CN110699749B - A kind of method for preparing large-area continuous single-layer single-crystal graphene film - Google Patents

Abstract

The invention discloses a method for preparing a large-area continuous single-layer single-crystal graphene film. The method comprises the following steps: and carrying out electrochemical polishing on the single crystal copper to obtain a single crystal copper substrate, and then carrying out growth of graphene on the single crystal copper substrate to obtain the continuous single-layer single crystal graphene film. At high temperature, a lot of silicon-oxygen compounds are attached to the surface of metal copper, which seriously affects the growth of graphene, and electrochemical polishing can remove the silicon-oxygen compounds, so that the surface of the copper becomes flat and clean. And then growing graphene by using the single-crystal copper to obtain single-layer graphene islands with the same orientation, and connecting the single-layer graphene islands to obtain the continuous single-layer single-crystal graphene film.

Description

A kind of method for preparing large-area continuous single-layer single-crystal graphene film

technical field

The invention relates to a method for preparing a large-area continuous single-layer single-crystal graphene film.

Background technique

Graphene is a two-dimensional atomic crystal with a hexagonal honeycomb structure and a thickness of one atomic layer. It was prepared by the mechanical stripping method jointly by physicists Andrei Geim and Konstantin Novoselov of the University of Manchester in the United Kingdom in 2004, and has since received widespread attention from scientists around the world. Graphene has unique and excellent properties, including good electrical and thermal conductivity, excellent mechanical properties and light transmittance, etc. These properties make it widely used in field effect transistors, flexible transparent electrodes, supercapacitors, graphene paper and other fields. application prospects.

At present, the main methods for preparing graphene are: mechanical exfoliation method, SiC epitaxial growth method, liquid phase exfoliation method and chemical vapor deposition (CVD) method. Chemical vapor deposition has become the most promising preparation method for industrialization due to its suitability for mass production and relatively low price. Among them, the preparation method using metal catalysts such as copper and nickel as the substrate is the main method. Since single-layer high-quality graphene can be controllably obtained on copper, chemical vapor deposition using metallic copper as a substrate has become a hot research topic in recent years. However, graphene grown using copper substrates has many problems. Most of the current commercially available copper foils are polycrystalline copper foils, on which graphene is grown as a polycrystalline graphene film, and there are many grain boundaries on it, which will seriously affect its electrical properties. Recently, there have been reports of successful preparation of single-crystal copper foils and growth of large-sized single-crystal graphene films. However, it can be concluded from the experiment that a large amount of silicon oxide compound will be attached to the surface of the copper foil, which will damage the graphene film. In addition, small double-layer nucleation sites are also introduced during the growth process, so that there are many double-layer islands after film formation, which affects the transport of electrons. Whether these problems can be solved becomes the basis and prerequisite for the realization of graphene applications.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for preparing a large-area continuous single-layer single-crystal graphene film.

The method for preparing a continuous single-layer single-crystal graphene film provided by the present invention comprises the following steps:

The single-crystal copper is electrochemically polished to obtain a single-crystal copper substrate, and then graphene is grown on the single-crystal copper substrate to obtain the continuous single-layer single-crystal graphene film.

In the electrochemical polishing of the above method, the polishing liquid is composed of water, phosphoric acid, ethanol, isopropanol and urea; the dosage ratio of the water, phosphoric acid, ethanol, isopropanol and urea is 250mL: 125mL: 125mL: 25mL: 4g ;

The voltage of the electrochemical polishing is 3-5V; the current is 2-4A; the polishing time is 2-5min; specifically, 3min.

In the graphene growth step, the carbon source is selected from at least one of methane, carbon monoxide, methanol, acetylene, ethanol, benzene, toluene, cyclohexane and phthalocyanine;

The reducing gas used is hydrogen;

The inert gas used is argon and/or nitrogen;

The growth temperature is 1070-1080°C; specifically 1075°C;

The reaction time is 0.5-1.5h; specifically 40-60min;

The flow rate of the inert gas is 100-400sccm; specifically 250sccm;

The flow rate of reducing gas is 25-200sccm;

The carbon source flow rate is 1-10 sccm; specifically 3-5 sccm; more specifically 3.5 sccm.

