Surface Acoustic Wave Hydrogen Sensors Based on Nanostructured Pd/WO₃ Bilayers.

Dana Miu,Ruxandra Birjega,Cristian Viespe

doi:10.3390/s18113636

Abstract

The effect of nanostructure of PLD (Pulsed Laser Deposition)-deposited Pd/WO3 sensing films on room temperature (RT) hydrogen sensing properties of SAW (Surface Acoustic Wave) sensors was studied. WO3 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 WO3 c-axis nanocolumnar growth, which promotes the diffusion of hydrogen but only once H2 has been dissociated in the nanoporous Pd layer. XRD (X-ray Diffraction) analysis indicates texturing of the WO3 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 WO3 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/WO3 films and of 0.1 Hz/ppm for bilayer films with a nanocolumnar WO3 structure. SAW sensors based on such layers compare favorably with WO3-based hydrogen detectors, which use other sensing methods, and with SAW sensors with dense Pd/WO3 bilayers.

Highlights

The development of sensitive gas sensors is an active research domain due to the serious safety hazard problems implied by its use [1,2,3,4]
In the case of hydrogen sensing, satisfactory results are obtained only when combined with catalytic metals such as Pt [5] or Pd [6,7], which lead to the dissociation of H2 and generates H+ ions and electrons, which diffuse into the WO3 layer
In the case of Pd/WO3 layers, Surface AcousticWaves (SAW) sensor results are better than the single WO3 due to the acoustoelectric interaction added to the mass effect, the results for dense Pd/WO3 bilayers are not satisfactory at temperatures close to room temperature (RT) [10]

Summary

Introduction

The development of sensitive gas sensors is an active research domain due to the serious safety hazard problems implied by its use [1,2,3,4]. In certain conditions, lead to an improved sensor response through increased acoustoelectric effects [11,12]. This is achieved by ensuring that the conductivity of the sensor is in the optimal range and by optimizing film thicknesses [13,14]. In the case of Pd/WO3 layers, SAW sensor results are better than the single WO3 due to the acoustoelectric interaction added to the mass effect (which is small for H2 ), the results for dense Pd/WO3 bilayers are not satisfactory at temperatures close to RT (room temperature) [10]. We have studied the effect of the nanostructure of pulsed-laser deposited Pd/WO3 bilayer sensing films on the H2 sensing properties of SAW sensors. The resulting SAW sensors compare favorably with WO3 -based hydrogen detectors, which use other sensing methods, and with SAW sensors with dense Pd/WO3 bilayers

Materials and Methods

Film Morphology and Structure

SEM images of the surfaces of layer is deposited at

Findings

Discussion and Conclusions