Palladium and phthalocyanine bilayer films for hydrogen detection in a surface acoustic wave sensor system

Presented here are the results concerning a hydrogen sensor based on two various bilayer structures in a surface acoustic wave dual-delay line system. The sensor material consists of two layers produced in two different vapour deposition processes. The first one is a 110 nm free phthalocyanine (H2Pc) or 230 nm nickel phthalocyanine (NiPc) layer, whereas the second is a 20 nm thin palladium (Pd) film. These structures were formed in one of the dual delay line systems on a LiNbO3 Y-cut Z-propagation substrate, while the other serves as a reference, permitting an easy detection of the arising differential frequency Deltaf. In such a bilayer structure we can detect hydrogen in a medium concentration range (from 1 to 4% in nitrogen), even at room temperature. The achieved results are compared with a previous structure of 720 nm copper phthalocyanine covered by 20 nm Pd film. The best results from the point of view of interaction stability have been achieved for the "old" structure (CuPc + Pd), whereas the biggest response is observed in the case of the new structure H2PC + Pd. The response was almost five times bigger than in the case of the CuPc + Pd structure and similar to12 times bigger than in the NiPc + Pd structure of these same hydrogen concentrations (from 2 to 4% in nitrogen) at 30degreesC. For all the structures the interaction response depends on temperature, and decreases with the increase of the interaction temperature. The comparison of responses for 1 and 3% of H-2 in N-2 has shown a difference of about one order of magnitude. (C) 2003 Elsevier B.V. All rights reserved.