Lewis Acid–Base-Driven Anisotropic Crystal Growth of Pyrochlore Pb ₂ Ti ₂ O _5.4 F _1.2 with Enhanced Visible-Light H ₂ Evolution Activity

Mixed-anion compounds offer unique functionalities unattainable with single-anion materials, yet rational morphology control remains largely unexplored. Here, we report a Lewis acid–base-driven strategy that enables low-temperature, solution-phase morphology control of the oxyfluoride photocatalyst Pb2Ti2O5.4F1.2 (PTOF). A microwave-assisted solvothermal method with monoethanolamine (MEA) was used to tune the particle morphology and size via the precursor-solution pH, which was adjusted by the addition of formic acid or acetic acid. PTOF, an A2B2X6X′0.5-type pyrochlore with intrinsic anion vacancies (X′2, 4d site), has exposed {111} facets composed of alternating Pb-rich and Ti-rich layers. Lewis basic MEA is proposed to bind selectively to undercoordinated, strongly acidic Pb2+ sites adjacent to Ti4+ and vacancies on Ti-rich {111} facets, suppressing growth along the surface direction and stabilizing these facets, thereby driving anisotropic crystal growth and forming plate-like nanoparticles. At pH 10 (formic acid), PTOF nanoparticles (∼30 nm) with a specific surface area of 37 m2 g–1 were obtained. Compared with an analogous PTOF synthesized by a conventional solid–state reaction, the optimized sample exhibited ∼29-fold higher H2 evolution activity in an aqueous solution containing dissolved disodium ethylenediaminetetraacetate under visible-light (λ > 400 nm) with the aid of a Pt cocatalyst. Lewis acid–base-directed facet stabilization is thus shown to be a promising approach for the rational morphological design of mixed-anion oxyfluorides via solution processing.