Applying the principle of water decomposition over photoelectrochemical cells to heterogeneous photocatalytic systems using powdered semiconductors is now a field of great interest, encouraging new fundamental investigations into chemical reactions which take place at the electrode surfaces in the electrochemical cells. In the present review, we have focused on systems, which can convert solar energy into chemical energy by using TiO 2 photocatalysts. The photocatalytic decomposition of water under UV light irradiation has been achieved with systems using various nanoparticle photocatalysts such as TiO 2 . However, recently, visible lightresponsive TiO 2 thin films photocatalysts have been successfully prepared by a radio frequency magnetron sputtering (RF-MS) deposition method. These thin film photocatalysts were found to have enough potential for the separate evolution of H 2 and O 2 from water under solar light. The TiO 2 thin films were prepared on metal substrates by RF-MS deposition and mounted on H-type containers filled with water. This unique system enabled the separate evolution of H 2 and O 2 from water under sunlight irradiation, opening new opportunities for the practical on-site production of pure and clean H 2 from water using abundant and clean sunlight in a safe, environmentally harmonious way.
The effect of chemical etching by HF solution on the photoelectrochemical performance and photocatalytic activity of visible light-responsive TiO 2 (Vis-TiO 2 ) thin films prepared by a radio-frequency magnetron sputtering method has been investigated. It was found that VisTiO 2 thin films treated with HF solution (HF-Vis-TiO 2 ) exhibit a remarkable enhancement of the photoelectrochemical performance not only under UV but also visible light irradiation as compared to untreated Vis-TiO 2 . The incident photon to current conversion efficiencies reached 66 and 9.4% under UV (k = 360 nm) and visible light (k = 420 nm), respectively. The HF-Vis-TiO 2 thin films have a larger surface area and higher donor density than Vis-TiO 2 , indicating that the remarkable increase in the photocurrent may be due to the short diffusion length of the photoformed holes in reaching the solid-liquid interface as well as to the high conductivity. Moreover, the HF-Vis-TiO 2 thin films were found to act as efficient photocatalysts for the decomposition of water with the separate evolution of H 2 and O 2 from H 2 O under visible or sunlight irradiation.
A flame aerosol method has been employed to prepare spherical TiO 2 nano-particle photocatalysts with controlled anatase/rutile phase ratios without calcination at higher temperatures. This method was found to have important advantages since the main factors in achieving high photocatalytic activity such as the particle size, crystallinity and the anatase/rutile phase ratios could be easily controlled. In particular, the incorporation of small amounts of bimetals, such as Fe and Zn, were found to initiate the formation of well-crystalline, small and uniform spherical nano-size particles with a well-defined anatase/rutile phase ratio of around 60/40, similar to P-25 TiO 2 . This suppressed the recombination of the photoformed charge carriers leading to a significant increase in the photocatalytic reactivity of the TiO 2 nano-particles. The incorporation of very small amounts of mono-metals, such as Fe, Cr and Zn (around 1 at.%), within the TiO 2 nano-particles led to a slight increase in the photocatalytic activity of the TiO 2 nano-particle photocatalysts for the complete oxidation of 2-propanol dissolved in water into CO 2 and H 2 O as compared with the unincorporated pure TiO 2 . The incorporation of bimetals of Fe and Zn within TiO 2 (Fe/Zn-TiO 2 ) nanoparticles, on the other hand, led to a remarkable enhancement in the photocatalytic activity as compared with the unincorporated and mono-metal incorporated TiO 2 .
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