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Solid State Ionics 177
Hydrothermal synthesis and lithium-intercalation properties of MoO2
nano-particles with different morphologies
Yongguang Liang, Zonghui Yi, Shuijin Yang, Liqun Zhou, Jutang Sun *, Yunhong Zhou
Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
Received 22 April 2005; received in revised form 2 December 2005; accepted 2 December 2005
Abstract
Different morphological MoO2 nano-sized particles were synthesized by a low temperature hydrothermal reaction as a novel method. The
physical characterizations were carried out by IR, TG/DTA, XRD, XPS, SEM, TEM and SAED. XRD and XPS data indicate the as-prepared
samples present at pure phase MoO2 with monocline symmetry. Spherical, sheet-like and bar-shaped nano-particles are observed by SEM,
respectively. The lithium-intercalation properties of spherical MoO2 powders were investigated by XRD, TEM and SAED in the light of bulk
MoO2. The results showed an irreversible phase transition after the initial discharge process, which is obviously different from the bulk
sample.
D 2005 Elsevier B.V. All rights reserved.
Keywords: MoO2; Nanomaterials; X-ray diffraction; Electron microscopy; Phase transition; Lithium intercalation properties
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1. Introduction
Since the introduction of nanochemistry in past decades,
numbers of materials in nano-scale have been synthesized via
many methods [1]. Currently, primary object of nanochemistry
is the development of new methods for preparing and
characterizing chemical structures within the size range from 1
nm to 100 nm in one, two or three dimensions [2]. Dimension-
ality is a key factor in determining the properties of nanomater-
ials [3] and the control of size and shape is very meaningful with
regard to specific applications as nanodevices [4].
Molybdenum dioxide powders have been widely used in the
fields of catalysts [5], sensors [6], recording materials [7],
electrode materials [8] and nano-sized materials with different
morphologies are expected to display unique properties.
Traditionally, molybdenum dioxide is prepared by reducing
molybdenum trioxide with hydrogen at high temperature and
the product is coarse-grained [9,10]. Manthiram et al. [11] and
Liu et al. [12] have reported the preparation of MoO2 powders
by a reduction process in solution reaction routes, but their
0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ssi.2005.12.001
* Corresponding author. Tel.: +86 27 87218494; fax: +86 27 68754067.
E-mail address: [email protected] (J. Sun).
results showed a limited success because only nano-sized
powders in zero dimension were prepared. It is difficult for
traditional methods to prepare such nanostructures in one, two
or three dimensions.
In this paper, we introduced a novel route to the preparation
of MoO2 nano-particles by a hydrothermal reaction route. The
results show the resulting spherical, sheet-like and bar-shaped
samples present a single phase MoO2 with monocline
symmetry. The characteristic lithium-intercalation properties
of spherical MoO2 powders imply there is a correlation
(2006) 501 – 505
ww
4000 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)3500
Fig. 1. IR spectrum of the precursor.
Wei
ght
%
End
o D
TA
(uV
/mg)
Exo
0 100 200 300 400 500 600 70080
84
88
92
96
100
-1.8
-1.2
-0.6
0
0.6
1.2
Temperature (°C)
DTATG
Fig. 2. TG and DTA curves of the MoO2Ix H2O precursor.
-ED
-NO
Mo -EG
(011
)
(11-
1)
(002
)
(220
)
(310
)
(131
)
(022
)
(11-
2)
(40-
2)
(011
)
(110
)
(-11
2)
(-31
2)
(031
)
(-23
1)
(-22
2)
(-21
1)
(-40
2)
(130
)
(110
)
(011
)
(-21
1)
(022
)
(-23
1)
(220
)
(-11
2)
(-13
2)
(-31
3)
2θ (degrees)
Inte
nsity
(a.
u.)
10 30 40 50 60 70 8020
O2
MoO2
MoO2
Fig. 4. XRD patterns of the as-prepared MoO2 samples.
Y. Liang et al. / Solid State Ionics 177 (2006) 501–505502
between their nano-sized structure and an irreversible phase
transition after the initial discharge process.
