Tunable and sustainable photocatalytic activity of photochromic Y-WO3under visible light irradiation

Article abstract of DOI:10.1039/d0ra09714d

Although photochromic and photocatalytic performance are the most significant features of WO3, the effects of photochromism on photocatalytic activities have not been investigated further. Herein, a novel gear-shaped WO3, with high coloration efficiency, fast reversibility, and remarkable photocatalytic performance was successfully prepared via a facile hydrothermal method. The influence of photochromic effects on its photocatalytic properties was evaluated under visible light irradiation. The results showed that the yellow WO3 sample exhibited higher photocatalytic efficiencies toward tetracycline hydrochloride (TCH), oxytetracycline (OTC), rhodamine B (RhB), and ciprofloxacin (CIP) (94.3%, 87.9%, 76%, and 68.6%, respectively, in 60 min). Further research found that the redox conversion between W6+ and W5+ played a key role in separating e-/h+ pairs. Importantly, the rapid and reversible conversion between W6+ and W5+ could be realized through light radiation or H2O2 treatment. Therefore, the gear-shaped WO3 possessed tunable and sustainable photocatalytic properties and maintained a high level of activity after recycling ten times under visible light irradiation. This work provides new insights into practical WO3 applications for environmental remediation based on photochromic regulation. This journal is

Full text of DOI:10.1039/d0ra09714d

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Tunable and sustainable photocatalytic activity of
photochromic Y-WO under visible light
3
Cite this: RSC Adv., 2021, 11, 1147
irradiation†
a
a
a
a
a
Qiansheng Li, Hui Zhang, Yunhui Yan, Zhijun Yang, Yingling Wang,*
bc
ab
Guoguang Liu
and Tianjun Ni
*
Although photochromic and photocatalytic performance are the most significant features of WO
effects of photochromism on photocatalytic activities have not been investigated further. Herein, a novel
gear-shaped WO , with high coloration efficiency, fast reversibility, and remarkable photocatalytic
performance was successfully prepared via facile hydrothermal method. The influence of
photochromic effects on its photocatalytic properties was evaluated under visible light irradiation. The
results showed that the yellow WO sample exhibited higher photocatalytic efficiencies toward
3
, the
3
a
3
tetracycline hydrochloride (TCH), oxytetracycline (OTC), rhodamine B (RhB), and ciprofloxacin (CIP)
(
94.3%, 87.9%, 76%, and 68.6%, respectively, in 60 min). Further research found that the redox
6+
5+
ꢀ
+
conversion between W and W played a key role in separating e /h pairs. Importantly, the rapid and
6
+
5+
reversible conversion between W and W could be realized through light radiation or H
Therefore, the gear-shaped WO possessed tunable and sustainable photocatalytic properties and
maintained a high level of activity after recycling ten times under visible light irradiation. This work
2 2
O treatment.
Received 16th November 2020
Accepted 16th December 2020
3
DOI: 10.1039/d0ra09714d
provides new insights into practical WO
photochromic regulation.
3
applications for environmental remediation based on
rsc.li/rsc-advances
9
,10
applied in photocatalysis.
a slow self-bleaching rate (>24 h), low solar light utilization, and
Photochromism can be dened as the reversible transformation coloration only under ultraviolet rays have limited the applica-
However, low solar modulation,
1
. Introduction
11
accompanying electron-transfer or redox reactions between tion of WO
different existing states, which have different optical properties
To resolve these issues, several methods, including coupling
such as absorption, luminescence and refractive index under with inorganic semiconductor materials or organic compounds,
3
in the photochromic and photocatalytic domains.
1,2
specic light irradiation. Photochromic materials can be doping with noble metals and the introduction of oxygen
divided into organic and inorganic materials. Compared to vacancies have been investigated.
3–5
12–22
However, these tradi-
organic photochromic materials, inorganic semiconductor tional methods involving the selection of doping elements,
4
materials possess good structure stability and low cost. Among composite proportions, and exacting conditions, increase
the numerous semiconductor materials like WO , MnO , TiO , design difficulties and costs. At present, it remains a challenge
3
3
2
3
ZnO, etc., WO has been widely used as a photochromic mate- to develop a single WO component with efficient, controllable,
3
rial due to its good stability, non-toxicity, low cost, good and reusable photochromic and photocatalytic properties via
memory, high coloration efficiency, excellent reversibility and a simple method.
