Preparation method of three-dimensional graphene

Abstract

The invention relates to a preparation method of three-dimensional graphene, and belongs to the technical field of nano materials. The preparation method comprises the following steps: firstly, mixing metal nitride and metal amino compound in an inert atmosphere, and filling a stainless steel tube with the mixture; then putting into a stainless steel ultrahigh vacuum reactor, and vacuumizing at room temperature; the reactor is heated by a heating wire to form metal nitrogen hydride. And then carrying out chemical vapor deposition reaction on the obtained metal nitrogen hydride and carbon monoxide gas to obtain a solid product, and carrying out acid pickling and centrifugation to obtain the three-dimensional graphene. The invention provides a new way for preparing the semi-hydrogenated graphene, which possibly plays an important role in catalysis, semiconductors and other applications.

Description

Technical Field

[0001] The present invention relates to the technical field of nanomaterials, and in particular to a method for preparing three-dimensional graphene. Background Art

[0002] Materials composed of light elements are of great importance due to their diverse physical and chemical properties, resulting from their short atomic distances. Carbon is the most representative of these elements, as its compounds have been extensively discovered and developed, such as carbon nanotubes, fullerenes, graphene, diamond, and diamond-like materials. Graphene, a single layer of carbon atoms arranged in a honeycomb network, has garnered widespread attention for its unique physical properties, including large surface area, superior electrical conductivity, excellent mechanical flexibility, and high thermochemical stability. Modulating graphene's electronic and crystalline structure is crucial for its chemical functionalization, particularly for applications in nanoelectronics. Hydrogen can alter the hardness, chemical inertness, and optical properties of amorphous carbon. Therefore, the preparation and industrialization of hydrogenated graphene, ranging from depleted to semi-hydrogenated to fully hydrogenated, is crucial.

[0003] The formation of fully hydrogenated graphene is called graphane. It was first studied theoretically and synthesized experimentally, proving that graphene can react with hydrogen atoms and that the hydrogenation of graphene is a reversible process. However, no atoms or molecules can penetrate into perfect graphene, so the hydrogenation of graphene can only occur on the top basal plane of graphene. Furthermore, single-sided hydrogenation of graphene is thermodynamically stable at low hydrogen coverage. In addition, DFT calculations show that half-hydrogenated graphene sheets (C2H) are stable at room temperature. Since half-hydrogenated graphene has important applications in the field of ferromagnetic semiconductors, it is urgent to explore a simple and fast method for the direct synthesis of half-hydrogenated graphene. So far, the only method mentioned above to synthesize half-hydrogenated graphene is to remove single-sided hydrogen from graphane.

[0004] Patent US9853104 discloses that hydrogenated graphene exhibits strong temperature dependence of semiconductor materials.

[0005] Patent CN103864046 uses a fluorinated graphite dispersion prepared by suspending fluorinated graphite in liquid amine, which is then added dropwise to a mixed solution of potassium-sodium alloy and anhydrous liquid amine. After 1-15 hours, a fatty alcohol is added to quench the reaction and produce hydrogenated graphene. However, this method has a long reaction time, multiple steps, and a complex process.

[0006] Patent US9853104 describes a method for hydrogenating graphene by preparing graphene sheets or thin films on silicon dioxide or silicon wafers using chemical vapor deposition (CVD). This method involves exposing the graphene sheets or CVD-grown graphene to hydrogen plasma. This method requires a complex process involving graphene preparation followed by hydrogenation.

[0007] Currently, most methods for producing semi-hydrogenated or partially hydrogenated graphene involve first preparing graphene and then partially hydrogenating it using hydrogen plasma under an electric field. However, this requires a two-step process, which reduces production efficiency, increases manufacturing costs, and makes it difficult to control the hydrogen content in the graphene. To date, no method has been disclosed for producing semi-hydrogenated graphene in a single step. Summary of the Invention

[0008] To address the above technical issues, the present invention provides a method for preparing three-dimensional graphene. This method utilizes chemical vapor deposition (CVD) to prepare three-dimensional graphene in a single step. This method involves a CVD reaction of carbon monoxide gas with a metal hydrocarbon hydride, where the heat released by the reaction creates a CVD atmosphere to produce semi-hydrogenated graphene and partially hydrogenated graphene.

