Thermodynamic stability of Borophene, \(\mathrm{B_2O_3}\) and other \(\mathrm{B_{1-x}O_x}\) sheets

• Published in: arXiv.org 2019-11
• Main Authors: Arnold, Florian M, Seifert, Gotthard, Kunstmann, Jens
• Format: Article
• Language: English
• Subjects: Allotropy; Boron; Borophene; Composition; Coordination numbers; Disproportionation; First principles; Organic chemistry; Oxidation; Phase diagrams; Physical properties; Sheets; Two dimensional materials
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1911.02950

The recent discovery of borophene, a two-dimensional allotrope of boron, raises many questions about its structure and its chemical and physical properties. Boron has a high chemical affinity to oxygen but little is known about the oxidation behavior of borophene. Here we use first principles calculations to study the phase diagram of free-standing, two-dimensional \(\mathrm{B_{1-x}O_x}\) for compositions ranging from \(x=0\) to \(x=0.6\), which correspond to borophene and \(\mathrm{B_2O_3}\) sheets, respectively. Our results indicate that no stable compounds except borophene and \(\mathrm{B_2O_3}\) sheets exist. Intermediate compositions are heterogeneous mixtures of borophene and \(\mathrm{B_2O_3}\). Other hypothetical crystals such as \(\mathrm{B_2O}\) are unstable and some of them were found to undergo spontaneous disproportionation into borophene and \(\mathrm{B_2O_3}\). It is also shown that oxidizing borophene inside the flakes is thermodynamically unfavorable over forming \(\mathrm{B_2O_3}\) at the edges. All findings can be rationalized by oxygen's preference of two-fold coordination which is incompatible with higher in-plane coordination numbers preferred by boron. These results agree well with recent experiments and pave the way to understand the process of oxidation of borophene and other two-dimensional materials.