Sol–Gel Pt-VO2 Films as Selective Chemoresistive and Optical H2 Gas Sensors

In this work, VO₂ (M1/R) thin films were exploited as H₂ gas sensors. A flat film morphology, obtained by furnace annealing, was compared with a laser-induced nanostructured one. The combination of the environmentally friendly sol-gel approach with the ultrafast laser crystallization allows for significant reductions in energy consumption and related emissions during the fabrication of VO₂ sensors. By decorating the sensors' surface with Pt nanoparticles (NPs), the sensor response was enhanced exploiting the hydrogen spillover effect. The Pt/VO₂ sensors, tested at operating temperatures between 20 and 200 °C and for concentration of H₂ from few ppm to 50000 ppm, offered a dual chemoresistive and optical sensing mode. Low operating temperatures of 150 °C were achieved, along with a detection limit as low as 2 ppm and a perfect baseline recovery. Both sensors guaranteed specific selectivity toward H₂, without response to NO₂ or humidity, and long-term stability over 500 h. The H₂ sensing mechanism, for both the monoclinic and rutile VO₂ phases, was investigated through in operando X-ray Diffraction and in situ X-ray Photoelectron Spectroscopy tests. The interaction was found to be based on the reversible formation of H_xVO₂ bronze, along with the reversible variations in the oxidation state of V.