Modern Research in Catalysis, 2013, 2, 13-18
http://dx.doi.org/10.4236/mrc.2013.23A003 Published Online September 2013 (http://www.scirp.org/journal/mrc)
Synthesis, Characterization, and Activity of Tin Oxide
Nanoparticles: Influence of Solvothermal Time on
Photocatalytic Degradation of Rhodamine B
Zuoli He1*, Jiaqi Zhou2
1Electronic Materials Research Laboratory, School of Electronic and Information Engineering,
Xi’an Jiaotong University, Xi’an, China
2School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an, China
Received April 26, 2013; revised June 9, 2013; accepted July 8, 2013
Copyright © 2013 Zuoli He, Jiaqi Zhou. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
The SnO2 spheres-like nanoparticles have been successfully synthesized by a microwave solvothermal method, in
which SnCl2·2H2O, poly(vinylpyrrolidone) PVP, H2O2 and NaOH as raw materials. The as-synthesized products have
been characterized by X-ray diffraction, scanning electron microscope, and UV/Vis/NIR spectrophotometer. Photocata-
lytic activities of the samples have been evaluated by the degradation of rhodamine B (RhB) under UV-light illumina-
tion. Results showed that these products with diameter about 1 - 2 µm, and when the reaction time prolong, the surface
of the SnO2 spheres will change to rough and then smooth when the time even longer. The product with nanorods on its
surface shows the higher photocatalytic activity and red shift in the UV-vis absorption, which are relative to the unique
structure. At last we studied the electron transfer reactions during photo-oxidation of RhB.
Keywords: SnO2; Nanoparticles; Photocatalytic; Solvothermal
1. Introduction
Environmental problems, especially, the sustained pollu-
tions of water by various organic and metallic ion con-
taminants have been one of the most serious problems.
And many efforts are dedicated to the remediation of en-
vironmental pollution [1-3]. For instance, photodegrada-
tion of organic compounds provides an available way to
deal with the water pollution. Nanostructured semicon-
ductors (such as TiO2, ZnO, SnO2 and so on) are proved
to be an excellent photocatalyst which can degrade many
kinds of persistent organic pollution [4-10].
Tin oxide (SnO2), as one of the most important semi-
conductor oxides, has been used as photocatalyst for pho-
todegradation of organic compounds. The results indicate
that SnO2 has exhibited photoactivity toward degradation
of dye and other organic compounds [11,12]. However,
just like other transition metal oxides photocatalysts such
as TiO2 and ZnO, SnO2 suffer from low photocatalytic
efficiency because of its wide-bandgap (energy of the
band gap is about 3.6 eV) [13] and high recombination
rates of photogenerated electron-hole pairs. This defects
hinder SnO2 photocatalyst using widely and practically in
the environmental application [14]. To overcome this pro-
blem, the fabrication of nanostructures provides an ef-
fective way.
The synthesis of TiO2-based photocatalysts has been
reported in our previous papers [15-18]. Recently, SnO2-
based photocatalysts were also synthesized by various
approaches and have exhibited attractive performances
[19-22]. In this work, we describe the synthesis of SnO2
nanoparticles via a microwave solvothermal method and
discuss the effect of solvothermal time on the morpholo-
gies and nanostructures. The as-synthesized SnO2 nano-
particles were well characterised and their photocatalytic
activities were evaluated by the photodegradation of Rho-
damine B (RhB).
2. Experimental
2.1. Preparation
All the reagents used in this experiment were analytical
grade and were used without further purification. In a
typical preparation procedure, first, 1.353 g of SnCl2·2H 2O
was dissolved into 40 mL distilled water under conti-
*Corresponding author.
C
opyright © 2013 SciRes. MRC

