过渡金属碳化物,氮化物和碳氮化物(MXenes)最近在摩擦学和,特别是固体润滑,由于其低剪切强度和形成低摩擦的能力,耐磨摩擦层。由于其固有的亲水性,它们作为润滑剂添加剂的使用仅显示出有限的成功。导致与纯基础油的低相相容性和分散稳定性。为了克服这一缺点并提高MXenes作为复合材料中的润滑剂添加剂和增强相的摩擦学性能,它们的可调谐表面化学成为当前研究的焦点。源于化学蚀刻合成MXenes,它们的外表面包含各种表面终端,其可以通过共价接枝/官能化充当分子的锚定点。通过采取适当的功能化策略,这个,反过来,可以帮助定制MXenes\'疏水性,分散稳定性,重堆趋势,或抗氧化性。这直接影响了它们在基础油中的分散稳定性,提高了它们与复合材料中其他基体材料的相相容性,从而导致增强的摩擦学性能。因此,这篇综述简要总结了现有的关于MXenes共价官能化的最新技术,特别强调摩擦学特性和需求,一个话题,尚未进行全面审查。第一章阐明了现有的合成方法,并详细了解了所产生的表面终止,这对随后的化学官能化至关重要。在总结了增加层间距离以改善化学功能化的可及性的策略以及有关MXene摩擦学的现有技术之后,我们批判性地讨论了使用不同偶联剂(有机硅烷,有机磷药剂,芳基重氮剂,除其他外)。随后,我们强调均匀分布的-OH表面终止的关键作用,以保证功能化方法的整体成功,并提高所得的摩擦学性能。最后,我们解决了现有的挑战,并得出了未来的研究方向。我们预计我们的文章可以作为MXenes化学官能化的优秀指南,其可用于各种应用,包括摩擦学,从而为MXenes的增强的物理和化学性质铺平了道路。
Transition metal carbides, nitrides and carbonitrides (MXenes) have recently attracted notable attention in
tribology and, particularly solid lubrication, due to their low shear strength and ability to form low-friction, wear-resistant tribo-layers. Their use as lubricant additives has only shown limited success due to their inherent hydrophilic character, causing a low phase-compatibility and dispersion-stability with pure base oils. To overcome this shortcoming and to boost MXenes\' tribological performance as lubricant additive and reinforcement phase in composites, their tuneable surface chemistry moves into the focus of current research. Originating from chemical etching to synthesize MXenes, their outer surface contains a variety of surface terminations, which can function as anchoring points for molecules via covalent grafting/functionalization. By adopting an adequate functionalization strategy, this, in turn, can help to tailor MXenes\' hydrophobicity, dispersion stability, restacking tendency, or oxidation resistance. This directly affects their dispersion stability in base oils and improves their phase compatibility with other matrix materials in composites, thus resulting in an enhanced tribological performance. Therefore, this
review concisely summarizes the existing state-of-the-art regarding MXenes\' covalent functionalization with a particular emphasis on tribological properties and needs, a topic, which has not been holistically reviewed yet. The first chapter sheds light on the existing synthesis approaches with detailed insights regarding the resulting surface terminations, which are crucial for the subsequent chemical functionalization. After summarizing strategies to increase their interlayer distance to improve the accessibility for chemical functionalization and the existing state-of-the art regarding MXene
tribology, we critically discuss the existing functionalization strategies using different coupling agents (organosilanes, organophosphorus agents, aryldiazonium agents, among others). Subsequently, we emphasize on the crucial role of homogeneously distributed -OH surface terminations to guarantee the overall success of the functionalization approach and to boost the resulting tribological performance. Lastly, we address the existing challenges and derive future research directions. We anticipate that our article can serve as an excellent guide for MXenes\' chemical functionalization, which can be useful in various applications including
tribology thus paving the way towards enhanced physical and chemical properties of MXenes.