CN104651802A - Method for directly synthesising nitrogen-doped graphene by simply using solid nitrogen source - Google Patents

Abstract

The invention belongs to the technical field of nano-carbon material preparation, and provides a new method for synthesizing nitrogen-doped graphene by using chemical vapor deposition under normal pressure conditions, with melamine as the main nitrogen and carbon source and methane as the auxiliary carbon source. The method does not use other catalysts other than metal substrates, does not use conventional gas or liquid nitrogen sources, has low cost, is non-toxic, is easy to operate, and is convenient for process application. Moreover, the reaction process has no special requirements on equipment, does not need other heating devices other than the main heating device, has strong operability, and is suitable for practical promotion. Firstly, the temperature of copper is raised slowly in the low temperature stage, and when the reaction temperature is reached, the nitrogen source and the carbon source are simultaneously introduced into the tube furnace at one time, and then the temperature is rapidly lowered to room temperature after the reaction is completed. At this time, a layer of nitrogen-doped graphene film has been formed on the metal surface. The doping amount and doping type of nitrogen in graphene, as well as the thickness and crystallinity of graphene can be controlled by optimizing the parameters of nitrogen source, temperature matching of front and rear temperature zones, reaction time, reaction temperature and other parameters.

Description

A simple method for directly synthesizing nitrogen-doped graphene using a solid nitrogen source

technical field

The invention belongs to the technical field of carbon nanomaterial preparation, and relates to a method of synthesizing nitrogen-doped graphene in one step in a CVD process using melamine as a nitrogen source without adding a catalyst. The instrument used in the reaction process is simple, the process is simple, and no main heating equipment CVD synthesis of graphene methods other than heating equipment.

Background technique

Graphene is a unique sp ² hybridized two-dimensional single-layer material, and its stable hexagonal lattice structure endows graphene with many excellent physical and chemical properties (Geim A K.science, 2009, 324, 1530 .). For example, its thermal conductivity (3000W/(m K)) is 10 times that of copper (397W/(m K)), Young's modulus reaches 1.0TPa, and its hardness is greater than that of diamond. It is currently the hardest material in nature. In addition, graphene also has ultra-high electron mobility (about 15000cm ² /(V s)) at room temperature, which is higher than that of single crystal silicon, and its resistivity is about 10 ^-6 Ω·cm, which is lower than that of copper or silver. material with the lowest rate. Moreover, single-layer graphene has very good optical properties, and its light absorption rate is only 2.3% in a wide wavelength band. This makes it have broad application prospects in many high-tech fields, which also makes the preparation of graphene a recent research focus (Li Xu. Materials Herald, 2008, 22(8), 48.).

However, graphene also has two major disadvantages. First, graphene itself has no energy band gap, which largely limits the application of graphene in electrical devices. Secondly, there are no activated functional groups on the surface of graphene, which is not conducive to the compounding with other materials (such as metal particles and organic functional small molecules), which also affects the research and promotion of graphene in many application fields. Scientific research has found that heteroatom doping in graphene can introduce energy band gaps and provide graphene surface reaction sites to enhance the chemical activity of graphene while ensuring the excellent electrical properties of graphene to a large extent. It can be more conveniently applied to many electrochemical fields, such as fuel cells (Zheng B, et al. Electrochemistry Communications, 2013, 28, 24.), sensors (Guo H L, et al. Sensors and Actuators B: Chemical, 2014, 193,623.), Supercapacitor (Su Peng. Acta Physicochemical Sinica, 2012,28,2745.).

Nitrogen-doped graphene can be divided into three types according to the bonding mode of CN, namely graphite-type N, pyridine-type N and pyrrole-type N. Among them, the factors affecting the electrical properties of nitrogen-doped graphene are mainly the nitrogen doping method and the amount of nitrogen doping, and the relative proportion of each nitrogen doping method plays a leading role. For example, studies have shown that the relative content of graphite-type N plays a leading role in the electrocatalytic performance of nitrogen-doped graphene (Wang Z, et al. Journal of Materials Chemistry C, 2014, 2, 7396.), and the doped Nitrogen graphene and the previous report (Lu Y F, et al. ACS Nano, 2013, 7, 6522.), the nitrogen doping amount of the nitrogen doped graphene combined by the two subjects is 5.6%, but the nitrogen doping type of Wang The graphitic nitrogen reaches more than 40%, and the electron mobilities of the final nitrogen-doped graphene are 74cm ² V ^-1 s ^-1 and 5cm ² V ^-1 s ^-1 respectively, which are quite different.

