Method for producing quantum dots of iron-containing sulfide and method for producing aqueous dispersion of quantum dots of iron-containing sulfide

Abstract

Provided is a method for producing quantum dots of an iron-containing sulfide, the method being characterized by having: a step for preparing a mixed solution in which at least ferrous hydroxide is present in 0-30°C water; a step for preparing a green rust-containing suspension by oxidizing the mixed solution to form green rust crystals in the mixed solution; a step for synthetizing quantum dots of the ferrite in the suspension by raising the temperature of the suspension to 50-100°C in an oxygen-free gas flow; a step for recovering the quantum dots of the ferrite in the suspension; and a step for treating the quantum dots of the ferrite with hydrogen sulfide gas at 110-150°C, thereby substituting the oxygen ions O2− of the ferrite with sulfide ions S2− and preparing quantum dots of an iron-containing sulfide.

Description

Method for producing quantum dots of iron-containing sulfide and method for producing aqueous dispersion of quantum dots of iron-containing sulfide

[0001] The present invention relates to a method for producing quantum dots of iron-containing sulfide and a method for producing an aqueous dispersion of quantum dots of iron-containing sulfide.

[0002] [Application of quantum dots to electronics and energy industries] Quantum dots are nanomaterials with a size of about 5 to several tens of nanometers. They have a large specific surface area, a wide size, short ion / electron transport pathways, low cost, tunable photoluminescence, and ease of surface functionalization. Therefore, quantum dots have recently attracted attention for their potential applications in various photoelectrochemical devices, energy conversion, and energy storage in batteries (e.g., Non-Patent Documents 1 and 2).

[0003] In particular, as environmental problems become more serious due to global warming and the consumption of fossil fuels, the development and use of quantum dots (QDs) in the fields of energy storage and conversion is accelerating.

[0004] [Quantum Dot Composites for Energy Industry] Quantum dots can be used as current collector and active electrode composites because of the abundance of heteroatom functional groups on their surfaces, providing a wide range of active sites. This can significantly improve the performance of electrochemical energy storage devices, such as excellent ionic conductivity, high speed, large capacity, and cycle stability.

[0005] Furthermore, the performance of batteries, which store electrochemical energy, is highly dependent on the electrode material, and utilizing the properties of quantum dots can significantly improve battery performance. Chakrabarty et al. reported CeO2 / Ce2O3 quantum dots that enhance the electrocatalytic activity of reduced graphene oxide (RGO) electrodes in supercapacitors (Non-Patent Document 3). Carbonaceous materials are the most promising candidates for the anode of sodium-ion batteries (SIBs), and Liu et al. constructed CQD-modified Na3V2(PO4) microspheres and reported a sodium-ion battery cathode with ultra-high capacity and high rate performance (Non-Patent Document 4). Sun et al. reported that by inducing interfacial Bi-C bonds, the use of rGO-rivet bismuth oxychloride as a high-performance anode for SIBs significantly improved capacity and cycling stability compared to other metal halide oxide anodes (Non-Patent Document 5). Additionally, the unique electron donor and acceptor properties and excellent electron transfer properties of QDs make them highly efficient and stable photocatalysts for hydrogen evolution, oxygen reduction, and oxygen evolution reactions.

[0006] As such, the market for quantum dots in electrochemical and photocatalytic systems in the energy industry, energy storage (electrochemical capacitors, lithium / sulfur batteries), and photocatalysis (hydrogen generation) is expanding, and there has been active development and practical application of important technologies, such as methods for synthesizing quantum dots, quantum dots with special functions, and methods for combining them.

[0007] [Iron sulfide quantum dot electrode material for lithium-ion batteries] Iron sulfide quantum dots are an important cathode material for lithium-ion batteries because they have high theoretical capacity and specific energy density, low cost, and meet the requirements for high-power storage devices. The small particle size, large surface area, and tunable surface functions of these quantum dots allow for short transmission paths and high conductivity, which has led to the development of an excellent synthesis method for iron sulfide quantum dots.

