**4. Conclusions**

In conclusion, we have demonstrated an e ffective, reproducible and high-yield exfoliation process, obtained by enhanced low energy ball milling and sonication. Specifically, the two-step exfoliation followed by drop casting the centrifuged suspension leads to the deposition of thin films of well-packed and interconnected WS2 flakes with controlled and reproducible microstructure over large areas, thus representing a fast, simple and scalable method, compatible with standard microelectronic fabrication techniques. We found that a spontaneous oxidation of WS2 leading to the formation of amorphous WO3 on the surface of the exfoliated WS2 takes place, addressing the crucial drawback of surface oxidation of TMDs. We also found that by pre-annealing the WS2 films at 180 ◦C, a reproducible surface oxidation of WS2 to amorphous WO3 takes place, which stabilize from further oxidation the WS2 layers. Reproducible gas sensing responses to NO2 and H2 and humidity at 150 ◦C operating temperature were achieved with detection limits of 200 ppb and 5 ppm to NO2 and H2, respectively. The cross-sensitivity test highlighted a weak interference played by NO2 to the H2 gas response. Water vapor at 40% RH also resulted in having no interference to the measure of NO2 and H2 gases, attesting promising characteristics of WS2 exfoliated films for gas sensing applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/10/1363/s1, Figure S1: Schematic illustration of the exfoliation process, Figure S2: Schematic illustration of the gas sensing equipment, Figure S3: XPS spectra of S 2p core level, Figure S4: Grazing incidence XRD spectra of the as-exfoliated WS2; as-exfoliated WS2—200 ◦C annealed for 1 h, Figure S5: The electrical response of WS2 post-annealed at 180 ◦C at di fferent operating temperatures and 800 ppb NO2 in dry air, Figure S6: Selectivity response of WS2 post-annealed at 180 ◦C at 150 ◦C operating temperature.

**Author Contributions:** Conceptualization, L.O., C.C., V.P. and S.M.E.; methodology, L.O. and C.C.; validation, L.O. and C.C.; formal analysis, V.P.; investigation, V.P. and S.M.E.; resources, C.C., L.O. and M.N.; data curation, V.P. and S.M.E.; Supervision, L.O. and C.C.; writing—original draft preparation, V.P., S.M.E. and C.C.; writing—review and editing, C.C., L.O. and V.P.

**Funding:** This research was funded by REGIONE ABRUZZO Dipartimento Sviluppo Economico, Politiche del Lavoro, Istruzione, Ricerca e Università Servizio Ricerca e Innovazione Industriale for financial support through progetto POR FESR Abruzzo 2018–2020 Azione 1.1.1 e 1.1.4—"Studio di soluzioni innovative di prodotto e di processo basate sull'utilizzo industriale dei materiali avanzati" CUP n. C17H18000100007.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.
