Fabrication of WO2.72/RGO nano-composites for enhanced photocatalysis

Article abstract of DOI:10.1039/c6ra26416f

WO_2.72 nanoparticles with sizes below 10 nm have been synthesized via controllable alcoholysis followed by thermal treatment. Through an in situ loading process, homogeneous composites containing WO_2.72 nanoparticles and reduced grap

Full text of DOI:10.1039/c6ra26416f

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Fabrication of WO2.72/RGO nano-composites for
enhanced photocatalysis
Cite this: RSC Adv., 2017, 7, 2606
ab
Bo Ma,a Erwei Huang,a Guangjun Wu,a Weili Dai,a Naijia Guanab and Landong Li
*
WO2.72 nanoparticles with sizes below 10 nm have been synthesized via controllable alcoholysis followed by
thermal treatment. Through an in situ loading process, homogeneous composites containing WO2.72
nanoparticles and reduced graphene oxide, i.e. WO2.72/RGO, can be fabricated. The physico-chemical
properties of the as-prepared WO2.72 and WO2.72/RGO composites are investigated by means of XRD,
Raman, XPS, TGA, SEM, TEM and UV-vis spectroscopy. The RGO support, as a very good electron
conductor, can efficiently transfer the photo-generated electrons formed on WO2.72 and suppress their
recombination. As a result, WO2.72/RGO nano-composites exhibit remarkable photocatalytic activity in
both the oxygen evolution from water splitting and selective oxidation of benzyl alcohol, several times
higher than pristine WO2.72. The better light harvesting properties of WO2.72/RGO nano-composites than
WO2.72 nanoparticles can be directly demonstrated by photocurrent response analysis.
Received 7th November 2016
Accepted 16th December 2016
DOI: 10.1039/c6ra26416f
www.rsc.org/advances
knowledge, few researches are focused on WO2.72 nanoparticles,
especially in the sizes below 10 nanometers.
Introduction
Graphene, a at monolayer of sp2 hybridized carbon atoms
Since Fujishima and Honda discovered photocatalytic water
splitting on a TiO2 electrode in the 1970s,1 semiconductor
photocatalysis has received a great deal of attention due to its
promising applications in solar energy harvesting, e.g. photo-
catalytic water splitting2–5 and photocatalytic reduction of
carbon dioxide.6–10 Tungsten sub-oxides (WO3ꢀx) with a band
gap of 2.4–3.0 eV are recognized as important n-type semi-
conductors, which have been successfully applied as gas
sensors, electrochromic devices and photocatalysts.11,12 Among
all the tungsten sub-oxides, monoclinic WO2.72, also expressed
as W18O49, has received considerable attention due to its
distinctive oxygen defect structure and intense near-infrared
photo-absorption.13–16
For semiconductors, morphology control is known as an
effective strategy to modifying their photocatalytic proper-
ties.17,18 Till now, WO2.72 samples with various morphologies,
e.g. nanowires, nanorods, nanosheets and hierarchical struc-
tures, have been successfully synthesized and applied as robust
semiconductor photocatalysts.14–16,19,20 Compared with bulk
materials, nanoparticles generally possess higher specic
surface areas and stronger adsorption ability toward reactants,
both of which are good for photocatalysis. Thus, it is important
to nd a simple process to obtain semiconductor nanoparticles
for efficient photocatalysis.21,22 However, to the best of our
arranged in a two-dimensional honeycomb lattice, with a large
specic surface area, exible structure, high intrinsic electron
mobility, and good thermal conductivities,23–25 has triggered
great research interests since graphene sheets were rst sepa-
rated from graphite in 2004.26 Graphene oxide (GO) is known as
a precursor for graphene preparation and it can be produced by
the strong oxidation/intercalation of graphite, which introduces
various oxide-containing functional groups on the carbon basal
plane and edges.27 Consequently, GO can be dispersed in water
on the molecular scale.28 Based on these advantages of GO, the
efforts of utilizing GO or reduced graphene oxide (RGO) as
support material have been devoted to the fabrication of effi-
cient photocatalysts.29 Especially, composites of RGO and stoi-
chiometric tungsten oxide have been recently prepared and
applied in different elds.30–36
In this study, sub-10 nm WO2.72 nanoparticles are synthe-
sized via controllable alcoholysis followed by thermal treat-
ment. Aer compositing with RGO, the homogeneous WO2.72
/
RGO exhibits greatly enhanced photocatalytic activity than
pristine WO2.72 in both oxygen evolution from water splitting
and alcohol selective oxidation, due to the synergetic effects
between WO2.72 and RGO.
