Fast Gelation of Ti₃C₂T_x MXene Initiated by Metal Ions

Abstract

Graphene faces the same problem, but its self-assembly into a 3D structures can effectively solve these problems. [5,6] The gelation of graphene oxide (GO) is the mostly used method to prepare a 3D assembled structure, and this process is triggered by the surface chemistry changes of GO NSs in solution. However, the gelation of the other 2D materials, such as transition metal dichalcogenides (TMDs), transition metal oxides (TMOs), and h-BN, [7][8][9][10] is difficult to achieve because of the fewer functional groups on their surface and their poor dispersion in an aqueous solution.Different from the above 2D materials, MXenes prepared by selective etching contain a number of functional groups on their surfaces and edges (F − , O, and OH) [11] show good dispersion in aqueous solution and have the potential to build 3D MXene-based assemblies by solution-based techniques. With the GO as the linker, 3D graphene/MXene hybrid monoliths have been assembled in solution with a low temperature heating. [12,13] In the assembly processes, the laminated MXene domains interacted with the π-conjugated GO sheets which then guided the formation of the 3D structure. The use of surfactants or polymers as the linkers to realize side-by-side joining of MXene NSs also has the potential to build 3D MXene-based monoliths. [14][15][16] However, all these strategies involve a complex process in which the MXenes are easily oxidized due to their high chemical activity. In addition, the above assembly process is different from the GO assembly that relies on self-gelation and has many advantages for achieving structure control by simply changing the solvent or the drying process. [17,18] Until now, the gelation of MXene in a similar manner to that of GO has been difficult to realize.Here, we report the fast gelation of MXene in an aqueous solution initiated by divalent metal ions. In a typical process, Fe 2+ ions are introduced in the MXene solution to destroy the electrostatic repulsive forces between the NSs and link them together, forming a stable 3D MXene hydrogel, similar to the GO hydrogel and its gelation process. [19] The use of metal ions as the joining sites enhances the interlinking of MXene NSs to form a 3D network due to their strong binding energy with the OH groups on the MXene surface. As a result, the formed hydrogel effectively suppresses the restacking of MXene NSs Gelation is an effective way to realize the self-assembly of nanomaterials into different macrostructures, and in a typical use, the gelation of graphene oxide (GO) produces various graphene-based carbon materials with different applications. However, the gelation of MXenes, another important type of 2D materials that have different surface chemistry from GO, is difficult to achieve. Here, the first gelation of MXenes in an aqueous dispersion that is initiated by divalent metal ions is reported, where the strong interaction between these ions and OH groups on the MXene surface plays a key role. Typically, Fe 2+ ions are introduced in the MXene dispersion whic...

MXene-based 3D porous macrostructures for electrochemical energy storage

“…One of the first concerns with the assembly of MXene architectures is their weak gelation capability (see GO) [54], which reduces the connections between sheets and hinders their mechanical stability. Therefore, if the concentration is too low, for example, a freeze-dried MXene colloid collapses into a powder of aerogel-like porous particles [53].…”

“…Therefore, if the concentration is too low, for example, a freeze-dried MXene colloid collapses into a powder of aerogel-like porous particles [53]. To overcome this problem, additives such as ethylenediamine (EDA) [55], thiourea dioxide (thiox) and ammonia [56], PAA [57], and divalent metal ions [54], have been used to functionalize the Ti 3 C 2 T x sheets and enhance their interconnectivity during the gelation process [54][55][56][57]. The mechanism that induces gelation depends on the additive used and can be via a chemical reduction of the surface of the sheets (when using mild reducing agents, such as EDA, thiox and ammonia, or PAA), or simple electrostatic interactions of the surface functional groups and the cross-linking agent (e.g.…”

“…Other functionalization strategies to improve gelation and mechanical resistance were also reported [54,56,57]. Wang et al produced a Ti 3 C 2 T x aerogel using thiox and ammonia solution to prevent oxidation of the sheets after hydrolysis and to enhance the formation of interconnected nanosheets [56].…”

2 D Materials for Electrochemical Energy Storage: Design, Preparation, and Application

“…prepared a 3 D N‐doped graphene framework which generated a high specific capacitance in supercapacitors and also displayed an obvious electrocatalytic activity for ORR . Aside from graphene, recent reports have shown that MXenes can also been assembled to form 3 D architectures . Deng et al.…”

“…To date, various2Dm aterials have been tested in supercapacitors, owing to their suitable physicochemical properties. [56] Amongt hem, graphene-based materials have drawn extensive attention. [57] They can be used as active materials directly in EDLCs owing to their large specific surfacea rea and high conductivity.…”