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 WS 2 , MoS 2 , 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.

Green Solvents for the Liquid Phase Exfoliation Production of Graphene: The Promising Case of Cyrene

“…However, this solvent is theoretically not ideal for graphite exfoliation, since both the surface tension and Hansen solubility parameters are higher than desired (especially the δ H and δ P components). As a proof of that, a very low yield (2.4%) was reported ( Paolucci et al, 2020 ). Methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (Rhodiasolv Polarclean) is a polar solvent that was also tested in sonication-assisted LPE of graphite, MoS 2 and WS 2 .…”

“…In the single report currently available on the use of TEA, the authors obtained dispersions of crystalline graphene flakes with high stability (over 9 months) ( Chen et al, 2017 ). However, TEA can induce detrimental chemical modifications in the flakes, possibly due to functionalization ( Song et al, 2015 ; Ryu et al, 2017 ; Paolucci et al, 2020 ). Furthermore, its very high viscosity precludes the deposition of the dispersion by versatile techniques, such as inkjet printing (usually requiring 1–10 cP) ( Paolucci et al, 2020 ).…”

“…However, TEA can induce detrimental chemical modifications in the flakes, possibly due to functionalization ( Song et al, 2015 ; Ryu et al, 2017 ; Paolucci et al, 2020 ). Furthermore, its very high viscosity precludes the deposition of the dispersion by versatile techniques, such as inkjet printing (usually requiring 1–10 cP) ( Paolucci et al, 2020 ). As a next alternative, urea aqueous solution has also been tested and led to crystalline flakes and 15-days stability ( He et al, 2015 ; Hou et al, 2019 ).…”

“…Methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (Rhodiasolv Polarclean) is a polar solvent that was also tested in sonication-assisted LPE of graphite, MoS 2 and WS 2 . It showed good performance, with a ∼350% higher amount of few-layer nanosheets (<5 nm thickness) and 10 times lower defect density with respect to NMP ( Paolucci et al, 2020 ). However, its high boiling point represents a pivotal drawback.…”

Aerosol-Printed MoS2 Ink as a High Sensitivity Humidity Sensor

“…Alternatively, the use of aqueous dispersions as inks for printing on multiple devices can be an approach to replace nonsustainable and expensive methods. 31 , 32 However, the development of new inks composed of functional materials, with different readily available properties, that are both sustainable and inexpensive still is in its infancy. 33 Recently, several works in the literature have shown the development of water-based inks of 2D materials.…”