Introduction:
Graphene, a miraculous material, because of its excellent physical and chemical properties, so it has a wide range of applications in many fields, such as energy and electronics, and its prospects are also very impressive.
Graphene in a microscopic perspective
The preparation process of graphene has undergone many technological innovations, starting with mechanical stripping methods, and gradually changing to the current chemical vapor deposition method, in addition to mechanical shearing methods.
Either way, it reflects the continuous innovation and improvement on the road of graphene preparation, which not only improves the demand for future preparation, but also plays a major role.
The preparation methods of graphene are mainly as follows.
Comparison of graphene preparation methods
First, the preparation method of graphene oxide
The redox method used in the preparation of graphene generally needs to achieve the effect of graphene oxide by oxidizing graphite. Graphene is then obtained by reducing graphene oxide. The specific methods and experimental steps of these two steps will be introduced below.
First of all, in the preparation of graphene oxide, most of the oxidants are used to oxidize the materials of graphene first, so as to form the synthesis of graphene oxides.
For example, potassium permanganate and nitric acid are some oxidants often used in the preparation of graphene. However, nitric acid is the most widely used one.
The following are the specific steps of the nitric acid oxidation method:
Experimental steps:
1. Add graphite powder to concentrated nitric acid and beat it up with ultrasonic waves to form a uniform suspension. The mass of graphite powder should be proportional to the volume of concentrated nitric acid, generally 1g of graphite powder added to 10mL of concentrated nitric acid.
2. Place the suspension on a blender and stir it so that it oxidizes evenly. The oxidation reaction time is generally 12-24 hours and can be adjusted as needed.
Graphite powder
3. The oxidized graphene oxide is repeatedly washed with water to remove concentrated nitric acid, nitrate and other residues. The number of repeated washes depends on the specific needs.
In the preparation process of graphene oxide, some key formulas are involved, which are described below.
Oxidation reaction formula
The preparation method of graphene oxide is usually obtained by chemical reactions of graphite oxide precursors.
The following is the equation for oxidation reactions:
In this reaction, glucose (C6H12O6) and nitric acid (HNO3) react to produce the oxidation of a precursor of graphene, which is called "graphene alkyd"
Redox reactions
Formula for calculating the degree of oxidation
The oxidation degree of grapheneenalic acid (GO) can be calculated using the following formula:
Oxidation degree formula
where $(\rm C/O){\rm GO}$ represents the atomic ratio of carbon to oxygen in graphenolenic acid (GO), and $(\rm C/O){\rm graphite}$ represents the atomic ratio inside.
Reductive reaction formula
Graphene oxide can be reduced graphene by reduction reaction. The chemical equations for the reduction reaction are as follows:
Reductive reaction formula
In this reaction, hydrazine hydrate is a widely used reducing agent in this field.
Restore the diagram
Second, the preparation method of reduced graphene oxide
If you want to prepare graphene by this method, you must rely on the reducing agent to oxidize it to form graphene.
Here are the specific steps of hydrazine hydrate reduction:
Experimental steps:
1. Add graphene oxide powder to deionized water to form a uniform suspension. The mass of graphene oxide should be proportional to the volume of deionized water, generally 1 g of graphene oxide added to 10 mL of deionized water.
2. Add hydrazine hydrate to the suspension and beat it up with ultrasonic waves to form a homogeneous mixture. The amount of hydrazine hydrate is generally 1-2 times that of graphene oxide.
Chemical formula for hydrazine hydrate reduction
3. Heat the mixture to about 80 °C and react for 2-3 hours. During the reaction, the mixture gradually turns black.
4. Precipitate the reduced graphene and wash it repeatedly with deionized water to remove the residual hydrazine hydrate, reduction products and other substances. The number of repeated washes depends on the specific needs.
5. Dry the washed graphene with a vacuum dryer or oven. The drying temperature should generally not exceed 60 °C.
Graphene drying process
It should be noted that in the process of graphene oxide and reduction, it is generally necessary to clean and clean the container and the operation interface first, so as to avoid the influence of some impurities and pollution sources on graphene.
In addition, in the process of reducing the reaction, it is also important to control the time and temperature of the reaction.
Therefore, in order to avoid adverse reactions in this process and cause graphene to lose its structural properties, the temperature and time in the reaction should be controlled.
It can be seen that the chemical redox method is a more common method in the graphene preparation method, which not only has fewer steps than other methods, but also has better cost and yield than other methods, which can meet the needs of graphene in different fields.
Redox reactions
In the process of this experiment, we must pay attention to safety, in addition, in order to avoid discrepancies in the preparation process, resulting in a decline in the quality of graphene, everyone needs to follow the operation steps to carry out the experiment.
In the preparation method of reduced graphene oxide, the formulas involving reduction reaction and structural characterization of graphene are introduced below.
Reduction reaction formula:
Reductive reaction formula
where $x$ and $y$ represent the atomic numbers of carbon and oxygen in graphene, respectively. During the reaction, hydrogen reduces oxygen atoms from graphene oxide to water, and also produces graphene materials.