6
–8
fatigue resistance. Besides, the optical absorption of WO
3
In terms of designing the single-component material, the
(
2.2–3.0 eV) in the visible light region makes it possible to be hydrothermal methods are facile, controllable and efficient for
the large-scale synthesis of semiconductor materials, in which
hydrothermal temperature, pH values and structure regulators
have signicant effects on the particle size, crystal structure and
morphology. It has been also found that structural modication
a
School of Basic Medical Science, Xinxiang Medical University, Xinxiang 453003,
China. E-mail: wang2002yl@126.com; tjni@xxmu.edu.cn
b
School of Environment, Henan Normal University, Xinxiang 453007, China
c
Faculty of Environmental Science and Engineering, Guangdong University of could inuence the photocatalytic and photochromic activities
Technology, Guangzhou 510006, China
Electronic supplementary information (ESI) available: SEM and TEM images of
B-WO ; SEM (a); TEM (b and c); HRTEM (d), full XPS spectra of the as-synthesized
3
of WO by providing special tunnels and large surface areas for
electron-transfer and light-harvesting.
†
4,23,24
3
The nature of photochromic WO may be attributed to the
3
samples, photodegradation activities of RhB under different pH values, the
adsorption–desorption equilibrium of TCH in dark with the inserts for the color
4
transition of different valence states in tungsten. Conse-
quently, there must be some intimate correlation between the
3
of Y-WO . See DOI: 10.1039/d0ra09714d
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2021 The Author(s). Published by the Royal Society of Chemistry
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photochromic effect and photocatalytic activity. The valence spectroscopy (XPS, Thermo Scientic, EscaLab 250Xi spec-
6
+
5+
state of tungsten changes from W to W during the photo- trometer) was utilized to analyze valence states of elements. The
ꢀ
+
chromic process when photoinduced electrons (e ) are optical properties were characterized via a Shimadzu UV2600
consumed to some extent, and the remaining holes (h ) form UV-vis spectrophotometer over 200–1400 nm. The Fourier
25
additional ROSs. Besides, the conversion between the oxida- transforms infrared spectrometer (FT-IR) spectra were per-
6
+
5+
tive state of W and the reductive state of W is a redox formed on a Shimadzu IRTracer-100 FT-IR spectrometer. The
process, which is conducive to the oxidation of organic pollut- photoluminescence (PL) spectra were obtained by a PL spec-
ants. Therefore, an effective strategy toward achieving efficient trometer (FLS1000, Edinburgh Instruments). The photo-
and sustainable photodegradation might be through the rapid electrochemical (PEC) properties were tested by an
6
+
5+
and reversible conversion between W and W . However, to electrochemical workstation (Princeton, VersaSTAT 3).
the best of our knowledge, the inuences of the photochromic
effect for WO on photocatalytic activity has not been further
3
2.4. Photodegradation experiments
investigated as yet.
Herein, we adopted a facile one-step hydrothermal method
3
for the synthesis of gear-shaped WO . Its rapid and reversible
photochromic properties made it a rechargeable material,
which played a critical role in the determination of photo-
catalytic activity and recyclability. A yellow (Y-WO ) and blue (B-
A 500 W gold halide lamp furnished with 420 nm cut-off lters
acted as the visible light source. In each experiment, the Y-WO
3
ꢀ1
or B-WO (50 mg) was dispersed in 50 mL RhB (10 mg L ), CIP
3
ꢀ
1
ꢀ1
ꢀ1
(
20 mg L ), OTC (20 mg L ) or TCH (20 mg L ) solutions,
respectively, with the pH values of 4, which was placed in the
dark with a 30 min stirring to obtain the good adsorption–
desorption equilibrium. The photoreaction conditions were
3
WO ) samples were both used as photocatalysts to degrade
3
pollutants including RhB, TCH, CIP, and OTC under visible
light irradiation. Through characterization and comparison, the
physical–chemical properties of the two samples were system-
atically investigated. This work focused principally on the
ꢁ
maintained at room temperature (20 ꢂ 1 C) under visible light
irradiation. The photodegradation activities on contaminants
were analyzed by the UV-2600 spectrophotometer.
inuences of the photochromic effects of WO
applications for environmental remediation.
