Method for producing graphene quantum dots, and graphene quantum dots

Abstract

Nanoparticles made of a metal oxide are heated to a temperature of from 70°C to less than 350°C. Graphene quantum dots are deposited on the surface of the nanoparticles made of a metal oxide by bringing a carbon-containing compound into contact with the nanoparticles made of a metal oxide while maintaining this heated state, thereby forming graphene-nanoparticle composites having the nanoparticles made of a metal oxide and graphene quantum dots deposited thereon.

Description

Method for manufacturing graphene quantum dots, graphene quantum dots

[0001] This disclosure relates to a method for manufacturing graphene quantum dots and to graphene quantum dots themselves.

[0002] Due to its excellent electrical conductivity, graphene is being actively researched for applications in electronic and energy-related devices such as field-effect transistors. However, because graphene lacks a band gap, its optical applications have been limited. When graphene is scaled down to a pseudo-zero-dimensional scale (for example, less than 10 nm in diameter), the quantum confinement effect emerges, introducing a band gap. Such materials are called "graphene quantum dots" and are attracting attention as new element materials for light-emitting devices and optoelectronic devices. Graphene quantum dots have the advantage of being less toxic than metal quantum dots such as CdSe, and are expected to have applications in bioimaging sensors.

[0003] Patent Document 1 discloses a method for preparing two-dimensional nanosheets. In Patent Document 1, a two-dimensional nanosheet refers to a two-dimensional nanoflake or a two-dimensional quantum dot. Patent Document 1 discloses the steps of preparing a nanoparticle template, growing a two-dimensional (2D) nanosheet on the surface of the nanoparticle template, removing the two-dimensional (2D) nanosheet from the surface of the nanoparticle template, and separating the nanoparticle template from the two-dimensional (2D) nanosheet.

[0004] Special Publication No. 2020-514221

[0005] Patent Document 1 hardly discusses the manufacturing of graphene quantum dots. The main interest of Patent Document 1 is WS ２ , WSe ２ MoS ２ MoSe ２ This involves the formation of two-dimensional nanosheets of transition metal dichalcogenide (TMDC) materials such as [material name missing]. In fact, as an example in Patent Document 1, MoS [material name missing] on a ZnO template ２ Nanosheet template-assisted growth, MoSe on ZnO template ２Template-assisted growth of nanosheets, WS on ZnO template ２ There is template-assisted growth of nanosheets, but there are no examples of graphene nanosheets. Therefore, further research and development on a method enabling the production of graphene quantum dots has been demanded.

[0006] The present disclosure has been made to solve the above-described problems, and an object thereof is to provide a method for producing graphene quantum dots and graphene quantum dots having an oxygen content of 6 wt% or less produced thereby.

[0007] The method for producing graphene quantum dots according to the present disclosure comprises depositing graphene quantum dots on the surface of the nanoparticles by bringing a carbon-containing compound into contact with the nanoparticles while heating the nanoparticles of the metal oxide at 70°C or higher and lower than 350°C to obtain a graphene-nanoparticle composite.

[0008] Other features of the present disclosure will be clarified below.

[0009] Graphene quantum dots can be produced.

[0010] It is a flowchart showing a method for producing graphene quantum dots. It is a TEM image of the produced graphene quantum dots. C ２ H ２ Result of monitoring the weight change of TiO ２ accompanied by the introduction of by TG. TiO at 250°C ２This is the CVD-TG curve. This is the N2 adsorption / desorption curve of TiO2_HT_250C30m. These are photographs of GQD_250C15m dispersed in EtOH and GQD powder. This is the XRD pattern of GQD_250C15m. This is the UV-vis spectrum of the GQD solution (GQD_250C15m). Figure 9a shows that the prepared GQD solution redisperses, and Figure 9b shows that GQD_R does not redisperse. These are the PL spectra of GQD_250C15m and GQD_R excited at 360 nm. These are PL spectra measured at different excitation wavelengths. This is the FT-IR spectrum. These are the TPD results from room temperature to 1800°C. These are the TPD results from room temperature to 1800°C. This is a table summarizing the gas generation amount and the amount of edge sites. Figures 16a-f are AFM images. Figures 16g, h, and i are SEM images. Figures 17a and b are AFM images. Figures 17c and d are SEM images. These are the XPS results for TiO2 / C_250C15m and R_TiO2 / C_250C15m. TiO2 at 250°C ２ C due to catalytic action ２ H ２ This is the in-situ IR spectrum during decomposition. ２ TiO ２ This is the in-situ IR spectrum of / C. ２ TiO ２ This is the in-situ IR spectrum of / C. ２ TiO ２ This is the in-situ IR spectrum of / C. This is the TPD profile. This is a table summarizing the gas generation and edge site amounts. Figure 25a is the UV spectrum. Figures 25b, 25c, and 25d are the PL spectra. These are the QY values ​​in EtOH and toluene for the three samples. Figure 27a is an image of each sample. Figure 27b is the measured UV spectrum. Figure 27c is a diagram showing the calculated band gap. Figure 27d is the ESR spectrum of each sample. This is a diagram summarizing the corresponding band gaps. Figure 29a is the TG-CVD curve. Figure 29b is a magnified view of its initial period. C ２ H ２ C's reaction when it was introduced ２ H ２This is the result of Mass, which shows the amount of C. Figure 31a is C ２ H ２ The gas generation rate upon introduction is shown. Figures 31b, c, and d are in-situ FT-IR spectra, respectively. Figures 32a, 32b, 32c, and 32d are H ２ CO, C ６ H ６ ,CH ４ This figure shows the incidence rate at 150°C and 600°C. ２ H ２ This figure shows the gas generation intensity when the system is introduced. TiO ２ Catalyst C above ２ H ２ This figure shows the decomposition mechanism scheme. Figures 35a and 35b show TiO2 after one and two heat treatments at 350°C for 90 minutes, respectively. ２ N ２ This shows adsorption / desorption. Figures 35c-f are TG curves. Figure 36a is the UV-vis spectrum, and Figures 36b-d are the PL spectra. This is a table summarizing the QY for each sample. This is a table summarizing the yield of GQDs formed at different reaction temperatures. These are characteristic diagrams of GQDs produced using aluminum oxide and zinc oxide as metal oxide nanoparticles. Figures 40A and 40C are photographs of GQDs obtained using hafnium oxide and zirconium oxide as templates. Figure 40B shows the decomposition start temperature of zirconium oxide. These are the analysis results for GQDs obtained using cerium oxide as a template. Figure 42A shows the TPD profile of GQDs obtained using cerium oxide as a template, and Figure 42B shows the amount of gas generated and the amount of edge sites when cerium oxide is used as a template. This is a diagram showing the decomposition temperatures of propylene and ethylene. This is a table summarizing the yield of GQDs formed at different reaction times. This is a diagram showing the effect of TMS doping density on GQDs. This is a diagram showing the PL spectra of the samples in Figure 45. TMS-doped SiO ２ This figure shows the weight change of nanoparticles during the CVD process. GQD / SiO ２This is an image of the ethanol dispersion of the complex taken under UV light irradiation. This figure shows the relationship between reaction time and UV-vis absorption spectrum. This figure shows the PL spectra of the four samples in Figure 49. This figure shows the relationship between reaction time and UV-vis absorption spectrum. This figure shows the PL spectra of the five samples in Figure 51. This is a photograph of the toluene dispersion of GQD taken under UV light irradiation and under indoor lighting. This figure shows the UV-vis absorption spectrum and PL spectrum for the same sample as in Figure 53. This figure shows the weight change of the sample.

[0011] Embodiment. The method for producing graphene quantum dots according to the embodiment includes obtaining a graphene-nanoparticle composite by growing graphene quantum dots on the surface of metal oxide nanoparticles by CVD. For example, the graphene quantum dots can be dispersed in a solvent by adding a solvent to the graphene-nanoparticle composite and performing sonication. The method for producing such graphene quantum dots will be described below. Hereafter, graphene quantum dots may be referred to as GQD.

[0012] Figure 1 is a flowchart showing a method for manufacturing graphene quantum dots according to an embodiment. First, in step S10, metal oxide nanoparticles (hereinafter sometimes referred to as nanoparticles or metal oxide nanoparticles) are prepared. Metal oxide nanoparticles include, for example, titanium oxide (TiO2). ２ (sometimes called titania), cerium oxide (CeO) ２ It includes zirconium oxide, hafnium oxide, aluminum oxide, or zinc oxide (also called ceria). The metal oxide nanoparticles can be any one of these materials. The size of the nanoparticles does not affect the size of the graphene quantum dots produced, so in that sense it is not particularly limited. However, the larger the overall surface area of ​​the nanoparticles, the more graphene quantum dots can be grown. In terms of optical properties, if a certain amount or more of graphene quantum dots can be produced, a spectrum as a quantum dot can be obtained. For example, the substrate nanoparticles are 100 m ２ It may have a specific surface area of ​​approximately / g.

[0013] Next, the process proceeds to step S12. In step S12, carbon is deposited on the nanoparticles. The deposited carbon is graphene quantum dots. Graphene quantum dots are sometimes called carbon quantum dots or carbon quantum dots. In step S12, a carbon-containing compound is brought into contact with the metal oxide nanoparticles while heating them to a temperature of 70°C or higher and less than 350°C. The carbon-containing compound includes, for example, acetylene or acrylonitrile. However, the carbon-containing compound is not particularly limited to these and can be any carbon source. For example, such a CVD process can be carried out with an argon-acetylene mixed gas at atmospheric pressure, with a partial pressure of acetylene of 1% to 20%.

[0014] When heated nanoparticles are brought into contact with a carbon-containing compound, the carbon-containing compound reacts on the nanoparticles, causing carbon to precipitate on the nanoparticle surface. In other words, graphene quantum dots can be precipitated on the surface of the nanoparticles. By forming graphene on nanoparticles using a controlled CVD method, only a single layer of graphene can be grown on the nanoparticles. Approximately one layer of graphene can be grown on the nanoparticles. The single layer or approximately one layer of grown graphene is, for example, in the form of a sheet of 2 nm or less. In another example, multiple layers of graphene can be grown. In this way, a graphene-nanoparticle composite containing nanoparticles and graphene quantum dots is obtained. For example, a graphene-nanoparticle composite can be obtained by the decomposition of acetylene and its precipitation on the titanium oxide surface.

