Photocatalytic composites based on Zn2SnO4 and carbon nanotubes

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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 Zn2SnO4/CNTs composite catalysts exhibit higher MB degradation activity than add Zn2SnO4.

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Nội dung Text: Photocatalytic composites based on Zn2SnO4 and carbon nanotubes

JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2014, Vol. 59, No. 7, pp. 144-149 This paper is available online at http://stdb.hnue.edu.vn PHOTOCATALYTIC COMPOSITES BASED ON Zn2 SnO4 AND CARBON NANOTUBES Nguyen Cao Khang1 , Vu Quoc Trung2 , Lam Thi Hang3 , Nguyen Thi Thu Ha3 , Nguyen Thi Lien1 , Doan Thi Thuy Phuong1 , Kieu Thi Bich Ngoc1 and Nguyen Van Minh1 1 Center for Nanoscience and Technology, Hanoi National University of Education 2 Faculty of Chemistry, Hanoi National University of Education 3 Hanoi University of Natural Resources and Environment Abstract. Zn2 SnO4 /CNTs photocatalytic composite were synthesized with multi - walled carbon nanotubes (MWCNTs) and Zn2 SnO4 (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 Zn2 SnO4 /CNTs composite catalysts exhibit higher MB degradation activity than add Zn2 SnO4 . Keywords: Zn2 SnO4 , CNTs, composite, photocatalyst. 1. Introduction Zinc stannate is an n-type semiconductor with an inverse spinel structure [1, 2]. Zn2 SnO4 is known for having high electron mobility, high electrical conductivity and attractive 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 electrode material for Li-ion batteries [3-8]. In addition, Zn2 SnO4 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 45% of visible light. Recently, 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 the 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: khangnc@hnue.edu.vn 144
Photocatalytic composites based on Zn2 SnO4 and carbon nanotubes but also stabilized the charge separation by trapping the electrons transferred from the semiconductor thereby hindering a charge recombination [10]. Their outstanding charge transfer abilities can favor the excited electron in the conduction band of nanocrystal semiconductors to migrate into the CNTs, thereby decreasing the ability of recombination of the electron-hole pairs and increasing photocatalytic activity under visible light. In this study, we report on the synthesis of Zn2 SnO4 /CNTs composites and their improved photocatalytic effect under visible light 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 Zn2 SnO4 /CNTs composites. 2. Content 2.1. Experiments Preparation of Zn2 SnO4 /CNTs composites: A Zn2 SnO4 precursor was synthesized using a simple hydrothermal process. Then the white Zn2 SnO4 precursor was mixed with MWCNTs at a ratio of 2/1 and 4/1 and named ZC1 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 establish an adsorption-desorption equilibrium. After that, the MB concentration 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-Zn2 SnO4 composite were revealed by TEM investigation. TEM image of the composite in Figure 1 shows that ZTO nanoparticles about 10 nm in size are attached to the sidewall of the MWCNTs. However, ZTO nanoparticles did not cover the entire surface of the MWCNTs, a composite made up of a random mixture of nanoparticulate ZTO and CNTs. UV-Vis diffuse reflectance spectra of MWCNTs (C), Zn2 SnO4 (Z), Zn2 SnO4 /MWCNTs with mZn2 SnO4 : mCNTs = 8 : 2 (ZC2), and Zn2 SnO4 /MWCNTs with mZn2 SnO4 : mCNTs = 7 : 3 (ZC1) 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 spectra, we recognize that ZC1 have 30% MWCNTs absorption, better than the ZC2 with 20% MWCNTs. Thus, the absorption increases with the increasing mass of CNTs. 145
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 Photocatalytic efficiency was evaluated by intensity peak at 665 nm in absorption spectra of MB solution. The percent degradation of MB solution was calculated as follows: A − Ao D= .100% Ao with D the percent degradation, Ao and A the maximum absorbance at 665 nm in the absorption spectra of an initial and constant MB solution, respectively. Absorbance spectral changes of methylene blue solution in the presence of ZC1, 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 - Zn2 SnO4 composite catalyst exhibits higher MB degradation activity than neat Zn2 SnO4 . The presence of CNTs in Zn2 SnO4 photocatalysts can raise photocatalytic activity from 10% to 25%. For composite catalysts with different MWCNTs mass, the activity increases with the increase in MWCNT mass, and the ZC1 with the biggest MWCNT mass shows the maximum effect. Degradation MB result over CNTs corresponds to previous research. To rely on a mechanism enhancement of photocatalytic properties of CNTs-TiO2 composite, we propose two mechanisms to explain the enhancement of photocatalytic properties of CNTs-Zn2 SnO4 . Table 1. Percent degradation of MB solution over different solids after 4 h of irradiation ZC1 ZC2 Z 75% 80% 90% 146
Photocatalytic composites based on Zn2 SnO4 and carbon nanotubes In the first, a high energy photon excites an electron from the valence band to the conduction band of Zn2 SnO4 . Photogenerated electrons are transferred into the CNTs and holes remain on the Zn2 SnO4 to take part in redo reactions. Figure 7 shows this mechanism. In the second, the CNTs act as sensitizers and transfer electrons to the Zn2 SnO4 . The photogenerated electron is injected into the conduction band of the Zn2 SnO4 , enabling 147
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 Zn2 SnO4 leaving a hole. The now positively charged Zn2 SnO4 can then react with adsorbed water to form hydroxyl radicals. A diagram of this mechanism is shown in Figure 7b. Figure 7. The proposed mechanisms for the CNT(tube)-mediated enhancement of photocatalysis 3. Conclusion Zn2 SnO4 /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-Zn2 SnO4 composite catalysts, which exhibit higher photocatalytic activity in comparison with neat Zn2 SnO4 . 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. 148
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