Specifically, the single crystal copper may be single crystal Cu(111).

The method for preparing the single crystal Cu(111) may include the following steps: performing several treatments on the polycrystalline copper foil to obtain the single crystal Cu(111);

Each treatment includes polishing and annealing;

After each annealing, the copper foil was naturally cooled to room temperature.

In the above method for preparing single crystal Cu(111), each treatment is polishing and annealing;

The number of times is at least 3 times; specifically, 3 times or 4 times.

In the polishing step, the polishing method is electrochemical polishing.

In the electrochemical polishing step, the polishing liquid used is composed of water, phosphoric acid, ethanol, isopropanol and urea; in the polishing liquid, the dosage ratio of water, phosphoric acid, ethanol, isopropanol and urea is 250mL:125mL:125mL: 25mL: 4g;

The voltage is 3V-5V;

The current is 2A-4A;

Polishing time is 2min-5min.

The annealing is carried out in the presence of an inert gas and a reducing gas; the inert gas is argon or nitrogen;

The reducing gas is hydrogen.

The flow rate of the inert gas is 100sccm-400sccm;

The flow rate of reducing gas is 50sccm-200sccm;

Annealing temperature is 1070℃-1083℃;

Annealing time is 30min-180min.

In addition, the continuous single-layer single-crystal graphene film prepared according to the above method also belongs to the protection scope of the present invention.

The method for preparing a continuous single-layer single-crystal graphene film provided by the present invention selects single-crystal copper as a growth substrate. Single crystal copper eliminates the grain boundaries and planes that are abundant on the surface of commercially available copper, resulting in highly oriented graphene islands when growing graphene. These graphene islands continue to grow until they are joined together, resulting in a single-crystal graphene film. However, a very large amount of silicon oxide compounds are attached to the surface of the single crystal copper (111) after annealing, and these silicon oxide compounds form protrusions and make the surface of single crystal copper uneven; silicon oxide compounds cannot catalyze the decomposition of carbon sources to grow graphite Therefore, the resulting graphene film is broken and discontinuous. Not only that, the graphene grown with other copper foils can observe a non-conductive white particle in the center, which is presumed to be a silicon oxide compound. Therefore, the elimination of silicon-oxygen compounds attached to single-crystal copper is crucial for the preparation of high-quality continuous graphene films.

The single crystal copper (111) surface obtained by annealing is electrochemically polished in advance and then graphene is grown, which can remove the pollutants on the surface of the single crystal copper and the attached silicon oxide compound, so that the surface of the single crystal copper becomes very clean and flat. The graphene grown by using the single crystal copper not only has a very clean surface, but also the obtained film is very complete without any voids and damages; all graphene islands are single-layer single-crystal graphene with the same orientation. When single-crystal copper without electrochemical polishing is used for growth, the graphene obtained under the same conditions is mostly multi-layer graphene.

Scanning electron microscope (SEM) characterization shows that there are many small white particles on the surface of the single crystal copper obtained after annealing, with different shapes and sizes. It is white because it is not conductive. Energy dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy (AES) showed that the substance contained Si and O, and it could be determined to be a silicon-oxygen compound. After electrochemical polishing, it can be seen on the SEM that all white particles have disappeared, and the surface of the copper sheet is very flat and clean.

It can be seen under SEM that after the graphene is grown, there are still many particles on the single-crystal copper that has not been electrochemically polished, and exist on the surface of the graphene. When treated with hydrofluoric acid (HF), it can be clearly seen under SEM that there are holes in the original silicon oxide compound, which proves that silicon oxide compound will hinder the growth of graphene, and make it damaged and discontinuous, thus affecting the The quality of graphene. After electrochemical polishing, it can be seen under the optical microscope (OM) that there are no more protrusions on the graphene, and the surface is very flat and clean. In addition, under the same conditions, many multi-layer regions will be generated before electrochemical polishing, and almost all of them are single-layer graphene after electrochemical polishing. Bad also plays a decisive role.

Description of drawings

FIG. 1 is a scanning electron microscope photograph of a single crystal copper (111) surface obtained after annealing at 1075°C.

