Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Scientific studies on photocatalysis started about two and a half decades ago. Titanium dioxide (TiO 2 ), which is one of the most basic materials in our daily life, has emerged as an excellent photocatalyst material for environmental purification. In this review, current progress in the area of TiO 2 photocatalysis, mainly photocatalytic air purification, sterilization and cancer therapy are discussed together with some fundamental aspects. A novel photoinduced superhydrophilic phenomenon involving TiO 2 and its applications are presented.

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Titanium dioxide photocatalysis

Abstract Scientific studies on photocatalysis started about two and a half decades ago. Titanium dioxide (TiO 2 ), which is one of the most basic materials in our daily life, has emerged as an excellent photocatalyst material for environmental purification. In this review, current progress in the area of TiO 2 photocatalysis, mainly photocatalytic air purification, sterilization and cancer therapy are discussed together with some fundamental aspects. A novel photoinduced superhydrophilic phenomenon involving TiO 2 and its applications are presented.

Titanium dioxide as photocatalysis

TiO2 photocatalysis and related surface phenomena

Photoinduced reactivity of titanium dioxide

Photochromic materials, which change their colours reversibly in response to light, can be applied to smart windows, displays and memories1,2,3. Conventional photochromic materials respond in a monochromatic way, so that multicolour photochromism has required several different materials or filters combined appropriately. If multicolour photochromism could be achieved with a simple material, photochromic devices would be find a greater number of applications, including a rewritable colour copy paper or electronic paper4 and a high-density multi-wavelength optical memory. Here we report multicolour photochromism of TiO2 films loaded with silver nanoparticles by photocatalytic means. Its colour, initially brownish-grey, changes under monochromatic visible light to almost the same colour as that of the light; the apparently uniform Ag–TiO2 film can be almost any colour. Behaviour similar to persistent hole-burning effects5,6 is also observed. The colour reverts to brownish-grey under ultraviolet light, and these processes are repeatable.

Multicolour photochromism of TiO2 films loaded with silver nanoparticles.

The photocatalytic decomposition efficiency of gaseous 2-propanol was studied using a titanium dioxide thin film under very weak UV light; the incident UV light intensity was 36 nW−45 μW·cm-2. Under such low-intensity UV illumination, the value of the quantum yield (QY) increased gradually with decreasing number of absorbed photons and finally saturated (28%) for a number of absorbed photons less than 4 × 1011 quanta·cm-2·s-1 for an initial 2-propanol concentration of 1000 ppmv. Thus, purely light-limited conditions were reached. For lower initial concentrations, the QY values decreased, but the same maximum QY value as that for 1000 ppmv was also approached with decreasing light intensity. We discuss these results in terms of the normalized absorbed photon number (Inorm/s-1), a parameter that we have defined as the ratio of the number of absorbed photons ([photon]ab) to the number of adsorbed 2-propanol molecules ([M]ad). When all of the experimental QY values were plotted as a function of Inorm, all of ...

Kinetics of photocatalytic reactions under extremely low-intensity uv illumination on titanium dioxide thin films

The photocatalytic degradation of bisphenol A (BPA), a representative endocrine disruptor, was carried out in TiO2 aqueous suspension. The main purposes were to confirm the total mineralization of BPA and to evaluate the estrogenic activity in the treated water during the photocatalytic reaction. An initial BPA concentration of 175 microM in water was totally degraded to carbon dioxide by TiO2-photocatalyzed reactions under UV irradiation of 10 mW cm-2 for 20 h. Four HPLC peaks indicating intermediate products appeared in chromatograms monitored at 275 nm, but the heights relative to that of the initial BPA were very low, at most 0.04 in the time period 5-10 h after the start of UV irradiation. All of the peaks finally disappeared after 20 h. For the treated water, the transcriptional estrogenic activity in response to human estrogen receptor in a yeast hybrid assay decreased drastically to less than 1% of the initial BPA's activity within 4 h. On the basis of these results, we conclude that TiO2 photocatalysis could be a useful technology for the purification of water containing BPA without generating any serious secondary pollution.

Degradation of bisphenol A in water by TiO2 photocatalyst

Ag−TiO2 films exhibiting multicolor photochromism were prepared by photoelectrochemical reduction of Ag+ to Ag nanoparticles in nanoporous TiO2 films under UV light. Color of the Ag−TiO2 film, initially brownish-gray, changes under a colored visible light to the color of the light and reverts to brownish-gray under UV light. Their chromogenic properties were improved by simultaneous irradiation for Ag deposition with UV and blue lights to suppress the formation of anisotropic Ag particles. Nonvolatilization of a color image was also achieved by removing Ag+ that was generated during the irradiation with a colored light. Once nonvolatilized, the image can be reproduced by UV light, even after the image is discolored under white light. This new effect evidenced that nanopores in the TiO2 film determine the resonance wavelengths of the Ag particles, as their molds. In addition, solvatochromic behavior of the Ag−TiO2 film proved that nanospaces left around the Ag nanoparticles affect the resonance wavelengths...

TiO2 films loaded with silver nanoparticles: control of multicolor photochromic behavior.

TiO2 coatings are known to protect some metals, including type 304 stainless steel, from corrosion on the basis of its reductive energy generated under UV irradiation. A TiO2 coating is coupled with a WO3 coating as an electron pool, in which the reductive energy can be stored. A WO3 film on a type 304 stainless steel plate can be charged by a UV-irradiated TiO2 coating on the same plate, in a 3 wt % NaCl aqueous solution, pH 5. The charged WO3 coating can protect the stainless steel plate from the corrosion for a while even after the UV light is turned off. Thus, the TiO2 coating protects the plate and charges the WO3 coating during the day, and the charged WO3 coating protects the plate during the night. The charge−discharge cycles are repeatable. A TiO2−WO3 composite coating also has the same effects.

Yoshihisa Ohko

Papers

Multicolour photochromism of TiO2 films loaded with silver nanoparticles.

Kinetics of photocatalytic reactions under extremely low-intensity uv illumination on titanium dioxide thin films

Degradation of bisphenol A in water by TiO2 photocatalyst

TiO2 films loaded with silver nanoparticles: control of multicolor photochromic behavior.

TiO2−WO3 Photoelectrochemical Anticorrosion System with an Energy Storage Ability