Study On Surface Modification Of ZnO Nano-materials And Their Photoelectrochemical Performances

Environment problems（water and air pollution）and energy issues are the two widespread problems in our country,which is desirable to be resolved.It is found that when light irradiates on some semiconductor materials,the electron-hole pairs could be generated,which could serve as active matter to degrade the organic dyes in water or generate O₂ and H₂ under the specific device.The generated H₂ could replace the fossil fuels and relieve the air pollution for its cleanness.These researches are cutting-edge sectors for resolving the environment and energy issues and offer a most ideal solution for people.With the gradually serious environment problems and energy crisis,it is desirable for scientists to search a semiconductor photocatalytic material with high efficient utilize of solar energy.ZnO is a proper photocatalytic material for its suitable band gap,high electron mobility,low cost and pollution-free.However,the high recombination efficiency of photo-generated electron-hole pairs and relative wide band gap restrict the further applicationa in solar light.In our subject research,we successfully enhanced the efficiency of photodegradation of organic dyes and photoelectrochemical（PEC）performance by morphology controlling and surface modification of graphene and noble metal nanoparticles.The composites exhibit PEC performance of degrading MB and PEC water splitting under irradiation were evaluated,which offer theoretical directions on application research.The main results of this work are as following:1.Synthesizing hexagonal ZnO（HZO）by solution method and then coated with graphene using electrostatic force-induced self-assembly method to form HZO@Gr structure.The HZO@Gr composite shows decrease recombination efficiency of photo-generated electron-hole pairs for the photoelectrons could transfer from HZO to graphene,and the band gap of the composite is also reduced.The composite displays high efficiency of photo-degrading methyl blue（MB）,reaching to 98.2%in 50 min.Compared with HZO,thephotocurrent density are enhanced 10 times at 0.8 V（vs.Ag/AgCl）.2.Successfully measuring the contact potential difference（CPD）of graphene,±（0001）plane of HZO and HZO@Gr by using atomic force microscope（AFM） and Kelvin probe force microscope（KPFM）module.The existence of internal electric field（IEF）was also demonstrated and estimated to be 9.9 x10⁴ V·cm^-1.Further,by calculating the work function and Fermi level of graphene,±（0001） plane of HZO and HZO@Gr,the transfer mechanisms of photogenerated electrons at the interfaces of HZO,Gr/Zn,Gr/O and MB_（aq）/HZO@Gr under IEF were also inferred.Finally,we obtained the photocatalytic mechanism of HZO based on the above conclusions.3.The ink slab-like ZnO/Gr photocatalytic composite was synthesized by one-step solution method.The composite could provide larger IEF for the ultrathin ink slab bottom with the thickness of 30 nm,which is beneficial for the separation of photo electron-hole pairs and migration of photo charger.As a result,the photocatalytic efficiency of MB reached to 96.5%in 30 min.Compared with HZO@Gr,the photocurrent density are enhanced 25%at 0.8 V（vs.Ag/AgCl）.4.To resolve the problem of low coverage of ZnO powders on FTO substrate,we have synthesized the ZnO nanorod（ZNR）arrays using hydrothermal method.The graphene was coating on ZNR surface by electrostatic force-induced self-assembly method to obtain ZNR@Gr composite.The PEC test exhibited that the electrical conductivity of the ZNR@Gr photoelectrode significantly improved,leading to the photocurrent density reaching to 1.98 mA·cm^-2,which was 6 times compared with bare ZNR.What’s more,the PEC stability enhanced from 50.5%to 87.5%,indicating an effective function to resist photocorrosion.Further,we also analyzed the electron transfer mechanism and explained the reason of the enhancement of PEC performance and stability when modified with graphene and AuPd nanoparticles from the perspective of energy level.5.To further improve the PEC performance and stability of ZNR@Grphotoelectrode,we have modified the surface of ZNR@Gr with AuPd nanoparticles using solution reduction method.The surface plasmon（SP）effect of AuPd nanoparticle helps the ZNR@Gr/AuPd excited in visible light,thus improve the utilization of incident light.The excited photoelectrons in AuPd nanoparticles could further be injected to graphene,increasing the density of electrons in graphene.As a result,the photocurrent density reached to 2.28mA·cm^-2,which reflect an excellent capability for PEC water splitting to generate H₂.What’s more,the PEC stability enhanced from 87.5%to 91.3% compared with ZNR@Gr,exhibiting a good potential for application.