*5.4. Frameworks (3D)*

In recent years, there have been a great number of works reporting a new generation of composite photocatalysts with 3D frameworks [115]. Actually, the 3D photocatalysts with well-deigned frameworks show great advantages such as a large specific surface area, high adsorptive capacity, good structure stability, good mass transfer ability, and a large number of exposed active sites, which make them promising candidates for the highly efficient photodegradation of contaminants in water. In general, the 3D composite photocatalysts could be obtained via two approaches, which are to directly construct a photocatalyst with 3D frameworks (type I), or compositing the photocatalysts with a template with 3D frameworks (type II).

Through employing the commonly reported synthesis methods of various 3D frameworks, such as the sol–gel process, in situ assembly, and template methods, various 3D photocatalysts with different characteristics have be fabricated [116]. The sol-gel process is a well-developed strategy to synthesize aerogels, and some of the produced aerogels, such as the SiO2 aerogels, have been commercialized [117].

In general, there are two stages for the sol-gel process method: first, a precursor (e.g., metal alkoxide) is subjected to the hydrolysis and condensation reactions to form a wet gel, during which time, numerous networks are generated between the alkoxide groups; subsequently, the formed wet gels are sufficiently dried to obtain aerogels. As a matter of course, a photocatalytic aerogel can be obtained using a metal alkoxide precursor with an appropriate photocatalytic activity. For example, Dagan et al. [118] prepared a series of highly porous TiO2 aerogels via the sol-gel method and they also proved that the photocatalytic degradation performance of the TiO2 aerogels for organic contaminants is much better than that of a commercial TiO2 (P25). Besides, various photoactive metal oxides, metal silylamide, or their composite aerogels have been developed. However, due to the limitation of sol-gel processes, some metal oxides or metal chalcogenides are not able to be synthesized into aerogels, and the obtained aerogel photocatalysts usually suffer from low crystallinity. Therefore, a new generated strategy, namely an assembly method, has been invented to construct aerogels based on various nanoscale units with different morphologies and chemical properties. As reported before [116], there are three typical steps for the assembly process: (i) fabrication of the building blocks, (ii) preparing the dispersion of the building blocks with appropriated concentration, and (iii) solidified the suspension of building blocks to form a 3D monolith. Based on this principle, Heiligtag et al. [119] developed a 3D framework Au-TiO2 photocatalysts with a preformed TiO2 nanoparticles as the blocking units without using any templates. Through modifying the surface of anatase TiO2 nanoparticles with trizma, the nanoparticles undergo an oriented attachment process during gelation and finally result in well-bonded networks. Moreover, based on the above-mentioned aerogel synthesis methods, various phototcatalytic aerogels can also be prepared via employing the preformed aerogels as the templates, such as a C3N4 aerogel that was fabricated by Kailasam and co-workers [120] via preparing a C3N4/SiO2 composite aerogel based on the sol-gel method at first, and then remove the SiO2 via treating the composite in 4 M NH4HF2 (Figure 22).

**Figure 22.** Schematic illustration indicating the synthesis process of porous carbon nitride and silica aerogels based on the sol-gel method and the digital photos of corresponding aerogels. Adapted with permission from Reference [120]. Copyright (2011) Royal Society of Chemistry.

As for the fabrication of type II 3D photocatalysts, an appropriate 3D porous substrate should be prepared before loading the active substance on its frameworks. Considering the requirements for high photoreactivity and good service performance, the aerogels/hydrogels derived from ceramics or carbon are mostly preferred. For example, Li et al. [121] fabricated a ternary magnetic composite of Fe3O4@TiO2/SiO2 aerogel via combining the sol-gel process and a hydrothermal treatment. During the fabrication process, Fe3O4 microspheres were first synthesized via the hydrothermal method; after that, Fe3O4@TiO2 core shell microspheres were fabricated via an in situ reaction method. The used SiO2 aerogel was derived from the industrial fly ash via a common sol-gel method. Finally, the as-prepared

Fe3O4@TiO2 core shell microspheres and SiO2 aerogel were combined via the hydrothermal method. According to their report, the obtained Fe3O4@TiO2/SiO2 aerogel exhibited an enhanced photocatalytic activity for the degradation of rhodamine B dye under visible light irradiation, and the aerogel could be facilely collected after the reaction due to its good magnetic separation performance. Interestingly, as shown in Figure 23, Jiang et al. [122] recently developed a separation-free PANI/TiO2 3D hydrogel for the continuous photocatalytic degradation of various contaminants in water. In their studies, the PANI hydrogel with 3D frameworks was synthesized via the polymerization of aniline. During the gelling process, the TiO2 nanoparticles (P25) were incapsulated in the hydrogels. As a result, the obtained PANI/TiO2 composite hydrogel exhibited an intriguing capacity for removing organic contaminants from water, which was mainly caused by the synergistic effect of adsorption enrichment of hydrogel and the in situ photocatalytic degradation of TiO2. Moreover, the presented separation-free characteristics in the obtained bulk materials indicate a good recyclability of the composite hydrogel.

**Figure 23.** Schematic illustration demonstrating the synthesis process of the 3D PANI/TiO2 composite hydrogel. Adapted with permission from Reference [122]. Copyright (2015) Wiley.

### **6. Summary and Perspectives**

In summary, in order to address the worldwide concerned issues of water pollutions, various photocatalysis processes based on different photocatalysts have been developed; meanwhile, numerous efforts have been made to further improve the photocatalytic activity of the catalysts based on the semiconducors. In this review, the recent progress in the development of composite semiconductor photocatalysts for wastewater treatment is presented, including the most-used strategies to narrow the band gap of semiconductors, to retard the recombination of the photo-generated electron-hole pairs, to enhance the visible light adsorption capacity, as well as to increase the reaction ratio between the photocatalysts and contaminants. Moreover, the composite catalyts with different morphologies and the corresponding photocatlytic performance were also summarized.

Although great development of the photocatalysis process has been obtained, there are still several problems yet to be addressed to further improve the practical application performance of the photocatalysis. Therefore, some plausible perspectives for the developing trend of composite photocatalysts for the wasterwater treatment are proposed based on the presented studies: (i) the existing synthesis methods are relative complex, high cost, and harmful to the environment to some degree, thus a more facile, highly efficent, and green method is anticipated; (ii) the mechanism of the composite semicondutor photocatalysts are still confusing and some of them are unpersuasive, therefore much more effort is needed for the basic studies of the catalytic mechanisms; and (iii) the practical use are limitted because the collection and reuse of the catalysts in water are still inconvenient due to their small size and poor mechanical property, therefore novel photocatalysts with easy collection property or new hybrid devices based on the composite of photocatalysts with selected

substrates (e.g., polymers, metals) are proposed. Finally, we anticipate that this review can provide some useful guidance for the design of next generation of photocatlysts for the wastewater remediation.

**Funding:** This work was supported by the Technology Development Program (S2598148) funded by the Ministry of SMEs and Startups (MSS, Korea) and the Commercialization Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science and ICT (MSIT) [2018\_RND\_002\_0064, Development of 800 mAh/g pitch carbon coating.

**Acknowledgments:** This work was supported by the Technology Development Program (S2598148) funded by the Ministry of SMEs and Startups (MSS, Korea) and the Commercialization Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science and ICT (MSIT) [2018\_RND\_002\_0064, Development of 800 mAh/g pitch carbon coating.

**Conflicts of Interest:** The authors declare no conflict of interest.
