**5. Synthesis of mesoporous TiO2**

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

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

For a well-succeeded action of the directing agent in the synthesis, the model and the inorganic

: cationic surfactants are used as structural drivers for inorganic species negatively

: anionic surfactants are used as structural drivers for positively charged inorganic

solids with almost uniform surface [60, 61].

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**3. Mesoporous nanostructured TiO2**

surfactant employed in the synthesis process [21, 71].

species need to interact [71]:

**1.** S+ I-

**2.** S-I+

charged;

species;

Among the methodologies of synthesis, the methods of sol-gel and homogeneous precipitation best favor the control of morphology [58, 72, 73]. The term sol-gel involves several synthetic methodologies based on hydrolytic processes [19, 20, 59, 74]. Hydrolysis and condensation reactions involving the precursor allows the formation of colloidal particles (sol) and subse‐ quent formation of three-dimensional networks (gel). This process ensures a good homoge‐ neity for the reaction product, leading, in general, to metastable phases, amorphous or not [50]. In the case of amorphous materials, a step of crystallization for oxides formed is required.

The use of solvothermal conditions is an alternative for conversion of the amorphous material to crystalline without the use of high temperatures [60, 73]. The solvothermal method consists in the dissolution of a metallic precursor in anhydrous benzyl alcohol and heating at temperatures above 423K. Under these conditions, the alcoholic hydroxyl group connects partially to the metal ion starting a polycondensation reaction. The steric hin‐ drance, due to the greater volume of the organic group, tends to control the formation of nanoparticles which tend to stabilize in smaller sizes and narrower size dispersions than the obtained in the sol-gel process [75].

The hydrothermal method is a particularity of the solvothermal method in which the solvent is water. Under hydrothermal conditions the solubility of the amorphous particles increases significantly and the crystallization can occur concurrently with processes of re-dissolution and re-precipitation in the crystalline core [50, 76, 77]. This method has been widely used, since small variations in the main synthesis parameters (pH, concentration of precursor, among others) can cause significant changes in morphology, size, chemical constitution, between other properties of the synthesized nanostructures.

The homogeneous precipitation method has also been studied enough due to its great versatility. In this case, chlorinated precursors of titanium or even titanium tetraisopropoxide react with a base (sodium or ammonium hydroxide) forming titanium hydroxide. This compound is converted through the loss of water in amorphous TiO2, which, under different thermal treatments, gives rise to the three known crystalline forms [78].

The combination between these strategies of synthesis and the use of molecular templates warrants a better control in obtaining ordered mesoporous materials. In this sense, two approaches can be highlighted: the soft template and hard template methods [60].

**Figure 3.** Representative scheme of the Soft-templating (A) and Hard-templating (B) methods, used in the synthesis of mesoporosos materials.

The soft-templating method consists in the use of a template in the liquid phase, being widely used in the sol-gel processes based on titanium precursors. Due to its high chemical reactivity these precursors tend to suffer hydrolysis, giving rise to dense precipitates very quickly. The self-structuration of surfactants can produce mesostructures at the sol particle. This occurs by the spontaneous organization of molecules around automounted micelle structures, usually in aqueous medium, giving rise to colloidal nanocrystals.

Studies have suggested that two mechanisms are related to the formation of mesoporous materials via soft-templating method [71, 79-81]. The first, known as Cooperative Self-Assembly (CSA), and the second Liquid Crystal Template (LCT). In CSA, there is a simulta‐ neous aggregation between the inorganic precursor and the surfactant, with the formation of a liquid crystal-like phase with cubic, hexagonal or lamellar arrangement, containing both micelles and the inorganic precursor. On the other hand, in the LCT mechanism the concen‐ tration of surfactant is so high that a liquid-crystalline phase is formed without the presence of inorganic precursors. In both mechanisms, after formation of the material is necessary to remove the template for the obtaining of porosity. For this, the procedure most commonly used is the calcination [71].

Chiola, Ritsko and coworkers, reported in 1971 the obtaining of mesoporous silica [82]. However, the special characteristics of the produced materials did not were at first explained

The combination between these strategies of synthesis and the use of molecular templates warrants a better control in obtaining ordered mesoporous materials. In this sense, two

**Figure 3.** Representative scheme of the Soft-templating (A) and Hard-templating (B) methods, used in the synthesis of

The soft-templating method consists in the use of a template in the liquid phase, being widely used in the sol-gel processes based on titanium precursors. Due to its high chemical reactivity these precursors tend to suffer hydrolysis, giving rise to dense precipitates very quickly. The self-structuration of surfactants can produce mesostructures at the sol particle. This occurs by the spontaneous organization of molecules around automounted micelle structures, usually

