Capping agents are the essential factor in nanoscience and nanotechnology. However, the types of capping agents are greatly limited. Defying conventional beliefs, here is shown that metal cations can also be considered as capping agents for oxide nanoparticles, particularly in maintaining their colloidal stability and controlling their facets. Here the general stabilizing effects of multivalent cations for oxide nanoparticles, and the facet controlling role of Al3+ ions in the growth and ripening of Cu2 O octahedra, are demonstrated. This discovery broadens the view of capping agent and opens doors for nanosynthesis, surface treatment, and beyond.