J Mater Sci (2008) 43:985–988
DOI 10.1007/s10853-007-2265-7

Thermochromism of vanadium–titanium oxide prepared fromperoxovanadate and peroxotitanate
Isao Tsuyumoto Æ Kentaro Nawa

Received: 7 June 2007 / Accepted: 26 October 2007 / Published online: 15 November 2007
Ó Springer Science+Business Media, LLC 2007

Abstract Vanadium–titanium complex oxides were prepared through a new synthetic route from peroxovanadate
and peroxotitanate solutions, and their thermochromism
were investigated. Reflectance spectra for V0.48Ti0.52O2
pellet showed a reversible reflectivity increase in thewavelength region of 700–1,000 nm above 45 °C. The
reflectivity was increased gradually from 45 °C to ca.
80 °C unlike the conventional VO2 which shows an abrupt
increase at 67 °C. The mechanism was discussed using
powder XRD patterns, SEM images and element distribution maps.

Introduction
Thermochromic smart windows, which automaticallyshade the sunlight at high temperature have attracted muchattention from the viewpoint of energy saving. Many
researchers have tried to apply the semiconductor-to-metaltransition of some metal transition oxides to the smart
windows, because it accompanies an abrupt reflectivity
change [1]. For example, the monoclinic form of VO2 istransformed into the rutile form at Tc = 67 °C [2], and thissemiconductor-to-metal transition brings about an abrupt
reduction of transmittance at near-infrared region [3]. On
the other hand, we have found a new nonstoichiometricorthorhombic titanium oxide (TiOx, x = 1.85–1.94) [4] andhave investigated its thermochromic properties in nearinfrared region [5]. In this study, we focus on the preparation of vanadium–titanium complex oxides and theirthermochromism.
Doping tungsten or molybdenum in VO2 lowers the Tcof the phase transition [6, 7], and the switching performance of double-doped films in the V1-x-yWxMoyO2 andV1-x-zWxTizO2 systems has been precisely investigated[8]. Phase transitions of V1-xTixO2 system have been also
investigated using the conductivity and DTA measurements [9, 10]. The DTA measurement suggested that thetransition temperature of V0.60Ti0.40O2 was 48 °C. Their
optical switching properties at 2.5 lm have been reported
using thin films deposited from alkoxides [11]. From thepractical point of view, a wet process using solutions issuitable for fabricating thin films because it is feasible toproduce a large-area film at a low cost. It is highly desirable to develop a vanadium–titanium solution process toachieve the smart window.
In the present study, we studied the reflectance spectrachange of V1-xTixO2 system as well as their structures andmorphologies. The samples of V1-xTixO2 were preparedthrough a new solution process from peroxovanadate andperoxotitanate aqueous solutions. The spectra changes inthe near-infrared region above 45 °C were presented.

Experimental
Preparation of materials

I. Tsuyumoto (&)  K. Nawa
Department of Environmental Chemistry,
College of Architecture and Environmental Engineering,
Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi,
Ishikawa 921-8501, Japane-mail: tsuyu@neptune.kanazawa-it.ac.jp

Peroxotitanate solution (ammonium citratoperoxotitanate
aqueous solution) was prepared by mixing metallic titanium powder (0.25 g) with the mixture of hydrogen
peroxide solution (30 wt% aq, 20 g), ammonia solution123986

Optical measurement
Temperature dependence of the reflectance spectra in thewavelength range 750–2,500 nm was measured using aspectrophotometer (Ubest V-570DS, JASCO corporation).
The incident angle was 5°.

3.590°C80°C

3.0

Reflectivity (%)(28 wt% aq, 5 g), and citric acid (0.25 g), according to theliterature [12]. A vigorous reaction took place mainly
between the metallic titanium and the hydrogen peroxidewith emission of oxygen gas to yield a transparent lightyellow peroxotitanate solution. Although it has been generally recognized that to prepare titanium aqueous solution
is technically difficult, titanium aqueous solution containing about 0.20 mol/kg of titanium was successfully
prepared though the present procedure.
Similarly, peroxovanadate solution was prepared bymixing metallic vanadium 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). This reaction yielded a transparent orange solution containing the complex ion of vanadium.
These two solutions were mixed and dried at room
temperature. We obtained a yellow residue and used this asa precursor of vanadium–titanium complex oxide. The
residue was pressed into pellet and heated at 400 °C for 2 hin hydrogen atmosphere. The pellet transformed fromyellow to black by reduction. The composition of thereduced sample was estimated as V0.48Ti0.52O2. The two
types of pellet after and before reduction were used for
optical thermochromic measurement.

J Mater Sci (2008) 43:985–988

70°C
60°C
50°C
47°C
46°C
45°C
40°C

50°C

2.5

40°C

1.9
1.8

2.0

1.7
1.6

1.5
1.0

1.5
750

750

1000

1250

1500

800

1750

850

900

λ (nm)

2000

950

1000

2250

2500

λ (nm)

Fig. 1 Reflectance spectra for V0.48Ti0.52O2 pellet after reduction in
H2 at 400 °C. Reflectivity in the range 750–1,000 nm was increased
with increasing temperature from 40 to 90 °C. Inset is reflectancespectra from 40 to 50 °C. The reflectivity begins to increase at 45 °C
and continues increasing gradually over 45 °C. These changes werereversible

