Many-Body Green’s Function Theory Studies On The Reaction Mechanism Of Water Photolysis Catalyzed By Titanium Dioxide

With global energy consumption and the great concern on environmental pollution,it is imperative to develop new,clean and renewable energy sources.Solar energy has attracted a great deal of interest as a sustainable and clean energy source.Solar-driven water splitting provides a promising method to store the abundant solar energy and to produce clean hydrogen and oxygen.Water splitting using semiconductor photocatalysts is currently a simple and economical method for large-scale solar hydrogen production.Titanium dioxide(TiO2)has been widely studied since it is low-cost,abundant,non-toxic and highly stable.In addition to the widely studied bulk-phase TiO2,two-dimensional TiO2 nanosheets are gradually coming into the spotlight.Due to its unique structural properties,it has become a popular research topic for photocatalytic water splitting.However,its reaction mechanism is still an open topic.The photocatalytic efficiency of TiO2 is hindered by its intrinsic defects,and metal ion doping is one of the most effective strategies to improve its efficiency.Nevertheless,transition metals are generally used as the dopants,while alkaline earth metals are rarely used.In recent years,it has been found in experiments that doping by the alkaline earth metal Mg can eliminates the inherent defective states of TiO2,which is undoubtedly beneficial to the photocatalytic reaction.Hence,it is necessary to study the mechanism of photocatalytic water splitting reaction on the Mg-doped TiO2.Many-Body Green’s Function Theory(MBGFT)is one of the most advanced and the most accurate theoretical methods in calculating the electronic structure.In this thesis,we used the GW method in MBGFT and the density functional theory(DFT)to investigate the reaction mechanism of photocatalytic water splitting on the two-dimensional L-TiO2 nanosheets and the Mg-doped anatase(101)surface,which can provide some theoretical basis for further understanding on the TiO2-photocatalyze water splitting reaction and for the design of photocatalysts.The main contents and results of this thesis are as follows.Firstly,the reaction mechanism of water splitting photocatalyed by two-dimensional LTiO2 nanosheets was investigated using DFT and MBGFT,revealing the reaction barriers and the hole transfer pathways in this reaction.Compared with the bulk-phase rutile TiO2,the advantage of L-TiO2 nanosheets in the photocatalytic hydrolysis reaction lies in its superior hole-trapping capability for the key reaction intermediate Ti-OH,which is extremely favorable for the oxidation half-reaction.However,the poorer stability of Ti-OH relative to its precursor is a negative factor limiting the water splitting reaction of L-TiO2 nanosheets and hence has a dramatic impact on the photocatalytic efficiency of the nanosheets.In addition,we found that introduction of Ti vacancies into L-TiO2 nanosheets would turn the nanosheets into p-type semiconductors,which could make the bridging oxygen atoms(Obr)become active sites for the water splitting.At Obr,the energy of the whole photocatalytic reaction tends to decrease,so that water splitting could proceed more easily.Therefore,the introduction of Ti vacancies may be one of the effective way to improve the photocatalytic efficiency of L-TiO2 nanosheets.Secondly,chemistry components of the Mg-doped TiO2 surface have been experimentally analyzed by the X-ray photoelectron spectroscopy(XPS),which indicates the presence of Mg atoms in the toppest three to five monolayers at least.This result was also confirmed by our DFT calculations.We found that the deeper the doping position of Mg is,the more unstable the system becomes.In addition,compared with the undoped anatase TiO2,the water dissociation site changes from the five-coordinated Ti atom(Ti5c)to Obr after Mg doping,with completely different dissociation products which are more stable.