The properties research and application of nitrogen-doped graphene depend on its cheap and large-scale preparation. Since graphene was first prepared by the tape stripping method, new methods have been emerging for the preparation of nitrogen-doped graphene as people pay attention to its electrocatalytic activity, such as: graphene oxide co-heating method, chemical vapor deposition method , hydrothermal method, plasma treatment method, etc.

Wherein the graphene oxide co-heating method requires the use of concentrated sulfuric acid in the process, and then with NH ₃ (Zhang LS, et al.PhysicalChemistry Chemical Physics, 2010,12,12055.) or organic nitrogen source (such as melamine) (Sheng ZH, et al. al.ACSNano, 2011, 5, 4350.) was prepared by co-heating. Hydrothermal methods often use metal nitrogen salts to react with organic molecules, such as Li ₃ N/CCl ₄ (Deng D, et al. Chemistry of Materials, 2011, 23, 1188.). The plasma method uses graphene oxide to react with N ₂ (Shao Y, et al. Journal of Materials Chemistry, 2010, 20, 7491.) to prepare nitrogen-doped graphene.

Since the graphene oxide method requires the use of concentrated sulfuric acid, the safety of the reaction limits the practical promotion of this method; the hydrothermal method limits its development due to its high cost and poor reproducibility; the plasma treatment method has higher requirements on the reaction device , which is not conducive to practical application. In contrast, the research on the synthesis of nitrogen-doped graphene by chemical vapor deposition has achieved fruitful results. Among them, the research on copper as the catalytic substrate is relatively mature. Regarding the choice of nitrogen source, they are gaseous nitrogen source, liquid nitrogen source and solid Nitrogen source. NH ₃ (Wei D, et al. Nano Letters, 2009, 9, 1752.) is the most researched gaseous nitrogen source, and its specific operation is to use NH ₃ mixed with CH ₄ and then pass it into the reaction device; the liquid nitrogen source mainly uses Macromolecular nitrogen-containing compounds, such as pyridine (Jin Z, et al. ACS Nano, 2011, 5, 4112.), the nitrogen-doped graphene synthesized by this method is of high quality, but due to the high price of pyridine and certain toxicity , is not conducive to production promotion.

At present, there are few studies on solid nitrogen sources. The main achievements are: Sun’s research group uses melamine dissolved in PMMA solution as carbon and nitrogen sources. The reaction process needs to be carried out under low pressure conditions, and photolithography is required after spin coating. Assisted in the synthesis of graphene (Sun Z Z, et al. Nature, 2010, 468, 549.), the synthetic nitrogen doping amount is 2%. Wang's research group used melamine as a nitrogen source (Wang Z, et al. Journal of Materials Chemistry C, 2014, 2, 7396.), and the nitrogen doping amount of the synthesized product was 5.6%, but the process used low pressure conditions and used additional heating device, the process is more complicated.

In this study, melamine was directly used as the nitrogen source, and special pretreatments such as solvation of melamine and coating on the surface of the catalyst were not required. The reaction process was carried out under normal pressure, and the reaction cost was low, and no heating equipment other than the main heating equipment was required. , low requirements on the reaction device, convenient operation, simple process and easy production and popularization.

Contents of the invention

The synthesis of nitrogen-doped graphene adopts the solid nitrogen source method, which simplifies the reaction process and reduces the cost. The cheap solid nitrogen source melamine is used to directly synthesize nitrogen-doped graphene by CVD process under normal pressure conditions. In addition to the catalytic metal for graphene growth, the process of the method does not require additional catalysts, does not require special treatment of solid melamine (such as being formulated into a solution, or coated on the surface of an external catalytic material), and does not require other catalysts except for the use of the main heating device. heating equipment.

