Abstract
The interaction of hydrogen gas with the palladium/rutile titanium dioxide model system was investigated with strong focus on the interface and its vicinity. Palladium (Pd) films were deposited on pretreated rutile titanium dioxide (r-TiO$_2$) single crystals by using magnetron sputtering. The proposed procedure results in an atomically sharp interface. The orientation relationship between Pd and r-TiO$_2$ was derived by x-ray and high resolution transmission electron microscopy investigations (HRTEM). In situ investigations were carried out on the Pd/r-TiO$_2$ system applying different hydrogen gas partial pressures in an environmental transmission electron microscope (ETEM). Here, especially the energetic structure of the interface and its vicinity was studied utilizing electron energy loss spectroscopy (EELS) on the titanium L$_{3,2}$ edge. Density functional theory (DFT) calculations were carried out, using a hybrid functional, to understand the origin of the observed changes in the EELS measurements. The EELS experiments suggest the accumulation of defects at the interface and its vicinity. A combination of EELS and DFT suggests high defect concentrations at the interface and its vicinity in a hydrogen gas environment. The DFT calculations predict this defect to be the hydrogenated oxygen vacancy. In addition, high resolution scanning TEM (HRSTEM) and positron annihilation spectroscopy (PAS) was applied to reveal changes in the vacancy and the dislocation density in the Pd films as a function of the hydrogen partial pressure uncovering a strong pressure dependency.
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