[0009] The present invention utilizes conventional chemical vapor deposition (CVD) technology to produce unilateral hydrogenated graphene, a stable three-dimensional structure in which sp2 and sp3 hybridizations form carbon-carbon single bonds through hydrogenation. This new solid-state nanomaterial, based on a diamond-like carbon (DLC) framework, is produced using chemical vapor deposition (CVD). Chemical vapor deposition (CVD) involves activating reactive gases with external energy, promoting chemical reactions and deposition of the reactants on the substrate surface.

[0010] The present invention is achieved through the following technical solutions:

[0011] The object of the present invention is to provide a method for preparing three-dimensional graphene, comprising the following steps:

[0012] (1) preparing metal nitrogen hydrides;

[0013] (2) subjecting the metal nitrogen hydride obtained in step (1) to a chemical vapor deposition reaction with carbon monoxide gas to obtain a solid product, which is then acid-washed and centrifuged to obtain the three-dimensional graphene.

[0014] In one embodiment of the present invention, the three-dimensional graphene includes semi-hydrogenated graphene and partially hydrogenated graphene.

[0015] In one embodiment of the present invention, the metal nitrogen hydride is selected from one or more of Li2NH, Mg2NH2 and Ca2NH2.

[0016] In one embodiment of the present invention, the metal nitrogen hydride is obtained by the following preparation method:

[0017] The metal nitride and the metal amino compound are mixed under an inert atmosphere and placed in a stainless steel tube; then placed in a stainless steel ultra-high vacuum reactor and evacuated at room temperature; the reactor is heated by an electric heating wire to form a metal nitrogen hydride.

[0018] In one embodiment of the present invention, the metal nitride is selected from one or more of lithium nitride, magnesium nitride and calcium nitride.

[0019] In one embodiment of the present invention, the metal amide compound is selected from one or more of lithium amide, magnesium amide and calcium amide.

[0020] In one embodiment of the present invention, the purity of the metal nitride is >99.9%;

[0021] And / or, the purity of the metal amide compound is ≥95%.

[0022] In one embodiment of the present invention, the molar ratio of the metal nitride to the metal amide is 1:3-3:1.

[0023] In one embodiment of the present invention, the heating condition is: heating at 350° C.-450° C. for 1 h-5 h, preferably 2 h.

[0024] In one embodiment of the present invention, the pressure of the chemical vapor deposition reaction is 150 psi-250 psi.

[0025] In one embodiment of the present invention, the temperature of the chemical vapor deposition reaction is 350°C-450°C.

[0026] In one embodiment of the present invention, the chemical vapor deposition reaction time is 10 min-3 h, preferably 1 h.

[0027] In one embodiment of the present invention, the pickling is performed by using 36.5 wt % hydrochloric acid.

[0028] The above technical solution of the present invention has the following advantages over the prior art:

[0029] (1) The present invention provides a method for preparing three-dimensional graphene, wherein semi-hydrogenated graphene and partially hydrogenated graphene are prepared by chemical vapor deposition of carbon monoxide and metal hydrocarbon hydrides. The chemical vapor deposition reaction product is in the form of a solid mixture. After acid washing and centrifugation, the obtained three-dimensional graphene is separated from other products, which are mainly metal cyanides, metal oxides, and metal carbonates. The reaction product is confirmed to be semi-hydrogenated graphene (C2H) by XRD and elemental analysis.

