# Synthesis of Nanometer-SizedWO3 Particles by Facile Wet Process and Their Photocatalytic Properties

Authors: Mitsunobu Iwasaki; Won-kyu Park
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Nanometer-sizedWO3 particles were prepared from alkaline alcohol-water mixture with WCI6 and NH3 under reflux. The resultant particles at various NH3/W mole ratios were well crystallized ranging 5–20 nm in diameter and the surface area was in the range of 13.70–32.50 m2g−1. Samples synthesized at NH3/W = 3.2 and 12.9 ratio (mol/mol) had an absorption edge in the visible light range and showed the photocatalytic activity under the irradiation of visible light, and the surface area and the crystallinity of those powders had a strong effect on the photocatalytic degradation. When ln(C0/C)(C0: initial concentration of acetaldehyde, C: final concentration of acetaldehyde after irradiation) was plotted against photoirradiation time, the degradation velocity constant (k) against acetaldehyde estimated from the slope of lines was 12.4 h−1g−1 at NH3/W = 3.2 and 13.9 h−1g−1 at NH3/W = 12.9 ratio (mol/mol), respectively.


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## Body

## 1. Introduction

In recent, a semiconductive
photocatalyst like titanium oxide (TiO2)
has a great attention in the viewpoint that its clean and infinite photo energy can decompose
the versatile volatile organic compounds (VOCs) such as NOx and SOx.
TiO2 as the representative semiconductor photocatalyst, in which the
electric potential of valance band is +3.0 eV [1], has a strong oxidation
function by a hole produced owing to electron’s transition from valence band to
conduction band. However, its energy band gap has a high value of 3.2 eV and
its absorption edge is laid on the UV light region. Therefore, the photocatalytic
property of TiO2 can exhibit outdoors but not indoors.Meanwhile, tungsten oxide (WO3), in which the electric
potential of valance band is +3.1 eV [2], has almost the same oxidation
capability as TiO2. Moreover, its energy band gap of 2.7 eV is relatively
small and its absorption edge locates on the visible region. Thus it is anticipated
to exhibit the photocatalytic ability even indoors. Also, TiO2/WO3 composite materials have been reported as fascinating ones: high active
photocatalyst [3], photocatalytically inactive materials [4], energy storage
photocatalyst [5]. At present, a thermal decomposition of tungsten acid has
been reported as a general method to prepare WO3 particles [6]. However,
the method has some problems that the high quantum efficiency was not expected
because its particle size was over submicron order. On the contrary, if synthesis
of nanometer-sized WO3 particle could be accomplished, it will be
used as a photocatalytic material in enormous applied fields.We have
already reported the low-temperature
synthesis of nanocrystalline metal oxides particles (ZnO [7, 8], TiO2 [9], Fe2O3 [10], Co2+-doped TiO2 [11, 12],
SnOx (x=1,2) [13] MgO-ZnO [14], and YMO4; Eu2+ (M=V,P) [15]) through facile wet process, so-called NAnometer-sized Crystal
Formation in Alcoholic Solutions method (NAC-FAS), in
which an alcohol-water mixture with metal salts is refluxed without the
deliberate addition of surfactants or adsorbing ligands. In this report, the low-temperature (<100°C) preparation of nanometer-sized WO3 single crystals using the NAC-FAS method was
investigated and its visible photocatalytic properties were estimated.


