Ultrasonic spray pyrolysis assembly of a TiO2-WO3-Pt multi-heterojunction microsphere photocatalyst using highly crystalline WO3 nanosheets: Less is better

Article abstract of DOI:10.1039/c5nj02981c

Multi-heterojunctions are more promising than single-heterojunctions in photocatalysis due to the availability of more interfaces between each component. However, photocatalytic activity is highly dependent on the contact mode of individual components. In this work, we assemble TiO₂-WO₃-Pt multi-heterojunction microspheres using ultrasonic spray pyrolysis and focus on their contact mode governed photocatalytic activity. The results reveal that using highly crystalline WO₃ nanosheets as building blocks could particularly enhance the photocatalytic activity of a TiO₂/WO₃ system toward the degradation of gaseous acetaldehyde and isopropyl alcohol. Furthermore, loading Pt nanoparticles onto WO₃ nanosheets could facilitate a more prominent enhancement of activity than that of TiO₂/Pt, benefiting from the two-electron reduction of oxygen at the interface of WO₃/Pt. Meanwhile, the high crystallinity of WO₃ nanosheets allows for a loading amount of Pt as low as 0.04 wt% in the TiO₂-WO₃-Pt system, which reduces the catalyst cost in comparison with that of the conventional amount of 1 wt%.

Full text of DOI:10.1039/c5nj02981c

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Page 1 of 18
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Ultrasonic spray pyrolysis assembly of TiO -WO -Pt
2
3
multi-heterojunction microspheres photocatalyst by highly
crystallized WO nanosheet: less is better
3
1
**
1,2**
1*
1
1
Han Zheng, Changhua Wang,
Xintong Zhang, Lina Kong, Yingying Li,
3
1
Yunyu Liu, Yichun Liu
1
Center for Advanced Optoelectronic Materials Research, and Key Laboratory of UVꢀEmitting
Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin
Street, Changchun 130024, China
2
College of Chemistry and Biology, Beihua University, Jilin 132103, China
3
College of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun 130024,
China
Abstract: Multiꢀheterojunction is more promising than singleꢀheterojunction in
photocatalysis due to the availability of more interfaces between each component.
However, its photocatalytic activity is highly dependent on the contact mode of
individual component. In this work, we assemble TiO
microspheres by ultrasonic spray pyrolysis and focus on their contact mode governed
photocatalytic activity. The results reveal that highly crystalline WO nanosheets as
building blocks could particularly enhance the photocatalytic activity of TiO /WO
system toward degrading gaseous acetaldehyde and isopropyl alcohol. Furthermore,
loading of Pt nanoparticles on WO nanosheets could facilitate to a more prominent
enhancement of activity than that of TiO /Pt, benefiting from the twoꢀelectron
2 3
ꢀWO ꢀPt multiꢀheterojunction
3
2
3
3
2
reduction of oxygen at the interface of WO
nanosheets allows a loading amount of Pt as low as 0.04 wt% in TiO
which reduces the catalyst cost in comparison with the conventional amount of 1wt%.
3 3
/Pt. Meanwhile, high crystallinity of WO
2
3
ꢀWO ꢀPt system,
1
. Introduction
TiO
2
continues to gain research interest in photocatalysisꢀrelated
*
Corresponding author. Tel./Fax: +86 431 85099772; Eꢀmail: xtzhang@nenu.edu.cn
*
*
These authors contributed equally to the work.
1

