Oxygen-Vacancy-Mediated Photocatalysis over Bi2Sn2O7: Exceptional Catalytic Activity and Selectivity

Article abstract of DOI:10.1002/cctc.201900454

A series of Bi₂Sn₂O₇ photocatalysts was developed with the aim to regulate defect chemistry as well as electronic structure by modulating the charge states of tin species for boosting photocatalytic selective oxidation of alcohols performance. Defective centers, such as oxygen vacancies (OVs), were beneficial to enhancing the adsorption and activation of molecular oxygen. Meanwhile, the valence band edge potential of Bi₂Sn₂O₇ with higher OVs content exhibited a distinct downshift, improving the thermodynamical driving force of photooxidation. Additionally, OVs contributed to promoting the separation efficiency of photogenerated carriers, which can be confirmed by the surface photovoltage measurement. By optimizing the electronic structure as well as defect chemistry, the optimal benzyl alcohol conversion efficiency was 76.0 % with selectivity toward benzaldehyde being about 100 %. It's found that photogenerated holes and ^.O₂ ^? active species played critical role in photocatalytic aerobic oxidation. This work may provide a novel insight to clarify the role of defect chemistry during the selective benzyl alcohol oxidation over Bi₂Sn₂O₇ photocatalyst.

Full text of DOI:10.1002/cctc.201900454

Accepted Article
Title: Oxygen Vacancies Mediated Photocatalysis over Bi2Sn2O7:
Exceptional Catalytic Activity and Selectivity
Authors: Shushu Huang, Xin Kou, Dan He, Chunfang Du, Xiaojing
Wang, and Yiguo Su
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Oxygen Vacancies Mediated Photocatalysis over Bi2Sn2O7:
Exceptional Catalytic Activity and Selectivity
Shushu Huang,[a] Xin Kou,[a] Dan He,[a] Chunfang Du*[a], Xiaojing Wang[a] and Yiguo Su*[a]
[13]
Abstract: A series of Bi2Sn2O7 photocatalysts was developed with attributes to strong oxidizability of the photogenerated holes
.
the aim to regulate defect chemistry as well as electronic structure Hence, numerous works put their efforts on engineering the
by modulating the charge states of tin species for boosting photocatalysts that hold mild oxidation and have a strong
photocatalytic selective oxidation of alcohols performance. interaction with O2, in order to achieve high conversion efficiency
Defective centers, such as oxygen vacancies (OVs), were and selectivity, by regulating the band structure and surface
beneficial to enhancing the adsorption and activation of molecular properties [7, 14, 15]
oxygen. Meanwhile, the valence band edge potential of Bi2Sn2O7
.
Heterobimetallic oxide Bi2Sn2O7 with unique pyrochlore structure
exhibits excellent photocatalytic property on remediating the
with higher OVs content exhibited a distinct downshift, improving
thermodynamical driving force of photooxidation. Additionally,
OVs contributed to promoting the separation efficiency of
photogenerated carriers, which can be confirmed by the surface
photovoltage measurement. By optimizing the electronic structure
as well as defect chemistry, the optimal benzyl alcohol conversion
efficiency was 76.0% with selectivity toward benzaldehyde being
environmental issues [16]. And the appropriate band gap structure
endows its benign oxidizing ability
recombination of photogenerated charge carriers restricted the
widespread application of Bi2Sn2O7 as efficient photocatalysts
[17]
. Whereas, rapid
[18-
20]. Driven by the need of solar light harvesting and efficient charge
separation, efforts have beed dedicated to modulating the lattice
structure, electronic structure and surface/interface chemistry, since
these intrinsic features often greatly influence the photocatalytic
activity. For instance, incorporation of Ti3+ into TiO2 or Sn2+ into
SnO2 can bring out the generation of oxygen vacancies (OVs)
because of the unbalanced charge [21-23]. Meanwhile, OVs can serve
as the electrons trap center in order to prevent the recombination of
photogenerated charge carriers, which enabled holes entirely to
participate in the photocatalytic oxidation, enhancing the
photocatalytic activity [24]. As typical reducible oxides, tin-based
oxides show tunable physic-chemical properties by controlling the
charge states of tin species with off-stoichiometric chemical
compositions, which predict the improved separation of hole-
electron pairs and the subsequent photocatalytic activity.
