TiO2 is known to be a kind of photoactive material, capable of exhibiting several interesting properties
such as self-cleaning, and pollution abatement. These were operated via a photocatalysis mechanism.
However, due to the considerably wide band gap energy of TiO2 (3.2 eV), the photocatalytic
activity of TiO2 is limited. It is excited by UV light, which accounts for 4–6% of the solar spectrum.
In this regard, for some applications requiring the degradation of organic compound under visible light
irradiation and/or even in the dark, the catalytic activity of TiO2 has yet to be further enhanced. Efforts
have been made to develop efficient visible light-activated photocatalysts. One possible strategy was
to reduce the band gap energy value of TiO2 by doping it with either metals or non-metallic materials.
Work by Chiarakorn et al. [1], for example, showed that a nanocomposite film made of Ag doped TiO2
incorporated into as-synthesized MCM-41 (with an Ag/Ti/Si ratio of 0.1/1/2) exhibited the highest
photocatalytic decolorization of methylene blue, with an efficiency of 81% under UV, and 30% under
visible light irradiation. Apart from this, the uses of other metals as a dopant for TiO2 have also been
reported, including noble metals such as Pt [2], Au [3] and Cu [4]. Ngaotrakanwiwat et al. also demonstrated
that it was possible to induce charge–discharge behavior and catalytic activity of metal oxides
in the dark by coupling TiO2 with some transition metal oxides such as WO3 [5] and V2O5. Recent
work by Ngaotrakanwiwat and Meeyoo [6] also showed that by using TiO2 in combination with TiO2–
V2O5 compound, with a Ti:V molar ratio of 1:0.11, and calcined at 550 C, superior energy storage ability
of the system was obtained. The TiO2/(TiO2–V2O5) compound exhibited about 1.7-times higher
capacity and 2.3-times higher initial charging rate compared to WO3. The above effects could be
explained in the light of the lower band gap energy and the energy storage ability of the transition
metal oxides, i.e. the material is capable of self-discharging in the absence of light. In this regard, photocatalytic
activity of the hybrid metal oxides in the dark might also be expected. Unfortunately, to the
best of our knowledge, a study on the catalytic activity of the hybrid metal oxide (TiO2/TiO2–V2O5) has
not been carried out and reported in any open literature.