How are nanomaterials made?
Release time:
2023-06-08
Nanomaterials are one of the decisive factors underpinning the development of modern technology. They refer to materials in which at least one dimension in three-dimensional space is on the nanoscale (0.1–100 nm) or composed of such units as their basic building blocks—roughly equivalent to the scale at which 10 to 100 atoms are closely packed together. Their applications are incredibly broad, touching nearly every aspect of our lives—from medicine and food to housing and transportation. These include nanoelectronic materials, nano-optoelectronic materials, nanobiomedical materials, nanosensitive materials, and nanostorage materials. The liquid-phase synthesis method is an effective approach for preparing nanomaterials. It is characterized by its simplicity, mild reaction conditions, low cost, minimal environmental pollution, and the ability to precisely control the purity, morphology, and particle size of the resulting products.
Nanomaterials are one of the decisive factors underpinning the development of modern technology. They refer to materials in which at least one dimension in three-dimensional space is on the nanoscale (0.1–100 nm) or composed of such units as their basic building blocks—roughly equivalent to the scale at which 10 to 100 atoms are closely packed together. Their applications are incredibly broad, touching nearly every aspect of life—from medicine and food to housing and transportation. These include nanoelectronic materials, nano-optoelectronic materials, nanobiomedical materials, nanosensitive materials, and nanostorage materials. The liquid-phase synthesis method is an effective approach for preparing nanomaterials. It is characterized by its simplicity, mild reaction conditions, low cost, minimal environmental pollution, and the ability to precisely control the purity, morphology, and particle size of the resulting products.
The hydrothermal method was first studied in the mid-19th century by geologists who sought to simulate natural ore-forming processes. After 1900, scientists developed the theory of hydrothermal synthesis and subsequently shifted their focus toward the study of functional materials. The hydrothermal method, also known as the hydrothermal process, falls under the category of liquid-phase chemical methods. It refers to an effective technique for preparing and synthesizing inorganic materials by using aqueous solutions as the reaction medium in specially designed, sealed reactors—commonly called autoclaves. By precisely controlling the temperature within these reactors, a high-temperature and high-pressure environment is created, enabling the synthesis of various inorganic materials. Depending on the type of reaction involved, hydrothermal reactions can be classified into hydrothermal oxidation, hydrothermal reduction, hydrothermal precipitation, hydrothermal synthesis, hydrothermal hydrolysis, and hydrothermal crystallization. Among these, hydrothermal crystallization is the most widely used. Hydrothermal technology has two key characteristics: first, its relatively low operating temperature; and second, the fact that the reactions take place in sealed vessels, thereby preventing the volatilization of reactants. In the hydrothermal process, due to the high temperature and pressure conditions, the solvent water enters either a critical or supercritical state, significantly enhancing its reactivity. In the synthesis reactions, water plays a dual role: it serves as a pressure-transmitting medium and as a chemical reaction medium. Under high pressure, most reactants can fully (or partially) dissolve in water, allowing the reaction to proceed under near-homogeneous conditions and thus accelerating the reaction rate. During this process, parameters such as temperature, pressure, and solution concentration interact with and constrain each other, exerting a profound influence on the morphology of the resulting samples. Therefore, only through strict and precise control of all experimental conditions during sample preparation can nanoscale samples with specific morphologies be successfully synthesized.
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The primary reasons why the hydrothermal method has attracted widespread attention are as follows: (1) The hydrothermal method employs moderate-temperature liquid-phase control, resulting in relatively low energy consumption and broad applicability. It can be used not only for the preparation of ultrafine particles but also for synthesizing large-sized single crystals and fabricating inorganic ceramic thin films. (2) The raw materials are relatively inexpensive and readily available, and the reaction proceeds via rapid convection in the liquid phase, yielding high product yields, uniform phases, high purity, excellent crystallinity, and controllable shapes and sizes. (3) During the hydrothermal process, by adjusting factors such as reaction temperature, pressure, heat treatment time, solution composition, pH value, type of precursor, and type of mineralizer, it is possible to effectively control the reaction and crystal growth characteristics. (4) Since the reaction takes place in a sealed container, the reaction atmosphere can be precisely controlled to create favorable redox conditions, enabling the formation of certain special phases. This approach is particularly advantageous for synthesis reactions involving toxic systems, thereby minimizing environmental pollution as much as possible. Given that nanomaterials prepared via the hydrothermal method typically exhibit uniform phases, high purity, well-defined crystal structures, monodispersity, and controllable shapes and sizes, hydrothermal technology has been widely adopted for the preparation of nanomaterials. However, the hydrothermal method also has significant limitations. A notable drawback is that it is often applicable only to the preparation of oxides or a few sulfides that are insensitive to water, while it is unsuitable for preparing other compounds that are sensitive to water. Against this backdrop, researchers have developed the solvothermal technique.
