxyiodide. Considering the fact that
the intensity of this peak is proportional to the relative
concentration of edge-shared MnO6 octahedra with respect to
corner-shared one,23 it can be supposed that the crystal structure
of nanocrystalline MnO2 contains corner-shared MnO6 octahedra
in significant concentration. In contrast, the observed sharp peak
C in LiMn1.8Cr0.2O4 and K-Li-Mn-O-I clarifies that there
are only edge-shared MnO6 octahedra, which is in good
agreement with their crystal structures, that is, spinel- and
layered-type.10,12c From the present XANES results, we are able
to obtain a measure for identifying the local structure of
nanocrystalline manganese oxide. Among various crystal struc-
tures of MnO2 such as R-, â-, γ-, ꢀ-, and λ-form (spinel-type)
and layered phase, the latter three structures consisting of the
network of edge-shared MnO6 octahedra16,24 cannot be adopted
as a local structure of nanocrystalline MnO2 because the weak
and broad peak C underlines the presence of corner-shared
MnO6 octahedra. Therefore, the remaining candidates are R-,
â-, and γ-MnO2-type structure.16,17 However, on the basis of
the present XANES results, we were unable to evaluate the
validity of these models. To solve this problem, we have
performed EXAFS fitting analyses with these structures.
structure allows us to reproduce well two FT peaks correspond-
ing to (Mn-Mn) shells. The best consistency in k-space data
was also obtained with R-MnO2 structure (Figure 3b). This
observation can be regarded as a strong evidence for R-MnO2-
type local structure in nanocrystalline MnO2. In fact, R-MnO2
structure having large 2 × 2 pores is most favorable for the
repeated Li insertion-disinsertion.15 To determine the structural
parameters such as coordination number (CN), bond distance
(R), and Debye-Waller factor (σ2), these FT peaks were isolated
by inverse FT to a k space and then curve-fitted.25 The
experimental and fitted FT and inverse FT spectra of nanocrys-
talline MnO2 are represented in panels a and b of Figure 4,
respectively, in comparison with those of nanocrystalline
K-Li-Mn-O-I and spinel LiMn1.8Cr0.2O4 and the curve-
fitting results are collected in Table 1. On the basis of R-MnO2
structure, a good fit was obtained for nanocrystalline MnO2 in
the R-range up to 3.6 Å. The best-fit bond distances of (Mn-
O) and (Mn-Mn) shells are well consistent with the crystal-
lographic values,15 confirming the reliability of fitting analysis.
In contrast to nanocrystalline MnO2, no FT peak appears around
3 Å for nanocrystalline K-Li-Mn-O-I and spinel LiMn1.8
-
For the purpose of evaluating the validity of these structural
models, first we have simulated theoretically the EXAFS data
on the basis of R-, â-, and γ-MnO2 structures. The experimental
and simulated spectra in R- and k-space are shown in panels a
and b, respectively, of Figure 3. In Fourier transform (FT)
diagram (Figure 3a), the first peak at ∼1.5 Å corresponds to
(Mn-O) bonding pair, whereas the following peaks at ∼2.5
and ∼3.1 Å originate from (Mn-Mn) shells in the edge- and
corner-shared MnO6 octahedra pairs, respectively. The main
difference among the three structural models is the relative
concentration of corner-sharing to edge-sharing of MnO6
octahedra. As can be seen clearly from Figure 3a, the R-MnO2
Cr0.2O4 because they do not have corner-shared MnO6 octahedra.
