Out-of-plane polarization engineering: Optimizing exciton dissociation and solar-to-hydrogen efficiency in photocatalytic water splitting

Photocatalytic water splitting is a promising route for clean energy production, yet remains constrained by poor carrier utilization and low solar-to-hydrogen (STH) efficiency. Here, we demonstrate the role of out-of-plane polarization in addressing these limitations by using 1H phase Cr-based Janus monolayers. The out-of-plane polarization optimizes band edges to match water redox potentials at pH = 0, and the out-of-plane piezoelectric electron redistribution via the Poisson effect extends photocatalytic activity to full pH levels. The Janus homojunction constructed from these monolayers inherits the optimized overpotential and naturally forms a type-II band alignment, and dipole-induced electric field further drives the segregated carriers to opposite sides participating in the respective half-reactions, facilitating exciton dissociation. Remarkably, out-of-plane polarization can break traditional bandgap lower limits of 1.23 eV, which enhances light absorption and boosts STH efficiency to 38.1% for the CrSSe homojunction at pH = 0 and 37% for the −2.5% CrSeTe homojunction in the pH range of 7–11. This work highlights the out-of-plane polarization as a powerful strategy to enhance photocatalytic water splitting performance, offering insight for designing high-efficiency photocatalysts.