KR102335973B1 - Process for Preparing Heteroatom-Doped Graphene - Google Patents

Abstract

The present invention relates to a method for preparing graphene doped with heteroatoms. More specifically, the present invention provides a method comprising the steps of: (i) heating a mixture of a heteroatom precursor and an alkali metal source under an inert atmosphere; and (ii) separating the graphene doped with a heteroatom, wherein the heteroatom precursor does not include a hydroxyl group. It's about use.

Description

Method for preparing heteroatom-doped graphene {Process for Preparing Heteroatom-Doped Graphene}

The present invention relates to a method for preparing graphene doped with heteroatoms. More specifically, the present invention comprises the step of heating a mixture of a heteroatom precursor and an alkali metal source under an inert atmosphere, wherein the heteroatom precursor does not contain a hydroxyl group, graphene doped with a heteroatom It relates to a method for manufacturing a fin and to the use of the graphene thus produced.

Graphene has attracted great attention in various research fields due to its unique structure and protruding properties (Novoselov, K. S. et al. A roadmap for graphene. Nature 490, 192.200 (2012)).

For example, using graphene with a two-dimensional structure in electronic devices (Wang, H. &Dai, H. Strongly coupled inorganic-nano-carbon hybrid materials for energy storage. Chem. Soc. Rev. 42, 3088.3113 (2013)), nano Medicines (Mao, HY et al. Graphene: Promises, Facts, Opportunities, and Challenges in Nanomedicine. Chem. Rev. 113, 3407.3424 (2013)), sensors (Bo, Z. et al. Green preparation of reduced graphene oxide for sensing) and energy storage applications. Sci. Rep. 4, 4684 (2014)), catalysts (Dai, L. Functionalization of Graphene for Efficient Energy Conversion and Storage. Acc. Chem. Res. 46, 31.42 (2012)), supercapacitors ( supercapacitor) (Yoon, J.-C., Lee, J.-S., Kim, S.-I., Kim, K.-H. & Jang, J.-H. Three-Dimensional Graphene Nano-Networks with High Quality and Mass Production Capability via Precursor-Assisted Chemical Vapor Deposition. Sci. Rep. 3, 1788 (2013)), and lithium ion batteries (Kan, J. & Wang, Y. Large and fast reversible Li-ion storages in Fe2O3 -graphene sheet-on-sheet sandwich-like nanocomposites Sci. Rep. 3, 3502 (2013); Kim, H. et al. Scalable functionalized grap hene nano-platelets as tunable cathodes for high-performance lithium rechargeable batteries. Sci. Rep. 3, 1506 (2013)) are being widely studied.

In order to find new functions and potential applications of graphene, by tuning the morphology of graphene, various types of graphene such as zero-dimensional graphene quantum dots, two-dimensional graphene nanoribbons, and three-dimensional graphene structures getting architecture.

Chemical doping is believed to be another effective method for changing the electrical properties and chemical activity of graphene, since the spin “I” degree and atomic charge density are affected by the dopant.

Various elements (B, N, P, S, etc.) were selected to be introduced into the carbon framework to obtain heteroatom-doped graphene.

According to prior research, the graphene doped with nitrogen is a direct synthesis method such as chemical vapor deposition (CVD), arc-discharge, segregation growth, and solvothermal method, And it is manufactured by various methods that can be classified into two types of post treatment such as heating method, hydrazine hydrate method, and plasma treatment method (Wang, H., Maiyalagan, T. & Wang, X. Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis, Characterization, and Its Potential Applications.ACS Catal.2, 781.794 (2012);Wei, D. et al.Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties. Nano Lett. 9, 1752.1758 (2009); Panchakarla, LS et al. Synthesis, Structure, and Properties of Boron- and Nitrogen-Doped Graphene. Adv. Mater. 21, 4726.4730 (2009); Zhang, C. et al. Synthesis of Nitrogen-Doped Graphene Using Embedded Carbon and Nitrogen Sources. Adv. Mater. 23, 1020.1024 (2011)).

Graphene doped with nitrogen shows different properties depending on the synthesis method thereof, because the doping amount of nitrogen and the type of nitrogen-doping are different.