[0030] (2) The present invention confirmed the synthesis of semi-hydrogenated graphene by the reaction of carbon monoxide with lithium imide based on thermodynamic predictions by volumetric method, temperature spectrum, X-ray diffraction and gas chromatography. The synthesized graphene was obtained after treatment with hydrochloric acid and characterized by various techniques such as X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Raman and FTIR spectroscopy, carbon, hydrogen and oxygen elemental analysis, mass spectrometry and gas chromatography. Elemental analysis found that the carbon-hydrogen ratio of the sample obtained at 350°C was 2:1, which is consistent with its formula C2H. The effect of hydrogen on graphene was studied by FTIR and Raman spectroscopy. On the other hand, the thermal stability of semi-hydrogenated graphene was detected by mass spectrometry and gas chromatography, and it was found that hydrogen was released at around 450°C. It can be seen that the present invention provides a new way to produce semi-hydrogenated graphene, which may play an important role in applications such as catalysis and semiconductors. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0032] Figure 1 Schematic diagram of the vacuum deposition apparatus of the present invention: Description of the accompanying drawings: 1, mechanical pump; 2, diffusion pump; 3, carbon monoxide; 4, argon; 5, 6 and 10 are valves; 7, pressure gauge; 8, reaction tube; 9, electric heating furnace; 11, thermocouple; 12, temperature monitor;

[0033] Figure 2 The temperature distribution of the solid reactant bed when lithium imide prepared from a mixture of lithium nitride and lithium amide in a molar ratio of 2:1 reacts with carbon monoxide in the present invention; wherein (a) 350°C; (b) 400°C; (c) 450°C;

[0034] Figure 3 The change in the conversion of CO with reaction time when lithium imide prepared from a mixture of lithium nitride and lithium amide having a molar ratio of 3:1, 2:1, 1:1, 1:2 and 1:3 is reacted with carbon monoxide at 350°C for 1 hour;

[0035] Figure 4 The X-ray diffraction patterns of lithium imide prepared by mixing lithium nitride and lithium amide at different molar ratios before carbon monoxide is introduced into the reaction chamber in the present invention are as follows: (a) 1:3; (b) 1:2; (c) 1:1; (d) 2:1; (e) 3:1;

[0036] Figure 5The X-ray diffraction patterns of the lithium imide products prepared by reacting carbon monoxide with lithium nitride and lithium amide at different molar ratios at 350°C for 1 hour are shown in the following figure: (a) 1:3; (b) 1:2; (c) 1:1; (d) 2:1; (e) 3:1;

[0037] Figure 6 The X-ray diffraction patterns of the lithium imide products prepared by the reaction of carbon monoxide with lithium nitride and lithium amide at different molar ratios at 400°C for 1 hour are (a) 1:3; (b) 1:2; (c) 1:1; (d) 2:1; (e) 3:1;

[0038] Figure 7 The X-ray diffraction patterns of the lithium imide products prepared by reacting carbon monoxide with lithium nitride and lithium amide at different molar ratios at 450°C for 1 hour are (a) 1:3; (b) 1:2; (c) 1:1; (d) 2:1; (e) 3:1;

[0039] Figure 8 The present invention is an X-ray diffraction pattern of the solid residue after acid washing of the reaction product of lithium imide prepared by carbon monoxide with lithium nitride and lithium amide at different molar ratios at 350°C for 1 hour; wherein, (a) 1:3; (b) 1:2; (c) 1:1; (d) 2:1; (e) 3:1;

[0040] Figure 9 TEM image (A), electron diffraction pattern (B) of HG-2-350, TEM image (C) of HG-2-400, and TEM image (D) of HG-2-450 in the present invention;

[0041] Figure 10 These are the SEM images and EDS results of HG-2-350 in the present invention;

[0042] Figure 11 The Raman spectra of the solid material remaining after the reaction of lithium nitride and lithium amide at molar ratios of 1:1, 2:1 and 3:1 at 350° C. for 1 hour and the treatment of carbon monoxide and lithium amide with HCl in the present invention;

[0043] Figure 12 FTIR spectra of the products obtained under different conditions in the present invention; (a) HG-2-350; (b) HG-2-400; (c) HG-2-450;

[0044] Figure 13 is the relationship between the G position and the I(D) / I(G) ratio and the hydrogen content in the present invention;

[0045] Figure 14This is a mass spectrometry analysis of the thermal decomposition of HG-2-350 in the present invention;

[0046] Figure 15 This is a gas chromatogram of thermal decomposition of graphene after carbon monoxide reacts with lithium imide at 350° C. in a mixture of lithium nitride and lithium amide at different molar ratios for 1 hour. DETAILED DESCRIPTION

[0047] The present invention will be further described below with reference to the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. However, the embodiments are not intended to limit the present invention.

