Two-dimensional (2D) materials are promising for applications in a wide range of fields because of their unique properties. Hydrogen boride sheets, a new 2D material recently predicted from theory, exhibit intriguing electronic and mechanical properties as well as hydrogen storage capacity. Here, we report the experimental realization of 2D hydrogen boride sheets with an empirical formula of HB, produced by exfoliation and complete ion-exchange between protons and magnesium cations in magnesium diboride (MgB) with an average yield of 42.3% at room temperature. The sheets feature an sp-bonded boron planar structure without any long-range order. A hexagonal boron network with bridge hydrogens is suggested as the possible local structure, where the absence of long-range order was ascribed to the presence of three different anisotropic domains originating from the 2-fold symmetry of the hydrogen positions against the 6-fold symmetry of the boron networks, based on X-ray diffraction, X-ray atomic pair distribution functions, electron diffraction, transmission electron microscopy, photo absorption, core-level binding energy data, infrared absorption, electron energy loss spectroscopy, and density functional theory calculations. The established cation-exchange method for metal diboride opens new avenues for the mass production of several types of boron-based 2D materials by countercation selection and functionalization.

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A recent experiment demonstrated that ultrasonication of MgB2 in water yields Mg-deficient hydroxyl-functionalized boron nanosheets at room temperature. Herein, we examined the mechanism of nanosheet formation. Analysis of the reaction products and temporal variation in pH and H2 production shows that the reaction between MgB2 and water comprises two steps: (i) an ion-exchange process between protons and a part of Mg cations of MgB2 with its exfoliation and (ii) the hydrolysis reaction between Mg-deficient boron hydride and water to produce H2 and Mg-deficient hydroxyl-functionalized boron sheets. The sheets with a stacking periodicity of 0.70 nm were obtained as the supernatant of the reaction product of water with MgB2. The stacking sheets can be further exfoliated if the reaction is conducted under ultrasonication. The derived nanosheets are composed of sp2-bonded boron framework and possess a disordered structure containing hydroxyl species and oxidized magnesium.

Hydrogen borides adopt a variety of structures because of the electron-deficient character of boron. Recently, we reported the synthesis of a layered hydrogen boride via a soft chemical route. Here, we ascertain the atomic arrangements in the layered hydrogen boride by using pair-distribution functions: the material dominantly consists of a corrugated B network decorated with three-center, two-electron B-H-B bridging bonds as well as ordinary two-center, two-electron B-H terminal bonds. The material is locally ordered but amorphous by diffractometry. This discrepancy can be accounted for by geometrical frustration caused by the positions of terminal B-H bonds located on one of two equivalent B atoms in the B-H-B bridging bonds. This material is electrically conductive (0.13 S cm À1 below 10 C) rather than ion conductive, and its B-H-B bonds are cleaved by the adsorption of molecules; this dynamic chemical nature originates from the frustrated structure and leads to unique hydrogen boride functionalities.

Hydrogen boride (HB) or hydrogenated borophene sheets are recently realized two-dimensional materials that are composed of only two light elements, boron and hydrogen. However, their catalytic activity has not been experimentally analyzed. Herein, we report the catalytic activity of HB sheets in ethanol reforming. HB sheets catalyze the conversion of ethanol to ethylene and water above 493 K with high selectivity, independent of the contact time, and with an apparent activation energy of 102.8 ± 5.5 kJ/mol. Hence, we identify that HB sheets act as solid-acid catalysts.