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.