2.3. ZnO immobilization by organic molecules: poly (acrylic acid)

photocatalyst surface with S-H, C-O, and hydroxyl groups (OH) creates defect sites advantageous in photocatalysis. Silver-experienced redox processes during PD functionalization, for

Figure 6. FTIR (a) and TEM image (b) of the as-synthesized photocatalyst functionalized ZnO by silver nanoparticles. Schematic illustration of the functionalization by PD method. (c). Histogram of silver particle size (d). Photocatalytic

This method favors insertion of ionic silver Ag+ into ZnO crystalline structure perceived as an expansion of lattice parameters measured by XRD. The new attached silver possibly anchors on the surface defect sites of ZnO [25, 26]. The role of the metallic modifier in Ag/ZnO is to promote

visible light, in our case, evidenced as an absorption in the visible region by UV-Vis spectroscopy. The IMP functionalization mechanism is different in the sense that silver nanoparticles and ZnO interaction is a function of the reaction time for an optimum of 2 h. For instance, the sample 1%Ag/ZnO-IMP11,2 synthesized using 1 wt.% Ag at pH 11 and 2 h under vigorous

, +0.799 eV vs. SHE) by photogenerated holes (+2.75 eV, vs. SHE)

) separation and to increase the photocatalyst sensitivity toward

) by photoexcited electrons.

instance, oxidation (Ag+

66 Photocatalysts - Applications and Attributes

the pair electron-hole (e/h+

/Ag0

degradation of bisphenol A (e), triclosan (f), and RhB (g).

and free radicals. They reduce again into the zero-valence form (Ag0

The utilization of powder photocatalyst may end unpractical for industrial scales because of the technical challenges like efficient dispersion and finally difficult separation of photocatalyst after the reaction that may entail important energetic costs and sometimes even producing a secondary pollution. Besides, photocorrosion is an important drawback in photocatalysis, and the anchoring of silver has been proved to control its progress. The immobilization of ZnO into organics tends to solve both the difficulty of photocatalyst dispersion and recuperation for a cyclic usage together with an improvement of the photostability [30]. The organics that have been reported for immobilization of TiO2 comprise polyaniline [31, 32] and polypyrrole [33] that are hydrophobic and opaque, then it is incompatible for aqueous applications. Hydrogels, based on acrylic polymers, allow effective transport of water and other dissolved molecules due to their hydrophilicity and high swelling capacity. This kind of polymers shows stimuli-responsive properties to pH, temperature, solvent composition, and ionic strengths. Hydrogels come to be an ideal choice for Ag/ZnO photocatalyst immobilization since they are colorless and visually transparent, which permits penetration of light (Figure 7e).

The general mechanism for the immobilization of Ag/ZnO into a poly (acrylic acid) (PAA) matrix involves the use of GLYMO as photocatalyst stabilizer and coupling agent. Infrared analysis (FTIR) shows characteristic bands at 916 and 1260 cm<sup>1</sup> for free epoxide functional groups, in pure GLYMO and Ag/ZnO-GLYMO, meaning that epoxide do not react with Ag/ZnO surface,

methoxysilyl groups (CH3O) of GLYMO, indicates two processes: hydrolysis of methoxy group and reaction with Ag/ZnO surface by weak bond such as hydrogen bonds. In addition, the peak at 1030 cm<sup>1</sup> corresponds to the formation of self-assembled monolayer Si-O-Si on Ag/ZnO surface, and confirms the efficient silanization of the photocatalyst (Figure 7a). Lastly, the interaction between the free tail of Ag/ZnO-GLYMO (highly reactive epoxy groups) and the carboxylic groups of PAA gives strong interaction and immobilization into the polymeric matrix.

Two possible arrangements of the photocatalyst bonded to the polymer matrix are proposed: (i) a photocatalyst pending from the PAA chain (Figure 7c) and (ii) a cross-linked–like struc-

Environmental SEM chamber allow us for first time to obtain images of 3D network of crosslinked PAA under low vacuum that gives us a time-window of approximately 30 minutes for the analysis before dehydration (Figure 7b). SEM-EDX confirms the homogeneous dispersion of photocatalyst within the PAA matrix on 8%Ag/ZnO-PAA composites. The swelling capacity of the composite increases with photocatalyst content (5–13 wt.%) due to the high hydrophilicity of Ag/ZnO (Figure 7d). In addition, the composite photostability after 16 h of UV

This enhanced photochemical stability has the potential of use as resistant composite packing material for continuous treatment of water under UV irradiation. This last aspect was first tested in batch experiments toward the degradation of bisphenol-A. Thus, Figure 7g shows the time evolution of the bisphenol-A photodegradation under visible light (>450 nm, 8 uW/cm<sup>2</sup>

