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 Bi 5 O 7 Br ultrafine hollow nanotubes (HNTs) with a wall thickness of ∼1 nm as an efficient force-sensitive piezocatalyst for water dissociation. Compared to symmetric [Bi 2 O 2 ]-constructed BiOBr, the Bi 5 O 7 Br HNTs built by axially oriented asymmetric polar [Bi 5 O 7 ] 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. Bi 5 O 7 Br also furnishes a more favorable active Bi site with easy H* desorption for H 2 evolution due to the upshifted p-band center (ε p ) of the Bi 6p orbital. Furthermore, mechanical strain amplifies the advantages of asymmetric polar [Bi 5 O 7 ] units, allowing Bi 5 O 7 Br 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 Bi 5 O 7 Br, 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 H 2 evolution reaction kinetics and thermodynamics. As a result, Bi 5 O 7 Br HNTs deliver an ultrahigh piezocatalytic H 2 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.