Ceramics International 42 (2016) 17717–17722

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Ceramics International
journal homepage: www.elsevier.com/locate/ceramint

Tailored synthesis of SnO2@graphene nanocomposites with enhanced

photocatalytic response
Haitao Chen a,b,n, Xuemei Pu a, Ming Gu a, Jun Zhu a, Liwen Cheng a
a
College of Physics Science and Technology, Yangzhou University, Yangzhou, 225002 People's Republic of China
b
National Laboratory of Solid State Microstructures, Laboratory of Modern Acoustics of MOE, Nanjing University, Nanjing, 210093 People's Republic of China

art ic l e i nf o a b s t r a c t

Article history: Hierarchical SnO2@graphene nanocomposites were synthesized by impregnating different weight per-
Received 29 June 2016 centages of Sn2 þ with graphene oxide nanosheets using a simple hydrothermal method. The precursor
Received in revised form ratio of Sn2 þ to graphene oxide plays a decisive role in tailoring the phase structure and composition,
5 August 2016
which thereby influences the photocatalytic performance to degrade the organic contaminants. The
Accepted 16 August 2016
SnO2@graphene nanocomposites exhibited greatly enhanced abilities for photocatalytic degradation of
Available online 16 August 2016
MO dye compare with the pure SnO2. It is believed that the tight heterojunction structure and efficient
Keywords: charge separation play important roles in facilitating interfacial electron transfer and reducing self-ra-
Nanocomposite diative recombination of charges, which accordingly enhance the photocatalytic performance.
Photocatalytic property
& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Charge separation

1. Introduction unstable and easily to recombine, dissipating their input energy as

radiative recombination, which would decrease the photocatalytic
Since persistent organic compounds and toxic water con- response [6]. Therefore, many attempts have been made to reduce
taminants produced by some industries are greatly harmful to the recombination of photogenerated electrons and holes and
animals, plants and human beings, scientists hope to find some further improve the photocatalytic response of SnO2 nanos-
environment-friendly methods to overcome such environmental tructures. One strategy is to form the heterostructures with two
problems. Photocatalysis provides an effective way to eliminate band-matched semiconductors which are bound at their surfaces.
the toxic chemical compounds in our environment using the solar This coupled band structure would permit the photogenerated
energy [1]. Over the years, a large number of photocatalytic ma- electrons and holes transfer between two semiconductors and
terials have been synthesized and their photocatalytic properties realize a better electron-hole separation [7]. However, two kinds of
have been investigated. TiO2 is undoubtedly the most commonly semiconductors in the structure are often not tightly bound to
studied photoctatalyst due to its stability and the ability to break each other and easily to separate and agglomerate in the photo-
down the organic pollutants. However, the low electron mobility degradation process. As a result, the photogenerated carriers
cannot effectively transfer through the interface and the charge
and rapid recombination of photogenerated carriers limit its ap-
separation is rather low [8]. Recently, one promising strategy de-
plication in practical. Tin dioxide (SnO2) similar to TiO2, possesses
veloped is to anchor the SnO2 nanoparticles onto the graphene
the highly chemical stability and does not bring the secondary
sheets. Graphene is an allotrope of carbon in the form of two-di-
pollution in the photocatalytic process. Furthermore, the two or-
mensional, atom scale thick and possesses the unprecedented
ders of electron mobility magnitude and oxidation ability of holes
electron and heat conductivity, mechanical and chemical stability
higher than that of TiO2 endows SnO2 a potential to be an ideal
and ultrahigh surface area. Graphene oxide (GO) is artificially
photocatalyst [2–4]. But, there are several fundamental issues need created by exfoliating graphene layer with strong oxidizers. Many
to be solved before it would be used in practical application. For oxygen-containing functional groups are introduced in the oxi-
example, the photocatalytic response of SnO2 nanostructures with dizing process which providing the active sites for hybridization
small size would decrease in their photodegradation process for with SnO2 materials. To form the SnO2@graphene composite could
self-aggregation derived from their high surface energy [5]. In be an excellent strategy to resolve the problems by means of the
addition, the photogenerated carriers in the excited states are high electron mobility and large specific surface areas of graphene
nanosheets. Additionally, with the proper synthesis technique,
n
Corresponding author at: College of Physics Science and Technology, Yangzhou
Sn2 þ would be oxidized by the oxygen groups in the GO and the
University, Yangzhou 225002, People’s Republic of China. resultant SnO2 particles are firmly bound on the graphene na-
E-mail address: htchen@yzu.edu.cn (H. Chen). nosheets. Such kind of connection brings several benefits: the

http://dx.doi.org/10.1016/j.ceramint.2016.08.095
0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
17718 H. Chen et al. / Ceramics International 42 (2016) 17717–17722

