Exceptional Cocatalyst-Free Photo-Enhanced Piezocatalytic Hydrogen Evolution of Carbon Nitride Nanosheets from Strong In-Plane Polarization.

Utilizing mechanical energy to produce hydrogen is emerging as a promising way to generate renewable energy, but is challenged by low efficiency and scanty cognition. In this work, graphitic carbon nitride (g-C3 N4 ) with an atomically thin sheet-like structure is applied for prominent piezocatalytic and photo-enhanced piezocatalytic H2 production. It is revealed that the anomalous piezoelectricity in g-C3 N4 originates from the strong in-plane polarization along the a-axis, contributed by the superimposed polar tri-s-triazine units and flexoelectric effect derived from the structured triangular cavities, which provides powerful electrochemical driving force for the water reduction reaction. Furthermore, the photo-enhanced charge transfer enables g-C3 N4 nanosheets to reserve more energized polarization charges to fully participate in the reaction at the surface reactive sites enriched by strain-induced carbon vacancies. Without any cocatalysts, an exceptional photo-piezocatalytic H2 evolution rate of 12.16 mmol g-1 h-1 is delivered by the g-C3 N4 nanosheets, far exceeding that of previously reported piezocatalysts and g-C3 N4 photocatalysts. Further, high pure-water-splitting performance with production of the value-added oxidation product H2 O2 via photo-piezocatalysis is also disclosed. This work not only exposes the potential of g-C3 N4 as a piezo-semiconductor for catalytic H2 evolution, but also breaks a new ground for the conversion of solar and mechanical energy by photomediated piezocatalytic reaction.