A method for preparing sol-gel in-situ doped rare earth element ZnO quantum dots

Abstract

This invention relates to the field of nano-optoelectronic material preparation technology, specifically disclosing a sol-gel method for in-situ doping of ZnO quantum dots with rare earth ions. The method uses zinc acetate as the zinc source, preparing a zinc source solution in an anhydrous ethanol system. By controlling the dropping rate of the alkaline solution and the reaction temperature, Zn²⁺ undergoes hydrolysis and dehydration reactions to generate ZnO quantum dots. Simultaneously, rare earth ions such as Eu, Tb, Pr, or Dy are introduced during the reaction to achieve in-situ doping. This invention does not require high-temperature and high-pressure equipment, has mild process conditions, and is simple to operate. By adjusting the type and doping ratio of rare earth elements, the particle size, lattice structure, and optical properties of ZnO quantum dots can be controlled, resulting in controllable changes in their absorption and emission peaks. The obtained quantum dots have uniform particle size, good dispersion, and stable luminescence properties.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterial preparation technology, specifically relating to a method for preparing ZnO quantum dots by in-situ doping of different rare earth elements into sol-gel. Background Technology

[0002] Zinc oxide (ZnO) is a wide bandgap semiconductor material with a bandgap of approximately 3.37 eV and a relatively large exciton binding energy (approximately 60 meV), showing broad application prospects in optoelectronic devices, ultraviolet detectors, light-emitting diodes, photocatalysis, and biolabeling. With the development of nanotechnology, ZnO quantum dots, due to their quantum confinement effect, exhibit size-dependent optical and electronic properties, becoming a hot research topic in recent years.

[0003] Currently, the main methods for preparing ZnO quantum dots include hydrothermal methods, solvothermal methods, precipitation methods, and sol-gel methods. Among them, hydrothermal methods typically require high-temperature and high-pressure reaction environments, have high equipment requirements, and are complex processes, making them unsuitable for large-scale preparation; precipitation methods, although simple to operate, are sensitive to reaction conditions and have difficulty controlling particle size distribution; solvothermal methods often require long reaction times and have high energy consumption.

[0004] The sol-gel method is widely used in the preparation of ZnO quantum dots due to its relatively simple process, mild reaction conditions, and low equipment requirements. However, ZnO quantum dots prepared by the existing sol-gel method still have the following shortcomings: (1) The particle size control precision is limited, and agglomeration is prone to occur, resulting in unstable optical performance; (2) The luminescence performance is mainly affected by its own defect structure, and the means of controlling the luminescence wavelength and intensity are limited; (3) The optical properties of quantum dots lack designability and controllability, making it difficult to meet the application requirements of multicolor emission and fine control.

[0005] To improve the optical properties of ZnO quantum dots, researchers have attempted to modify them by introducing rare earth elements. Rare earth elements possess a unique 4f electron shell structure, enabling narrow-band emission peaks and rich energy level structures, which are beneficial for achieving multicolor luminescence and energy transfer modulation. For example, Eu³⁺ ions produce red light, Tb³⁺ ions produce green light, Dy³⁺ ions produce blue-white light, and Pr³⁺ ions exhibit multi-level transition characteristics. By appropriately doping with rare earth ions, it is hoped that the luminescence efficiency of ZnO quantum dots can be improved and their luminescence range expanded.

[0006] However, existing methods for rare-earth-doped ZnO quantum dots still have the following problems: (1) The doping process usually requires high-temperature annealing, which increases energy consumption; (2) Rare earth ions are prone to surface adsorption or aggregation, making it difficult to achieve uniform lattice doping; (3) Inaccurate control of the doping ratio leads to limited range of luminescence intensity and emission peak position adjustment; (4) Some methods and processes are complicated and have poor repeatability.

[0007] Therefore, there is an urgent need to provide a method for preparing rare-earth-doped ZnO quantum dots that is simple in process, mild in conditions, controllable in doping, and capable of achieving tunable optical properties, so as to solve the above-mentioned problems in the existing technology.