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1
ꢀ4
applications. Recent study reveals that its solar conversion efficiency could be
further improved when TiO meets second metal oxide in terms of type II
2
5
ꢀ15
14
heterojunction. For example, Zhang et al. prepared the WO
.0% WO /TiO composite showed far superior photoactivity as compared to pure
TiO , Pꢀ25 and pure WO in the degradation of methyl orange and 2,4ꢀdichlorophenol
3 2
/TiO composites and
5
3
2
2
3
15
2 6 2
in UV and visible light. Liu et al. developed Bi MoO nanosheet/TiO nanobelt
heterostructure and observed an excellent photodegradation performance under UV
and visible light irradiation. Moreover, such a heterostructure possessed high
ꢀ1
ꢀ1
photocatalytic oxygen production with a rate of 0.668 mmol h g . However, among
part of type II heterojunctions, conduction band of secondary oxide in type II
heterojunction is lower than that of TiO
side of secondary oxide are difficult for onset of singleꢀelectron reduction of oxygen.
As such, photoꢀgenerated electrons flowed from TiO to foreign component across
2
and thereby photogenerated electrons on the
2
interface of heterojunction usually tend to seriously aggregate on the side of
secondary metal oxide and result in unfavorable “back electron transfer”
recombination.
To overcome the difficulty in initiation of oxygen reduction by photoꢀgenerated
electrons, promotion of multiꢀelectron transfer is found to be an alternative mean in
16ꢀ18
order to boost the photocatalytic activity of aforementioned metal oxides.
For
example, the potential of conduction band of Ag PO cannot match the redox
3
4
potentials of singleꢀelectron reduction of oxygen. However, the twoꢀelectron
reduction of oxygen over Ag PO can be initiated and result in remarkable
3
4
19
20
photooxidation ability.
Similarly, Abe et al.
reported that PtꢀWO
3
photocatalytically decomposed isopropyl alcohol with ca. 100ꢀfold enhanced CO
2
generation rate, while only 4ꢀfold enhancement was observed for Ptꢀ(NꢀTiO ). The
2
reason for high activity of PtꢀWO
reduction of O on the Pt cocatalyst rather than singleꢀelectron reduction. Hashimoto
et al. grafted Cu(II) or Fe(III) species on a series of oxides, such as Ti1ꢀ3x
3
was due to the promotion of multiꢀelectron
2
21
x 2
W Ga2xO ,
22
3+
23
24
(Sr1−yNa
y
)(Ti1ꢀxMo
x
3
)O ,
Ti selfꢀdoped TiO
2
and Fe doped TiO
2
.
The grafted
species consistently enhance photocatalytic activity toward degradation of isopropyl
2

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alcohol induced by interfacial charge transfer (IFCT) process and thereafter
consumption in multiꢀelectron reduction of O mediated by the surface clusters.
2
On the basis of above considerations, integration of advantages of type II
heterojunction (wide light absorption range and improved charge separation) and
cocatalyst (multiꢀelectron induced surface reaction) into multiꢀheterojunction
becomes a very promising way to access to high solar conversion efficiency. With
regard to multiꢀheterojunction, contact mode between individual components is highly
critical. For instance, Hoffmann et al. observed high hydrogen production rate about
ꢀ3
ꢀ1 ꢀ1
(
6ꢀ9) ×10 mol h g over CdS/(PtꢀTiO ), which is higher by a factor of 3ꢀ30 than
2
2
5
that of Ptꢀ(CdS/TiO
2
). Wang et al. showed a synergistic enhancement of H
2
x
evolution over the Pt/SnO
x
/TiO heterostructures formed by anchoring Pt NPs at SnO
2
sites, whereas the rate of H evolution over Pt/TiO /SnO counterparts prepared by
2
2
x
2
6
27
grafting SnO
TiO /WO
of noble metal on desired surface of TiO or WO . Major differences were observed
x
species on Pt/TiO
2
was significantly decreased.
Pap et al. obtained
2
3
/noble metal (Au or Pt) composites by means of selective photodeposition
2
3
in photocatalytic activity toward degradation of oxalic acid and production of
hydrogen over catalysts wherein only the position of the noble metal was changed.
We have fabricated (WO
nanorods and Pt and TiO
multiꢀheterojunction under visible light was found dependent on the location of the
loaded Pt nanoparticles. Pt surface loading on WO , as opposed to loading on the TiO
3 2 3
ꢀPt)/TiO multiꢀheterojunction photocatalyst based on WO
28
2
nanoparticles. The photocatalytic performance of the
3
2
surface, was more beneficial in maximizing the photocatalytic activity. Therefore,
contact mode of chosen building blocks should not be overlooked and
multiheterojunction should be smartly designed for photocatalysis.
Inspired by these reports, in this work, we continue to focus on
2 3
multiꢀheterojunction assembled from TiO , WO and Pt as well as their contact mode
dependent photocatalytic activities. Furthermore, ultrasonic spray pyrolysis technique
is employed to assemble multiꢀheterojunction due to fact that production of porous
microspheres where light trapping via multiple scattering at airꢀsolid interface is
favored. Highly crystallized WO
3
nanosheets are chosen as building blocks for the
3

