We report a new synthetic approach for the preparation of small FeOx nanoparticles, formed on the surface of hollow N,O-doped carbon nanoshells through an "inside-out" mechanism. By control of the pyrolysis conditions, iron species can ex-solve from inside N,O-doped carbon nanoshells onto the external carbon surface, where they form stable small FeOx nanoparticles. The location (endo- or exohedral) of the iron species is correlated with an observed switch of the product selectivity (from H2O2 to H2O) in the electrocatalytic oxygen reduction reaction.
