Boosting the photocatalytic performance of (001) BiOI: Enhancing donor density and separation efficiency of photogenerated electrons and holes

Article abstract of DOI:10.1039/c6cc00903d

Herein, we demonstrate a simple strategy to boost the photocatalytic performance of BiOI by introducing oxygen defects into the BiOI. The oxygen-deficient BiOI exhibits superior photocatalytic performance for the degradation of formaldehyde gas. The enhancement of photocatalytic activity is due to the enhanced separation and migration efficiency of photogenerated electrons and holes.

Full text of DOI:10.1039/c6cc00903d

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Boosting photocatalytic performance of (001) BiOI: enhance
donor density and separation efficiency of the photogenerated
electrons and holes
Received 00th January 20xx,
Accepted 00th January 20xx
Wenjie Fan a, Haibo Li a, Fengyi Zhao a, Xujing Xiao a, Yongchao Huang a, Hongbing Ji a* and Yexiang
DOI: 10.1039/x0xx00000x
Tong a,b
*
www.rsc.org/
Herein, we demonstrate
a simple strategy to boost the various experimental and theoretical calculation results
photocatlytic performance of BiOI by introducing oxygen defects revealed that the surface oxygen defects played a vitally
into the BiOI. These oxygen-deficient BiOI exhibit suprior important role in the mobility and separation efficiency of
photocatalytic performance for degradation of formaldehyde gas. photo-generated carriers.22 For examples, Lei et al.
The enhancement of photocatalytic activity is due to the demonstrated that oxygen vacancies enhance the water
enhanced separation and migration efficiency of photogenerated splitting efficiency of ultrathin In2O3 experimentally and by
electrons and holes.
theoretical calculation results.23 Furthermore, Niu et al.
showed that nitrogen vacancies carbon nitride enhances
visible light absorption and suppress recombination of photo-
generated electron and hole, showing superior photocatalytic
activities.18 Our recent work revealed that oxygen-deficient
BiOI nanosheets have an excellent photocatalytic performance
due to increasing the visible light absorption and donor
density.24 Some methods such as inert gases annealing,25
electroreduction,24 H2 treatment at high temperatures26 have
been used to introduce oxygen vacancies into BiOI. However,
these methods need high temperature which is dangerous to
some extent. Therefore, developing a simple, safe and feasible
progress to introduce oxygen defects into BiOI is still need for
practical applications.
Formaldehyde (HCHO) is considered to be one of the major
indoor pollutions, which originated from the adhesives,
coatings and various wood-based materials.1,2 Long term
disclosure to HCHO is harmful for human, causing pulmonary
diseases and nasopharyngeal cancer.3 Photocatalysis has been
admitted to be one of the most prospective methods to
overcome this environmental problem.4-10 The main
bottleneck of this process is to develop efficient, stable, and
cost-effective catalysts that can operate under solar light.
Bismuth oxyiodide (BiOI) is an attractive semiconductor, which
possesses a layered crystal structure and suitable band gap
(1.7-1.8 eV).11-13 However, the fast recombination of charges
and the poor conductivity bring about its low photocatalytic
efficiency, which prevent its practical applications.14 In order
to enhance the photocatalytic performance of BiOI, various
strategies have been carried out, such as heterojunction,
doping, and structural designing.15-17 Recently, introducing
oxygen defects into the lattice of BiOI has become new
developing approach, which can superbly enhance the
photocatalytic activity of BiOI.
In this study, we demonstrated
a simple, safe and
practicable method to introduce oxygen defects into BiOI
nanosheets via low-frequency ultrasound device at room
temperature. The oxygen defective BiOI possessed excellent
photocatalytic performance for HCHO gas oxidation, which
was 3 times higher than that of the untreated BiOI under the
visible light irradiation. Such superb activity was due to the
formation of oxygen defects in BiOI, which enhanced the
donor density and increased the separation efficiency of the
photogenerated electrons and holes. Furthermore, it showed
outstanding stability performance after 5 cycles, revealing that
it can be used for practical applications.
