Synthesis And Visible-Light-Driven Photocatalytic Activity Of Carbon Modified TiO₂ And Monolithic TiO₂

Photocatalytic degradation of pollutants has attracted much attention because it makes use of solar energy. TiO₂, as one kind of catalyst, has been studied widely. However, two bottleneck problems should be conquered before the wide spread of this technology. The first one is that the band gap energy of TiO₂ is so large that it can only be excited by UV light. The second one is that most studies focus on preparation and application of TiO₂ powder, which could not be used in every situation. In this study, we tried to resolve these two problems by applying carbon modification and large-dimension shape control to TiO₂. The major research contents and results are listed as follows.(1) Carbon modified TiO₂ was successfully synthesized by using carbon element in tetrabutyl titanate as carbon source. Tetrabutyl titanate and water were used as reactants, and the amount of these reactants were controlled in order to realize incomplete hydrolysis of tetrabutyl titanate. The resulted amorphous TiO₂ contained large amount of alkoxy and 1-butanol. In situ carbon modification could be realized by controlling the following calcination condition. The as-synthesized catalyst was characterized and the results indicate that carbon modifies TiO₂ in two ways: doping as interstitial carbon in the crystal lattice and coating as graphite-like carbon on the surface. It is found that the interstitial carbon could narrow the band gap, and the graphite-like carbon could serve as sensitizer. The sensitization effect of graphite-like carbon is found to contribute greatly to the excellent photocatalytic performance of carbon modified TiO₂. The formation mechanism of the carbon modified TiO₂ was discussed according to nuclear magnetic resonance, Raman spectroscopy and X-ray photoelectron spectroscopy.(2) Making use of tunable chemical property of graphene oxide was firstly proposed to control photoelectronic and photocatalytic performance of graphene oxide/TiO₂ composites. This could be realized by changing concentration of graphene oxide in starting solution. Graphene oxide/TiO₂ composites were prepared by using TiCl₃ and graphene oxide as reactants by self-assembly method. The as-synthesized composites were characterized and the results show that the concentration of graphene oxide in starting solution plays an important role in photoelectronic and photocatalytic performance of graphene oxide/TiO₂ composites, and crystalline quality and chemical states of carbon elements in composites. Either p-type or n-type semiconductor was formed by graphene oxide in graphene oxide/TiO₂ composites. These semiconductors could be excited by light with wavelength larger than 510 nm and acted as sensitizer in graphene oxide/TiO₂ composites. When graphene oxide formed p-type semiconductor, p-n heterojunction could be found in composites. The chemical doping and stress induced by TiO₂ to graphene oxide are the reasons that cause difference in photocatalytic and photoelectronic properties among graphene oxide/TiO₂ composites.(3) Monolithic TiO₂ was successfully synthesized in 7 days under modest drying conditions. The methods combined sol-gel method, phase separation and in situ C, N modification together. The effect of molar ratio of tetrabutyl titanate to water in starting solution on the porous structure and photoactivity of monolithic TiO₂ has been studied. It has been found that when molar ratio of tetrabutyl titanate to water was 1:22, the as-prepared monolithic TiO₂ possessed the best mechanical strength, specific surface area, adsorption property and visible light photocatalytic activity. The porous structure is also found to play important role in photocatalytic performance of monolithic TiO₂. The monolithic TiO₂ prepared under optimized condition was further characterized and the results show that TiO₂ exists as anatase, and both nitrogen and carbon elements exist in monolithic TiO₂ and result in its visible light photocatalytic activity. The preparation mechanism of monolithic TiO₂ has been discussed. Acetylacetone and HCl controlled the hydrolysis and condensation speed. Formamide could control pH value and phase separation, and water could control the trend of phase separation.(4) The photocatalytic performance of the catalysts mentioned above in degradation of environmental pollutants was studied. Bisphenol A was used as target. Scavengers of·OH and O₂·^- were used to test the active species during reaction. It has been found that when irradiated for 45 min, 180 min, and 8 h, carbon modified TiO₂, graphene oxide/TiO₂ composite and monolithic TiO₂ could degrade 99.5%, 96.2% and 89% of bisphenol A, respectively. During photocatalytic reaction of carbon modified TiO₂, photogenerated hole and O₂·^- acted as active species. During photocatalytic reaction of monolithic TiO₂, photogenerated hole,·OH and O₂·^- acted as active species. During photocataytic reaction of graphene oxide/TiO₂ composite, O₂·^- acted as active species. The degradation mechanisms for bisphenol A with these catalysts were tentatively proposed.