*3.2. Photodegradation*

Organic dyes are used on a large scale in modern industries. Wastewater polluted with such substances subsequently leads to environmental problems [66,67]. This is due to the fact that most organic dyes consist of biodegradable aromatic structures and azo-groups. Adsorption and catalytic oxidation remain the most e ffective among the methods for treating wastewater from dyes [68–70]. For photocatalytic degradation of organic compounds, in most cases, TiO2, in the doped form, is used.

Graphene has a large specific surface area and electrical conductivity. The presence of such properties in graphene is of interest to scientists in the preparation of a graphene/nano-TiO2-based photocatalyst [71–73]. After irradiation of a TiO2-based photocatalyst, electrons move into the graphene structure. Based on the author's results [74], it helps to avoid recombination between charge carriers. TiO2/graphene-based photocatalyst can be obtained by creating either a chemical bond between them or via the approach of their mechanically mixing [75,76]. Due to existence of chemical bond, the electron is transferred unhindered from the photocatalyst to graphene, reducing the probability of recombination. This is the main explanation for the increased activity of the photocatalyst with graphene.

Composites based on TiO2/graphene attract scientists' attention not only as highly e fficient photocatalysts due to light absorption in a wide range and charge separation, but also taking in account its high adsorption capacity to pollutants. However, TiO2/graphene-based composite produced by the hydrothermal method cannot serve as an e ffective photocatalyst. The reason is the agglomeration of graphene layers, which adversely a ffects the adsorption and photocatalytic properties. The solution to this problem is described in [77], where graphene-based aerogel was used as an auxiliary material for TiO2 [78,79]. Aerogel was obtained by thermal reduction of graphene oxide. Table 2 lists some reports on the use of composite TiO2/graphene based photocatalysts to remove various organic compounds (pollutants and dyes) from water.



Before testing photocatalytic activity, it was necessary to saturate the materials in the dark conditions. If saturation is not performed at dark conditions, a decrease in the concentration of the target pollutant or the color intensity change of the used in experiment dye will be associated not only with photocatalysis, but also with adsorption and photocatalytic degradation, so the e ffectiveness of the photocatalyst will be incorrect. X. Sun et al. [77] compared two samples of a photocatalyst of the same mass: hydrothermally obtained TiO2-reduced graphene oxide (rGO), and aerogel based on TiO2-rGO (Figure 7a). As can be seen, despite the equal mass of both samples, the TiO2-rGO based aerogel has a larger specific volume than that of the second sample. For further characterization of their adsorption, both samples were used to adsorb methylene blue in the dark. During the experiment, the absorption intensity was analyzed. The results showed that it took 2 min for TiO2-rGO based aerogel to achieve adsorption saturation, while for TiO2-rGO powder, it took more than 10 min (Figure 7b). Figure 7c,d demonstrates the color change of methylene blue, which proves the high absorption coe fficient of visible light by TiO2-rGO aerogels. According to the authors, the adsorption rate also a ffects the efficiency of photocatalysts.

Photocatalytic activity directly depends on the number of active centers on the surface of the photocatalyst. Photolithography is a block of technological processes of photochemical technology aimed at creating the relief in the film, as well as a film of metal deposited on a substrate [87,88]. Using this method, a group of scientists [89] managed to obtain TiO2 films with lattice, square, and hexagonal structures (Figure 8) and investigate the influence of a such surface textures on the photocatalysis. Obtained results showed that the activity of photocatalyst in the form of a film is not improved by increasing the values of specific surface area. Surface texture also has an effect on mass transfer during photocatalysis.

**Figure 7.** (**a**) Photograph of 80 mg TiO2-reduced graphene oxide (rGO) powder (left) and aerogel (right); (**b**) methylene blue (MB) dark adsorption results of the TiO2-rGO aerogel/powder; color changing of MB solution during dark adsorption with TiO2-rGO powder (**c**), and aerogel (**d**). All figures are reprinted from [77], with permission from Elsevier, 2020.

**Figure 8.** SEM images of (**a**) planar TiO2 film; (**b**) grating-structured TiO2 film; (**c**) square-structured TiO2 film; (**d**) hexagon-structured TiO2 film. All figures are reprinted from [89], with permission from Elsevier, 2019.

