Identifying polar and non-polar octahedron units in isomorphic layered perovskites towards efficient piezocatalytic H2 evolution

Mechanical energy-driven water splitting for H₂ evolution is one of the promising strategies to achieve carbon neutrality, but so far the development of efficacious piezocatalysts and the in-depth understanding of the piezocatalytic mechanism still have many deficiencies. Here, two isomorphic layered perovskites, Bi₃TiNbO₉ and SrBi₂Nb₂O₉ constructed by [Bi₂O₂]²⁺ layers and [BiTiNbO₇]²⁻ or [SrNb₂O₇]²⁻ layers along the c-axis, are developed as H₂-evoluting piezocatalysts. Bi₃TiNbO₉ exhibits a piezocatalytic H₂ production rate of 1025.5 µmol g⁻¹ h⁻¹ with glucose as the sacrificial agent, while the H₂ evolution rate of SrBi₂Nb₂O₉ is 586.1 µmol g⁻¹ h⁻¹, respectively. The experimental analyses and theoretical calculations on microstructural information, such as octahedron distortion index and bond angle variance, demonstrate that TiO₆ octahedra have a higher degree of distortion compared to NbO₆ octahedra, which results in more excellent properties in piezoelectricity, polarity and degree of polarity change, carrier concentration and separation efficiency of Bi₃TiNbO₉. In-depth analysis reveals that, under tensile or compressive strain, the a-axis alters significantly and the b-axis underwent a relatively minor change in Bi₃TiNbO₉ due to the fact that perovskite layers show more pronounced response to the stress than the [Bi₂O₂]²⁺ layers. Interestingly, it is found that in the building units of layered perovskites, only the octahedra at specific positions exhibit dipole moment field that contributes to piezocatalysis, while the effect of others can be neglected. Ultimately, a reasonable analysis and explanation of the piezocatalytic process is carried out. This work provides a forward-looking perspective into the design of high-performance piezocatalysts on the basis of accurately distinguishing intrinsic polar units.