DOI: 10.14457/tu.the.2022.598
title: Liquid-phase exfoliation of graphite using the serum from skim natural rubber latex

description: This work aimed at increasing the yield of exfoliated graphite products from the green exfoliation of graphite using the serum from skim natural rubber latex (SNRL) with the left-over ammonia. The rubber particles were coagulated with 0.7% w/v cationic polyacrylamide solution and the remaining serum containing ammonia was used as an exfoliating medium. The study considered two groups of variations: process parameters and raw material parameters. Process parameters included the extension of a previous study applying ultrasonication and using a homogenizer probe instead of ultrasonication. Previously, in one-step ultrasonication-assisted exfoliation, 25 mg graphite/ml serum was sonicated for 2 h at 29-53°C in an ultrasonic bath at 40 kHz and was left standing for 2 h at room temperature. The top part was centrifuged at 2000 rpm for 30 min, and then the top section of this was centrifuged at 10500 rpm for 30 min to collect the solid and the yield was determined to be 0.28%. In this research, three effects were investigated for ultrasonication-assisted process, which were the application of multi-step exfoliation by repeating the one-step exfoliation up to three times, prolonged sonication time in one-step exfoliation, and varying centrifugal time and speed in the first step of separation. Moreover, shear-assisted exfoliation was also investigated. For the study of raw material parameters, the amount of graphite in the same volume of serum was varied and the effect of size distribution of raw graphit was also investigated. Raman spectroscopy was used to evaluate the quality of the exfoliated graphite samples and scanning electron microscopy was used to observe their morphologies. Raman spectroscopy results for the precursor graphite and commercial graphene were used as references for comparisons. It was found that the values of I2D/IG ratios were 0.35 and 0.51, the values of full width at half maximum (FWHM) of 2D peak were 78.3 and 73.8 cm-1, and the position of 2D peak located at 2706.0 and 2686.2 cm-1 for the precursor graphite and commercial graphene, respectively. All these three results indicated that commercial graphene was multilayer graphene and used as three criteria for comparison. Firstly, the process variables were studied. In the multi-step ultrasonication-assisted exfoliation, it was found that the Raman spectroscopy results showed that exfoliation occurred at some degrees and the sample comparable to commercial graphene was obtained after third-step of exfoliation. The yield of this sample was 0.32 %, similar to the yield of 0.28% above but the quality of the product was improved. When the sonication time was extended to 6 h in one-step exfoliation, the quality of the exfoliated product from the top part of the last centrifugation was close to, but not yet comparable to, commercial graphene with an increasing yield of 0.68%. Later, the step of first separation using centrifugal speed of 2000 for 30 min was changed to 1000 rpm for 20 and 2000 rpm for 10 min, it was found the top part of the suspension from centrifugation at 2000 rpm for 10 min gave the product comparable to commercial graphene with an increasing yield of 0.48%. In addition, one-step exfoliation of graphite with 25 mg graphite/ml serum was done with a homogenizer probe instead of ultrasonication. while using a homogenizer probe at 5400 rpm for 20 min instead of an ultrasonic bath could increase the yield up to 10.5% and if using a homogenizer probe at 8100 rpm for 30 min, it could increase the yield up to 13.23% but the quality of products were still not close to that of commercial graphene. Subsequently, the raw material variables were studied. The amount of the precursor graphite per ml of the serum was varied to be 10, 12,5, 15, 17.5, 20.0, 22.5 and 25 mg/ml. Upon decreasing graphite concentration, the yield was first decreased from 0.43% (G-0.250) to 0.27% (G-0.200) and then increased to 0.70% (G-0.100), but there was no sample meeting the three criteria of commercial graphene, only close to for some samples such as G-0.100. For the effect of graphite size, it was obviously seen that when using lighter-weighted graphite obtained from the suspended part after 15-second and 45-second settlings, the yield could be increased to 0.74% and 1.56%, respectively. The quality of the products did not yet meet the quality of commercial graphene, only getting closer. Lastly, SEM micrographs, showed that the products were multilayer graphene with more than 5 layers. Some selected samples were also checked with sedimentation and methylene blue adsorption. It was found that the high-quality exfoliated sample showed better dispersion in water, resulting in 99% turbidity after 20-min sedimentation and yielded high adsorption capacity.