The technical solution adopted by the present invention to solve its technical problem is: utilize double-temperature-zone tubular furnace to place melamine in a special quartz sample boat in the direction of the upper airflow, and place the metal catalytic substrate (such as copper foil) in the direction of the lower airflow. On the quartz tray with a magnet handle, install the flanges on both sides of the tube furnace, turn on the gas mixing system and adjust the gas flow required in CVD in turn, and then under the protection of the carrier gas, adjust the temperature control device to turn the tube The furnace was heated up to the undetermined reaction temperature at a rate of 10-20°C/min. In this method, in order to solve the problem of thermal volatilization of melamine, the melamine sample is always placed outside the heating zone of the tube furnace before reaching the reaction temperature, and the sample is sent to the heating furnace after reaching the predetermined temperature. Using the gas flow direction of the carrier gas/reaction gas, the vaporized carbon source and nitrogen source are transported to the catalytic metal substrate in the downward gas flow direction to realize the decomposition of the carbon source/nitrogen source, and the deposition of carbon atoms/nitrogen atoms on the surface of the catalytic metal, Diffusion/dissolution, nucleation, growth and other processes. After the reaction is finished, the catalytic metal in the downflow direction (the second temperature zone) is removed from the heating temperature zone, and the temperature is rapidly lowered at a rate of 20-50° C./min. After cooling to room temperature, the metal substrate was taken out, and nitrogen-doped graphene was successfully synthesized on the catalytic metal surface.

The beneficial effect of the present invention is that the reaction pressure is atmospheric pressure, no catalyst other than the metal catalytic substrate is needed in the process, and no conventional gas or liquid nitrogen source is used, the method is low in cost, non-toxic, easy to operate and convenient for process application. There is no special requirement for the reaction equipment, no heating device other than the main heating device is needed, the operability is strong, and it is suitable for practical promotion. The doping amount and doping type of nitrogen in graphene, as well as the thickness and crystallinity of graphene can be controlled by optimizing the parameters of nitrogen source, temperature matching of front and rear temperature zones, reaction time, reaction temperature and other parameters.

Description of drawings

The present invention will be further described below in conjunction with accompanying drawings and examples.

Fig. 1 is a schematic diagram of a reaction device for synthesizing nitrogen-doped graphene in the present invention.

Fig. 2 is a schematic diagram of temperature values of nitrogen-doped graphene synthesized in different time periods in the present invention.

Figure 3 is the first example of using melamine as the nitrogen source, _CH4 as the carbon source, copper foil as the substrate, a constant temperature of 950 ° C, and a _H2 flow rate of 10 sccm to generate nitrogen-doped graphene, in which the temperature was raised to 1050 ° C during the reaction Annealing, and then cooling down to the reaction temperature for reaction. After the reaction is completed, the nitrogen-doped graphene formed by cooling down rapidly is transferred to the SiO ₂ (thickness 300nm)/Si surface by a conventional wet chemical method. Optical microscope test pictures and Raman spectrum pictures.

Fig. 4 is according to the condition of first example, produces the XPS spectrum of nitrogen-doped graphene. It was confirmed that the synthesis of nitrogen-doped graphene was successfully achieved using this technique.

Figure 5 is the second example of the low crystallinity graphene produced by the reaction using melamine as the nitrogen source, _CH4 as the carbon source, and copper foil as the substrate at a low temperature of 500-1000 ° C and reducing carrier gas _H2 . Optical microscope test pictures and typical Raman spectra transferred to SiO ₂ (thickness 300nm)/Si surface by conventional wet chemical method.

Figure 6 The third example uses melamine as the nitrogen source, _CH4 as the carbon source, and copper foil as the substrate. The temperature is set at 1000°C, and the _H2 flow rate is 10-50sccm to react to form nitrogen-doped graphene. When the temperature reaches 1000°C The carbon source and the nitrogen source are introduced at the same time. After the reaction is completed, the nitrogen-doped graphene formed by rapid cooling is transferred to the SiO ₂ (thickness 300nm)/Si surface by the conventional wet chemical method. Optical microscope test pictures and Raman spectra.

Detailed ways

In the present invention, the tube furnace with dual temperature zones reaches the undetermined temperature first, and _H2 is introduced during the heating process to prevent the copper foil from being oxidized. The nitrogen and carbon sources on the side of the temperature zone in the airflow direction of the furnace are introduced into the tube furnace, and the high temperature is used to decompose and dissociate the carbon and nitrogen-containing substances. Lowering the temperature causes nitrogen-doped graphene to be formed on the surface of the copper foil. Among them, the metal substrate used in the process of synthesizing nitrogen-doped graphene by CVD method itself acts as a catalyst, so no additional catalyst is needed here. Moreover, compared with the traditional method that does not use gas or liquid nitrogen sources, the reaction process can be completed with only one main heating device and no other heating devices are required. Therefore, our method is simpler, cheaper and suitable for promotion.

