Correlation between Surface Chemistry and Optical Properties in Colloidal Cu₂O Nanoparticles

Nanostructured Cu₂O can catalyze or photocatalyze various organic transformations and is a model photocathode for the reduction of protons or carbon dioxide. The performance of Cu₂O nanomaterials in these applications depends crucially on their surface chemistry and optical properties. Here, we demonstrate that the absorption spectrum of colloidal Cu₂O nanocrystals depends strongly on the oxidation states of surface copper species. Pristine as-synthesized colloidal Cu₂O nanoparticles exhibit an absorption spectrum with a resonance at λ = 570 nm that disappears upon exposure to air or water. Both ground-state and transient absorption spectra of pristine Cu₂O nanoparticles are very similar to the analogous spectra of colloidal metallic Cu nanoparticles and are therefore consistent with the behavior expected of a localized surface plasmon resonance (LSPR). X-ray photoelectron and Auger spectroscopy indicate that the observation of an LSPR feature in pristine Cu₂O nanoparticles correlates with the presence of Cu⁰ atoms on their surfaces. This work demonstrates the significant role that surface redox chemistry plays in determining the optical properties of colloidal Cu₂O nanocrystals. Additionally, the correlation between the localized redox chemistry of Cu₂O nanoparticles and their optical spectra revealed here is distinct from the behavior of Cu₂E (E = S, Se, Te) nanoparticles and will allow for a deeper understanding of the mechanistic aspects of their catalytic and photocatalytic behaviors.