Method for preparing spherical nano barium fluoride based on temperature response type surfactant-free microemulsion

Abstract

The invention discloses a method for preparing spherical nano barium fluoride, spherical barium fluoride smaller than 5nm is synthesized by a microemulsion reaction method, and the microemulsion is temperature-responsive and does not contain a surfactant. Controllable preparation of the spherical barium fluoride with the ultra-small size smaller than 5 nm under the surfactant-free condition is achieved for the first time. A ternary surfactant-free microemulsion system composed of diethylene glycol monohexyl ether, octane and water is constructed, the amphiphilic solvent characteristic of diethylene glycol monohexyl ether is used for replacing a traditional surfactant, and the dependence of an existing template method on the surfactant is successfully broken through. The particle size of the obtained BaF2 product is only smaller than 5 nm and enters a strong quantum confinement scale range, and the preparation blank of a barium fluoride material in the field of sub-5-nm ultra-small solid spheres is filled.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterial preparation technology, specifically, it relates to a method for preparing ultra-small particle size, surfactant-free nanosphere barium fluoride and the resulting product. Background Technology

[0002] Barium fluoride (BaF2), as an important semiconductor functional material, has attracted much attention due to its broad application prospects in catalysts, novel memories, optical devices, and biosensors. Numerous studies have shown that the performance of BaF2 largely depends on its size, morphology, and dimensionality. Therefore, the preparation of BaF2 nanomaterials with uniform size and specific morphology is crucial for achieving application goals such as improving its photocatalytic efficiency and optimizing battery performance. Considering current application requirements, the preparation of BaF2 nanospheres with ultra-small size, regular solid spherical morphology, and clean, residue-free surfaces is essential for leveraging its quantum size effect and improving photocatalytic efficiency. -The ionic conductivity and the assurance of infrared optical transmittance are of significant theoretical and practical value. However, the core technical challenge of achieving highly reproducible and controllable synthesis of BaF2 nanomaterials, particularly controlling the size to below 10 nm while simultaneously obtaining solid spherical morphology and avoiding surface contamination, remains largely unresolved. To address this challenge, traditional methods such as microemulsion methods relying on surfactants or templates (JAm. Chem. Soc., 2003, 125, 37, 11196) and hydrothermal methods (RSC Adv, 2021, 11, 40051) have been extensively studied and have successfully prepared BaF2 with various morphologies, including sheets, nanowires, hollow spheres, and cubes. However, existing technologies have the following inherent drawbacks: First, research is mostly focused on hollow or heterogeneous structures, with less attention paid to the controllable synthesis of ultra-small solid spherical BaF2; Second, the surfactants introduced by traditional template methods are difficult to completely remove, and their residues are firmly adsorbed on the material surface, causing characteristic absorption loss of CH bonds in infrared optics applications, hindering the interfacial migration of F⁻ ions in ionic conductor applications, and covering surface active sites in catalytic applications, severely restricting the intrinsic properties of the material; Third, the size of BaF2 particles prepared by existing methods is usually limited to the range of tens to hundreds of nanometers (RSC Adv, 2021, 11, 40051), making it difficult to break through the size bottleneck below 10 nm, which restricts in-depth exploration in cutting-edge fields such as quantum effects and ultra-high conductivity. There have been reports on the preparation of nano-barium fluoride using the traditional microemulsion method. The morphology of the obtained products is mainly rod-shaped (J Solid State Chem, 2006, 179, 955) and hollow spheres (CrystEngComm, 2010, 12, 1945), and the particle size is mostly concentrated above 50 nm (Langmuir 2023, 39, 1181). There is no clear explanation of the controllable preparation and reproducibility of ultra-small solid spheres smaller than 5 nm.

[0003] Traditional methods rely on surfactants to construct the microreaction environment, inevitably introducing organic residues. Furthermore, existing methods struggle to simultaneously achieve the triple control objectives of ultra-small size, solid spherical morphology, and narrow particle size distribution, and none clearly demonstrate whether the reported methods can accurately and reproducibly synthesize spherical BaF2 with a size less than 5 nm and uniform morphology. In addition, traditional template methods utilize large amounts of surfactants, resulting in complex post-processing and significant organic wastewater discharge, among other environmental problems. Therefore, it is necessary to develop a method for preparing ultra-small spherical BaF2 materials to address the key issues of surfactant residues, difficulty in size control, and heavy environmental burden in existing technologies. Summary of the Invention

[0004] Purpose of the invention: To address the core challenge in existing technologies of achieving controllable preparation of ultra-small spherical barium fluoride under surfactant-free conditions, while simultaneously ensuring that the product surface is free of organic residues, has uniform particle size and height, and exhibits good reproducibility, this invention provides a highly reproducible synthesis method for preparing spherical barium fluoride smaller than 5 nm based on temperature-responsive surfactant-free microemulsions.

