Electronic and Magnetic Properties of Double Metal MXenes
Authors:   , Yang Yizhou , May Steven J. 
Published: November 2022

MXene are a rapidly-expanding family of 2D transition metal carbides and nitrides with chemical formula of M_[n+1]X_nT_x where M is one or more early transition metals, X is carbon or/and nitrogen, and T_x represents surface species. The freedom allowed within the chemical composition of MXenes is an enabling factor in their wide range of applications such as supercapacitors, EMI shielding, catalysts, and water deionization. This dissertation presents an investigation into their fundamental electronic properties with emphasis on understanding how the M-site affects the MXene electronic behavior in three different types of double-M MXenes. The first type is the out-of-plane ordered double-M MXenes M'₂M"C₂T_x. The contributions of surface (M' site) and sub-surface (M" site) Ti atoms to the electronic structure of four Ti-based MXenes (Ti₂CT_x, Ti₃C₂T_x, Cr₂TiC₂T_x, and Mo₂TiC₂T_x) is studied using soft x-ray absorption spectroscopy, revealing minimal changes in the spectral features between the parent MAX phase and its MXene counterpart when no surface Ti atoms are present in the MXene, such as Mo₂TiC₂T_x and Cr₂TiC₂T_x. In contrast, for MXenes with surface Ti atoms, here Ti₃C₂T_x and Ti₂CT_x, the Ti L-edge spectra are significantly modified compared to their parent MAX phase compounds. The second type is solid solution MXene Ti_yNb_[2-y]CT_x where Ti and Nb randomly occupy the M site. To understand the intrinsic and extrinsic contributions to the macroscopic electronic transport properties, a systematic study controlling compositional and structural parameters was conducted within the solid solution Ti_yNb_[2-y]CT_x system. In particular, I investigate the different roles played by M-site composition, flake size and d-spacing on macroscopic transport. Hard x-ray photoemission spectroscopy and ellipsometry were employed to identify changes to electronic structure induced by the M-site alloying. A statistical analysis of measured transport data was used to find the dependence, correlation, and p-value for the low- and room-temperature conductivities and effective carrier mobility as a function of the compositional and structural contributions. The results provide guidance for designing and engineering MXenes with increased conductivities. The third type is secondary metal incorporated Ti₃C₂T_x, M-Ti₃C₂T_x. Ni and Co are incorporated into delaminated Ti₃C₂T_x by mixing dilute NiCl₂ or CoCl₂ solutions with delaminated Ti₃C₂T_x solutions. The ratio of Ni:Ti = 0.024 and Co:Ti = 0.071 is determined by nano x-ray fluorescence (XRF). The incorporated Ni and Co are uniformly distributed in the sample as imaged by nano XRF. DC magnetization measurements confirm that as-synthesized delaminated Ti₃C₂T_x is paramagnetic with a low susceptibility of 2.0 x 10⁻⁷ emg/g·Oe. The room temperature magnetic susceptibility for Ni-Ti₃C₂ is 10 times larger than d-Ti₃C₂, and Co-Ti₃C₂ is 100 times larger than the d-Ti₃C₂. The work provides a straightforward means of enhancing magnetic responses of MXenes using post-synthesis incorporation of magnetic transition metals.