Synthesis and Characterization of Amino-Functionalized Ti₃C₂ MXene with Extremely High Conductivity

MXenes are a family of 2D transition metal carbides, nitrides, and carbonitrides represented by the formula M_n+1X_nT_x, where M is an early transition metal, X is carbon and/or nitrogen, and T_x represents terminal groups. One of the main advantages of MXenes is that their surfaces are easy to modify, given the abundance of ligand-exchangeable terminal groups such as –OH. Altering the inherent surface functional groups through ligand exchange is a promising method for tuning the physical and chemical properties of MXene. For instance, introducing new functional groups onto the MXene surface via ligand substitution can enhance oxidation resistance, improve dispersibility in organic solvents, and modulate the material’s work function.^[1] Introducing amino groups on the MXene surface is also a notable modification, which provides many reactive sites for biomolecules, organic molecules, and polymers. Additionally, amino groups allow for modulating the generally negatively-charged surface potential of MXene.^[2,3] There have been many reported methods for introducing amino groups onto the MXene surface, most of which involve silane coupling agents (SCAs). However, amino-functionalization using SCAs has significant drawbacks. The high reactivity of SCAs leads to reactions resulting in Ti-OH formation, but self-polymerization may also occur among the SCAs themselves, resulting in formation of polysiloxanes on the MXene surface that significantly reduce the high conductivity MXenes typically possess.<br/>Herein, we successfully achieved amino-functionalized MXenes using phosphonic acid as a ligand. The resultant material demonstrated a remarkably high conductivity of approximately 4,000 S/cm. Through various analyses, it has been revealed that dehydrating condensation occurs between Ti-OH on the MXene and P-OH in the phosphonic acid ligand, forming versatile Ti-O-P bonds on the final product. Beyond this, we will present other factors that are hypothesized to contribute to the improvement in conductivity seen for amino-functionalized MXene.<br/><br/><b>References</b><br/>^[1] Daesin Kim et al., <i>ACS Nano</i> 2019, 13, 13818.<br/>^[2] Hossein Riazi et al., <i>Adv. Mater. Interfaces</i> 2020, 7, 1902008.<br/>^[3] Saurabh Kumar et al., <i>Biosensors and Bioelectronics</i> 2018, 121, 243.