[0005] To achieve the above-mentioned technical objectives, the technical solution of the present invention is as follows: This invention first constructs a temperature-responsive surfactant-free microemulsion system. The surfactant-free microemulsion is a ternary system composed of diethylene glycol monohexyl ether as an amphiphilic solvent, octane as the oil phase, and water. At 25°C, the single-phase region of this surfactant-free microemulsion accounts for 32.44% of the total area; upon heating to 35°C, the single-phase region decreases to 24.37%, indicating that the system exhibits good temperature response characteristics. Within the single-phase region of this microemulsion system, a sample point located in the W / O region and exhibiting temperature responsiveness (i.e., situated between the 25°C and 35°C phase boundaries) is selected as the reaction medium.

[0006] The microemulsion contains 50-60% diethylene glycol monohexyl ether, 15-25% octane, and 20-30% aqueous phase.

[0007] The method for preparing spherical barium fluoride nanoparticles includes the following specific steps: 1) Preparation of microemulsion A: Prepare a BaCl2 aqueous solution as the aqueous phase, mix it with diethylene glycol monohexyl ether and octane to prepare W / O type microemulsion A. The obtained microemulsion A is a microemulsion with BaCl2 aqueous solution as the aqueous phase. 2) Preparation of microemulsion B: Prepare an aqueous solution of NH4F as the aqueous phase, mix it with diethylene glycol monohexyl ether and octane to prepare W / O type microemulsion B. The obtained microemulsion B is a microemulsion with an aqueous phase of NH4F aqueous solution. 3) Add microemulsion B dropwise to microemulsion A and stir the reaction at room temperature; 4) The microemulsion obtained in step 3) is heated to perform demulsification treatment. 5) The product obtained after demulsification was purified to obtain barium fluoride nanoparticles.

[0008] Furthermore, the concentration of the BaCl2-containing aqueous solution is 18.325-73.3 mmol / L, and the concentration of the NH4F-containing aqueous solution is twice the concentration of the BaCl2-containing aqueous solution. The molar ratio of the two is 1:2 to ensure that BaCl2... 2+ With F - The reaction proceeds completely at a stoichiometric ratio of 1:2.

[0009] Furthermore, in step 3), the dropping rate is 0.5~2 mL / s, and the reaction is continuously stirred at room temperature for 30~60 minutes.

[0010] Furthermore, in step 4), the demulsification temperature is 35°C, and the holding time is 10-30 minutes, so that the microemulsion system is completely destroyed and the generated BaF2 nanoparticles are released.

[0011] Furthermore, the method also includes purifying the product obtained after demulsification: the mixture after demulsification is centrifuged, the obtained solid is washed with deionized water and methanol 2-3 times in sequence, and finally dried under vacuum at 50-60°C for 6-12 hours to obtain pure spherical BaF2 powder smaller than 5 nm.

[0012] Compared to the common challenges of existing surfactant-free microemulsion (SFME) nanoparticle preparation technologies, where the micro-region size of SFME systems typically reaches around 10 nm but fails to effectively confine particle growth, the synthesized product size generally far exceeds the microemulsion droplet diameter. The microemulsion droplets serve only as initial mixing sites, losing their confinement function as nanoreactors. This invention exhibits a completely unexpected technical effect: the constructed SFME system microemulsion droplets, detected by DLS, have an average diameter of approximately 9 nm, but the final obtained BaF2 nanoparticles have an average particle size of less than 5 nm, achieving a "reverse confinement" effect where the product size is much smaller than the micro-region size. This result contrasts sharply with the common pattern of "product size far exceeding micro-region size" in existing technologies, proving that this invention successfully suppresses the Ostwald ripening-driven size growth mechanism. It achieves true nano-confined synthesis in a surfactant-free microemulsion system for the first time, breaking through the inherent bottleneck of SFME technology's difficulty in preparing 5 nm ultra-small materials, and achieving an unprecedented balance between green synthesis and ultra-small size control.

[0013] Beneficial effects: 1) This invention achieves, for the first time, the controllable preparation of ultrasmall spherical barium fluoride particles smaller than 5 nm under surfactant-free conditions. By constructing a ternary surfactant-free microemulsion system composed of diethylene glycol monohexyl ether, octane, and water, the amphiphilic solvent properties of diethylene glycol monohexyl ether are utilized to replace traditional surfactants, successfully overcoming the dependence on surfactants in existing template methods. The resulting BaF2 product has a particle size of less than 5 nm, entering the strongly quantum confinement scale range, filling the gap in the preparation of barium fluoride materials in the sub-5 nm ultrasmall solid sphere field.

