Controllable Synthesis OfO Metal Xide Micro/Nano-materials And Their Photoelectronic Properties

Dye-sensitized solar cell (DSSC) and photocatalysis are two photoelectronicprocesses usually using semiconductors as media to achieve the photon-to-electronconversion. Based on the abundant and clean solar energy, these strategies could beadopted to handle the two major crises confronted with human society: energyshortage and environmental deterioration. ZnO and TiO₂are two star semiconductorsinvolved in these photon-electron conversions because of their chemical stability andlow cost. However, there are still two drawbacks preventing them from makingfurther progress in both research and application territories: the narrow absorptionspectrum in the visible region and the fast recombination of the charge carriers. In thispaper, homogenous precipitation and solvothermal method were respectively appliedto prepare series of ZnO and TiO₂micro/nano structures and hetero-materials. In orderto improve the power conversion of DSSC and the performanceof photocatalyst, threestrategies including heterostructuring, morphology controlling and selective exposingof specific crystal facet were used to achieve effective separation and transfer of thecharge carriers. To reveal the underling mechanism, different photoelectronic analyticmethods such as UV-vis absorption spectrum, electronic impedance spectrum (EIS)and Mott-Schottky analysis were adopted to analysis the separation, transport andrecombination of photo-induced charge carriers. The main contents of this thesis arelisted as below:Firstly, ZnO hierarchical structures assembled from sheet-like sub-units were fabricated via a facial low temperature deposition. PVP assisted hydrothermal methodwas then adopted to wrap TiO₂layers onto the surface of ZnO hierarchical structures.The structural properties and DSSC performance of these ZnO/TiO₂materials wereintensively studied. The results show that this surface modification could greatlyenhance the dye loading and reduce the recombination of photo-induced electronswith the oxidized dye molecules and species in electrolyte. Therefore the shortcircuit current density (J_sc) is improved from7.80to10.79mA cm^-2. Moreover, thismodification could elevate the quasi-Fermi energy level of the hybrid film, whichleads to enhancement of open circuit voltage (V_oc) from598to642mV. Therefore, theoverall conversion efficiency (η) improves from1.96to3.60%。Secondly, monodisperse TiO₂hierarchical hollow spheres composed of two typesof primary crystals (nano-grains:10-20nm in diameter and nano-spindles:15-30nmin width,50-150nm in length) were prepared using commercial P25, hydrogenperoxide and ammonia as starting materials. According to time-dependent trials,intermediate amorphous colloid spheres with different diameters were firstlygenerated by solvothermal treating the peroxotitanate precursor. Then,well-crystallized hollow spheres were formed after a two-step Ostwald ripening.Moreover, the effects of solvent composition, reaction temperature and ammoniadosage over the morphologies of final TiO₂products were investigated in detail.Subsequently, the TiO₂hollow spheres were fabricated as scattering layer on top of asemi-transparent adsoption layer made of nano-size TiO₂spindles.The thicknesses ofthe scattering layer were tuned and a power conversion efficiency of6.97%is achieved, which is enhanced from the cell (5.32%) made of semi-transparentfilm by33%. These primary units play different roles to promote the conversion efficiency byimproving the dye loading amount and electron transport rate and the hollow spherescould aslo effectively scatter and trap the incident light, which gives rise to higherlight harvest and short-circuit current density of the device.Thirdly, H+exchanged potassium titanate nanowires (H-KTNW) was used asprecursor to fabricate TiO₂truncated octahedrons via hydrothermal process usingammonium fluoride as stabilizer for the {001} facets. The size of the products couldbe tuned by adjusting the precuor dosage and the NH4F concertration. The DSSC testshows, compared with the TiO₂octahedrons exposing {101} facets (6.68%) andcommercial P25(6.36%), the truncated octahedrons exhibit higher conversionefficiency7.39%. based on the dynamic test, the sample with well-defined crystalfacets exhibit higher recombination resistance and electron life time, which leads toimproved conversion efficiency. Moreover, the IPCE, EIS and OCVD methods wereaslo utilized to reveal the mechanism of efficiency enhancement.Fourthly, Zn(OH)₂octahedrons with tunable size distribution were prepared viahomogenous precipitation in low temperature aqueous solution. Because of theamphoteric habit of these template materials, an ion-exchange method was applied tosynthesize series of transitional metal sulfides taking advantage of the soluabiltydifferencies. Moreover, the hydrophilic groups distributed on the template couldenrich the metal ions and facilitate the later mineralization along the surface. Based onthese special qualifications, sol-gel method and homogenous precipitation were respectively adopted to fabricate SiO₂and CeO₂octahedral hollow structures. Inaddition, the Zn(OH)2could be converted into ZnO via low temperature calcination,which could be used to generate a group of ZnO based hetero-materials. It isreasonable to believe that this template method could be adopted as a general processto effectively prepare hollow and hetero-materials with unique octahedral shape.Fifthly, a low temperature method based on ion-exchange was used to constructZnS layers on the surface of ZnO micro/nano rods. ZnO/ZnS hetero-, core/shell andhollow structures were respectively prepared via altering the concentration of Na2Ssolution and the formation mechanism was studied by time-dependent trails. It wasfound the dissolving of ZnO core also obeys the anisotropic growth. Thephotocatalysis results show the ZnO/ZnS heterostructures and ZnS tubes haveenhanced photocatalytic properties than pure ZnO. The structural properties of thesematerials were intensively investigated to reveal the effect of band gap alignment onthe charge separation and degradation efficiency. Controlled experiments confirm thephotocatalysis efficiency was greatly affected by the surface area, charge transfer andlight capture ability of as-prepared materials.