The two-dimensional (2D) MXene Ti3C2Tx is functionalized by surface groups (Tx) that determine its surface properties for, e.g. electrochemical applications. The coordination and thermal properties of these surface groups has, to date, not been investigated at the atomic level, despite strong variations in the MXene properties that are predicted from different coordinations and from the identity of the functional groups. To alleviate this deficiency, and to characterize the functionalized surfaces of single MXene sheets, the present investigation combines atomically resolved in situ heating in a scanning transmission electron microscope (STEM) and STEM simulations with temperature-programmed x-ray photoelectron spectroscopy (TP-XPS) in the room temperature to 750 °C range. Using these techniques, we follow the surface group coordination at the atomic level. It is concluded that the F and O atoms compete for the DFT-predicted thermodynamically preferred site and that at room temperature that site is mostly occupied by F. At higher temperatures, F desorbs and is replaced by O. Depending on the O/F ratio, the surface bare MXene is exposed as F desorbs, which enables a route for tailored surface functionalization.

Extensive research has been invested in two-dimensional (2D) materials, typically synthesized by exfoliation of van der Waals solids. One exception is MXenes, derived from the etching of constituent layers in transition metal carbides and nitrides. We report the experimental realization of boridene in the form of single-layer 2D molybdenum boride sheets with ordered metal vacancies, Mo4/3B2-xTz (where Tz is fluorine, oxygen, or hydroxide surface terminations), produced by selective etching of aluminum and yttrium or scandium atoms from 3D in-plane chemically ordered (Mo2/3Y1/3)2AlB2 and (Mo2/3Sc1/3)2AlB2 in aqueous hydrofluoric acid. The discovery of a 2D transition metal boride suggests a wealth of future 2D materials that can be obtained through the chemical exfoliation of laminated compounds.

charge storage, [7,8] electromagnetic interference shielding, [9] filtering, [10] and a range of additional applications. [7] MXenes constitute a large and growing family of 2D materials [11,12] that are obtained from the laminated M n+1 AX n (MAX) phases (M is a transition metal, A is a group A element-mostly groups 13 and 14-and X is C and/or N) [13] by chemical etching of the atomically thin A element layers that separate sheets of M n+1 X n . As the A element is removed, the MXene surfaces are immediately functionalized by surface terminating species, T x . [6,14] Hence, the proper MXene formula is M n+1 X n T x . Accordingly, the MXene properties can be tuned through structure, intrinsic composition, and surface terminations. The structure is inherited from the parent MAX phase (hexagonal, space group P6 3 /mmc), but compositional tuning displays an extraordinary toolbox for property tuning through MXenes based on single M and X elements, as well as alloys on both M and X. [12,15] In addition, there are reports on MXenes forming out-of-plane [16] and in-plane [17] double-M elemental ordering, as well as vacancy-ordered structures. [18,19] Manipulation of the surface terminations constitutes the final and most powerful variable for property tuning. [20] Despite several theoretical investigations, [21][22][23] noninherent terminations have remained experimentally unexplored. Currently, the MXene preparation dictates that T x is inherent to the etchant and predominantly a combination of O and F, where OH has also been considered as a minor [24] or even negligible contribution. [25] In the area of carbon capture (CC), MXenes are predicted to be highly efficient for capturing CO 2 , enabling capture of 2-8 mol CO 2 kg −1 . [20,21] However, the MXene surfaces were assumed to be termination free, an experimentally unrealistic starting point, given the current wet-chemical preparation routes for MXenes. To unlock the MXene potential for noninherent terminations or adsorption of other molecules, such as CO 2 , we have subjected the archetype Ti 3 C 2 T x MXene to a novel approach. Using in situ environmental transmission electron microscopy (ETEM), single Ti 3 C 2 T x sheets were subjected to an initial high-temperature treatment to desorb F, [25] and a subsequent H 2 exposure to remove the persistent O from the surfaces. The thereafter termination-depleted MXene was subsequently exposed to CO 2 gas, resulting in the first MXene to be terminated by a noninherent molecule. Additionally, termination-depleted MXene surfaces were exposed to N 2 gas after which no N adsorption was observed, Global warming caused by burning of fossil fuels is indisputably one of mankind's greatest challenges in the 21st century. To reduce the everincreasing CO 2 emissions released into the atmosphere, dry solid adsorbents with large surface-to-volume ratio such as carbonaceous materials, zeolites, and metal-organic frameworks have emerged as promising material candidates for capturing CO 2 . However, challenges remain because of limited CO...

