Direct Evidence of the Exfoliation Efficiency and Graphene Dispersibility of Green Solvents toward Sustainable Graphene Production

Abstract

Achieving a sustainable production of pristine high-quality graphene and other layered materials at a low cost is one of the bottlenecks that needs to be overcome for reaching 2D material applications at a large scale. Liquid phase exfoliation in conjunction with N-methyl-2-pyrrolidone (NMP) is recognized as the most efficient method for both the exfoliation and dispersion of graphene. Unfortunately, NMP is neither sustainable nor suitable for up-scaling production due to its adverse impact on the environment. Here, we show the real potential of green solvents by revealing the independent contributions of their exfoliation efficiency and graphene dispersibility to the graphene yield. By experimentally separating these two factors, we demonstrate that the exfoliation efficiency of a given solvent is independent of its dispersibility. Our studies revealed that isopropanol can be used to exfoliate graphite as efficiently as NMP. Our finding is corroborated by the matching ratio between the polar and dispersive energies of graphite and that of the solvent surface tension. This direct evidence of exfoliation efficiency and dispersibility of solvents paves the way to developing a deeper understanding of the real potential of sustainable graphene manufacturing at a large scale.

Figure 1

(a) Digital images of graphite platelets (150 μm; G150) and powder (50 μm; G50); BET surface area evaluation of both. (b) Centrifuge tubes with a fixed amount of exfoliation product containing graphite aggregates and graphene dispersed in NMP and IPA, respectively. When the exfoliation product contains the same amount of graphene, solvents with a higher graphene dispersibility result in a higher concentration of graphene collected from the supernatant. (c) For the conventional method, the concentration characterized is limited by the graphene dispersibility in the solvent. Due to the lower graphene dispersibility in solvent A, the exfoliated graphene restacks and sediments during centrifugation. This results in a lower amount of graphene being collected from the supernatant, and thus, a lower concentration is measured, despite the same initial amount of graphene content as in solvent B. (d) The NMP-R method makes use of the higher graphene dispersibility of NMP to minimize graphene restacking so that the graphene produced remains well-separated in the supernatant. Note that the black and red graphs depict the data for G150/GR150 and G50/GR50, respectively.

Figure 2

Mass of GR150 and GR50 graphene produced per surface area of G150 and G50 graphite, respectively, exfoliated in various exfoliation solvent media (x-axis), determined through the conventional method (without re-dispersion), NMP-R, and GS-R. (a) Conventional method in comparison with the (b) NMP-R. The solvents on the x-axes of the graphs (a,b) are arranged in the order of decreasing solvent exfoliation efficiency. The data for NMP-exfoliated graphene in (a) is shown in (b) instead to compare with the NMP-R of the NMP-exfoliated graphene. (c) Conventional method in comparison with the (d) GS-R. The solvents on the x-axes of the graphs (c,d) are arranged in the order of decreasing dispersibility. (a1–d1) Digital images of graphene dispersion presented in (a–d), respectively, and arranged according to the solvent sequence in the x-axis of the graphs. The NMP-R graphene dispersions shown in (b1) are diluted five times for better contrast.

Figure 3

Yield of (a) GR150 and (b) GR50 in different solvent media, which is the combined contributions of solvent exfoliation efficiency and dispersibility. The graphene yield is determined through the conventional method. Exfoliation efficiency and graphene dispersibility are determined using the NMP-R and GS-R techniques, respectively. The unit of the axes is in mg/m². (c) Concentration of GR50 exfoliated in EA as compared to its re-dispersion in IPA and EtOH/D.I. water (1:1). Inset: digital images of dispersions correspond to the x-axis arrangement. (d) Owens–Wendt–Rabel and Kaelble model plot for graphite surface energy determination. The interfacial contact angle measurement data evaluated by the Washburn method of the graphite starting materials G150 and G50 are shown in (e,f), respectively, and the exfoliation solvent media listed in the x-axis.

Figure 4

Raman spectroscopy analysis for GR150 and GR50 before and after NMP-R (a) on type of defects (D/D′ intensity ratio) for NMP, IPA/Ace (1:1), EA, IPA, and EtOH, (b) and defect density (D/G intensity ratio) against disorder (G peak fwhm). AFM thickness analysis performed on (c) GR 150 and (e) GR 50 exfoliated in IPA and displayed in (d,f), respectively.