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2
sake of not only good contact with TiO and Pt nanoparticles but also minimization of
defects at the interface. Structural features and photocatalytic activities of
asꢀassembled multiꢀheterojunction will be systematically discussed.
2
. Experimental Section
2
.1 Materials and Characterization
.1.1 Materials
TiO powders (Degussa P25) were obtained from Nihon Aerisol Co.
PtCl ·6H O was purchased from Aladdin in analytical grade purity. Acetaldehyde
) and CO (2 vol% in N ) were obtained from Beijing Chemical Co.
powders were purchased in analytical purity from Chinese National
2
2
H
2
6
2
(2 vol% in N
2
2
2
Bi , WO
2
O
3
3
Medicine Group Chemical Reagent Co. All chemical reagents were used as received,
without further purification.
2
3
.1.2 Preparation of WO nanosheets
WO nanosheets were synthesized by stoichiometric amounts of Bi O and
3
2 3
WO to 800 °C for 16 h. The obtained powders were ground and then treated with 6M
3
HCl solution for 3 days at room temperature to promote the proton exchange. The
obtained light yellow suspension was centrifuged, washed thoroughly with deionized
o
water to remove the residual acid, and finally dried at 80 C in air overnight. The
obtained powders(0.4 g) was immersed into 100 ml tetramethylammonium hydroxide
aqueous solution (0.0028 M) for 8 days with continues stirring. As a result, a colloidal
29
suspension with a light green appearance was obtained successfully, as references .
o
After centrifugation and drying at 80 C, the obtained products were WO
3
nanosheets.
2
3
.1.3 Preparation of WO /Pt hybrids
The WO
3
nanosheet sample (100 mg) was added into 10 ml water and
ultrasonically dispersed for 10 min, and then, aqueous H PtCl solution (256ꢁl , 0.1 M)
2
6
and 1.1 mL methanol was added. The solution was stirred continuously under UV
3
0
light (340ꢀ420 nm) irradiation for 2 h. The asꢀobtained Ptꢀloaded WO
were centrifuged and dried at 343 K at ambient condition. The stoichiometric ratio of
Pt on WO is 1 wt%.
3
nanosheets
3
4

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2.1.4 Preparation of TiO
2 3
ꢀWO ꢀPt multiꢀheterojunction
TiO ꢀWO ꢀPt microspheres were prepared by the ultrasonic spray pyrolysis
2
3
method. Typically, P25 TiO powders (3 g) and preꢀsynthesized WO ꢀPt composite
2
3
(0.12 g) were dispersed into 1000 ml of water to form a mixture suspension.
Subsequently, the suspension was atomized by an ultrasonic nebulizer, and the formed
mist was passed through a quartz tube heated at 1073 K. Then, powders were
collected at an electrostatic collecting device connected to the end of the quartz tube.
Finally, the asꢀobtained powders were annealed at 773 K for 30 min under ambient
conditions.
2.1.5 Characterization
Xꢀray diffraction (XRD) characterization was carried out on a Rigaku
D/maxꢀ2500 Xꢀray diffractometer. Photoluminescence (PL) spectra were recorded on
+
a JobinꢀYvon HR800 instrument with an Ar laser source of 325 nm wavelength in a
macroscopic configuration. Morphology and composition analyses were carried out
on a FEI quanta 250 field emission scanning electron microscopy (FESEM) and a
high resolution electronic micrographs were acquired using a JEOL JEMꢀ2100
transmission electron microscope (TEM) operated at an acceleration voltage of 200
kV. UVꢀVis diffusion reflectance (DR) spectra were recorded on a Lambda 900 UV–
vis–NIR spectrophotometer, using BaSO
.1.6 Photocatalytic Test
4
as reference.
2
Photocatalytic properties of samples were investigated by measuring the
photodegradation of gaseous acetaldehyde and isopropanol under solar light
irradiation. 0.05 g microsphere powders were uniformly spread in a sample holder
ꢀ2
with a geometric area of 0.25 cm , which was placed on the bottom of a 500 mL
cylinderꢀtype Pyrex glass vessel. The glass vessel was flushed with O (20%)/N gas
to remove carbon dioxide from the system, and the relative humidity of atmosphere
inside the vessel was controlled to 45% by passing the O /N gas through chilled
2
2
2
2
water. 5 mL acetaldehyde was introduced into the reaction vessel using a
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PressureꢀLock syringe to reach a concentration of 200 ppmv. After keeping dark for
0 min, the glass vessel was irradiated from top by a 150 W xenon lamp (Hayashi
3
UV410) which emit light of wavelength range of 350ꢀ700 nm. The light intensity
2
irradiated on the samples was determined to be 1 mW/cm at 365 nm by light
intensity meter from Beijing Normal University. The degradation of acetaldehyde and
the generation of carbon dioxide were monitored using a gas chromatograph
(
SPꢀ2100A, BFRL Co.), equipped with a 2 m PorapakꢀQ column and a flame
ionization detector. Isopropanol and acetone were monitored on gas chromatograph
GC2014C，SHIMADZU Co.), equipped with a WonderCap Wax column and a flame
ionization detector.
(
3
. Results and Discussion
3
In our work, firstly, we synthesize highly crystalline WO nanosheet, employ
3
photochemical method to load platinum (Pt) on the WO nanosheet to form a noble
metalꢀsemiconductor heterostructure. The reason to choose highly crystalline WO
3
nanosheets instead of traditional WO nanopaticles is that highly crystalline WO3
3
nanosheet has few defects and inhibits recombination of electrons and holes. In
addition, lamellar material has more reaction sites which are helpful for forming a
good contact between Pt nanoparticles and WO
3
nanosheets. Subsequently，we
combine pure TiO (P25) with Pt loaded WO
2
3
nanosheets to form
multiꢀheterojunction microspheres with ordered contact by a ultrasonic spray
pyrolysis method. The multiꢀheterojunction microspheres microspheres have follow
advantages：(1) porous microspheres are facile for adsorption of organic compounds
and improve the photocatalytic activity due to their large surface areas. (2) The
multiple heterogeneous structure photocatalyst with ordering contact mode can
migrate the photogenerated electron and holes and suppress recombination more
effectively than pure TiO
2 2 3
and TiO ꢀWO binaryꢀheterogeneous structure. (3) Only
rarely little platinum nanoparticles (0.04%，relative to the amount of TiO
2
) are loaded
in the multiple heterostructure photocatalyst, which not only results in activity
enhancement but also save cost in comparison with traditional value of 1%.
6

