Synthesis and thermal stability of W-doped VO2 nanocrystals

Article abstract of DOI:10.1016/j.materresbull.2011.06.030

Pure and W-doped vanadium dioxide nanocrystals have been synthesized by using V₂O₅ and oxalic acid as precursors via a thermolysis method. The VO₂ nanocrystals have a nearly spherical morphology with size ranging from 50 to 100 nm. The metal-insulator transition (MIT) temperature of the nanocrystals decreases with increasing W-doping content. The successive heat-induced fatigue character of the MIT in W-doped VO₂ nanocrystals was investigated by DSC analysis together with structural study, and a high stability upon heating-cooling cycles was found with respect to MIT temperature, peak temperature and latent heat of the phase transition.

Full text of DOI:10.1016/j.materresbull.2011.06.030

Materials Research Bulletin 46 (2011) 2100–2104
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Synthesis and thermal stability of W-doped VO2 nanocrystals
*
F.Y. Kong, M. Li, S.S. Pan, Y.X. Zhang, G.H. Li
Key Laboratory of Materials Physic, Anhui Key Laboratory of Nanomaterial and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Pure and W-doped vanadium dioxide nanocrystals have been synthesized by using V2O5 and oxalic acid
as precursors via a thermolysis method. The VO2 nanocrystals have a nearly spherical morphology with
size ranging from 50 to 100 nm. The metal-insulator transition (MIT) temperature of the nanocrystals
decreases with increasing W-doping content. The successive heat-induced fatigue character of the MIT in
W-doped VO2 nanocrystals was investigated by DSC analysis together with structural study, and a high
stability upon heating–cooling cycles was found with respect to MIT temperature, peak temperature and
latent heat of the phase transition.
Received 27 January 2011
Received in revised form 15 June 2011
Accepted 24 June 2011
Available online 2 July 2011
Keywords:
A. Oxides
ß 2011 Elsevier Ltd. All rights reserved.
B. Chemical synthesis
D. Crystal structure
1. Introduction
into commercial production lines with a short cycle period and
simple operation. The thermal stability of the VO2 nanopowder is
The metal–insulator transition (MIT) in VO2 takes place at about
68 8C [1], accompanying structural transition from high-
an important issue that should address clearly for practical
applications. Nevertheless, only few reports focus on the thermal
stability of VO2 [22].
In this paper, we report the synthesis of pure and W-doped VO2
nanocrystals by a thermolysis method and study of the thermal
stability of the nanocrystals by DSC analysis. It was found that the
transition quality of the VO2 powder is much stable without
degradation up to successive 100 heating–cooling cycles.
a
temperature rutile VO2 (R) to low-temperature monoclinic VO2
(M). It displays an about 105 decrease in resistivity and a large
change in transparency in the infrared region as VO2 (M) converts
to VO2 (R). These characteristics makes VO2 a promising material
for a wide variety of technological applications, such as field effect
transistor-like or uncooled microbolometers [2,3], holographic
storage systems [4–6], optical switching devices [7,8], sensors
[9,10] and smart windows [11,12].
Much efforts have been devoted to refining growth techni-
ques [13], to enhancing the optical transmittance by doping [14]
or developing multilayer structure with TiO2 or SiO2 [15,16], to
developing a better understanding of the size effects in the
structural phase transition [17] and to controlling the MIT
temperature. The MIT temperature can be well modified by
strain control or elements incorporating in VO2, such as W, Mo
and Cr.
Because of suitable for coating the substrate surface with a large
dimension and/or complex morphology, the VO2 nanopowder has
attracted much attention recent years. Different synthetic tech-
niques have been developed, such as hydrothermal method, liquid-
phase hydrolysis method, and the evaporative decomposition of
solutions technique [18–21], and among them the thermolysis
method is a most promise technique that can be easily integrated
2. Experimental
2.1. Sample synthesis
In a typical synthesis procedure, oxalic acid (C2H2O4ꢀ2H2O)
(3.56 g) was first resolved into 100 mL ion-exchange water with
constant stirring. Then 2.0 g of V2O5 powder was added to the
solution to make a yellow suspension at a temperature of about
60 8C. The solution turned green and then blue with final pH value
between 2 and 3, indicating the reduction of V5+ to V4+. Finally, the
solution was dried at 75 8C for 2 days to obtain the precursor
powders. The obtained powders were heated in a horizontal
vacuum tube furnace at 550 8C for 1 h under a flow air gas of about
15 sccm. VO2 nanopowders were then obtained through the
thermolysis. The W-doped VO2 powders were prepared in the
same way. Ammonium tungstate was added into the blue solution
and stirred before drying in the oven. The samples with different
W-doping were defined as V0 (un-doped), V1 (0.5 at%), V2 (1.5 at%)
and V3 (2.5 at%).
