The Investigation On The Photocatalytic Characteristics Of ZnO

ZnO semiconductor material has aroused great interests due to its great development potential in many fields such as photocatalysis,solar cells,and optoelectronic devices.Especially in the field of photocatalysis,the photocatalytic activity and the effective use of visible light are limited by the wide bandgap of ZnO and the large recombination ratio of electron-hole pairs.The photocatalytic performance of ZnO bulk structure can be effectively improved by doping and surface modification.The doping of iron?Fe?atom plays an important role in the photocatalytic activity of ZnO system under visible light irradiation,and ZnO?0001?surface exhibits better photocatalytic activity than pristine bulk ZnO and the other three low Miller index surfaces driven by the visible light.Therefore,it is necessary to further investigate the mechanism of the adsorption and doping of Fe atom improving the photocatalytic activity of ZnO?0001?surface.In addition,the doping of rare earth metal yttrium?Y?atom can elevate the light absorption of ZnO in the visible light region,and Y tends to affect the photoelectric properties of ZnO together with the defects in ZnO.ZnO monolayer?ZnO-ML?has become a research hotspot because of its large surface area to volume ratio.Therefore,Y and defect co-doped ZnO-ML system has become a potential material for enhancing photocatalysis.Based on the GGA+U correction,the first principle calculation is used to study Fe adsorbed and doped ZnO?0001?surface system,Y doped as well as Y and defect co-doped ZnO-ML system.The details are as follows:1.The Fe atom adsorbed ZnO?0001?surface systems are calculated,considering three high-symmetry adsorption sites?H₃,T₄ and Top?.The calculated adsorption energy shows that the adsorption energy of the system is the smallest?-5.665 eV?when Fe is adsorbed at the H₃ site compared with T₄ and Top sites.For the Top site,compared with the pristine ZnO?0001?surface,the absorption peak located at 1.17 eV has a red shift,and the elevation of the absorption coefficient is more pronounced in the visible-light region,because of the Fe-related levels located in the forbidden band and near the Fermi level,which will improve the separation rate of photogenerated electrons and holes.The calculated electrostatic potential reveals that the work function of the ZnO?0001?surface is significantly decreased from2.340 to 1.768 eV when iron is adsorbed on the Top site.Furthermore,the degradation mechanism based on the band structure is analyzed.The adsorption of iron will promote the photogenerated electrons transferred to the Fe adatoms.2.The Fe atom doped ZnO?0001?surface systems are calculated by considering the substitutional sites?P₁,P₂ and P₃?in the three relaxed Zn-O bilayers and the interstitial site?P_i?in the centre of the octahedron surrounded by Zn atoms.The calculated formation energy indicates that the iron atom energetically prefers to occupy the Zn site on the topmost Zn-O bilayer of ZnO?0001?surface.Compared with the pristine ZnO?0001?surface,Fe-3d state will generate several impurity levels in the forbidden band of the systems when Fe atom is doped at the P₁,P₂ and P₃ sites,and the conduction band shifts to the lower energy region.When Fe doped at the P_i site,the band gap of system is reduced to 1.434 eV.The calculated electron density difference confirms that the situation of chemical bond between iron atom and the neighboring O or Zn atoms.Comparison of the absorption coefficients of the pristine and Fe doped ZnO?0001?surface systems shows that light absorption is significantly enhanced in the range from 0 to 3.5 eV.The absorption peak of the substitutional sites shifts to lower energy?<1.63eV?,whereas the absorption peak of the interstitial site shifts to 2.23eV.3.The Y doped with three different doping concentrations?x=6.25 at.%,11.11 at.%,25at.%?as well as Y and Zn vacancy(V_Zn)or O vacancy?V_O?co-doped ZnO-ML systems are calculated.The calculated formation energy indicates that the higher the Y doping concentration is,the greater formation energy is and the more unstable system is.The band gap of Y doped ZnO-ML systems are larger than that of the pristine ZnO-ML?4.056 eV?,and the higher doping concentration is,the larger the band gap is.There are Y-related levels located in forbidden band?near the Fermi level?in the spin-up channel,which can effectively promote the separation of photogenerated electron-hole pairs.The calculated Mulliken atomic populations and bond populations confirmed the situation of chemical bond and charge transfer between Fe and the adjacent O atoms.Correspondingly,the absorption edge of Y doped ZnO-ML systems are all red-shift to the lower energy region,and a new absorption peak are arised in the visible light region.The higher Y-doped concentration is,the more significant light absorption enhancement of the system is.Furthermore,compared with Y doped ZnO-ML,the light absorption coefficients of the Y and V_O co-doped ZnO-ML systems in the visible and near-ultraviolet regions are further increased compared with Y doped ZnO-ML.This is because that the band gap of Y and V_O co-doped ZnO-ML?4.27 eV?is smaller than that of the Y doped ZnO-ML system?4.45 eV?,and the impurity level of the co-doped system?E_v+1.33eV?is closer to valence band maximum compared with that of Y doped system?E_v+4.0eV?.The research could provide some theoretical references for improving the photocatalytic activity of ZnO-based materials.