[0015] Next, the process proceeds to step S14. In step S14, the graphene quantum dots are dispersed in the solvent by adding a solvent to the graphene-nanoparticle composite and performing sonication. The graphene-nanoparticle composite is dispersed in an organic solvent such as ethanol, acetone, or toluene, and then subjected to sonication. For example, the metal oxide nanoparticles after CVD are placed in a vial together with the solvent to be dispersed (ethanol, acetone, toluene, etc.), and subjected to sonication at 28 kHz for 1 hour using an ultrasonic device such as the W-113A manufactured by Honda Electronics Co., Ltd. The graphene quantum dots are dispersed in the organic solvent by the sonication. This can be described as a peeling process that removes the graphene quantum dots from the graphene-nanoparticle composite. It has been found that if water is used instead of an organic solvent for sonication, the graphene quantum dots do not disperse in the water.

[0016] As mentioned above, a single layer or approximately one layer of graphene may be formed on the nanoparticles, and sheet-like graphene quantum dots of, for example, 2 nm or less in size are deposited on the surface of the nanoparticles. The exfoliation process in this embodiment only requires ultrasonic treatment and does not include the well-known chemical intercalation process. In other words, graphene quantum dots can be exfoliated from nanoparticles by ultrasonic treatment alone without going through a chemical intercalation process. On the other hand, if two or three layers of graphene are deposited on the nanoparticles, it is thought that the graphene quantum dots cannot be exfoliated from the nanoparticles by ultrasonic treatment alone, and an intercalation process is necessary. Figure 1 of Patent Document 1 (JP 2020-514221) discloses that two 2D sheets are formed by epitaxial growth of a 2D layer on a template, and the 2D sheets are exfoliated from the template via chemical intercalation. In other words, the 2D sheets cannot be exfoliated by ultrasonic treatment alone, and an intercalation process is essential for exfoliation. By the way, ultrasonic treatment is one example of a process for exfoliating graphene quantum dots from a nanoparticle template. In other words, the removal of the active growth material may include the application of mechanical tools, mechanical or ultrasonic cleaning, chemical reagents, heat, or other external energy sources to the surface of the nanostructure and / or growth substrate. For example, graphene quantum dots can be detached from the nanoparticles by applying any mechanical force other than ultrasound to the graphene-nanoparticle composite.

[0017] Next, the process proceeds to step S16. In step S16, the used metal oxide nanoparticles are removed from the organic solvent. For example, the metal oxide nanoparticles precipitate in the solution. The metal oxide nanoparticles may be recovered from the solvent by filtration, decantation, or centrifugation.

[0018] Next, the process proceeds to step S18. In step S18, the metal oxide nanoparticles recovered in step S16 are made reusable. Since the recovered metal oxide nanoparticles are covered with carbon, the metal oxide nanoparticles can be heated, for example, in air to burn off the remaining carbon. For example, heating to 350°C can remove the carbon on the titanium oxide. The metal oxide nanoparticles from which the carbon has been removed are in a reusable state. Since the metal oxide nanoparticles function as a catalyst for decomposing carbon-containing compounds to obtain graphene quantum dots, the metal oxide nanoparticles are not consumed and are reusable as described above. The inventors have found that metal oxide nanoparticles (TiO ２ It was experimentally confirmed that the material can be reused at least three times. Step S18 is an optional step and can be omitted.

[0019] Next, the process proceeds to step S20. Step S20 is a step in which the organic solvent is removed to obtain powdered graphene quantum dots. For example, powdered graphene quantum dots can be obtained by volatilizing the organic solvent. Step S20 is optional. If step S20 is omitted, a dispersion containing graphene quantum dots is obtained. It was confirmed that this solution fluoresces when exposed to UV light.

[0020] Figure 2 shows a transmission electron microscope (TEM) image of the manufactured graphene quantum dots. The size of the graphene quantum dots was measured to be approximately 3 nm to 6 nm. The diameter of the GQDs formed using nanoparticles with particle sizes of several tens of nanometers was generally 3 nm or less. It was confirmed that these graphene quantum dots of approximately 3-6 nm size fluoresce. For example, the oxygen content of the graphene quantum dots manufactured by the above method is 6% by weight or less. This oxygen content is lower than that of conventional products, which have an oxygen content of 9.6% by weight. Because the graphene quantum dots of this embodiment have a low oxygen content, they are highly hydrophobic. Therefore, they are poorly soluble in water but can be dispersed in organic solvents. On the other hand, commercially available products with a high oxygen content have low hydrophobicity. Therefore, they are easily soluble in water but do not disperse in organic solvents. The manufacturing method for commercially available GQDs involves adding acid to large graphite to produce small GQDs, so they contain many oxygen functional groups such as hydroxyl groups (-OH) and carboxyl groups (-COOH). On the other hand, the manufacturing method for GQDs of this disclosure is the CVD method, so it does not contain many oxygen functional groups. This is the reason for the differences in dispersibility in organic solvents and water. It has been found that GQD produced by the method disclosed herein and commercially available GQD have similar luminescence properties, but differ in their dispersibility in solvents.

[0021] Conventionally, graphene quantum dots with atomically defined structures have been synthesized using organic synthesis methods that utilize small molecules as raw materials, as disclosed in publications such as the Journal of the American Chemical Society, 2011, 133, 15221. However, conventional manufacturing methods are complex, making industrial-scale synthesis difficult. As a result, commercially available graphene quantum dots are currently very expensive, costing around 500,000 yen per gram for the cheapest options. In contrast, the method for producing graphene quantum dots described in this embodiment uses only carbon-containing compounds such as acetylene gas as raw materials, and the nanoparticles used as catalysts are reusable. Furthermore, the conventional C ２ H ２Considering that the reaction temperature for CVD is 600°C, the heat treatment temperature in this embodiment is low, for example, 250°C, and the subsequent process only requires ultrasonic dispersion. Therefore, the method for manufacturing graphene quantum dots according to this embodiment is suitable for cost reduction. The manufacturing cost of graphene quantum dots when this method is implemented industrially is estimated to be several hundred yen per gram of graphene quantum dots. Considering that current commercially available products cost around 500,000 yen per gram, a significant reduction in manufacturing costs can be expected with the method of this embodiment.

[0022] <1. Experimental Method> 1.1 TiO2, a graphene-nanoparticle composite ２ / C formation TiO ２ Place it in the vertical furnace, and add 225 ml / min to the vertical furnace. －１ Ar was introduced at the flow rate. Then the temperature was changed from room temperature to 10°C / min. －１ CVD was performed by raising the temperature to a specific temperature at the following heating rate. After maintaining the target temperature for 30 minutes, Ar was added to the mixed gas (C ２ H ２ 45 ml / min －１ and Ar 180 ml / min －１ The furnace temperature was lowered to room temperature by switching to a different setting for a certain period of time. Depending on the reaction temperature and reaction time, the obtained samples were given names such as TiO2 / C_200C30m, TiO2 / C_250C15m, TiO2 / C_250C30m, TiO2 / C_300C30m, etc. TiO with different exposed surfaces ２ To obtain a single crystal, the single crystal is placed in a horizontal furnace and Ar is introduced while the temperature is set to 10°C min. －１ The temperature was raised to 250°C. After maintaining this state for 30 minutes, Ar was added to the mixed gas (C ２ H ２ 100 ml / min －１ and Ar 400ml / min －１ ) was switched to for a certain period of time. The material obtained was TiO ２Based on the exposed surface, they were named TiO2 / C_110, TiO2 / C_001, and TiO2 / C_100, respectively. To investigate the carbon coating and GQD formation process on the 110 surface, the TiO2 / C_110 surface was exposed. ２ C is applied to single crystals at different reaction temperatures and reaction times. ２ H ２ - CVD was executed.

[0023] 1.2 Preparation of GQD Obtained TiO ２ The C material was dispersed in EtOH and sonicated for 1 hour (28 Hz). After removing larger particles by filtration, a GQD solution was obtained. The obtained GQD solutions were named GQD_200C30m, GQD_250C15m, GQD_250C30m, GQD_300C30m, etc., depending on the reaction temperature and reaction time in the CVD process. The powder remaining after sonication was named R_TiO2 / C_250C15m, R_TiO2 / C_250C30m, etc.

[0024] 1.3 Evaluation of Characteristics TiO ２ The carbon concentration of / C was determined by thermogravimetric analysis (TG, TGA-50, Shimadzu Corporation) in an air atmosphere at 10°C / min. －１ The heating temperature was investigated from 25°C to 900°C. ２ The surface properties of C were analyzed by X-ray photoelectron spectroscopy (XPS, JEOL, JPS-9200) using Al Kα rays. ２ The specific surface area (SSA) of C was measured at -196°C. ２ Characterization was performed by adsorption isotherm. Total pore volume was calculated using P / P0 = 0.96. Crystallinity of GQD was characterized using powder X-ray diffraction (XRD) analysis (MiniFlex 600, Rigaku Inc.) with CuKα radiation generated at 40 kV and 15 mA. The size and shape of GQD were evaluated by transmission electron microscopy (TEM, JEM-2010, JEOLL Ltd.). Functional groups of GQD were analyzed in the wavenumber range of 500–4000 cm⁻¹. －１The characteristics were evaluated using a Fourier transform infrared spectrometer (FT / IR-6600, JASCO Corporation). Total amount of edge sites (N ｅｄｇｅ ) can be calculated using the following formula: N ｅｄｇｅ = 2N Ｈ２ +N Ｈ２Ｏ +N ＣＯ +N ＣＯ２ TiO ２ The height of the GQD on the single crystal was recorded using an atomic force microscope (AFM, Park NX-10). ２ The morphology of the carbon coating on the single crystal was characterized by a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi).

