Determining the oxidation state of small, hydroxylated metal-oxide nanoparticles with infrared absorption spectroscopy

Metal-oxide nanoparticles (MONPs) are of broad interest as catalysts and catalyst supports in automotive, industrial, and biomedical applications. In water-containing environments, MONPs adsorb hydroxyl groups at nanoparticle corners, edges, and faces. While adsorbed surface groups will influence MONP properties, including oxidation state and the availability and accessibility of catalytically active sites, determination of the density, distribution, and effects of such groups is experimentally challenging. Here, focusing on CeO₂ nanoparticles (CNPs) as a representative MONP system, we report computational and experimental results showing that, for small, hydroxylated CNPs, the presence or absence of strongly infrared (IR) absorbing O-H stretching modes associated with O-H-OH complexes present on fully oxidized CNPs can be used to probe the configuration of surface-adsorbed hydroxyl groups and the CNP net oxidation state. As the vibrational modes of O-H-OH complexes are distinct from those of molecular water (3240 vs 3404 cm^-1), these results demonstrate that IR absorption spectroscopy can be used to determine the oxidation state of small, hydroxylated CNPs, as well as other MONPs, more generally, in water-containing environments.