Considering typical experimental settings, a Cu2O nanoparticle with the diameter of 20 nm 37 , at pH 8 and U = -0.3 VRHE, should be reduced to pure Cu. To simulate the high roughness of the surface, a 3240 atom Cu slab derived by removing all oxygen from a 30 atomic layers Cu2O(111)-p(9×9) was used. After equilibrium, the slab structure (equivalent to 4.4 nm NP) forms a loss configuration with average coordination number (with a decay controlled by an error function ) of 9.08. 2861 active sites are detected via Delaunay triangulation sampling of the last frame of the simulation, and there are 16 unique active sites based on the isomorphism analysis as shown in Figure . The last 9 unique active sites together represent only 0.45% of the number of total active sites, thus they are categorized to 1 class (H) for analysis. Therefore, 8 classes of active sites are considered. A, B, and C categories account for the most, 13.28%, 49.98%, and 27.19% populations, corresponding to atop, fcc/hcp, and bridge sites, respectively, and the Cu atoms in these sites could be under-coordinated. Class F (2.48%) also represents three coordinated sites, but unlike the hcp/fcc sites, two of the Cu atoms are not interconnected. D(4.89%), E(1.12%), and G(0.59%) categories represent four coordinated sites, where E is similar to the hollow site on (100), while the other two categories are connected in different ways.