Correlation between Surface Chemistry and Optical Properties in Colloidal Cu<sub>2</sub>O Nanoparticles
Mehrin Tariq, Melissa D. Koch, Jordan W. Andrews, Kathryn E. Knowles
Abstract
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.