Interlayer Covalently Enhanced Graphene Materials: Construction, Properties, and Applications

The development of large-scale and controlled graphene production lays the foundation for macroscopic assembly. Among the diverse assembly strategies, modulating the interlayer interaction of graphene nanosheets is of vital importance because it determines the mechanical, electrical, thermal, and permeation properties of the macroscopic objects. Depending on the nature and strength of the interlayer interaction, covalent and noncovalent bondings, such as hydrogen bonding, ionic interaction, pi-pi interaction, and van der Waals force, are classified as two main types of interlayer connection methods, which solely or synergistically link the individual graphene nanosheets for practical macroscopic materials. Among them, the covalent bonding within the interlayer space renders graphene assembly adjusted interlayer distance, strong interlayer interaction, a rich diversity of functionalities, and potential atomic configuration reconstruction, which has attracted considerable research attention. Compared with other noncovalent assembly methods, covalent connections are stronger and thus more stable; however, there are some issues that remain. First, the covalent modification of the graphene surface depends on the defects and/or functional groups, which becomes difficult for graphene films free of surface imperfections. Second, the covalent connection partly alters the sp(2) hybrid carbon atoms to sp(3), resulting in a deteriorated electrical conductivity. Thus, the electrical properties of the macroscopic assembly are far inferior to those of the constituent nanosheets, thereby restricting their applications. Lastly, covalent bonding is naturally rigid, rendering high modulus and strength to the graphene assembly while impairing the toughness. As in certain applications, both high strength and toughness are required; thus, a balanced covalent and noncovalent interaction is required. In this review, we discuss the recent progress in the construction method, properties, and applications of the interlayer covalently connected graphene materials. In the construction method, graphene is classified according to the synthesis method as oxidation-reduction and chemical vapor deposition method, wherein the latter represents graphene without abundant surface bonding sites and is hard to be covalently connected. For the former graphene produced by the oxidation-reduction method, the paper and fiber assembly forms are discussed. Then, the influence of covalent bonding on the mechanical and electrical properties is studied. Note that both the enhancement and potential impairments caused by covalent bonding are addressed. Finally, the applications in electrical devices, energy storage, and ion separation are summarized. The interlayer covalently connected macroscopic graphene material unifies the exceptional properties of graphene and the advantages of assembly strategy and will find applications in related fields. Moreover, it will also inspire the assembly of other graphene-like two-dimensional materials for a richer diversity of applications.

Automated summary

The development of large-scale and controlled graphene production is crucial for the macroscopic assembly of graphene. Covalent and noncovalent bondings are two main types of interlayer connection methods that play significant roles in graphene assembly. However, covalent connection of graphene faces challenges like surface imperfections, deteriorated electrical conductivity, and impaired toughness.