Surface oxygen defect is very important for photocatalytic
performance, which affects the physical-chemical properties
and electronic structure of semiconductors.18-21 In particular,
BiOI nanosheets rich in defects (R-BiOI) were obtained
using low-frequency ultrasound of the untreated BiOI at room
temperature. In order to study the structural information of
the R-BiOI nanosheets, XRD and Raman spectroscopy
aMOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of
Environment and Energy Chemistry; The Key Lab of Low-carbon Chem & Energy
Conservation of Guangdong Province, Sun Yat-Sen University, 135 Xingang West
Road, Guangzhou 510275 (P. R. China).
bDepartment of Chemistry, Shantou University, Guangdong 515063, P. R. China
E-mail: Email:jihb@mail.sysu.edu.cn (H.B.Ji); chedhx@mail.sysu.edu.cn (Y.X.T)
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Scheme 1 Scheme suggesting the formation of oxygen vacancies induced by low-
frequency ultrasound.
were first carried out. The XRD patterns of two samples can be
indexed as tetragonal BiOI (PCPDF no.: 10-0445) (Fig. S1a,
ESI†). Surprisingly, the peak (001) facets of R-BiOI is higher
than that of BiOI, showing that sonicating the untreated BiOI
can expose more (001) facets. The Raman peaks at 85 cm-1 and
147 cm-1 belong to the Bi-I vibration of BiOI (Fig. S1b, ESI†).27
Furthermore, the FT-IR (Fig. S1c, ESI†) and XPS survey spectra
(Fig. S1d, ESI†) revealed that the structural of BiOI did not
changed after low-frequency ultrasound. All these results
demonstrated that the structural was maintained after using
low-frequency ultrasound on the untreated BiOI sample.
However, it is worthy to mention that the morphologies of the
samples were different, which can be revealed by the SEM and
TEM (Fig. 1). It can be seen that the thickness of BiOI
nanosheets was about 20 nm and displayed high single-crystal
structure. It can be observed that BiOI demonstrated high
(001) facets. The nanosheets broke into smaller fragments
after ultrasound, which may offer a high reactive surface area
for photocatalytic processes. The interplanar distances of the
samples coincided well with (001) and (110) facets,
respectively. Additionally, the results of the BET analysis
Fig. 2 (a) EPR spectra, (b) Normalized Bi 4f XPS spectra, (c) O 1s XPS spectra, (d)
UV−vis absorpꢀon spectra of BiOI and R-BiOI samples.
demonstrating the successful introduction of oxygen defects
into BiOI nanosheets. Furthermore, the X-ray photoelectron
spectroscopy (XPS) measurements offered ancillary evidence
of the existence of oxygen defects. Compared to the BiOI, the
Bi 4f5/2 and Bi 4f7/2 peaks of R-BiOI displayed a shift of 0.3 eV
toward lower binding energy, which revealed that the R-BiOI
had more low-charge Bi ions and more oxygen defects than
the BiOI.28 The existence of oxygen defects caused distinct
coordinations of oxygen species, which can be confirmed in
the O 1s XPS (Fig. 2c). There are two peaks at 529.7 (lattice
oxygen) and 531.4 eV (surface active oxygen). The intensity of
peak (531.5 eV) of R-BiOI was increased, verifying the oxygen
defects were successfully introduced into BiOI. All these results
revealed that the oxygen defects were successfully introduced
into the BiOI through low-frequency ultrasonication. Fig. 2d
shows the absorption spectra of BiOI and R-BiOI samples.
Significantly, the absorption spectra of two samples are similar,
while the absorbed intensity of R-BiOI is higher compared to
the BiOI nanosheets. This reveals that the R-BiOI sample can
absorb more visible light, which may be one of the reasons for
the excellent photocatalytic performance of R-BiOI nanosheets.
The photocatalytic degradation of HCHO gas under visible
light was carried out to assess the photocatalytic efficiencies of
BiOI and R-BiOI nanosheets, which is shown in Fig. 3. After 120
min only 4% of HCHO was degraded under the visible light
irradiation. So the self-degradation of HCHO gas could be
ignored. Significantly, the BiOI sample could remove 35%
HCHO after 120 min under visible light, while more than 99%
was achieved for the R-BiOI nanosheets. This result
indisputably confirms that the R-BiOI sample demonstrated
superior activity than the BiOI sample and revealed that the
photoactivity of BiOI could be dramatically enhanced by
introducing the oxygen defects. The effect of the
ultrasonication time on the photocatalytic performance was
investigated (Fig. S2). It can be seen that the ultrasound time
has obvious effect on the photocatalytic performance. The
BiOI with ultrasound time at 15 h has the best photocatalytic
performance of degradation of HCHO. Surprisingly,
revealed that the surface area of R-BiOI (7.04 m
larger than that of BiOI (5.92 m2 g-1).
⋅
2 g-1) was little
⋅
In order to probe the presence of oxygen defects after
ultrasound, the electron spin resonance measurement was
performed. A broad peak at g=2.004 is identified as the
electrons trapped on oxygen vacancies,23 which is shown in
Fig. 2a. The intensity of the R-BiOI was higher than that of BiOI,
Fig. 1 (a) SEM images, (b) TEM image of BiOI nanosheets. (c) SEM images, (d)
TEM image of R-BiOI nanosheets.
