Visible light-driven decomposition of gaseous benzene on robust Sn 2+-doped anatase TiO2 nanoparticles

Article abstract of DOI:10.1039/c4ra05904b

This work shows the efficient degradation of benzene over robust Sn ²⁺-doped TiO₂ nanoparticles prepared by a facile sol-gel route under visible light irradiation. The structure, optical properties and chemical states of Sn species incorporated into anatase TiO₂ were carefully characterized by X-ray diffraction, transmission electron microscopy, Raman, UV-vis diffuse reflectance, X-ray photoelectron, X-ray absorption, and electron spin resonance (ESR) spectroscopy. The maximal conversion rate of benzene achieved is up to 27% over the Sn/TiO₂ with a Ti/Sn atomic ratio of 40 : 1 and remains constant for a cyclic run of six days, indicating the high photo-stability for the decomposition of benzene. The characterization results reveal that the Sn²⁺-doping narrows the band gap energy of anatase TiO₂, leading to a visible-light response. The photocatalytic degradation pathway of benzene was proposed based on the results of ESR and Fourier transform infrared spectra. These results offer a full comprehension of the visible light photocatalysis of Sn²⁺-doped TiO₂ for degradation of volatile organic pollutants. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra05904b

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PAPER
Visible light-driven decomposition of gaseous
2
+
benzene on robust Sn -doped anatase TiO
2
Cite this: RSC Adv., 2014, 4, 34315
nanoparticles†
Huaqiang Zhuang, Quan Gu, Jinlin Long,* Huan Lin, Huaxiang Lin and Xuxu Wang*
2+
This work shows the efficient degradation of benzene over robust Sn -doped TiO
by a facile sol–gel route under visible light irradiation. The structure, optical properties and chemical states
of Sn species incorporated into anatase TiO were carefully characterized by X-ray diffraction, transmission
2
nanoparticles prepared
2
electron microscopy, Raman, UV-vis diffuse reflectance, X-ray photoelectron, X-ray absorption, and
electron spin resonance (ESR) spectroscopy. The maximal conversion rate of benzene achieved is up to
2
27% over the Sn/TiO with a Ti/Sn atomic ratio of 40 : 1 and remains constant for a cyclic run of six days,
indicating the high photo-stability for the decomposition of benzene. The characterization results reveal
2+
Received 18th June 2014
Accepted 29th July 2014
2
that the Sn -doping narrows the band gap energy of anatase TiO , leading to a visible-light response.
The photocatalytic degradation pathway of benzene was proposed based on the results of ESR and
DOI: 10.1039/c4ra05904b
Fourier transform infrared spectra. These results offer a full comprehension of the visible light
2+
www.rsc.org/advances
2
photocatalysis of Sn -doped TiO for degradation of volatile organic pollutants.
1,3,7–13
under UV light irradiation,
but such photocatalysts were
1
. Introduction
severely restricted in practical application due to several
Benzene is a notorious pollutant released largely from shoe- intrinsic drawbacks: (1) the light response of those photo-
making, chemical industries, house paints, and so forth. Expo- catalysts is limited in UV range of solar spectrum, consequently
sure to the hazardous compound, even at a low level of 1 ppm, causing a low quantum efficiency; (2) the photooxidation of
increased risk of leukemia and induces some fatal illnesses, benzene exhibits commonly a quick deactivation due to a large
1,2
such as cancer, teratogenicity, etc. The chemical, physical, and amount of carbon deposit on the surface of photocatalysts; (3) a
biological treatments including catalytic combustion, adsorp- large part of exploited photocatalysts are composed of rare
tion, and absorption have been applied widely for removal of metals, besides TiO
benzene from the environment. However, these cleanup tech- broaden the light response into visible light, Li et al. prepared a
2
, and thus greatly increase the use-cost. To
3
nologies are inefficient for a ppm concentration of benzene in series of nanocomposites by coupling of TiO
exhaust gas. Deep removal of benzene and its derivatives is still a with some visible-light responsive vanadates, such as InVO
formidable challenge for environmental scientists.
BiVO , LaVO , and Ag VO , and showed a promising efficiency
Photocatalytic oxidation by light-excited semiconductors has for the degradation of benzene under visible light.
nanocrystalline
2
,
4
4
4
3
4
14–17
But the
been shown as a promising technology to treat air and water photoactivity of the optimal photocatalyst lasted only for 30–50
contaminations derived from volatile organic compounds hours. The high cost and quick deactivation are two crucial
4–6
(
VOCs), due to its environmental friendly and low cost. Much disadvantages for the practical application. Therefore, consid-
effort has been devoted to the development of efficient photo- ering the large-scale application of the photocatalytic tech-
catalysts for removal of VOCs. The plenty of inorganic materials nology in environmental purication and remediation, the
including TiO
ZnSn(OH) , Cd
shown to be photocatalytic active for the oxidation of benzene focus of research in photocatalysis community.
