We report on the fabrication, the morphological, structural, andnchemical characterization, and the study of the electrical response tonNO2 and other gases of resistive type gas sensors based on liquidnchemically exfoliated (in N-methyl pyrrolidone, NMP) MoS2 flakesnannealed in air either at 150 degrees C or at 250 degrees C. The activenmaterial has been analyzed by scanning electron microscopy (SEM), andnmicro Raman and X-ray core level photoemission spectroscopies. SEM showsnthat MoS2 exfoliated flakes are interconnected between electrodes of thensensing device to form percolation paths. Raman spectroscopy of thenflakes before annealing demonstrates that the flakes are constituted byncrystalline MoS2, while, annealing at 250 degrees C, does not introducena detectable bulk contamination in the expected form of MoO3. The sensornobtained by thermal annealing in air at 150 degrees C exhibits anpeculiar p-type response under exposure to NO2. In line with core levelnspectroscopy evidences, this behavior is potentially ascribed tonnitrogen substitutional doping of S vacancies in the MoS2 surfacen(nitrogen atoms being likely provided by the intercalated NMP). Thermalnannealing the MoS2 flakes in air at 250 degrees C irreversibly sets annn-type behavior of the gas sensing device, with a NO2 detection limit ofn20 ppb. This behavior is assigned, in line with core level spectroscopyndata, to a significant presence of S vacancies in the MoS2 annealednflakes and to the surface co-existence of MoO3 arising from the partialnoxidation of the flakes surface. Both p-and n-type sensors have beenndemonstrated to be sensitive also to relative humidity. The n-typensensor shows good electrical response under H-2 exposure. (C) 2014nElsevier B.V. All rights reserved.