*3.2. Electrical Properties*

The sheet resistivity of the hybrid film was measured by the Hall effect (Ecopia HMS-3000). To demonstrate the reproducibility of the experimental results, three specimens with square shape (10 × 10 mm) were cut from different locations of the hybrid film and tested. The average value was reported with standard deviation.

Table 2 lists the electrical resistivity and conductivity of the hybrid film with different weight percentages of GNP. It can be seen that the conductivity of the hybrid film is increasing with the increase of the content of GNP as shown in Figure 5. While both MWCNTs and GNPs are highly conductive, GNPs are more conductive for two reasons. Firstly, their two-dimensional nature results in a better connectivity and so a greater choice of conductive paths for electrons to flow through. Secondly, their planar nature allows them to pack more closely than MWCNTs, giving lower porosity [35]. Thus, GNP is the dominant factor on the electrical property of the hybrid film. A remarkable increase in the electrical conductivity from 47.72 S/cm to 192.60 S/cm was observed when the GNP content was increased from 0 to 50 wt.%. The conductivity of the hybrid film GNP-50 was enhanced by 304% in comparison with the GNP-0, due to the formation of 3D conductive networks [36]. The hybrid film exhibits a well-stacked layered structure throughout the cross section. The MWCNT network bridges the gap between the GNPs. Larger lateral dimension of GNP acts as a strong holder while MWCNT serves as a wire to connect GNP. The conductivity of the hybrid film depends on the conductive network formed by the MWCNTs and the inherent conductivity of GNPs. At a low weight fraction of GNP, MWCNT and GNP are not close-packed to form effective conductive pathways in the hybrid film. The overlap of MWCNTs introduces larger interfacial resistance that further decreases the conductivity of the hybrid film with too much MWCNTs in the grapheme layer. When the fractions of GNPs were increased, the percolated network of MWCNTs and GNPs was formed

which provided efficiently conductive pathways for electron transfer in the hybrid film. The decrease in the sheet resistivity by incorporation of GNPs demonstrates that two-dimensional GNPs provide a more efficient percolating network than one-dimensional MWCNTs. Furthermore, GNP worked as strong holders with a large surface area to support contact between the MWCNT and GNP, resulting in a further reduction of the contact resistance. In the MWCNT-dominated hybrid film, a pronounced synergistic effect on conductivity can be observed. The electrical conductivity is related to both in-plane and through-thickness conduction of electrons. It is clear to see from Figure 2 that 1-D MWCNTs act as bridges to connect 2-D GNPs and provide additional channels for the electron transfer within the hybrid film. This leads to a decreased electrical resistance and may be considered as the major reason for the synergistic effect of the MWCNT and GNP hybrid films. In addition, high electrical conductivity of GNP in the basal plane enhances the synergistic effect on electrical conductivity.


**Table 2.** Electrical properties of hybrid film with different weight percentage of GNP.

**Figure 5.** Electrical conductivity of MWCNT/GNP hybrid films with different weight percentages of GNP.
