Dissociative Binding of Carboxylic Acid Ligand on Nanoceria Surface in Aqueous Solution: A Joint In Situ Spectroscopic Characterization and First-Principles Study

Carboxylic acid is a common ligand anchoring group used to functionalize nanoparticle surfaces. Its binding structure and mechanism as a function of the oxidation states of metal oxide nanoparticle surfaces are not well characterized. Here we present an in situ sum-frequency generation vibrational spectroscopy (SFG-VS) and spin-polarized density functional theory (DFT) study on the binding of deuterated acetic acid on ceria nanoparticles (CNPs) in aqueous solution. SFG spectra from the CNP surface revealed that the binding modes of the carboxylate group depend on the oxidation states of the cerium on the surface. SFG polarization analysis suggested that bidentate chelating and bridging binding modes coexist on partially reduced ceria surfaces, while oxidized ceria surfaces are dominated by the bidentate bridging mode. DFT calculations show consistent results that dissociative adsorption is more thermodynamically stable than molecular state adsorption. Dissociative adsorption to a highly undercoordinated corner site on the model ceria cluster is more favorable compared to the sites on a flat surface. This direct spectroscopic evidence from SFG spectra of a ceria nanoparticle and computational results on a model ceria cluster help to clarify the binding structures and the dissociative binding mechanism on ceria nanoparticles. These results demonstrate that the SFG method is applicable to directly characterize reactions and binding chemistry at the nanoparticle surface buried in aqueous solution.