Studies have suggested that two mechanisms are related to the formation of mesoporous materials via soft-templating method [71, 79-81]. The first, known as Cooperative Self-Assembly (CSA), and the second Liquid Crystal Template (LCT). In CSA, there is a simulta‐ neous aggregation between the inorganic precursor and the surfactant, with the formation of a liquid crystal-like phase with cubic, hexagonal or lamellar arrangement, containing both micelles and the inorganic precursor. On the other hand, in the LCT mechanism the concen‐ tration of surfactant is so high that a liquid-crystalline phase is formed without the presence of inorganic precursors. In both mechanisms, after formation of the material is necessary to remove the template for the obtaining of porosity. For this, the procedure most commonly

Chiola, Ritsko and coworkers, reported in 1971 the obtaining of mesoporous silica [82]. However, the special characteristics of the produced materials did not were at first explained

in aqueous medium, giving rise to colloidal nanocrystals.

mesoporosos materials.

92 Solar Radiation Applications

used is the calcination [71].

approaches can be highlighted: the soft template and hard template methods [60].

**Figure 4.** Formation of mesoporous structures: (A) Cooperative self-assembly process (CSA), and (B) Liquid–crystal templating process (LCT).

because the absence of adequate methods of analysis. In 1990, scientists from Mobil Oil Corporation [83], reported obtaining of mesoporous silicate molecular sieves, denoted by M41s, synthesized using a supramolecule-templating process, giving rise to a new and fascinating research field. Subsequently, efforts have been concentrated in studies related to synthesis and applications of mesoporous materials [9, 72, 84-91].

In 1995, the preparation of mesoporous TiO2 using sol-gel method associated with a surfactant as structural template were at first reported [92]. Since then, efforts have been spent on the proposition of methods of synthesis of ordered mesoporous TiO2, aiming to contribute to the improvement of the photocatalytic activity of this material, aiming its different applications [93-96].

Nanometric structures of mesoporous TiO2 were produced using the sol-gel methodology, under acidic (pH 3 to 4) and alkaline (pH 7 to 9) media, using cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl benzenesulphonate (DBS) and nonylphenol ethoxylate as surfactants (NPE) [72]. The effect of the surfactant and pH on morphology, particle size, surface area, and pore size distribution was studied being the synthesized oxides evaluated consid‐ ering the degradation of naphthalene. Using CTAB under acidic pH the nanometric material is 100% anatase, while under alkaline pH the material was produced in two crystalline phases, anatase (84.7%) and brookite (15.3%). In the presence of DBS, under acidic pH, 83% of the crystalline phase was anatase and 17% rutile, while under alkaline pH, 100% was anatase. However, using NPE a mixture 94.7% anatase and 5.3% rutile was produced under acidic pH, while under alkaline pH the phases are anatase and brookite (8.28%).The authors suggested that the good result obtained for the degradation of naphthalene (97% up to 4 h under irradiation in the visible - λ > 400 nm) using the mesoporous material obtained in the presence of CTAB in acidic medium is due to the surface area (86.7 cm3 g-1) and to the mean pore volume (0.0569 cm3 g-1).

A variant of the Soft Templating method is the Evaporation Induced Self Assembly (EISA) [97, 98]. It was originally designed for the preparation of fine films of mesostructured silica [79], and has been successful in obtaining different mesoporous materials (TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, WO3, HfO2, SnO2, etc) [79, 80]. In this method, organic solvents such as ethanol, propanol, etc, are used as reaction medium. By virtue of the use of non-aqueous medium and block copolymer surfactants, the rate of hydrolysis well as redox reactions and phase trans‐ formation are minimized. EISA presents advantages when compared to aqueous sol-gel reaction, such as more control and homogeneity in the formation of particles, high crystallinity in reaction temperatures between 373 and 573K.

**Figure 5.** Scheme presenting the most representative steps of Evaporation Induced Self-Assembly (EISA).

A methodology capable of producing monodisperse spheres of TiO2 at room temperature, using titanium (IV) butoxide (TBT) mixed to ethylene glycol, used as template, was proposed in [99]. The mixture was subsequently subjected to a conventional hydrothermal treatment. Thus, it was possible to obtain nanoparticles with surface area up to 233.2 m2 g-1 and mean pore volume of 0.27 cm3 g-1. The photocatalytic activity of the synthesized material was evaluated by degradation of methylene blue, Red MX-5B and phenolphthalein under UV irradiation, when were obtained kinetic constants of degradation of 6.7, 13.2, and 14.7 w% min-1, respectively, attributed to the greater surface area of the catalyst. The synthesis of TiO2 spheres based on EISA, using titanium (IV) tetra isopropoxide and Triton X-100 has been also reported, aiming their application in DSSCs [100]. The particle size could be controlled changing the Ti/H2O molar ratio and adjusted between 500 and 1100 nm, with a surface area of 56-151 cm2 g-1.