[0008] On the other hand, iron sulfide as a negative electrode material is Fe3S4, which is an alkali ion (Na ＋， Li ＋) It is attracting attention as a promising anode material for batteries. ＋ Ion batteries are important as next-generation secondary batteries to cope with the future challenges of lithium ion resource depletion and rising prices, and the development of Fe3S4 as a negative electrode material is actively progressing. There is a large market for Fe3S4 as an environmentally friendly and cost-effective alternative to conventional lithium ion batteries. The performance reported so far is that Fe3S4 electrodes can achieve 0.2Ag －１ 548mAhg －１ It exhibits a high reversible specific capacity of 20 Ag and also exhibits excellent cycling stability. －１ 275mAhg after 3500 cycles －１ is known to hold.

[0009] Furthermore, in recent years, due to its high electronic conductivity and excellent electrochemical properties, Fe3S4 and its composites have attracted attention as new active materials for lithium / sodium ion batteries. Because of their high sulfur content, Fe3S4 and its composites are expected to have better cycling performance than FeS-based composites. For example, at 5C, they achieved ~169.5 mAhg －１ It has been reported that the capacity of the Fe3S4 material is high and that it exhibits good cycle stability. Furthermore, it is expected that the performance can be significantly improved by using the Fe3S4 material as quantum dots.

[0010] [Quantum dot synthesis methods] Generally, methods for producing materials are classified into top-down and bottom-up methods. Top-down methods typically involve preparing a precursor, followed by chemical etching, microwave irradiation, ultrasonic treatment, electrochemical methods, hydrothermal / solvothermal methods, and hot injection methods.

[0011] Top-down quantum dot synthesis requires a balance between nucleation and growth to control the QD size and size distribution. For example, reagents may be injected into a hot reaction solution at room temperature to induce burst homogeneous nucleation, followed by slow growth of the quantum dots. The synthesis must begin with burst nucleation, followed by slow growth to achieve precise size control. Therefore, the reaction temperature and reactivity of the monomers play a crucial role in the nucleation and growth of quantum dots in top-down methods.

[0012] On the other hand, bottom-up methods are chemically synthesized from organic and inorganic precursors. The quantum dots synthesized by this method exhibit unique chemical and physical properties due to strong quantum confinement and edge effects, resulting in zero-dimensional materials with wide band gaps, ultrasmall sizes, and high surface-to-volume ratios. Furthermore, the size, structure, functional groups, and heteroatom parameters can be controlled during the synthesis of quantum dots, improving the active sites per unit mass, physicochemical tunability, and adaptability for hybridization with other nanomaterials.

[0013] K. Agarwal, H. Rai and S. Mondal, Quantum dots: an overview of synthesis, properties, and applications, Mater. Res. Express 10 (2023) 06 2001M. Liu, N. Yazdani, M. Yarema, M. Jansen, V. Wood and E. H. Sargent, Nature Electronics volume 4, pages 548-558 (2021)N. Chakrabarty, A. Dey, S. Krishnamurthy and Amit K. Chakraborty (2021) CeO2 / Ce2O3 quantum dot decorated reduced graphene oxide nanohybrid as electrode for supercapacitor. Appl Surf Sci 536:147960S. Liu, X. Cao, Y. Zhang, K. Wang, Q. Su, J. Chen, Q. He, S. Liang, G. Cao, A. Pan (2020) Carbon quantum dot modified Na3V2(PO4)2F3 as a high performance cathode material for sodium-ion batteries. J Mater Chem A 8: 18872-18879.H. Sun, H. Ji, E. Ju, Y. Guan, J. Ren and X. Qu (2015) Synthesis of Fluorinated and Nonfluorinated Graphene Quantum Dots through a New Top-Down Strategy for Long-Time Cellular Imaging. Chem Eur J 21:3791-3797

[0014] [Challenges of using iron sulfide in high-performance lithium-ion batteries] Iron sulfide is theoretically the most economically advantageous material for improving the performance of lithium-ion batteries. However, when sulfur is used, it undergoes a series of compositional and structural changes during cycling, resulting in the formation of soluble polysulfides and insoluble sulfides. This leads to challenges such as low electrical conductivity of sulfur and its discharge products (Li2S2 / Li2S), shuttle of intermediate polysulfides during the recharge / discharge process, and large volume expansion of sulfur.

[0015] To solve these problems, quantum dots can be used to absorb polysulfides with a high specific surface area and many surface functional sites, eliminating the polysulfide shuttle and simultaneously addressing the expansion of the volume of sulfur particles, thereby achieving high sulfur loading.

