Large-scale emission of atmospheric carbon dioxide
has now become a global environmental issue due to
the greenhouse effect. Many researchers have demonstrated
that CO2, in the presence of water, can be photocatalytically
converted to methane and/or methanol
over TiO2 and copper doped TiO2 [1–3] and other metal
oxide catalyst [4]. It is likely that the efficiency and selectivity
of the product depend on the type of catalyst. The
bandgap of TiO2 (3.0 and 3.2 eV for rutile and anatase,
respectively), make it perfect to be activated by
near ultraviolet photon, though it is not the main fraction
found in the solar ray.
It is therefore assumed we might be able to manage a
photocatalytic process to produce useful organic compounds,
such as methanol as a renewable fuel, from
CO2 with only input of cheap photon source at ambient
temperature. In principle, such an idea is viable but for
practical concern several problems need to be resolved
first. The most crucial problem is a low quantum yield
in the photo catalysis process due to electron and positive
hole recombination, hence producing only limited
reduction species on catalyst surface.
In order to solve low quantum yield and selectivity
problems many researchers had modified TiO2 by doping
it with metal impurities. Many transition metal ions
have been demonstrated to enhance the rate of photocatalytic
oxidation or reduction by changing the dynamics
of electron–hole recombination and interfacial
charge transfer [5]. In CO2 photoreduction, Yamashita
et al. [2] reported that addition of copper (II) into the
TiO2 matrix could improve the efficiency and selectivity
of producing methanol. It was suggested that copper at
the first oxidation species (CuI) may play a significant
role in the formation of methanol. Tseng et al. [1] also
observed that the formation of methanol was found to