A method for preparing iron-doped carbon dots from coal gasification slag for use in photoexcitation devices.

Abstract

This invention belongs to the fields of photoexcitation device fabrication, solid waste recycling, and graphene powder technology. It provides a method for preparing iron-doped carbon dot composite materials for photoexcitation devices using coal gasification slag as raw material, and a device using the obtained carbon dots as a gain medium. The method involves washing and settling the coal gasification slag, combined with magnetic separation to screen out carbonaceous components with high iron content. The iron-doped carbon dot composite material is then obtained through steps such as oxidation, depolymerization, and exfoliation with hydrogen peroxide solution, followed by condensation and reflux, filtration, and drying. Specific pretreatment of the raw material enhances the particle size and fluorescence performance of the fluorescent carbon dots. By adjusting selective oxidation, depolymerization, and exfoliation conditions such as hydrogen peroxide concentration and temperature, the solid waste raw material can be used to prepare iron-doped carbon dot composite materials. The carbon nanoparticles prepared by this invention, after further modification, can be applied to fields such as photoamplification devices or fluorescence sensing.

Description

Technical Field

[0001] This invention belongs to the fields of photoexcitation device fabrication, solid waste recycling, and graphene powder technology, specifically relating to a method for preparing iron-doped carbon dots from coal gasification fine slag used in photoexcitation devices. Background Technology

[0002] Optical excitation devices primarily refer to key components that utilize the stimulated emission effect of lasers to amplify optical signals or generate coherent light through an optical amplification process. The most crucial component is the gain medium. Under external energy excitation, the gain medium achieves population inversion. When particles in the gain medium are excited by external energy, a large number jump from low energy levels to high energy levels, forming a non-equilibrium distribution where the number of particles at high energy levels far exceeds that at low energy levels. If incident light of a specific frequency triggers this, stimulated emission avalanche effects are induced, thereby achieving exponential amplification of the optical signal and ultimately outputting a laser beam with high brightness, high directionality, high monochromaticity, and high coherence. The performance of the gain medium directly determines the laser's output power, beam quality, wavelength coverage, and other core parameters. Finding superior and cost-effective laser gain media has been a continuous research focus for researchers.

[0003] Coal gasification fine slag, a traditional solid waste (including fine particulate residue discharged from the bottom of the gasifier, as well as fly ash that flows with the flue gas inside the gasifier, is discharged from the top of the furnace, and is collected by subsequent dust removal), is one of the main solid wastes generated during the coal gasification process. It is characterized by its small particle size, large specific surface area, and high residual carbon content (reaching approximately 40-60% after treatment). Since this residual carbon can be used to prepare high-value products such as catalysts, porous materials, and rubber and plastic fillers, the existing technology for enriching and separating residual carbon from coal gasification fine slag is the primary prerequisite and core task for realizing its resource utilization and high-value application.

[0004] Currently, the main processing technologies for enriching residual carbon in coal gasification slag include flotation, gravity separation, and oil agglomeration separation. However, existing processes generally have technical defects that make it difficult to meet the needs of high-value industrial utilization. Specifically: First, during the separation process, key process parameters such as slurry pH and solid-liquid ratio lack precise control methods, resulting in the failure to fully utilize the differences in surface properties between residual carbon and ash, leading to poor separation selectivity and affecting the carbon enrichment effect. Second, there is a lack of efficient surface modification technology, resulting in small differences in the physicochemical properties of residual carbon and ash, further increasing the difficulty of separation. Third, the separation methods are relatively simple, mostly using single sedimentation or centrifugal separation modes, which cannot achieve fine classification and separation of particles with different densities and particle sizes, resulting in low carbon enrichment efficiency and the final residual carbon product purity (carbon content) failing to meet the requirements for high-value utilization. Fourth, the various separation stages are independent of each other, failing to build a synergistic system, resulting in low overall processing efficiency and high energy consumption, making it difficult to adapt to the needs of large-scale continuous industrial production. Furthermore, when the target utilization direction of coal gasification fine slag is the final product that needs to be combined with Fe element, the existing process is difficult to retain residual carbon and iron-rich components at the same time, and cannot effectively and cost-effectively remove redundant ash that is low in carbon and low in iron, which further limits its targeted high-value application.

[0005] Taking existing typical processes as examples: the flotation separation process has poor targeting, insufficient adjustment precision, and slow response when adjusting the pH value of the slurry. At the same time, it cannot dynamically adjust parameters according to the mineral composition of the raw material ash, making it difficult to achieve both carbon recovery rate and carbon content, and thus failing to achieve the optimal separation effect. Although the oil agglomeration separation process has certain advantages in separating extremely fine particles, the stability of agglomerate formation is poor, and the subsequent separation precision is insufficient, which easily leads to residual carbon loss or ash contamination. The air classification process is significantly affected by the matching degree of the inlet gas velocity and turbine speed, and is prone to particle agglomeration, which seriously affects the separation stability and separation effect.

[0006] Carbon dot composites (also known as carbon nanoparticles or carbon dots, abbreviated as CDs) are a new type of carbon nanomaterial with a size smaller than 10 nm (typical particle size 2-10 nm). They were first discovered in 2004 by researchers such as Xu et al. while purifying single-walled carbon nanotubes (the related research results were published in a top journal, representing the first recognized report in the field of carbon dots). This material, with its excellent optical properties, low toxicity, and tunable luminescence characteristics, shows broad application prospects in various fields such as biomedical applications, optoelectronic device fabrication, and environmental sensing. Iron-doped carbon dots can be used as gain media in photoexcitation devices. Graphene quantum dots in carbon dot composites, constructed into graphene sheets, fall under the category of graphene powder.

[0007] Currently, the main synthesis methods for carbon nanodots can be divided into two categories: top-down and bottom-up methods. The core principle of the top-down method is to exfoliate a fixed carbon source under specific process conditions, transforming it into nanoscale carbon dots. The bottom-up method synthesizes carbon dots through the polymerization reaction of small molecule compounds under specific conditions. Based on the preparation of carbon dot composite materials, heteroatom doping modification technology can effectively control the chemical and band structures of carbon dots, achieving customized functionalization and expanding their application range.

[0008] Although various carbon dot preparation methods have been reported in the prior art, how to prepare carbon dots using cheaper and simpler processes, and simultaneously achieve efficient doping of specific impurity elements, remains a core goal that those skilled in the art continue to explore.

[0009] Chinese invention patent publication CN121227346A discloses a method for preparing multi-inorganic element-doped carbon quantum dot materials using coal gangue. This method uses inexpensive coal gangue as raw material to prepare reinforced carbon quantum dot materials doped with inorganic elements such as Al and Si, which can meet the application needs of optoelectronic devices and environmental sensing fields. However, the raw material for this technical solution is screened and configured coal gangue, where Si and Al are associated mineral phases. The composition and structure of coal gasification slag are fundamentally different from those of coal gangue. Besides a very small amount of iron ore associated with coal, iron in the coal gasification slag also includes iron-containing catalysts added during gasification. During gasification, iron elements melt or gasify to form particles, which are then captured by a bag filter and ultimately enter the gasification slag. Therefore, this existing method cannot be directly applied to the preparation of carbon dot composite materials based on coal gasification slag.