[0014] 2) The spherical BaF2 particles smaller than 5 nm obtained by this invention have a high-purity surface state, avoiding performance degradation caused by surfactant residue. Since the synthesis process does not introduce any traditional surfactants, the surface of the resulting product has no adsorption of organic molecules, eliminating the need for cumbersome post-washing treatment, and has significant advantages in ion conductors, infrared optics, and catalytic applications.

[0015] 3) The microemulsion system constructed in this invention has temperature-responsive characteristics, providing a new dimension for the fine control of morphology and size. By utilizing the phase boundary changes in the temperature range of 25℃ to 35℃, the W / O microregion composition suitable for the synthesis of BaF2 spherical particles smaller than 5 nm can be precisely locked. The synthesis method has good controllability and high reproducibility.

[0016] 4) The spherical BaF2 particles smaller than 5 nm obtained by this invention have ultra-small size, uniform morphology and good dispersibility. The particle size distribution is highly uniform and there is no agglomeration, which lays the material foundation for its application in high-end optics, catalysis and ion conductors.

[0017] 5) The method of this invention is green, environmentally friendly, low-cost, and simple. The diethylene glycol monohexane and octane used are commercially available reagents, the system composition is simple, and there is no need to synthesize complex surfactants or template agents; the entire preparation process is carried out under mild conditions with low energy consumption; the absence of surfactants not only reduces raw material costs but also simplifies the post-processing procedures, with almost no significant emissions of waste, meeting the requirements of green chemistry and sustainable development, and has good prospects for industrial application. Attached Figure Description

[0018] Figure 1 This is a ternary phase diagram of diethylene glycol monohexyl ether (amphiphilic molecule), octane (oil phase), and water in Example 1, where the component contents of the system are expressed as mass fractions. As can be seen from the diagram, the ternary phase diagram is divided into two parts: the blank part is the single-phase region; the shaded part is the multiphase region.

[0019] Figure 2 These are microemulsion phase diagrams at different temperatures of 25℃ and 35℃.

[0020] Figure 3 These are pictures of emulsions that did not break at 25℃ and those that broke at 35℃.

[0021] Figure 4 This is the XRD pattern of spherical BaF2 particles smaller than 5 nm prepared using a surfactant-free microemulsion in Example 3.

[0022] Figure 5 This is a TEM image of the spherical BaF2 obtained in Example 3 using a W / O type microemulsion as a precursor and the microemulsion as a solvent. Detailed Implementation

[0023] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments and accompanying drawings. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0024] Example 1: Construction of a surfactant-free microemulsion precursor of diethylene glycol monohexyl ether / octane / water Weigh 1 g of a binary mixture of diethylene glycol monohexyl ether and octane in mass ratios of 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, and 1:9, and place it in a 10 mL glass bottle. Then, place the bottle in a constant-temperature water bath at 25 ± 1 °C. Add water dropwise with magnetic stirring, recording the total mass of water used when the solution changes from clear to cloudy. Repeat this process three times and take the average value. Plot a ternary phase diagram based on the mass of water corresponding to each ratio, as shown below. Figure 1 As shown. The clarified liquid is the single-phase region of the desired surfactant-free microemulsion.

[0025] Example 2: Temperature response of surfactant-free microemulsion The water bath temperature was changed to 35°C according to the method in Example 1, and other operations were the same as above. The final three-phase diagrams at different temperatures (25°C and 35°C) are shown below. Figure 2 As shown. According to the phase diagram at 25℃, 6.22 g of water, 13.975 g of diethylene glycol monohexyl ether, and 4.8050 g of octane were mixed to obtain a clear and transparent water-in-oil (W / O) microemulsion. Figure 3 A), when the temperature is increased from 25℃ to 35℃, demulsification and oil-water separation can be observed. Figure 3 B). To facilitate observation of the oil-water interface before and after demulsification, Figure 3 The microemulsion was pre-stained with a small amount of the water-soluble dye methylene blue.

