Structural tailoring for piezo-phccotocatalysis enhances ferroelectric polarization

Intergrowth ferroelectric semiconductors, owing to their intrinsic strong spontaneous polarization electric fields, can continuously and effectively facilitate the separation and migration of charge carriers. This property makes them highly promising as piezo-photocatalytic candidate materials. Nevertheless, traditional intergrowth ferroelectrics suffer from limitations such as high-temperature synthesis conditions and difficult morphology control, greatly restricting their sensitivity to stress response and exposure of active sites, which in turn impairs their catalytic performance.

In a new study published in Advanced Powder Materials, a team of researchers from China and Singapore reported new perspectives and directions for initiating and investigating the mechanisms of high-performance intergrowth piezo-photocatalysts.

“Although structural tailoring is recognized as a method to modulate polarization strength, the atomic-scale mechanisms linking lattice distortion to polarization remain unclear,” says co-corresponding Hongjian Yu. “Hence, deeper investigation into the enhanced polarization mechanisms resulting from structural tailoring is essential to fully exploit its modulatory potential.”

In the study, two-dimensional ferroelectric intergrowth Bi₇Ti₄NbO₂₁ (BT-BTN) nanosheets were successfully synthesized via a one-step hydrothermal method.

“We managed to characterize their morphology and ferroelectric properties in detail,” shares Yu. “Compared with bulk BT-BTN prepared using high-temperature solid-state processes, the two-dimensional thin-layered nanosheets exhibit more excellent mechanical force sensitivity and higher specific surface area for enhanced CO₂ adsorption.”

Moreover, the intergrowth structure of BT-BTN nanosheets exhibit a stronger remanent polarization, inducing stronger piezoelectric polarization field under same stress compared to parent phases Bi₃TiNbO₉ (BTN) and Bi₄Ti₃O₁₂ (BT), which can effectively enhance charge carrier separation and migration.

Density functional theory (DFT) calculations reveal that intergrowth ferroelectric BT-BTN possesses a unique multi-layer superlattice structure where (Bi₂O₂)²⁺ layers are sandwiched by different layers of perovskite-like (BiTiNbO₇)²⁻ and (Bi₂Ti₃O₁₀)^2-.

“To accommodate this layered mismatch, the octahedra in perovskite-like layers undergo lattice adjustment, leading to self-tilting and self-rotation,” explains Yu. “Specifically, the increase tilting angle along a-axis and rotation angle within the a-b plane of the octahedra could induce stronger spontaneous polarization electric field and higher concentrations of polarization charges.”

Furthermore, when periodic mechanical stress was further applied, two-dimensional thin layers generated larger strain with exacerbating rotation and tilting, resulting in a super-strong piezoelectric polarization electric field.

“This thin-layer structure and powerful spontaneous polarization field not only utilize mechanical energy to promote chemical adsorption and activation of CO₂ molecules on the surface, but also effectively drive the separation and migration of charges from the bulk to the surface, thus improving the photocatalytic reaction activity,” says first author Jingren Ni.

Consequently, the BT-BTN nanosheets demonstrated significantly enhanced CO generation (426.97 μmol g^-1 h^-1), compared with BT-BTN bulks, BT and BTN. “Our work offers new perspectives for high-performance intergrowth piezo-photocatalysts.” adds Ni.