*2.2. Synthesis of CeO2 and PDA@CeO2*

Nanopolyhedral CeO2 was synthesized by sol-gel followed by thermal decomposition according to the procedure reported in 15. Three composite materials made of cerium dioxide nanoparticles coated with PDA were prepared by reacting, under stirring, a weighed amount of cerium dioxide nanoparticles with 10 mL of a dopamine hydrochloride solution: specifically, 100 mg CeO2 with 0.01 M dopamine (hereafter Sample 1), 150 mg CeO2 with 0.05 M dopamine (Sample 2) or with 0.1 M dopamine (Sample 3). A suitable amount of NaOH 0.1 M was added to keep pH at 8.5 to achieve a polydopamine film on the nanoparticles. Subsequently, the dispersion was stirred for 24 h at room temperature in dark conditions. The powder was then recovered by centrifugation, washed with water several times and then dried at 80 ◦C overnight. These composite materials will hereafter be indicated as PDAx@CeO2, where x is the PDA weight percentage in the composite.
