Fabrication And Photocatalytic Properties Of One-dimensional Gallium Nitride Nanomaterials

Energy shortage and environmental deterioration are two globally concerned issues, urging human beings to seek renewable energy suppliestogether with new initiatives to reduce carbon dioxide(CO2) emission. Photocatalysis using semiconductor and sunlight is widely regarded as an ideal green technology, which can generate clean energy hydrogen from water splitting and reduce CO2 into hydrocarbon fuels.Among various semiconductor photocatalysts, gallium nitride(Ga N), a typical III-V group compound, has been attracting much attention in recent years. GaN is a direct semiconductor with a bandgap of 3.4 eV, which is suitable for photocatalytic water splitting and CO2 reduction. Recently, one-dimensional(1-D) nanomaterials have been paid great attention in photocatalysis field due to their large aspect ratio and surface area as well as controllable surface engineering to create more reactive sites. So far, 1-D GaN nanomaterials in photocatalytic applications are mostly prepared via metal-organic chemical vapor deposition(CVD) and plasma-assisted molecular beam epitaxy(MBE), both of which are, however, toxic, costly, uneasy of operation and low-yielding. To promote the application of GaN in photocatalysis and photoelectrochemistry, it is necessary to develop a simple and economic method to synthesize high purity 1-D GaN nanomaterials.In this thesis, the research focuses on controllable synthesis of various 1-D GaN nanomaterials by a simple and efficient CVD method. Their preparation, morphologies and structures, optical properties, growth mechanism and especially photocatalytic performance are investigated systematically.Firstly, 1-D GaN nanocrystals were synthesized via Au-catalyzed vapor-liquid-solid process. The influences of substrate, reaction temperature/time, ammonia flow, gallium precursor, carrier gas on the morphology of GaN and the corresponding mechanisms have been studied carefully.Secondly, the straight and smooth nanowires under the optimal growth parameters were used in photocatalytic degradation of isopropanol, hydrogen generation and carbon dioxides reduction. It was found that the rate of acetone and CO2 decomposed from isopropanol over this type of nanowires was nearly 200 times than that over GaN powder. The production of hydrogen generation over GaN nanowires was 250 times than that over powder. Selectivity in CO2 reduction(CH4 and CO) has been realized when loading different cocatalysts, i.e. Pt and Rh, on the nanowires.Thirdly, highly crystallized zigzag nanowires were successfully synthesized by combination of a sol-gel method and the carbon thermal assisted technique. The study on the intrinsic structural characteristics suggested that the alternation of Ga-terminated facet and N-terminated facet of the zigzag nanowires was induced by the reduction of electrostatic interaction energy. After modifying the nanowires with Rh2-yCryO3, stoichiometric water splitting has been realized over the zigzag GaN nanowires but not over the smooth ones. Photoelectrochemical response further confirmed the superior charge carrier separation and transportation of the zigzag GaN nanowires. The present study has provided inspiration for developing new photocatalysts for water splitting.