The larger activity of
the WO3−x film compared to that of the TiO2 film under UVA light
is explained in terms of the band gap structure of the two semiconductors.
The energy of the UVA lamp used in the experiments
(3.4 eV) is close to the band gap of anatase TiO2 (3.2 eV), a detriment
to an efficient adsorption of photons, whereas it largely exceeds the
band gap of WO3−x (2.6 eV) [22].