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3
.1 Structure and morphology
Fig.1: Structural characterization of WO
image; (c) SAED pattern.
XRD and HRTEM analysis are performed to characterize preꢀsynthesized WO3
3
nanosheets: (a) XRD pattern; (b) TEM
nanosheets. As shown in Fig.1a, all diffraction patterns can be well indexed to
orthorhombic phase (JCPDS No.20ꢀ1324). TEM image in Fig. 1b shows that the
products are nanosheets with side length of ca. 400 nm. In HRTEM image (Fig. 1c) of
a WO
3
crystallite, interplanar distances corresponding to d(020) (0.384 nm measured;
0
.3783 nm theoretical) and d(002) (0.392 nm measured, 0.3894 nm theoretical) are
separated by a 89.6° ± 0.5° measured angle (90° theoretical). Corresponding selected
area diffraction pattern shows a typical orthorhombic symmetry consisting of clear
3
spots with strong diffraction, indicating the high crystallinity of WO nanosheet. The
clear spots can be labeled with planes of (002), (200) and (220) of orthorhombic WO
respectively.
3
,
SEM images of different samples synthesized by ultrasonic spray pyrolysis are
shown in Fig. 2. As can be seen in Fig. 2a, pristine TiO samples display the
2
morphology of polydispersed microspheres with diameters from 0.5 to 2 ꢁm. The
surface of the spheres is not smooth, suggesting that the resulting microspheres are
formed by the aggregation of nanoparticles driven by the drying of mist droplets.
Similar phenomenon has been observed in ultrasonic spray pyrolysis prepared
31
32
33
34
microspheres such as TiO2, Fe O , ZnS and SiO2. On the contrary, pure WO3
2
3
samples in Fig. 2d show broken and hollow spheres with smooth surface, probably
resulting from the difficult agglomeration of the primary largeꢀsized nanosheets. After
incorporating WO
3
nanosheets or WO
3
/Pt hybrids with TiO
2
nanoparticles, porous
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microspheres can be well maintained as shown in Fig. 2b and c. The edges of WO
3
nanosheets can not be distinguished in the microspheres, mainly due to their tendency
of curling or wrapping in TiO nanoparticles.
2
2
Fig.2: SEM images of (a) pristine TiO , (b) TiO
2
3
/WO ,
(
c) TiO
2
ꢀWO
3
ꢀPt and (d) WO
3
samples
TiO2
WO -0%
3
(a)
(b)
A: TiO2 (c)
B: WO3
WO -2%
A
TiO -WO
3
2
3
WO -4%
TiO -WO -Pt
3
2
3
WO -10%
3
WO -20%
3
A
B
B
10
20 30 40 50 60 70 80 20 30 40 50 60 70 80 20
25
30
35
2θ
(deg.)
2θ (deg.)
2θ (deg.)
2
Fig.3: (a) XRD patterns of pure TiO , typical TiO
2
ꢀWO
3 2 3
and TiO ꢀWO ꢀPt samples;
(
b and c) XRD patterns of different TiO ꢀWO ꢀPt multiꢀheterojunctions and
2 3
enlargement of the region between 20ꢀ35 degrees.
8

