Hollow nanomaterials have attracted intense attention due to their special structures and potential applications in many fields. In this paper, we report a surfactant free synthesis of hollow Cu 2 O nanocubes by reducing Cu 2+ precursors using Cl À ions as the morphology regulator at room temperature. It is found that in the presence of Cl À ions, hollow Cu 2 O nanocubes can be easily synthesized by directly reducing Cu 2+ precursors with ascorbic acid. Through well-designed experiments, we propose that, in this surfactant free synthetic route, the formation of hollow Cu 2 O nanocubes results from a reaction activated Kirkendall diffusion process of cubic CuCl intermediates, which are formed in the reaction process and act as self-sacrificial templates. The amounts of Cl À ions and NaOH are two key factors to determine whether hollow Cu 2 O nanocubes are formed or not. † Electronic supplementary information (ESI) available: SEM images and XRD patterns of precursors, and SEM and TEM images of Cu 2 O obtained with different amounts of NaCl and reaction times. See