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

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 MoAl_1-xB, 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).