Experimental
Preparation of WO2.72 and WO2.72/RGO composites
aSchool of Materials Science and Engineering, National Institute for Advanced
Materials, Nankai University, Tianjin, 300350, P. R. China
All the chemical reagents employed in this study are of analyt-
ical grade from Alfa Aesar and used as received without further
purication. Graphene oxide (GO) was synthesized by the
bKey Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin,
300071, P. R. China. E-mail: lild@nankai.edu.cn
2606 | RSC Adv., 2017, 7, 2606–2614
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
oxidation of natural graphite powder using Hummers method type xenon lamp (PLS-SXE, wavelength: 300–780 nm). In
with some modications.37,38 (NH4)xWO3ꢀy was synthesized at a typical experiment, 0.1 g photocatalyst and 0.2 g AgNO3 (as
the ambient atmosphere by a solvent reuxing method. In sacricing reagent) were sequentially added into 100 mL
a typical procedure, 1 g of WCl6 was dissolved in 31.0 mL ultra-pure water in the reactor under magnetic vigorous
ethanol to get the W6+ concentration of ꢁ0.08 M, then 5 mL stirring to obtain a well-mixed suspension. Aer being
ammonia solution (25 wt%) was added under vigorous stirring. evacuated for 30 min, the reactor cell was irradiated by the Xe
The mixture was stirred in air for 15 min, followed by reuxing lamp. During the experiment, the temperature of the reactor
at 373 K for 3.5 h in a ask. 100 mg GO was dispersed in 100 mL system was kept at 298 K by owing temperature-controlled
ethanol and mixed with the calculated amount of (NH4)xWO3ꢀy cooling water. The amount of O2 evolved from water split-
and stirred for 4 h at room temperature. Aer solvent being ting was determined by an on-line gas chromatograph (Var-
evaporated, the brown mixture was dried at 353 K overnight. ian CP-3800, equipped with a thermal conductivity detector
The obtained solid was further pyrolyzed under argon atmo- and a 4 m 5A molecular sieve column; nitrogen as carrier
sphere at the temperature of 723 K for 2 h to derive the nal gas).
product WO2.72/RGO-x, where
percentage of RGO in the composite.
x
represents the weight
The photocatalytic oxidation of benzyl alcohol was per-
formed in a top-irradiation-type double-walled quartz cell
For reference, pure WO2.72 powders were obtained via the cooled by water with a Xe lamp (200 W, wavelength: 320–780
same procedure but without adding GO solution and pure WO3 nm) as the light source. In each experiment, the catalyst of 0.1 g
powders were pyrolyzed under air atmosphere instead of argon. was dispersed in the 60 mL benzotriuoride solvent containing
25 mmol benzyl alcohol in the quartz cell under stirring. The
suspension was then irradiated with oxygen bubbled in at 20
mL minꢀ1. The organic products were analyzed by GC (Shi-
madzu GC-2010 Plus) and GC-MS (Shimadzu GCMS-QP2010
SE). The apparent quantum yield of benzyl alcohol oxidation
was measured under the same photocatalytic reaction condi-
tions but using 350 nm monochromatic light obtained by band-
pass lters. The light intensity is measured to be 3.2 mW cmꢀ2
with an area of 7.1 cm2. The apparent quantum yield (QY) of
photocatalytic benzyl alcohol oxidation was calculated accord-
ing to the follow equations:
Sample characterization
The X-ray diffraction (XRD) patterns of samples were recorded
using a Bruker D8 advance powder diffractometer with Cu-Ka
radiation (l ¼ 0.1542 nm) at a scanning rate of 5ꢂ minꢀ1
.
The specic surface areas of samples were determined
through N2 adsorption/desorption isotherms at 77 K collected
on a Quantachrome iQ-MP gas adsorption analyzer.
The thermogravimetric analysis (TGA) of samples was per-
formed on a TA SDT Q600 analyzer in air at a heating rate of 10
K minꢀ1
.
nreacted
electrons
The Raman spectra of samples were recorded on a Renishaw
InVia Raman spectrometer with the green line of an Ar-ion laser
(514.53 nm) in micro-Raman conguration.
Diffuse reectance ultraviolet-visible (UV-Vis) spectra of
samples were recorded against BaSO4 on a Varian Cary 300 UV-
Vis spectrophotometer.
Scanning electron microscopy (SEM) images of samples were
taken on a eld emission scanning electron microscope (FE-
SEM, Hitachi S-4800). Transmission electron microscopy
(TEM) images of samples were taken on a FEI Tecnai G2 F20
electron microscope at an acceleration voltage of 200 kV.
X-ray photoelectron spectra (XPS) and ultraviolet photoelec-
tron spectra (UPS) of samples were recorded on a Thermo
Scientic ESCALAB 250Xi spectrometer, which were employed
with monochromatic exciting Al-Ka X-ray (1486.6 eV) and He I
(21.2 eV), respectively.
