Ultrasound exfoliation of graphite in biphasic liquid systems containing ionic liquids: A study on the conditions for obtaining large few-layers graphene

Long-term stable solid concentrated graphene dispersion assisted by a highly aromatic ionic liquid

HYPOTHESIS

The sonochemical exfoliation of graphite in solution has been demonstrated as a promising and easy technique for producing graphene dispersions. This is usually done in organic solvents and leads to unstable dispersions with very low graphene concentration. Ionic liquids (ILs) represent a versatile and safe alternative to traditional organic solvents. A few recent studies reported the use of commercial ILs with bulky anions, such as bis(trifluoromethylsulfonyl)imide, and aromatic cations, such as imidazolium, which favour the exfoliation of graphite through π-π and cation-π interactions. Although recently investigated, the role of aromatic groups on imidazolium cations is still controversial and systematic studies are still necessary. Besides, these studies were limited to liquid dispersions at room temperature.

EXPERIMENTS

Herein, we prepared four highly aromatic imidazolium-based ILs, including the newly reported 1-(naphthylmethyl)-3-benzylimidazolium bis(trifluoromethanesulfonyl)imide, [(Np)(Bn)im][NTf₂]. These ILs were used for the sonochemical exfoliation of graphite and compared with a commercial benchmark, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Bmim][NTf₂].

FINDINGS

Interestingly, [(Np)(Bn)im][NTf₂] allowed reaching solid dispersions at room temperature containing thin few layer graphene sheets with long-term stability (up to 2 years) and high concentration (3.6 mg/mL). Such graphene dispersion combines long-term stability in the solid-state and high processability in the liquid state, by a simple heating above 60 °C.

Polyaromatic cores for the exfoliation of popular 2D materials

Two-dimensional (2D) nanomaterials have attracted interest from the scientific community due to their unique properties. The production of these materials has been carried out by diverse methodologies, the liquid phase exfoliation being the most promising one due to its simplicity and potential scalability. The use of several stabilizers allows to obtain dispersions of these 2D nanomaterials in solvents with low boiling points. Herein we describe a general exfoliation method for different 2D materials employing a biphasic water/dichloromethane system and two different (poly)aromatic hydrocarbons (PAHs). This method allows us to obtain dispersions of the exfoliated 2D materials with high concentrations in the organic solvent. Due to the low boiling point of dichloromethane, and therefore its easy removal, the obtained dispersions can be employed as additives for different composites. We corroborate that the exfoliation efficiency is improved due to the π-π and van der Waals interactions between the PAHs and the layers of the 2D materials.

Heterogeneous electro-Fenton process for degradation of bisphenol A using a new graphene/cobalt ferrite hybrid catalyst

A simple, efficient, environmentally friendly, and inexpensive synthesis route was developed to obtain a magnetic nano-hybrid (GH) based on graphene and cobalt ferrite. Water with a high content of natural organic matter (NOM) was used as solvent and a source of carbon. The presence of NOM in the composition of GH was confirmed by FTIR and Raman spectroscopy, which evidenced the formation of graphene, as also corroborated by XRD analyses. The diffractograms and TEM images showed the formation of a hybrid nanomaterial composed of graphene and cobalt ferrite, with crystallite and particle sizes of 0.83 and 4.0 nm, respectively. The heterogeneous electro-Fenton process (EF-GH) achieved 100% degradation of bisphenol A (BPA) in 50 min, with 80% mineralization in 7 h, at pH 7, using a current density of 33.3 mA cm^-2. The high catalytic performance was achieved at neutral pH, enabling substantial reduction of the costs of treatment processes. This work contributes to understanding the role of NOM in the synthesis of a magnetic nano-hybrid based on graphene and cobalt ferrite, for use in heterogeneous catalysis. This nano-hybrid has excellent potential for application in the degradation of persistent organic pollutants found in aquatic environments.

Solvent-tuned ultrasonic synthesis of 2D coordination polymer nanostructures and flakes

Herein, a new 2-dimensional coordination polymer based on copper (II), {Cu2(L)(DMF)2}n, where L stands for 1,2,4,5-benzenetetracarboxylate (complex 1) is synthesized. Interestingly, we demonstrate that both solvent and sonication are relevant in the top-down fabrication of nanostructures. Water molecules are intercalated in suspended crystals of complex 1 modifying not only the coordination sphere of Cu(II) ions but also the final chemical formula and crystalline structure obtaining {[Cu(L)(H2O)3]·H2O}n (complex 2). On the other hand, ultrasound is required to induce the nanostructuration. Remarkably, different morphologies are obtained using different solvents and interconversion from one morphology to another seems to occur upon solvent exchange. Both complexes 1 and 2, as well as the corresponding nanostructures, have been fully characterized by different means such as infrared spectroscopy, x-ray diffraction and microscopy.

When graphene meets ionic liquids: a good match for the design of functional materials

Graphene is an attractive material that is characterized by its exceptional properties (i.e. electrical, mechanical, thermal, optical, etc.), which have pushed researchers to attach high interest to its production and functionalization processes to meet applications in different fields (electronics, electromagnetics, composites, sensors, energy storage, etc.). The synthesis (bottom-up) of graphene remains long and laborious, at the same time expensive, and it is limited to the development of this material in low yield. Hence, the use of graphite as a starting material (top-down through exfoliation or oxidation) seems a promising and easy technique for producing a large quantity of graphene or graphene oxide (GO). On the one hand, GO has been extensively studied due to its ease of synthesis, processing and chemical post-functionalization. One the other hand, "pristine" graphene sheets, obtained through exfoliation, are limited in processability but present enhanced electronic properties. Both types of materials have been of great interest to design functional nanomaterials. Ionic liquids (ILs) are task-specific solvents that exhibit tunable physico-chemical properties. ILs have many advantages as compared with conventional solvents, such as high thermal and chemical stability, low volatility, excellent conductivity and inherent polarity. In the last decade, ILs have been widely employed for the preparation and stabilization of various nanomaterials. In particular, the combination of ILs and graphene, including GO and pristine graphene sheets, has been of growing interest for the preparation, processing and functionalization of hybrid nanomaterials. Understanding the structure and properties of the graphene/IL interface has been of considerable interest for a large panel of applications ranging from tribology to energy storage.

The production of graphene using impinging jet exfoliation in a binary system of CO2 and N-methyl pyrrolidone

High quality and high quantity few-layer graphene was successfully prepared using a new impinging jet method. Natural graphite flakes were first agitated in N-methyl pyrrolidone (NMP) with the assistance of supercritical CO₂, then the half-exfoliated graphite was further stripped using the shear stress derived from the impinging jets. After the energy conversion and stress analysis of the graphite particles during the whole exfoliation process, it was revealed that the size of the target mesh, the distance between the nozzle and the target, the decompression rate, and the size of the raw materials had a significant influence on the exfoliation process. Additionally, a microscopic view of the exfoliation and dispersion mechanism of graphene in the CO₂-NMP system was investigated using molecular dynamics simulation, and CO₂ was found to be beneficial for the penetration of NMP into the graphite sheets. Finally, the concentration and quality characteristics of the prepared graphene were characterized using ultraviolet-visible spectroscopy, transmission electron microscopy, Raman spectroscopy, and atomic force microscopy. The maximum concentration was as high as 0.689 mg ml^-1, the thickness of 68% of the product was less than 2.5 nm, and the lateral dimension was from 0.5 to 3.0 μm. These results indicate that this impinging jet method is promising for large-scale industrial production.