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and valence band XPS spectra as shown in Fig. S4. It illustrates
that
Fig. 3 (a) Photocatalytic performance of BiOI and R-BiOI samples for degradation of
HCHO under visible light irradiation. (b) Cycling performance of degrading HCHO over
R-BiOI sample under visible light irradiation.
photocatalytic performance decreased as the ultrasound time
more than 15 h. This is because more oxygen vacancies will
generated as the photoelectrons and holes recombination
center, hence, decreasing the photocatalytic performance. In
addition, recycling experiments for the degradation of HCHO
were carried out to study the stability of the photocatalytic
activity of R-BiOI. It is noteworthy that after 5 cycles the
photocatalytic performance of R-BiOI was retained without
any decay, suggesting that R-BiOI has excellent cycling stability.
Besides this, the morphology and the structure of the R-BiOI
didn’t have any change after 5 cycles, further affirming the
stability of R-BiOI nanosheets (Fig. S3, ESI†). All the results
above convincingly show that oxygen defects could availably
enhance the photoactivity of BiOI nanosheets.
Fig.
4 (a) PL spectra with an excitation wavelength of 400 nm, (b) Visible light
photocurrent responses, (c) Nyquist plots under visible light, (d) Mott −Schoꢁky plots
of BiOI and R-BiOI samples.
the oxygen defects R-BiOI nanosheets exhibit a narrowed band
gap from 1.81 to 1.74 eV compared with the BiOI nanosheets.
Surprisingly, the valence band of R-BiOI and BiOI was the same.
This indicates that the oxygen defects contribute to shifting
the onset light absorption and then improve the photocatalytic
performance. Photocatalytic mechanism scheme of R-BiOI
catalysts under visible light irradiation was shown in Scheme
S1. All the results above demonstrate that introducing oxygen
defects can suppress the recombination of photogenerated
electrons and holes and improve the photocatalytic
performance.
To understand why introducing oxygen defects could affect
the photocatalytic performance of BiOI, photoluminescence
(PL) measurements, photocurrent responses, electrochemical
impedance spectra were conducted. Fig. 4a shows PL spectra
of BiOI samples with excitation wavelength of 400 nm. The
BiOI sample shows a strong emission band at 615 nm, which is
due to the high recombination of photoelectrons and holes.29
While the intensity of this peak of R-BiOI decreased obviously,
indicating that the introduced oxygen defects can greatly
suppress the recombination of photoelectrons and holes of
BiOI. Furthermore, the photocurrent response can directly
reflect the separation and migration efficiency of
photogenerated electrons, which is showed in Fig. 4b.
Obviously, the photocurrent density of R-BiOI under visible
light irradiation is much higher than that of BiOI, which
suggests that the R-BiOI sample exhibit the excellent
performance. Moreover, the charge separation efficiency was
studied by the electrochemical impedance. As we know, the
small arc radius indicates that the efficiency of charge transfer
is higher.11, 30 Fig. 4c shows that the radius of R-BiOI is smaller
than that of BiOI in any case, either under the dark or under
the visible light irradiation. This implies that the charge
transfer efficiency of R-BiOI is higher than that of BiOI. The
Mott-Schottky plots of the two samples are displayed in Fig.
4d, and the R-BiOI sample demonstrates smaller slope
comparing to the BiOI. This result suggests that introducing
oxygen defects could significantly increase the donor density,
which can facilitate the separation of photoelectrons and
holes. In order to understand the CBM and VBM level of R-BiOI
and BiOI, we measured the UV−vis diffuse reflectance spectra
In summary, we demonstrated that the photocatalytic
performance of BiOI can be improved by introducing the
oxygen defects. Under the visible light irradiation, the R-BiOI
sample could remove more than 99% HCHO in 120 min, which
was 3 times considerably higher than that of BiOI nanosheets.
Additionally, the R-BiOI sample has an attractive cycling
stability after 5 cycles. The separation and migration efficiency
of photogenerated carriers caused its enhanced photocatalytic
performance of degrading HCHO. This study not only offers
new sight into how the oxygen defects affected the
photocatalytic activity, but also paves the way for designing
photocatalytic systems with efficient visible light.
Acknowledgements
This work was preliminarily supported by the National Science
Fund for Distinguished Young Scholars (21425627), the
Natural Science Foundation of China (21461162003 and
21476271), and Natural Science Foundation (2014KTSCX004
，
2014A030308012 and S2013030013474) of Guangdong
Province, China. We thank Middle School Student Talent Plan
and Jianxiang Liao for his kind assistance in our experiment.
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