Researchers use natural surfactant for efficient liquid-phase exfoliation of graphene

Researchers from India's SRM Institute of Science and Technology have reported on a natural, environmentally friendly and sustainable surfactant for exfoliating graphene in water.

Aqueous graphene dispersions hold potential for many applications such as printed and paper electronics and polymer composites, but the water’s surface energy tends to be too 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. 

Sapindus Mukorossi (SM) surfactant is a natural, low-cost (0.02 $/g), non-toxic, and biodegradable non-ionic surfactant compared with commercially available and most studied surfactants. The team demonstrated its use for defect-free, ultra-thin and micron-sized (lateral length) graphene nanosheets at high concentrations and stabilized in water.

The concentration of exfoliated graphene reached >1 mg/ml within one hour of exfoliation. The exfoliated graphene dispersions were systematically studied and the quality of nanosheets was estimated through fractional mass analysis using a combination of electron microscopy, Raman spectroscopy and absorption spectroscopy. The fractional mass of graphene was standardized with defined
defects, and lateral dimensions were evaluated, which is crucial for knowing the graphene quality for specific applications.

The team reported that the nanosheets are defect free, pure sp2 carbon, transparent and thin. The total yield of few-layered graphene was approximately 85%.

The researchers started by subjecting natural graphite 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.

This method with a novel biodegradable precursor signifies the effective use of starting materials to get a maximal 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.