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

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.

Development of Pd/TiO2 Porous Layers by Pulsed Laser Deposition for Surface Acoustic Wave H2 Gas Sensor

“…The SAW sensors used are based on quartz substrates, with ST-X-cut and with a parallelogram geometry with 45 • angle, to reduce the influence of the SAW unwanted reflections. The advantage of the quartz substrate is good stability at RT [12]. It is a delay line type sensor, having an oscillating frequency of~69 MHz [35].…”

“…Through the acusto-electrical interactions, the surface acoustic wave is very sensitive at changes in electrical conductivity [37], and these interactions could be enhanced in bilayers [12]. The acusto-electrical effect comes from the interaction of electrical potential that accompanies the SAW propagation with the mobile charges in the sensing film [12]. The variation of the electrical conductivity can lead both to a decrease and to an increase of the center frequency [38].…”

“…In order to obtain various morphology and properties of the materials through the PLD technique, the deposition pressure is an important parameter influencing this feature [3,10,11]. As the depositing pressure increases, the porosity of the material also increases [12].…”

“…Whereas, hydrogen is an odorless, colorless and vastly combustible gas: flammable between 4%-75% in air, with high diffusion coefficient in air (0.60 cm 2 /s), and low ignition energy (0.002 mJ), the failure to detect it in time can lead to an explosion [22,23]. Over time, several types of hydrogen sensing sensors have been developed, including chemiresistive gas sensors [24], SAW sensors [12], metal-oxide semiconductor gas sensors [23]. Different types of sensing materials have been used for hydrogen detection: metal oxide thin films (ZnO, TiO 2 , WO 3 , SnO 2 , In 2 O 3 ) [25,26], metal thin films (Pd, Pt) [27], multilayer of metal oxides and metals (Pd/WO 3 , Pd/V 2 O 5 , Pd/SnO 2 , Pd/ZnO) [12,23,28,29], composite materials [30], nanowires [31].…”

“…Over time, several types of hydrogen sensing sensors have been developed, including chemiresistive gas sensors [24], SAW sensors [12], metal-oxide semiconductor gas sensors [23]. Different types of sensing materials have been used for hydrogen detection: metal oxide thin films (ZnO, TiO 2 , WO 3 , SnO 2 , In 2 O 3 ) [25,26], metal thin films (Pd, Pt) [27], multilayer of metal oxides and metals (Pd/WO 3 , Pd/V 2 O 5 , Pd/SnO 2 , Pd/ZnO) [12,23,28,29], composite materials [30], nanowires [31]. Some of these types of sensors have the ability to detect hydrogen at temperatures above 200 • C, which is inappropriate for hydrogen detection because the risk of the explosion becomes higher especially at high hydrogen concentrations [30].…”