Multi-Step Topochemical Pathway to Metastable Mo₂AlB₂ and Related Two-Dimensional Nanosheet Heterostructures

Abstract

The rational synthesis of metastable inorganic solids, which is a grand challenge in solid-state chemistry, requires the development of kinetically controlled reaction pathways. Topotactic strategies can achieve this goal by chemically modifying reactive components of a parent structure under mild conditions to produce a closely related analogue that has otherwise inaccessible structures and/or compositions. Refractory materials, such as transition metal borides, are difficult to structurally manipulate at low temperatures because they generally are chemically inert and held together by strong covalent bonds. Here, we report a multistep low-temperature topotactic pathway to bulk-scale Mo2AlB2, which is a metastable phase that has been predicted to be the precursor needed to access a synthetically elusive family of 2-D metal boride (MBene) nanosheets. Room-temperature chemical deintercalation of Al from the stable compound MoAlB (synthesized as a bulk powder at 1400 °C) formed highly strained and destabilized MoAl1–x B, which was size-selectively precipitated to isolate the most reactive submicron grains and then annealed at 600 °C to deintercalate additional Al and crystallize Mo2AlB2. Further heating resulted in topotactic decomposition into bulk-scale Mo2AlB2–AlO x nanolaminates that contain Mo2AlB2 nanosheets with thickness of 1–3 nm interleaved by 1–3 nm of amorphous aluminum oxide. The combination of chemical destabilization, size-selective precipitation, and low-temperature annealing provides a potentially generalizable kinetic pathway to metastable variants of refractory compounds, including bulk Mo2AlB2 and Mo2AlB2–AlO x nanosheet heterostructures, and opens the door to other previously elusive 2-D materials such as 2-D MoB (MBene).

Lithium nickel borides: evolution of [NiB] layers driven by Li pressure

“…[15][16][17][18][19][20] MBenes were predicted to have similarly fascinating properties [21][22][23][24] but have not yet been obtained experimentally, despite several prominent attempts. [24][25][26][27][28] Recent theoretical investigation of the Li-Ni-B system 29 revealed a new Li-rich layered compound Li 2 NiB, whose structure is derived from RT-LiNiB. According to the calculations, only a little extra energy is needed to accommodate an additional Li layer in the structure of the LiNiB parent phase.…”

Computationally Driven Discovery of a Family of Layered LiNiB Polymorphs

“…Borophane sheets were obtained by exfoliation of a layered MgB 2 superconductor, while to the best of our knowledge there are no experimental reports about the synthesis of 2D MBenes. Recent studies on the partial topochemical deintercalation of Al from layered MoAlB or complete removal of In to produce a layered structure TiB from Ti 2 InB 2 signifies the importance of the judicial choice of suitable precursors for the preparation of novel 2D MBenes.…”

Computationally Driven Discovery of a Family of Layered LiNiB Polymorphs

“…Borophane sheets were obtained by exfoliation of a layered MgB 2 superconductor, while to the best of our knowledge there are no experimental reports about the synthesis of 2D MBenes. Recent studies on the partial topochemical deintercalation of Al from layered MoAlB or complete removal of In to produce a layered structure TiB from Ti 2 InB 2 signifies the importance of the judicial choice of suitable precursors for the preparation of novel 2D MBenes.…”