## 3 Results and Discussion
### Surface Vibrational Properties by FTIR  
FTIR spectra of CO adsorbed at liquid nitrogen temperature, at decreasing coverages up to the residual pressure of 4 \(\times\) 10\({}^{-4}\) Torr, on the surface of TiO\({}_{2}\) n-sh (pretreated at 873 K), of MoS\({}_{2}\)/TiO\({}_{2}\) n-sh (36 mL MoS\({}_{2}\)) (pretreated at 673 K), and of MoS\({}_{2}\)/TiO\({}_{2}\) n-sh (Mo 3% wt/wt) (pretreated at 673 K) are shown in Figure 6. The main feature at 2159 cm\({}^{-1}\) on the TiO\({}_{2}\) n-sh sample (Figure 6a) is due to CO molecules adsorbed on the Ti\({}^{4+}\) sites located on (1 \(\times\) 4) reconstructed (001) surfaces, together with a minor feature observed at 2179 cm\({}^{-1}\), which has been assigned to CO molecules adsorbed on (101) surfaces, less extensive than (001), according to the data and models reported in the literature [35]. Moreover, a weak shoulder at ca. 2155 cm\({}^{-1}\), which is hidden within the envelope of the band on the low frequency side, due to CO species interacting with the residual surface OH groups, can be detected. Notice that the band at 2159 cm\({}^{-1}\) is fully reversible upon CO outgassing, whereas the peak at \(\sim\)2179 cm\({}^{-1}\) remains almost unaffected at the temperature of the experiment. Furthermore, the maximum of the 2159 cm\({}^{-1}\) band undergoes a quite negligible upward frequency shift upon decreasing CO pressure, approaching the singleton v(CO), plausibly due to the fading away of weak lateral interactions (dynamic and static in type) within the CO adlayer. The low upshift of the singleton mode with respect to the CO stretching mode in the gas phase (2143 cm\({}^{-1}\)) together with the complete reversibility upon CO outgassing are explained with the low electrophilicity/reduced acidity of the Ti\({}^{4+}\) sites on (001) surfaces, due to a screened electrostatic potential at these Ti sites, which are strongly bound to two oxygens. This agrees with the weakness of their interaction with CO [19]. Surprisingly, moving to the spectra of CO adsorbed on the surface of MoS\({}_{2}\)/TiO\({}_{2}\) nanosheets obtained by ex situ approach (Figure 6b), no vibrational modes associated with molybdenum species are observed. The absorption bands are mainly due to the CO interaction with the support, which can be explained with the very low concentration of Mo\({}^{\times+}\) species. However, a general decrease in intensity of the main IR features compared to pure TiO\({}_{2}\) n-sh is noted. It is plausible to assume that a few MoS\({}_{2}\) slabs cover small surface regions on both [001] and [101] faces. This causes a lower number of available Ti\({}^{4+}\) sites to interact with CO probe molecules, hence a decreased intensity of the adsorption signals on (101) and (001) surfaces. Finally, from the CO spectra on MoS\({}_{2}\)/TiO\({}_{2}\) nanosheets (Mo 3 wt.%) obtained by the in situ approach (Figure 6c), significant changes can be ob  
Figure 6: FTIR spectra of CO (30 Torr) adsorbed at 77 K at decreasing coverage on the surface of: (**a**) TiO\({}_{2}\) anatase nanosheet (n-sh) pretreated at 873K, (**b**) MoS\({}_{2}\)/TiO\({}_{2}\) n-sh obtained by the ex situ method, and (**c**) MoS\({}_{2}\)/TiO\({}_{2}\) n-sh obtained by the in situ method. served. At higher frequencies, the features at 2179 and 2159 cm\({}^{-1}\), already assigned on previous systems to CO molecules adsorbed on TI\({}^{4+}\) sites on (101) and (001) surfaces, respectively, are also observed, but with a different intensity ratio. Considering the asymmetry on the low frequency side of the band centered at 2159 cm\({}^{-1}\), modes due to CO physisorption on heterogeneous surface sites, including residual OH groups and/or sulfur anions on the basal planes of crystalline MoS\({}_{2}\) cannot be ruled out [55]. However, in the 2130-2050 cm\({}^{-1}\) range, two significant bands, at 2117 and 2066 cm\({}^{-1}\), are observed which are absent on the MoS\({}_{2}\)/TiO\({}_{2}\) n-sh sample obtained by the ex situ method (Figure 6b).