Titanium dioxide (TiO ) is the most promising photocatalyst
2
, Zn
2
GeO
4
, b-Ga
2
O
Sb O
3
, GaOOH, In(OH)
3
, InOOH, development of cheap photocatalysts with robust photostability
have been reported and for visible light-driven oxidation of gaseous benzene remains a
6
2
Sb
2
O
6.8, and Sr
2 2 7
2
for solving current environmental pollution, due to its strong
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy oxidizing ability, high chemical stability, nontoxicity and rela-
18
and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, People's tively low cost. Unfortunately, owing to the large band gap of
Republic of China. E-mail: jllong@fzu.edu.cn; xwang@fzu.edu.cn; Web: http://chem.
3
.2 eV, it only can be excited by UV light to generate electron–
fzu.edu.cn/szdw/teacherinfo.aspx?id¼40; Fax: +86-591-83779121; Tel: +86-591-
19
hole pairs. Metal and nonmetal doping has been well-
established to be the most simple and effective way to extend
its excitation wavelength to visible light. Commonly, doping
8
†
1
3779121
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra05904b
20
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2
TiO with nonmetal elements such as F, B, P, N, S, C, and I can were uniformly dispersed into 40 mL absolute ethanol by
create signicant absorptions in visible light region by intro- ultrasonication for 5 min, and a certain amount of SnCl $2H O
2
2
ducing a deep energy level in valence band structure of the was added in 40 mL distilled water under vigorous magnetic
semiconductor, attributing to the substitution for lattice oxygen stirring. Then, the tetrabutyl titanate ethanol solution was
21–27
atoms.
2
But the metal doping is a more popular method to added dropwise into the SnCl aqueous solution under vigor-
prepare photofunctional materials, because it oen leads to a ously stirring at room temperature and the yellow solid was
red shi of the whole band edge. Among these oen-used metal precipitated. Aer 1 h of the magnetic stirring, the suspension
ꢀ
dopants including Cr, Mn, Co, V, In, Fe, Bi, Sn, Ni, Ce, and other solution was heated at 90 C on oil bath for 1 h. Finally, the
2
8–34
rare earth (RE) metals,
only Bi was found to be effective for obtained yellow solid was washed with distilled water and then
ꢀ
the photocatalytic decomposition of benzene under visible light dried at 100 C for 12 h. The resultant solid samples were denoted
31
irradiation. La-doped anatase TiO was only UV light active for as TS-x (where x represents the atomic ratio of Ti/Sn, x ¼ 100, 60,
2
35
the decomposition. Relative to Bi, V, Cr, Co, Mn, and RE, Fe 50, 40, 30, 20, and 5, respectively). N-doped TiO as a reference
2
36–39
ꢀ
and Sn are two cheaper transitional metals.
A number of sample was synthesized by calcining anatase TiO
studies showed that the incorporation of Sn into anatase TiO under dry NH ow 10 h, which was designated as N-TiO
signicantly enhanced the photocatalytic activity for decolor-
2
NPs at 450 C
4
+
2
3
2
.
40–43
ation of dyes and hydrogen production under UV light.
Venkata et al. found that Sn/TiO prepared by using SnCl as a
2 2
Sn source showed excellent photocatalytic activity for degrada-
tion of RhB and cresol under visible light irradiation. Anna
et al. suggested that Sn species in Sn/TiO2 contributed
2.2 Catalyst characterization
44
XRD measurement was taken on a Bruker D8 Advance X-ray
˚
diffractometer with Cu Ka radiation (l ¼ 1.5406 A). Raman
spectra were surveyed with 785 nm excitation at room temper-
ature on a Renishaw Raman microscope. UV-vis DRS spectra
were recorded on a Varian Cary 500 Scan UV-vis-NIR spectrom-
45
2+
mainly to the visible light photocatalysis. A few reports also
studied the incorporation of Sn into tantalates and niobates.
2
+
46
eter with BaSO
4
as the reference. The Brunauer–Emmett–Teller
2
+
Recently, we also found that the Sn dopant can induce
hydrogen production over SnO nanoparticles (NPs) under
visible light irradiation. These works inspired us to exploit the
(
7
BET) specic surface areas were determined by N adsorption at
7 K on a Micromeritics ASAP 2020. XPS spectra were carried out
2
2
47
on a VGESCALAB 250 XPS system with a monochromatized Al Ka
X-ray sources (15 kV 200 W 500 mm pass energy ¼ 20 eV). All
binding energies were referenced to the C 1s peak at 284.6 eV of
surface adventitious carbon. HRTEM images were obtained by a
JEOL model JEM 2010 EX instrument at an accelerating voltage
of 200 kV. XAFS spectra were surveyed at Beamline BL14W1 of
Shanghai Synchrotron Radiation Facility (SSRF), with the use of
a Si (311) double-crystal monochromator. The FTIR experiments
were performed in a special IR cell in conjunction with a vacuum
system, and the catalyst powders were rst pressed into a self-
supporting IR disk (35–40 mg, 18 mm diameter), subsequently
the disk was placed in the sample holder that could be moved
vertically along a cell tube. Before the FTIR measurement was
2
+
new application of Sn -doped semiconductor photocatalysts in
degradation of VOCs.
