High Photocatalytic Performance On Interfacial Modified Nano ZnO By Built-in Electric Field And External Field

Solar energy is one of the most abundant clean energy,with the development of artificial solar energy utilization technology,solar energy in people’s daily life in the production of more and more forms of use.In the direct use of solar energy programme,photocatalytic technology using sustainable solar energy resources to activate related chemical reactions can be considered as a sustainable green development programme and has received widespread attention.At present,the main semiconductor photocatalytic materials include titanium dioxide（TiO₂）,tungsten trioxide（WO₃）,Zinc oxide（ZnO）,cadmium sulfide（CdS）and so on.Among them,ZnO has many advantages such as large reserves,simple preparation,low cost,nontoxic,and so on.Pure ZnO usually exists in the form of white hexagonal crystal or white powder,also known as zinc white product,its crystal structure is mostly hexagonal wurtzite.ZnO is a typical direct bandgap material,which can be easily excited by ultraviolet irradiation to generate electrons and holes with high photoelectric efficiency.However,the direct bandgap semiconductor properties of the material also make the photogenerated electrons and holes recombination rate very high,which limits the photocatalytic efficiency.Most of the research on ZnO photocatalytic materials focuses on expanding the optical absorption range of ZnO and improving the photogenerated carrier separation efficiency.The particle size,synthesis process parameters,surface properties and other properties of ZnO nanoparticle have important effects on the catalytic performance.Controlling the micro-nano size growth of semiconductor materials is conducive to improve its photoresponse and photocatalytic performance,but nanoscale particles will affect the separation efficiency of solid and liquid systems,resulting in secondary problems such as the reuse of the materials.On the other hand,most semiconductor materials rely on hydrothermal synthesis or multi-step complex synthetic methods,and simple,efficient,low-cost synthesis of highly photoresponsive ZnO materials can meet the requirements of different applications,meanwhile simple,efficient,low-cost synthetic method is also one of the main development directions of the material.This thesis addresses the problem of high activity and low separation characteristics of nano-sized ZnO photocatalyst,focusing on ZnO morphology and interfacial modulation.On the basis of maintaining high catalytic activity,the separation characteristics of the catalyst in water are regulated by self-assembly.The photocatalytic performance of ZnO based samples is enhanced by in situ bonded sacrificial agent structure,monolayer interfacial heterojunction and built-in electric field,and external field modulation.The main content of this thesis is as follows:1.One-step synthesis of TEOA surface-modified ZnO self-assembled microspheres and their photocatalytic reduction properties of heavy metal ionsAlthough the use of metal materials has opened a new era of human civilization,the widespread metal utilization in modern society has caused a wider range of secondary heavy metal pollution problems.Heavy metal pollution is a common water pollution problem,and the treatment process is low efficiency and difficult.Heavy metal pollution is easy to form the surface water,groundwater,soil and other multimedia complex pollution problems,ecological restoration is particularly difficult.Therefore,in this thesis,the formation of uniformly ZnO microparticles were precipitated by the burst nucleation process using organic alkali variable temperature hydrolysis,self-assembly of the crystal nucleus.Triethanolamine（TEOA）was in situ coordinated on the surface of the crystal nucleus,and the modified ZnO microparticles was validated for application in fast photocatalytic reduction deposition of Cr（Ⅵ）in water.It was found that TEOA acts both as a synthetic auxiliary to modulate specific morphology and surface modification of the ZnO samples.The experimental and theoretical results show that the interfacial bonded TEOA makes the coordination between the non-shared electron pairs of N atoms and surface Zn atoms,which affects the electron density distribution around Zn atoms and facilitates the capture of heavy metal ions by ZnO,while TEOA directly participates in the hole oxidation reaction at the interface as a sacrificial agent,and this process does not require diffusion through the solid-liquid interface,which can achieve rapid photogenerated hole consumption Therefore,the photocatalytic reduction of Cr（Ⅵ）can be achieved within 5 min,and the photoreduction efficiency of Cr（Ⅵ）can be increased by about 6 times compared with that of ZnO microspheres without TEOA modification.The photocatalytic performance of TEOA-modified ZnO can be renewed by a simple TEOA solution immersion process,and the photocatalytic performance of ZnO microspheres is "resurrected".In addition,the highly active ZnO self-assembled microspheres have excellent solid-liquid separation properties,avoiding the problem of secondary contamination during environmental water treatment.This TEOA-modified ZnO microsphere has the advantage of simple batch synthesis,which can provide a rapid solution for water pollution and environmental water treatment remediation under emergency conditions,and has excellent practical application prospects.2.Highly active and selective photocatalytic reduction of CO₂ by calcined CoPcS/ZnO/rGO under simulated sunlight radiationZnO surface modification and internal electric field construction can prolong the life of photogenerated electron hole pairs and improve the photocatalytic activity of the materials.Cobalt sulfonated phthalocyanine（CoPcS）is a highly efficient metalorganic compound（MOF）catalyst with large π-electron structure and good photoresponse of Co（Ⅱ）phthalocyanine conjugated rings.Through anion exchange with ZnO surfaces,CoPcS monomolecular bonded composites can be