High-Quality and Efficient Liquid-Phase Exfoliation of Few-Layered Graphene by Natural Surfactant

Converting thick and large natural graphite flakes into thin, defect-free graphene at high yield is challenging. Aqueous graphene dispersions have a unique advantage for many technological applications such as printed and paper electronics and polymer composites, but the water’s surface energy is high to stabilize them. However, surfactants provide adequate repulsive potential against restacking of exfoliated nanosheets in colloidal dispersions to overcome the van der Waal’s attractive forces. The major problem with reported and commercial surfactant resources is their inability to produce micrometer-sized graphene with few defects at high yield with using a minimal amount of economical and sustainable surfactants. Inspired by the natural surfactant Sapindus mukorossi (SM) used for ages due to its medicinal benefits, we report the exfoliation and processing of graphene nanosheets in natural surfactant/water. The nanosheets are defect free, pure sp2 carbon, transparent, thin, and lateral with micrometer-sized dimensions confirmed by electron microscopy, Raman spectroscopy, and XPS techniques. The total yield of few-layered graphene is approximately 85%. When starting, natural graphite is subjected to ultrasonication in water/natural surfactant for 17 h in 10 recycles of exfoliation. The average value of the recycled graphene Raman ID/IG is 0.17 ± 0.04 with a high yield in the recycling process. Our method with a novel biodegradable precursor signifies the effective use of starting materials to get the maximum output, thereby decreasing the chemical waste. The natural surfactant used here to stabilize graphene is a complex molecule with high molecular weight and a nonionic surfactant with aglycone, acting as a hydrophobic tail, and glycone, a sugar moiety hydrophilic head producing steric repulsive forces against aggregation. Exfoliated, few-layered graphene is impregnated in low-density polyurethane foam with open-shell structures and used for strain sensor and oil–water separation applications. These results signify the importance of naturally available surfactants for producing atomically thin materials using industrially viable techniques.