[0008] This invention is proposed against this background. By improving the sol-gel process, in-situ doping of rare earth ions is achieved, and the particle size, lattice structure and optical properties of ZnO quantum dots are adjusted by controlling the doping ratio, thereby realizing the controllable adjustment of the optical performance of quantum dots. Summary of the Invention

[0009] This invention aims to provide a method for synthesizing rare-earth element-doped ZnO quantum dots using a sol-gel method. The technical problem to be solved is to simplify the preparation process while ensuring that the optical properties of the resulting quantum dots vary with the type and proportion of the rare-earth element used for doping. The technical solution of this invention is as follows: The sol-gel method for synthesizing rare-earth element-doped ZnO quantum dots includes the following steps: Step 1, preparation of zinc source solution. Zinc salt is selected as the zinc source. The acid radical ions generated after decomposition can be removed by volatilization or water washing, resulting in high chemical purity of the synthesized ZnO QDs. Step 2, preparation of alkaline solution. An alkaline environment can rapidly promote the hydrolysis and dehydration reaction of Zn²⁺ to generate Zn(OH)₂ intermediate, which is then further dehydrated under alkaline conditions to form ZnO QDs. This process can usually be carried out at room temperature or under hydrothermal conditions, without the need for high-temperature and high-pressure equipment, and is simple to operate. Step 3, quantum dot solution synthesis. A cooled alkaline solution is slowly added to the zinc source solution, and the mixture is stirred thoroughly. Heating is stopped immediately after a color change occurs, and the solution is cooled to prevent particle growth, thus obtaining nano-sized quantum dots. Step four: Rare earth element doping. Solutions containing rare earth elements are added dropwise to the quantum dot synthesis solution according to different concentrations, and the mixture is heated and stirred appropriately. In one embodiment, the alkaline solution preparation in step two includes the following steps: First, 1g of KOH is weighed using a balance, and 30ml of anhydrous ethanol is measured using a graduated cylinder and poured into a beaker and heated to 78°C; Second, KOH is added to anhydrous ethanol, stirred to dissolve, and a potassium hydroxide ethanol solution is obtained, which is then cooled to room temperature for later use. In one embodiment, the zinc source solution preparation in step one includes the following: First, 100ml of anhydrous ethanol is measured using a graduated cylinder and poured into a reflux condenser and heated to 80°C. Second, 2.2g of zinc acetate is weighed using a balance, and the weighed zinc acetate is added to anhydrous ethanol and stirred to dissolve, resulting in a zinc acetate ethanol solution. In step four, the rare earth element doping includes the following: doping the ZnO quantum dot solution with Eu, Dy, Pr, and Tb elements, with a rare earth element:Zn molar ratio of 0–7%. The heating is oil bath heating; the stirring is magnetic stirring.Compared with existing technologies, this invention has the following advantages: 1. This invention uses the sol-gel method, carrying out the reaction in an anhydrous ethanol system, eliminating the need for high-temperature and high-pressure equipment. The preparation conditions are mild, the operation is simple, and energy consumption and equipment costs are reduced. 2. By controlling the concentration of the alkaline solution and the dropping rate, the particle size of ZnO quantum dots can be effectively controlled, inhibiting particle agglomeration and obtaining nanoscale quantum dots with uniform particle size and good dispersion. 3. By introducing different rare earth elements (Eu, Dy, Pr, Tb) and adjusting their doping ratio (0–7 mol%), the optical properties of ZnO quantum dots can be controlled, allowing the emission peak position and emission intensity to change with the type and ratio of dopant. 4. The introduction of rare earth elements improves the ZnO lattice structure and defect state, promotes the energy transfer process, and improves the emission intensity and stability of quantum dots. 5. The method of this invention is highly versatile and applicable to doping with various rare earth ions, providing a technical basis for the preparation of multicolor luminescent materials and optoelectronic functional materials. 6. The ZnO quantum dots obtained by this invention have good dispersion and high stability, making them suitable for subsequent film formation or optoelectronic applications. Attached Figure Description Figure 1 The absorption spectrum of ZnO:Eu quantum dots prepared in the embodiments of the present invention; Figure 2 The emission spectrum of ZnO:Eu quantum dots prepared in the embodiments of the present invention; Figure 3 Transmission electron microscope image of ZnO:Eu quantum dots prepared in an embodiment of the present invention; Figure 4 The absorption spectrum of ZnO:Tb quantum dots prepared in the embodiments of the present invention; Figure 5 The emission spectrum of ZnO:Tb quantum dots prepared in the embodiments of the present invention; Figure 6 Transmission electron microscope image of ZnO:Tb quantum dots prepared in an embodiment of the present invention; Figure 7 The absorption spectrum of ZnO:Pr quantum dots prepared in the embodiments of the present invention; Figure 8 The absorption spectrum of ZnO:Pr quantum dots prepared in the embodiments