Materials Research Bulletin 39 (2004) 1737–1744

Nonstoichiometric orthorhombic titanium oxide, TiO2d andits thermochromic properties
Isao Tsuyumoto*, Hiroshi Uchikawa
Department of Environmental Chemistry, College of Environmental Engineering and Architecture,
Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japan
Received 25 June 2003; received in revised form 26 April 2004; accepted 28 April 2004

Abstract
Redox relationship among anatase, rutile, and a new nonstoichiometric orthorhombic titanium oxide waselucidated. Single crystal of nonstoichiometric orthorhombic titanium oxide TiO1.94 was prepared by reducing asingle crystal of rutile TiO2 at 950 8C in H2 for 1 h based on this relationship. The single crystallinity wasdemonstrated by both the Laue and the Debye–Scherrer methods. Temperature dependence of the conductivity
and the reflectance spectrum was measured for the single crystal of TiO1.94, and it was found that the reflectivity
in near-infrared region over 1700 nm gradually increased with increasing temperature. ‘‘Smart windows’’ using
this titanium oxide was proposed.
# 2004 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides; C. X-ray diffraction; D. Electrical properties; D. Optical properties

1. Introduction
Window glass whose reflectivity increases with increasing temperature controls room temperature
automatically because it shades sunlight at high temperature. This is called ‘‘smart window’’ and many
researchers have tried to apply thermochromism of some transition metal oxides accompanied by asemiconductor-to-metal transition. For example, the monoclinic form of VO2 is transformed into therutile form at Tc ¼ 67 8C, and this semiconductor-to-metal transition brings about an abrupt reductionof transmittance at near-infrared region. Kudo and co-workers [1] have reported that the thin film of
VO2 was prepared by a wet process of sol–gel techniques, and that the Tc of 67 8C was lowered bydoping tungsten or molybdenum.

*

Corresponding author.
E-mail address: tsuyu@neptune.kanazawa-it.ac.jp (I. Tsuyumoto).
0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2004.04.037

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The reflectivity change due to thermochromism is explained by using the plasma frequency op asfollows.
4pne2
(1)
m
where n is the density of conduction electrons and m is the mass of electron. In general, light below thefrequency of op is reflected by the substance, and thus it is effective to increase op, i.e., n to expand thewavelength range in which light is reflected. There are two methods to increase n. One is to use asemiconductor-to-metal transition. This brings about an abrupt increase in n, i.e., op at Tc. The reported
reflectivity change of VO2 in near-infrared region at Tc is based on this idea. The other is to use gradualincrease in n with increasing temperature, and this results in a gradual increase in reflectivity. Here we
paid attention to nonstoichiometric compounds and their electrical properties.
The structure and electrical properties of nonstoichiometric compounds such as TinO2n1 (n ¼ 210),VnO2n1 (n ¼ 39), KxWO3 (x ¼ 0:170:33) have been intensively investigated because the latticedefect accompanied by the nonstoichiometry is closely associated with various functional properties:
ionic conduction, gas-sensitivity, magnetism, etc. [1–3]. Tremendous reports have appeared on TiO2 withregard to photocatalyst, and the structure and electrical properties of its reduced form TinO2n1 have been
the subject of many previous researches [4–6]. The Tc of TinO2n1 have been reported to be 660, 450,150, 125, and 133 K for n ¼ 2, 3, 4, 5, 6, respectively. Thus, we have investigated synthesis of new
titanium oxide with reduced form TiO2d, and have reported that a new titanium oxide of orthorhombic
system with a nonstoichiometric composition of TiO1.94 is obtained by reducing anatase TiO2 in H2 at
950 8C [7]. It is highly desirable to use an environment-friendly material like titanium oxides to keep mild
room temperature automatically and at the same time to decompose harmful chemical species. The goalof this study is to develop thermochromic windows using the new titanium oxide. Here we present thepreparation of its single crystal, and its electrical and optical properties.
o2p ¼

