Applications of Titanium Dioxide Materials

*Xiaoping Wu*

### **Abstract**

Titanium dioxide (TiO2) is a stable, non-toxic inorganic material. Because of very high refractive index, TiO2 has been widely used as a white pigment. The optimal particle sizes of TiO2 for pigment applications are around 250 nm. The pigmentary applications of TiO2 can be found in many common products such as paints, plastics, paper and ink. Global titanium dioxide pigment sales have reached several million tons annually. Titanium dioxide is also a semiconducting material. When excited by photons which have energy equal to or higher than the band gap of TiO2, electron/hole pairs can be generated. The dynamics of the photogenerated electron/hole pairs of TiO2 is fundamentally important to its photocatalytic properties. More recently, nano-structured TiO2 has raised a great deal of interests in research after the discoveries of the important potentials for applications. The enormous efforts have been put in the preparation, characterization, scientific understandings, and modifications of the photocatalytic properties of TiO2. The applications of nano-structured TiO2 can be now found in a wide range of areas including electronic materials, energy, environment, health & medicine, catalysts, etc. This chapter has discussed and highlighted the development of the applications of titanium dioxide materials in many of those areas.

**Keywords:** Titanium dioxide, Applications, Pigment, Nano-structured

### **1. Introduction**

Titanium dioxide (TiO2) is an inorganic substance that is used extensively as a white pigment. Compared with many other inorganic pigments, TiO2 has the advantages of high stability, being non-toxic, and low cost. TiO2 have three polymorphs: anatase, rutile and brookite, but only anatase and rutile crystal forms have been useful as pigment. Both anatase and rutile crystals have very high refractive indices, and their particles can scatter visible light almost completely [1]. The optimum particle size of TiO2 for pigment applications is around 250 nm. In the early application of TiO2 as pigment, it was found that paint faded more rapidly than others when painted films were exposed to the Sun and ultraviolet (UV) light. Coating with inorganic compounds such as alumina or silica suppressed the catalytic activity on the surface and improve the weather resistance, leading titanium dioxide in wide applications as white pigment. Global titanium dioxide pigment sales were about 6 million tons in 2017 and the growth trend of global titanium dioxide pigment sales is continuing over the recent years [2].

In the 60s of the last century, scientists studied the photo-induced phenomena on the solids of TiO2 and ZnO under UV light irradiation [3–5]. In the early 1970s, research on photocatalysis by TiO2 got wide attention due to the historic discovery of the electrochemical water splitting by use of TiO2 [6]. In the 1990s, photocatalytic research of TiO2 had made progress in the practical applications of TiO2 in the decomposition of harmful organic materials [7, 8]. A function of super hydrophilicity of TiO2 was also discovered [9]. Since the beginning of this century, nano-structured TiO2 has attracted extensive interests. When the particle sizes of TiO2 are reduced down to the nano-meter scale (generally in 1–100 nm), the surface characteristics and surface areas of TiO2 have changed dramatically. The new or enhanced physical and chemical properties of nano-structured TiO2 begin to emerge. The photocatalytic property of nano-structured TiO2 has been greatly enhanced because of the changes in the surface characteristics and surface areas. Quantum effects of nano-structured TiO2 can also have a role to play, affecting its photocatalytic, optical or electronic properties. As the results of academic and industrial research in recent years, enormous progresses have been made in the preparation, characterization, and scientific understandings of nano-structured TiO2. Nano-structured TiO2 have begun to find applications in a wide range of areas including electronic materials, energy, environment, health & medicine, sensors, catalysts, etc.

TiO2 pigment is industrially produced from titanium containing ores by using a Chloride or Sulfate process [1, 10]. Nano-structured TiO2 are made in the different ways, depending on the material characteristics required for the specific applications. A number of innovative fabrication methods of nano-structured TiO2 materials have been developed and are used for different applications. These methods can be broadly classified as the liquid phase or the gas phase methods. Nanoparticles, nano-wires, nano-tubes, two or three-dimensional nano-structured TiO2 materials can also be fabricated for different applications [11–18].

This chapter will discuss and highlight the recent development of applications of titanium dioxide as pigment and as functional materials in the areas of energy, environment, catalyst, and biomedicine.

#### **2. Application of titanium dioxide as pigment**

#### **2.1 Light scattering of pigmentary TiO2**

The main use of titanium dioxide is white pigment, because it absorbs almost no incident light in the visible region of the spectrum (380–700 nm). Titanium dioxide has a strong light scattering power, and scatters incident light in three ways: surface reflection, refraction and diffraction in the crystal [1]. When the refractive index difference between titanium dioxide and medium increases, the reflected light increases and complies with Eq. (1):

$$\mathbf{R} = \frac{\left(\mathbf{n\_{p}} - \mathbf{n\_{m}}\right)^{2}}{\left(\mathbf{n\_{p}} + \mathbf{n\_{m}}\right)^{2}} \tag{1}$$

np and nm are the refractive index of pigment and medium, respectively [1]. Titanium dioxide has a high refractive index (refractive index of rutile and anatase titanium dioxide is 2.70 and 2.55 respectively) [19]. These high refractive index values enable the rutile and anatase TiO2 pigments to have much greater hiding power in coatings or in plastics, making TiO2 to be a much better pigment than the other chemical substances. Therefore, under the same conditions, only less titanium dioxide is needed to form a coating, which is white and opaque. Studies have shown that the optical properties of titanium dioxide pigments are related to their particle size, and the optimum of particle size of pigmentary titanium dioxide is around 250 nm [1].
