Ultrasonic Exfoliation of Xenes

Xenes are 2D monoelemental nanomaterials with extraordinary properties such as very high surface area, anisotropic physical / chemical properties including superior electric conductivity or tensile strength. Ultrasonic exfoliation or delamination is an efficient and reliable technique to produce single-layer 2D nanosheets from layered precursor materials. Ultrasonic exfoliation is already established for the production of high-quality xenes nanosheets on industrial scale.

Xenes – Monolayer Nanostructures

Xenes are monolayer (2D), monoelemental nanomaterials, which feature a graphene-like structure, intra-layer covalent bond, and weak van der Waals forces between layers. Examples for materials, which are part of the xenes class are borophene, silicene, germanene, stanene, phosphorene (black phosphorus), arsenene, bismuthene, and tellurene and antimonene. Due to their single-layer 2D structure, xenes nanomaterials are charcterized by a very large surface as well as improved chemical and physical reactivities. This structural characteristics give xenes nanomaterials impressive photonic, catalytic, magnetic, and electronic properties and make these nanostructures very interesting for numerous industrial applications. The picture left show SEM images of ultrasonically exfoliated borophene.

Production of Xenes Nanomaterials using Ultrasonic Delamination

Liquid Exfoliation of Layered Nanomaterials: Single-layer 2D nanosheets are produced from inorganic materials with layered structures (e.g., graphite) that consist in loosely stacked host layers that display layer-to-layer gallery expansion or swelling upon the intercalation of certain ions and/or solvents. Exfoliation, in which the layered phase is cleaved into nanosheets, typically accompanies the swelling due to the rapidly weakened electrostatic attractions between the layers which produce colloidal dispersions of the individual 2D layers or sheets. (cf. Geng et al, 2013) In general it is known that swelling facilitates exfoliation through ultrasonication and results in negatively charged nanosheets. Chemical pretreatment also facilitates exfoliation by means of sonication in solvents. For example, functionalization allows the exfoliation of layered double hydroxides (LDHs) in alcohols. (cf. Nicolosi et al., 2013)
For ultrasonic exfoliation / delamination the layered material is exposed to powerful ultrasonic waves in a solvent. When energy-dense ultrasound waves are coupled into a liquid or slurry, acoustic aka ultrasonic cavitation occur. Ultrasonic cavitation is characterized by the collapse of vacuum bubbles. The ultrasound waves travel through the liquid and generate alternating low pressure / high pressure cycles. The minute vacuum bubbles arise during a low pressure (rarefaction) cycle and grow over various low pressure / high pressure cycles. When a cavitation bubble reaches the point where it cannot absorb any further energy, the bubble implodes violently and creates locally very energy-dense conditions. A cavitational hot-spot is determined by very high pressures and temperature, respective pressures and temperature differentials, high-speed liquid jets, and shear forces. These sonomechanical and sonochemical forces push the solvent between the stacked layers and break-up layered particulate and crystalline structures thereby producing exfoliated nanosheets. The image sequence below demonstrates the exfoliation process by ultrasonic cavitation.

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.
© Tyurnina et al. 2020 (CC BY-NC-ND 4.0)

Modeling has shown that if the surface energy of the solvent is similar to that of the layered material, the energy difference between the exfoliated and reaggregated states will be very small, removing the driving force for re-aggregation. When compared to alternative stirring and shearing methods, ultrasonic agitators provided a more effective energy source for exfoliation, leading to the demonstration of ion intercalation–assisted exfoliation of TaS₂, NbS₂, and MoS₂, as well as layered oxides. (cf. Nicolosi et al., 2013)

TEM images of ultrasonically liquid exfoliated nanosheets: (A) A graphene nanosheet exfoliated by means of sonication in the solvent N-methyl-pyrrolidone. (B) A h-BN nanosheet exfoliated by means of sonication in the solvent isopropanol. (C) An MoS2 nanosheet exfoliated by means of sonication in an aqueous surfactant solution.
(Study and pictures: ©Nicolosi et al., 2013)

Ultrasonic Liquid-Exfoliation Protocols

Ultrasonic exfoliation and delamination of xenes and other monolayer nanomaterials has been extensively studied in research and was successfully transferred to industrial production stage. Below we present you selected exfoliation protocols using sonication.

