Investigation On Structure And Physical Properties Of Several Photocatalysts Under High Pressure

With the increasing attention to environmental protection and the demands for clean energy,photocatalysis semiconductor materials attract more and more attention and research.Semiconductor materials are widely used in the fields of photocatalysis,energy conversion and storage.Many of These applications requires these materials to have strong light absorption and good electron transport capacity.In recent years,research in high-pressure(HP)physics has proved that the properties of materials can be significantly improved by using pressure treatment.Thus,using high pressure to change the crystal structure,electronic structure and electrical conductivity of materials is of great significance for the development and application of semiconductor photocatalysts.In this thesis work,the crystal structures,the electronic structures,the electrical transport,the light absorption and the photoelectric properties of several hydrogenated titania and two novel oxysulfides at high pressures were studied systematically by using relevant high-pressure techniques.Firstly,in this thesis research,rutile TiO2 was hydrogenated at different temperatures via solid-gas phase reactions,producing black titania.In addition,two oxysulfides with narrow band gap were synthesized.All the produced materials exhibit strong light absorption.We studied the pressure dependences of the structure change,the electrical conductivity as well as the light adsorption of black titania using HP X-ray diffraction,Raman/UV-Vis spectroscopy,and electrical transport measurements.It is found that in compression of TiO2 reduced by H2 at 500? and 800?,there is a phase transition from rutile to baddeleyite at?17.0 GPa and?21 GPa,respectively.The latter transforms to TiO2-OI again at?27.0 GPa.Accompanying the HP phase transition,the electrical conductivity exhibits complex variations with the pressure,in good accord with the bandgap changes of involved HP-phases as a function of pressure.This confirms that pressure affects the electrical conductivity of black titania via controlling the number of charger carriers.This work provides a fundamental understanding of the pressure-induced structure-property relationship in black titania and will have important implications for tuning the photocatalytic activity via pressure and for developing new applications such as pressure sensors.Synchrotron X-ray diffraction,Raman spectroscopy,AC-impedance measurements,photocurrent measurements and UV-Vis absorption spectroscopy were used to investigate the structures and properties of the two synthesized oxysulfides.Results show that the orthorhombic phases of La2Ta2ZrS2O8 and La2Ta2TiS2O8 transform to two new high-pressure phases at?23 and 25 GPa,respectively.Although their bandgaps show similar trends in variations with the pressure,their pressure-dependences of the electrical resistances and photocurrents are significantly different.For samples of similar sizes,the resistance of La2Ta2ZrS2O8 is?2-3 orders of magnitudes higher than that of La2Ta2TiS2O8,and the former exhibits a sharp decrease with pressure after the phase transition,while the latter presents only steady decrease with increasing pressure.This difference arises from their different responses in the carrier mobility under compression,which is related to their different compressibility at high pressure(bulk modulus of La2Ta2ZrS2O8 is 122.5 GPa and that of La2Ta2TiS2O8 is 132.6 GPa).Both the electrical conductivity and defect formation at high pressure can affect the photoelectric properties,making the La2Ta2TiS2O8 compound possess photocurrents superior to the La2Ta2ZrS2O8 one.The findings from this work show that the La2Ta2TiS2O8 compound is a better candidate than La2Ta2ZrS2O8 for developing applications in photocatalysis,photovoltaics,photoelectric devices,etc.