Synthesis of WO3·H2O nanoparticles by pulsed plasma in liquid

Article abstract of DOI:10.1039/c4ra01551g

Pure orthorhombic-phase WO₃·H₂O nanoparticles with sizes of about 5 nm were synthesized by pulsed plasma in deionized water, in which tungsten electrodes provide the source of tungsten and the water is the source of oxygen and hydrogen. The quenching effect and liquid environment inherent in this pulsed plasma in liquid method resulted in ultra-small particles with lattice lengths (a = 5.2516 A, b = 10.4345 A, c = 5.1380 A) larger than those of reference lattices. The emission lines of W I atoms, W II ions and H I atoms were observed by an optical emission spectrum in order to gather information on the synthetic mechanism. These nanoparticles showed higher absorption in the visible region than did ST-01 TiO₂ and Wako WO₃ nanoparticles. The WO₃·H₂O nanoparticles displayed more activity in the photocatalytic test than did the commercial TiO₂ sample (ST-01). Also, the absorption edge of WO ₃·H₂O shifted to longer wavelengths in the UV-Vis absorption pattern relative to that of the anhydrous tungsten oxide.

Full text of DOI:10.1039/c4ra01551g

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Synthesis of WO $H O nanoparticles by pulsed
3
2
plasma in liquid
Cite this: RSC Adv., 2014, 4, 28673
a
a
b
c
Liliang Chen,† Tsutomu Mashimo,* Hiroki Okudera, Chihiro Iwamoto
d
and Emil Omurzak
3 2
Pure orthorhombic-phase WO $H O nanoparticles with sizes of about 5 nm were synthesized by pulsed
plasma in deionized water, in which tungsten electrodes provide the source of tungsten and the water is
the source of oxygen and hydrogen. The quenching effect and liquid environment inherent in this pulsed
plasma in liquid method resulted in ultra-small particles with lattice lengths (a ¼ 5.2516 A˚ , b ¼ 10.4345 A˚ , c
¼
5.1380 A˚ ) larger than those of reference lattices. The emission lines of W I atoms, W II ions and H I atoms
were observed by an optical emission spectrum in order to gather information on the synthetic
mechanism. These nanoparticles showed higher absorption in the visible region than did ST-01 TiO
and Wako WO nanoparticles. The WO $H nanoparticles displayed more activity in the
photocatalytic test than did the commercial TiO sample (ST-01). Also, the absorption edge of
WO $H O shifted to longer wavelengths in the UV-Vis absorption pattern relative to that of the
anhydrous tungsten oxide.
2
Received 22nd February 2014
Accepted 4th June 2014
3
3
2
O
2
DOI: 10.1039/c4ra01551g
3
2
www.rsc.org/advances
Although less studied than anhydrous tungsten oxide, the
2
association of water with tungsten oxide to form the WO $H O
1
Introduction
3
There is a keen need for new forms of ‘green’ energy. The hydrate is considered to be important for the chemical prop-
development of solar energy, such as that produced using erties of the resulting molecular species, and has been shown to
7
photocatalysts, will have great long-term benets since this cause a change in optical absorption. This indicates that
form of energy is affordable, inexhaustible and clean. The most hydration may modify the band structures of WO
prevalently used photocatalyst, TiO
, is relatively inefficient, may alter its photocatalytic properties for specic applications,
however, at utilizing visible light. Numerous efforts to modify even though the hydrate has lower photocatalytic efficiency
its band structure by doping have not been very successful than that of unhydrated WO
because dopants usually act as recombination centers between
In this study, we carried out and analyzed the results of a
3
, and thus
2
1
.
3
the photogenerated electrons and holes, which markedly one-step synthesis of WO
3
8,9
$H
2
O nanoparticles, using the pulsed
2
reduce the photocatalytic activity.
plasma in liquid method.
As a potential substitute, WO , which possesses a small
3
band-gap energy of 2.4–2.8 eV, stable physicochemical proper-
3
ties and resilience to photocorrosion effects, has strong pho-
tocatalytic activity in the visible-light region. However, its low
conduction band level limits the photocatalyst to react with
4
electron acceptors and then increases the recombination of
photogenerated electron–hole pairs. Thus, much effort has
been focused on the particles modied with other components,
5,6
such as Ag.
a
Institute of Pulsed Power Science, Kumamoto University, Kumamoto 860-8555, Japan.