[0026] Example 3: Preparation method of spherical BaF2 material 1) Add 0.0521 g of BaCl2 to 6.2200 g of water to prepare an aqueous BaCl2 solution. Use the resulting BaCl2 salt solution as the new aqueous phase, replacing the pure water in the microemulsion. Mix this solution with 13.9750 g of diethylene glycol monohexyl ether and 4.8050 g of octane. After preparation, stir at room temperature for 30 min to obtain W / O type microemulsion A. The composition ratio of this microemulsion is consistent with the previously selected W / O type microemulsion precursor, i.e., aqueous phase mass fraction 24.88%, diethylene glycol monohexyl ether mass fraction 55.90%, and octane mass fraction 19.22%. 2) Add 0.0185 g of NH4F to 6.2200 g of water to prepare an NH4F aqueous solution. Use the resulting NH4F salt solution as the new aqueous phase, replacing the pure water in the microemulsion. Mix this solution with 13.9750 g of diethylene glycol monohexyl ether and 4.8050 g of octane. After preparation, stir at room temperature for 30 min to obtain W / O type microemulsion B. The composition ratio of this microemulsion is consistent with the previously selected W / O type microemulsion precursor, i.e., aqueous phase mass fraction 24.88%, diethylene glycol monohexyl ether mass fraction 55.90%, and octane mass fraction 19.22%. 3) Add microemulsion B obtained in step (2) dropwise to the microemulsion containing BaCl2 obtained in step (1) and react for 1 hour with stirring at room temperature; 4) After the reaction is complete, the microemulsion containing the sample is placed in a 35°C water bath. After the layers are separated, the gray precipitate is obtained by filtration. The gray precipitate is washed three times each with water and methanol. Then the obtained sample is placed in a 50°C oven and dried for 18 hours to obtain BaF2.

[0027] XRD diffraction was performed. Figure 4 ; Figure 4 All diffraction peaks were in good agreement with the standard spectrum of spherical BaF2, and showed good consistency with the reported data (JCPDS NO: 04-0452). No diffraction peaks of other impure phases were found in the XRD pattern; the BaF2 prepared above was redispersed in methanol and its TEM was measured, as shown in the figure. Figure 5 As shown.

[0028] Example 4: Effect of different reactant concentrations on BaF2 particle size: To examine the flexibility of the method described in this invention in controlling product size, BaF2 was synthesized in this embodiment at different concentrations of BaCl2 and NH4F. The specific operating steps were the same as in Example 3, except for the initial concentrations of the BaCl2 and NH4F aqueous solutions to achieve different final concentrations in the microemulsion.

[0029] Concentration design and conversion: The mass fractions of water, diethylene glycol monohexyl ether, and octane in microemulsions A and B are kept constant (the aqueous phase always accounts for 24.88%). The reactant concentration in the microemulsion is obtained by multiplying the molar concentration of the reactant in its aqueous solution by the mass fraction of the aqueous phase. To obtain the target concentration, a salt solution of the corresponding concentration needs to be prepared.

[0030] 0.0260 g, 0.1041 g, and 0.2082 g of BaCl2 were weighed and added to 6.2200 g of water to prepare BaCl2 aqueous solutions. The resulting BaCl2 salt solution replaced the pure water in the microemulsion as a new aqueous phase and was mixed with 13.9750 g of diethylene glycol monohexyl ether and 4.8050 g of octane to prepare W / O type microemulsion A. The composition ratio of the microemulsion was consistent with the previously selected W / O type microemulsion precursor, i.e., aqueous phase mass fraction 24.88%, diethylene glycol monohexyl ether mass fraction 55.90%, and octane mass fraction 19.22%. 0.0093 g, 0.0370 g, and 0.0741 g of NH4F were weighed and added to 6.2200 g of water to prepare NH4F aqueous solutions. The resulting NH4F salt solution replaced the pure water in the microemulsion as a new aqueous phase and was mixed with 13.9750 g of diethylene glycol monohexyl ether and 4.8050 g of octane to prepare W / O type microemulsion B. The composition ratio of the microemulsion was consistent with the previously selected W / O type microemulsion precursor, i.e., aqueous phase mass fraction 24.88%, diethylene glycol monohexyl ether mass fraction 55.90%, and octane mass fraction 19.22%. Microemulsion preparation and reaction: Following the calculations described above and referring to the steps in Example 3, aqueous solutions of BaCl2 and NH4F of corresponding concentrations were prepared. Then, strictly following the steps in Example 3, microemulsions A and B with different concentrations were prepared, and subjected to mixing, demulsification, and purification processes.

[0031] The obtained BaF2 product was characterized by TEM to observe its particle size and morphology. The results showed that: 1) When BaF2 nanoparticles containing 0.0260g of BaCl2 microemulsion A and 0.0093g of NH4F microemulsion B are obtained, the average particle size is less than 5 nm, the morphology is uniform, and they are spherical.

