The Modulation Of The Structure,Electronic And Photocatalytic Properties For G-C₃N₄ By Surface Modification

Nowadays,the two global crisises including energy shortage and environmental pollutions are seriously threating the survival of human and the development of society.Therefore,both governments and scientists around the world have paid lots of efforts to meet the challenges.Photocatalytic technology has been proved to be one of the promising ways to solve these problems because it can not only transfer solar energy to hydrogen energy but also degrade organic pollutions.In past several decades,people have made many achievements,however,the actual photocatalytic activity cannot meet the demand of large-scale production,various modification measures have been taken to improve the photocatalytic performance,besides,looking for new photocatalysts with high efficient is another important way.In recent years,graphitic carbon nitride?g-C₃N₄?,a new metal-free semiconductor material,has attracted much attentions due to its numerous merits,such as high stability,low cost,absorbing visible wavelength,etc.In fact,this polymeric material has several prominent drawbacks like high electron-hole recombination rate,small specific surface area and less active sites,which restrict the photocatalytic efficiency,however,the enthusiasm of researchers has never reduced.People adopt many modification scheme to improve the photocatalytic performance,for example,optimizing the preparation procedure,decreasing material size to nano-scale,chemical doping and physical adsorption,these measures can indeed improve the reaction activity at different level.However,some underlying mechanisms are not clear which need further investigation.In present dissertation,we systematically studied two adsorptive kinds of g-C₃N₄ based photocatalysts by first-principles calculations.The main framework of the present dissertation is as follows:Chapter 1:A brief introduction of research progress for semiconductor photocatalysts.The research background as well as current development of g-C₃N₄ are also given.Chapter 2:The description of theoretical framework for density functional theory?DFT?and the software package adopted in present researches.Chapter 3:We theoretical investigate the underlying mechanism behind the high photocatalytic performance of C60 adsorbing g-C₃N₄.By building a variety of possible adsorption configurations,we systematically studied the change of structure,stability,electronic properties as well as light absorption of g-C₃N₄ before and after C60 adsorption.The results show that the adsorption of C60 can lead an irreversible structure distortion of g-C₃N₄,and the structural distortion plays a crucial role in improving photocatalytic performance of g-C₃N₄.The structure with distortion is more stable,the optical adsorption is enhanced,and the VBM shifts down with a stronger photooxidation capacity.Besides,we qualitatively analyze the relationship between distortion degree with energy and electronic properties.This theoretical research provides an innovative interpretation to experimental phenomenon.Chapter 4:In this chapter,we investigate the structural change and electronic properties of g-C₃N₄ monolayers with individual Pt adatoms by theoretical calculation.The results show that the adsorption of Pt can partly reduce the recombination of photo-generated carriers.Pt atom can be used as an active site of oxidation reaction,which facilitates the photocatalytic reaction.The chemical adsorption can not only enhance the optical absorption of composite,but also magnify the scope of optical absorption,improve the utilization of sunlight.Chapter 5:Summarizing the whole content of the dissertation and giving a preview of development in future.