[0048] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials and reagents used are all commercially available unless otherwise specified.

[0049] The present invention proposes a method for preparing semi-hydrogenated graphene and LiC2N2 by exothermic reaction of carbon monoxide with lithium imide. This reaction is based on the following thermodynamic prediction: the changes in the Gibbs free energy and enthalpy of the reaction are very negative, indicating that they are thermodynamically favorable exothermic reactions. Therefore, the present invention conducts research in the following three aspects: (1) exploring the thermodynamic feasibility of synthesizing semi-hydrogenated graphene and lithium cyanide from carbon monoxide; (2) synthesizing graphene and lithium cyanide using carbon monoxide and lithium imide as raw materials; and (3) characterizing the resulting material to verify that it is semi-hydrogenated graphene.

[0050] Under certain conditions, a chemical reaction is driven by the difference in Gibbs free energy between products and reactants, as shown in the following equation:

[0051] ΔG=ΔH-T×ΔS

[0052] Where ΔH is the change in enthalpy, ΔS is the change in entropy, and T is the temperature. A negative change in Gibbs free energy indicates that the reaction is thermodynamically favorable. Furthermore, a negative change in enthalpy corresponds to an exothermic reaction. This criterion can determine whether a chemical reaction is thermodynamically feasible and exothermic.

[0053] The synthesis of single-sided hydrogenated graphene (C2H) from carbon monoxide requires consideration of hydrogen-containing materials, including gases such as water vapor and ammonia, and solids such as lithium imide. Water vapor and ammonia are used as hydrogen sources instead of hydrogen because studies have shown that the hydrogenation of graphene is produced by the H ions generated by the fragmentation of water vapor and ammonia. + and H radicals, rather than reactions with hydrogen. Their thermodynamic data are shown in Table 1:

[0054] Table 1 Thermodynamic data

[0055]

[0056] Therefore, if carbon monoxide and water vapor can generate graphene, the following reaction should occur: 2CO (g) +0.5H2O (g) =C2H (s) +1.25O 2(g)

[0057] Thermodynamic calculations show that the reaction has a positive Gibbs free energy change (ΔG° = 546.2 kJ / mol). Therefore, the reaction is thermodynamically unfavorable. The process of carbon monoxide and ammonia forming graphene can be written as:

[0058] 2CO (g) +1 / 3NH 3(g) =C2H (s) +1 / 6N 2(g) +O 2(g)

[0059] However, the Gibbs free energy of this reaction also has a positive change (ΔG° = 437.4 kJ / mol). This indicates that ammonia cannot be used to generate graphene from carbon monoxide, so lithium imide can be used as a hydrogen source because it is a very attractive compound for hydrogen storage. If lithium imide is used as a hydrogen source for the reaction of carbon monoxide to generate graphene, the reaction equation should be expressed as:

[0060] 2CO (g) +2Li2NH (s) =C2H (s) +N 2(g) +0.5H 2(g) +2Li2O (s)

[0061] The reaction is thermodynamically favorable due to the negative change in Gibbs free energy (ΔG° = -350.6 kJ / mol). Furthermore, the reaction between carbon monoxide and lithium imide can also produce lithium cyanide due to the negative change in the Gibbs free energy of the reaction (ΔG° = -350.1 kJ / mol):

[0062] CO (g) +2Li2NH (s) =Li2CN 2(s) +Li2O (s) +H 2(g)

[0063] Therefore, the above thermodynamic calculations predict that the reaction of carbon monoxide with lithium imide will be a new method to produce hydrogenated graphene and lithium cyanide.

[0064] Example 1

[0065] In order to verify the thermodynamic analysis of the synthesis of semi-hydrogenated graphene by carbon monoxide and lithium imide, this embodiment carried out a method for preparing semi-hydrogenated graphene by the reaction of carbon monoxide and lithium imide in an ultra-high vacuum closed system; the schematic diagram of the vacuum deposition equipment used is shown in FIG. Figure 1 The specific preparation method is as follows:

[0066] Lithium nitride (Li3N, >99.9% purity) and lithium amide (LiNH2, 95% purity) were prepared by mixing 0.3g of lithium nitride and lithium amide powders at molar ratios of 1:3, 1:2, 1:1, 2:1, and 3:1 in an argon-filled glove box. The mixture was then placed into a stainless steel tube with an outer diameter of 6.35mm and a length of 3.5cm. The tube was then placed into a stainless steel ultra-high vacuum reactor with an outer diameter of 28.6mm and a length of 65cm. The reactor was evacuated at room temperature for 1 hour. The reactor was then heated at 350°C for 2 hours using a heating wire to form lithium imide.