In this study, we report the rst demonstration of visible-
light photocatalytic decomposition of gaseous benzene using
relatively cheap Sn-doped anatase TiO
materials with different Ti/Sn atomic ratios were prepared by a
2
facile method of using tetrabutyl titanate and SnCl as Ti and
Sn sources. The structural, optical, and photocatalytic prop-
erties of Sn-doped anatase TiO NPs were systematically char-
acterized by X-ray diffraction (XRD), N2 physical adsorption,
transmission electron microscopy (TEM), X-ray absorption ne
structure analysis (XAFS), Raman, X-ray photoelectron (XPS),
UV-vis diffuse reectance (UV-vis DRS), electron spin resonance
ESR), and Fourier transform infrared (FTIR) spectroscopies.
The characterization results reveal that Sn species doped into
2
anatase TiO are co-present in the form of Sn and Sn . With
decreasing Ti/Sn atomic ratios, the absorption band edge of the
material appears a signicant red-shi, corresponding to the
decrease in band gap energy. It was found that the Sn-
2
nanoparticles. Such
4
+
2
+
2
ꢁ
4
carried out, the disk was treated under dynamic vacuum (10
(
Torr) at 333 K for 2 h. Aer the disk cooled to room temperature,
benzene (10 mL) was introduced into the cell with a syringe via
the septum. Aer 30 min of benzene adsorption, the sample disk
was then irradiated with a 500 W Xe-arc lamp equipped an UV-
cutoff (l > 420 nm) as the visible light source. During illumina-
tion, the infrared spectra of the disk were recorded regularly on a
Nicolet 670 FTIR spectrometer with a deuterated triglycine
sulfate (DTGS) detector at a resolution of 4 cm and 32 scans.
ESR spectra were measured using a Bruker model A300 spec-
trometer with a 500 W Xe-arc lamp equipped with an UV-cutoff
2
+
4+
incorporated TiO
2
photocatalyst can not only work under
visible light, but has robust photostability for the photocatalytic
decomposition of benzene. It can keep very constant photo-
reactivity aer a cyclic run of six days. A possible molecular
pathway for the visible light photocatalytic decomposition of
benzene over Sn-incorporated TiO was proposed based on the
ESR and FTIR characterizations.
ꢁ
1
(
l > 420 nm) as the visible light source. Typically, the water
2
solution of DMPO was rst added to 5 mg catalysts. The DMPO–
catalyst mixture was sampled using a capillary tube and placed
in an ESR tube for the ESR experiments.
2. Experimental
2.1 Catalyst preparation
2.3 Photocatalytic activity measurements
2
+
The Sn -doped TiO
2
NPs were synthesized by a sol–gel route. In The photocatalytic degradation of gaseous benzene was carried
the typical preparation, rstly, 4 g tetrabutyl titanate (Ti(OBu)
4
)
out in a xed-bed photoreactor. The photocatalyst (0.45 g, 60–80
34316 | RSC Adv., 2014, 4, 34315–34324
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mesh) was loaded in a 2 cm-width, 3 cm-long quartz glass
reactor connected to a gas chromatograph (GC, HP6890). The
experiment was performed in a closed circulation reaction
system and a 500 W Xe arc lamp equipped with a UV cutoff (l >
420 nm) was used as the visible light source. Benzene was kept
in ice water to maintain the system temperature and diluted in a
pure oxygen stream. The ow rate of the reaction system was
ꢁ
1
kept at 20 mL min . Meanwhile, the equilibrium concentra-
tion of benzene was determined to be 200 ppm and the initial
concentration of CO was 0 ppm. The concentration of benzene
2
and CO2 was determined by an online gas chromatograph
equipped with a ame ionization detector, a thermal conduc-
tivity detector, and a Porapak R column. Benzene was found to
be stable in the catalyst loaded reactor without visible light
illumination, and no degradation of benzene was observed
when it was illuminated in the absence of catalyst.
3. Results and discussion
3.1 Structural and optical characteristics of the TS-x samples
The XRD patterns of the undoped TiO and Sn-doped TiO
2
2
samples (denoted as TS-x) with different Ti/Sn atomic ratios are
showed in Fig. 1A. For the TS-x samples, they exhibit only
diffraction peaks of anatase TiO
assigned to SnO, Sn or SnO are discernable, showing that
the incorporation of Sn species doesn't alter the crystalline
structure of TiO
conrms the conclusion. All of diffraction peaks for the TS-x Insert shows the plots of [F(R)hn] vs. photon energy (hn).
samples don't shi compared to the undoped TiO , implying
that Sn species may be highly dispersed into bulk of anatase
TiO or on the surface. In addition, it appears that the diffrac- TS-x samples also originates from the Sn incorporation, which
2
, and no diffraction peaks
2
O
3
2
Fig. 1 (A) The XRD patterns of as-prepared TS-x samples and undoped
2
. The Raman study (Fig. S1,† see ESI) further TiO . (B) UV-vis DRS spectra of undoped TiO and TS-x samples. The
2 2
1/2
2
2
+
2
2
+
tion peaks of the TS-x samples are weaker and broader maybe gives rise to a hybridized group (Sn 5s–O 2p) at the
compared to the undoped TiO
sample, showing the lower valence band level in the TiO , or the defects formed by the
crystallinity and smaller particle size. As seen from Table 1 doping.
