The preparation of nanopowders is generally categorized into Physical Methods and Chemical Methods. Below is a detailed comparison list highlighting their characteristics:
Comparison Table: Physical vs. Chemical Synthesis of Nanopowders
|
Feature |
Physical Methods (Top-down) |
Chemical Methods (Bottom-up) |
|
Basic Principle |
Top-down: Bulk materials are broken down into nanoparticles using physical energy (mechanical, thermal, etc.). |
Bottom-up: Atoms, molecules, or ions are assembled into nanoparticles through chemical reactions. |
|
Typical Techniques |
Mechanical ball milling, Thermal/Vacuum evaporation, Laser ablation, Plasma processing, Sputtering. |
Sol-gel process, Hydrothermal/Solvothermal synthesis, Chemical precipitation, Microemulsion, CVD. |
|
Particle Size Control |
Difficult to achieve precise control. Usually results in a broad size distribution. |
Highly precise. Size and morphology can be tuned by adjusting reaction time, pH, and surfactants. |
|
Purity & Homogeneity |
High risk of contamination from grinding media (in milling). Physical vapor methods yield high purity. |
High chemical homogeneity at the molecular level. However, precursors or by-products may remain as impurities. |
|
Morphology (Shape) |
Particles are often irregular or random in shape. |
Highly customizable (spheres, rods, tubes, sheets, etc.) by controlling growth kinetics. |
|
Cost |
High equipment cost (e.g., lasers, vacuum systems), but uses cheaper raw bulk materials. |
Low equipment cost (for solution-based methods), but high cost for pure precursors and solvents. |
|
Scalability |
Methods like ball milling are very easy to scale for industrial mass production. |
Scaling up is challenging because maintaining uniform reaction conditions (heat/stirring) is difficult in large volumes. |
|
Dispersity |
Particles tend to agglomerate physically; low surface activity during processing. |
Better dispersion can be achieved by adding capping agents or surfactants during the reaction. |
|
Environmental Impact |
Generally "Greener" (solvent-free), but highly energy-intensive and noisy. |
Significant environmental footprint due to the use of organic solvents, acids, and toxic chemical waste. |
In addition, a mixed preparation method that combines the two can also be used.
(1) Evaporation condensation method under inert gas
Generally, it is formed by particles with clean surfaces and particle sizes between 1-100 nm under high pressure, which is also necessary for the sintering process of nanoceramics. Various nano solid materials have been successfully synthesized both domestically and internationally using techniques such as inert gas evaporation, including metals and alloys, ceramics, amorphous ionic crystals, and semiconductors.
Yan Hongge et al. studied the changes in metal evaporation rate, yield, particle size, and morphology of ultrafine powders by changing the evaporation process parameters. They designed and researched a preparation device for ultrafine powders, which melted and evaporated the metal in the crucible under Ar gas pressure of 50-1000 Pa using medium frequency induction heating, and captured the powder through a water-cooled solenoid, ultimately obtaining fine copper powder of 180-560 nm.
(2) Hydrothermal method
The hydrothermal method is generally used to synthesize nanoparticles in fluid systems such as aqueous solutions or steam under high temperature and high pressure conditions, by separation, heat treatment, and other operations. The hydrothermal method is a simple, economical, pollution-free, and energy-saving process suitable for industrial production.
Liang et al. synthesized an amphiphilic molybdenum disulfide nanosheet (KH550-MoS2) by hydrothermal method. The ultra-low concentration of KH550-MoS ₂ nanofluid reduced the IFT to 2.6 mN/m, changed its contact angle from 131.2 ° to 51.7 °, and significantly improved the stability of lotion. Through core displacement experiments, the ultra-low concentration KH550-MoS2 nanofluid can increase oil displacement efficiency by 14% after water flooding.
(3) Complex decomposition method
The double decomposition method refers to the method of preparing nanoparticles by reacting easily soluble metal ion salts (such as CaCl2, MgCl2, etc.) with easily soluble salts (such as NH4HCO ∝ or Na2CO ∝, etc.) under appropriate process conditions, at a certain temperature, pH value, and other reaction conditions. In the reaction, by regulating the concentration of reactants, the supersaturation of nanoparticles, and studying factors such as the concentration and type of crystal control agents, nanoparticles with different sizes, concentrated distributions, and diverse morphologies can be obtained.
In recent decades, a large number of scholars have adopted the method of complex decomposition to prepare various methods for preparing nano calcium carbonate. The particle size distribution of nano calcium carbonate particles prepared by the double decomposition method is generally around 20-100 nm. Zhao Lina used the precipitation reaction method of soluble salts and polyacrylic acid as a crystal control agent to prepare butterfly shaped aragonite shaped calcium carbonate particles with special morphology by controlling factors such as temperature and pH value.
(4) Microemulsion method
Two immiscible solvents form lotion under the action of surfactant and precipitate solid nanoparticles from lotion. Chen Liping et al. used the three components of CTAB/cyclohexanol/corresponding salt aqueous solution to form three typical microemulsion systems, namely, O/W, W/O and oil-water continuous two-phase systems. The results indicate that in the W/O system, the nucleation and growth range of BaSOx crystals are limited to water nuclei of fixed size, resulting in 15 nm cubic or rectangular particles; In the oil-water continuous biphasic system, oil-water forms a network structure, resulting in particles with a size of about 700 nm. As the salt concentration gradually increases, the shape of the particles begins to shift from "fishbone spines" to flower shapes; In the O/W system, the particle size is approximately 1 μ m, and as the salt concentration increases, the particle size increases and the morphology changes from flower shaped to "fishbone spine". Different micro lotion systems have different effects on the size and morphology of BaSO ₄ particles.
Ding Yang and others can not only obtain environmentally friendly microemulsion system, but also improve the dispersion of dispersed phase in the microemulsion system by changing the type of cosurfactant, which not only effectively inhibits the growth of CaCO3 particles, but also controls the growth direction of CaCO3 particles, and has a regulatory effect on the size of nano-CaCO3particles.
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