Synthesis And Characterization Of Metal And Semiconductor Homojunction/heterojunction Aiiomateirals

Metal and/or semiconductor homojunctions or heterojunction nanostructures have drawn worldwide attention due to their novel structures, unique electrical and optical properties, and potential applications in future optoelectronic devices. However, metal/metal homojunction and metal/semiconductor heterojunction nanomaterials synthesized to date have non-uniform size, defected interface, are difficult to operate, which restricts its scale-up preparation and wide applications. Based on growth kinetics and anisotropy of nanocrystals, this thesis mainly focused on the solution-phase preparation of metal/metal homojunction and metal/semiconductor heterojunction nanomaterials with uniform sizes, smooth and defect-free interfaces through varying metal or semiconductor monomer concentrations, reaction temperature, and capping agent concentration. Meanwhile, the solution-phase nucleation and growth mechanism between metal and metal or semiconductor monomers for synthesizing metal/metal homojunction and metal/semiconductor heterojunctions is explored. Major results of my thesis are as follows:1. A synthetically directed self-assembly strategy to the aqueous-phase synthesis of twinned crystalline silver/silver homojunction nanoparticles (Ag/Ag HJNPs) is demonstrated. In the self-assembly, ethylenediamine tetraacetic acid disodium (EDTA) and solution pHvalues play a crucial role in the formation of Ag/Ag HJNPs while the sizes of Ag nanoparticles (NPs) in the Ag/Ag HJNPs depend on the reductant concentrations of ascorbic acid. Surface-enhanced Raman scattering (SERS) measurements indicate that the SERS intensity acquired from the Ag/Ag HJNP colloidal solution is about200times stronger than that obtained from isolated Ag NP colloid solution.2. Ag/Ag homojunction nanowires were fabricated in large quantity by the reduction of silver nitrate with ethylene glycol, in which the tips of two or more Ag nanorods were connected at a certain angle. The geometrical and crystalline structures and surface Plasmon resonance property of the Ag/Ag homojunction nanowires are demonstrated by transmission electron microscopy (TEM), high-resolution (HRTEM), selected-area electron diffraction (SAED), X-ray diffraction (XRD), and UV-visible (UV-vis) spectroscopy. Moreover, our studies indicate that reaction temperature and capping concentration play an important role in the formation of Ag/Ag homojunction nanowires. Compared with Ag nanorods and nanospheres, the Ag/Ag homojunction nanowires are a better SERS substrate.3. Pt/Pt homojunction microparticles with fist-like shape were synthesized through the reduction of H2PtCl6with absolute ethanol under hydrothermal atmosphere, whose diameters and lengths can be effectively controlled by varying H2PtCl6concentrations. Unlike previous methods for only synthesizing pt nanoparticles of about10nm or even smaller, the use of the hydrothermal and surfactant-free system provides a shortcut strategy to prepare micron-sized platinum particles. SERS investigation demonstrates that the microfists have SERS activity and similar SERS enhancement to an electrochemically roughened Pt electrode.4. Au/MWCNT heterojunction nanomaterials were synthesized in high yield by a sulfhydryl-functionalized self-assembly strategy. The component, size, structure, morphology and bond mode of the Au/MWCNT heterojunctions were investigated and demonstrated by TEM, SEM, XRD, energy-dispersive x-ray spectroscopy (EDS), Fourier-transform infrared (FTIR) and UV-vis measurements. Cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) studies indicate that the MWCNT/Au heterojunctions have a novel electron transfer property, which retards electron transfer of the horseradish peroxidase or the ferricyanide in the underlying electrodes.5. Ag/Ag2S heterojunction nanowires were prepared by Ag-nanowire-mediated aqueous-phase method, which were characterized by TEM, HRTEM, XRD, and EDS. In the Ag/Ag2S heterojunction nanowires, the Ag2S nanoparticle sizes and positions in the nanowires and the distance between Ag2S nanoparticles can be effectively controlled by changing reaction conditions. These Ag/Ag2S heterojunction nanowires can be transferred to other metal/semiconductor heterojunction nanomaterials through cation exchange reaction. More importantly, Ag/CdS heterojunctions can be also acquired from the Ag/Ag2S heterojunction nanowires through cation exchange reaction.6. Ag/ZnO core/shell heterostructured nanomaterials were synthesized by Ag nanoparticle-mediated method, using which a novel hydrogen peroxide biosensor could be constructed. TEM and XRD measurements reveal that the Ag/ZnO heterojunctions are successfully acquired. Moreover, their sizes and shapes depended on Ag nanoparticles concentrations. Electrochemical measurements indicate that electrochemical properties of the Ag/ZnO-x-based biosensors also associate with the Ag nanoparticle content and the Hb-Nafion-Ag/ZnO-2biosensor has the best electrochemical and electrocatalytic properties.