Investigation of sub-nm binary oxidic surface modifications on mixed ionic electronic conductors with ToF-SIMS: Oxidic overlayer stability and ionic interdiffusion behavior

Abstract

Modifications of mixed ionic electronic conductor (MIEC) surfaces are a promising approach to improve oxygen exchange reaction (OER) kinetics and can have a tremendous impact on surface charges and secondary ion yields. Utilizing time-of-flight secondary ion mass spectrometry (ToF-SIMS), we examined degradation, segregation, and cation interdiffusion behaviors on La<inf>0.6</inf>Sr<inf>0.4</inf>CoO<inf>3-δ</inf> (LSC) and Pr<inf>0.2</inf>Ce<inf>0.8</inf>O<inf>2-δ</inf> (PCO) thin films modified with ∼0.5 nm CaO, TiO<inf>2</inf>, and SnO<inf>2</inf> overlayers after annealing at 700 °C and 800 °C, respectively. Surface profiles (AFM; ToF-SIMS) and depth profiles (ToF-SIMS) revealed structural and chemical transformations, including particle formation and surface roughening. The overlayer stability varied significantly for different overlayers on the same material and for the same overlayer on LSC and PCO. All oxidic overlayers were more stable on PCO than LSC. Depth profiling indicated that ⁴⁰Ca⁺ ions penetrated the entire LSC and PCO layers (30 nm). ⁴⁸Ti⁺ aligns more accurately with a typical analytical solution to Fick's diffusion equation for finite systems, showing higher diffusion coefficients in LSC than PCO. ¹²⁰Sn⁺ exhibited minimal penetration, indicating high stability on the surface and less intermixing with either MIEC. These results hint at the need to differentiate between the stability of the binary oxides on the surface and the bulk diffusivity.