It should be noted here that the nanocrystalline K-Li-Mn-
O-I crystallizes with rhombohedral layered structure,10 which
is dissimilar to the case of the present MnO2 nanocrystal. Such
a structural difference between both nanocrystalline compounds
would be ascribed to the presence of potassium ion that can act
as pillar species for stabilizing the layered structure. It is also
worthwhile to mention that nanocrystalline MnO2 shows small
but distinct FT peaks in 4-6 Å region, which are absent in
nanocrystalline K-Li-Mn-O-I. This implies that the struc-
tural coherence is greater for the former than for the latter, as

5794 J. Phys. Chem. B, Vol. 107, No. 24, 2003
Hwang and Choy
TABLE 1: Results of Nonlinear Least Square Curve
Fittings for the Mn K-Edge EXAFS Spectra of
Nanocrystalline MnO2, K-Li-Mn-O-I, and Spinel
LiMn1.8Cr0.2O4
sample
bond
CN R (Å) σ2 (10-3 × Å2)
nanocrystalline
Mn-Ob
Mn-Ob
Mn-Ob
Mn-Ob
Mn-Mnb
Mn-Mnb
Mn-Ob
Mn-Ob
Mn-Mnb
Mn-Ob
Mn-Ob
Mn-Ob
2.0 1.89c
1.0 1.89c
1.0 1.90
2.0 1.91
2.0 2.85
2.0 2.89
1.0 3.39
1.0 3.41
4.0 3.42
2.0 3.42
2.0 3.43
1.0 3.54
3.64
3.64
3.64
3.64
5.78
5.78
2.86
2.86
5.12
2.86
2.86
2.86
a
MnO2
nanocrystalline
K-Li-Mn-O-I Mn-Mn
Mn-O
6.0 1.90
4.2 2.86
4.89
6.22
spinel
LiMn1.8Cr0.2O4
Mn-O
6.0 1.91
Mn-Mn,Cr 6.0 2.90
4.59
4.29
a The curve fitting analysis was performed for the range of 0.859 e
R e 3.59 Å and 2.85 e k e 13.95 Å-1 b To avoid unnecessary
.
computation, we have taken into account only single scattering (SS)
paths for present fitting analysis. In fact, there is no multiple scattering
(MS) path with focusing effect (i.e., bond angle of 180°), and hence,
the contribution of any MS path is below 10% of the first shell
maximum peak amplitude. c According to the crystallographic data of
R-MnO2, these bond distances are slightly different by 0.005 Å.
Figure 6. k3-weighted Cr K-edge (a) EXAFS spectra and (b) their
FTs for (i) the nanocrystalline CrO2 compound, (ii) Cr2O3, and (iii)
spinel LiMn1.8Cr0.2O4. The range over which the Fourier filtering has
been made is shown by the arrows. In panel b, the solid lines and empty
circles represent the fitted and experimental data, respectively.
fairly weak in intensity for LiMn1.8Cr0.2O4 and Cr2O3 with
trivalent chromium ion in the octahedral site. In the case of
nanocrystalline CrO2, a rather intense peak P is also detected,
suggesting the tetrahedral symmetry around chromium. This is
inconsistent with the crystallographic data of crystalline CrO2
in which Cr is stabilized in slightly distorted CrO6 octahedra
with the bond distances of 1.9006 (×4) and 1.9048 Å (×2).18
In this regard, we have carried out Cr K-edge EXAFS analyses
to exactly determine the local structure around chromium.
The k3-weighted Cr K-edge EXAFS spectra for nanocrystal-
line CrO2, reference Cr2O3, and spinel LiMn1.8Cr0.2O4 are plotted
in Figure 6a and the corresponding FTs in Figure 6b. The
results of curve fitting are collected in Table 2. In contrast to
Cr2O3 and spinel LiMn1.8Cr0.2O4, CrO2 shows no well-defined
FT features except the first peak corresponding to (Cr-O)
shell, evidencing its nanocrystalline character. We tried to fit
the spectrum of nanocrystalline CrO2 for the estimation of
structural parameters. Unfortunately, the attempt based on the
crystallographic structure data of CrO218 failed. And other trials
with Cr2O3 or CrO3 structure19,20 were also found to be
unsuccessful. Alternatively, we constructed mixture models by
combining the crystal structures of Cr2O3 and CrO3, and by
combining those of CrO2 and CrO3. As shown in Figures 6b
and 7, an excellent fit could be obtained with a mixture model
of Cr2O3 and CrO3 structures,27 clarifying that the chromium
ions in nanocrystalline CrO2 are located not only in tetrahedral
site with the average (Cr-O) bond distance of 1.75 Å but also
in octahedral site with the average (Cr-O) bond distance of
1.98 Å. The relative concentration of Cr2O3- and CrO3-type
structures was evaluated to be 0.56 and 0.44, respectively, with
the average Cr oxidation state of 4.3. This is in good agreement
with the edge position in XANES spectra. The other mixture
Figure 5. Cr K-edge XANES spectra for the nanocrystalline CrO2
compound (s), in comparison with those for the references Cr2O3
(- - -), CrO3 (-‚ -), and spinel LiMn1.8Cr0.2O4 (‚‚‚). The inset
presents corresponding second derivative spectra for preedge region
of 5985-5995 eV.
suggested from XRD analyses. These phenomena can be
rationalized by taking into account their different synthetic
methods in which the former was prepared by dropwise addition
of KBH4 solution to KMnO4 solution, while the latter was
obtained by rapid addition of LiI to KMnO4 solution. The slower
mixing of reactants gives rise to a more effective crystal growth
of MnO2 crystallites.
