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MXenes are a rapidly growing class of 2D transition metal carbides and nitrides, finding applications in fields ranging from energy storage to electromagnetic interference shielding and transparent conductive coatings. However, while more than 20 carbide MXenes have already been synthesized, Ti 4 N 3 and Ti 2 N are the only nitride MXenes reported so far. Here by ammoniation of Mo 2 CT x and V 2 CT x MXenes at 600 °C, we report on their transformation to 2D metal nitrides. Carbon atoms in the precursor MXenes are replaced with N atoms, resulting from the decomposition of ammonia molecules. The crystal structures of the resulting Mo 2 N and V 2 N were determined with transmission electron microscopy and X-ray pair distribution function analysis. Our results indicate that Mo 2 N retains the MXene structure and V 2 C transforms to a mixed layered structure of trigonal V 2 N and cubic VN. Temperature-dependent resistivity measurements of the nitrides reveal that they exhibit metallic conductivity, as opposed to semiconductor-like behavior of their parent carbides. As important, room-temperature electrical conductivity values of Mo 2 N and V 2 N are three and one order of magnitude larger than those of the Mo 2 CT x and V 2 CT x precursors, respectively. This study shows how gas treatment synthesis such as ammoniation can transform carbide MXenes into 2D nitrides with higher electrical conductivities and metallic behavior, opening a new avenue in 2D materials synthesis. 

Abstract MXenes, two‐dimensional (2D) transition metal carbides and/or nitrides, possess surface termination groups such as hydroxyl, oxygen, and fluorine, which are available for surface functionalization. Their surface chemistry is critical in many applications. This article reports amine functionalization of Ti₃C₂T_xMXene surface with [3‐(2‐aminoethylamino)‐propyl]trimethoxysilane (AEAPTMS). Characterization techniques such as X‐ray photoelectron spectroscopy verify the success of the surface functionalization and confirm that the silane coupling agent bonds to Ti₃C₂T_xsurface both physically and chemically. The functionalization changes the MXene surface charge from −35 to +25 mV at neutral pH, which allows for in situ preparation of self‐assembled films. Further, surface charge measurements of the functionalized MXene at different pH values show that the functionalized MXene has an isoelectric point at a pH around 10.7, and the highest reported positive surface charge of +62 mV at a pH of 2.58. Furthermore, the existence of a mixture of different orientations of AEAPTMS and the simultaneous presence of protonated and free amine groups on the surface of Ti₃C₂T_xare demonstrated. The availability of free amine groups on the surface potentially permits the fabrication of crosslinked electrically conductive MXene/epoxy composites, dye adsorbents, high‐performance membranes, and drug carriers. Surface modifications of this type are applicable to many other MXenes. 

Abstract Control of surface functionalization of MXenes holds great potential, and in particular, may lead to tuning of magnetic and electronic order in the recently reported magnetic Cr₂TiC₂T_x. Here, vacuum annealing experiments of Cr₂TiC₂T_xare reported with in situ electron energy loss spectroscopy and novel in situ Cr K‐edge extended energy loss fine structure analysis, which directly tracks the evolution of the MXene surface coordination environment. These in situ probes are accompanied by benchmarking synchrotron X‐ray absorption fine structure measurements and density functional theory calculations. With the etching method used here, the MXene has an initial termination chemistry of Cr₂TiC₂O_1.3F_0.8. Annealing to 600 °C results in the complete loss of F, but O termination is thermally stable up to (at least) 700 °C. These findings demonstrate thermal control of F termination in Cr₂TiC₂T_xand offer a first step toward termination engineering this MXene for magnetic applications. Moreover, this work demonstrates high energy electron spectroscopy as a powerful approach for surface characterization in 2D materials.