Electrochemical Routes To Synthesize Metal Oxide Semiconductors And Their Properties Characterizations

Metal oxide semiconductor (MOS) nanostructures have powerful applications in optics, catalysis, and photovoltaics due to their semiconducting properties, structural varieties, ability to change valence, co-existence of mixed valences, nonstoichiometic composition, and stability. The research on MOS nanostructures involve crystal growth, nanoscience, crystal structure, catalysis, electricity, optics, and surface science, which presenting a great challenge. Therefore, synthesizing MOS nanostructures and investigating the related properties or seeking new features are high of significance both in theory and in application. Dissertation is developed centered around the synthesis, structure characterization, formation mechanism discussion, and properties study of MOS nanostructures, including SbxOy, ZnO, CuO, and their composites with Ag. The synthesizing method is mainly based on electrochemical route, the studied properties are mainly related to optics and catalysis.(1) The potential ofφ(Cu/Cu2+) is higher than that ofφ(Sb/Sb3+). However, the successful occurrence of Cu foil to replace Sb3+ to form a Sb coating (a typical chemical plating method) is mainly due to no Cu2+ before reaction and the much lower concentration of Cu2+ during reaction. A regular fourteen-faced polyhedron shape of Sb three-dimensional structure is obtained by this facile method. The structure presents a loose feature which is similar to a cage. The cage is formed by the ordered intertexture of Sb nanowires. Sb cage can be oxidized under moderate hearting in O2 atmosphere with morphology conservation but with poor crystallinity. The cage grown on a passivation substrate can be oxidized much facilely due to the interaction with the copper oxide species in the passivation coating. Such a loose structure may find promising application in catalysis.(2) Various morphologies of ZnO nanostructures can be obtained through a novel method, incorporating electrochemical corrosion with three modes: liquid membrane and above and below the water line in partial immersion. The mechanism of the growth of one-dimensional-based nanostructures is proposed as electrochemical corrosion and oriented attachment, which occur in a liquid membrane or in a vapor membrane or in solution for partial immersion. The evolution of ZnO nanostructures such as nanorods, nanowires, nanopins, nanoparticles, comb-like structures, nanodentrites, and hierarchical structures is observed, and the influence of concentration, reaction time, additives, state of substrate, membrane thickness, and solvent on the morphology of ZnO is investigated. Optical properties of ZnO nanostructures are studied by using UV-visible absorption spectra and photoluminescence (PL). Their optical gaps vary from different morphologies. Among the studied samples, short nanorods show the largest optical gap, while big nanorods present the smallest value of optical gap. PL properties demonstrate that peaks of near-band emission and defect-related luminescence are basically in the same position. However, intensities for different morphologies are of different values, and short nanorods exhibit the best near-band emissions.(3) ZnO nanowires can grow during heat treatment from the micropores of Ag nanoclusters deposited on a Zn substrate. A Ag-modified ZnO nanowires superstucture is obtained by electrodepostion. The elctrochemical behavior of Ag(I) reducing on the surfaces of ZnO nanowires is unusual. The intensity of light absorption in the ultraviolet region almost keeps, but the optical bandgap of ZnO nanowires is red shifted, and the photoluminescence (PL) intensity decreases greatly, which are mainly due to the charge transfer between ZnO and Ag. Moreover, many split Raman peaks appear resulting from surface plasmon resonance (SPR) coupling. Additionally, extraordinary strong light absorption is obtained primarily arising from the interaction of SPR, SPR coupling, nonlinearities, charge transfer, and plasmon waveguides occurring on the superstructure.(4) ZnO nanostructure-modified Ag sub-micron particles composite is prepared by association of chemical plating and electrochemical corrosion. By controlling reaction time, composites with different coverage of ZnO on the surfaces of Ag sub-micron particles such as mosaic structures, core/shell structures, and ball cactus-like structures are obtained. The optical properties and photoelectron response on different composites are also studied. Only mosaic structures present the broad UV-vis absorption and exhibit enhanced photovoltaic effects mainly arising from the relatively large size of Ag particles which can store much more photoinduced electrons, the configuration of many ZnO nanostructres sharing one big Ag particle which promoting the efficiency of charge transfer, and the occurrence of SPR on Ag which enhancing the charge transfer between Ag and ZnO.(5) Copper oxide nanoparticles, floc-like structures, sub-micron brick-like structures, and nanorods and nanofilms with different features in shapes are assembled by an electrochemical corrosion route. The successful obtainment of these nanostructures are mainly based on the control of the growing conditions, including concentration, growing time, additives, growing modes, and temperature. In addition, the formation mechanism of copper oxide nanostructures with different shapes, different surface states, different structures, and different distributions are also investigated.(6) The oxygen adsorption properties of copper oxide nanostructures are studied via X-ray photoelectron spectroscopy (XPS) and Raman spectra (RS). The behavior of oxygen adsorption on different copper oxide nanostructures is investigated. The results show that the oxygen adsorption ability of copper oxide nanostructures depends on the quantities of defect structures, the crystallinity, and the specific surface area. The samples exhibit stronger oxygen adsorption ability after exposure to O2 atmosphere. The adsorbed oxygen can oxidize surface adsorbed carbon species. The degree of the oxidation of the carbon species depends on the reaction condition. Generally, the stronger the ability of oxygen adsorption, the higher the catalytic activities toward the oxidation of the carbon species. Moreover, high temperature enhances catalytic reactions.(7) The catalytic activity toward ethanol electrooxidation on copper oxide nanostructure is studied. The results show that copper oxide nanostruture demonstrates high catalytic activity toward ethanol electrooxidation with high catalytic kinetics and unobvious poisoning effects. The high catalytic activity is mainly derived from the copper oxide nanostructure which presenting the strong oxygen adsorption ability, the catalysis of the high-valence copper species, and the nanoscale of copper oxide catalyst. Without introducing a promoter, copper oxide nanostructure itself can improve its catalytic activity by changing surface structure. Copper oxide nanorods with large quantities of defect structures can exhibit much higher catalytic activity, which is mainly due to its one-dimensional configuration that benefiting the adsorption and desorption of reactants and products, the transportation of carriers, the oxidation of CuO, and the contact of CuO with solution in deep extent. Copper oxide nanostructure can be severed as an electrocatalyst in the anode material in direct ethanol fuel cells.