Enhanced photocatalytic activity of Pt/WO3 photocatalyst combined with TiO2 nanoparticles by polyelectrolyte-mediated electrostatic adsorption

Article abstract of DOI:10.1039/c4cy01075b

An electrostatic adsorption approach was used to realize a composite structure in which larger metal oxide crystalline particles were surrounded by metal oxide nanoparticles. Poly(diallyldimethylammonium chloride) (PDDA) and poly(sodium 4-styrenesulfonate) were alternately adsorbed onto crystalline tungsten trioxide (WO₃) particles (ca. 200 nm) via a layer-by-layer assembly, followed by adsorption of TiO₂ particles (ca. 6 nm) on the surfaces covered by PDDA. After calcination to remove the adsorbed polymer layers, Pt particles were dispersed on the composite structure by photodeposition. The resulting Pt/WO₃-TiO₂ composite photocatalyst showed a higher rate of activity towards the photocatalytic decomposition of gaseous acetone under visible light irradiation (λ > 420 nm) compared with that of Pt/WO₃. Pt/WO₃-TiO₂ also converted acetone to CO₂ almost completely, whereas the amount of CO₂ produced over Pt/WO₃ was much smaller than that expected for the complete oxidation of acetone. The enhanced activity of Pt/WO₃-TiO₂ was ascribed to hole transfer from the valence band of WO₃ to that of TiO₂, which likely suppressed electron-hole recombination and enabled the oxidation reaction to take place on the surface of the TiO₂ particles.

Full text of DOI:10.1039/c4cy01075b

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Enhanced photocatalytic activity of Pt/WO3
photocatalyst combined with TiO2 nanoparticles by
polyelectrolyte-mediated electrostatic adsorption
Cite this: DOI: 10.1039/c4cy01075b
Tomomi Ohashi, Takashi Sugimoto, Kaori Sako, Shinjiro Hayakawa,
*
*
Kiyofumi Katagiri and Kei Inumaru
An electrostatic adsorption approach was used to realize a composite structure in which larger metal oxide
crystalline particles were surrounded by metal oxide nanoparticles. Poly(diallyldimethylammonium chloride)
(PDDA) and poly(sodium 4-styrenesulfonate) were alternately adsorbed onto crystalline tungsten trioxide
(WO3) particles (ca. 200 nm) via a layer-by-layer assembly, followed by adsorption of TiO2 particles
(ca. 6 nm) on the surfaces covered by PDDA. After calcination to remove the adsorbed polymer layers,
Pt particles were dispersed on the composite structure by photodeposition. The resulting Pt/WO3–TiO2
composite photocatalyst showed a higher rate of activity towards the photocatalytic decomposition of
gaseous acetone under visible light irradiation (λ > 420 nm) compared with that of Pt/WO3. Pt/WO3–TiO2
also converted acetone to CO2 almost completely, whereas the amount of CO2 produced over Pt/WO3
was much smaller than that expected for the complete oxidation of acetone. The enhanced activity of
Pt/WO3–TiO2 was ascribed to hole transfer from the valence band of WO3 to that of TiO2, which likely
suppressed electron–hole recombination and enabled the oxidation reaction to take place on the surface
of the TiO2 particles.
