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Titanium dioxide (TiO2) is a standard white pigment. However, when TiO2 is exposed to ultraviolet light, it will catalyze the degradation of the surrounding organic matrix. Surface coating of TiO2 is an effective method for reducing the catalytic effect of TiO2. It can also improve the dispersion of TiO2 in an organic matrix. This review critically introduces recent results on the surface coating of TiO2. First, the main features of TiO2, including processes, structure, and final properties, are described briefly. Second, this chapter reports and discusses different surface coating methods for TiO2 with inorganic oxides and organic matter. Inorganic oxides, such as Al2O3, SiO2, and ZrO2, would form a continuous dense film and block the defects of the TiO2 lattice. They can give TiO2 excellent weather resistance. The organic matter available for surface treatment includes the surfactant, the coupling agent, and the macromolecule. They can improve the affinity of TiO2 with various organic matrices. Surfactant treatment is relatively simple. Coupling agents can give TiO2 more novel properties, such as thermal stability. Macromolecules can increase the volume of TiO2 particles through steric hindrance and improve the dispersion of TiO2 in an organic matrix. However, coating TiO2 in a single matter is challenging to meet the increasing performance requirements. Therefore, it is necessary to study further the effect of co-coating with different inorganic oxides and organic matter on the structure and properties of TiO2.
School of New Materials and Shoes and Clothing Engineering, Liming Vocational University, Quanzhou, China
Hui Xiao
School of New Materials and Shoes and Clothing Engineering, Liming Vocational University, Quanzhou, China
Huaxin Wang
School of New Materials and Shoes and Clothing Engineering, Liming Vocational University, Quanzhou, China
*Address all correspondence to: jingleewong@gmail.com
1. Introduction
Since the discovery of titanium in 1791, TiO2 has been used commercially for over 100 years. Compared with other white pigments, such as ZnS, BaSO4, ZnO, etc., TiO2 is a nontoxic, stable pigment [1]. It shows high hiding power, refractive index, whiteness, and other excellent physical properties (Table 1) [2]. As a result, TiO2 is widely used in printing ink, plastics, paper, coating materials, and cosmetics. It is also an indispensable raw material for the light industry, electronic industry, and other fields [3]. TiO2 is produced by the sulfate and chloride process. The quality of TiO2 produced by the two processes is different (Table 2) [2, 4]. Upmarket TiO2 is mainly produced by the chloride process [5].
(1) Titanium concentrate: low price, stable, can be obtained directly from mining; (2) acid-soluble titanium slag: relatively high price, good quality, need to be chemically processed
(1) Titanium concentrate/white titanium: low price, stable, high process technology requirements; (2) rutile: relatively high price, low process technology requirements; (3) Titanium chloride slag and artificial rutile: higher price, low process technology
Auxiliary raw materials
H2SO4
Cl2
Price
Low
High
Type
Anatase, middle-end rutile
High-end rutile
H2SO4 recovery/%
13
75
Flow
Long and complex
Short and simple process
Technology
Mature
Domestic immaturity
Control accuracy
Low
High
Quality
Coverage and yellowing resistance are weaker than chloride methods but cheaper and less used in specific areas such as papermaking and chemical fiber
High purity, good comprehensive performance, high price
Energy consumption
(1) Pressure on environmental protection, but recycling of waste by-products can be improved; (2) large consumption of coal, natural gas, steam, water, and electricity
(1) Less three wastes, small pressure for environmental protection, but the treatment of ferric chloride in solid waste is difficult; (2) consumption is relatively small
TiO2 shows a regular lattice structure. There are three crystalline forms of TiO2 in nature: anatase, rutile, and brookite [6]. Anatase has a tetragonal crystal system. However, it will slowly transform into rutile after heating at about 610°C [7] and completely transform into rutile at 915°C. The latter also has a tetragonal crystal structure, each unit contains six atoms, and its oxygen atoms are densely packed, so rutile shows the highest stability [8]. Compared with anatase, rutile exhibits higher density, hardness, refractive index, and dielectric constant.
