M. Sun et al.: Nanocrystalline tungsten oxide thin film: Preparation, microstructure, and photochromic behavior
7
ditch hypothesized that the partial ionization of water in
prevent tungstenic acid microparticles from coagulation
+
the films could produce H ions, which diffuse to the
so as to form the nanocrystalline WO films with different
3
cathode and become H during electrochromism; then
WO is reduced to the blue-colored WO3−m(OH) . There
particle sizes. The oxalic acid concentrations of the so-
lutions were 0.1, 0.2, 0.3, 0.4, 0.6, and 0.8 M, which were
numbered as 01, 02, 03, 04, 05, and 06, respectively.
In group two, a series of WO и 2H O colloid solutions
3
m
was a similar process for photochromism of WO .
3
8
Faughnan et al. proposed a double-charge-injection
3
2
(
DCI) model for the formation of the tungsten bronze by
with different pH was prepared. As a comparison, solu-
tion 03 in group one was used as the original solution.
The pH value of the colloid solution was adjusted from 1
to 7 by adding 1:1 diluted NH и H O drop by drop, and
the oxalic acid concentration in the solutions was kept
constant at 0.3 M. These solutions were designated as P1,
P2, P3, P4, P5, P6, and P7 for pH 1, pH 2, pH 3, pH 4,
pH 5, pH 6, and pH 7, respectively.
A small drop of each colloid solution was applied onto
formvar films supported by copper grids to obtain the
TEM film sample. The microstructure of these films was
observed from the photographs taken at a magnification
of 100,000× using a JEOL JEM-200CX TEM.
a simultaneous injection of ions and electrons into the
film. Schirmer et al. reported that a small polaron might
9
be generated just below the conduction band and cause
the blue color. From these studies, it is generally recog-
nized that the intervalent electron transfer between tung-
sten ions with different valence states is a possible
photochromic mechanism. Although considerable efforts
have been made by these researchers, the mechanism of
photochromism of tungsten oxide, especially nanocrys-
talline WO films made by colloid chemistry method,
remains unclear. It has not been clarified yet, for ex-
ample, where the H comes from, and how the photo-
3
2
3
+
chromic property depends on the structure of the films.
In this work, we prepared a large area of nanocrystal-
Two groups of tungsten oxide films were formed by
casting the colloid solutions on quartz substrate at room
temperature. All the film samples we used in the experi-
ment were made from freshly prepared colloid solutions
without any heat treatment.
The photochromic experiments were carried out using
a 500-W mercury lamp as the light source. All absorption
measurements were made on an ultraviolet–visible spec-
trophotometer (Shimadzu UV-1601PC). The XRD pat-
terns were recorded on MAC SCIENCE M18AHF XRD
line WO thin film by using colloid chemistry method
3
and succeeded in modifying the microstructure and pho-
tochromic property of the film by controlling the solution
process. With transmission electron microscope (TEM)
and x-ray diffraction (XRD) studies the growth process,
microstructure, and constituent phase of the film were
observed and analyzed. By means of ultraviolet–visible
absorption spectra (UVAS) and x-ray photoelectron
spectra (XPS), the photochromic behavior and electronic
valence state variation of the film during photochromism
were investigated to find some new photochromic phe-
spectrometer using a Cu K radiation ( ס
1.5418 Å).
␣
XPS were measured with an ESCA LAB-5 photoelectron
spectrometer using monochromatic Mg K␣ anode
(1253.6 eV). Binding energies were referred to the C1s
line, the energy of which was accepted to be 284.6 eV.
The instrumental resolution was about 0.2 eV. Samples
were maintained at room temperature and at a pressure of
nomena of the nanocrystalline WO thin film and to give
3
some reasonable explanation for the photochromic proc-
ess of this film.
−9
about 2 × 10 mbar.
II. MATERIALS AND EXPERIMENTAL METHOD
A series of transparent colloid solutions of tungsten
III. RESULTS AND DISCUSSION
trioxide dihydrate WO и 2H O was prepared based on
3
2
1
0
A. Microstructure of the film
the colloid chemistry method. Tungstenic acid precipi-
tate was prepared first by dissolving a certain amount of
Na WO и 2H O in high-purity water and adding con-
To obtain good photochromism properties and high
stability, WO films are expected to be nanocrystalline,
2
4
2
3
centrated HCl drop by drop without stirring until the
solution became acidic (pH ≈ 4). This white, gelatinous
precipitate was then dissolved by adding a concentrated
uniform, and compact. Therefore, it is of the utmost im-
portance to prepare the films meeting the above require-
ments by choosing an appropriate tungstate colloid
solution. By means of a TEM we observed the micro-
3
solution of oxalic acid and diluted to 100 cm to form a
transparent colloid solution of WO и 2H O. The pH of
structure of WO films prepared from tungstate colloid
3
2
3
this original colloid solution was lower than 1.
solution (group one) with different oxalic acid concen-
To investigate the influence of solution process on
microstructure and photochromic property of the film we
prepared two groups of colloid solutions of WO и 2H O.
In group one, we produced a set of colloid solutions with
constant pH (≈1) but different oxalic acid concentration,
in which oxalic acid, as a surfactant, could be used to
tration (see Fig. 1). The size of particles in the WO films
3
is on the nanometer scale and very uniform, and the
shape was regularly rectangular. The selected-area elec-
tron diffraction patterns of the films indicated that the
3
2
WO films were crystalline (when oxalic acid concentra-
3
tion was less than 0.8 M).
9
28
J. Mater. Res., Vol. 15, No. 4, Apr 2000
Downloaded: 28 May 2014
IP address: 128.233.210.97