Strain‐Tunable Bandgaps, Strong Light Absorption, and Active Hydrogen Evolution Reaction in Light Metal‐Decorated Borophene

Current strategies for stabilizing 2D borophene sheets and compensating for boron's electron deficiency primarily rely on transition metals, many of which are heavy and costly, thereby limiting their practical applications. In contrast, the use of light‐metal elements to stabilize borophene has remained largely unexplored. This work demonstrates that the light‐metal beryllium (Be) can effectively stabilize 2D boron sheets. Through swarm‐intelligence structural searches combined with first‐principles calculations, three stable monolayers of beryllium tetraboride (BeB 4 )—the α‐, β‐, and γ‐phases—are predicted. The α‐BeB 4 phase is metallic, while β‐ and γ‐BeB 4 are indirect semiconductors with bandgaps of 0.39 and 1.65 eV, respectively. Notably, biaxial strain can reversibly tune β‐BeB 4 between semiconducting and metallic states. Furthermore, γ‐BeB 4 exhibits a high visible‐light absorption coefficient (3.80 × 10 5 cm − 1 ) and an out‐of‐plane negative Poisson's ratio, indicating auxetic behavior. Remarkably, α‐BeB 4 demonstrates a near‐zero Gibbs free energy for hydrogen adsorption, suggesting excellent catalytic activity for the hydrogen evolution reaction. These unique properties position BeB 4 monolayers as promising, lightweight, and cost‐effective candidates for next‐generation energy conversion and optoelectronic applications.