A surface science perspective on TiO2 photocatalysis

The prototypical photocatalyst TiO2 exists in different polymorphs, the most common forms are the anatase- and rutile-crystal structures. Generally, anatase is more active than rutile, but no consensus exists to explain this difference. Here we demonstrate that it is the bulk transport of excitons to the surface that contributes to the difference. Utilizing high –quality epitaxial TiO2 films of the two polymorphs we evaluate the photocatalytic activity as a function of TiO2-film thickness. For anatase the activity increases for films up to ~5 nm thick, while rutile films reach their maximum activity for ~2.5 nm films already. This shows that charge carriers excited deeper in the bulk contribute to surface reactions in anatase than in rutile. Furthermore, we measure surface orientation dependent activity on rutile single crystals. The pronounced orientation-dependent activity can also be correlated to anisotropic bulk charge carrier mobility, suggesting general importance of bulk charge diffusion for explaining photocatalytic anisotropies.

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Why is anatase a better photocatalyst than rutile?--Model studies on epitaxial TiO2 films.

Electrochromics for smart windows: Oxide-based thin films and devices

Defect states in reduced and n-type doped titania are of fundamental importance in several technologically important applications. Still, the exact nature of these states, often referred to as “Ti3+ centers”, is largely unclear and a matter of debate. The problem is complicated by the fact that electronic structure calculations based on density functional theory (DFT) in the local density approximation (LDA) or semilocal generalized gradient approximation (GGA) provide results that do not account for many of the experimentally observed fingerprints of the formation of Ti3+ centers in reduced TiO2. Here, we investigate the properties of at least four different types of Ti3+ centers in bulk anatase, (1) 6-fold-coordinated Ti6c3+ ions introduced by F- or Nb-doping, (2) Ti6c3+−OH species associated with H-doping, (3) undercoordinated Ti5c3+ species associated with oxygen vacancies, and (4) interstitial Ti5c3+ species. The characterization of these different kinds of Ti3+ centers is based on DFT+U and/or hybri...

Reduced and n-Type Doped TiO2: Nature of Ti3+ Species

Transparent conducting oxides for electrode applications in light emitting and absorbing devices

The development and properties of titanium dioxide (TiO 2 )-based transparent conducting oxides (TCO), which exhibit properties comparable to those of In 2-x Sn x O 3 (ITO), are reviewed in this article. An epitaxial thin film of anatase Ti 0.94 Nb 0.06 O 2 exhibited a resistivity (p) of 2.3 x 10 -4 Ω cm and internal transmittance of ~95% in the visible light region. Furthermore, we prepared polycrystalline films with p of 6.4 x 10 -4 Ω cm at room temperature on glass substrates by using sputtering. We focus on characteristics unique to TiO 2 -based TCO, such as a high refractive index, high transmittance in infrared, and high stability in reducing atmospheres. Possible applications of TiO 2 -based TCOs, as well as the mechanism of the transparent conducting properties found in this d-electron-based TCO, are discussed in this review.

Properties of TiO2‐based transparent conducting oxides

Nb-doped anatase TiO2 [Ti0.94Nb0.06O2 (TNO)] films with high electrical conductivity and transparency were fabricated on nonalkali glass using pulsed laser deposition and subsequent annealing in a H2 atmosphere. The amorphous films as deposited on unheated substrates were found to crystallize, forming polycrystalline films at around 350°C. The films annealed at 500°C showed resistivity down to 4.6×10−4Ωcm at room temperature and optical transmittance of 60%–80% in the visible region, which are comparable to those of epitaxial films. These results indicate that TNO films have the potential to be practical transparent conducting oxides that could replace indium tin oxide.

Fabrication of highly conductive Ti1- xNbxO2 polycrystalline films on glass substrates via crystallization of amorphous phase grown by pulsed laser deposition

The γ-phase of copper(I) iodide (γ-CuI) is a p-type semiconductor with a wide bandgap (Eg ≈ 3.1 eV). Conventionally, γ-CuI thin films have been synthesized by the iodination of Cu thin layers with iodine vapor. However, γ-CuI films fabricated by this method have a rough surface and thus a frosted-glass-like appearance, which make it difficult to apply this material to transparent electronics. In this paper, a simple new method is proposed for the synthesis of truly transparent p-type γ-CuI films. The chemical reaction between Cu3N thin films and solid-phase iodine at 25 °C was found to yield highly transparent polycrystalline γ-CuI films with shiny appearance. The γ-CuI films fabricated by this method had root-mean-square roughness values of 8–12 nm, which are less than one-third of those for γ-CuI films synthesized by the conventional method. As a result, specular transmittance of >75% in the visible region was attained. An as-prepared film had a resistivity (ρ) of 3.1 × 10–2 Ω cm, hole density (nh) of 8...

Truly Transparent p-Type γ-CuI Thin Films with High Hole Mobility

We have investigated electronic band structure of a transparent conducting oxide, Nb-doped anatase TiO2 (TNO), by means of first-principles band calculations and photoemission measurements. The band calculations revealed that Nb 4d orbitals are strongly hybridized with Ti 3d ones to form a d-nature conduction band, without impurity states in the in-gap region, resulting in high carrier density exceeding 1021 cm-3 and excellent optical transparency in the visible region. Furthermore, we confirmed that the results of valence band and core-level photoemission measurements are consistent with prediction by the present band calculations.

https://iopscience.iop.org/article/10.1143/APEX.1.111203/pdf

Naoomi Yamada

Papers

Properties of TiO2‐based transparent conducting oxides

Fabrication of highly conductive Ti1- xNbxO2 polycrystalline films on glass substrates via crystallization of amorphous phase grown by pulsed laser deposition

Truly Transparent p-Type γ-CuI Thin Films with High Hole Mobility

Electronic Band Structure of Transparent Conductor: Nb-Doped Anatase TiO2

Study on the species of heavy metals in MSW incineration fly ash and their leaching behavior