Photocatalytic splitting of H2O-to-H2O2 by BiOI/g-C3N4/CoP S-scheme heterojunctions

Abstract

Photocatalytic H₂O-to-H_2(g) and –H₂O_2(aq) (2H₂O_(l) → H_2(g) + H₂O_2(aq)) by a two-electron reaction is kinetically feasible. The proton-coupled two-electron transfer induced a photocatalytic oxygen reduction reaction (ORR) from oxygen and water is also an accessible path for the safe production of H₂O₂. It is thus desirable to synthesize effective photocatalysts that yield H₂O₂ by the coupling of two-electron transfer water oxidation (+1.23 V_NHE) and ORR (+0.68 V_NHE). In this work, new g-C₃N₄/BiOI S-scheme heterojunctions with co-catalyst cobalt phosphide (CoP) (BiOI/g-C₃N₄/CoP) were prepared for the photocatalytic H₂O-to-H₂O₂ reactions. The BiOI/g-C₃N₄/CoP heterojunctions can enhance visible light absorption and reduce electron–hole pair recombination rates. Because of the water splitting by two-electrons and reduction of oxygen, the BiOI/g-C₃N₄/CoP heterojunctions have a better photocatalytic H₂O-to-H₂O₂ yield than with the BiOI/g-C₃N₄ heterojunction by 3.0–8.4 times. With more CoP (i.e., 19 wt%) on the BiOI/g-C₃N₄, the BiOI/g-C₃N₄/(CoP)_0.19 heterojunction generates more photocatalytic H₂O₂ with higher formation rate constant (k_f), equilbrium constant (K), and a lower decomposition rate constant (k_d). This new, low-cost and ready-prepared BiOI/g-C₃N₄/CoP S-scheme heterojunction has a high efficiency for the reversible photocatalytic H₂O-to-H₂O₂ reaction, which could be feasibly used for applications in H₂O₂-fuel cells for electricity using solar energy.