COMMUNICATION
Novel synthesis of homogenous CsxWO3 nanorods with excellent NIR
shielding properties by a water controlled-release solvothermal process†
a
a
Chongshen Guo, Shu Yin, Peilin Zhang,a Mei Yan,b Kenji Adachi,c Takeshi Chonanc and Tsugio Satoa
*
Received 22nd June 2010, Accepted 2nd August 2010
DOI: 10.1039/c0jm01972k
Nanosize homogenous rod-like tungsten bronze CsxWO3 with
excellent NIR shielding ability was successfully synthesized by
a novel and facile water controlled-release solvothermal process
(WCRSP).
There has been great demand for materials which shield near-infrared
(NIR, wavelength of 780 to 2500 nm) radiation (heat rays) by
Fig. 1 (A) The structure of CsxWO3 framework projected on a–b planes;
employing transparent coating on the windows of automobiles,
buildings, etc.,1 in order to reduce energy consumption of air condi-
(B) the Tyndall effect of the as-prepared sample dispersed in (a) water, (b)
ethanol and (c) hexane.
tioning and thereby decrease the emission of carbon dioxide. For the
application as heat ray shielding materials, an excellent shielding
ability of NIR rays as well as high visible light transparency is
AxWO3+x/2 to AxWO3. Such heat treatment resulted in the increase in
required. It is well known that nanoparticles of noble metals,2 black
the grain size and inhomogenity of the morphology of the particles.
compounds,3 rare-earth hexaborides,4–6 tin-doped indium oxide
In this paper, a new facile method to synthesize tungsten bronze
(ITO)7.8 etc., can shield NIR due to the effect of the plasma vibration
type CsxWO3 nanorods possessing excellent NIR shielding ability
of the free electrons. However, each of them has its own drawbacks.
was developed by employing a water controlled-release solvothermal
The films consisting of noble metal or black compounds show low
process (WCRSP), where after mixing WCl6 ethanol solution and
visible light transparency.2,3 The rare-earth hexaborides shield only
CsOH ethanol solution, the mixed solution and a desired amount of
certain wavelengths of IR rays and do not shield the whole extent of
CH3COOH was placed in a Teflon-lined autoclave and heated at
NIR rays.4–6 In addition, high temperature (ca. 1500 ꢀC) and vacuum
conditions are necessary for the syntheses of rare-earth hexaborides.
It exhausts much energy to crush the rare-earth hexaborides particles
because of their hardness. Tin-doped indium oxide (ITO) film is a
famous transparent conductive film with heat-ray shielding effect,9–12
however, it can only shield the IR rays of wavelength longer than
1500 nm.10 In addition, indium is one of the expensive rare metal
resources.
200 ꢀC for 20 h. The product could be dispersed well in solvents and
showed Tyndall effect (Fig. 1(B)), indicating the existence of
nanoparticles.
All the XRD peaks of the sample could be indexed as the hexa-
gonal caesium tungsten bronze Cs0.32WO3 (JCPDS file No. 831334)
and no impurity peak was observed (Fig. 2(a)). The product showed
the average particle size of 85 nm with a narrow particle size
distribution (Fig. 2(b)).
Adachi et al. reported that the nanoparticles of caesium bronze
(CsxWO3) with a hexagonal structure (Fig. 1(A)) showed promising
performance as solar filters.5 They prepared the hexagonal caesium
tungsten bronze nanoparticles by traditional solid state reaction in
H2–N2 gas atmosphere around 800 ꢀC, followed by milling for long
Fig. 3 shows the TEM, HRTEM, SED and EDS of the product.
The typical images showed the softly agglomerated homogenous
nanorods with 15 nm in diameter and 40–50 nm in length. The
chemical composition analysis based on the EDS (Fig. 3d)
indicated the existence of Cs and W elements with the atomic ratio of
Cs : W ¼ 0.317 : 1, which is very close to the typical hexagonal
tungsten bronze structure of Cs0.32WO3.
times. It was also reported that the hexavalent tungstate with
13,14
a general formula of AxWO3+x/2
could be prepared by hydro-
thermal reactions. The compound showed very similar XRD profiles
to tungsten bronze (AxWO3), however, did not show NIR absorption
ability because of the lack of free electrons. Therefore, the additional
heat treatment under H2 atmosphere was necessary to reduce
aInstitute of Multidisciplinary Research for Advanced Materials, Tohuko
University, 2-1-1 Katahira,Aoba-ku, Sendai, Japan. E-mail: shuyin@
tagen.tohoku.ac.jp
bDepartment of Chemistry, Graduate school of Science, Tohuko University,
Aoba-ku, Sendai, Japan
cIchikawa Research Laboratory, Sumitomo Metal Mining Co., Ltd, Japan
† Electronic supplementary information (ESI) available: Experimental
methods, SEM and TEM images, NMR results; the reflectance profiles
of CsxWO3 and ITO glass; the transmittance spectra of ITO powder.
See DOI: 10.1039/c0jm01972k
Fig. 2 (a) XRD pattern and (b) particle size distribution of the prepared
sample (reference: Cs0.32WO3, JCPDS No. 831334).
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 8227–8229 | 8227