Study On The Control Of ZnO Nanostructures & Morphology And The Surface Photoelectronic Properties

Zinc oxide(ZnO)is an important oxide semiconductor,which has a wide band gap of 3.37eV at room temperature.Its exciton binding energy is 60 meV,much larger than that of other semiconductors such as GaN(25 meV)and ZnSe(22 meV).This energy is 2.3 times of room temperature thermal energy(26 meV),which besteads the excitons of ZnO with high stability.Furthermore,exciton-exciton scattering-induced stimulated emission occurs at a threshold lower than that for the electron-hole plasma recombination.These essences make ZnO have dramatic potential applications in the light emitting diodes,ultraviolet laser,detectors,solar cells and high-frequency and high-power devices.Due to the polar surface,ZnO presents in various nanostructures,including nanobelts,nanorings,nanowires(nanorods),nanoflowers,nanotubes,nanosheets and nanohelixes.Different structures have found different applications.So,controllable synthesis of ZnO nanostructures is of great significance.It has been found that ZnO displays strong intereactions with S-species and acts as a well-known sulfur adsorbent. It is rational to make use of S-contained ligands for controllable growth of ZnO nanostructures.In this dissertation,thiols and Na₂S₂O₃ have been applied into the ZnO nanostructure fabrication,the main contents include the following parts.(1)ZnO nanostructures were synthesized in a controlled manner by using mercapto-compounds(C_nH_2n+1SH,n = 5,6,8,10 and 12)as ligands.With dodecylthiol as a representative example,the derived ZnO nanocrystals were characterized with X-ray diffraction,energy dispersion X-ray analysis, transmission electron microscopy,and photo-luminescence spectroscopy techniques.The experimental data confirmed the following mechanism.Heating the two phase system composing by thiol and aqueous solution of Zn(NO₃)₂ and hexamethyltetramine,primary ZnO clusters were generated and they were spontaneously capped by thiol molecules dissolved in the aqueous phase.When a critical size was reached,the thiol-capped particles transferred from aqueous to organic phase because of the hydrophobicity of the alkyl chains and their growth was stopped there.After calcining the thiol-capped precursor particles at elevated temperature,ZnO nanoparticles were obtained.The reaction system behaved differently from others in the case of n = 5,it changed from a two-phase to a uniform suspension after heating the mixture.A unique octahedron nanostrucutre was obtained,which was theoretically predicted as the intergradation of tetrapod ZnO nanocrystals but had never been observed before this report.This observation is helpful to the elucidation of the generation of ZnO nanostructures in various morphologies.Influence of dodecythiol on the system of zinc foil-formamide was studied.With the aid of dodecythiol,untra-thin ZnO nanosheets and unique bird-net structures were generated through self-assembling process on the surface of zinc foils.(2)Using a simple hydrothermal strategy,a novel porous arc-shaped S-doped ZnO nanowires were fabricated by reaction zinc foil in an aqueous solution of sodium hyposulfite at 65℃.The radius and length of nanowires were about 100 nm and 5μm, and diameter of nanopores on nanowires is about 10 nm.Through a two-step process, ZnO nanorods array was transferred to ZnO nanoleaves at mild conditions.Moreover, hexagonal form ZnS with wurtzite structure was obtained with a facile solution under 65℃.Surface photovoltage spectroscopy(SPS)and phase investigations indicated that the obtained ZnS was an n-type semiconductor,and the intrinsic relationship between the field-induced SPS(FISPS)and surface photocurrent spectroscopy(SPC) of ZnS nanoparticles was also established by combinational studis.(3)By in situ assmbling of ZnO nanorods and T-PTCDI[N,N-di(4-thiophenol)perylene-3, 4,9,10-tetracarboxylic diimide],ZnO/T-PTCDI hybrid was synthesized. SPS and FISPS measurement results showed that hybridization dramatically improved the disassociation of the photogenerated electron-hole pairs thereby the surface photovoltaic response enhanced by several tens times.The sufficient quenching of the fluorescence of the hybrids further confirmed the efficient photo-induced electron transfer from T-PTCDI to ZnO component.And the electronic structure data also suggested an energetically favorable electron transfer process.(4)Zinc-aluminum layered double hydroxide(Zn-A1 LDH)thin films were prepared by direct precipitation on the surface of porous anodic alumina oxide(AAO) template.The effects of nano-sized wall-edges on Zn-A1 LDH growth were studied and it was found that the Zn-A1 LDH films can be grown more efficiently on porous AAO membrane than those on flat bulk alumina oxide surface.The water contact angle of the as-prepared Zn-A1 LDH films was measured to be 122°,but after calcination at 600℃for 2h,it became 65°,indicating the surface transformed from a hydrophobic to a hydrophilic one,the thermal-induced reconstruction and the removal of inter-layer anions were ascribed to the changes of surface wettability.Photoacoustic properties were studied and the nanopores of AAO showed enhanced the photocoustic response,the enhancement was inversely proportional to the radius of nanopores of AAO template.