Photocatalytic composites based on Zn2SnO4 and carbon nanotubes

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.

This paper is available online at http://stdb.hnue.edu.vn

PHOTOCATALYTIC COMPOSITES BASED ON Zn2SnO4

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

1Center for Nanoscience and Technology, Hanoi National University of Education

2Faculty of Chemistry, Hanoi National University of Education

3Hanoi University of Natural Resources and Environment

Abstract.Zn2SnO4/CNTs photocatalytic composite were synthesized with multi

-walled carbon nanotubes (MWCNTs) and Zn2SnO4(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 Zn2SnO4/CNTs composite catalysts exhibit higher MB degradation

activity than add Zn2SnO4

Keywords: Zn2SnO4, CNTs, composite, photocatalyst.

1 Introduction

Zinc stannate is an n-type semiconductor with an inverse spinel structure [1, 2]

Zn2SnO4 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, Zn2SnO4has 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

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 Zn2SnO4/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 Zn2SnO4/CNTs composites

2 Content

2.1 Experiments

Preparation of Zn2SnO4/CNTs composites: A Zn2SnO4 precursor was synthesized using a simple hydrothermal process Then the white Zn2SnO4precursor 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-Zn2SnO4 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), Zn2SnO4 (Z),

Zn2SnO4/MWCNTs with mZn2SnO4 : mCNTs = 8 : 2 (ZC2), and Zn2SnO4/MWCNTs with mZn2SnO4 : 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

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:

D = A − A o

A o .100%

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 - Zn2SnO4 composite catalyst exhibits higher MB degradation activity than neat Zn2SnO4 The presence of CNTs in Zn2SnO4 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-Zn2SnO4

Table 1 Percent degradation of MB solution over different solids

after 4 h of irradiation

In the first, a high energy photon excites an electron from the valence band to the conduction band of Zn2SnO4 Photogenerated electrons are transferred into the CNTs and holes remain on the Zn2SnO4 to take part in redo reactions Figure 7 shows this mechanism

In the second, the CNTs act as sensitizers and transfer electrons to the Zn2SnO4 The photogenerated electron is injected into the conduction band of the Zn2SnO4, enabling

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 Zn2SnO4 leaving a hole The now positively charged Zn2SnO4 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

Zn2SnO4/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-Zn2SnO4 composite catalysts, which exhibit higher photocatalytic activity in comparison with neat

Zn2SnO4 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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