[0016] Furthermore, by creating a composite material of quantum dots and graphene, not only can these issues be resolved, but also a high C rate and high capacity are expected. However, conventional synthesis methods, such as the bottom-up and top-down methods, have their limitations, and a completely new synthesis method is required.

[0017] That is, an object of the present invention is to provide a novel method for producing quantum dots of iron-containing sulfide that is different from conventional methods.

[0018] In order to achieve the above object, the present invention provides a method for producing iron-containing sulfide quantum dots, which includes the steps of: preparing a mixed solution in which at least ferrous hydroxide is present in water at a temperature of 0°C to 30°C; oxidizing the mixed solution to form green rust crystals in the mixed solution, thereby preparing a green rust-containing suspension; elevating the temperature of the suspension in an oxygen-free air stream to a temperature of 50°C to 100°C, thereby synthesizing the ferrite quantum dots in the suspension; recovering the ferrite quantum dots in the suspension; and treating the ferrite quantum dots with hydrogen sulfide gas at a temperature of 110°C to 150°C, thereby converting the oxygen ions O of the ferrite. ２－ sulfide ions S ２－and a step of substituting the iron-containing sulfide compound with the iron-containing sulfide compound to produce the iron-containing sulfide quantum dots.

[0019] Such a method for producing quantum dots of iron-containing sulfide is a novel and simple method for producing quantum dots of iron-containing sulfide.

[0020] In this case, the ferrite quantum dots can be oriented and associated in the step of synthesizing the ferrite quantum dots. Also, the ferrite quantum dots can be aggregated and precipitated in the step of synthesizing the ferrite quantum dots.

[0021] In the method for producing quantum dots of iron-containing sulfide of the present invention, such association and condensation methods can be employed to obtain agglomerated particles.

[0022] The present invention also provides a method for producing an aqueous dispersion of iron-containing sulfide quantum dots, comprising the steps of: mixing iron-containing sulfide quantum dots produced by any of the above-described methods for producing iron-containing sulfide quantum dots with water; and ultrasonically dispersing the iron-containing sulfide quantum dots mixed in the water to prepare the dispersion of the iron-containing sulfide quantum dots.

[0023] In the present invention, an aqueous dispersion of iron-containing sulfide quantum dots can be prepared in this manner.

[0024] In this case, the ultrasonic dispersion can be carried out in the presence of a dispersant mixed in the water.

[0025] In this way, dispersion can be carried out more simply and reliably by performing dispersion using ultrasonic waves in the presence of a dispersant.

[0026] The present invention provides a novel and simple method for producing quantum dots of iron-containing sulfide, which is different from conventional methods.

[0027] 1 is an XRD chart of the iron-containing sulfide obtained in Example 1. FIG. 2 is a transmission electron microscope photograph of an ultrasonically dispersed sample of the iron-containing sulfide.

[0028] The present invention will be described in detail below, but the present invention is not limited thereto.

[0029] The present inventors have intensively developed a method for synthesizing iron-containing sulfide quantum dots from green rust compounds of iron hydroxide as a completely new synthesis method that combines the advantages and disadvantages of innovative methods for synthesizing quantum dots.

[0030] The present invention is a method for producing iron-containing sulfide quantum dots, which includes the steps of: preparing a mixed solution in which at least ferrous hydroxide is present in water at a temperature of 0°C to 30°C; oxidizing the mixed solution to form green rust crystals in the mixed solution, thereby preparing a green rust-containing suspension; elevating the temperature of the suspension in an oxygen-free air stream to 50°C to 100°C, thereby synthesizing the ferrite quantum dots in the suspension; recovering the ferrite quantum dots in the suspension; and treating the ferrite quantum dots with hydrogen sulfide gas at a temperature of 110°C to 150°C, thereby converting the oxygen ions O of the ferrite. ２－ sulfide ions S ２－ and producing iron-containing sulfide quantum dots. In the description of the present invention, the size of the quantum dots is preferably less than 50 nm, and quantum dots with a particle size of 2 to 10 nm are particularly preferred. Furthermore, quantum dots with a particle size of 2 to 5 nm are also possible.