[0032] 2) When the average particle size of the BaF2 nanoparticles obtained by the microemulsion A containing 0.1041g of BaCl2 and the microemulsion B containing 0.0370g of NH4F is also less than 5 nm, there is no significant difference in particle size from the product under the condition of 5 mmol / L, and it is consistent with the results of Example 3.

[0033] 3) When the average particle size of BaF2 nanoparticles obtained by mixing 0.2082g of BaCl2 microemulsion A and 0.0741g of NH4F microemulsion B increases significantly to about 30 nm, the particle morphology still remains spherical.

[0034] This embodiment verifies that the method of the present invention can stably prepare ultra-small spherical BaF2 particles smaller than 5 nm within a specific concentration range, i.e., when the mass of BaCl2 in the corresponding microemulsion (6.2200 g water, 13.9750 g diethylene glycol monohexyl ether, and 4.8050 g octane) does not exceed 0.1041 g. When the reactant concentration exceeds this range (e.g., BaCl2 mass 0.2082 g), the nucleation and growth kinetics within the microemulsion droplets change, leading to a significant increase in particle size. This provides clear process parameter guidance for further controlling the product size; that is, while maintaining a constant aqueous phase ratio in the microemulsion, the concentration of reactants within the microemulsion droplets can be precisely controlled by adjusting the initial concentration of the salt solution, thereby achieving controllable preparation of spherical BaF2 particles ranging from smaller than 5 nm to 30 nm.

[0035] The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Any person skilled in the art can easily make changes or substitutions to the components after the present invention is disclosed, and all technical improvements that do not depart from the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing spherical barium fluoride nanoparticles, characterized in that, Spherical barium fluoride particles smaller than 5 nm were synthesized using a microemulsion reaction method. The microemulsion was temperature-responsive and did not contain surfactants.

2. The method for preparing spherical barium fluoride nanoparticles according to claim 1, characterized in that, The microemulsion is a temperature-responsive water-in-oil microemulsion composed of diethylene glycol monohexyl ether, octane, and an aqueous phase.

3. The method for preparing spherical barium fluoride nanoparticles according to claim 2, characterized in that, The microemulsion contains 50-60% diethylene glycol monohexyl ether, 15-25% octane, and 20-30% aqueous phase.

4. The method for preparing spherical barium fluoride nanoparticles according to claim 3, characterized in that, At 25°C, the area of ​​the single-phase region of the microemulsion is 32.44%; when the temperature is increased to 35°C, the area of ​​the single-phase region decreases to 24.37%.

5. The method for preparing spherical barium fluoride nanoparticles according to claim 4, characterized in that, The specific steps include the following: 1) Preparation of microemulsion A: Prepare an aqueous solution of BaCl2 as the aqueous phase, mix it with diethylene glycol monohexyl ether and octane to prepare W / O type microemulsion A. The obtained microemulsion A is a microemulsion with an aqueous phase of BaCl2. 2) Preparation of microemulsion B: Prepare an aqueous solution of NH4F as the aqueous phase, mix it with diethylene glycol monohexyl ether and octane to prepare W / O type microemulsion B. The obtained microemulsion B is a microemulsion with an aqueous phase of NH4F aqueous solution. 3) Add microemulsion B dropwise to microemulsion A and stir the reaction at room temperature; 4) The microemulsion obtained in step 3) is heated to perform demulsification treatment; 5) The product obtained after demulsification was purified to obtain barium fluoride nanoparticles.

6. The method for preparing spherical barium fluoride nanoparticles according to claim 5, characterized in that, The concentration of the BaCl2-containing aqueous solution is 18.325-73.3 mmol / L, and the concentration of the NH4F-containing aqueous solution is twice the concentration of the BaCl2-containing aqueous solution.

7. The method for preparing spherical barium fluoride nanoparticles according to claim 5, characterized in that, The dropping rate in step 3) is 0.5~2 mL / s.

8. The method for preparing spherical barium fluoride nanoparticles according to claim 5, characterized in that, The reaction time for step 3) is 0.5-1 hour.

9. The method for preparing spherical barium fluoride nanoparticles according to claim 5, characterized in that, In step 4), the demulsification temperature is 35°C, and the holding time is 10-30 minutes.

10. The method for preparing spherical barium fluoride nanoparticles according to claim 5, characterized in that, The purification process in step 5) involves centrifuging the mixture, washing the resulting solid with deionized water and methanol 2-3 times in sequence, and finally drying it under vacuum at 50-60°C for 6-12 hours to obtain pure spherical BaF2 powder smaller than 5 nm. After filtration, washing and drying, barium fluoride nanoparticles are obtained.

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