FIG. 2 is an energy scattering X-ray spectroscopic analysis diagram of white particles on the surface of single crystal copper (111) in Example 1. FIG.

FIG. 3 is an Auger electron spectrum analysis diagram of white particles on the surface of single crystal copper (111) in Example 1. FIG.

FIG. 4 is a scanning electron microscope photograph of the single crystal copper (111) surface after electrochemical polishing in Example 1. FIG.

FIG. 5 is an optical microscope photograph of graphene grown on a single crystal copper (111) surface after electrochemical polishing in Example 1. FIG.

FIG. 6 is a scanning electron microscope photograph of graphene grown on the (111) surface of single-crystal copper without electrochemical polishing in Example 1. FIG.

FIG. 7 is a scanning electron microscope photograph of graphene grown on a single crystal copper (111) surface without electrochemical polishing after treatment with hydrofluoric acid in Example 1. FIG.

8 is an optical microscope photograph of graphene grown on a single crystal copper (111) surface without electrochemical polishing in Example 1. FIG.

FIG. 9 is an optical microscope photograph of individual graphene islands before film formation in Example 2. FIG.

FIG. 10 is an optical microscope photograph of the graphene thin film etched after film formation in Example 2. FIG.

Detailed ways

The method of the present invention will be described below through specific embodiments, but the present invention is not limited thereto, and any modifications, equivalent replacements and improvements made within the spirit and principle of the present invention, etc., shall be included in the scope of the present invention. within the scope of protection.

The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials can be obtained from commercial sources unless otherwise specified.

Embodiment 1,

First, prepare a single crystal copper (111) plane

In the first step, a 100 μm thick commercial copper foil (purity 99.9%) was cut to a size of about 4cm×6cm, and electrochemical polishing was performed in an ethanol/phosphoric acid system (wherein, the specific composition of the polishing solution was: deionized water ( mL), phosphoric acid (mL), ethanol (mL), isopropanol (mL) and urea (g) in a ratio of 250:125:125:25:4; the conditions for electrochemical polishing are: the maintaining voltage is 5V, the current is 2A, and the polishing time is 3min.) Clean with deionized water to remove the residual polishing liquid on the surface, and dry with nitrogen.

In the second step, the copper foil obtained in the first step was placed in a high-temperature tube furnace, and 200 sccm of argon gas and 100 sccm of hydrogen gas were introduced to start heating. The tube furnace was heated to 1075°C, and the heating time was 45min. Hold for 1 h for annealing. After cooling to room temperature naturally, the sample was taken out.

In the third step, the copper foil obtained in the second step is subjected to electrochemical polishing treatment and high temperature annealing process again until the surface of the sample is completely transformed into a single crystal copper (111) surface. Figure 1 is an SEM photograph of the single crystal copper (111) surface obtained after annealing, from which it can be seen that there are many white particles on the surface, covering the surface of the copper foil. Figures 2 and 3 are EDX characterization and AES characterization, respectively, which both confirm that the white particles are silicon oxides, and there is no silicon oxide signal in the flat surface area, indicating that they only exist in the protrusions.

Second, prepare a single-layer continuous graphene film

The single-crystal copper (111) obtained in the third step was electrochemically polished, placed in the center of a tube furnace, 250 sccm of argon gas and 25 sccm of hydrogen gas were introduced, and the temperature was started. The heating time was 45min. When the temperature reached 1075℃, 3.5sccm of methane was introduced and grown for 1h. After the reaction was completed, it was naturally cooled to room temperature. Figure 4 is a SEM photograph of the single crystal copper (111) surface after electrochemical polishing, on which no traces of white particles can be seen, and the surface is a flat and clean copper (111) surface. Figure 5 is the OM image of the grown graphene, there are no protrusions on the graphene, and the surface is very flat and clean.

As a control, graphene was grown under the same conditions using single crystal copper (111) without electrochemical polishing.