Received 19th August 2014,
Accepted 9th October 2014
DOI: 10.1039/c4cy01075b
www.rsc.org/catalysis
band (CB) of WO3 move to Pt particles and multi-electron
reduction reactions proceed on the Pt, achieving visible light
Introduction
Nanostructural control of composite materials to achieve
enhanced functions has been of great interest in the fields of
catalyst and photocatalyst design.1,2 Various nanomaterials,
such as nanoparticles,3–5 nanofibers,6,7 and nanoporous
materials, have been investigated.8–12 The design of compos-
ite materials is also a promising strategy to achieve novel
functions in this area, and nanocomposite structures are
important subjects of recent studies on photocatalysts.9–12 For
example, composite photocatalysts of TiO2 combined with
mesoporous silica showed molecule selective photocatalytic
activity toward the decomposition of organic molecules in
water.11,12 Furthermore, Z-scheme photocatalysis has been
achieved by combining different metal oxides to create systems
in which one metal oxide passes electrons to the other.13,14
WO3 has recently attracted much attention as a visible
light-sensitive photocatalyst. Abe et al. found that platinum-
loaded WO3 (Pt/WO3) showed high photocatalytic activity for
the oxidation of organic molecules under visible light irradia-
tion.15 In this system, electrons excited to the conduction
sensitivity and high photocatalytic activity. Composite mate-
rials containing WO3 and TiO2 have recently been investi-
gated extensively.16–22 In the early stages of such research,
Miyauchi and Hashimoto et al. reported a WO3–TiO2 bilayer
structure showing high photoinduced hydrophilic proper-
ties.21 In this system, holes generated in the valence band
(VB) of WO3 transfer to the VB of TiO2 and electrons excited
to the CB of TiO2 transfer to the CB of WO3, enhancing the
efficiency of charge separation and the hydrophilic properties
of the surface. A Pt/WO3/TiO2 trilayer structure prepared by
sputtering has been applied to the photocatalytic decomposi-
tion of organic molecules in air.22 The thin film photocatalyst
showed high activity because of enhanced charge separation
efficiency. An important process in these systems is the trans-
fer of electrons and holes through the interfaces between
WO3 and TiO2. Thus, the design of such interfaces is a funda-
mental and interesting aspect of achieving enhanced photo-
catalytic performance.
In this study, we constructed a nanostructure in which
well-crystallized TiO2 nanoparticles (ca. 6 nm) were adsorbed
onto the surface of larger WO3 crystals (ca. 200 nm). The
WO3–TiO2 interfaces were formed by an electrostatic adsorp-
tion technique. Pt was deposited on the composite to form a
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima
University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan.
E-mail: kktgr@hiroshima-u.ac.jp, inumaru@hirohsima-u.ac.jp
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Pt nanoparticle/WO3 crystal/TiO2 crystalline particle structure.
The photocatalytic performance of the composite material
was investigated and is discussed in terms of the nanostruc-
ture of and interfaces between the two different metal oxides.
Characterization
X-ray powder diffraction patterns were measured using a D8
Advance diffractometer (Bruker AXS) with Cu Kα radiation
(Ni filtered). An in-house made X-ray fluorescence spectrome-
ter was used to determine the TiO2 content of the sample.
SEM and TEM images were taken using an S-4800
microscope (Hitachi) and a JEM-2010 microscope (JEOL),
respectively. ζ Potentials were measured with an ELSZ-2
(Otsuka Electric Co.).
Experimental
Materials
Na2WO4·2H2O, 20 wt% poly(diallyldimethylammonium chloride)
(PDDA) and poly(sodium 4-styrenesulfonate) (PSS) were pur-
chased from Sigma Aldrich. Hydrochloric acid (35 wt%), methanol
and acetone were purchased from Nacalai Tesque (Tokyo,
Japan). Tartaric acid was obtained from Kishida Chemical
Co., Ltd. (Tokyo, Japan). TiO2 sol (TKS-203, anatase form,
20.1 wt%) was kindly supplied by Tayca Co., Ltd. (Osaka, Japan).
All of the reagents were used without further purification.
Photocatalytic test
The procedure used to measure the photocatalytic decompo-
sition of gaseous acetone has been described in detail else-
where.24,25 A glass vessel (Nichiden-Rika Glass Co., Ltd.
SV-100, 113.8 cm3) was used as the reactor. The photocatalyst
sample (30 mg) was placed on an area of 1.5 cm × 2.0 cm of
a glass plate, and the plate was placed in the reactor. The
reactor was then irradiated with a 500 W Xe lamp (JASCO Co.)
overnight to completely convert organic species in the reactor
to CO2. The lamp was turned off and 2.71 × 10−6 mol of ace-
tone, equivalent to 583 ppm, was injected into the reactor.
The gas phase in the reactor was sampled with a syringe and
analyzed using a GC equipped with an FID detector
(Shimadzu GC-2014). After the adsorption had reached equi-
librium, the reactor was irradiated again with the Xe lamp.
An L42 filter was used to cut out light with wavelength
shorter than 420 nm. The time courses of the concentrations
of acetone and CO2 were monitored with the GC. A
methanizer (Shimadzu MTN-1) was used for analysis of CO2.
