**3. Mesoporous nanostructured TiO2**

Organized mesoporous nanostructured titanium dioxide (TiO2) is one of the most promising semiconductor materials, considering its current technological applications [19-21, 44, 60, 62-70]. The presence of pores in nanostructured materials tends to enhance their physical and chemical properties. Compared with bulk TiO2, the uniform channels in mesoporous TiO2 do not only increase the density of active sites with high accessibility but also facilitate the diffusion of reactants and products; the high surface area and large pore volume provides enhanced capability for dye loading and pollutant adsorption [60]. It is also possible to produce mesoporous TiO2 using template-free methods. However, these materials usually present disordered mesostructures as a result of the irregular packing of building blocks [60].

#### **4. Surfactants as drive agents in obtaining mesoporous ordered materials**

Certain surfactants are capable to act as drive agents, as structural templates, in the synthesis of a semiconductor oxide, promoting its organization. This process occurs due to the sponta‐ neous organization of the critical nuclei of the oxide formed around the structures defined by the surfactant, which control the growth of the microstructures initially formed, resulting in ordered mesoporous materials. The structural control and manipulation of the physical characteristics of a mesoporous material depends on the nature and concentration of the surfactant employed in the synthesis process [21, 71].

For a well-succeeded action of the directing agent in the synthesis, the model and the inorganic species need to interact [71]:


The concentration of surfactant in the reaction medium must be higher than the critical micelle concentration (CMC), which favors the auto mounting of micelles, originating bi and threedimensionalarraysresponsibleforemptyspacesthatwillproducethenanoporousmaterial[71].
