Study On Blue Light Excitation Performance Of Doped TiO₂ Photocatalyst Thin Films By Plasma Processing

TiO₂ has been regarded as one of the most extensively investigated materials in a large variety of applications including solar cells and photocatalysis due to its wide set of interesting properties,such as non-toxic,low-cost and chemically stable,etc.However,many of the practical applications are limited by the inactivity in the visible light region as its wide energy gap is about 3.2 eV.Doping with rare earth（RE）elements having unique electronic configurations has been considered as one of the effective methods in stretching the applications of TiO₂ with improved performances because of their wide range of optical,electronic,magnetic and photocatalytic characteristics.Recently,numerous studies both experimentally and theoretically indicate that photocatalytic activity of TiO₂ can be significantly enhanced by doping with RE elements.In this thesis,series of TiO₂ thin films have been synthesized using magnetron sputtering which belongs to low temperature plasma physical vapor deposition,including pure TiO₂ thin films and Er-doped TiO₂ thin films.The as-synthesized samples are characterized using X-ray diffraction（XRD）,scanning electron microscope（SEM）,atomic force microscopy（AFM）,X-ray photoelectron spectroscopy（XPS）,ellipsometry and ultraviolet-visible（UV-Vis）for the measure of crystal structure,morphology,films thickness,surface composition and transmission respectively.The various characterizaton methods have been used to investigate the effect of power frequency、power supply and annealing on the deposition of thin films as well as the the influence of Er doping on the chemical composition,deposition rate,crystal structure,surface morphology and optical properties of TiO2 thin films.In addition,the plasma characteristics of Er doped TiO2 thin films deposited by magnetron sputtering were studied by using the optical emission spectroscopy and the retarding field energy analyzer in this thesis.The main results are as follows:（1）The deposition rate of 13.56 MHz magnetron sputtering is consistent with the deposition rate of typical RF magnetron sputtering,and the surface of TiO₂ film is granular,uniform and compact.However,the deposition rate of 60 MHz magnetron sputtering is an extremely low deposition rate,which is suitable to be used as a power source for doping.The annealing treatment is helpful to the transition from amorphous state to crystalline state,which can improve the quality of the thin film.（2）Er-doped TiO₂ thin films have been prepared by 13.56 MHz and 60 MHz dual-frequency magnetron co-sputtering technology.It is found that Er doping can inhibit the growth of TiO₂ grains,reduce the grain size,change the band structure of TiO₂ and finally achieve the tunable band gap.（3）Under blue light,TiO₂ thin films prepared by 13.56 MHz and 60 MHz power supply showed no photocurrent,while Er-doped TiO₂ thin films prepared by 13.56 MHz and 60 MHz dual-frequency magnetron co-sputtering generated photocurrent with the large amount of Er doping,which indicate the increase of photogenerated electron hole pairs and thus the photocatalytic properties of thin films become better.（4）Plasma properties of 13.56 MHz and 60 MHz dual-frequency magnetron co-sputtering were measured by retarding field energy analyzer and optical emission spectroscopy.It is found that the sputtering rateβfollows the equationβ=γJ_Ar+(whereγis the sputtering yield,J_Ar+is the Ar⁺ion flux),when 13.56 MHz power supply remains the power of 200 W and the power of 60 MHz increases from 100 W to 250 W,the ion flux remained almost unchanged while the ion energy decreases from 12.5 eV to 11.09 eV,which leads to a lower sputtering deposition rate.The obtained Er-doped TiO₂ thin films show a higer Ra when the ion energy is lower,which can be attributed to the ions impinging the substrate that inhibits the diffusion of atoms and blocks the formation of smooth films.A larger surface roughness leads to a larger surface area which enables the simultaneous adsorption of more molecules and therefore a higher efficiency of photocatalysis.