Computational discovery of bilayer transition metal monoxides exhibiting Dirac fermions, superior carrier mobility, and efficient photocatalytic CO₂ reduction

Transition metal monoxides (MOs) have long been recognized for their versatile electronic and catalytic properties, yet their two-dimensional (2D) forms remain largely unexplored. In this study, we theoretically design a bilayer MO sheet featuring a graphene-like hexagonal lattice. Through systematic screening of various transition metals, we identify nine stable MO bilayers. Detailed band structure analyses categorize these into five semiconductors, two metals, and two Dirac semimetals. Remarkably, the Dirac semimetal TiO exhibits Fermi velocities up to 3.8 × 10⁵ m s^-1, while the semiconducting CdO demonstrates exceptional carrier mobility of 7848.33 cm² s^-1 V^-1, surpassing many well-known 2D materials such as MoS₂ and WS₂. Optical studies reveal strong visible-light absorption in CdO and ScO, highlighting their promise as efficient light-harvesting materials. Crucially, CdO and ZnO show energetically favorable catalytic activity for the reduction of CO₂ to CH₄, with rate-limiting energy barriers of 1.09 eV and 0.18 eV, respectively, outperforming benchmark photocatalysts like g-C₃N₄ and MoS₂. Additionally, the competing hydrogen evolution reaction is effectively suppressed, ensuring high selectivity and catalytic efficiency for CO₂ conversion. The unique combination of Dirac fermions, outstanding carrier mobility, robust visible-light absorption, and superior CO₂ reduction capability position these MO bilayers as highly promising candidates for next-generation nanoelectronic and photocatalytic applications.