2
2
listing their average particle size, which was calculated using
the Scherrer formula, the average particle size of the TS-x TiO
The morphologies and structural details of the undoped
and TS-40 samples are further observed by TEM and
2
samples slightly decreases due to the incorporation of Sn HRTEM, as shown in Fig. 2. The Fig. 2A and D show their TEM
species. N adsorption results (Fig. S2,† see ESI) show that the images, a large quantity of NPs with a diameter of 5–7 nm are
2
doping greatly changes pore size distribution and increases BET discernable, corresponding to the XRD results reported above.
specic surface area. Therefore, it can be concluded that the Sn The HRTEM images (Fig. 2B and E) display the typical lattice
incorporation has a considerable inuence on crystallinity, fringes of d-spacing of about 0.35 nm, which is in good agree-
particle size, and textural properties of anatase TiO
changes don't contribute to their visible-light activity, which corresponding to metallic Sn and Sn oxides are not observed for
will be conrmed later.
the TS-40 sample, but the corresponding EDX pattern conrms
2
. But these ment with the (101) plane of anatase TiO . Any crystal structures
2
The Sn incorporation also greatly changes the optical prop- the existence of tin as shown by the weak uorescence signal
erties of anatase TiO , as shown in Fig. 1B representing the UV- (Fig. 2F). The EDX mapping spectra (Fig. 2G–J) show homoge-
2
vis DRS spectra of the TS-x samples and undoped TiO2.
The undoped TiO
exhibits a band-edge absorption in UV These results conclusively indicate that the Sn dopant is highly
neous distribution of Sn, Ti, O elements into the TS-40 sample.
2
19
region corresponding to the bang-gap energy of 3.2 eV. Aer dispersed into TiO
doping Sn, the band-edge clearly shis towards visible light
.
2
region. As shown in the inset in Fig. 1B, the band gap narrows
gradually with increasing Sn content. The TS-5 sample with the
highest Sn content displays the lowest band gap energy of ca.
3
.2 Visible-light photocatalytic activity for benzene
photodegradation
2
.95 eV, decreasing by 0.25 eV as compared with the undoped The photocatalytic activity of these as-prepared TS-x samples
2
+
TiO . This is consistent with the results of Sn -doped Sn Ta O , was evaluated by the gas-phase photocatalytic decomposition of
2
2
2 7
Sn
ture.
2
Nb
2
O
7
, SnO
2
, TiO
2 2 2 7
, and Ca Ta O reported in litera- benzene under visible light irradiation and compared with that
4
4,46,47
Thus, it is certain that the visible light response of the of N-doped TiO
2
(N-TiO
2
) as the reference photocatalyst, as
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Table 1 The physicochemical characteristics of as-prepared TS-x samples and undoped TiO
2
Entry
TiO
0
228.0
6.5
0
2
TS-100
TS-60
TS-50
TS-40
TS-30
TS-20
TS-5
Sn content (wt%)
1.5
255.7
5.9
28.6
6.4
2.5
236.8
5.9
21.3
10.8
3.0
248.4
5.9
20
10.5
3.7
248.8
5.6
18.5
17.9
5.0
260.7
5.8
13.7
15.0
7.4
263.6
5.7
9.3
11.9
29.7
257.1
5.4
3.1
7.6
2
ꢁ1
S
D
BET (m g
)
b
A
(nm)
a
Ti/Sn (at%)
Sn /Sn (at%)
2+
4+a
—
a
b
Calculated from the XPS results. The size of anatase particle calculated from the XRD results.
Fig. 2 TEM and HRTEM images of undoped TiO
2
(A and B) and TS-40
(
D and E) samples. The figures (C and F) show the EDX patterns of
2
undoped TiO and TS-40 samples, respectively. The Figures (G–J)
show the EDX mapping images of Sn, Ti and O elements in the TS-40
sample.
shown in Fig. 3. All of the samples are visible light active for the
photocatalytic decomposition of benzene. It appears that upon
light on, the conversion rate of benzene quickly rises and rea-
ches a maximum, along with the considerable decrease. All
photocatalysts eventually achieve a steady-state conversion of
benzene aer more than 200 min of light illumination (Fig. 3A),
while the mineralization of benzene into CO basically keeps an
2
initial constant (Fig. 3B). More importantly, all of the TS-x
samples exhibit a higher conversion of benzene and a higher
concentration of produced CO
seen from Fig. 3C that with decreasing Ti/Sn atomic ratio, the
conversion of benzene and the concentration of CO produced
2 2
, compared to N-TiO . It can be
2
increase, and reach a maximum at Ti/Sn atomic ratio of 40,
along with the decrease. But the TS-40 sample with the highest
conversion of benzene gives a moderate mineralization of about
86%, less than that (92.6%) of the TS-20 sample. Considering
this high mineralization, the TS-20 sample was chosen as a
representative to further examine the photostability of catalysts
for the reaction, which is the most pivotal assessment for the
practical application of photocatalysts. As shown in Fig. S3† (see
Fig. 3 (A) The conversion of benzene and (B) the yield of produced
CO over N-TiO TS-20, TS-30, TS-40 and TS-100 samples,
respectively. (C) The change of benzene degradation with different Ti/
2
2
,
Sn atomic ratios.