* Corresponding author. Tel.: +86 551 5591437; fax: +86 551 5591434.
E-mail address: ghli@issp.ac.cn (G.H. Li).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.06.030

F.Y. Kong et al. / Materials Research Bulletin 46 (2011) 2100–2104
2101
2.2. Sample characterization
be indexed to R-phase VO2 (JCPDS Card No. 44-0253, P42/mnm).
The difference of XRD peak position between M- and R-phase is
very small, to prove the formation of the R-phase, the enlargement
of the three strongest diffraction peaks for samples V0 and V3 were
shown in Fig. 2(c)–(e). From these figures one can clearly see that
the diffraction peaks positions for sample V3 (curve 2) either
slightly shift to left (Fig. 2(c) and (e)) or right (Fig. 2(d)) as
comparison with sample V0 (curve 1), and match well with R-
phase. This result shows that the R-phase VO2 with room
temperature phase transition temperature can be obtained using
present synthesis method.
Fig. 3 shows SEM and TEM images of sample V1. One can see
that VO2 nanopowders have a nearly spherical morphology with
size ranging from 50 to 100 nm. The corresponding SAED pattern
shown in Fig. 3(b) clearly indicates that VO2 nanopowders have a
monoclinic structure. The HRTEM image shown in Fig. 3(c) proves
that the VO2 (M) nanopowder is a single crystalline. The fringe
spacing of 0.25, 0.22 and 0.45 nm match well with the (2 0 0),
(0 2 0) and (2 1 0) interplanar spacing of VO2 (M) nanocrystals,
The valence state and the chemical composition of the samples
were studied by X-ray photoelectron spectroscopy (XPS; Perkin-
Elmer PHI-5600ci). The microstructure was determined by X-ray
diffraction (XRD; Philips X’Pert Pro MPD and Cu K
a radiation at
˚
1.540 56 A, at temperature of about 26 8C), and transmission
electron microscope (TEM; JEOL Model 2010). Thermal gravimetric
analysis (TGA) was carried out with a Perkin-Elmer Pyris 1 Thermal
Gravimetric Analyzer with a heating rate of 10 8C/min in a flowing
nitrogen atmosphere. The phase transition behavior and the
thermal stability were analyzed by differential scanning calorime-
try (Netzsch DSC-204) at a constant heating rate of 10 8C/min from
0 to 100 8C.
3. Results and discussion
3.1. Morphology, structure and component analysis
The reaction of V2O5 powder with H2C2O4 solution can be
expressed by following eqation [23]:
with is in according well with the XRD spectra (2
u
ꢂ 378, 39.88,
42.28). SEM observations indicate that the as-prepared VO2 (M)
nanocrystals have a spherical shape with partially agglomeration,
see Fig. 3(d), in which the primary grain size is in the range of 50–
100 nm and is good agreement with TEM result.
V2O5 þ 3H2C2O2 ! VOC2H4 þ 3H2O þ 2CO2
(1)
in which the obtained precursor powders is vanadyl oxalate after
dry. To understand the decomposition process of the vanadyl
oxalate powders, the thermal gravimetric analysis (TGA) was
conducted in flowing N2 atmosphere, as shown in Fig. 1. From the
TGA curve, one can see there are four weight losses in temperature
range of 50–700 8C: ꢁ5.92% form 50 8C to 131 8C, which is related
to the elimination of adsorption water [24], ꢁ7.96% from 131 8C to
221 8C, related to the procedure of dehydration, ꢁ38.16% from
221 8C to 329 8C and ꢁ2.95% form 329 8C to 499 8C, related
respectively to the decomposition and oxidation of the precursor,
which is a complex decomposition procedure, including the
oxidation of carbon and hydrogen. No further weight loss was
observed at temperature higher than 500 8C, indicating that the
precursor vanadyl oxalate powders have completely thermal
decomposed, and thus the 550 8C was selected as thermal
treatment temperature to prepare VO2 samples.