[0010] Furthermore, after the high-temperature gasification process, the carbonaceous components in the coal gasification slag exhibit a more regular microcrystalline arrangement and a more uniform particle size distribution. Most importantly, the abundant iron elements dispersed and embedded in the slag are not simply impurities, but rather excellent in-situ doping sources. Compared to exogenous doping, the unique carbon-iron symbiotic structure of the coal gasification slag provides a unique microenvironment for constructing stable coal-based carbon dots, possessing great potential to replace traditional synthetic iron-doped carbon dots and achieve high-value applications in the optoelectronic field.

[0011] Researchers in this field are considering using coal gasification slag as raw material to prepare carbon dot composite materials. This technical solution requires targeted modifications to existing pretreatment processes for coal gasification slag, such as washing, sedimentation, and magnetic separation, to obtain carbon-rich and iron-rich raw materials that meet the requirements for preparing carbon dot composite materials. To date, no relevant technologies have been reported in the existing field, and no feasible technical reference can be provided.

[0012] Therefore, there is an urgent need to develop an efficient method for preparing iron-doped carbon dot composite materials using coal gasification slag as raw material, and then using the obtained carbon dots as the gain medium of photoexcitation devices, so as to solve many defects in the existing technology, promote the high-value utilization of coal gasification slag and the low-cost preparation of carbon dot materials. Summary of the Invention

[0013] To address the aforementioned technical problems, this invention aims to provide a method for preparing iron-doped carbon dots from coal gasification fine slag for photoexcitation devices, and the resulting photoexcitation device. The method involves enriching the coal gasification fine slag with high residual carbon through composite separation to form raw materials that meet the requirements for carbon dot preparation. Fe-doped carbon nanoparticles (carbon dot composite materials) are then prepared using a simple water bath method for photoexcitation devices. This method and the resulting photoexcitation device are then used to prepare iron-doped carbon dots. Specifically, this invention treats the coal gasification fine slag to form raw materials suitable for carbon dot preparation, then directly heats and oxidizes it under hydrogen peroxide conditions. During the water bath stirring process, condensation and reflux occur. The hydroxyl radicals generated by the hydrogen peroxide during this process break down weak bonds in the carbon molecules of the coal structure. Simultaneously, the original Fe-containing mineral phase components in the carbonaceous components of the coal gasification fine slag cannot be oxidized, thus obtaining water-soluble Fe-doped carbon nanoparticles. This method allows for the control of product particle size and photoluminescence properties.

[0014] The specific technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0015] A method for preparing iron-doped carbon dots from coal gasification fine slag for use in photoexcitation devices, comprising the following steps:

[0016] (1) Washing: Mix 20-30 parts by weight of coal gasification fine slag with 70-80 parts by weight of water to obtain an initial slurry. Stir the initial slurry and continuously add surfactant (or surface modifier) ​​during the stirring process. The amount of surfactant added is 0.2-0.5 wt.% of the dry weight of coal gasification fine slag. During the stirring process, control the solid-liquid ratio to be (20-30):(70-80) and the pH value to be 7.0-8.0. The stirring and washing time is 20-30 min to obtain a washing slurry.

[0017] (2) Settling: The washing slurry obtained in step (1) is transported to the settling area and intermittently stirred for 1 to 3 minutes every 10 to 12 minutes. The intermittent stirring time is 40 to 60 minutes. Then, it is allowed to stand for 10 to 15 minutes to allow the solid material to settle and stratify. The upper part is enriched residual carbon particles, and the bottom part is rich mineral phase particles (mainly aluminosilicate mineral phase).

[0018] (3) Separation: The slurry layered in step (2) is extracted by multi-stage gradient suction to remove the residual carbon particles in the upper part (using an existing adjustable height suction device, which sucks the material away through the suction pipe without disturbing the bottom material), and then the mineral-rich particles at the bottom are removed.

[0019] (4) Magnetic separation: The residual carbon particle slurry extracted by multi-stage gradient suction in step (3) is magnetically separated and screened to obtain magnetic residual carbon particle slurry; then it is placed in a vacuum drying oven and dried at 100~120℃ to obtain high residual carbon iron-containing coal gasification fine slag.

[0020] This magnetic separation process can not only remove a small number of Fe-free particles from residual carbon particles, but also remove residual oily impurities from liquids.

[0021] (5) Condensation and reflux: The high residual carbon iron-containing coal gasification fine slag dried in step (4) is mixed with hydrogen peroxide at a solid-liquid ratio of 1g:(50~500)ml (more preferably 1g:200ml). Then, it is placed in a condensation and reflux device heated by a water bath at 40~120℃ (more preferably 40~100℃) and condensed and refluxed for 1~12h (more preferably 2~12h) under the condition of magnetic rotor stirring.

[0022] (6) Filtration: The liquid obtained after condensation and reflux in step (5) is filtered using an organic filter membrane of 0.45 or 0.22 micrometers, or a dialysis membrane can be used for filtration to obtain an Fe-doped carbon dot solution that has passed through the dialysis membrane.

[0023] The liquid passing through the dialysis membrane, namely the carbon nanoparticle solution, exhibits a clear concentration-dependent luminescence characteristic. The photofluorescence behavior varies significantly depending on the concentration of carbon dots; generally, optimal excitation is achieved at 445 nm, and optimal emission at 525 nm. Furthermore, this carbon nanoparticle solution possesses photocatalytic reaction properties and, after subsequent modification, can be directly used for the photocatalytic degradation of organic matter.

[0024] (7) Modification: Add disodium ethylenediaminetetraacetate to the Fe-doped carbon dot solution obtained in step (6) and stir the reaction at 50~70℃ for 8~13 min.

[0025] (8) Drying: The obtained liquid is dried to remove moisture, and iron-doped carbon dot composite material is obtained.

[0026] Preferably, the amount of disodium ethylenediaminetetraacetate added in step (7) is 0.35 to 0.65 times (more preferably 0.5 times) the mass of the high residual carbon iron-containing coal gasification fine slag.

[0027] Preferably, the drying in step (8) is freeze drying or rotary evaporation.

[0028] Preferably, in step (1), the coal gasification slag is coal gasification slag with an average particle size ≤ 80 μm.

[0029] Preferably, in step (1), when the solid-liquid ratio is higher than 30:70 during the stirring process, water is added to the slurry so that the solid-liquid ratio of the slurry is 20:80, and the solid-liquid ratio is maintained at (20~30):(70~80); when the pH value is lower than 7.0, a sodium hydroxide solution with a concentration of 5~8wt.% is added to the slurry; when the pH value is higher than 8.0, a hydrochloric acid solution with a concentration of 3~5wt.% is added to the slurry to maintain the pH value of the slurry at 7.0~8.0.