−
about 100%. It’s found that photogenerated holes and •O2 active
species played critical role in photocatalytic aerobic oxidation. This
work may provide a novel insight to clarify the role of defect
chemistry during the selective benzyl alcohol oxidation over
Bi2Sn2O7 photocatalyst.
Introduction
Over past decades, photocatalysis is regared as a significant
process in clean energy production, environmental modification
and fine chemical synthesis [1-5]. Among them, selective oxidation
of alcohols that triggered by solar energy and used O2 as the
primary oxidant is one of the fundamental organic transformations
of industrial chemistry, attracting enormous attention in recent
years [6]. The corresponding carbonyl compounds (such as
aldehydes and ketones) are the versatile precursors for the
Herein, heterobimetallic oxide Bi2Sn2O7 photocatalysts were
systematically prepared in order to regulate oxygen vacancies
(OVs) content by controlling the charge states of tin species with
the aim to tune electronic structure and charge separation
efficiency for boosting photocatalytic performance. Selective
benzyl alcohol oxidation was selected as model reaction to explore
the effect of defect chemistry on photocatalytic activity over
heterobimetallic oxide Bi2Sn2O7.
[7,
synthesis of pharmaceuticals and fine chemicals 8]. And the
utilization of O2 as oxidizing agent avoids the drawback of the
traditional oxidants such as permanganate and dichromate which
are environmental unfriendly [9]. Although the selective oxidation
of alcohol into corresponding carbonyl compounds using noble
metals or semiconductor complexes has been successfully achieved,
still restricting by the high price, low conversion efficiency and
poor selectivity under visible light irradiation [10, 11]. Generally, the
interaction of O2 with the surface of photocatalysts, especially with
Results and Discussion
the defective surface, plays a key role in determining the
The corresponding XRD patterns of all samples (Fig. S1 and Fig.
1a) illustrated that single phase of Bi2Sn2O7 (JCPDS: 01-088-0496)
was obtained with advisable Sn(II) content. However, a larger
amount of Sn(II) gave birth to a tiny characteristic diffraction peak
of SnO2 in XRD patterns and the peaks of Bi12O17Cl2 (JCPDS:00-
037-0702) were existed in the XRD pattern of BS10 because of
lower Sn(II) content. For comparison, SnO2 (JCPDS: 01-077-0447)
and Bi2O3 (JCPDS: 98-000-6263) were also prepared as illustrated
in Fig. 1a. SEM and TEM images (Fig. 1b and Fig. 1c)
demonstrated that BS7 presented a regular flower-like morphology
constituted of tiny nanoparticles with fine crystallinity.
Additionally, the morphology of the prepared sample displayed an
[12]
conversion efficiency of selective alcohol oxidation
.
Simultaneously, the poor selectivity of semiconductors always
[a]
S Huang, X Kou, D He, Prof. C. Du, Prof.Dr. X. Wang, Prof.Dr. Y.
Su
College of Chemistry and Chemical Engineering
Inner Mongolia University
Hohhot 010021 (P. R. China)
E-mail: cesyg@imu.edu.cn, cedchf@imu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201900454
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minor change along with the variation of Sn(II) content (Fig. S2).
Meanwhile, as illustrated in Fig. S3, the particle size of Bi2Sn2O7
sample, which was calculated by Scherrer Formula, gradually
decreased with the reduction of Sn(II) content. According to
previous report, the decrease of particle sizes predicted that more
exposed surface atoms would be existed, leading to the generation
of surface defect dipoles which may have an effect on the lattice
parameters [25]. As HRTEM image (inset of Fig. 1c) showed that
the measured lattice spacing was 0.314 nm which was closed to
that of 0.310 nm for (2 2 2) plane of Bi2Sn2O7. The UV-vis spectra
(Fig. 1d) showed that BS7 and Bi2O3 both had a strong absorption
in visible region, whereas SnO2 exhibited unobservable visible
light absorption (Fig. 1d). On account of Bi2Sn2O7 to show an
indirect electronic transition (Fig. S4), the Eg (band gap energy)
can be estimated by the following equation: αhν = A(hν-Eg)1/2, in
which α, ν, Eg, and A are the absorption coefficient, incident light
frequency, band gap, and constant, respectively [26]. As illustrated
in the inset of Fig. 1d, the band gap energies as well as the visible
light absorption feature (Fig. S5) of all as-prepared samples were
nearly identical with the variation of Sn(II) content, which can be
identified by the comparison of DFT calculated band structure of
Bi2Sn2O7 and Sn(II)-Bi2Sn2O7 (Fig. S6 and S7), proving that the
content of Sn(II) in Bi2Sn2O7 had minor impact on the visible light
absorption capability.