CO2 concentration was expressed after subtraction of its con-
centration at time zero (ca. 1000 ppm).
Preparation of photocatalysts
Plate-like WO3 crystals were prepared according to the litera-
ture.23 0.99 g of Na2WO4·2H2O and 2.09 g of tartaric acid
were added to 15 cm3 of 35 wt% HCl mixed with 30 cm3 of
pure water. The resulting solution was ultrasonically treated
for 1 h, charged into an autoclave, and then heated at 433 K
for 24 h. The obtained precipitate was separated by centrifu-
gation and alternately washed with water and ethanol three
times. The resulting solid was then calcined in air at 623 K
(heating rate 5 K min−1) for 4 h. WO3 particles with irregular
morphology were also prepared by another method. A solu-
tion of 1 M Na2WO4·2 H2O (25 cm3) and a solution of 3 M
HCl (225 cm3) were combined at 373 K and was stirred for
30 min. The precipitate was filtered, dried and calcined at
773 K for 2 h. This sample was designated as WO3-ir.
The obtained WO3 crystals were dispersed in water
(20 g dm−3). Solutions of each of poly(diallyldimethylammonium
chloride) (PDDA) and poly(sodium 4-styrenesulfonate) (PSS)
were prepared using an aqueous 0.5 mol dm−3 NaCl solution.
The concentrations of the polymers were adjusted to 1 mg cm−3.
5 cm3 of the WO3 dispersion was separated by centrifugation,
and 5 cm3 of the PDDA solution was added to the centrifuge
vessel, followed by vigorous mixing using a vortex mixer.
After standing for 15 min, the solid was separated by centri-
fugation and washed with water three times. Next, 5 cm3 of
the PSS solution was added to the solid. The two procedures
were repeated several times for the layer-by-layer deposition
of the polymers. Finally, 5 cm3 of dilute TiO2 sol (1 wt%) was
added to the solid covered by PDDA for electrostatic adsorp-
tion of TiO2. The solid was then washed with water three
times, dried and calcined at 773 K for 2 h (heating rate
1 K min−1). Pt was loaded on the resulting samples by
photodeposition. 0.1 g of sample was dispersed in 10 cm3 of
pure water and outgassed using bubbling N2 gas for 2 h.
13.3 × 10−3 cm3 of a 0.193 M aqueous solution of H2PtCl4 and
1.1 cm3 of CH3OH were then added to the suspension. The
mixture was photoirradiated with a 500 W Xe lamp. For com-
parison, the Pt photodeposition was carried out on an addi-
tional sample using an L42 cut filter.
Estimation of quantum efficiency
The light power irradiated to the photocatalyst through the
L42 filter was measured with a radiometer (Topcon UVR-300
with a detector UD400; sensible wavelength range 360–490 nm).
The value was 2.2 mW cm−2. Considering the area of the
photocatalyst in the reactor (3.0 cm2) and the minimum photon
energy that WO3 can absorb (2.6 eV; 477 nm), the maximum
number of photons having energy larger than 2.6 eV irradiated
to the photocatalyst was roughly estimated. Based on this value,
the approximate apparent quantum efficiency was calculated.
Results
Fig. 1 shows the variation in the ζ potential of the samples
during the layer-by-layer adsorption process. The pristine
WO3 crystals had a negative potential of ca. −40 mV. After the
first addition of PDDA, the ζ potential became positive, indi-
cating electrostatic adsorption of PDDA molecules on the sur-
face of the WO3. Then, PSS and PDDA were alternately
adsorbed on the surface of the sample to form five layers of
the polymers in total (3 layers of PDDA and 2 layers of PSS).
During these processes, the sign of the ζ potential alternated
correspondingly. The second and the third PDDA adsorption
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magnification image confirmed that close contact between
WO3 and TiO2 was maintained after the calcination.