of CTAB in acidic medium is due to the surface area (86.7 cm3 g-1) and to the mean pore volume

A variant of the Soft Templating method is the Evaporation Induced Self Assembly (EISA) [97, 98]. It was originally designed for the preparation of fine films of mesostructured silica [79], and has been successful in obtaining different mesoporous materials (TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, WO3, HfO2, SnO2, etc) [79, 80]. In this method, organic solvents such as ethanol, propanol, etc, are used as reaction medium. By virtue of the use of non-aqueous medium and block copolymer surfactants, the rate of hydrolysis well as redox reactions and phase trans‐ formation are minimized. EISA presents advantages when compared to aqueous sol-gel reaction, such as more control and homogeneity in the formation of particles, high crystallinity

**Figure 5.** Scheme presenting the most representative steps of Evaporation Induced Self-Assembly (EISA).

A methodology capable of producing monodisperse spheres of TiO2 at room temperature, using titanium (IV) butoxide (TBT) mixed to ethylene glycol, used as template, was proposed in [99]. The mixture was subsequently subjected to a conventional hydrothermal treatment. Thus, it was possible to obtain nanoparticles with surface area up to 233.2 m2 g-1 and mean pore volume of 0.27 cm3 g-1. The photocatalytic activity of the synthesized material was evaluated by degradation of methylene blue, Red MX-5B and phenolphthalein under UV irradiation, when were obtained kinetic constants of degradation of 6.7, 13.2, and 14.7 w% min-1, respectively, attributed to the greater surface area of the catalyst. The synthesis of

(0.0569 cm3

94 Solar Radiation Applications

g-1).

in reaction temperatures between 373 and 573K.

Using the EISA strategy excellent catalysts based on CeO2, capable to oxidize harmful gases, as CO, NOx and hydrocarbons, in non-toxic gases, were synthesized [81]. The tribloc copolymer P123 and polyvinyl pyrrolidone (PVP) were used as templates. In this way, were obtained catalysts with different pore sizes (about 40 μm, 2 μm and < 0.3 μm) and specific surface area of 32.5 m2 g-1.

Damages to the mesoporous structure can occur due to the presence of residual templates, as for example the blocking of pores or even poisoning of catalytic sites on the surface of the mesoporous material. Calcination has been pointed as a way to promote the elimination of some surfactants [101]. Non-ionic surfactants, as Triton X-100, can be easily removed by washing of the oxide formed [102].

In the hard-templating method the solid is formed around a template, usually zeolites and meso and macroporous silica, via interactions between the template and the precursor [60]. This methodology is more complex when compared to the soft-templating method. However, it is highly effective in obtaining mesoporous structures [103]. By applying this methodology nanostructures are formed on surface of the templates [104].

The obtaining of uniform nanospheres of TiO2 based on hard-templating methodology, using silica as template and (NH4)2TiF6 as precursor, has been proposed [105]. This material pre‐ sented surface area of 54.2 m2 g-1 and pore volume of 0.15 cm3 g-1. The photocatalytic activity of this material was evaluated regarding the degradation of the dyes methylene blue, rhoda‐ mine B and methyl-orange. The levels of degradation were respectively 95%, 95% and 90%. This result was attributed to the high surface area and charge separation in these monodisperse materials.

In reference [106] the obtaining of mesoporous TiO2 is proposed using the combination of hard-and soft-tamplating methods. Using mesoporous silica in the first step (hard-templat‐ ing) and Pluronic P123 as driver agent in the soft-templating stage, to control of morphol‐ ogy of the particles, was possible the production of three-dimensionally ordered mesopores with surface area of 145 m2 g-1 and pore volume of 0.246 cm3 g-1. Due to the porous of the material obtained and the three-dimensional periodicity of the spheres, the authors highlight the possible applications that these materials can have, as in environmental photocataly‐ sis, solar cells and in lithium rechargeable batteries. Finally, the authors highlight the ease with which this methodology can be extended for the synthesis of other mesoporous metal oxides.

Figure 6 presents the main differences between the hard and soft template methods, while Table 1 presents the structure of the main surfactants suggested as driver agents for the synthesis of mesoporous materials.

**Figure 6.** Comparison between hard and soft templating [107].


**Figure 6.** Comparison between hard and soft templating [107].

Polyethylene glycol H

O O O

OH

OH H

H H H OH

Triton X-100

96 Solar Radiation Applications

Ethyl cellulose

**Surfactant Structure References**

O H

O

O H

O

n

O

OH

H H

HOH

H

OH

[108, 109]

[77, 113, 114]

n [109-112]

n

O

**Table 1.** Main surfactants suggested as driver agents for the synthesis of mesoporous materials and their chemical structures.
