Babak Anasori

Electromagnetic interference shielding with 2D transition metal carbides (MXenes)

Materials with good flexibility and high conductivity that can provide electromagnetic interference (EMI) shielding with minimal thickness are highly desirable, especially if they can be easily processed into films. Two-dimensional metal carbides and nitrides, known as MXenes, combine metallic conductivity and hydrophilic surfaces. Here, we demonstrate the potential of several MXenes and their polymer composites for EMI shielding. A 45-micrometer-thick Ti3C2Tx film exhibited EMI shielding effectiveness of 92 decibels (>50 decibels for a 2.5-micrometer film), which is the highest among synthetic materials of comparable thickness produced to date. This performance originates from the excellent electrical conductivity of Ti3C2Tx films (4600 Siemens per centimeter) and multiple internal reflections from Ti3C2Tx flakes in free-standing films. The mechanical flexibility and easy coating capability offered by MXenes and their composites enable them to shield surfaces of any shape while providing high EMI shielding efficiency.

Pseudocapacitive Electrodes Produced by Oxidant-Free Polymerization of Pyrrole between the Layers of 2D Titanium Carbide (MXene)

Heterocyclic pyrrole molecules are in situ aligned and polymerized in the -absence of an oxidant between layers of the 2D Ti3C2Tx (MXene), resulting in high volumetric and gravimetric capacitances with capacitance retention of 92% after 25,000 cycles at a 100 mV s(-1) scan rate.

A Critical Review of the Oxidation of Ti2AlC, Ti3AlC2 and Cr2AlC in Air

Of all the M n+1AX n phases, the most resistant to oxidation in air in the 900–1,400°C temperature range are Ti2AlC, Ti3AlC2 and Cr2AlC. A literature review, however, shows that while many claim the oxidation kinetics to be parabolic, others claim them to be cubic. Whether the kinetics are parabolic or better is of vital practical importance. By carefully re-plotting the results of others and carrying out one oxidation run for ≈3,000 h at 1,200°C on a Ti2AlC sample, we conclude that the oxidation kinetics are better described by cubic kinetics and that even that conclusion is an approximation. Lastly, we present compelling evidence that the rate-limiting step during the oxidation of Ti2AlC is diffusion down the alumina scale grain boundaries.

All-MXene (2D titanium carbide) solid-state microsupercapacitors for on-chip energy storage

On-chip energy storage is a rapidly evolving research topic, opening doors for the integration of batteries and supercapacitors at the microscale on rigid and flexible platforms. Recently, a new class of two-dimensional (2D) transition metal carbides and nitrides (so-called MXenes) has shown great promise in electrochemical energy storage applications. Here, we report the fabrication of all-MXene (Ti3C2Tx) solid-state interdigital microsupercapacitors by employing a solution spray-coating method, followed by a photoresist-free direct laser cutting method. Our prototype devices consisted of two layers of Ti3C2Tx with two different flake sizes. The bottom layer was stacked large-size MXene flakes (lateral dimensions of 3–6 μm) serving mainly as current collectors. The top layer was made of small-size MXene flakes (∼1 μm) with a large number of defects and edges as the electroactive layer responsible for energy storage. Compared to Ti3C2Tx micro-supercapacitors with platinum current collectors, the all-MXene devices exhibited a much lower contact resistance, higher capacitances and better rate-capabilities. Areal and volumetric capacitances of ∼27 mF cm−2 and ∼357 F cm−3, respectively, at a scan rate of 20 mV s−1 were achieved. The devices also demonstrated excellent cyclic stability, with 100% capacitance retention after 10 000 cycles at a scan rate of 50 mV s−1. This study opens up a plethora of possible designs for high-performance on-chip devices employing different chemistries, flake sizes and morphologies of MXenes and their heterostructures.

Experimental and theoretical characterization of ordered MAX phases Mo2TiAlC2 and Mo2Ti2AlC3

Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases—Mo2TiAlC2 and Mo2Ti2AlC3—synthesized by mixing and heating different elemental powder mixtures of mMo:(3-m)Ti:1.1Al:2C with 1.5 ≤ m ≤ 2.2 and 2Mo: 2Ti:1.1Al:2.7C to 1600 °C for 4 h under Ar flow. In general, for m ≥ 2 an ordered 312 phase, (Mo2Ti)AlC2, was the majority phase; for m < 2, an ordered 413 phase (Mo2Ti2)AlC3, was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo2TiAlC1.7 and Mo2Ti1.9Al0.9C2.5, respectively. High resolution scanning transmission microscopy, XPS and Rietveld analysis of powder X-ray diffraction confirmed the general ordered stacking sequence to be Mo-Ti-Mo-Al-Mo-Ti-Mo for Mo2TiAlC2 and Mo-Ti-Ti-Mo-Al-Mo-Ti-Ti-Mo for Mo2Ti2AlC3, with the carbon atoms occupying the octahedral sites between the transition metal layers. Consistent with the experimental results, the theoretical calculations clearly show that M l...

