Effects of high-temperature heating of hydrogen boride sheets under high-hydrogen partial pressure

<p>Two-dimensional (2D) materials are used in various fields such as electrode catalysts, photocatalysts, electronics, optoelectronics, inserts and lubricants because of its superior properties. In 2017, experimental realization of the hydrogen boride sheets (HB sheets) have been reported as a new 2D materials¹⁾. HB sheets are composed of boron and hydrogen with stoichiometric 1:1. HB sheets theoretically take the structure of a six-membered ring of boron bridged by hydrogen, but it is only a local structure. In fact, it is reported that HB sheets have two types of bonds: one is three-center, two-electron (3c-2e) bond (B-H-B bond) and the other is two-center, two-electron (2c-2e) bond (B-H bond)²⁾. HB sheets are expected to be used as a hydrogen storage material. It is estimated to exhibit a hydrogen storage of 8.5 wt% and releases hydrogen by heating. Not only itself but also complexes with HB sheets are expected to exhibit a high level of hydrogen storage. For example, Li doped HB sheets theoretically indicate up to hydrogen storage of 11.57 wt%³⁾. However, hydrogen dissociation temperature of HB sheets is very wide because of its structural inhomogeneity, so it is important to control its structure and evaluate hydrogen dissociation characteristics. This attempt may also contribute to other functionalities of HB sheets such as a catalyst or reductant. In this study, HB sheets were heated under extreme conditions such as high-hydrogen partial pressure or ultrahigh pressure to induce structural changes while preventing the decomposition and desorption of hydrogen from the HB sheets.</p><p></p><p>The purpose of this study is to examine the effects of heating the HB sheets and to clarify how the pressure and temperature conditions affects the physical properties, especially hydrogen dissociation characteristics of the HB sheets.</p><p></p><p>We have conducted heating of HB sheets under 573~1073 K temperature condition and 5~11 MPa hydrogen partial pressure. We have also conducted heating under ultrahigh pressure (3500 and 5000 MPa) without hydrogen.</p><p></p><p>Figure 1 shows M/z=2 thermal desorption spectroscopy (TDS) of normal HB sheets and heated HB sheets. Intensity values are normalized with the maximum value as 1. An interesting point is that the peak of hydrogen dissociation shifts toward higher temperatures as the heating temperature increases. Fourier-transformed infrared absorption (FTIR) spectroscopy measurements were also conducted to examine changes in structure, and there were not significant spectral differences between the samples under different heating conditions in terms of peak positions of B-H-B and B-H vibrational modes. The only observed difference is peak positions changes of B-H-B caused B-B distance change²⁾. In addition, X-ray diffraction patterns of B₂O₃ Ⅰ and B₂O₃ Ⅱ were detected in sample heated at ultrahigh pressure.</p><p></p><p>References:</p><p>1) H. Nishino, et al., J. Am. Chem. Soc. 139 (2017) 13761-13769.</p><p>2) S. Tominaka, et al., Chem. 6 (2020) 406-418.</p><p>3) L. Chen, et al., Phys. Chem. Chem. Phys. 20 (2018) 30304-30311.</p>