Theoretical Study Of The Carrier-transfer Properties In GeS-based Composite Photocatalytic Systems

With the rapid development of industrial and agricultural technology,energy and environmental issues have become serious gradually.It is urgent to seek a feasible method to solve these problems.At present,semiconductor photocatalytic technology is considered as one of the most effective methods to deal with the environmental problems and energy crises due to the advantages of low energy consumption,simple operation,mild reaction conditions and less secondary pollution.Therefore,it is increasingly concerned by the researchers in energy and environment related fields.The layered group Ⅳ monosulfide GeS with an orthorhombic structure has received much attention due to its unique physical and chemical properties.The moderate band gap and band edge positions make single-layer GeS suitable for hydrogen reduction reaction in photocatalytic water splitting.However,the carriers in GeS monolayer are easy to recombined and show weak oxidation capacity,which leads to the low photocatalytic efficiency.Therefore,it is needed to regulate the GeS monolayer to obtain a stronger photocatalytic performance.Constructing GeS-based heterostructures with different photocatalysts can provide more possibilities for the carriers to transfer and separate.Especially the introduction of polar materials plays an important role in the regulation of carriers.In addition,various defects formed during the preparation process of the material are expected to improve the band gap and affect the distribution of carriers.Therefore,introducing the built-in electric field and the interfacial defects into heterostructures can regulate the transfer properties of carriers,thereby deeply affects the carriers recombination rate and the photocatalytic efficiency.Based on first-principles calculations,the effects of the built-in electric field in sandwich heterostructure of GeS and polar hexagonal GeS,and the interfacial defects in GeS/Bi OB heterostructure on the electronic structures and the carriers-transfer properties are investigated.The main contents and conclusions are as follows:（1）The GeS-based sandwich heterostructures are constructed by introducing the hexagonal GeS between two GeS monolayers to investigate the effects of different built-in electric fields on the carriers-transfer properties,optical absorption properties and redox abilities.The involvement of the hexagonal GeS generates a vertical electric field in heterostructure,and the electric field enhances until convergences as the thickness increases.Under the action of the vertical electric field,electrons and holes transfer to the upper and lower surfaces of the heterostructures,respectively.Subsequently,the carriers transfer to the edges to participate in the photocatalytic reactions under the promotion of the parallel electric field in GeS layers.Meanwhile,with the increase of the intermediate hexagonal GeS layers,the band gaps of the heterostructures decrease and eventually disappear.When hexagonal GeS monolayer or bilayer is used to form the sandwich heterostructure,the optical absorption of the composite system is enhanced in the visible and near-infrared region,and the photocatalytic full water splitting is also expected at p H=7.Furthermore,the thickness of the hexagonal GeS has a slight impact on the redox abilities of the heterostructures.This study provides an efficient heterostructure system for photocatalytic full water splitting with better carriers-transfer properties.（2）The geometrical structures,electronic structures,and carriers-transfer properties of the GeS/BiOBr heterostructure,GeS/V_O-BiOBr and GeS/V_Br-BiOBr heterostructures with O vacancy and Br vacancy defects are investigated,and the influences of these properties on the photocatalytic performances are analyzed.The band gap of GeS/BiOBr heterostructure is reduced compared with the two components,which improves the optical absorption intensity in the low energy range and widens the optical absorption range.The existence of interface electric field promotes the Z-type transfer and the spatial separation of carriers.Meanwhile,the redox abilities of the heterostructure are also improved.Considering the common O vacancy and Br vacancy in BiOBr,the defected GeS/V_O-BiOBr and GeS/V_Br-BiOBr heterostructures are further constructed.Compared with the light absorption in pure heterostructure,an obvious red shift is found in the defected heterostructure systems.Moreover,the appearance of the impurity energy levels is beneficial to the Z-type transfer of the carriers and the redox abilities of the heterostructures.Especially,compared with the pure heterostructure,higher redox abilities and reduction ability are obtained in GeS/V_Br-BiOBr and GeS/V_O-BiOBr heterostructures,respectively.By investigating the electronic and photocatalytic properties of pure and defected GeS/BiOBr heterostructures,this thesis provides a new way to improve the photocatalytic properties of GeS materials from a theoretical point of view.