QY ½%ꢃ ¼
ꢄ 100%
nincident
photons
nbenzyl
consumed ꢄ 2
photons
alcohol
¼
ꢄ 100%
nincident
The transformation of one benzyl alcohol molecule
consumes two photoelectrons:
photocatalyst
ƒƒƒƒƒƒƒƒ!
hn
hþ þ eꢀ
2C6H5CH2–OH + 4h+ / 2C6H5–CHO + 4H+
4H+ + 4eꢀ + O2 / 2H2O
Photoluminescence (PL) spectra were recorded on a Spex
FL201 uorescence spectrophotometer. The samples were dry-
pressed into self-supporting wafers and then illuminated
by 325 nm He–Cd laser as excitation source at ambient
temperature.
Electrochemical measurements
The electrochemical measurements were measured using
a three-electrode cell electrochemical workstation (IVIUM
CompactStat) in 0.5 M Na2SO4 solution. The saturated Ag/AgCl
and platinum foil (1 ꢄ 2 cm2) were used as the reference elec-
trode and the counter electrode. The sample of 1 mg was
dispersed in 1 mL anhydrous ethanol and then evenly grinded
Photocatalytic evaluations
Photocatalytic water oxidation reactions were performed in to slurry. The slurry was spread onto ITO glass and the exposed
a top-irradiation-type Pyrex cell connected to a closed gas area was kept at 0.15 cm2. The prepared ITO/samples were dried
circulation and evacuation system with external light irradi- overnight under ambient condition and then used as the
ation. The light source was a 300 W high pressure integrated working electrode for electrochemical measurements.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 2606–2614 | 2607

View Article Online
RSC Advances
Paper
Results and discussion
The XRD patterns of RGO, WO2.72 and WO2.72/RGO-5 samples
are shown in Fig. 1. All of the diffraction peaks of WO2.72 sample
can be indexed to JCPDS 36-101, indicating the formation of
39
pure phase of monoclinic WO2.72
. For RGO sample, the weak
and broad diffraction peak at ꢁ25ꢂ corresponds to the (002)
peak of graphene is observed.40 For WO2.72/RGO-5, the diffrac-
tion peaks corresponding to monoclinic WO2.72 are observed,
while the presence of the RGO support (5% weight loading)
signicantly decreases the intensity of diffraction peaks. The
average crystallite size of WO2.72 in pure WO2.72 and WO2.72
/
RGO composite are calculated to be 9 and 7 nm by using the
Scherrer equation. These results indicate that the growth of
WO2.72 nanoparticles on the RGO sheets could be restricted
to some extent, in consistence with the TEM observations
(vide infra).
Raman spectroscopy is an effective means to characterize
carbon materials with usual characteristics of G and D bands.
The Raman spectra of the GO, RGO and WO2.72/RGO are shown
in Fig. 2. In the case of GO, the G-band peak at around 1595
cmꢀ1 is attributed to a well ordered sp2 carbon-type structure
and the D-band at around 1345 cmꢀ1 is attributed to the
structural defects of the hexagonal graphitic structure.41,42 The
intensity ratio D-band to G-band, i.e. ID/IG, is calculated to be
1.0, indicating the existence of abundant surface defects in GO.
Upon annealing to RGO, the ID/IG decreases signicantly to 0.8
due to the elimination of some structural defects. While for
WO2.72/RGO-5, a lower ID/IG of 0.7 is obtained, revealing that
some graphitic structure defects in RGO are further eliminated
through compositing with WO2.72. Moreover, a noticeable shi
of D-band from 1345 to 1360 cmꢀ1 is observed due to the
interaction between WO2.72 nanoparticles and defects in RGO.
XPS analysis is performed to reveal the chemical composi-
Fig. 2 Raman spectra of GO, RGO and WO2.72/RGO.
RGO. In the survey spectra of the WO2.72/RGO, the existence
of W, O and C could be clearly identied (Fig. 3a). In the high-
resolution C 1s XPS of the WO2.72/RGO (Fig. 3b), binding energy
values at 284.6, 285.7, 287.1 and 289.0 eV are observed. The
binding energy value at 284.6 eV is assigned to the graphitic
carbon (C–C bonds), while the another three binding energy
values at 285.7, 287.1 and 289.0 eV are assigned to C–O, C]O,
and O–C]O bonds, respectively.43 In the O 1s XPS (Fig. 3c), two
binding energy values are observed, i.e. 530.6 eV due to W–O
bonds in WO2.72 and 531.9 eV due to H–O or C–O bonds from
residual oxygen-containing groups. The C 1s and O 1s XPS
indicate the existence of a small amount of oxygen-containing
tion and the element existing states in the as-prepared WO2.72
/
Fig. 1 XRD of RGO, WO2.72 and WO2.72/RGO.
