## 3 Results and Discussion
### Micromorphology and Surface Chemical Composition Analysis  
The dispersed state and micromorphology of HNTs before and after surface modification were observed by FE-SEM, as presented in Figure 1. Natural HNTs shows nanotube structures with massive block impurities stacked together (Figure 1a), and these unique nanotube structures are completely preserved and easily recognized after the purification of acid-heat pretreatment (Figure 1b), which is the same as the study to treat HNTs with sulfuric acid [41]. Figure 1c,d exhibit the morphological changes of PDA decorated HNTs, and it is observed that the tubular structure of the PDA-HNTs and VM/PDA-HNTs become thicker than that of the P-HNTs, while the dispersibility is improved, indicating that PDA is well anchored on the HNTs surfaces [42; 43]. After grafting with ODA, the hydrophilic surface properties of P-HNTs were transformed by the hydrophobic long-chain alkyl groups in ODA molecules, and excitingly, a large number of single continuous or intact HNTs fibers distinctly appeared in Figure 1e,f. The agglomeration of HNTs fibers was notably decreased. The micromorphology images of single halloysite clay nanotube (HNT) samples are presented in Figure 2. As shown in Figure 2a, it could be clearly observed that pure HNTs were tubular in shape with a smooth surface and comprised a distinct hollow space running longitudinally along the tube, revealing the double-layered and open-ended characteristics [44]. The external diameters and lengths of P-HNTs were about 30-60 nm and 0.15-1.0 \(\upmu\)m, respectively. After surface modified with PDA, the hollow space was difficult to be distinguished and the cylinder wall of the PDA coated HNT was significantly thicker than that of pure HNT (Figure 2b). This may be due to the self-polymerized PDA layers in both internal and external surfaces [32]. When compared to the PDA-HNTs, the edges and surfaces of ODA-HNTs became irregularity, which was ascribed to the long-chain alkylamine grafting onto the PDA-HNTs surfaces (Figure 2c). The XRD patterns of pure HNTs and surface modified HNTs were observed, as shown in Figure 3. The typical characteristic diffraction peaks of HNTs located at the 20 angles of 12.2\({}^{\circ}\), 18.0\({}^{\circ}\), 19.8\({}^{\circ}\), 24.8\({}^{\circ}\), 26.6\({}^{\circ}\), 30.0\({}^{\circ}\), 34.8\({}^{\circ}\), 38.3\({}^{\circ}\), 52.5\({}^{\circ}\), 55.4\({}^{\circ}\) and 62.3\({}^{\circ}\) were all found in P-HNTs, which demonstrated that the crystalline structure of HNTs can hardly be damaged after acid purification [31; 45]. Apparently, the following process of PDA and ODA surface modification had no obvious effects on these typical crystal diffraction peaks of P-HNTs with respect to both the shapes and 20 angles, implying that the original crystal structure of HNTs was not altered. Figure 1: Field emission scanning electron microscopy (FE-SEM) images of the (**a**) pristine halloysite clay nanotubes (HNTs); (**b**) pure HNTs (P-HNTs); (**c**) polydopamine (PDA) decorating HNTs (PDA-HNTs); (**d**) PDA-HNTs assisted with Vitamin M (VM/PDA-HNTs); (**e**) octadecylamine (ODA) grifting HNTs (ODA-HNTs); and (**f**) ODA-HNTs assisted with Vitamin M (VM/ODA-HNTs). The FTIR spectra of the P-HNTs, PDA-HNTs, ODA-NHTs, and VM/ODA-HNTs were investigated to confirm the structures and chemical groups of pure and surface modified HNTs as shown in Figure 4. In the spectra of P-HNTs, two strong absorption peaks appeared at 3698 cm\({}^{-1}\) and 3625 cm\({}^{-1}\), which were ascribed to the stretching vibration of AlO-H bonds. As for the HNTs spectra in fingerprint region ranging from 2000 to 500 cm\({}^{-1}\), the peaks at 1091 cm\({}^{-1}\) and 1031 cm\({}^{-1}\) represented the in-plane stretching vibration of Si-O bonds and symmetric stretching of Si-O-Si was observed at 793 cm\({}^{-1}\). The spectral bands at 910 cm\({}^{-1}\) and 690 cm\({}^{-1}\) correspond to the stretching vibration of Al-OH bonds.