Unveiling Strong Electric Fields of Ultrafine Hollow Nanotubes Axially Orienting Asymmetric Polar [BiO] 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 BiOBr ultrafine hollow nanotubes (HNTs) with a wall thickness of ∼1 nm as an efficient force-sensitive piezocatalyst for water dissociation. Compared to symmetric [BiO]-constructed BiOBr, the BiOBr HNTs built by axially oriented asymmetric polar [BiO] 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. BiOBr also furnishes a more favorable active Bi site with easy H* desorption for H evolution due to the upshifted p-band center (ε) of the Bi 6p orbital. Furthermore, mechanical strain amplifies the advantages of asymmetric polar [BiO] units, allowing BiOBr to undergo larger structural distortion with substantially increased ΔU. Under strain, a large upward shift of ε of the Bi 6p orbital occurs for BiOBr, 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 evolution reaction kinetics and thermodynamics. As a result, BiOBr HNTs deliver an ultrahigh piezocatalytic H production rate of 2456.48 μmol g h 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.