Enhanced Photocatalytic Performance Of Nano-ZnO By An Alternating Piezoelectric Field

Due to the advantages of simple preparation,low cost,non-toxicity,strong redox ability,etc.,ZnO has broad application in the fields of pollutant degradation,photocatalytic hydrogen evolution.etc.However,in the field of photocatalysis,ZnO suffers from poor photocatalytic activity due to its low separation efficiency of photogenerated charge carriers,which seriously hinders the industrial application of ZnO photocatalysts.Based on the piezoelectric and photocatalytic properties of ZnO,this paper proposes to use the mechanical energy in the application environment to induce an alternating piezoelectric electric field in the ZnO crystal to continuously improve the separation efficiency of photogenerated charge carriers.On the one hand,an alternating piezoelectric electric field was induced in ZnO with different morphologies and structures to separate carriers by ultrasound with high energy density or water flow with low energy density;on the other hand,in order to improve the piezo-photocatalytic efficiency of ZnO,ZnO was modified and the interaction of piezoelectric potential between Schottky barrier,oxygen vacancy or doping was studied.By testing the light absorption spectrum,piezo-photocatalytic activity and photoelectrochemical performance of the sample,and using first-principles and finite element methods,the mechanism of dynamic piezoelectric field for improving the photocatalytic activity of ZnO is explained.The main conclusion is as follows:Firstly,ultrasonic waves with high energy density are used to generate an alternating piezoelectric electric field in ZnO nanoparticles.ZnO porous microspheres composed by nanosheets was prepared via hydrothermal method.Then,ZnO microspheres(V-ZnO)with a high oxygen vacancy concentration was prepared by healing treatment under vacuum.By applying ultrasonic waves in catalytic process,an alternating piezoelectric electric field in the ZnO crystal continuously drives the majority carriers(electrons)in ZnO to migrate to the surface,and makes ZnO possesses piezoelectric catalytic activity in dark.Comparing the photocatalytic,piezo-photocatalytic and piezoelectric catalytic performance of ZnO microspheres with different treating atmospheres,it is found that as the oxygen vacancy concentration increases,more majority carriers(electrons)could migrate to the surface of the microspheres to participate in the catalytic reaction by dynamic piezoelectric electric field,and the piezoelectric catalytic activity of ZnO porous microspheres is continuously enhanced.Secondly,water flow energy with low-frequency and low-energy density in the photocatalytic environment is used to induce an alternating piezoelectric electric field in ZnO.ZnO nanorod arrays(ZnO NRA)were grown on two-dimensional fluorine-doped SnO2 flat plate(FTO)and three-dimensional ZrO2 porous ceramic foams by hydrothermal method.The effects of water movement mode and substrate structure on the piezoelectric field and piezo-photocatalytic activity of ZnO NRA were studied.It is found that in the process of reciprocal shaking water flow through the two-dimensional substrate or unidirectional water flow through the three-dimensional substrate,with the changing of the direction and intensity of the flowing water on the ZnO nanorods,an alternating piezoelectric electric field is induced in the ZnO nanorods.Due to the separation effect of photogenerated charge carriers by the dynamic piezoelectric electric field,the photocatalytic activity of ZnO NRA is improved.And with the increase of the intensity and change frequency of the dynamic piezoelectric electric field,the piezo-photocatalytic activity of ZnO NRA is continuously enhanced.Thirdly,in order to improve the piezo-photocatalytic activity of ZnO NRA,a V-doped ZnO nanorod array(ZnO:V NRA)was synthesized by a one-step hydrothermal method.It is found that with the increase of V doping concentration,the size of ZnO nanorods first increased and then decreased,and the aspect ratio remained.V doping in ZnO nanorods not only forms the doped energy level in ZnO,but also improves the dynamic piezoelectric electric field strength of ZnO nanorods,reduces the recombination of photogenerated charge carriers,accelerates the migration of photogenerated charge carriers.When the concentration of V is 1 mol%,the piezo-photocatalytic activity of ZnO:V NRA is the highest.The piezo-photocatalytic efficiency of ZnO:V NRA for methyl orange reaches 61.6%in 2 hours and the degradation rate constant k is 0.0076 min-1.Finally,in order to further improve the piezo-photocatalytic activity of ZnO NRA,a piezoelectric reduction method was used to prepare Ag-supported ZnO NRA(Ag/ZnO NRA).The electrons are enriched on the surface of the ZnO nanorods to participate in the reduction of Ag+by an alternating piezoelectric electric field,so that Ag nanoparticles with a diameter of about 30 nm were loaded on the surface of the ZnO nanorods.Due to the coupling effect of piezoelectric electric field and Schottky barrier,the piezo-photocatalytic degradation of Ag/ZnO NRA for Methyl orange reaches 94.3%in 2 h and the degradation rate constant k is 0.0227 min-1.When Ag and ZnO form a Schottky barrier,free electrons are transferred from Ag to ZnO NRs,which makes the piezoelectric catalytic degradation of Ag/ZnO NRA for Rhodamine B reaches 58.8%,the degradation rate constant k is 0.0073 min-1 and the catalytic activity retention rate is 85.8%after 10 cycles.