[0025] 1.4 Optical Spectroscopy The ultraviolet-visible (UV-vis) absorption spectrum of the GQD solution was obtained using a JASCO V-770TWK in the wavelength range of 200–800 cm². －１ The fluorescence (PL) emission spectrum of the GQD solution was recorded. The fluorescence (PL) emission spectrum of the GQD solution was measured by JASCO. For comparison, a commercially available GQD solution (Sigma-Aldrich, 900560) prepared by the top-down method was used as a reference and is referred to as GQD_R. For the calculation of the quantum yield (QY) of GQD, QY is given by 0.1 M H ２ SO ４ Using the quinine sulfate inside as a standard substance, the following formula was used for the calculation (QY = 0.54 at 360 nm):

[0026]

[0027] Here, I represents the integrated fluorescence emission intensity, n represents the refractive index of the solvent, and A represents the light absorption rate. The subscripts X and ST represent the sample and standard substance, respectively.

[0028] 1.5 Investigation of the mechanism 1) In-situ CVD-TG TiO250°C ２ Catalyst C on the surface ２ H ２ The deposition process was investigated using in-situ CVD-TG. First, TiO ２Put it into the TG chamber (STA2500 Regulus, NETZSCH Japan), and then introduce the mixed gas (20 ml / min －１ C ２ H ２ and 80 ml / min －１ Ar). The weight change was monitored up to 250 °C at a heating rate of 1 °C / min －１ . 2) In-situ IR C ２ H ２ -The surface change of TiO during the CVD process ２ was investigated by in-situ IR (Shimadzu) analysis using the diffuse reflectance infrared Fourier transform (DRIFT) mode. First, the chamber containing TiO ２ (S.T. Japan Inc., Heat Chamber Type-1000 °C) was heated to 250 °C under an Ar flow (200 mL / min －１ ) and maintained for 30 minutes. Next, the mixed gas (2 mL / min －１ C ２ H ２ and 198 mL / min －１ Ar) was introduced and maintained for 30 minutes. After cooling to room temperature, an air flow (200 mL / min －１ ) or H ２ was introduced. 3) In-situ CVD-Mass C ２ H ２ -The gas evolution during the CVD process was investigated by in-situ MASS (BEL). First, TiO ２ (about 200 mg) was placed in a tube furnace, and a helium (He) flow at a flow rate of 200 ml / min －１ was introduced. The sample was heated to a specific temperature at 10 °C / min ２ until the pressure of H －１ O stabilized. After maintaining for 40 minutes, the He flow was changed to a mixed gas (2 mL / min －１ C ２ H ２ and 198 mL / min －１The gas was replaced with He, and the pressure of the resulting gas was recorded using MASS. Next, the pressure of the resulting gas was converted to molecular weight.

[0029] <Results and Discussion> 2.1 Formation of GQD and Optical Properties The inventors found that the formation of oxygen vacancies (Vo) on the template surface was due to C ２ H ２ It is known that this has a significant impact on the decomposition initiation temperature. ２ and CEO ２ In this case, oxygen atoms readily escape from the template surface as CO to form Vo. The formed Vo acts as an active site, C ２ H ２ It is advantageous for adsorption of C ２ H ２ This lowers the decomposition start temperature of TiO. ２ Because it has a high ability to form Vo, C ２ H ２ - It has the potential to be used as a CVD template. To clarify the starting temperature, TiO2 levels increase with temperature. ２ The weight change was continuously monitored using in situCVD-TG. Figure 3 shows C ２ H ２ TiO ２ This is the result of monitoring the weight change of TiO using TG. As shown in Figure 3, TiO ２ The weight of starts to increase from 70°C. This means that at 70°C C ２ H ２ This means that decomposition occurs at low temperatures. ２ H ２ - The high potential of CVD was demonstrated. Furthermore, TiO at 250°C ２ Carbon growth on TiO was investigated by in-situ TG-CVD (Figure 4 shows TiO at 250°C). ２ (This is the CVD-TG curve of TiO ２ Based on the SSA (calculated from the N2 adsorption / desorption curve shown in Figure 5), the number of carbon layers (N ｓｔａｃｋｉｎｇ It was converted to ). Figure 5 shows TiO ２This is the N2 adsorption / desorption curve for HT250C30m. Carbon growth exhibits a two-step process, which is CeO ２ C to ２ H ２ It is similar to CVD. The initial rapid carbon growth up to about 0.5 layers is C ２ H ２ TiO for decomposition ２ This is due to the excellent catalytic effect of the surface, which significantly slows down subsequent decomposition.

[0030] Due to its ability to grow carbon at low temperatures, GQD solutions can be prepared by CVD followed by sonication. After carbon coating at 250°C for 15 minutes, the resulting TiO2 / C_250C15m was dispersed in EtOH and sonicated for 1 hour. The suspension was then centrifuged to obtain the GQD solution. Due to the localized strong hot spots generated by sonication, the GQD is formed from TiO2. ２ It is separated from C and dispersed in EtOH. The resulting GQD solution appeared yellow under room light and exhibited distinct blue fluorescence under 365 nm UV light (see Figure 6). Figure 6 illustrates GQD_250C15m dispersed in EtOH and GQD powder. After evaporation of the solvent, a brown GQD powder called GQD_250C15m was collected. The XRD pattern of GQD_250C15m (Figure 7) shows a weak 002 peak at approximately 20°, which is attributed to the stacking of graphene nanosheets. Compared to other graphene materials where the 002 peak is at approximately 24°, the 002 peak of GQD is at a lower position, indicating that the spacing between layers is greater than 0.34 nm. On the other hand, the size of the fabricated GQD was small and lacked the long-range ordered lattice structure of graphene, so the 10 peak was not observed.

[0031] The absorption characteristics of the obtained GQD solution (GQD_250C15m) were analyzed using UV-vis spectroscopy (Figures 8 and 9a show the redispersion of the prepared GQD solution, and Figure 9b shows the redispersion of GQD_R). GQD_250C15m showed absorption peaks at 230 nm and 270 nm, which gradually decreased in the visible region. The absorption peak at 230 nm was due to a π→π* transition, the absorption peak at 270 nm was due to an n→π* transition arising from C=O / C-O, and the long tail was related to the functional groups on the surface. This was then compared with commercially available GQD (GQD_R) prepared by the top-down method. Compared with GQD_250C15m, GQD_R did not have an absorption peak at 270 nm, but showed a strong absorption peak at 360 nm, which was due to an n→π* transition. This may be because the top-down approach introduces a large number of -OH and N atoms, resulting in different positions for the n orbitals of GQD_R.

[0032] Next, the photoluminescence (PL) spectra of GQD_250C15m and GQD_R excited at 360 nm were measured. The results are shown in Figure 10. GQD_R shows two PL centers at 445 nm and 513 nm. In comparison, GQD_250C15m showed a wider emission range with three emission peaks centered at 417, 470, and 520 nm. Furthermore, PL spectra were measured at different excitation wavelengths. The results are shown in Figures 11A and 11B. Figure 11A shows the PL spectra of GQD_250C15m at different excitation wavelengths, and Figure 11B shows the PL spectra of GQD_R at different excitation wavelengths. GQD_250C15m showed a weak PL intensity at an excitation wavelength of 280 nm. As the excitation wavelength increased from 280 nm to 360 nm, the PL intensity increased, showing the highest emission intensity at 360 nm. Furthermore, as the excitation wavelength increased from 360 nm to 400 nm, the PL intensity decreased along with the redshift. Moreover, while the emission of GQD_R is independent of the excitation wavelength, GQD_250C15m exhibits excitation-dependent emission characteristics. This may be due to the lack of particle size separation in GQD_250C15m. Additionally, the QY at 360 nm was measured using a relative method with quinine sulfate as the standard substance. The QY of GQD_R is H ２Using oxygen as the solvent, the QY of the prepared GQD_250C15m was calculated to be 10.2%, and it showed a QY of 8.7% in EtOH and 10.5% in toluene, comparable to that of GQD_R. The higher QY in toluene than in EtOH may be due to the fewer oxygen-containing functional groups, which is thought to contribute to the advantage of luminescence in nonpolar solvents.

[0033] To investigate the chemical structures of the two GQDs, the FT-IR spectra shown in Figure 12A were measured. GQD_250C15m was measured at 2966, 2940, and 2870 cm⁻¹. －１ Three peaks centered around, and 1460 and 1380 cm －１ It has two peaks centered around C. ２ H ２ This is due to the vibration of sp3 hybridized carbon produced by the decomposition of . Furthermore, 1730 cm －１ The stretching vibration in the vicinity is due to -C=O, and is 1100-1200 cm. －１ Several peaks in the vicinity are attributed to -CO. In contrast, 3000 and 1400 cm －１ No nearby peaks were observed in GQD_R. However, in GQD_R, peaks belonging to -OH and -NH were observed at 3500 and 3300 cm⁻¹, respectively. －１ The wide peaks in the vicinity and the 1650 and 1550 cm peaks caused by C=O / C=N －１ A strong peak centered on was observed. The high concentrations of -OH and -C=O / C=N are thought to be due to the use of a strong acid during preparation. Based on the above analysis, it is thought that GQD_250C has lower concentrations of oxygenated species such as CO / C=O and -OH because a strong acid was not used during preparation. Figure 12B shows the FT-IR spectra of GQD_300C30m and GQD_R. The results for Figure 12B are the same as those for Figure 12A, so the explanation is omitted.

