Colloidal Cu₂O Nano-Crystal Photoelectrodes: The Role of Interfacial Chemistry

Organic ligands are integral constituents of colloidal nanocrystals (NCs) and play an important role in their synthesis, their compatibility with different solvents and the electronic properties. We have studied the role of the interfacial chemistry in the charge transfer and passivation against photocorrosion of octadecylamine(ODA)-coated cuprite (Cu₂O) NCs for solar water splitting applications. ODA ligands are very efficient surfactants for the synthesis of nanoscale octahedral Cu₂O NCs with narrow size distribution. In addition, the long alkyl-chain ODA molecules form a highly hydrophobic monolayer on top of the NCs that reduces the contact with the aqueous solution and may improve the passivation of the NCs towards photocorrosion, which is a major challenge of Cu₂O for solar water splitting applications. Photoelectrochemical measurements of Cu₂O NCs thin layers on Au electrodes show, though, that the ODA molecules that cover the NCs impede the transport of the photogenerated charges to the liquid and, thus, suppress the photoresponse under solar illumination. Ligand exchange procedure is, then, carried out with more conductive molecules in order to improve the photo-induced charge separation and transport across the Au-NCs-liquid interfaces and to increase the photoresponse of the NCs.