# Fabrication of Eco-Friendly Graphene Nanoplatelet Electrode for Electropolishing and Its Properties
## 3 Results and Discussion  
GNP was noncovalently functionalized by melamine to form highly oriented liquid crystals. We used a one-step ball milling process for the melamine functionalization process. The ball milling process for graphene functionalization has advantages on mass production for defect-free graphenes [11]. The functionalization of GNPs was verified by spectroscopic analysis. The XPS and Raman spectra of the functionalized GNP are presented in Figure 1. Peaks corresponding to C and O elements were observed for pristine GNPs, and after melamine functionalization, additional N element was observed in a survey scan (Figure 1a). The C1s peak region in the XPS spectra of the GNPs (Figure 1b,c) was deconvoluted into five peaks at about 284.6, 285.2, 286.2, 286.58, and 289.08 eV, which corresponded to C=C, C\(-\)C, C\(-\)O, C\(-\)N, and \(\pi-\pi^{*}\) shake-up entities, respectively. The peak at 284.6 eV was attributed to the sp\({}^{2}\) graphitic structure with a \(\pi-\pi^{*}\) transition peak at 289.08 eV, indicating interaction between GNPs. The peak at 286.2 eV was attributed to defects in the GNPs (Figure 1b). After non-covalent functionalization, an additional peak at 286.58 eV was exhibited, corresponding to the C\(-\)N bonds of melamine (Figure 1c) [12; 13]. The Raman spectra of pristine GNPs and M-GNP showed D-, G-, and 2D bands at 1345, 1574, and \(\sim\)2700 cm\({}^{-1}\) (Figure 1d), respectively. GNP and M-GNP had similar I\({}_{\text{D}}\)/I\({}_{\text{G}}\) ratios because noncovalent processes do not affect the intrinsic properties of GNP. The I\({}_{\text{D}}\)/I\({}_{\text{G}}\) ratio of the pristine GNP was 0.906; that of M-GNP was 0.95. However, a significant shift and shape difference in the 2D band was observed between GNP and M-GNP. Compared with that of pristine GNPs (2688 cm\({}^{-1}\)), the 2D band of the M-GNPs was upshifted to 2690 cm\({}^{-1}\)[14]. The increase in the number of \(\pi-\pi\) interactions among the hexagonal rings of graphenes split the \(\pi\) and \(\pi^{*}\) bands due to the frequency of the scattering phonons, which is determined by the double-resonance condition [15]. As a result, the increased number of \(\pi-\pi\) interactions resulted in an upshift of the 2D bands. Therefore, the upshifted 2D band of M-GNP indicates that melamine molecules interacted with the GNP surface via \(\pi-\pi\) interactions [11]. To observe the liquid crystal phase transition of M-GNP from isotropic to nematic, a 20 mg/mL M-GNP suspension was observed by polarized optical microscopy (POM) (Figure 2a). The birefringence domains were observed at 20 mg/mL of M-GNP, which is a common characteristic of lyotropic liquid crystal materials. Then, these nematic-phase gels were spread on a PTFE substrate with direct doctor blades printing. By applying shear forces to nematic-phase gels, M-GNP particles created highly aligned parallel structures of pre-standing films. The M-GNP film was fabricated into a rectangular shape with dimensions of 5 cm \(\times\) 2 cm; the thickness of the M-GNP film was 15.59 \(\pm\) 0.493 \(\upmu\)m (n = 6) (Figure 2b) [16]. We observed that M-GNP was highly oriented in the lateral direction, and micrometer-scale voids and defects disappeared. This highly oriented structure offers a larger sized graphene domain and induces both high load transfer and modulus properties. Micro-voids and defects often cause stress concentration during deformation of the films. Therefore, the lateral orientation of the graphene particle is oneof the key elements enhancing the mechanical and electrical properties of the films [17]. Within this highly oriented structure, the M-GNP film exhibited both higher modulus (4.2 \(\pm\) 0.71 GPa, n = 6) and 161 higher strength (70.5 \(\pm\) 10.8 MPa, n = 6) than those values of vacuum-filtered GNP films (Figure 2c). Ideal GNP has a high mobility beyond 200,000 cm\({}^{2}\)/Vs, but M-GNP films were fabricated using multiple lateral crystals, for which the electrical conductivity of the film depends on the tunneling model [18].