Sol–Gel Pt-VO<sub>2</sub> Films as Selective\nChemoresistive and Optical H<sub>2</sub> Gas Sensors

In this work, VO<sub>2</sub> (M1/R) thin films were exploited\nas\nH<sub>2</sub> gas sensors. A flat film morphology, obtained by furnace\nannealing, was compared with a laser-induced nanostructured one. The\ncombination of the environmentally friendly sol–gel approach\nwith the ultrafast laser crystallization allows for significant reductions\nin energy consumption and related emissions during the fabrication\nof VO<sub>2</sub> sensors. By decorating the sensors’ surface\nwith Pt nanoparticles (NPs), the sensor response was enhanced exploiting\nthe hydrogen spillover effect. The Pt/VO<sub>2</sub> sensors, tested\nat operating temperatures between 20 and 200 °C and for concentration\nof H<sub>2</sub> from few ppm to 50000 ppm, offered a dual chemoresistive\nand optical sensing mode. Low operating temperatures of 150 °C\nwere achieved, along with a detection limit as low as 2 ppm and a\nperfect baseline recovery. Both sensors guaranteed specific selectivity\ntoward H<sub>2</sub>, without response to NO<sub>2</sub> or humidity,\nand long-term stability over 500 h. The H<sub>2</sub> sensing mechanism,\nfor both the monoclinic and rutile VO<sub>2</sub> phases, was investigated\nthrough in operando X-ray Diffraction and in situ X-ray Photoelectron\nSpectroscopy tests. The interaction was found to be based on the reversible\nformation of H<sub><i>x</i></sub>VO<sub>2</sub> bronze,\nalong with the reversible variations in the oxidation state of V.