The practical significance of nano titanium dioxide in the cosmetics field.

In the natural environment we are in, there are a large number of microorganisms such as bacteria and fungi. Some of them can directly or indirectly cause various diseases in humans, posing significant health risks and even serious threats to our well-being.

The commonly used sterilization methods in the past, such as ultraviolet sterilization and high-temperature sterilization, can indeed eliminate a certain number of bacteria. However, these methods also have many drawbacks. For instance, the cost is often relatively high, and during use, some may cause secondary pollution, adversely affecting human health, while also consuming a considerable amount of energy.

Among the various inorganic dopants, nano TiO₂stands out with many remarkable advantages, such as excellent UV resistance that can effectively block UV damage to related substances; outstanding photocatalytic performance that can participate in various chemical reactions under light; good biocompatibility that shows better adaptability when in contact with biological entities; strong chemical stability that maintains its performance in different chemical environments; and being non-toxic and harmless, not causing adverse effects on the surrounding environment and human health; it also possesses antibacterial properties that can inhibit the growth and reproduction of bacteria. For these reasons, nano TiO₂has received widespread attention and favor from relevant researchers in recent decades, becoming the 'darling' of the research field.

Nano titanium dioxide TiO₂is a semiconductor metal oxide with multiple crystal forms. In nature, it mainly exists in three crystal forms: anatase (tetragonal system), rutile (tetragonal system), and brookite (orthorhombic system). The structure of these three crystal forms consists of [TiO₆] octahedra, differing in the connection methods of [TiO₆] octahedra. Brookite and rutile types are composed of [TiO₆] octahedra sharing vertices and edges, while anatase type is composed of [TiO₆] octahedra sharing edges.

The differences in the structures of these three crystal forms lead to significant differences in their properties. Among them, the most stable crystal form is rutile type, which has a dense structure with higher density, hardness, refractive index, and dielectric constant than brookite and anatase types, thus having a wide range of applications and the highest demand; brookite transforms into stable rutile type under high-temperature calcination at 650°C. The transformation temperature from anatase to rutile depends on the additives used during calcination. Anatase has a relatively stable crystal structure and good photocatalytic properties at room temperature, along with good product performance and wide applications. However, brookite has poor thermal stability and thus has no practical value in industry, leading to less research.

Preparation of nano titanium dioxide

With the widespread application of nano TiO₂, researchers have also conducted studies on its preparation methods and have now developed various structures of nano TiO₂, including nanospheres, nanoflowers, nanofibers, nanotubes, and nanorods.

(1) Sol-gel method The sol-gel method is widely used because it can control the structure and surface properties of composite oxides. It has advantages such as low reaction temperature, simple process, low cost, and uniform particle distribution. This method is mainly based on the hydrolysis and polycondensation of a metal alkoxide to ultimately generate hydroxides or oxides under certain conditions. It mainly includes four steps: precursor synthesis, sol formation, gel formation, and high-temperature calcination. The nano titanium dioxide prepared by the sol-gel method is essentially amorphous and requires high-temperature calcination to obtain rutile-type nanoparticles.

(2)Hydrothermal synthesis method The hydrothermal synthesis method involves using water or other solutions as solvents in a polytetrafluoroethylene reaction vessel placed in heating equipment. Under high temperature and pressure conditions, the material first dissolves then crystallizes before finally separating to obtain products. The precursors for preparing nano titanium dioxide via hydrothermal methods are usually tetrabutyl titanate, titanium tetrachloride, or isopropanol titanium with deionized water or hydrochloric acid as solvents; nanoparticles produced by this method are not prone to aggregation with small grain sizes.

(3) Hydrolysis method The principle of the hydrolysis method is to allow titanium salts to undergo direct hydrolysis reactions at low temperatures to generate TiO₂. Subsequently, it requires sequential centrifugation operations to separate impurities from the target product; washing to remove residual impurities; drying to eliminate excess moisture; then calcining to further stabilize and shape the product's properties; finally grinding it to achieve suitable particle size. In this way, the target product can be directly obtained.

In the hydrolysis method, commonly available titanium salts include titanium isopropoxide, tetrabutyl titanate, and titanium tetrachloride. Among them, titanium tetrachloride tends to hydrolyze too quickly during hydrolysis processes making it difficult to control the entire process. In contrast, using metal alkoxides as precursors for sample preparation results in samples with small particle sizes and more uniform distributions making it an ideal choice for titanium sources.

(4) Uniform precipitation method The uniform precipitation method utilizes chemical reactions to promote crystals in solution to form gradually in a uniform and slow manner. When preparing nano titanium dioxide, inorganic titanates or organic titanates are selected as titanium sources while NaOH solution, ammonia water or urea serve as precipitating agents. Under their interactions, corresponding chemical reactions occur leading to the generation of TiO(OH) precipitates.

