This is because the CuO content
is produced in proportion to Cu content in Cu-TiO2/GF photocatalysts
(as also explained in the XRD results). CuO acts as an
intermediate agent for the transfer of photo-generated electrons
from the valence band to the conduction band of TiO2, leading
to increased visible light absorption of Cu-TiO2/GF photocatalysts.
Therefore, visible light absorption of Cu-TiO2/GF increased with
increasing Cu content. However, at high Cu content, such as 7.5
and 10% of Cu in Cu-TiO2/GF, Cu strongly defects into the TiO2
lattice resulting in too many oxygen vacancies in the TiO2 lattice.
This was also observed in the XPS analysis. Oxygen vacancies act as