Facet‐Engineering in Cs ₃ Bi ₂ Br ₉ Vacancy‐Ordered Perovskite With Built‐In Anisotropic Charge Separation for Selective Photocatalytic Toluene Oxidation

ABSTRACT Controlling the exposure of anisotropic crystal facets provides a promising strategy to direct the migration of photogenerated charges and improve photocatalytic performance. Nevertheless, the underlying charge dynamics resulting from facet engineering in 2D Cs 3 Bi 2 Br 9 vacancy‐ordered perovskite remain poorly understood. Herein, we synthesize two distinct morphologies of Cs 3 Bi 2 Br 9 single crystals (nanosheets and nanostrips) with different ratios of exposed (001) to elucidate this mechanism. Surface photovoltage mapping, selective photodeposition, and DFT calculations jointly confirm the existence of anisotropic charge separation driven by an intrinsic facet junction electric field (∼180 meV). This electric field directs photogenerated holes to accumulate on the (001) facets for toluene activation, while electrons migrate to the (010) facets. Benefiting from this intrinsic facet junction, medium‐sized Cs 3 Bi 2 Br 9 nanosheets with optimal (001) facet exposure achieve a high benzaldehyde production rate of 692 µmol·g −1 ·h −1 with 87% selectivity, significantly outperforming other morphologies. This work provides the first demonstration of facet engineering in Cs 3 Bi 2 Br 9 , establishing a crystal‐orientation design principle for efficient and selective photocatalytic organic transformations.