Engineering Laboratory of Nuclear Energy Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China.
Nanoscale. 2018 May 10;10(18):8763-8771. doi: 10.1039/c8nr01292j.
MXenes represent a new family of two-dimensional materials that have attracted considerable attention in recent years. Because of the remarkably different structures of Sc2C(OH)2 and Sc2CO2 MXene and their recently reported properties, this study explored the structural evolution and mechanism of chemical conversion between these two MXenes. Using first-principles density functional theory (DFT), the mechanism for dehydrogenation/hydrogenation is investigated by gradually removing/adding surface hydrogen atoms for Sc2C(OH)2/Sc2CO2 supercells. Employing three different supercells (2 × 2 × 1, 3 × 3 × 1 and 4 × 4 × 1), intermediate states Sc2C(OH)xO2-x with varying hydrogen content x (0.0625≤x ≤ 1.94) are obtained. The results show that the trend is to minimize the difference in the number of hydrogen atoms and the distance between them on the two sides of the monolayer. This feature is found to be generally applicable to other functional groups of MXenes during surface conversion. Analysis of these structures shows that all the oxygen, carbon and scandium atoms remain in essentially the same locations as in Sc2C(OH)2 until atoms rearrange in the carbon layer at sufficiently low x. Regarding the electronic properties, the behavior of the rearranged configurations is found to depend on the structure, moving beyond the conventional model of p-type doping induced by dehydrogenation. Bipolar magnetic semiconductors (BMSs) are identified from these rearranged configurations by the inhomogeneous distribution of hydrogen atoms on the different sides and x values approximately in the range of 0.188 ≤ x ≤ 0.812. Findings from this study suggest that the intrinsic spin-polarized semiconducting characteristics of Sc2C(OH)xO2-x are expected to be experimentally observable if samples are prepared as nanoscale flakes. The current results indicate that Sc-based MXene may be a promising material for nanoscale spintronic devices.
MXenes 代表了一类新型二维材料，近年来引起了广泛关注。由于 Sc2C(OH)2 和 Sc2CO2 MXene 的结构显著不同，以及它们最近报道的性质，本研究探索了这两种 MXene 之间的结构演变和化学转化机制。本研究采用第一性原理密度泛函理论（DFT），通过逐步去除/添加表面氢原子来研究 Sc2C(OH)2/Sc2CO2 超胞的脱氢/加氢机制。使用三个不同的超胞（2×2×1、3×3×1 和 4×4×1），获得了具有不同氢含量 x（0.0625≤x ≤ 1.94）的中间态 Sc2C(OH)xO2-x。结果表明，趋势是最小化单层两侧氢原子数量和它们之间距离的差异。发现这一特征通常适用于 MXenes 表面转化过程中的其他官能团。对这些结构的分析表明，所有的氧、碳和钪原子在本质上都保持在 Sc2C(OH)2 中的相同位置，直到在足够低的 x 值下，碳原子层中的原子重新排列。关于电子性质，发现这些重排构型的行为取决于结构，超出了传统的脱氢诱导 p 型掺杂模型。通过不同侧氢原子的不均匀分布以及 x 值大约在 0.188 ≤ x ≤ 0.812 的范围内，从这些重排构型中确定了双极磁半导体（BMS）。如果样品制备为纳米级薄片，预计 Sc2C(OH)xO2-x 的本征自旋极化半导体特性将在实验中可观察到。目前的结果表明，基于 Sc 的 MXene 可能是用于纳米级自旋电子器件的有前途的材料。
