Elucidating facet dependent electronic and electrochemical properties of Cu₂O nanocrystals using AFM/SCEM and DFT

Cuprous oxide (Cu₂O) has extensively been studied owing to its excellent optical, magnetic, and catalytic properties. Many of these properties are facet-dependent and have not been well elucidated. This work synthesized cubic, cuboctahedral, octahedral, and rhombic dodecahedral shaped Cu₂O nanocrystals of ∼300 nm in size to evaluate the facet-dependent electrochemical activities. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were firstly used to reveal the average electrochemical activities at the ensemble level. Atomic force microscopy-scanning electrochemical microscopy (AFM-SECM) was further used to assess the electrochemical activities of different Cu₂O nanocrystals at the facet level. Hexaammineruthenium (III) chloride ({Ru(NH₃)₆}Cl₃) was employed as the probe molecules that reacted with four different Cu₂O nanocrystals under –400 mV and yielded ∼300 pA current between the probing tip and the nanocrystal surface. The tip-current mapping results indicate that rhombic dodecahedral Cu₂O exhibits higher electrocatalytic activity than other shaped Cu₂O, due to the presence of dominant exposed facet of {110} as indicated by the relatively high tip current. Density-functional theory (DFT) calculations confirmed the facet dependence of local surface energy and electronic structure of Cu₂O nanocrystals. Besides electrochemical activity, the surface work function and adsorptive properties were both observed to vary with the shape and dominant exposed facets of Cu₂O. This study presented a unique experimental and computational chemistry approach to analyze surface electrochemical properties of Cu₂O crystals at a crystalline facet level.