Mechanism for Liquid Phase Exfoliation of MoS2
Mechanism for Liquid Phase Exfoliation of MoS2
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DOI：
10.1021/acs.chemmater.5b04224
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发表时间：
2016-01-12
影响因子：
8.6
通讯作者：
Vaia, Richard A.
中科院分区：
文献类型：
--
作者：
Jawaid, Ali;Nepal, Dhriti;Vaia, Richard A.
高效、可重复且可扩展的 MoS2 剥离方法对于利用这些新兴材料（从涂层和复合材料到可打印设备）至关重要。无添加剂技术，例如通过超声处理进行溶剂辅助剥离，被认为是最可行的方法，其中 N-甲基-2-吡咯烷酮 (NMP) 是最有效的溶剂。然而，人们对剥离机制以及 NMP 在此过程中所起的关键作用的了解一直难以捉摸，并且对产品产量和质量的有效提高提出了挑战。在这里，我们报告了系统实验，通过阐明与 NMP 相关的超声波化学作用来了解溶剂辅助剥脱的机制。已证实，在 O-2(g) 存在的情况下，NMP 中溶解的水分在超声处理过程中起着至关重要的作用。水分含量越高，去角质过程就越有效。相反，当在惰性气氛下使用干燥溶剂进行剥离时，反应产率会降低。这是由于通过自氧化途径原位形成了氧化还原活性物质，该途径通过氢过氧化物中间体将 NMP 转化为 N-甲基琥珀酰亚胺。这些高反应性物质似乎有助于通过反应性边缘部位的氧化来剥落；充电会在相邻片材之间产生库仑斥力，从而破坏层间基面键合，并使高偶极溶剂中的颗粒实现静电稳定。根据这些见解，证明了在先前报道的惰性溶剂（例如乙腈）中以及在没有探针超声处理的情况下的剥落。这些发现表明，MoS2 的剥落（可能还有一般 TMD 的剥落）可以通过了解表面-溶剂界面发生的化学反应来介导。
A highly efficient, reproducible, and scalable approach for exfoliation of MoS2 is critical for utilizing these emerging materials from coatings and composites to printable devices. Additive-free techniques, such as solvent-assisted exfoliation via sonication, are considered to be the most viable approach, where N-methyl-2-pyrrolidone (NMP) is the most effective solvent. However, understanding the mechanism of exfoliation and the key role NMP plays during the process have been elusive and challenges effective improvements in product yield and quality. Here, we report systematic experiments to understand the mechanism of solvent-assisted exfoliation by elucidating the sonolysis chemistries associated with NMP. It is confirmed that in the presence of O-2(g) dissolved moisture in NMP plays a critical role during sonication. The higher the moisture content, the more efficient the exfoliation process is. Conversely, when exfoliations are carried out with dried solvents with an inert atmosphere, reaction yields decrease. This is due to redox-active species formed in situ through an autoxidation pathway that converts NMP to N-methyl succinimide by hydroperoxide intermediates. These highly reactive species appear to aid exfoliation by oxidation at reactive edge sites; the charging creates Coulombic repulsion between neighboring sheets that disrupts interlayer basal plane bonding and enables electrostatic stabilization of particles in high-dipole solvents. From these insights, exfoliation in previously reported inactive solvents (e.g., acetonitrile), as well as in the absence of probe sonication, is demonstrated. These findings illustrate that exfoliation of MoS2, and possibly TMD's in general, can be mediated through understanding the chemistry occurring at the surface-solvent interface.