**4. α-Fe2O3@TiO2, MOF@TiO2 and others TiO2 nanocomposites**

TiO2 application as a photocatalyst is of great interest because this compound is nontoxic, economically viable, chemically inert, photostable to corrosion, besides having high thermal stability and intense photocatalytic activity and oxidation power [22, 109]. The main crystalline structures of titanium dioxide are anatase, rutile and brookite, but the last one is difficult to be synthesized in laboratory [110]. However, not all crystalline structures have the same efficiency in the absorption of light for catalysis, and rutile, although the polymorph thermodynamically more stable has reduced photocatalytic activity in comparison to anatase [111]. This occurs possibly due to the high temperature required for its preparation, resulting in an increase in the particle size, lower electron mobility in relation to anatase

and high rate of electron/vacancy recombination, that cause a reduced number of hydroxyl groups on the surface [10].

However, despite all these favorable features, TiO2 has some limitations that affects its large-scale application in industrial processes, such as the recombination of photogenerated charges due to defects, impurities or other imperfections on crystal surface, which reduce the photocatalytic efficiency. In addition, the band gap energy required for the formation of the electron/vacancy pair is equal to 3.2 eV, which restricts this catalyst to the use of UV-light [112]. Since only 5% of sunlight wavelengths are in the UV region, an alternative energy source is necessary, making the process more expensive when this semiconductor is applied [23]. Furthermore, the reactive oxygen species (ROS) are formed on the surface of the titanium dioxide, mainly due to the reaction and capture of an electron of the water molecule by the vacancy present on this surface or by the electron donation on the CB. Accordingly, the higher density of radicals is close to the semiconductor surface. Therefore, strategies that improve the incidence of the molecules on the surface of the photocatalyst, like adsorption, would increase the probability of attack and degradation of these molecules by ROS. This can enhance the selectivity and the photocatalytic capacity of this advanced oxidation process.

Therefore, many efforts have been performed to improve the adsorption capacity and photocatalytic efficiency of TiO2. Structural modifications of TiO2 have been proposed to improve the photocatalysis, such as metal deposition, doping with nonmetals, functionalization with organic molecules or coupling of other metals with TiO2 [25, 113]. Different methods have been adopted to synthesize TiO2 nanoparticles such as sol-gel, solvothermal, hydrothermal, sonochemical and mechanochemical, which were described in the above sections [114–116].

Cheng et al. [117] synthesized a Fe2O3@TiO2 nanocomposite with high adsorption and photocatalytic activity by solvothermal method. Titanium glycolate precursor and (NH4)2Fe(SO4)2•6H2O were dispersed by vigorous sonication in deionized water. This reaction mixture was autoclaved for 6 h at 180°C in a Teflon-lined autoclave. The combined effect of adsorption and photocatalysis were evaluated by Rhodamine B degradation under visible light. It was observed that the TiO2@Fe2O3 exhibited improved adsorption (RhB removal ≈ 20%) and photocatalytic (RhB degradation ≈ 75%) capability compared to pure TiO2 and Fe2O3 nanoparticles.

Fe2O3@TiO2 nanocomposite was prepared via mechanochemical process by [118]. Commercial P25® and different amounts of magnetic Fe3O4 nanoparticles were mixed by a mechanical process and the mixtures were calcined at 450°C for 2 h. XRD characterizations revealed that the samples are formed basically for TiO2 and α-Fe2O3 after calcination, with high purity. MB adsorption efficiency is near 15% for 5%-Fe2O32TiO2 with photocatalytic degradation approximately equal to 65% after 80 min under visible light.

TiO2@MIL-101 core-shell structure was synthesized by [119] under hydrothermal method and investigated for adsorption and photocatalytic degradation of methyl orange. MIL-101 was synthesized under hydrothermal conditions and added with titanium butoxide into ethanol solution under stirring. Then, ultrapure water was added into the solution that was transferred into a Teflon-lined bomb sealed and heated at 220°C for 3 h to produce TiO2@MIL-101 core-shell composites. The results revealed that the TiO2@-MIL-101 core-shell composite possessed excellent adsorption of MO (removal ≈ 70%), probably due to π-π interaction between benzene rings in MO and MIL-101, and also showed inspiring property on the degradation of MO, reaching a removal efficiency equal 97% after 50 min under UV-light radiation.