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ESI), the catalyst maintains a benzene conversion rate of ca. the Mossbauer spectroscopy. Aer calcination in air, the color
1.7% and a CO2 yield of ca. 130 ppm during continuously of these TS-x photocatalysts transforms from yellow into white,
1
running for 6 days. It is important to note that no obvious losing the visible light absorption. These results further indi-
deactivation is observed for the catalyst, indicating the prom- cate that the visible light response of the as-prepared TS-x
2
+
ising application in the photocatalytic air purication. catalysts results from Sn species and defects. It is judged from
2
+
Compared to all of the visible light photocatalysts reported so the intensity of Sn 3d XPS peaks that the Sn species are minor
4
+
far, the TS-x materials are the most robust visible-light photo- in all of the TS-x samples, while the Sn species are predomi-
catalysts for the degradation of benzene. nant. As listed in Table 1, the TS-40 sample gives the largest
2
+
4+
Sn /Sn atomic ratio of 17.9%, corresponding to the highest
conversion of atomic ratio as demonstrated in Fig. 4D. The near
linear relationship further conrms that the visible light
decomposition of benzene is closely related to the Sn dopant.
Interestingly, the Ti/Sn atomic ratio determined by XPS for all of
the TS-x samples is far lower than the theoretical value that is
calculated by feedstock, conclusively indicating that a majority
3
.3 Chemical states of Sn species
2
+
To reveal the origin of visible-light photocatalysis, XPS was rst
applied to characterize the chemical states of Sn dopants in
TiO . Fig. 4 shows the XPS spectra of Ti 2p, O 1s, and Sn 3d for
2
the undoped TiO and TS-x samples. The Ti 2p3/2 and Ti 2p1/2
binding energies locate at 458.5 and 464.3 eV, respectively,
which can be attributed to Ti in TiO (Fig. 4A). The O 1s XPS
spectra for all the TS-x samples are consisted of two peaks at
approximately 530 and 531.6 eV (Fig. 4B). The main peak at ca.
2
of Sn species are richened on the surface layer of anatase TiO
2
4
+
NPs. This is to say, the Sn species represent a gradient distri-
bution into TiO2 NPs, gradually decreasing from outside to
inside. This result also implies that the surface layer of NPs is
the main contributor to the visible light photocatalysis.
2
530 eV is assigned to the lattice oxygen atoms in TiO
2
, and the
The geometric structure of Sn species doped into TiO
further characterized by XAFS. Sn K-edge XAFS data were
obtained for SnO, SnO
, and TS-x (x ¼ 5, 40, and 60). The XAFS
data of SnO and SnO2 are used as references therein. The
normalized Sn K-edge XANES spectra of SnO, SnO , TS-5, TS-40,
and TS-60 are shown in Fig. 5A. The Sn K-edge positions of TS-5,
-40, and -60 are closer to that of the reference SnO , instead of
SnO, indicating that the majority of tin doped into TiO2 is
present in the form of +4 oxidation state. According to the E
2
was
shoulder at 531.6 eV is attributed to the surface hydroxyls of
TiO
be tted with four peaks. The set of peaks centered at 485.8 and
20
2
. The Sn 3d XPS spectra of the TS-x samples (Fig. 4C) can
2
2
+
494.6 eV belongs to Sn 3d5/2 and Sn 3d3/2 core levels of Sn in
2
tin-oxides doped into TiO , and another set of binding energies
locating at 486.6 and 495.0 eV is corresponding to Sn in tin-
oxides doped into TiO
2
coexistence of Sn and Sn in the Sn-doped TiO materials by
2
4
+
4
8,49
50
2
2
.
Moumita et al. also proved the
2
+
4+
0
2
+
position (Fig. 5B), it can be concluded that the Sn content
decreases in the order of TS-40 > TS-5 > TS-60. These results are
in line with the XPS results.