3d3/2 at 494.9 eV. Only typical XPS binding energies of Sn(IV)
was observed for SnO2, where two peaks of Sn(IV) 3d5/2 and 3d3/2
appeared at the binding energies of 486.4 eV and 494.8 eV [27, 28]
.
It indicated that Sn(II) and Sn(IV) were coexisted in BS7 catalyst,
which can be confirmed by the XPS spectra of Sn 3d for the
obtained Bi2Sn2O7 photocatalysts with different initial content of
SnCl2·2H2O (Fig. S8). The generation of Sn(II) in Bi2Sn2O7
samples may be due to the utilization of SnCl2·2H2O as precursor
which can be verified by the comparison with the sample
prepared by using SnCl4·5H2O (Fig. S9). Meanwhile, Sn 3d
orbits of Bi2Sn2O7 samples which were achieved by adding
different molar ratio of SnCl2·2H2O and SnCl4·5H2O as precursor
also can be split into four peaks for Sn(Ⅱ) and Sn(Ⅳ) (Fig. S10).
And the Sn(II) content in sample was approximately reduced
along with the increased dosage of SnCl4·5H2O during synthetic
process (Fig. S11), proving the reason of the generation of Sn(II).
Furthermore, it was observed that the content of Sn(II) for BS7
sample was larger than others (Fig. 2c). In combination of XPS,
thermogravimetric spectrometry (TG, Fig. S12) and ICP analyses,
the chemical composition of BS7 sample was estimated to be
Bi1.14Sn(Ⅱ)0.66Sn(Ⅳ)0.34O3.05·0.44H2O (S13). Obviously, BS7
sample exhibited a nonstoichiometric feature, which might
produced abundant defective centers, which would be confirmed
by the XPS spectra of O 1s and the electron spin resonance (ESR)
measurements. As depicted in the O 1s spectra for SnO2, Bi2O3
and BS7 (Fig. 2b), the binding energies at ~530.0 eV and ~531.2
eV were assigned to the lattice oxygen atoms (OL) and surface
hydroxyl groups (Os), respectively [29]. The peak located at
~532.5 eV for Bi2O3 and BS7 might be related to the surface
adsorbed O2, corresponding to surface defects such as oxygen
vacancies (OVs) [30], which was also confirmed by ESR data (Fig.
S14). And O 1s high resolution spectra of all samples also had the
characteristic signal of OVs as shown in Fig. S15 and Fig. S16.
Meanwhile, Fig. 2c showed that BS7 owned a higher content of
OVs along with higher Sn(II) content, which can be verified by
the stronger ESR signal of BS7 than that of BS4 and BS10 (Fig.
S17). Additionally, the valence band XPS (VB-XPS) was also
given to illustrate the inflence Sn(II) and defect chemistry on the
electronic structure. Apparently, the valence band potential with
respect to Fermi level varied with the alteration of Sn(II) and
OVs. It’s noted that an obvious downshift for the valence band
edge of BS7 could be observed in Fig. 2d. This result suggested
that the valence band potential can be modulated by the variation
of Sn(II) and OVs, which may have great impacts on the
photocatalytic activity of the as-prepared Bi2Sn2O7 photocatalyts.
In principle, the radius of ion may play a critical role in the cell
parameters of lattice structure [31]. Hence, Rietveld refinement of
the diffraction was taken to study the effect of Sn(II) content on
the lattice parameter of Bi2Sn2O7. Typical refinement results of
Bi2Sn2O7 samples were presented in Fig. S18 using a primitive
cubic unit cell (space group Fd-3m). The reasonable values of the
goodness-of-fit parameters (Rwp, Rp and χ2) in Table S1 were
achieved to get the best quality of refinement. Fig. S18 showed
that cell volume of the as-prepared samples was greatly related to
the Sn(II) content. The higher Sn(II) content in Bi2Sn2O7 sample
accompanied with a larger cell volume (Fig. S19), which might
due to the ion radius of Sn(II) was larger than that of Sn(Ⅳ),
leading to an expansion of lattice structure. Hence, the Rietveld
Fig. 1 XRD patterns of BS7, SnO2 and Bi2O3 samples (a), SEM image of
BS7 (b), TEM image (Inset is HRTEM image) of BS7 (c) and UV-vis diffuse
reflectance spectra of BS7, SnO2 and Bi2O3 (the inset is the band energy of
BS1-BS10) (d).
