K+, Ni and carbon co-modification promoted two-electron O2 reduction for photocatalytic H2O2 production by crystalline carbon nitride

79.6 μM of H 2 O 2 was produced in water after 60 min over K + , Ni and carbon-co-modified g-C 3 N 4 which was obtained by calcination of Ni 2+ -anchored and PVP-encapsulated melamine-cyanuric acid supramolecules, then re-calcination with KCl–LiCl.