2608 | RSC Adv., 2017, 7, 2606–2614
Fig. 3 Survey (a), C 1s (b), O 1s (c) and W 4f (d) XPS of WO2.72/RGO-5.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
carbon groups in the WO2.72/RGO.44 The core-level W 4f XPS
(Fig. 3d) show the presence of two sets of peaks at 35.7/37.8 and
35.2/37.2 eV, respectively. The strong peaks with binding energy
values of 35.7 and 37.8 eV are assigned to W 4f7/2 and W 4f5/2 of
the characteristic W6+ oxidation state, while the small peaks at
35.2 and 37.2 eV can be assigned to W 4f7/2 and W 4f5/2 of W5+
oxidation state in tungsten oxides.45
Fig. 4 shows the TGA curves WO2.72/RGO composites with
different RGO loadings. The weight loss observed from 300 to
473 K from WO2.72/RGO is mainly due to the loss of water. While
the largest weight loss occurs at temperature from 673 to 873 K
for all WO2.72/RGO samples should be due to the oxidative
destruction of the carbon skeleton, i.e. carbon-based materials
oxidized to CO2 in owing oxygen. According to the weight loss
of WO2.72/RGO composites, the weight loading of RGO in the
Fig. 5 SEM images of WO2.72 (a) and WO2.72/RGO (b) samples.
composites is determined to be 3.1, 4.7 and 6.5% for WO2.72
RGO-3, WO2.72/RGO-5 and WO2.72/RGO-7, respectively.
/
The SEM images of WO2.72 and WO2.72/RGO-5 are shown in
Fig. 5. The pure WO2.72 appears as aggregation of nanoparticles
with size of ꢁ10 nm (Fig. 5a). The uniform nanoparticles ob-
tained demonstrate that thermal decomposition of (NH4)x-
WO3ꢀy under inert atmosphere is an effective strategy to
prepare WO2.72. For WO2.72/RGO composite, the curled and
corrugated layered structure of RGO nanosheets with tiny
nanoparticles spread over the surface could be identied
(Fig. 5b). These tiny nanoparticles, i.e. WO2.72, are homoge-
neously distributed with sizes below 10 nm, slightly larger than
that obtained from XRD analysis (Fig. 1). Besides, the close
contact between WO2.72 nanoparticles and RGO support can be
illustrated, which is very important for the photocatalytic
performance of WO2.72/RGO composite (vide infra).
The morphology and composition of the WO2.72/RGO-5
composite are further characterized by TEM and the represen-
tative images as shown in Fig. 6. In the overview TEM image
(Fig. 6a), uniform WO2.72 nanoparticles, with size of 3–10 nm,
are evenly deposited on RGO nanosheets. The average particle
size is calculated to be 6.7 nm for supported WO2.72 (Fig. 6b), in
Fig. 6 TEM image of WO2.72/RGO (a) and WO2.72 particle size distri-
bution (b) on graphene; HR-TEM image of WO2.72/RGO with SAED
patterns shown inset (c) and the enlarged view of lattice fringe (d) in
a single WO2.72 nanoparticle.
good agreement with XRD analysis (Fig. 1). In the preparation
process, the hydrolysis rate of tungsten salt could be well
controlled by the existence of ammonia, which releases
hydroxyl ions slowly in ethanol with increasing temperature.
Since ammonia molecules tend to strongly chelate with tung-
sten ions, they can work as capping agents to hinder the particle
growth. In the HR-TEM image of WO2.72/RGO-5, WO2.72 parti-
cles of 5–8 nm could be clearly identied (Fig. 6c), and the
selected area electron diffraction patterns (Fig. 6c, inset) reveal
the good crystallinity of WO2.72 particles. For a single WO2.72
particle on RGO (Fig. 6d), clear lattice-fringe spacing of 0.38 nm
is measured, corresponding to the (010) planes of monoclinic
WO2.72 in WO2.72/RGO composite. The TEM analysis results
provide us with direct evidence on the formation of sub-10 nm
WO2.72 nanoparticles on the surface of RGO sheets.
Fig. 7 shows the N2 adsorption–desorption isotherms of the
samples WO2.72 and WO2.72/RGO-5. Both of the WO2.72 and
WO2.72/RGO samples show similar adsorption isotherms of type
Fig. 4 TGA curves of WO2.72/RGO with different RGO loadings.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 2606–2614 | 2609

View Article Online
RSC Advances
Paper
such circumstances, it is impossible to calculate the band gap of
WO2.72 by using Kubelka–Munk equation, which is commonly
employed in the literature.48–50 While for WO2.72/RGO
composite, light absorption in the entire UV-vis region could be
observed, similar to the case of WO2.72. It can be expected from
UV-vis spectra that WO2.72 and WO2.72/RGO composite might be
good photocatalysts under UV-vis light.