[0031] The present invention is a method for producing quantum dots of iron-containing sulfide. The iron-containing sulfide produced by the present invention is not limited to those consisting only of iron and sulfur, but can also contain metals other than iron. That is, it can be Fe3S4 or MxFe3-xS4 (where M is a divalent transition metal ion). In this case, the quantum dots of ferrite, which is the intermediate product, can be ferrite represented by the general formula MFe2O4. The divalent transition metal ion can be Fe, ２＋ In addition, Zn ２＋ , Co ２＋ etc.

[0032] More specifically, this method can be performed as follows.

[0033] First, a mixed solution is prepared by adding at least ferrous hydroxide to water at a temperature of 0°C to 30°C (Step 1). By adding divalent transition metal ions other than iron, a ferrite containing metals other than iron and oxygen can be prepared. In particular, this mixed solution can be an aqueous solution in which the molar ratio of divalent metal ions M to Fe is M / Fe = 0 to 0.5.

[0034] Next, the mixed solution prepared in step 1 is oxidized to form green rust crystals in the mixed solution (step 2). This produces a green rust-containing suspension. ２＋ A part of the Fe ２＋ and Fe ３＋ It becomes an intermediate in which both coexist.

[0035] Next, the temperature of the green rust-containing suspension prepared in step 2 is raised to 50°C or higher and 100°C or lower in an oxygen-free air stream, thereby synthesizing ferrite quantum dots in the green rust-containing suspension (step 3). In this way, by raising the temperature of the green rust-containing suspension, quantum dots can be synthesized without an oxidation reaction.

[0036] The ferrite quantum dots can be aligned and aggregated in the step of synthesizing the ferrite quantum dots in step 3. This alignment and aggregation can be achieved by leaving the green rust-containing suspension (for example, for one day or more).

[0037] Furthermore, the ferrite quantum dots can be aggregated and precipitated in the step of synthesizing the ferrite quantum dots in step 3. This aggregation and precipitation can be carried out, for example, by evaporating the water contained in the green rust-containing suspension.

[0038] Next, the ferrite quantum dots synthesized in the green rust-containing suspension are recovered in step 3. As a recovery method, known means such as filtration and drying can be used.

[0039] Next, the ferrite quantum dots obtained in step 3 are treated with hydrogen sulfide gas at 110°C or higher and 150°C or lower to convert the oxygen ions O ２－ sulfide ions S ２－ to prepare iron-containing sulfide quantum dots (step 4).

[0040] The method of the present invention can produce quantum dots such as pyrite (FeS). There have been no previous examples of pyrite synthesis using a hydrogen sulfide gasification reaction at such low temperatures, between 110°C and 150°C (e.g., 120°C), making this a novel approach. For particles larger than this, with particle sizes of 50 to 300 nm, it was not possible to synthesize FeS using conventional gasification processes at such low temperatures, such as 120°C. However, this problem was resolved by reacting particles of quantum dot size (particle size less than 50 nm, particularly 2 to 10 nm).

[0041] Furthermore, if the reaction temperature is set to a high temperature of 150 to 170°C or higher, gray guide (Fe3S4) can be synthesized, and the particle size can be 2 to 100 nm. Furthermore, gray guide can be synthesized even at 200 to 300°C or higher.

[0042] In the present invention, by selecting the reaction temperature and quantum dot particle size, it is possible to synthesize both pyrite and greyguide using the same gasification reactor, thereby simplifying the industrial production process and enabling the production of two products.

[0043] The present invention further provides a method for producing an aqueous dispersion of iron-containing sulfide quantum dots. This method includes a step of mixing iron-containing sulfide quantum dots produced by the above-mentioned method for producing iron-containing sulfide quantum dots with water, and further includes ultrasonically dispersing the iron-containing sulfide quantum dots mixed in water to prepare an aqueous dispersion of iron-containing sulfide quantum dots. Here, ultrasonic dispersion can be performed in the presence of a dispersant mixed in water.

[0044] More specific examples of the present invention will now be described.

