Two-dimensional carbides and nitrides of transition metals, known as MXenes, are a fast-growing family of materials that have attracted attention as energy storage materials. MXenes are mainly ...prepared from Al-containing MAX phases (where A = Al) by Al dissolution in F-containing solution; most other MAX phases have not been explored. Here a redox-controlled A-site etching of MAX phases in Lewis acidic melts is proposed and validated by the synthesis of various MXenes from unconventional MAX-phase precursors with A elements Si, Zn and Ga. A negative electrode of Ti
C
MXene material obtained through this molten salt synthesis method delivers a Li
storage capacity of up to 738 C g
(205 mAh g
) with high charge-discharge rate and a pseudocapacitive-like electrochemical signature in 1 M LiPF
carbonate-based electrolyte. MXenes prepared via this molten salt synthesis route may prove suitable for use as high-rate negative-electrode materials for electrochemical energy storage applications.
Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesizing a series of Zn-based MAX phases ...and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition-metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this replacement reaction between the Zn element from molten ZnCl2 and the Al element in MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC), novel MAX phases Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC were synthesized. When employing excess ZnCl2, Cl-terminated MXenes (such as Ti3C2Cl2 and Ti2CCl2) were derived by a subsequent exfoliation of Ti3ZnC2 and Ti2ZnC due to the strong Lewis acidity of molten ZnCl2. These results indicate that A-site element replacement in traditional MAX phases by late transition-metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach. In addition, this is the first time that exclusively Cl-terminated MXenes were obtained, and the etching effect of Lewis acid in molten salts provides a green and viable route to preparing MXenes through an HF-free chemical approach.
The properties of two-dimensional (2D) materials depend strongly on the chemical and electrochemical activity of their surfaces. MXene, one of the most recent additions to 2D materials, shows great ...promise as an energy storage material. In the present investigation, the chemical and structural properties of individual Ti3C2 MXene sheets with associated surface groups are investigated at the atomic level by aberration corrected STEM-EELS. The MXene sheets are shown to exhibit a nonuniform coverage of O-based surface groups which locally affect the chemistry. Additionally, native point defects which are proposed to affect the local surface chemistry, such as oxidized titanium adatoms (TiO x ), are identified and found to be mobile.
Global warming caused by burning of fossil fuels is indisputably one of mankind's greatest challenges in the 21st century. To reduce the ever‐increasing CO2 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 CO2. However, challenges remain because of limited CO2/N2 selectivity and long‐term stability. The effective adsorption of CO2 gas (≈12 mol kg−1) on individual sheets of 2D transition metal carbides (referred to as MXenes) is reported here. It is shown that exposure to N2 gas results in no adsorption, consistent with first‐principles calculations. The adsorption efficiency combined with the CO2/N2 selectivity, together with a chemical and thermal stability, identifies the archetype Ti3C2 MXene as a new material for carbon capture (CC) applications.
A combination of in situ heating and H2 exposure of single 2D transition metal carbide (MXene) sheets using environmental transmission electron microscopy (ETEM) shows that surface terminations inherent to the MXenes are depleted and enable new terminations. Accordingly, depleted 2D Ti3C2 MXene is demonstrated as an effective adsorber of CO2.
Intercalated layered materials offer distinctive properties and serve as precursors for important two-dimensional (2D) materials. However, intercalation of non-van der Waals structures, which can ...expand the family of 2D materials, is difficult. We report a structural editing protocol for layered carbides (MAX phases) and their 2D derivatives (MXenes). Gap-opening and species-intercalating stages were respectively mediated by chemical scissors and intercalants, which created a large family of MAX phases with unconventional elements and structures, as well as MXenes with versatile terminals. The removal of terminals in MXenes with metal scissors and then the stitching of 2D carbide nanosheets with atom intercalation leads to the reconstruction of MAX phases and a family of metal-intercalated 2D carbides, both of which may drive advances in fields ranging from energy to printed electronics.
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
−3
and excellent stability. Finally, a flexible PSC with a high average transmittance of over 33.5% was successfully constructed by all-solution 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.
MXene based all-solution processed semitransparent flexible photovoltaic supercapacitor was fabricated by integrating the flexible organic photovoltaic with MXene as the electrode and transparent MXene supercapacitors in the vertical direction.
The integration and cooperation of mechanoreceptors, neurons and synapses in somatosensory systems enable humans to efficiently sense and process tactile information. Inspired by biological ...somatosensory systems, we report an optoelectronic spiking afferent nerve with neural coding, perceptual learning and memorizing capabilities to mimic tactile sensing and processing. Our system senses pressure by MXene-based sensors, converts pressure information to light pulses by coupling light-emitting diodes to analog-to-digital circuits, then integrates light pulses using a synaptic photomemristor. With neural coding, our spiking nerve is capable of not only detecting simultaneous pressure inputs, but also recognizing Morse code, braille, and object movement. Furthermore, with dimensionality-reduced feature extraction and learning, our system can recognize and memorize handwritten alphabets and words, providing a promising approach towards e-skin, neurorobotics and human-machine interaction technologies.
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 Ti3C2Tx MXene, inherently terminated by F and O, are defluorinated by heating in vacuum and subsequentially exposed to O2 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 TiO2 and desorbs surface bound H2O and CO2 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.
The surface functionalization of 2D materials is critical for tuning properties and application performance. In the case of MXenes, oxygen‐terminated surfaces are critical for applications in energy storage and catalysis. Here, a route for exclusively functionalizing MXene surfaced by oxygen is shown and pushed to the limit.
Abstract
The exploration of two-dimensional solids is an active area of materials discovery. Research in this area has given us structures spanning graphene to dichalcogenides, and more recently 2D ...transition metal carbides (MXenes). One of the challenges now is to master ordering within the atomic sheets. Herein, we present a top-down, high-yield, facile route for the controlled introduction of ordered divacancies in MXenes. By designing a parent 3D atomic laminate, (Mo
2/3
Sc
1/3
)
2
AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, we show evidence for 2D Mo
1.33
C sheets with ordered metal divacancies and high electrical conductivities. At ∼1,100 F cm
−3
, this 2D material exhibits a 65% higher volumetric capacitance than its counterpart, Mo
2
C, with no vacancies, and one of the highest volumetric capacitance values ever reported, to the best of our knowledge. This structural design on the atomic scale may alter and expand the concept of property-tailoring of 2D materials.
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 (Mo2/3Y1/3)2AlC, the resulting MXene can be either: i) (Mo2/3Y1/3)2C with in‐plane elemental order through selective removal of Al atoms or ii) Mo1.33C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo2/3Y1/3)2C (ideal stoichiometry) is used as an electrode in a supercapacitor—with KOH electrolyte—a volumetric capacitance exceeding 1500 F cm−3 is obtained, which is 40% higher than that of its Mo1.33C counterpart. With H2SO4, the trend is reversed, with the latter exhibiting the higher capacitance (≈1200 F cm−3). 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.
Using atomically resolved electron microscopy, it is shown that the chemically ordered atomic laminate (Mo2/3Y1/3)2AlC can be used to derive either i) an in‐plane vacancy ordered Mo1.33C MXene or ii) (Mo2/3Y1/3)2C MXene with in‐plane elemental order. These materials have different electrochemical properties, with in one case a capacitance value exceeding 1500 F cm−3.
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