Performance Modulation Of ZnO-based Photocatalysts Through Built-in Electric Field

ZnO has attracted much attention in the field of photocatalytic degradation of pollutants and energy conversion due to its excellent photoelectric property,piezoelectricity,and thermal stability.The introduction of built-in electric fields（including built-in electric fields in heterojunction and built-in piezoelectric fields,etc.）can promote the separation of photo-generated carrier on the premise of preserving the characteristics of the semiconductor itself,so as to improve the photocatalytic activity of the semiconductor.Although many works on improving the photocatalytic performance of ZnO through built-in electric fields by heterojunction have been reported,it is still very important to flexibly design different types of built-in electric fields in heterojunction based on the conductive properties and energy band structure to improve the photocatalytic activity of ZnO.In addition,when subjected to stress such as environmental disturbance（including water flow,wind movement,etc.）,ZnO will be polarized due to its piezoelectric property,generating a built-in piezoelectric field（referred to as a piezoelectric field）.It is promising research to use the ZnO piezoelectric field to improve the separation of photogenerated carriers,which can not only improve the conversion efficiency of light energy but also utilize the mechanical energy in the environment.Based on the above discussion,different types of built-in electric fields in heterojunction and built-in piezoelectric fields are designed in this paper to improve the separation ability of photogenerated charge carriers to obtain excellent photocatalytic performance.The effects of built-in electric fields on carrier separation and transfer are studied by means of spectroscopy and electrochemistry.The specific contents of this paper are as follows:1.The two-dimensional/one-dimensional（2D/1D）BiOI/ZnO p-n heterojunction was constructed with different conductive types of semiconductors,which achieved a double improvement of light absorption and carrier separation efficiency.The results of electrochemical and fluorescence spectra show that the built-in electric field of BiOI/ZnO p-n heterojunction promotes carrier separation and transfer.In addition,narrow-gap semiconductor BiOI（1.97 e V）was combined with broad-gap semiconductor ZnO（3.3 e V）to broaden the spectral absorption range of.Moreover,2D/1D BiOI/ZnO branch-leaf-like spatial structure has a light limiting effect,which jointly improves the light absorption efficiency.Compared with pure BiOI and pure ZnO,the degradation rate constant k of methyl orange（MO）over BiOI/ZnO photocatalyst is increased by 0.23 times and 2.37 times.2.The two-dimensional/zero-dimensional（2D/0D）Zn In₂S₄/ZnO Z-scheme heterojunction was built with the same conducting type semiconductor,which realized the dual optimization of energy level structure and spatial structure.In terms of energy level structure,density functional theory calculation results show that there is a Fermi energy level difference between Zn In₂S₄ and ZnO,which is opposite to that of the type II energy band structure,reversing the direction of the built-in electric field,thus avoiding the transfer of carriers to the energy band with lower energy,and preserving the strong redox ability of Zn In₂S₄ and ZnO.In terms of spatial structure,there are a lot of tiny interfaces between 2D Zn In₂S₄ and 0D ZnO,and the transmission channel of photogenerated carriers is shortened,which is conducive to carrier separation.Compared with pure Zn In₂S₄ and pure ZnO,the degradation rate constant k of tetracycline over Zn In₂S₄/ZnO photocatalyst is increased by 4.36 times and 1.78 times.3.A p-n Co₃O₄/ZnO nanorod（ZnO NR）piezo-photocatalytic system was designed to realize the coupling between the piezoelectric field and the built-in electric field of p-n heterojunction,thus improving the photocatalytic performance of ZnO.With the increase of strain,the MO degradation performance of Co₃O₄/ZnO NR increases gradually,and the increase is significantly higher than that of Co₃O₄/ZnO nanoparticles with a weak piezoelectric property,indicating that the piezoelectric effect of Co₃O₄/ZnO NR contributes more to the increase of MO degradation than mass transfer.The reaction rate constants k of photocatalysis,piezoelectric catalysis,and piezo-photocatalysis were solved respectively,and the coordination coefficient I_s=k_piezo+photo/(k_piezo+k_photo)was evaluated.It is found that the I_s of ZnO NR and Co₃O₄/ZnO NR is greater than 1,and that of Co₃O₄/ZnO NR is greater than ZnO NR.These results demonstrate that piezoelectric and photocatalytic processes do not occur independently but cooperatively in this system,and p-n heterojunction promotes the synergy of the two catalytic processes.4.The ZnO NR/PVDF-HFP flexible porous membrane,a photocatalytic system assisted by two piezoelectric fields,was constructed to achieve high photocatalytic performance and application flexibility.The composite of PVDF HFP and ZnO NR forms two piezoelectric fields under the disturbance of water flow.The results of steady-state fluorescence spectra and transient fluorescence spectra show that these two piezoelectric fields inhibit the luminescence of ZnO NR,that is,promote the separation of photogenerated carriers,and thus improve the photocatalytic activity of ZnO NR.When the stirring rate is increased from 200 rpm to 1000rpm,the reaction rate constant k of ZnO NR/PVDF-HFP porous membranes for MO degradation is increased by 2.95 times.In addition,the ZnO NR/PVDF-HFP membrane can float,so it can be recovered without complicated centrifugation and precipitation treatment,which has the advantages of flexibility and portability in practical applications.