1,000–2,500 nm is slightly changed above 80 °C asreported for the conventional vanadium oxides in the earlier researches. Our critical temperature Tc is 45 °C, muchlower than that of the reported vanadium oxide VO2,67 °C. It should be noted that our reflectivity change
occurs gradually over Tc, while the reflectivity change ofthe reported VO2 takes place abruptly at Tc.
Reflectance spectra of the yellow sample pellet before
reduction were also measured for comparison (Fig. 2). The
baseline was changed with increasing temperature due todesorption of hydrated water and thus relative comparison
among these curves is impractical. However, it is important
to note that the gradual reflectivity change in 700–
1,000 nm was also observed for the sample pellet before

Structural aspects

Results and discussion
Reflectance spectra of the reduced black sample pellet
V0.48Ti0.52O2 in 750–2,500 nm at 40–90 °C are shown inFig. 1. The reflectivity in 700–1,000 nm is gradually
increased with increasing the temperature. The increase
starts at 45 °C and saturates at 80 °C. We also confirmedthat this reflectivity change was reversible. In addition tothis gradual change in 700–1,000 nm, the reflectivity in

123

2.5
2.0

Reflectivity (%)

Powder XRD (X-ray diffraction) patterns were taken tocheck crystal structures of the powder samples before andafter the reduction in hydrogen atmosphere. SEM and EDX
observations (SEMEDX Type N, Hitachi) were performedon the reduced powder sample at the acceleration voltageof 25 kV.

1.540°C
50°C
60°C
70°C
80°C
90°C

1.0
0.5
0.0
750

1000

1250

1500

1750

2000

2250

2500

λ (nm)

Fig. 2 Reflectance spectra for the precursor of V0.48Ti0.52O2 before
reduction. Reflectivity increase peaking around 820 nm was also
observed for the pellet before reduction. The baseline was changed
due to desorption of hydrated water, and thus relative comparison
among these curves is inappropriate

J Mater Sci (2008) 43:985–988

987

reduction. Reduction from pentavalent vanadium to tetravalent was found to be unnecessary for the present samples.
Figure 3a and b shows powder XRD patterns of the
reduced and unreduced samples, respectively. The reduced
sample contained V2O3 [13] and amorphous phase. The
V2O3 has been reported to show thermochromism around
150 K, much lower than the present measurement. The
yellow powder before reduction contained ammonium
vanadium oxide ((NH4)4V2O11) [14] along with amorphousphase.
SEM and EDX analysis were performed to provide
details on the amorphous phase. Figure 4a shows theSEM image of the reduced sample, and Fig. 4b and cshow the element distribution map of the same sample fortitanium and vanadium. Figure 5a–c shows more magnified images. The distributions of the two elements were
heterogeneous, and the compounds consisted of multiphase with various vanadium/titanium ratios. The regions

I (a. u.)

(a)

(b)

with high vanadium content contained lower titanium, andvice versa.
Our experimental results suggested that the amorphous
phase was the thermochromic phase, and that the gradualincrease was due to the variety of the vanadium/titanium
ratios. More studies are needed for details. For example,analysis of radial distribution functions (RDFs) of theamorphous phases will provide insight into the thermochromism of the present samples, because long-range
periodicity was observed in the XRD patterns for the two
samples around 2h = 3 or 7° (Cu Ka) corresponding to dvalues of 2.9 or 1.3 nm. In practical point of view, it will
be possible to prepare the smart window which shades
sunlight at 45 °C by purifying an amorphous phase with acertain vanadium/titanium ratio. In the design of the smart
window, high reflectivity in near-infrared region and hightransmittance in far-infrared region are simultaneously
essential for preventing temperature rise, because heat ray
from the sunlight consists of near-infrared rays and visible
light, and substance cools by emitting far-infrared rays.
The reflectivity change in 700–1,000 nm observed for thepresent material is highly appropriate for the smart window. It is expected that the smart window will be
developed by fabricating thin films of this material on glass
windows. Taking advantage of the solution process, fabrication of large area thin film will be possible by the spin
coating method.

Conclusions
20100

4060802θ (de gree ) / Cu K αFig. 3 Powder XRD patterns for (a) V0.48Ti0.52O2 and (b) theprecursor before reduction. Sharp peaks are due to (a) V2O3 and (b)(NH4)4V2O11. Long-range periodicities were observed for both
samples (indicated by arrows)

V0.48Ti0.52O2 was prepared through a new synthetic routefrom peroxovanadate and peroxotitanate solutions. The
reflectance spectra of V0.48Ti0.52O2 pellet showed a gradualreflectivity increase in the wavelength region of 700–
1,000 nm above 45 °C, along with a slight reflectivity
increase in 1,000–2,500 nm above 80 °C. The gradualFig. 4 (a) SEM image forV0.48Ti0.52O2 and its element
distribution maps of (b) Ti and(c) V123

988

J Mater Sci (2008) 43:985–988

Fig. 5 (a) SEM image forV0.48Ti0.52O2 and its element
distribution maps of (b) Ti and(c) V with higher magnification
than Fig. 4reflectivity increase in 700–1,000 nm was also observed
for the sample pellet before reduction as prepared from themixed solution of peroxovanadate and peroxotitanate.
Powder XRD patterns, SEM images and element distribution maps suggested that an amorphous phase containing
vanadium and titanium played an important role in thethermochromism. Analysis of the amorphous phase ishighly desirable to elucidate the reflectivity changemechanism. Purification of the amorphous phase will be
useful in practical use as well as in the analysis.

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