Preparation Of TiO₂/Ti₃C₂ And G-C₃N₄/CaCO₃ Hybridized Photocatalysts And The Mechanisms On Photocatalysis

The excessive consumption of traditional fossil energy has led to the sharp rise of the concentration of greenhouse gas CO₂ and air pollutants（such as NO_X）,and the energy shortages and environmental problems are becoming increasingly serious.Therefore,it is of great significance to develop efficient,economical and eco-friendly technologies to solve these problems.Semiconductor photocatalysis has received increasing attention because it can be driven by solar energy for the oxidation/reduction of toxic pollutants.TiO₂ and g-C₃N₄ are the most representative inorganic and organic semiconductor photocatalysts respectively,which have the merits of physiochemical stability,low cost and ecofriendliness.However,they still suffer from the rapid recombination of charge carriers and insufficient utilization of solar energy.Therefore,given the limitations of TiO₂ and g-C₃N₄,the high efficiency composite catalysts TiO₂/Ti₃C₂ and g-C₃N₄/CaCO₃ are constructed.Their photocatalytic enhancement mechanism and structure-activity relationship are also elucidated.The research contents are shown as follows:PART Ⅰ:Heterojunctions of Ti₃C₂ and TiO₂ nanosheets with high-energy facet for efficient CO₂ photoreduction.Ti₃C₂ MXene is a novel graphene-like material with two-dimensional layered structure,which has been widely used as a co-catalyst due to its excellent conductivity.It is of great importance to explore the photocatalytic enhancement mechanism of Ti₃C₂ as a co-catalyst on TiO₂ for CO₂ photoreduction to produce solar fuel.The preparation methods of high-energy TiO₂nanosheets/Ti₃C₂ composite photocatalytic materials usually face the problems of complicated operation procedures and large amount of hydrofluoric acid.In this dissertation,high-energy TiO₂ nanosheets/Ti₃C₂ composite photocatalytic material was successfully prepared by a facile one-step hydrothermal method.Besides,the effects of hydrofluoric acid content on the morphology and CO₂ photoreduction activity of the composites were systematically investigated.It was found that（1）by treating 2.0 g Ti₃AlC₂ with 3-5 ml HF,and then adding water,TiO₂ nanosheets could be grown in-situ on Ti₃C₂via a hydrothermal process;（2）the optimized TiO₂/Ti₃C₂ sample has superior photocatalytic activity with the CO₂photoreduction rate of 13.45μmol·g^-1·h^-1,which is higher than that of commercial P25 TiO₂(10.95μmol·g^-1·h^-1).Noteworthy,it likewise has excellent selectivity for CH₄ production（76.4%）.The reason for the enhanced photocatalytic activity is that Ti₃C₂ with lower work function can capture the photogenerated holes of TiO₂ under light irradiation,promote its rapid migration,and improve the separation efficiency of photogenerated carriers,ensuring the highly efficient process of CO₂ photoreduction.Accordingly,this work opening a new avenue for design and fabricate hybrid photocatalysts for high-efficiency CO₂ photoredcution.PART Ⅱ:g-C₃N₄/CaCO₃ heterojunctions for efficiently photocatalytic NOx purification.In view of the semiconductor modification method,this work adopted the method of building insulator based heterojunction to induce the photoexcited charge of g-C₃N₄ transfer to insulator,so as to improve the overall photocatalytic efficiency.The g-C₃N₄/CaCO₃composite was prepared by a hydrothermal treatment and calcination using dicyandiamine and calcium chloride as ingredient.The effect of the mass ratio of calcium chloride and dicyandiamine on for the visible light photocatalytic NO_X removal of g-C₃N₄/CaCO₃ was studied.It was found that（1）when the mass ratio of calcium chloride to dicyandiamine is 1:20,the photocatalytic NO_X removal rate of g-C₃N₄/CaCO₃ samples can reach 51.18%,which is significantly higher than that of g-C₃N₄ nanosheets（34.05%）and bulk g-C₃N₄（19.47%）.The reason for the enhanced activity is that the visible light absorption of g-C₃N₄ is improved by CaCO₃ modification.On the other hand,CaCO₃ can be used as the receptor of g-C₃N₄photogenerated hole,which can effectively inhibit the recombination of photogenerated charge carriers of g-C₃N₄.Therefore,the carrier could efficiently migrate to the surface of catalyst,and participate in photocatalytic reaction.（2）Based on the in-situ DRIFT characterization,we clarify that the photocatalytic mechanism of NO removal by g-C₃N₄/CaCO₃ is NO→NO⁺→NO₃^-/NO₂^-.This study lays a foundation for the application of insulators in the field of photocatalysis.