Metal oxide nanostructures and nanocomposites for energy related applications
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Metal oxide nanomaterials are being actively investigated for energy and sustainable development, from energy generation and storage, to water and environment decontamination. This research concerns the study of TiO2 nanostructures for solar cells and photocatalysis, and VO2 for electrochemical supercapacitors. As a wide bandgap semiconductor, TiO2 is being investigated for dye sensitized solar cells (DSSCs), while the dye loading and photocarrier transport are still bottlenecks for high solar energy conversion efficiency. Hydrothermal method was used to synthesize a mixed nanostructure by infiltrating the TiO2 nanoparticles (NPs) onto the walls of highly ordered TiO2 nanotubes (NTs). The aim is to combine the merits of the NP’s high dye loading and high light harvesting capability with the NT’s straight carrier transport path and high electron collection efficiency to improve DSSC performance. Compared with the bare NT structure, dye loading of this mixed NT and NP structure is more than doubled. To improve photocatalytic efficiency of TiO2, nanocomposite structures of TiO2/graphene were investigated with the aim to enhance electron-hole separation. Hydrothermal method was employed to synthesize graphene-TiO2 nanowire composite (GNW) and graphene-TiO2 nanoparticle composite (GNP) with graphene serving as the conductive supporting framework. The photocatalytic performance and related properties of TiO2 nanoparticles (NPs), TiO2 nanowires (NWs), GNP and GNW were comparatively evaluated based on photodegradation of methylene blue (MB) under visible light. The result reveals that the relative photocatalytic activity of GNW is much higher than GNP and pure NWs or NPs. To employ the pseudocapacitance of transition metal oxides to achieve much higher energy density for supercapacitor applications, VO2 was studied as electrode oxide. VO2 was found to be promising to serve as the electrode materials of supercapacitors, while the high resistivity significantly inhibits its performance. A facile method was employed to dramatically reduce VO2 resistivity by almost 3 orders of magnitude by thermal treatment of VO2 powder in hydrogen environment. The fabricated supercapacitor showed considerably enhanced specific capacitance and energy density compared with the pristine VO2, along with excellent stabilization and high efficiency over the long-term cycling measurement.