Sustainable Liquid-Phase Exfoliation of Layered Materials with Nontoxic Polarclean Solvent

Abstract

Liquid-phase exfoliation is the most suitable platform for large-scale production of two-dimensional materials. One of the main open challenges is related to the quest of green and bioderived solvents to replace state-of-the-art dispersion media, which suffer several toxicity issues. Here, we demonstrate the suitability of methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (Rhodiasolv Polarclean) for sonication-assisted liquid-phase exfoliation of layered materials for the case-study examples of WS₂, MoS₂, and graphene. We performed a direct comparison, in the same processing conditions, with liquid-phase exfoliation using N-methyl-2-pyrrolidone (NMP) solvent. The amount of few-layer flakes (with thickness <5 nm) obtained with Polarclean is increased by ∼350% with respect to the case of liquid-phase exfoliation using NMP, maintaining comparable values of the average lateral size, which even reaches ∼10 μm for the case of graphene produced by exfoliation in Polarclean, and of the yield (∼40%). Correspondingly, the density of defects is reduced by 1 order of magnitude by Polarclean-assisted exfoliation, as evidenced by the I(D)/I(G) ratio in Raman spectra of graphene as low as 0.07 ± 0.01. Considering the various advantages of Polarclean over state-of-the-art solvents, including the absence of toxicity and its biodegradability, the validation of superior performances of Polarclean in liquid-phase exfoliation paves the way for sustainable large-scale production of nanosheets of layered materials and for extending their use in application fields to date inhibited by toxicity of solvents (e.g., agri-food industry and desalination), with a subsequent superb impact on the commercial potential of their technological applications.

Figure 1

(a) Plain and (b) ball-and-stick representations of the atomic structure of methyl 5-(dimethylamino)-2-methyl-5-oxopentanoate (Polarclean).

Figure 2

Yield of the Polarclean-assisted LPE as a function of centrifugation speed for WS₂, MoS₂, and graphene.

Figure 3

(a) BF-STEM micrograph with different overlapped flakes of WS₂ transferred on a lacey carbon grid. (b) Atomic-resolution HAADF-STEM micrograph on the side of the same sample in panel a, in correspondence of an isolated flake. A ball-and-stick representation of the WS₂ atomic structure is overlapped to the experimental micrograph, with W and S atoms depicted in blue and green, respectively, while the unit cell is indicated by red lines. The contrast in intensity of W and S sites is due to their different atomic number. The inset reports the fast Fourier transform (FFT) of the micrograph. (c) Raman spectra for bulk WS₂ and for nanosheets exfoliated in the liquid phase using Polarclean solvent. (d) Absorbance spectrum in the 400–800 nm range, showing the A exciton.

Figure 4

(a) Representative high-resolution SEM image of typical WS₂ flakes. (b) Analysis of lateral size distribution of WS₂ flakes determined from SEM images. (c) Representative AFM image of WS₂ flakes. The height profile along the white solid line is reported in the inset. (d) Analysis of thickness distribution determined from AFM measurements.

Figure 5

(a) S 2p and (b) W 4f core-level spectra of powder and Polarclean-exfoliated and NMP-exfoliated WS₂ samples.

Figure 6

Representative SEM image of isolated exfoliated flakes of (a) MoS₂ and (b) graphene. Statistical analysis of (c, d) lateral size and (e, f) thickness of (c, e) MoS₂ and (d, f) graphene flakes, respectively.

Figure 7

Raman spectrum for graphene exfoliated with Polarclean solvent (brown curve). For the sake of comparison, we report also Raman spectra for NMP-assisted (green curve) and Cyrene-assisted (red curve) LPE exfoliation of graphene (data taken from ref (79)) and, moreover, bulk graphite (black curve). See Figure S12 in the Supporting Information for a comparison extended to other solvents.