3
on photocatalytic
3
. Results and discussion
3.1. Morphology analysis
2
. Experimental
To further investigate the morphology and crystalline structure
of the as-synthesized samples, SEM, TEM, and HRTEM
measurements were conducted. As shown in Fig. 1 and S1,† The
SEM and TEM images of the two samples both exhibited
a typical 3D gear-like structure composed of numerous nano-
rods, which endowed it with a large surface area. The HRTEM
2
.1. Materials
All the chemicals used in the experiments were analytical
reagents (AR). Sodium tungstate dihydrate (Na WO $2H O) was
purchased from Macklin. Concentrated hydrochloric acid (HCl,
6%) and hydrogen peroxide (30%) were purchased from the
Zhengzhou Paini Chemicals Reagent Factory.
2
4
2
3
(Fig. 1f) images show clear lattice fringes with an interplanar
spacing of 0.39 nm, which could be indexed to the (001) crystal
26
plane of WO
3
.
2.2. Synthesis of the samples
Y-WO
3
was synthesized via a similar but improved hydro- 3.2. Structure and elements analysis
26
2 4 2
thermal method. Firstly, 40 mM Na WO $2H O (80 mL) was
From the XRD patterns of Y-WO
3 3
and B-WO (Fig. 2a), one can
prepared in deionized water and then added dropwise
concentrated hydrochloric acid into pH ¼ 2.1. Then, the
precursor solution was transferred into a 100 mL Teon-lined
see that the XRD patterns of these two samples were remarkably
ꢁ
autoclave for hydrothermal treatment at 180 C, 14 h. Aer
cooled down to room temperature, the obtained products were
washed several times with water and ethanol. Finally, the
precipitate was further treated by immersing it in a 0.2 M H
2
O
2
ꢁ
solution in dark at 100 C for 0.5 h. B-WO
3
was prepared by
exposing Y-WO to the visible light for 1 h.
3
2.3. Characterizations
The morphology of as-synthesized samples was determined by
the scanning electron microscopy (SEM, FEI, Quanta FEG 450)
and transmission electron microscopy (TEM, FEI, Tecnai
G220S-Twin). The lattice fringe and crystal structure were
investigated on a high-resolution electron microscopy (HRTEM,
FEI, Tecnai G220S-Twin) and an X-ray diffractometer (XRD,
3
Fig. 1 SEM and TEM images of Y-WO samples; SEM (a–c); TEM (d and
Bruker, D8 Advance), respectively. X-ray photoelectron e); HRTEM (f).
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Fig. 3 The UV-vis-NIR absorption in the region from 200 nm and
Fig. 2 XRD (a); FT-IR spectra (b); XPS of O 1 s and W 4f (c and d).
1
400 nm with the inset for band gaps (a); PL spectra (b); the trans-
mittance spectra with inserts for the colors (c); the cycle times of the
transmittance at 1300 nm prior to and following coloration (d).
similar and corresponded to a hexagonal phase (JCPDS no. 33-
27
1
387). This demonstrated that the stable phase structures of
the as-synthesized samples did not change following exposure
B-WO exhibited a broader optical absorption in the visible and
3
ꢁ
ꢁ
ꢁ
ꢁ
to visible light. The typical peaks at 14.0 , 22.5 , 28.1 , 36.4 ,
NIR regions due to localized surface plasmon resonance (LSPR)
absorption that was dependent on the oxygen vacancies, which
ꢁ
and 55.3 were ascribed to the (100), (001), (200), (201), and
(
2
221) planes. While the diffraction peak intensity of B-WO
2.5 decreased due to the decline of crystallinity, as shown in
3
at
5+
11,33
was in keeping with the appearance of W in the XPS.
