Preparation and application of nanometer titanium dioxide (titanium dioxide) powder
June 12, 2022
1 Introduction Nanomaterials are an emerging material and generally refer to ultrafine particles with a particle size of less than 100 nm. Such ultra fine particles have a large surface area, high surface activity, good catalytic properties, it has both metallic and non-metallic with a specific energy. With the rapid development of modern science and technology, the application of nanomaterials has become more and more extensive, and its requirements are getting higher and higher. Nano-titanium dioxide on, since it has a great volume effect, surface effect, optical characteristics, color effects, it is displayed in the optical, electrical and catalysis its special nature, it as a new material The field of application is increasingly widespread.
Preparation of 2 nanometer TiO 2 powder Since nanometer TiO 2 has many excellent properties and its use is quite extensive, its preparation has received great attention at home and abroad. At present, there are two main methods for preparing nano TiO 2 powder: physical method and chemical method.
2.1 Physical methods The physical methods for preparing nano-TiO 2 powders are sputtering, thermal evaporation and laser evaporation. The physical method for preparing nanoparticles is the earliest method. Its advantage is that the equipment is relatively simple, easy to operate and easy to analyze particles, can prepare high-purity particles, and can also prepare films and coatings. Its output is large, but the cost is higher.
2.2 Chemical method The chemical methods for preparing nano TiO 2 powder mainly include liquid phase method and gas phase method. The liquid phase method includes a precipitation method, a sol-gel method and a W/O microemulsion method; and a gas phase method mainly includes a TiCl 4 gas phase oxidation method. The liquid phase method has a long reaction period and a large amount of three wastes. Although amorphous particles can be obtained first, and crystal transformation occurs at high temperature, the calcination process is liable to cause sintering or agglomeration of the particles. The gas phase oxidation method has low cost and wide source of raw materials. It can quickly form anatase, rutile or mixed crystal TiO 2 particles with simple post-treatment and high degree of continuity. However, this method requires higher technology and equipment.
2.2.1 Preparation of nanometer TiO 2 by uniform precipitation method
The precipitation and formation of nanoparticles from the liquid phase involves two processes: one is the formation process of the nucleus, called the nucleation process; the other is the growth process of the nucleus, called the growth process. When the nucleation rate is less than the growth rate, it is advantageous to generate large and small coarse particles; when the nucleation rate is greater than the growth rate, it is advantageous for the formation of nanoparticles. Therefore, in order to obtain nanoparticles, it is necessary to ensure that the nucleation rate is greater than the growth rate, that is, to ensure that the reaction proceeds at a higher supersaturation.
The uniform precipitation method for preparing nano-TiO 2 is to slowly and uniformly release OH - in solution by using CO(NH 2 ) 2 . The basic principles mainly include the following reactions:
CO(NH 2 ) 2 +3H 2 O=2NH 3 •H 2 O+CO 2 ↑ NH 3 •H 2 O=NH 4 + +OH-TiO 2 + +2OH - =TiO(OH) 2 ↓ TiO(OH 2 = TiO 2 + H 2 O
In this method, the precipitant which is not added to the solution directly reacts with TiOSO 4 , but the precipitate is slowly formed throughout the solution by a chemical reaction. The addition of a precipitant directly to the solution tends to cause the local concentration of the precipitant to be too high, so that impurities are trapped in the precipitate. In the homogeneous precipitation method, since the precipitant is slowly generated by a chemical reaction, as long as the rate of generating the precipitant is controlled, the concentration unevenness can be avoided, and the supersaturation can be controlled within an appropriate range, thereby controlling the particles. The growth rate of the nanoparticles is uniform, compact, easy to wash, and high in purity. The method has low production cost, simple production process and convenient industrial production. [next]
2.2.2 Sol - Gel Method Sol - Gel method is an important method for preparing nano powder. It has its unique advantages, in which the mixing of the components in the reaction is carried out in the molecule, so the particle size of the product is small and the homogeneity is high; the reaction process is easy to control, and some products which are difficult to obtain by other methods can be obtained, and the reaction is It is carried out at low temperature to avoid the appearance of high temperature heterophase and high purity of the product. However, the disadvantage is that the sol-gel method uses metal alkoxide as a raw material, and its cost is high, and the process flow is long, and hard agglomeration is easily generated in the post-treatment of the powder. The nano-TiO 2 powder is prepared by the sol-gel method, and the titanium alkoxide is used as a raw material. It was originally formed into a transparent sol by hydrolysis and polycondensation, and then converted into a gel structure by adding an appropriate amount of deionized water. The gel was aged for a while and then dried in an oven. After the gel is completely turned into a dry gel, it is ground and calcined to obtain a uniform nano TiO 2 powder. The chemical reaction is as follows: In the sol-gel method, the structure of the final product has been initially formed in the solution, and the subsequent process is directly related to the properties of the sol, so the quality of the sol is very important. The hydrolysis and polycondensation of alkoxides is the root cause of the conversion of homogeneous solutions into sols. The conditions for controlling the hydrolysis and polycondensation of alkoxides are the key to the preparation of high quality sols. Therefore, the choice of solvent is a prerequisite for the preparation of the sol. At the same time, the pH value of the solution has an effect on the formation and agglomeration state of the colloid. The amount of water added affects the structure of the alkoxide hydrolyzed polycondensate. The length of the aging time changes the growth state of the crystal grains, and the change of the calcination temperature is on the powder. The phase structure and grain size effects. In summary, in the process of preparing TiO 2 powder by sol-gel method, there are many factors affecting the formation and properties of the powder. Therefore, the process conditions should be strictly controlled to obtain nano-TiO 2 powder with excellent performance.
