Environmental nanotechnologyadvanced oxidation nanotechnologies

As discussed in Section 6.1, the crystallographic, electronic, and structural properties of TiO2 are of importance with respect to UV light utilization, catalytic sites for the hydroxyl radical generation, and reactant accessibility to/from TiO2. Recently, in order to control the physicochemical properties of TiO2 at the nano level and thus achieve maximum photocatalytic performance in versatile environmental applications, nano-technology has been introduced in this research area, especially in the field of synthesis of new catalytic TiO2 materials. This results in more efficient TiO2-based advanced oxidation nanotechnologies (AONs) for water treatment, purification, recycle, and reuse. Among the synthetic routes, modified sol—gel methods employing pore templating agents including block copolymers and surfactant molecules have been attractive during the last decade as promising approaches for the tailor-designing of TiO2 structure [49,54,74,76,86-91].

Sol-gel preparation methods refer to room temperature wet chemistry-based formation of solid inorganic materials from molecular precursors [91,92]. The technology is applied for the versatile preparation of powders, catalytic films, inorganic membranes, monoliths, fibers, reactive coatings, sensors, and optics [92-94]. Fig. 6 shows the surfactant template-based solgel synthesis of TiO2. The approach utilizes self-assembled surfactants as a pore template or particle growth template. Finally, TiO2 inorganic materials with highly porous network or well-defined TiO2 nanoparticles are formed.

Based on the synthesis approach, by changing the type, chain length, and concentration of surfactant, it is possible to control the physicochemical properties of TiO2 (i.e., morphological structure, crystal phase, defect structure, impurities, BG energy, hydrophilicity) and thus improve its organic adsorption capacity and catalytic activity [49,54,74,76,88]. Especially, this technique is very useful in fabricating TiO2 films and

•o* O TiO2

—m •

• •


• m-

-• Tio2 m—




Figure 6 Synthesis approaches of engineered TiO2 via sol-gel method employing surfactant self-assembly as (a) a pore template and (b) particle growth template. (a) Synthesis of TiO2 with mesoporous inorganic network: (i) surfactant molecules are self-organized in water-rich environment, forming surfactant head group outside towards water molecules and its tail group inside free from water, (ii) titanium alkoxide precursor is hydrolyzed and condensed to form TiO2 inorganic network around the self-assembled surfactant, forming a surfactant organic template-embedded TiO2 inorganic matrix, and (iii) porous TiO2 inorganic network is formed after removal of the organic template by thermal treatment or organic extraction. (b) Synthesis of TiO2 nanoparticles: (i) surfactant molecules are self-organized in water-poor environment (bulk hydrophobic solvent (HS) with small portion of water), forming surfactant head group inside towards water molecules and its tail group outside towards HS, (ii) titanium alkoxide precursor is hydrolyzed and condensed to form TiO2 inorganic network in the water phase, inside of self-assembled surfactant, forming TiO2 inorganic core/surfactant organic shell structure, and (iii) well-defined TiO2 nanopar-ticles are formed after removal of the organic template.

membranes with engineered properties to improve their performance. In order to minimize the hydraulic resistance through a membrane, an asymmetric mesoporous TiO2 membrane was fabricated by changing the concentration of a surfactant in a sol-gel synthesis of TiO2 [76]. The membrane showed a hierarchical change in pore diameter and porosity from 2—6nm and 46.2%, 3—8nm and 56.7% to 5—11 nm and 69.3% from the top to the bottom layer, and exhibited improved water permeability without sacrificing organic retention and photocatalytic activity. Interestingly, a nitrogen-containing surfactant (dodecylammonium chloride) as a pore templating material to tailor-design the structural properties of TiO2 and as a nitrogen dopant to narrow its BG (as discussed in Section 6.4) was introduced in a sol-gel synthesis of TiO2 [49]. Nitrogen atoms in the surfactant were diffused and incorporated into the crystal lattice of TiO2 during calcination. The synthesis of mesoporous TiO2 and in situ nitrogen-doping of the TiO2 were concurrently achieved.

The nanotechnological approach for the preparation of TiO2 with engineered functionalities and properties for environmental applications is interdisciplinary, integrating environmental engineering and science, chemical engineering, materials science, and chemistry. We believe that the TiO2-based AONs have tremendous potential to profoundly change current science and engineering in the field of water and wastewater treatment.

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