Covalent Functionalization and Cross-Linking of 2H MoS2 and MXene

The hybridization of two-dimensional (2D) materials exhibits unique properties and opens up opportunities for new device designs and fabrication. Extensive efforts have been made to realize the stacking of different 2D material in a layer-by-layer fashion, forming van der Waals heterostructures[1]. To control the distance between flakes and construct 3D networks, the connection between different 2D material layers has also been achieved through covalent cross-linking. However, despite the well-developed synthetic strategies for carbon-based materials such as graphene, graphene oxide and carbon nanotube, covalent cross-linking remains underexplored for transition metal dichalcogenides (TMDs), especially for the semiconducting phases, such as 2H MoS2. This is due to the lack of dangling bonds on the TMD’s basal planes, making them less reactive than their metallic counterparts and carbonaceous materials, and thus hindering the functionalization for further cross-linking steps. Functionalization of metallic MoS2 with organohalides and diazonium salts has been demonstrated [2,3], but a further annealing step is required in these processes to restore the semiconducting phase. While functionalization with organic thiols has been a widely accepted strategy for 2H MoS2, the nature of the interaction between the thiols and the MoS2 surface, whether physisorbed or covalently bonded, remains a subject of debate[4]. More recently, direct chemical functionalization of 2H MoS2 grown by chemical vapor deposition (CVD) has been reported, although this was limited to small-scale flakes on wafers.<br/>In this study, we activated 2H MoS2 by introducing sulfur vacancies, which facilitated the grafting of aryl carboxylic groups using diazonium salts, resulting in a 13% surface coverage. The functionalized 2H MoS2 was then cross-linked with amino-decorated Ti3C2Tx MXene via a straightforward EDC/NHS reaction in aqueous media. The resulting 3D structure effectively prevents the restacking of these 2D materials and enhances the stability of MXene in aqueous environments by obstructing oxygen and water molecules through the cross-linked network. This work introduces a novel strategy for the functionalization of semiconducting TMDs, enabling their use as building blocks for further cross-linking with zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) materials, as well as biomolecules. The resulting hybrid materials hold promise for applications in sensors, energy storage and photodetectors.<br/>Geim, Andre K., and Irina V. Grigorieva. Nature 499.7459 (2013): 419-425.<br/>Voiry, Damien, et al. Nature chemistry 7.1 (2015): 45-49.<br/>Knirsch, Kathrin C., et al. ACS nano 9.6 (2015): 6018-6030.<br/>Chen, Xin, et al. Angewandte Chemie 128.19 (2016): 5897-5902.