E-mail: mashimo@gpo.kumamoto-u.ac.jp; Fax: +81 96 342 3293; Tel: +81 96 342 3295
b
Faculty of Natural System, Institute of Science and Engineering, Kanazawa University,
Kanazawa 920-1192, Japan
c
Faculty of Engineering, Kumamoto University, Kumamoto 860-8555, Japan
d
Priority Organization for Innovation and Excellence, Kumamoto University,
Kumamoto 860-8555, Japan
†
Present address: The Peac Institute of Multiscale Sciences, Chengdu, Sichuan,
10207, China.
Fig. 1 Schematic of the system to generate pulsed plasma in deion-
ized water by use of two tungsten electrodes.
6
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acetaldehyde was prepared in the vessel by injection of satu-
rated gaseous acetaldehyde. Irradiation was conducted at room
2
Experimental
As shown in Fig. 1, two metallic electrodes with a diameter of temperature aer equilibrium between the gaseous and adsor-
mm and 99.95% tungsten purity (Rare Metallic Co., Ltd.) bed acetaldehyde had been reached, which was ascertained by
hereaer called simply “tungsten electrodes”) were submerged monitoring the concentration by a gas chromatograph approx-
5
(
in degassed deionized water in a quartz beaker without adding imately every 30 minutes. Methane and CO byproducts were
conducting salts (pH value is about 7). Pulses with the same negligible. The excited light source was an LED lamp with the
ꢁ
2
single-pulse duration of about 15 ms, as shown in Fig. 2, were parameters of 455 nm and 1 mW m at room temperature. The
generated between the two rod tips at a voltage of 100 V and photocatalytic results were compared with a commercial TiO
current of 50 A when the electrolysis of water occurred. This nanoparticle (ST-01, anatase, Ishihara Sangyo Kaisha, Ltd.) and
process utilized deionized water as a source of oxygen and commercial WO (Wako Pure Chemical Industries, Ltd.). UV-Vis
2
3
hydrogen, and tungsten electrodes as the source of tungsten. spectra of the synthesized sample, ST-01 and Wako WO
The whole synthetic system only contained the elements W, O taken using a JASCO V-550 UV/VIS spectrometer.
and H (from tungsten rods and water) without contamination
were
3
from other metal cations. The microsecond duration of pulsed
3
Results and discussion
plasma and the surrounding cool liquid helped to quench the
growth of the small-sized nanoparticles. Aer a one-hour reac-
tion, the yellow-green nanoparticles were carefully collected
3.1. Characterization of WO $H O nanoparticles
3 2
The crystal structure of the sample was checked by XRD (Fig. 3).
ꢀ
aer drying in air at 120 C for 2 hours.
The crystal yielded diffraction peaks at 2q ¼ 16.5, 19.3, 23.8, 25.7,
Nanoparticle phase purity and structure were determined by
X-ray diffraction (XRD) (Rigaku RINT 2000/PC) using Cu Ka
radiation (40 kV, 200 mA). Crystal parameters were calculated by
Rietveld renement. Morphology and microstructural charac-
terization of prepared samples were observed using high-reso-
lution transmission electron microscopy (HRTEM) (Philips
Tecnai F20). The powders for HRTEM were prepared by putting
them into ethanol and deaggregating them by sonication for 30
min. Optical emission spectra were obtained using an optical
probe placed adjacent to the beaker, and data were transmitted
via an optical ber to an SEC2000-UV-VIS spectrometer. A
HITACHI F-2500 luminescence spectrophotometer was used to
evaluate photocatalytic activity by the photodecomposition of
3
6
4
0.5, 33.4, 34.2, 35.0, 37.7, 38.9, 42.8, 45.9, 49.2, 52.8, 54.3, 56.3,
2.8 and 66.2 , corresponding well with the JCPDS card number
3-0679. This diffraction conrmed that the obtained nano-
ꢀ
particles were the WO
tungsten indicated the high purity of the WO
particles synthesized by the pulsed plasma in liquid method.
