Sandwiched Epitaxy Growth of Single-Crystalline Hexagonal Bismuthene Nanoflakes for Highly Selective Electrocatalytic CO2-to-formate Conversion

Two-dimensional (2D) bismuthene material was predicted to possess intriguing physical and electrical properties, such as high-temperature quantum-spin Hall effect, topological edge state, high carrier mobility, and tunable band gap. However, epitaxial growth of single-crystalline 2D bismuthene nanoflakes inevitably requires a high vacuum environment, primarily due to the high surface energy constraints of Bi. Herein, we report the growth of 2D single-crystalline hexagonal bismuthene nanoflakes on Cu foil substrate at atmospheric pressure by chemical vapor deposition. Based on first-principles calculations, the structural transformation of Bi on Cu foil can be suppressed by introducing the top h-BN layer, which potentially compensates for the charge transfer from Bi to the Cu (1 1 1) surface. The sandwich structure is identified by cross-sectional SEM and EDS characterization, demonstrating that bismuthene nanoflakes are sandwiched between the h-BN film and Cu foil. Benefiting from the encapsulation of the top h-BN layer, bismuthene nanoflakes also exhibit excellent thermal stability in ambient air even after annealing at 500 °C for 10 min. For further practical application, bismuthene nanoflakes are utilized for electrochemical CO2 reduction reactions (CO2RR). These bismuthene nanoflakes demonstrate remarkable ability in converting CO2 to formic acid with a Faradaic efficiency of 96.3% at ‒1.0 V (vs. RHE) and exhibit great catalytic stability with a Faradaic efficiency of over 90% in 15 h CO2RR tests. The ultrathin 2D feature of as-prepared bismuthene nanoflakes may result in abundant CO2 adsorption sites and stabilize the intermediate *OCHO, finally favoring the formation of HCOOH. We provide an effective strategy to simultaneously synthesize and passivate 2D single-crystalline bismuthene nanoflakes towards CO2RR, which is expected to be applied to other 2D materials with strong metallic growth behavior.