Diagram
Third, chemical vapor deposition method
In addition, in addition to the above preparation methods, there are other preparation methods, such as chemical vapor deposition is one of them. The principle of preparing graphene by CVD method is that graphene is grown on a metal substrate by gas-phase reaction at high temperature and pressure. Its main steps include:
Experimental steps:
- The metal substrate (generally copper, nickel, etc.) is put into tetrahydrofuran (THF) to remove impurities such as surface oxides.
Chemical vapor deposition
2. Put the cleaned substrate into an easy place, and then add the appropriate amount of carbon source gas. (generally methane, ethylene, etc.), and the surface temperature of the metal substrate reaches the optimal temperature for graphene growth (generally 1000-1100 °C) by heating.
3. A graphene film is formed on the surface of the metal substrate, and the film thickness can be controlled by reaction time and gas flow. The general reaction time varies from 30 minutes to several hours.
4. Take out the metal substrate, corrode the metal substrate with a chemical solution, and transfer the graphene film to other substrates. Transfer methods generally include mechanical peeling method, chemical peeling method, etc., and can be characterized and analyzed by SEM, TEM and other instruments after transfer.
Schematic diagram of the reaction process
It should be noted that the preparation of graphene by CVD method requires control of reaction parameters, such as temperature, gas flow, reaction time, etc., and there are also certain requirements for substrate selection and pretreatment. During the experiment, it is necessary to follow laboratory safety specifications and strictly operate to ensure the preparation of high-quality graphene.
In this process, some key formulas are involved, which are described below.
Deposition reaction formula
The chemical equations for the deposition reaction in the CVD method are as follows:
In this reaction, methane (CH4) acts as a carbon source molecule to produce carbon atoms and hydrogen by pyrolysis. Carbon atoms form a single layer of graphene on the surface of the metal substrate.
Thermodynamic equilibrium formula
The process of the deposition reaction in the CVD method is controlled by thermodynamic equilibrium. The change in chemical potential during the reaction can be calculated by the following formula:
Atomic layer deposition rate formula
In the CVD method, the graphene layer on the surface of the metal substrate is deposited layer by layer by carbon atoms. The atomic layer deposition rate can be calculated using the following formula:
where $R_{\rm ALD}$ represents the atomic layer deposition rate, $V_{\rm eff}$ represents the effective volume of reactant molecules, and $\theta_{\rm surf}$ represents surface coverage. This formula shows that the atomic layer deposition rate has a nonlinear relationship with the surface cover, and the deposition rate gradually slows down as the surface coverage increases.
4. Mechanical peeling method
The mechanical peeling method uses mechanical force to peel graphite flakes into a single layer of graphene. This peeling method usually requires several steps to try during the preparation process, and the specific steps are as follows:
1. Preparation of graphite crystals
First of all, the mechanical peeling method will first prepare some high-quality graphite crystals when preparing graphene. Graphite crystals are usually synthesized by high-temperature graphitization reactions, and the formula involved is:
2. Mechanical stripping
The prepared graphite crystal is placed on an appropriate substrate, and a force perpendicular to the substrate plane is applied using a mechanical peeling device to gradually peel off the graphite layer into a single layer of graphene. The mechanical stripping process involves relatively few formulas, mainly involving mechanical formulas in Newtonian mechanics.
3. Characterization
The resulting graphene is characterized, including structural and property characterization. Structural characterization can use scanning electron microscopy, transmission electron microscopy, and other techniques to observe the morphology and structure of graphene.
Schematic diagram of mechanical peeling
Property characterization can analyze the physical and chemical properties of graphene using techniques such as photoelectron spectroscopy, Raman spectroscopy, etc.
Mechanical peeling method is a simple and effective method for preparing graphene, which does not require complex experimental devices and expensive chemical reagents, so it has high practicability.
The mechanical stripping process mainly involves the mechanical formulas of Newtonian mechanics, but in the experiment, it is necessary to finely control the parameters such as force and speed to ensure that high-quality single-layer graphene is obtained.
Schematic diagram of mechanical peeling
Summary:
On the road to exploring graphene preparation, mankind has gone through a long and tortuous road. From the initial mechanical peeling method, to the later graphene oxide reduction method, chemical vapor deposition method, etc., each preparation method embodies the unremitting efforts and continuous innovation of scientists.
Graphene has attracted attention because of its unique structure and properties, and is considered "the miracle of carbon atoms".
It not only has excellent electrical, optical, thermal and other properties, but also has extremely high mechanical strength and flexibility, and has a wide range of application prospects. From electronics and energy to biomedicine, graphene has great potential.
Schematic diagram of graphene stripping method
However, the preparation and application of graphene still faces many challenges. One of the biggest challenges is how to produce high-quality graphene on a large scale and at low cost.
In future research, scientists need to continue to explore new preparation methods and application areas to promote the development and application of graphene technology.
Resources:
"Discussion on the preparation of graphene"
《Graphene preparation method and overview》
"Graphene, the new favorite of new materials"
"The Making and Discovery of Graphene"