In the method of the present invention, the metal substrates all refer to the clean substrates after ultrasonic 5min successively with acetic acid, acetone and isopropanol; the gas ratios are all standard volume flow ratios;

Specifically, we use melamine as the main nitrogen and carbon source, and _CH as an auxiliary carbon source The method example of using CVD method to synthesize graphene includes the following steps:

Figure 1 and Figure 2 show that _the present invention uses melamine as a nitrogen source, CH as a carbon source, a schematic diagram of the method for CVD synthesis of graphene and the temperature values of the main heating equipment in different time periods. In the figure, before _T1 temperature is a slow temperature rise stage, and then divided into two cases of annealing and no annealing treatment of the catalytic metal according to the experimental needs. Among them, in the case of no annealing treatment, the nitrogen source and carbon source are introduced immediately after the temperature zone rises _to T ₁ temperature, and the temperature is rapidly lowered after a period of reaction; Anneal at high temperature for a certain period of time, after cooling down to the reaction temperature _T3 , introduce nitrogen source and carbon source to react for a certain period of time, and then rapidly cool down. Wherein the change of the carrier gas flow rate in the whole process is that before the reaction starts, the carrier gas flow ratio remains unchanged, and the carrier gas is a mixed gas containing only Ar and H ₂ , and when the reaction stage is reached, the carbon source and the nitrogen source are simultaneously Introduced to participate in the reaction. In this stage, the carrier gas also contains _the auxiliary carbon source CH ₄ in addition to the mixed gas of Ar and H ₂ . After the reaction is completed, it is a rapid cooling stage. gas. We have successfully prepared nitrogen-doped graphene by this method.

Figure 3 shows an example of nitrogen-doped graphene produced in the experimental process after annealing the copper foil as shown in Figure 2 . In this example, the carrier gas flow ratio is Ar/H ₂ =50/1, and the reaction temperature is selected as 950°C. It can be seen from the results that under this condition, there is better continuity, and the Raman 2D peak intensity is better, so nitrogen-doped graphene with better quality is produced.

Shown in Figure 4 is the XPS test of generating nitrogen-doped graphene as shown in Figure 3 . The test results show that the characteristic peak of N1s appears at the position of binding energy of 399.34eV, which fully shows that nitrogen-doped graphene can be successfully synthesized by this technology.

FIG. 5 shows an example of an annealing group as shown in FIG. 2 . This example includes the effect of nitrogen-doped graphene at different reaction temperatures. Among them, the flow rate of Ar in group ae is 400-600 sccm, the flow rate of H ₂ is 20 sccm, and the reaction temperatures are 500, 800, 900, 950, and 1000°C respectively. It can be seen from the observation of the Raman spectrum that due to the 2D peak (near 2700cm ^-1 ) It is strongest at 1000°C, has a weaker 2D peak at 950°C, and no 2D peak appears below 900°C. Therefore, nitrogen-doped graphene with low crystallinity can be grown below 900 °C. Compared with the optical microscope images, the continuity is poor when the reaction temperature is 1000°C, while the continuity is better at 950°C. Therefore, our technology can obtain nitrogen-doped graphene with different crystallinity through temperature regulation.