charge carriers generating from the SnO2 could be easily trans-

ferred through the graphene nanosheets; the self-aggregation of
SnO2 particles and the restacking of the graphene nanosheets
could be greatly inhibited. Therefore, well synthesized
SnO2@graphene composites could be expected to show a high and
stable degradation response [9].
In the past years, many efforts have been made to fabricate
hierarchical SnO2@graphene composites. For example, Sham et al.
synthesized SnO2@graphene nanocomposites by microwave-as-
sisted hydrothermal method using SnCl2  2H2O and GO as the
precursor [10]. Paek et al. prepared the SnO2@graphene nano-
composite by two steps. Firstly, SnO2 sol was prepared through
hydrolysis SnCl4 with NaOH. Secondly, the SnO2 sol was dropped
into the prepared graphene dispersion in ethylene glycol to form
the SnO2@graphene nanocomposite [11]. Xue et al. prepared
SnO2@graphene nanocomposite using the electrophoretic de-
position and magnetron sputtering techniques [12]. The as-pre-
pared SnO2@graphene nanocomposites mentioned above ex-
Fig. 1. XRD patterns of sample SG0.005 (a), SG0.01 (b), SG0.02 (c), SG0.03 (d).
hibited the excellent behaviors in their fields. However, the
synthesis process was complex, multisteps, time-consuming, and
3. Results and discussion
additives needed. Furthermore, most of the reported
SnO2@graphene nanocomposites are mainly used in the ion-bat- 3.1. XRD analysis
tery [10,11], gas sensor [13], and supercapacitor [14], and very
limited reports to focus on their photocatalytic applications. Fig. 1 shows the XRD patterns of the product prepared with
Therefore, one simple and efficient method to synthesize the different Sn2 þ contents. All diffraction peaks can be assigned to
SnO2@graphene composites and further deeper understanding of the standard tetragonal SnO2 phase (JCPDS No: 41-1445) when the
their photocatalytic properties are necessary [15]. Sn2 þ concentration is less than 0.03, indicating that all Sn2 þ ions
In this work, we prepared SnO2@graphene nanocomposites are fully oxidized into SnO2. When the Sn2 þ concentration is 0.03,
using one facile in situ hydrothermal method. The phase structure there appear the new diffraction peaks of SnO, suggesting only a
and composition of the as-prepared samples as well as their part of Sn2 þ are oxidized into the Sn4 þ for the insufficient of GO.
The results indicated that the Sn2 þ can be directly oxidized into
photocatalytic properties were discussed. The introduction of SnO2
the SnO2 by GO nanosheets. And the composition of the as-pre-
nanoparticles firmly bound on the graphene nanosheets sig-
pared samples can be modulated through controlling the amount
nificantly enhanced the photocatalytic performance for photo-
ratio of Sn2 þ /GO. Adequate GO nanosheets are necessary for get-
degradation of MO in comparison with the pure SnO2 powders. ting the pure fully oxidized of the Sn2 þ .

3.2. Morphology and composition

2. Experimental procedure Fig. 2a shows a general view of GO nanosheets. It can be seen

that the GO nanosheets were rippled and folded with each other,
SnO2@graphene nanocomposites were prepared through oxi- which confirms the single layer feature of the GO sheets. They are
dation reduction reaction between Sn2 þ and GO nanosheets, and transparent and exhibit a very stable nature under the electron
GO nanosheets were purchased from Nanjing XFNANO Materials beam. Fig. 2b exhibits one smooth graphene sheet which is fully
Tech Co. Typically, 20 ml SnCl2  H2O solution was added to 20 ml dispersed with SnO2 nanoparticles and the clear lattice fringes of
0.08 M NaOH aqueous solution. Then, 16 mg GO was added to the the nanoparticles can be seen in Fig. 2c. The high-resolution TEM
above suspension under ultrasonication. The as-obtained mixture image, in Fig. 2d, reveals that the lattice spacing of SnO2 is
was transferred into a 50 ml Teflon-lined stainless steel autoclave 0.33 nm, corresponding to the (110) planes. The selected area
and kept at 180 °C for 6 h. The resulting precipitation was washed electron diffraction pattern, inserted in Fig. 2d, taken from one
with ethanol and distilled water several times. To investigate the single nanoparticle shows that the concentric rings are composed
of bright discrete diffraction spots, which indicates the poly-
effect of amount ratio of Sn2 þ to graphene oxide on the compo-
crystalline nature of the SnO2 nanoparticles. The diffraction rings
sition and the resulting photocatalytic properties, the concentra-
from the center are corresponding to (110)/(101), (200)/(002),
tion of Sn2 þ varied at 0.005, 0.01, 0.02, and 0.03 M under the same
(211)/(112), (220), and (321) SnO2 [16,17].
of GO's mass. The final products were marked as SG0.005, SG0.01, The chemical configuration and interaction of SnO2@graphene
SG0.02, and SG0.03 respectively. For comparison, the pure SnO2 were recorded by XPS technique. Fig. 3 shows the typical XPS
powder was also prepared under the same condition except no GO spectra of as-synthesized SG0.01 nanocomposite. From the survey
used. XPS scan, Fig. 3a, it can be found that the as-prepared SG0.01 na-
The structures and morphologies the products were char- nocomposite is composed of C, O and Sn elements. No other ele-
acterized by X-ray diffraction (XRD, Shimadzu 7000), high-re- ments can be found, indicating the purity of the as-prepared
solution transmission electron microscopy (HRTEM, FEI, Tecnai G2 sample. The high resolution C 1s spectrum, as shown in Fig. 3b, can
F30 S-TWIN). The optical properties were conducted on a double be deconvoluted into three sub-bands corresponding to the carbon
beam UV–vis absorption spectrophotometer (Cary 5000, Varian, atoms in different carbon-containing functional groups: graphitic
USA). sp2 carbon atoms at 284.8 eV, carbon in C–O at 285.8 eV, and π–π*
transitions in aromatic system at 289.4 eV [18]. Fig. 3c shows the
 H. Chen et al. / Ceramics International 42 (2016) 17717–17722 17719

