TiO2/SnO2 double-shelled hollow spheres-highly efficient photocatalyst for the degradation of rhodamine B

Article abstract of DOI:10.1016/j.catcom.2014.11.032

TiO₂/SnO₂ double-shelled hollow spheres are successfully synthesized by two-step liquid-phase deposition method using carbon sphere templates. The formation process of TiO₂/SnO₂ hollow spheres is discussed. The

Full text of DOI:10.1016/j.catcom.2014.11.032

Catalysis Communications 60 (2015) 129–133
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
TiO2/SnO2 double-shelled hollow spheres-highly efficient photocatalyst
for the degradation of rhodamine B
a
b
b
c,
a
a
Jujun Yuan a, , Xianke Zhang , Hongdong Li , Kai Wang , Shiyong Gao , Zhen Yin , Huajun Yu ,
⁎
⁎
Xiurong Zhu a, Zuzhou Xiong a, Yingmao Xie a
School of Physics and Electronics, Gannan Normal University, Ganzhou 341000, PR China
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 October 2014
Received in revised form 28 November 2014
Accepted 29 November 2014
Available online 2 December 2014
TiO2/SnO2 double-shelled hollow spheres are successfully synthesized by two-step liquid-phase deposition
method using carbon sphere templates. The formation process of TiO2/SnO2 hollow spheres is discussed. The
samples are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy,
high-resolution transmission electron microscopy and UV–vis absorption spectroscopy. The behavior of
photogenerated charges in the TiO2/SnO2 heterojunction structures has been investigated through surface
photovoltage spectroscopy. The TiO2/SnO2 hollow spheres which are realized show significantly enhanced pho-
tocatalytic activities, with respect to the cases of SnO2 and TiO2 hollow spheres. Furthermore, TiO2/SnO2 hollow
spheres show good recyclable photocatalytic activities.
Keywords:
Hollow spheres
Liquid-phase deposition
Photocatalysis
SnO2
© 2014 Elsevier B.V. All rights reserved.
TiO2
1. Introduction
convenient way for photogenerated carriers to transfer to the reaction
surfaces. For example, TiO2/SnO2 composite nanoparticles had been fabri-
Environmental problems, such as organic pollutants and toxic water
pollutants, provide the impetus for fundamental and applied research in
the environmental area. Semiconductor photocatalysts exhibit several
properties that are strongly sensitive to their structures, and are consid-
ered to be important materials for environmental applications [1–4].
However, the relatively low quantum yield that is normally caused
due to rapid recombination of photogenerated electrons and holes hin-
ders the commercialization of this technology [5]. In order to enhance
the quantum yield, the application of heterojunction oxide structures,
such as ZnO/SnO2 [6,7], TiO2/SnO2 [8,9], and WO3/TiO2 [10,11], have
been further introduced because the heterojunction could provide a po-
tential driving force (the internal electrostatic potential in the space
charge region) to reduce the recombination of photogenerated charge
carriers.
cated by one-step flame to get improved photocatalytic activity [14]. Effi-
cient bicomponent TiO2/SnO2 nanofiber photocatalysts were prepared by
electrospinning method [15,16]. Sieve-like SnO2/TiO2 nanotubes also
showed enhanced photocatalytic efficiency, which had been fabricated
by assembling sieve-like macroporous Sb-doped SnO2 film on vertically
aligned TiO2 nanotubes through a block copolymer soft-template method
[17]. However, there are few studies on TiO2/SnO2 hollow spheres as effi-
cient and low-cost photocatalysts.
In this paper, we report the SnO2/TiO2 double-shelled hollow
spheres synthesized by a two-step liquid-phase deposition (LPD) meth-
od using carbon spheres as the templates for the first time. The SnO2/
TiO2 hollow spheres, which have been achieved, show highly efficient
photocatalytic activities for the decomposition of rhodamine B (RhB).
Among these semiconductor oxides, TiO2 and SnO2, well-known wide
direct band gap n-type semiconductors, are considered as the most
promising functional materials due to their unique physical and chemical
properties [12,13]. In the past decade, more and more TiO2/SnO2
nanomaterials have been reported to improve photocatalytic efficiency,
because these nanomaterials possess heterojunction structure and a
high surface area, which can provide both effective driving force and
2. Experimental
2.1. Fabrication of SnO2/TiO2 hollow spheres
Carbon spheres were prepared by the hydrothermal method accord-
ing to a reported procedure [18]. In a typical procedure, 4 g of glucose
was dissolved in 30 mL of deionized water, and then the resulting solu-
tion was transferred to a 50 mL Teflon-lined autoclave and maintained
at 180 °C for 20 h. The products were centrifuged and rinsed for several
times with distilled water and finally dried at 80 °C for 12 h. For the
⁎
Corresponding authors.
E-mail addresses: yuanjvjun@aliyun.com (J. Yuan), gaoshiyong@hit.edu.cn (S. Gao).
http://dx.doi.org/10.1016/j.catcom.2014.11.032
1566-7367/© 2014 Elsevier B.V. All rights reserved.