For further information on the band edge positions of
WO2.72 and WO2.72/RGO composite, UPS and Mott–Schottky
plots were performed, as shown Fig. 9. The Mott–Schottky plots
of samples show positive slopes corresponding to n-type semi-
conductors, and, under such circumstances, the conduction
band potential is very close to the at-band potential, which can
be obtained from the x intercept of the linear region in Mott–
Schottky plots. The calculated conduction band bottoms of
WO2.72 of WO2.72/RGO-5 are 0.22 and 0.04 eV, respectively,
versus normal hydrogen electrode (Fig. 9; le chart). From UPS,
the valence band tops of WO2.72 and WO2.72/RGO-5 are deter-
mined to be 2.83 and 2.92 eV, respectively, versus normal
hydrogen electrode (Fig. 9; right chart). On the basis of the
results from Mott–Schottky and UPS analysis, the band gaps of
WO2.72 and WO2.72/RGO-5 are calculated to be 2.61 and 2.88 eV,
respectively. Obviously, compositing with RGO results in
a noticeable change in the band structure of WO2.72. Consid-
ering its band gap and band edge positions, the as-fabricated
WO2.72/RGO-5 should be promising photocatalyst in photo-
catalytic oxidation reactions under UV light.
Fig. 7 Nitrogen adsorption–desorption isotherms WO2.72 and WO2.72
RGO-5.
/
IV with a hysteresis loop. The values of the BET specic surface
areas of WO2.72 and WO2.72/RGO sample are calculated to be 58
and 76 m2 gꢀ1, respectively. Obviously, the presence of RGO
distinctly increases the surface area of the composite due to its
intrinsic high surface area and its support effects on the
dispersion of WO2.72 area.
The UV-vis spectra of WO3, WO2.72, RGO and WO2.72/RGO-5
are shown in Fig. 8. Stoichiometric WO3 appears as dark
yellow powder and its absorption edge occurs at ꢁ460 nm
originated from the band-to-band transitions (band gap of
2.7 eV).3 Due to the existence of a large number of oxygen
vacancies,46,47 the sub-stoichiometric WO2.72 appears as black
powder and it shows a broad absorption in the entire UV-vis
region (200–700 nm), which should be due to the formation
of new discrete energy bands below the conduction band. Under
The light harvesting properties of WO2.72 and WO2.72/RGO
composites are investigated in two typical photocatalytic reac-
tions, i.e. oxygen evolution from water splitting and selective
oxidation of benzyl alcohol.
As shown in Fig. 10, with AgNO3 as sacricial reagent, oxygen
evolution from water splitting could be achieved with tungsten
oxides semiconductors under irradiation. The sub-10 nm
tungsten oxides employed in this study should be more active
than their bulk counterparts because smaller particle size
generally means easier transfer of charge-carrier and higher
photocatalytic efficiency. With similar particle size, the sub-
stoichiometric WO2.72 (oxygen evolution rate: 151 mmol g hꢀ1
)
is more active than stoichiometric WO3 (oxygen evolution rate:
98 mmol g hꢀ1) under UV-vis light since the recombination of
Fig. 8 UV-vis spectra of RGO, WO3, WO2.72 and WO2.72/RGO-5.
2610 | RSC Adv., 2017, 7, 2606–2614
Fig. 9 Mott–Schottky plots and UPS of WO2.72/RGO-5 sample.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Fig. 10 Photocatalytic O2 evolution from water splitting over different
samples under UV-vis light. Reaction conditions: 0.1 g photocatalyst;
0.2 g AgNO3 in 100 mL pure water; T ¼ 298 K.
photogenerated electron–hole pairs can be effectively sup-
pressed upon the introduction of oxygen vacancies.35 RGO
exhibits neglectable activity in the photocatalytic oxygen
evolution, however, it can signicantly promote the photo-
catalytic oxygen evolution when compositing with WO2.72. The
promotion effects of RGO are very much dependent on its
weight loading and a volcano-shaped curve is observed. Typi-
cally, 5% weight load of RGO in WO2.72/RGO is optimized and
an oxygen evolution of 420 mmol g hꢀ1 is obtained for WO2.72
RGO-5, which is about 2.8 times higher than WO2.72
With suitable incident photons, electron–hole pairs can be
created in WO2.72, followed by the separation and migration of
photo-generated electrons and holes to the solid–solution
interface. The photo-generated holes can oxidize water to
oxygen at the interface, while the photo-generated electron
/
Fig. 11 Photocatalytic oxidation of benzyl alcohol to benzaldehyde
over different samples under UV-vis light and the recycling test of
WO2.72/RGO-5 sample. Reaction conditions: 0.1 g photocatalyst;
25 mmol benzyl alcohol in 60 mL benzotrifluoride as solvent; T ¼
298 K.
.
cannot reduce water to hydrogen owing to the insufficient
Fig. 11. The selective photocatalytic oxidation of benzyl alcohol
can be achieved on all WO2.72-based catalysts with 91% selec-
tivity toward benzaldehyde (small quantities pf benzoic acid
and benzyl benzoate detected as dominating by-products,
Table 1). Similar to those observed in the photocatalytic
oxygen evolution (Fig. 10), the sub-stoichiometric WO2.72 is
exhibits slightly higher photocatalytic activity in benzyl alcohol
selective oxidation than stoichiometric WO3, and the presence
of RGO support can enhance the photocatalytic activity of
WO2.72 to a great extent. Typically, WO2.72/RGO-5 exhibits the
highest photocatalytic activity with a benzyl alcohol conversion
51
positive conduction band position of WO2.72
.
