Preparation And Photoelectrochemical Properties Of Carbon And Nitrogen-doped TiO₂ Thin Films

Titanium dioxide (TiO2) is among the most promising materials for photocatalytic and photoelectrochemical applications due to its low cost, photostability, chemical inertness and nontoxicity. In photocatalysis, it has been used to purify wastewater or air by the organic compounds decomposition and to produce hydrogen by water splitting. In photoelectrochemistry, it has been used dye-sensitized in solar cells. However, the application of TiO2 is hindered because it is active only under ultraviolet (UV) light (about 4% of the solar energy) illumination due to its wide band gap of 3.2-3.5 eV. Therefore, much effort has been made to modify its band gap energy in order to enhance its absorption in the visible light region. In this thesis, carbon or nitrogen doped TiO2 thin films were prepared by direct current (DC) reactive magnetron sputtering, anodization and electrophoretical deposition methods. The as-prepared films were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoemission spectroscopy, UV-Vis spectrophotometry and photoelecrochemical measurements. The main results and conclusions can be summarized as following:1. Nitrogen-doped titanium dioxide thin films with visible light photoresponse were prepared by oxidation of sputtered TiNx films, whose nitrogen contents can be easily changed by controlling the volume rate of N2/(Ar+N2) during DC reactive magnetron sputtering process. The reference TiO2 sample was also deposited by the same method under Ar/O2 gas mixture. The formation of anatase type TiO2 is confirmed by XRD. SEM measurement indicates a rough surface morphology with sharp, protruding modules after annealing treatment. Optical properties reveal an extended tailing of the absorption edge toward the visible light region due to the presence of nitrogen. The band gap of the N-doped sample is reduced from 3.36 eV to 3.12 eV compared with the undoped one. All the N-doped samples show red shift in photoresponse towards visible region and improved photocurrent density under visible irradiance is observed for the N-doped samples from 0.29μA/cm2 to 10.71μA/cm2,37 times larger than that of the undoped sample, when the volume rate of N2/(Ar+N2) was controlled at 33%.2. Mesoporous carbon-doped TiO2 thin films with different contents of carbon were prepared by anodization of TiCx thin films deposited by DC magnetron sputtering using a graphite-embedded titanium target. A mesoporous surface morphology of the films is revealed by SEM. The content of carbon in the carbon-doped TiO2 thin films can be controlled by adjusting the area ratio of graphite to titanium in the target, which is revealed by EDS. The absorption edge of the carbon-doped sample is shifted from 380 nm to 554 nm compared with the undoped one. About three fold increase in photocurrent density is observed at the sample with 4.21 at% carbon compared with the pristine sample under visible light illumination.3. Carbon-doped TiO2 thin films were also synthesized by annealing of titanium carbide (TiC) thin films which were electrophoretically deposited on Ti surface. The XRD patterns confirm the deposition of TiC films on Ti substrates and formation of both anatase and rutile structure of TiO2 after the annealing treatment. XPS results demonstrate the formation of Ti-C bond in the carbon doped TiO2 films. The influence of deposition voltage and annealing temperature on the photoelectrochemical response of the samples under visible illumination is studied, which suggests higher photo-response can be observed for the samples deposited at 13.5 V and 350℃. Photoelectrochemical results show enhancement (about 4.2 times) in photocurrent for the as-prepared carbon-doped TiO2 electrodes under visible light illumination.