Basic Definition and Characteristics of Nano-Silica
Nano-silicon dioxide (also known as white carbon black) is an inorganic chemical material, commonly known as white carbon black. It is an anhydrous or hydrated silicon dioxide or silicate substance in the form of a fine powder or ultrafine particles. Depending on the preparation method, it can be divided into two categories: fumed silica and precipitated silica.

Main Characteristics:
Physical Properties: Amorphous white powder, non-toxic, odorless, and non-polluting, with a small particle size (7-40nm), high purity, low density, and a large specific surface area (up to 1000). m²/g), good dispersion properties.
Chemical Properties: High chemical stability, acid resistance (except hydrofluoric acid), alkali resistance (stable at room temperature), high melting point (approximately 1713°C), high hardness (Mohs hardness 7).
Optical Properties: High reflectivity for UV and visible light, with a reflectivity of 70%-80% for short-wave UV (200-280nm) and over 85% for visible light (400-800nm).
Surface Properties: The surface contains numerous unsaturated residual bonds and hydroxyl groups in various bonding states, resulting in high reactivity.

Main Synthesis Methods of Nano-Silica: Vapor Phase Method: Using organosilane halide as the raw material, it undergoes a hydrolysis reaction with hydrogen and oxygen at temperatures of 1200-1600°C, producing extremely fine SiO₂ particles in the form of smoke. The product purity can reach 99% and the particle size can reach 10-20 nm. Advantages include high product purity, small particle size, and good dispersibility; disadvantages include stringent equipment requirements, complex processes, high energy consumption, and high production costs.

Precipitation method: Nano-SiO₂ is produced by reacting silicates with acids. This method features simple processes, low energy consumption, readily available raw materials, and low cost. However, the product particle size is affected by factors such as the type and concentration of the acidifier and stirring speed, making the morphology difficult to control. The pore size distribution is wide, and aggregates are easily formed.

Sol-gel method: Using tetraethyl silicate or sodium silicate as the silicon source, the product is obtained through hydrolysis, aging, drying, and heat treatment. The process is controllable, but the raw material cost is high and the preparation time is long.

Other methods include microemulsion, reverse microemulsion, chemical vapor condensation, and ion exchange. Among them, the biosynthetic method uses biological cells or their extracts as reducing and capping agents to produce nano-SiO₂, offering environmental and economic advantages.