Sankalp Kota

Two-dimensional Mo 1.33 C MXene with divacancy ordering prepared from parent 3D laminate with in-plane chemical ordering

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, (Mo2/3Sc1/3)2AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, we show evidence for 2D Mo1.33C 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, Mo2C, 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.

Atomic structure and lattice defects in nanolaminated ternary transition metal borides

ABSTRACT We use analytical aberration-corrected high-resolution scanning transmission electron microscopy to image the atomic structure of the layered ternary transition metal (M) borides, Cr2AlB2, Fe2AlB2, and MoAlB. In these ternaries, MB layers and Al single or double atomic layers are interleaved. The atomic positions of the M elements and Al are clearly resolved by Z-contrast images. The following structural defects are also found and described herein: a 90° twist boundary along [010] in Cr2AlB2, a tilt boundary in Fe2AlB2, and Mo2AlB2-like stacking faults in MoAlB, where some of the MB-based structures are intercalated by one (instead of two) Al layer(s). GRAPHICAL ABSTRACT IMPACT STATEMENT Atomic structures of ternary transition metal borides Cr2AlB2, Fe2AlB2, and MoAlB are observed for the first time by HRSTEM. Some structural defects are also found in those borides.

Tailoring Structure, Composition, and Energy Storage Properties of MXenes from Selective Etching of In-Plane, Chemically Ordered MAX Phases

The exploration of 2D solids is one of our times 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 (Mo2/3 Y1/3 )2 AlC, the resulting MXene can be either: i) (Mo2/3 Y1/3 )2 C with in-plane elemental order through selective removal of Al atoms or ii) Mo1.33 C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo2/3 Y1/3 )2 C (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 Mo1.33 C counterpart. With H2 SO4 , 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.

Pressure-induced shear and interlayer expansion in Ti3C2 MXene in the presence of water

The interlayer spacing of the material Ti3C2 MXene expands under pressure due to intercalation of water. Pseudo-negative compressibility in layered materials is a phenomenon typically limited to in situ high-pressure experiments in some clay minerals and carbon-based materials. We show that the MXene Ti3C2Tx expands along its crystallographic c direction when compressed in the presence of H2O. This expansive effect occurs when a mixture of powders and excess water is quasi-hydrostatically compressed in a diamond anvil cell; it also occurs to a much larger extent when powders are pressed uniaxially into discs and, notably, persists after pressure is released. We attribute the expansion to the insertion of H2O molecules and have identified shear-induced slipping of the nanosheets comprising multilayered MXene particles as a possible cause of this behavior in the latter case. This both has implications for the processing of MXenes and contributes to the field of materials with pseudo-negative compressibility by adding a new member for further investigation.