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Fig. 3 depicts XRD patterns for the samples prepared by ultrasonic spray pyrolysis.
◦
◦
◦
◦
2
For pure TiO , diffraction peaks (Fig. 3a) at 2θ = 25.22 , 37.78 , 47.94 , 54.96 , and
◦
6
2.69 can be respectively indexed to the (101), (004), (200), (211), and (204) planes
of anatase (JCPDS No. 21ꢀ1271). In addition, diffraction peaks attributed to rutile are
found, as shown by the appearance of its (110) reflection at ∼27.4° of 2θ (JCPDS No.
2
1ꢀ1276). There is no any other characteristic peak in the XRD patterns, which
confirms that other new phases are not formed during the preparation processes.
Hence, the crystalline structure of pure TiO is the mixture of anatase and rutile phase.
For the WO ꢀTiO ꢀPt composites, no diffraction peaks of WO can be found until the
content of WO reaches 20 wt% (Fig.3b and c), resulting from either the low amount
2
3
2
3
3
3 2
or high dispersity of WO component on TiO . In curve Fig.3b and c, asꢀdetected
◦
◦
diffraction peaks located at 23.70 and 28.77 can be attributed to orthorhombic phase
of WO (JCPDS No. 20ꢀ1324). No diffraction peaks of noble metal Pt are detected in
3
2 3
all TiO ꢀWO ꢀPt samples, which is mainly due to the ultraꢀsmall size of Pt
nanoparticles and their quite low amount.
The structure of the asꢀprepared heterojunctions was further studied by TEM and
HRTEM. Fig.4a shows the HRTEM for platinum loaded WO nanosheet samples. It is
3
shown that Pt nanoparticles have a close contact to WO
3
nanosheet and WO
3
/Pt
heterostructure is well formed. In typical TEM image of WO
3
ꢀTiO ꢀPt microspheres
2
shown in Fig. 4b, uniform distribution of pores in microspheres is once again
observed, confirming above observation in SEM images. HRTEM image (Fig. 4c)
clearly shows an interface between WO
.63 Å can be indexed to (101) plane of anatase TiO
orthorhombic WO , respectively.
3
and TiO
2
. The fringe spacing of 3.545 Å and
2
2
, and (220) plane of
3
3
Fig. 4: (a) HRTEM image of WO ꢀPt heterojunction, (b,c) TEM and HRTEM
images of TiO
2
ꢀWO
3
ꢀPt multiꢀheterojunction microspheres.
9

