## 3 Results and Discussion
### Surface Vibrational Properties by FTIR
The appearance of these features, downshifted with respect to the CO free molecule (2143 cm\({}^{-1}\)), indicates the formation of surface sites characterized by \(\pi\)-backdonation character [56]. They have already been assigned [57] to carbonyl species anchored on a multiplicity of coordinatively unsaturated Mo\({}^{x+}\) centers (x \(<\) 4) on highly defective situations (Mo\(-\)S) [48; 58]. In particular, the band at 2117 cm\({}^{-1}\) can be ascribed to the Mo\({}^{x+}\) species (x \(<\) 4) located on defective sites (e.g., edges), while the 2066 cm\({}^{-1}\) one could be due to the interaction of CO with Mo\({}^{x+}\) species (x \(<\) 4) located on highly coordinatively unsaturated (cus) sites such as corners [41]. Such bands are the last to disappear by outgassing and this is a further confirmation of the presence of isolated and cus molybdenum species on defective sites associated with the formation of sulfur vacancies [45]. FTIR spectra of CO adsorbed at 77 K at maximum coverage on TiO\({}_{2}\) n-sh, MoS\({}_{2}\)/TiO\({}_{2}\) n-sh (via ex situ method), and MoS\({}_{2}\)/TiO\({}_{2}\) n-sh (via in situ method) are compared in Figure 7 (black, blue, and red curves, respectively) aiming to investigate the effects of the different synthesis methods on the surface properties of the two-system MoS\({}_{2}\)/TiO\({}_{2}\) n-sh, as compared to the pure TiO\({}_{2}\)-nsh sample. First, it appears that (i) the spectra of both the MoS\({}_{2}\)/TiO\({}_{2}\) n-sh samples are characterized by a lower intensity of the typical signals at 2179 and 2159 cm\({}^{-1}\) due to the adsorption of CO on the (101) and (001) surfaces, respectively; and (ii) only for the MoS\({}_{2}\)/TiO\({}_{2}\) n-sh sample prepared via the in situ method, the signals in the range 2130-2050 cm\({}^{-1}\), associated with reduced Mo\({}^{x+}\) species (x \(<\) 4), are observed. The general decrease in intensity can be associated with the presence of new Mo\({}^{x+}\) species masking the TI\({}^{4+}\) sites, which are no longer available for interactions with CO. Such a phenomenon is more evident on MoS\({}_{2}\)/TiO\({}_{2}\) n-sh prepared via the in situ approach, due to the higher MoS\({}_{2}\) concentration, than on the sample obtained via the ex situ method. Furthermore, the absence of the components assigned to Mo\({}^{x+}\) species in reduced states (x \(<\) 4) for the sample obtained by the ex situ method can be explained by the fact that the Mo\({}^{x+}\) species are present only on defective sites belonging to thin and uniformly distributed MoS\({}_{2}\) slabs. Indeed, the HRTEM images (Figure 3) have shown thinner and more defective MoS\({}_{2}\) slabs for the sample prepared by the in situ approach. Conversely, preparation via the ex situ approach gives rise to a heterogeneous and minor amount of MoS\({}_{2}\) slabs anchored to the surface of TiO\({}_{2}\) nanosheets, which are characterized by a higher level of stacking and are therefore less reactive. These remarks on different defects are in agreement with what has been observed with Raman (regarding the band at ca. 227 cm\({}^{-1}\)) and XRD analyses.