Preparation Of Red TiO₂ And The Separation Properties Of Photogenerated Charges

Photocatalysis can effectively utilize solar energy to decompose water into hydrogen,degrade pollutants,and reduce carbon dioxide,which is one of the most potential means to alleviate energy and environmental problems,including the greenhouse effect.The key goal of photocatalysis is to develop low-cost photocatalysts with high solar energy conversion efficiency and high stability.According to the basic steps of the photocatalytic reaction,light absorption is a prerequisite and directly determines the upper limit of the conversion efficiency;the separation of photogenerated carriers is the central link in controlling the photocatalytic activity.The research work of this thesis focuses on the application of TiO2-based photocatalysts for water splitting.The bottlenecks of TiO2 are no visible light response and insufficient driving force for photo-generated charge separation.To solving these problems,on the one hand,the homogeneously doped red TiO2 is obtained by adjusting the spatial distribution of the dopant to achieve band-to-band strong absorption in the whole spectrum of visible light;on the other hand,by precisely controlling the dopant concentration to adjust the surface band bending of TiO2,the migration behavior of the carriers can be regulated.Finally,by constructing a heterostructure to modulate the photogenerated carriers in the red TiO2,the separation ability of the photogenerated charges and the stability of the red TiO2 are further enhanced.It provides a basis for regulating the visible light absorption and photo-generated charge transfer of wide band gap semiconductors.Study on the preparation of homogeneously B/N co-doped red TiO2 photocatalyst and its performance on water splitting.Boron doped TiO2 microspheres were prepared by a hydrothermal method,and the followed nitridation was conducted by low-temperature pyrolysis of urea in an inert atmosphere.Homogeneous nitrogen doping elevated the valence band maximum,and the photo-generated holes have better migration ability.The obtained homogeneously B/N co-doped red TiO2 has the characteristic of band-to-band visible light absorption edge,and the absorption edge has a red-shift up to 680 nm.Under the irradiation of monochromatic light of 550 nm wavelength,it has the acti vity of oxidizing water to release oxygen.By changing the homogeneous N doping concentration,the photo-generated charge separation and migration characteristics of red TiO2 can be adjusted to realize the preference of hydrogen or oxygen evolution under visible light.The concentration of N doping is further controlled by the heat treatment temperature.The N doping concentration can significantly affect the surface band-bending of TiO2,which in turn affects the migration and separation of carriers and thus the surface reaction process.At a low nitrogen doping concentration,the photo-generated electrons are easy to migrate out for reduction reaction,while at heavy nitrogen doping concentration,photo-generated holes are beneficial to migrate out for oxidation reaction,this realizes the switching of the preference of hydrogen or oxygen evolution from water splitting under visible light.The effective regulation of redox reaction provides some important implication for the ultimate realization of overall water splitting under visible light.Constructing a heterostructure of Ti0.91O2 nanosheet/red TiO2.The coating of Ti0.91O2 ultra-thin nanosheets on the red TiO2 microspheres was conducted by a flocculation method.Because of the relatively large work function of the nanosheets,an upward energy band bending is formed at the interface of the heterostructures.In this case,the internal photogenerated electrons need to overcome a certain energy barrier to migrate to the surface,while holes facilitate migration out,thereby promoting the photo-generated charge separation.The coating of the nanosheets also changes the reaction sites and enhances the photocatalytic water oxidation performance and stability of the red TiO2.