Photocatalytic properties of nano-structured TiO2-carbon films obtained by means of electrophoretic deposition

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Abstract

Recent studies have shown that the light-absorption and photocatalytic efficiencies of TiO2 can be improved by coupling TiO2 nano-particles with nonmetallic dopants, such as carbon. In this paper, we describe the electrophoretic preparation of a novel TiO2-carbon nano-composite photocatalyst on a glass indium thin oxide (ITO) substrate. The objective is to take better advantage of the (e/h+) pair generated by photoexcitation of semiconducting TiO2 particles. The transfer of electrons (e) into adjacent carbon nano-particles promotes reduction of oxygen to produce hydrogen peroxide (H2O2) which, in the presence of iron ions, can subsequently form hydroxyl radicals (radical dotOH) via the Fenton reaction. At the same time, radical dotOH is formed from water by the (h+) holes in the TiO2. Thus, the radical dotOH oxidant is produced by two routes. The efficiency of this photolytic-Fenton process was tested with a model organic compound, Orange-II (OG-II) azo dye, which is employed in the textile industry.

Introduction

Heterogeneous photocatalysis for water-treatment technologies has attracted the attention of many research groups around the world [1]. Among advanced oxidation processes (AOPs), photocatalytic treatment offers a capability for destroying organic contaminants by oxidation with hydroxyl radicals (radical dotOH) that are generated under mild conditions [2], [3]. The best-known photocatalyst is titanium dioxide (TiO2). The band-gap value of anatase TiO2 is around 3.2 eV, which enables UV light of wavelengths smaller than 400 nm to activate the catalyst [4]. The mechanism of photocatalysis involves the promotion of an electron (e) from the valence band (VB) into the conduction band (CB) of the semiconducting oxide, creating a hole (h+) in the VB according to Eq. (1).TiO2+hνecb+hvb+

In water, this process is followed by the formation of radical dotOH at the semiconductor surface as well as direct oxidation of organic compounds (R), according to the following reactions [5]:hvb++H2OOH+H+hvb++OHOHadhvb++RadR+

Meanwhile, the electrons that are promoted to the CB can react with electron acceptors, such as oxygen, present in the solution [5]:ecb+O2O2

In recent years, many studies have been made in order to improve the catalytic efficiency of TiO2. The addition of other materials, for example carbon, has been of interest because this material can promote the adsorption of organic compounds to be transformed [6], [7]. The effect of carbon on the photocatalytic efficiency of TiO2 has been tested [8], [9], and improved photocatalytic effectiveness has been reported by several authors [10], [11], [12]. Other workers have found that the recombination of holes and electrons, the major cause of inefficiency, can be retarded by applying a small positive potential to the TiO2, and consequently the degradation of organic compounds through photoelectrocatalysis is enhanced [13]. According to research done by Egerton and Christensen [14], the method of preparation of the electrodes is the most important determinant factor for controlling the photocatalyst activity [15], [16], [17].

In this work, we report a preliminary study in which two approaches to water treatment are combined, photocatalysis and Fenton's reaction, which involves the homogeneous conversion of peroxide to radical dotOH. Vulcan carbon has been incorporated with nanoscale TiO2 particles in a mixed surface film, resulting in a nano-structured composite TiO2-carbon photocatalyst for oxidative wastewater treatment. The overall reaction scheme is illustrated in Fig. 1. The electrons ejected by UV illumination to the CB can reduce dissolved oxygen (ORR) on the carbon surface to produce hydrogen peroxide. Ferrous ion in solution subsequently catalyzes the conversion of peroxide to radical dotOH, an effective oxidant. At the same time radical dotOH is also produced directly from water by the holes in the semiconductor. With this scheme, we hope to enhance the degradation of organic compounds present in the water. In this study, the efficacy of the composite photocatalyst is tested by observing the decolorization of Orange-II (OG-II) azo dye as a model organic compound.

Section snippets

Reagents and instruments

Nano-particulate TiO2 (P25, 80% anatase, 20% rutile, average particle diameter 20 nm) was purchased from Degussa, and Vulcan carbon XC-72R (nominal average particle diameter 5 nm) was provided by CABOT. Conductive glass plates, TEC15, coated with SnO2 films doped with indium (ITO), were supplied by Hartford Glass, USA. Analytical grade OG-II dye (C16H11N2NaO4S) was obtained from Aldrich, and used as received. Na2SO4, FeSO4·7H2O, H2SO4, and 2-propanol 99.97% obtained from J.T. Baker were of

Structure of the surface films

Representative SEM images of the experimental surfaces are shown in Fig. 2. Fig. 2A shows the fresh ITO surface. Figs. 2B and C present the films of TiO2, and the composite TiO2-carbon films, respectively, both prepared by ED with a deposition time of 40 s, and sintering. The difference between the simple TiO2 film, and that containing carbon suggests the formation of aggregates that we ascribe to carbon particles in the TiO2 film, and both are much rougher than the ITO substrate. Fig. 3 shows

Conclusions

This work presents a preliminary study of nano-structured, composite TiO2-carbon photocatalysts. According to the results, it is possible to synthesize the composite films by ED. The resulting material was shown to promote photocatalytic oxygen–reduction, and our results indicate that, with an optimal ratio of carbon to TiO2, it is possible to generate H2O2 in solution at a level 50% higher than that obtained by TiO2, without carbon. With respect to the band-gap energies, the action spectra

Acknowledgments

The authors thank the Mexican Council for Science and Technology (CONACyT), and the Council for Science and Technology of Guanajuato (CONCyTEG) for financial support of this work (Grant GTO-04-C02-68). J.M.P.H also acknowledges CONACyT for a graduate fellowship. T.W.C. is a Cooperante supported by U.S. Peace Corps under agreement with CONACyT.

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