Morphology, local stoichiometry, and photoexcited states in Cu@Cu2O nanostructured systems grown by physical synthesis

Cuprous oxide (Cu2O) is a p-type semiconductor with promising applications as a photocathode material in photoelectrochemical cells for hydrogen production, owing to its visible-range bandgap and suitable band edge positions for water splitting. The incorporation of metallic Cu nanoparticles can further enhance light absorption and extend the absorption range of Cu2O toward the red and near-infrared due to the excitation of localized surface plasmon resonances within the metal nanoparticles (NPs). Starting from molecular-beam-epitaxy-grown metallic Cu NPs, post-growth thermal treatments under oxidizing and reducing conditions can be tuned to obtain either Cu@Cu2O core@shell NPs or Cu2O nanostructured films. The surface stoichiometry of the samples is investigated using x-ray photoelectron spectroscopy and Auger electron spectroscopy. Transmission electron microscopy, coupled with electron energy-loss spectroscopy, provides insights into the morphology and the local oxidation state of the NPs with nanometric resolution. Steady-state and time-resolved optical characterization of the samples confirms the presence of optical features related to localized surface plasmon resonances in the Cu cores and to exciton formation in the Cu2O shell.