The Study Of Structure And Properties Of Gd Doped TiO₂ Thin Films

In this work, Ag thin films with different thickness on TiO₂ films prepared by sol-gel are deposited by RF magnetron sputtering, and their structure and properties are analyzed. TiO₂ films doped with 0.3～0.6 at% Gd are deposited by reactive DC magnetron co-sputtering of Ti-Gd metallic target and annealed at 1000 ℃. Structural, surface morphological, optical and photocatalytic properties of Gd doped TiO₂ films relative to pure TiO₂ films are characterized by electron probe microscopy analysis （EPMA）, X-ray diffraction （XRD）, scanning electron microscope （SEM）, atomic force microscope （AFM）, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy （XPS） and room temperature photoluminescence （PL）, respectively. The results are as follows:1. Phase transformation of TiO₂ thin films prepared by sol-gel after annealing at different temperatures is from anatase （400℃） to mixture of anatase and rutile （550℃） and to rutile （700℃）. Two kinds of growth behavior of Ag particle films on the TiO₂ thin films are observed, one is mainly the Ag islands growth with Ag depositon time of 20-55s and characteristic size increasing from 32 to 37 nm; the other is the Ag islands growth that focused on merging with others into big islands in which depositon time of Ag island is 90-200s, with characteristic size 37 nm and maximum size ranging from 72 to 170 nm. Photocatalytic efficiency of Ag/TiO₂ films in the decreasing order is Ag（200s）/TiO₂^ Ag（55s）/TiO₂、Ag（20s）/TiO₂、Ag（90s）/TiO₂ and TiO₂.2. Gd-doped TiO₂ films promote the anatase-to-rutile phase transformation （ART）, and the phase transformation process in view of Gd doping is from pure anatase （Gd:0 at%） to mixture of anatase and rutile（Gd:0.3 at%） to pure rutile（Gd:0.4～0.6 at%）. XPS analysis confirms that Gd ions are in the +3 oxidation state, Ti ions are in the +4 oxidation state, and Gd doping promotes the increasing of oxygen vacancy concentration. Optical band gap for annealed films with Gd content of 0,0.3,0.4 and 0.6 at.% is 3.32±0.03,3.25±0.03,2.94± 0.03, and 2.97±0.03 eV, respectively. Moreover, via PL measurement further exhibits their energy band structure, especially the indirect transition, which is dependent on the occurrence of anatase （Gd:0 at%, at 3.26eV） or rutile phase （Gd:0.4±0.6 at%, at 3.07-3.1 eV） in films due to Gd doping, while the intraband transition at 3.61 and 3.41 eV is also observed in both pure TiO₂ and 0.3 at.% Gd-doped TiO₂ films. PL peak at 2.71 eV is assigned to oxygen vacancy, indicating the increase in oxygen vacancy concentration in the Gd-doped TiO₂ films with Gd content increasing, in accordance with XPS analysis. As-deposited films with appropriate content of Gd doping （0.3-0.4 at%） exhibit an improvement in photocatalytic activity, but photocatalytic activity of the annealed films decreases with increasing Gd content.