XPS technique was carried out to specify the chemical
composition and the defective feature of the obtained samples.
The high resolution signal of Sn 3d for BS7 sample (Fig. 2a) was
fitted by four characteristic peaks as Sn(II) 3d5/2 at 485.8 eV,
Sn(II) 3d3/2 at 494.2 eV, Sn(IV) 3d5/2 at 486.5 eV and Sn(IV)
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refinement results revealed that Sn(II) was successfully imported
into Bi2Sn2O7 photocatalysts, which might be of importance for
modulating the photocatalytic performance of samples.
performance of Bi2Sn2O7 samples which were gained by using
different molar ratios of SnCl2·2H2O and SnCl4·5H2O as
precursor (Fig. S23). As shown in Fig. 3c, the
conversionefficiency was decreased with the increased content of
Sn(IV), which demonstrated that the content of Sn(Ⅱ) in
Bi2Sn2O7 had a significant impact on the catalytic performance of
benzyl alcohol oxidation. Furthermore, Fig. 3d presented that the
Fig. 3 The conversion efficiency of benzyl alcohol oxidation over all prepared
samples (a), aromatic alcohols oxidation over BS7 sample (b) and benzyl
alcohol oxidation over obtained samples with different initial content of Sn2+
and Sn4+ (c). Benzyl alcohol conversion efficiency and cell volume of samples
and the corresponding content of Sn(II) and OVs (d).
Fig. 2 High resolution regional XPS spectra of Sn 3d (a) and O 1s (b). The
content of Sn(II) and OVs (oxygen vacancies) on sample surfaces
determined by XPS analyses (c). Valence band XPS (VB-XPS) spectra of
samples (d).
The selective photocatalytic oxidation of benzyl to benzaldehyde
under visible light irradiation (λ ≥ 420 nm) utilizing O2 as
oxidizing agent was selected as model reaction to investigate the
photocatalytic performance of photocatalysts in this work. GC
spectrum of the photocatalytic oxidation reaction was provided in
Fig. S20. Apparently, benzyl alcohol conversion efficiency was
increased prolonged with irradiation time (Fig. S21). Fig. 3a
illustrated the benzyl alcohol conversion efficiency over all
obtained photocatalysts. It was noted that all prepared samples
exhibited photocatalytic reactivity toward benzyl alcohol
oxidation. Meanwhile, the conversion efficiency was optimized
as a function of Sn(II) and OVs content, showing a maximum
value of 76.0% (BS7) (Fig. S22). Further increase of initial Sn(II)
content caused a significant decrease in the conversion efficiency.
Obviously, the conversion efficiency of most Bi2Sn2O7 samples
was higher than that of their monometallic oxide (SnO2 and
Bi2O3). Apart from the high conversion efficiency, BS7 also
displayed near 100% selectivity toward benzaldehyde (Table S2).
Fig. 4 The recycling experiment for photocatalytic oxidation benzyl alcohol
over BS7 sample (a). XRD (b), high solution XPS spectra of Sn 3d (c) and O
1s (d) of BS7 before and after the cycling test.
Simultaneously, BS7 exhibited
a higher benzyl alcohol
conversion efficiency compared with previously reported works
(Table S3). Aerobic oxidation of a series of aromatic was chosen
to estimate the widespread applicability of Bi2Sn2O7
semiconductor, illustrated in Fig. 3b. It is observed that Bi2Sn2O7
owned high conversion efficiency and selectivity toward aromatic
alcohols oxidation, which implied unprecedented application of
Bi2Sn2O7 catalysts. Moreover, we also examined the catalytic
cell volume, OVs content as well as benzyl conversion efficiency
were regulated by the Sn(Ⅱ) content in sample. Meanwhile, the
similar morphology of all samples (Fig. S24) demonstrated that the
morphology had a negligible effect on the photocatalytic activity.
