Research On The Designing Preparation And Properties Of Micro/Nanostructured Zn₂GeO₄-based Phostocatalytic Materials

Production of clean hydrogen fuel from water and biomass derivatives using sunlight has been considered as a promising solution to energy and environmental problems. The photocatalytic material plays a vital role in photocatalytic reactions. The photocatalytic activity of photocatalysts is highly dictated by their compositions, grain sizes, crystal orientations, shapes, crystal facets, and hierarchical micro/nanostructures. Of Zn2GeO4 semiconductor material, its unique multi-dimensional channel structure leads to large mobility of photo-excited electrons and facilitates the separation of photo-generated charge carriers. Of particular note is the remarkable photocatalytic stability of the material, therefore, the Zn2GeO4-based nanomaterials are very important for photocatalytic materials research in the photacatalytic fields.In this dissertation, based on effect of crystal structure on the growth habits of Zn2GeO4 crystals, a series of high-quality the Zn2GeO4-based semiconducting nanomaterials were synthesized by using as hydrothermal synthetic route well as solvothermal synthetic route and in situ high temperature nitriding technique. The photocatalytic properties of such materials will be studied systemically to establish the intrinsic correlations between the micro/nanostructures and surface structures of Zn2GeO4-based photocatalytic materials and their photocatalytic activities. The details are summarized as follows:(1) Nanorod-shaped Zn2GeO4, an efficient heterogeneous photocatalytic hydrogen-evolving catalyst, has been facilely prepared in high yield by a simple hydrothermal synthesis technique. Each nanorod owns its individual prism-like structure, which shows a highly enhanced UV-light-driven activity in the photocatalytic decomposition of water-methanol solution to hydrogen. In comparison with Zn2GeO4 nanorods prepared in a non stoichiometric ratio, the prismatic Zn2GeO4 nanorods have a beneficial surface structure and superb crystal quality as well as 1-D nanostructure with a high aspect ratio, which lead to significantly high photocatalytic activity for the photocatalytic decomposition of water/biomass derivatives to produce hydrogen under UV irradiation.(2) Well-defined Zn2GeO4 hexagonal nanorods and nanofibers with high aspect ratios have been readily realized in high yield by a simple and general hydrothermal synthesis method free of any surfactant or template. Detailed investigations indicated that the prismatic Zn2GeO4 nanocrystals are uniform single crystal with the longitudinal direction along [001] and were dominated by (110) and (-1-10) surfaces. Based on first-principles and the periodical bond chain (PBC) theory, the planes horizontal to the c-axis such as (110) and (100) are energetically more stable than (001) in the phenacite-type Zr2GeO4. Thus the renascent nanocrystals of Zn2GeO4 would preferentially grow along direction of [001]. As an important wide-band-gap photocatalyst, the products of regular Zn2GeO4 nanocrystals with hexagonal 1-D structure exhibit superior photocatalytic activities for the photocatalytic decomposition of water-methanol solution to hydrogen under UV irradiation. The enhanced photocatalytic activity of the hexagonal Zn2GeO4 nanorods can be a result of a synergistic effect of the beneficial surface structures, large surface area and 1-D nanostructure with a high aspect ratio.(3) Crystal orientation-ordered hexagonal Zn2GeO4 nanorod-bundles have been obtained via a synergistic self-assembly route in a binary triethanolamine/water solvent system using a solvothermal route. Each nanorod unit of the bundles maintains its individual hexagonal rod-shaped structure and is dominated by (110) and (-1-10) facets, and all of them are aligned parallelly to the growth direction in a high density. An ordered self-assembly of anisotropic nanorods growth mechanism was suggested to elucidate the formation of the unique bundle structures. The novel structure of Zn2GeO4 bundles assembled by hexagonal nanorods leads to a high UV-light-driven activity for the photocatalytic decomposition of water-methanol solution to hydrogen. The correlation of structure and photocatalytic activity of the ordered nanorod-bundles was discussed, it is showed that the superstructures may favor improved separation of the photo-generated electron-hole pairs and decrease the electron-hole recombination rate.(4) Hierarchical Zn2GeO4 hollow spheres were fabricated via a synergistic self-assembly route in a binary triethanolamine/water solvent system using a solvothermal route. Each building subunit of the spheres owns a hexagonal rod shape and is dominated by (110) facet. The sequential nucleation and growth of the assembled hollow structures is mainly determined by the cooperative effect of TEA and NaOH. In comparison with the Zn2GeO4 nanorods, the hierarchical hollow spheres owns larger surface area and stronger light absorption as well as unique superstructure, which lead to significantly high photocatalytic activity for the photocatalytic decomposition of water and biomass derivatives to hydrogen.(5) N-graphene/Zn1.231Ge0.689N1.21800.782 (N-graphene/ZnGeON) nanocomposites were self-assembled via electrostatic interaction of positively charged Zn2GeO4 nanorods with negatively charged graphene oxide (GO), followed by a high temperature in situ nitridation technique. During this two-step chemistry process, N-graphene and the intimate interfacial contact between ZnGeON and the N-graphene sheets are readily achieved. The as-prepared N-graphene/ZnGeON nanocomposites could not only catalyze the evolution of CH4 from photocatalytic reaction of CO2 and H2O, but also catalyze the evolution of aldehyde from photocatalytic reaction of alcohols and O2. It is suggested that the enhanced light absorption and photo-electrochemical performances in our current work is not the main reasons leading to photocatalytic reduction of CO2. However, in comparison with the ZnGeON, the N-graphene/ZnGeON hybrid showed a higher conversion for the photocatalytic oxidation of benzyl alcohol under visible light and performed with 100% selectivity. The enhanced photocatalytic activities could be attributed to ultra high charge mobility of N-graphene for oxidation of benzyl alcohols.