To understand size effect on the reduction degree of OD-Cu, slabs of different thicknesses (equivalent to spherical nanoparticles of a diameters between 17.5 and 4.1nm) were simulated. Figure shows that the final state differs significantly. Large nanoparticles keep low oxygen quantities if any, while the smallest one retains 15% of O-atoms, and the overall reaction energy is slightly endergonic, Table . The lower the S.S.A., the larger amount of energy (per atom) is released to reach the same reduction level. On the contrary, the higher the S.S.A., the more oxygen is retained in the most stable structure. This phenomenon can be explained by the lower surface energy of Cu2O(111) (0.665 J/m 2 ) compared to Cu(111) (1.339 J/m 2 ) , and the reconstruction of Cu2O surface could significantly reduce the energy of the system as shown in Section S2.2. This observation is in line with a recent experimental study using Atomic-scale STEM and electron energy loss spectroscopy techniques in which the small Cu nanoparticles were completely oxidized to Cu2O while the large Cu nanoparticles formed Cu@Cu2O structures. 6