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• We prepared all-inorganic mixed perovskite CsPbBr 3-x I x using an efficient light-assisted method. • We showed that CsPbBr 3-x I x particles have the I-concentration gradient from the surface to the interior. • We showed that CsPbBr 3-x I x /Pt powders are a good photocatalyst for the H 2 evolution in aqueous HBr/KI solution. • We showed that the excellent photocatalytic performance of CsPbBr 3-x I x arises from the bandgap funnel structure. A simple and efficient light-assisted method is employed to prepare powder samples of all-inorganic mixed-perovskite CsPbBr 3-x I x from CsPbBr 3 by ion exchange in aqueous HBr/KI solution such that the concentration of I in a sample particle decreases on going from the surface to the interior. CsPbBr 3-x I x /Pt, namely, CsPbBr 3-x I x samples loaded with Pt nanoparticles, shows a high performance for the hydrogen evolution under visible-light irradiation in aqueous HBr solution saturated with CsPbBr 3. The H 2 evolution rate of the CsPbBr 3-x I x /Pt powders (200 mg) is determined to be 224 μmol h−1, under 120 mW cm-2 visible-light (λ ≥ 420 nm) illumination. The CsPbBr 3-x I x samples have a high stability, with no apparent decrease in the catalytic activity after 50 h of repeated H 2 evolution experiments. The apparent quantum efficiency of CsPbBr 3-x I x /Pt is determined to be 2.15% under the irradiation of 450 nm light. [ABSTRACT FROM AUTHOR]