In order to efficiently manufacture graphene doped with sulfur, graphene oxide may be heat-treated as a sulfur source. Various sulfur-containing molecules (SO ₂ , H ₂ S, CS ₂ , and benzyl disulfide) have been studied as sulfur sources, and the catalytic activity of graphene doped with sulfur for oxygen reduction reaction has been studied.

However, the development of a simple, low-cost, large-scale, heteroatom-doped graphene preparation method remains an important problem. In addition, different doping methods are required for each element.

Stride et al. prepared graphene through a solvothermal method using ethanol as a carbon source (Choucair, M., Thordarson, P. & Stride, JA Gram-scale production of graphene based on solvothermal synthesis and sonication. Nat. Nanotechnol. 4, 30.33 (2009)), the reaction is relatively mild and straightforward.

Japanese Patent Application Laid-Open No. 2012-153555 discloses a sorbothermal of an alkali metal and a precursor compound having at least one heteroatom selected from the group consisting of boron, silicon, nitrogen, phosphorus, sulfur, and fluorine, and at least one hydroxyl group. Disclosed is a method for producing graphene containing heteroatoms by thermally decomposing a reactant. However, in the Japanese Patent Application Laid-Open No. 2012-153555, a precursor containing a hydroxyl group must be used as a starting material and sodium metal must be used, and there is a difference in configuration from the present invention.

The present inventors have completed the present invention by confirming that graphene doped with heteroatoms can be prepared by using a material not containing a hydroxyl group and using only NaOH rather than sodium metal.

An object of the present invention is a method for producing graphene doped with heteroatoms, comprising heating a mixture of a heteroatom precursor and an alkali metal source under an inert atmosphere, wherein the heteroatom precursor does not contain a hydroxyl group is to provide

Another object of the present invention is to provide a use for using the graphene doped with heteroatoms prepared through the above method as a negative electrode material for a lithium ion battery and as a catalyst for an oxygen reduction reaction for energy storage.

The above-described object of the present invention comprises the step of heating a mixture of a heteroatom precursor and an alkali metal source under an inert atmosphere, wherein the heteroatom precursor does not contain a hydroxy group, graphene doped with a heteroatom This can be achieved by providing a manufacturing method.

The heteroatom may be selected from S, N, B, Si, P or F. In addition, the heteroatom precursor may be dimethylsulfoxide or dimethylformamide. Moreover, the alkali metal source may be Na or NaOH.

In one embodiment of the present invention, when the heteroatom precursor is dimethyl sulfoxide, graphene doped with sulfur may be obtained.

In another embodiment of the present invention, when the heteroatom precursor is dimethylformamide, graphene doped with nitrogen may be obtained.

In the method of the present invention, the heating temperature of step (i) may be 150 ℃ to 300 ℃.

The graphene doped with heteroatoms prepared by the method prepared according to the method of the present invention can be used as a negative electrode material for a lithium ion battery.

In addition, the graphene doped with heteroatoms prepared by the method prepared according to the method of the present invention can be used as a catalyst for the oxygen reduction reaction for energy storage.

According to the method of the present invention, graphene doped with heteroatoms having a high content of heteroatoms can be prepared in large quantities through a simple process and mild reaction conditions.

The graphene doped with heteroatoms prepared according to the method of the present invention exhibits superior electrochemical properties compared to graphene prepared by the conventional method due to the high heteroatom content.

1a is a schematic diagram illustrating the formation of S-doped graphene, FIG. 1b is a TEM picture of the S-doped graphene, FIG. 1c is a high-resolution TEM picture of the S-doped graphene, and FIG. 1d is a dark field TEM photograph of the S-doped graphene, and FIG. 1E is a sulfur element mapping result of the S-doped graphene.
Figure 2a is Raman spectra of S-doped graphene, N-doped graphene and solvothermal graphene, and Figure 2b is S-doped graphene, N-doped graphene annealed at 900 °C. and N ₂ -adsorption/desorption isotherms of solvothermal-synthesized graphene.
3 is an XRD pattern of S-doped graphene, N-doped graphene, solvothermal-synthesized graphene, and graphite.
Figure 4 shows the pore distribution of S-doped graphene, N-doped graphene, S-doped graphene-900 (annealed at 900°C), and N-doped graphene-900 (annealed at 900°C). shows
5a is XPS spectrum of S-doped graphene, N-doped graphene and solvothermal graphene, and FIG. 5b is S-doped graphene, N-doped graphene and solvothermal graphene. the C1s XPS spectra of the pin, Figure 5c _{S1 (S2p 3/2),} S2 (S2p 1/2) and S3 having the (oxidized sulfur) S- doped yes and S2p XPS spectra of the pin, Figure 5d N1 A high-resolution N1s XPS spectrum of N-doped graphene with (pyridinic-N), N2 (pyrrolic-N) and N3 (graphitic-N).
6 shows the cycle performance and Coulombic efficiency of S-doped graphene, N-doped graphene, and solvothermal-synthesized graphene at a current density of 200 mAg ^{−1 .}
7 is a galvanostatic charging of (a) solvothermal graphene, (b) S-doped graphene, and (c) N-doped graphene during the initial 5 cycles at a current density of 200 mAg ^{−1 .} and discharge profiles, and cyclic voltammograms of (d) solvothermal graphene, (e) S-doped graphene, and (f) N-doped graphene at a scan rate of 0.1 mVs−1 ( cyclic voltammogram), and galvanostatic charge and discharge profiles of (g) solvothermal graphene, (h) S-doped graphene, and (i) N-doped graphene at various current rates am.
8 shows the potential-dependent electron transfer numbers of graphene, N-doped graphene, and S-doped graphene.

Hereinafter, the present invention will be described in more detail with reference to the following examples or drawings. However, the description of the following examples or drawings is only intended to specifically describe specific embodiments of the present invention, and is not intended to limit or interpret the scope of the present invention to the contents described therein.

Example 1. S- doped graphene synthesis

A mixture solution of 2 g of NaOH and 10 mL of dipalesulfoxide (DMSO) was added to a three-necked flask equipped with a reflux condenser and heated under a flow of nitrogen gas. The colorless mixture boiled at about 250°C and gradually increased in viscosity. After boiling for about 24 hours, the liquid turned into a black, cake-like substance. Finally, the product was washed several times with hydrochloric acid (10 wt%) and deionized water and dried in an oven at 90°C.

A two-dimensional sheet-like structure formed of carbon and sulfur atoms was obtained (FIG. 1). The surface morphology of the S-doped graphene was analyzed using an atomic force microscope (AFM). Looking at the AFM photograph, a crumpled silk veil-like structure with a thickness of about 1 nm is shown.

Example 2. N- doped graphene synthesis

50 mL of dimethylformamide (DMF) and 25 g of sodium metal were added to 100 mL of a Teflon-lined autoclave, sealed and heated at 190° C. for 72 hours. After cooling to room temperature, the autoclave was opened. The product was immediately transferred to a beaker containing 10 wt % hydrochloric acid. The product was further washed with deionized water and dried in an oven at 90°C.

comparative example One. in solvothermal by graphene synthesis

50 mL of methanol and 25 g of sodium metal were added to 100 mL of Teflon-lined autoclave, sealed and heated at 190° C. for 72 hours. After cooling the autoclave to room temperature, the solids were annealed at 500° C. under a flow of nitrogen gas. The product thus obtained was washed several times with hydrochloric acid (10 wt%) and deionized water, and dried in an oven at 90°C.

Example 3. Characterization

TEM pictures and TEM-EDS pictures were obtained using JEOL JEM-1010 and JEM-2100F, respectively. AFM pictures were obtained using a Vecco atomic force microscope (AFM). Raman spectral measurements were performed using a Dongwoo DM500i Raman spectrometer using green (514.5 nm) laser excitation. BET surface area and BJH pore size distribution were measured using Micromeritics Tristar 3000. XPS was performed using a Kratos AXIX-HSi. Elemental analysis was performed using a Thermo Scientific Flash 2000 analyzer.

To analyze the distribution of elemental dopants, TEM energy-dispersive X-ray spectroscopy (EDS) mapping was performed. Figure 1d shows the homogeneous distribution of S element in the graphene sheet. Raman spectra of S-doped graphene, N-doped graphene, and graphene prepared by a solvothermal method were investigated. Broad D and G bands were observed at about 1350 and 1600 cm ^{−1 , respectively ( FIG. 2 ).} The spectrum of graphene synthesized in the examples of the present invention is described in Choucair, M., Thordarson, P. & Stride, JA Gram-scale production of graphene based on solvothermal synthesis and sonication. Nat. Nanotechnol. 4, 30.33 (2009 )), which was in good agreement with that reported. X-ray diffraction (XRD) patterns of the solvothermal graphene and graphite were analyzed ( FIG. S4 ). Graphite showed a sharp peak at 26.3°, which corresponds to the (002) diffraction peak (d-spacing = 0.34 nm). The (002) peaks of the S-doped graphene, N-doped graphene and solvothermal graphene were shown at 23.2°, 25.5° and 23.4°, respectively. The interlayer distance of the S-doped graphene is 0.40 nm, which is greater than the interlayer distance of the N-doped graphene (0.35 nm) and the interlayer distance of the solvothermal-synthesized graphene (0.38 nm). bigger The conductivity of the S-doped graphene and the N-doped graphene is 0.607×10 ⁻³ Sm ⁻¹ and 0.416×10 ⁻³ Sm ⁻¹ , respectively, which is the conductivity of the solvothermal-synthesized graphene (0.121). Sm ^-1 ). These results show that the resistance of graphene is increased by doping with S or N.

_{The N 2} adsorption and desorption isotherms and pore size distributions of graphene doped with heteroatoms were analyzed ( FIGS. 2B and S5 ). Brunauer-Emmett-Teller (BET) surface areas of the S-doped graphene and N-doped graphene were 754 m ² g ^-1 and 317 m ² g ^{-1 ,} and the pore volume (PV) was 0.82 cm, respectively. ³ g ⁻¹ and 0.91 cm ³ g ⁻¹ . After annealing at high temperature (900° C.) under an inert atmosphere, the respective BET surface areas of the S-doped graphene and N-doped graphene were 1564 m ² g ⁻¹ (PV = 1.53 cm ³ g ⁻¹ ) and 2255 It was increased by ^{1 (PV = 1.73 cm 3 g} -1) - m 2 g.

To elucidate the chemical structure of the S-doped graphene, X-ray photoelectron spectroscopy (XPS) was adopted. The XPS spectrum of the solvothermal-synthesized graphene derived from methanol confirmed the fact that only carbon and oxygen atoms were present. However, signals of sulfur and nitrogen atoms were clearly observed in the XPS spectra of the S-doped graphene and N-doped graphene, respectively (FIG. 3a). A high-resolution Cls peak corresponding to sp ² -hybridization (CC bond, 284.5 eV) was observed, and the samples showed small peaks corresponding to CO bond, carbonyl (C=O), and carboxylate (OC=O). was seen (Fig. 3b). The high-resolution S2p spectrum of the S-doped graphene can be deconvoluted into three different peaks (FIG. 3c). Lines at 163.9, 165.1 and 168.5 eV corresponds to the _{2p 3/2, 2p 1/} 2 , and oxidized sulfur functional groups, respectively. In addition, there was an invisible peak corresponding to the thiol group (162.0 eV). In addition, the high-resolution N1s spectrum of the N-doped graphene can also be assigned to pyridine N (pyridinic N: 398.0 eV), pyrrolic N (400.1 eV), and graphite N (graphitic N: 401.3 eV). It can be resolved into three different peaks (Fig. 3d). These results suggest that the S or N is covalently bonded to the graphene network. According to elemental analysis, the sulfur content of the S-doped graphene was 22.83 wt%, and the nitrogen content of the N-doped graphene was 12.25 wt%. The sulfur content of the S-doped graphene has been previously reported (Yang, Z. et al. Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction. ACS Nano 6, 205.211 (2012); Yang , S. et al. Efficient Synthesis of Heteroatom (N or S)-Doped Graphene Based on Ultrathin Graphene Oxide-Porous Silica Sheets for Oxygen Reduction Reactions. Adv. Funct. Mater. 22, 3634.3640 (2012)) < 5%) is much greater than

Example 4 . Electrochemical characterization (for battery use)

For battery application, samples were prepared by heat treatment at 600° C. under _{N 2 .} For the preparation of the working electrode, a slurry in which the synthesized graphene, Super P and poly(vinylidene fluoride) (weight ratio 70:10:20) were well mixed in N-methyl-2-pyrrolidone was coated on a copper current collector. After drying, the electrodes were pressed and dried again at 120° C. under vacuum. In a glove box filled with argon, using the prepared working electrodes and lithium foil (as counter electrode and reference electrode), a two electrode 2016-type coin cell was fabricated. _{1.0 M LiPF 6} dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 5:5) was used as the organic electrolyte. In this embodiment, all voltage ranges for the battery test were set to 3.0 to 0.01 V (Li ⁺ /Li). The specific capacity was calculated only according to the weight of the active material and the weight of additives (binder and conductive agent) was not included.

In order to evaluate the potential applications of the S-doped graphene, N-doped graphene and solvothermal graphene as anode materials, an electrochemical cell with graphene electrodes was tested with a current density of 200 at room temperature. Charging and discharging were performed galvanostatically with mAg ^{-1 (FIG. 4).} The reversible capacity of the S-doped graphene, N-doped graphene and solvothermal ^- synthesized graphene is reasonably high (500-800 mAhg -1 ). In the S-doped graphene and solvothermal graphene, the specific capacity decreases during the first 30 cycles and gradually increases as the number of cycles increases. In addition, this activation process can also be observed in other graphene-based electrodes (Ai, W. et al. A novel graphene-polysulfide anode material for high-performance lithium-ion batteries. Sci. Rep. 3, 2341 (2013)) ;Li, X. et al. Superior cycle stability of nitrogen-doped graphene nanosheets as anodes for lithium ion batteries. Electrochem. Comm. 13, 822.825 (2011)). The S-doped graphene, N-doped graphene, and solvothermal synthesized graphene showed high cyclic stability without capacity fluctuation that may occur during a long cycle. The initial Coulombic efficiencies of the S-doped graphene, N-doped graphene, and solvothermal synthesized graphene are 65.8%, 58.3%, and 45.8%, respectively. The average Coulombic efficiency in the 30th to 150th cycle of the solvothermal graphene was 97.2%, but the average Coulombic efficiency of the S-doped graphene and N-doped graphene was much higher (99.8%, respectively). and 98.5%). It has been shown that half-cell Coulombic efficiency can affect cycle retention in complete Li-ion cells. Also, in the voltage profile, a plateau in the course of the first discharge occurring at about 1.0-0.7 V (more evident in the CV curve) is the decomposition of the electrolyte and the formation of a solid electrolyte interphase (SEI) layer. (Fig. S6), the high plateau is prominent in the solvothermal synthesized graphene electrode. This finding means that if graphene is doped with nitrogen and sulfur, side reactions including electrolyte decomposition can be suppressed. The shape of the charge and discharge profiles in the N-doped graphene and solvothermal-synthesized graphene is different in the case of other graphenes (Ritter, KA & Lyding, JW The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons) Nat. Mater. 8, 235.242 (2009); Li, X. et al. Superior cycle stability of nitrogen-doped graphene nanosheets as anodes for lithium ion batteries. Electrochem. Comm. 13, 822.825 (2011)). , the profile of the S-doped graphene is slightly different. In the CV curve of the S-doped graphene, a positive peak (cathodic peak: ~1.4 V) and a negative peak (anodic peak: ~2.4 V) were additionally observed, which are polysulfide conversion to lithium sulfide and lithium sulfide, respectively. is due to the conversion of polysulfides of The N-doped graphene electrode shows the highest rate capability. The S- doped graphene, N- doped graphene, and solvothermal synthesis graphene electrodes showed a specific capacity of each of 270.5, 278.9 and 267.5 in the mAhg ^{-1 -1} 2500 mAg current density, which is 200 mAg ^At a current density of -1, this corresponds to a capacity of 44.9%, 40.7% and 53.1%, respectively. _{Through the formation of LiC 3} under the condition that all graphene layers exist as a single layer, the theoretical capacity of graphene is 744 mAhg ^-1 . It has been reported that the lithium storage properties of graphene are sensitively dependent on many factors such as synthesis methods and conditions, interlayer spacing, disorder degree, and surface functional groups. The reversible capacity of graphene ^{may vary from about 300 mAhg -1} to 1500 mAhg ^-1 or more. The S-doped graphene and N-doped graphene show high cycle stability with excellent Coulombic efficiency, compared to other graphene electrodes previously reported.

Example 5. Electrochemical characterization ( electrocatalytic motion)

For electrocatalytic applications, samples were prepared by heat treatment at 600° C. under _{N 2 .} Electrochemical measurements were performed using an Autolab PGSTAT 101 in a standard 3-compartment electrochemical cell. In all experiments, an Ag/AgCl reference electrode was used as a reference electrode, and a Pt wire was used as a counter electrode. All potentials mentioned in this oxygen reduction reaction (ORR) experiment were converted to a pH-independent reversible hydrogen electrode (RHE) using a hydrogen oxidation reaction. Three types of graphene samples (5 mg) and 20 μL of 5 wt% Nafion ionomer were each sonicated for 10 minutes and dispersed in 0.4 mL of 2-isopropyl alcohol. For the rotating-disk electrode (RDE), 7 μL of catalyst ink was charged onto the RDE using a ^{0.000196 m 2 glassy carbon disk.} The electrode rotation speed was 1600 rpm (scan speed, 5 mVs ⁻¹ ), 0.1 M KOH electrolyte. To calculate the electron transfer number (0.6 V _RHE ), RDE measurements were performed using different rotational speeds of 400, 900, 1200 and 1600 rpm.

To investigate the electrocatalytic activity of the S-doped graphene and N-doped graphene on the ORR and confirm the doping effect, 0.1 M KOH electrolyte (20 wt% Pt/C commercial E-TEK catalyst) Rotating-disk electrode (RDE) voltammetry was performed in The N-doped graphene exhibits higher ORR activity than the solvothermal-synthesized new graphene. In addition, N-doped graphene has an onset potential (0.9 V _RHE ) similar to that of a platinum electrode. The activity of doped carbon for the ORR was improved by a combination of various factors, but it was not clearly revealed. However, the enhancement factor can be explained by doping, for example the presence of N, B, and S in the carbon. These results mean that the electrocatalytic activity of the catalyst for the ORR increases with the introduction of a dopant having an electronegativity different from that of carbon. Furthermore, by the Koutechy-Levich method of varying the rotational speed, it was revealed that the _{electron transfer number at 0.2 V RHE} changed from 2.8 of solvothermal new graphene to 3.9 of N-doped graphene. lost (FIG. S7). To emphasize the importance of the effect of proper doping on ORR activity in practical applications, the potential (0.6 V _RHE ) for comparing electron transport numbers was selected in Fig. 5b, which is a very high potential from a general point of view. In the case of solvothermal-synthesized graphene, the electron transport number means a dominant two-electron pathway that is inefficient for ORR. However, upon doping with elements such as N and S, the ORR pathway involves both 2-2-electron and 4-electron transport, which are more affinity for the ORR. Heteroatom (N, S)-doped graphene by solvothermal method was synthesized by a bottom-up method. The bottom-up method is different from the top-down method and contains a significant amount of metallic impurities that can affect the activity. However, in the present invention, NaOH or Na metal that does not affect the activity is used. It was confirmed by the XPS results that there were no other impurities in the graphene synthesized by the method of the present invention.

Claims (7)

heating the mixture of the heteroatom precursor and the alkali metal source under an inert atmosphere;
The heteroatom precursor is characterized in that it does not contain a hydroxyl group,
The heteroatom is S, N, B, Si, P and F, characterized in that selected from the group consisting of, graphene doped with a heteroatom manufacturing method. delete The method of claim 1, wherein the heteroatom precursor is dimethylsulfoxide or dimethylformamide. The method of claim 1 , wherein the alkali metal source is Na or NaOH. The method of claim 1, wherein the heating temperature is 150°C to 300°C. delete delete

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