The exploration of 2D solids is one of our time's generators of materials discoveries. A recent addition to the 2D world is MXenes that possses a rich chemistry due to the large parent family of MAX phases. Recently, a new type of atomic laminated phases (coined i-MAX) is reported, in which two different transition metal atoms are ordered in the basal planes. Herein, these i-MAX phases are used in a new route for tailoriong the MXene structure and composition. By employing different etching protocols to the parent i-MAX phase (Mo Y ) AlC, the resulting MXene can be either: i) (Mo Y ) C with in-plane elemental order through selective removal of Al atoms or ii) Mo C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo Y ) C (ideal stoichiometry) is used as an electrode in a supercapacitor-with KOH electrolyte-a volumetric capacitance exceeding 1500 F cm is obtained, which is 40% higher than that of its Mo C counterpart. With H SO , the trend is reversed, with the latter exhibiting the higher capacitance (≈1200 F cm ). This additional ability for structural tailoring will indubitably prove to be a powerful tool in property-tailoring of 2D materials, as exemplified here for supercapacitors.

MXenes are a rapidly growing family of 2D materials that exhibit a highly versatile structure and composition, allowing for significant tuning of the material properties. These properties are, however, ultimately limited by the surface terminations, which are typically a mixture of species, including F and O that are inherent to the MXene processing. Other and robust terminations are lacking. Here, we apply high-resolution scanning transmission electron microscopy (STEM), corresponding image simulations and first-principles calculations to investigate the surface terminations on MXenes synthesized from MAX phases through Lewis acidic melts. The results show that atomic Cl terminates the synthesized MXenes, with mere residual presence of other termination species. Furthermore, in situ STEM-electron energy loss spectroscopy (EELS) heating experiments show that the Cl terminations are stable up to 750 °C. Thus, we present an attractive new termination that widely expands the MXenes' functionalization space and enable new applications.

Tuning and tailoring of surface terminating functional species hold the key to unlock unprecedented properties for a wide range of applications of the largest 2D family known as MXenes. However, a few routes for surface tailoring are explored and little is known about the extent to which the terminating species can saturate the MXene surfaces. Among available terminations, atomic oxygen is of interest for electrochemical energy storage, hydrogen evolution reaction, photocatalysis, etc. However, controlled oxidation of the surfaces is not trivial due to the favored formation of oxides. In the present contribution, single sheets of Ti 3 C 2 T x MXene, inherently terminated by F and O, are defluorinated by heating in vacuum and subsequentially exposed to O 2 gas at temperatures up to 450 °C in situ, in an environmental transmission electron microscope. Results include exclusive termination by O on the MXene surfaces and eventual supersaturation (x > 2) with a retained MXene sheet structure. Upon extended O exposure, the MXene structure transforms into TiO 2 and desorbs surface bound H 2 O and CO 2 reaction products. These results are fundamental for understanding the oxidation, the presence of water on MXene surfaces, and the degradation of MXenes, and pave way for further tailoring of MXene surfaces.

Solar energy, although it has the highest power density available in terms of renewable energy, has the drawback of being erratic. Integrating an energy harvesting and storage device into photovoltaic energy storage modules is a viable route for obtaining self-powered energy systems. Herein, an MXene-based all-solution processed semitransparent flexible photovoltaic supercapacitor (PSC) was fabricated by integrating a flexible organic photovoltaic (OPV) with Ti 3 C 2 T x MXene as the electrode and transparent MXene supercapacitors with an organic ionogel as the electrolyte in the vertical direction, using Ti 3 C 2 T x thin film as a common electrode. In the quest for a semitransparent flexible PSC, Ti 3 C 2 T x MXene was first used as a transparent electrode for OPV with a high power conversion efficiency of 13.6%. The ionogel electrolyte-based transparent MXene supercapacitor shows a high volumetric capacitance of 502 F cm À3 and excellent stability. Finally, a flexible PSC with a high average transmittance of over 33.5% was successfully constructed by allsolution processing and a remarkable storage efficiency of 88% was achieved. This strategy enables a simple route for fabricating MXene based high-performance all-solution-processed flexible PSCs, which is important for realizing flexible and printable electronics for future technologies.

Two-dimensional (2D) transition metal carbides and/or nitrides (MXenes) are a new class of 2D materials, with extensive opportunities for property tailoring due to the numerous possibilities for varying chemistries and surface terminations. Here, Ti2AlC and Nb2AlC MAX phase epitaxial thin films were deposited on sapphire substrates by physical vapor deposition. The films were then etched in LiF/HCl solutions, yielding Li-intercalated, 2D Ti2CTz and Nb2CTz films, whose terminations, transport and optical properties were characterized. The former exhibits metallic conductivity, with weak localization below 50 K. In contrast, the Nb-based film exhibits an increase in resistivity with decreasing temperature from RT down to 40 K consistent with variable range hopping transport. The optical properties of both films were determined from spectroscopic ellipsometry in the 0.75 to 3.50 eV range. The results for Ti2CTz films confirm the metallic behavior. In contrast, no evidence of metallic behavior is observed for the Nb2CTz film. The present work therefore demonstrates that one fruitful approach to alter the electronic and optical properties of MXenes is to change the nature of the transition metal.