Surface acoustic wave hydrogen gas sensor based on layered structure of palladium/metal-free phthalocyanine

A layered sensor structure of metal-free phthalocyanine H2Pc (∼160 nm) with a very thin film of palladium (Pd∼20 nm) on the top, has been studied for hydrogen gas-sensing application at relatively low temperatures of about 30◦C and about 40◦C. The layered structure was obtained by vacuum deposition (first the phthalocyanine Pc and than the Pd film) onto a LiNbO3Ycut Z-propagating substrate, making use of the Surface Acoustic Wave method, and additionally (in this same technological processes) onto a glass substrate with a planar microelectrode array for simultaneous monitoring of the planar resistance of the layered structure. In such a layered structure we can detect hydrogen in a medium concentration range (from 0.5 to 3% in air) even at about 30◦C. At elevated temperature up to about 40◦C the differential frequency increases proportionally (almost linearly) to the hydrogen concentration and the response reaches its steady state very quickly. The response times are about 18 s at the lowest 0.5% hydrogen concentration to about 42 s at 4% (defined as reaching 100% of the steady state). In the case of the investigated layered structure a very good correlation has been observed between the two utilized methods – the frequency changes in the SAW method correlate quite well with the decreases of the layered structure