**3. Physical and chemical properties of NS TiO2**

Titanium dioxide is one of the most studied and well-researched compounds in materials science, due to its outstanding and exceptional properties which include stability of its chemical structure, biocompatibility, physical, optical, and electrical properties, nontoxicity, corrosion resistance, and low cost [51–53]. Generally, the morphology and physical/chemical properties of TiO2 nanostructures depend on the synthesis process, precursor type, and concentration, use of capping agents, synthesis temperature, pressure, and time [31]. Titanium dioxide, CI 77891, also known as Titanium (IV) oxide or Titania, CAS No: 13463-67-7 is a naturally occurring oxide with the chemical formula TiO2 and a molecular weight of 79.87 g mol−1. It belongs to the family of transition metal oxides [54]. The most important titanium minerals are rutile (TiO2), ilmenite (FeTiO3), and titanite (CaTiSiO5) [54]. In nature, titanium dioxide occurs mainly in three crystalline forms: rutile, anatase, and brookite. In addition, other polymorphs have also been reported (**Figure 2**) [32]. In addition, there are at least 3 reported non-crystalline TiO2 phases: a low-density amorphous TiO2 and two high-density amorphous TiO2 types. TiO2 (II) and TiO2 (H) are high-pressure forms that have been synthesized from the rutile phase [31, 54–56].

In various technologically relevant applications, nano-size-scaled materials have shown beneficial properties related not only to their chemical composition but also to the small dimensions and the large surface-to-volume ratio. Generally, a material is defined as a nanomaterial when it has a specific surface area by volume greater than 60 m2 cm−3, excluding materials consisting of particles with a size lower than 1 nm [57].

The high surface area brought about by small particle size is a crucial parameter for the high performance of many TiO2-based devices. It provides more active sites and a large interface for any type of reaction/interaction between the device and the interacting media. Thus, the performance of TiO2-based devices is largely influenced by the size of TiO2 building units. For example, high surface area TiO2 nanomaterials can guarantee good accessibility and contact with the electrolyte in lithium-ion batteries. Small primary crystals offer short diffusion paths for lithium and are beneficial for short charging–discharging times in batteries. Anatase, which has a

### **Figure 2.**

*Structures of TiO2 phases: (a) rutile, (b) anatase, (c) brookite, (d) TiO2 (B), (e) TiO2 (II), (f) TiO2(R), (g) TiO2 (II), (h) baddeleyite TiO2, (i) TiO2-OI, (j) TiO2-OII and (f) cubic TiO2 [32].*

greater surface area than its counterparts, is widely used as a photocatalyst in photon–electron transfer, whereas rutile is used for light scattering [57]. Surface charge is an important property of nanoparticle dispersions. When nanoparticles are dispersed in an aqueous solution, surface ionization and adsorption of cations or anions generate a surface charge, creating an electric potential between the particle surface and the bulk of the dispersion medium [58]. Depending on the measurement technique, the surface charge can be expressed either as surface charge density (potentiometric

*Nanostructured Titanium Dioxide (NS-TiO2) DOI: http://dx.doi.org/10.5772/intechopen.111648*

titration) or zeta potential (electrokinetic method). The point where the surface charge density is zero is defined as the point of zero charges (ZPC), and the point where the zeta potential is zero is defined as the isoelectric point (IEP) [58].

The surface of TiO2 nanoparticles dispersed in aqueous media or humid atmosphere can react immediately with water molecules, and reasonable amounts of hydroxyl groups are formed as shown in Eq. 1 [30, 58].

$$\text{Ti}^{\text{IV}} + H\_2O \to \text{Ti}^{\text{IV}} - OH + H^\* \tag{1}$$

When the surface of TiO2 is fully hydroxylated, the oxide ions in the oxide and water absorbed on the surface would distribute electrons and form equal quantities of two types of hydroxyl groups [30].

The surface charge of titania is a function of solution pH, which is affected by the reactions that occur on the particle surface as shown in Eqs. 2 and 3.

$$\text{Ti}^{\text{IV}} - \text{OH} + \text{H}^\* \rightarrow \text{Ti}^{\text{IV}} - \text{OH}\_2^{\text{-}} \tag{2}$$

$$\text{Ti}^{\text{IV}} - \text{OH} \rightarrow \text{Ti}^{\text{IV}} - \text{O}^{\text{-}} + \text{H}^{\text{+}} \tag{3}$$

A variety of nanostructured TiO2 materials with fascinating morphologies have been reported. The synthesis methods used for the fabrication of these nanostructures have a significant effect on their dimensions. In general, nanostructure forms of TiO2 have been classified into 0D (nanospheres, quantum dots), 1D (nanowires, rods, and tubes), 2D (layers and sheets), and 3D (nanoparticles, nanoflowers, etc.) architectures, which are summarized in **Figure 3** [61, 62].

Dissolution is defined as the dynamic process during which constituent molecules of the dissolving solid migrate from the surface to the bulk solution through a diffusion layer. The thermodynamic parameter that controls this process is described as solubility and along with the concentration gradient between the particle surface and the bulk, the solution acts as the driving force of particle dissolution [36]. Both solubility and rate of dissolution are dependent on a particle's chemical and surface properties such as surface area, surface morphology, and surface energy, as well as size. Crystallinity and crystal structure also need to be considered. They depend also on the possible adsorbed species, and the state of aggregation of the nanoparticles

**Figure 3.** *Categorization of hierarchical TiO2 nanostructure form [59, 60].*

and are further impacted by the surrounding media (properties of the diffusion layer and the possible solute concentration) [36, 63].

Studies have shown that TiO2 nanoparticles tend to aggregate and their aggregation has a strong influence on nanoparticle behavior due to the nature and size of the aggregates (i.e., the packing density of the nanoparticles), and aggregation can potentially impact their reactivity, nanoparticle-cellular interactions, and toxicity [43]. There are two types of aggregations: homo-aggregation and hetero-aggregation. Homoaggregation refers to the aggregation of two particles of identical characteristics (i.e., NP–NP attachment). Heteroaggregation refers to the aggregation of particles with different physical or chemical characteristics (e.g., NP–clay particle attachment). In the natural environment of aquatic systems, the state of aggregation of the nanoparticles is greatly influenced by diverse conditions such as ionic strength (IS), ionic composition, co-existing colloids, natural organic matter (NOM) (e.g., humic y fulvic and humic substances), pH, and other physicochemical factors [64].
