First-principles Study Of Bi₂O₃、CuO、NiO Semiconductors And Cu Electrode

Both photocatalytic decomposition of H20 and photocatalytic reduction of CO₂ are effective ways to solve the energy crisis and global warming.However,most photocatalysts are powder,which leads to the high cost of the above photocatalysis processes due to hard recycle.Magnetic photocatalysts are easy for recycle and reuse.Thus we turn our eay on the photocatalytic materials with magnetic properties,and hope that this kind of magnetic materials with proper redox potential for water decomposition or CO₂ reduction.Additionally,we also explore the photocatalytic materials without magnetic properties,and it is expected that after magnetic modification,this kind of photocatalytic materials can have magnetic properties.Cu electrode has been the focus because it is capable of producing hydrocarbon products with suitable hydrogen evolution potential and adsorption ability of CO.However,the Cu electrode suffers poor activity and stability.So mang numerous efforts have been made to improve the performance of Cu electrode.In this paper,semiconductors bismuth oxide,nickel oxide and copper oxide,have been considered as main research objects.First principles theory is used to investigate the effect of doping on the space configuration,electronic structure and magnetic properties,absorption spectrum and transition state.Based on reasonable physical and chemical models,we are devoted to the investigation of the relationship between structure and native electronic properties of materials,which will shed light on understanding the physical and chemical properties of materials and thus benefit the design of novel and highly efficient nano-structured material The detail research works are as following:1.In the first two chapters,we introduce the material background and the theory of first principles.Target materials mainly include the semiconductors Bi₂O₃,CuO and NiO,and electrochemical material Cu.In the part of first principles,the density functional theory,exchange-correlation function,plane wave solution for schrodinger equation,the theory serch for transition state,the projector-augmented wave（PAW）are mainly introduced.2.In the third chapter,we are devoted to the design of Bi₂O₃ with high activity by single metallic doping,metallic and non-metallic codoping approachs.First-principles density functional theory was performed to investigate the atomic structures,electronic structure,absorption spectra of dopant complexes involving Cu,Ag,Pb,Pd,Sn five kind of metal single doping of Bi₂O₃,and codoping with nonmetal N.A shallow impurity state level was shown in Ag and Cu and Pb Single doping system respectively,deep impurity state levels were shown in both Pd and Sn single doping system.In the five codoping systems,the 2p state of N influenced all of the electronic structures.The calculation results of hole effective mass were shown that there was a shup increase of conductivity when N doped in Cu single doped system.3.In Chapter 4,the effect of doping on the magnetic properties of some photocatalysts that are easily recycled was studied using the local spin density approximation（LSDA）+U method on typical divalent metal oxide semiconductors CuO,NiO,Ni-doped CuO,and Cu-doped NiO.A new implementation of the LSDA+U model based on the PAW method has been presented to calculate the effect of Ni and Cu doping on the spatial structure,magnetic properties,and electronic structure of CuO and NiO,respectively.This is an all-electron method without any shape approximation for the potential or charge density.It is found that the influence on the spatial structure of CuO doped by Ni and that of NiO introduced by Cu doping are negligible.Additionally,in the Ni doped CuO system,the VB and CB are clearly spin-split,corresponding to a net effective magnetic moment of about μeff= 1.66 μB.Compared to the perfect CuO system,with a net effective magnetic moment of about μeff=0.66μB,the enhanced magnetism may benefit the improvement in photocatalytic activity and recy cling ability of these catalytic materials.Moreover,in the Cu doped NiO system,the appearance of Cu 3d states near to the Fermi level increases the width of the VB and narrows the band gap compared with pure NiO.The emergence of the Cu 3d states within the band gap leads to the presence of two energy levels around the Fermi level,which may effectively separate the electron-hole pair and also result in enhanced absorption of visible light and infrared light.Thus,we can conclude that the change in the band structure for the two doped semiconductors may lead to improved photocatalytic activity and simultaneously have a net magnetic moment for simple recycling and reuse.4.In Chapter 5,CINEB calculation method for searching transition state was used to investigate the influence of oxygen defects on Cu electrode for CO₂ reduction.The rate-determing step on copper electrode for CO₂ electrochemical reduction is the process of the adsorption state of CO to be reduced to CHO with the adsorption state as well.The higher reaction energy barrier made this step reaction hard to happen.Thus we tried to reduce the energy barrier for the step on the surface of Cu electrode through modification.From a theoretical perspective,we used first principles density functional theory to establish slab models,and compared the transition state from CO reduction to CHO on a perfect Cu（111）surface and oxygen defects modified Cu（111）surface.Then the influence of oxygen defect on the reaction energy barrier was analyzed.The calculation results show that the existence of oxygen defect indeed reduces the reaction energy barrier.