Annealing enhanced hydrogen absorption in nanocrystalline Pd∕Au sensing films

Abstract

The enhanced hydrogen absorption and sensitivity of nanocrystalline Pd(60at.%)∕Au(40at.%) thin films were realized through the development of a thermal annealing process and a determination of its underlying enhancement mechanism. 20-nm-thick films were deposited by magnetron sputtering and then annealed at temperatures ranging from 100 to 400 °C. Optical reflectance and x-ray diffraction (XRD) analyses were utilized to investigate the H2 response and microstructure characteristics of the as-deposited and annealed films as a function of the annealing temperature. The as-deposited films exhibited a consistently low H absorption, evidenced by a small reflectance-signal change at even a 4% H2 concentration, while displaying a 5-second response time. The combined stress and composition analyses by XRD indicate that the as-deposited film is under a compressive stress of ∼560MPa and has an unexpectedly low Pd (40%) content in the ∼7-nm nanocrystallites, in contrast to the overall film Pd concentration (60%), with Pd enrichment occurring in the disordered grain boundary. The low H absorption characteristics can be overcome by a thermal annealing process, and it was determined that a 200 °C annealing temperature was most desirable. The annealing process resulted in a 4× to 6× enhancement of the signal change, with a minimal effect on the response time. The annealing also stabilized the microstructure, allowing for enhanced sensing stability, reliability, and durability. The underlying mechanism for hydrogen absorption enhancement is comprised of three heat-promoted events within the film microstructure: grain growth, a reduction of the internal compressive stress, and atomic intermixing of Pd from its enrichment and disordered state at the grain boundary into the Pd∕Au grain lattice.