# Effect of MXene Nanosheet Sticking on Supercapacitor Device Performance
## 3 Factors Influencing Sticking in MXene Electrodes
### Surface Chemistry of MXene Flakes
#### 3.2.2 Hydrophobicity  
Hydrophobicity, which is the tendency of a material to repel water, is another critical aspect of the surface chemistry of MXene flakes. MXenes are generally hydrophilic, meaning they have an affinity for water due to the presence of oxygen-containing functional groups. The hydrophilicity of MXene surfaces can contribute to sticking by facilitating water-mediated inter-particle interactions and promoting agglomeration. To reduce sticking, researchers have focused on modifying the surface chemistry of MXene flakes to enhance their hydrophobicity. The introduction of hydrophobic functional groups, such as alkyl chains, can reduce the affinity of MXene surfaces towards water. This hydrophobic modification reduces water-mediated interactions and disfavors agglomeration, mitigating sticking issues in MXene electrodes. The authors in [54] have reported functionalization of Ti\({}_{3}\)C\({}_{2}\) with silylation reagents ((3-aminopropyl))triethoxysilane (APTES)) to tailor its hydrophilicity. The stability of Ti\({}_{3}\)C\({}_{2}\) was improved by APTES functionalization by blocking the contact between water and MXenes. This improvement was also achieved for hexadecyltrimethoxysilane and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FOTS) (Figure 8). The results indicated the change in surface properties from hydrophilic to hydrophobic. Two superhydrophobic self-assembled monolayers (SAMs) [(3-chloropropyl)trimethoxysilane (CPTMS)] and FOTS were used for surface modification of MXenes via formation of covalent bonds between -OH and different surface functional groups. The water contact angle findings revealed a superhydrophobic of 156\({}^{\circ}\) and 96\({}^{\circ}\) (not shown) for the fluorine-containing (Ti\({}_{3}\)C\({}_{2}\)-F) and chlorine-containing (Ti\({}_{3}\)C\({}_{2}\)-Cl) films, respectively, whereas the pristine Ti\({}_{3}\)C\({}_{2}\) is inherently hydrophilic due to the presence of -O and -OH functional groups (exhibiting a water contact angle of 33\({}^{\circ}\)). Therefore, modifying Ti\({}_{3}\)C\({}_{2}\) with superhydrophobic agents like FOTS and CPTMS could serve as an effective strategy to impede the degradation of MXene materials. Figure 7: Ions transport in two distinct configurations of (**a**) horizontally stacked and (**b**) vertically aligned. In horizontally stacked MXene films, ion transport takes place predominantly within the gaps between the flakes. The ions diffuse horizontally along the layers of MXene, facilitated by the open channels and interlayer spaces between the stacked sheets [51]. Recently, hydrophilic MXenes were transformed into hydrophobic forms through low-energy N\({}^{+}\) ion irradiation at varying fluences [56]. The characterization techniques used confirmed the removal of surface functional groups after irradiation. Additionally, DFT simulations validated the significant impact of -OH and -O-OH groups on the hydrophilic properties of pristine MXene. Analysis of adsorption energy data indicated that the adsorption of water molecules on surface-terminal-group-free MXenes was energetically unfavorable, suggesting a hydrophobic nature. Ion irradiation was employed to selectively eliminate surface-terminating functional groups, thereby rendering MXenes hydrophobic. Furthermore, an impressive enhancement in electrical conductivity was observed with increasing ion fluence, which can be attributed to the preferential sputtering of functional groups and structural changes in MXene. Following ion irradiation at varying significant alterations in both morphology (Figure 9) and composition were observed. As the ion fluence increased, MXene layers gradually fused together, culminating in the formation of a densely interconnected structure at higher fluences. Analysis of the energy-dispersive X-ray spectroscopy (EDS) spectrum for irradiated samples at an ion fluence of 5 \(\times\) 10\({}^{16}\) ions cm\({}^{-2}\) revealed a reduction in the relative peak intensity for fluorine.