2
The k -weighted Fourier transforms (absolute part) of Sn K-
edge EXAFS spectra of the TS-x photocatalysts are displayed in
Fig. 5C. There is only one maximum between 1.0 and 2.0 A˚ ,
which is assigned to the oxygen atoms surrounding the tin
absorber in the rst shell (without phase correction). Only one
weak back-scattering arising from Sn–Sn coordination shell is
˚
discernable in the radial distribution function at 2.8 A, indi-
cating the presence of oligonuclear tin–oxo species. A model
consisting of a single tin absorber coordinated to one shell of
oxygen atoms and one shell of tin atoms was used to t the
Fourier-ltered EXAFS spectra. The best ts are shown in
Fig. S4† (see ESI), and the t results are given in Table 2. The
sum of the O neighbors is approximately 5 for the TS-x (x ¼ 5,
˚
4
0, 60) samples. The average Sn–O distance of 2.03 A in the TS-x
˚
samples is far lower than that (2.20 A) of SnO, closer to those
˚
(
2.04 and 2.05 A) of SnO
2
. Moreover, each tin atom is coordi-
nated, on average, to approximate one tin atom with an average
˚
Sn–Sn distance of ca. 3.15 A. Also, the Sn–Sn distance is far
˚
lower than the Sn–Sn1 distance (3.52 A) of SnO, closer to that
˚
(
3.17 A) of SnO . It can be thus concluded that Sn species
2
present as SnO
x
nanoclusters into TiO
2
. Unluckily, the
detailed composition of these SnO
further claried due to the intrinsic limitation of the XAFS
technique.
x
nanoclusters can't be
Fig. 4 The high resolution XPS spectra of Ti 2p, O 1s and Sn 3d (A, B
and C, respectively) for the undoped TiO and TS-x samples. (D) The
plot of conversion of benzene vs. Sn /Sn atomic ratio for the TS-x
2
4+
2+
samples.
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Fig. 5 (A) Normalized Sn K-edge XANES spectra of SnO, SnO
TS-60 samples in normalized Sn K-edge XANES spectra. (C) Fourier transforms of the Sn K-edge k -weighted EXAFS spectra of the samples.
2
, TS-5, TS-40 and TS-60 samples. (B) The E
0
positions of SnO
2
, TS-5, TS-40 and
2
Table 2 Fit results from EXAFS spectra shown in Fig. 5
vacancies at g ¼ 2.004 is lack of change, indicating that no
oxygen vacancies are formed on the surface with visible light
irradiation. This is to say, surface oxygen vacancies are not
responsible for the visible light excitation, consequently it
doesn't contribute to the visible light-driven decomposition of
benzene.
2
2
˚
s (A )
0
DE (eV)
(10%)
˚
Samples
Shell
CN (15%)
R (A) (10%)
(10%)
SnO
2
Sn–O1
Sn–O2
Sn–Sn1
Sn–Sn2
Sn–O
Sn–Sn1
Sn–Sn2
Sn–Sn3
Sn–O
Sn–Sn
Sn–O
Sn–Sn
Sn–O
4.0
2.0
2.0
4.1
4.0
4.0
4.0
4.0
5.2
0.7
5.0
0.8
5.0
1.3
2.04
2.05
3.17
3.73
2.20
3.52
3.66
3.80
2.03
3.15
2.03
3.15
2.03
3.16
0.0078
0.0004
0.0039
0.0015
0.0067
0.0044
0.0045
0.0054
0.0050
0.0030
0.0049
0.0041
0.0045
0.0064
13.2
10.7
8.4
3+
The evolution of the F and Ti lines with irradiation time
16.8
15.6
14.9
14.8
14.5
10.7
ꢁ5.4
11.0
ꢁ5.7
11.4
ꢁ5.2
was observed for the TS-40 catalyst, as shown in Fig. 6C. Their
intensity increases exponentially with the irradiation time, and
tends to a plateau aer more than 200 s of irradiation. It is
noticeable that there is no relationship between the two ESR
signals. It can be thus concluded that the electrons trapped on
SnO
TS-5
TS-40
TS-60
Sn–Sn
3.4 ESR study on the visible light photo-excitation of TS-x
samples
To further comprehend the visible light-excited mechanism and
the probable pathway of electron transfers under visible light
irradiation, ESR spectroscopy was employed to survey the TS-x
samples at low temperature (100 K). As shown in Fig. 6A, all of
the TS-x samples, including the undoped TiO , show one weak
2
ESR line at g ¼ 2.004. It is denitely attributable to oxygen
51
vacancy created near the surface based on literature. But it is
noticeable that aer 120 s of visible light irradiation (Fig. 6B),
there occur several new ESR signals at g ¼ 1.998, 1.988, and
52–54
1.958 for all of the TS-x samples. According to literature,
the
set of ESR signals at g ¼ 1.988 and 1.958 is unambiguously
3
+
assigned to surface Ti species originated from the electron-
4
+
trapping of surface-layer Ti cations, and the line at g ¼
.998 belongs to a bulk localized defect, such as an electron
trapped on a lattice site (F center), O species formed by hole-
1
Fig. 6 (A) ESR spectra of the undoped TiO
without light irradiation. (B) ESR spectra of all samples after 120 s of
visible light irradiation at 100 K. (C) The evolution of the F and Ti lines
2
and the TS-x samples
+
ꢁ
3+
trapping. The g ¼ 1.998 line is not observed on the undoped
TiO
2
under visible light irradiation, showing that it originates with irradiation time under visible light illumination. (D) ESR signals of
the DMPO–cOH spin adducts for the TS-40 and undoped TiO
samples without irradiation and with visible light irradiation.