[0034] To investigate the types and amounts of functional groups, high-sensitivity vacuum TPD (vacuum temperature-controlled desorption) was performed from room temperature to 1800°C (Figures 13 and 14). Figure 13A shows the TPD pattern of GQD_250C15m, Figure 13B shows the TPD pattern of GQD_300C30m, and Figure 14 shows the TPD pattern of GQD_R. The corresponding gas generation amounts and edge site amounts are summarized in the table in Figure 15. Figure 15A shows the results of calculating the gas generation amounts and Nedge for GQD_250C15m and GQD_R from the TPD. The total gas generation amount of GQD_250C15m was 4.5 mmol / g, which is much lower than the 8.3 mmol / g of GQD_R, indicating that GQD_250C15m has fewer edge sites. CO below 800°C ２ and H ２ O emission was much lower than in GQD_R, suggesting that -COOH formation by low-temperature CVD and sonication was lower than in the top-down method, and is consistent with FT-IR. On the other hand, CO emission from C=O was observed around 300°C to 600°C, with maximum gas emission being C. ２ H ２ H related to hydrogen termination edge sites due to decomposition ２ This was the case. CO and H of GQD_250C15m and GQD_R. ２ It is noteworthy that the desorption temperatures of both are lower than those of carbon materials with continuous graphene domains. This is thought to be because the smaller domain size of GQD, which does not have a continuous carbon layer structure, results in lower desorption energy at the edge sites. Furthermore, the low decomposition temperature is due to the sp of GQD_250C15m, which can be observed by FT-IR. ３ This could be due to a high CH content. In Figure 15A, the H content [wt%] is the gas containing desorbed hydrogen (H ２ H ２ The hydrogen content is calculated from the total of (O), and the O content [wt%] is the gas containing the desorbed oxygen (CO, CO). ２ H ２This refers to the oxygen content calculated from the total of O). As shown in Figure 15A, the H content and oxygen content of GQD_250C15m are 0.6% and 4.1%, respectively, which are significantly lower than GQD_R, which has a hydrogen content of 1.3% and an oxygen content of 9.6%. This indicates that the concentrations of -COOH and -OH on the surface of GQD_250C15m are low. This leads to excellent dispersion characteristics in nonpolar solvents. As shown in Figure 9a, GQD_250C15m can be uniformly redispersed in toluene after solvent removal. The good dispersion in nonpolar solvents is thought to be due to the low density of -COOH and -OH on the surface. Figure 9b shows that the commercially available GQD_R does not disperse when the water is evaporated from a dispersed state in water and placed in toluene. Figure 15B shows the results of calculating the gas generation amount and Nedge of GQD_300C30m from TPD.

[0035] As mentioned above, the oxygen content of GQD_250C15m is 4.1 wt%, and the oxygen content of GQD_300C30m is 5.7 wt%, which is lower than the oxygen content of the commercially available GQD_R, which is 9.6 wt%. The oxygen content of the GQD according to this embodiment is lower than 9.6 wt%, and in one example it is 6 wt% or less, and in another example it is 5 wt% or less.

[0036] 2.2 Mechanism of GQD formation TiO ２ To investigate carbon coating on the surface and the formation of GQD, multiple TiO2 single crystals with different exposed crystal planes were subjected to carbon deposition at 250°C. ２ H ２ It was used in CVD. According to the inventors' experiments, TiO ２ The formation rate of the GQDs shown above is related to the exposed crystal planes, and the 110 plane was suitable for GQD production because it grew quickly and uniformly. Therefore, the process of GQD formation on the 110 plane at 250°C was investigated by varying the reaction time and observed with AFM and SEM (Figure 16). Figures 16a-f show the results at 250°C. ２ H ２ -TiO after CVD ２(110) AFM images of single crystal plates. CVD times were 15 minutes (Figures 16a, d), 30 minutes (Figures 16b, e), and 60 minutes (Figures 16c, f). Figures 16g, h, i show C at 250°C. ２ H ２ -TiO after CVD ２ These are SEM images of single crystal plates, with deposition times of 15 minutes (Figure 16g), 30 minutes (Figure 16h), and 60 minutes (Figure 16i), respectively. The insets in Figures 16a-c show the results at 250°C. ２ H ２ -TiO after CVD ２ (110) Images of single crystal plates, with deposition times of 15 minutes (Figure 16a), 30 minutes (Figure 16b), and 60 minutes (Figure 16c), respectively.

[0037] The single crystal plate is light brown in color, and the color deepens with reaction time (inset in Figure 16a-c). When the reaction time is 15 minutes, TiO ２ The uniform carbon deposits shown above were observed in a dot morphology. Subsequently, as the reaction time increased, the density of the carbon deposits gradually increased, and some of the dot-like carbon particles connected, resulting in a linear, island-like distribution. This island-like distribution was also observed by SEM. When the reaction time was increased to 60 minutes, the density of the dot-like carbon particles increased, and the island-like distribution became more pronounced. Furthermore, according to the inventors' experiments, the precipitation of a substance considered to be GQD was confirmed by CVD on a 1 cm × 1 cm titania single crystal plate. Therefore, although a titania surface is necessary for the formation of GQD, the size of the metal oxide nanoparticles is not particularly limited.

[0038] This unique dot-shaped growth is C ２ H ２ TiO for decomposition ２ Catalytic behavior and TiO ２ This is thought to be related to the distribution of the active sites shown above. C ２ H ２ When introduced at 250°C, TiO ２ It tends to adsorb and decompose around the active site, but TiO2 is far from the active site. ２It is difficult to decompose on the surface. The catalytic effect around the active site decreases as carbon deposition continues, but this is because of CH ４ - This is similar to how the graphene formation rate decreases as the carbon layer increases in CVD. Therefore, as shown in the previous TG-CVD curve, the reaction is rapid C ２ H ２ The process begins with decomposition, followed by a decrease in rate due to the weakening of the catalyst. Furthermore, rutile-type TiO ２ Prior art has shown that Vo on the 110-plane is easily formed uniformly, and as oxygen release increases, it grows linearly, which coincides with carbon growth. ２ Upper C ２ H ２ Decomposition is thought to be directly related to the formation of Vo.

[0039] However, when the reaction temperature was raised to 450°C, a different growth behavior was observed. Unlike when the reaction temperature was 250°C, C ２ H ２ As the decomposition rate increased, a visible black carbon layer was observed on the plate after CVD (insets in Figures 17a and 17b). Figures 17a and 17b show the results at 450°C. ２ H ２ -TiO after CVD ２ (110) AFM images of single crystal plates. The CVD time was 30 minutes for Figure 17a and 60 minutes for Figure 17b. Figures 17c and d show CVD at 250°C. ２ H ２ -TiO after CVD ２ (110) SEM images of single crystal plates. The CVD time was 30 minutes for Figure 17c and 60 minutes for Figure 17d. The insets in Figures 17a and 17b show CVD at 450°C. ２ H ２ -TiO after CVD ２ (110) Images of single crystal plates, with deposition times of 30 minutes and 60 minutes, respectively. No significant height changes were observed in the AFM images for both 30-minute and 60-minute reaction times (Figures 17a, b), and only a flat surface was observed in the SEM images (Figures 17c, d). Therefore, TiO ２ The uniform graphene growth shown above was presumably carried out at 450°C. The formation of GQDs was performed at low temperatures under TiO２ It was demonstrated that this is due to the unique carbon deposition behavior on the surface.

[0040] To investigate why GQDs are removed by sonication, the chemical structures of TiO2 / C_250C15m and R_TiO2 / C_250C15m were analyzed by XPS and are shown in Figure 18. Figure 18a shows the C1s spectrum of TiO2 / C_250C15m, which can be decomposed into four peaks centered at 284.4 (C=C), 285.0 (-CO), 285.96 (-C=O), and 288.4 eV (-COOH). After sonication, the intensity of the peak originating from -CO decreased from 32.2% to 21.4% (Figure 18b). Furthermore, no peaks attributable to the Ti-C bond were observed. ２ H ２ This suggests that the C-O-Ti is not deposited and bonded to the Ti atoms, but rather absorbed to the surface via C-O-Ti. Subsequently, this C-O-Ti is broken down during sonication, and GQD becomes TiO ２ It peels off from the surface.

[0041] To investigate the origin of oxygen-containing edge sites on GQD, TiO was used during the CVD process at 250°C. ２ Surface changes, C ２ H ２ The system was continuously monitored using in-situ IR spectroscopy while (1 v% with Ar) was flowing. Figure 19 shows TiO2 at 250°C. ２ C due to catalytic action ２ H ２ These are in-situ IR spectra during decomposition. Figures 20, 21, and 22 show H ２ TiO ２ These are in-situ IR spectra of C, taken at room temperature, 250°C for 30 minutes, and 250°C for 2 hours, respectively. As shown in Figure 19, ~1430 cm⁻¹ was observed throughout the entire process. －１ The vibration band centered around TiO ２ This is due to TiO-Ti oscillations. When the temperature is raised from room temperature to 250°C while Ar is flowing, TiO ２ Because hydroxyl groups are removed from the surface, 1630 cm －１ The vibration peak gradually disappears. C２ H ２ With the introduction of this, 2970, 2930, 2870 cm －１ Three new peaks centered around C are increasing in number, and this is C ２ H ２ sp generated from decomposition ３ It originates from C-H. However, no oscillation peaks assigned to -CO or C=O were observed. ２ H ２ After stopping the introduction of the airflow and allowing it to cool to room temperature, the airflow was switched to the reaction holder. 1650 cm －１ A peak originating from -C=O appeared at the position. Therefore, most of the oxygen-related edge sites of GQD are edge sites and O when exposed to air. ２ This is thought to be due to the reaction. Several radicals are thought to be present at the edges of the carbon atoms after CVD, and they readily react with air to form edge sites.

[0042] After that, C ２ H ２ When the introduction was stopped and the room was cooled to room temperature, H ２ The solution was introduced into the reaction holder (1 v% with Ar) and observed by in-situ IR spectroscopy. As shown in Figure 20, at 1650 cm⁻¹ －１ A peak appears centered at -C=O, which is due to the formation of carbon-coated TiO. ２ Before the upper edge radical comes into contact with air, H ２ When exposed to H ２ It was demonstrated that the reaction between edge radicals and the GQD surface changes the type and concentration of oxygen-containing functional groups. Next, at 250°C, C ２ H ２ After introducing H into the reaction holder ２ (1v% withAr) was introduced, and the introduction time was changed. H ２ After stopping the introduction and cooling, H ２ It was reintroduced. As shown in Figure 21, it was heated at 250°C for approximately 30 minutes. ２ When H was introduced, no peak appeared. After cooling to room temperature, H was used again. ２ When this is introduced, 1650cm －１A peak located at -C=O is observed, which is attributable to H ２ When introduced at 250°C for 2 hours, no new peak appeared, and when cooled, H ２ Even after reintroducing -C=O, no peaks belonging to -C=O were observed. Therefore, H ２ While it tends to react with radicals at the ends of GQD, it is thought to form different types of oxygen-containing functional groups via different pathways.

