Key Engineering Materials
ISSN: 1662-9795, Vol. 485, pp 283-286
doi:10.4028/www.scientific.net/KEM.485.283
© 2011 Trans Tech Publications, Switzerland
Online: 2011-07-04
Solvothermal Synthesis of WO3 Photocatalysts and
Their Enhanced Activity
Yuji Kondo and Shinobu Fujiharaa
Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi,
Kohoku-ku, Yokohama 223-8522, Japan
a
[email protected]
Keywords: Tungsten trioxide, Morphology control, Photocatalyst, Solvothermal synthesis
Abstract. Tungsten trioxide (WO3) is known as a visible light responsive photocatalyst, but its
photocatalytic activity is relatively low, as compared to that of anatase titanium dioxide (TiO2). To
enhance the activity, high specific surface areas are necessary. In this study, WO3 particles with a
hierarchical architecture, which was assemblies of spherical particles 20 – 30 nm in diameter, were
synthesized by the solvothermal method. The hierarchical WO3 particles had high specific surface
areas and their photocatalytic activity was found to be 2.5 times higher than that of the commercial
WO3.
Introduction
Recently, semiconductive photocatalysts have received much attention from the viewpoint of a
technology for environmental purification. For example, anatase TiO2 is used for the degradation of
harmful organic substances such as volatile organic compounds. However, bandgap energy of TiO2
is 3.2 eV and its absorption edge is located at the UV light region. Therefore, the photocatalytic
activity of TiO2 can only be attained under the UV light. As the UV light accounts for only about
4% of the solar energy on the earth’s surface, the usage efficiency of the solar light is relatively low.
With the purpose of utilization of a wider solar spectrum, a visible light responsive photocatalyst
such as WO3 is quite promising. Bandgap energy of WO3 is 2.7 eV and its absorption edge is
located at the visible region [1]. As a result, WO3 can absorb the visible light below 480 nm.
However, the photocatalytic activity of WO3 is lower than that of anatase TiO2 at present.
To enhance the photocatalytic activity, high specific surface areas and high crystallinity of the
material are necessary because the former increases photocatalytic reaction sites and the latter
decreases crystal lattice defects which cause the electron-hole recombination. A solvothermal
synthesis, which utilizes a solvent to increase the solubility of solids and enhance the reaction rate
under high pressure and temperature, is one of the methods for generating specific morphologies
with high specific surface areas [2,3]. For example, the morphology control of cobalt microcrystals
has been studied by the solvothermal synthesis employing different kinds of solvents [4].
In this study, we synthesized WO3 particles with a hierarchical architecture by the solvothermal
method under various conditions to enhance the photocatalytic activity. The degradation of
rhodamine B in an aqueous solution was utilized to evaluate their photocatalytic activity.
Experimental
Preparation of WO3 particles. Sphere-like WO3 with a hierarchical architecture was synthesized
using a simple solvothermal method in methanol (MeOH) – ethanol (EtOH) mixed solvents. All
chemical regents used in this experiment were of analytical grade and used without further
purification. Tungsten hexachloride (WCl6, 2 mmol) was dissolved in the MeOH–EtOH mixture (50
mL) under vigorous stirring at room temperature. MeOH to EtOH volume ratios of the solvents
were varied between 100 : 0 (pure MeOH), 75 : 25, 50 : 50, 25 : 75, and 0 : 100 (pure EtOH). The
resultant yellow solutions were poured into 60 mL Teflon lined autoclaves, which were heated at
100 – 180 ºC for 1 – 10 h. After cooling to room temperature, products were separated by
centrifugation, washed several times with ethanol, and then dried in air at 90 ºC for 24 h. The
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Electroceramics in Japan XIV
samples thus obtained were finally calcined at 600 ºC for 30 min in air.
Characterization. The phase and purity of the samples were examined by powder X-ray diffraction
(XRD) patterns. The crystallite size was calculated from full width at half-maximum of the
strongest diffraction peak at 2θ = 24.4º using the Scherrer’s equation. Morphology of the samples
was observed by a field-emission scanning electron microscope (FE-SEM) and a field-emission
transmission electron microscope (FE-TEM). Brunauer-Emmett-Teller (BET) specific surface area
was determined from nitrogen adsorption. Before the measurement, the samples were evacuated at
393 K for 2 h.
Photocatalytic activity. The photocatalytic activity of the samples was evaluated by the
degradation of rhodamine B (RhB) in an aqueous solution under the irradiation with UV light (365
nm) [2]. The synthesized WO3 powder (0.05 g) was poured into 20 mL of the RhB aqueous solution
(4 mg L–1) in a plastic case (the bottom surface area was 26 cm2) at room temperature. Before
turning on light, the solution was kept still in a dark condition for 6 h to ensure the establishment of
an adsorption–desorption equilibrium of RhB. The concentration of RhB during the degradation
was monitored by colorimetry using a UV-vis spectrophotometer. Commercial WO3 (47357A,
Soekawa Chemical) was also tested for comparison.
