The synthesis of refractory materials usually relies on high-temperature conditions to drive diffusion-limited solid-state reactions. These reactions result in thermodynamically stable products that are rarely amenable to low-temperature topochemical transformations that postsynthetically modify subtle structural features. Here, we show that topochemical deintercalation of Al from MoAlB single crystals, achieved by room-temperature reaction with NaOH, occurs in a stepwise manner to produce several metastable Mo-Al-B intergrowth phases and a two-dimensional MoB (MBene) monolayer, which is a boride analogue to graphene-like MXene carbides and nitrides. A high-resolution microscopic investigation reveals that stacking faults form in MoAlB as Al is deintercalated and that the stacking fault density increases as more Al is removed. Within nanoscale regions containing high densities of stacking faults, four previously unreported Mo-Al-B (MAB) intergrowth phases were identified, including MoAlB, MoAlB, MoAlB, and MoAlB. One of these deintercalation products, MoAlB, is identified as the likely MAB-phase precursor that is needed to achieve a high-yield synthesis of two-dimensional MoB, a highly targeted two-dimensional MBene. Microscopic evidence of an isolated MoB monolayer is shown, demonstrating the feasibility of using room-temperature metastable-phase engineering and deintercalation to access two-dimensional MBenes.

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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).

Colloidal hybrid nanoparticles integrate two or more nanocrystal domains into a single architecture that can have properties not found in, or enhanced relative to those of, the individual components. These hybrid nanomaterials are typically constructed using multistep seeded-growth reaction sequences, which are conceptually analogous to the total synthesis approaches used in molecular synthesis. Here, we discuss in detail the synthetic protocols that lead to the formation of three-component Ag−Pt−Fe 3 O 4 and Au−Pt−Fe 3 O 4 heterotrimers. These instructive model systems highlight the important synthetic details that underpin successful hybrid nanoparticle reactions. We provide detailed, step-by-step protocols for generating these materials, focusing on describing and rationalizing the key reaction parameters that need to be rigorously controlled to minimize unwanted nanoparticle byproducts. The importance of comprehensive analysis using a suite of materials characterization tools is highlighted, as such efforts are useful for diagnosing subtle chemical and morphological features that can lead to synthetic bottlenecks throughout the course of the reaction sequences. Finally, we offer strategies for circumventing these commonly encountered problems as well as insights that can lead to increased hybrid nanoparticle yields and improved sample-to-sample reproducibility. Although this work specifically details the synthesis of Ag−Pt−Fe 3 O 4 and Au−Pt−Fe 3 O 4 heterotrimers, these synthetic strategies and protocol guidelines are generally applicable to many other hybrid nanoparticle systems.