With the pres-
ence of Ag+ in the reaction system, the photo-generated elec-
trons can reduce Ag+ to Ag at the interface, which close the cycle
and initiate the photocatalytic reaction, i.e. oxygen evolution
from water splitting with Ag+ ions as sacricial reagents. RGO,
similar to graphene, is known as a very good electron conductor.
When compositing with WO2.72, RGO can efficiently transfer the
photo-generated electrons away from the WO2.72 to react with
electron acceptor Ag+, leaving the photo-generated holes on
WO2.72 to oxidize water to oxygen. In such a way, the presence of
RGO support can efficiently suppress the recombination of
photo-generated electrons and holes, and, therefore, promote
the photocatalytic activity.24 Increasing the loading of RGO in
WO2.72/RGO should facilitate the transfer of photo-generated
electrons, however, it would also reduce the light absorbance
due to the light shielding effect of RGO.52,53 As a balance of these
two factors, an optimized RGO loading of 5% is obtained for
efficient oxygen evolution.
ꢀ1
rate of 10.8 mmol gcat hꢀ1, four times higher than WO2.72
ꢀ1
(2.7 mmol gcat hꢀ1). The apparent QY for benzyl alcohol
photocatalytic oxidation over WO2.72/RGO-5 is calculated to be
as high as 68.9% under monochromatic UV light at 350 nm. In
addition to its remarkable photocatalytic activity, WO2.72/RGO-5
exhibits good stability in the oxidation of benzyl alcohol. As
shown in Fig. 11, no obvious activity loss can be observed in ve
recycling tests, demonstrating its great potential in practical
application.
The photocatalytic performance of the WO3, WO2.72 and
WO2.72/RGO composites are further evaluated in the photo-
catalytic oxidation benzyl alcohol, and the results are showed in
PL spectroscopy is very useful to disclose the efficiency of charge
carrier trapping, immigration and transfer in semiconductor
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 2606–2614 | 2611

View Article Online
RSC Advances
Paper
Table 1 Photocatalytic oxidation of benzyl alcohol over WO2.72/RGO compositesa
Product selectivity (%)
Photocatalyst
Benzaldehyde
Benzoic acid
Benzyl benzoate
C balance (%)
WO3
WO2.72
79
91
94
91
92
93
92
6
4
2
5
4
3
3
15
5
4
4
4
91
94
93
95
92
91
92
WO2.72/RGO-1
WO2.72/RGO-3
WO2.72/RGO-5
WO2.72/RGO-7
WO2.72/RGO-9
4
5
a
Reaction conditions: 0.1 g photocatalyst, 25 mmol benzyl alcohol in 60 mL benzotriuoride as solvent, T ¼ 298 K, reaction time ¼ 8 h.
materials. Generally, the PL emissions are originated from the
radiative recombination of photogenerated electrons and holes.
Room temperature emission PL spectra of WO2.72 and WO2.72/RGO
samples are measured and shown in Fig. 12. For all samples, PL
signals at the wavelength of 400–700 nm are observed, due to the
surface and bulk irradiative recombination.54,55 It is observed that
compositing with RGO signicantly decreases the PL intensity of
WO2.72, conrming the role of RGO in promoting the separation of
photogenerated electron–hole pairs on WO2.72. For WO2.72/RGO
composites with different RGO loadings, the normalized PL
intensity are relevant with the photocatalytic activity (Fig. 10 and
11). That is, the lower PL intensity, the higher photocatalytic
intensity.
The light harvesting properties of WO2.72 and WO2.72/RGO-
5 are nally examined by means of photocurrent responses,
and the results are shown in Fig. 13. Pure WO2.72 show
signicant current responses at bias of 0.2–1.0 eV (vs. Ag/AgCl)
under UV-vis light. WO2.72/RGO-5 shows signicant current
responses at bias of ꢀ0.1–1.0 eV, which are several times
higher than WO2.72. The presence of 5% RGO support can
greatly improve the separation of photo-generated electrons
and holes, and accordingly lead to enhanced photocurrent
Fig. 13 Photocurrent responses of WO2.72 and WO2.72/RGO-5 under
UV-vis light.
responses (Fig. 13) and photocatalytic activity in different
reactions (Fig. 10 and 11).
Conclusions
In this study, sub-10 nm WO2.72 particles have been prepared
via a controllable alcoholysis route followed by thermal treat-
ment under inert atmosphere. With the in situ addition of GO
solution, WO2.72/RGO nano-composites containing WO2.72
nanoparticles and RGO nanosheets in close contact can be
successfully fabricated. This is a very simple and reproducible
strategy that can be extended to other nano-composite systems.