Figure 5 illustrates the Cr K-edge XANES spectrum of
nanocrystalline CrO2, together with those of the references
Cr2O3, CrO3, and spinel LiMn1.8Cr0.2O4. The edge energy of
nanocrystalline CrO2 is lower than that of CrO3 but higher than
those of Cr2O3 and LiMn1.8Cr0.2O4, highlighting the tetravalent
oxidation state of chromium in this compound. In contrast to
the reference CrO3 with tetrahedral Cr+VI ions,20 the preedge
peak P related to the dipole-forbidden 1s f 3d transition is

Local Atomic Arrangement and Electronic Structure
J. Phys. Chem. B, Vol. 107, No. 24, 2003 5795
TABLE 2: Results of Nonlinear Least Square Curve
Fittings for the Cr K-Edge EXAFS Spectra of
we have found that the tetravalent manganese ions are stabilized
in octahedral sites with R-MnO2-type local structure. The
presence of large 2 × 2 pores in R-MnO2-type structure would
allow the present nanocrystalline MnO2 to accommodate the
lithium ion in a reversible way and to become a promising
electrode material for lithium secondary battery. On the other
hand, Cr K-edge XAS study reveals that there are two kinds of
chromium ions in nanocrystalline CrO2, a trivalent Cr ion in
octahedral symmetry with Cr2O3-type local structure and a
hexavalent Cr ion in a tetrahedral site with CrO3-type local
structure. The latter species is presumed to exist on the surface
of particles, resulting in an easy grafting process of lithium ion
and an improved electrochemical activity. The present work
demonstrates that XANES/EXAFS is a very powerful tool not
only for determining the local crystal and electronic structures
of nanocrystalline materials but also for understanding their
physicochemical properties.
Nanocrystalline CrO2, Cr2O3, and Spinel LiMn1.8Cr0.2O4
sample
bond
CN
R (Å) σ2 (10-3 × Å2)
nanocrystalline Cr-Ob
1 × 0.44d 1.66e
1 × 0.44d 1.66e
2 × 0.44d 1.83
3 × 0.56d 1.93
3 × 0.56d 2.02
1.93
1.93
1.93
1.28
1.28
Cr-Ob
Cr-Ob
Cr-Oc
Cr-Oc
a
CrO2
Cr2O3
Cr-O
Cr-O
3.0
3.0
1.94
2.03
1.14
1.14
LiMn1.8Cr0.2O4 Cr-O
6.0
1.98
2.89
2.58
3.22
Cr-Mn,Cr 6.0
a The curve fitting analysis was performed for the range of 0.920 e
R e 2.117 Å and 3.75 e k e 13.5 Å-1 b These three shells originate
.
from the Cr-O coordination sphere in CrO3 phases. c These two shells
originate from the Cr-O coordination sphere in Cr2O3 phases. d The
present coordination numbers are multiplied by the relative concentra-
tion of Cr2O3 and CrO3 phases. e According to the crystallographic data
of CrO3, these bond distances are slightly different by 0.004 Å.
Acknowledgment. This work was supported by the Faculty
Research Fund of Konkuk University in 2002 and in part by
the Korean Ministry of Science and Technology through the
NRL project ‘99. Authors are also grateful to Prof. M. Nomura
for helping us to get the XAS data in the Photon Factory.
References and Notes
(1) (a) Treuil, N.; Labruge`re, C.; Menetrier, M.; Portier, J.; Campet,
G.; Deshayes, A.; Frison, J. C.; Hwang, S. J.; Song, S. W.; Choy, J. H. J.
Phys. Chem. B 1999, 103, 2100. (b) Kang, S. H.; Goodenough, J. B.;
Rabenberg, L. K. Electrochem. Solid-State Lett. 2001, 4, A49.
(2) (a) Polzot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.
M. Nature 2000, 407, 496. (b) Kwon, C. W.; Campet, G.; Portier, J.; Poquet,
A.; Fourne`s, L.; Labruge`re, C.; Jousseaume, B.; Toupance, T.; Choy, J.
H.; Subramanian, M. A. Int. J. Inorg. Mater. 2001, 3, 211.
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A.; Kim, J. Chem. Mater. 1998, 10, 2895. (c) Kim, J.; Manthiram, A.