TiO2 has excellent physical and chemical properties, however, TiO2 surface has a photocatalytic active site (Figure 1). After absorbing ultraviolet light energy, electron–hole pairs (the charge carrier) are generated [10]. The valence band hole (h+) is highly oxidizing while the conduction band electron (e−) is highly reducing [11]. The h+ oxidizes H2O or OH− ion to the hydroxyl radical (OH•), the e− reduces adsorbed oxygen (O2) species to superoxide (O2•) and then undergoes a series of reactions to give the OH• radical. These radicals will react with surrounding organic substances, resulting in the decomposition of the organic matrix [12]. What’s worse, TiO2 particles are easy to agglomerate due to the high special surface area, causing poor dispersion in the organic matrix [13].
Figure 1.
The main photocatalytic process of TiO2 [9].
To overcome the drawbacks of TiO2 mentioned above, one can use coatings. The coating of TiO2 by inorganic oxides, such as alumina, silica, and zirconia [14], can effectively inhibit the oxidative degradation of the organic matrix, finally improving the light and weather resistance [15, 16]. The poor dispersion of TiO2 can also be effectively solved by coating [17]. Therefore, it is of great social significance and economic value to study the coating of TiO2 to improve the physical stability and dispersion, extending new applications of TiO2. In this chapter, we introduce the modification strategies of TiO2 to the readers. To fully describe the modification mechanism, processes and properties of modified TiO2 will be discussed.
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 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.
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].
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).
Figure 3.
Scheme of silica coating process.
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.
Zhang et al. [25] prepared binary Al2O3/SiO2-coated rutile TiO2 composites by a liquid-phase deposition method starting from Na2SiO3·9H2O 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.
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].
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 essential directions to study further the co-coating of various inorganic oxides and the regulation and process of coating structure.
Modifying TiO2 by organic agents is realized by coating them with organic substances such as surfactants, coupling agents and polymers. It can improve the affinity of TiO2 particles with organic matrices, resulting in better dispersion of TiO2. Thus, the pigmentary properties of TiO2, such as tint-reducing power, hiding power, and whiteness, are shown.
In the modification, there are two mechanisms: physical adsorption of organic agents on the surface of TiO2 and chemical bonding between TiO2 and organic agents [30]. The principle of physical adsorption is that the hydrophilic group of the organic coating agent is adsorbed on the surface of TiO2 particles. In contrast, the oleophilic group is outwardly affinity to the surrounding polymer matrix. Therefore, the polymer chains can penetrate the TiO2 aggregates and separate the TiO2 particles, finally improving the dispersion of TiO2. For chemical bonding, the hydroxyl groups on the surface of TiO2 particles act as active sites, which will react with organic coating agents and form covalent bonds. As a result, the TiO2 particles change from hydrophilic to hydrophobic. Several kinds of organic agents can be used for the surface modification of TiO2, including surfactants, coupling agents and polymers.
3.1 Surfactants
Surfactants can be divided into cationic, anionic, and nonionic surfactants. One can use surfactants singly or together to modify TiO2 particles to evaluate the performance of TiO2. Li et al. [31] chose anionic sodium dodecyl sulfate (SDS) and nonionic nonylphenol ethoxylate (NPEO, Tergitol NP-9) to study the effect of surfactants on the behaviors of TiO2 in aqueous solution. The results showed that both surfactants could be adsorbed onto the surface of nano-TiO2 but that only SDS can significantly decrease the zeta potential of TiO2. Both surfactants reduced the aggregation of TiO2 and retarded the aggregate sedimentation at surfactant concentrations ≥0.015% (w:v). In addition, SDS exerted a more substantial reductive effect than NP-9.
Wei et al. [32] used different surfactants, such as cetyltrimethylammonium bromide (CTAB), sodium dodecylbenzene sulfonate (SDBS), and diethanolamine (DEA), to modify TiO2 particles. The crystal type of TiO2 has no noticeable change with the addition of different surfactants, but the morphology, size, and dispersion of the TiO2 particles have changed to some extent. Among the three surfactants, CTAB is beneficial in reducing TiO2 particle size and improving TiO2 dispersion and agglomeration. And this CTAB-coated TiO2 had the greatest photostability in methyl orange degradation.
Wittmar et al. [33] prepared modified TiO2 particles by adding a cationic imidazolium salt solution. It was found that an increase in the alkyl chain length was beneficial, leading to a narrowing of the particle size distribution and a decrease of the agglomerate size in dispersion. The smallest average nanoparticle sizes in dispersion were around 30 nm.