Results and Discussion
WO3 photocatalysts by the solvothermal synthesis. The phase and purity of the samples were
first examined by XRD. Results (not shown) indicated that all the samples were identified as a
single phase of monoclinic WO3. Fig. 1 shows FE-SEM images of the samples synthesized at 140
or 180 ºC for 10 h with pure MeOH or EtOH. Morphologies of the particles are very different from
each other. The shape of the particles synthesized at 140 ºC with MeOH is spherical 500 – 800 nm
in diameter (Fig. 1a). The magnified image (Fig. 1b) shows that such the secondary spherical
particles have a hierarchical architecture, which is assemblies of primary spherical nanoparticles
approximately 50 nm in diameter. In contrast, the shape of the particles synthesized at 140 ºC with
EtOH is irregular assemblies of primary plate-like nanoparticles giving the overall diameter of 300
– 400 nm (Fig. 1c). When the temperature is increased to 180 ºC, the shape of the particles with
EtOH turns into spherical assemblies of spherical nanoparticles (Fig. 1d). This suggests that the
shape of the primary particles with EtOH should change from the plate to the sphere with increasing
the solvothermal treatment temperature.
Fig. 2a shows the BET specific surface area of the samples synthesized at the different
solvothermal treatment temperatures for 10 h with MeOH or EtOH. The surface area gradually
increases with increasing the temperature in both cases. Fig. 2b shows the dependence of the
crystallite size on the temperature. Contrary to the surface area, the crystallite size gradually
decreases with the temperature. Fig. 2c compares the photocatalytic activity for the decomposition
of RhB. It is seen that the activity gradually increases with the temperature. These results
demonstrate that the photocatalytic activity is governed mainly by the specific surface area.
(a)
(b)
1 µm
(d)
(c)
200 nm
1 µm
1 µm
Fig. 1 FE-SEM images of the samples synthesized (a, b) at 140 ºC with MeOH, (c) at 140 ºC
with EtOH, and (d) at 180 ºC with EtOH for 10 h.
dV/dlog(D) pore volume / cm3 •g 1 •nm
(a)
－
15
10
5
0
60
(b)
40
(a)
100 °C
2.0E-01
120 °C
140 °C
160 °C
180 °C
1.0E-01
0.0E+00
0
20
－
－1
Decomposition rate / ppm•h
285
20
dV/dlog(D) pore volume / cm3 •g 1 •nm
Crystallite size / nm
Specific surface area / m2 •g
－1
Key Engineering Materials Vol. 485
0
(c)
0.3
0.2
0.1
0
80
100
120
140
160
180
Solvothermal treatment temperature /
(b)
50
Pore size / nm
100
100 °C
4.0E-01
120 °C
140 °C
160 °C
180 °C
2.0E-01
0.0E+00
200
0
oC
Fig. 2 (a) The specific surface area, (b)
the crystallite size, and (c) the
decomposition rate of RhB in the samples
synthesized with MeOH (circle) and
EtOH (square) as the solvent.
50
100
Pore size / nm
Fig. 3 The BJH desorption pore size distribution
of the samples synthesized with (a) MeOH and
(b) EtOH.
Fig. 3 shows the corresponding Barett-Joyner-Halenda (BJH) pore size distribution of the
samples. The amount of pores 20 – 30 nm in size increases with increasing the temperature. This
indicates that the increase in the pore amount leads to the increase in the specific surface area (Fig.
2a). In addition, the pore amount of the samples synthesized with EtOH is larger than that of the
samples with MeOH. This agrees with the fact that the samples synthesized with EtOH exhibits the
larger specific surface area than the samples with MeOH.
