Overview Of The Preparation Of Graphene
At present, there are many methods for the preparation of graphene. This paper is divided into physical and chemical methods.
1 physical method for the preparation of graphene
The physical method is usually based on cheap graphite or expanded graphite as raw material, through mechanical stripping, orientation epiphysees, liquid or gas direct stripping method to prepare single or multi-layer graphene. These methods are easy to obtain raw materials, the operation is relatively simple, synthetic graphene high purity, less defects.
1.1 mechanical stripping method
Mechanical stripping or micromachining is one of the simplest ways to directly peel a graphene sheet from a larger crystal. Novoselovt et al. Succeeded in stripping and observing monolayer graphene from highly oriented pyrolytic graphite with a very simple micromachined stripping method in 2004, demonstrating the independent presence of monolayer graphene. The specific process is as follows: first use oxygen plasma in the 1 mm thick highly oriented pyrolytic graphite surface ion etching, when the surface etching 20 μm wide and 2 μm deep micro-slot, with photoresist will It is glued to the glass substrate, and then tear tape is repeatedly removed with a transparent tape, and then the excess highly oriented pyrolytic graphite is removed and the glass substrate with the microcapsules is placed in the acetone solution for ultrasonic, and finally the monocrystalline silicon wafer Into the acetone solvent, the use of van der Waals force or capillary force will be a single layer of graphene "remove".
However, this method has some drawbacks, such as the size of the product obtained is not easy to control, can not reliably prepare a long enough graphene, and therefore can not meet the industrial needs.
1.2 orientation epiphytic method - crystal growth
Peter W.Sutter et al. Used rare metal ruthenium as the growth matrix, using the atomic structure of the matrix "species" out of graphene. The C atoms are first infiltrated into ruthenium at 1150 ° C and then cooled to 850 ° C. Before the large amount of carbon atoms are absorbed, they float to the surface of the ruthenium, forming a monolithic carbon atom "island" on the surface of the substrate, Island "gradually grow up, eventually grow into a layer of complete graphene. After the first layer coverage rate of 80%, the second layer began to grow, the bottom of the graphene and the matrix there is a strong interaction between the second layer after the formation of the former layer and the substrate is almost completely separated, leaving only weak coupling, Thus, a monolithic graphene sheet was prepared. However, the graphene sheets produced by this method tend to be uneven in thickness, and the adhesion between graphene and the matrix affects the properties of the prepared graphene flakes.
1.3 Liquid phase and gas phase direct stripping method
Liquid phase and gaseous direct stripping method refers to the direct release of graphite or expanded graphite (EG) (usually by rapid temperature rise to 1000 ° C above the surface of oxygen-containing groups removed to get) added to an organic solvent or water, with ultrasound , Heating or air flow to produce a certain concentration of single or multi-layer graphene solution. Coleman et al. Dispersed graphite in N-methyl-pyrrolidone (NMP) in the same manner as liquid-phase peeling of carbon nanotubes. The yield of monolayer graphene was 1% after 1 hour of ultrasound, and prolonged ultrasound (462 h) So that the graphene concentration of up to 1.2 mg / mL. The results show that the interaction between the solvent and the graphene can balance the energy required to peel off the graphene when the solvent matches the surface energy of the graphene, and the surface tension of the graphene can be 40 ~ 50mJ / M2. The effect of stripping graphite sheet can be improved by the impact of airflow. Janowska et al. Used expanded graphite as raw material and microwave irradiation to improve the total yield of graphene (~ 8%) with ammonia as solvent. In-depth studies have shown that ammonia produced by solvent decomposition at high temperatures can penetrate into the graphite sheet and peel off the graphite when the air pressure exceeds a certain value enough to overcome the van der Waals force between the graphite sheets.
Because of the cheap graphite or expanded graphite as raw material, the preparation process does not involve chemical change. The preparation of graphene by liquid phase or gas phase direct stripping method has the advantages of low cost, simple operation and high product quality, but there is also monolithic graphene yield High, lamellar agglomeration serious, need to further remove the stabilizer and other defects.