**2. Porosity and porous materials**

The porosity, related to the presence of cavities, channels or interstices is of great importance since itis related to the ability of materials to interact with atoms, ions, molecules, and nanopar‐ ticlesnot onlyattheir surfacesbut also throughoutthebulk[51, 52].Thus,the control ofporosity is very important if the objective is the development of new materials [53, 54].

The pores are classified as closed or open, considering the ability of porous materials to interact with their neighborhoods. Figure 1 shows various types of closed (a) or opened (b, c, f) pores.

**Figure 1.** Schematic representation of a porous solid, presenting the most common types of pores: (a) closed; (b, c, d) opened; (e) interconnected; (f) surface roughness. Font: [55].

Open pores have continuous channels that communicate to the outer surface of the material, generating a cross-linked structure. Closed pores are inactive for the flow of liquids, gases and other substrates, being totally isolated from their neighborhood. These pores are related to macroscopic properties such as mechanical resistance and thermal conductivity [55]. The pores can also be interconnected, (e). Otherwise used to classify the pores takes into account its format: bottleneck, (b), cylindrical, (c), and funnel type, (d). The surface roughness, represented by (f), is also a type of porosity [55].

As technological applications for TiO2 can be cited its use in ultraviolet radiation absorbing filters [3, 23], in chemical sensors for gases [24-26], as a bactericide [27], in biomaterials for bone implants [28], in environmental photocatalysis [8, 24, 29, 30], in the photocatalytic hydrogen

The photocatalytic efficiency of TiO2 depends on its structural and morphological character‐ istics, which are related to the method of synthesis used in the preparation of nanoparticles [18, 21, 29, 45]. To be photoactive, favoring the photocatalysis process, besides being mainly consisting of anatase crystalline phase, the TiO2 must possess high specific surface area, good porosity, with high sized pores [35, 45, 46]. In this context, the search for TiO2 particles that have differentiated features, with catalytic properties potentiated, constitutes a field of intense

The porosity, related to the presence of cavities, channels or interstices is of great importance since itis related to the ability of materials to interact with atoms, ions, molecules, and nanopar‐ ticlesnot onlyattheir surfacesbut also throughoutthebulk[51, 52].Thus,the control ofporosity

The pores are classified as closed or open, considering the ability of porous materials to interact with their neighborhoods. Figure 1 shows various types of closed (a) or opened (b, c, f) pores.

**Figure 1.** Schematic representation of a porous solid, presenting the most common types of pores: (a) closed; (b, c, d)

Open pores have continuous channels that communicate to the outer surface of the material, generating a cross-linked structure. Closed pores are inactive for the flow of liquids, gases and

is very important if the objective is the development of new materials [53, 54].

evolution [17, 31-36], in dye-sensitized solar cells [21, 37-44], among other.

activity [19, 20, 47-50].

88 Solar Radiation Applications

**2. Porosity and porous materials**

opened; (e) interconnected; (f) surface roughness. Font: [55].

**Figure 2.** Representation of the six main types of isotherm: I, microporous solids; II, non-porous solids; III, macropo‐ rous solids; IV and V, mesoporous solids; VI, non-porous solids with uniform surface.

IUPAC recommends a quantitative division of the pores in three classes according to their mean diameter: micropores, lower than 2 nm; mesopores, between 2 and 50 nm; macropores, higher than 50 nm [56]. This pore size classification is based on measurements of adsorptiondesorption of N2 in boiling temperature and in the statistical width of layers of N2 adsorbed. The analysis of these data is usually done by using the Brunauer/Emmett/Teller method (BET), proposed by Stephen Brunauer, Paul Hugh Emmett e Edward Teller [57-59]. Applying BET, it is possible to describe the form of the adsorption and desorption isotherms for a specific solid. Knowing the format of the isotherm it is possible to define its porosity [60], Figure 2.

Microporous solids usually present a type I isotherm, whereas the isotherms II and III are related to non-porous solids finely divided or macroporous solids. Already the isotherms type IV and V present a hysteresis loop, a characteristic of mesoporous materials. The type IV hysteresis represents materials with uniform porosity, while type V hysteresis is referred to pores with non-defined forms and sizes. Finally, the type VI hysteresis is related to non-porous solids with almost uniform surface [60, 61].
