Titanium dioxide (TiO2), as an important semiconductor metal oxide, has been widely investigated in the field of photocatalysis. The properties of TiO2, including its light absorption, charge transport and surface adsorption, are closely related to its defect disorder, which in turn plays a significant role in the photocatalytic performance of TiO2. Among all the defects identified in TiO2, oxygen vacancy is one of the most important and is supposed to be the prevalent defect in many metal oxides, which has been widely investigated both by theoretical calculations and experimental characterizations. Here, we give a short review on the existing strategies for the synthesis of defective TiO2 with oxygen vacancies, and the defect related properties of TiO2 including structural, electronic, optical, dissociative adsorption and reductive properties, which are intimately related to the photocatalytic performance of TiO2. In particular, photocatalytic applications with regard to defective TiO2 are outlined. In addition, we offer some perspectives on the challenge and new direction for future research in this field. We hope that this tutorial minireview would provide some useful contribution to the future design and fabrication of defective semiconductor-based nanomaterials for diverse photocatalytic applications.

Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications

Nitrogen-doped titanium dioxide as visible-light-sensitive photocatalyst: designs, developments, and prospects.

Second-generation TiO2-xDx photocatalysts doped with either anions (N, C, and S mostly) or cations have recently been shown to have their absorption edge red-shifted to lower energies (longer wavelengths), thus enhancing photonic efficiencies of photoassisted surface redox reactions. Some of the studies have proposed that this red-shift is caused by a narrowing of the band gap of pristine TiO2 (e.g., anatase, Ebg = 3.2 eV; absorption edge ca. 387 nm), while others have suggested the appearance of intragap localized states of the dopants. By contrast, a recent study by Kuznetsov and Serpone (J. Phys. Chem. B, in press) has proposed that the commonality in all these doped titanias rests with formation of oxygen vacancies and the advent of color centers (e.g., F, F+, F++, and Ti3+) that absorb the visible light radiation. This article reexamines the various claims and argues that the red-shift of the absorption edge is in fact due to formation of the color centers, and that while band gap narrowing is not an...

Is the Band Gap of Pristine TiO2 Narrowed by Anion- and Cation-Doping of Titanium Dioxide in Second-Generation Photocatalysts?

Titanium dioxide represents an effective photocatalyst for water and air purification and for self-cleaning surfaces. Additionally, it can be used as antibacterial agent because of strong oxidation activity and superhydrophilicity. TiO 2 shows relatively high reactivity and chemical stability under ultraviolet light (λ 400 nm) should allow the main part of the solar spectrum, even under poor illumination of interior lighting, to be used. Visible light-activated TiO 2 could be prepared by metal-ion implantation, reducing of TiO 2 , non-metal doping or sensitizing of TiO 2 with dyes. This paper reviews preparation methods of doped-TiO 2 with metallic and nonmetallic species, including various types of dopants and doping methods currently available. The mechanism of heterogeneous photocatalysis in the presence of TiO 2 is also discussed.

Doped-TiO2: A Review

A visible-light (vis)-active titanium oxide photocatalyst was prepared by a simple wet process: the calcination of the hydrolysis product of Ti(SO 4 ) 2 with ammonia using an ordinary electric furnace in dry air at 400 °C. The color of this photocatalyst was vivid yellow and absorbed light in the blue (400 nm) to bluish-green (550 nm) region exclusively. Its structure was characterized to anatase with oxygen-deficient stoichiometry by XRD and X-ray photoelectron spectroscopy. Nitrogen was also detected, but only in trace amounts. Using blue-light-emitting diodes as a light source, 540 ppm of acetone was decomposed within 36 h and a stoichiometric yield of CO 2 was obtained. From the results of the crystalline size D , it was found that vis-activity could be realized on polycrystalline particles and the grain-boundaries (GBs) are thought to be important, since oxygen vacancies are easily created in GBs, which could form a GB state. Finally, we concluded that oxygen-deficient sites formed in GBs are important to emerge vis-activity, and nitrogen doped in the part of oxygen-deficient sites are important as a blocker for reoxidation.

Visible-light-active titanium oxide photocatalyst realized by an oxygen-deficient structure and by nitrogen doping

In order to realize a visible (vis)-light-active TiO2 photocatalyst, reduction treatment using low-temperature hydrogen plasma with elevated temperatures was applied on anatase TiO2 powder of ST-01 without causing a phase transition to rutile. According to plasma-heat treatment, oxygen was evacuated from ST-01 particles and the colored ST-01, ivory to beige, was obtained. XPS results showed non-stoichiometric, expressed as TiO2−x
} and x was increased with the increase of treating time. Plasma-heat treated ST-01 showed new ESR signal at g = 2.003 assigned to electrons trapped at the oxygen-defect site. This signal was strengthened when vis-light illumination was applied. Results of vis-light activity tests, evaluated by photocatalytic oxidation of benzoic acid in liquid phase and 2-propanol in gas phase using vis-light illumination (>406 nm), showed excellent activity while low ST-01 showed almost no activity on both tests.

Preparation of a visible-light-active TiO2 photocatalyst by RF plasma treatment

The existing nano-indentation method and developmental micro-tensile testing method are more attractive in accurately defining the irradiation effect on the mechanical behavior and adopted in this study to identify the detailed irradiation behavior of pure tungsten. The pure tungsten (A.L.M.T. Corp., Japan) produced by the powder metallurgy and hot-rolling process has a purity of 99.99 wt%. The irradiation was conducted to a maximum of 18.0 dpa by the TIARA (Takasaki Ion Accelerators for Advanced Radiation Application) at 500 and 800 ℃. The micro-tensile specimens with the gauge section of about 0.8 µm (width) × 0.8 µm (thickness) × 10 µm (length) were machined by a focused ion beam device. The nano-indentation results showed the saturation of irradiation hardening and its irradiation temperature dependence. In the case of 500℃, the indentation hardness of pure tungsten irradiated in TIARA increased with increasing dose, and it reached 6.5 GPa after irradiation to 1.0 dpa. Beyond 1.0 dpa, indentation hardness approached a constant. In the case of 800℃, indentation hardness reached 8.1 GPa by irradiation to 3.0 dpa with an irradiation hardening saturation. Moreover, with the stress vs. strain curve by the micro-tensile tests, we found a similar tendency to the nano-indentation results. Additionally, tungsten micro specimens showed a ductile fracture at all conditions in these tests. The total elongation was independent of irradiation dose and irradiation temperature. 

M. Ando

Papers

Preparation of a visible-light-active TiO2 photocatalyst by RF plasma treatment

Mechanical Properties of Self-Ion Irradiated Pure Tungsten using Nano-Indentation Test and Micro-Tensile Test