Crystallinity offers countless opportunities for the controlled synthesis of multimetallic 2D and 3D nanomaterials. Herein we have successfully synthesized 2D Pt-Ag ultrathin nanosheets through the oxidative etching of twin seeds and 3D Pt-Ag-Cu tetrapods via altering the crystallinity through the incorporation of copper into the Pt-Ag alloy. A better electrocatalytic activity is obtained for the oxidation of formic acid which is 3.8 times higher than that of a commercial platinum catalyst as the stepped surface atom densities are higher on the nanosheets.