Sensing mechanism of hydrogen sensors based on palladium-loaded tungsten oxide (Pd–WO3)

This paper presents a study on hydrogen sensing mechanism of Pd-loaded tungsten oxide (Pd–WO3). WO3 nanoparticles (sphere-like) were prepared and characterized by transmission electron microscopy (TEM), scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. The synthesized WO3 nanoparticles was loaded with palladium (Pd) catalyst at 1 at% and annealed in air at 400 °C. The chemical composition of the elements and their spatial distribution on the surface were determined by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) respectively. Pd–WO3 thick film was prepared by screen-printing on alumina substrates fitted with gold electrodes and a platinum heater. The responses to 25–200 ppm hydrogen (diluted in air) were measured at operating temperatures ranging from 200 to 280 °C in dry air and with 50% relative humidity. The changes in the optical properties (on the surface) and crystal phases (in bulk) of Pd–WO3 in contact with hydrogen were investigated by UV–vis–NIR and XRD respectively. The in situ characterizations were carried out in different conditions, 3% H2 diluted in synthetic air/nitrogen at temperatures ranging from 25 to 200 °C. The changes in the crystal phases, formation of hydrogen tungsten bronze, on the surface or bulk were correlated with the electrical responses. A sensing mechanism of Pd–WO3 for H2 is proposed and discussed.