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J. Yuan et al. / Catalysis Communications 60 (2015) 129–133
preparation of SnO2 shell, 0.3 g of carbon spheres was dispersed and im-
mersed in the aqueous solution of SnF2 (0.02 M) under stirring at 50 °C
for 8 h. The products were centrifuged and dried at 80 °C for 12 h. For
the fabrication of TiO2 shell, carbon spheres with SnO2 shell are
dispersed and immersed in the aqueous solution of ammonium
hexafluorotitanate (0.05 M) and boric acid (0.15 M) under stirring at
50 °C for 40 min. The resulting products were dried at 80 °C for 12 h,
followed by calcination at 550 °C for 4 h in air. TiO2/SnO2 double-
shelled hollow spheres were finally obtained. For comparison, SnO2
and TiO2 hollow spheres are also fabricated with the same process to
obtain the SnO2 shell and TiO2 shell, respectively.
2.3. Photocatalytic evaluation
The photocatalytic activities of TiO2/SnO2 hollow spheres were
investigated by measuring the photodegradation rate of RhB aqueous
solution. The reaction suspension was prepared by adding the sample
(10 mg) into a 10 mL of RhB (10 mg L−1) solution. The suspension
was stirred in the dark for 1 h to ensure an adsorption/desorption equi-
librium prior to UV irradiation. The suspension was then irradiated by
an Hg lamp (500 W) under continuous stirring. The remaining amounts
of RhB in solution were determined by measuring the absorption inten-
sity of the main peak at 554 nm by UV–vis spectrophotometer
(KD723PC).
2.2. Characterization
3. Result and discussion
The structure and morphologies of the samples were characterized
by means of X-ray diffraction (XRD), scanning electron microscopy
(SEM), transmission electron microscopy (TEM) and high-resolution
transmission electron microscopy (HRTEM). The element signatures of
the samples were characterized by energy-dispersive X-ray (EDX).
The UV–vis transmission optical properties were texted by UV-3150
double-beam. The specific surface area was determined by using the
standard Brunauer–Emmett–Teller (BET) method. The separation char-
acteristics of photogenerated charge carriers were tested by a lock-in-
based surface photovoltage (SPV) measurement system.
The crystallinity of composite hollow spheres was examined by
powder XRD measurements (Fig. S1). It can be seen that there are two
sets of diffraction peaks. One set of diffraction peaks is indexed to the ru-
tile SnO2 phases (JCPDS file.03-1114). The other set of peaks is connect-
ed to the anatase TiO2 phases (JCPDS file.02-387). No characteristic
peaks for impurity were observed, which further confirmed the compo-
sition of SnO2 and TiO2 in the heterostructure.
Fig. 1a shows the typical SEM image of carbon spheres. The average
diameter is about 460–520 nm. From Fig. 1b, it can be seen that the
Fig. 1. Low and high (inset) magnifications of SEM images of carbon sphere templates (a) and TiO2/SnO2 hollow spheres (b). (c) EDX spectrum of TiO2/SnO2 hollow spheres. (d) TEM im-
ages of TiO2/SnO2 hollow spheres. HRTEM image of SnO2 shell (e) and TiO2 shell (f).