[0030] Preferably, the surfactant in step (1) is a microcapsule, which encapsulates hexadecyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB), hexadecyltrimethylammonium chloride (CTAC) or a polyacrylamide derivative, and the microcapsule has a particle size of 0.5~1.5 mm.

[0031] Under stirring, the microcapsule skin slowly dissolves and releases surfactants, selectively altering the wettability of the residual carbon surface and increasing the difference in physical properties between the residual carbon particles and other particles.

[0032] Preferably, in step (2), the intermittent stirring is electromagnetic stirring using a combination of pulsed electric field and weak magnetic field. The voltage of the pulsed electric field is 30-50V and the pulse frequency is 50-100Hz. The magnetic field strength of the weak magnetic field is adjusted to 0.1-0.3T, the current intensity of the electromagnetic coil is 2-3A, and the number of coil turns is 800-1200. The pulsed electric field promotes particle aggregation, and the weak magnetic field promotes the stratification of paramagnetic particles, thereby accelerating the preferential sedimentation of mineral-rich phase particles.

[0033] Preferably, in steps (2) and (3), the height ratio of the enriched residual carbon particles at the top to the height of the rich mineral phase particles at the bottom is (7.0~8.5):(1.5~3.0). (After research and analysis, it was found that this height setting can select out the enriched residual carbon particles as completely as possible.)

[0034] In essence, after the settling process, there is no very obvious stratification. Only carbon-rich particles tend to accumulate at the top, while mineral-rich particles preferentially accumulate at the bottom. By removing approximately 20% of the bottom material, most of the mineral-rich phase is removed, resulting in a significant increase in the residual carbon ratio in the remaining coal gasification slag. Through multi-stage gradient suction, approximately 80% of the upper material is retained in the settling layer while the bottom 20% is removed, thus removing the mineral-rich particles through washing and settling. The removed mineral-rich particles can then be used directly as building material raw materials. Multi-stage gradient suction effectively avoids disturbance to the settling layer caused by manual scooping, ensuring a stable residual carbon content in the light-phase slurry. The multi-stage gradient suction system used in this invention is a known existing device.

[0035] Preferably, in step (4), the magnetic field strength of the magnetic separation is 280~320mT (more preferably 300mT).

[0036] Preferably, the magnetic residual carbon particle slurry obtained by separation accounts for 58~82 wt.% of the residual carbon particle slurry (since magnetic separation has a certain solid-liquid separation effect, the solid matter in the residual carbon particle slurry obtained by magnetic separation actually accounts for about 68~88 wt.%).

[0037] Preferably, in step (4), the drying time is 1.5 to 2.5 hours, and the resulting high residual carbon coal gasification fine slag has a residual carbon content of 20 to 46%, a moisture content of ≤10%, and the microstructure of the coal gasification fine slag exhibits highly interlocked characteristics.

[0038] Preferably, the hydrogen peroxide in step (6) is a hydrogen peroxide solution with a concentration of 10~30 wt.%.

[0039] Preferably, the molecular weight of the dialysis membrane in step (7) is 1500~5000 Da.

[0040] An iron-doped carbon dot composite material, wherein the iron-doped carbon dot composite material is prepared by the above method, and the iron-doped carbon dot composite material powder contains Fe element doping.

[0041] The technical advantages of this invention are as follows:

[0042] 1. This invention does not aim to propose a carbon dot composite material with superior performance, but rather to propose a method for preparing a qualified carbon dot composite material suitable for photoexcitation devices using specific solid waste (fine coal gasification slag) as raw material. This provides a relatively low-cost raw material and preparation method for carbon dot composite materials, thereby significantly increasing the added value of the specific solid waste. This invention, through sequential washing, sedimentation separation, magnetic separation, and drying processes on the solid waste (fine coal gasification slag) generated during the coal gasification process, achieves efficient classification and enrichment of different components based on the principles of material density differences, surface property differences, and phase stability. The lighter phase with high residual carbon content (mainly composed of carbonaceous components used to prepare carbon dots and Fe elements highly intercalated with carbon) is generally located in the upper layer of the system due to its lower relative density, while the mineral-rich phase (mainly mineral-rich particles rich in SiO2, Al2O3, etc.) settles at the bottom of the system due to its higher density. This refining process essentially achieves the goal of "enriching useful components and removing as many useless impurities as possible" in coal gasification fine slag. It efficiently enriches key elements such as carbon and iron required for the preparation of carbon dot composite materials, and reduces the adverse effects of impurities on the crystallinity, optical properties and stability of carbon dots. As a result, the treated coal gasification fine slag can be used as the main raw material for carbon dot composite materials. If unwashed and unsedimented coal gasification fine slag is used directly, it cannot be used directly in the field of high-end carbon material preparation due to its high impurity content. The specific settings of washing, sedimentation, separation and magnetic separation processes in this invention realize the high-value-added resource utilization of industrial solid waste, which has both environmental and economic benefits.

[0043] 2. In the washing process, this invention precisely controls the solid-liquid ratio and pH value while stirring. On the one hand, this disrupts the adsorption balance on the surface of the coal gasification fine slag particles, allowing soluble impurities adsorbed on the surface of residual carbon and iron-rich phases to fully dissolve in the washing liquid, achieving preliminary removal of impurities. On the other hand, the specific stirring prevents the fine slag particles from agglomerating, ensuring sufficient solid-liquid contact and laying the foundation for subsequent phase separation. Simultaneously, by adding surfactants, the specific interaction between surfactant molecules and the surfaces of different components in the fine slag is utilized to change the surface charge properties and wettability of each component. Particles rich in residual carbon should ideally be placed in the upper layer due to their low density, but their surfaces are rich in oxygen-containing functional groups (such as -COOH, -OH), which are highly hydrophilic and difficult to stably accumulate in the upper layer. However, by setting specific surfactants, they can be adsorbed onto the particle surface, shielding the hydrophilic groups, significantly improving hydrophobicity, making them easier to aggregate and remain in the upper layer of solid sediments. This ensures efficient aggregation of the top carbon phase and the bottom mineral-rich phase, avoiding excessive cross-mixing of different useful components and providing a crucial foundation for the efficient implementation of subsequent sedimentation and separation processes. In a preferred embodiment, a microcapsule-encapsulated surfactant is used. Utilizing the slow-release properties of the microcapsules, a cationic surfactant is slowly released under stirring. This surfactant can slowly and selectively adsorb onto the surface of the carbon particles, directionally altering the wettability of the carbon surface. This enhanced surfactant activity significantly improves the efficiency and precision of subsequent phase separation and reduces the loss of useful components.