Laminated and Two-Dimensional Carbon-Supported Microwave Absorbers Derived from MXenes

Microwave absorbers with layered structures that can provide abundant interfaces are highly desirable for enhancing electromagnetic absorbing capability and decreasing the thickness. The atomically thin layers of two-dimensional (2D) transition-metal carbides (MXenes) make them a convenient precursor for synthesis of other 2D and layered structures. Here, laminated carbon/TiO2 hybrid materials composed of well-aligned 2D carbon sheets with embedded TiO2 nanoparticles were synthesized and showed excellent microwave absorption. Disordered 2D carbon layers with an unusual structure were obtained by annealing multilayer Ti3C2 MXene in a CO2 atmosphere. The minimum reflection coefficient of laminated carbon/TiO2 composites reaches -36 dB, and the effective absorption bandwidth ranges from 3.6 to 18 GHz with the tunable thickness from 1.7 to 5 mm. The effective absorption bandwidth covers the whole Ku band (12.4-18 GHz) when the thickness of carbon/TiO2/paraffin composite is 1.7 mm. This study is expected to pave the way to the synthesis of carbon-supported absorbing materials using a large family of 2D carbides.

Mo2Ga2C: a new ternary nanolaminated carbide

We report the discovery of a new hexagonal Mo2Ga2C phase, wherein two Ga layers – instead of one – are stacked in a simple hexagonal arrangement in between Mo2C layers. It is reasonable to assume this compound is the first of a larger family.

Tunable Magnetism and Transport Properties in Nitride MXenes

Two-dimensional materials with intrinsic and robust ferromagnetism and half-metallicity are of great interest to explore the exciting physics and applications of nanoscale spintronic devices, but no such materials have been experimentally realized. In this study, we predict several M2NTx nitride MXene structures that display these characteristics based on a comprehensive study using a crystal field theory model and first-principles simulations. We demonstrate intrinsic ferromagnetism in Mn2NTx with different surface terminations (T = O, OH, and F), as well as in Ti2NO2 and Cr2NO2. High magnetic moments (up to 9 μB per unit cell), high Curie temperatures (1877 to 566 K), robust ferromagnetism, and intrinsic half-metallic transport behavior of these MXenes suggest that they are promising candidates for spintronic applications, which should stimulate interest in their synthesis.

Solid Solubility and Magnetism upon Mn Incorporation in the Bulk Ternary Carbides Cr2AlC and Cr2GaC

Herein, we report on the bulk synthesis of (Cr 1−xMn x)2AlC and (Cr1−yMny)2GaC MAX phases. Scanning electron and transmission electron microscopy, in combination with energy-dispersive X-ray spectroscopy performed locally on MAX phase grains, revealed x and y to be 0.06 (3 at%) and 0.3 (15 at%), respectively. The introduction of Mn into the structure did not result in appreciable changes in the c-lattice constants. Vibrating sample magnetometry measurements suggest that bulk (Cr0.7Mn0.3)2GaC may be magnetic.

Rational Design of Two-Dimensional Metallic and Semiconducting Spintronic Materials Based on Ordered Double-Transition-Metal MXenes

Two-dimensional (2D) materials that display robust ferromagnetism have been pursued intensively for nanoscale spintronic applications, but suitable candidates have not been identified. Here we present theoretical predictions on the design of ordered double-transition-metal MXene structures to achieve such a goal. On the basis of the analysis of electron filling in transition-metal cations and first-principles simulations, we demonstrate robust ferromagnetism in Ti2MnC2Tx monolayers regardless of the surface terminations (T = O, OH, and F), as well as in Hf2MnC2O2 and Hf2VC2O2 monolayers. The high magnetic moments (3-4 μB/unit cell) and high Curie temperatures (495-1133 K) of these MXenes are superior to those of existing 2D ferromagnetic materials. Furthermore, semimetal-to-semiconductor and ferromagnetic-to-antiferromagnetic phase transitions are predicted to occur in these materials in the presence of small or moderate tensile in-plane strains (0-3%), which can be externally applied mechanically or internally induced by the choice of transition metals.