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.