The chemical state and composition of the W-doped VO2
nanocrystals was investigated by XPS analysis. The XPS spectra
Fig. 2 shows the XRD patterns of the as-prepared VO2 samples
with and without W-doping after thermal treatment at 550 8C.
From Fig. 2(a), one can see that for the samples V0, V1 and V2, all
peaks can be indexed to M-phase VO2 (JCPDS Card No. 44-0252,
monoclinic, P21/C 14), and no other vanadium oxides, such as V2O3,
V2O5, were observed. No peaks for ammonium tungstate were
observed, suggesting the formation of a VO2–WO3 solid solution.
While for sample V3 shown in Fig. 2(b), all the diffraction peaks can
Fig. 2. XRD patterns of sample: (a) V0 (curve (1)), V1 (curve (2)), V2 (curve (3)) and
that from JCPDS Card No. 44-0253, (b) V3 and that from JCPDS Card No. 44-0252;
(c)–(e) part enlargement of the XRD peaks for samples V0 (curve (1)) and V3 (curve
(2)).
Fig. 1. Typical TGA and DTG curves of the precursor powders.

2102
F.Y. Kong et al. / Materials Research Bulletin 46 (2011) 2100–2104
Fig. 3. (a) TEM image and (b) corresponding SAED pattern; (c) HRTEM images of a single nanocrystal and (d) SEM image for sample V2.
were calibrated by the C1s peak (284.6 eV) from adventitious
hydrocarbon contamination on the sample surface. Only four
elements C, V, W and O can be observed in the wide energy range
survey spectrum. The high-resolution analyses of the O1s and V2p3/2
peaks, as well as their deconvolution analysis based on Gaussian
function areshowninFig. 4(a). One canseethatthemainfitting peak
of V2p3/2 is centered at 516.46 eV, which is consistent with binding
energy of V4+ ion [19], the small peak at binding energy of 514.53 eV
corresponds to V3+ ions, indicating there is some V2O3 phase in the
sample, which is consistent with the deconvolution of the V3p peak
shown below. The O1s peak can be separated into two components
relating to a peak with a binding energy of 530.57 eV, and an
unknown peak witha binding energyof 528.17 eV. Deconvolution of
the V3p peak indicates that the V ions present in V4+ and V3+ with a
binding energy of 41.30 and 38.14 eV, respectively, as shown in
Fig. 4(b). Quantitative calculation indicate that the main existence
chemical state is V4+, and there is only 6.5% in the form of V3+, which
indicating the dominant existing form of V ions is V4+. It should be
noted that this assignment has a measure of uncertainty because of
the overlap between the W peaks and the V2p3/2. The present of the
V3+ is helpful to the reduction of the MIT transition temperature due
to the disruption of the V4+–V4+ unit in VO2 (M) by formation of V3+
and causing a destabilization of the monoclinic phase of VO2 [25,26].
From the deconvolution of the W4f peak (Fig. 4(b)), one can see that
the W is present as W6+, with W4f5/2 and W4f7/2 binding energies of
34.46 and 36.30 eV, respectively. This result indicates that the W is
VI value state after incorporating into the VO2 lattice. The XPS
analyses also show the existence of both V4+ and V3+ in pure VO2
nanocrystals.
The preparation of VO2 by the decomposition of the vanadyl
oxalate is a controlled oxidation process, and the reaction can be
described as:
2VOC2O4 þ O2 ¼ 2VO2 þ 4CO2
(2)
The controlling of the flow rate of air in the thermal treatment is
essential in obtaining VO2 phase. It was found that V2O3 phase will
Fig. 4. High resolution scans of XPS peaks of (a) V2p and O1s, (b) V3p and W4f and the
corresponding Gassian fittings.

