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To synthesize nanomaterials with specific morphologies, the hydrothermal method is a commonly used and effective approach. In the hydrothermal process, the amount of raw materials and the subsequent washing and recovery are two crucial steps that significantly affect the morphology and properties of the nanomaterials.
Feeding quantity factor
Dosage and Morphology Control: The dosage has a significant impact on the morphology of nanomaterials. For instance, in the synthesis of CeO₂ nanomaterials, the concentration of Ce⁴⁺ affects its morphology. When the molar ratio of water to Ce⁴⁺ exceeds a certain value, the desired flower-like morphology may not be formed. Therefore, precise control of the dosage is necessary to achieve the target morphology.
Avoid excessive feeding amount: An excessive feeding amount may lead to a high concentration in the reaction system, causing the growth rate of nanoparticles to be too fast, making it difficult to control the morphology. In addition, too high a concentration may also trigger agglomeration, disrupting the monodispersity. Therefore, the feeding amount should be optimized according to the specific reaction system and target morphology.
Post-washing recovery 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 helps reduce the interaction force between the solvent and the nanoparticles, thereby reducing the possibility of agglomeration. For example, for nanoparticles with hydroxyl groups on their surface, water or alcohols can be selected as the washing solvent.
Washing method and degree: During the washing process, excessive washing should be avoided to prevent the destruction of the formed specific morphology. Excessive washing may lead to the removal of active sites on the surface of nanoparticles or cause nanoparticles to agglomerate. Therefore, a gentle washing method should be adopted, and the number of washing cycles and duration should be controlled.
Protection during the recycling process: During the recycling process, measures should be taken to protect the morphology and properties of nanomaterials. For example, surfactants can be used to stabilize nanoparticles and prevent agglomeration. At the same time, the introduction of impurities or contaminants during the recycling process should be avoided.
Precautions
Temperature and pressure control: The temperature and pressure of the hydrothermal reaction are crucial factors influencing the morphology of nanomaterials. The reaction temperature and pressure conditions should be optimized based on the specific reaction system and the desired morphology.
Reaction time: Reaction time also affects the morphology and properties of nanomaterials. Too short a reaction time may lead to incomplete growth of nanoparticles, while too long a reaction time may cause agglomeration or morphological changes. Therefore, reaction time should be precisely controlled.
Safe operation: When operating the hydrothermal reactor, it is recommended that the external heating temperature not exceed 220℃, and the heating and cooling rates should be ≤5℃/min. After the reaction is complete, the reactor lid can only be opened after the reactor body has naturally cooled down to room temperature. Dismantling under pressure is strictly prohibited. The filling level in the reactor should not exceed 80%, and as the hydrothermal reactor does not have an overpressure relief device, it is strictly prohibited to use it under excessive temperature or pressure conditions.