[0067] Finally, at a reactor temperature of 350°C, carbon monoxide gas (ultra-high purity 99.9%) was injected into the reactor at 180 psi, causing the carbon monoxide and lithium imide to immediately react. The carbon monoxide pressure change was monitored using a digital pressure gauge. After 1 hour of reaction, a solid product, including crystalline and amorphous materials, was obtained. Furthermore, to remove the crystalline products (Li2O, LiOH, Li2CO3, and Li2CN2), the solid product was treated with 36.5% wt% hydrochloric acid (HCl) and then washed with pure water. The resulting solid was separated from the water by centrifuge and then dried overnight at approximately 80°C. The dried solid materials were labeled HG-nT (where n is the molar ratio of lithium nitride to lithium amide and T represents the reactor temperature), namely HG-0.3-350, HG-0.5-350, HG-1-350, HG-2-350, and HG-3-350.

[0068] Example 2

[0069] This example provides a method for preparing semi-hydrogenated graphene by reacting carbon monoxide with lithium imide. The method is similar to Example 1, differing only in that heating with an electric heating wire at 400°C yields solid materials HG-0.3-400, HG-0.5-400, HG-1-400, HG-2-400, and HG-3-400. To facilitate comparison of the effects at different temperatures, sample HG-2-400, with a molar ratio of 2:1, was used for analysis.

[0070] Example 3

[0071] This example provides a method for preparing semi-hydrogenated graphene by reacting carbon monoxide with lithium imide. The method is similar to Example 1, differing only in that heating is performed at 450°C using a heating wire to obtain solid materials HG-0.3-450, HG-0.5-450, HG-1-450, HG-2-450, and HG-3-450. To facilitate comparison of the effects at different temperatures, HG-2-450, a sample with a 2:1 molar ratio, was used for analysis.

[0072] Test Case

[0073] (1) Figure 2 As shown in the figure, the temperature changes of the solid reactant bed during the reaction were monitored using thermocouples located in a mixture of lithium nitride and lithium amide in a 2:1 molar ratio. When CO enters the reactor at 350°C (a), the bed temperature immediately jumps to nearly 500°C. Furthermore, when the reactor temperature is 450°C (c), the bed temperature can reach as high as 625°C. After 10 minutes, the transition temperature drops back to the reactor's initial temperature. Note that there is still a slight temperature drop, which may be due to carbon deposits on the iron reactor. This temperature curve confirms that carbon monoxide can immediately ignite the reaction at temperatures of 350°C or higher.

[0074] (2) When lithium imide prepared from a mixture of lithium nitride and lithium amide with a molar ratio of 3:1, 2:1, 1:1, 1:2 and 1:3 was reacted with carbon monoxide at 350°C for 1 hour, the CO conversion rate changed with the reaction time as shown in the following figure: Figure 3 As shown in the figure, the CO reaction of carbon monoxide and lithium imide at different molar ratios at a reactor temperature of 350°C changes with time. It can be seen that when CO is introduced, the reaction can start immediately, and as the reactor temperature increases ( Figure 3 ), the amount of CO consumed increases. In the first minute, almost 0.2 g of CO is consumed to form other compounds. At this temperature, the CO consumption rate appears to be the same for all molar ratios.

