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 (Ga₂O₃) 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 Ga₂O₃. 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 O₁ or O₂ sites significantly narrows the bandgap, through synergistic upward valence band maximum shifting and built-in field modulation, whereas O₃ 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 Ga₂O₃-based photocatalysts and optoelectronic devices.