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Solid State Ionics 179 (2008) 1227 – 1229
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Thermochromism of titanium–vanadium oxide thin films prepared fromperoxotitanate and peroxovanadate solutions
Isao Tsuyumoto ⁎, Kentaro Nawa
Department of Environmental Chemistry, College of Architecture and Environmental Engineering, Kanazawa Institute of Technology, Japan
Received 11 June 2007; received in revised form 22 January 2008; accepted 27 January 2008

Abstract
Thin films of vanadium–titanium complex oxides were prepared on glass substrates from the mixture of peroxovanadate and peroxotitanatesolutions by the spin coating method. Temperature dependence of the reflectance spectra for the thin films were investigated, and it was found that
their reflectivities in the wavelength region from 700 to 1000 nm were reversibly increased over 45 °C. The changes in optical dielectric constants
were estimated and discussed from a viewpoint of the practical applications as thermochromic smart windows.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Vanadium; Titanium; Oxide; Thermochromism; Reflectivity

1. Introduction
Vanadium dioxide VO2 exhibits a semiconductor-to-metaltransition owing to a structural transformation from a monoclinic phase to a tetragonal rutile-type phase at Tc = 67 °C [1].
This transition accompanies an abrupt change in optical properties, i. e., a significant increase in reflectivities in infrared
region [2,3]. Much effort has been made to apply these opticalproperties to thermochromic smart windows which automatically shade the sunlight at high temperature.
A number of researchers have tried to lower the transition
temperature Tc by doping another element. Doping tungsten ormolybdenum in VO2 reduces the Tc [4,5], and the switching
performance of double-doped films in the V1 − x − yWxMoyO2
and V1 − x − yWxTiyO2 systems has been investigated [6]. Phasetransitions of V1 − xTixO2 system have been also investigatedusing the conductivity and DTA measurements [7,8]. The
optical switching properties have been reported using thin films
deposited from alkoxides [9]. A wet process using solutions issuitable for fabricating thin films from the practical viewpoint,
because it is easy to dope another element and to produce a⁎ Corresponding author.
E-mail address: tsuyu@neptune.kanazawa-it.ac.jp (I. Tsuyumoto).
0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ssi.2008.01.061

large-area thin film at a low cost. A vanadium–titanium solution
process is highly desirable to achieve the smart window.
We have found a new nonstoichiometric orthorhombic titanium oxide (TiOx, x = 1.85–1.94) [10] and have investigated its
thermochromic properties [11]. We have also reported thethermochromic properties of the solid pellet of vanadium–titanium oxide (V0.48Ti0.52O2) [12]. The pellet of V0.48Ti0.52O2
showed a reflectivity increase in the wavelength of 750–
1000 nm above 45 °C. The goal of this study is to fabricate thinfilms of vanadium–titanium oxide through a solution processusing peroxovanadate and peroxotitanate solutions and to evaluate their thermochromic properties.
2. Experimental
2.1. Preparation of solutions
Peroxotitanate solution was prepared by mixing metallic
titanium powder (0.25 g) with the mixture of hydrogen peroxide
solution (30 wt.% aq, 20 g), ammonia solution (28 wt.% aq,5 g), and citric acid (0.25 g), according to the literature [13].
A transparent light yellow peroxotitanate solution (0.20 mol/kgof Ti) was obtained by a vigorous reaction between the metallic titanium and the hydrogen peroxide. The peroxo and citric1228

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V-570DS, JASCO corporation). The temperature of the sample was controlled by a rubber heater. The incident angle
was 5°.
2.4. XPS measurement

Fig. 1. Reflectance spectra for thin films having the composition ofV0.48Ti0.52O2 prepared from the mixture of peroxovanadate and peroxotitanatesolutions. The reflectivity change was reversible.

