## 4 Discussion  
From the DFT analysis, it is assumed that the creation of a single vacancy by Cu in Cu\({}^{2+}\) (CuO) state only exists on the surface which does not contributes to the alteration of the band gap of the material at a very small mole percentage. On the other hand, dual vacancy created by the Cu dopant (Cu\({}^{+}\) state) does change the band gap of the material, as well as introducing various trap sites in between. This reduces the overall band gap energy, but the trap sites potentially serve as recombination centers. XPS analysis showed the co-existence of Cu in +1 (Cu\({}_{2}\)O) and +2 (CuO) oxidation states, where Cu\({}^{+}\) is the major species in the sample which necessarily committed in the variation of optical property of the sample which is also evident in the UV-vis absorption plot (Figure 8a). A possible photocatalytic disinfection mechanism is proposed based on the DFT, XPS and UV-vis results (Figure 12).  
On light illumination, electron-hole pairs are generated on the individual surfaces of TiO\({}_{2}\) and Cu\({}_{2}\)O. The photo-excited charge carriers have the ability to recombine or participate in the photocatalysis reaction. TiO\({}_{2}\) (a n-type semiconductor) has the conduction and valence band edge values at 3.1 eV and \(-\)0.1 eV, respectively [25], while the Cu\({}_{2}\)O (a p-type semiconductor) has the conduction and valence band edge potential at 1.91 eV and \(-\)0.26 eV, respectively [82]. Under light irradiation, both semiconductor surfaces are in contact, drives the electron transfer process (Figure 12a). As the electron from conduction band of Cu\({}_{2}\)O moves to that of TiO\({}_{2}\) and the hole migrates from the valence band of TiO\({}_{2}\) to that of Cu\({}_{2}\)O [83]. This results in the lowering of the Fermi level of Cu\({}_{2}\)O and rising of the TiO\({}_{2}\), such that to reach a pseudo equilibrium phase resulting in formation of internal electric field. Furthermore, this external field prevents the flow of charge carriers from the opposite sites and thus makes these charge carriers participate in the photocatalytic reaction. Apart from the p-n junction formed by the Cu\({}_{2}\)O and anatase TiO\({}_{2}\), there exists a considerable proportion of rutile phase in our samples. This anatase and rutile mixture forms a possible type 2 heterojunction which also  
Figure 11: Images of (**a**) _E. coli_ colonies and (**b**) _S. aureus_ colonies on agar plates after various time intervals of Cu-TiO\({}_{2}\) 650 \({}^{\circ}\)C.  
necessarily contributes to the improved catalytic activity (Figure 12b) [84]. The enhanced antimicrobial activity could also be attributed to the well-known bactericide activity of Cu ions in the sample [85].  
The charge carriers formed on various heterojunction sites (type 2 junction of anatase-rutile and p-n junction of Cu\({}_{2}\)O-anatase TiO\({}_{2}\)) reacts with the oxygen and water adsorbed on the catalyst surface to produce superoxide radicals and hydroxyl radicals, respectively. These reactive oxygen species (ROS) further interact with the bacterial cell wall which results in membrane disruption and outflow of intracellular materials; this results in cell lysis (Figure 12). In addition to this, the ROS produced has the ability to interact with the sugar phosphate groups present in the DNA of the bacteria to cause gene alteration. Furthermore, altering the protein expression responsible for cellular functioning leads to additional cell damage [11; 86].