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 AlB2-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 (MgB2), titanium diboride (TiB2), chromium diboride (CrB2), etc. The AlB2-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 AlB2-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.

AlB2-type layered metal diborides have sought renewed research attention in recent years on account of their ability to yield XBenestheir quasi-two-dimensional (2D) counterparts. These quasi-2D nanostructures present exciting avenues to utilize the rich science offered by metal diborides. While a range of approaches have been developed to exfoliate layered metal diborides, the ability to obtain XBenes in a scalable manner is in its incipient stagesthis is a critical bottleneck in translating their rich nanoscience into tangible technology. In this work, we present a scalable approach that employs high-energy ball milling to exfoliate titanium diboride (TiB2) into its quasi-2D counterparts at the gram scale. We first show that milling TiB2 crystals for an optimal duration (6 h) and at a specific balls-to-powder ratio (20:1) yields multi-layer-thick nanosheets (∼5–10 nm thick). Second, by using Rietveld refinement and Raman spectroscopy, we show that the chemical integrity of TiB2 is retained to a large extent upon exfoliationthere is an associated formation of defects within the crystal structure of TiB2 that evolve with milling. Finally, we show that upon milling, the native electrochemical activity of TiB2 is enhanced by several folds. This ability to obtain nanosheets of TiB2 in a scalable manner using a high-energy ball mill bridges a critical missing link in the fast-growing science on nanoscaling AlB2-type metal borides.

No abstract