Direct in situ detection of grain boundary reduction in nanocrystalline ceria

Abstract

Enrichment of the reduced Ce³⁺ species near grain boundaries in ceria is a widely established phenomenon which has previously been observed in ex situ experiments. Here, in situ X-ray absorption near-edge spectroscopy (XANES) is employed to detect and quantify grain boundary reduction under device-relevant conditions. Single-crystal and dense nanocrystalline films of undoped ceria were characterized by Ce L₃ XANES at high temperatures (615–845 °C) in humidified hydrogen. Nanocrystalline ceria (30–40 nm mean grain sizes) exhibited large enhancements in Ce³⁺ concentration, from 2.0× to 11× relative to bulk ceria. Implications for grain boundary reduction thermodynamics and anticipated conductivity enhancements are discussed.