The Cu+ ions 4 s0 orbitals correspond to the conduction band lower energy levels
and charge transfer can occur from the hybridized orbitals forming the
valence band to the empty Cu states at the conduction band. The optical
direct band gap value of ~2.4 eV, which was determined following
Tauc's equation [41], is higher than the band gap (~2 eV) measured in
Cu2O films grown by electrodeposition technique [42].