A theoretical analysis of the effect of the hydrogenation of graphene to graphane on its mechanical properties.

We investigated the effect of the hydrogenation of graphene to graphane on its mechanical properties using first-principles calculations based on density-functional theory. The hydrogenation reduces the ultimate strengths in all three tested deformation modes--armchair, zigzag, and biaxial--and the in-plane stiffness by 1/3. The Poisson ratio was reduced from 0.178 to 0.078, a 56% decrease. However, the ultimate strain in zigzag deformation was increased by 8.7%. The shear mode elastic constants are more sensitive than longitudinal ones to hydrogenation. The fourth and fifth order longitudinal mode elastic constants are inert to the hydrogenation, in contrast to a large decrease of those in second and third order. The hydrogenation does not change the monotonic decrement of the Poisson ratio with increasing pressure, but the rate is tripled. Our results indicate that graphene-graphane systems could be used for hydrogen storage with high speed of charge-discharge of hydrogen.