Effects of MeOH/EtOH volume ratios. The volume ratio of the solvents between MeOH and
EtOH influenced the morphology of the samples. Fig. 4 shows FE-SEM images of the samples
synthesized at 140 ºC for 10 h with the different volume ratio of the solvents. The shape of all the
particles is spherical assemblies of spherical nanoparticles. When the mixed solvents are used, the
(a)
(c)
(b)
1 µm
1 µm
(d)
1 µm
200 nm
Fig. 4 FE-SEM images of the samples synthesized at 140 ºC for 10 h with the different MeOH :
EtOH volume ratio of (a) 25 : 75, (b) 50 : 50, and (c, d) 75 : 25.
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Electroceramics in Japan XIV
(b)
(a)
WClm(OCH3)6–m
or
WClm(OC2H5)6–m
Nucleation
and
growth
Assembly
50 nm
500 nm
Fig. 5 Schematic diagrams with FE-TEM and FE-SEM images explaining the growth mechanism.
diameter of the primary spherical nanoparticles tends to be smaller than those found with the pure
solvents. In particular, the diameter is as small as 20 – 30 nm (Fig. 4d) when the sample is
synthesized with the MeOH : EtOH = 75 : 25 ratio. It was confirmed that the specific surface area
became larger as the diameter of the primary spherical nanoparticles was smaller. This sample (M :
E = 75 : 25) showed the specific surface area of 17.0 m2 g–1, the crystallite size of 32.7 nm and the
decomposition rate of RhB of 0.289 ppm h–1. In a comparative study, the commercial WO3 sample
showed the specific surface area of 2.5 m2 g–1, the Scherrer’s crystallite size of 35.4 nm and the
decomposition rate of 0.111 ppm h–1. This difference clearly indicates that our solvothermally
synthesized WO3 samples have the higher photocatalytic activity.
Growth mechanism of the hierarchical WO3 spheres. In order to clarify the morphological
evolution of the hierarchical WO3 spheres, the solvothermal treatment time was changed between 1
and 10 h while the MeOH : EtOH volume ratio was fixed at 75 : 25 with the constant temperature of
140 ºC. A growth process of the hierarchical WO3 spheres is proposed in Fig. 5, based on the
microstructural observation during the synthesis. WCl6 is dissolved in MeOH or EtOH to form a
6-fold coordinated WClm(OCH3)6–m or WClm(OC2H5)6–m complex species. The linking between the
complex species leads to the formation of WO3 by a condensation reaction in the solvothermal
process. Within 1 h, homogenous nucleation and growth of spherical nanoparticles takes place (Fig.
5a). When the reaction time is increased to 5 h, smaller spherical particles aggregate into larger
secondary particles to minimize the surface energy (Fig. 5b). Finally the particles shown in Fig. 4c
are obtained.
Conclusions
The spherical WO3 particles with the hierarchical architecture, which was assemblies of the
spherical primary nanoparticles, were synthesized by the solvothermal method with methanol and
ethanol as the solvents. As the solvothermal treatment temperature was increased, the specific
surface area was increased and then the photocatalytic activity could be enhanced. In particular, the
hierarchical WO3 sphere with the larger specific surface area was synthesized with the MeOH :
EtOH = 75 : 25 ratio at 140 ºC for 10 h. The general trend was also observed that the photocatalytic
activity was higher as the specific surface area became larger. Finally, the photocatalytic activity of
our WO3 was enhanced by 2.5 times higher than the commercial WO3.
References
[1] G. R. Bamwenda and H. Arakawa: Appl. Catal. A Vol. 210 (2001) p. 181.
[2] D. Chen and J. Ye: Adv. Func. Mater. Vol. 18 (2008), p. 1922.
[3] H.G. Choi, Y.H. Jung and D.K. Kim: J. Am. Ceram. Soc. Vol. 88 (2005), p. 1684.
[4] L. Duan, S. Jia and L. Zhao: Mater. Res. Bull. Vol. 45 (2010), p. 373.
Electroceramics in Japan XIV
10.4028/www.scientific.net/KEM.485
Solvothermal Synthesis of WO3 Photocatalysts and their Enhanced Activity
10.4028/www.scientific.net/KEM.485.283
DOI References
[1] G. R. Bamwenda and H. Arakawa: Appl. Catal. A Vol. 210 (2001) p.181.
10.1016/S0926-860X(00)00796-1
[3] H.G. Choi, Y.H. Jung and D.K. Kim: J. Am. Ceram. Soc. Vol. 88 (2005), p.1684.
10.1111/j.1551-2916.2005.00341.x
[4] L. Duan, S. Jia and L. Zhao: Mater. Res. Bull. Vol. 45 (2010), p.373. Assembly WClm(OCH3)6–m or.
10.1016/j.materresbull.2010.01.002