Fig. 3 shows the X-ray diffraction (XRD) patterns of the
samples prepared in this study. Fig. 3a is the diffraction pat-
tern of the powder obtained after drying the TiO2 sol. The
clear peaks were all assignable to the anatase TiO2 crystal
structure. The peaks in the XRD pattern of the pristine WO3
(not shown) were all ascribed to monoclinic WO3. A weak
peak of anatase TiO2 was observed in the XRD pattern of the
TiO2-adsorbed sample in addition to the diffraction peaks of
WO3 (Fig. 3b and its inset). X-ray fluorescence analysis
revealed that the sample contained a small amount of TiO2
(6 wt%). This is the reason why only the main XRD peak of
anatase was observed in the diffraction pattern of the TiO2-
adsorbed sample (Fig. 3b). According to these results, the
electrostatic adsorption technique successfully realized a
structure in which WO3 crystals were completely surrounded
by and in close contact with TiO2 nanoparticles.
The photocatalytic activity of the samples was tested
using the decomposition of gaseous acetone under visible
light irradiation. Fig. 4 presents the results obtained for the
Pt/WO3–TiO2 composite photocatalyst. For comparison, the
results for Pt/WO3 are also presented in the same figure. In
the time course of acetone concentration (Fig. 4a), adsorption
equilibrium was achieved at the initial stage before the
photoirradiation was begun. The amount of acetone intro-
duced to the reactor corresponded to 583 ppm; the differ-
ences between the equilibrium concentration and this value
Fig. 1 Variation of ζ potential during the electrostatic adsorption process.
resulted in similar positive potentials of ca. 50 mV, which
seemed to reach a stable level. This suggests that the PDDA
molecules fully covered the surface of the sample. It is well
known that the alternating adsorption of the polymers forms
a “precursor layer”, which is important to form effective sur-
face charge for well-reproducible electrostatic adsorption of
particles.26–28 Finally, the sample was subjected to adsorption
of TiO2 nanoparticles using the TiO2 sol. The potential
changed to a negative value of ca. −30 mV, indicating that
the surface of the sample was well covered by negatively
charged TiO2 nanoparticles.
Electron microscope images of the samples are presented
in Fig. 2. Fig. 2a and b show SEM and TEM images of the
pristine WO3 crystals, respectively. Thick plate-like crystals of
ca. 200 nm in width were observed. Fig. 2c shows a TEM
image of the sample after adsorption of TiO2 nanoparticles
(before calcination). The WO3 crystals were clearly completely
surrounded by TiO2 nanoparticles, which appeared to form
aggregates of ca. 20–50 nm in size. These aggregates were
closely attached to the surfaces of the WO3 crystals. The
sample was calcined at 773 K in air to remove the polymer
layers on the WO3 surfaces. Fig. 2d presents a TEM image
of the TiO2-adsorbed sample after calcination. The higher
Fig. 2 Microscope images of the samples. (a) SEM image of pristine
WO3, (b) TEM image of pristine WO3, (c) TEM images of WO3 after
adsorption of TiO2 nanoparticles, and (d) TEM image after removal of
organic polymer layers by calcination.
Fig. 3 X-ray powder diffraction patterns of (a) dried TiO2 sol and
(b) Pt/WO3–TiO2. The inset shows the magnified pattern, and the red
circle indicates the main diffraction peak of the anatase TiO2 phase.
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Fig.
5
First-order plot of acetone concentration during the
photocatalytic reaction. Solid circles, Pt/WO3–TiO2; solid triangles, Pt/WO3.
prepared and compared, where an L42 cutoff filter was used
during the photodeposition of Pt onto the composite. The
photocatalyst prepared without an L42 filter must have Pt
particles on both the WO3 crystal and TiO2 nanoparticles,
whereas for the photocatalyst prepared using an L42 filter,
the Pt must exist only on WO3. The photocatalytic activities
of the two different composite photocatalysts were very
similar (not shown) under photoirradiation through an L42
filter. This is reasonable because TiO2 particles cannot be
photoexcited under these conditions. The decomposition of
acetone over TiO2 under the same conditions was negligibly
slow. These results demonstrate that a combination of Pt/WO3
and TiO2 is indispensable to the enhanced activity observed in
the present work. Pt/WO3-ir–TiO2, a composite photocatalyst
prepared from WO3 having irregular morphology (WO3-ir),
showed lower activity compared to Pt/WO3–TiO2 but still have
higher activity than Pt/WO3, showing wide versatility of the
present method.
