## 3 Results and Discussion
Such viscosity change is known as shear thinning, which has been generally observed in nematic liquid  
Figure 1: (**a**) Schematics for preparation of graphene oxides (GOs) and larger-sized GOs (L-GOs) from graphite (top) and expanded graphite (bottom), respectively; (**b**) distribution of lateral flake sizes of GOs and L-GOs (top) and their scanning electron microscopy (SEM) images (bottom). Scale bars in SEM images are 100 \(\upmu\)m.  
crystal suspension [25]. For the L-GO suspension, clear shear-thinning behavior was observed even at the lowest concentration of 4 mg/mL, which suggested that L-GO suspensions could form the nematic liquid crystal phase at a much lower concentration than the GO suspension. Our observations indicated that this difference could mainly be attributed to the average flake size of the L-GOs, which is about four times larger than that of the GOs. In good agreement with [PERSON]'s theory, the four times larger GO flakes induced a four times lower critical concentration for formation of the liquid crystal. It was reported that the liquid crystal behavior of GO suspensions is also influenced by the surface charge of the GO flakes [12]. However, the zeta potentials of the GO (\(-31.64\pm 2.54\) mV) and L-GO (\(-32.96\pm 2.07\) mV) suspensions showed similar values, suggesting that the surface charges do not contribute to the difference in critical concentrations. In addition, lamellar domains in L-GO increased as the concentration of L-GO increased, and more continuous lamellar domains formed, like a hairy structure. As a result, the L-GO suspension showed a more ordered state than that of the GO suspension at the same concentration. Using the simple doctor blade technique, bulk-scale GO films were prepared from the GO and L-GO suspensions at a concentration of 20 mg/mL (Figure 3a). At this concentration, both GO and L-GO suspensions showed ordered nematic liquid crystal behavior; therefore, aligned GO flakes in the films were easily obtained by applying shear forces with the doctor blade (Figure 3b). The thicknesses of both the GO and L-GO films were around 30 \(\upmu\)m and many micrometer scale voids were observed in their cross-sections. After HI reduction, these microscale voids disappeared, and film thicknesses were reduced to around 3.96 \(\upmu\)m. These observations were consistent with our XRD results (Figure 3c). Due possibly to the removal of oxygen functional groups or remaining intercalants between GO flakes, the d-spacings of GO (9.40 A) and L-GO (9.88 A) films were remarkedly reduced to 3.72 and 3.68 A, respectively, by HI reduction. Figure 2: Polarized optical microscopy (POM) images of (**a**) GO and (**b**) L-GO suspensions according to their concentrations. Insets show schematics of GO and L-GO flakes in suspensions for each POM image. Scale bars are 200 \(\upmu\)m. (**c**) Plots of viscosity versus shear rate according to concentrations of GO and L-GO suspension. The chemical structures of graphene-based films made from GO and L-GO flakes were analyzed by FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible (UV-Vis) spectroscopy. The FTIR spectra of both GO and L-GO showed five typical peaks at 1050, 1130, 1620, 1720, and 3320 cm\({}^{-1}\), which corresponded to the C-O, C-O-C, C=C, and C=O stretching and O-H vibration modes, respectively (Figure 4a). Compared to GO, L-GO showed weaker O-H vibration and C=O stretching peaks; it also showed a slightly stronger C-O-C stretching peak. These differences may have resulted from size differences between the GO and L-GO flakes. It is known that oxygen functional groups on the basal planes of GOs are mainly epoxides, whereas carboxylic functional groups are most abundant at the edges of GOs [26; 27; 28]. Increasing the flake size means that the fraction of edges in the overall flakes decreases, resulting in a decreased fraction of carboxylic functional groups and an increase of epoxide groups.