As
ꢁ
shown in Fig. 3b, the PL spectra of as-synthesized samples
showed emission peaks at about 470 nm under the excitation at
3
Fig. 1f and S1d,† the lattice fringes of Y-WO (001) are clearer
than that of B-WO , which might be attributed to appearance of
3
3
325 nm. Y-WO performed decreased PL intensity aer visible
oxygen vacancies in B-WO3.
light irradiation, indicating that the electron-transfer interac-
tions existed in the coloration process under visible light irra-
The chemical bonds of the as-synthesized samples were
investigated by FT-IR spectra. As shown in Fig. 2b, the bands
6+
5+
diation and the conversion from the W to W could accelerate
ꢀ1
ꢀ1
appearing at 1600 cm and 3340 cm in Y-WO
3
were ascribed
ꢀ
+
16
the separation of e /h pairs.
to O–H bending vibration and stretching vibration of the
To further analyze the photochromic properties of as-
synthesized samples, the photochromic test was carried out
surface adsorbed water, respectively, besides, the bands below
ꢀ
1
1
000 cm can be attributed to the stretching vibration of O–W–
(Fig. 3c). The as-synthesized samples also demonstrated
28
O. Aer visible light irradiation, there was no evident change
a remarkable modulation efficiency of 69%, with the color
changing from yellow to blue under visible light irradiation for
in B-WO , indicating that no other chemical groups were
3
generated on the surface of the two samples.
1
h. Meanwhile, the B-WO could be converted back to Y-WO by
3 3
The typical survey XPS spectra of the as-synthesized samples
H O treatment. For its stability (Fig. 3d), the Y-WO3 was
2
2
(Fig. S2†) conrmed the co-existence of W and O. In the O 1 s
colored and bleached ten times and there was no obvious
change in the coloration efficacy and reversibility, indicating
that the as-synthesized samples were stable and reusable.
XPS spectra (Fig. 2c), both samples exhibited two peaks at
ꢃ530.8 eV and 532.4 eV corresponding to the binding energies
3
of W–O and –OH groups, respectively. Compared to Y-WO , the
3
ratio of –OH/W–O in B-WO decreased from 0.16 to 0.13, and
the W–O peaks showed a slight shi, which was attributed to
3
.4. Photocatalytic performance
ꢀ
+
a reaction during the photochromism process: WO + xe + xH
3
The photocatalytic performance of the as-synthesized samples
was evaluated via degrading RhB, CIP, OTC and TCH under
visible light irradiation. As illustrated in Fig. 4a, b, c and d,
under visible light exposure, the photocatalytic degradation
rates of TCH, OTC, RhB, and CIP in the presence of Y-WO
9
1
1
/
H WO . As for the W 4f peaks of Y-WO (Fig. 2d), the strong
x
3
3
peaks at 38.06 eV/W 4f5/2 and 35.90 eV/W 4f7/2 conrmed the
6+
29
3
existent of W . Further, the W 4f peaks of B-WO were tted to
6
+
5+
11,30
W
at 38.01 eV/35.87 eV and W at 36.70 eV/34.85 eV.
shied to lower binding energies and
3
were
The W 4f peaks of B-WO
3
4.3%, 87.9%, 76%, and 68.6%, respectively, in 60 min. While
5
+
6+
W
W
appeared, suggesting that W might be transformed to
3
the B-WO presented lower photocatalytic degradation effi-
5
+
31,32
under visible light irradiation.
ciencies of 17.1%, 32.1%, 34%, and 44.8%, respectively, in
0 min. In addition, we found that Y-WO performed signi-
cantly higher photocatalytic activity than that of the commercial
As shown in Fig. 3a, the two samples possessed a similar band WO (11.3%, 4.1%, 15.5% and 5.7%, respectively, in 60 min).
6
3
3.3. Photochromic performance
3
6
+
gap of ꢃ2.9 eV and revealed an intrinsic absorption edge at This could have been because a portion of the W was con-
5
+
6+
ꢃ480 nm. For the optical absorption beyond the band edge, the verted to W during the photochromism process and W
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Fig. 5 The DMPO electron spin resonance (ESR) spectra of as-
synthesized samples (a and b); PEC properties (c and d).
might be the primary reactive species in the process of photo-
catalytic. However, Y-WO performed signicantly higher pho-
3
tocatalytic activity than B-WO , which indicated the presence of
other active species.
3
Fig. 4 The photocatalytic efficiencies of TCH, OTC, RhB and CIP over
Y-WO
reusability of the as-synthesized samples (e); active species trapping
experiments of Y-WO under visible light irradiation (f).