[0043] H at different temperatures for oxygen-containing functional groups ２ To further investigate the detailed impact of the process on GQD, C ２ H ２ After decomposition is complete, leave at 250°C and room temperature for 2 hours. ２ Next, GQDs were collected by sonication and named GQD_250C15m_250C H2 and GQD_250C15m_H2, respectively. Then, the two GQDs were measured by TPD and compared with GQD_250C15m (see Figures 23 and 24). Figures 23a, b, and c are the TPD profiles of GQD_250C15m, GQD_250C15m_H2, and GQD_250C15m_250C H2, respectively. The corresponding gas generation and edge site amounts are summarized in the table in Figure 24.

[0044] Different amounts of H ２ After treatment, gas emissions increased. GQD_250C15m_250C H2 increased slightly from 4.5 mmol / g to 5.2 mmol / g, and GQD_250C15m_H2 increased to 7.1 mmol / g. ２ Compared to untreated GQD, the H and O content does not change significantly above 250°C, but the H content in the low-temperature range does not change significantly. ２ and H ２ The desorption and attachment of O increased. Also, H at room temperature ２ After the treatment, the H and O content clearly increased, and a new desorption peak appeared around 200-300°C. ２ After processing, H ２ It is noteworthy that the desorption temperature increases. This is because H ２ After processing, sp near the H-edge site ３This is thought to be because the -C decreases and the desorption energy at the H-end site increases.

[0045] Next, UV-vis spectra and PL spectra were measured to investigate the effect of different edge sites on the PL properties of GQD. Figure 25a shows the UV spectra of GQD_250C15m, GQD_250C15m_H2, and GQD_250C15m_250C H2 in EtOH. Figures 25b, 25c, and 25d show the PL spectra of GQD_250C15m, GQD_250C15m_H2, and GQD_250C15m_250C H2 in toluene, respectively. As shown in Figure 25a, H at room temperature ２ The UV-vis spectrum after treatment showed a stronger peak centered at 270 nm. This is due to the formation of n-π-CHO. * This is thought to be due to an increase in transitions. However, n-π * The peaks attributable to the transition decreased. Subsequently, the PL spectrum was measured at different excitation wavelengths. As shown in Figure 25b, at 280 cm⁻¹ －１ and 360cm －１ There are two emission peaks located at π-π, which are respectively π-π * Transitions and n-π * It originates from the luminescence of the transition. n-π * The intensity determined by the transition is H at room temperature. ２ After processing, it is enhanced (Figure 25c), and H at 250°C ２ The QY decreased significantly after treatment (Figure 25d), which is consistent with the UV spectrum. Figure 26 shows the calculated QY values ​​for these three samples in EtOH and toluene. The QY of GQD_250C15m_H2 increased by approximately 2%, from 8.67% to 10.6% in EtOH and from 10.49% to 12.47% in toluene. Furthermore, the QY of GQD_250C15m_250C H2 increased by approximately double, to 16.5% in EtOH and 23.1% in toluene. Therefore, H ２ The process is considered useful for improving QY.

[0046] Figure 25a illustrates that GQD_250C15m exhibits ultraviolet light absorption in toluene. Furthermore, Figure 25b shows that the same sample exhibits fluorescence in toluene. These findings support the fact that GQD_250C15m, as shown in Figure 9a, disperses well in toluene.

[0047] 2.3 TiO ２ Upper catalyst C ２ H ２ Decomposition mechanism As mentioned above, at low temperatures TiO ２ GQDs are generated around the active site on the surface, which is related to Vo formation. Therefore, TiO ２ The TiO is processed by introducing Ar at different temperatures. ２ Vo formation on the surface was investigated (Figure 27a). Figure 27a shows images of TiO2-pristine, TiO2_HT_250C30m, TiO2_HT_450C30m, and TiO2_HT_600C30m. ２ The color gradually darkens as the heat treatment progresses, changing from white for TiO2_pristine to gray for TiO2_600C30m. This is thought to be due to an increase in Vo density. Next, the UV spectra of all heat-treated samples were measured. As shown in Figure 27b, all samples showed strong absorption in the UV region, and the absorption intensified as the heat treatment temperature increased. The increase in absorption is attributed to the narrowing of the band gap due to the introduction of Vo. Next, the corresponding band gap was calculated as shown in Figure 27c, and the results are summarized in Figure 28. The band gap decreased from 3.07 eV for TiO2_pristine to 2.98 eV for TiO2_600C30m, confirming that the Vo density increased with increasing reaction temperature.

[0048] Vo density is C ２ H ２ To clarify the effect on decomposition, Ar was introduced before CVD to TiO ２ Heat it at various temperatures, and after the temperature drops to 250°C, C ２ H ２ We introduced C. ２ H ２ TiO in the implementation process２ The weight change was investigated using TG. Figure 29a shows TiO after heat treatment at different temperatures. ２ This is the TG-CVD curve at 250°C. Figure 29b is an enlarged view of the initial period. As shown in Figure 29a, all CVD-TG curves are TiO ２ Rapid C due to catalytic effect ２ H ２ This shows a two-stage process corresponding to decomposition and continued carbon growth after the catalytic effect weakens. Figure 29b is a magnified view of the initial stage. The higher the heat treatment temperature, the gradually shorter the duration of the first stage at the same growth rate. Therefore, TiO ２ Although the catalytic effect was not weakened, it was suggested that the number of active sites on the surface decreased as the surface Vo increased. Furthermore, the slope of the second stage gradually decreased with increasing heat treatment temperature. This is because TiO has different Vo densities. ２ Above is C ２ H ２ This is due to differences in their adsorption capabilities. C has a high electron density. ２ H ２ C ２ H ２ Rutile-type TiO ２ It is thought to donate electrons to it. Oxygen is removed and TiO ２ Because excess electrons remain on the surface, TiO ２ As the surface Vo density increases, TiO ２ The electron density above gradually improves. Therefore, as the heat treatment temperature increases, TiO ２ C to ２ H ２ Adsorption becomes difficult, and the growth rate in the second stage decreases. In summary, C ２ H ２ TiO before implementation ２ The increase in Vo density above does not affect the catalytic decomposition rate, but reduces the number of active sites. Therefore, TiO ２ Upper C ２ H ２ It is thought that the decomposition is catalyzed not by Vo itself, but by the Vo formation process.

[0049] Next, TiO ２ C above２ H ２ To investigate the detailed mechanism of decomposition, C ２ H ２ (1% in Ar) is introduced to release gas and C ２ H ２ The changes were observed using Mass. Figure 30 shows C under observation by Mass. ２ H ２ C's reaction when it was introduced ２ H ２ This shows the amount of C. As shown in Figure 30, ２ H ２ Decomposition began at 130°C, and the decomposition rate increased with the reaction temperature, reaching a maximum value at approximately 400°C. After that, the decomposition rate decreased slightly and stabilized at 15 umol / min. This is because the catalytic effect weakens with carbon coating. Next, the generation rate was calculated and is shown in Figure 31a. Figure 31a shows C under observation by Mass. ２ H ２ This indicates the gas generation rate upon installation. H ２ The formation process begins at 130°C, followed by the removal of CO, and then TiO ２ It has been shown that Vo is formed above. ２ The generation process is not based on the active site being Vo, but rather on the Vo generation process being based on C ２ H ２ This reaffirms that it promotes decomposition. After that, CH4 is heated from approximately 240°C. ４ and C ６ H ６ H is generated and reaches its maximum value at 380°C. After that, the generation rate decreases. ２ The release of C increases. This is due to C at high temperatures. ２ H ２ The rapid decomposition of H and the formation of graphene ２ This is thought to be because the amount of [substance] increases, suppressing other side reactions.

[0050] As can be seen from the previous TG-CVD curve shown in Figure 3, TiO ２ Upper C ２ H ２ The decomposition initiation temperature is approximately 70°C, but gas release is not clearly observed. Therefore, in order to clarify the changes before gas release, C was used at 100°C. ２ H２ By introducing TiO ２ The surface changes were observed using IR spectroscopy. Figures 31b, c, and d show the results at 100°C, 150°C, and 250°C, respectively. ２ H ２ This is the in-situ FT-IR spectrum after introducing the signal for 30 minutes. As shown in Figure 31b, at a reaction temperature of 100°C, the value is 2940 cm⁻¹. －１ and 2850cm －１ Two weak peaks appear located at -CH2-(-sp 3 Due to the formation of CH. 2820 cm －１ and 2720cm －１ There is no peak centered around CeO ２ Upper C ２ H ２ This is different from disassembly. TiO ２ Upper C ２ H ２ The induction period for decomposition is C ２ H ２ →CC ２ Instead of following H+H, other C ２ H ２ This suggests that they are connected and linked.

[0051] Next, the reaction temperature was increased to 150°C (Figure 31c). ２ H ２ Introducing this will result in 3000 cm －１ Four peaks appear below, corresponding to -CH2- and -CH3. Furthermore, 3030 cm －１ A small peak appeared at this location, suggesting the formation of -C=C. Then, when the reaction temperature rose to 250°C, the peak increased to 3060 cm⁻¹. －１ A peak located at C was observed. ６ H ６ The formation of was shown. In summary, C ２ H ２ This is a structurally altered TiO ２ It is thought that the -CH3 is adsorbed and forms -CH3 and -C=C-. Next, the -CH3 is released from the surface after obtaining H, and the -C=C- is C ２ H ２ And continued to react C ６ H ６It forms a matrix, and ultimately forms graphene.