3
$H
2
O hydrate. The absence of metallic
$H O nano-
3
2
Table 1 lists Rietveld renement parameters of WO $H O
3
2
obtained by pulsed plasma in deionized water, comparing them
with Szymanski's data and reference data in PDF# 43-0679. The
crystallographic structure of the synthesized WO $H O sample
3 2
10
is well rened in the orthorhombic Pnmb space group (see Fig. 4
and 5) in the region of 14 to 90 degrees. Observed data are
indicated by dots, and the calculated prole is indicated by a
solid line. Short vertical bars below the pattern represent the
positions of all possible Bragg reections, and the line below the
short vertical bars represents the difference between the
observed and calculated patterns. Because the hydrogen atoms
are very light, they were neglected and not calculated in this
acetaldehyde into CO . A Tedlar bag (AS ONE Co. Ltd.) was used
2
3
as the photo-reactor vessel with a volume of 125 cm . A mass of
1
3 2
00 mg of WO $H O powder was spread evenly on the bottom of
2
a glass dish (area: 9.6 cm ¼ irradiation area), which was placed
in the reaction vessel described above. Five hundred ppm of
3 2
Fig. 2 Plot of current versus time for a single-pulse duration gener- Fig. 3 XRD powder pattern of WO $H O nanoparticles by pulsed
ated by the pulsed plasma in liquid system. plasma in deionized water.
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Table 1 Structure parameters of WO
3
$H
2
O nanoparticles compared
with Szymanski's and PDF#43-0679 data
8
Sample
Plasma sample
Szymanski's data
PDF#43-0679
Space group
Pnmb
Pnmb
Pnmb
Lattice parameter
˚
a (A)
5.2516(8)
10.7345(1)
5.1380(7)
5.249(2)
10.711(5)
5.133(2)
5.238
10.704
5.12
˚
b (A)
˚
c (A)
W
Position
4c
0.25
0.2228(2)
0.0123(2)
4c
0.25
0.2209(8)
ꢁ0.0037(3)
4c
4c
4c
8d
x
y
z
Fig. 5 The crystal structure model of WO
3
$H
2
O nanoparticles by
O1
Position
x
y
z
pulsed plasma in deionized water.
4c
0.25
0.4293(2)
0.0620(9)
4c
0.25
0.436(2)
0.075(4)
renement, and were just added into the model of the structure
for creating the image. Compared with the cell parameters from
the other sources, the crystal parameters of the WO $H O
3 2
O2
Position
x
y
z
4c
0.25
0.0680(2)
ꢁ0.0578(9)
4c
0.25
0.066(2)
ꢁ0.064(4)
nanoparticles prepared by pulsed plasma in deionized water are
the largest. In our experiment, the plasma was produced in
liquid, and water was the only source of oxygen. It was highly
possible that the synthesized nanocrystals contain oxygen
11
O3
Position
vacancies which tend to increase the lattice lengths. The
quenching effect by the surrounding cool liquid during plasma
synthesis may also help to inhibit the crystal growth and result
8d
8d
x
y
z
0.4626(8)
0.2555(3)
0.2864(9)
0.495(8)
0.227(2)
0.249(5)
12
in lattice expansion of the nanocrystals. Furthermore, the
atomic positions are shied a little in the plasma sample (Table
1), which indicates that the pulsed plasma in liquid method can
introduce structural distortion in the synthesized nanoparticles.
High energy-density plasma commonly increases the
synthesis temperature, leading to nanoparticle growth, but low
energy-density plasma reduces the production rate. Thus, to
optimize the balance between them, experimental parameters
such as single-pulse duration and voltage were adjusted in
T overall
0.046758
Prole R factors
GOF
R
R
1.72
14.52
18.32
p
wp
3 2
order to prepare uniform small-sized WO $H O nanoparticles
in large amounts. Besides, the synthetic environment of the
pulsed plasma in liquid method helps to disperse nanoparticles
in the liquid. As shown in Fig. 6, the morphology and particle
size were investigated by HRTEM. The images indicated that
3 2
very small orthorhombic WO $H O nanoparticles with sizes of
about 5 nm were obtained by pulsed plasma in deionized water.
The calculated size according to the Scherer equation from the
peaks of the XRD patterns was about 250 um. Because we dried
the particles before carrying out the XRD experiments, the
results here are considered to be due to the aggregation of
particles. The crystal parameters of WO $H O according to PDF
3
2
˚
˚
˚
card #43-0679 are 5.238 A ꢂ 10.704 A ꢂ 5.12 A h90.0 ꢂ 90.0 ꢂ
9
0.0i. As shown for the example in Fig. 6a, some particles were
only about 1–2 nm in diameter, and contained only 1 or 2 unit
cells. We conclude that some nanoparticles were nearly as small
13
as the unit cell.
3 2
Fig. 4 Rietveld refinement plot of WO $H O nanoparticles by pulsed
plasma in deionized water using the orthorhombic space group Pnmb.
