Unveiling Strong Electric Fields of Ultrafine Hollow Nanotubes Axially Orienting Asymmetric Polar [Bi5O7] Units for Efficient Piezocatalytic Water Splitting

Exploiting efficient piezocatalytic systems for water splitting is a promising avenue to generate clean energy carriers, though it remains challenging. Here, we develop Bi₅O₇Br ultrafine hollow nanotubes (HNTs) with a wall thickness of ∼1 nm as an efficient force-sensitive piezocatalyst for water dissociation. Compared to symmetric [Bi₂O₂]-constructed BiOBr, the Bi₅O₇Br HNTs built by axially oriented asymmetric polar [Bi₅O₇] units demonstrate high chemical bond anisotropy and greater local electrostatic potential difference (ΔU) at all the [-Bi-Br-], [-Bi-O-] and [-Br-Br-] areas, rendering strong piezoelectricity and internal electric field. Bi₅O₇Br also furnishes a more favorable active Bi site with easy H* desorption for H₂ evolution due to the upshifted p-band center (ε_p) of the Bi 6p orbital. Furthermore, mechanical strain amplifies the advantages of asymmetric polar [Bi₅O₇] units, allowing Bi₅O₇Br to undergo larger structural distortion with substantially increased ΔU. Under strain, a large upward shift of ε_p of the Bi 6p orbital occurs for Bi₅O₇Br, which weakens the interaction between Bi sites and H*, bringing more favorable chemisorption and H* adsorption with a diminished energy barrier, thus resulting in improved H₂ evolution reaction kinetics and thermodynamics. As a result, Bi₅O₇Br HNTs deliver an ultrahigh piezocatalytic H₂ production rate of 2456.48 μmol g^-1 h^-1 from pure water in the absence of sacrificial agents, with a mechanical-to-hydrogen efficiency of 0.28%, as well as comparable activity in seawater and tap water. This work proposes a promising tactic for seeking efficient piezocatalysts by designing an ultrafine nanostructure incorporating favorably oriented asymmetric structural units.