[0044] 3. This invention solves the technical problems of insufficient stratification of light and heavy phases due to continuous stirring and low settling efficiency during the sedimentation separation process by setting an intermittent stirring mode. Intermittent stirring breaks up particle adsorption and agglomeration during the stirring stage, allowing fine slag particles to redisperse. During the settling stage when stirring stops, the light phase (residual carbon-enriched phase) and the heavy phase (rich mineral phase) are clearly stratified under gravity, allowing the denser rich mineral phase in the fine slag to gradually settle to the bottom. By precisely setting the interval of intermittent stirring, the rich mineral phase is ensured to settle to the bottom as completely as possible. Subsequently, by selecting a multi-stage gradient suction method, the upper residual carbon-enriched light phase is precisely retained, while the rich mineral phase particles at the bottom are removed. This process further increases the carbon and iron content in the raw materials (although the absolute amounts of carbon and iron remain unchanged or decrease slightly, the relative enrichment of carbon and iron is achieved by removing as much of the rich mineral phase as possible, thus increasing their content). This ensures that the carbon and iron content in the treated raw materials meets the stringent requirements for preparing carbon dot composite materials, especially providing a sufficient Fe source for the subsequent preparation of Fe-doped carbon dots, ensuring uniform doping and providing a raw material guarantee for the excellent performance of the carbon dot products. Furthermore, by setting specific magnetic separation steps, some light phase particles that do not contain Fe are further removed, thereby further enhancing the relative iron content and providing an excellent raw material foundation for the preparation of iron-doped carbon dot composite materials.

[0045] 4. The coal gasification slag enriched with residual carbon and iron after the specific processing of this invention contains microcrystalline particles that are all agglomerated together. This invention uses the refined coal gasification slag as raw material and adopts a synergistic effect of condensation reflux and hydrogen peroxide of specific concentration and dosage for oxidation and deagglomeration. In this system, hydrogen peroxide can generate a large number of highly active hydroxyl radicals (·OH), which have a high oxidation potential and can quickly and comprehensively destroy the bonds between microcrystalline particles in the refined slag. The bonds inside the microcrystals are stable π-π bonds, which are difficult to destroy. The agglomerated microcrystals will slightly agglomerate due to electrostatic forces to form carbon dots, and the microcrystals contain embedded Fe elements, so the carbon dots formed are Fe-doped carbon dots. At the same time, the existing condensation reflux device can effectively control the temperature of the reaction system, avoid the hydrogen peroxide decomposition too quickly leading to a decrease in oxidation efficiency, and also prevent the volatilization and loss of small molecule carbon fragments, ensuring the full utilization of the carbon source. The magnetic stirring action ensures uniform oxidation and depolymerization, enhancing the nucleation and growth of carbon quantum dots. Simultaneously, it promotes uniform dispersion of Fe within the particles, ensuring uniform Fe doping into the carbon quantum dot lattice and improving the optical and magnetic properties of the carbon dots. Subsequent dual filtration—vacuum filtration and dialysis membrane filtration—based on particle size sieving principles, removes incompletely depolymerized large carbon particles and impurities, while dialysis membrane filtration precisely controls the particle size distribution of the carbon quantum dots. This synergistic process further improves the uniformity of Fe doping, preventing performance fluctuations caused by localized Fe enrichment, and precisely controls the bulk particle size of the carbon quantum dots to approximately 2.5 nm. This particle size range closely matches the particle size requirements of carbon dot materials in optoelectronic device applications, ensuring excellent charge transport efficiency and optical response performance, thus meeting the practical application needs of the optoelectronic devices involved in this invention.

[0046] 5. In this invention, after obtaining the Fe-doped carbon dot solution, a modification operation was performed. Initially, the modification operation was only intended to suppress unintended oxidation and improve the homogeneity of the solution by adding a metal chelating agent. However, after the Fe-doped carbon dot solution was modified with disodium ethylenediaminetetraacetate and stirred at 50-70°C for 8-13 minutes, an unexpected red shift was found in the PL spectrum after modification. The UV-Vis emission spectrum also showed a change in the peak at the 360nm wavelength. That is, this invention achieved unexpected technical effects through this specific modification operation, which greatly improved the photoelectric conversion efficiency of the obtained iron-doped carbon dot composite material, broadened the light absorption range, and optimized the fluorescence emission characteristics. The above effects could not be directly derived or expected in the principle design stage, providing a new approach for modifying iron-doped carbon dots with significantly improved performance in this field. Attached Figure Description

[0047] Figure 1 The XRD pattern of the coal gasification fine slag obtained in step (4) of this invention is shown.

[0048] Figure 2 Transmission electron microscopy (TEM) images and corresponding particle size distribution diagrams of the iron-doped carbon dot composite material prepared by fine treatment of coal gasification slag according to the present invention.

[0049] Figure 3 This is a higher-resolution schematic diagram of carbon dots for this invention.

[0050] Figure 4 This is a high-resolution lattice fringe analysis diagram of the carbon dots in the iron-doped carbon dot composite material obtained in this invention.

[0051] Figure 5 This is a TEM-EDS image of the iron-doped carbon dot composite material obtained in this invention.

[0052] Figure 6 The present invention provides the carbon dot PL and PLE spectra of the iron-doped carbon dot composite material.

[0053] Figure 7 The UV-Vis spectra are for Comparative Example 2 and Example 1, respectively.

[0054] Figure 8 The images show the PL fluorescence spectra corresponding to Comparative Example 2 and Example 1, respectively.

[0055] Figure 9 This is a transmission electron microscope image of Comparative Example 3.

[0056] in Figure 2 In the middle, the upper right corner shows the particle size distribution statistics. Detailed Implementation

[0057] The process technology solution of the present invention will be further described below with reference to embodiments and accompanying drawings. Unless otherwise specified, each feature is merely one example of a series of equivalent or similar features. These embodiments are merely for the purpose of aiding understanding the present invention and should not be considered as specific limitations thereof.

[0058] Example 1

[0059] This embodiment illustrates the method of preparing iron-doped carbon dots from coal gasification slag for photoexcitation devices according to the present invention, comprising the following steps:

[0060] (1) Washing: 25 parts by weight of coal gasification fine slag and 75 parts by weight of water are mixed to obtain an initial slurry. The initial slurry is stirred, and surface modifiers of CTAB microcapsules (slow-release effect) are continuously added during the stirring process. The microcapsules are encapsulated with CTAB (hexadecyltrimethylammonium bromide). The average particle size of the microcapsules is 1 mm. The amount of surface modifier added is 0.3 wt.% of the dry weight of the coal gasification fine slag. During the stirring process, the solid-liquid ratio is controlled at (20~30):(70~80), the pH value is 7.0~8.0, and the stirring and washing time is 26 min to obtain a washing slurry. The coal gasification slag is coal gasification slag with an average particle size ≤80μm (including fine particulate residue discharged from the bottom of the gasifier, as well as fly ash that flows with the flue gas in the gasifier, is discharged from the top of the furnace, and is collected by subsequent dust removal); during the stirring process, when the solid-liquid ratio is higher than 30:70, water is added to the slurry to make the solid-liquid ratio of the slurry 20:80, and the solid-liquid ratio is maintained at (20~30):(70~80) throughout the washing process; when the pH value is lower than 7.0, a sodium hydroxide solution with a concentration of 7wt.% is added to the slurry, and when the pH value is higher than 8.0, a hydrochloric acid solution with a concentration of 3.5wt.% is added to the slurry to maintain the pH value of the slurry at 7.0~8.0.