XPS spectra were taken for two samples: the dry residue ofthe mixture of peroxotitanate and peroxovanadate solutions
and its reduced form heated in hydrogen stream at 400 °C for
2 h. This measurement was performed to compare the valancestates between before and after the hydrogen reduction, because
the hydrogen reduction was found to be unnecessary for oursample.
3. Results and discussion
3.1. Thermochromism measurement

groups are ligands of the titanium to form a stable complex ion,and the ammonium ion is a counter ion.
Peroxovanadate solution was prepared in a similar mannerby mixing metallic vanadium powder (0.25 g), hydrogen peroxide solution (30 wt.% aq, 20 g), ammonia solution (28 wt.%
aq, 5 g), and citric acid (0.25 g). This reaction yielded a transparent orange solution containing the complex ion of vanadium.
2.2. Fabrication of thin films
The mixture of the two solutions (peroxotitanate and peroxovanadate) was spin-coated on a soda-lime glass substrate at
the spinning rate of 8000 rpm, and a thin film of precursor ofV1 − xTixO2 was formed. In order to remove ligands, counter ion,and water, the film was heated in air at 400 °C for 2 h. Although
in the synthetic procedure used in the earlier report, reduction
in a hydrogen stream was essential to obtain VO2 [6], hydrogengas was unnecessary in our procedure to obtain thermochromic films. The composition of the thin film was estimated atV0.48Ti0.52O2.

Fig. 1 shows reflectance spectra for thin film of vanadium–titanium oxide on a glass substrate heated in air at 400 °Cfor 2 h. Reversible reflectivity change was observed distinctly in the wavelength region of 700–1000 nm and slightly
in the wavelength region of 1500–2500 nm. The reflectivity began to increase at 45 °C and ceased to increase around
80 °C. We found that the thin films having the composition of
V0.48Ti0.52O2 show reversible reflectivity increase above 45 °C.
These properties have not been observed for the VO2 samples,
which show an abrupt increase in reflectivity at Tc = 67 °C. The
starting temperature at which the reflectivity began to increase ismuch lower than that of the VO2. It is important to note thatthe reflectivity increase of the present thin films was gradualunlike the VO2. These thermochromic properties of the thinfilms are similar to those of powder compact of the present
vanadium–titanium oxides which we have already reported
previously [13]. The reflectivity of the powder compact waschanged in the range of 1.6–3.0%, whereas that of the thin

2.3. Measurement of thermochromic properties
The reflectance spectra in the wavelength range of 750−
2500 nm were measured using a spectrophotometer (UbestFig. 2. Optical dielectric constant of the thin film estimated from the reflectivity.

Fig. 3. XPS spectra (Ti 2p, V 2p, O 1 s) for the dry residue of the mixture ofperoxotitanate and peroxovanadate solutions and its reduced form heated inhydrogen stream at 400 °C for 2 h. The inset is the magnified figure of the V 2pspectrum.

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film was changed in the range of 8–11%. This is due to thedifference of surface roughness between the powder compact
and the thin films on glass substrate.
The change in optical dielectric constant of the thin film
was estimated from the reflectance spectra observed usings-polarized light (Fig. 2). The result suggests that the reflectivity
increase was attributed to the decrease in the dielectric constant.
In our previous study, we have concluded from SEM and EDX
observations that the present thermochromic property is owing
to an amorphous vanadium–titanium oxide phase generated by
the new method. The optical dielectric constant is considered to
be changed due to a gradual structural change in the amorphous
phase.
Thicker thin films were also fabricated by repeating a set ofspin coating and drying. However, increasing the thickness ofthe thin films did not bring about an increase in reflectivity.
The reflectivity is dependent on the optical dielectric constant, i.e.,complex refractive index, n + i k. Strategy for increasing thechange in n is required in order to enhance the reflectivity change.

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the same as those after the hydrogen reduction. This point isadvantageous because the reduction using explosive hydrogenis unnecessary.
4. Conclusions
Thin films having the composition of V0.48Ti0.52O2 wasprepared through a wet process using peroxovanadate andperoxotitanate solutions. The reflectance spectra for the thinfilms showed a reversible reflectivity increase in the wavelength
region of 700–1000 nm above 45 °C. The reflectivity change
is attributable to the change in the optical dielectric constant,which is considered to be changed owing to a gradual structuralchange in the amorphous phase. The XPS spectra showed thatthe valence state of vanadium in the dry residue of mixture of
the peroxovanadate and peroxotitanate solutions were tetravalent, same as that after hydrogen reduction at 400 °C for 2 h.
This is the reason why hydrogen reduction is unnecessary forthe present vanadium–titanium oxides to exhibit the thermochromism unlike the conventional vanadium oxide.

3.2. XPS analysis
References
XPS spectra for the dry residue of the mixture of peroxotitanate and peroxovanadate solutions and its reduced form
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