[0052] Furthermore, C under different reaction temperatures ２ H ２ TiO ２ Gas emissions from were measured and calculated. Figures 32a, 32b, 32c, and 32d show C at different temperatures, respectively. ２ H ２ H when it was introduced ２ CO, C ６ H ６ ,CH ４ This figure shows the incidence rate of H. As shown in Figure 32a, below 450°C H ２ This rarely occurs. ２ H ２ But initially C ２ This is consistent with the previous assumption that it does not decompose into H and H. Figures 33a and 33b show C at 150°C and 600°C, respectively, under observation by Mass. ２ H ２ This shows the intensity of gas generation upon introduction. As shown in Figure 33a, there was no significant increase in the raw intensity at m / z (mass-to-charge ratio) = 41, 42, and 78, while a slight increase was observed at m / z = 51. Two C ２ H ２ C ４ Only chemical species are produced, C ３ Chemical species and C ６ H ６ It has been suggested that it is not produced. Also, at 450°C, a small amount of H is produced during the first 10 minutes. ２ Formation is observed, and below 600°C, graphene is formed rapidly, resulting in a large amount of H ２ This was generated. H ２ Compared to C ６ H ６ and CH ４ Since it can be formed at temperatures above 150°C, at low temperatures the H atoms become CH ４ C ６ H ６ It is suggested that it is easily released as CH (Figures 32c-32d). ４ The formation of C is remarkably fast at 600°C, but C is produced at different reaction temperatures except 150°C. ６ H ６There was no significant difference in the generation of CO. Furthermore, CO generation was observed at all temperatures, and the generation rate increased with increasing temperature. The maximum generation rate was observed at 600°C, after which it decreased. Since CO is produced in a catalytic process, C ２ H ２ It was demonstrated that it only affects the initial stage of degradation. Furthermore, CO and CH ４ The generation patterns are similar, and the intensities of m / z=41 and 42 also show the same generation pattern (Figure 33b). ３ Chemical species, CO, CH ４ It is suggested that they are likely to be generated in the same step. Also, m / z=51 and 78 are H ２ It is suggested that the process is similar to generation and likely occurs in the same steps.

[0053] Based on the results so far, TiO ２ C above ２ H ２ The decomposition mechanism was estimated and is shown in Figure 34. Figure 34 shows TiO ２ Catalyst C above ２ H ２ This is the decomposition mechanism scheme. First, TiO ２ Upward C ２ H ２ Adsorption occurs. As mentioned above, C ２ H ２ is TiO ２ When adsorbed to TiO ２ It acts as an electron donor to the surface. Therefore, C atoms with high electron density become rutile-type TiO ２ It becomes easier for the upper oxygen atom to bond. Also, the angle between the C-H bonds changes, and the C-C bond becomes longer. This is because the electron density between the C atoms decreases, and after adsorption, C ２ H ２ This is because it becomes difficult to maintain the linear structure. As a result, the C-C bond changes from sp hybridization to sp ２ It changes to a hybrid, and then sp ３ Similar to a mixed formation, 3000 cm －１ It shows an IR peak of less than . On the other hand, absorbed C ２ H ２ The H atoms from TiO ２It combines with oxygen atoms on the surface to form -OH, or TiO ２ H ２ It may be released as O. ２ H ２ The absorption process is TiO ２ This involves Vo formation on the surface.

[0054] The next step is nucleation and chain growth. As shown in Figure 34b, the main pathway is another C ２ H ２ The adsorbed substance combines with C ４ It forms a seed, then C ６ H ６ This is how it forms. After that, the process continues, and a large amount of H ２ It forms graphene while releasing C. ２ H ２ It becomes saturated with H atoms, and then CH ４ It may be released as such. And the remaining C will be released as CO into TiO ２ Released from or more C ２ H ２ It is thought that it combines with other elements to form graphene.

[0055] 2.4 TiO ２ Reuse and large-scale synthesis of GQD To manufacture GQD on a large scale, 10g of TiO at a time ２ C in a rotary kiln ２ H ２ - Used in CVD. C ２ H ２ After introducing it at 250°C for 15 minutes, 9.75g of carbon-coated TiO ２ Next, 1 g of carbon-coated TiO ２ The mixture was dispersed in EtOH and sonicated for 1 hour. After centrifugation and evaporation of ethanol, 1.69 mg of GQD powder was obtained. Therefore, 10 g of TiO ２ It is estimated that 16.5 mg of GQD can be obtained in a single CVD operation using this method. By scaling up the reactor, it is possible to increase the weight of GQD obtained at one time.

[0056] Next, TiO ２ We attempted to reuse the template. C was used as the carbon source. ２ H ２ After preparing GQD at 250°C for 15 minutes using the same conditions for the second and third GQD preparations, the remaining carbon-coated TiO ２ Then, with air introduced, it is heat-treated at 350°C for 90 minutes to remove all of the coated carbon, and then TiO ２ We obtained the following. Lowering the heat treatment temperature not only removes all residual carbon, but also TiO ２ It also helps to suppress changes in SSA. Figures 35a and 35b show TiO after heat treatment at 350°C for 90 minutes, once and twice, respectively. ２ N ２ Adsorption / desorption is shown. As shown in Figures 35a and 35b, the SSA of TiO2 is (78.2m) before CVD. ２ ・g －１ ) and after the first heat treatment at 350°C (76.8m ２ ・g －１ ) and after the second heat treatment (75.9 m ２ ・g －１ There is almost no change between the two. TiO after heat treatment ２ C ２ H ２ - It was reused as a CVD template. This reuse process is recyclable, and the resulting carbon-coated TiO ２ This is the number of CVD recycling cycles, TiO2 / C_2 nd _250C15m and TiO2 / C_3 rd It was named _250C15m. Figures 36a and 36b show GQD_250C15m and GQD_2 in toluene, respectively. nd _250C15m, GQD_3 rd The UV-vis spectrum and the PL spectrum excited at 360 nm are for GQD_250C15m. The Cwt% (weight ratio of carbon component) of GQD_250C15m was 6.9 before sonication and 5.3 after sonication, with a yield of 23.2%. nd The Cwt% of _250C15m was 5.85 before sonication and 5.3 after sonication, with a yield of 9.4%. GQD_3rd The Cwt% of _250C15m was 5.47 before sonication and 5.0 after sonication, with a yield of 8.6%. nd The Cwt% of _250C2h was 5.3 before CVD, 9.4 before sonication, and 8 after sonication, with a yield of 34.1%. GQD_3 rd The Cwt% of _250C2h was 8.0 before CVD, 10.0 before sonication, and 9.3 after sonication, with a yield of 35%. Figures 36c and 36d show GQD_2, respectively. nd _250C15m, GQD_3 rd These are PL spectra of _250C15m excited at different wavelengths. As shown in Figure 36a, GQD_2 nd _250C15m and GQD_3 rd _250C15m exhibits a UV-vis spectrum similar to GQD_250C15m, showing two absorption peaks in the UV region attributable to π-π* and n-π*. As shown in Figure 36b, the PL spectra of all GQDs excited at 360 nm showed similar emission patterns. Furthermore, GQD_2 nd _250C15m and GQD_3 rd _250C15m exhibited excitation-dependent photoluminescence characteristics at multiple excitation wavelengths (Figures 36c, 36d).

[0057] Also, GQD _2 nd _250C15m and GQD _3 rd The QY values ​​for 250C15m were measured and summarized in the table in Figure 37. GQD _2 nd The QY content of _250C15m was calculated to be 13.3% in EtOH and 13.4% in toluene, and GQD _3 rd The QY of 250C15m was calculated to be 14.8% EtOH and 11.9% toluene. ２ It was found that the optical properties do not change significantly by reusing rutile-type TiO ２ It was suggested that recycling this material into the manufacturing of GQD could contribute to further cost reductions in GQD production.

[0058] 3. Summary: C at low temperatures ２ H ２- It was found that GQD solutions can be produced by CVD followed by ultrasonic treatment. This is because TiO ２ C above ２ H ２ This is due to the unique growth behavior of the decomposition. Furthermore, the formation of Vo, which acts as a catalytic process, leads to C ２ H ２ The decomposition of and subsequent graphene formation are promoted. The resulting GQD has fewer surface functional groups than commercially available GQD, and in particular, fewer oxygen-terminated edge sites. Also, the oxygen-terminated edge sites are H ２ Because it can be modified by processing, QY is significantly improved, especially when processed at 250°C. This method has a high potential to produce a large quantity of GQD at once, and furthermore, TiO used as a template ２ Because it can be recycled, manufacturing costs can be further reduced.

[0059] 4. Regarding the temperature range in which metal oxide nanoparticles can be manufactured using GQD, the inventors stated that TiO ２ We confirmed that when the temperature of the nanoparticles used to precipitate GQD on the surface of the nanoparticles is increased to 350°C, the yield of GQD becomes almost zero. As mentioned above, TiO ２ C above ２ H ２ Since the temperature at which weight increase begins when the solution is passed through is 70°C, the temperature range for metal oxide nanoparticles to precipitate GQD can be, for example, 70°C or more and less than 350°C. Figure 38 shows the yield of GQD formed at different reaction temperatures for 30 minutes. The yield of GQD is determined by low-temperature CVD. ２ This was calculated by the weight reduction of carbon due to ultrasonic treatment relative to the total amount of carbon grown on the surface. Figure 38 shows that GQD can be quantitatively synthesized at 200°C to 300°C. Therefore, the temperature range for metal oxide nanoparticles during CVD can also be 200-300°C.

[0060] As mentioned above, the reaction temperature for titania can be 200-300°C or between 70°C and 350°C. When cerium oxide was used as the metal oxide nanoparticle, it was confirmed that GQD could be produced at 300°C, but not at 150°C or 450°C. The reason GQD could not be synthesized at 150°C is that even if carbon components precipitate, graphene does not grow in this temperature range. The reason GQD could not be synthesized at 450°C is that the size of the graphene becomes too large. In this case, the graphene becomes, for example, larger than several tens of nanometers. It is thought that the reason why GQD cannot be obtained at 350°C in the case of titania is the same. When zinc oxide or aluminum oxide was used as the metal oxide nanoparticle, it was confirmed that GQD could be produced at 300°C. Figure 39 shows the results when aluminum oxide and zinc oxide were used as the metal oxide nanoparticles, and the carbon-containing compound was C ２ H ２ The properties of GQD produced by supplying the reaction mixture at a reaction temperature of 300°C for 30 minutes are illustrated. The solvent used was EtOH. Figures 39A and 39B are photographs of GQD formed using aluminum oxide and zinc oxide as metal oxide nanoparticles, respectively, irradiated with indoor light and UV light. Neither of the ultrasonically treated GQDs were transparent, and particles remained. Both GQDs emitted fluorescence when exposed to UV light. Furthermore, from Figures 39C and 39D, it was found that their UV-vis spectra were similar. Figure 39E is the PL spectrum, and it was found that the two GQDs had similar PL spectra. In both cases of metal oxide nanoparticles, the reaction temperature must be high enough for graphene to grow, but the temperature must be controlled so that the graphene does not become too large. A suitable temperature range for this is considered to be between 200°C and 300°C.

