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

Sonication using an ultrasonic probe is a efficient method to exfoliate monolayer graphene nanosheets and to disperse them uniformly. Ultrasonic functionalization allows to produce water-dispersable graphene.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.

Advantages:

  • high quality graphene
  • high yield
  • water-based dispersion
  • high concentration
  • high efficiency
  • rapid process
  • low cost
  • high-throughput
  • environmental-friendly

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7kW ultrasonic dispersing system for inline graphene production (Click to enlarge!)

Ultrasonic reactor with probe-type ultrasonicators for industrial-scale graphene exfoliation

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)

Ultrasonic graphene exfoliation in water

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

Ultrasonic exfoliation of water-dispersable graphene nanosheets (Unalan et al. 2015)

TEM images of pull-G for: (a) 10 min; (b) 20 min; (c) 30 min; (d) 60 min; (e) chit-G for 30 min; (f) alg-G for 30 min. (Unalan et al. 2015)

Ultrasonicators for Graphene Exfoliation

The industrial Ultrasonicator UIP6000hdT delivers 6000 watts of high-intensity ultrasound via probe (sonotrode) for homogenization, dispersion, emulsification and extraction applications.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:

Batch VolumeFlow RateRecommended Devices
10 to 2000mL20 to 400mL/minUP200Ht, UP400St
0.1 to 20L0.2 to 4L/minUIP2000hdT
10 to 100L2 to 10L/minUIP4000
n.a.10 to 100L/minUIP16000
n.a.largercluster of UIP16000

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Intense ultrasonication using an ultrasonic probe (sonotrode) is a highly effective method to delaminate and exfoliate graphene nano-sheets.

Mechanism of ultrasonic exfoliation of 2D nanosheets.
(study and graphic: Tyurnina et al., 2020)



Facts Worth Knowing

Graphene

Graphene sheet Graphene is a monolayer of sp2-bonded carbon atoms. Graphene offers unique material characteristics such as an extraordinary large specific surface area (2620 m2 g-1), superior mechanical properties with a Young’s modulus of 1 TPa and an intrinsic strength of 130 GPa, an extremely high electronic conductivity (room-temperature electron mobility of 2.5 × 105 cm2 V-1 s-1), very high thermal conductivity (above 3000 W m K-1), to name the most important properties. Due to its superior material properties, graphene is heavily used in the development and production of high performance batteries, fuel cells, solar cells, supercapacitor, hydrogen storages, electromagnetic shields and electronic devices. Furthermore, graphene is incorporated into many nanocomposites and composite materials as reinforcing additive, e.g. in polymers, ceramics and metal matrices. Due to its high conductivity, graphene is an important component of conductive paints and inks.
The rapid and safe ultrasonic preparation of defect-free graphene at large volumes at low costs allows for widening the applications of graphene to more and more industries.

Literature/References


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