of the present invention; Figure 9 Transmission electron microscope image of ZnO∶Pr quantum dots prepared in an embodiment of the present invention; Figure 10 The absorption spectrum of ZnO:Dy quantum dots prepared in the embodiments of the present invention; Figure 11 The absorption spectrum of ZnO:Dy quantum dots prepared in the embodiments of the present invention; Figure 12 Transmission electron microscope image of ZnO∶Dy quantum dots prepared in an embodiment of the present invention; Figure 13 Transmission electron microscope image of ZnO quantum dots prepared in an embodiment of the present invention; Figure 14 X-ray photoelectron spectrum of ZnO quantum dots prepared in an embodiment of the present invention; Figure 15 The average particle size and interplanar spacing of ZnO quantum dots doped with different rare earth elements prepared in the embodiments of the present invention; Figure 16 The images show the FTIR infrared spectra of ZnO quantum dots doped with different rare earth elements prepared in the embodiments of the present invention. Detailed Implementation To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. Example 1: A method for preparing ZnO quantum dots in sol-gel doped with rare earth ions according to an embodiment of the present invention includes the following steps: First, a zinc salt is selected to prepare a zinc source solution; simultaneously, an alkaline solution is prepared. Then, the alkaline solution cooled to room temperature is slowly added to the zinc source solution, and the mixture is stirred thoroughly. Simultaneously, the prepared rare earth element solution is slowly added. Heating is immediately stopped and the solution is cooled after a color change occurs to prevent particle growth, thereby obtaining nano-sized quantum dots. Example 2: Based on the method for preparing ZnO quantum dots with rare earth ions in situ using sol-gel according to Example 1 of this invention, a method for synthesizing rare earth Eu-doped ZnO:Eu quantum dots by sol-gel method is described below. The operation steps are as follows: Weigh 1g of KOH using a balance, measure 30ml of anhydrous ethanol into a beaker using a graduated cylinder and heat to 78°C. Add KOH to the anhydrous ethanol and stir to dissolve, obtaining a potassium hydroxide ethanol solution, and cool it to room temperature for later use. Simultaneously, weigh 2.2g of zinc acetate using a balance, measure 100ml of anhydrous ethanol into a reflux condenser and heat to 80°C. Add the weighed zinc acetate to the anhydrous ethanol and stir to dissolve, obtaining a zinc acetate ethanol solution. Slowly add the KOH solution cooled to room temperature to the zinc acetate ethanol solution, then add the prepared Eu solution and stir thoroughly. After the solution undergoes a color change, immediately stop heating and cool to prevent particle growth, thereby obtaining ZnO quantum dots doped with different Eu molar ratios. The absorption and emission spectra of ZnO:Eu quantum dots with different doping contents prepared in this embodiment are shown below. Figure 1and Figure 2 As shown, Eu doping causes a change in the absorption peak of ZnO quantum dots. Transmission electron microscopy images of ZnO:Eu quantum dots with different doping contents prepared in this embodiment are shown below. Figure 3 As shown, the synthesized quantum dot samples have a particle size of approximately 6.14 nm and are uniformly distributed. The quantum dot particles exhibit distinct lattice fringes, and a plane spacing of 0.265 nm can be clearly seen, corresponding to the (002) crystal plane of zinc oxide. Example 3: Based on the method for preparing ZnO quantum dots with rare earth ions in situ using sol-gel according to Example 1 of this invention, a method for synthesizing rare earth Tb-doped ZnO:Tb quantum dots by sol-gel method is described below. The operation steps are as follows: Weigh 1g of KOH using a balance, measure 30ml of anhydrous ethanol into a beaker using a graduated cylinder and heat to 78°C. Add KOH to the anhydrous ethanol and stir to dissolve, obtaining a potassium hydroxide ethanol solution, and cool it to room temperature for later use. Simultaneously, weigh 2.2g of zinc acetate using a balance, measure 100ml of anhydrous ethanol into a reflux condenser and heat to 80°C. Add the weighed zinc acetate to the anhydrous ethanol and stir to dissolve, obtaining a zinc acetate ethanol solution. Slowly add the KOH solution cooled to room temperature to the zinc acetate ethanol solution, then add the prepared Tb solution and stir thoroughly. After the solution undergoes a color change, immediately stop heating and cool to prevent particle growth, thereby obtaining ZnO quantum dots doped with different Eu molar ratios. The absorption and emission spectra of ZnO:Tb quantum dots with different doping contents prepared in this embodiment are as follows: Figure 4 and Figure 5 As shown, Eu doping causes a change in the absorption peak of ZnO quantum dots. Transmission electron microscopy images of ZnO:Tb quantum dots with different doping contents prepared in this embodiment are shown below. Figure 6As shown, the synthesized quantum dot samples have a particle size of approximately 5.73 nm and are uniformly distributed. The quantum dot particles exhibit distinct lattice fringes, and a plane spacing of 0.266 nm can be clearly seen, corresponding to the (002) crystal plane of zinc oxide. Example 4: Based on the method for preparing ZnO quantum dots with rare earth ions in situ using sol-gel according to Example 1 of this invention, a method for synthesizing rare earth Pr-doped