Distinguishing electronic contributions of surface and sub-surface transition metal atoms in Ti-based MXenes
Distinguishing electronic contributions of surface and sub-surface transition metal atoms in Ti-based MXenes
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DOI：
10.1088/2053-1583/ab68e7
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发表时间：
2020-02
期刊：
影响因子：
5.5
通讯作者：
Yizhou Yang;Kanit Hantanasirisakul;Nathan C Frey;B. Anasori;R. Green;P. Rogge;I. Waluyo;A. Hunt;P. Shafer;E. Arenholz;V. Shenoy;Y. Gogotsi;S. May
中科院分区：
文献类型：
--
作者：
Yizhou Yang;Kanit Hantanasirisakul;Nathan C Frey;B. Anasori;R. Green;P. Rogge;I. Waluyo;A. Hunt;P. Shafer;E. Arenholz;V. Shenoy;Y. Gogotsi;S. May
MXene 是一个快速扩展的二维过渡金属碳化物和氮化物家族，由于其在储能和电磁干扰屏蔽等应用中的优异性能而引起了人们的关注。许多其他电子和磁性特性已经通过计算预测，但由于获得均匀表面终端 (Tx) 的实验困难而尚未实现，因此需要独立于表面终端的 MXene 的新设计方法。在本研究中，我们使用软 X 射线吸收光谱区分了表面和次表面 Ti 原子对四种含 Ti MXene（Ti2CTx、Ti3C2Tx、Cr2TiC2Tx 和 Mo2TiC2Tx）电子结构的贡献。对于表面过渡金属层上没有 Ti 原子的 MXene，例如 Mo2TiC2Tx 和 Cr2TiC2Tx，我们的结果显示母体 MAX 相与其 MXene 之间的光谱特征变化最小。相比之下，对于表面具有 Ti 原子的 MXene（此处为 Ti3C2TX 和 Ti2CTx），Ti L 边光谱与其母体 MAX 相化合物相比发生了显着修改。第一原理计算提供了源自表面和次表面钛原子的部分态密度的类似趋势，证实了光谱测量。这些结果表明，源自次表面 M 位层的电子态在很大程度上不受表面终端的干扰，表明 Tx 终端改变与 Ti 原子相关的标称电子数的长度尺度相对较短，并表明所需的能带特征应该由次表面 M 位点承载，这些次表面 M 位点在电子上比其表面 M 位点对应物更稳健。
MXenes are a rapidly-expanding family of 2D transition metal carbides and nitrides that have attracted attention due to their excellent performance in applications ranging from energy storage to electromagnetic interference shielding. Numerous other electronic and magnetic properties have been computationally predicted, but not yet realized due to the experimental difficulty in obtaining uniform surface terminations (Tx), necessitating new design approaches for MXenes that are independent of surface terminations. In this study, we distinguished the contributions of surface and sub-surface Ti atoms to the electronic structure of four Ti-containing MXenes (Ti2CTx, Ti3C2Tx, Cr2TiC2Tx, and Mo2TiC2Tx) using soft x-ray absorption spectroscopy. For MXenes with no Ti atoms on the surface transition metal layers, such as Mo2TiC2Tx and Cr2TiC2Tx, our results show minimal changes in the spectral features between the parent MAX phase and its MXene. In contrast, for MXenes with surface Ti atoms, here Ti3C2Tx and Ti2CTx, the Ti L-edge spectra are significantly modified compared to their parent MAX phase compounds. First principles calculations provide similar trends in the partial density of states derived from surface and sub-surface Ti atoms, corroborating the spectroscopic measurements. These results reveal that electronic states derived from sub-surface M-site layers are largely unperturbed by the surface terminations, indicating a relatively short length scale over which the Tx terminations alter the nominal electron count associated with Ti atoms and suggesting that desired band features should be hosted by sub-surface M-sites that are electronically more robust than their surface M-site counterparts.