F.Y. Kong et al. / Materials Research Bulletin 46 (2011) 2100–2104
2103
Fig. 7. (a) Latent heat of phase transformation,
D
H; (b) peak temperature, Tp and
Fig. 5. DSC curves and phase transition temperature of VO2 with W-doping content
onset phase transition temperature, Tc, as a function of thermal cycle.
of (1) 0 at%, (2) 0.5 at%, (3) 1.5 at% and (4) 2.5 at%.
be formed without air flow, equaling to thermal treatment in
vacuum, and different amounts of V6O13 phase will be coexisted
with VO2 if the air flow rate is higher than 20 sccm. The V2O5 and
VO2 phase mixture will be obtained when the air flow rate is higher
than 200 sccm. The pure V2O5 phase can be obtained only when the
thermal treatment is performed in air atmosphere, as reported by
Pan and colleague using the same vanadium precursor [23].
room temperature, whereas others are monoclinic phase. One also
can see from Fig. 4 that the decrease in phase transition
temperature almost linear with increasing W content. The result
indicated that the reduction of phase transition temperature of
VO2 nanocrystals is about 16.5 8C per W at%, which is slightly lower
than the reported value (19 8C per 1 at%) by Troy D. Manning et al.
using a APCVD method [26]. Our results indicate that the phase
transition temperature of VO2 nanocrystals can be adjusted easily
by W-doping, and demonstrate promising application in future
smart windows.
3.2. Thermal stability of phase transition
The phase transition behaviors of the VO2 nanocrystals with
and without W-doping were characterized by DSC analysis, as
show in Fig. 5. In the heating cycle, a clear endothermic peak can
be seen in each of the DSC curves, which is attributed to the
monoclinic/rutile structural phase transition. Three parameters
can be obtained in a typical DSC curve: the onset transformation
temperature (Tonset), peak temperature (Tp) and latent heat of
The thermal stability of sample V3 was investigated by DSC
analysis with successive thermally induced MIT. As shown in
Fig. 6, all the DSC curves with different cycling numbers show
evident endothermic peaks and the peaks almost have the same
shape. The cycling number dependence of the latent heat of
phase transition (
D
H) obtained from the area of the endothermic
H has almost a
peak in DSC data are shown in Fig. 7(a), the
D
phase transformation (
D
H). In this study the Tonset was defined as
constant value of about 6.3 ꢃ 0.2 J/g and no attenuation was
detected, indicating the phase transition from high-temperature
rutile VO2 (R) to low-temperature monoclinic phases VO2 (M) is
stable. In addition, the phase transition temperatures of the onset
temperature (Tc) and peak temperature (Tp) also have a constant
value at different heating–cooling cycles, as shown in Fig. 7(b). The
Tc and Tp are about 23.9 ꢃ 0.3 and 39.4 ꢃ 0.2 8C, respectively. From
the analysis, one can clearly seen that there is no degradation in
the phase transition parameters, implying that the stoichiometry
of VO2 nanocrystals do not degraded under multiple thermally
induced transition, and the transition quality is well maintained
within successive 100 cycles and the phase transition is completely
reversible. Since the phase transition temperature varies with
different W contents, the doped W in the nanocrystals is proved to
be of excellent stability, and this high stability is considered due to
both the size effect and the existence of V3+ in the VO2 (M)
nanocrystals [26]. Accordingly, the W-doped VO2 nanocrystals
synthesized by the thermolysis method have excellent thermal
stability, which is very important and beneficial for the techno-
logical application of VO2 nanocrystals as smart windows.
the transition temperatures (Tc). The phase transition temperature
of the undoped VO2 nanocrystals is about 67 8C, which is close to
the reported value [21]. While for sample V3, the phase transition
temperature is at about 24.2 8C, close to room temperature. This
result is fully consistent with the XRD results, in which when
doped with 2.5 at% W, the VO2 nanocrystals become rutile phase at
4. Conclusion
Rutile and monoclinic VO2 nanocrystals were synthesized by
the thermolysis method with ammonium tungstate as a substitu-
tional dopant. The dopant is in the W6+ form, and there is a small
amount of V3+ in the VO2 nanocrystals. A significant correlation
Fig. 6. DSC curves of VO2 at cycle of (1) 1st, (2) 35th, (3) 75th and (4) 100th.
between phase transition temperature and W dopant was

2104
F.Y. Kong et al. / Materials Research Bulletin 46 (2011) 2100–2104
[8] W.X. Huang, X.G. Yin, C.P. Huang, Q.J. Wang, T.F. Miao, Y.Y. Zhu, Appl. Phys. Lett. 96
(2010) 261908.
[9] H.M. Liu, A. Rua, O. Vasquez, V.S. Vikhnin, F.E. Fernandez, L.F. Fonseca, O. Resto, S.Z.