Electrochem. Solid-State Lett. 1999, 2, 55.
(4) (a) Tsang, C.; Kim, J.; Manthiram, A. J. Solid State Chem. 1998,
137, 28. (b) Jeong, Y. U.; Manthiram, A. Electrochem. Solid-State Lett.
1999, 2, 421. (c) Jeong, Y. U.; Manthiram, A. J. Solid State Chem. 2001,
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(5) (a) Xu, J. J.; Kinser, A. J.; Owens, B. B.; Smyrl, W. H. Electrochem.
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Figure 7. Fourier filtered Cr K-edge EXAFS spectrum for the (a)
nanocrystalline CrO2 compound, in comparison with those for the
references (b) Cr2O3 and (c) spinel LiMn1.8Cr0.2O4. The solid lines and
empty circles represent the fitted and experimental data, respectively.
(6) Gummow, R. J.; Liles, D. C.; Goodenough, J. B. Mater. Res. Bull.
1993, 28, 1249.
model of CrO2 and CrO3 structures also gave an acceptable fit,
but the resulting (Cr-O) bond distances were determined to be
1.74 and 1.97 Å, which correspond to the bond distances of
(Cr+VI-O) and (Cr+III-O), respectively.19,20 This provides a
clear evidence on the coexistence of trivalent and hexavalent
chromium ions, as well as on the absence of tetravalent
chromium ion. Judging from our previous report on nanocrys-
talline LiMn2O4 showing the Mn oxidation state is higher for
the surface species,1a the hexavalent chromium ion would exist
on the surface of particles while the trivalent chromium would
be in the core of particles. This is further supported by the fact
that an incomplete coordination for surface species would favor
lower coordination number. As suggested from the “electro-
chemical grafting” concept,1a,2b,28 the easily reducible Cr+VI
species on the surface are presumed to provide active sites for
Li+ grafting, leading to an improved electrochemical activity
for this nanocrystalline CrO2 compound. In fact, this material
was known to exhibit better electrochemical performances
compared to the well-crystalline CrO2 compound.8,29
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We have elucidated the electronic and crystal structures of
X-ray amorphous MnO2 and CrO2 nanocrystals by applying
XAS analysis. From the Mn K-edge XANES/EXAFS results,

5796 J. Phys. Chem. B, Vol. 107, No. 24, 2003
Hwang and Choy
(21) The oxygen contents of the nanocrystals prepared under the same
reaction conditions were reported to be 1.89 for MnO2 and 2.03 for CrO2
(refs 4a and 8).
categories were set to be dependent on each other by using crystallographic
data of R-MnO2 because it is not possible to determine a small path length
difference (0.01-0.05 Å) from EXAFS analysis (ref 26).
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(27) To estimate the relative concentration of CrO3 and Cr2O3 domains,
2
the amplitude reduction factor (S0 ) was set as a variable while the
coordination number (CN) was fixed to the crystallographic values. All of
the bond distances (R), Debye-Waller factors (σ2), and energy shifts (∆E)
were allowed to vary under the constraint that σ2 and ∆E are set to be the
same for the shells belonging to each domain, that is, three (Cr-O) shells
corresponding to CrO3 domain and two (Cr-O) shells corresponding to
Cr2O3 domain. Also the (Cr-O) bond distances in each domain were
constrained to be dependent on each other by using crystallographic data
of CrO3 and Cr2O3, which is due to the limitation in the determination of
small path length differences.
(28) Campet, G.; Wen, S. J.; Han, S. D.; Shastry, M. C. R.; Portier, J.;
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(25) In the course of fitting analysis, the coordination number (CN) was
fixed to the crystallographic values while the amplitude reduction factor
2
(S0 ) was allowed to vary. The best-fit S02 of nanocrystalline MnO2 is well
consistent with that of spinel LiMn1.8Cr0.2O4 within 10% deviation. All of
the bond distances (R), Debye-Waller factors (σ2), and energy shifts (∆E)
were set as variables. However, because of the difficulty in quantitative
EXAFS fitting involving distant shells beyond the first one, we have applied
several constraints for these variables. First, σ2 and ∆E were set to be the
same for coordination shells within several categories, that is, four
(Mn-O) shells at ∼1.90 Å, two (Mn-Mn) shells at 2.85-2.89 Å, and
five (Mn-O) shells beyond 3.39 Å. This constraint can be rationalized from
the fact that these adjacent shells would possess very similar degree of
structural disorder and energy shift. Second, the bond distances in the