Defect generation in Pd layers by ‘smart’ films with high H-affinity

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.


STM measurements during hydrogen gas loading (in situ STM)
In situ STM measurements were carried out using an OMICRON UHV Micro-STM system (background pressure ≤ 2×10 -10 mbar). The STM images of the same area of interest were collected continuously at different hydrogen (5.6 gas purity) pressures (1×10 -9 mbar -1×10 -5 mbar) and exposure times (60 min -3000 min) for subsequent data evaluation. The hydrogen pressure was adjusted via a control unit that precisely regulates the gas leak rate upon continuous pumping of the vacuum chamber. By using this loading procedure, it is possible to keep the pressure constant during the long-term STM measurements. 2

XRD measurements during hydrogen gas loading (in situ XRD)
In situ XRD measurements were performed at synchrotron facilities in Grenoble (ESRF) at beam line BM20 and in Hamburg (Petra III, DESY) at beam line PO8. The portative vacuum chambers designed for in situ hydrogen gas loading experiments were used. Both chambers are equipped with automatic valves and pressure gauges which allows for a precise control of the pressure (background pressure ≤ 1×10 -6 mbar). 1,2 The measurements were performed in the conventional -2 geometry. The loading was carried out in continuous hydrogen flow, p H2 was increased stepwise from p H2 = 1 × 10 -6 mbar to p H2 = 1 mbar. The loading time was varied from 60 min to 300 min depending on the loading pressure and the film thickness. Usually, first changes in the XRD patters were observed at p H2 ≥ 1 × 10 -3 mbar.
For all the samples studied by XRD, the hydrogen gas-pressures were higher as compared to the pressure values that were required in the UHV-STM loading chamber to initiate the precipitation of the hydride phase in Nb-H thin films. We attribute this observation to a change of the sample surface or interface conditions occurring upon sample exposure to air, during the transfer to the XRD system. Comparing studies on similar Nb films covered with a 20 nm Pd film that were not exposed to air did not require higher pressures for Nb-hydride phase formation. 3

Peak broadening upon coherent phase transformations
XRD measurements on Pd20nm/Nb28nm/Al 2 0 3 did not show a separated peak related solely to the Nb hydride ( Figure 2C). Upon hydrogen loading the Nb (110)

ETEM measurements
A FEI Nova NanoLab 600 focused ion beam (FIB) instrument was used to prepare TEM lamellas by the lift-out technique. Hydrogen-induced microstructural changes in Pd/Nb/Al 2 O 3 systems were monitored using a FEI Titan 80-300 environmental transmission electron microscope (ETEM) operated at 300 keV, and equipped with energy dispersive X-Ray (EDX) and electron energy loss spectrometer (EELS). The hydrogen pressure was increased stepwise from 5×10 -7 mbar to 5 mbar.
During the pressure increase and hydrogen loading (about 1 h in total) the electron beam was blanked to minimize its influence on the structure of studied films.

Defect healing in Pd film upon hydrogen exposure
The experimental HRTEM images of the Pd12nm/Nb15nm/Al 2 O 3 sample at various hydrogen pressures up to 1 mbar are shown in Figure 2S. The hydride formation in the Nb film was not detected in the area of interest. Therefore, no interface changes occur at Pd/Nb interface. Instead, hydrogen induced defect healing is found in the Pd film. The stacking faults marked by the yellow arrows in Figure 2S (a-e) disappear in Figure 2S