DC electric field induced orientation of a graphene in water

Graphene, as a classical two-dimensional material, has various excellent physical properties, which can be further transferred into its nanocomposite. Under external fields, the nonspherical nanoparticles in liquid environment will exhibit various deterministic movements, among them is the orientation behavior. By realizing the orientation control of nanoparticles, we can, on one hand, increase the thermal conductivity of the system along the oriented direction, and on the other hand, fabricate novel nano-devices based on the nanoscale self-assembly, which may become the key components in NEMS and Lab-on-a-chip architectures. However, current studies mainly focus on the orientations of one-dimensional rod-shaped particles, like carbon nanotubes. For a two-dimensional nanoparticle, like graphene, the situation is more complex than the one-dimensional one, because two unit vectors should be defined to monitor the orientation behaviors. As far as we know, this part of research has not been extensively carried out. Thus, in this paper, the molecular dynamics method is used to study the orientation of a single uncharged rectangular graphene in water, induced by DC electric fields. We track the orientations of the normal and long-side vectors of graphene. The results show that at a relatively high electric strength of 1.0 V/nm, the graphene is preferred to orient its normal vector perpendicular and its long-side vector with a small angle (located between 0 degrees and 30 degrees) with respect to the electric direction, respectively. With the increase of the electric field strength, the orientation preference of the normal vector along the electric direction is increased. To explain this phenomenon, we calculate the orientation distribution of water molecules in the first hydration shell. The dipoles tend to be parallel to the electric direction, and the surfaces of water molecules tend to be parallel to the surface of graphene. These two combined effects result in the above orientation behavior of the normal vector. Another interesting phenomenon is that the decrease of the length to width ratio of graphene will cause both the orientation preferences of the normal vector and the long-side vector to decrease. By utilizing the Einstein relation, we can obtain the rotational diffusion coefficients of graphene around the normal vector and long-side vector. The qualitative results show that the orientation orders of the normal vector and long-side vector respectively have negative correlations with the rotational diffusion coefficients of the rotation around the long-side vector and the normal vector. The orientation behavior of the platelike graphene actually comes from the competing effects between its rotational Brownian motion and the external field. Increasing the strength of the external field or reducing the rotational diffusivity will both lead to an increased orientation order of the nonspherical nanoparticle.