*3.2. Dispersion State and Chemical Analysis of TOCNF-CNT and TOCNF-CNT@PANI Nanohybrids*

The microstructure morphologies of CNTs, TOCNF-CNT, and TOCNF-CNT@PANI-2 nanohybrids are shown in Figure 2a–c. Due to the hydrophobic surface and strong van der Waals forces between CNT fibers, it was difficult for the pristine CNTs to disperse well in water [13]. It was clear to see that pristine CNTs formed densely agglomeration and precipitation in water (Figure 2a and insert). After introducing TOCNFs into CNT suspension, as shown in Figure 2b, the CNTs (length: 500–700 nm, diameter: 20–30 nm) dispersed in an individual form without aggregation in water with the help of TOCNFs (length: 1 μm, diameter: 20–30 nm).

**Figure 2.** Transmission electron microscope (TEM) images of (**a**) CNTs, (**b**) TOCNF-CNT nanohybrids, (**c**) TOCNF-CNT@PANI nanohybrids, and the inset in each image represents the corresponding dispersion state of each suspension at the concentration of 0.5 wt%; (**d**) Ultraviolet-visible (UV-vis) spectra of neat CNTs, TOCNF-CNT, and TOCNF-CNT@PANI-2 nanohybrids in water. (**e**) Fourier transform infrared (FTIR) spectra and (**f**) X-ray diffraction (XRD) patterns of TOCNFs, CNTs, TOCNF-CNT, TOCNF-CNT@PANI-2 nanohybrids, and TOCNF-CNT@PANI/PVA-2 composite hydrogel.

Figure 2c shows nodular-structure. PANI particles were deposited on the surface of TOCNF-CNT nanohybrids, forming a continuous shell structure with a high aspect ratio. The in-situ deposited aniline fiber could grow a fresh polymer, then initiate the continuous growing process and form a block precipitate, and the initial seed morphology would be transcribed on a long scale [14]. TOCNF-CNT@PANI composite fiber with a "core-shell" structure was the prototype of the 3D hierarchical conductive network. The PANI grew along the TOCNF-CNT templates to form the "shell", which was carried by TOCNF-CNT fibers, and dispersed in water to construct a hierarchical 3D conductive network. It was noted that the continuous conductive networks fabricated by TOCNF-CNT@PANI composite fibers were obligatory to synthesize the ECHs with high conductivity and enhanced mechanical performances [15].

The UV-vis spectroscopy of CNT, TOCNF-CNT, and TOCNF-CNT@PANI-2 complexes are shown in Figure 2d. Due to the poor dispersibility and high aggregation, the absorption peak of pure CNT suspension was much weaker than that of the TOCNF-CNT nanohybrids at the same CNT concentrations (0.015 wt%), suggesting that TOCNFs improved significantly the dispersibility of CNTs in an aqueous medium. The absorption peak of TOCNF-CNT nanohybrids was around 262 nm in the UV-vis spectrum, which could be assigned to the π−π transitions of CNTs and showed a more uniform dispersion state of CNTs [16]. Two peaks of TOCNF-CNT@PANI-2 at 285 and 475 nm were from the transition of π−π inter-band and the polaron band of PANI, respectively. The end band (~800 nm) of the TOCNF-CNT@PANI-2 complexes was smooth because of the cross-linking of polymer chains, indicating that it was successful in doping PANI into the conductive state [17].