Fig. 7 shows the energy-dispersive X-ray spectroscopy (EDX)
pattern of a plasma sample, in which the element Cu is due to
the HRTEM grid and the carbon peak source is the membrane
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Fig. 6 High-resolution TEM images of WO
pulsed plasma in deionized water. (a) Image of a WO
an ultra-small size.
3
$H
2
O nanoparticles by
$H O crystal with
3
2
Fig. 8 Optical emission spectrum of the pulsed plasma in deionized
water with tungsten electrodes.
hydrogen. Soon aer, the atoms W I in the plasma discharge
zone lose electrons and transform into ions W II, which then
react with oxygen and hydrogen to quickly form a WO $H O
3 2
molecular complex. When these complexes aggregate into crys-
tallites and are quenched suddenly by the surrounding cool
3 2
liquid, ultra-small WO $H O nanoparticles appear. According to
the Balmer series, there should be additional emission lines due
to H atoms, but these lines at about 410 nm, 430 nm, 490 nm, are
masked by the strong emission peaks of W atoms.
3.3. Photocatalytic properties and UV absorption
Photocatalytic properties of pure tungsten hydrate has not been
garnering much attention. In the published literature, most of
the reported hydrates were mixed with other components such
6
,14
as WO
3
and Ag, and the hydrates that were investigated were
3 2
Fig. 7 EDX pattern of the WO $H O nanoparticles synthesized by
15–17
usually in the forms WO
3
$0.33H
2
O and WO
recently, Q. Zeng et al. reported the photocatalytic properties
of pure WO $H O hollow spheres. However, the synthesized
3
$0.5H
2
O.
Only
pulsed plasma in deionized water and shown in the HRTEM image.
Inset shows a table of the percentage content of the elements W and
O in these nanoparticles.
18
3
2
spheres only showed high photocatalytic efficiency in acid
solutions. Fig. 9 shows the dependence of the CO evolution
rate on the decomposition of acetaldehyde under visible-light
irradiation over pure WO $H O samples synthesized by pulsed
2
covering the Cu grid. There were no other contaminating
elements. The atomic percentages of W and O were 21.06% and
3
2
plasma method. The irradiated wavelength was controlled by
using various cut-off lters. The photocatalytic activity of
WO $H O nanoparticles irradiated by visible light at 455 nm
7
8.93%, respectively, closely tting with the ratio of these two
elements in WO $H O, which is equal to 1 : 4.
3
2
3
2
was higher than the photocatalytic activity of commercial TiO2
samples (ST-01) under any conditions. Aer 720 min, the
3.2. Mechanism of formation
The optical emission spectrum of the pulsed plasma during the amount of CO
experiment is shown in Fig. 8. The spectral lines were identied 28.4% larger than that from ST-01.
using reference data. The coexistence ofatoms W I, ions W II, and In order to explain the photocatalytic behavior of WO
atoms H I, apparent from the optical emission spectrum, allows nanoparticles made using plasma and compare their light
elucidation of the mechanism of formation of the WO $H absorption ability to that of ST-01 and of commercial Wako
2
generated from the plasma sample is about
3
2
$H O
3
2
O
nanoparticles as follows: it is assumed that, rst, very hot plasma WO , UV-Vis absorption spectra (Fig. 10) were measured for
3
with extreme energy ablates the tungsten metal in the tips of the these three species. The obvious red shi of the absorption edge
rods to form very active tungsten atoms W I; simultaneously, the and higher absorbance of the WO
deionized water is decomposed by plasma into oxygen and light region, compared with that of the ST-01 TiO
3
$H
2
O sample in the visible-
sample,
2
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mechanism of formation of the WO
3
$H
2
O particles was
analyzed using an emission spectrum. These nanoparticles,
with the ultra-small particle size of about 5 nm, exhibit higher
absorption than do TiO (ST-01) and Wako WO . The red shi of
2
3
the absorption edge in the UV-Vis absorption pattern of the
plasma-synthesized sample, compared with the absorption
edge of the anhydrous tungsten oxide, indicates the possible
band modication and the potential applications of WO
nanoparticles in the solar energy eld.
3 2
$H O
Acknowledgements
This work is supported by the China Scholarship Council and
the Global Center of Excellence Program of Japan.
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Fig. 9 Photocatalytic properties of WO $H O nanocrystals grown in
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6
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This journal is © The Royal Society of Chemistry 2014
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