ZnO:Pr quantum dots by sol-gel method is described below. The operation steps are as follows: Weigh 1g of KOH using a balance, measure 30ml of anhydrous ethanol into a beaker using a graduated cylinder and heat to 78°C. Add KOH to the anhydrous ethanol and stir to dissolve, obtaining a potassium hydroxide ethanol solution, and cool it to room temperature for later use. Simultaneously, weigh 2.2g of zinc acetate using a balance, measure 100ml of anhydrous ethanol into a reflux condenser and heat to 80°C. Add the weighed zinc acetate to the anhydrous ethanol and stir to dissolve, obtaining a zinc acetate ethanol solution. Slowly add the KOH solution cooled to room temperature to the zinc acetate ethanol solution, then add the prepared Pr solution and stir thoroughly. Once the solution changes color, immediately stop heating and cool to prevent particle growth, thereby obtaining ZnO quantum dots doped with different Eu molar ratios. The absorption and emission spectra of ZnO:Pr quantum dots with different doping contents prepared in this embodiment are shown below. Figure 7 and Figure 8 As shown, Eu doping causes a change in the absorption peak of ZnO quantum dots. Transmission electron microscopy images of ZnO:Pr quantum dots with different doping contents prepared in this embodiment are shown below. Figure 9 As shown, the synthesized quantum dot samples have a particle size of approximately 6.5 nm and are uniformly distributed. The quantum dot particles exhibit distinct lattice fringes, and a plane spacing of 0.267 nm can be clearly seen, corresponding to the (002) crystal plane of zinc oxide. Example 5: Based on the method for preparing ZnO quantum dots with rare earth ions in situ using sol-gel according to Example 1 of this invention, a method for synthesizing rare earth Dy-doped ZnO:Pr quantum dots by sol-gel method is described below. The operation steps are as follows: Weigh 1g of KOH using a balance, measure 30ml of anhydrous ethanol into a beaker using a graduated cylinder and heat to 78°C. Add KOH to the anhydrous ethanol and stir to dissolve, obtaining a potassium hydroxide ethanol solution, and cool it to room temperature for later use. Simultaneously, weigh 2.2g of zinc acetate using a balance, measure 100ml of anhydrous ethanol into a reflux condenser and heat to 80°C. Add the weighed zinc acetate to the anhydrous ethanol and stir to dissolve, obtaining a zinc acetate ethanol solution. Slowly add the KOH solution cooled to room temperature to the zinc acetate ethanol solution, then add the prepared Dy solution and stir thoroughly. Once the solution changes color, immediately stop heating and cool to prevent particle growth, thereby obtaining ZnO quantum dots doped with different Eu molar ratios. The absorption and emission spectra of ZnO:Dy quantum dots with different doping contents prepared in this embodiment are shown below. Figure 10and Figure 11 As shown, Eu doping causes a change in the absorption peak of ZnO quantum dots. Transmission electron microscopy images of ZnO:Dy quantum dots with different doping contents prepared in this embodiment are shown below. Figure 12 As shown, the synthesized quantum dot samples have a particle size of approximately 5.09 nm and are uniformly distributed. The quantum dot particles exhibit distinct lattice fringes, and a plane spacing of 0.258 nm can be clearly seen, corresponding to the (002) crystal plane of zinc oxide. Comparative Example 1: Based on the sol-gel in-situ doped rare-earth ion ZnO quantum dots preparation method provided in Example 1 of this invention, a method for synthesizing ZnO quantum dots using the sol-gel method is described below. The steps are as follows: Weigh 1g of KOH using a balance, measure 30ml of anhydrous ethanol into a beaker using a graduated cylinder, and heat to 78°C. Add the KOH to the anhydrous ethanol, stir to dissolve, and obtain a potassium hydroxide ethanol solution. Cool the solution to room temperature for later use. Simultaneously, weigh 2.2g of zinc acetate using a balance, measure 100ml of anhydrous ethanol into a reflux condenser, and heat to 80°C. Add the weighed zinc acetate to the anhydrous ethanol, stir to dissolve, and obtain a zinc acetate ethanol solution. Slowly add the KOH solution cooled to room temperature to the zinc acetate ethanol solution, stir thoroughly, and immediately stop heating and cool after the solution undergoes a color change to prevent particle growth, thus obtaining ZnO quantum dots. The transmission electron microscope image of the ZnO quantum dots prepared in this example is shown below. Figure 13 As shown, the synthesized quantum dot sample has a particle size of approximately 5.4 nm and is uniformly distributed. The quantum dot particles exhibit distinct lattice fringes, with a clearly visible interplanar spacing of 0.26 nm, corresponding to the (002) crystal plane of zinc oxide. The XRD image of the ZnO quantum dots prepared in this embodiment is shown below. Figure 14 As shown, the average size of its (002) crystal is calculated to be 5.3852264 nm using the Debye-Scherrer formula. Comparative Example 2: This invention provides a method for preparing ZnO quantum dots with in-situ rare-earth ion doping in sol-gel. This comparative example compares the average particle size, interplanar spacing, and FTIR infrared spectra of ZnO quantum dots doped with different ions, as shown... Figure 15 and Figure 16 As shown, rare earth ion doping effectively modulates the size, lattice structure, and surface chemical environment of ZnO quantum dots, providing important support for optimizing their optical performance.