## 2. Experimental Procedures

WO3 nanoparticles
were prepared as follows. Tungsten chloride (WCl6; TEOKA
Chemical Co. Inc.), purity >98%, 5.04 mmol was dissolved in 50 mL of ethanol. Under refluxing at 70°C, the mixture of ammonia water and ethanol, which concentration varied in the range of 0.016–0.26 mol/L, was dropwise added for 1 hour and
subsequently refluxed for 1 hour. After this operation, the mixture was cooled
to room temperature while stirring the mixture. Precipitates in the reaction
mixture were isolated by the centrifugal separation at a rotating rate of 13 000
r.p.m., washed with methanol, dried at room temperature overnight in a vacuum. Prepared
samples were heat treated at 400°C for 2 hours under atmosphere.Powder X-ray diffraction
(XRD: RIGAKU MiniFlex) was carried out using Ni-filtrated Cu-Kα radiation for determining
the crystal structure of products. The normal condition was set at 30 kV-15 mA
in the range of 5°–105° with 8°/min.
The crystallite size was calculated from the half-peak width at half height for
the WO3⁢(2θ=25°) diffraction peaks using the Scherrer equation. TG-DTA
measurement (RIGAKU Thermal analyzer TG 8010 D) for investigating the
temperature dependences of samples was carried out, and FT-IR spectrometer (Nippon Bunkou, FT/IR-470 plus)
was used to investigate the ammonium salt and the absorbed water that remained as
impurities on the surface of the as-prepared WO3 nanocrystals. Particle size of WO3 nanocrystals was observed using a transmission electron microscope (TEM: JEOL
JEM-3010) at an acceleration voltage of 300 kV. Specific surface area was measured
by Quantchrome Quantsorb
using the Brunauer-Enarmett-Teller (BET) method after samples were dried at 120°C. UV-visible spectra for WO3 nanocrystals were measured by a
spectrophotometer U-4000 (Hitachi Co., Ltd., Hitachi, Japan). Photocatalytic
activity of WO3 nanocrystals for the photo oxidation degradation of gaseous
acetaldehyde was measured at room temperature by gas chromatography (Shimatsu
GC-9A, FID detector) and the light source was Xenon lamp (Wacom Co. HX-500) with
a cut-off filter and the light intensity of λex>450 nm was 3.50 mW⋅cm-2.


