Theoretical Study On Photocatalytic Properties Of Silver/Silver Halides

Energy and environmental problems have gradually become hot issues with the progress of human society.In order to realize the sustainable development,we should improve the utilization ratio of clean and pollution-free renewable energy on one hand and on the other hand,explore how to effectively convert carbon dioxide and other greenhouse gases.Because of the direct use of solar energy to activate the catalyst,semiconductor-based photocatalytic technology has promising application prospects in solving the environmental and renewable energy problems.The development and utilization of novel visible-light photocatalyst is an important research direction to improve the efficiency of photocatalysis.Due to the combination of plasma resonance effect of noble metals,metal-semiconductor heterojunction structures,photocatalytic performance of semiconductors and etc.,the silver/silver halide catalysts show high catalytic activities in the visible-light region.In this thesis,the photocatalytic properties of silver/silver halides have been systematically investigated using density functional theory?DFT?,in order to clarify the effects of the surface microstructure,noble metal clusters adsorption and multiple halogen doping on the electronic properties of the photocatalysts,reveal the regulation mechanism of the band structures,get the better understanding of effects of semiconductor properties on the catalytic performance,and explain the relationship between composition-structure-catalytic properties of the photocatalysts.This study provides a theoretical basis for designing efficient visible light catalysts.The geometrical and electronic structures of the AgBr?100?and?110?surfaces have been investigated using density functional theory plus Hubbard U contributions.The results show that the AgBr?110?surface has the lower atomic coordination number with more danging bonds than the AgBr?100?surface.Therefore,the AgBr?110?surface is more strongly polarized and has a larger surface relaxation,and thus showing the smaller band gap and higher catalytic activity.We also investigate the adsorption properties of silver atoms on the AgBr?100?and?110?surfaces and catalytic performance of the photocatalysts for water splitting.The overall free-energy drop of water dissociation follows the order Ag/AgBr?110?>Ag/AgBr?100?>AgBr?110?>AgBr?100?,suggesting that Ag/AgBr?110?indeed has the highest catalytic activity.Upon the adsorption of the Ag_n?n=2¹3?clusters on the AgBr?110?surface,the metal induced gap states?MIGBs?are formed in the energy-gap region of the silver bromide,due to the electronic wave functions of Ag_n mixing with the orbitals of the silver ions in the silver bromide.The MIGBs stride across the Fermi level of the system and are the half filling band,which makes the whole system display a certain metallic property.The forming of MIGBs adjusts the energy band structure of the system,widens the light response range to the visible light region,even to the near infrared region,and enhances the absorption efficiency of the system.The edge potentials of the valence band and conduction band are studied,the results show that the edge potentials of Ag_n/AgBr?110?meet the potential conditions for the photocatalytic reaction of CO₂ and H₂O generating CH₃OH.With the enlarging the size of the Ag_n?n=2¹3?clusters,the geometric structure of the clusters changes from 2D to 3D,the light absorption range of the system is further red-shifted,even to the infrared light.The absorption intensity in the visible region is enhanced and the light absorption efficiency of the system is improved.Furthermore,the free energy change of photocatalytic CO₂ and H₂O reaction are studied,and the results show that the Ag₁₃/AgBr?110?surface has the highest catalytic activity among Ag_n?n=2¹3?/AgBr?110?systems.Due to the synergistic effect of gold and silver elements,electrons are transferred from the metal clusters into the semiconductor in the Ag_13-nAu_n/AgBr?110?system,while there is also a part of the electron transferred from the Ag atoms to the Au atoms in the metal clusters.The forming of MIGBs widens the light response range to the visible light region,even to the near infrared region.Inaddition,the electronic transitions from MIGBs and VB to CB are overlapped,because of the small gap between the two bands.The visible light absorption intensity of Ag_13-nAu_n/AgBr?110??n=1-3?is larger than that of Ag₁₃/AgBr?110?.With the increase of gold contents in the alloy clusters,the reduction potential of the reaction on Ag₁₀Au₃/AgBr?110?surface?n=1-5?decreases first,and then increases,and lowest reduction potential exists in the Ag₁₀Au₃/AgBr?110?surface.The free energy change of the photocatalytic CO₂ and H₂O reaction on Ag_13-nAu_n/AgBr?110?is studied and the reaction on Ag₁₂Au/AgBr?110?shows the least free energy increase.Considering the light absorption and catalytic performance,Ag₁₂Au/AgBr?110?has the highest catalytic activity among the considered systems.The Ag₄Br_4-n-n I_n?n=1-3?systems are also studied.The density of electronic states in valence band energy region is increased by iodine elements doping in AgBr.With the increase of the content of iodine,the absorption threshold of the absorption spectrum is red shifted,at the same time,the reduction potential of Ag₄Br_4-n-n I_n?n=1-3?first increases,and then decreases.In these systems,Ag₂BrI has the highest reduction potential.The surface energy of Ag₂BrI?110?is larger than that of Ag₂BrI?100?.Silver atom is more easily adsorbed on Ag₂BrI?110?surface.Due to the adsorption of Ag on the Ag₂BrI?110?surface,the MIGBs are formed in the energy-gap region of the silver iodobromide.The forming of MIGBs widens the light response range to the visible light region,enhancing the absorption efficiency of the system.The study on the reduction over potential for the photocatalytic reaction suggests that Ag/Ag₂BrI?110?has the highest catalytic activity in the research systems.