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The stability of Bi2Sn2O7 sample was further invistigated, which
played a vital role in widespread industiral application of
photocatalysts. Clearly, there exists no apparent reduction in
photocatalytic benzyl alcohol conversion efficiency and the
selectivity toward benzaldehyde after five reaction cycles, as
displayed in Fig. 4a. And XRD pattern (Fig. 4b) of BS7 after cyclic
experiment showed no obvious change compared with that of
before. Additionally, high solution XPS spectrum of Sn 3d after
cyclic test illustrated that Sn(Ⅱ) content in sample had no evident
decrease (Fig. 4c). Meanwhile, three kinds of oxygen species were
all maintained and OVs content showed a tiny change after five
cycles, as shown in Fig. 4d. Hence, the above analyses proved that
Bi2Sn2O7 photocatalyst presented an outstanding stability,
promising an extensive application in photocatalysis.
governs the reactivity of photocatalytic benzyl alcohol oxidation.
And the valence band (VB) edge potential of Bi2Sn2O7
photocatalysts can be estimated using ultraviolet photoelectron
spectroscopy (UPS), absorption spectra and Mott-Schottky
analyses. Mott-Schottky plots of BS7 were showed in Fig. S25, the
positive slopes revealed that Bi2Sn2O7 exhibited a characteristic of
n-type semiconductor and the conduction band (CB) edge of BS7
was calculated to be -0.45 V versus NHE Combined with the band
gap energy (Eg) in Fig. 1d and Mott-Schottky analyses in Fig. S26,
the CB and VB edges of semiconductors were calculated, as shown
in Fig. S27. Meanwhile, the VB edge potential of BS7 sample was
determined to be 1.92 V, which is close to the result (1.91 V)
derived from UPS data (Fig. S28). Moreover, it is seen that the VB
edge of BS7 exhibited a distinct downshift compared with other
samples, promising a higher thermodynamical driving force of
photooxidation.
Fig. 5 Nitrogen adsorption-desorption isotherms of the as-prepared samples.
Inset figure shows the BET surface area of all samples.
As the adsorption and activation of molecular oxygen on
photocatalyst surface played an important role in photocatalytic
selective alcohol oxidation, the adsorption energy of O2 on
Bi2Sn2O7 surface was investigated by DFT calculation. The results
of DFT calculation (Table S4) indicated that (2 2 2) plane
containing oxygen vacancies owned an improved adsorption ability
of O2 compared with the defect-free (2 2 2) plane. Hence, the
increased OVs content implies increasing O2 adsorption ability of
BS7, which promises the improvement of photocatalytic
performance. Meanwhile, the surface area had a significant
influence on the photocatalytic property, because the surface active
site was vital to photoctalytic performance of catalyst [27]. The
nitrogen adsorption-desorption isotherms of the obtained products
were presented in Fig. 5. It is clearly seen that Bi2Sn2O7 samples
exhibited a type Ⅳ character. And the surface area of BS7 was
42.40 m2/g, which was larger than that of other samples. In view of
surface chemistry, the larger surface area always owns a higher
phtocatalytic performance on account of more active sites on the
surface. Hence, more active sites on BS7 sample were benificial to
enhancing the adsorption of alcohols, leading to improved catalytic
activity. As is well known, the position of VB maximum which
determines the oxidizing capability of photogenerated holes,
Fig. 6 Surface photovoltage (SPV) spectra (a) and EIS Nyquist plots of
samples (b), Effects of scavengers on the conversion of benzyl alcohol (c),
EPR detection of in situ formed •O2− under different visible light irradiation time
(d).
Surface photovoltage (SPV) measurements were employed to
investigate the photogenerated carrier separation efficiency of
photocatalysts because higher signal intensity usually implies
higher charge separation efficiency [32]. It is noted that BS7 owned
a higher SPV signal intensity than that of other samples and SnO2
as well as Bi2O3 (Fig. 6a and Fig. S29), suggesting the enhanced
charge separation efficiency of BS7 photocatalyst. The
improvement of separation efficiency was also confirmed by
surface photocurrent spectroscopy (Fig. S30). Moreover, the
strongest photocurrent intensity of BS7 in Fig. S31 also clarified
the effective separation of photoinduced hole-electron pairs.
