Green Carbon Nanostructures for Functional Composite Materials

Abstract

Carbon nanostructures are widely used as fillers to tailor the mechanical, thermal, barrier, and electrical properties of polymeric matrices employed for a wide range of applications. Reduced graphene oxide (rGO), a carbon nanostructure from the graphene derivatives family, has been incorporated in composite materials due to its remarkable electrical conductivity, mechanical strength capacity, and low cost. Graphene oxide (GO) is typically synthesized by the improved Hummers' method and then chemically reduced to obtain rGO. However, the chemical reduction commonly uses toxic reducing agents, such as hydrazine, being environmentally unfriendly and limiting the final application of composites. Therefore, green chemical reducing agents and synthesis methods of carbon nanostructures should be employed. This paper reviews the state of the art regarding the green chemical reduction of graphene oxide reported in the last 3 years. Moreover, alternative graphitic nanostructures, such as carbons derived from biomass and carbon nanostructures supported on clays, are pointed as eco-friendly and sustainable carbonaceous additives to engineering polymer properties in composites. Finally, the application of these carbon nanostructures in polymer composites is briefly overviewed.

Keywords: clays; hydrothermal carbons; polymer composites; reduced graphene oxide; supported carbons.

Figure 1

The chemical production of graphene oxide from graphite.

Figure 2

Proposed mechanism for HTC of cellulose. Reproduced with permission from Falco et al. Green Chem. 2011, 13, 3273 published by the Royal Society of Chemistry [64].

Figure 3

Schematic representation of graphene-like materials formed inside sepiolite pores (endogenic regions) and epitaxially grown on the sepiolite surface (exogenic regions). Reproduced with permission from Ruiz-García et al., J. Mater. Chem. A 2017, 2009, published by the Royal Society of Chemistry [88].

Figure 4

Schematic representation of the assembly of hydrothermal carbon spheres (HCS) from chitosan (CS) and Ti₃C₂T_x MXene. Reproduced with permission from Wu et al., Chem. Eng. J. 2021, 426, 130776, published by Elsevier [93].