## 3 Results and Discussion
### Optical Properties by UV-vis Spectroscopy  
Due to the close relation between the optical properties of MoS\({}_{2}\)/TiO\({}_{2}\) nanosheets and their morphology/structure, UV-Vis-NIR spectra can give detailed information on the nature and properties of both the sulfided phase and the support, as well as on their electronic structure. In Figure 5, the UV-vis spectra of TiO\({}_{2}\) anatase nanosheets, MoS\({}_{2}\)/TiO\({}_{2}\) obtained by the in situ approach, and MoS\({}_{2}\)/TiO\({}_{2}\) obtained by the ex situ approach are compared to those of the MoS\({}_{2}\) bulk, used as a reference. All the spectra were recorded in the diffuse reflectance mode and converted to equivalent absorption Kubelka-Munk units. The curve of the TiO\({}_{2}\) nanosheets (black line) shows the typical absorption edge of TiO\({}_{2}\)-based systems, due to the transition from O\({}^{2-}\) antibonding orbital to the Ti\({}^{4+}\) lowest energy orbital [19]. On the basis of the literature data [51] and other previous results [40], it appears that the spectral features of the MoS\({}_{2}\) reference (grey line) can be assigned as follows: (i) a first absorption threshold at about 700 nm associated to a direct transition at the K point [52; 53]; (ii) two sharp peaks at 680 nm and at 622 nm, on the high energy side of the 700 nm threshold, assigned to A and B excitonic transitions, respectively, whose separation energy can be explained with spin-orbit splitting at the top of the valence band at the K point [54]; (iii) a second threshold at about 500 nm, due to a direct transition from the deep in the valence band to the conduction band; (iv) another two excitonic transitions at 482 nm (C), and at 399 nm (D), also associated with the 500 nm threshold transition [51]; and (v) a third threshold at about 350 nm due to transitions from deep in the valence band [52]. The curve of the MoS\({}_{2}\)/TiO\({}_{2}\) hybrid system obtained by the in situ approach (red line) shows a continuum and wide absorption over the UV-visible range, due to the presence above the valence band, of new electronic states arising from sulfur-oxygen exchange reactions at the surface of TiO\({}_{2}\) nanosheets. These states result from the mixing of Sulphur 3p atomic orbitals with the TiO\({}_{2}\) valence band [19]. Notice that reduced molybdenum species on TiO\({}_{2}\) defective nanosheets have to be considered, although hidden inside the wide and intense absorption. This could be further proof of the formation of thin MoS\({}_{2}\) platelets [40; 48]. The typical MoS\({}_{2}\) excitonic bands A and B are shifted to higher energies as compared to those of MoS\({}_{2}\) bulk, meaning a quantum confinement of the excitons, due to the low dimension of the MoS\({}_{2}\) particles, which is in agreement with the HRTEM and XRD results. The features in the 400-500 nm range, already explained with the C and D excitons of MoS\({}_{2}\), overlap inside a broad band, as expected for a highly dispersed supported material. Similar features are present in the MoS\({}_{2}\)/TiO\({}_{2}\) hybrid obtained through the ex situ approach (blue line), although slightly downward shifted, as compared to the in situ sample. Considering both the systems in more detail, obtained by in situ and ex situ approaches (red and blue lines, respectively), if compared to those of the reference MoS\({}_{2}\) bulk, it can be observed that the energy values of A and B excitons are only slightly upward shifted, whereas relevant shifts are observed for the C and D excitons envelope, because quantum size effects more significantly affect the C and D bands [52]. Notice that quantum size effects have been related to the low dimensionality of the MoS\({}_{2}\) particles along the c direction, which is due to a low number of layers, but the small dimension of the MoS\({}_{2}\) platelets along the \"in-plane\" a and b directions can also play a role [54].