In situ real-time investigation of hydrogen-induced structural and optical changes in palladium thin films

In situ X-ray reflectance (XRR) and X-ray diffraction (XRD) were employed to finely characterise palladium thin films of different thicknesses during a hydrogenation process. The derived information was combined with in-situ ellipsometric measurements to achieve a complete knowledge of both structural and optical changes during hydrogen exposure. The films expansion, crystalline structure and optical properties modifications were characterised by de-coupling the mutual effects. The structural analyses revealed that when the palladium hydride formation is completed, the films undergo a relative thickness expansion of about 8-9%. This value is considerably larger than the relative lattice parameter increase reported in the literature; in the present work this discrepancy is partly attributed to the Poisson effect. Combining such results with the ellipsometric measurements, the palladium and palladium hydride optical constants were determined. Such results provide a general framework for monitoring structural and optical modifications of catalytic materials in reactive environments. (C) 2017 Elsevier B.V. All rights reserved.