2.2.3 Reverse micelle or W/O microemulsion method Reverse micelle or W/O microemulsion method is a new method developed in the past ten years. The method is simple in equipment, easy to operate, and can artificially control the size of the synthetic particles, and has unique advantages in the preparation of ultrafine particles, especially nanoparticles. The reverse micelle means that the surfactant is dissolved in an organic solvent, and when its concentration exceeds CMC (critical micelle concentration), it forms a liquid particle structure in which the hydrophilic polar head faces inward and the hydrophobic chain faces outward. The anti-micelle core can solubilize water molecules to form water nucleus. When the particle diameter is less than 100, it is called reverse micelle. When the particle diameter is between 100~2 000 nm, it is called W/O type microemulsion. The reverse micelle or microemulsion system generally consists of a surfactant, a co-surfactant, an organic solvent and a H 2 O moiety. It is a thermodynamically stable system whose water core is equivalent to a "microreactor". This "microreactor" has a large interface in which various compounds can be solubilized and is a very good chemical reaction medium. The water core size of the reverse micelle or microemulsion is determined by the amount of solubilized water and increases with increasing water content. Therefore, when the chemical reaction is carried out in the water core to prepare the ultrafine particles, since the reactants are confined in the water core, the particle size of the finally obtained particles will be controlled by the size of the water core. Reverse micelle microemulsion or TiO 2 nanoparticles prepared using TBP (tributyl phosphate) as extractant, kerosene as diluent, titanium ions extracted at room temperature while controlling the conditions so as to form reverse micelles of the organic phase The solution is solution, the solution is back-extracted with ammonia water at room temperature, the amount and concentration of the ammonia water are controlled, and the obtained precipitate is washed and dried to obtain a nano-TiO 2 powder. The reverse micelle or microemulsion method can use the size of the micelle to control the particle size and has potential advantages in the preparation of nanoparticles. However, this method has just started, there are many basic research to be done, the type of anti-micelle or microemulsion, microscopic The law between structure and selectivity of particle preparation needs to be explored, and more new reverse micelle or microemulsion systems for ultrafine particle synthesis need to be sought.
2.2.4 TiCl 4 gas phase oxidation method [next]
The gas phase method for preparing nano TiO 2 is typically TiCl 4 gas phase oxidation. The method uses nitrogen as a carrier gas of TiCl 4 , uses oxygen as an oxidant, performs oxidation reaction in a high temperature tubular aerosol reactor, and obtains nano TiO 2 powder by gas-solid separation. During this process, the residence time and reaction temperature have an effect on the particle size and crystal form of TiO 2 . The principle of the reaction: In the gas phase reactor, the effect of reactant consumption on the nucleation rate of the particles has a greater influence on the growth rate because the nucleation rate is more sensitive to the supersaturation of the product monomer in the system. As the reaction proceeds, the supersaturation decreases rapidly. The initial stage of the reaction is mainly nucleation, and in the late stage of the reaction, the nucleation is terminated, and the surface growth is dominant. Generally, the reaction rate is extremely fast at high temperatures, and the residence time is prolonged, but the particle growth time is prolonged, so that the particle size of the product increases and the specific surface area decreases. At the same time, the residence time is prolonged, and the anatase cluster has sufficient time to transform into a rutile cluster to increase the rutile content. In addition, in the gas phase reactor, the ultrafine particle formation process includes a gas phase chemical reaction, a surface reaction, a homogeneous nucleation, heterogeneous nucleation, coagulation, and aggregation or sintering. The gas phase reaction rate is very fast at high temperature, so that the effect of temperature change on the nucleation rate is not significant, and the temperature rises, the single-molecule epitaxy and surface reaction rate on the surface of the particle increase, and the average degree of freedom of gas molecules increases. The collision is intensified, the rate of particle coagulation increases, and the particles tend to coagulate and grow. In addition, since the primary particles in the reactor are quite small, the surface energy of the particle boundary is very large, and the small particles are easily diffused gradually, and the large particles are fused to reduce the surface energy. The higher the reaction temperature, the faster the diffusion rate of the grain boundary, and the higher the sintering driving force. Large, resulting in a decrease in the specific surface area of the particles and an increase in the particle size.