Electrochemical impedance spectroscopy (EIS) is always applied
to measure the charge transfer efficiency and the electrical
conductivity for each corresponding sample. Generally, a smaller
diameter of Nyquist plot represents a smaller resistance of
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semiconductor [33]. As displayed in Fig. 6b the smallest charge-
transfer resistance of BS7 sample implied that a faster interfacial
charge transfer occurred at the surface/interface with irradiation [34,
35]. In brief, these results demonstrated that the improved
photocatalytic benzyl alcohol activity of BS7 was originated from
its higher Sn(Ⅱ) content and the subsequent higher oxygen
vacancies content, leading to the improvement of oxidizing ability,
O2 adsorption on the surface and the photogenerated charges
separation efficiency.
hydrothermal method by regulating the initial Sn(II) content. A
higher Sn(II) content as well as the subsequent higher oxygen
vacancies content, which were beneficial to enhancing O2
adsorption, downshift of valence band edge and efficient
photogenerated charges separation, were the most important
origination of the enhanced benzyl alcohol conversion efficiency
and selectivity toward benzaldehyde for BS7 sample. Controlled
experiments indicated that the active species in the photocatalytic
−
process were photogenerated holes and •O2 active species. This
work will offer a novel strategy for designing the surface properties
of Bi2Sn2O7 catalysts for photocatalytic activity.
In order to more thoroughly understand the origin of the enhanced
photocatalytic performance and selectivity for benzyl alcohol
oxidation over BS7, a series of controlled experiments was carried
out by adding different active species scavengers. The conversion
efficiency was inhibited significantly when benzoquinone (BQ) as
Experimental Section
−
a superoxide radical (•O2 ) scavenger was added (Fig. 6c).
Meanwhile, as the formic acid (HCOOH) was added as holes (h+)
scavenger the conversion efficiency and selectivity both were
decreased apparently (Table S5). In addition, when we replaced O2
with N2, the conversion efficiency was typically sluggish, which
supposed that O2 played an important role in photocatalytic benzyl
alcohol oxidation. Interestingly, aerobic oxidation of benzyl
alcohol was obviously obstructed by the addition CCl4 as electrons
(e-) scavenger. Because O2 cannot directly oxide benzyl alcohol, O2
activated by electrons was therefore vital for benzyl alcohol
oxidation, which can be confirmed by the control experiment on
the adding of CCl4 and N2 [36]. Since, the above observation
Catalysts preparation
Bi2Sn2O7 samples were produced via the simple hydrothermal method.
Briefly, 4.0mmol of Bi(NO3)3·5H2O was dissolved into 40 mL nitric acid
solution (1 M) to get solution A. And different amount of SnCl2·2H2O (4.0,
3.8, 3.5, 3.2, 3.0, 2.8, 2.6, 2.4, 2.2 and 2.0 mmol) was added into 10 mL
deionized water to get solution B. Then A was injected to solution B with
stirred vigorously and a yellow suspension was obtained. The pH value
of the suspension was adjusted to 10 by using 1 M NaOH solution and
further stirred for 1h at room temperature. Finally, the mixture was
transferred into a 100 mL Teflon-lined autoclave and heated at 200 oC for
12h. The resultant precipitates were washed with deionized water
o
−
consecutively and dried 12h at 80 C. The final products were named as
declared that the photogenerated holes and •O2 active species
BS1, BS2, BS3, BS4, BS5, BS6, BS7, BS8, BS9 and BS10, respectively.
Theoretical and experimental quality of the obtained samples was also
presented in Fig. S34. Additionally, SnO2 was prepared at the same
condition without Bi(NO3)3·5H2O and Bi2O3 was obtained without adding
SnCl2·2H2O.
contributed together for the photocatalytic benzyl oxidation activity.
Besides, the synergistic effect between phogenerated holes and
electrons was conducive to a high selectivity achieved. Meanwhile,
as illustrated in Fig. S32, the positive SPV signal indicated that
photogenerated holes accumulated at the surface participating in
the oxidation reaction and the added electron filed had no influence
Catalysts characterization
[37, 38]
on the transfer direction of photogenerated charge carries
Additionally, the EPR signal of •O2 (Fig. 6d) was increased with
prolonged irradiation time which gave a direct evidence of •O2 as
.
