Thio-based surfactant will do. If your material is not sensitive to S2-, you can even use Na2S. It is really quite complicated when your material is sensitive to S2-, some capping agent such as PEI is usually used for the surface modification.
If you add some water to the Cu2O suspension in oil , since the oil is lighter than water the naoparticles are transferred to the eqeous layer. You can also add stabilizing agent such as PVP in water first and stir the mixture.