JOURNAL OF SCIENCE OF HNUE
Mathematical and Physical Sci., 2014, Vol. 59, No. 7, pp. 144-149 This paper is available online at hup://stdb.hnue.edu-vn
PHOTOCATALYTIC C O M P O S I T E S BASED O N Zn2Sn04 AND CARBON NANOTUBES
N g u y e n C a o K h a n g \ V u Q u o c T r u n g ^ , L a m T h i H a n g ^ , N g u y e n T h i T h u H a \ N g u y e n T h i L i e n \ D o a n T h i T h u y P h u o n g \
K i e u T h i B i c h N g o c ^ a n d N g u y e n V a n M i n h ^
^Center for Nanoscience and Technology, Hanoi National University of Education '^Faculty of Chemistry, Hanoi National University of Education
^Hanoi University of Natural Resources and Environment Abstract. Zn2Sn04/CNTs photocatalytic composite were synthesized with multi - walled carbon nanotubes (MWCNTs) and Zn2Sn04 (ZTO) using a grinded method, The UV - V is diffuse reflectance spectra showing that the composite materials can absorb at higher wavelength and the absorption covers the entire range of the visible region. The application of the catalysts to photocatalytic degradation of methylene blue (MB) was tested under visible light irradiation. From photocatalytic result, we found that all Zn2Sn04/CNTs composite catalysts exhibit higher MB degradation activity than add Zn2Sn04.
Keywords: Zn2Sn04, CNTs, composite, photocatalyst.
1. Introduction
Zinc stannate is an n-type semiconductor with an inverse spinel sttucture [1, 2].
Zn2Sn04 is known for having high electton mobility, high electrical conductivity and atttactive optical properties, all of which make it suitable for a wide range of applications, such as photovoltaic devices, sensors for humidity and combustible gases, negative electtode material for Li-ion batteries [3-8]. In addition, Zn2Sn04 has been widely utilized as a photocatalyst because it is relatively safe, inexpensive and resistant to photocorrosion.
However, its wide-band gap (3.7 eV) can capture only UV light, 3 - 5% of the solar irradiance at the earth's surface, compared to the 4 5 % of visible light.
Recentiy, the authors indicated that the photocatalytic activity of some semiconductors can be improved by make a composite with CNTs [9]. It has been reported that tiie MWCNTs not only provided a large surface area support for the catalyst, Received October 2, 2014, Accepted October 25, 2014.
Contact Nguyen Cao Khang, e-mail address: [email protected]
but also stabilized the charge separation by ttapping the electtons ttansferred from the semiconductor thereby hindering a charge recombination [10]. Their outstanding charge ttansfer abilities can favor tiie excited electton in the conduction band of nanocrystal semiconductors to migrate into die CNTs, diereby decreasing the ability of recombination of die electton-hole pairs and increasing photocatalytic activity under visible hght.
In this study, we report on the synthesis of Zn2Sn04/CNTs composites and their improved photocatalytic effect under visible hght using a simple grinding method. The photocatalytic activities of samples were assessed using a photodegradation of methyl blue. This presentation is also to clarify the role of CNTs in the origin of visible light photocatalytic activity of Zn2Sn04/CNTs composites.
2. Content 2.1. Experiments
Preparation of Zn2Sn04/CNTs composites: A Zn2Sn04 precursor was synthesized using a simple hydrothermal process. Then the white Zn2Sn04 precursor was mixed with MWCNTs at a ratio of 2/1 and 4/1 and named ZCl and ZC2, respectively. The mixture was ground for 3 h in an agate mortar and dried at 100 °C in a vacuum for 4 h.
Photocatalytic test: Visible-light photocatalytic activities were evaluated using the decomposition rate of MB in aqueous solution. The experiments were carried out in a self-designed 200 mL reactor vessel equipped with a gas cooling and magnetic stirring system. Illumination in the visible region was carried out using a 100 W filament lamp. 50 mL of MB solution 10 ppm and 25 mg of the photocatalyst sample were added to the reactor vessel under constant stirring. Before turning on the light, the suspension containing MB and photocatalyst was magnetically stirred in a dark condition for 30 minutes to estabUsh an adsorption-desorption equilibrium. After that, the MB concenttation was considered to be the initial concentration. The light was then turned on and we started to count the reaction time.
2.2. Results and discussion
The morphologies of the MWCNTs-Zn2Sn04 composite were revealed by TEM investigation. TEM image of the composite in Figure 1 shows tiiat ZTO nanoparticles about 10 nm in size are attached to the sidewall of the MWCNTs. However, ZTO nanoparticles did not cover die entire surface of tiie MWCNTs, a composite made up of a random mixture of nanoparticulate ZTO and CNTs.