2. Experimental
2.1. Preparation of materials
Nonstoichiometric orthorhombic titanium oxide TiO1.94 (a ¼ 0:938 nm, b ¼ 0:919 nm,c ¼ 0:588 nm) was prepared according to our previous paper [7]. TiO2 powder with anatase form
was used as starting material. After grinding in an agate mortar, the sample powder was transferred in
an alumina boat and placed in a quartz tube. After purging the tube with N2 gas, H2 gas was introduced.
The temperature was increased at 15 8C/min, and heating was performed at 950 8C for 1 h. TiO2powder with rutile form and a single crystal (10 mm  10 mm  0:5 mm) with rutile form were also
used as starting materials and the products were investigated.
2.2. Structural aspects
Laue photographs were taken to verify the single crystallinity of the product from the rutile singlecrystal. Exposure time was 180 s under backscatter configuration (Cu Ka, 40 kV, 20 mA, MX-Labo,
MAC Science Co.).

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Single crystallinity of the product was also checked using a Debye–Scherrer diffractometer withmonochromatized Cu Ka radiation (Miniflex, Rigaku Corporation).
2.3. Conductometry
The resistivity was measured by the conventional d.c. four-probe method in N2 stream. A constantcurrent flow was made to flow from two outside electrodes with a current source (R6144, AdvantestCorporation), and the electric potential difference between the two inside electrodes induced by thecurrent was measured with an electrometer (HP34401A, Hewlett–Packard Corporation).
2.4. Optical measurement
The reflectance spectra in the wavelength range of 200–2500 nm was measured as a function oftemperature using a spectrophotometer (Ubest V-570DS, JASCO Corporation).

3. Results and discussion
3.1. Structure and redox properties
It was found that the orthorhombic titanium oxide was also obtained by reducing the rutile TiO2 in H2
at 950 8C for 1 h, and that heating the orthorhombic titanium oxide in air at 950 8C yields the rutileTiO2. In a word, the redox reaction between the orthorhombic titanium oxide and the rutile TiO2 wasreversible at 950 8C. Since it is well-known that heating the anatase TiO2 at 950 8C in air yields therutile TiO2, redox reactions among these three compounds are schematically depicted by a relationshipin Fig. 1.
Single crystals of rutile TiO2 are found in a wide variety of rocks and also produced artificiallyfor optical materials. We investigated a product obtained by reducing the rutile single crystal at
950 8C in H2, because the redox reaction between the two was reversible as for powder sample. A
dark blue opaque sample (201.1 mg, r ¼ 4:002 g/cm3) was formed by reducing a transparent
rutile single crystal (1 0 0) (10 mm  10 mm  0:5 mm, 203.7 mg, r ¼ 4:074 g/cm3). The color of
the product was similar to that of the powder sample, and the size was almost same as that of
the starting material. The chemical formula of the product was approximately estimated from theFig. 1. Redox reactions among the anatase, the rutile, and the orthorhombic titanium oxides.

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mass change in the reduction at TiO1.94. The product showed metallic shine after polishing thesurface.
Laue photographs were taken to verify the single crystallinity of the product. Fig. 2a and b show Lauephotographs for the rutile single crystal and its reduced form, respectively. Exposure time was 180 s
under backscatter configuration (Cu Ka, 40 kV, 20 mA, MX-Labo, MAC Science Co.). Definite spots
observed for the reduced form indicates single crystallinity similar to the rutile. Some spots observedwith streaks may reflect heterogeneous distribution of oxygen composition in depth direction.

Fig. 2. Laue photographs from the rutile single crystal (a) and its reduced form (b).

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Fig. 3. Debye–Scherrer X-ray diffraction patterns from the rutile single crystal (1 0 0) (a) and its reduced form (b).

