Tuning the Electronic Structure and Photocatalytic Properties in Polar Two‐Dimensional Gallium Oxide Through Interlayer Stacking and Sulfur Doping

ABSTRACT Two‐dimensional (2D) gallium oxide (Ga 2 O 3 ) holds great promise for photocatalysis due to its intrinsic out‐of‐plane polarization and built‐in electric field, which facilitate charge separation. However, its wide bandgap severely limits visible‐light absorption. Herein, first‐principles calculations are employed to explore two complementary strategies for bandgap engineering in polar 2D Ga 2 O 3 . In bilayers, reversing the polarization direction of one monolayer switches the interlayer band alignment from staggered to broken‐gap, enabling giant tunneling electroresistance for ferroelectric/antiferroelectric tunnel junctions. Nevertheless, this approach is insufficient for photocatalysis, as parallel polarization causes bandgap closure while antiparallel polarization yields only marginal bandgap reduction. To address the visible‐light limitation while preserving the built‐in field, site‐selective sulfur doping is introduced in the lowest‐energy FE‐ZB′ monolayer. Substitution at O 1 or O 2 sites significantly narrows the bandgap, through synergistic upward valence band maximum shifting and built‐in field modulation, whereas O 3 substitution widens it. The doped systems maintain strong surface potential differences, enabling spatial separation of photogenerated carriers and satisfying water redox potentials. Enhanced visible‐light absorption and favorable OER/HER overpotentials confirm their viability for overall water splitting. Strain engineering further demonstrates robust tunability. This work establishes a synergistic framework combining polarization control with atomic‐scale doping for high‐performance 2D Ga 2 O 3 ‐based photocatalysts and optoelectronic devices.