The as-prepared WO2.72/RGO nano-composites with
different RGO loadings are well characterized by a series of
techniques and applied as photocatalysts in two typical reac-
tions, i.e. oxygen evolution from water splitting and selective
oxidation of benzyl alcohol. With the existence of RGO support
as a good electron conductor, the photo-generated electrons on
WO2.72 can be transferred to RGO, and, therefore, the recom-
bination of photo-generated electrons and holes can be sup-
pressed to a great extent. Accordingly, WO2.72/RGO nano-
Fig. 12 PL spectra of WO2.72 and WO2.72/RGO composites.
2612 | RSC Adv., 2017, 7, 2606–2614
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
composites exhibit greatly enhanced photocatalytic activity 18 S. Liang, L. Wen, S. Lin, J. Bi, P. Feng, X. Fu and L. Wu,
than pristine WO2.72 in both the oxygen evolution from water Angew. Chem., 2014, 126, 2995–2999.
splitting and selective oxidation of benzyl alcohol. The promo- 19 G. Xi, J. Ye, Q. Ma, N. Su, H. Bai and C. Wang, J. Am. Chem.
tion effects of RGO support are dependent on RGO loadings and
the optimized RGO loading is determined to be 5% in WO2.72
Soc., 2012, 134, 6508–6511.
20 H. Bai, N. Su, W. Li, X. Zhang, Y. Yan, P. Li, S. Ouyang, J. Ye
and G. Xi, J. Mater. Chem. A, 2013, 1, 6125–6129.
/
RGO nano-composite. For WO2.72/RGO with RGO weight
loading of 5%, oxygen evolution rate of 420 mmol g hꢀ1 and 21 P. Wu, W. Zhu, A. Wei, B. Dai, Y. Chao, C. Li, H. Li and S. Dai,
ꢀ1
benzyl alcohol oxidation rate of 10.8 mmol gcat hꢀ1 can be
Chem.–Eur. J., 2015, 21, 15421–15427.
¨
obtained under UV-vis light, namely 2.8 and 4.0 times higher 22 F. Bottger-Hiller, R. Lungwitz, A. Seifert, M. Hietschold,
than pristine WO2.72 nanoparticles, respectively. The concept of
using RGO as a support with good electron conducting prop-
M. Schlesinger, M. Mehring and S. Spange, Angew. Chem.,
Int. Ed., 2009, 48, 8878–8881.
erties should be a general strategy to the fabrication of efficient 23 Y. Zhang, Y.-W. Tan, H. L. Stormer and P. Kim, Nature, 2005,
compositing photocatalysts.
438, 201–204.
24 A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.
25 M. J. Allen, V. C. Tung and R. B. Kaner, Chem. Rev., 2009, 110,
132–145.
Acknowledgements
This work is nancially supported by the National High Tech- 26 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang,
nology Research and Development Program of China
(2015AA033303), the National Natural Science Foundation of
Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov,
Science, 2004, 306, 666–669.
China (21421001 and 21303087), Municipal Natural Science 27 O. C. Compton and S. T. Nguyen, Small, 2010, 6, 711–723.
Foundation of Tianjin (13RCGFGX01124, 13JCQNJC05900 and 28 O. C. Compton, Z. An, K. W. Putz, B. J. Hong, B. G. Hauser,
14JCQNJC05700), and the Ministry of Education of China
(IRT13022).
L. C. Brinson and S. T. Nguyen, Carbon, 2012, 50, 3399–3406.
29 Q. Xiang, J. Yu and M. Jaroniec, Chem. Soc. Rev., 2012, 41,
782–796.
30 B. Chai, J. Li, Q. Xu and K. Dai, Mater. Lett., 2014, 120, 177–
Notes and references
181.
1 A. Fujishima, Nature, 1972, 238, 37–38.
2 A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253–278.
31 J. Qin, M. Cao, N. Li and C. Hu, J. Mater. Chem., 2011, 21,
17167–17174.
3 T. Hisatomi, J. Kubota and K. Domen, Chem. Soc. Rev., 2014, 32 M. Zhou, J. Yan and P. Cui, Mater. Lett., 2012, 89, 258–261.
43, 7520–7535.
33 J. Guo, Y. Li, S. Zhu, Z. Chen, Q. Liu, D. Zhang, W.-J. Moon
4 S. J. A. Moniz, Energy Environ. Sci., 2015, 8, 731–759.
and D.-M. Song, RSC Adv., 2012, 2, 1356–1363.
5 L. Li, J. Yan, T. Wang, Z.-J. Zhao, J. Zhang, J. Gong and 34 X. An, C. Y. Jimmy, Y. Wang, Y. Hu, X. Yu and G. Zhang, J.
N. Guan, Nat. Commun., 2015, 6, 5881.
6 S. Wang and X. Wang, Small, 2015, 11, 3097–3112.
7 S. Wang, W. Yao, J. Lin, Z. Ding and X. Wang, Angew. Chem.,
Int. Ed., 2014, 53, 1034–1038.
8 S. Wang, Z. Ding and X. Wang, Chem. Commun., 2015, 51,
1517–1519.
