Higher Ultrasonic Frequency Liquid Phase Exfoliation Leads to Larger and Monolayer to Few-Layer Flakes of 2D Layered Materials

Abstract

Among the most reliable techniques for exfoliation of two-dimensional (2D) layered materials, sonication-assisted liquid-phase exfoliation (LPE) is considered as a cost-effective and straightforward method for preparing graphene and its 2D inorganic counterparts at reasonable sizes and acceptable levels of defects. Although there were rapid advances in this field, the effect and outcome of the sonication frequency are poorly understood and often ignored, resulting in a low exfoliation efficiency. Here, we demonstrate that simple mild bath sonication at a higher frequency and low power positively contributes to the thickness, size, and quality of the final exfoliated products. We show that monolayer graphene flakes can be directly exfoliated from graphite using ethanol as a solvent by increasing the frequency of the bath sonication from 37 to 80 kHz. The statistical analysis shows that ∼77% of the measured graphene flakes have a thickness below three layers with an average lateral size of 13 μm. We demonstrate that this approach works for digenite (Cu9S5) and silver sulfide (Ag2S), thus indicating that this exfoliation technique can be applied to other inorganic 2D materials to obtain high-quality few-layered flakes. This simple and effective method facilitates the formation of monolayer/few layers of graphene and transition metal chalcogenides for a wide range of applications.

Graphene Oxide and Reduced Graphene Oxide Nanoflakes Coated with Glycol Chitosan, Propylene Glycol Alginate, and Polydopamine: Characterization and Cytotoxicity in Human Chondrocytes

“…Detailed AFM statistical analysis of exfoliated GO and rGO flakes was performed to analyze the distribution of the lateral size and thickness of the synthesized flakes throughout the wafer. Bath sonication at 80 kHz frequency (100 W) was used to form thinner and larger lateral size flakes, applying a technique based on sonication-assisted LPE recently proposed by our group [ 32 ].…”

“…Next, the mixture was centrifuged (12,000 rpm for 1 h) and washed with 5% hydrochloric acid (HCl) solution 2 times. Then, the mixture was extensively rewashed with distilled water and centrifuged (12,000 rpm for 1 h) until the solution reached pH = 7, and finally was dried in a vacuum furnace for 24 h. Finally, the obtained graphite oxide (brownish solid) was exfoliated to GO flakes after 2 h of sonication-assisted liquid-phase exfoliation (LPE) (Elmasonic P, bath frequency: 80 kHz, 100 W) [ 32 ].…”

“…Chemical vapor deposition (CVD) was used to reduce GO to rGO (black solid) [ 32 ]. Dried GO was loaded into a quartz tube and inserted into a Lindberg Blue furnace preheated to 900 °C for 2 h, and reduced by hydrogen flow (400 sccm) under an argon (100 sccm) atmosphere.…”

MXenes—A New Class of Two-Dimensional Materials: Structure, Properties and Potential Applications

“…In contrast to the amplitude, higher frequencies positively impact the MXene stability. They cause a high number of small bubbles with uniform sizes, preventing violent interparticle collapses and promoting the weakening of the interlayer Van der Waals forces [55]. Papers report different frequencies: from 6 kHz [56] to 40 kHz [57][58][59].…”

Phase-Dependent Photocatalytic Activity of Bulk and Exfoliated Defect-Controlled Flakes of Layered Copper Sulfides under Simulated Solar Light

“…Then, we studied the effect of gradual thermal annealing under an inert or reducing atmosphere on the phase transitions in the copper sulfide system. Subsequently, as we reported recently, we thinned down the bulk copper sulfides into few-layer flakes using liquid exfoliation via sonication at higher frequencies . To overcome the limitations of solvents with high boiling points, we used ethanol, which is defined as an environmentally friendly liquid due to its low toxicity, low boiling point, and easy availability.…”