[0075] (3) Before the introduction of carbon monoxide, the compounds obtained by heating the mixture of lithium nitride and lithium amide in different molar ratios were subjected to X-ray diffraction analysis, such as Figure 4 As shown. When the molar ratio of lithium nitride to lithium amide is 2:1 or 3:1, some lithium nitride is still detected in the compound. It should be noted that due to partial oxidation of the compound during heating, some lithium oxide is also detected by X-ray diffraction. After 1 hour of reaction, a dark brown solid product is obtained, which is crushed and then subjected to X-ray diffraction analysis. Typical spectrum ( Figure 5 、 Figure 6 and Figure 7) shows the XRD spectra of the products after the reaction of CO with lithium imide at 350℃, 400℃ and 450℃ for 1h. The results show that the peaks of lithium nitride or lithium amide all disappeared, while many new peaks appeared after the reaction. This shows that all lithium nitride and lithium amide were converted into other compounds by reacting with CO. In addition, a careful analysis of the peaks in the XRD spectrum ( Figure 5 ), peaks belonging to lithium cyanide are found at 26.2°, 34.4°, 40.3°, 49.4°, and 56.8°. No graphene peaks are found. This is reasonable, as hydrogenation may amorphize graphene, and graphene should have a small, broadened diffraction peak at 26.5°. The other main component in the product is lithium oxide. The mole fractions of each component were obtained through analysis, as shown in Table 2:

[0076] Table 2 Mole fractions of components in solid products

[0077]

[0078] By comparing the reactions with the same molar ratio of 2:1 at different temperatures, it was found that the higher the reaction starting temperature, the less lithium oxide was produced, and the corresponding lithium cyanide and lithium carbonate contents increased.

[0079] Regarding the gas phase at the end of the reaction, as predicted by thermodynamic analysis, nitrogen and hydrogen had already formed. Therefore, the gas phase was introduced into a GC using a Parapak Q column, which detected the two new gases formed during the reaction: hydrogen and methane. Their molar ratios in the gas phase at the end of the reaction are shown in Table 3:

[0080] Table 3 Reaction gas molar ratio calculated by GC

[0081]

[0082]

[0083] The molar ratio of hydrogen decreased as the amount of lithium amide in the original mixture decreased. Therefore, the detected hydrogen gas was consistent with the prediction. However, there was no evidence of nitrogen gas generation during the reaction. This is primarily because nitrogen atoms may be induced to form graphene sheets, as confirmed by elemental analysis described later.

[0084] (4) So far, both the gas phase and solid phase of the reaction of carbon monoxide with lithium imide have confirmed the feasibility of the predicted reaction. Whether this reaction can produce semi-hydrogenated graphene will be tested by the following different techniques. In order to find amorphous semi-hydrogenated graphene, aqueous hydrochloric acid solution is used to dissolve all the crystalline components of the product (Li2CN2, Li2O, LiOH, Li2CO3) at room temperature, and then solid-liquid centrifugation is performed (including pure water washing). After treatment, about 3-8wt% of solid matter remains, depending on the molar ratio of lithium nitride to lithium amide. The remaining solid material is further subjected to X-ray diffraction measurement. X-ray diffraction spectra of the solid obtained by reacting carbon monoxide with lithium imide at different temperatures for 1h ( Figure 8 ) shows that there is only a very broad peak at around 25°, which is a typical feature of graphite-like amorphous layer structure.

[0085] (5) The present invention further performs elemental analysis on the amorphous solid to provide the C / H / N / O weight ratio, as shown in Table 4:

[0086] Table 4 Elemental analysis results of hydrochloric acid treated amorphous solid product.

[0087] sample Cwt% Hwt% Nwt% 0wt% Molecular formula HG-0.3-350 76.49 3.63 4.02 14.83 <![CDATA[CN 0.045 H 0.569 O 0.145 ]]> HG-0.5-350 78.00 3.57 3.72 14.00 <![CDATA[CN 0.041 H 0.549 O 0.135 ]]> HG-1-350 79.66 3.74 3.05 12.31 <![CDATA[CN 0.033 H 0.563 O 0.116 ]]> HG-3-350 80.84 3.39 2.55 12.03 <![CDATA[CN 0.027 H 0.503 O 0.112 ]]> HG-2-350 80.62 3.32 2.62 12.87 <![CDATA[CN 0.028 H 0.494 O 0.120 ]]> HG-2-400 79.23 2.86 3.27 13.11 <![CDATA[CN 0.035 H 0.433 O 0.145 ]]> HG-2-450 81.60 2.12 4.70 10.19 <![CDATA[CN 0.049 H 0.311 O 0.094 ]]>

