Quasi-Two-Dimensional Nanostructures from AlB2-Type Metal Borides: Physicochemical Insights and Emerging Trends

Transition-metal diborides are a class of abundantly available ceramic materials that exhibit a landscape of rich properties such as extraordinary high strength, exceptional hardness, and high-melting points. To date, these materials have been investigated primarily for their bulk scale properties. Many of these transition-metal diborides adopt an AlB<inf>2</inf>-type structure. In this type of structure, the metal atoms are sandwiched between alternating layers of boron atoms. Examples of these types of borides include, magnesium diboride (MgB<inf>2</inf>), titanium diboride (TiB<inf>2</inf>), chromium diboride (CrB<inf>2</inf>), etc. The AlB<inf>2</inf>-type structure is appealing for a plethora of reasons, primary among them is the inherent presence of boron atoms arranged in a honeycomb pattern reminiscent of the graphenic arrangement. This provides an opportunity to access two-dimensional (2D) boron. Recent research trends indicate a rising interest in nanoscaling these borides to yield 2D nanostructures. Research groups across the globe are currently pursuing this very objective, and multiple reports have already emerged showing that it is possible to nanoscale these bulk AlB<inf>2</inf>-type borides into nanosheets. One of the routes that researchers have adopted to this end is the well-recognized, top-down exfoliation. This involves methods such as liquid-phase exfoliation, mechanical exfoliation, and chemically induced selective extraction. This review will chronicle the research evolution of top-down exfoliation of metal borides from a physicochemical context and present the reader with a summary of these methods and the associated findings. We will also briefly discuss the various areas where these nanoscaled metal borides have just started finding applications toward sustainable technologies and present our perspective on where the field is headed. We hope that this review will be a timely addition to the fast-evolving literature on nanoscaling metal borides.