Ultrasonic Exfoliation of Water-Dispersible Graphene

• Mono- and bi-layer graphene nanosheets can be produced rapidly via ultrasonic exfoliation with high throughput and at low cost.
• Ultrasonically exfoliated graphene can be functionalized with biopolymers in order to obtain water-dispersible graphene.
• By ultrasonic cavitation, the synthesized graphene can be further processed into a stable water-based dispersion.

Ultrasonic Exfoliation of High-Quality Graphene

Ultrasonication is a reliable method to produce graphene layers (mono-, bi- and few-layer graphene) from graphite flakes or particles. Whilst other common exfoliation techniques such as ball- and roll mills or high-shear mixers are linked to low quality and the use of aggressive reagents and solvents, the ultrasonic exfoliation method convinces by its high quality output, high process capacity and mild processing conditions.
Ultrasonic cavitation creates intense shear forces, which separate the stacked graphite layers into mono-, bi- and few-layers of defect-free graphene.

Water-Dispersable Graphene Sheets via Sonication

Ultrasonication is a an effective procedure with repeatable results to untangle carbonnanotubes in water or organic solvents.[/caption]Under normal conditions, graphene is hardly dispersable in water and forms aggregates and agglomerates when dispersed in aqueous medium. Since aqueous systems have significant advantages of being inexpensive, non-toxic, environmental-friendly, water-based graphene systems are highly attractive to graphene manufacturers and the downstream industry.
In order to obtain water-dispersable graphene nanosheets, the ultrasonically exfoliated graphene is modified with polysaccharides / biopolymers such as pullulan, chitosan, alginate, gelatine or gum arabic.

Protocol of Direct Exfoliation of Graphite using Ultrasonics

Non-ionic pullulan and anionic alginate (1.0 g) were separately dissolved in 20 ml of distilled water, whereas cationic chitosan (0.4 g) was dissolved in 20 ml of distilled water with 1 wt% acetic acid. Graphite powder was dispersed in the aqueous biopolymer solutions and treated using a probe-type ultrasonicator UP200S (maximum power 200 W, frequency 24 kHz, Hielscher Ultrasonics, Germany) equipped with a titanium sonotrode (micro tip S3, tip diameter 3 mm, maximum amplitude 210µm, acoustic power density or surface intensity 460 W cm-2) under the following conditions: 0.5 cycle and 50% amplitude, for a period of 10, 20, 30, and 60 min, respectively. Best results were obtained at 30min sonication. Sonication was applied at the power of 16.25 W for 30 minutes, with energy consumption (energy output per unit volume) of 731 Ws ml-1.
Subsequently, mixtures were centrifuged at 1500 rpm for 60 min to remove unexfoliated graphite particles and then washed 5 times and again centrifuged at 5000 rpm for 20 min to remove excess biopolymers. The resultant dark-gray solutions were vacuum-dried at 40ºC until no mass-loss. The resulting polymer–graphene powders were redispersed in water (1 mg ml-1 for pullulan and chitosan; 0.18 mg ml-1 for alginate) for characterization. Graphene sheets obtained by pullulan-, alginate-, and chitosan-assisted ultrasonication were indicated as pull-G, alg-G, and chit-G, respectively.
Out of the three systems, pullulan and chitosan were more effective in exfoliation of graphite than alginate. This method yielded exfoliated mono-, bi-, and few-layer graphene sheets with only low lateral (edges) defects. The adsorption of biopolymers on graphene surface affords a long-lasting stability (more than 6 months) of the aqueous dispersion.
(cf. Unalan et al. 2015)

A high-speed sequence (from a to f) of frames illustrating sono-mechanical exfoliation of a graphite flake in water using the UP200S, a 200W ultrasonicator with 3-mm sonotrode. Arrows show the place of splitting (exfoliation) with cavitation bubbles penetrating the split.
(study and pictures: © Tyurnina et al. 2020

Ultrasonicators for Graphene Exfoliation

Hielscher high-power ultrasonic processors are used worldwide for the successful exfoliation and dispersion of graphite and graphene. Our ultrasonic dispersers are available from lab and bench-top up to full industrial production units. Besides robustness, 24/7 operation and low maintenance, Hielscher ultrasonicators convince by high ease of processing and linear scalability.
Processes can be easily tested and optimized in the lab. Afterwards, all process results can be scaled completely linear to commercial production level. This makes sonication an effective and efficient production method for the high volume of high-quality graphene sheets.
Hielscher Ultrasonics industrial ultrasonic processors can deliver very high amplitudes. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation. For even higher amplitudes, customized ultrasonic sonotrodes are available. Matching ultrasound reactors ensure the capability of reliable and safe mass production of high quality graphene nanosheets as well as stable nanosheet dispersions.

The robustness of Hielscher’s ultrasonic equipment allows for 24/7 operation at heavy duty and in demanding environments.
The table below gives you an indication of the approximate processing capacity of our ultrasonicators: