Method for rapidly preparing graphene quantum dots in household microwave oven

Abstract

The invention provides a method for rapidly preparing graphene quantum dots (GQDs) by using a household microwave oven, aiming at the cost problem of large-scale preparation of the GQDs. According to the method, the characteristic that water molecules in a microwave oven vibrate at a high speed and can rapidly increase the temperature is utilized, a trace amount of deionized water is dropped into the precursor citric acid, the purpose of greatly shortening the synthesis time is achieved, and carbonization of the citric acid can be completed within 3 minutes. In addition, the GQDs solution is purified through centrifugal treatment of an ethanol solution, and the method is more convenient and faster than a dialysis bag purification method in the aspect of purifying the graphene quantum dots and is higher in purification efficiency. The method is suitable for large-scale preparation of GQDs. Results of a transmission electron microscope (TEM) and a high resolution transmission electron microscope (HRTEM) show that when the synthesis time is controlled to be 2.5-3 minutes, the obtained GQDs show excellent high crystallinity and size uniformity.

Description

Technical Field

[0001] The present invention relates to the science and engineering of inorganic nanomaterials, specifically novel methods for the efficient and rapid preparation of graphene quantum dots. More specifically, the invention relates to an innovative method for preparing graphene quantum dots using a household microwave oven. By improving existing processes, this method effectively simplifies the preparation and purification of graphene quantum dots (GQDs), shortening the production cycle. Furthermore, it produces graphene quantum dots with a size less than 1.3 nanometers and high crystallinity, opening up a new path for the mass production and widespread application of GQDs. Background Art

[0002] As graphene nanoclusters, GQDs (GQDs) are atomically thin sheets of graphene, one to three layers thick, with lateral dimensions less than 20 nanometers. Graphene quantum dots (GQDs) are a zero-dimensional nanomaterial with a structure that is composed of atomically thin sheets of graphene, one to three layers thick, and a lateral dimension less than 20 nanometers. Due to surface effects and quantum confinement, as well as their rich edge structure and band gap that varies with size and edge structure, they have become more attractive than graphene and carbon nanotubes. Furthermore, compared to traditional quantum dots, GQDs possess several attractive properties, such as good biocompatibility, low toxicity, low cost, strong water solubility, high fluorescence stability, high mobility, and low resistivity. Consequently, they are attracting increasing attention for new applications in energy conversion and storage, electro- / photo- / chemical catalysis, flexible devices, sensing, display, imaging, and biomedicine. For example, in energy conversion and storage devices, their high charge mobility allows them to be doped into battery electrodes and micro-supercapacitors, significantly improving electron transfer efficiency. They are ideal materials for solar cells, lithium-ion batteries, and supercapacitors. GQDs are used as charge-trapping media in high-performance organic nano-floating-gate memories, spin-coated onto device substrates to effectively enhance the memory's charge transport performance. Unlike two-dimensional graphene, graphene quantum dots do not have a zero bandgap. Their bandgap is limited by their size and edge structure, so different preparation methods can also affect their electrical properties. Due to their high mobility, low resistivity, and ease of integration, graphene quantum dots also have great potential in nanoelectronic devices.

[0003] GQD synthesis strategies: bottom-up and top-down. In the top-down process, bulk graphitized carbon materials (e.g., multi-walled carbon nanotubes (MWCNTs), graphene oxide, graphene, graphite, etc.) are used as the carbon source. The carbon source is then exfoliated and cut into the desired GQDs using chemical or physical methods. Top-down synthetic pathways include pulsed laser ablation, electrochemical cutting, and reduction / oxidation cutting. In the reduction / oxidation cutting process, appropriate chemicals (such as reducing or oxidizing agents) are used to cut the bulk carbon precursor like scissors and form small-sized GQDs. In contrast to the top-down approach, the bottom-up approach involves fusing small precursor molecules (such as 1,3,6-trinitropyrene, glucose, citric acid, etc.) into larger structures to produce GQDs. Hydrothermal, soft template, microwave-assisted, and step-by-step organic synthesis methods are the most common bottom-up synthetic methods. Among them, the microwave method can effectively shorten the precursor fusion time, thereby producing GQDs with uniform particle size distribution. In a foreign journal article entitled "Gram-scale synthesis of single-crystalline graphene quantum dots with superior optical properties", the nitration reaction of 1,3,6-trinitropyrene was successfully used to achieve gram-scale mass production of GQDs through hydrothermal treatment under alkaline aqueous solution conditions. The product showed good dispersibility and uniformity of lateral size, with an average lateral size of about 3.5 nanometers (nm). However, the synthesis of trinitropyrene is difficult, the preparation process is complicated, and the cost is high, which limits its competitiveness in commercial-scale applications. In the domestic journal article "Fluorescence Spectrum Study of Multi-Luminescence Bands of Graphene Quantum Dots Prepared by Laser Ablation", a pulsed laser ablation method was used to successfully prepare GQDs with high purity and an average size of about 3 nm, which improved the quality of graphene quantum dots, but the synthesis steps were relatively cumbersome. The international journal article "Controlled synthesis of blue luminescent graphene quantum dots from carbonized citric acid: Assessment of methodology, stability, and fluorescence in an aqueous environment" uses a microwave method with citric acid as a precursor. Experiments show that GQDs can be synthesized by microwave heating at 150W for 90 minutes. Although this method is simple to operate, it requires a long preparation time, which affects the synthesis efficiency.The international journal article "Fluorine Functionalized Graphene Quantum Dots as Inhibitor against hIAPP Amyloid Aggregation" developed a microwave-assisted hydrothermal method to synthesize highly fluorescent and dispersible fluorine-doped GQDs (GQDs) smaller than 3 nm. This material shows significant potential in the biomedical field, particularly in inhibiting amyloid aggregation. The domestic journal article "Preparation and Spectroscopic Characterization of Graphene Quantum Dots from Deashed Anthracite from Jiangou Mountain in Ningxia" employed a one-step chemical oxidation method using anthracite from Jiangou Mountain in Ningxia as a carbon source to successfully synthesize GQDs with uniform size distribution, good crystal structure, and stable fluorescence properties. The GQDs were relatively large, ranging from 6 to 10 nm. In the foreign journal article "Bottom-Up Fabrication of Single-Layered Nitrogen-Doped Graphene Quantum Dots through Intermolecular Carbonization Arrayed in a 2D Plane", a soft template method and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) were used as precursors to prepare single-layer nitrogen-doped GQDs with an average size in the range of 2-5nm. The product has high yield and excellent fluorescence properties.

[0004] On the other hand, Chinese patent document CN117431063A discloses a method for synthesizing GQDs by plasma chemical vapor deposition technology, which allows precise control of the size and band gap of GQDs, showing significant flexibility. Chinese patent CN116768201A discloses a method for preparing lignin-based GQDs, which, after a hydrothermal reaction, can adjust the fluorescence emission wavelength by further reaction of salicylic aldehyde derivatives for different fluorescence requirements. Chinese patent document CN117417248A discloses a new method for preparing hydrophilic single-layer graphene quantum dots, which obtains GQDs with a size of only 2.34 nm by organic synthesis technology. Due to their excellent hydrophilicity and fluorescence properties, these GQDs are very suitable for use in physiological water environments. In addition, Japanese patent document JP2021537415A provides a method for preparing GQDs of uniform size and shape by embedding and exfoliating graphite nanoparticles in an aqueous solution. Australian patent document AU2021106312A discloses a method for preparing GQDs of uniform size by ultrasonic-assisted hydrothermal method, which uses graphene oxide as raw material and cuts carbon-carbon bonds by ultrasonic mechanical stress under the strong oxidation of nitric acid. This method is simple and mild, and can obtain GQDs with high purity, small particle size and uniform distribution. U.S. patent document US201916551750A invented a biomass-based high-efficiency fluorescent GQDs and its preparation method. Through the hydrothermal reaction of a composite carbon source, nitrogen source, multivalent metal ions and water, GQDs with high fluorescence intensity were obtained. Among the current existing graphene quantum dot synthesis schemes, some have high synthesis costs, or have high requirements for equipment and reaction temperature, or the size of the synthesized graphene quantum dots is large. Therefore, there is an urgent need for a synthesis method suitable for large-scale, low-cost preparation of small-sized graphene quantum dots.

[0005] The present invention uses citric acid as a carbon source and sodium hydroxide as a dispersant to synthesize GQDs in a household microwave oven. The synthesis steps are simple and the cost is low. The synthesized graphene quantum dots are as small as 1.3 nm in size, with uniform size distribution and high crystallinity, and have good application prospects. A trace amount of deionized water is added to the citric acid, and the temperature is rapidly increased by the vibration of water molecules in the microwave, which greatly reduces the reaction time. At the same time, the synthesis of GQDs under different reaction times is discussed. Under a high-resolution transmission electron microscope, highly crystalline GQDs were observed in the characterization diagram of the synthesis time of 2.5-3 min. A simple centrifugal purification treatment of the graphene quantum dot solution with an ethanol solution is used to effectively improve the purification efficiency of the graphene quantum dots. A good synthesis scheme is provided for the large-scale preparation of graphene quantum dots. Summary of the Invention

[0006] This invention relates to a novel inorganic nanomaterial preparation technology. By heating citric acid in a household microwave oven, graphene quantum dots (GQDs) approximately 1.3 nm in size were successfully synthesized. Through a cleverly designed centrifugation process in an ethanol solution, GQDs were efficiently extracted from the GQD solution and subsequently obtained as a solid powder.

[0007] The present invention offers significant advantages, primarily in the following aspects: First, the method is low-cost and has simple and easy steps. Second, the synthesized graphene quantum dots are extremely small, down to 1.3 nanometers in size, exhibit uniform dispersion, and exhibit high crystallinity. Furthermore, the present invention allows for control of the synthesis time to regulate the growth of the graphene quantum dots, thereby optimizing their properties.

[0008] In terms of the purification process of graphene quantum dots, the present invention innovatively introduces a unique ethanol solution centrifugal purification method. Compared with traditional dialysis purification, this method can significantly improve the purification efficiency while reducing time and economic costs. Through the application of this method, the cycle of synthesizing graphene quantum dots can be shortened, making the entire production process more efficient, while reducing the overall production cost of graphene quantum dots, making their application in multiple fields possible. In addition, the introduction of this purification method undoubtedly enhances the competitiveness and practical value of the present invention in the field of graphene quantum dot preparation technology.

[0009] For the generated GQDs, we used transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) to conduct detailed structural characterization and testing. The test results showed that the obtained GQDs have extremely high crystallinity and excellent size uniformity, further confirming the excellent performance and stable and reliable product quality of the method of the present invention. The specific operation process of the present invention is as follows:

[0010] The present invention comprises the following specific steps:

[0011] 1. Preparation

[0012] 1. Mix 2 g of citric acid and 1 ml of deionized water in a 150 ml beaker.

[0013] 2. Place the mixture in a 700W household microwave oven, select medium heat, and heat for 3 minutes.

[0014] 3. Stop heating when the liquid turns orange-yellow and irritating gas is released.

[0015] 4. After cooling to room temperature, add 100 ml of 0.25 mol L -1 Sodium hydroxide solution was stirred vigorously for 30 minutes to finally obtain a graphene quantum dot solution.

[0016] 2. Purification

[0017] 1. Mix the ethanol solution and the graphene quantum dot solution in a ratio of 4:1.

[0018] 2. Centrifuge at 12000 rpm for 10 minutes.

[0019] 3. Take the sediment in the centrifuge tube and dry it at 60°C for 2 hours to obtain graphene quantum dot solid.

