Disclosed are methods for forming carbon nano-onions. The methods include annealing a carbon nanodiamond starting material in an inert atmosphere. The method can be carried out at ambient pressure. Also disclosed are methods for functionalizing carbon nano-onions. For instance, carbon nano-onions can be functionalized so as to be soluble in aqueous or organic solvents, as desired. Also disclosed are methods for separating mixtures of carbon nano-onions. In particular, mixtures of carbon nano-onions can be separated from one another based upon differences in electrochemical characteristics of the different nano-onions.