*2.3. Morphology*

Field emission scanning electron microscope (JSM-7600F, Jeol Ltd., Tokyo, Japan) was performed to characterize the surface morphology and cross-section view of the hybrid film. An accelerating voltage of 10 kV and a working distance of 5–10 mm were adopted to generate the field emission scanning electron microscope (FESEM) images of the hybrid film. The samples were sputter-coated with a conductive gold layer before taken the image.

A typical MWCNT/GNP hybrid film is shown in Figure 1 It can be seen that the film is highly flexible, which can be rolled up or bear small radius bending without any damage or fracture. The surface morphology and cross-section view of the hybrid film with 0 wt.% (GNP-0), 20 wt.% (GNP-20) and 50 wt.% (GNP-50) of GNP are presented in Figure 2. The surface morphology of the film GNP-0 (0 wt.% GNP and 100 wt.% MWCNT) exhibits homogenous and densely packed mass of randomly oriented MWCNTs without any agglomeration, and this orientation gives rise to its isotropic properties as shown in Figure 2a. For the hybrid film GNP-20 (20 wt.% GNP and 80 wt.% MWCNT), most of the GNPs are covered by the MWCNTs as shown in Figure 2c. As the content of GNP increases, some of the GNPs can be observed on the top of MWCNTs as shown in Figure 2e for hybrid film GNP-50 (50 wt.% GNP and 50 wt.% MWCNT). GNPs and MWCNTs are uniformly dispersed and highly entangled with each other. From the cross-section SEM images shown in Figure 2b,d and f, MWCNTs and GNPs are successfully deposited to form densely packed film with layered structure. Similar layered structure with MWCNTs distributed between GNP sheets in the flexible GNP/MWCNT film using as a high performance supercapacitor was also reported by Lu et al. [11]. It can be attributed to the filtration-induced directional flow during the fabrication process. Under the vacuum filtration pressure, the 2D GNP tended to self-adjust their basal planes parallel to the filter membrane plane due to the large aspect ratio of GNP sheets, resulting in significant alignment of GNP sheets [32]. Clearly, GNP sheets served as the supporters to hold the MWCNTs in-between, generating a more compact and aligned structure of hybrid films. Graphene sheets uniformly spread on MWCNTs and alternately stacked layer structure are observed. With the increase of GNP content, the long and tortuous MWCNTs are embedded between the GNP layers, which can prevent the aggregation of GNPs. It appears that MWCNTs were preferentially oriented and bridged the gap between the GNP layers.

*Nanomaterials* **2018**, *8*, 786

**Figure 2.** SEM images of hybrid films (**a**) surface morphology of GNP-0; (**b**) cross-section view of GNP-0; (**c**) surface morphology of GNP-20; (**d**) cross-section view of GNP-20; (**e**) surface morphology of GNP-50; (**f**) cross-section view of GNP-50.