9 S. Wang, Y. Hou and X. Wang, ACS Appl. Mater. Interfaces,
2015, 7, 4327–4335.
10 S. Wang and X. Wang, Angew. Chem., Int. Ed., 2016, 55, 2308–
2320.
Mater. Chem., 2012, 22, 8525–8531.
35 X. Li, S. Yang, J. Sun, P. He, X. Xu and G. Ding, Carbon, 2014,
78, 38–48.
36 B. Li, X. Shao, T. Liu, L. Shao and B. Zhang, Appl. Catal., B,
2016, 198, 325–333.
37 W. S. Hummers Jr and R. E. Offeman, J. Am. Chem. Soc., 1958,
80, 1339.
38 D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii,
Z. Sun, A. Slesarev, L. B. Alemany, W. Lu and J. M. Tour,
ACS Nano, 2010, 4, 4806–4814.
11 D. Chen, L. Gao, A. Yasumori, K. Kuroda and Y. Sugahara, 39 Z.-F. Huang, J.-J. Zou, L. Pan, S. Wang, X. Zhang and
Small, 2008, 4, 1813–1822.
L. Wang, Appl. Catal., B, 2014, 147, 167–174.
12 J. Yan, T. Wang, G. Wu, W. Dai, N. Guan, L. Li and J. Gong, 40 L. Tang, Y. Wang, Y. Li, H. Feng, J. Lu and J. Li, Adv. Funct.
Adv. Mater., 2015, 27, 1580–1586. Mater., 2009, 19, 2782–2789.
13 Z.-F. Huang, J. Song, L. Pan, F. Lv, Q. Wang, J.-J. Zou, 41 D. K. Padhi and K. Parida, J. Mater. Chem. A, 2014, 2, 10300–
X. Zhang and L. Wang, Chem. Commun., 2014, 50, 10959–
10962.
14 G. Xi, S. Ouyang, P. Li, J. Ye, Q. Ma, N. Su, H. Bai and
C. Wang, Angew. Chem., Int. Ed., 2012, 51, 2395–2399.
15 D. Wang, J. Sun, X. Cao, Y. Zhu, Q. Wang, G. Wang, Y. Han,
100312.
42 D. K. Padhi, K. Parida and S. K. Singh, J. Phys. Chem. C, 2015,
119, 6634–6646.
43 J. Paredes, S. Villar-Rodil, A. Martınez-Alonso and J. Tascon,
´
Langmuir, 2008, 24, 10560–10564.
G. Lu, G. Pang and S. Feng, J. Mater. Chem. A, 2013, 1, 8653– 44 M. Fernandez-Merino, L. Guardia, J. Paredes, S. Villar-Rodil,
8657.
P. Solis-Fernandez, A. Martinez-Alonso and J. Tascon, J. Phys.
Chem. C, 2010, 114, 6426–6432.
45 D. Lu, J. Chen, J. Zhou, S. Deng, N. Xu and J. Xu, J. Raman
Spectrosc., 2007, 38, 176–180.
16 Z.-F. Huang, J. Song, L. Pan, X. Jia, Z. Li, J.-J. Zou, X. Zhang
and L. Wang, Nanoscale, 2014, 6, 8865–8872.
17 F. E. Osterloh, Chem. Soc. Rev., 2013, 42, 2294–2320.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 2606–2614 | 2613

View Article Online
RSC Advances
Paper
46 G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, 51 R. Abe, T. Takata, H. Sugihara and K. Domen, Chem.
R. C. Fitzmorris, C. Wang, J. Z. Zhang and Y. Li, Nano Lett.,
2011, 11, 3026–3033.
47 T. R. Gordon, M. Cargnello, T. Paik, F. Mangolini,
Commun., 2005, 3829–3831.
52 H.-I. Kim, S. Kim, J.-K. Kang and W. Choi, J. Catal., 2014, 309,
49–57.
R. T. Weber, P. Fornasiero and C. B. Murray, J. Am. Chem. 53 K. Chang, Z. Mei, T. Wang, Q. Kang, S. Ouyang and J. Ye, ACS
Soc., 2012, 134, 6751–6761. Nano, 2014, 8, 7078–7087.
48 A. E. Morales, E. S. Mora and U. Pal, Rev. Mex. Fis. S, 2007, 53, 54 W. Feng, G. Wu, L. Li and N. Guan, Green Chem., 2011, 13,
18–22. 3265–3272.
49 D. K. Padhi, K. Parida, G. K. Pradhan and S. K. Singh, Chem. 55 J. Yan, G. Wu, N. Guan and L. Li, Appl. Catal., B, 2014, 152–
Eng. J., 2014, 255, 78–88.
153, 280–288.
50 D. K. Padhi, K. Parida and S. K. Singh, Inorg. Chem. Front.,
2016, 3, 1582–1596.
2614 | RSC Adv., 2017, 7, 2606–2614
This journal is © The Royal Society of Chemistry 2017