Controlling the morphology and size of alumina hinges on precisely controlling the precursors, additives, reaction conditions (temperature/pH/time), and post-processing during preparation. Different methods can achieve the directional synthesis of diverse structures, ranging from nanoparticles to micron-sized sheets, spheres, and fibers.
1. Precursor selection and preparation method determine the basic morphology.
Different synthetic routes determine the initial growth pattern and structural characteristics of alumina:
Precipitation method: By controlling the ratio of aluminum salts (e.g., aluminum nitrate) to precipitants (ammonia, ammonium bicarbonate) and the feeding rate, the nucleation rate can be controlled, resulting in uniformly sized spherical or near-spherical particles. For example, rapid precipitation promotes the formation of small particles, while slow dropwise addition promotes ordered crystal growth.
Sol-gel method: Utilizing the hydrolysis and condensation of aluminum alkoxides (e.g., aluminum isopropoxide), high-purity, ultrafine, and well-dispersed nano-alumina can be prepared by adjusting the pH, solvent type, and hydrolysis temperature. Acidic conditions typically inhibit rapid nucleation, favoring the formation of uniform spherical particles.
Hydrothermal/Solvothermal Method: In a closed autoclave, precise control of one-dimensional or two-dimensional structures such as nanorods and nanosheets can be achieved by adjusting the reaction temperature (120–220℃), holding time, and mineralizing agent (NaOH). For example, extending the holding time helps crystals preferentially grow along specific crystal planes, forming needle-like or plate-like structures.
Molten Salt Method: Using chloride or sulfate salts as the reaction medium, the reaction is carried out at 400–900℃. Molten salts not only lower the activation energy of the reaction but also effectively limit anisotropic crystal growth. Studies have shown that adding Zn²⁺ can induce hexagonal plate-like products, while Ti⁴⁺ alters the growth trend, increasing the thickness.
2. Precise Control Through Additives and Surfactants
Crystal Growth Promoters: Appropriate addition can significantly improve particle regularity. For example, adding specific organic additives to an ammonium bicarbonate precipitation system can yield hexagonal alumina particles with a size of 1–2 μm and a regular shape, with a specific surface area of approximately 2 m²/g.
Surfactants: CTAB, PEG, etc., can adsorb onto specific crystal faces, inhibiting their growth and thus controlling the final morphology. Using CTAB and PEG under acidic conditions can obtain more uniformly sized, lamellar alumina; while SDBS may cause secondary nucleation, affecting product purity.
Pore-forming agents/pore expanders: Organic substances such as starch, PEG, and carbon black, after calcination, lose their pores, forming controllable pore structures, suitable for preparing hollow spherical or porous microspheres, improving specific surface area and mass transfer efficiency.
