Modification Of TiO₂-based Semiconductor Materials For Photocatalysis

TiO₂ has long been applied in photocatalysis for its nontoxicity, low cost, superior photocatalytic activity and long-term chemical stability. However, TiO₂ only absorbs light in the ultraviolet region（about 3⁴% of solar energy） due to its large band gap（3.0-3.2 eV）, visible light occupied more than 40% of solar energy and cannot be utilized, which has critically restrained its practical applications. Moreover, the recombination of the photo-generated electron-hole pairs dissipates the input energy in the form of heat or emitted light, which is one of the main reasons for the low efficiency of photocatalysis. To obtain high-performance photocatalysts, TiO₂-based semiconductor materials with various heterostructures and dopants were designed and fabricated, the effect of heterojuncion and dopants on the TiO₂ photocatalytic activity have been studied. The main contents were summarized as follows:I: Construction of TiO₂-based heterojunctions and interface strutures（1） The composite structure of Si（thin film）/TiO₂（nanotube arrays） and Si/TiO₂ heterojunctions have been fabricated via chemical vapor deposition Si on TiO₂ nanotube arrays synthesized by anodizing titanium foil. The construction of Si/TiO₂ heterojunctions efficiently enhanced the visible light absorption ability and extended the light absorption range of TiO₂ nanotube arrays. Moreover, the Si（thin film）/TiO₂（nanotube arrays） structure could multiplereflect the incident lights and prelong their travelling paths to further improve the light absorption intensity. The photocatalytic reduction of CO₂ to hydrocarbon（such as methenol） results show that the efficiency of TiO₂ nanotube arrays was increased by 39.2% with Si thin film coated. The enhanced photocatalytic activity of Si/TiO₂ composites can be contributed to the formation of Si/TiO₂ heterojunction, which improved the seperation of photo-generated electrons and holes.（2） Si/TiO₂ heterojunction nanotube arrays were successfully fabricated by a two-step process of anodization and electrodeposition method followed by annealing. Characterization results show that the TiO₂ nanotubes were ⁵00 nm in length, ⁸0 nm in diameter and ¹6 nm in the tube wall thickness. Si nanoparticles were successfully filled into the TiO₂ nanotubes to form Si/TiO₂ heterojunction. From the result of photo-decomposition of Rhodamine B（Rh. B）, it was found that the Si/TiO₂ nanotube composites show higher photocatalytic activity（¹.78 times in kinetic constants） compared with TiO₂ nanotubes.（3） Ag/TiO₂ nanocomposites were successfully synthesized via a facile one-step hydrothermal process. The construction of Ag/TiO₂ Schottky junction enhanced the visible light absorption intensity. Photocatalytic degradation of Rh.B results show that the kinetic constant of Ag/TiO₂ was 3.88 times of that of TiO₂ nanosheets. The improved photocatalytic activity should be contributed to the construction of Ag/TiO₂ Schottky junciton, which efficiently improve the separation rate of photo-generated electrons and holes.（4） TiO₂/GQDs heterojunction has been constructed via a facile method combining hydrothermal reaction and post calcination treatment in Citric acid environment, and greatly improves the visible light absorption ability of TiO₂. Photocatalytic degradation of Rh.B results show that TiO₂/GQDs exhibit improved photocatalytic activity under both UV and Vis light irradiation compared with the bare TiO₂ nanosheets. The enhanced photocatalytic property of TiO₂/GQDs nanocomposites could be ascribed to two facets: under UV light irradiation, the formation of TiO₂/GQDs heterojunction could improve the separation efficiency of the photo-generated carries, prolong their longevity; moreover, under visible light irradiation, the GQDs could absorb visible light and be excited to generate electron-hole pairs, which will transfer to conduction band of TiO₂ to photodegrade RhB, resulting in a high-performance photocatalytic activity.II: TiO₂-based photocatalysts