Breaking centrosymmetry induces piezo-pyroelectric coupling in sulfur-vacancy Bi₂S₃ for enhanced hydrogen evolution

Piezo-pyroelectric coupling catalysis enables efficient harvesting of ambient mechanical and thermal energy for green hydrogen production, yet its activation in intrinsically centrosymmetric materials remains a major challenge. Herein, we propose a sulfur vacancy engineering strategy to break the inherent centrosymmetry of orthorhombic Bi₂S₃, successfully unlocking synergistic piezo-pyroelectric effects for enhanced hydrogen evolution. Structural characterization studies and theoretical calculations confirm that sulfur vacancy introduction induces localized lattice distortion and charge redistribution, generating sizable net dipole moments to transform the non-polar host into a polar architecture. Crucially, the emergence of a pronounced second-harmonic generation (SHG) signal with a distinct four-lobed pattern provides direct optical evidence for the symmetry breaking. The defective Bi₂S₃ exhibits markedly boosted piezoelectric and pyroelectric responses and delivers exceptional hydrogen production performance under ultrasonic excitation driven by coupled piezo-pyroelectric polarization. Control experiments further clarify the dominant contribution of the pyroelectric effect to the overall catalytic activity. Moreover, the catalyst demonstrates outstanding cycling stability without obvious structural degradation. This work provides a defect-mediated symmetry-breaking paradigm to activate piezo-pyroelectric coupling in layered chalcogenides for sustainable energy conversion.