Organic Functionalization of Ti3C2Tx MXene for Urea Adsorption in Aqueous Solutions

Two-dimensional MXene materials with the composition Ti3C2Tx, where T represents a surface termination species, have become popular for many applications due to their large surface areas, unique mechanical and electrochemical properties, and the ability to create thin single-layer systems. The high surface area and hydrophilicity of this material render it a viable option as a molecular adsorber in aqueous solutions. In recent studies, using a simplified synthesis method called minimally intensive layer delamination, we have shown how the surface functionalization (T) affects the material's ability to adsorb urea. Here, we present a new approach to enhancing urea adsorption by functionalizing the surface with amino acid and "amino acid-like" moieties such as glutamic acid, lysine, and l-dopa. The differences between these functionalized MXenes are probed using PXRD, XPS, and FTIR/Raman and solid-state 13C MAS NMR spectroscopy, which appear to emanate from distinct steric bonding configurations between each amino acid and the MXene surface, thus facilitating a variety of organic–urea interactions in these regions. In particular, solid-state 13C MAS NMR offers a detailed picture of each amino acid's bonding configuration based on peak shifting/broadening due to paramagnetic interactions with the Ti3+ positions within the MXene surface. Furthermore, corresponding solid-state 7Li MAS NMR measurements verified that the surface functionalization does, in fact, deintercalate Li+ ions, not only from Cu functionalization derivatives but also for particular organic-functionalized systems. It was found that glutamic acid-functionalized MXene has a maximum urea adsorption capability of 23.5 mg/g in aqueous media. This adsorption capacity was found to be superior compared to that of pristine MXene used in previous studies for urea adsorption.