Fig. 2. (a) TEM image of GO nanosheets; (b,c) TEM image of SnO2@graphene composite; (d) High-resolution TEM image of SnO2 nanoparticles on a graphene sheets. The
inset shows the corresponding SAED pattern.

binding energy of Sn 3d3/2 and Sn 3d5/2 in SG0.01 nanocomposite 3.3. Absorption properties
at 495.8 eV and 487.5 eV, respectively. Compared with the position
of Sn 3d5/2 peak in SnO2 nanoparticle (486.4 eV), the binding In order to evaluate the light absorption properties, the ab-
energy of Sn 3d5/2 in SG0.01 shifts toward larger binding energy. sorption spectra of SnO2@graphene nanocomposites and pure
Peng et al. reported that the oxygen deficiency could decrease the SnO2 nanoparticles were recorded, as shown in Fig. 4. It can be
binding energy of Sn [19]. The larger binding energy of Sn in SG0.01 seen that there is almost no visible-light absorption for pure SnO2
nanocomposite indicates the rich oxygen in the sample, which nanoparticles because of the wide band-gap. However, all the
corresponding to the XRD result. The peak separation of Sn 3d5/2 SnO2@graphene nanocomposites show the continuous visible-
light absorption. This can be attributed that the graphene na-
and Sn 3d3/2 in SG0.01 is 8.4 eV as the same as pure SnO2, which
nosheets harvest the visible-light and thus extend the absorption
confirms the couple of SnO2 nanoparticles on graphene sheets
range for the SnO2@graphene nanocomposites [23]. So, introduc-
[20]. As shown in Fig. 3d, the O 1s can be deconvoluted into two
tion of the graphene nanosheets in the samples, not only increases
sub-bands at 531.4 and 532.5 eV. The band at 531.4 eV corresponds
the amount of light absorption but also extends the light absorp-
to the lattice oxygen in SnO2 species, and the band at 532.5 eV tion range.
corresponds to the residual oxygen-containing functional groups
of the GO nanosheets and H2O molecules adsorbed [21]. XPS re- 3.4. Photocatalytic properties
sults further confirm that SnO2 nanoparticles have effectively
bound with the graphene nanosheets, and this would facilitate the To investigate the adding of the graphene and amount ratio of
electron transfer through the interface between SnO2 and gra- GO to Sn2 þ on the transfer process of photogenerated carriers, the
phene in the photodegradation processes as discussed in the fol- photocatalytic activity of MO over different photocatalysts under
lowing part [22]. the same mass (fifty micrograms) is evaluated. Fig. 5a shows the
17720 H. Chen et al. / Ceramics International 42 (2016) 17717–17722

Fig. 3. XPS analysis for SnO2@graphene composites: the survey spectrum (a), the high-resolution spectra for C1s (b), Sn 3d (c), and O1s (d), respectively.

degradation curves of MO solution (20 mg L  1) with different

photocatalysts of SG0.005, SG0.01, SG0.02, SG0.03, and SnO2 nano-
particles under UV-light irradiation. The characteristic absorption
of MO at 462 nm was chosen to monitor the photocatalytic con-
version percentage. The photocatalytic degradation efficiency was
calculated form the following expression:
C0 − Ct
Degradation= ×100%
C0