A photocatalytic adsorbent, N-doped TiO2 nanoparticles encapsulated in MIL-100(Fe) cages was developed by [120] for the adsorption and photocatalytic

**73**

*Photocatalytic Adsorbents Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.79954*

(≈1.93 nm) and high surface area (≈1400 m<sup>2</sup>

alumina and silica.

increased adsorption capacity.

degradation efficiency equal 99.1% for MB and 93.5% for RhB.

degradation, enhances methylene blue (MB) and rhodamine B (RhB) under visible light. MIL-100(Fe) was synthesized under hydrothermal technique from a mixture of Fe powder, H3BTC, HF, HNO3 and H2O that was stirred and transferred into a Teflon-lined stainless steel autoclave at 150°C for 12 h. The samples of MIL-100(Fe) encapsulating N-TiO2 nanoparticles were prepared by impregnating MIL-100(Fe) in a dilute suspension that contained different concentrations of as-prepared neutral N-TiO2 nanoparticles. This mixture was stirred for 3 h at ambient temperature, dried and calcinated at 150°C for 4 h. All the samples showed adsorption efficiencies greater than 80% under dark conditions, probably related to MOFs pore size

•g<sup>−</sup><sup>1</sup>

32%N-TiO2@MIL-100(Fe) sample was the highest of all samples evaluated, with an

However, to improve the adsorption capacity and to enhance the photocatalytic capacity, researchers have evaluated the inclusion of distinct organic and inorganic compounds, such as cyclodextrins and their derivatives, noble metals (silver), lanthanides (cerium, samarium, lanthanum and neodymium), graphene, zeolites,

In this context, Dal'Toé et al. [121] investigated the incorporation of plasmonic Ag nanoparticles on the physicochemical and photocatalytic properties of La-doped TiO2 nanostructure. The nanocrystalline La-doped TiO2 powder was prepared by an ultrasound-assisted wet impregnation method. La(NO3)3•6H2O and commercial TiO2 P25® were dispersed in distilled water and stirred for 30 min. Then, the solution was ultrasonically processed for 3 h. After the ultrasound processing, the solution was heated to the boiling point and left evaporating for 40 min. The as-obtained paste was dried overnight at 120°C. The resultant solid was ground with a pestle to obtain a fine powder, which was calcined at 500°C for 1 h. Then, La/TiO2 sample was dispersed into distilled water and aliquots of AgNO3 solution was added according to the desired Ag molar ratio (0.5–5%). The mixture was then photoirradiated under 80 W Hg vapor lamp and dried. The results revealed an increase in the adsorption capacity of the nanoparticles when the Ag molar ratio is between 2 and 4%, with efficiencies more than 65%, which proportionally increased the photocatalytic activity. Thus, the enhancements achieved in the photocatalytic decolorization (>95% in 30 min) of MB by Ag-La/TiO2 materials are directly related to the

On the other hand, organic molecules functionalization to enhance the photocatalytic activity of TiO2 was proposed by [122]. Carboxymethyl-β-cyclodextrin (CMCD) functionalization of TiO2 doped with lanthanum was evaluated to MB adsorption and degradation under UV-light radiation. TiO2-La nanoparticles were synthesized as described by [121]. The CMCD was synthesized by the dissolution of β-CD and NaOH aqueous ClCH2COOH solution, which was maintained at 50°C in a jacketed reactor for 5 h. A white precipitate (CMCD) was obtained by addition of methanol and acetone to the solution. The CMCD functionalized catalysts were prepared TiO2–La dispersed in distilled water. This solution was added to CMCD along with cyanamide and maintained at 90°C for 4 h. The results showed that adsorption efficiency increase for CMCD@TiO2-La sample, with a removal near 15% after 60 min under dark conditions. Accordingly, the improvement of the photocatalytic activity achieved for this nanoparticle is also related to the adsorption of the MB by CMCD, although the mass transfer is low due to the reduced concentration of this oligosaccharide at the TiO2 nanoparticles surface. This increase in degradation efficiency occurs because the CMCD has the function of enhancing the density of the dye at the semiconductor and solution interface, where the ROS are formed. Usually these species return to a thermodynamically

). The photocatalytic activity of

#### *Photocatalytic Adsorbents Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.79954*

*Advanced Sorption Process Applications*

hydroxyl groups on the surface [10].

