1.4 Methods of synthesis of TiO2-RE

There are several methods used for the preparation of TiO2 doped with rare earth ions, they exist from the most complex and expensive to the simplest and cheapest. These methods vary depending on the final structure that is desired, for example, to obtain thin films or coatings, the following methods are more used: Micro-arc oxidation [34], magnetron sputtering [35], spin coating [36] and dip coating [37]. To prepare powders with defined nanostructures are electrospinning (nanofibers) [38], anodization (nanotubes) [39], microemulsion (spheres) [40], hydrothermal (nanowires) [41], state solid reaction (amorphous) [42], impregnation (amorphous) [43], and sol-gel (different structures) [44]. All of them can be modified, combined with each other or coupled to different energy sources, such as microwaves [45] and ultrasounds [46], to create doped materials with unique photocatalytic properties.

#### 1.4.1 Sol-gel

The sol-gel method has been the most used process for the synthesis of TiO2 doped with rare earth ions due to the modifications that can make it, at its low cost and easy operation. With this method, crystalline titania can be prepared on a nanometric scale, with a high purity at low temperatures, stoichiometrically controlling the composition when dopants are inserted. This technique is also used for the synthesis of materials with spectral response in the visible region. In this process, the monomers in solution are hydrolyzed and polycondensed to form a polymer network in gel form (during this step, most of the authors report the insertion of doping ions), which contains a liquid phase and a solid phase. After the formation of the gel, the xerogel is formed by removing the solvents in the medium, and then thermal stability is provided by calcining the xerogel at temperatures above 200°C, until obtaining a dense solid with the desired crystalline structure [44].

#### 1.4.2 Impregnation

Wet chemical impregnation is the very simple preparation method to implement to synthesize titania doped with rare earth ions; its process is very easy to perform; it employs mild working conditions and a low energy cost. It provides a uniform distribution of the dopant with the surface of the TiO2, generating an excellent adhesion between both, which allows controlling the structure, morphology, and particle size simply by modifying conditions such as rotation speed or agitation, contact time, system pH, and nature of solvents. This procedure can be summarized in three simple steps: (1) Place the titania in contact with an aqueous solution containing dissolved dopant precursors for a certain time in constant agitation. (2) Remove excess water in the system, and (3) Activate the material obtained with thermal treatments at elevated temperatures. The variables that mostly influence the preparation of titanium oxide doped with this method are listed below: morphology and structure of TiO2, amount of dopant material and its disposition with titania, types of solvents used, system pH and type of treatment thermal employee. Under the control of these conditions, this process allows being constantly reproducible [47].

#### 2. Materials and methods

#### 2.1 Characterization techniques of TiO2-RE

TiO2 doped with rare earth ions were characterized by XRD, N2 physisorption, Raman spectroscopy, scanning electron microscopy, and Uv-Vis spectroscopy with diffuse reflectance to describe the electronic, structural, and morphological properties.

### 2.1.1 UV-Vis (DRS)

UV-Vis spectra were used to estimate the band gap energy (Eg) for each catalyst, if the absorption coefficient (α) is zero, according to Eq. (1). This was performed in a UV-Vis spectrophotometer equipped with an integrating sphere for diffuse reflectance (Varian model Cary 300) using BaSO4 as a reference [48].

$$\boldsymbol{\alpha}(\mathbf{h}\mathbf{v}) = \mathbf{A} \left(\mathbf{h}\mathbf{v} - \mathbf{E}\mathbf{g}\right)^{\frac{m}{2}} \tag{1}$$

## 2.1.2 X-ray diffraction (XRD)

A Bruker model D8 advance X-ray diffractometer with anode of Cu Kα radiation (λ = 1.5418 Å) was used. The samples were measure/d in the range of 2θ = 20–70 with a 0.02° step at a rate of 1 s/point at room temperature. To obtain the crystallographic planes of the crystal structures in the samples; they were identified through the library of the Joint Committee on Powder Diffraction Control Standards (JCPDS). The crystal size was calculated in nm (D) of the crystalline phases with the Scherrer Eq. (2) [48]:

$$\mathbf{D} = \frac{\mathbf{K}\lambda}{\mathfrak{B}\cdot\mathbf{C}\alpha\ \mathfrak{B}}\tag{2}$$

The percentage of the Rutile phase (%R) was determined using Spurr Eq. (3) [49]:

Photocatalytic Treatment of Pesticides Using TiO2 Doped with Rare Earth DOI: http://dx.doi.org/10.5772/intechopen.84677

$$\mathbf{R} = \left(\frac{\mathbf{1}}{\mathbf{1} + \left(\mathbf{0.8\*} \frac{\mathbf{1}\_{\mathbf{A}}}{\mathbf{1}\_{\mathbf{R}}}\right)}\right) \* \mathbf{100} \tag{3}$$