[0061] 5. Examination of various metal oxide nanoparticles TiO ２ The lowest reaction temperature at which GQD can be produced is 70°C. Experiments conducted by the inventors showed that CeO2 is produced as a metal oxide nanoparticle. ２ Al ２ O ３When using ZnO, it was found that GQD can be produced from 113°C, 150°C, and 180°C, respectively. At lower reaction temperatures, C is present in the initial stage. ２ H ２ The decomposition rate is low. GQDs made from all metal oxide nanoparticles emitted similar blue fluorescence. In this experiment, CeO ２ Al ２ O ３ The particle size of ZnO was approximately 20 nm. Among these metal oxide nanoparticles, TiO ２ Only this could be easily separated from the GQD solution. The metal oxide nanoparticles were hafnium oxide (HfO) with a particle size of 30 nm. ２ When using ), acetylene (C ２ H ２ The decomposition start temperature of ) was confirmed to be 108°C. This suggests that GQD can be produced from low temperatures of 108°C or higher using hafnium oxide. Figure 40A is a photograph of GQD obtained using hafnium oxide as a template for metal oxide nanoparticles. This GQD was synthesized using a vertical furnace, with hafnium oxide as the base material and acetylene as the carbon-containing compound, maintaining a reaction temperature of 300°C for 2 hours. As shown in Figure 40A, it was found that GQD exhibiting fluorescence to UV light can be obtained. Zirconium oxide (ZrO) was used as the metal oxide nanoparticle. ２ When using ), as shown in Figure 40B, acetylene (C ２ H ２ The decomposition start temperature of ) was confirmed to be 100°C. This suggests that GQD can be produced from low temperatures of 100°C or higher using zirconium oxide. Figure 40C is a photograph of GQD obtained using zirconium oxide as a template for metal oxide nanoparticles. This GQD was synthesized using a vertical furnace, with zirconium oxide as the base material and acetylene as the carbon-containing compound, maintaining a reaction temperature of 300°C for 2 hours. As shown in Figure 40C, it was found that GQD exhibiting fluorescence to UV light was obtained. Figure 41 shows ceria (CeO) ２ This is the analysis result for GQD obtained using ) as a template. Figure 41A shows CeO ２This is a photograph of a GQD solution formed by irradiating it with indoor light or UV light (365 nm). It was found to emit blue fluorescence under UV light. Figure 41B is the UV-VIS spectrum of the same sample. Figure 41C is the photoluminescence spectrum of the same sample. From this experiment, it was found that GQD can be produced using ceria as a template. Figure 42A is a diagram showing the TPD profile of GQD obtained using ceria as a template. This TPD profile is shown in Figure 13B as TiO ２ The TPD profile is similar to that of the GDQ obtained using the template, suggesting that the functional groups on the surface are similar. Figure 42B shows the amount of gas generated and the amount of edge sites when ceria is used as the template. The results in Figure 42B can be compared with the measurement and calculation results for each sample in Figure 15. As negative data, metal oxide nanoparticles were used as hydroxyapatite and calcium carbonate, respectively, at a reaction temperature of 300°C. ２ H ２ Although we provided the necessary materials, we were unable to manufacture GQD. Using metal oxide nanoparticles as MgO, we reacted at a temperature of 400°C with C ２ H ２ Although we provided the materials, we were unable to manufacture GQD. The materials were metal oxide nanoparticles and SiO ２ Even in that case, GQD could not be manufactured. MgO, SiO ２ In the case of acetylene, the temperatures at which carbon deposition occurs are high (317°C and 525°C), making it impossible to obtain GQD. The yield for each metal oxide nanoparticle was investigated. The yield is the ratio of GQD recovered by sonication to the total carbon precipitated by CVD. The material that could not be recovered by sonication is the carbon component that did not detach from the metal oxide nanoparticles. TiO ２ The yields are shown in Figure 38. The yield of cerium oxide was below the limit of quantification, generally less than 1 wt%.

[0062] 6. Examination of carbon-containing compounds as carbon sources. The ability to synthesize GQD depends on the carbon source. Acetylene has a low decomposition temperature on oxides, but methane has a high decomposition temperature of generally 700°C or higher on oxides, making GQD synthesis impossible. In addition, it is known that GQD synthesis is not possible with acetonitrile, but is possible with acrylonitrile. Figure 43A shows C as a carbon source. ３ H ６ This figure shows the results of an investigation into the decomposition temperature of (propylene). ２ The decomposition temperature of propylene was investigated, and the result was found to be 383°C. When attempting to synthesize graphene quantum dots (GQDs) using propylene as a carbon source at 400°C, it was found that GQDs could not be obtained. Figure 43B shows C as a carbon source. ２ H ４ This figure shows the results of an investigation into the decomposition temperature of ethylene. ２ The decomposition temperature of ethylene was investigated, and the result was found to be 400°C. When attempting to synthesize graphene quantum dots (GQDs) at 400°C using ethylene as the carbon source, it was found that GQDs could not be obtained. In these GQD synthesis experiments, a vertical furnace was used, titanium dioxide was used as the base material, propylene or ethylene was used as the carbon-containing compound, and the reaction temperature was maintained at 400°C for 2 hours.

[0063] 7. Possibilities of Reduced CVD Time and Number of GQD Layers Figure 4 illustrates that extending the CVD time under titania, acetylene CVD, and 250°C conditions increases the average number of layers. The inventors confirmed that under these conditions, GQD could be obtained with a yield of 20-25% of the total precipitated carbon when the CVD time was from 5 min to 60 min. The experimental results are shown in Figure 44A. The CVD time does not affect GQD synthesis as long as it is at least 60 min. However, for example, to coat a single layer, it is necessary to adjust the CVD time. In this example, it is necessary to keep it within about 60 minutes. It was found that even with a small number of carbon layers, there are no problems in GQD production, and for example, GQD can be produced even with a CVD time of 5 minutes. Therefore, GQD can be produced from a single layer of carbon that is not multi-layered. This suggests the possibility of shortening the time required for GQD production. Figure 44B shows the UV-VIS spectra of each sample. From this spectrum, the strongest absorption peaks appear at 230 nm and 270 nm in GQD_250C15m, corresponding to the π→π* transition and n→π* transition, respectively. Figure 44B shows that GQD_250C15m is optimal.

[0064] 8. SiO ２ Production of GQD using nanoparticles SiO modified with silyl groups ２ While heating the nanoparticles to a temperature between 500°C and 900°C, the SiO ２ A carbon-containing compound is brought into contact with nanoparticles. For example, the carbon-containing compound may include acetylene or acrylonitrile. This results in SiO ２ A graphene-nanoparticle composite can be obtained by depositing graphene quantum dots on the surface of the nanoparticles. In this way, the inherently inert SiO ２By activating the surface of nanoparticles with silyl groups, GQDs can be synthesized by CVD using carbon-containing compounds. Furthermore, the graphene quantum dots can be detached from the nanoparticles by applying mechanical tools, mechanical or ultrasonic cleaning, chemical reagents, heat, or other external energy sources to the graphene-nanoparticle composite. After providing the graphene-nanoparticle composite in an organic solvent, the graphene quantum dots can be detached from the nanoparticles. For example, by sonicating the organic solvent to which the graphene-nanoparticle composite has been provided, the graphene quantum dots disperse in the organic solvent. Then, by removing the organic solvent, powdered graphene quantum dots can be obtained. Used SiO ２ The nanoparticles can be recovered by filtration, decantation, or centrifugation, and reused by heating them in air to burn off any remaining carbon.

[0065] As one example, the following experiment was conducted: TMS-modified SiO ２ GQD synthesis on nanoparticles: SiO modified with trimethylsilyl group (TMS) ２ Nanoparticles were prepared. SiO₂ modified by TMS. ２ The nanoparticles are sometimes referred to as TMS-SPS150-10. SPS150-10 refers to porous SiO2. ２ This means SiO ２ C ２ H ２ CVD was performed using SiO as the carbon source. Specifically, SiO ２ Place the nanoparticles in a horizontal furnace and add 500 mL of Ar. －１ Introduce at this flow rate, then raise the temperature in the furnace to 10°C min. －１ The mixture was heated to 600°C at the following heating rate. After reaching 600°C, it was held for 30 minutes, and then Ar was added as a mixed gas (C ２ H ２ 20 mL min －１ Ar 480 mL min －１ The CVD was performed by switching to a mixed gas of [compound name]. After the CVD was completed, Ar was passed through the furnace again while it cooled to room temperature. The obtained GQD / SiO ２The complex is denoted as TMS-SPS150-10_x_y, where x is the TMS doping density [μmol m －２ ], where y represents the CVD time [min]. The obtained graphene-nanoparticle composite was dispersed in toluene, and GQD isolation was attempted by sonication.

[0066] Figure 45 shows the results of an investigation into the effect of TMS doping density on GQD. Figure 45 shows five liquids A, B, C, D, and E, each with a TMS doping density of 3.8 μmol m －２ , 0 μmol·m －２ , 0.8 μmol·m －２ , 2.0 μmol·m －２ , 3.8 μmol·m －２ Porous SiO ２ This is an ethanol dispersion of graphene-nanoparticle composites obtained by GQD synthesis using the same CVD reaction conditions (600°C, 20 min). Figure 45 shows images taken after irradiating these liquids with UV light (365 nm). Sample A is C ２ H ２ TMS-modified SiO without introducing ２ The nanoparticles were heat-treated at 600°C, and the resulting dispersion did not show fluorescence. Sample B was not TMS modified, meaning its TMS doping density was 0 μmol·m. －２ of SiO ２ This was done using [unmodified SiO2], and the resulting dispersion showed very weak blue fluorescence under UV light irradiation. ２ C ２ H ２ This is because the catalytic activity for decomposition is very low, and the carbon deposition rate is extremely slow. Samples C, D, and E had TMS doping densities of 0.8 μmol·m³, respectively. －２ , 2.0 μmol·m －２ , 3.8 μmol·m －２ Porous SiO ２ This is an example using SiO. ２ As the TMS doping density to SiO2 increased, the fluorescence color changed continuously from pale blue to yellow and then to orange-red. This is because of the high doping density of SiO2. ２This suggests that GQDs generated on a substrate have larger particle sizes.