[0088] It can be concluded that the amorphous product is nitrogen-doped graphene because the carbon-hydrogen molar ratio of the graphene obtained by the reaction of carbon monoxide with a mixture of lithium nitride and lithium amide in molar ratios of 2:1 and 3:1 at 350°C is 2:1, which is consistent with its formula C2H. From a thermodynamic point of view, the Gibbs formation energy of graphene sheets is negative (-18.9 kJ / mol), while the Gibbs formation energy of graphene is positive (157.4 kJ / mol), which shows that graphene sheets are more stable than graphene, so the obtained sample should be a graphene sheet. However, from a kinetic point of view, according to previously reported theoretical work, the nucleation barrier for the formation of graphene (fully hydrogenated graphene) is larger than the nucleation barrier for the formation of graphene (semi-hydrogenated graphene) because the nucleation energy increases with the increase of the amount of hydrogen. Therefore, even if graphene is more stable, graphene sheets are easier to form. TEM image of HG-2-350 ( Figure 9 A) can be observed semi-hydrogenated graphene. Electron diffraction shows that the obtained semi-hydrogenated graphene has an amorphous structure. In addition, by comparing the different reaction starting temperatures at a molar ratio of 2:1, it is found that the higher the reaction starting temperature, the lower the hydrogen content in the hydrogenated graphene, indicating that the increase in reaction temperature leads to hydrogen loss and graphene stacking. This stacking can be seen from the TEM images of HG-2-400 and HG-2-450 ( Figure 9 C and D) are confirmed.

[0089] Figure 10Representative SEM images of graphene obtained by reacting lithium nitride with lithium amide in a 2:1 molar ratio at 350°C for 1 hour, followed by carbon monoxide and lithium imide. The graphene's morphology supports the conclusions drawn from the TEM images. The corresponding EDS results are consistent with elemental analysis, indicating that carbon is the primary component of the graphene, but the nitrogen content is too low to be detected.

[0090] Considering its local structure, the hydrogenation of graphene changes the bond structure of graphene compounds because carbon has the ability to adapt to different possible states of sp1, sp2 and sp3 hybridization. Sp2-dominated graphite compounds have different degrees of aromatic clusters and different ordered structures within the graphene plane. A typical Raman spectrum of hydrogenated graphene is shown in Figure 1. Figure 11 As shown, all samples can be seen at 1570 cm -1 ~1580cm -1 There are strong G lines on the left and right, at 1350cm -1 There are strong D lines on the left and right, which are similar to the Raman spectrum of graphite carbon, corresponding to the E and C sp2 atoms respectively. 2g Phonon stretching modes and a 1g Symmetric k-point phonon breathing modes. The latter mode is only observed in the presence of disorder, which means that hydrogenation of the graphene network increases the degree of disorder. In addition, at 700 cm -1 No out-of-plane CC vibrations were found at 2200 cm-1, which may be due to the low nitrogen content. -1 The band at the bottom is attributed to the C≡N stretching vibration, which is caused by the low nitrogen content. The corresponding FTIR spectrum is as follows Figure 12 Obviously, E g The symmetric pattern makes Raman spectroscopy infrared active. The main difference between FTIR spectroscopy and Raman spectroscopy is the overlap of the G and D bands. Another difference is that the 2200 cm -1 It can be seen that as the reaction temperature decreases, this band becomes more intense, resulting in a higher hydrogen content. In addition, in FTIR, 2900cm -1 and 3500cm -1 The bands are always attributed to CH or NH vibrations. When the reactor temperature is increased to 450 °C, these bands are not observed, which is consistent with the elemental analysis results that higher temperatures reduce the hydrogen content.