[0045] Example 1 First, at 25°C, 3 g of ferrous sulfate was dissolved in 150 ml of zinc chloride (Zn / Fe = 0.01) in water (previously degassed with oxygen by passing N2 gas through it), and the pH was adjusted to 8.5 with 0.05 M NaOH aqueous solution under an N2 gas atmosphere to prepare a hydroxide suspension (a mixture containing ferrous hydroxide and zinc co-hydroxide). This suspension was immersed in a 90°C hot water bath, and the suspension was heated to 75°C and reacted (oxidized) for 3 minutes. The reaction was then stopped by immersing in 4°C cold water. 50 ml of 0.01 M sodium acetate (pH 3.5) solution (4°C) was added to dissolve and recover the reaction residue. The precipitate was then collected by settling under an N2 atmosphere and dispersed in 2 L of water degassed with N2 gas. The mixture was left for 2 days, and the precipitate, which had aggregated due to orientation association, was collected using a centrifuge. XRD analysis of this recovered material confirmed that zinc ferrite crystals were oriented in the

[110] plane. Furthermore, from a transmission electron microscope photograph of the crystals, it was observed that the zinc ferrite crystal particles of the quantum dots had a particle size of 5 nm, and 3 to 10 particles were oriented and associated to form agglomerates.

[0046] The collected precipitate slurry was thinly applied to a 2.5 cm x 7 cm glass plate, and hydrogen sulfide gas was passed through it at 3 ml / min at 120°C for 24 hours. A product that had slightly turned yellowish-black was obtained, and its X-ray diffraction (XRD) was measured (see Figure 1). This showed that the oxygen ions (O ２－ ) is a sulfide ion (S ２－ ) of zinc ferrite substituted with oxygen ions (O ２－ ) is a sulfide ion (S ２－ It was confirmed that the iron-containing sulfides (zinc ferrite structure sulfides) substituted with ZnO were associated in the

[110] orientation.

[0047] Example 2 0.1 g of quantum dot aggregates of iron-containing sulfide (zinc ferrite structure sulfide) obtained in Example 1 above, which were aligned in a

[110] orientation, was subjected to ultrasonic cavitation treatment in 50 ml of water for 15 minutes to obtain a dispersion solution. A transmission electron microscope photograph was taken ( FIG. 2 ), and it was observed that the zinc ferrite structure sulfide had a crystal particle diameter of 5 nm, with 3 to 10 particles aligned in an orientation, forming agglomerates.

[0048] [Example 3] 0.5 g of the sample obtained in Example 2 was left in air at room temperature for one and a half years, and the electrical conductivity was measured during that time. As a result, the initial value of 150 Ωcm was maintained, and it was confirmed that the iron sulfide was a stable compound that was not oxidized.

[0049] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any configuration that is substantially identical to the technical idea described in the claims of the present invention and that provides similar effects is included within the technical scope of the present invention.

Claims

1. A method for producing iron-containing sulfide quantum dots, comprising the steps of: preparing a mixed solution containing at least ferrous hydroxide in water at a temperature of 0°C to 30°C; oxidizing the mixed solution to form green rust crystals in the mixed solution, thereby preparing a green rust-containing suspension; elevating the temperature of the suspension to 50°C to 100°C in an oxygen-free air stream, thereby synthesizing the ferrite quantum dots in the suspension; recovering the ferrite quantum dots in the suspension; and treating the ferrite quantum dots with hydrogen sulfide gas at a temperature of 110°C to 150°C, thereby converting the oxygen ions O of the ferrite. ２－ sulfide ions S ２－ and a step of substituting the iron-containing sulfide compound with the iron-containing sulfide compound to produce iron-containing sulfide quantum dots.

2. The method for producing iron-containing sulfide quantum dots according to claim 1, characterized in that in the step of synthesizing the ferrite quantum dots, the ferrite quantum dots are oriented and associated.

3. The method for producing iron-containing sulfide quantum dots according to claim 1, characterized in that in the step of synthesizing the ferrite quantum dots, the ferrite quantum dots are aggregated and precipitated.

4. A method for producing an aqueous dispersion of iron-containing sulfide quantum dots, comprising the steps of: mixing iron-containing sulfide quantum dots produced by the method for producing iron-containing sulfide quantum dots according to any one of claims 1 to 3 with water; and dispersing the iron-containing sulfide quantum dots mixed in the water using ultrasound to prepare a dispersion of the iron-containing sulfide quantum dots.

5. The method for producing an aqueous dispersion of iron-containing sulfide quantum dots according to claim 4, characterized in that the ultrasonic dispersion is carried out in the presence of a dispersant mixed in the water.

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