Surface Acoustic Wave Hydrogen Sensors Based on Nanostructured Pd/WO(3) Bilayers

The effect of nanostructure of PLD (Pulsed Laser Deposition)-deposited Pd/WO(3) sensing films on room temperature (RT) hydrogen sensing properties of SAW (Surface Acoustic Wave) sensors was studied. WO(3) thin films with different morphologies and crystalline structures were obtained for different substrate temperatures and oxygen deposition pressures. Nanoporous films are obtained at high deposition pressures regardless of the substrate temperature. At lower pressures, high temperatures lead to WO(3) c-axis nanocolumnar growth, which promotes the diffusion of hydrogen but only once H(2) has been dissociated in the nanoporous Pd layer. XRD (X-ray Diffraction) analysis indicates texturing of the WO(3) layer not only in the case of columnar growth but for other deposition conditions as well. However, it is only the predominantly c-axis growth that influences film sensing properties. Bilayers consisting of nanoporous Pd layers deposited on top of such WO(3) layers lead to good sensing results at RT. RT sensitivities of 0.12–0.13 Hz/ppm to hydrogen are attained for nanoporous bilayer Pd/WO(3) films and of 0.1 Hz/ppm for bilayer films with a nanocolumnar WO(3) structure. SAW sensors based on such layers compare favorably with WO(3)-based hydrogen detectors, which use other sensing methods, and with SAW sensors with dense Pd/WO(3) bilayers.