with Ti³⁺ and N doping（1） Ti³⁺-Si@TiO₂ core-shell heterojunction nanostructure was synthesized via hydrothermal reaction with tetrabutyl titanate and Si powder which was prepared from the magnesiothermic reduction of SiO₂ st?ber nanospheres. Ti³⁺ doping was formed from partial reduction of TiO₂ by H2, which was produced from the reaction between Si and HF in the hydrothermal condition. It was found that the Ti³⁺-Si@TiO₂ core-shell heterojunction nanostructure imporved the visible light absorption ability and possessed much higher photocatalytic activity than individual Si and TiO₂ samples for the CO₂ conversion and degradation of RhB（2.1 times in photocatalytic kinetic constant）. This excellent performance could be attributed to the enhanced light absorption ability and high separation efficiency of photo-generated carriers due to the elaborate construction of Si/TiO₂ heterojunction and Ti³⁺ doping. The concentration of Ti³⁺ doping and quantity of Si/TiO₂ heterojunctions could be tuned to improve the photocatalytic activity by changing the hydrothermal temperature and additive of Si. The results showed that Ti³⁺-Si@TiO₂ core-shell heterojunction nanostructure exhibits the highest photocatalytic activity with adding 0.3 g Si nanospheres at 180 oC in the hydrothermal reaction.（2） Ti³⁺ doped TiO₂ single anatase crystals with truncated bipyramidal nanostructure and high photocatalytic activity were obtained via hydrothermal reaction of Ti nanopowder, HCl and HF. Ti³⁺ doping extends the light absorption from the UV into the visible range and results. Moreover, the obtained Ti³⁺ self-doped TiO₂ sample possesses much higher photocatalytic activities than undoped TiO₂-HF and TiO₂-HCl samples for degradation of MB. This excellent performance should be attributed to the advantages of anatase pahse, improved light absorption ability and enhanced concentration of photo-generated carriers due to doping of Ti³⁺ ions. The adding of HF was found to be responsible for the formation of anatase phase, truncated bipyramidal nanostructure and Ti³⁺ doping.（3） Ti³⁺-TiO₂/GQDs sample was obtained via the slightly reduction of TiO₂ by GQDs after high temperature annealing treatment, which possesses both TiO₂/GQDs heterojunctions and numerous vacancy pits in the TiO₂ nanosheets. Photocatalytic results show that Ti³⁺-TiO₂/GQDs exhibit s the improved photocatalytic activity under both UV and Vis light irradiation compared with the TiO₂/GQDs heterosturcture materials. This improved phtocatalytic activity should be attributed to the existence of TiO₂/GQDs with photo-generated carriers separation effect, GQDs with visible light response, and especially the Ti³⁺ doping, which could improve the hydrophilic property of TiO₂ surfaces to enhance their RhB adsorption ability and be excited by visible light to generate photo-induced electrons to conduction band of TiO₂ for photocatalytic reaction.（4） Ti³⁺ and N co-doped porous TiO₂ nanosheets were obtained from energetic material（labeled as HMX） explosion treatment. The doping concentration of Ti³⁺ and N could be tuned by changing the addition of HMX. Ti³⁺ and N co-doping greatly enhanced the visible light absorption ability and photocatalytic degradation of RhB under visible light irradiation. Ti³⁺ and N co-doped porous TiO₂ nanosheets exhibit the highest photocatalytic activity only when the Ti³⁺ and N are in the doping concentrations, for that excess Ti³⁺ doping will form defect center and trap photo-generated carriers to be recombined. The enhanced photocatalytic activity of Ti³⁺ and N co-doped porous TiO₂ nanosheets under visible light irradiation can be attributed to the formation of mid-energy level between the conduction band and valance band of TiO₂, which narrowing the bandgap of TiO₂ and can be excited by visible light to generate photo-induced carriers for photocatalytic reaction. Moreover, the formation of Ti³⁺ and porours morphology effectively improved the adsorpotion ability of RhB, which is benefical for the improvement of TiO₂ photocatalytic activity.