Weisz, Sensors 5 (2005) 185–198.
[10] E. Strelcov, Y. Lilach, A. Kolmakov, Nano Lett. 9 (2009) 2322–2326.
[11] I.P. Parkin, T.D. Manning, J. Chem. Educ. 83 (2006) 393–400.
[12] T.D. Manning, I.P. Parkin, M.E. Pemble, D. Sheel, D. Vernardou, Chem. Mater. 16
(2004) 744–749.
confirmed. The room temperature stable rutile VO2 was obtained
with 2.5 at% W doping with onset phase transition temperature of
24.2 8C. It was found the transition character of VO2 nanocrystals is
very stable upon 100 heating–cooling cycles in the temperature
range of 0–100 8C, and this excellent thermal stability is beneficial
for applications as thermochromic material.
[13] M. Soltani, M. Chaker, E. Haddad, R.V. Kruzelesky, J. Vac. Sci. Technol. A 24 (2006)
612–617.
[14] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Appl. Phys. Lett. 95 (2009)
171909.
Acknowledgements
[15] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Solar Energy Mater. Solar Cells 93
(2009) 1685–1687.
[16] R. Lopez, L.A. Boatner, T.E. Haynes, J.R.F. Haglund, L.C. Feldman, Appl. Phys. Lett. 85
(2004) 1410–1412.
[17] R. Lopez, T.E. Haynes, L.A. Boatner, L.C. Feldman, R.F. Haglund, Phys. Rev. B 65
(2002) 224113.
[18] C.X. Cao, Y.F. Gao, H.J. Luo, J. Phys. Chem. C 112 (2008) 18810–18814.
[19] C.L. Xu, L. Ma, X. Liu, W.Y. Qiu, Z.X. Su, Mater. Res. Bull. 39 (2004) 881–886.
[20] S.A. Lawton, E.A. Theby, J. Am. Ceram. Soc. 78 (1995) 104–108.
[21] R. Li, C.Y. Liu, Mater. Res. Bull. 45 (2010) 688–692.
[22] G. Fu, A. Polity, N. Volbers, B.K. Meyer, Thin Solid Films 515 (2006) 2519–2522.
[23] A. Pan, J.-G. Zhang, Z. Nie, G. Cao, B.-W. Arey, G. Li, S.-Q. Liang, J. Liu, J. Mater. Chem.
20 (41) (2010) 9193–9199.
This work was financially supported by the National Basic
Research Program of China (2009CB939903), and innovation
project of the Chinese Academy of Sciences (KJCX2-YW-H2O).
References
[1] F.J. Morin, Phys. Rev. Lett. 3 (1959) 34–36.
[2] C. Chen, Z. Zhou, Appl. Phys. Lett. 91 (2007) 011107.
[3] S. Chen, H. Ma, S. Wang, N. Shen, J. Xiao, H. Zhou, X. Zhao, Y. Li, X. Yi, Thin Solid
Films 497 (2006) 267–269.
[4] W.R. Roach, Appl. Phys. Lett. 19 (1971) 453–455.
[5] A.A. Bugayev, M.C. Gupta, Opt. Lett. 28 (2003) 1463–1465.
[6] K.I. Zemskov, M.A. Kazaryan, V.G. Mokerov, G.G. Petrash, A.G. Petrova, Sov. J.
Quant. Electron. 8 (1978) 245–247.
[7] M. Rini, Z. Hao, R.W. Schoenlein, C. Giannetti, F. Parmigiani, S. Fourmaux, J.C. Kieffer,
A. Fujimori, M. Onoda, S. Wall, A. Cavalleri, Appl. Phys. Lett. 92 (2008) 181904.
[24] B. Tang, J.C. Ge, C.G. Wu, L.H. Zhuo, J.Y. Niu, Z.Z. Chen, Z.Q. Shi, Y.B. Dong,
Nanotechnology 15 (2004) 1273–1276.
[25] C. Tang, P. Georgopoulos, M.E. Fine, J.B. Cohen, M. Nygren, G.S. Knapp, A. Aldred,
Phys. Rev. B 31 (1985) 1000–1011.
[26] T.D. Manning, I.P. Parkin, J. Mater. Chem. 14 (2004) 2554–2559.