[0061] (2) Settling: The washing slurry obtained in step (1) is transported to the settling area and intermittently stirred for 2 minutes every 11 minutes. The total duration of the intermittent stirring process is 55 minutes. Then, it is left to stand for 12 minutes to allow the solid material to settle and stratify. The upper part is enriched residual carbon particles and the bottom part is rich mineral phase particles (there is no very obvious dividing line, but the upper part is basically considered as enriched residual carbon particles). The intermittent stirring is electromagnetic stirring using a combination of pulsed electric field and weak magnetic field. The voltage of the pulsed electric field is 40V and the pulse frequency is 80Hz. The magnetic field strength of the weak magnetic field is adjusted to 0.2T, the current intensity of the electromagnetic coil is 2.6A, and the number of coil turns is 1000. The pulsed electric field promotes particle aggregation, and the weak magnetic field promotes the migration of paramagnetic ash particles, thus accelerating ash settling.

[0062] (3) Separation: The height ratio of the mineral-rich phase slurry settling at the bottom to the residual carbon phase slurry is determined to be 25%:75%, that is, the heavy phase is basically 1 / 4 of the height, while the top 3 / 4 is considered to be the residual carbon phase slurry. The residual carbon phase slurry at the top is extracted by multi-stage gradient suction of the slurry from step (2), and then the mineral-rich phase slurry at the bottom is removed.

[0063] (4) Magnetic separation: The residual carbon particle slurry extracted by multi-stage gradient suction is magnetically separated and screened using an XCGS type magnetic separator tube. The residual carbon particle slurry is fed into the inlet of the magnetic separator pipe, the magnetic separator is started, and the magnetic field strength is set to 300mT. As the residual carbon particle slurry flows downward along the pipe, the magnetic particles are retained, while the non-magnetic particles and liquid flow downward and are removed. In this embodiment, the magnetic particles obtained after magnetic separation account for about 80% of the initial total residual carbon particles. The magnetic residual carbon particle slurry is then separated and placed in a vacuum drying oven at 110°C for drying to obtain high residual carbon iron-containing coal gasification fine slag.

[0064] The results of industrial and chemical analysis of the coal gasification slag are shown in Table 1. XRD analysis of the slag yielded the following results: Figure 1 The results are shown in Table 1. Table 1 shows the industrial and chemical composition of the high residual carbon coal gasification slag obtained in Example 1.

[0065] Table 1

[0066]

[0067] Where: M is moisture; A is ash; V is volatile matter; FC is fixed carbon; ad is air-dried basis. Furthermore, in compositional analysis, due to the highly complex forms of elements in the slag, they are generally expressed using oxides. Therefore, the inorganic elements in Table 1, except for carbon, are converted to oxides and do not necessarily represent their existence in oxide form.

[0068] Analysis of the results in Table 1 shows that the C content in the organic matter is as high as 35.17 wt.%, which meets the raw material requirements for carbon dot materials.

[0069] The obtained XRD pattern is as follows Figure 1 As shown, through Figure 1As can be seen from the peak positions, intensities, and overlaps, the characteristics of each phase can be clearly distinguished. The strong peak region (2θ≈20°~30°) is the highest intensity peak group in the spectrum, with SiO2 and CaFe2Si2O6 phases as the core contributors. The strongest single peak around 2θ≈26° belongs to the characteristic diffraction of SiO2 and is one of the main phases in the sample. The adjacent CaFe2Si2O6 peak partially overlaps with the CaCO3 peak, indicating that calcium, iron, and silicon elements form a composite silicate phase, accompanied by a small amount of calcium carbonate. The peaks in the medium-strong peak region (2θ≈30°~50°) are relatively dispersed, dominated by the CaFe2Si2O6 peak, superimposed with a small amount of weak peaks from SiO2 and CaCO3. The characteristic peaks around 2θ≈35° and 40° are typical diffraction peaks of CaFe2Si2O6, proving that this iron-calcium silicate phase is the core metal silicate phase in the sample. The intensity of the diffraction peaks in the weak peak region (2θ > 50°) is significantly reduced, mainly consisting of weak diffraction peaks of CaFe2Si2O6, with no Al peaks appearing. 13 The distinct characteristic peak of Fe4 indicates that CaFe2Si2O6 is the main phase in the refined coal gasification slag. Simultaneously, through... Figure 1 It can also be seen that the overall baseline is stable, with no obvious interference from impurity peaks, indicating that the purity of the coal gasification slag after fine treatment is high (without a large amount of amorphous matter); and the slight broadening of some peaks is a consequence of the grain refinement due to the presence of nanocrystals. That is, as shown in Table 1 and... Figure 1 Analysis shows that the coal gasification slag obtained after the above steps (1) to (4) meets the requirements for preparing carbon dot composite materials.

[0070] (5) Reflux: Weigh 1 g of the coal gasification fine slag powder obtained in step (5) and place it in a 500 mL round mouth flask. Add 200 mL of H2O2 solution with a concentration of 30 wt.% and stir the mixture thoroughly in a water bath at 60 °C for 3 h. Then reflux for 8 h.

[0071] (6) Filtration: After the above reaction was completed, the solution was allowed to cool to room temperature and then filtered using a 0.22-micron organic filter membrane. The filtrate was retained and then filtered through a dialysis membrane with a molecular weight of approximately 3000 Da. The liquid that passed through the dialysis membrane was an Fe-doped carbon dot solution. Thus, an aqueous solution containing carbon nanoparticles was successfully obtained. The particle size was 2.67 ± 0.70 nm, i.e., the average particle size was 2.67 nm. Characterization of the luminescence properties of the nanoparticle solution showed that the sample obtained the strongest excitation at around 445 nm, and the emission peak in its emission spectrum was around 525 nm.

[0072] (7) Modification: Fe-loving solvent was used for modification. Specifically, 0.5 g of disodium ethylenediaminetetraacetate (EDTA-2Na) was added to the obtained Fe-doped carbon dot solution and stirred for 10 min at 60 °C.

[0073] (8) Drying: The obtained liquid is freeze-dried to obtain the modified iron-doped carbon dot composite material.

[0074] Energy dispersive spectroscopy (EDS) analysis was performed on the obtained iron-doped carbon dot composite material, and the results are shown in Table 2. Figure 5 The results are shown in Table 2. Table 2 is the energy dispersive spectroscopy atomic ratio table of the iron-doped carbon dot composite material obtained in Example 1.

