Hierarchical Transitional Metal Oxides: Controllable Synthesis, Catalytic And Gas Sensing Properties

Research on transition metal oxide has been one of the most important branches ofinorganic functional materials due to their excellent physical and chemical properties andpotential applications in industry and daily life. Transition metal oxide with hierarchicalmicro-/nano-structures, which was built by nanosized building blocks, attractedresearcher’s attention. Controllable synthesis of micro/nanomaterials with novel micro-and nano-structure (e.g., size, crystal planes, exposed facets, hollow, porous structure,etc.) is significant for understanding the relationships between their structures andproperties, and further exploring the application of these materials. Typical transitionmetal oxides, such as TiO2, Co3O4, NiCo2O4and ZnO were widely studied and applied inthe field of optical, electrical and magnetic. In this work, metal alkoxide precursormethod was used to prepare transition metal oxide materials with controlled structures.We developed a solvothermal method without any additions of surfactants or templates inpreparation of various transition metal (Ti, Co, Ni and Zn) alkoxide precursors in amixture of glycerol and isopropanol. Then, transition metal oxides with differenthierarchical structure and morphology were obtained by a simple heat-treatment and usedin water splitting and gas sensor, respectively.The main parts of this thesis including:1. A new approach was developed for the preparation of TiO2nanomaterials with {010}facet without the assistance of F ions. Porous anatase TiO2microspheres composed of{010}-faceted nanobelts were prepared by using a hierarchical flower-like titaniumglycerolate as the starting material. Through a simple thermal treatment, the precursorwas directly converted into the desired TiO2material with a large surface area. Theas-prepared TiO2nanomaterial was shown to serve as an efficient photocatalyst for H2evolution, and its activity was more than twice that of the benchmark P25TiO2. The clean {010} active facet, porosity, and especially the high photocatalytic activity of the TiO2material all highlight the importance of this precursor strategy.2. We report the facile synthesis of hollow Co3O4microspheres composed of porous,ultrathin (<5nm), single-crystal-like nanosheets via a novel “self-template” route. Theas-obtained hollow Co3O4nanomaterial possesses a high BET surface area (~180m2/g),and can serve as an active and stable water oxidation catalyst under both electrochemicaland photochemical reaction conditions, owing to its unique structural features. In theelectrochemical water oxidation, this catalyst affords a current density of10mA/cm2(avalue related to practical relevance) at the overpotential of~0.40V. Moreover, with theassistance of sensitizer [Ru(bpy)3]2+(bpy=2,2’-bipyridine), this nanomaterial cancatalyze water oxidation reaction under visible light irradiation (>450nm) with an O2evolution rate of~12218μmol g-1h-1. Our results suggest that delicate nanostructuringcan offer unique advantage for developing efficient water oxidation catalysts.3. Hollow NiCo2O4microspheres as a noble-metal-free solid-state bimetallic wateroxidation catalysts, were successful prepared by a similar approach as that of preparingCo3O4. The as-obtained hollow NiCo2O4microspheres were composed of porous,ultrathin (<3nm) nanosheets, with a large BET surface area of~180m2/g. With theassistance of sensitizer [Ru(bpy)3]2+, this nanomaterial can also catalyzed water oxidationreaction under visible light irradiation (>420nm) with an O2evolution rate of~843μmolg-1min-1, and its activity was more than twice that of the hollow Co3O4microspheres,seven times than that of the NiO nanosheets. NiCo2O4has better performance, owing tothe bimetallic synergy of Co and Ni and its unique structural features. This work isexpected to open up new synthetic avenues towards the preparation of other advancedbimetallic inorganic nanomaterials with with unique structures and excellent properties.4. Wurtzite ZnO microspheres composed of radially aligned porous nanorods wereprepared through simple thermal treatment of a zinc monoglycerolate precursor.Isopropanol solvent can greatly decrease the formation temperature of zincmonoglycerolate (85oC), and also influence the morphology of zinc monoglycerolate. The as-prepared ZnO material was used as a highly sensitive sensing material for ethanoldetection.