In principle, the solvothermal synthesis technique is very similar to the hydrothermal method. By replacing water with organic solvents, solvothermal synthesis significantly expands the application scope of the hydrothermal method and represents its further development. Non-aqueous solvents also serve as pressure-transmitting agents, reaction media, and mineralizing agents. Solvothermal synthesis possesses several unique advantages that cannot be replicated by other methods. First, solvothermal synthesis effectively prevents hydrolysis and oxidation of precursors and products, thereby facilitating the smooth progression of synthesis reactions. Second, the solvothermal system provides an important means for controlling material morphology. The low temperature, high pressure, and solution conditions of the solvothermal system favor the formation of crystals with well-defined crystallographic orientations, while ensuring high purity of the synthesized products. By carefully selecting and controlling reaction temperature and solvent, nanomaterials of different particle sizes can be prepared. In particular, when combined with polymers or surfactants in the solvothermal system, effective control over material shape becomes possible. Third, solvothermal synthesis is a crucial approach for preparing special phases. In the solvothermal system, reactants exist in molecular or colloidal states, exhibiting high reactivity. This allows solvothermal synthesis to replace certain solid-phase reactions, promoting the development of low-temperature and mild chemical processes and enabling new chemical reactions. Moreover, due to the distinct chemical environment of the system, metastable phase products that cannot be obtained under conventional conditions may be formed.
Under hydrothermal and solvothermal conditions, the main factors influencing the morphology of the resulting products are: (1) the properties of the solvent: these primarily include the solvent’s viscosity, its coordination ability with metals, its solubility for salts and metals, and its acidity or basicity. Studies have shown that a solvent’s strong coordination ability is crucial for synthesizing one-dimensional nanocrystalline materials. The solvent’s coordination capability not only affects the morphology of the product but also influences reaction activity and phase composition. Therefore, selecting an appropriate solvent can facilitate the synthesis of metastable phases. Moreover, even in the absence of a coordinating solvent, if the reactants themselves contain substances with coordination capabilities, it is still possible to obtain materials with one-dimensional morphologies. (2) The effect of temperature: generally speaking, as the temperature increases, the size of the nanoparticles tends to grow. Reaction temperature not only affects reaction kinetics but also induces phase transformations in the product. (3) Template effects: The templates commonly used in template-assisted synthesis are typically hard templates, which exploit the voids within the structure of the matrix material for nanomaterial synthesis. Such structural matrices include glass, molecular sieves, macromolecular ion-exchange resins, and polymers. However, soft templates are also frequently employed. Under hydrothermal conditions, both types of templates can be quite effective. For example, polyamines are considered soft templates for the synthesis of one-dimensional nanocrystals. In addition to the major factors mentioned above, other factors such as reaction time, pH value, catalysts, and the molar ratio of reactants can also influence the preparation of the materials.
As a result, the hydrothermal and melt-solution methods demonstrate broad applicability in the synthesis of inorganic nanocrystals. Since the reactions are carried out in sealed containers, they prevent the release of toxic substances during the process, making the reaction procedure relatively simple and highly reproducible. In short, this technology was developed in response to the demands of industrial production; as it has evolved, it has also greatly promoted advances in other scientific and technological fields. As a straightforward nanosynthesis technique, it is bound to achieve even greater development in the future.
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How are nanomaterials made?
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