**2. Inorganic modification of TiO2**

The purpose of the inorganic modification is to coat TiO2 with a layer of the inorganic hydrated oxide film. This film can block and cover the lattice defects of TiO2 and reduce the connecting possibility between organic matrix and active groups of TiO2. Such films comprise alumina, silica, zirconia, etc.

## **2.1 Alumina**

Alumina (Al2O3) is a suitable electron acceptor, which can annihilate the photoelectrons generated by TiO2 after ultraviolet absorption and excitation, inhibiting subsequent active groups' generation [18]. In addition, Al2O3 can also reflect ultraviolet from natural light [19]. Thus, Al2O3 is one of the most used materials for the inorganic coating of TiO2.

A variety of chemicals, such as sodium metaaluminate (NaAlO2) and aluminum sulfate (Al2(SO4)3), have been used for TiO2 coating. These metal salts are added into TiO2 suspension at various pH, and the positively charged OH-Al hydrolyzed by soluble salt is adsorbed and wrapped on the surface of TiO2 particles to form hydrated alumina. The structure of hydrated alumina will change at different pH values. It shows an amorphous structure at pH 5, a floccular false boehmite structure at pH 8– 10, and a flaky gibbsite structure at pH above 10.

Zhang et al. [20] reported the preparation of compact amorphous Al2O3 film on the TiO2 under the molar ratio NaAlO2/TiO2 of 1/22 at 80°C in pH 5. After being coated by Al2O3 films, the whiteness and brightness of the modified TiO2 samples increased with the increase of the Al2O3 loading, while the relative light scattering index depended on the alumina loading.

Dong et al. [21] synthesized alumina-coated rutile TiO2 samples using the chemical liquid deposition method under various pH and aging temperatures. The results showed that this film-coating process should mainly be attributed to chemical bonding and physical adsorption (**Figure 2a**). The higher aging temperature was in favor of

### **Figure 2.**

*(a) Schematic diagram of physical adsorption and chemical bonding of Al2O3 coated rutile; dispersion stability of Al2O3-coated rutile TiO2 samples at different pH values (b) and aging temperatures (c) [21].*

the elevation of the boehmite content of the coating film, causing the enhancement of dispersion stability. It contributed to the increase of steric hindrance and electrostatic repulsion. The coated TiO2 exhibited well dispersion stability at pH 9 (**Figure 2b**) and aging temperature 200°C (**Figure 2c**), respectively.

Wu et al. [22] discussed the mechanism of the film-coating process of hydrated alumina on TiO2 particles in an aqueous solution. The effects of temperature, pH value, and Al2(SO4)3 solution were investigated. It is found that TiO2 particles promote the hydrolysis of Al2(SO4)3 in both acidic and basic solutions and adsorb positively charged OH-Al species in slurries. When the OH-Al species or TiO2 particles have enough energy to cross the repulsion threshold, the hydroxyl groups on the surface of the TiO2 particles will condense with the OH-Al species, leading to the coating of OH-Al species on the surface of the TiO2 particles. As a result, the Al2O3 film is formed.

### **2.2 Silica**

Silica coating shows a similar function as alumina. Compared with alumina, silica film gives more chemical stability to TiO2. TiO2 suspension is added to water-soluble silicon compound in base condition. Silicon is deposited on TiO2 particles as Si(OH)4 through physical adsorption and chemical bonding between Si(OH)4 and TiO2. The deposited Si(OH)4 is further condensed into a silica gel, finally realizing the coating of TiO2 particles (**Figure 3**).

Liu et al. [23] prepared SiO2-coated TiO2 powders by the chemical deposition method starting from rutile TiO2 and Na2SiO3. The evolution of island-like and uniform coating layers depended on the ratio of Na2SiO3 to TiO2, reaction temperature, and pH. The result showed that the whiteness and brightness of the TiO2 product increased with the loading of SiO2.

Lin et al. [24] studied the surface characteristics of hydrous silica-coated TiO2 particles. Different analytical techniques were used to characterize the silica oxide coatings on TiO2 particles. Analyses showed that hydrous silica is continuously coated on the surface of TiO2 particles. The hydrous silica film coating can improve the durability of pigment weather and dispersion properties.