Figure 6 shows an example of the situation without annealing shown in Figure 2 . This example includes the influence on the quality of synthesized nitrogen-doped graphene under different carrier gas flow ratio conditions. Among them, the Ar flow rate in the ad diagrams is 400-600 sccm, and the H ₂ is 50, 20, 15, and 10 sccm respectively. After the reaction, the samples were tested by optical microscopy and Raman spectroscopy, and the graphene transfer process was carried out after the graphene was transferred to the SiO ₂ (thickness 300nm)/Si surface by a wet transfer method. From the optical microscope image we can see that the _H2 flow at 20 sccm produces a film of relatively good quality, and the continuity of the film becomes poor when the _H2 is changed. Observing the Raman spectrum, when _H2 changes from 50sccm to 10sccm, it has a 2D peak (near 2700cm ^-1 ), and the generated carbon film has a certain D peak (near 1350cm ^-1 ), which proves that the doped Nitrogen graphene (the standard form of graphene will produce a D peak of the degree of reaction defects as N atoms enter the graphene lattice). Moreover, as can be seen from the color contrast of the optical microscope, the thickness and continuity of the resulting film are greatly affected by the hydrogen flow rate. Therefore, our technology can obtain nitrogen-doped graphene with different thicknesses through the regulation of reaction conditions such as hydrogen, and it is even expected to synthesize regularly distributed nitrogen-doped graphene quantum dots.

The above examples and characterization results prove that the method we invented can use melamine as a solid nitrogen source at a relatively low temperature, and the reaction can generate nitrogen-doped graphene. Due to the use of CVD technology, this technology can easily realize the doping amount and doping type of nitrogen in graphene, as well as the thickness, Control of structural features such as degree of crystallinity.

Claims (10)

1. A method for synthesizing nitrogen-doped graphene with melamine as a nitrogen source is characterized in that, under a certain reaction pressure, no other catalysts other than the metal substrate are used, no other heating equipment other than the main heating equipment is needed, and at a certain composition Under the protection of the carrier gas with high flow rate, let the metal substrate heat up slowly, and then introduce the nitrogen source and carbon source outside the heating zone into the reaction tube furnace at one time after reaching the reaction temperature. The decomposition products of melamine and the carbon source are introduced into the surface of the metal substrate to react with nitrogen-doped graphene. After the reaction is completed, the temperature is rapidly lowered to room temperature. At this time, a layer of nitrogen-doped graphene has been formed on the surface of the metal substrate. 2. The certain reaction pressure according to claim 1 comprises reaction pressures such as normal pressure and vacuum used in the chemical vapor deposition method. 3. metal substrate according to claim 1 comprises the used metal film, metal foil, metal flake of chemical vapor deposition method synthetic graphene. 4. other catalyzers except the metal base according to claim 1, comprise any other form except the necessary metal base of chemical vapor deposition synthesis nitrogen-doped graphene can make the organic or inorganic matter of melamine activation, or industrial and artificial Prepare the catalyst. 5. Other heating equipments other than the main heating equipment according to claim 1, comprising heating bands other than the equipment providing the necessary temperature for chemical vapor deposition, other temperature zones other than the metal base placed in the multi-temperature zone tube furnace, heating Covers and any other equipment that can provide heating functions. 6. temperature of reaction according to claim 1 comprises enough to make melamine decompose and reduce and generate the temperature value in the temperature interval of nitrogen-doped graphene or amorphous carbon, and make the product and carbon source after melamine thermal decomposition generate on metal catalyst surface Temperature values in the temperature range of nitrogen-doped graphene. 7. The composition and flow rate of carrier gas according to claim 1, composition comprises reaction gas, reducing gas, carrier gas or several mixtures, flow rate comprises the gas that the carbon product that low temperature generation can be successfully converted into nitrogen-doped graphene Matching. 8. The cooling according to claim 1 includes fast and slow cooling at a certain rate, and a method of program-controlled cooling at different rates at different times. 9. taking melamine according to claim 1 as the specific method of nitrogen source chemical vapor deposition synthesis nitrogen-doped graphene comprises the steps: 1) Place the cleaned metal catalytic substrate in the reaction chamber of the heating equipment, and feed the carrier gas. 2) Utilize the heating equipment to raise the temperature to the temperature interval described in claim 6, while keeping the carrier gas passing through, pass through melamine, _CH4 and reducing gas and keep it for a period of time at this temperature, and use it on the metal catalyst substrate Nitrogen-containing species after thermal decomposition of melamine react to form carbon nitrogen compounds. 3) In the temperature interval described in right 6, keep for a period of time to fully react. 4) Use a certain cooling method to reduce the temperature of the metal catalytic substrate to normal temperature as required. 10. The carrier gas according to claim 1 comprises: reducing carrier gas such as hydrogen, ammonia and other reducing gases; inert gas such as argon, nitrogen, neon and other stable gases; reducing carrier gas Mixed gas with inert carrier gas.