Single crystallinity of the product was also investigated using an X-ray diffractometer withmonochromatized Cu Ka radiation. Fig. 3a and b show Debye–Scherrer X-ray diffraction patterns fromthe rutile single crystal and its reduced form, respectively. The plane of incidence was normal to (1 0 0),and thus single crystals exhibit diffraction spots definitely from planes parallel to (1 0 0). The rutile
single crystal showed only two diffraction peaks from (2 0 0) and (4 0 0) planes except for those fromsample holders (Fe, Ni, Al), and the lattice parameter a was calculated at 0.460 nm. Similarly the
reduced form showed only two diffraction peaks from (2 0 0) and (4 0 0) planes, and the lattice
parameter a was calculated at 0.931 nm.
These results indicate that the rutile single crystal was transformed into the single crystal of its
reduced form, orthorhombic titanium oxide TiO2d. The lattice parameters of powder sample of theorthorhombic titanium oxide TiO1.94 were a ¼ 0:938 nm, b ¼ 0:919 nm, c ¼ 0:588 nm, as reported
previously [7]. It is interesting to note that the calculated lattice parameter of the reduced form,
a ¼ 0:931 nm, was close to a ¼ 0:938 nm for the powder sample rather than b ¼ 0:919 nm. In a word,
an increase in lattice parameter by reduction occurred normal to the surface where hydrogen gas was incontact. The directivity of the reduced single crystal appears to be dependent on the direction ofdiffusion of hydrogen.
Furthermore, scanning electron microscope (SEM) observation of the reduced form showed smoothsurface with almost no cracks as shown in Fig. 4. This indicates that the transformation between therutile and the reduced form occurs through topochemical process such as diffusion of hydrogen oroxygen.
3.2. Electrical and optical properties
Temperature dependence of the conductivity of the single crystal sample of TiO1.94 is shown in Fig. 5.
The conductivity at 25 8C was 17.2 S m1, and it increased monotonously with increasing temperature.
The temperature dependence was semiconductive in the range of 25–300 8C, and the activation energy
was estimated at 2.75 kJ/mol (2:85  102 eV). The semiconductor-to-metal transition was notobserved in the temperature range. This result predicts that the sample shows a gradual reflectivity
change in the temperature range.

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Fig. 4. SEM picture of the reduced form obtained by heating the rutile single crystal in H2 at 950 8C for 1 h.

Reflectance spectra for the single crystal samples of the rutile and of the orthorhombic TiO1.94 areshown in Fig. 6a and b, respectively. Absorption at 400 nm observed for the rutile is due to thephotoexcitation of electron–hole pairs, which contribute to the photocatalytic function. This peak at
400 nm disappeared after reduction because high electrical conductivity of the reduced form depresses
the generation of the electron–hole pairs. This suggests that the photocatalytic function is unexpected asFig. 5. Conductivity of the single crystal of TiO1.94 in the range of 25–300 8C.

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Fig. 6. Reflectance spectra for the single crystal samples of the rutile (a) and of the orthorhombic TiO1.94 (b).

for the orthorhombic TiO1.94. It is interesting to note that the reflectivity of the orthorhombic TiO1.94
increases with increasing wavelength above 900 nm, whereas that of the rutile decreases. Near-infrared
light from the sun largely affects temperature rise in room. Glass window which transmits visible lightand simultaneously shades the near-infrared light is desirable because heat ray can be shaded withtransparency.
Reflectance spectra of the orthorhombic TiO1.94 at 25–120 8C are shown in Fig. 7. It was found thatthe reflectivity in near-infrared region over 1700 nm gradually increased with increasing temperature,
whereas it was almost the same below 1700 nm. In general, optical properties are strongly correlatedwith electrical properties, and thus this spectrum change is correlated with the increase of theconductivity, i.e., the density of conduction carriers shown in Fig. 5. Theoretical study on thecorrelation between the spectrum and the carrier density is needed to provide details. In application to‘‘smart window’’, it is essential to reflect near-infrared light at high temperature, because nearinfrared light from the sun contributes greatly to the temperature rise in the room. Our results indicatethat the orthorhombic titanium oxides satisfy this requirement. It can be said that ‘‘smart windows’’
are achieved by fabricating thin film of the orthorhombic titanium oxides on glass or dispersing thefine powders in glass matrix. It will be possible to achieve a drastic resistivity change of the material
by preparation of complex oxides with the other transition metal oxides which exhibit phasetransitions.

Fig. 7. Reflectance spectra for the single crystal sample of the orthorhombic TiO1.94 at 120, 80, 60, and 25 8C from top.

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Acknowledgements
The authors thank Dr. Yoshinobu Nakamura, the University of Tokyo, Japan, for Laue measurement.
Part of this work was supported by a grant from the Nissan Science Foundation.

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