2. Experimental
2.1. Materials synthesis
The precursors were synthesized by a low-temperature
hydrothermal route. 5.29 g (NH4)6Mo7O24I4H2O, was fully
dissolved in 60.0 mL second-distilled water and the solution
was divided into three parts. Two parts were added with proper
amounts ethylene glycol (EG) and ethylene diamine (ED) as
surfactants, respectively. All of them were slowly dropped with
10.0 mL N2H4IH2O (86 mass%). Every solution was sealed in a
closed autoclave for 12 h at 140 -C, then cooled in the air and
filtrated carefully. The filter cakes were washed with distilled
water and absolute alcohol successively. After dried under
vacuum at 100 -C, the precursors were treated at 400 -C in
argon atmosphere for 6 h to give three black MoO2 powders,
noted as MoO2-NO (using no surfactant), MoO2-EG (using EG
as surfactant) and MoO2-ED (using ED as surfactant),
respectively.
2.2. Physical characterization
The thermal stability of the precursors was examined by
means of TG and DTAwith a Netzsch STA 449 thermal analysis
Inte
nsity
(a.
u.)
10 30 40 50 60 70 802θ (degrees)
100 °C
200 °C
380 °C
(011
) (1
1-1)
(002
)
(220
)
(310
)
(131
)(022
)
(11-
2)
(40-
2)
300 °C
20
Fig. 3. XRD patterns of the resulted samples at different temperatures.
Table 1
Unit cell parameters of three MoO2 samples
MoO2-NO MoO2-EG MoO2-ED
Crystal system monoclinic monoclinic monoclinic
Space group P21/n (no. 14) P21/c (no. 14) P21/c (no. 14)
Unit cell
dimensions
a =5.6054(2) A a =5.6036(3) A a =5.6110(2) A
b =4.8587(1) A b =4.8554(2) A b =4.8562(4) A
c =5.5385(1) A c =5.6087(4) A c =5.6291(2) A
b =119.400(4)- b =120.859(4)- b =120.960(5)-
V =131.3877(6) A3 V =130.9983(9) A3 V =131.5130(7) A3
Z 4 4 4
Density,
calculated
6.468 g/cm3 6.487 g/cm3 6.462 g/cm3
system at a heating rate of 10 -C/min from 25 to 700 -C in a
flow of argon. Infrared spectrum was recorded using KBr
pellets on a Nicolet AVATAR-360 spectrometer in range of
400–4000 cm�1. Identification of phases and structures was
carried out on a Shimadzu 6000 X-ray diffractometer at a
scanning rate of 2-/min and a step of 0.02-, using Cu Ka
radiation (k =1.54056 A). XPS measurements were performed
on a KRATOS XSAM800 photoelectron spectrometer with an
exciting source of Mg Ka=1253.6 eV. The particle sizes and
morphological features were observed by a scanning electron
microscope (Hitachi SEM X-650). TEM and SAED of
electrodes were tested by a transmission electron microscope
(JEOL JEM2010FEF).
The electrochemical cell consisted of a MoO2 working
electrode and a lithium foil counter electrode. Electrodes
were prepared by mixing MoO2 powders with 15% acetylene
black and 5% PTFE, compressing the mixture onto a nickel
gauze current collector. A 1 molIL�1 solution of LiClO4
dissolved in EC/DEC (1:1) was used as the electrolyte. A
Celgard 2400 microporous membrane was used as a separator.
The cell was discharged and charged between 2.0 and 0.01 V vs.
metallic lithium at a constant current density of 100 mAIg�1.
3. Results and discussion
IR spectrum of the precursor is consistent with the standard
spectra of molybdenum dioxide. As shown in Fig. 1, the
intense band at 925 cm�1 are assigned to m(MofO), while the
238 236 234 232 230 228 226
Binding energy (eV)
-ED
-NO
Mo -EGO2
MoO2
MoO2Inte
nsity
(a.
u.)
Fig. 5. XPS spectra of the Mo 3d doublet in the resulting MoO2 samples.
Vol
tage
(V
)
0
1.0
2.0
3.0
Capacity (mAh.g-1)0 750 1250 1500
---....
500 1000
1st charge
2nd discharge
1st discharge
-BulkMoO2
-NOMoO2
250
Fig. 7. The discharge and charge curves of MoO2-NO and bulk MoO2 test cells.
Y. Liang et al. / Solid State Ionics 177 (2006) 501–505 503
prominent bands in the range of 500–850 cm�1 are attributed
to m(Mo–O–Mo). In addition, the broad band at 3453 cm�1
and 1596 cm�1 corresponds to the absorbed water.