W Hampton Bay lamp, home-made reactor in Figure 7f) at 50C. On the other hand, 47% of the initial concentration of 10 mg/L is degraded within 7 h by 8%Ag/ZnO-PAA composite, it represents a decrease of 50% compared with Ag/ZnO-GLYMO. The composite was reused in a second consecutive cycle without washing that results in an improvement of 40% compared with the first cycle. It is necessary to highlight that the sorption of bisphenol-A in dark conditions was not important; therefore, regeneration of the composite is not necessary in continuous water treatments. Ag/ZnO-PAA composites like those synthesized in this study

Bismuth oxyhalides BiOCl (X:Cl, Br, and I) are a new class of semiconductors that have recently attracted attentions in the photocatalytic process due to their relatively slow electronhole recombination process. BiOXs are conformed by Bi3+, O2, and halide (X) ions stacked in [X-Bi-O-Bi-X]n layers, giving a tetragonal structure with no linkers interactions with halide

, corresponding to C-H from the

Modified Metallic Oxides for Efficient Photocatalysis http://dx.doi.org/10.5772/intechopen.80834

, 8

69

as desired. Disappearance of bands at 823 and 780 cm<sup>1</sup>

ture, where the photocatalyst is the bridge between two PAA chains.

(365 nm) exposure was corroborated by FTIR and TGA analysis.

are less sensitive to saturation compared to zeolites and carbon materials.

3. Bismuth oxychloride (BiOCl)

Figure 7. FTIR (a) and E-SEM image (b) of the as-synthesized Ag/ZnO-PAA. Schematic representation of the ligands between photocatalyst pending from the PAA chain and its photocatalytic role (c). Semi-swell Ag/ZnO-PAA composite (d) and reaction vessel containing photocatalyst dissolved in the pollutant solution (e).Home made photocatalytic reactor operated at room temperature and pressure (f). Photocatalytic degradation of bisphenol-A (g).

The general mechanism for the immobilization of Ag/ZnO into a poly (acrylic acid) (PAA) matrix involves the use of GLYMO as photocatalyst stabilizer and coupling agent. Infrared analysis (FTIR) shows characteristic bands at 916 and 1260 cm<sup>1</sup> for free epoxide functional groups, in pure GLYMO and Ag/ZnO-GLYMO, meaning that epoxide do not react with Ag/ZnO surface, as desired. Disappearance of bands at 823 and 780 cm<sup>1</sup> , corresponding to C-H from the methoxysilyl groups (CH3O) of GLYMO, indicates two processes: hydrolysis of methoxy group and reaction with Ag/ZnO surface by weak bond such as hydrogen bonds. In addition, the peak at 1030 cm<sup>1</sup> corresponds to the formation of self-assembled monolayer Si-O-Si on Ag/ZnO surface, and confirms the efficient silanization of the photocatalyst (Figure 7a). Lastly, the interaction between the free tail of Ag/ZnO-GLYMO (highly reactive epoxy groups) and the carboxylic groups of PAA gives strong interaction and immobilization into the polymeric matrix.

Two possible arrangements of the photocatalyst bonded to the polymer matrix are proposed: (i) a photocatalyst pending from the PAA chain (Figure 7c) and (ii) a cross-linked–like structure, where the photocatalyst is the bridge between two PAA chains.

Environmental SEM chamber allow us for first time to obtain images of 3D network of crosslinked PAA under low vacuum that gives us a time-window of approximately 30 minutes for the analysis before dehydration (Figure 7b). SEM-EDX confirms the homogeneous dispersion of photocatalyst within the PAA matrix on 8%Ag/ZnO-PAA composites. The swelling capacity of the composite increases with photocatalyst content (5–13 wt.%) due to the high hydrophilicity of Ag/ZnO (Figure 7d). In addition, the composite photostability after 16 h of UV (365 nm) exposure was corroborated by FTIR and TGA analysis.

This enhanced photochemical stability has the potential of use as resistant composite packing material for continuous treatment of water under UV irradiation. This last aspect was first tested in batch experiments toward the degradation of bisphenol-A. Thus, Figure 7g shows the time evolution of the bisphenol-A photodegradation under visible light (>450 nm, 8 uW/cm<sup>2</sup> , 8 W Hampton Bay lamp, home-made reactor in Figure 7f) at 50C. On the other hand, 47% of the initial concentration of 10 mg/L is degraded within 7 h by 8%Ag/ZnO-PAA composite, it represents a decrease of 50% compared with Ag/ZnO-GLYMO. The composite was reused in a second consecutive cycle without washing that results in an improvement of 40% compared with the first cycle. It is necessary to highlight that the sorption of bisphenol-A in dark conditions was not important; therefore, regeneration of the composite is not necessary in continuous water treatments. Ag/ZnO-PAA composites like those synthesized in this study are less sensitive to saturation compared to zeolites and carbon materials.
