Research On The Synthesis Of Germanate Compounds Nanostructures And Their Photocatalytic Reduction Of CO₂

Reduction of CO2to valuable hydrocarbons using solar energy is one of the best solutions to both the global warming and the energy shortage problems. The unique multi-dimensional channel structures of Metal germinates photocatalysts facilitates the separation of photogenerated charge carriers with good photocatalytic performance. The photocatalytic activity of photocatalysts is highly dictated by their sizes, shapes and crystal facets. In this dissertation, some germanate compounds nanomaterials were synthesized in a binary ethylenediamine (En)/water solvent system using a solvothermal route. The formation mechanism and photocatalytic activity of CO2reduction of these nanomaterials were studied. The details are summarized as follows:(1) Sheaf-like, hyperbranched Zn2GeO4nanoarchitectures were successfully synthesized in this mixed-solvent system. These structures may be assigned to the splitting crystal growth mechanism, resembling some minerals observed in nature. Addition of increasing amounts of En was found to enhance the degree of crystal splitting. Nitridation of the resulting Zn2GeO4superstructures under NH3flow produced yellow Zn1.7GeN1.8O solid solution, which allows photocatalytically converse CO2into CH4in the presence of H2O at ambient conditions under visible light irradiation. (2) Single-crystalline Zn2GeO4nanobelts with thickness as thin as～7nm (corresponding to five repeating cell units) and aspect ratio up to10,000has been synthesized using this solvothermal route. En can adsorb on solid surfaces and selectively bind to some specific panels to control the velocity and direction of crystal growth, which was termed as the solvent-coordination molecular-template mechanism. The ultralong and ultra-thin geometry of the Zn2GeO4nanoribbon proves to improve the photocatalytic activity toward reduction of CO2in the presence of water vapor into CH4.(3)A novel, highly crystalline indium germinate hybrid subnanowire, which we denote as In2Ge2O7(En), with general diameters of2-3nm and lengths up to hundreds of nanometres was synthesized in this En/water solvent system. The hybrid ultrathin nanowire exhibits an ultraviolet photoluminescence emission, a dramatic blue shift by more than100nm relative to pure inorganic In2Ge2O7nanowire andmicrotubes. The In2Ge2O7(En) ultrathin nanowire performs selectively the photocatalytic reduction of CO2into CO in the presence of water vapor. With reference to our Zn2GeO4nanoribbon photocatalyst, which was recently used to produce CH4under the same photocatalytic conditions, this work is a significant sign that the greenhouse gas, CO2, can be ameliorated into desirable kinds of renewable fuels using different germinate catalysts.(4) Well-defined,3D hierarchical microcrystals united with ultrathin ZnGa2O4nanosheets with over99.6%exposed{110} facets with highly active surfaces were deposited in this (En)/water solvent system. The{110}-dominant ZnGa2O4nanosheets proves4-time conversion efficiency of CO2reduction higher than cubic nanocrystals with{100} and octahedral nanocrystals with{111} surfaces. The enhancement of the photocatalytic efficiency is strongly associated with inherent catalytic behavior of {110} surface with high density of cations for CO2absorption and low work function for enhanced CO2activation. In addition, the unique3D hierarchical architecture also promotes the photocatalytic performance through high surface area, ultrathin thickness which allows charge carriers to move rapidly from the interior to the surface to participate in the photoreduction reaction, and improvement of light scattering to be in favor of enhancing the light absorption.