Synthesis, Characterization, and Activity of Tin Oxide Nanoparticles: Influence of Solvothermal Time on Photocatalytic Degradation of Rhodamine B

doi:10.4236/mrc.2013.23A003

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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

Email: *[email protected], *[email protected]

Received April 26, 2013; revised June 9, 2013; accepted July 8, 2013

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.

Z. L. HE, J. Q. ZHOU

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

(a) (b)

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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