[0020] This method can be implemented in a variety of ways and is not limited to the specific steps listed above. Several alternative implementations, along with their TEM and HRTEM results, are shown in the accompanying figures. The figures show fluctuations in the shape, size, and crystallinity of the resulting GQDs, demonstrating effective control of the graphene quantum dots under varying reaction conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 These are the transmission electron microscopy image (a), high-resolution transmission electron microscopy image (b), and particle diameter statistics (c) of graphene quantum dots synthesized under a reaction time of 3 minutes in Example 1.

[0022] Figure 2 These are the transmission electron microscopy image (a), high-resolution transmission electron microscopy image (b) and particle diameter statistics graph (c) of graphene quantum dots synthesized under a reaction time of 2.5 minutes in Example 2.

[0023] Figure 3 These are the transmission electron microscopy image (a), high-resolution transmission electron microscopy image (b) and particle diameter statistics graph (c) of graphene quantum dots synthesized under a reaction time of 3.5 minutes in Example 3. DETAILED DESCRIPTION

[0024] Implementation method one:

[0025] 1. Mix 2 g of citric acid and 1 ml of deionized water and heat in a microwave for 3 minutes.

[0026] 2. After cooling to room temperature, add 100ml 0.25mol L -1 The sodium hydroxide solution was stirred vigorously for 30 minutes to obtain the graphene quantum dot solution.

[0027] 3. The ethanol solution and the graphene quantum dot solution were mixed in a ratio of 4:1 and centrifuged. The sediment was dried at 60°C for 2 hours to obtain a graphene quantum dot solid.

[0028] 4. Graphene quantum dots were ultrasonically dispersed in an ethanol solution for 30 minutes, then dropped onto a carbon-coated copper mesh and characterized using TEM. The GQDs were observed to be uniformly distributed spherical particles with an average diameter of 1.3 ± 0.5 nm. HRTEM images revealed clear lattice spacing between the quantum dots.

[0029] Implementation method 2:

[0030] 1. Mix 2 g of citric acid and 1 ml of deionized water and heat in a microwave for 2.5 minutes.

[0031] 2. Prepare graphene quantum dot solution and solid according to implementation method one.

[0032] 3. TEM characterization showed smaller, uniformly sized elliptical GQDs with an average size of approximately 0.17±0.1 nm. However, only fuzzy lattice fringes were visible in the HRTEM image.

[0033] Implementation method three:

[0034] 1. Mix 2 g of citric acid and 1 ml of deionized water and heat in a microwave for 3.5 minutes.

[0035] 2. Prepare graphene quantum dot solution and solid according to implementation method one.

[0036] 3. The resulting GGQDs are irregularly shaped and clearly stacked, with HRTEM images confirming distinct lattice fringes. The nanoparticle diameters range from 4 to 18 nm, with a primary concentration of 9 to 12 nm.

[0037] The accompanying charts illustrate TEM and HRTEM characterizations of graphene quantum dots synthesized under various implementation methods, as well as a statistical plot of particle diameter. By varying the heating time, a household microwave oven was used to address the size control issue during the graphene quantum dot preparation process, significantly improving process efficiency.

[0038] This invention provides a new solution for synthesizing graphene quantum dots in the field of preparing inorganic nanomaterials. Due to the high crystallinity and size uniformity of GQDs, they are suitable for applications in optoelectronic devices, nanoelectronic devices and other fields. Figure 1 、 Figure 2 、 Figure 3 etc. confirmed the stability and consistency of product quality.

Claims

1. A method for preparing graphene quantum dots, characterized in that: Place a certain amount of citric acid in a beaker and add an appropriate amount of deionized water. Then, place the beaker in a microwave oven on medium heat and heat for several minutes until the liquid turns orange-yellow and irritating gases are released. Then, stop heating. After cooling the beaker to room temperature, add an appropriate amount of sodium hydroxide solution and stir vigorously to obtain a graphene quantum dot solution.

2. A method for purifying graphene quantum dots, characterized in that: The ethanol solution and the graphene quantum dot solution are mixed in a certain ratio and then centrifuged at an appropriate speed. The sediment in the centrifuge tube is removed and dried in an environment with a suitable temperature to obtain a graphene quantum dot solid powder.

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