3 Nano-TiO 2 Application Due to the special properties of nano-ultrafine particles, it has broad application prospects in various fields.
3.1 Utilization in the chemical industry Catalysis is one of the important areas of nano-ultrafine particle applications. The high specific surface area and high activity of nano-ultrafine particles can significantly improve the catalytic efficiency, and has been researched and developed internationally as a fourth-generation catalyst. Nano TiO 2 has high chemical activity, good heat resistance and chemical resistance, and can be used as a catalyst, a catalyst carrier and an absorbent with excellent properties. For example, nano-TiO 2 exhibits a relatively high catalytic activity when catalyzing the removal of S by H 2 S. In addition, inorganic or organic composite materials of nano-SiO 2 and TiO 2 have special functions, and these nano materials are under development.
3.2 Applications in Electronic Industry Products Nano-TiO 2 is an important component of many electronic materials and can be used to make nano-sensitive materials and nano-ceramic functional materials. Due to its small size, large specific surface area and high surface activity, the nanoparticles are suitable as gas-sensitive materials. For example, nano-TiO 2 can be used to make gas sensitive components with high sensitivity. Meanwhile, since the nanophase ceramic molding plastic deformation can be achieved, people use a shape deformation of TiO 2 nanoparticles formed of nano-TiO 2 ceramics, the ceramic ultrafine grain size and retain their properties.
3.3 Application in environmental protection The photocatalytic action of nano-TiO 2 particles has a wide range of applications in environmental protection. There are many reports at home and abroad that report progress in this area. London and Ontario Nuclear Technology Environmental Company have developed a novel ambient temperature photocatalytic technology that uses artificial light and nano-titanium dioxide catalysts to decompose polychlorinated biphenyls in industrial waste and contaminated groundwater. When the polluted water passes through the titanium dioxide coating network, as long as it is irradiated by low-measurement ultraviolet light, a reaction occurs, and an extremely active hydroxyl radical is generated, and the organic poison is rapidly decomposed into carbon dioxide and water. In addition, the use of nano-TiO 2 materials as photocatalysts can also catalyze the degradation of dye contaminants emitted by the textile printing and dyeing industry and the photographic industry. With the development of social economy, people pay more and more attention to the improvement of quality of life and health. Antibacterial, antiseptic, deodorizing, purifying the air and optimizing the environment will become people's pursuit. At present, the world is facing serious environmental pollution, and nano-TiO 2 as a long-time photocatalyst has been applied to various environmental problems other than water and air purification. Relevant information indicates that nano-TiO 2 is useful for destroying microscopic bacteria and odors. In addition, cancer cells can be inactivated, and odor can be controlled, which is very effective for curing nitrogen and for removing oil. [next]
3.4 Application in the cosmetics industry Nano TiO 2 has excellent UV shielding properties, plus its transparency (no white residue on the skin, thick smear) and non-toxic (does not irritate the skin to cause inflammation) It has become an ideal raw material for sunscreen cosmetics. According to industry reports, a certain amount of nano-TiO 2 has been added as a raw material for sunscreens, cosmetic bases and lipsticks every year in Japan.
3.5 Applications in the fields of medicine and food processing Nanostructures are not only strong, but also have their own ability to resist external impurities, and are not easy to combine with external impurities. At the same time, nano-sized particles or small organic molecules will be more beneficial to the body's absorption and improve the efficacy of the drug. Therefore, nano TiO 2 has broad application prospects in health and food industry. It has been reported that TiO 2 thin film ceramics having antibacterial and purification properties have been developed. In addition, nano-TiO 2 has been used in the food industry, such as an additive for Robust milk. In addition, nano TiO 2 is widely used in plastics, coatings and other industries, and can be used as raw materials for plastic fillers, advanced paints and coatings.
4 Conclusions Nanomaterials are the most dynamic research areas in today's new materials research and have a significant impact on future socio-economic development. As an important member of nano-TiO 2 , it has been a hot topic in the research and development of domestic and foreign competition in recent years. Its preparation method is becoming more and more perfect, and its application field is expanding. However, in the preparation process of ultrafine particles, particle agglomeration is needed. A big problem solved. At present, there have been many reports on the mechanism of agglomerate formation and the agglomeration state during the preparation of oxide ultrafine powder by wet chemical method, and some progress has been made in this aspect. In the preparation of nano-TiO 2 , agglomeration may occur in processes such as precipitation, drying, calcination, etc. Therefore, in order to control the agglomeration state of the powder, it is necessary to control the whole process of powder preparation, thereby obtaining good dispersion. , nano TiO 2 powder with excellent performance.
