Hydrogen diffusion in cerium oxide thin films fabricated by pulsed laser deposition

Cerium oxide (CeO2) is well known to be reducible by hydrogen (H-2), yet the diffusion and solution properties of hydrogen in ceria at elevated temperatures have remained challenging to evaluate. We therefore fabricated nanometer-thin (similar to 100 nm) cerium oxide films on Si(111) substrates by pulsed laser deposition (PLD) and quantitatively investigated the H depth distributions therein by means of resonant H-1(N-15,alpha gamma g)C-12 nuclear reaction analysis (NRA) before and after annealing in H-2 gas at 773e973 K. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) reveal that the as-deposited films exhibit single phase CeO2 structure and partially reduced stoichiometry (CeO1.69). H-2 annealing does not largely change the H content of the as-deposited films; in all conditions several atomic percent of hydroxyl (OH) are found to exist in a thin (similar to 4 nm) surface layer, whereas stably bound hydrogen in the bulk of the films is almost uniformly distributed and of much smaller quantity (similar to 0.2 at.%) than the oxygen vacancy concentration in the partially reduced ceria. Its low concentration and high thermal stability identify this bulk H species as likely being strongly bound to defects in the polycrystalline films rather than as a hydride species that interacts weakly with O-vacancies. The H diffusion coefficient and activation energy in the ceria films are determined as > 10(-18) m(2) s(-1) at 773e973 K and <1.69 eV, respectively. The observed diffusion activation energy is somewhat larger than theoretical predictions for thermal diffusion of H in stoichiometric bulk CeO2, suggesting that defects and oxygen vacancies in the PLD-fabricated ceria thin films possibly have an impact on the H mobility. (c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.