Zhang et al. [34] discussed the influence of different surfactants on the thermal stability, weather fastness, and pigmentary properties of TiO2 particles. The results are collected in Table 3. Compared with neopentyl glycol (NPG), polyethylene glycol (PEG) and trimethylolethane (TME), trimethylolpropane (TMP) can bring the highest whiteness (97.64 of L) to TiO2 particles only in the content of 0.3 wt%.
Coupling agents are amphoteric structural compounds, which can be divided into silane coupling agents, titanate coupling agents, aluminate coupling agents, etc. One end group of the coupling agent can react with the hydroxyl group on the surface of TiO2 particles to form a strong chemical bond. The other can react with the polymeric matrix. Consequently, two kinds of materials of different polarity, TiO2, and a polymer, are closely combined to give the composite material excellent comprehensive performance.
Silane coupling agents were first developed and used widely to modify TiO2. In the reaction, organic silicon is adsorbed on TiO2, and the molecule part reacts with the hydroxyl group on the surface of TiO2 to prevent the aggregation of particles. Wang et al. [35] reported the modification of TiO2 by three kinds of silane coupling agents (KH550, KH570, and HDTMS). The chemical structure and the reaction mechanism with TiO2 are shown in Figure 5(a). The results showed that TiO2 modified by silane coupling agent had small particle size, improved hydrophobicity, and low surface energy (Figure 5(b, c)). Furthermore, compared with raw TiO2 and KH550 coated TiO2, HDTMS-coated TiO2 and KH 570 coated TiO2 had excellent dispersion stability as white pigments in blue light curing inks (Figure 5(d)).
Figure 5.
(a) The scheme of reaction between silane coupling agents with different chemical structures and TiO2 particle, (b) particle size distribution of raw and modified TiO2, (c) contact angle of raw and modified TiO2-water interface, (d) dispersion of raw and modified TiO2 [35].
Sabzi et al. [36] used aminopropyl trimethoxyl silane (APS) as a coupling agent to modify TiO2. The results showed that silane coupling agents could significantly improve the dispersion of TiO2 in polyurethane composite and the mechanical properties of composite. Xuan et al. [37] reported the modification of TiO2 by vinyltrimethoxyl silane (A171) and the reinforcement of modified TiO2 on wheat straw fiber/polypropylene composite. The modified TiO2 could effectively improve the tensile, flexural, and impact resistance as well as the UV light stability of the composite. However, the thermal stability of the coupling agents is poor. This leads to the degradation of the organic layer on the surface of TiO2. Finally, the color and whiteness of TiO2 are changed.
3.3 Polymer
The above two organic treatment methods depend on the reaction of small molecular modifiers with the surface groups of TiO2. In contrast, a modification with a macromolecule uses the polymer to coat the TiO2 particles directly or the reactive monomer to polymerize on the surface of TiO2 particles. In the coating with polymers, there is no interaction between the polymeric groups and TiO2, but the polymer induces a steric hindrance [38]. As a result, the dispersion of TiO2 in the subsequent polymer matrix is improved. TiO2 shows good pigmentary properties. The reaction mechanism and classification of polymer coating modification are collected in Table 4.
Method
Mechanism
Classification
Microcapsule method
The continuous and dense polymer capsule is formed by in situ polymerization with active monomer or adsorbing polymers directly using the Van der Waals force
Active monomers are adsorbed on the surface of TiO2 and then polymerized.
Surface active points of TiO2 are stimulated and initiate the polymerization of the monomer.
Polymer chains are adsorbed directly to form a dense film on the surface of TiO2 by using Van der Waals force
Surface grafting modification
The surface of inorganic particles was pretreated first, and graft polymerization was initiated. There are two different pretreatments: coupling agent pretreatment and surfactant pretreatment
Surface coupling reaction with polymerizable organic monomers.
Introducing free radical-producing compounds to graft polymerizable organic monomers.
