Engineering surface dipoles on mixed conducting oxides with ultra-thin oxide decoration layers

Abstract

Improving materials for energy conversion and storage devices is deeply connected with an optimization of the surfaces of these materials. Surface modification has therefore emerged as a very promising strategy on the way to enhance modern energy technologies. This study shows that surface modification with ultra-thin oxide layers allows for a systematic tailoring of surface properties, in particular of the surface dipole and the work function, and it introduces the ionic potential of surface cations as a readily accessible descriptor for these effects. The combination of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) illustrates that basic oxides with a lower ionic potential than the host material induce a positive surface charge and reduce the work function of a mixed conducting host material and vice versa. The emerging surface dipoles are caused by changes in the energy landscape, leading to redistribution of electronic charge density and/or a geometric reorientation of the surface. As a proof of concept that this strategy is widely applicable to tailor surface properties, we examined the effect of ultra-thin decoration layers on the oxygen exchange kinetics of pristine thin films of structurally and chemically different mixed conducting oxides in very clean conditions by means of in-situ impedance spectroscopy during pulsed laser deposition (i-PLD). The extended study confirms that basic decorations with a reduced surface work function lead to a substantial acceleration of the oxygen exchange on a material's surface. Computational results suggest that the underlying mechanism of the kinetic improvement relies on a modification of the energetics of charged O2 adsorbates and substantiate their importance for the oxygen exchange reaction.