Tetsuya Yamaki

Band gap narrowing of titanium dioxide by sulfur doping

Titanium dioxide (TiO2) doped with sulfur (S) was synthesized by oxidation annealing of titanium disulfide (TiS2). According to the x-ray diffraction patterns, TiS2 turned into anatase TiO2 when annealed at 600u200a°C. The residual S atoms occupied O-atom sites in TiO2 to form Ti–S bonds. The S doping caused the absorption edge of TiO2 to be shifted into the lower-energy region. Based on the theoretical analyses using ab initio band calculations, mixing of the Su200a3p states with the valence band was found to contribute to the band gap narrowing.

Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculations

Abstract The electronic structures of titanium dioxide (TiO 2 ) doped with 3d transition metals (V, Cr, Mn, Fe, Co and Ni) have been analyzed by ab initio band calculations based on the density functional theory with the full-potential linearized-augmented-plane-wave method. When TiO 2 is doped with V, Cr, Mn, Fe, or Co, an electron occupied level occurs and the electrons are localized around each dopant. As the atomic number of the dopant increases the localized level shifts to lower energy. The energy of the localized level due to Co is sufficiently low to lie at the top of the valence band while the other metals produce midgap states. In contrast, the electrons from the Ni dopant are somewhat delocalized, thus significantly contributing to the formation of the valence band with the O p and Ti 3d electrons. Based on a comparison with the absorption and photoconductivity data previously reported, we show that the t 2g state of the dopant plays a significant role in the photoresponse of TiO 2 under visible light irradiation.

Sulfur-doping of rutile-titanium dioxide by ion implantation: Photocurrent spectroscopy and first-principles band calculation studies

Sulfur (S)-doped titanium dioxide (TiO2) was synthesized by ion implantation and subsequent thermal annealing. The S ions were implanted into the single crystals of rutile TiO2 at a fluence of 8×1015 ions/cm2. According to the results of Rutherford backscattering spectroscopy and ion channeling analysis, the irradiation damage recovered by annealing at 600u2009°C in air. In the annealed crystal, the S atoms occupied oxygen sites for form Ti-S bonds, as confirmed by x-ray photoelectron spectroscopy. Compared to the pure TiO2, a photocurrent was observed in the lower-energy regions for the S-doped TiO2. Based on the theoretical analyses by the first-principles band calculations using the full potential linearized augmented plane-wave methods within the generalized gradient approximation, the mixing of the S 3p states with the valence band (VB) was found to contribute to the increasing width of the VB. This leads to the band gap narrowing in the S-doped TiO2. Therefore, the photon-to-carrier conversion was induc...

Fluorine-doping in titanium dioxide by ion implantation technique

Abstract We implanted 200 keV F+ in single crystalline titanium dioxide (TiO2) rutile at a nominal fluence of 1xa0×xa01016 to 1xa0×xa01017 ionsxa0cm−2 and then thermally annealed the implanted sample in air. The radiation damage and its recovery process during the annealing were analyzed by Rutherford backscattering spectrometry in channeling geometry and variable-energy positron annihilation spectroscopy. The lattice disorder was completely recovered at 1200 °C by the migration of point defects to the surface. According to secondary ion mass spectrometry analysis, the F depth profile was shifted to a shallower region along with the damage recovery and this resulted in the formation of an F-doped layer where the impurity concentration steadily increased toward the surface. The F doping proved to provide a modification to the conduction-band edge of TiO2, as assessed by theoretical band calculations.

Preparation of anatase and rutile thin films by controlling oxygen partial pressure

Abstract Titanium dioxide (TiO 2 ) films on glass substrates were prepared by pulsed laser deposition (PLD) technique. The dependence of crystal structure, morphology and photocatalytic activity on oxygen partial pressure were investigated using X-ray diffraction (XRD), atomic force microscope (AFM) and UV–VIS spectrophotometer. By increasing oxygen partial pressure, the surface morphology changed from smooth to rough, and the crystal structure changed from rutile to anatase at fixed substrate temperature of 773xa0K. The change of the surface morphology and the formation of anatase structure by variation of oxygen partial pressure resulted in an increase of photocatalytic activity.