[0075] Table 2

[0076]

[0077] By analyzing Table 2 and Figure 5 Analysis revealed that, besides Cu, the iron-doped carbon dot composite material had the highest Fe content, reaching 12.28 at.%. This demonstrates that the present invention yielded an iron-doped carbon dot composite material. (During transmission electron microscopy (TEM) of the sample, the sample solution needs to be dropped onto an ultrathin carbon film. This ultrathin carbon film contains copper and is coated with an amorphous carbon layer. Therefore, in the energy dispersive spectroscopy (EDS) structure image, the copper is generated by the carbon film on which the sample was placed during TEM. Thus, although Cu is present, it is not inherent to the material itself but rather comes from the carbon film during the detection process. The carbon element may originate from the carbon film or carbon dots, but one thing is certain: the original carbon on the carbon film will not have lattice fringes. Therefore, carbon with lattice fringes corresponds to carbon quantum dots.)

[0078] Transmission electron microscopy analysis was performed on the obtained iron-doped carbon dot composite material, and the results were as follows: Figure 2 The results shown are obtained through Figure 2 The results show a large number of uniformly distributed nanoscale particles with varying grayscale values ​​on their surfaces. The scale bar (20 nm) indicates that all particles are at the nanoscale. The particle size distribution in the upper right corner shows a unimodal distribution, indicating good particle size uniformity. The majority of particles are concentrated in the 1.5–3.5 nm range, with an average size of approximately 2.3–2.7 nm. This particle size range represents the dominant particle size of the iron-doped carbon dot composite powder. The D50 is approximately 2.5 nm (median particle size), the D10 is approximately 1.7 nm (meaning very few particles are smaller than 1.7 nm), and the D90 is approximately 3.3 nm (particles larger than 3.3 nm are also rare). TEM analysis confirms the good uniformity of the nanoparticle size, with no obvious agglomeration or impurity particles.

[0079] TEM analysis of the carbon dots was performed to obtain the following results: Figure 3 The results are shown (scale bar is 5 nm). (By...) Figure 3It can be seen that the composite material exhibits a distinct carbon dot distribution, with clearly visible spherical carbon dots, approximately 2.5 nm in size. Clear lattice fringes are visible within these carbon dot particles, indicating that they possess a partially crystalline or quasi-crystalline structure, rather than being completely amorphous. Furthermore, the particles are nearly spherical with clear edges and no obvious agglomeration; the partially crystalline structure is a crucial structural basis for the excellent fluorescence and catalytic properties exhibited by these carbon dots. Figure 2 and Figure 3 It can be seen that the carbon dot sample has excellent characteristics such as small size, narrow distribution, high dispersibility, and partial crystallization, making it suitable for application research in fields such as catalysis, bioimaging, and sensing.

[0080] Further high-resolution lattice fringe analysis was performed on the carbon dots of the iron-doped carbon dot composite material, yielding the following results: Figure 4 The results are shown. Figure 4 The left image shows the Fourier Transform (FFT) spectrum. The centrally symmetrical bright spots correspond to the diffraction signals of the carbon dots' crystal planes, indicating that the carbon dots have a periodic crystal structure, rather than being completely amorphous. The middle image shows the reconstructed image from the Inverse Fourier Transform (IFFT). By selecting specific diffraction spots in the FFT and performing the inverse transform, a filtered reconstruction of the lattice fringes is obtained, showing a periodic structure. By eliminating noise interference, the fringes are made clearer, and the periodicity of the fringes directly corresponds to the interplanar spacing of the carbon dots. The right image shows the analysis of the lattice fringe period (interplanar spacing) (line profile analysis). The horizontal axis represents length (nm), and the vertical axis represents intensity (reflecting the lattice periodicity). 0.525nm corresponds to the spacing of a certain crystal plane in the carbon dot; 2.101nm represents a long-period structure; and 2.627nm represents another long-period signal, thus revealing a more macroscopic structural repetition or interface effect. The most crucial short period of 0.525 nm is direct evidence of carbon dot crystallization. This differs from the typical interplanar spacing of graphitic carbon (e.g., (002)≈0.34 nm, (100)≈0.21 nm), indicating that the carbon dot is a doped structure, thus proving it is an Fe-doped carbon dot. Combined with... Figure 4 and Figure 3 The carbon dots exhibit uniform size with an average diameter of approximately 2.5 nm and a narrow distribution. Individual particle sizes are generally between 2.5 and 3.0 nm. The long-period signals (2.101 / 2.627 nm) match the particle size, indicating that the structural period is consistent with the overall scale. In terms of structural distribution, the particles are well-dispersed, uniform in size, and exhibit complete crystallization without significant defects. The FFT diffraction spots are clear and symmetrical, and the IFFT fringes are uniform, demonstrating good structural uniformity. These carbon dots are not completely amorphous but possess a partially crystalline quasi-crystalline structure with a core interplanar spacing of approximately 0.525 nm. The presence of long-period structures at 2.101 nm and 2.627 nm indicates the presence of an Fe-doped induced superlattice, which will further modulate their optical and electrical properties.

[0081] Comprehensive photoluminescence spectral analysis of carbon dot composite materials yielded the following results: Figure 6 The results shown are obtained through Figure 6 The photoluminescence (PL) and photoluminescence excitation (PLE) spectra show that under 445 nm excitation, the emission peak is located at 525 nm, corresponding to green fluorescence. When monitoring emission at 525 nm, the optimal excitation peak is located in the approximately 450-460 nm range, which highly coincides with the aforementioned 445 nm excitation wavelength. The Stokes shift Δλ = 525 nm - 445 nm = 80 nm indicates that a larger Stokes shift effectively avoids overlap between excitation and emission light, reduces self-absorption, and thus improves the signal-to-noise ratio of fluorescence detection. Analysis of the 525 nm emission peak reveals a sharp and symmetrical peak shape, indicating a single fluorescence emission center and uniform energy distribution. Figure 2 and Figure 3 HRTEM results show that its carbon dots have uniform particle size and are partially crystalline; the size uniformity and ordered lattice are important structural bases for the narrow fluorescence peak; while Fe 3+ / Fe 2+ The introduction of iron can modulate electronic energy levels through heteroatom doping, surface defect states, or metal-carbon coordination structures, causing the fluorescence to redshift to the green region (pure carbon dots typically emit blue light). The excitation peak at approximately 450 nm shows no overlap with the emission peak, indicating that the fluorescence originates from molecular state / defect state transitions rather than simple interband transitions. Iron doping introduces intermediate energy levels (doped / defect levels), resulting in strong absorption of the carbon dots in the visible light region (445 nm), achieving more efficient visible light excitation, which is crucial for applications such as bioimaging and photocatalysis. The emission spectrum shows a stable baseline in the 550–700 nm range with no obvious impurity peaks, indicating high sample purity and the absence of fluorescence interference from impurities, thus demonstrating the stability of the preparation results obtained by the process method of this invention. The excitation spectrum shows a rapid decrease in intensity in the long-wavelength region (>500 nm), which is also a typical characteristic of carbon dot excitation spectra, further proving that the preparation process of this invention can obtain carbon dot composite materials that meet the requirements. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the iron-doped carbon dots obtained in Example 1 of this invention clearly show that, under 445 nm excitation, the sample exhibits strong and narrow green fluorescence emission at 525 nm, with the optimal excitation peak located at approximately 450 nm and a Stokes shift of 80 nm. Combining the results of TEM and HRTEM, it can be concluded that the uniform small size (approximately 2.5 nm), the partially crystalline quasi-crystalline structure, and the defects / doping energy levels introduced by iron doping collectively regulate the electronic transition process, achieving efficient visible light excitation and stable green fluorescence emission, laying the foundation for its application in fields such as bioimaging, fluorescence sensing, and photocatalysis.

