▎ 摘　　要

In this Letter, we found that the growth of graphene on Cu oxide foil is significantly affected by the concentration of oxygen. The grain size of graphene grown on a Cu substrate with a relatively high oxygen concentration is much smaller than that on the substrate with lower oxygen concentration. By controlling the oxidation of the Cu substrate at a proper degree, we can obtain millimeter scale graphene single crystals at a growth temperature of 1050 degrees C. On the basis of our experimental observations, the dual role of oxygen in the CVD growth of graphene was revealed: (i) Oxygen on a Cu surface can contribute to the decomposition of hydrocarbon feedstock and decrease the graphene growth barrier, resulting in an increased growth rate and a larger grain size of graphene; (ii) excess oxygen in the Cu substrate leads to etching of the graphene edge. Our research provides insights to obtain large-area and single-crystalline graphene by choosing a proper Cu oxide substrate.