Photodegradation of phenol in a polymer-modified TiO2 semiconductor particulate system under the irradiation of visible light

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Abstract

Preparation methods and photocatalytic activity of the photocatalyst nanometer TiO2/poly-(fluorene-co-thiophene) (PFT) were studied under GaI3 lamp irradiation in the present research. The degradation rate of phenol using the target photocatalyst was much higher than it was using TiO2/rhodamine B. Diffuse reflectance spectra showed that the absorbance range of TiO2/PFT was expanded from 387 nm (TiO2) to about 500 nm, and fluorescent emission spectra showed that the spectral shape of TiO2/PFT was identical with that of pure PFT. The fluorescent quantum efficiency of TiO2/PFT was estimated at 3.8% while that of PFT film was about 14.7%, which indicated that electrons transferred from the PFT into the conduction band of TiO2.

Introduction

Dye-sensitized TiO2 solar cells [1], [2] and dye-sensitized TiO2 photodegradation of organic substances have been the subject of much research since Honda and Fujishima reported photochemical splitting of water by a TiO2 electrode [3]. The abilities of large band gap semi-conducting metal oxides such as TiO2 to mediate complete mineralization of organic pollutants in water and wastewater under UV irradiation have been thoroughly studied [4], [5], [6], [7], [8], [9]. However, the wide band gap (3.2 eV) of TiO2 limits the application of solar light since UV light in solar light is less than 4%. To extend the photoresponse of TiO2 to the visible region, surface modification and dye sensitization have been reported [10], [11], [12], [13]. Three basic processes are involved in the dye sensitizing semiconductor. First, dye sensitizer adsorbs onto the surface of TiO2; second, adsorbed dye is excited by visible light; and finally, electrons are injected from the excited dye with high mobility into the conduction band of TiO2[14], [15].

In our study, a conjugated polymer was used as a photosensitizer to modify TiO2. Conjugated polymers have already been widely used as sensitizers in photovoltaic devices such as solar cells [16], [17], [18]. Photoinduced electron transfer from π-conjugated polymers to nanocrystalline TiO2 was experimentally demonstrated by photoinduced absorption (PIA) spectroscopy and photoinduced electron spin resonance (ESR) [19]. Although there are plenty of conjugated polymers, so far little work has been done on using conjugated polymer modified TiO2 to degrade organics. This may be due to two challenging problems: (1) the stability of conjugated polymers is poorer than that of dyes (for example, poly (p-phenylenevinylene) (PPV) is readily oxidized under room light in the air [20]). (2) The electron mobility of conjugated polymers is much lower than that of dyes [21]. The thiophene oligomer was reported to have high electron mobility [22]. Additionally, polythiophene has been used to degrade fungicide [23], which indicates that polythiophene is quite stable under photoirradiation in the air. Therefore, we chose a copolymer consisting of fluorene and thiophene.

Section snippets

Materials

TiO2 (Degussa P25: 80% anatase, 20% rutile; 50 m2/g) and Al2O3 (Degussa, 100 m2/g) were both pre-dried at a temperature of 393 K under vacuum. The conjugated polymer poly-(fluorene-co-thiophene), whose chemical structure is presented in Fig. 1, was synthesized according to the literature [24]. Purified water was obtained from a Millipore filtration system (Millipore ZLXS50020). All other chemicals were analytical grade and used without further purification.

Measurements

A UV-2501 UV–visible spectrophotometer

Diffuse reflectance spectra (DRS)

Fig. 2 shows the DRS of different powders. The photosensitizer used in our study was the co-polymer of fluorene/thiophene (70/30 in molar ratio) with maximum adsorption at a wavelength of about 400 nm [24]. As was shown in Fig. 2, there is no absorption above 400 nm for pure TiO2, while TiO2/PFT shows light absorption from 360 to about 500 nm. Similar regularity is observed also for pure Al2O3 and Al2O3/PFT. The spectra of TiO2/PFT and Al2O3/PFT were the spectra of polymer PFT adsorbed onto the

Conclusion

The results of the present study demonstrate that the synthesized conjugated polymer poly(fluorene-co-thiophene), which has strong absorbance in the visible region, can photosensitize TiO2 to catalyze the degradation of phenol under visible light. Compared with the photocatalytic activity of the PFT surface-modified non-conducting material alumina, an electron transition was likely to occur in the TiO2/PFT system. Oxygen plays an important role during the degradation degradation chain reaction

Acknowledgement

This work was financed by the New Century Excellent Talent Program of MOE (NCET-04-0790), PR China.

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