[0082] This demonstrates that the specific processing of raw materials, implementation methods, and modification steps of this invention can yield iron-doped carbon dot composite materials that can be practically applied.

[0083] Comparative Example 1

[0084] This comparative example demonstrates a comparative test of the extraction of the upper residual carbon particle slurry without using a multi-stage gradient suction method. The difference from Example 1 is that in step (3), the upper 75% of the solid material was scooped up for separation. It was found that when scooping the upper solid material, due to the disturbance of the water, both the upper and lower materials experienced strong agitation, resulting in re-turbidity and re-entry into the water, thus leading to poor accuracy of the scooped material. This proves that after the specific washing and sedimentation processes of this invention, the suction method used to extract the upper solid material under the specific circumstances of this invention can achieve a highly efficient separation effect.

[0085] Comparative Example 2

[0086] This comparative example is used to demonstrate a comparative experiment without modification. The difference between this comparative example and Example 1 is that the modification operation in step (7) was not performed; instead, the Fe-doped carbon dot solution obtained in step (6) was directly subjected to the drying treatment in step (8) to obtain an iron-doped carbon dot composite material. All other settings are exactly the same as in Example 1. By performing UV-Vis absorption detection and PL fluorescence analysis on the obtained iron-doped carbon dot composite material, and comparing it with the modified iron-doped carbon dot composite material obtained in Example 1, the results are as follows: Figure 7 and Figure 8 The results are shown.

[0087] Through analysis Figure 7 and Figure 8Comparative Example 2 showed the highest absorbance near 250 nm (approximately 0.095 au), which decreased rapidly with increasing wavelength, typical of π→π* transition absorption in a carbon dot / conjugated system. A weak absorption shoulder peak appeared at approximately 370–380 nm, indicating the presence of certain surface state / defect state energy levels, corresponding to n→π* or surface defect transitions. The absorbance approached 0, indicating almost no absorption, suggesting very weak absorption in the visible light region (>400 nm). In contrast, Comparative Example 1 showed a significant decrease in ultraviolet absorption intensity, with absorbance at 250 nm dropping from approximately 0.095 to approximately 0.125, but the overall decay was faster, and the absorbance in the 300–400 nm range was generally lower than that of Comparative Example 2. The weak bulge at 350–400 nm completely disappeared, and the curve smoothly decreased, indicating that surface defects / surface states were significantly passivated or eliminated. In other words, the modification in Example 1 improved the optical purity and stability of the material compared to Comparative Example 2, making it more suitable for applications such as photoluminescence / photocatalysis. Compared to Comparative Example 2, Example 1 eliminated the defect absorption shoulder peak in the 350-400 nm range, indicating that the modification effectively reduced dangling bonds, functional groups, or structural defects on the carbon dot surface. The steeper absorption profile in the ultraviolet region indicates a more ordered π-electron conjugated system and reduced defect channels for energy dissipation. Therefore, when the product obtained in Example 1 is used in the field of photoluminescence, its defects are reduced, nonradiative recombination is decreased, fluorescence quantum yield is likely to be increased, and luminescence is more stable and monochromatic. When used in the field of ultraviolet sensing, the modified absorption is sharper, and the sensitivity and selectivity in the ultraviolet region may be improved, making it more suitable for narrowband ultraviolet detection.

[0088] Combination Figure 8The comparison shows that, for the product in Comparative Example 2, the photoluminescence spectrum at an excitation wavelength of 320 nm has a main peak at 449.6 nm (blue-violet band) with a fluorescence intensity of approximately 90 a.u. A weak fluorescence peak (intensity approximately 60 a.u.) exists between 650 and 670 nm, typically corresponding to emission from surface defect states or long-wavelength trapped states. The relatively wide main peak indicates some uneven energy level distribution, with surface and defect states contributing significantly. For Example 1, the main peak is found to be at 451.8 nm (a redshift of approximately 2.2 nm compared to the unmodified product in Comparative Example 2). This redshift indicates a narrowing of the band gap, meaning the material can absorb and utilize photons with slightly lower energy and longer wavelengths. In photocatalytic applications, this helps to more effectively utilize the blue-violet portion of the solar spectrum, improving solar energy capture efficiency. Fine-tuning of the energy level structure optimizes the separation and migration paths of photogenerated electron-hole pairs. In photocatalytic applications, more matched energy levels can promote specific reduction or oxidation reactions, improving catalytic selectivity. The approximately 2.2 nm redshift directly reflects the precise modulation of the material's internal electronic structure. This indicates that the modification not only passivated surface defects but also precisely modulated the conjugated structure of the carbon core, achieving a narrower band gap. This combination of a more regular structure and a slightly narrower band gap provides new optimization possibilities for the material's applications in photocatalysis and other fields. This conclusion was unexpected, as the modification was intended to suppress unintended oxidation, but a redshift was observed, leading to the deduction that the modification effectively modulated the conjugated structure of the carbon core, resulting in an unexpected technical effect.

[0089] Combination Figure 7 Analysis showed that the modified Example 1 eliminated the defect absorption shoulder peaks in the 350-400 nm range, indicating that surface defects / dangling bonds were significantly passivated and the π-conjugated structure became more regular. Figure 8 The fact that the position of the main peak on the meter remained almost unchanged indicates that the intrinsic luminescence center (carbon nucleus / conjugated π system) was not destroyed, and the luminescence source was still the intrinsic state of the carbon point. That is, through the specific modification in Example 1, the surface structure defects reflected in UV-Vis were significantly reduced, making the π-conjugated system more regular and the absorption spectrum purer. When this composite material is used in the field of ultraviolet photocatalysis / ultraviolet sensing, the reduced defects due to the modification in Example 1 facilitate carrier separation, resulting in sharper absorption and improved selectivity.

