Abstract
In this paper, we demonstrate that the microstructure and the surface of a thin palladium (Pd) film can be intentionally altered by the presence of a subjacent niobium (Nb) film. Depending on the thickness of the Nb film and on the hydrogen gas pressure, defects in the Pd film can be healed or created. To demonstrate this effect, Pd/Nb/sapphire (Al2O3) stacks are studied during hydrogen gas exposure at room temperature by using scanning tunneling microscopy (STM), X-ray diffraction (XRD) and environmental transmission electron microscopy (ETEM). STM shows that hydrogen-induced topography changes in the Nb films depend on the film thickness which affects the height of the Nb surface corrugations, their lateral size and distribution. XRD measurements show that these changes in the Nb hydride film influence the microstructure of the overlaying Pd film. ETEM reveals that the modifications of the Pd film occur due to the precipitation and growth of the Nb hydride phase. The appearance of new defects, interface and surface roughening is observed in the Pd film above locally grown Nb hydride grains. These results can open a new route to design ‘smart’ catalysts or membranes, which may accommodate their microstructure depending on the gaseous environment.
Highlights
Nano-sized metal films are widely used as a key element for hydrogen storage[1, 2], hydrogen gas sensors[3,4,5], gas purification membranes[6, 7] and heterogeneous catalysts[8, 9]
The mean hydrogen solubility depends on the Pd-particle size or the Pd film thickness and, on the microstructure[25,26,27]
We demonstrate experimentally that the lattice expansion during hydrogen absorption in Nb-films modifies the microstructure of an overlying Pd layer in the Pd/Nb/ sapphire (Al2O3) system
Summary
The results of the XRD measurements suggest that the Pd layer undergoes microstructural changes resulting from the phase transformation occurring within the Nb-H film (marked by yellow color in Fig. 2(a) and (b)). It shows HRTEM images of the Pd12nm/Nb15nm/Al2O3 taken before the hydrogen exposure (Fig. 4a) and during hydrogen exposure at pH2 = 1 mbar (Fig. 4b) in a sample area without border regions. Morphological changes of Pd film may be enhanced by decreasing the thickness ratio dPd/dNb. A particular example with dPd/dNb = 2 nm/55 nm = 0.04 is shown, which summarizes STM measurements performed during hydrogen gas exposure of a Pd2nm/Nb55nm/Al2O3. The microstructural features can be designed to be pronounced for the applications, such as recovery of surface activity of catalysts
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