Fig. 4 Time courses of acetone and CO2 concentrations during the
photocatalytic reaction.
are attributed to the adsorption of the acetone onto the
surface of the photocatalyst. Pt/WO3–TiO2 showed a larger
adsorption of acetone than Pt/WO3. This is probably because
the considerable surface area of the TiO2 nanoparticles
contained in Pt/WO3–TiO2 meant that a larger amount of
acetone was removed from the gas phase than in the case of
Pt/WO3. After the photoirradiation, Pt/WO3–TiO2 decomposed
acetone faster than Pt/WO3 did. The difference in the reac-
tion rates is clearly demonstrated in the first-order plots of
acetone concentration (Fig. 5).
The difference between the activities of the two
photocatalysts is more obvious in the time course of CO2 con-
centration (Fig. 4b). For Pt/WO3, CO2 generation was slow
and the concentration produced was much lower than that
expected for the complete oxidation of acetone (1749 ppm),
even when acetone had disappeared from the gas phase
(700 min after the start of photoirradiation). In contrast,
Pt/WO3–TiO2 generated CO2 much more quickly, and the con-
centration of CO2 reached the level close to that expected for
complete oxidation of acetone when acetone in the gas phase
disappeared. That is, Pt/WO3–TiO2 with TiO2 nanoparticles
decomposed acetone completely to CO2, and its photocata-
lytic activity was much higher than that of Pt/WO3. For com-
parison, TiO2 sol was dried and tested for the photocatalytic
decomposition of acetone. TiO2 showed very low activity
under the same test conditions (not shown), demonstrating
that a combination of Pt/WO3 and TiO2 is indispensable to
the enhanced activity under visible light irradiation. Further-
more, an additional type of composite photocatalyst was
Discussion
Electrostatic adsorption was successfully used to prepare
WO3 crystals surrounded by TiO2 nanoparticles, as shown in
Fig. 2c. The primary particles of the TiO2 were ca. 6 nm in
size but formed aggregates of ca. 20 nm in size (Fig. 2d).
These aggregates closely contacted the surfaces of the WO3
crystals.
The composite catalyst Pt/WO3–TiO2 showed enhanced
photocatalytic activity for the photodecomposition of gaseous
acetone compared with that of Pt/WO3. In particular, the
amount of CO2 generated over Pt/WO3 was much less than
that over Pt/WO3–TiO2. In other words, Pt/WO3–TiO2 showed
superior performance in the total oxidation of acetone,
whereas Pt/WO3 could not decompose acetone completely to
CO2. It was confirmed that TiO2 alone could not decompose
acetone under the same conditions. These results have dem-
onstrated that the combination of Pt/WO3 and TiO2 is essen-
tial for enhanced photocatalytic activity in complete acetone
oxidation. It has been claimed that a combination of WO3
and TiO2 allows transfer of holes from the VB of WO3 to that
of TiO2.21 In the present study, light with wavelength longer
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recombination and enabled the oxidation reaction to
occur on the surface of the TiO2. This work highlights a
promising strategy for constructing nanostructured composite
photocatalysts by using an electrostatic adsorption approach.
Acknowledgements
This work was supported by the Advanced Catalytic Trans-
formation Program for Carbon Utilization (ACT-C), Japan
Science and Technology Agency (JST), by Asahi-Glass Founda-
tion, and by Grants-in-Aid for Scientific Research (B) from
Japan Society for the Promotion of Science, and partially by a
Grant-in-Aid for Scientific Research (no. 22107011) on the
Innovative Areas: “Fusion Materials: Creative Development
of Materials and Exploration of Their Function through
Molecular Control” (area no. 2206) from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT).
The authors thank Dr. M. Maeda for TEM measurements.
Fig. 6 Schematic illustration of the composite photocatalyst.
than 420 nm was used for the photocatalytic reaction. Thus,
the WO3 was able to absorb photons and generate excited
electrons in its CB and holes in its VB, while the TiO2 could
not absorb the light. If the holes in the VB of WO3 trans-
ferred to the VB of TiO2, the recombination of electrons and
holes would have been suppressed (Fig. 6) and the holes
would contribute to the oxidation reaction on the surface of
the TiO2 particles.
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