## 3 Results and Discussion
### Application Analysis of Superhydrophobic HNTs  
Phase change thermal storage technology, using phase change material (PCM) to store and release the intermittent or surplus heat, is one of the most efficient methods for solar thermal storage [53]. Many Organic materials, such as paraffin, PEG, and stearic acid (SA), dominate the PCMs utilization due to their superior merits of high capacity and good chemical stability [54]. However, the leakage occurring in solid-liquid phase change transition limits their practical applications. To combat this drawback, various porous materials have been exploited as package substrates for preparing form-stable PCM composites [55]. Halloysite as a kind of natural nanomaterial has the advantages of low cost, easy accessibility, and high porosity, which can be used as a packaging material to absorb PCMs [56]. However, its original hydrophilic property makes it incompatible with the hydrophobic organic compounds. As shown in Figure 9 and Table 2, pure HNTs (P-HNTs) can only absorb stearic acid with the maximum mass fraction of 46.2%. In this case, the latent heats of melting and cooling of SA/P-HNTs were 68.12 J/g and 60.75 J/g, respectively. After bio-inspired superhydrophobic surface modification, superhydrophobic HNTs (S-HNTs) based SA composites (SA/S-HNTs) presented the higher maximum package mass fraction (58.6%) than SA/P-HNTs, as well as the melting (88.42 J/g) and the cooling (79.37 J/g) latent heats, which were improved by 22.94%, 29.80%, and 30.65%, respectively. This may be attributable to the good interface affinity between SA and S-HNTs. Besides, the resultant form-stable PCM composite also showed an excellent thermal stability. The latent heats of SA/HNTs  
Figure 8: Environment durability performance of superhydrophobic HNTs. (**a**) contact angles (CAs) for HNTs samples immersed into different chemical solutions for 24 h; (**b**) CAs as a function of time of ultrasonic washing in water; (**c**) CAs as a function of outdoor solar radiation exposure time. Figure 7: Water wettability properties characterization. (**a**) Contact angle testing results as a function of time for HNTs samples; and optical photographs of deionized water droplets on (**b**) P-HNTs; (**c**) PDA-HNTs; (**d**) VM/PDA-HNTs; (**e**) ODA-HNTs; and (**f**) VM/ODA-HNTs. were 85.23 J/g for melting and 75.61 J/g for cooling, presenting a decrease of only 3.19 J/g and 3.76 J/g, respectively, which showed a good potential application in thermal energy storage. These results were close to the research about superhydrophobic HNTs based phase change material composites via the surface modification of polydimethylsiloxane [6]. Oil/water separation technique is one of the most favorable methods of wastewater treatment [35; 37]. Due to the abundant pore structures and high surface area, HNTs may be used as an absorbent substrate to separate water from the oil/water mixture. However, the natural HNT has no selective absorption for oil and water. Hence, it can only be used to remove water-soluble cationic and anionic contaminants from wastewater [57]. Excitingly, the resultant superhydrophobic HNTs (S-HNTs) that have unique water resistance property and strong oil-absorbing capacity are promisingly suitable as a filter for oil/water separation. As shown in Figure 10, trichloromethane (TCM, dyed with Oil Red O)/water mixture was poured onto the facility apparatus, then TCM penetrated through the S-HNTs layer spontaneously, and was finally dripped into the collecting conical flask. The separated water was blocked by the S-HNTs, remained inside the funnel, and then poured back into the beaker. The complete oil/water separation process can be achieved with a simple way and without demand of extra force, which showed that S-HNTs have great potential for use in oil/water separation.