## 1 Introduction
Considering the favorable optical properties of MoS\({}_{2}\)--that is, excitonic absorption bands in the visible spectra together with the occurring of efficient charge transfer phenomena between MoS\({}_{2}\) nanoparticles and TiO\({}_{2}\) nanosheets [26]--recently, highly dispersed MoS\({}_{2}\) nanoparticles have been prepared through different methods, including ex situ and in situ approaches. It has been reported that, from ex situ synthesis at low cost and the scalable production of highly heterogeneous MoS\({}_{2}\)/TiO\({}_{2}\)-based composites, weak interface interactions are obtained [5; 19]. On the contrary, composites with strong interface contacts can be achieved, although at a low yield, from in situ strategies [5]. It is known that from in situ methods, atomic scale assembling and then growth of well-defined MoS\({}_{2}\) slabs can be obtained, thus providing effective chemical interactions at the interface. Concerning the in situ strategies, chemical solution methods are mainly used to prepare MoS\({}_{2}\)/TiO\({}_{2}\)-based composites, where MoS\({}_{2}\) nanoplatelets grow on TiO\({}_{2}\) basal planes [5]. As a result, 2D-2D MoS\({}_{2}\)/TiO\({}_{2}\) nanosheets exhibit high activity due to the large contact area, although it was reported that heterostructures coming from solution chemical methods show a low amount of exposed active edge sites [5]. Notwithstanding the great interest in MoS\({}_{2}\)/TiO\({}_{2}\) systems, there is a limited number of studies on TiO\({}_{2}\) nanosheets with exposed {001} facets decorated with MoS\({}_{2}\) slabs obtained from either ex situ or in situ approaches. Among these studies, different structures including 2D nanojunctions [8; 27], MoS\({}_{2}\) nanoparticles or nanosheets decorating TiO\({}_{2}\) nanosheets [28; 29; 25; 30], and MoS\({}_{2}\) quantum dots (QDs) at the surface of TiO\({}_{2}\) sheets were found very effective for enhanced photocatalytic H\({}_{2}\) production and dyes photodegradation. Furthermore, it is known that in MoS\({}_{2}\)/TiO\({}_{2}\) hybrid nanosheets' interfacial charge transfer mechanisms [31], together with improved separation efficiency of photogenerated e\({}^{-}\)h\({}^{+}\) pairs caused by the presence of MoS\({}_{2}\) as cocatalysts [25; 30], have a role in increasing the photocatalytic activity. According to some authors, TiO\({}_{2}\) nanosheets are preliminarily prepared by using a fluorine-based morphology controlling agent, which gives rise to the formation of Ti-F bonds on the exposed TiO\({}_{2}\)[001] facets [32; 33; 34]. The surface fluorine species, due to their instability during hydrothermal treatment, tend to be replaced by hydroxyl groups, which in turn absorb Mo precursors, such as MoO4\({}^{2-}\) and MoS4\({}^{2-}\) anions, which are then reduced in situ by the S source to form 2D-2D MoS2/TiO2 nanosheets [5; 8; 35]. Based on these published results and on the lack of comparison between such layered systems prepared by in situ and ex situ approaches, in the present work, 2D-2D MoS2/TiO2 hierarchical nanostructures have been obtained either via an ex situ approach, which implies a preliminary exfoliation/fragmentation under solvent assisted ultrasonication of bulk MoS2, followed by subsequent wet impregnation of TiO2 nanosheets and via an in situ approach, from a molybdenum oxide precursor in a sulfiding atmosphere (H2S), which gives rise to the formation of highly dispersed and strongly anchored MoS2 slabs on TiO2 nanosheets. In this paper, the morphology, structure, vibrational, and optical properties of the samples obtained from the ex situ and in situ strategies are compared. Particular attention was paid to the different stacking degrees, the size distribution and dispersion of the MoS2 nanosheets, decorating the TiO2 supports, together with investigation of the surface sites on the main exposed faces, by means of adsorption of a suitable probe molecule. In this regard, we have used for the first time CO molecules for probing the surface of MoS2/TiO2 combined heterostructures.