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4-01
Hydrogen Gasochromism of WO3 Films Prepared by
Reactive Sputtering
S. Yamamoto, K. Kawaguchi, M. Sugimoto and M. Yoshikawa
Environment and Industrial Materials Research Division, QuBS, JAEA
The detection of gaseous hydrogen leakage below the
lower explosive limit (LEL) of 4% by volume ratio of
hydrogen in air, the lack of electric sparking possibilities in
explosive environments, is an important technology.
Gasochromic materials, coloration by gases, have
considerable promise as the optical gas sensing materials.
Hydrogen gasochromism of Tungsten trioxide (WO3) films
coated with noble metal (Pd, Pt) catalysts, the color changes
reversibly from grayish semi-transparent to deep blue, has
been investigated for the application in optical hydrogen gas
sensors. In this study, we investigated the effects of
crystalline structure on gasochromism of WO3 films.
WO3 films were prepared by a reactive rf magnetron
sputtering from a W (purity 99.9%) target in an argon and
oxygen mixture. WO3 films with thicknesses of
approximately 300 nm were deposited on mirror-polished
SiO2 and -Al2O3 ( 0112)
substrates. The substrates
temperatures were maintained at 200 °C for amorphous
WO3 film, and at 600 °C for oriented and epitaxial WO3 films. Polycrystalline WO3 films were prepared by
annealing of amorphous WO3 films at a temperature of
400 C in air for 2 hours using an electric furnace. The
films were characterized by X-ray diffraction (XRD),
Rutherford backscattering spectroscopy (RBS). To
examine the hydrogen gasochromic performance of WO3 films, the films were coated with a Pd layer (15 nm) by
sputtering. And then, the transmittance at a wavelength of
645 nm was measured using a spectrometer while an argon
gas including 1% hydrogen.
Figure 1 shows XRD patterns for the WO3 films on SiO2 and -Al2O3 ( 0112)
substrates. The film prepared on SiO2 substrate at 200 °C has an amorphous structure, and after
that the film annealed at 400 °C in air becomes a
polycrystalline structure. The characteristic peaks of the
XRD pattern of the annealed film can be attributed to a
monoclinic WO3 phase, as referred to in the JCPDS 43-1035
file. The WO3 film on SiO2 and -Al2O3 ( 0112)
substrates
deposited at 600 °C show the growth of (010)-oriented and
epitaxial WO3(001) films. The hydrogen gasochromic
performance of WO3 films with various structures was
examined by optical transmission measurements. The
change in optical transmittance, T/T0 at 645 nm of Pd/WO3 films between the initial state (T0) and the colored state (T),
was measured. Optical response curves of coloration by
1% H2 in Ar for amorphous, polycrystalline, (010)-oriented
and epitaxial WO3 (001) films coated with a Pd layer were
summarized in Fig 2. The normalized transmittance, T/T0
JAEA-Review 2010-065
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of the (010)-oriented and amorphous WO 3 films on SiO 2
substrates are rapidly changing within a few minutes by
exposure to 1% hydrogen gas. On the contrary, the ratio
T/T0 of the polycrystalline and the epitaxial WO 3 (001) film
shows a slight change. The hydrogen gasochromic
performance of amorphous and (010)-oriented WO3 films is
superior to that of polycrystalline and epitaxial WO3 (001)
films. It indicates that gasochromic performance of WO3
films is influenced by the structure of the films.
20 30 40
2 (deg.)
50 60
Fig. 1 XRD patterns for the WO 3 films: (a) amorphous
WO3, (b) polycrystalline WO 3, (c) (010)-oriented
WO 3 and (d) epitaxial WO 3 (001).
T/T0
Intensity (a.u.)
1
0.5
(002)
(002)
(020)
-Al 2O 3
(020), (200)
(112)
(202), (220)
0
0 200 400 600
Time (s)
(d)
(b)
(d)
(b)
Fig. 2 Optical response curves of coloration by 1% H 2 in
Ar for (a) Pd/(010)-oriented WO3 film, (b)
Pd/amorphous WO3 film, (c) Pd/epitaxial WO 3 (001)
film and (d) Pd/polycrystalline WO3 film, respectively.
(222)
(004)
-Al 2O 3
(c)
(a)
(c)
(a)