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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

Tra

nsm

ittan

ce (

%)

20

40

60

80

100

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: jtsun@whu.edu.cn (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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