Radiation grafting of styrene into crosslinked PTEE films and subsequent sulfonation for fuel cell applications

Abstract Proton exchange membranes were prepared by the radiation-induced grafting of styrene into crosslinked polytetrafluoroethylene (PTFE) films and subsequent sulfonation. The degree of grafting was controlled in the range of 5–75% by the crosslinking density of the PTFE matrix as well as the grafting conditions. The resulting membranes showed a large ion exchange capacity reaching 2.6xa0meqxa0g −1 , which exceeded the performance of commercially available perfluorosulfonic acid films such as Nafion. This will encourage the use of our ion exchange membranes in solid polymer electrolyte fuel cells.

Photo-induced surface charge separation of highly oriented TiO2 anatase and rutile thin films

Surface charge separation behavior of photo-generated carriers in highly oriented TiO2 anatase and rutile films was investigated using a technique in which the transient surface charge is observed by laser pulse irradiation without metal contacts and an externally applied field. According to the measurements, the quantum efficiency of photo-generated holes transported toward the surface was determined as a function of incident laser energy. The photo-generated holes in anatase can be transported toward the surface for irradiation at the photon energy of its bandgap. The holes transported toward the rutile surface, however, were generated close to the surface for irradiation at the photon energy much higher than its bandgap.

Characterization of epitaxial TiO2 films prepared by pulsed laser deposition

Abstract Epitaxial titanium dioxide thin films with anatase and rutile structures have been deposited by pulsed laser deposition (ArF excimer laser) under a controlled O 2 atmosphere. The anatase TiO 2 (0xa00xa01) films have been successfully prepared on LaAlO 3 (0xa00xa01), LSAT (0xa00xa01), SrTiO 3 (0xa00xa01) and YSZ (0xa00xa01) substrates. Also, high-quality epitaxial rutile TiO 2 (1xa00xa00) films were grown on α -Al 2 O 3 (0xa00xa00xa01) substrates. The substrate temperature during the deposition was in the range from 349°C to 608°C under 6×10 −3 xa0Torr O 2 gas pressure. The quality of films and crystallographic relationships were assessed by X-ray diffraction, X-ray pole figures and Rutherford backscattering spectroscopy (RBS)/channeling. The optical properties were characterized in the ultraviolet-visible region by optical absorption measurement. The optical band gaps for anatase and rutile TiO 2 epitaxial films were evaluated to be 3.22 and 3.11xa0eV, respectively.

Preparation of epitaxial TiO2 films by PLD for photocatalyst applications

Abstract Pulsed laser deposition (PLD) with a KrF excimer laser was used to prepare epitaxial TiO 2 films on a (0xa00xa00xa01) sapphire substrate. The crystalline structure and surface morphology of the films were then investigated as a function of the laser fluence in the PLD. X-ray diffraction and Raman spectroscopy results showed that the films were composed of the (1xa00xa00) oriented rutile phase with a small amount of the (0xa00xa01) oriented anatase. The anatase-to-rutile ratio in such phase-mixed films was controlled by the laser fluence; the ratio increased as the fluence decreased. From atomic force microscopy observations, the film deposited at the lowest fluence was found to have a rough surface consisting of coarse grains. This film exhibited the best photocatalytic performance during the decomposition of the organic dyes possibly due to the maximum relative content of anatase and the large surface area.

UV-ray photoelectron and ab initio band calculation studies on electronic structures of Cr- or Nb-ion implanted titanium dioxide

Abstract The electronic structure of TiO 2 doped with Cr or Nb by ion implantation is investigated by UPS and ab initio band calculations. Rutile TiO 2 was implanted with 150 keV Cr or Nb ions and subsequently annealed in an air, and the recovery of radiation damage and substitution of implanted metals for Ti atoms was confirmed. UPS spectra for both of these oxides revealed a signal corresponding to the electron-occupied level in the band gap. For Cr-doped TiO 2 , this signal appeared close to the edge of the valence band (VB), while Nb-doped TiO 2 exhibited a small signal far from the VB edge. According to band calculations, the midgap levels of Cr- and Nb-doped TiO 2 consist of the electronic states localized in the Cr t 2g orbital and delocalized over the Ti t 2g and Nb t 2g orbitals, respectively. Consequently, the electronic character of both of these oxides is determined by the energy of the t 2g states of Ti and the dopant.