Z. L. HE, J. Q. ZHOU 14
nuous magnetic stirring to form white slurry. Then 1.44 g
of NaOH and 5 mL of 30% H2O2 were introduced to the
well-stirred mixture at room temperature with simulta-
neous vigorous agitation until NaOH dissolved comp-
letely. When the solution became transparent, 1.2 g of
poly (vinylpyrrolidone) PVP (MW 30,000) was introduc-
ed. After several minutes of stirring, Subsequently, the
obtained solution was transferred into five 100 mL teflon
autoclaves, which was treated in a MDS-8 microwave
hydrothermal system (manufactured by Shanghai Sineo
Microwave Chemistry Technology Co. Ltd.) at 180˚C for
30 min, 60 min, 90 min, respectively, and allowed to
cool to room temperature naturally. The resulting white
powder was collected from the bottom of the Teflon con-
tainer after decanting the supernatant, washed several
times with absolute ethanol and distilled water. Subse-
quently, the products were dried in vacuum at 60˚C for
12 h for further characterization.
2.2. Characterization
Morphologies of the samples were observed by using a
high-resolution field emission environmental scanning
electron microscope (JSM-6700). All the images were
obtained under high vacuum mode without sputter coat-
ing. X-Ray Diffraction (D/max-2200, Diffractometer
with Cu Ka radiation) was used to verify crystal phase
and estimate the crystal sizes of as-synthesized SnO2 na-
noparticles. Absorption spectrum was measured on a
UV/Vis/NIR spectrophotometer (LAMBDA-950) in the
wavelength range of 200 - 800 nm.
2.3. Photocatalytic Activity Measurement
The photocatalytic activity of as-synthesized SnO2 nano-
particles was evaluated through the degradation of 5
mg/L Rhodamine B (RhB) in a BL-GHX-V multifunc-
tional photochemical reactor (Shanghai Bilon Experi-
ment Equipment Co. Ltd., Shanghai, China). The volume
of the reaction solution was 250 mL (8 test tubes of 30
mL) into which 25 mg of photocatalyst was added and
stirred for 30 min. The solution was dispersed by soni-
cation, and then transferred to test tubes. Irradiation was
provided by a medium-pressure Hg lamp (300 W), and
the reaction temperature was kept about 25˚C. Stirring
was performed at all the times during the reaction. Sam-
pling was also performed at regular intervals (every 15
min). The residual concentration of RhB was determined
by measuring its absorbance at 554 nm using an UV/
Vis/NIR spectrophotometer (LAMBDA-950).
3. Results and Discussion
3.1. SEM and XRD Analysis
XRD pattern of the as-prepared product is shown in Fig-
ure 1. All the diffraction peaks are quite similar to those
Figure 1. XRD patterns of the SnO2 nanoparticles synthe-
sized with different reaction time: (a) 30 min; (b) 60 min; (c)
90 min.
of SnO2, which can be indexed as the tetragonal rutile
structure of SnO2 with lattice constants of a = 4.738 Å
and c = 3.187Å, which is in good agreement with the
JCPDS file of SnO2 (JCPDS 41 - 1445) [22]. No impu-
rity diffraction peaks are observed, indicating the high
purity of the final products. In additionally, when treated
for 30 min, the diffraction peaks were broaden and wea-
ken, due to the relative lower crystallinity and size-quan-
tization effect of nanomaterials (shown in Figure 1(a)).
The increase of crystallinity was corresponding to the re-
action times during the solvothermal process, it clearly
show when reacted 90 min, the sample has the most high
crystallinity among these samples. According to the
Scherrer equation,

cosDK

 , the average crys-
tallite sizes of SnO2 calculated from the main diffraction
peak are about 9, 16 and 17 nm, respectively.
Scanning electron microscopy (SEM) image of the
as-grown product were shown in Figure 2, the nanopar-
ticles were seem to spheres, and the diameters about 1 - 2
μm are clearly observed. Higher magnification SEM im-
age shown in Figure 2(b) demonstrate the detailed
structural information of the sample prepared during 30
min heat treatment. As can be seen in Figure 2(b), the
observed SnO2 spheres are seem to be soft and have
some holes, due to the effect of PVP. So we can indicate
that the process of formation of the sphere is not com-
pleted and this is just a middle product. When the time
prolong to 60 min, the spheres made up of numerous
one-dimensional tetragonal prism nanorods with an ave-
rage diameter of about 100 nm were clearly seen in Fig-
ure 2(d). Interestingly, when continue prolong the time
to 90 min, the surface become smooth, but the nanorods
also can be seen (shown in Figure 2(f)). I think the nano-
sized particles (as shown in Figure 2(d)) around the
sphere will gather on the surface of the sphere, and close
the hole between nanorods. This process was simply
shown in Figure 3, the nanoparticles will show this pro-
cess in the solvothermal process, and when this will con-