65% after 80 min under visible light.

and high rate of electron/vacancy recombination, that cause a reduced number of

However, despite all these favorable features, TiO2 has some limitations that affects its large-scale application in industrial processes, such as the recombination of photogenerated charges due to defects, impurities or other imperfections on crystal surface, which reduce the photocatalytic efficiency. In addition, the band gap energy required for the formation of the electron/vacancy pair is equal to 3.2 eV, which restricts this catalyst to the use of UV-light [112]. Since only 5% of sunlight wavelengths are in the UV region, an alternative energy source is necessary, making the process more expensive when this semiconductor is applied [23]. Furthermore, the reactive oxygen species (ROS) are formed on the surface of the titanium dioxide, mainly due to the reaction and capture of an electron of the water molecule by the vacancy present on this surface or by the electron donation on the CB. Accordingly, the higher density of radicals is close to the semiconductor surface. Therefore, strategies that improve the incidence of the molecules on the surface of the photocatalyst, like adsorption, would increase the probability of attack and degradation of these molecules by ROS. This can enhance the selectivity

and the photocatalytic capacity of this advanced oxidation process.

mechanochemical, which were described in the above sections [114–116].

Therefore, many efforts have been performed to improve the adsorption capacity and photocatalytic efficiency of TiO2. Structural modifications of TiO2 have been proposed to improve the photocatalysis, such as metal deposition, doping with nonmetals, functionalization with organic molecules or coupling of other metals with TiO2 [25, 113]. Different methods have been adopted to synthesize TiO2 nanoparticles such as sol-gel, solvothermal, hydrothermal, sonochemical and

Cheng et al. [117] synthesized a Fe2O3@TiO2 nanocomposite with high adsorption and photocatalytic activity by solvothermal method. Titanium glycolate precursor and (NH4)2Fe(SO4)2•6H2O were dispersed by vigorous sonication in deionized water. This reaction mixture was autoclaved for 6 h at 180°C in a Teflon-lined autoclave. The combined effect of adsorption and photocatalysis were evaluated by Rhodamine B degradation under visible light. It was observed that the TiO2@Fe2O3 exhibited improved adsorption (RhB removal ≈ 20%) and photocatalytic (RhB degradation ≈ 75%) capability compared to pure TiO2 and Fe2O3 nanoparticles. Fe2O3@TiO2 nanocomposite was prepared via mechanochemical process by [118]. Commercial P25® and different amounts of magnetic Fe3O4 nanoparticles were mixed by a mechanical process and the mixtures were calcined at 450°C for 2 h. XRD characterizations revealed that the samples are formed basically for TiO2 and α-Fe2O3 after calcination, with high purity. MB adsorption efficiency is near 15% for 5%-Fe2O32TiO2 with photocatalytic degradation approximately equal to

TiO2@MIL-101 core-shell structure was synthesized by [119] under hydrothermal method and investigated for adsorption and photocatalytic degradation of methyl orange. MIL-101 was synthesized under hydrothermal conditions and added with titanium butoxide into ethanol solution under stirring. Then, ultrapure water was added into the solution that was transferred into a Teflon-lined bomb sealed and heated at 220°C for 3 h to produce TiO2@MIL-101 core-shell composites. The results revealed that the TiO2@-MIL-101 core-shell composite possessed excellent adsorption of MO (removal ≈ 70%), probably due to π-π interaction between benzene rings in MO and MIL-101, and also showed inspiring property on the degradation of MO, reaching a removal efficiency equal 97% after 50 min under UV-light radiation. A photocatalytic adsorbent, N-doped TiO2 nanoparticles encapsulated in MIL-100(Fe) cages was developed by [120] for the adsorption and photocatalytic