[0091] It is worth noting that hydrogenation of the carbon grid leads to a series of defects: bond angle disorder, long string disorder, and hybridization. Due to the relatively low concentration of nitrogen, the amount of hydrogen will cause changes in the G and D peaks. One of the changes is the position of the G mode, which is at 1500 cm -1 -1600cm -1The G mode intensity ratio (I(D) / I(G)) varies between 0.1 and 0.1, and is commonly used to identify sp3 bonding sites because the G mode decreases with increasing sp3 bonding. Furthermore, the intensity ratio of the D and G modes, I(D) / I(G), is relative to the grain size. The location of the G mode and I(D) / I(G) can be correlated with the hydrogen content. Figure 13 The graph shows how the G position and I(D) / I(G) ratio change with hydrogen content. Note that the G position increases with decreasing hydrogen content, which is consistent with the fact that hydrogenation increases the sp3 bond content. The observation that the I(D) / I(G) ratio is almost independent of the H content indicates that hydrogenated graphene is amorphous and therefore hydrogenation does not affect the I(D) / I(G) ratio.

[0092] (6) In order to study the thermal stability of semi-hydrogenated graphene, the TPMS of the gas products during the decomposition of graphene from room temperature to 800℃ was recorded at a heating rate of 10℃ / min. Taking HG-2-350 as an example, the typical characteristics of graphene thermal decomposition are as follows: Figure 14 As shown. The fragments after decomposition are H2O, CO, CO2 and C=NH. However, the signals of these peaks are very small, which can be attributed to the low content of nitrogen and oxygen in the sample. Note the presence of C=NH, which, from the perspective of the termination group > C=NH, indicates that the N atom may terminate the aromatic ring. Except for HO, CO, CO and C=NH, no hydrogen that may be produced during the decomposition of graphene was detected. Based on the results of TPMS and elemental analysis, the formation of hydrogen can be reasonably predicted. To confirm this, gas chromatography was used for verification because it can detect all gases produced during the thermal decomposition of graphene. The gas chromatograph recorded the thermal decomposition process of graphene from room temperature to 900℃ with a heating rate of 10℃ / min, and then kept at 900℃ for 1h. A typical spectrum is shown below. Figure 15 As shown in the figure, the first peak belongs to absorbed water, and the other very large peak starting at 450°C is hydrogen released during thermal decomposition, which was confirmed by Parapak Q column gas chromatography. This decomposition curve also confirms that the sample is not hydrogenated amorphous carbon (HAC), but hydrogenated graphene, because HAC releases hydrogen when heated at 600K (230°C), while hydrogenated graphene dehydrogenates at around 450°C, which is consistent with the results of the present invention.

[0093] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for preparing three-dimensional graphene, characterized in that: The following steps are involved: (1) preparing metal nitrogen hydrides; (2) subjecting the metal nitrogen hydride obtained in step (1) to a chemical vapor deposition reaction with carbon monoxide gas to obtain a solid product, which is then acid-washed and centrifuged to obtain the three-dimensional graphene.

2. The preparation method according to claim 1, characterized in that The metal nitrogen hydride is selected from one or more of Li2NH, Mg2NH2 and Ca2NH2.

3. The preparation method according to claim 2, characterized in that The metal nitrogen hydride is obtained by the following preparation method: The metal nitride and the metal amino compound are mixed under an inert atmosphere and placed in a stainless steel tube; then placed in a stainless steel ultra-high vacuum reactor and evacuated at room temperature; the reactor is heated by an electric heating wire to form a metal nitrogen hydride.

4. The preparation method according to claim 3, characterized in that The metal nitride is selected from one or more of lithium nitride, magnesium nitride and calcium nitride.

5. The preparation method according to claim 3, characterized in that The metal amide compound is selected from one or more of lithium amide, magnesium amide and calcium amide.

6. The preparation method according to claim 3, characterized in that The purity of the metal nitride is greater than 99.9%; And / or, the purity of the metal amide compound is ≥95%.

7. The preparation method according to claim 3, characterized in that The molar ratio of the metal nitride to the metal amide is 1:3-3:

1.

8. The preparation method according to claim 3, characterized in that The heating conditions are: heating at 350° C.-450° C. for 1 h-5 h.

9. The preparation method according to claim 1, characterized in that The pressure of the chemical vapor deposition reaction is 150 psi-250 psi.

10. The preparation method according to claim 1, characterized in that The temperature of the chemical vapor deposition reaction is 350°C-450°C; And / or, the chemical vapor deposition reaction time is 10 minutes to 3 hours.

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