Photocatalytic activity of TiO2 based composite films by porous conjugated polymer coating of nanoparticles

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Abstract

To improve the photocatalytic activity of titanium dioxide (TiO2), one possible approach consists of extending light absorption to visible range by using conjugated polymers together with TiO2 nanoparticles in order to enhance light harvesting. The use of appropriate polymers would facilitate the charge separation at the contact surface of the materials and hence effectively increases the photocatalytic reaction, allowing a more efficient oxidation of organic pollutants. In this work, we proposed an improved synthesis method for obtaining conjugated polymer/TiO2 composite thin films having a porous morphology, which can potentially increase the contact surface between materials. A polyvinyl alcohol/titanium dioxide (PVA/TiO2) precursor was prepared and deposited as thin films on glass substrates. The films were then thermally treated by a controlled protocol, resulting in the transformation of PVA into a conjugated polymer coating the TiO2 particles. The morphology of the obtained composite films showed a porous aspect indicating that the polymer and the nanoparticles were intimately mixed. Photocatalytic activities of the porous composite films (P-PVA/TiO2) were examined through the degradation processes of a phenol solution under visible light irradiation. The results showed that, by their particular morphology, which increased the polymer-nanoparticle contact surface, P-PVA/TiO2 composites easily and efficiently degraded phenol under visible light.

Introduction

Photocatalysis has attracted worldwide research interest because of its potential ability solar energy [1]. Titania as one of the most promising photocatalysts has been investigated for its attractive applications in water splitting and mineralization of toxic organic substances [2], [3], [4]. Photocatalysts using this oxide have several advantages such as high efficiency, good stability, ready availability, and nontoxicity. However, because of its large band gap (3.2 eV), titania absorbs only UV light and this limits its optical performance. To extend its absorption to the visible range of the solar spectrum, attempts to synthesize new materials with large absorption bands have been carried out [5], [6].

The use of conjugated polymer doped TiO2 has showed that the photocatalytic performance of TiO2 could be improved under visible light [7], [8], [9]. The conjugated polymer/TiO2 composites are usually obtained by in-situ polymerization, and seldom loaded on support, limiting the usage as well as the re-use of the obtained photocatalyst [10]. On the other hand, composites with a porous structure are interesting for their, photocatalytic activity because of the high surface/volume ratio, which enhances the photocatalytic performance [11], [12], [13].

In this paper, a new method for preparing porous conjugated unsaturated polymer/TiO2 composite deposited on glass substrates is developed by using anatase TiO2 sol and polyvinyl alcohol (PVA) as a precursor. The TiO2 sol was prepared with tetra-butyl titanate at low temperature (70 °C), and by combining freeze drying and heat-treatment techniques, porous-polyvinyl alcohol/titanium dioxide (P-PVA/TiO2) composites with visible light activity were prepared and characterized. The photocatalytic activity of the P-PVA/TiO2 composites was investigated by studying the degradation of phenol solution.

Section snippets

Experimental

Tetrabutyl titanate (TBOT), absolute ethanol (EtOH), acetic acid (AcOH), acetylacetone (Acac), and phenol were purchased at Chengdu Kelong Chemical Reagents Factory (China) and used as received. Polyvinyl alcohol (PVA) with polymerization degree of 1700 and hydrolytic degree of 99% was supplied by Sichuan Vinylon Factory (Chongqing, China).

100 ml deionized water and 0.3 ml concentrated HCl were mixed and stirred at 70 °C in a three-necked flask. A mixture solution of EtOH, AcOH and TBOT was then

Characterization results

Fig. 2 shows the SEM micrographs of P-PVA/TiO2. It can be seen that the obtained composite films show a porous aspect and the polymer and the nanoparticles were intimately mixed (Fig. 2a). The surface of the composite film (Fig. 2b) contains numerous small cavities, which favor the adsorption and degradation of the dye solution.

The X-ray diffraction patterns (XRD) of P-PVA/TiO2, PVA/TiO2 and pure TiO2 are presented in Fig. 3. For TiO2 spectrum, the peaks at 2θ=25.3°, 37.8°, 47.8°, 54.2° and

Conclusion

We have investigated composites using TiO2 nanoparticles in a conjugated PVA matrix to be used in photocatalytic applications. The composite materials have been synthesized by a simple method combing freeze-drying and heat-treatment using TBOT as titanium source and PVA as precursor of conjugated polymer. We have shown that using P-PVA/TiO2 composites extended the photo-response of TiO2 from UV to visible part of sunlight and significantly improved the absorption of the material. In addition,

Acknowledgments

The authors are grateful to the National Natural Science Foundation of China (Grant nos. 50573052, and 51173116) for supporting this research. They also thank R. Michaud for technical assistance in measuring the film thickness.

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