MXene Surface Terminations: A Perspective

Two-dimensional (2D) transition metal carbides and nitrides are among the newest, and probably largest, families of 2D materials that have been discovered. They have shown promise in many applications, such as electromagnetic shielding, energy storage devices like batteries and supercapacitors, transparent conductive electrodes, as catalysts for hydrogen production, and several others. Their general formula is Mn+1XnTz (n = 1–4) or M1.33XTz, where M is an early transition metal, X is carbon and/or nitrogen, and Tz represents various terminations present upon etching the Al layer. The first MXene discovered, Ti3C2Tz, was synthesized by etching aluminum (Al) from the nanolaminated MAX phase Ti3AlC2 using hydrofluoric acid (HF). Since the discovery of Ti3C2Tz in 2011, more than 30 other MXene chemistries have been reported with different M and X elements. It has been observed that, as expected, changing the MXene chemistry also influences the properties in the range of applications listed above, allowing us to tailor them according to what suits the application of interest the best. Unlike the success in tailoring the M and X elements over the years, attempts to engineer Tz was, until recently, a challenge. A series of recent discoveries and an improved understanding of MXene surface properties has now rendered the customization of Tz possible. This progress added a new “knob” that can be turned to further tune MXene properties. The majority of this Perspective is focused on surface terminations: their engineering, characterization, and effect on MXene properties.