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3
.2 Optical properties
Fig. 5 depicts the typical diffuse reflectance spectra of pureꢀTiO , TiO ꢀWO and
2
2
3
2
ꢀ
4+
TiO
2
ꢀWO
3
ꢀPt. All spectra are dominated by the edge due to the O ꢀTi charge
transition in anatase TiO
2
at 300ꢀ380 nm, which is comparable to the excitation of
electrons from the valence band to the conduction band. UVꢀvis spectra performed in
the diffuse reflectance mode (R) are transformed to the KubelkaꢀMunk function F(R)
to separate the extent of light absorption from scattering. The value of band gap is
1
/2
obtained from the plot of the Kubelka–Munk function (F(R)E) versus the energy of
the absorbed light E. Accordingly, the band gap (Eg) for pure TiO is calculated to be
ca. 3.07 eV, which fall within the range of mixed phases of anatase and rutile.
Different from previously reported TiO ꢀWO composites, no redꢀshift of the
2
2
3
absorption edge of TiO ꢀWO and TiO ꢀWO ꢀPt in comparison with pristine TiO is
2
3
2
3
2
observed, mainly resulting from the low content of WO
3
and its high dispersion in the
3
1,35
composite.
3
Hence, it can be deduced that asꢀincorporated WO components in
multiꢀheterojunction have no influence on the absorption properties of TiO . In other
2
words, in spite of narrower band gap of WO
activity of either TiO ꢀWO or TiO ꢀWO ꢀPt may not be influenced by means of
improvement of absorption property.
3 2
than that of TiO , the photocatalytic
2
3
2
3
100
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
1
1
1
.4
.2
.0
TiO2
3.07eV
3.07eV
TiO -WO
2
3
TiO
-WO
-Pt 3.08eV
2
3
0.8
0.6
0.4
0.2
0.0
2
.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
Band gap(eV)
300
400
500
600
700
800
Wavelength(nm)
Fig.5: UVꢀVisꢀDRS spectra of different samples
Photoluminescence (PL) spectroscopy has been useful in the field of
36ꢀ39
photocatalysis.
Information such as surface oxygen vacancies and defects, as well
10

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as the efficiency of charge carrier trapping, immigration and transfer can be obtained.
For pure TiO , as illustrated in Fig. 6, it exhibits a broad emission band in the visible
2
light region. It is believed that the peaks result from the electronic transition mediated
40
by the defect levels such as oxygen vacancies in the band gap. After incorporating
with WO nanosheets, the intensity of TiO is significantly decreased. Considering
3
2
that the PL emission is the result from the free charge carrier recombination, the lower
PL intensity indicate that the TiO ꢀWO photocatalyst has a lower recombination rate
of photoꢀgenerated electrons and holes, which is due to the fact that the electrons
and holes are separated by the charge transfer at the heterojunction interfaces of WO
nanosheets and TiO nanoparticles. More importantly, TiO ꢀWO ꢀPt
2
3
41
3
2
2
3
multiꢀheterojunction shows drastic quenching of PL intensity, suggesting a markedly
enhanced charge separation coꢀpromoted by WO /Pt and WO /TiO heterojunction.
3
3
2
3 3
This result confirms significant participations of WO nanosheets and WO /Pt
heterojunction in the decrease of recombination of photoꢀgenerated electronꢀhole
pairs.
TiO2
TiO -WO
2
3
TiO -WO -Pt
2
3
400
500
600
700
Wavelength (nm)
Fig.6: PL spectra of different samples
.3 Photocatalytic performance
The photocatalytic test was investigated against degradation of acetaldehyde
3
ꢀ2
2
under solar light with intensity of 1 mw/cm at 365 nm. For comparison, TiO ꢀPt (the
same method of photochemical Pt loading, the quantity of Pt is 0.04 wt % relative to
the entire photocatalyst microspheres) were also fabricated by ultrasonic spray
pyrolysis method. Fig. 7a,b shows typical plots of decrease of acetaldehyde and
11

