TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Nanoparticles: In the compositionally modulated nanoparticle the catalytically active material comprises platinum and the modifier material comprises a transition metal. The first particle shape is substantially icosahedral. The compositionally modulated nanoparticle has an atomic composition of platinum (55%), iron (17%), and the remainder gold. The shape controlled seed material modifies the catalytic durability of the compositionally modulated nanoparticle relative to the catalytically active material. The compositionally modulated nanoparticle has a core portion consisting essentially of the shape controlled seed material; and an interfacial region disposed between the core portion and the shell portion, where the interfacial region comprising an alloy of the mixture and the shape controlled seed material. Preferred Electrocatalyst: The electrocatalyst further comprises an interfacial portion disposed between the core portion and the Pt-bimetallic shell portion, where the interfacial portion comprises an alloy of gold and a bimetallic shell material. The several nanoparticles have an atomic composition of less than 55% platinum. The Pt-bimetallic shell portion forms an epitaxial coating over the core portion. A surface layer of the Pt-bimetallic shell portion consists essentially of platinum. The second metal comprises iron and the surface layer is electronically altered by the iron to increase the catalytic activity of the surface layer. The core portion comprises a shape configured to suppress dissolution of the platinum from the surface layer in an oxygen reduction reaction environment. The electrocatalyst is integrated into a cathode of a fuel cell. Preferred Method: In the method for forming several durable core/shell particles having high catalytic activity, each of several nanoparticles has a surface portion enveloping the shell portion, and the topmost surface portion consists essentially of platinum. The method further involves adjusting at least one of the amounts of the gold nanoparticles, the platinum precursor and the alloying metal precursor to selectively define the thickness of the shell portion; and incorporating the nanoparticles into a carbon substrate for use in a fuel cell. The thickness of the shell portion is substantially uniform and is 1-1.5 nm. Preferred Components: The alloying metal in the method for forming several durable core/shell particles is selected from Fe, Co and Ni. EXAMPLE - Au nanoparticles were prepared from a solution of tetralin (10 ml), oleylamine (10 ml), and HAuCl43H2O (0.1 g) in air at 15 degrees C and magnetically stirred under N2 flow. Tert-butylamine-borane complex (0.5 mmol) was dissolved in tetralin (1 ml) and oleylamine (1 ml) and injected into the precursor solution. The reaction initiated instantaneously and solution changed to a deep purple color within 5 seconds. The mixture was aged at 15 degrees C for 1 hour before acetone (60 ml) was added to precipitate Au nanoparticles. The Au nanoparticles, with diameter of 7 nm, were collected by centrifugation (8500 rpm, 8 minutes), washed with acetone and re-dispersed in hexane. Core/shell Au/FePt3 nanoparticles were prepared by mixing 7 nm Au nanoparticles (30 mg) with octadecene (10 ml), Pt-acetoacetonate(acac)2 (0.1 g), oleylamine (1 ml) and oleic acid (1 ml) at 120 degrees C. Fe(CO)5 (0.03 ml) was added under N2 atmosphere, then temperature was raised to 200 degrees C. The solution was cooled to room temperature after 30 minutes. Iso-propanol (50 ml) was added to precipitate the nanoparticles and the product was collected by centrifuge (6000 rpm, 5 minutes). The resulting Au/FePt3 nanoparticles, which had a 1.5 nm shell over the 7 nm core, were washed with ethanol and re-dispersed in hexane.