Fig. 9. The effects of (a–d) hydroxide and (e–h) epoxide group density on the mechanical properties of GO upon a tensile loading along zigzag direction with the tensile strain of (a) 11.2% (b) 12.2% (c) 13.2% (d) 15.2 (e) 0% (f) 9.8% (g) 12.2% (h) 14.4% and R = 12.5%.