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increase of CO
2
concentration as a function of irradiation time on different
photocatalysts. According to the LangmuirꢀHinshelwood kinetics model, the apparent
rate for acetaldehyde degradation can be expressed as:
1
.0
.8
.6
.4
.2
TiO2
200
150
3.5
3.0
2.5
2.0
TiO2
TiO -WO
TiO
2
2
3
TiO -WO
0
2
3
TiO -WO -Pt
TiO -WO
2
3
2
3
TiO -WO -Pt
2
3
TiO -Pt
TiO -WO -Pt
2
2
3
0
TiO -Pt
2
100
TiO -Pt
2
1
.5
.0
.5
0.0
0
1
50
0
0
(
a)
(
80
b)
(c)
0
0.0
0
20
40
60
80
0
20
40
60
0
20
40
60
80
Time(min)
Time (min)
Time
(min)
Fig.7: (a) Degradation curves of acetaldehyde over different samples, (b) CO
2
evolution curves over different samples, (c) reaction kinetic curves over different
samples.
ln(C
reaction, t is the reaction time, and C
apparent first order rate constant, k, are given by the slope of ln(C
0
/C) = kt, where C is the concentration of acetaldehyde during photocatalytic
0
is the initial concentration of acetaldehyde. The
0
/C) versus t, and
the results are 0.01422, 0.01325, 0.02491 and 0.0379 min for TiO , TiO ꢀPt,
TiO ꢀWO and TiO ꢀWO ꢀPt microspheres, respectively (Fig.7c). For TiO ꢀWO
photocatalyst, it is more efficient in degradation of acetaldehyde than that of pure
TiO , which agrees well with previous TiO ꢀWO system. Notably, loading of Pt
ꢀ1
2
2
2
3
2
3
2
3
2
2
3
nanoparticles on the surface of P25 hardly exhibits improvement of activity in terms
of either decrease rate of acetaldehyde or evolution rate of CO . Different from most
2
study of TiO
photocatalytic activity of TiO
amount of Pt loading as low as 0.04%. Interestingly, when the same amount of Pt is
loaded on WO , asꢀobtained TiO ꢀWO ꢀPt multiꢀheterojunction displays a dramatic
decrease of acetaldehyde and increase of CO concentration at initial 60 min. About
0% of acetaldehyde was oxidized into CO after 60 min, showing the most excellent
photocatalytic activity of TiO ꢀWO ꢀPt multiꢀheterojunction. Herein, it should be
2
/Pt photocatalyst, our results suggest a poor improvement of the
2
after Pt loading, which is mainly due to the very few
3
2
3
2
9
2
2
3
noted that Pt loading percentage in previous Pt/metal oxide composite photocatalyst is
12

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DOI: 10.1039/C5NJ02981C
usually 1% in most reported results. In our case, the significant enhancement of
activity induced by incorporation of less amount of Pt of 0.04% in
multiꢀheterojunction can be explained as following: Pt nanoparticles are inꢀsitu
photodeposited on the surface of WO
3
nanosheets. As demonstrated in
aforementioned XRD and HRTEM results, WO nanosheets are highly crystallized.
3
3
Under UV light irradiation, high crystallization of WO nanosheets can ensure easy
immigration of photoꢀexcited electrons from crystal inner to surface, less trapping of
electrons by surface defects states and inhibition of recombination of electrons and
holes. As such, H
WO nanosheets and hence Pt nanoparticles are uniformly deposited on WO
without aggregation. Therefore, WO /Pt interface with high amount and good quality
are produced, even if the Pt loading amount is lower than conventional value of 1%.
Benefiting from the electrons flowing from WO to Pt and onset of multiꢀelectron
2 6
PtCl is effectively photoreduced by electrons on the surface of
3
3
surface
3
3
reduced of oxygen, the photocatalytic performance of multiꢀheterojunction is
obviously boosted.
1
200
000
4
3
2
1
0
(
a)
(b)
1
TiO2
TiO2
8
00
00
00
00
TiO -WO
2
TiO -WO3
2
3
TiO -WO -Pt
TiO -WO -Pt
2 3
2
3
6
4
2
0
0
20
40
60
80
0
20
40
60
Time (min)
Time (min)
1
1
1
400
200
000
800
600
400
200
0
1
80
50
20
(c)
(d)
TiO2
1
TiO -WO
2
3
1
TiO -WO -Pt
2
3
9
6
3
0
0
0
0
TiO2
TiO -WO3
2
TiO -WO -Pt
2
3
0
20
40
60
80
0
20
40
60
80
Time (min)
Time (min)
Fig. 8: (a) Degradation curves of isopropanol over different samples, (b) reaction
kinetic curves over different samples, (c) CO evolution curves over different samples,
d) concentration curves of acetone intermediate over different samples.
2
(
13