[0090] Comparative Example 3

[0091] This comparative example is used to demonstrate a comparative experiment on the preparation of carbon dot composite materials from coal gasification fine slag that has not undergone the washing, sedimentation, separation, and magnetic separation treatment of this invention. This comparative example does not perform steps (1) to (4) of the example, but instead directly performs ordinary washing, removes the ash, and then directly performs condensation reflux treatment; the other steps are exactly the same as in Example 1. The obtained product was analyzed by transmission electron microscopy to obtain the following results: Figure 9 The results are shown. (By...) Figure 9 It can be seen that although there are some carbon dots within the field of view, the distribution density within the same field of view is significantly lower than that in Example 1. That is, the performance of the carbon dot composite material obtained from coal gasification fine slag that has not undergone the washing, sedimentation, separation, and magnetic separation treatment specifically set in this invention is unsuitable for most application scenarios.

[0092] The technical principles of the present invention have been described above with reference to specific embodiments. These descriptions are merely for explaining the principles of the invention and should not be construed as limiting the scope of protection of the invention in any way. Based on this explanation, those skilled in the art can readily conceive of other specific embodiments of the invention without inventive effort, and these embodiments will all fall within the scope of protection of the present invention.

Claims

1. A method for the preparation of iron-doped carbon dots from coal gasification fine slag for optically excited devices, characterized by, Includes the following steps: (1) Washing: Mix 20-30 parts by weight of coal gasification fine slag with 70-80 parts by weight of water, stir the initial slurry and continuously add surfactant. The amount of surfactant added is 0.2-0.5 wt.% of the dry weight of coal gasification fine slag. During the stirring process, control the solid-liquid ratio to be (20-30):(70-80) and the pH value to be 7.0-8.

0. The stirring and washing time is 20-30 min to obtain the washing slurry. (2) Settling: The obtained washing slurry is settled. Intermittent stirring is performed for 1 to 3 minutes every 10 to 12 minutes. The total duration of intermittent stirring is 40 to 60 minutes. Then, it is allowed to stand for 10 to 15 minutes to allow the solid material to settle and separate into layers. The upper part is enriched residual carbon particles, and the bottom part is rich mineral phase particles. (3) Separation: The residual carbon particles in the upper part of the slurry are extracted by a multi-stage gradient suction method, and then the mineral-rich particles at the bottom are removed; the multi-stage gradient suction method is to use a suction device with adjustable height for suction. (4) Magnetic separation: The residual carbon particle slurry extracted by multi-stage gradient suction is magnetically separated and screened to obtain magnetic residual carbon particle slurry; then it is placed in a vacuum drying oven and dried at 100~120℃ to obtain high residual carbon iron-containing coal gasification fine slag. (5) Condensation and reflux: The high residual carbon iron-containing coal gasification fine slag is mixed with hydrogen peroxide at a solid-liquid ratio of 1g:(50~500)ml, and placed in a condensation and reflux device heated by a water bath at 40~120℃. The mixture is stirred by a magnetic rotor and condensed and refluxed for 1~12h. (6) Filtration: The liquid obtained in step (5) is filtered using an organic filter membrane of 0.45 or 0.22 micrometers and then filtered using a dialysis membrane to obtain an Fe-doped carbon dot solution that has passed through the dialysis membrane. (7) Modification: Add disodium ethylenediaminetetraacetate to the Fe-doped carbon dot solution and stir the reaction at 50~70℃ for 8~13 min; (8) Drying: The obtained liquid is dried to remove moisture, and the modified iron-doped carbon dot composite material is obtained.

2. The method for preparing iron-doped carbon dots from coal gasification slag for photoexcitation devices according to claim 1, characterized in that, The amount of disodium ethylenediaminetetraacetate added in step (7) is 0.35 to 0.65 times the mass of the high residual carbon iron-containing coal gasification fine slag; The drying process described in step (8) is freeze drying or rotary evaporation.

3. The method for preparing iron-doped carbon dots from coal gasification slag for photoexcitation devices according to claim 1 or 2, characterized in that, In step (1), the coal gasification fine slag is coal gasification fine slag with an average particle size ≤80μm; In step (1), when the solid-liquid ratio is higher than 30:70 during the stirring process, water is added to the slurry to make the solid-liquid ratio of the slurry 20:80, and the solid-liquid ratio is maintained at (20~30):(70~80); when the pH value is lower than 7.0, a sodium hydroxide solution with a concentration of 5~8wt.% is added to the slurry; when the pH value is higher than 8.0, a hydrochloric acid solution with a concentration of 3~5wt.% is added to the slurry to maintain the pH value of the slurry at 7.0~8.

0.

4. The method for preparing iron-doped carbon dots from coal gasification fine slag for photoexcitation devices according to claim 1, characterized in that, The surfactant in step (1) is a microcapsule, which encapsulates hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride or a polyacrylamide derivative, and the particle size of the microcapsule is 0.5~1.5 mm.

5. The method for preparing iron-doped carbon dots from coal gasification fine slag for photoexcitation devices according to claim 1, characterized in that, In step (2), the intermittent stirring is electromagnetic stirring using a combination of pulsed electric field and weak magnetic field. The voltage of the pulsed electric field is 30-50V and the pulse frequency is 50-100Hz. The magnetic field strength of the weak magnetic field is adjusted to 0.1-0.3T, the current intensity of the electromagnetic coil is 2-3A, and the number of coil turns is 800-1200. The pulsed electric field promotes particle aggregation, and the weak magnetic field promotes the stratification of paramagnetic particles, thereby accelerating the sedimentation of mineral-rich phase particles.

6. The method for preparing iron-doped carbon dots from coal gasification slag for photoexcitation devices according to claim 1, characterized in that, In steps (2) and (3), the height ratio of the upper enriched residual carbon particles to the bottom rich mineral phase particles is (7.0~8.5):(1.5~3.0).

7. The method for preparing iron-doped carbon dots from coal gasification slag for photoexcitation devices according to claim 1, characterized in that, In step (4), the magnetic field strength of the magnetic separation is 280~320mT; the proportion of magnetic residual carbon particles in the separated slurry is 58~82wt.%; In step (4), the drying time is 1.5 to 2.5 hours, and the resulting high residual carbon iron-containing coal gasification fine slag has a residual carbon content of 20 to 46% and a moisture content of ≤10%, and the microstructure of the coal gasification fine slag exhibits highly interlocked characteristics.

8. The method for preparing iron-doped carbon dots from coal gasification slag for photoexcitation devices according to claim 1 or 2, characterized in that, The hydrogen peroxide in step (5) is a hydrogen peroxide solution with a concentration of 10~30 wt.%.

9. An iron-doped carbon dot composite material, characterized in that, The iron-doped carbon dot composite material is prepared by the method described in any one of claims 1 to 8, wherein the powder of the iron-doped carbon dot composite material contains Fe element doping.

10. A photoexcitation device, characterized in that, The photoexcitation device includes a laser gain medium, which is an iron-doped carbon dot composite material prepared by any one of claims 1 to 8.

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