A typical TG and DTA curves of MoO2IxH2O precursor
using no surfactant are shown in Fig. 2. The trace shows two
major events. The mass loss of 12.8% occurs in the region of
72–347 -C, corresponding to the loss of almost one
molecular absorbed water (theoretical value 12.4%). The
endothermic peak and exothermic peak at about 182 -C and
378 -C on the DTA curve give two thermal behaviors. The
broad endothermic peak on the DTA curve at about 182 -Ccorresponds the release of the adsorbed water [12]. The sharp
exothermic peak at 378 -C is associated with the formation of
crystalline MoO2 (Fig. 3), agreeing the data in literature
[11,12]. The MoO2IxH2O precursors using EG and ED as
surfactants provide similar TG and DTA curves, and the
Fig. 6. SEM images of MoO2 samples: (a) MoO2-NO,
values of x are equal to about 0.50, 0.30 according to 6.40%,
3.84% mass loss, respectively.
Fig. 4 illustrates the powder XRD patterns of the as-
prepared samples. A first determination of cell parameters was
made using auto-indexing programs TREOR [14] and DIC-
VOL91 [15] embedded in the CRYSFIRE suite [16] and
monoclinic lattice was suggested. The lattice constants
obtained were further refined using the CELREF program
[17], which gave the values of the refined parameters listed in
Table 1. The data of MoO2-NO, MoO2-EG and MoO2-ED are
consistent with JCPDS card 32-0671, JCPDS card 78-1073 and
JCPDS card 86-0315. The indexation of major diffraction
peaks was listed in the XRD patterns.
Fig. 5 presents the Mo 3d spectra for MoO2-NO, MoO2-
EG and MoO2-ED. The 3d5/2 peak of MoO3 is almost
coincident with 3d3/2 peak of MoO2, thus leading to the
(b) MoO2-EG, (c) MoO2-ED and (d) bulk MoO2.
a
10 20 30 40 50 60 80
0.01 V
0.01 V
1.78 V
2.00 V
2.00 V
1.46 V
1.46 V
b
Inte
nsity
(a.
u.)
Inte
nsity
(a.
u.)
2θ (degrees)
2θ (degrees)70
10 20 30 40 50 60 8070
Fig. 8. XRD patterns of MoO2 electrodes at different states during the initial
charge process. (a) MoO2-NO and (b) bulk MoO2.
Fig. 9. TEM images and SAED of MoO2 electrodes at different states during the init
electrode charged at 1.46 V, (c) bulk MoO2 electrode discharged at 0.01 V and (d)
Y. Liang et al. / Solid State Ionics 177 (2006) 501–505504
characteristics of three-peak shape [18]. The spectral lines at
232.9 eV and 229.5 eV are assigned to the 3d3/2, 3d5/2 peaks
for Mo(VI), respectively. In the separate XRD patterns in Fig.
2, the primary MoO2-NO is found to be pure MoO2. It
indicates that the MoO2-NO powders were actually oxidized
at the surfaces with the exposure to air, leading to the
presence of MoO3 phase. It is consistent with the SEM image
of MoO2-NO that near nano-sized particles with porous state
provide much activity. The latter two samples display relative
inertia and the 3d5/2 peak of MoO3 is weaken to a plateau,
giving two-peak shape of 3d3/2, 3d5/2 peaks for Mo(VI). The
spectral lines at 232.1 eV and 229.1 eV are assigned to the
3d3/2, 3d5/2 peaks for MoO2-EG, respectively. The spectral
lines at 232.4 eV and 229.6 eV are attributed to the 3d3/2,
3d5/2 peaks for MoO2-ED, respectively.
Fig. 6 show the SEM images of the as-prepared MoO2
samples. As seen from the morphology (Fig. 6a), spherical
MoO2-NO grains with particles sizes from 80 nm to 250 nm
aggregate and loosely stacked with a porous state, which
provides high specific surface area. It is different with coarse-
grained molybdenum dioxide prepared by high-temperature
reactions [9,10] and nano-sized ones obtained through solution
routes [11–13]. The sheet-like MoO2-EG particles in Fig. 6b
show an average diameter of about 250 nm and thickness less
than 100 nm. The SEM image of Fig. 6c reveals MoO2-ED
bars ordering with a shape of rectangle and showing smooth
surface. The length of the bars can reach up to 15 Am with
breath less than 400 nm. But there is less than 100 nm in
some direction obviously. The morphologies and character-
istics of these nano-sized materials are apparently controllable
depending on the surfactants used. This new adaptable
ial charge process. (a) MoO2-NO electrode discharged at 0.01 V, (b) MoO2-NO
bulk MoO2 electrode charged at 1.46 V.
