Preparation And Photocatalytic Properties Of Three Dimensional Porous P-NiO/n-ZnO Nano Heterojunction Materials

Semiconductor photocatalysis technology had broad application prospects in solving environmental issues because of safe,efficient and low-cost.ZnO is considered to be a very important semiconductor photocatalytic material because the valence band position of ZnO is low,so the photogenerated hole has good oxidation ability.At the same time,it has high conduction band position and strong reduction ability of photogenerated electrons.However,the photocatalytic efficiency of ZnO was seriously affected by the high recombination of electrons and holes.Although the specific surface area of nano catalyst is higher than that of bulk catalyst,which can provide more active sites,the specific surface energy of nano materials is relatively high,which is easy to agglomerate in the photocatalytic reaction and difficult to recover when suspended in the liquid surface.The reaction efficiency and performance of photocatalyst were seriously affected.In view of the above problems,three-dimensional porous NiO/ZnO nanojunction materials were prepared by freeze-drying method.Heterojunction formed by p-NiO and n-ZnO to enhance photogenerated electron and hole separation ability;Nanostructures can increase specific surface area and active sites;The macroporous structure is used to enhance the mass transfer process of liquid-phase and gas-phase reactions.At the same time,its large-size characteristics were conducive to sedimentation and improve the recovery performance of the catalyst.The specific research contents are as follows:（1）Using the polymer-assisted freeze-drying method,p-NiO/n-ZnO heterojunctions with three-dimensional（3D）porous self-supporting nanostructure,including micropore,mesopore and macropore,were successfully synthesized.Besides,the influence of polymer properties on the microstructure of heterojunction was also investigated.Firstly,the precursor solution containing polymer（PVP）,Ni（CH₃COO）₂,Zn（CH₃COO）₂ and water was frozen to make the water become ice crystal.Then the ice crystals were removed by freeze dryer to form macropores.Finally,PVP was removed by high temperature calcination to form micropores and mesopores.Therefore,NiO/ZnO heterojunctions with micropore size of 1～2nm,mesoporous size of 2～50 nm and macropore size of 50 nm～10μm were obtained.The experimental results showed that with the increase of PVP content in the precursor solution,the size of the pore structure decreases from 10μm to 2μm,which proved that the pore structure can be adjusted.（2）The photocatalytic efficiency of three-dimensional porous p-NiO/n-ZnO heterojunction was investigated by using Rh B degradation as a model.The three-dimensional porous NiO/ZnO heterojunction exhibited more excellent photocatalytic activity for Rh B degradation than that of NiO and ZnO with same structure.The degradation rate of NiO/ZnO heterojunction was respectively 2.1 times that of ZnO and 47 times that of NiO,which can be attributed to the formation of heterojunction reducing the recombination of photogenerated carriers.In addition,the photocatalytic efficiency of porous NiO/ZnO heterojunction was 3times that of powder NiO/ZnO heterojunction.This is due to the fact that the porous structure was more conducive to light collection and mass transfer,and also promotes the charge transfer.Especially,because of its large size and good stability,the material was easy to settlement and recycle.（3）Furthermore,the photocatalytic reduction of CO₂ by three-dimensional porous p-NiO/n-ZnO heterojunction was studied.Similar to the degradation of Rh B,the CO₂reduction efficiency of porous NiO/ZnO nanoheterostructure was 1.5 times that of powder NiO/ZnO nanoheterostructure,which was attributed to the enhanced chemisorption on the catalyst surface.In addition,the photocatalytic efficiency under thermal assistance is much higher than the accumulation of thermal catalysis efficiency and photocatalytic efficiency at room temperature.This was due to the fact that the photoelectrons in discrete energy levels can be partially converted into high-energy hot electrons at high temperature,which accelerated the conduction of photoelectrons to CO₂.