FIG. 6 is an SEM photo of the obtained graphene, and the silicon oxide compound is embedded in the graphene, which has an influence on the inherent texture direction of the graphene. It was then subjected to HF treatment, and the results are shown in FIG. 7 . The place where the silicon oxide compound originally existed turned into a hole, which damaged the graphene. It was confirmed that the silicon oxide compound on the surface of the single crystal copper would seriously affect the quality of the graphene, making it damaged and incomplete. Fig. 8 is the OM photo. Compared with Fig. 5, it can be seen that under the same conditions, the graphene grown on the single-crystal copper without electrochemical polishing has many multi-layer structures, while Fig. 5 is a uniform single layer, indicating that for graphite The regulation of the number of alkene layers depends not only on the conditions, but also on the quality of the substrate.

Example 2. Verification of single crystal properties of graphene films

In order to verify the single crystallinity of the obtained graphene film, the judgment of the same orientation rate of graphene islands before film formation and the experiment of local etching of the film after film formation were carried out. The results are shown in Figure 9 and Figure 10, respectively. In the first step in this example, the growth time was shortened from 1 h to 40 min after the introduction of methane, and the result in FIG. 9 can be obtained. In this operation step, after the growth, the flow rates of argon and hydrogen were kept unchanged, the methane was turned off, the temperature was kept constant for 5 minutes, and then the temperature was naturally cooled to obtain the data in Fig. 10 . In Figure 9, most of the graphene islands have the same orientation, which means that they have the same boundary pointing in the same direction, and all the graphene islands are hexagonal single crystals, which ensures that the formed graphene films are also single crystal films. In the etching behavior, the obtained holes have the same orientation as the grown graphene single crystal, so the orientation of the grown single crystal graphene can be determined by the orientation of the etched holes, so as to determine the orientation of the grown single crystal graphene. Single crystallinity of the resulting graphene films. As can be seen in Figure 10, the obtained holes all have the same orientation and are standard regular hexagons, which proves that the graphene islands are single-crystal graphene with the same orientation, and the side confirms that the graphene film is single-crystal graphene.

Claims (1)

1. A method for preparing a continuous single-layer single-crystal graphene film comprises the following steps:

carrying out electrochemical polishing on the single crystal copper to obtain a single crystal copper substrate, and then carrying out graphene growth on the single crystal copper substrate to obtain the continuous single-layer single crystal graphene film;

in the electrochemical polishing, polishing solution consists of water, phosphoric acid, ethanol, isopropanol and urea; the dosage ratio of the water, the phosphoric acid, the ethanol, the isopropanol and the urea is 250 mL: 125 mL: 125 mL: 25mL of: 4g of the total weight of the mixture;

the voltage of the electrochemical polishing is 3-5V; the current is 2-4A; polishing for 2-5 min;

in the graphene growing step, a carbon source is selected from at least one of methane, carbon monoxide, methanol, acetylene, ethanol, benzene, toluene, cyclohexane and phthalocyanine;

the reducing gas is hydrogen;

the inert gas is argon and/or nitrogen;

the growth temperature is 1070-1080 ℃;

the reaction time is 0.5-1.5 h;

the flow rate of the inert gas is 100-400 sccm;

the flow rate of the reducing gas is 25-200 sccm;

the flow rate of the carbon source is 3-5 sccm;

the single crystal copper is single crystal Cu (111);

a method for preparing the single crystal Cu (111), comprising the steps of: processing the polycrystalline copper foil for a plurality of times to obtain the single crystal Cu (111);

each treatment comprises polishing and annealing;

after each annealing, naturally cooling the copper foil to room temperature;

each treatment is polishing and annealing;

the number of times is at least 3 times;

in the polishing step, the polishing method is electrochemical polishing;

in the electrochemical polishing step, the polishing solution consists of water, phosphoric acid, ethanol, isopropanol and urea; in the polishing solution, the dosage ratio of water, phosphoric acid, ethanol, isopropanol and urea is 250 mL: 125 mL: 125 mL: 25mL of: 4g of the total weight of the mixture;

the voltage is 3V-5V;

the current is 2A-4A;

polishing for 2-5 min;

the annealing is carried out in the presence of an inert gas and a reducing gas; the inert gas is argon or nitrogen;

the reducing gas is hydrogen;

the flow rate of the inert gas is 100sccm-400 sccm;

the flow rate of the reducing gas is 50sccm-200 sccm;

the annealing temperature is 1070-1083 ℃;

the annealing time is 30min-180 min.

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