**72**

degradation, enhances methylene blue (MB) and rhodamine B (RhB) under visible light. MIL-100(Fe) was synthesized under hydrothermal technique from a mixture of Fe powder, H3BTC, HF, HNO3 and H2O that was stirred and transferred into a Teflon-lined stainless steel autoclave at 150°C for 12 h. The samples of MIL-100(Fe) encapsulating N-TiO2 nanoparticles were prepared by impregnating MIL-100(Fe) in a dilute suspension that contained different concentrations of as-prepared neutral N-TiO2 nanoparticles. This mixture was stirred for 3 h at ambient temperature, dried and calcinated at 150°C for 4 h. All the samples showed adsorption efficiencies greater than 80% under dark conditions, probably related to MOFs pore size (≈1.93 nm) and high surface area (≈1400 m<sup>2</sup> •g<sup>−</sup><sup>1</sup> ). The photocatalytic activity of 32%N-TiO2@MIL-100(Fe) sample was the highest of all samples evaluated, with an degradation efficiency equal 99.1% for MB and 93.5% for RhB.

However, to improve the adsorption capacity and to enhance the photocatalytic capacity, researchers have evaluated the inclusion of distinct organic and inorganic compounds, such as cyclodextrins and their derivatives, noble metals (silver), lanthanides (cerium, samarium, lanthanum and neodymium), graphene, zeolites, alumina and silica.

In this context, Dal'Toé et al. [121] investigated the incorporation of plasmonic Ag nanoparticles on the physicochemical and photocatalytic properties of La-doped TiO2 nanostructure. The nanocrystalline La-doped TiO2 powder was prepared by an ultrasound-assisted wet impregnation method. La(NO3)3•6H2O and commercial TiO2 P25® were dispersed in distilled water and stirred for 30 min. Then, the solution was ultrasonically processed for 3 h. After the ultrasound processing, the solution was heated to the boiling point and left evaporating for 40 min. The as-obtained paste was dried overnight at 120°C. The resultant solid was ground with a pestle to obtain a fine powder, which was calcined at 500°C for 1 h. Then, La/TiO2 sample was dispersed into distilled water and aliquots of AgNO3 solution was added according to the desired Ag molar ratio (0.5–5%). The mixture was then photoirradiated under 80 W Hg vapor lamp and dried. The results revealed an increase in the adsorption capacity of the nanoparticles when the Ag molar ratio is between 2 and 4%, with efficiencies more than 65%, which proportionally increased the photocatalytic activity. Thus, the enhancements achieved in the photocatalytic decolorization (>95% in 30 min) of MB by Ag-La/TiO2 materials are directly related to the increased adsorption capacity.

On the other hand, organic molecules functionalization to enhance the photocatalytic activity of TiO2 was proposed by [122]. Carboxymethyl-β-cyclodextrin (CMCD) functionalization of TiO2 doped with lanthanum was evaluated to MB adsorption and degradation under UV-light radiation. TiO2-La nanoparticles were synthesized as described by [121]. The CMCD was synthesized by the dissolution of β-CD and NaOH aqueous ClCH2COOH solution, which was maintained at 50°C in a jacketed reactor for 5 h. A white precipitate (CMCD) was obtained by addition of methanol and acetone to the solution. The CMCD functionalized catalysts were prepared TiO2–La dispersed in distilled water. This solution was added to CMCD along with cyanamide and maintained at 90°C for 4 h. The results showed that adsorption efficiency increase for CMCD@TiO2-La sample, with a removal near 15% after 60 min under dark conditions. Accordingly, the improvement of the photocatalytic activity achieved for this nanoparticle is also related to the adsorption of the MB by CMCD, although the mass transfer is low due to the reduced concentration of this oligosaccharide at the TiO2 nanoparticles surface. This increase in degradation efficiency occurs because the CMCD has the function of enhancing the density of the dye at the semiconductor and solution interface, where the ROS are formed. Usually these species return to a thermodynamically

stable state without reacting with organic molecules due to their short lifetimes and because the contaminants are dispersed in the solution. Thus, when the dye is adsorbed, the transfer of electrons between the ROS and the contaminant becomes more probable, increasing the efficiency of the process.