The FTIR spectra of TOCNFs, CNTs, TOCNF-CNT nanohybrids, TOCNF-CNT@PANI-2 composite, and TOCNF-CNT@PANI/PVA-2 hydrogel is shown in Figure 2e. For all the samples, the broad bands at around 3330 cm−<sup>1</sup> and sharp bands at around 2900 cm−<sup>1</sup> were generally due to O-H stretching of the hydrogen bonds and asymmetrically stretching vibration of C–H in the CH2 group, respectively [18–21]. The peaks around 1000 cm−<sup>1</sup> and 1600 cm−<sup>1</sup> were ascribed to the C–O–C vibration and the carbonyl functional groups of cellulose [22–24]. For the spectra of neat CNTs, the characteristic absorption around 1640 cm−<sup>1</sup> was assigned to the quaking of the carbon skeleton [25,26]. For the spectra of TOCNFs, the peaks at 1430 cm−<sup>1</sup> and 1325 cm−<sup>1</sup> were assigned to the hydrogen bonding and CH2 wagging [27]. The typical absorption peak of TOCNF-CNT-2 nanohybrids was analogous to that of TOCNFs, which was due to the absorption peak of TOCNFs covering the absorption peak of the CNTs in the FTIR spectra, revealing the CNTs were successfully combined with TOCNF bio-templates [28]. Comparing with the spectra of pure CNTs, the C–O–C bending band shifted from 1100 cm−<sup>1</sup> to 1000 cm−<sup>1</sup> (TOCNF-CNT-2 nanohybrids), confirming the existence of hydrogen bonding between TOCNFs and CNTs [29].

After in situ polymerization, the peaks at 1430 and 1325 cm−<sup>1</sup> were assigned to the C = C vibration deformation of the quinoid ring and benzenoid ring, respectively [30]. For TOCNF-CNT@PANI-2 complexes, the N−H bonding vibration caused the major peaks at 3330 cm−<sup>1</sup> shifted to lower wavenumbers, indicating that the TOCNF-CNT nanohybrids were coated with PANI [31]. Further, the vanishing of the sharp band at 1600 cm−<sup>1</sup> from the carbonyl functional groups of cellulose revealed that PANI coated onto TOCNF-CNT nanohybrids successfully. For TOCNF-CNT@PANI/PVA-2, the distinct peaks at 1430 and 1325 cm−<sup>1</sup> were attributed to the asymmetric B−O−C bonding, 840 cm−<sup>1</sup> was assigned as B–O bonding of free B(OH)4 <sup>−</sup>, and 660 cm−<sup>1</sup> was ascribed to B–O–B bonding in the borate molecule networks, suggesting the presence of borax and borate [32], which further confirmed the existence of borate cross-linking network between the PVA molecule chains, TOCNF−CNT@PANI nanohybrids, and borate within the hydrogels [33].

Figure 2f shows the XRD diffraction patterns of CNTs, TOCNFs, TOCNF-CNT nanohybrids, TOCNF-CNT@PANI-2 nanocomposite, and TOCNF-CNT/PVA-2 hydrogel samples. A diffraction peak of CNTs at 2θ = 26◦ arose from interlayer spacing (002), reflecting the characteristic of graphite. The diffraction peak at 2θ = 43◦ arose from in-plane crystal lattice (100) [34]. In the XRD diffraction patterns of TOCNFs, a sharp peak and a broad peak at 2θ = 22.2◦ and 15.0◦, attributed to (002) and (101)

planes, suggested the crystallization from cellulose I [22]. Comparing with the XRD profile of pure CNTs, the TOCNF-CNT nanohybrids showed two additional peaks at 2θ = 15.0◦ and 22.2◦, reflecting the characteristic of cellulose I. These observations suggested that CNTs and TOCNFs were combined and remained integrality [35].

For the XRD spectra of TOCNF-CNT@PANI-2 nanocomposite, due to in situ polymerization of ANI monomers, the peaks located at around 2θ = 15.0◦ and 22.2◦ were wider than that of TOCNF-CNT nanohybrids, which could be attributed to the overlapping of diffraction peaks at 2θ = 19.4◦ and 14.9◦ from (020) and (011) crystal planes of PANI [36]. The peak intensity at 2θ = 25.8◦ was enhanced, which was due to the overlapping of diffraction peaks of π-π stacking corresponding to the co-facially stacked conjugated backbones from the polymer chains of PANI [37]. After the incorporation of PVA hydrogel, the peaks of CNFs at 2θ = 15◦ disappeared, and a broad new peak emerged at 2θ = 22.2◦, which contained the diffraction peaks at 2θ = 19.4◦ corresponding to the orthogonal lattice from PVA with semi-crystalline structure. All these revealed the strong interactions between PVA, borax, and TOCNF-CNT@PANI-2 nanocomposite and built a 3D network in the composite hydrogels [38].
