Deciphering the impacts of symmetric and asymmetric structures in heptazine units over g-C₃N₄ on piezo-photocatalysis for H₂O₂ production from pure water

The vacancy-engineering approach is showing promise in modulating graphitic carbon nitride (g-C₃N₄) structures to accelerate catalytic yield. Nonetheless, current studies mainly focus on the catalytic influences of carbon or nitrogen monovacancies in the materials’ structures, resulting in the elusive roles of cluster vacancies, where the co-presence of nitrogen and carbon vacancies can create asymmetric or symmetric structures in the heptazine units of g-C₃N₄, influencing the total catalytic outcomes under light or/and ultrasound irradiation. In this work, taking the piezo-photocatalytic production of H₂O₂ from pure water as a model reaction and utilizing positron annihilation techniques, we realize the hiding roles of cluster vacancies in g-C₃N₄. The results show that asymmetric structures can help overcome adverse charge compensation effects. The disorders will act as trapping states to store excited electrons and avoid charge neutralization processes between photo- and piezo-induced processes. In contrast, the symmetry in a local heptazine unit will enhance the piezocatalytic activity but degrade the piezo-photocatalytic outcomes under light irradiation. Therefore, this study unveils the roles of symmetric and asymmetric structures in response to visible light, UV light, and ultrasound over g-C₃N₄−based catalysts, which would be necessary for material designs and developments at a molecular level.