After generating precipitates, a series of subsequent processing steps are required: first filtering out the precipitate from the solution; then washing to remove attached impurities; followed by drying to eliminate excess moisture; finally through calcination steps leading to obtaining rutile phase nano titanium dioxide.

It is worth noting that throughout the reaction process, as long as the speed of precipitate generation can be precisely controlled, there is a chance to obtain products with relatively high purity, small particle size, and a very uniform particle size distribution, achieving a more ideal quality.

(5) Microemulsion method: Microemulsion is a transparent and thermodynamically stable system composed of four parts: water or solution, oil, surfactant, and co-surfactant. Depending on the continuous phase and dispersed phase, it can be divided into forward water-in-oil microemulsion method (O/W) and reverse oil-in-water microemulsion method (W/O). The reverse microemulsion method is often used in the preparation of nano-titanium dioxide, characterized by its ability to solubilize water in its core, forming a water core where chemical reactions occur. When the particles in the water core grow to a certain size, surfactant molecules will adsorb onto the surface of the nanoparticles, stabilizing them and preventing further growth.

(6) Spray pyrolysis method: Titanium alkoxide is first selected as the precursor, and then devices such as electrostatic and ultrasonic are used to atomize the reactants. Through their action, the reactants are transformed into extremely small droplets. Next, these tiny droplets enter the reactor along with the carrier gas and undergo hydrolysis within the reactor. After this series of steps, we can finally obtain the desired target product.

Applications of nano-titanium dioxide

(1) Application of photocatalytic properties: In daily life, emissions from vehicle exhaust and industrial production generate various gases such as nitrogen oxides and sulfides. These gases enter the air, causing numerous environmental issues.

For TiO₂, its photocatalytic effect can come into play. It can oxidize gases that cause air pollution, and during rainfall, these oxidized pollutants are removed along with rainwater, thus purifying the air.

Moreover, under ultraviolet light irradiation, hydroxyl radicals will form on the surface of TiO₂. These radicals have strong destructive power; they can damage organic substances and participate in the lipid oxidation process of cell membranes. Once the cell membrane is degraded, they can further degrade substances such as cytoplasm within cells. It is precisely because of this characteristic that TiO₂ possesses antibacterial properties, which last for a relatively long time and are safe and non-toxic. Therefore, it is widely used in various fields such as antibacterial ceramics and antibacterial coatings.

(2) Application of ultraviolet shielding properties: Nano TiO2 particles have a small particle size and their energy band gap width falls within the ultraviolet range, giving them scattering and absorption capabilities for ultraviolet light. Based on this performance advantage, they can act as UV shielding agents added to products such as cosmetics, plastics, and coatings. In cosmetics, they can provide whitening and sun protection effects; in plastics and coatings, they help improve product aging resistance and weather resistance, allowing these products to better withstand external environmental factors during use, extending their lifespan and enhancing overall quality.

(3) Application of high refractive index properties: TiO2 is commonly used as an inorganic component for preparing high refractive index nanocomposites due to its high refractive index and good thermal stability. High refractive index nanocomposites combine the characteristics of organic matrices and inorganic components not only exhibit excellent optical performance but also improve light extraction efficiency in organic light-emitting diodes (OLEDs), with wide applications in display lighting, anti-reflective coatings, and optical lenses.

(4) Other applications: Solar energy is an inexhaustible clean energy source in the world. Research on solar energy utilization can effectively address energy shortages. Nano TiO2 not only has an appropriate band gap width but also exhibits good photoelectrochemical stability and simple manufacturing processes, making it suitable as electronic collection and transmission materials.

In recent years, perovskite solar cells have received significant attention in solar cell research due to their low manufacturing costs and high efficiency. In perovskite solar cells, nano TiO2 is typically used to create hole blocking layers and electron transport layers.

Nano titanium dioxide differs from pigment-grade titanium dioxide with particle sizes of 200–400nm; due to its particle size being less than 100nm, it exhibits surface effects, volume effects, quantum size effects typical of general nanomaterials. Therefore, nano titanium dioxide possesses excellent optical, thermal, electrical, mechanical, and physicochemical properties. In recent years, with deeper research into its unique properties has gained widespread attention and development for applications in coatings, plastics, cosmetics, batteries, etc.

Reference sources:

1. Li Huaqian, Qiu Guibao et al. Research Progress on Titanium Dioxide Preparation Technology
2. Yin Jinkun. Study on Antibacterial Properties of Element-Doped Nano Titanium Dioxide
3. Tian Yue. Research on Synthesis of Cosmetic-Grade Nano Titanium Dioxide Using Microreaction Technology

4. Shi Ruixue. Study on Preparation and Application Performance of Nano Titanium Dioxide Dispersion