Claims

1. A method for preparing rare earth element-doped ZnO quantum dots in situ using a sol-gel method, characterized in that, The process includes the following steps: Step 1, preparing a zinc source solution by dissolving zinc salt in anhydrous ethanol to form a homogeneous solution; Step 2, preparing an alkaline ethanol solution by dissolving hydroxide in anhydrous ethanol to form an alkaline solution; Step 3, under stirring conditions, slowly adding the alkaline ethanol solution dropwise to the zinc source solution to induce hydrolysis and dehydration of Zn²⁺ to generate ZnO quantum dots; Step 4, adding a rare earth element salt solution during the ZnO quantum dot generation process to allow rare earth ions to be in-situ doped into the ZnO lattice; Step 5, cooling the solution after the reaction to obtain a rare earth element-doped ZnO quantum dot solution.

2. The method according to claim 1, characterized in that, The zinc salt is selected from one or more of zinc acetate, zinc nitrate, or zinc chloride.

3. The method according to claim 1, characterized in that, The hydroxide is potassium hydroxide or sodium hydroxide.

4. The method according to claim 1, characterized in that, The rare earth element is selected from one or more of Eu, Tb, Pr, and Dy.

5. The method according to claim 1, characterized in that, In step three, the alkaline ethanol solution is added slowly over a period of 5–60 minutes.

6. The method according to claim 1, characterized in that, The reaction temperature in step three is from room temperature to 80°C.

7. The method according to claim 1, characterized in that, In step four, the molar ratio of rare earth elements to Zn is 0–7%.

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