Where C0 is the initial concentration of MO, and Ct is the con-

centration of MO at a certain time t. From the Fig. 5a, it can be
found that the samples with different Sn2 þ /Sn4 þ contents show
different catalytic performance at a certain time. The degradation
efficiency is significantly better than the previous reports for the
samples without GO under the same conditions [24]. For the
convenience of compare, the degradation of pure tetragonal SnO2
is also listed and it can be seen that there is almost no degradation
ability for pure SnO2 even under UV light irradiation. It is espe-
cially noted that the degradation reaches best for sample SG0.01
Fig. 4. UV–vis absorption spectra of the as-synthesized SnO2@graphene and the MO is almost completely degraded in 120 min. While
composites. further increasing the mass of Sn4 þ in the composite, the photo-
catalytic activity decreases monotonously. However, the degrada-
tion percentage can still reach 60% in 120 min for sample SG0.03. It
suggests that the graphene has played a crucial role in the
 H. Chen et al. / Ceramics International 42 (2016) 17717–17722 17721

the ultrahigh specific surface area, which greatly extended the

light harvest area, range and dye adsorption areas. The Fig. 4 has
proved that SnO2@graphene nanocomposites successfully realized
the optical response shifting from UV to the visible-light range.
Secondly, the strong coupling between SnO2 and graphene na-
nosheets facilitates the charge transfer and suppresses the re-
combination of electrons and holes. The SnO2 nanoparticles an-
chored in the graphene networks are synthesized through oxida-
tion of Sn2 þ by GO. There is an intimate interaction between SnO2
nanoparticles and graphene nanosheets, which effectively facil-
itate interfacial electron transfer during the photocatalytic pro-
cesses. At the same time, the graphene nanosheet also plays a role
as electron acceptor layer that give rise to suppress the electron-
hole recombination and improve the transportation. Thirdly, the
dispersion of SnO2 nanoparticles on graphene nanosheets greatly
prevents the self-aggregation of SnO2 nanoparticles and the re-
stacking of graphene nanosheets, which not only increase the
contact area between the active materials but also provides the
fast transport pathways for the photogenerated carriers. As for the
sample SG0.01 showing a better photocatalytic performance than
sample SG0.005, it is because that the sample SG0.01 provides more
active sites and absorbed more light than sample SG0.005. With
further increasing the content of SnO2, the content of SnO2 in the
SnO2@graphene heterocomposites increases and gradually fully
covers the surface of graphene sheets. As a result, there provide
more active sites for the photogenerated electrons to combine
with the holes in the charge transfer process. As a consequence,
the photocatalytic performance decreases from sample SG0.01 to
SG0.03.
Fig. 6 gives the synthesis procedure of SnO2@graphene nano-
composites and the photocatalytic degradation process under UV
light irradiation. First, the Sn2 þ mixed with GO dispersion reacted
in autoclave and Sn2 þ are transformed into SnO2 nanoparticles on
graphene sheets, in which GO provides the oxygen for the for-
mation of SnO2 nanoparticles. Second, irradiation under UV light
results in separation of photogenerated electrons and holes in
conduction and valence band of SnO2 nanoparticles. Third, the
Fig. 5. (a) Degradation curves of samples SS0.005, SG0.01, SG0.02, SG0.03, and SnO2, photogenerated electrons of SnO2 transfer from the conduction
respectively; (b) Recycling test performance of sample SG0.01. band to graphene. The valence band of SnO2 and graphene act as
active sites of the photogenerated holes and electrons accom-
enhancement of photocatalytic activity in the SnO2@graphene panied by a series of reactions to generated corresponding oxygen
systems. Fig. 5b shows the recycling performance of sample SG0.01 active species such as  OH and  O2–. The oxygen active species are
by MO degradation for three runs. It can be seen that the efficiency the oxidation agents, which are favorable for MO degradation [25].
after the third run is almost the same as that of the fresh sample,
suggesting the good stability in the photo-degradation process.
What are the reasons for the increased photocatalytic capacity 4. Conclusions
of SnO2@graphene heterocomposites? Firstly, high dye adsorption
ability and extended light absorption range for the In summary, we synthesized the SnO2@graphene nanocompo-
SnO2@graphene composites are believed one important factor. sites via an oxidation–reduction hydrothermal reaction between
Graphene is a 2D, single atom thick monolayer structure showing GO and Sn2 þ . The SnO2@graphene nanocomposites displayed the

Fig. 6. Illustration of the band configuration and interfacial charge transfer between SnO2 and graphene under light irradiation.
17722 H. Chen et al. / Ceramics International 42 (2016) 17717–17722

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