To evaluate the activity of photocatalysts, scientists conducted an experiment, in which the dye degradation occurred in water as a result of its exposure with UV (254 nm) on methyl orange. As shown in Figure 9, the microstructured TiO2 films exhibit a more active photocatalytic activity than TiO2 films with a flat surface. According to scientists, this is due to the presence of reaction centers on the surface of microstructured films. The highest photocatalytic activity was observed for TiO2 films with a square microstructure. The experimental results showed that the efficiency of photocatalysts is influenced not only by the surface area, but also by the type of their microstructure. It was revealed, that the TiO2 film with a grating structure, despite its low specific surface area, possessed photoactive properties similar to the TiO2 film with a hexagonal structure [89].

A significant role during photocatalysis is played by the surface microstructure, which influences the mass transfer of degradable organic compounds by the diffusion to the surface of the photocatalyst. It is important to take into account the fact that the fluid flowing over the surface with protrusions meets more resistance from the side of the walls, compared to the fluid flowing through the flat surface. As a result, such protrusions can adversely affect the degradation efficiency of organic pollutants.

**Figure 9.** Photocatalytic degradation of MO under ultraviolet light (254 nm) irradiation. C and C0 represent the real-time and initial concentration of methyl orange solution. This figure is reprinted from [89], with permission from Elsevier, 2019.

TiO2 doped with Fe2O3 is one of the best-known e ffective photocatalysts. Due to the narrow bandgap of Fe2O3 (2.2 eV), its doping leads to a redshift of the light response of the photocatalyst. The phenomenon of decreasing of photoactivity of TiO2 based photocatalyst doped with non-metals during heating, the high cost of some metals and the availability of Fe2O3 in large quantities increases the attractiveness of Fe2O3 over other dopants [90]. J.-J. Zhang et al. [91] used Fe2O3 nanoparticles, which served as a doping agen<sup>t</sup> for obtaining the TiO2/graphene aerogel (GA) based photocatalyst with a 3D structure (Figure 10). Due to the narrow bandgap (2.0 eV), Fe2O3 can easily generate electron-hole pairs, thereby contributing to the photodegradation of rhodamine B even in visible light. The results showed that the aqueous solution containing rhodamine B (RhB) was purified to 97.7% (Figure 10 b). Fe3O4 can also be used as a doping agen<sup>t</sup> for the photocatalysis [92]. For example, F. Soltani-Nezhad et al. [93] presented a method for producing a GO/Fe3O4/TiO2-NiO-based photocatalyst, which is able to e fficiently degrade imidacloprid (pesticide).

**Figure 10.** (**a**) The rate constants for the adsorption and the collective removal of rhodamine B (RhB) over TiO2-GA and the Fe2O3-TiO2-GA composites; (**b**) stability of the Fe2O3-TiO2-GA (25%) composites in the removal of RhB dye. Both figures are reprinted from [91], with permission from Elsevier, 2018.

To increase the e fficiency of the process of degradation of organic pollutants by photocatalysis, attempts have been made to combine radiation with ultrasonic cavitation. S. Rajoriya et al. [94] reported the photodegradation of 4-acetamidophenol to 91% using Sm (samarium) and N-doped TiO2 photocatalysts, in which a combination of UV radiation, hydrodynamic cavitation, and ultrasound was applied. In another work, the use of N and Cu-doped TiO2@CNTs in sono-photocatalysis for purification of pharmaceutical wastewater is discussed [95]. The results showed that when using a commercial Xe lamp (50 W) and ultrasound for pharmaceutical wastewaters treatment, the photocatalyst removal efficiency within 180 min were 100, 93, and 89% for sulfamethoxazole, chemical oxygen demand, and total organic carbon, respectively.

The grea<sup>t</sup> interest in the use of ultrasonic action during photocatalysis is justified by the enhancement of electronic excitations, which leads to an increase in the density of pairs of charged particles. Under the influence of ultrasound, the aggregate photocatalysts are dispersed, contributing to the rapid renewal and expansion of the boundaries of heterogeneous reactions, which improves the mass transfer and the course of chemical reactions [96,97].