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Meanwhile, isopropanol was also employed as substance to further verify above
4
2,43
photocatalytic results.
isopropanol proceeds via formation of acetone as an intermediate followed by further
decomposition of acetone to the final products CO and H O. Fig.8 shows time
courses of the concentration of products generated from 2ꢀpropanol over TiO2,
TiO ꢀWO and TiO ꢀWO ꢀPt under solar light irradiation. For bare TiO , either
generation rate of acetone intermediate or evolution rate of CO is slower than that of
TiO ꢀWO and TiO ꢀWO ꢀPt samples, suggesting the less efficiency of TiO in
degradation of isopropanol (Fig. 8aꢀd). For TiO ꢀWO and TiO , the higher amount of
acetone intermediate and lower evolution rate of CO at the late stage indicate their
insufficient oxidation ability. For TiO ꢀWO ꢀPt multiꢀheterojunction, it shows a
comparable generation rate of acetone to that of TiO ꢀWO at the initial 40 mins, and
It is well known that the photocatalytic decomposition of
2
2
2
3
2
3
2
2
2
3
2
3
2
2
3
2
2
2
3
2
3
whereas a slower rate to that of TiO
TiO ꢀWO ꢀPt multiꢀheterojunction displays higher evolution rate of CO
TiO and TiO ꢀWO during the whole photocatalytic reaction (Fig. 8c). In this regard,
2 3
ꢀWO at the last 40 mins (Fig. 8d). Importantly,
2
3
2
than that of
2
2
3
it can be deduced that TiO
oxidation of isopropanol to acetone and further mineralization of acetone to CO
Therefore, in combination with the photocatalytic results of degradation of
acetaldehyde and isopropanol, it is safe to conclude that TiO ꢀWO ꢀPt
multiꢀheterojunction sample displays superior photocatalytic activity to that of pure
TiO and TiO ꢀWO
Mechanism for the enhancement of photocatalytic activity of the TiO ꢀWO ꢀPt
2 3
ꢀWO ꢀPt multiꢀheterojunction is highly active for both
2
.
2
3
2
2
3
.
2
3
can be explained by Scheme 1. Since the conduction band edge of WO
3
is lower than
that of TiO , photoꢀgenerated electrons can be transferred to and accumulated in the
2
conduction band of WO . Furthermore, owing to the highꢀcrystallinity of WO
3
3
3 2 3
nanosheet, high quality of WO ꢀTiO and WO ꢀPt interface is obtained and
recombination of photoꢀgenerated electrons and holes induced by defects can be
44
avoided. Accumulated electrons then quickly transfer to the surface of platinum
nanoparticles and hence separation of electrons and holes can be promoted. Besides
that, photoꢀgenerated electrons injected into Pt nanoparticles act as electron pools to
14

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DOI: 10.1039/C5NJ02981C
initiate multiꢀelectron reductions of the oxygen molecules, which further strengthen
the photooxidation ability of organic substances.
Scheme 1: Mechanism for the enhancement of photocatalytic activity of the
TiO ꢀWO ꢀPt multiꢀheterojunction.
2
3
4
. Conclusions
TiO
prepared via a facile ultrasonic spray pyrolysis method. Multiꢀheterojunction
assembled via preloading Pt nanoparticles on highly crystallized WO nanosheets is
found more active than that via nonselective deposition of Pt nanoparticles. The
photocatalyst activity of TiO ꢀWO ꢀPt multiꢀheterojunction (0.04 wt% of Pt) is 2.86
2 3
ꢀWO ꢀPt multiple heterostructure porous microspheres was successfully
3
2
3
times higher than TiO ꢀPt heterostructure composite with the same loading amount of
2
Pt. Furthermore, the improvement of photocatalytic performance can be confirmed by
2
1
.66 times and 1.52 times higher in acetaldehyde degradation, and also 1.8 times and
.03 times higher in isopropanol degradation, than that of TiO and TiO ꢀWO
2
2
3
2 3
composite, respectively. The asꢀassembled porous TiO ꢀWO ꢀPt microspheres
multiꢀheterojunction photocatalyst has a good application prospect on gasꢀphase
photocatalytic degradation system. The method of selection of highly crystallized
WO nanosheets providing brilliant availability to noble metal nanoparticles for
3
separation of photoexcited electronꢀhole is a good inspiration for synthesis of other
multiple heterostructure photocatalyst.
15

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Acknowledgement:
This work was supported by the Natural Science Foundation of China (Grant Nos.
1072032, 91233204, 51372036, and 51102001), the Key Project of Chinese Ministry
5
of Education (No 113020A), the National Basic Research Program (2012CB933703),
the 111 project (No. B13013), and the International Science & Technology
Cooperation Program of China (2013DFG50150), China Postdoctoral Science
Foundation
Key Laboratory for UVꢀEmitting Materials and Technology of Ministry of Education
130026505).
(2014M551156),
Open
Project
of
(
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Graphic Abstract
2 3
In spite of the less amount of Pt (0.04 wt%), the activity of TiO -WO -Pt multi-heterojunction is
still 2.66 times and 1.52 times higher in acetaldehyde degradation than that of TiO
2 2 3
and TiO -WO
composite, respectively.