J. Yuan et al. / Catalysis Communications 60 (2015) 129–133
131
Fig. 2. Schematic of the steps for forming the TiO2/SnO2 hollow spheres.
hollow spheres have average diameters of ~550–700 nm and some
spheres show open features with two shells, indicating that TiO2/SnO2
hollow spheres have been fabricated successfully. Fig. 1c shows the
chemical analysis of the TiO2/SnO2 hollow spheres by EDX analysis.
From the EDX result, it can be observed that the dominant elements in
the hollow spheres are stannum, titanium, and oxygen. A more detailed
view of the morphology and microstructure of hollow spheres has been
obtained by means of TEM. TEM images of the spheres clearly depict the
remarkable contrast between the peripheral and internal parts (Fig. 1d).
It is evident the hollow spheres are formed with two shells. Close exam-
ination shows that the inner shell (SnO2) has a thickness of ~30–35 nm
and outer shell (TiO2) has a thickness of ~60–70 nm. Fig. 1e and f shows
HRTEM images of SnO2 and TiO2 shells respectively. The clear and
aligned lattice fringes, with a mean interplanar spacing of about
0.33 nm (Fig. 1e), correspond to the (110) plane of tetragonal SnO2.
The distance between the adjacent lattice fringes corresponds to the
interplanar distance of the tetragonal TiO2 (101), which is d (101) =
0.35 nm (Fig. 1f).
Fig. 3a. The Eg values of the SnO2, TiO2 and TiO2/SnO2 hollow spheres
are 3.59, 3.36 and 3.45 eV, respectively, by extrapolating the linear por-
tion to the photon energy axis. This experimental result shows that the
presence of TiO2 in the TiO2/SnO2 hollow spheres contributed to a red
shift of the absorption edge compared with the SnO2 hollow spheres.
It is evident that the formation of TiO2/SnO2 heterojunctions modified
the electronic structures of the pristine semiconductors.
The results of surface photovoltage spectroscopy (SPS) of the sam-
ples are shown in Fig. 3b. SPS presents the SPV amplitude as a function
of the incident wavelength. SPS is a well-established contactless and
nondestructive technique for semiconductor characterization that relies
on analyzing illumination-induced changes in the surface voltage. It is
The N2 adsorption–desorption isotherms of the calcined TiO2/SnO2
hollow spheres at liquid nitrogen temperature (77 K) have been plotted
in Fig. S2. The isotherm exhibits a type IV with type H4 hysteresis loop,
which is characteristic of mesoporous solid materials [19,20]. As re-
vealed from the Barrett–Joyner–Halenda (BJH) pore size distribution
(the inset of Fig. S2), the average mesopore size is about 3.5 nm and
pore volume is about 0.2 cm3/g. The BJH analysis indicates that the over-
all BET surface area of TiO2/SnO2 hollow spheres is 46.1 m2/g.
The above results show that we have successfully prepared TiO2/
SnO2 hollow spheres using a simple two-step LPD approach. Fig. 2 illus-
trates schematically the steps for the formation of the hollow spheres.
The process starts with growing carbon spheres by a hydrothermal
method. This is followed by the deposition of SnO2 on the carbon
spheres. The reaction is summarized as the following [21]:
SnF2 þ H2O þ 1=2O2↔SnO2 þ 2Hþ þ 2F−
:
ð1Þ
In the first step of synthesis system, the SnO2 is formed on the carbon
spheres by a hydrolysis of SnF2. For the formation of TiO2 shell, the reac-
tions are described as the following [22,23]:
−
TiF6 þ 2H2O↔TiO2 þ 4HF þ 2F−
ð2Þ
ð3Þ
2
H3BO3 þ 4HF↔BF4− þ 3H2O þ Hþ:
In the synthesis system, the TiO2 is formed by hydrolysis of Ti-fluoro
complex ions as in Eq. (2). As shown in Eq. (3), the added boric acid acts
as a scavenger for F−. Lastly, the TiO2/SnO2 hollow spheres can be ob-
tained by wiping off carbon spheres during the process of calcination.
The UV–visible absorption spectrum of TiO2/SnO2 hollow spheres is
shown in Fig. 3a. For comparison, the absorption spectrum of SnO2 and
TiO2 hollow spheres are also presented. In the direct transition semicon-
ductor, the optical absorption coefficient (α) and the band gap energy
(Eg) are related by αhv = D(hv − Eg)1/2, where h is the Planck's con-
stant, v is the frequency of the incident photon, Eg is the optical band
gap and D is constant. The variations of (αhv)2 versus the photon energy
hv in the fundamental absorption region are plotted in the inset of
Fig. 3. (a) UV–vis absorption spectrum of TiO2/SnO2, SnO2, and TiO2 hollow spheres; the
inset is the corresponding plot of (hv)2 versus photon energy. (b) SPS of the TiO2/SnO2,
SnO2, and TiO2 hollow spheres.

