SYNTHESIS AND CHARACTERIZATION OF HYDROPHOBIC Pd/W03 THIN FILMS FOR HYDROGEN SENSING

Abstract

Hydrogen is a clean energy source with a potential to be used as a fuel for industrial applications. 1-lowever, it is explosive at room temperature and therefore its leak detection becomes an important area of research. In the present work the structural, optical and hydrogen sensing properties of nanostructured Pd-capped tungsten oxide (Pd/W03) thin films. The Structural parameters have been tailored at varying substrate temperature (100-400°C) and oxygen partial pressure (P02= 0.5-1.2 Pa) to obtain the optimized parameter. The micro-structural, morphological and structural analyses of the as-deposited films have been done by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) ,Atomic force Microscopy (AFM). Hydrophobicity was found to be a direct function of surface roughness. High hydrophobicity was observed for the samples deposited at higher substrate temperature. The samples were hydrogenated at 2 bar hydrogen pressure in an operating temperature range 27- 150°C. Optical transmittance spectra was observed transparent for W03 thin films deposited at P02 = 0.5 Pa while it drastically decreases to 50% for hydrogenated Pd/W03 thin films. A transition from hydrophilic to hydrophobic surface was observed with increase in substrate temperature. The hydrophobicity was observed as the main reason for increase in sensor response with increase in substrate temperatures. Using AFM analysis, it was observed that the surface roughness followed the same trend as hydrophobicity. Hydrophobicity played a keen role in limiting the effect of water vapor during recovery. To increase the hydrophobicity further to increase the sensor response of the samples and study the effect of hydrophobicity on hydrogen sensing properties. To study this effect Pd/W03 thin films were prepared at different oxygen partial pressure. The contact angle and roughness were found maximum for the films deposited at P02 0.5 Pa. Recovery time of the sample was observed to improve with hydrophobicity. Increase in hydrophobicity implies limitation in the wettability of the samples. High hydrophobicity gives rise to surface tension which repels water from the surface. Reduced energy barrier combined with the hydrophobicity raised the sensor response of the film substantially making it a suitable choice for hydrogen sensing. Films deposited at P02 = 0.5 Pa showed best response time of I sec and recovery time of 8 min in the presence of 2 bar hydrogen pressure at 500C.