Guo Yuqi, Gupta Adway, Gilliam Matthew S, Debnath Abhishek, Yousaf Ahmed, Saha Sanchari, Levin Mark D, Green Alexander A, Singh Arunima K, Wang Qing Hua
Materials Science and Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, USA.
Nanoscale. 2021 Jan 28;13(3):1652-1662. doi: 10.1039/d0nr07971e.
Liquid phase exfoliation (LPE) is a method that can be used to produce bulk quantities of two-dimensional (2D) nanosheets from layered van der Waals (vdW) materials. In recent years, LPE has been applied to several non-vdW materials with anisotropic bonding to produce nanosheets and platelets, but it has not been demonstrated for materials with strong isotropic bonding. In this paper, we demonstrate the exfoliation of boron carbide (B4C), the third hardest known material, into ultrathin nanosheets. B4C has a structure consisting of strongly bonded boron icosahedra and carbon chains, but does not have anisotropic cleavage energies to suggest that it can be readily cleaved into nanosheets. B4C has been widely studied for its very high melting point, high mechanical strength, and chemical stability, as well as its zero- and one-dimensional nanostructured forms. Herein, ultrathin nanosheets are successfully prepared by sonication of B4C powder in organic solvents and are characterized by microscopy and spectroscopy. Density functional theory (DFT) simulations reveal that B4C can be cleaved along several different crystallographic planes with similar energetic favourability, facilititated by an unexpected mechanism of breaking boron icosahedra and forming new boron-rich cage structures at the surface. Atomic force microscopy (AFM) shows that the nanosheets produced by LPE are as thin as 5 nm, with an average thickness of 31.4 nm and average area of 16 000 nm2. Raman spectroscopy shows that many of the nanosheets exhibit additional carbon-rich peaks that change with laser irradiation, which are attributed to atomic rearrangements and amorphization at the nanosheet surfaces, consistent with the diverse cleavage planes. High-resolution transmission electron microscopy (HRTEM) demonstrates that many different cleavage planes exist among the exfoliated nanosheets, in agreement with DFT simulations. This work elucidates the exfoliation mechanism of 2D B4C and suggests that LPE can be applied to generate nanosheets from a variety of non-layered and non-vdW materials.
液相剥离法（LPE）是一种可用于从层状范德华（vdW）材料中大量制备二维（2D）纳米片的方法。近年来，LPE已应用于几种具有各向异性键合的非vdW材料以制备纳米片和薄片，但对于具有强各向同性键合的材料尚未得到验证。在本文中，我们展示了将已知第三硬的材料碳化硼（B4C）剥离成超薄纳米片。B4C具有由强键合的硼二十面体和碳链组成的结构，但没有各向异性的解理能表明它可以很容易地裂解成纳米片。B4C因其非常高的熔点、高机械强度和化学稳定性以及其零维和一维纳米结构形式而受到广泛研究。在此，通过在有机溶剂中对B4C粉末进行超声处理成功制备了超薄纳米片，并通过显微镜和光谱进行了表征。密度泛函理论（DFT）模拟表明，B4C可以沿着几个不同的晶面以相似的能量优势进行裂解，这是由一种意想不到的机制促成的，即破坏硼二十面体并在表面形成新的富硼笼状结构。原子力显微镜（AFM）显示，通过LPE制备的纳米片薄至5 nm，平均厚度为31.4 nm，平均面积为16000 nm2。拉曼光谱表明，许多纳米片表现出随激光照射而变化的额外富碳峰，这归因于纳米片表面的原子重排和非晶化，与不同的解理面一致。高分辨率透射电子显微镜（HRTEM）表明，剥离的纳米片中存在许多不同的解理面，这与DFT模拟结果一致。这项工作阐明了二维B4C的剥离机制，并表明LPE可应用于从各种非层状和非vdW材料中生成纳米片。