−
XRD (X-ray power diffraction) spectra the obtained products were
recorded on a Rigaku DMAX2500 X-ray diffractometer using Cu Kα
radiation. SEM (Scanning electron microscopy) on a HITACHI S-4800
apparatus was used to characterize the micro-morphologies of the as-
prepared catalysts. TEM (Transmission electron microscopy) was
performed on a FEI Tecnai G2 F20 S-TWIN field emission microscope
apparatus using acceleration voltage of 200 kV. Perkin Elmer UV/VS/NIR
Lambda 750 s spectrometer was carried out to investigate the UV-vis
DRS spectra of samples. XPS (X-ray Photoelectron Spectroscopy)
−
a dominated active species during photocatalytic process.
Based on the above analyses, a feasible reaction mechanism for the
benzyl alcohol oxidation over Bi2Sn2O7 photocatalyst was
presented in Fig. S33. Upon visible light illumination, the
photogenerated electrons are excited into the conduction band (CB)
of Bi2Sn2O7, leaving the positively charged holes at the valence
band (VB). OVs on Bi2Sn2O7 surface could serve as active sites to
capture electrons and adsorb oxygen molecule, contributing to the
separation of photogenerated carriers and the activation of oxygen.
Then, the electrons are able to reduce the molecular oxygen
measurements were recorded using an ESCALab220i-XL with
a
monochromatic Al Kα and charge neutralizer. All calculations based on
density functional theory (DFT), using the CASTEP code. The
generalized gradient approximation (GGA) with the Perdew-Burke-
Ernzerhof (PBE) was adopted for treating models. The plane-wave cutoff
energy was set at 300 eV. The total energies were converged to less
than 10-5 eV. The atomic positions and lattice constants were relaxed
until the residual forces became less than 0.01 eV/Å. The transient
photocurrent response of all samples was detected by using Metrohm
Autolab (PGST AT302N) under white (neutral) light irradiation (LED 690
lm). EIS (Electrochemical impedance spectroscopy) spectra were
obtained in the frequency range of 0.1 to 100 KHz. Mott-Schottky plots of
Bi2Sn2O7 were measured with the frequency of 500 Hz, 1000 Hz and
1500 Hz in dark.
-
absorbed on the surface into •O2 species, whereas the holes
remained in VB participated in oxidizing the benzyl alcohol to an
active carbocations intermediate. Then the carbocations react with
-
•O2 species to produce the final product, corresponding to
benzaldehyde [39-41]
.
Conclusions
In summary, a series of flower-like Bi2Sn2O7 photocatalysts
containing oxygen vacancies (OVs) was synthesized by
Evaluation of photocatalytic activity
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The photocatalytic activity of as-prepared samples was estimated in a
quartz tube under visible light irradiation (300 W Xe lamp, λ ≥ 420 nm).
Briefly, 0.05 g of the products was placed in 10 mL methylbenzene
solution involving a certain amount of aromatic alcohol (1 mM) to form a
suspension with persistently stirred at 40 oC in water bath. Before
illumination, anaerobic and aerobic reactions were performed by bubbling
with pure N2 and O2 for at least 1 hour, respectively. After illuminating 10
hours, the suspension was centrifuged to remove the powder and
measured the concentration of the benzyl alcohol and products by GC-
FID (Shimadzu GC-2014C).
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This work is financially supported by the National Natural
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Keywords: Bi2Sn2O7 • oxygen vacancy • band edge •
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This article is protected by copyright. All rights reserved.

10.1002/cctc.201900454
ChemCatChem
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A series of Bi2Sn2O7 photocatalysts
was developed with the aim to
regulate defect chemistry as well as
electronic structure by modulating the
charge states of tin species for
Shushu Huang,[a] Xin Kou,[a] Dan He,[a],
Chunfang Du*[a], Xiaojing Wang[a] and
Yiguo Su*[a]
Page No. – Page No.
boosting
photocatalytic
selective
Oxygen Vacancies Mediated
Photocatalysis over Bi2Sn2O7:
Exceptional Catalytic Activity and
Selectivity
oxidation of alcohols performance.
Photocatalytic benzyl alcohol activity
of Bi2Sn2O7 was originated from its
higher Sn( Ⅱ ) content and the
subsequent higher oxygen vacancies
content, leading to the improvement of
oxidizing ability, O2 adsorption on the
surface and the photogenerated
charges separation efficiency.
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Author(s), Corresponding Author(s)*
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Title
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This article is protected by copyright. All rights reserved.