[0067] Figure 46 shows the emission (PL) spectrum of the sample from Figure 45 at an excitation wavelength of 365 nm. The redshift in this PL spectrum indicates a high doping density of SiO ２ This supports the tendency for GQD particles to have larger particle sizes when generated on a substrate.

[0068] By the way, the inventors conducted this experiment under the same conditions as the experiment with SiO ２ TiO ２ When we also investigated the case using a substrate, the resulting powder turned completely black, and no fluorescent GQDs were obtained at all. This is because of TiO ２ TMS-doped SiO ２ This is thought to be because it has much higher catalytic activity than [another substance], causing excessive carbon deposition. Therefore, the variability of the fluorescence color obtained in this study is due to TMS-doped SiO ２ A specific type of C on the substrate ２ H ２ It can be concluded that this is due to a decomposition mechanism.

[0069] Figure 47 shows TMS-doped SiO ２ This figure shows the results of investigating the weight change of nanoparticles during the CVD process. In this experiment, the TMS doping density was 0.8 μmol·m. －２ , 2.0 μmol·m －２ , 3.8 μmol·m －２ Regarding the three TMS-SPS150-10 models, C ２ H ２ C due to a mixed gas flow of (20 vol%) and Ar ２ H ２ -This is the result of monitoring the weight change during the CVD process (600 °C) using TG measurement. From these CVD-TG measurement results, it can be seen that the reaction rate slows down as the TMS doping density decreases, and in particular, when the doping density is 0.8 μmol·m －２ In the sample, carbon deposition was found to be significantly slower. This trend is observed in CH on TMS-SPS150-10. ４This also correlates well with the degradation rate. This suggests that the fluorescence properties of GQD can be controlled by adjusting the TMS doping density and reaction time (CVD time). To test this hypothesis, TMS-SPS150-10 with different TMS doping densities was treated with CVD at 600°C for different durations, and the effect of reaction time was investigated.

[0070] Figure 48 shows the GQD / SiO2 obtained at different TMS doping densities and reaction times. ２ This image shows the ethanol dispersion of the complex under UV light (365 nm) irradiation. The upper left photograph in Figure 48 shows a TMS doping density of 3.8 μmol m －２ Porous SiO ２ Regarding the case using [the specified method], the upper right figure shows a TMS doping density of 2.0 μmol m －２ Porous SiO ２ Regarding the case using [this method], the figure below shows a TMS doping density of 0.8 μmol m －２ Porous SiO ２ This concerns the case using [specific TMS doping density]. Regardless of the TMS doping density, the fluorescence color changed in accordance with the reaction time. The lower the TMS doping density, the more clearly the change in fluorescence color appeared with increasing reaction time. For example, with a TMS doping density of 0.8 μmol m －２ Porous SiO ２ When using [the specified method], reaction times of 10 min, 20 min, 30 min, 40 min, and 50 min each exhibited different fluorescent colors: blue, light blue, light yellow, orange, and orange-red.

[0071] Figure 49 shows a TMS doping density of 2 μmol m －２ This figure shows the relationship between the reaction time and the UV-vis absorption spectrum of the sample. Figure 49 shows the UV-vis absorption spectra of the following four samples with reaction times of 10 min, 20 min, 30 min, and 40 min. TMS-SPS150-10_2.0 μmol m -2 _10 TMS-SPS150-10_2.0 μmol m -2 _20 TMS-SPS150-10_2.0 μmol m -2_30 TMS-SPS150-10_2.0 μmol m -2 _40 From Figure 49, the TMS doping density is 2 μmol m －２ In the sample, the UV-vis absorption band gradually shifted to longer wavelengths as the reaction time was extended, suggesting that the conjugated system within the GQD expanded. Figure 50 shows the PL spectra of the four samples in Figure 49 at an excitation wavelength of 365 nm. In Figure 50, a redshift was observed that coincided with the change in fluorescence color in the upper right photograph of Figure 48.

[0072] Figure 51 shows a TMS doping density of 0.8 μmol m －２ This figure shows the relationship between the reaction time and the UV-vis absorption spectrum of the sample. Figure 51 shows the UV-vis absorption spectra of the following five samples, with reaction times varied at 10 min, 20 min, 30 min, 40 min, and 50 min. TMS-SPS150-10_0.8 μmol m -2 _10 TMS-SPS150-10_0.8 μmol m -2 _20 TMS-SPS150-10_0.8 μmol m -2 _30 TMS-SPS150-10_0.8 μmol m -2 _40 TMS-SPS150-10_0.8 μmol m -2 _50 From Figure 51, the TMS doping density is 0.8 μmol m －２ In the sample, the redshift of the absorption peak with increasing reaction time was observed at a TMS doping density of 2.0 μmol m －２ The reaction was more gradual than in the previous case. Figure 52 shows the PL spectra of the five samples in Figure 51 at an excitation wavelength of 365 nm. In the PL spectra as well, it was observed that the emission peaks gradually shifted toward longer wavelengths. In Figure 52, a redshift was observed that corresponded to the change in fluorescence color in the lower part of the photograph in Figure 48. These results revealed that under conditions of low TMS doping density, the size and optical properties of the GQD can be controlled more precisely by adjusting the reaction time.

[0073] Figure 53 shows photographs of a toluene dispersion of GQD taken under UV light (365 nm) irradiation and under indoor lighting. GQD / SiO ２ The complex is TMS-SPS150-10_0.8 μmol m -2 _20, that is, a TMS doping density of 0.8 μmol m －２ The CVD time for this sample is 20 min. ２ The composite was sonicated in toluene and then centrifuged to remove SiO ２ The toluene dispersion of GQD obtained by removing [unspecified substance] was photographed under UV light irradiation and under indoor lighting. The obtained toluene dispersion of GQD showed clear blue emission under UV light irradiation. Figure 54 shows the UV-vis absorption spectrum and the PL spectrum at an excitation wavelength of 365 nm for the same sample as in Figure 53. The UV-vis absorption spectrum is shown on the left side of Figure 54. A clear emission peak was observed in the PL spectrum shown on the right side of Figure 54. Therefore, TMS-modified SiO ２ Even when using TiO ２ As in similar cases, it was shown that GQDs can be isolated by sonication.

[0074] Next, TMS-doped SiO ２ We investigated the temperature range for CVD in GQD manufacturing using nanoparticles. First, we used TMS-SPS150-10_0.8 μmol m -2 Place it in a TG chamber (STA2500 Regulus, NETZSCH Japan), then add the mixed gas (20 ml / min) －１ C ２ H ２ and 80ml / min －１ Ar was introduced. 2℃・min －１ The weight change was monitored up to 600°C at the following heating rate. This is almost the same method as described in paragraph 0028 above, and this time, TMS-SPS150-10_0.8 μmol m -2 C on the surface ２ H ２ The deposition initiation temperature was investigated by in-situ CVD. The investigation results are shown in Figure 55.

[0075] Figure 55 shows TMS-SPS150-10_0.8 μmol m -2 This figure shows the weight change of TMS-SPS150-10_0.8 μmol m -2 The weight of starts to increase from 500°C. ２ H ２ This means that decomposition and the accompanying carbon deposition will occur. Also, TMS-SPS150-10_0.8 μmol m -2 C ２ H ２ When CVD was performed at 900°C for 20 minutes using as the carbon source, a black powder was obtained. This means that large-sized graphene was formed on the surface, and in fact, this sample did not show fluorescence under UV light, so GQD could not be produced. From these results, TMS-doped SiO ２ In GQD manufacturing using this method, the CVD temperature range can be 500°C or higher and less than 900°C.

Claims

1. A method for producing graphene quantum dots, comprising heating metal oxide nanoparticles to a temperature of 70°C or higher and less than 350°C while contacting the nanoparticles with a carbon-containing compound to precipitate graphene quantum dots on the surface of the nanoparticles, thereby obtaining a graphene-nanoparticle composite.

2. The manufacturing method according to claim 1, further comprising detaching the graphene quantum dots from the nanoparticles by applying a mechanical tool, mechanical or ultrasonic cleaning, chemical reagents, heat, or other external energy source to the graphene-nanoparticle composite.

3. The manufacturing method according to claim 2, characterized in that the nanoparticles are recovered by filtration, decantation, or centrifugation, and the nanoparticles are reused by heating them in air and burning off the remaining carbon.

4. The manufacturing method according to any one of claims 1 to 3, wherein the metal oxide comprises titanium oxide, cerium oxide, zirconium oxide, hafnium oxide, aluminum oxide, or zinc oxide.

5. The manufacturing method according to any one of claims 1 to 3, wherein the carbon-containing compound comprises acetylene or acrylonitrile.

6. A manufacturing method according to any one of claims 1 to 3, wherein the graphene-nanoparticle composite is provided in an organic solvent, and then the graphene quantum dots are peeled off from the nanoparticles.

7. Graphene quantum dots with an oxygen content of 6% by weight or less.

8. SiO modified with a silyl group ２ A method for producing graphene quantum dots, comprising heating nanoparticles to a temperature of 500°C or higher and less than 900°C, contacting the nanoparticles with a carbon-containing compound, thereby depositing graphene quantum dots on the surface of the nanoparticles to obtain a graphene-nanoparticle composite.

9. The manufacturing method according to claim 8, comprising detaching the graphene quantum dots from the nanoparticles by applying a mechanical tool, mechanical or ultrasonic cleaning, chemical reagents, heat, or other external energy source to the graphene-nanoparticle composite.

10. The manufacturing method according to claim 9, characterized in that the nanoparticles are recovered by filtration, decantation, or centrifugation, and the nanoparticles are reused by heating them in air to burn off any remaining carbon.

11. The manufacturing method according to any one of claims 8 to 10, wherein the carbon-containing compound comprises acetylene or acrylonitrile.

12. A manufacturing method according to any one of claims 8 to 10, wherein the graphene-nanoparticle composite is provided in an organic solvent, and then the graphene quantum dots are peeled off from the nanoparticles.

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