*2.2. Film Preparation*

MWCNTs and GNPs have strong tendencies to form bundles and aggregate together because of their high surface area and the strong van der Waals interaction. They are also hydrophobic and have poor solubility in aqueous solutions [30]. In this study, the MWCNT/GNP hybrid films were prepared with the aid of surfactant Triton X-100. The molecular structure of surfactant Triton X-100 (Big Sun Chemical Corp., New Taipei City, Taiwan) contains a hydrophilic polyethylene oxide group and hydrocarbon lipophilic (or hydrophobic group), which improves the dispersibility of MWCNTs and GNPs in aqueous solution [31]. The hybrid films with different weight ratios of MWCNT and GNP were prepared using the following process. The total mass of MWCNTs and GNPs was held constant at 0.16 g. Surfactant Triton X-100 with the weight of 5 g were dissolved in 500 ml deionized water and dispersed by a sonicate tip (Q700, Qsonica L.L.C., Newtown, CT, USA) for 30 min at 30 W. The sonicator was operated at pulse mode (10 s on and 20 s off). Then, a total mass 0.16 g of MWCNTs and GNPs with a desired weight ratio were added to the suspension and dispersed by a sonicate tip for 3h at 30 W. Upon completion of the dispersion process, the MWCNT/GNP suspension was filtered through a Polytetrafluoroethylene (PTFE) microporous membrane (pore size 0.45 μm, diameter 90 mm) by a vacuum filtration. The experimental setup of the vacuum filtration is shown in Figure 1. After filtration, the hybrid film was peeled off from the filter membrane and washed by a large amount of isopropyl alcohol to remove any residual surfactant. The film was dried in a vacuum oven preheated to 40 ◦C for 12h. The typical thickness of the hybrid film was 60–80 μm. Following the same process, a series of hybrid films with GNP weight percentage ranging from 0 wt.% to 50 wt.% were fabricated to investigate the effect of GNP on the mechanical and electrical properties. In this study, hybrid films with GNP weight percentages of 0%, 10%, 20%, 30%, 40% and 50% were denoted as GNP-0, GNP-10, GNP-20, GNP-30, GNP-40 and GNP-50, respectively.

**Figure 1.** Experimental setup of the vacuum filtration and as-prepared hybrid film.