Ultrasonic Exfoliation of Phosphorene Nanoflakes

Phosphorene (also known as black phosphorus, BP) is a 2D layered, monoelemental material formed from phosphorus atoms.
In the research of Passaglia et al. (2018), the preparation of stable suspensions of phosphorene − methyl methacrylate by sonication-assisted liquid-phase exfoliation (LPE) of bP in the presence of MMA followed by radical polymerization is demonstrated. Methyl methacrylate (MMA) is a liquid monomer.

Protocol for Ultrasonic Liquid Exfoliation of Phosphorene

MMA_bPn, NVP_bPn, and Sty_bPn suspensions were obtained by LPE in the presence of the sole monomer. In a typical procedure, ∼5 mg of bP, carefully crushed in a mortar, was put in a test tube and then a weighted quantity of MMA, Sty, or NVP was added. The monomer bP suspension was sonicated for 90 min by using a Hielscher Ultrasonics homogenizer UP200St (200W, 26kHz), equipped with sonotrode S26d2 (tip diameter: 2 mm). The ultrasonic amplitude was maintained constant at 50% with P = 7 W. In all cases, an ice bath was used for improved heat dissipation. The final MMA_bPn, NVP_bPn, and Sty_bPn suspensions were then insufflated with N2 for 15 min. All the suspensions were analyzed by DLS, showing rH values really close to that of DMSO_bPn. For example, the MMA_bPn suspension (having about 1% of bP content) was characterized by rH = 512 ± 58 nm.
While other scientific studies on phosphorene report sonication time of several hours using ultrasonic cleaner, high boiling point solvents, and low efficiency, the research team of Passaglia demonstrate a highly efficient ultrasonic exfoliation protocol using a probe-type ultrasonicator (namely the Hielscher ultrasonicator model UP200St).

Ultrasonic Exfoliation of Monolayer Nanosheets

To read more specific details and exfoliation protocols for borophene and ruthenium oxide nanosheets, please follow the links below:
Borophene: For sonication protocols and results of ultrasonic borophene exfoliation, please click here!
RuO2: For sonication protocols and results of ultrasonic ruthenium oxide nanosheet exfoliation, please click here!

Ultrasonic Exfoliation of Few–Layer Silica Nanosheets

Few–layer exfoliated silica nanosheets were prepared from natural vermiculite (Verm) via ultrasonic exfoliation. For the synthesis of exfoliated silica nanosheets the following liquid–phase exfoliation method was applied: 40 mg silica nanosheets were dispersed in 40 mL absolute ethanol. Subsequently, the mixture was ultrasonicated for 2 h using a Hielscher ultrasonic processor UP200St, equipped with a 7 mm sonotrode. The amplitude of ultrasound wave was kept constant at 70%. An ice bath was applied to avoid overheating. Unexfoliated SN were removed by centrifugation at 1000 rpm for 10 min. Finally, the product was decanted and dried at room temperature under vacuum overnight. (cf. Guo et al., 2022)

High-Power Ultrasound Probes and Reactors for Exfoliation of Xenes Nanosheets

Hielscher Ultrasonics designs, manufactures, and distributes robust and reliable ultrasonicators at any size. From compact lab ultrasonic devices to industrial ultrasonic probes and reactors, Hielscher has the ideal ultrasonic system for your process. With long-time experience in applications such as nanomaterial synthesis and dispersion, our well-trained staff will recommend you the most suitable setup for your requirements. Hielscher industrial ultrasonic processors are known as reliable work horses in industrial facilities. Capable to deliver very high amplitudes, Hielscher ultrasonicators are ideal for high-performance applications such as synthesis of xenes and other 2D monolayer nanomaterials such as borophene, phosphorene or graphene as well as a reliable dispersion of these nanostructures.
Extraordinarily powerful ultrasound: 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.
Highest Quality – Designed and Made in Germany: All equipment is designed and manufactured in our headquarter in Germany. Before delivery to the customer, every ultrasonic device is carefully tested under full load. We strive for customer satisfaction and our production is structured to fulfil highest quality assurance (e.g., ISO certification).

The table below gives you an indication of the approximate processing capacity of our ultrasonicators: