Graphene-like nanoscale gas sensors from material synthesis to applications

A very widespread area of application for nanomaterials is that of gas sensors. Until now, in fact, the measurement of pollutants in the air or in gaseous currents, has always been carried out through the use of traditional techniques, very effective, but long and often complicated. The use of solid-state gas sensors, devices capable of transforming chemical information, such as the concentration of a pollutant, into an electrical signal through the combined action of a receptor and a transducer, is considered the future in the field of environmental monitoring, as it allows simple, immediate measures and involves the use of small, even portable devices. In this work nanomaterials have been used as gas sensors, thus exploiting their properties to significantly modify the electrical resistance following exposure to small concentrations of oxidizing and reducing gases. Material’s evolution has been followed from the synthesis to the final application as gas sensors. In particular Transition Metal Dichalcogenides exfoliation process to produce high quality and high quantity of mono to few layer flakes has been set up, and once the final product has been tested, his performances have been optimized by combination with other materials. The design of the exfoliation process has been developed in light of the possibility of massive production and eventually the scalability of the process itself. Morphological characterization of materials was carried out by means of Electron Microscopy (TEM and SEM), Atomic Force Microscopy, X-ray Diffraction and X-ray Photoelectron Spectroscopy. The electrical characterizations have been carried out by a laboratory equipment. Target gases such as H2 and NO2 with concentrations like those found in pollute environments were investigated. This PhD work has developed through relationships with various research groups including the Departments of Physics and Industrial Engineering of the University of L'Aquila, the Department of Industrial Engineering of the University of Padova and the Department of Chemistry of the University of Sassari. In particular, this research has been aimed to: - Exfoliation of mono to few layered 2D Transitional Metal Dichalcogenides (TMDs) by grinding assisted probe sonication technique and optimization of exfoliation process; - Investigation of the effect of the exfoliation on the surface chemistry of WS2; - The morphological, microstructural and electrical characterization of synthesized sensing materials to define factors influencing gas sensing properties; - Investigation on the gas sensing properties of above-mentioned nanostructured semiconductors to target gases such as H2, NO2, and humidity under simulated conditions. - Synthesis and investigation of gas sensing properties of traditional semiconductor Metal Oxide Sensors (MOS). The main achievements earned during this PhD program have been: - Design of a reproducible and controllable exfoliation procedure to fabricate pristine mono and few layered WS2, by means of grinding assisted probe sonication yielding suspension of flake of average lateral size of 100 nm and thickness of 6 nm (less than 10 layers per flake) - Control of the surface oxidation and thermal stability of exfoliated WS2 flakes - Application of the exfoliated material to produce gas sensors, with complete understanding of the role of WS2/WO3, able to detect NO2 at concentration of 200 ppb and H2 5 ppm. - Improvement of of the gas sensing performances of WS2 by the combination of graphene oxide to enhance conductivity and improvement of the response/recovery of the sensor by using light irradiation - Improvement of of the gas sensing performances of traditional metal oxide sensors by adding different doping elements during synthesis.