TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preparation (Claimed): Preparation of hollow platinum nanoparticles, comprises: (a) placing multi-metal nanoparticles on a carbon support of a metal compound of formula (PtxM/C) in a device configured to subject the nanoparticles to a sequence of polarizations; and (b) carrying out sequence of polarizations on the nanoparticles between (i) a reducing potential comprised of 0-0.5 V/relative hydrogen electrode (RHE) (i.e. measured relatively to a reversible hydrogen electrode) and (ii) an oxidizing potential comprised of 0.9-3 V/RHE (i.e. measured relatively to a reversible hydrogen electrode), until single-crystal hollow platinum nanoparticles are obtained.x = 0.2-10; andM = a metal less noble than platinum.Preferred Method: The step (b) is carried out until single-crystal hollow platinum nanoparticles are obtained with a diameter of 3-10 nm and the shell of which is dense with a thickness of 0.277-0.831 nm. The multi-metal nanoparticles on the carbon support are obtained by (a1) preparing colloidal nanoparticles of a metal (M) on a carbon support, i.e. M/C type, from M(z+) metal salts, and (b1) preparing a platinum deposit on the nanoparticles obtained at the end of step (a1). The method comprises (c) cleaning the nanoparticles in an acidic solution. The method further comprises: (c1) carrying out post-treatment in an oven at a controlled temperature of 100-600 degrees C for 10 minutes to 10 hours, where the step (c1) is carried out either directly at the end of step (b) if the method does not comprise step (c), or at the end of step (c) if the method comprises step (c). The post-treatment is an electrochemical post-treatment. The post-treatment is carried out in a sequence of polarizations in an electrolytic solution that is different from the electrolytic solution of step (b). The step (c1) is carried out under a controlled atmosphere which is neutral, oxidative or reductive. Preferred Components: The central cavity has a diameter of 2-10 nm. The nanoparticle comprises a number of atoms with low coordination that is less than 10% of the total number of atoms of surface. The sequence of polarizations of step (b) is carried out with: a succession of linear ramps of potentials (rising and falling potentials) at a potential scanning rate of 1-1000 mVs-1; a succession of plateaus of potentials with a duration of 1-36000 seconds; or a mixture of ramps and plateaus for rising (and respectively falling) potential phases. The sequence of polarizations has a rectangular profile, for which: the reducing potential is 0.05-0.5 V/RHE, preferably 0.2-0.4 V/HRE; and the oxidizing potential is 0.60-1.4 V/RHE, preferably 0.8-1 V/RHE. The device configured for submitting the nanoparticles to a sequence of polarizations, comprises an electrode or a membrane-electrodes assembly of a fuel cell, and a rotary conducting drum. EXAMPLE - None given.