Pentagonal Janus SiXY monolayers: a new frontier for 2D photocatalysis with high solar-to-hydrogen efficiency

Photocatalytic water splitting represents a cornerstone technology for sustainable hydrogen production, with two-dimensional (2D) materials offering unparalleled potential due to their vast specific surface area and unique electronic structures. This work introduces, through first-principles calculations, a new family of 2D Janus pentagonal SiXY (penta-SiXY, X, Y = P, As, and Sb) monolayers as highly promising photocatalysts. Our calculations confirm their exceptional dynamic, thermal, and mechanical stability. These penta-SiXY monolayers are intrinsic semiconductors with suitable band gaps ideally suited for solar energy harvesting (1.28-2.33 eV) and exhibit remarkable visible-light absorption coefficients (up to 3.6 × 10⁵ cm^-1). Critically, we demonstrate that biaxial strain engineering not only preserves the requisite band edge alignment for overall water splitting across a wide pH range but also significantly enhances photocatalytic performance. Under a modest biaxial strain, the theoretical limit value of solar-to-hydrogen (STH) efficiency can be boosted to an impressive 29.27%, coupled with visible light absorption reaching 4.06 × 10⁵ cm^-1. This study identifies Janus penta-SiXY monolayers as a class of superior photocatalysts and establishes strain engineering as a potent and scalable strategy for optimizing the photo-efficiency of novel 2D materials.