Y. Liang et al. / Solid State Ionics 177 (2006) 501–505 505
structure prompted us to construct different morphologies in
which sizes and chemical environments could be varied. Fig.
6d displays a bulk MoO2 sample obtained via traditional high
temperature solid phase reaction [9] in order to investigate the
difference of lithium-intercalation from spherical MoO2-NO
powders.
The discharge-charge profiles of spherical porous MoO2-
NO and bulk MoO2 with a current density of 100 mAIg�1 are
presented in Fig. 7. There is no obvious potential plateau in the
initial charge curve and the following charge curves, which is
different with the reported results [19,20]. According to
previous findings, there are two constant potential plateaus in
rutile MoO2 at 1.42 and 1.74 V on charge. Moreover, the
MoO2-NO test cell delivers 1486 mA hIg�1 and 784 mA hIg�1
capacities two times higher than bulk MoO2 powders during
the initial discharge and charge process, respectively. Clearly,
different lithium-intercalation reaction took place during the
process.
In order to evaluate any structural changes of MoO2-NO
electrodes during Li ion extraction, a group of cells were
stopped at different states during the initial charge process,
respectively. These cells were opened in an argon-filled glove
box to recover the electrodes, and the electrodes were
subsequently rinsed in EC to remove the residual LiClO4 and
finally dried under vacuum. The dried electrodes were
subjected to XRD. Fig. 8 presents the changes of XRD
patterns in the electrodes. The electrode presents an amorphous
phase at 0.01 V at end of the first discharge process. The
spherical MoO2-NO remains amorphous phase whereas bulk
MoO2 represents a transition of phase between the orthogonal
symmetry and monoclinic symmetry when passing through a
reflect point during the initial charge process [8,20]. For further
investigation of the phase transition, both MoO2-NO and bulk
MoO2 at 0.01 V and 1.46 V in the initial charge process were
studied by SAED and TEM. Both Fig. 9a and b present
amorphous phases. More phases appear during lithium-
extraction from 0.01 V to 1.46 V according the characteristics
of SAED micrograph. However, bulk MoO2 electrode at
0.01 V displays co-existence of amorphous phase and crystal-
line phase. Average particle size of bulk MoO2 is obviously
smaller than the primitive sample. The electrode at 1.46 V in
Fig. 9d show poly-crystalline phases clearly. The results of
SAED are consistent with XRD data above.
In fact, the physical properties of MoO2-NO provide a full
Li ionic diffusion and electron-exchange. One cannot expect
the above features in similar compounds prepared via
conventional reaction [18–21], such as submicronic grains
and porous state, which are very desirable for a material to be
employed as electrode-active material in rechargeable lithium-
containing batteries. As mentioned above, a different lithum-
intercalation reaction took place in porous MoO2-NO. More-
over, the crystalline structure changes irreversibly leading to
amorphous phase in the following cycles. On contrast, to large
particles of coarse-grained MoO2 powders, the rate of ionic
diffusion through the particle is slow relative to the effective
current density (the rate of charge transfer at the surface). This
would result in a radial lithium concentration gradient with the
particle. Only the outer layers of the rutile-type crystals actively
involved in lithium intercalation and a phenomenon of
‘‘electrochemical grinding’’ [22] took place in electrode. There
is a correlation between their nanostructural properties and an
irreversible phase transition after the initial discharge process in
MoO2-NO electrode.
4. Conclusions
Different morphological MoO2 nano-particles were synthe-
sized by a hydrothermal reaction route. The morphologies and
characteristics of these nano-sized materials are apparently
controllable depending on the surfactants used. The character-
istic lithium-intercalation properties of spherical MoO2 pow-
ders imply there is a correlation between their nano-sized
structures and an irreversible phase transition after the initial
discharge process.
Acknowledgement
The authors would like to thank the National Natural
Science Foundation of China (20471044) for financial support.
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