3 3
and B-WO under visible light irradiation (a–d); the stability and
To further verify the key reactive species in the photocatalytic
ꢀ
1
system, various scavengers including IPA (600 mg L ) for cOH,
3
ꢀ
1
ꢀ
ꢀ1
+
BQ (80 mg L ) for cO
2
and AO (700 mg L ) for h were utilized
35
under visible light irradiation. As shown in Fig. 4f, aer the
addition of AO, IPA and BQ, respectively, the degradation effi-
possessed high oxidative properties for the oxidation of organic
contaminants. Besides, the reductive conversion process
accompanied by the consumption of photogenerated electrons
ciencies of TCH were reduced from 94.3% to 8.1%, 80.3% and
+
8
7.2%, indicating that h was the primary reactive species in the
ꢀ
25
process of photocatalytic. cOH and cO
2
radicals also contrib-
facilitated the separation of holes. Therefore, achieving the
rapid and reversible conversion from B-WO to Y-WO was an
uted to the photodegradation, which was corresponding to the
results of ESR measurements.
3
3
effective strategy to realize sustainable photocatalytic degrada-
PEC tests including the photocurrent response (PCR) and
electrochemical impedance spectroscopy (EIS) were investi-
gated to evaluate the properties of photoinduced charge
carriers. As shown in Fig. 5c and d, the Y-WO3 exhibited
signicantly higher photocurrent intensity and a smaller
tion activity. In the process of photocatalytic, Y-WO3 would
convert to B-WO
while B-WO could be back to Y-WO
the recovered photocatalytic efficiency. Stability and reusability
tests for Y-WO were conducted under visible light irradiation.
3
with the decreased photocatalytic activity,
3
3
by H treatment with
2 2
O
3
semicircle radius than B-WO , which indicated a higher rate of
3
As illustrated in Fig. 4e, aer recycling 10 times, there was no
signicant change in the photocatalytic degradation of TCH,
which indicated that the as-synthesized samples were stable
and reusable.
34
3
interfacial charge transfer in the Y-WO .
Based on the above results, a potential photocatalytic
mechanism for the as-synthesized samples was proposed
3
(Fig. 6). Under visible light irradiation, the Y-WO is excited and
the photoinduced electrons transfer from the valence band (VB)
to the conduction band (CB). On one hand, a portion of the
3
.5. Photocatalytic mechanism
ꢀ
photoinduced electrons can convert O to cO for the degra-
2
2
The DMPO electron spin resonance (ESR) measurements were
applied to the further demonstration of ROSs under visible light
irradiation. As shown in Fig. 5a, DMPO-cOH could be conrmed
in both samples due to the signal intensity of the four charac-
teristic peaks with a ratio of 1 : 2 : 2 : 1. The characteristic
peaks for DMPO-cO
dation of pollutants. Conversely, the insertion of photo-
generated electrons and hydrogen ions at the tungsten sites of
6+
5+
WO
WO
leaves the h in VB for the direct oxidation of pollutants or
3
causes the reduction from W to W and the formation of
25
H
x
3
with the color conversion from yellow to blue. This
3
4
+
ꢀ
2
could be observed in both samples
5+
indirect oxidation of H
2
O to cOH, while the reductive W can be
(Fig. 5b). In contrast, the signal intensity of DMPO-cOH and
6+
reversibly converted back to W by an H O treatment; thus,
ꢀ
2 2
DMPO-cO
2
3 3
for B-WO was at the same level as Y-WO , sug-
forming a rechargeable cycle between Y-WO and B-WO . The
3
3
gesting that both samples could generate almost the same
amount of cOH and cO2 radicals under visible light irradiation.
Furthermore, the DMPO-cOH signal intensity for both samples
performed much stronger than DMPO-cO
6+
5+
rapid and reversible conversion between the W and W redox
sites not only signicantly promoted the separation of e /h
ꢀ
ꢀ
+
ꢀ
2
, implying that cOH
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4
5
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degradation of TCH, OTC, RhB, and CIP under visible light
irradiation. The characterization results demonstrated that the
6
+
5+
redox conversion between the W and W during the photo-
chromism process played a key role in enabling highly efficient
and sustainable photodegradation. This work provided a new
strategy in the environmental remediation via achieving the
1
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2
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3
Conflicts of interest
4
The authors declare no conict of interest.
1 S. Yamazaki, H. Ishida, D. Shimizu and K. Adachi, ACS Appl.
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