Nano-TiO2 can not only absorb ultraviolet rays, but also reflect and scatter ultraviolet rays, as well as transmit visible light. It is a physical shielding ultraviolet protectant with superior performance and promising development. Due to its small particle size and high activity, nano-titanium dioxide can not only reflect and scatter ultraviolet rays, but also absorb ultraviolet rays, so that it has a stronger blocking ability against ultraviolet rays. Compared with some organic UV protection agents of the same dosage, nano-titanium dioxide has a higher absorption peak in the ultraviolet region. Unlike organic UV protection agents, it only absorbs UVA or UVB. More importantly, it is also a broad-spectrum shielding agent. Titanium dioxide can absorb ultraviolet energy higher than 420nm to stimulate the transition of inner electrons in the crystal structure of titanium dioxide. Utilizing the transparency and ultraviolet absorption capacity of nano-TiO2 can also be used as food packaging films, inks, coatings, textile products and plastic fillers, can replace organic ultraviolet absorbers, and used in coatings to improve the aging resistance of coatings.

1.3 Self-cleaning function

Nano-TiO2 has strong "super-hydrophilicity", it is not easy to form water droplets on its surface, and nano-TiO2 can act on hydrocarbons under visible light irradiation. Using such an effect, a thin layer of nano-TiO2 can be coated on the surface of glass, ceramics and ceramic tiles. The photocatalytic reaction of titanium oxide can decompose the organic pollutants adsorbed on the surface of titanium oxide into CO2 and O2. Inorganic substances can be washed away by rain water together to achieve self-cleaning function. Japan has successfully developed self-cleaning ceramic tiles in the laboratory. This new product has a thin layer of nano-TiO2 film on the surface. Any matter that sticks to the surface, including oil stains and bacteria, is exposed to light, due to the catalysis of nano-TiO2. It can further oxidize these hydrocarbons into gases or easily wiped off substances. Nano-TiO2 photocatalysis makes it easy to clean high-rise building glass, kitchen tiles that are prone to greasy dirt, car rearview mirrors and front window glass.

2 Dispersion performance of nano-TiO2

2.1 Nano TiO2 dispersion mechanism

Hu Jie et al. found that the van der Waals force, electrostatic repulsion force, steric hindrance caused by the adsorption layer, etc. are the main factors in the water dispersion system of nano-titanium dioxide. In addition, the dispersion mechanism of nano-TiO2 can also be explained from the electron double layer theory. The double-electron layer theory refers to the fact that the inside of the particle is called the particle nucleus and is generally negatively charged to form a negative ion layer, and its exterior forms a positive ion layer due to electrical attraction (the counter ion layer includes inactive ion layer and diffusion layer) It is called electric double layer. In general, this theory relies on adjusting the pH value or adding electrolytes to generate charges on the particle surface to increase the thickness of the electric double layer and the potential value of the particle surface, so that repulsive forces are generated between the particles, thereby achieving particle dispersion.

2.2 The influence of pH on the dispersion of nano-TiO2

Titanium dioxide can be positively charged, negatively charged or neutral in aqueous solution due to different pH values. When the pH value is low, TiO2-OH2+ is formed on the surface of nano-TiO2, which leads to positive charge on the surface of the particles; when the pH value is in the middle value, Ti-OH bonds are formed on the surface of the particles. At this time, the particles are neutral. Dispersion performance is the worst; when the pH value is higher, OH-O- bonds are formed on the particle surface, which makes the particle surface negatively charged. At this time, the greater the pH value, the more negative charge on the particle surface, and the greater the thickness of the electric double layer. Thicker, the greater the repulsive potential energy between particles, the better the particle dispersion effect. Guo Wenlu et al. believe that in the study of the stability of nano-titanium dioxide water dispersion system, the dispersion effect is the best when pH=10.

2.3 The influence of electrolyte on the dispersion of nano-TiO2

The dispersion of nano titanium dioxide is related to the electrolyte. When pure water is used as a dispersant, the uneven distribution of nano-titanium dioxide is caused by electrostatic attraction between particles. Studies have shown that when an organic solvent is used as a dispersant in an aqueous solution, nano-titanium dioxide has better dispersibility. Ren Xijuan et al. proved through experiments that the higher the electrolyte ion value, the stronger the aggregation effect, and a small amount of electrolyte will also cause the dispersion to agglomerate. Therefore, the mixing and adhesion of inorganic salts should be avoided in normal use and storage.

2.4 The influence of surfactants on the dispersion of nano-TiO2

Polymer surfactants can be adsorbed by nanometer titanium dioxide through hydrogen bonding, van der Waals force, and electrostatic attraction. For example, longer molecular chains (polyacrylamide PAM, polymethacrylic acid PMAA, etc.) can provide steric hindrance shielding, thereby hindering the agglomeration of nano titanium dioxide particles. However, the amount of polymer surfactant added needs to be strictly controlled, otherwise it will be counterproductive.

The compound surfactant has a better dispersibility effect on titanium dioxide. Ionic surfactants and non-ionic surfactants are mixed in the solution to form micelles. After the non-ionic surfactant molecules are inserted into the micelles, the electrical properties between the "ionic heads" of the original ionic surfactants The repulsive force is weakened, so that the CMC in the mixed solution decreases.

2.5 The influence of dispersing equipment on the dispersibility of nano-TiO2

Nano titanium dioxide has semiconductor properties, and has excellent characteristics such as high stability, high transparency, and high activity. In the current industrial research and application, a large number of nanometer titanium dioxide will be used. As a new nanometer material, it is very necessary to improve its dispersibility. Different dispersing equipment will show different degrees of effect on its dispersion. At present, the commonly used dispersing equipment are: ultrasonic oscillation disperser, electric stirring instrument, and vertical stirring mill.

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