Khazaei Mohammad, Ranjbar Ahmad, Esfarjani Keivan, Bogdanovski Dimitri, Dronskowski Richard, Yunoki Seiji
Computational Materials Science Research Team, RIKEN Advanced Institute for Computational Science (AICS), Kobe, Hyogo 650-0047, Japan.
Departments of Mechanical and Aerospace Engineering, Physics, and Materials Science and Engineering, University of Virginia, 122 Engineer's Way, Charlottesville, VA 22904, USA.
Phys Chem Chem Phys. 2018 Mar 28;20(13):8579-8592. doi: 10.1039/C7CP08645H.
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.
将MAX相化学剥离成二维（2D）MXene可被视为新型二维体系合成中的一项重大突破。为深入了解MAX相的剥离可能性，并确定哪些MAX相是成功剥离成二维MXene的有前景的候选者，我们进行了广泛的电子结构和声子计算，并确定了82种不同实验合成的晶体MAX相到MXene的力常数、键强度和静态剥离能。我们的结果表明力常数和键强度之间存在明显的相关性。由于相邻原子贡献给一个“A”原子的总力常数越小，剥离能就越小，从而使剥离更容易。我们提出了37种MAX相可成功剥离成二维Ti2C、Ti3C2、Ti4C3、Ti5C4、Ti2N、Zr2C、Hf2C、V2C、V3C2、V4C3、Nb2C、Nb5C4、Ta2C、Ta5C4、Cr2C、Cr2N和Mo2C MXene。此外，我们还探索了电荷注入对MAX相的影响。我们发现注入的电荷，包括电子和空穴，主要被过渡金属接收。这是由于MAX相的电子性质，即费米能级附近的态主要由过渡金属的d轨道主导。对于带负电的MAX相，注入的电子会导致结构膨胀和沿c轴的键长伸长，从而削弱了结合力。另一方面，对于带正电的MAX相，键会变得更短更强。因此，我们预测通过电化学或门控技术进行电子注入可以显著促进MAX相剥离成二维MXene的可能性。
