Supercritical fluid provides new ideas for the industrial production of graphene

2021-07-21


The current methods for preparing graphene include mechanical exfoliation, SiC epitaxial growth, oxidation-reduction graphite, chemical vapor deposition (CVD), supercritical fluid exfoliation, etc. According to the editor's understanding, these preparation methods all have their advantages and disadvantages. For example, mechanical peeling method can obtain a few or multiple layers of graphene with complete crystal structure, but its production efficiency is not high and cannot be applied on a large scale. The oxidation-reduction method is to oxidize graphite into graphite oxide and disperse it in aqueous medium, and then reduce it to obtain graphene; This method can be used for industrial large-scale production of graphene, but the structure of graphene is greatly damaged and there are many defects in graphene. The SiC epitaxial growth method can obtain single-layer or multi-layer graphene with larger dimensions, but its production equipment requires high requirements, high costs, and the defects of graphene are uncontrollable and the thickness is uneven. The CVD method can achieve large-scale preparation of graphene, but the cost is high and the process is complex. In contrast, the method of preparing graphene by supercritical fluid exfoliation can obtain high-quality single-layer or few layers of graphene, and has the advantages of simple operation process, green preparation process, low pollution, low energy consumption, and low cost, which has been favored by some researchers.
In fact, graphene already exists in nature, but it is difficult to peel off a single layer structure. Graphene layer by layer is graphite, with a thickness of 1 millimeter containing approximately 3 million layers of graphene. A pencil lightly scratches on the paper, leaving traces that may be several layers or even just one layer of graphene.
In 2004, Andre Geim and Konstantin Novoselov, two scientists from the University of Manchester in the UK, found that they could get thinner and thinner graphite sheets in a very simple way. They peel off graphite flakes from highly oriented pyrolytic graphite, and then stick two sides of the flakes on a special tape. Tear off the tape to split the graphite flakes in two. Continuously operating in this way, the sheets became thinner and thinner, and then they obtained a sheet composed of only one layer of carbon atoms, which is graphene. Since then, new methods for preparing graphene have emerged one after another. In 2009, Andre Geim and Konstantin Novoselov found the integer quantum Hall effect and the quantum Hall effect at room temperature in the single-layer and double-layer graphene systems, respectively, and they won the 2010 Nobel Prize in Physics. Before the discovery of graphene, most physicists believed that thermodynamic fluctuations did not allow any two-dimensional crystals to exist at finite temperatures. So, its discovery immediately shocked the academic community of condensed matter physics. Although both theoretical and experimental communities believe that perfect two-dimensional structures cannot exist stably at non absolute zero degrees, single-layer graphene can be prepared in experiments.
On March 31, 2018, China's first full-automatic mass production graphene organic solar optoelectronic device production line was launched in Heze, Shandong Province. The project mainly produces graphene organic solar cell (hereinafter referred to as graphene OPV) that can generate electricity in low light, and solves three major solar power generation problems: application limitations, angle sensitivity, and difficulty in modeling. On June 27, 2018, the China Graphene Industry Technology Innovation Strategic Alliance released the newly formulated group standard "Naming Guidelines for Products Containing Graphene Materials". This standard specifies the naming method for new products related to graphene materials.
Characteristics of Supercritical Fluids
Supercritical fluid (SCF) refers to a fluid whose temperature and pressure are above the critical point. In supercritical fluids, the boundary between liquid and gas disappears, and the physical properties of supercritical fluids combine liquid and gas properties. Their density is two orders of magnitude higher than that of gas, close to the density of liquids; The viscosity is smaller than that of liquids, but the diffusion speed is about 2 orders of magnitude faster than that of liquids, indicating good fluidity and mass transfer performance; Its dielectric constant changes sharply with pressure. At the same time, supercritical fluids also have characteristics that distinguish them from gases and liquids: near the critical point, the density of the fluid is very sensitive to temperature and pressure, especially pressure. Small changes can cause significant changes in the density of the fluid, leading to significant changes in multiple properties of the fluid, such as viscosity, dielectric constant, diffusion coefficient, and solubility. Therefore, the physical and chemical properties of supercritical fluids can be controlled by adjusting pressure and temperature.
The Principle of Supercritical Fluid Stripping of Graphite
The principle of graphite stripping using supercritical fluid (SCF) is introduced using SC CO2 (critical temperature TC=31.1 ℃, critical pressure PC=7.38MPa) as an example, as shown in Figure 1. Graphite is a layered structure that can be seen as a single layer of graphene stacked layer by layer through van der Waals forces (Figure 1A). The high dispersibility and strong permeability of supercritical fluids make it easy to enter the interlayer of graphite, forming an intercalation structure (Figure 1B); When rapid pressure relief occurs, SC CO2 undergoes significant expansion, releasing a large amount of energy to overcome the interlayer forces of graphite (Figure 1C), resulting in a single or few layers of graphene (Figure 1D). It is understood that this method is simple to operate, easy to achieve under conditions, and does not use strong acids or bases during the preparation process, making it green and environmentally friendly. Partial Study on Preparation of Graphene by SCF Stripping.
Among numerous supercritical fluids, supercritical CO2 is widely used in practical production and research processes. This is because supercritical CO2 not only has high diffusion and permeability, but also has relatively low critical temperature (304.1K) and critical pressure (7.38 Mpa). Moreover, its chemical properties are not active, non-toxic, odorless, and odorless, with moderate cost, and can be reused repeatedly. At the same time, due to the solubility of organic molecules in supercritical CO2, it can also serve as an effective "entrainer" to carry certain small molecules into the interior of the material, achieving intercalation and modification of layered materials.
Research has shown that supercritical fluids can assist in the preparation of graphene. Rangappa et al. reported a method for one-step exfoliation of graphene using supercritical fluids (ethanol, NMP, DMF). Firstly, the graphite is uniformly dispersed into the corresponding solvent through ultrasound, and then the dispersion is placed in a high-pressure reactor, rapidly heated to a supercritical state. After 1 hour of reaction, 90% -95% graphene sheets with less than 8 layers can be obtained, with a single-layer graphene content of 6% -10% (Figure 2).
Liu et al. first used supercritical DMF to exfoliate expanded graphite to obtain fewlayer graphene (FG), and then treated FG with supercritical DMF again to obtain single-layer graphene after separation. They also studied the impact of supercritical fluid conditions on the exfoliation effect.
Pu et al. reported a method for preparing graphene using supercritical CO2 gas intercalation of graphite. Firstly, the graphite was soaked in supercritical CO2 for 30 minutes, and then rapidly deflated in an aqueous solution containing sodium dodecyl sulfonate (SDS) to expand and peel the graphite. SDS can prevent the re accumulation of graphene layers. The yield of graphene obtained by this method can reach 30-40%, which has the advantages of simple operation and low cost. However, there are many graphene layers (-10 layers) prepared.
On the basis of their previous work, Jang et al. further utilized supercritical ethanol and sodium pyrenesulfonate (1-PSA) to strip graphite, achieving one-step stripping and modification of graphene. Sodium pyrene sulfonate can not only prevent the re aggregation of graphene, but also facilitate the detachment process. Research has found that with the increase of sodium pyrene sulfonate dosage, the stripping efficiency of graphite significantly improves. When the carbon atom ratio of sodium pyrene sulfonate to graphite is 1:1, the yield of single-layer or double-layer graphene can reach 60%.
The supercritical fluid stripping method for preparing graphene has achieved controllable preparation of graphene layers, with simple process, low cost, and low equipment requirements. It has good potential in large-scale production of graphene, which will provide a new path for industrial production of graphene.

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