The free radicals of the particle itself capture the polymer chain to achieve graft polymerization
“Anchor positioning” coating
The functional group of the polymer can anchor on the surface of TiO2, and the solvent chain of the polymer extends in a nonaqueous system to provide steric stability
Man et al. [40] used the microcapsule method to modify TiO2. The in situ polymerization of acrylic monomer on the surface of TiO2 particles obtained the core-shell structure of modified TiO2. This core-shell structure TiO2 showed improved dispersion in organic media and excellent UV shielding ability. Olad et al. [41] used polyaniline (PANI) to modify TiO2 through in situ polymerization. The results showed that PANI was successfully implanted on the surface of TiO2, effectively inhibiting the aggregation of TiO2 nanoparticles.
In the “Anchor positioning” coating method, the polymers used are named hyper-dispersant, which Schofield first proposed in the 1980s. Compared with the structure of traditional dispersants, such as surfactant SDS, with the hydrophilic and lipophile groups, hyper-dispersants have two completely different groups: anchoring group and solvent group. The anchoring groups are anchored on the particles’ surface by single-point or multipoint anchoring or co-anchoring with a surface synergist (Figure 6). At the same time, the solvent chain is extended in a nonaqueous system to provide steric stability. Therefore, the particles are stably dispersed (Figure 7). So, the hyper-dispersants have a unique dispersion effect on the nonaqueous system.
Figure 6.
The anchorage form of hyper-dispersant on particle surface: (a) single-point, (b) multipoint, and (c) co-anchoring with a surface synergist.
Figure 7.
The scheme of particle dispersion by using hyper-dispersant.
There are a lot of different anchoring groups, and solvent chains can be designed to synthesize hyper-dispersant (Tables 5 and 6). Thus, hyper-dispersants with different effects can be designed and synthesized by selecting different anchoring groups and solvent chains.
Table 5.
Electronegativity and section width of the anchoring group [42].
Table 6.
Solubility parameters of some polymer structural units [42].
Schaller et al. [43] modified TiO2 with poly(acrylic acid)-polystyrene block copolymer hyper-dispersants. It is proved that the end group of polymers will form some bond interactions with TiO2 particles, which improves the stability between polymer and TiO2 particles and then improves the dispersion of TiO2 in water.
Zhang et al. [44] synthesized three hyper-dispersants: nonterminated, carboxyl-terminated, and polyethylene imine-grafted poly(hydroxyl carboxylic acid) ester. It is found that polyethylene imine-grafted hyper-dispersant has the best dispersion performance in nano-TiO2/resin solution dispersion systems.
A novel acrylic polyester hyper-dispersant containing methacrylic acid (MAA), butyl acrylate (BA), and 3-pentadecylphenyl acrylate (PDPA) was polymerized by Liu et al. [45]. This hyper-dispersant was used to disperse TiO2 in a nonpolar solvent system. The results showed that the viscosity and particle size of suspensions were affected by monomer ratio and molecular weight. The optimum monomer ratio and molecular weight were MAA: BA: PDPA = 1:10:1.2 (wt%) and 6000, respectively. Liu et al. [46] further reported the effects of acrylic polyester hyper-dispersant on the dispersion of TiO2 in different organic solvents. The results showed that acrylic polyester hyper-dispersant adsorption onto TiO2 is spontaneous and physical.
As a critical pigment, TiO2 has a considerable application market. However, due to the crystal defects, there is a photocatalytic active site on the surface of TiO2. After absorbing ultraviolet light, free radicals are produced, causing organic compound degradation in the surrounding TiO2. Besides, TiO2 particles are easy to agglomerate and are dispersed poorly in the organic matrix due to the high specific surface area. The surface inorganic/organic modification of TiO2 is an excellent choice to overcome the drawbacks of TiO2. The coating films on the surface of TiO2 can effectively inhibit the oxidative degradation of the organic matrix and improve the dispersion of TiO2, finally improving pigmentary properties of TiO2, such as whiteness, hiding power, light resistance, and weather resistance. With the development of the economy, the demands for applying TiO2, such as high weather resistance, light resistance, and dispersion stability, are gradually increased. Thus, the coating treatments of TiO2 are an essential strategic development direction in the future.
The authors gratefully acknowledge the support by the Scientific Foundation of the Liming Vocational University (grant no. LMPT 202101, LZB202101).
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Written By
Jingyi Wang, Hui Xiao and Huaxin Wang
Submitted: 27 November 2022Reviewed: 18 April 2023Published: 23 May 2023
Open access peer-reviewed chapter