Z. L. HE, J. Q. ZHOU 15
Figure 2. FESEM images of the SnO2 nanoparticles synthe-
sized by microwave solvothermal method with different re-
action time: (a) (b) 30 min, (c) (d) 60 min, (e) (f) 90 min.
Figure 3. Schematic diagram of the microwave solvother-
mal process with time increase.
tinue occur when the time even longer. And this will also
lead to the size of nanocrystals and diameter of the sphere
increase. The BET surface areas of the sample were ob-
tained from N2 adsorption/desorption isotherms determin-
ed at liquid nitrogen temperature on an automatic ana-
lyzer (Micromeritics, ASAP 2010), and the BET surface
area of the samples are 45.1, 56.5, 50.9 m2/g, respec-
tively.
3.2. UV-Vis Analysis
Figure 4 shows the absorption spectra of the samples,
which are all nearly identical, indicating that their optical
band gaps were also almost the same. The fundamental
absorption edge of SnO2 located in the UV region at
about 325 nm. But they also show some differences, as
shown in Figure 4 inset curves. The fundamental absor-
ption edges were 327.5, 328.5 and 352.0 respectively.
The products prepared during 60 min were shown a little
red shift due to the unique nanostructures and creation of
oxygen vacancies [15]. It will narrow the band gap and
enhance the UV absorption.
Figure 4. UV-visible absorption spectra of the SnO2 nano-
particles synthesized by microwave solvothermal method
with different reaction time: (a) 30 min; (b) 60 min; (c) 90
min. Inset was show the absorption at the range of 250 - 450
nm.
3.3. Evaluation of Photocatalytic Activities
Photocatalytic activity of the synthesized SnO2 products
was evaluated by monitoring the change in optical ab-
sorption of an RhB solution at ~554 nm during its photo-
catalytic decomposition process. The kinetics of this re-
action can be monitored by UV-vis spectroscopy as seen
from UV-vis spectra measured at different times shown
in Figures 5(a)-(c). It demonstrates RhB shows a strong
absorption band at 554 nm and the addition of SnO2
products leads to a decrease of the absorption band with
time. The color of the dispersion disappeared, indicating
that the chromophoric structure of the dye was destroyed.
The order rate kinetics with respect to the RhB concen-
tration could be used to evaluate the photocatalytic rate
as done previously. As it clearly demonstrated the SnO2
photocatalysts show higher photocatalytic activity. Fig-
ure 5(d) shows a comparison of the photocatalytic ac-
tivities among SnO2 photocatalyst treated in different
times. Additional experiments in the absence of photo-
catalyst, was also present in Figure 5(d). It can be noted
that a trend of the photocatalytic activity is as follow
order: b > a > c. The photocatalytic can be attributed to
UV absorption, and the products prepared during 60 min
shows a red shift and shows the higher photocatalytic
activity. The existing of the nanorods also increases the
surface area and there is more reaction place during
photocatalytic process [18].
At last, we study the mechanism for photocatalytic de-
gradation of RhB, and the electron transfer reactions in-
volved in the selective photooxidation of RhB with oxy-
gen are proposed in Figure 6. When the SnO2 photo-
catalyst is excited under light irradiation with greater
energy than its band gap energy, it will cause the forma-
ion of the hole-electron pair in the SnO2, Subsequently, t

Z. L. HE, J. Q. ZHOU 16
(a) (b)
(c) (d)
Figure 5. Photocatalytic degradation of RhB solutions under UV irradiation by the presence of SnO2 nanoparticles synthe-
sized in different time: (a) 30 min, (b) 60 min, (c) 90 min. The concentration of the reactants was as follow: [RhB] = 5 mg/L,
[SnO2] = 100 mg/L.
Hole (h+) with high activity may react with H2O or hy-
droxyl groups adsorbed on the surface of the SnO2, the
formed hydroxyl radicals also have strong oxidizing ac-
tivity Hole (h+) and Electron (e-) can react with the dye
molecule in favor of its degradation directly and follo-
wing mineralization. In the process, the RhB can interact
with the photogenerated holes in the valence band (VB),
and provides a direct chemical reaction between the dye
and the photocatalyst [23].
4. Conclusion
Figure 6. Electron transfer reactions with RhB.
In summary, we studied the SnO2 photocatalyst pre-
paraed via microwave solvothermal process. When the
reaction time prolong, the surface of the SnO2 spheres
redox reactions can occur superoxide ions and hydroxyl
radicals which are nonselective strong oxidizing agents,

Z. L. HE, J. Q. ZHOU 17
will change to rough and then smooth when the time
even longer. We also propose the modeling to explain
this interesting phenomenon. The product with nanorods
on its surface shows the higher photocatalytic activity
and red shift in the Uv-vis absorption, which are relative
to the unique structure.
5. Acknowledgements
This work was supported by the Research Fund for the
Doctoral Program of Higher Education of China under
grant 20120201130004, the Science and Technology De-
veloping Project of Shaanxi Province (2012KW-11), and
the Fundamental Research Funds for the Central Univer-
sities.
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