## 3. Results and Discussions

Figure1 shows
the XRD patterns for as-prepared samples at the different NH3/W mole
ratio, which was 3.2, 6.5, 12.9, 25.7, and 52.4, respectively, and XRD patterns
of samples heat treated at 400°C were demonstrated in Figure 2. As shown in Figure 1, diffraction peaks attributed
to H2WO4 crystal were identified at NH3/W=3.2 mole ratio and in other samples prepared at the range of NH3/W = 6.5–52.4 mole ratio, the crystalline peaks of (NH4)6W12O39 were observed. Meanwhile, after heat treatment at 400°C, only WO3 peak in all samples was observed as demonstrated in Figure 2. The main peak of XRD
patterns which was
laid around 2θ=25° was broad in the case of sample prepared at NH3/W=3.2 mole ratio,
but this peak became sharper for other samples over NH3/W=6.5 mole
ratio. From the calculation using Scherrer equation, the crystalline size of WO3 at NH3/W=3.2 mole ratio was estimated to be 6 nm and it became over
20 nm beyond NH3/W=6.5 mole ratio. As a result, at the higher NH3/W mole ratio, it might be possible to synthesize the nanometer-sized WO3 particles with high crystallinity using this NAC-FAS method.Figure 1
XRD patterns of as-prepared samples at various
NH3/W mole ratios: (a) 3.2, (b) 6.5, (c) 12.9, (d) 25.7, and (e) 52.4 (mol/mol), respectively.Figure 2
XRD patterns of samples heat treated at 400°C
for 2 hours at various NH3/W ratios: (a) 3.2, (b) 6.5, (c) 12.9,
(d) 25.7, and (e) 52.4 (mol/mol), respectively.The results of TG-DTA
measurement for as-prepared samples at NH3/W = 12.9 mole ratio were
shown in Figure 3. The endothermic
peak around 70°C was attributed to the evaporation of absorbed
water and the exothermic peak in the range between 320°C and 370°C
was owing to the remained organic component. Another exothermic peak was observed
around 400°C and it is accompanied with the abrupt weight decrease due to the
decomposition of (NH4)6W12O39.Figure 3
TG-DTA
curves of as-prepared sample at NH3/W=12.9 ratio (mol/mol).Meanwhile, Figure4 demonstrated FT-IR spectra of the above same sample. In this figure, the absorption
peak around 3000 cm−1 was assigned to the stretching vibration of
N–H bond and the one around 1400 cm−1 was attributed to the rotational
vibration of N–H bond, respectively. However, after heat treatment at 400°C,
these peaks disappeared and the absorption peak’s intensity attributed to the stretching
vibration of W–O bond became stronger. From these FT-IR and TG-DTA results, it was
clear that (NH4)6W12O39 decomposed
and transformed to WO3 crystal in the as-prepared samples by the
heat treatment at 400°C.Figure 4
FT-IR spectra of samples prepared at NH3/W=12.9 ratio (mol/mol).Figure5 shows
TEM images of WO3 nanocrystalline particles prepared at various
different NH3/W mole ratios, which was 3.2, 6.5, 12.9, 25.7, 52.4,
and commercial products, respectively. The average particle size of commercial
product as shown in Figure 5(f) was
found to be about 100 nm, while, the average particle size of synthesized WO3 powder in this work was observed to be in the range of 5–20 nm even though
it showed the slight aggregation. In addition, the surface area of commercial
WO3 powder was measured to be 3.0 m2g−1, while
that of WO3 powder prepared by NAC-FAS method at the different NH3/W mole ratios, which were 3.2,
6.5, 12.9, 25.7, and 52.4, was measured to be 32.50 m2g−1,
12.40 m2g−1, 18.20 m2g−1, 13.70 m2g−1,
16.80 m2g−1, respectively, and it showed higher values of
4–10 times comparing
to that of commercial product.TEM images of WO3 nanocrystalline particles prepared at various NH3/W ratios: (a) 3.2. (b) 6.5, (c) 12.9, (d) 25.7, (e) 52.4 (mol/mol),
and (f) commercial product, respectively.
(a)(b)(c)(d)(e)(f)Figure6 demonstrates the UV-visible spectra of commercial WO3 product and WO3 nanocrystalline particles prepared at NH3/W⁢=3.2 and 12.9 mole
ratio. Crossing point of two dotted lines in the figure indicates the original
location of absorption edge at NH3/W=12.9 mole ratio assuming that the
impurity is not present in the sample. As a result, the basic absorption edge
of WO3 nanocrystalline particles prepared at NH3/W=3.2
and 12.9 mole ratio showed the red shifting comparing to commercial WO3 product and had an absorption edge in the visible light range. Therefore, the impurity
energy level owing to oxygen deficiency might be formed in from the result that
new absorption peak appeared below 600 nm wavelength [16]. Next,
we
investigated the property of the photocatalytic degradation of gaseous acetaldehyde
for these samples.Figure 6
UV-visible
spectra of WO3 nanocrystalline particles: (a) 3.2, (b) 12.9 NH3/W
ratio (mol/mol), and (c) commercial product.Figure7 shows
the photocatalytic degradation against gaseous acetaldehyde of commercial WO3 powder and WO3 nanocrystalline one prepared at NH3/W=3.2
and 12.9 mole ratio. It is very clear that all WO3 sample had a photocatalytic activity of the
visible light from the result that the acetaldehyde concentration decreased by the
photoirradiation time. Meanwhile, when ln (C0/C) (C0: initial
concentration of acetaldehyde, C: final concentration of acetaldehyde after
irradiation) was plotted against photoirradiation time, a linear relation was made
for all samples. The degradation velocity constant (k) against acetaldehyde estimated from the slope of lines was 12.4 h−1g−1 at NH3/W=3.2 and 13.9 h−1g−1 at NH3/W = 12.9 ratio (mol/mol), respectively, and they showed higher
value than that of commercial WO3 product, which value was 7.80 h−1g−1.
These results reveal that the surface area and the crystallinity of WO3 powder have a strong effect on the photodegradation of gaseous acetaldehyde.Figure 7
Change in
acetaldehyde concentration against irradiation time under visible light (I=3.5 mWcm−2): (a) 3.2, (b) 12.9 NH3/W ratio (mol/mol), and (c)
commercial product.


## 4. Conclusions

Nanometer-sized
WO3 particles have been successfully prepared from alkaline
alcohol-water mixture with WCl6 and NH3 under reflux. The resultant particles at various NH3/W
mole ratios were well crystallized ranging 5–20 nm in diameter and the surface area was in the
range of 13.70–32.50 m2g−1 and
showed the high crystallinity at the higher NH3/W mole
ratio. Especially, samples synthesized at NH3/W=3.2 and 12.9 (mol/mol) had an absorption edge in the visible light range and showed high
photocatalytic activity under the irradiation of visible light comparing to the
commercial WO3 products, and experimental
results reveal that the surface area and the crystallinity of those powders had
a strong effect on the photodegradation.



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*Source: 169536-2008-05-13.xml*