SiO2 can be easily deposited on TiO2 surfaces. However, SiO2 coating layers with a lower polarity cannot significantly enhance the dispersibility of TiO2 in a polar solvent. Moreover, the hydrogen bond interaction between the hydrated SiO2 will lead to thixotropy. Al2O3 coating layers with many –OH groups not only improve the dispersibility of TiO2 powders in polar solvents but also provide abundant active sites for further organic modification. However, Al2O3 coating layers tend to anchor loosely at TiO2 surfaces. Therefore, various reports are about the formation of binary Al2O3/ SiO2 films on the TiO2 surface.

**Figure 3.** *Scheme of silica coating process.*

Zhang et al. [25] prepared binary Al2O3/SiO2-coated rutile TiO2 composites by a liquid-phase deposition method starting from Na2SiO39H2O and NaAlO2. The formation of continuous and dense binary Al2O3/SiO2 coating layers depended on the pH value of the reaction solution and the alumina loading. The coated TiO2 particle had a high dispersibility in water. Compared with SiO2-coated TiO2 samples, the whiteness and brightness of the binary Al2O3/SiO2-coated TiO2 particles were higher.

To improve the dispersion and reduce the photocatalytic activity of TiO2, Godnjavec et al. [26] modified TiO2 by the SiO2/Al2O3 films on the surface of particles and incorporated modified TiO2 into the polyacrylic coating. The results showed that surface treatment of TiO2 with SiO2/Al2O3 could improve the dispersion of TiO2 in the polyacrylic matrix, and the UV protection of the clear polyacrylic composite coating was enhanced.

## **2.3 Zirconia**

Zirconia (ZrO2) has a high refractive index (2.170) and weak ultraviolet light absorption. Therefore, the ZrO2 coating considerably reduces UV absorption causing higher photostability [27] and increasing the glossiness of TiO2 particles. This coating can increase the amount of hydroxyl groups on the surfaces of the TiO2 particles, which improves the dispersibility of TiO2 powders in aqueous media and provides more active sites for the subsequent organic modification.

The TiO2 powders are dispersed in distilled water with ultrasonic treatment to obtain TiO2 suspension, and the zirconium salt solution is added as follows. The zirconium salt hydrolyzes rapidly, and the zirconia nanoparticles grow and form aggregates on the surface of TiO2 through Zr–O–Ti bonds. The zirconia nanoparticles will grow and form a continuous and dense film.

Zhang et al. [28] reported that the ZrO2-coated rutile TiO2 could be prepared by the chemical liquid deposition method starting from rutile TiO2 and ZrOCl2. The formation of zirconia coating depended on pH value of reaction solution and the mole ratio of ZrOCl2 to TiO2. When the pH value reached to 9 with a mole ratio of ZrOCl2 to TiO2 of 1:51, the zirconia aggregates with an average particle size of about 4 nm coated on the surface of the TiO2 particles (**Figure 4a**, **b**). Compared with the exposed rutile TiO2, the dispersibility, whiteness, brightness, and relative light scattering index of the ZrO2-coated TiO2 were significantly improved.

Li et al. [29] prepared ZrO2-coated TiO2 by a precipitation method. The Zr(SO4)2 solution was added to TiO2 suspension at the pH of 5.2 at 50°C. The mass ratio of ZrO2 to TiO2 was 1.0%, and a dilute NaOH solution was used to adjust the pH value. The results showed that supersaturation of the Zr(SO4)2 solution is one of the key factors influencing the type of nucleation in the zirconia coating. Lower supersaturation benefits the heterogeneous nucleation of zirconia on the surface of TiO2 particles, while higher supersaturation leads to the homogeneous nucleation of zirconia itself. A suitable ZrO2 content is about 1.0 wt.%, and this thick and continuous film gives better pigmentary properties.

To sum up, the function of the inorganic oxide-coated film of TiO2 is to form a barrier, reducing the photoactivity of TiO2 and the production of free radicals on the surface of TiO2. As a result, the coated TiO2 has good pigmentary properties, including weather and light resistance. However, using a single inorganic oxide coating is often not sufficient to meet the requirements of several applications. So, it is one of the

### **Figure 4.**

*TEM micrographs of bare rutile TiO2 and ZrO-coated TiO2 at T = 80°C, ZrOCl2:TiO2 = 1:51 with different pH (a) and with a different molar ratio of ZrOCl2 to TiO2 (b) [28].*

essential directions to study further the co-coating of various inorganic oxides and the regulation and process of coating structure.