formed.rGO is also a two-dimensional conductive material with excellent electron conductivity.CoPcS/ZnO/rGO complexes have excellent CO₂ photoreduction activity and high CO selectivity.The material can be synthesized by simple methods such as hydrothermal and calcination,in which the conjugated ring of Co（Ⅱ）phthalocyanine,rGO and ZnO plays a key role in the separation of photogenic electron holes.The hydrophilic and hydrophobic micro-differentiators on the surface of the material,and the selective adsorption of gaseous CO₂ and H₂O also become the key factors for its high photocatalytic activity.SEM,TEM,PL and ESR results confirmed the formation of interfacial monolayer structure heterojunction and the ability of efficiently separating photogenerated electron hole pairs.Compared with pure ZnO material,the composite material significantly inhibited the ability of ZnO to generate OH free radicals,and the free radicals generated were converted to·O₂-.The photocatalytic mechanism of the material was mainly a type Ⅱ heterojunction reaction mechanism process.The Co（Ⅱ）phthalocyanine conjugated ring on the material,as the photogenerated electron transfer and gaseous CO₂ adsorption reaction center,has extremely high CO selectivity and high photocatalytic reduction activity.CO selectivity is up to 99.0%,and the yield is up to 26.15μmol·g-1·h-1.The reaction process can adapt to C1 industry’s large-scale industrial production of green carbon product supply,and is expected to become an important basic support technology for achieving carbon neutrality and developing ecological civilization.3.Self-assembled heterostructures of ZnO nanoparticles on CdS nanorods and their internal electric field for efficient carrier separationThe interface-built electric field based on the heterostructure is an important scheme to enhance the photocatalytic performance of semiconductors.Based on the synthesis of ZnO nanoparticles with self-assembled properties,the self-assembled properties are utilized to form a heterostructure with CdS nanorods to give full play to the carrier modulation of the interface-built electric field and the spectral response range expansion of CdS materials with visible light response to achieve efficient solar photocatalytic hydrogen production reaction.However,both single CdS and ZnO are limited by the high photogenerated carrier complexation rate,and therefore,solving their carrier separation problems is fundamental for the construction of efficient composites.In this work,the composite materials with "sphere-rod" structure were constructed in situ on CdS nanorods by using the self-assembly property of TEOAmodified ZnO nanoparticles.The hydrogen production performance of the composite CdS-ZnO materials was investigated at different ratios,and it was found that the loading of ZnO had a significant effect on the performance.1 wt%of ZnO loading could increase the photocatalytic hydrogen production performance by more than 50%compared with that of ZnO or CdS alone,and 10-20 wt%of ZnO loading showed the optimal photocatalytic performance,and the hydrogen production efficiency was more than 10 times that of ZnO or CdS alone.The hydrogen production efficiency is more than 10 times that of ZnO or CdS alone.The superoxide radical and hydroxyl radical in the photocatalytic system were characterized by ESR,and the type-II heterostructure of CdS and ZnO was demonstrated by the signal strength of both radicals.The electric field built into the interface of the heterostructure promotes the separation of photogenerated carriers in CdS and ZnO,resulting in the enrichment of a large number of photogenerated electrons in the conduction band of ZnO,which makes the redox potential of photogenerated electrons more compatible with hydrogen reduction,and thus enhances the catalytic reaction utilization of photogenerated electrons.These CdS-ZnO heterostructured materials not only expand the spectral utilization range but also enhance the separation efficiency of photogenerated carriers,which have strong application prospects in photocatalytic hydrogen production and other related energy conversion.4.Morphology and photocatalytic properties of ZnO nanomaterials underultrasonic piezoelectric fieldIn addition to the intrinsic properties of materials,the external field control has become a method to control the photocatalytic properties based on the intrinsic properties of materials,but not completely limited to the intrinsic properties of materials-The ZnO nanomaterials in this study are traditional piezoelectric materials,which can generate polarized electric field from structural deformation under mechanical force.This in-situ polarized electric field can regulate the photogenerated carrier separation of ZnO nanomaterials,and then enhance the photocatalytic performance.In this paper,the hexagonal wurtzite phase ZnO with different morphologies,such as nanorods,nanowires,nanosheets and self-assembled microspheres,was prepared by solvothermal reaction under different conditions,based on the analysis of photocatalytic degradation of model pollutant Rhodamine B in water,the typical piezoelectric properties of hexagonal wurtzite ZnO were investigated,the effects of morphology and exposure surface on photogenerated carrer separation are discussed.In the process of photocatalysis,ZnO-mediated piezoelectric electric field was introduced into the material by non-contact ultrasonic driving,and the polarization electric field was established in the material.The improvement of photocatalytic efficiency of ZnO nanowire is the most obvious,which indicates that the morphology-dependent piezoelectric photoelectron effect has a positive effect on the separation of photogenerated charge carriers,more exposed active surfaces are beneficial to the utilization of charge carriers.The research results provide a direction for further development of new ZnO photocatalytic materials,morphology and modification methods,and provide a research basis for ultrasoundcontrolled photocatalytic materials.