Hydrogen boride nanosheets (HB sheets) are facilely synthesized via ion-exchange treatment on magnesium diboride (MgB2) in an acetonitrile solution. Optical absorption and fluorescence spectra of HB sheets indicate that their bandgap energy is 2.8 eV. According to first-principles calculations, optical absorption seen at 2.8 eV is assigned to the electron transition between the σ-bonding states of B and H orbitals. In addition, density functional theory (DFT) calculations suggest the other allowed transition from the σ-bonding state of B and H orbitals to the antibonding state with the gap of 3.8 eV. Significant gaseous H2 release is found to occur only under photoirradiation, which causes the electron transition from the σ-bonding state to the antibonding state even under mild ambient conditions. The amount of H2 released from the irradiated HB sheets is estimated to be 8 wt%, indicating that the sheets have a high H2-storage capacity compared with previously reported metal H2-storage materials.

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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.

Boron-based two-dimensional materials are of interest for use in electronic devices and catalytic applications, for which it is important that they are chemically stable. Here, we explore the chemical stability of hydrogen boride nanosheets in water. Experiments reveal that mixing hydrogen boride and water produces negligible amounts of hydrogen, suggesting that hydrolysis does not occur and that hydrogen boride is stable in water, which is in contrast to most boron hydride materials. First-principles calculations reveal that the sheets interact weakly with water even in the presence of defects and that negatively charged boron prevents the onset of hydrolysis. We conclude that the charge state of boron and the covalent boron-boron bond network are responsible for the chemical and structural stability. On the other hand, we found that proton exchange with hydrogen boride nanosheets does occur in water, indicating that they become acidic in the presence of water.