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To synthesize nanomaterials with specific morphologies, hydrothermal method is a commonly used and effective approach. In the hydrothermal process, the feeding amount and later washing and recovery are two key links that have a significant impact on the morphology and properties of nanomaterials.
Feed quantity factor
Feeding amount and morphology control: The feeding amount has a significant impact on the morphology of nanomaterials. For example, in the synthesis of CeO ₂ nanomaterials, the concentration of Ce ⁴⁺ can affect their morphology. When the ratio of water to Ce ⁴⁺ exceeds a certain value, the desired flower like morphology may not be generated. Therefore, precise control of the feeding amount is required to obtain the target morphology.
Avoid excessive feeding amount: Excessive feeding amount may lead to high concentration in the reaction system, causing the growth rate of nanoparticles to be too fast and difficult to control the morphology. In addition, excessive concentration may also cause agglomeration and disrupt monodispersity. Therefore, the feeding amount should be optimized based on the specific reaction system and target morphology.
Post washing and recycling factors
Selection of washing solvent: The polarity of the washing solvent should match the surface properties of the nanoparticles to ensure good dissolution and dispersion effects. Choosing a solvent with lower surface tension can help reduce the interaction force between the solvent and nanoparticles, thereby reducing the possibility of agglomeration. For example, for nanoparticles with hydroxyl groups on their surface, water or alcohols can be chosen as washing solvents.
Washing method and degree: During the washing process, excessive washing should be avoided to avoid damaging the specific morphology that has already formed. Excessive washing may result in the removal of active sites on the surface of nanoparticles or cause aggregation of nanoparticles. Therefore, a gentle washing method should be adopted, and the washing frequency and time should be controlled.
Protection during the recycling process: Measures should be taken to protect the morphology and properties of nanomaterials during the recycling process. For example, surfactants can be used to stabilize nanoparticles and prevent aggregation. At the same time, impurities or pollutants should be avoided from being introduced during the recycling process.
Precautions
Temperature and pressure control: The temperature and pressure of hydrothermal reactions are important factors affecting the morphology of nanomaterials. The reaction temperature and pressure conditions should be optimized based on the specific reaction system and target morphology.
Reaction time: Reaction time can also affect the morphology and properties of nanomaterials. A too short reaction time may lead to incomplete growth of nanoparticles, while a too long reaction time may cause agglomeration or morphological changes. Therefore, the reaction time should be precisely controlled.
Safe operation: It is recommended that the external heating temperature of the hydrothermal reactor should not exceed 220 ℃ during operation, and the heating and cooling rate should be ≤ 5 ℃/min. When the reaction is complete, the lid of the kettle can only be opened after the kettle has naturally cooled to room temperature. Disassembly under pressure is strictly prohibited. The filling degree inside the kettle should not exceed 80%, and the hydrothermal kettle does not have an overpressure unloading device. It is strictly prohibited to use it at excessive temperature or pressure.
