Prepareation And Mechenism Study Of Two Surface-modified TiO₂ Catalysts

In recent years, much effort has been made to development visible-light activated TiO₂-based materials. However, as the visible-light activity of titania could be affected by many factors, the mechanism study of some visible-light activated ion-modified TiO₂ catalysts are always in depute. In this study, we developed two kinds of surface-modified TiO₂ catalysts through a facile low-temperature refluxing method, both of which have obtained an effective visible-light activity in the degradation of methyl orange (MO). Through a serious of experiments, we investigated the substances which caused the visible light activity of titania, and tentatively discussed the degradation mechanism of MO under visible light on these two catalysts.1. A visible-light responsive anatase TiO₂ was synthesized through a peptizing-reflux method, using tetrabutyl titanate as the titanium precursor. The catalyst is mainly anatase, and composed of spherical-shaped nanoparticles (4～5 nm). We observed that in the synthetic process of TiO₂ catalysts, the nitric acid not only helps to peptize the titania, promote the formation of surface hydroxyl groups, but also inhibited the over-hydrolysis of tetrabutyl titanate; the presence of acid and hydrolysis products (n-butanol) cooperatively lead to the retention of more alkoxyl groups on the anatase surface, which is proved to be the main cause of the visible light activity.2. A fluorine-modified nanosized TiO₂ (F-TiO₂) was prepared through a precipitation -fluorination-reflux method. The results showed the small particle size (5～8 nm) of ellipsoidal shaped F-TiO₂ samples. The presence of fluorine not only suppressed the formation of brookite phase, but also improved the crystallinity of anatase phase. The fluorine atoms mainly distributed on the surface of TiO₂, and existed in both forms of chemical- adsorption and interstitial-doping. Compared to the pure titania, the fluorine-modified TiO₂ powder showed much higher degradation efficiency of methyl orange (MO) under visible light. Further investigation showed that the increased degradation rate of MO under visible light caused by the synergistic effect of chemical-adsorption and interstitial-doping fluorine atoms. 3.The visible-light degradation of MO on the two kinds of modified TiO₂ catalysts was actually caused by surface states. On the alkoxyl-derived surface states, excited electrons could be generated and transfered easily through a ligand-to-metal charge transfer (LMCT) process under visible light irradiation. However, the degradation of MO on F-TiO₂ samples was actually caused by the enhanced self-degradation of dye on the surface states formed by fluorine atoms. Therefore, in the research of visible-light activited TiO₂ catalysts, the surface states of titania formed in the synthetic process should not be neglected; besides, the investigation on the degradation efficiency of different substrates is also essential to fully understand the intrinsic property of surface-modified TiO₂ catalysts.