Microstructure and Characteristics of TiO2 Thin Films with Sn and Nb Doping Prepared by DC/RF Co-sputtering

• Main Authors: Hsiao-Chiang Yao, 姚曉強
• Other Authors: Fuh-Sheng Shieu
• Format: Others
• Language: en_US
• Online Access: http://ndltd.ncl.edu.tw/handle/59518525922649297355

博士 === 國立中興大學 === 材料科學與工程學系 === 96 === Titanium dioxide (TiO2) thin films were deposited on Si wafer and glass substrates by direct current (dc) magnetron sputtering, by which the photocatalytic phase, anatase, was obtained under the optimum deposition conditions of O2/(O2+Ar) flow ratios and substrate biases. TiO2 thin films doped with Sn or Nb were prepared on glass substrates by dc/rf (radio frequency) magnetron co-sputtering, in which dc and rf were utilized for Ti and Sn or Nb targets, respectively, and the samples were post-annealed at temperatures ranging from 473 to 773 K for 1 h in ambient air. Microstructure, surface morphology, chemistry, and optical properties of the un-doped, Sn-doped, and Nb-doped TiO2 films were characterized in details by glancing incidence X-ray diffraction (GIXD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS), differential thermal analysis/thermogravimetry (DTA/TG), atomic force microscopy (AFM), UV/visable spectrophotometry, ellipsometry, and X-ray photoelectron spectroscopy (XPS). In addition, the photocatalytic activity was evaluated by measurement of the degeneration of methylene blue under UV/visible irradiation. The analytical results of GIXD show that the anatase phase is the dominant phase at an O2/(O2+Ar) flow ratio of over 40 %, whereas the films exhibit the rutile phase dispersed in amorphous matrix at an O2/(O2+Ar) flow ratio of 20 %. Microstructure and optical properties of the TiO2 films are dependent closely upon the rf power applied to the substrates as a bias. The anatase phase with columnar structure was obtained for the films prepared at rf powers of 0 and 5 W; whereas the films exhibit a rutile phase with equi-axial crystals at rf powers of 15 and 25 W. The rf-bias-assisted TiO2 films have a smoother surface than that without rf bias, and the absorbance edges shift to long wavelengths. The TiO2 films with a rutile phase have a higher refractive index, whereas better photocatalytical properties were observed for the films having anatase phase. TiO2 films with various Sn or Nb doping contents were investigated on the microstructure, optical properties, and photocatalytic activities. All the as-deposited TiO2 films on glass substrate were amorphous. The concentration of Sn in rf powers of 100 and 200 W and Nb in rf powers of 200, 300, and 400 W were 6.65 and 16.4 at.% for Sn and 1.98, 3.56, and 4.9 at.% for Nb, respectively, by X-ray photoelectron spectroscopy. GIXD revealed a polycrystalline phase for the Nb-doped films post-annealed at temperature 523 K, in contrast to the un-doped one that has to be annealed at temperature 723 K, indicating that Nb dopant can enhance the crystallization of amorphous TiO2. Furthermore, the Nb-doped film post-annealed at 673 K was found to have an anatase-dominanted phase with a fine grain microstructure, while the Sn-doped film post-annealed at 673 K was found to have a coexistence of anatase/rutile duplex and amorphous structure, as observed by using TEM. Heat treatment also induces a change in the surface morphology of the TiO2 films examined by FE-SEM. The optical properties of the TiO2 films were characterized by UV/visible spectrophotometry. The average transmittance of the Sn-doped and Nb-doped films is higher than 85%. Absorption zone in visible regime and optical band gap of the Sn-doped or Nb-doped TiO2 films increase with the doping contents. Under visible light irradiation, all the Nb-doped TiO2 films exhibit better photocatalytic activity than that of un-doped and Sn-doped ones. Among them, the Nb-doped TiO2 film with rf power of 400 W post-annealed at temperature 673 K shows the best photocatalytic performance.