UV-Vis diffuse reflectance spectta of MWCNTs (C), Zn2Sn04 (Z), Zn2Sn04/MWCNTs witii mz„,sn04 : HICNTS - 8 : 2 (ZCl), and Zn2Sn04/MWCNTs with mznsSnO^ : mcNTs = 7 : 3 (ZCl) are shown in Figure 2. The composite materials can absorb from 430 nm to 800 nm and the absorption covers the whole range of the visible region. From absorption spectta, we recognize that ZCl have 30% MWCNTs absorption, better tfian tiie ZC2 with 20% MWCNTs. Thus, die absorption increases widi tiie increasing mass of CNTs.
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Nguyen Cao Khang, Vu Quoc Trung, Lam Thi Hang, Nguyen Thi Thu Ha, Nguyen Thi Lien, Doan Thi Thuy Phuong, Kieu Thi Bich Ngoc and Nguyen Van Minh
Figure L SEM image of Zn^n04/CNTs composite
300 4 X 500 600 700
Wavelength (mn) Figure 2. UV- Vis diffuse reflectance
absorption spectra of (a) C, (b) ZCl, (c) ZCl and (d) Z Photocatalytic efficiency was evaluated by intensity peak at 665 nm in absorption spectta of MB solution. The percent degradation of MB solution was calculated as follows:
D = ^--^.im%
with D the percent degradation, Ao and A the maximum absorbance at 665 nm in the.
absorption spectta of an initial and constant MB solution, respectively.
Absorbance specttal changes of methylene blue solution in the presence of ZCl, ZC2 and Z are shown in Figures 3,4 and 5. Figure 6 and Table 1 show the photocatalytic degradation of MB over synthesized samples under visible light irradiation.
It is evident that all of the MWCNTs - Zn2Sn04 composite catalyst exhibits higher MB degradation activity than neat Zn2Sn04. The presence of CNTs in Zn2Sn04 photocatalysts can raise photocatalytic activity from 10% to 25%. For composite catalysts witii different MWCNTs mass, the activity increases with the increase in MWCNT mass, and the ZCl with the biggest MWCNT mass shows die maximum effect. Degradation MB result over CNTs corresponds to previous research.
To rely on a mechanism enhancement of photocatalytic properties of CNTs-Ti02 composite, we propose two mechanisms to explain the enhancement of photocatalytic properties of CNTs-Zn2Sn04.
Table h Percent degradation of MB solution over different solids after 4 h of irradiation
ZCl 75%
ZC2 80%
Z 90%
(% )
i »«rn
•
^\^ ^ .
0 1 2 3 4
Time (h) A\\\\
300 400 500 600 700 V\fevelength (nm)
^
T ' " "
n SC
•-....^^
^"~"^.
^ •
T i m e (h)
b 5h at dark 2 h jW 4 h M
300 400 500 600 700 Wavelength (nm)
Figure 3. Absorbance spectral changes Figure 4. Absorbance spectral changes of methylene blue solution of methylene blue solution
in the presence of ZCl in the presence ofZC2
1.2
0.0
i f
Time (h) l o S h
2 h /
1,
/
ndarit
\
\ '•^
300 400 500 600 700
Wavelength (nm) Time (h)
Figure 5. Absorbance spectral changes Figure 6. Photocatalytic degradation of methylene blue solution of methylene blue over synthesized samples
in the presence ofZ under visible light irradiation In the first, a high energy photon excites an electton from the valence band to the conduction band of Zn2Sn04. Photogenerated electtons are ttansferred into the CNTs and holes remain on the Zn2Sn04 to take pari in redo reactions. Figure 7 shows this mechanism.
In the second, the CNTs act as sensitizers and ttansfer electtons to the Zn2Sn04. The photogenerated electton is injected into the conduction band of the Zn2Sn04, enabling
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Nguyen Cao Khang, Vu Quoc Trung, Lam Thi Hang, Nguyen Thi Thu Ha, Nguyen Thi Lien, Doan Thi Thuy Phuong, Kieu Thi Bich Ngoc and Nguyen Van Minh
the formation of superoxide radicals by adsorbed molecular oxygen. Once this occurs, the positively charged nanotubes remove an electron from the valence band of the Zn2Sn04 leaving a hole. The now positively charged Zn2Sn04 can then react with adsorbed water to form hydroxyl radicals. A diagram of this mechanism is shown in Figure 7b.
Electron transfer
Electron baclc transfer
Figure 7. The proposed mechanisms for the CNT(tube)-mediated enhancement of photocatalysis
3. Conclusion
Zn2Sn04/CNTs composite photocatalysts containing MWCNTs with different mass were prepared using a grinding method. The composite materials can absorb at higher wavelength and the absorption covers the whole range of visible region. The photocatalytic degradation of methylene blue was observed over MWCNTs-Zn2Sn04 composite catalysts, which exhibit higher photocatalytic activity in comparison with neat Zn2Sn04. We propose that the origin of the enhancement of photocatalytic efficiency of the composite is the presence of MWCNTs which decrease the ability of recombination of the electron-hole pairs and increases photocatalytic activity under visible light.
Acknowlegements. This work was supported by the Hanoi National University of Education project. No. SPHN-13-362TD.
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