Elucidation of the synergistic effects of 3d metal (M = Cu, Co, and Ni) dopants and terminations (T = -O- and -OH) of Ti₃C₂T_x MXenes for urea adsorption ability via DFT calculations and experiments

Dialysis is an artificial process to remove excess urea toxins from the body through adsorption or conversion. Urea adsorption by emergent 2D materials such as MXenes is one probable approach. Based on density functional theory (DFT) studies, the surface of Ti₃C₂T_x (T = -O- and -OH) MXenes is not optimum for urea adsorption. Therefore, functionalization with 3d metal dopants (Cu, Co, and Ni) is proposed to improve their urea adsorption ability. DFT calculations indicate that oxygen-terminated Ti₃C₂O₂ has a much better urea adsorption ability when doped with Cu, Co, and Ni, with adsorption energy (E_ads) values of −2.11 eV, −1.90 eV and −1.72 eV, respectively. These adsorption energies are much more favourable than that of the undoped one (E_ads = −0.52 eV). To verify the calculation results, MILD Ti₃C₂T_x, or MXenes synthesized via the safer and easier minimally intensive layer delamination (MILD) method, were utilized to simulate Ti₃C₂O₂ since they have -O- termination as the dominant species. Experimentally, the adsorption studies found that low concentration of Cu, Co, and Ni on MILD Ti₃C₂T_x showed a urea removal efficiency of 21.9%, 6.0% and 0.2%, respectively, much better than 0% removal efficiency of unfunctionalized Ti₃C₂T_x. Therefore, both DFT calculations and experiments showed that various metal functionalized MXenes have a similar trend for urea adsorption, highlighting the feasibility of using the computational approach to predict urea adsorption and further opening a new promising direction for the urea adsorption. Finally, this study is also the first to examine synergistic effects of metal dopants and surface terminations on MXenes for urea adsorption.