Insights into exfoliation possibility of MAX phases to MXenes
Insights into exfoliation possibility of MAX phases to MXenes
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DOI：
10.1039/c7cp08645h
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发表时间：
2018-04-07
影响因子：
3.3
通讯作者：
Yunoki, Seiji
中科院分区：
文献类型：
--
作者：
Khazaei, Mohammad;Ranjbar, Ahmad;Yunoki, Seiji
将MAX相化学剥离成二维MXenes可以被认为是合成新型2D体系的一项重大突破。为了深入了解MAX相剥离的可能性，并确定哪些MAX相有希望成功地剥离成2D MXenes，我们进行了广泛的电子结构和声子计算，并确定了82个不同实验合成的晶体MAX相的MAX相到MXenes的力常数、键强度和静态剥离能。结果表明，力常数与成键强度之间存在明显的相关性。当相邻原子贡献的A原子的总力常数较小时，剥离能变小，从而更容易剥离。我们提出了成功剥离成2D Ti2C、Ti3C2、Ti4C3、Ti5C4、Ti2N、Zr2C、Hf2C、V2C、V3C2、V4C3、Nb2C、Nb5C4、Ta2C、Ta5C4、Cr2C、Cr2N和Mo2C MXen的37个最大相。此外，我们还探讨了电荷注入对MAX相的影响。我们发现注入的电荷，包括电子和空穴，主要由过渡金属接收。这是由于MAX相的电子性质，费米能级附近的态主要由过渡金属的d轨道主导。对于带负电荷的MAX相，注入的电子导致结构膨胀和沿c轴的键距延长，从而削弱了结合。另一方面，对于带正电的最大相，键变得更短和更强。因此，我们预测，通过电化学或门控技术的电子注入可以显著地促进MAX相向2D MXenes的剥离。
Chemical exfoliation of MAX phases into two-dimensional (2D) MXenes can be considered as a major breakthrough in the synthesis of novel 2D systems. To gain insight into the exfoliation possibility of MAX phases and to identify which MAX phases are promising candidates for successful exfoliation into 2D MXenes, we perform extensive electronic structure and phonon calculations, and determine the force constants, bond strengths, and static exfoliation energies of MAX phases to MXenes for 82 different experimentally synthesized crystalline MAX phases. Our results show a clear correlation between the force constants and the bond strengths. As the total force constant of an A atom contributed from the neighboring atoms is smaller, the exfoliation energy becomes smaller, thus making exfoliation easier. We propose 37 MAX phases for successful exfoliation into 2D Ti2C, Ti3C2, Ti4C3, Ti5C4, Ti2N, Zr2C, Hf2C, V2C, V3C2, V4C3, Nb2C, Nb5C4, Ta2C, Ta5C4, Cr2C, Cr2N, and Mo2C MXenes. In addition, we explore the effect of charge injection on MAX phases. We find that the injected charges, both electrons and holes, are mainly received by the transition metals. This is due to the electronic property of MAX phases that the states near the Fermi energy are mainly dominated by d orbitals of the transition metals. For negatively charged MAX phases, the electrons injected cause swelling of the structure and elongation of the bond distances along the c axis, which hence weakens the binding. For positively charged MAX phases, on the other hand, the bonds become shorter and stronger. Therefore, we predict that the electron injection by electrochemistry or gating techniques can significantly facilitate the exfoliation possibility of MAX phases to 2D MXenes.