2
from the Sn doping. The line intensity of surface oxygen
34320 | RSC Adv., 2014, 4, 34315–34324
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+
4+
2ꢁ
55
F centers and Ti cations as well as the holes trapped by O
electrons, respectively. The DMPO spin-trapping ESR tech-
2
+
ions should result from the photoexcited Sn dopant. The nique was employed to monitor the formation of these reactive
visible light excitation for these TS-x catalysts can be described oxygen species, as shown in Fig. 6D. Upon irradiation of the
as follow.
catalyst–water–DMPO suspension with visible light, four char-
acteristic ESR lines belonging to the DMPO–cOH adduct are
clearly observed for the TS-40 photocatalyst, while the undoped
TiO doesn't appear any ESR signals. It indicates that water can
2
be really oxidized by the irradiated TS-x catalysts to form
CBꢁ
4+
+
+ hVB visible light irradiation
TS-x + hv / e
(1)
(2)
(3)
(4)
ꢁ
3+
eCB + Ti / Ti trapped electron
hydroxyl radicals. Moreover, when the catalyst is dispersed into
CBꢁ
+
e
+ F / F trapped electron
methanol solution and irradiated with visible light, six weak
ꢁ
+
2ꢁ ꢁ
ESR lines corresponding to the DMPO–c O adduct are
2
h
VB + O / O trapped hole
discernable (Fig. S6,† see ESI), suggesting the smooth transfer
of photogenerated electrons from TS-x NPs to adsorbed O .
2
Based on the ESR line intensity of the two adducts, it can be
concluded that hydroxyl radicals are the main active species,
2
+
According to the results reported above, the Sn species are
heterogeneously doped into the bulk of TiO as nanoclusters
2
with a low nuclearity or isolated atoms. The electronic transi-
tion for the TS-x catalysts can be regarded as the band-gap
transition from the valence band consisting of Sn 5s and O 2p
orbitals to the conduction band consisting of Ti 3d orbitals. The
Mott–Schottky result shows no any changes in conduction-band
which play a crucial role in mineralization of benzene into CO
and H O.
2
2
To further clarify the details of the photocatalytic oxidation
for benzene, in situ FTIR spectroscopy was used to investigate
the adsorption and photo-reactivity of benzene on the surface of
the optimal catalyst TS-40. Some important intermediates were
clearly identied by the technique. Fig. 7 shows the FTIR spectra
of the TS-40 surface under benzene atmosphere during visible
light irradiation. Aer heat treatment under dynamic vacuum at
2
+
structure of Sn -doped TiO , which still illuminates the n-type
2
semiconducting feature and the at-band potential of ꢁ0.31 V
vs. NHE (Fig. S5,† see ESI). Together with the UV-vis DRS results,
it can be concluded that the valence band level of the TS-x
catalysts become more negative, relative to that of the pure
3
cm
33 K for 2 h, the FTIR spectrum in the region of 1100–2200
2
TiO , because it is controlled by the contribution degree of Sn 5s
orbitals as shown in Scheme 1. When the TS-x photocatalysts
ꢁ
1
of the TS-40 catalyst exhibits several vibration bands
ꢁ
1
(Fig. 7A-a). The absorption band at 1628 cm , corresponding to
are illuminated by visible light, the photogenerated electrons
2
+
4+
the bending vibration of water molecules, is the most prom-
inent, indicating that a considerable number of water mole-
cules remain on the catalyst surface. The three bands occurring
at 1125, 1071, and 1040 cm are corresponding to character-
istic absorption peaks for the OR group of titanium tetrabut-
are quickly transferred from the Sn chromophores to the Ti
3
+
+
neighbors to form Ti species, or to F centers. The photo-
generated holes are trapped by nearby lattice oxygen ions to
ꢁ
1
ꢁ
form O species, which are capable of oxidizing H
2
O to generate
hydroxyl radicals.
56,57
oxide, which is the precursor of the catalyst.
Upon
introduction of benzene, there appears a sharp band at
ꢁ
1
3
.5 Mechanism of visible light-driven decomposition of
1478 cm attributed to the C–C stretching vibration of the
ꢁ
1
benzene over TS-x samples
aromatic ring and two weak bands at 1825 and 1968 cm
assigned to the C–H out-of-plane bending vibration of
In general, the photocatalytic decomposition of VOCs by semi-
conductors mainly results from active oxygen species including
ꢁ
hydroxyl radicals (cOH) and superoxide anion radicals (O c ),
2
which are produced by the oxidation of water by photo-
generated holes and the reduction of O by photogenerated
2
Fig. 7 (A) FTIR spectra of the TS-40 photocatalyst after (a) degassing
ꢀ
at 60 C for 2 h, (b) adsorption of benzene for 30 min at room
temperature, and with visible light irradiation for (c) 10, (d) 60, (e) 120
and (f) 180 min. (B) The subtracting FTIR spectra between TS-40
adsorbing benzene and TS-40 sample: (a) adsorption of benzene for
30 min, (b) 10, (c) 60, (d) 120 and (e) 180 min.
