Preparation of polyaniline-modified TiO2 nanoparticles and their photocatalytic activity under visible light illumination
Introduction
Photocatalytic degradation is an efficient and economical method to decompose organic pollutions into less dangerous matter. As a photocatalyst, TiO2 has the advantages of high chemical stability, high photocatalytic activity to oxidize pollutants in air and water, relative low-price and nontoxicity [1]. However, the wide band gap of TiO2 (3.2 eV) only allows it to absorb the ultraviolet light (<387 nm) that occupies only a small fraction (3–5%) of the solar photons, which limits its wide use. It is of paramount importance to improve the photocatalytic efficiency of TiO2 by shifting its optical response from the UV to the visible range without the decrease of photocatalytic activity. In recent years, several groups have reported investigations to convert TiO2 absorption from the ultraviolet to the visible region by doping TiO2 with transition metals such as Cr, Mn, Fe, V, etc. [2], [3], [4], [5], [6]. Recently, doping TiO2 with nonmetal atoms has received much attention. For example, the doping of nitrogen [7], [8], [9], carbon [10], [11], [12], [13], sulfur and iodine [14], [15] in TiO2 can lower its band gap and shift its optical response to the visible light region. In addition, a variety of dyes can be used to photosensitize TiO2 particles by absorbing visible light photons and inject electrons to conduction band (CB) of TiO2, so as to improve the efficiency of visible light [16], [17].
At present, attention is paid to conducting polymers, which are used as photosensitizers to modify TiO2 nanoparticles [18]. Conducting polymers have already been widely used in photovoltaic devices such as solar cells, light-emitting diodes and corrosion-protecting paint. Although there are plenty of conducting polymers, little work has been done on using conducting polymer to modify TiO2 to degrade organic pollutions. As a typical conducting polymer, polyaniline (PANI) has unique electrical, optical and photoelectric properties. And most importantly, it is cheaper than other conducting polymers. Recently, Li reported hybrid composites of conductive polyaniline and nanocrystalline TiO2, which were prepared by self-assembling and graft polymerization [19]. Compared with neat TiO2 nanoparticles, the nanocomposites showed better photocatalytic activity in photodegradation of methyl orange under sunlight. Zhang has got polyaniline-anatase TiO2 nanocomposites powders and investigated their solid-phase photocatalytic degradation [20].
In the present study, a series of polyaniline-TiO2 nanocomposite powders with different PANI/TiO2 ratios were prepared by ‘in situ’ chemical oxidative polymerization of aniline. In addition, their photocatalytic degradation of phenol was investigated. Results indicate that the introduction of PANI to TiO2 nanoparticles can enhance the photocatalytic efficiency of TiO2 under visible light irradiation.
Section snippets
Reagents and materials
Phenol, aniline, ammonium peroxydisulfate and hydrochloric acid were obtained from Tianjin Chemical Reagents Company. All these reagents were of AR grades and used without further purification with the exception of aniline that had been distilled before used.
Nanoparticulate TiO2, which was mostly anatase, with an average particle size of ca.15 nm and BET specific surface area of ca.70 m2/g, was prepared by sol–gel hydrolysis and condensation of ethanol solutions of tetrabutyl titanate (Ti(OC4H9)4
TEM images
The nanoparticles of neat TiO2 and polyaniline-modified TiO2 are clearly displayed on their images in Fig. 1. From the TEM images, we find that PANI-modified TiO2 does not change the size of neat TiO2 significantly, as shown in Fig. 1(a) and (b). The sizes of both modified and neat TiO2 are monodisperse about 10–20 nm. Moreover, the crystal lattice line can be clearly found in the TEM images. The aggregations of both kinds of particles are caused by high surface energy; however, the
Conclusion
PANI-modified TiO2 composites were prepared by ‘in situ’ chemical oxidative polymerization of aniline in the TiO2 suspension. As a photosensitizer to TiO2, PANI can improve the photocatalytic efficiency of nano-TiO2 catalyst. The degradation of phenol in an aqueous solution under visible light was carried out to evaluate the photocatalytic activity. It is found that PANI-modified TiO2 shows significantly higher photocatalytic activity than neat TiO2 under visible light illumination. The
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