Ultrathin Single Crystalline MgO(111) Nanosheets**

Synthesizing high-quality two-dimensional nanomaterials of nonlayered metal oxide is a challenge, especially when long-range single-crystallinity and clean high-energy surfaces are required. Reported here is the synthesis of single-crystalline MgO(111) nanosheets by a two-step process involving the formation of ultrathin Mg(OH)₂ nanosheets as a precursor, and their selective topotactic conversion upon heating under dynamic vacuum. The defect-rich surface displays terminal -OH groups, three-coordinated O²⁻ sites and low-coordinated Mg²⁺ sites, as well as single electrons trapped at oxygen vacancies, which render the MgO nanosheets highly reactive, as evidenced by the activation of CO molecules at low temperatures and pressures with formation of strongly adsorbed red-shifted CO and coupling of CO molecules into C₂ species.