## 3 Results and Discussion  
### Preparation Process and Reaction Mechanism  
The detailed mechanism of preparing the superhydrophobic HNTs via mussel inspired surface modification is shown in Scheme 1. HNTs can be easily decorated with catechol groups when immersed into an alkalescent dopamine (DA) solution at room temperature. In this case, polydopamine (PDA) was synthesized via the oxidation of polymerization of DA, and firmly adhered on the HNTs surfaces, forming a robust, nanometer-thick, and smooth film, which was determined by [PERSON] and his co-workers [31]. The secondary reaction to graft long-chain and anti-water alkylamine on PDA layers can introduce rough structure into HNTs substrates, which was the crucial factor for building superhydrophobic surfaces. However, the smooth PDA coatings obtained by typical method can hardly form primary obvious hierarchical structure, thus with the following secondary reaction, only hydrophobic surfaces would be achieved [35]. Various nanoparticles including noble metal (Ag) [36], metallic oxide (Fe\({}_{3}\)O\({}_{4}\)) [37], and silicon oxide [38] were incorporated through the catechol coordinate bonds and hydrogen bonds to create primary hierarchical structures on HNTs surfaces, however, this most-used approach suffered from a lot of disadvantages, such as high cost, time-consuming, and poor stability. Therefore, a direct way to control the roughness of PDA coatings without incorporation of nanoparticles was urgently needed.  
Fortunately, it has been reported that bio-tetramers can serve as nano-templates for the preparation of nanomaterials [39]. In a basic environment, Vitamin M (VM) that was abundant in fresh vegetables can physically assemble into a tetramer through hydrogen bonds, and finally create columnar structures via \(\pi\)-\(\pi\) interactions [40]. As for the fabrication process of roughness PDA coatings, VM columnar structures disturbed the formation of continuous PDA film, while acted as structure-direction agents to help polymerize scattered PDA nanoparticles along the vertical direction [35]. According to this, we chose VM as an accelerator to control the procedure of PDA coating, which formed primary nano hierarchical structures followed by grafting ODA, and eventually transformed the surfaces properties of HNTs with robust superhydrophobicity.  
To date, long chain alkylsilanes were frequently used to produce superhydrophobic surfaces. Nevertheless, this modified process only changed the external surface properties, and the hydrophilic internal surfaces still remained because of the incompatibilities between the gibbsite octahedral sheet groups and alkylsilanes [20]. In addition, the natural nanostructure may suffer the risk of beingdamaged due to the harsh reaction conditions. Herein, the mussel inspired surface modification was carried out under mild environment, which changed the hydrophilic properties both on the internal and external surfaces without damaging the original nanotube structures. The hydrophobic external surfaces improve the dispersion of HNTs, meanwhile the hydrophobic internal surfaces enhanced the absorption ability for oil and anti-water compounds. The resultant superhydrophobic surfaces exhibited prominent stability in numerous harsh environments owing to the superior adhesion of catechol groups.  
**Scheme 1.** The production process and reaction mechanism of superhydrophobic halloysite nanotube (HNTs). **(a)** Surface modification procedure of HNTs; **(b)** Chemical reaction mechanism: 1 Illustration of oxidation polymerization and probable connection mode of dopamine on HNTs [33]; 2 The mechanism of grafting octadecylamine (ODA) to obtain roughness surface via Michael addition and Schiff based reactions; 3 The formation of Vitamin M (VM) tetramer and the resultant columnar structure.