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J. Yuan et al. / Catalysis Communications 60 (2015) 129–133
an effective tool for the investigation of the photoinduced charge-
transfer properties in TiO2/SnO2 hollow spheres and may provide useful
information for understanding the catalytic mechanism of the sample.
From Fig. 3b, it can be seen that an apparent SPV response ranging
from 300 to 390 nm is observed for TiO2/SnO2 and TiO2 hollow spheres,
whereas a quite weak SPV response is observed for SnO2 hollow
spheres. Compared with the TiO2 hollow spheres, TiO2/SnO2 hollow
spheres show a distinctly stronger SPV response, indicating that the
photogenerated charges are more effectively separated for the latter
material.
RhB, a chemically stable and poorly biodegradable dye contaminant
in wastewater, is used to evaluate the photocatalytic ability of the TiO2/
SnO2 hollow spheres. Fig. 4a shows the temporal absorption spectral
changes of RhB in photo-degradation over the TiO2/SnO2 hollow sphere
catalysts under UV-light irradiation. The result indicated that the
absorption peak at 554 nm disappeared after irradiation for 15 min.
The photocatalytic activity of TiO2/SnO2 hollow spheres is plotted in
Fig. 4b, evaluated by the degradation examination of RhB solution
under UV-light illumination. For comparison, the photocatalytic exper-
iments were further carried out with the SnO2 and TiO2 hollow spheres.
After 15 min illumination, the degradation of RhB in a direct photolysis
process without any photocatalyst is negligible. The degradation effi-
ciency of RhB by a photocatalytic process was 100% when TiO2/SnO2
hollow spheres existed. 80% and 9% of RhB were photocatalytic degrad-
ed when using TiO2 and SnO2 hollow spheres, respectively, under the
same conditions. It is obvious that the TiO2/SnO2 hollow spheres exhib-
ited a much higher photocatalytic activity. The stability of the TiO2/SnO2
heterojunction photocatalyst was also demonstrated by the recycling
test (Fig. 4c). After four cycles of photocatalytic degradation, no signifi-
cant loss of activity was represented. Based on the above photocatalytic
results, it is demonstrated that the improved photocatalytic ability of
the TiO2/SnO2 heterojunction photocatalyst could be attributed to the
enhanced quantum yield promoted by the heterojunction, which was
consistent with the observation in the SPV measurements. Additionally,
the composite hollow spheres with high surface areas were in favor of
the transfer of electrons and holes generated inside the crystal to the
surface, and facilitated the degradation of RhB.
The charge separation mechanism of TiO2/SnO2 hollow spheres is
shown in Fig. 4d. The whole process consists of the formation of charge
carriers, the dissociation of charge carriers into free carriers, the transfer
of free carriers to the surface, and the generation of active oxygen spe-
cies such as •OH or •O−2 . Under UV luminescence, the photogenerated
carriers of electrons and holes appear in the conduction band (CB) and
valence band (VB) for each semiconductor, respectively. As we know,
the CB edges of SnO2 and anatase TiO2 are situated at +0.07 and
−0.34 V versus normal hydrogen electrode (NHE) at pH 7, respectively.
The VB edge of SnO2 (+3.66 V) is more positive than that of anatase
TiO2 (+3.02 V) [8,24,25]. Under the potential driving force of TiO2/
SnO2 heterojunction, the electrons (holes) transfer from the CB of TiO2
(VB of SnO2) to the CB of SnO2 (VB of TiO2). Due to the presence of a
TiO2/SnO2 heterojunction, the probability of the recombination of elec-
tron-hole pairs is significantly reduced. Consequently, the highly efficient
active oxygen species processes would occur for the degradation. It is pro-
posed that the hole-related redox process plays a major role for the de-
composition of RhB, because the CB potential of SnO2 seems too low for
the electron to reduce O2 to generate •O−2 (the E b theta N (O2/•O2−) is
about −0.046 eV).
4. Conclusions
In conclusion, the TiO2/SnO2 hollow spheres have been fabricated by
a facile two-step LPD method to attain excellent photocatalytic activities
for the decomposition of RhB. The TiO2/SnO2 heterojunction structure
can effectively promote an increase in the charge separation of the
Fig. 4. (a) Time-dependent UV–vis absorption spectra of the RhB solution in the presence of TiO2/SnO2 hollow spheres. (b) Photocatalytic decomposition of RhB solution with TiO2/SnO2
hollow spheres, SnO2 hollow spheres, TiO2 hollow spheres, and without photocatalyst for reference. (c) Recyclability test of photocatalytic decomposition for TiO2/SnO2 hollow spheres.
(d) Schematic band diagram of TiO2/SnO2 hollow spheres at thermal equilibrium and the transfer processes of photogenerated carriers (e−, electrons; h+, holes).

J. Yuan et al. / Catalysis Communications 60 (2015) 129–133
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Acknowledgments
This work was financially supported by the National Natural Science
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
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