Scheme 1 Proposed energy diagram for the TS-x material.
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Scheme 2 Proposed reaction pathways for photo-oxidation of benzene on TS-x materials.
58,59
benzene,
indicating that benzene molecules are absorbed on ultimately oxidized to CO
2
and H
2
O via a series of the involve-
the catalyst surface (Fig. 7A-b). Visible light irradiation gives rise ment of hydroxyl radicals. According to the ESR results-reported
to tremendous changes in FTIR spectra of the catalyst (Fig. 7A- above that the TS-40 catalyst represents weak ESR signals
c–f). Some new bands attributed to products and intermediates belonging to superoxide anion radicals, Process I is certainly
appear in the FTIR spectra and increase in intensity with irra- minor in the visible-light-driven decomposition of benzene over
2
+
diation time. The subtracting spectra between TS-40 adsorbing robust Sn -doped TiO
benzene and TS-40 are able to more clearly illuminate the suggest that the crucial intermediate, phenol, comes predomi-
changes, as shown in Fig. 7B. An intense negative absorbance nantly from process II and the reaction of O molecules with
c , because the control experiment proves that the pres-
gesting the substitution of benzene for part of adsorbed water ence of O molecules is obligatory for the photocatalytic
molecules. The six new bands at 1258, 1348, 1364, 1442, 1536, decomposition of gaseous benzene. This may be the main
2
catalysts. It was more reasonable to
2
ꢁ
1
+
occurs at 1628 cm aer adsorbing benzene (Fig. 7B-a), sug-
6 5
C H
2
ꢁ
1
1
595 and 1640 cm increase in intensity with increasing irra- origin of high photostability for the benzene photooxidation
diation time, indicating that they are the products or interme- over the TS-x materials, which needs be further studied.
diates arising from the benzene photooxidation. According to
59–61
ꢁ1
literature,
the three bands at 1258, 1364 and 1595 cm are
4
. Conclusions
consistent with the characteristic peaks of adsorbed phenol,
ꢁ
1
and the three bands at 1348, 1442 and 1536 cm result from
the formation of carboxylates on the catalyst surface. The 1640
2+
In this work, we rst demonstrate that the Sn -doped TiO2
nanoparticles are excellent and promising photocatalysts for
the visible-light-driven decomposition of benzene with high
photostability. An optimal Ti/Sn atom ratio is achieved to be
ꢁ
1
cm
band can be attributed to carbonyl-containing adsor-
59
ꢁ
bates. Although these organic molecules containing –COO
and C]O groups can't be identied completely by the FTIR
technique, a possible molecular pathway can be proposed to
elucidate the visible light photocatalysis for the decomposition
of benzene, together with the previously-reported results by
40 : 1 for the photocatalyst. The detailed characterizations
2+
x
reveal that the Sn dopant presents as SnO nanoclusters with a
low nuclearity. The doping doesn't affect the conduction-band
structure of anatase TiO , but introduces a hybridized group
2
11
62,63
64
Hisahiro, Thuan,
and Zhong. As demonstrated in Scheme
2+
(Sn 5s–O 2p) at the valence band level of anatase TiO2,
2
, the photooxidation of benzene mainly involves the oxygen
consequently the considerable decrease in optical band gap of
the TS-x materials. The products and intermediates are well-
identied by in situ FTIR and ESR spectroscopies, and show
some important information on the photooxidation process.
The involvement of hydroxyl radicals formed by the hole-
oxidation of water should be the main origin of photocatalytic
decomposition of benzene under visible light.
65–67
and hole transfers from catalyst to reactive substrates.
incorporated Sn species can be excited with visible light to
The
2
+
produce electron–hole pairs. Electrons are efficiently captured
4
+
3+
ꢁ
2 2
to O c .
by Ti species to form the Ti species, which reduce O
Holes not only react with the absorbed benzene to generate the
+
6 5
benzene radical cation (C H c ), but also oxidize directly the
adsorbed water molecules to form cOH. There are three
68
possible pathways to produce the phenol intermediates from
+
ꢁ
the C
6
H
5
c . The rst one should be the reaction of O
2
c
or O
2
Acknowledgements
+
with C H c to form a peroxide, which can be further converted
6
5
This work was nancially supported by the NSFC (Grants no.
21373051 and U1305242), the Science and Technology project of
the Education Office of Fujian Province of P. R. China (JA12017),
and National Basic Research Program of China (973 Program,
to phenol by reductive processes (denoted as process I and II,
62,63
respectively).
Another pathway is the direct reaction of
+
hydroxyl radicals (cOH) with C
6 5
H c to generate phenol, which is
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no. 2012CB722607). We thank Professor Yuying Huang and 27 X. Hong, Z. Wang, W. Cai, F. Lu, J. Zhang, Y. Yang, N. Ma and
Professor Zheng Jiang (SSRF) for the XAFS experiments.
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