*5.1. Nanoparticles (0D)*

Generally, the 0D materials are characterized as spherically shaped with nano-scaled dimensions. Nanoparticles as a typical 0D material have been widely used in the area of photocatalysis with the merits of large surface area, simple synthesis methods, and easy to be functionalized [90]. Up to now, several synthesis approaches have been invented, among which the sol-gel method, hydrothermal method, and solvothermal method could be the most-used techniques for the fabrication of 0D composite photocatalysts.

#### 5.1.1. Sol-Gel Method

The sol-gel process is a commonly used and effective strategy for the preparation of various inorganic materials, especially for the metal oxides based on the corresponding precursors, and it has several merits including being low cost, processed at low-temperature, and the fine control of the product's chemical composition. Therefore, the sol-gel process is one of the most-used techniques for the preparation of composite semiconductor photocatalysts [58]. For example, Vaiano et al. [91] immobilized the N-doped TiO2 nanoparticles (NPs) on glass spheres via the sol-gel method. Through regulating the synthesis conditions, and employing the Triton X-100 as the surface active agent, the obtained N-doped TiO2 NPs/glass spheres exhibited a good photocatalytic activity for methylene blue and eriochrome black-T in water under UV and visible light irradiation. Moreover, the composite catalyst was easy to be separated from the reaction mixture with a good durability. Recently, Chen et al. [92] prepared a Ni-Cu-Zn ferrite@SiO2@TiO2 composite via a simple sol-gel method. With the immobilization of Ag and magnetic ferrite, the composite photocatalysts exhibited comparatively good photodegradation performance for the methylene blue under a visible light source with lower power. Moreover, the composite catalysts can be easily separated using a magnet and can be reused well without significant loss of photocatalytic activity (Figure 17).

**Figure 17.** (**a**) The cycling test of the catalytic performance. (**b**) Dye removal capacity by adsorption and degradation for different cycles. (**c**) Digital photos demonstrating the recyclability by using an external magnetic field. Adapted with permission from Reference [92]. Copyright (2017) Elsevier.

#### 5.1.2. Hydrothermal Methods

The hydrothermal method is a wet-chemistry method for synthesizing single crystals. Through the hydrothermal method, a great deal of crystalline phases that are not stable at the melting point can be obtained [93]. As a result, numerous semiconductor nanoparticles with different surface morphologies and compositions can also be prepared using the hydrothermal method. As a

representative work, Wu et al. [94] fabricated the F-doped flower-like TiO2 nanoparticle on the surface of Ti via a low-temperature hydrothermal process. They reported that the presence of HF in water and the hydrothermal reaction time play an important role in the formation of the F-doped flower-like TiO2 nanostructures. Through regulating the synthesis parameters, the obtained F-doped TiO2 flower-like nanomaterials exhibited a superior photoelectrochemical activity for the photodegradation of organic pollutants compared with P-25. They also demonstrated that the improved photoelectrochemical activity of the F-doped TiO2 flower-like nanomaterials was mainly due to the larger surface area and the enhanced visible light harvest capacity. Additionally, magnetic composite photocatalysts can also be synthesized using the hydrothermal method, such as the magnetic CoFe2O4/Ag/Ag3VO4 photocatalysts fabricated by Jing and co-workers [95]. During this study, the as-prepared CoFe2O4 nanoparticles were dispersed in the solutions with AgNO3 and Na3VO4, and the mixture suspensions were hydrothermally treated to prepare CoFe2O4/Ag/Ag3VO4 composites. Through controlling the weight ratios of CoFe2O4 in the composite system, the optimal CoFe2O4/Ag/Ag3VO4 composite exhibited significantly improved photocatalytic activity toward the degradation of various contaminants including methyl orange, tetracycline, and could even kill *Escherichia coli* solely under the driving of visible light. Moreover, with the advantage of having a good magnetic response property, the corresponding CoFe2O4/Ag/Ag3VO4 composite could be facilely collected from the water by applying an extra magnetic field (Figure 18).

**Figure 18.** (**a**) Magnetic separation performance of the CoFe2O4/Ag/Ag3VO4 composite. (**b**) The absorption spectra of methyl orange solutions over time in the presence of CoFe2O4/Ag/Ag3VO4 (under visible light λ ≥ 440 nm) and the UV–vis absorption spectrum of CoFe2O4/Ag/Ag3VO4. (**c**) The evolution of the absorption spectra of methyl orange solutions over time in the presence of CoFe2O4/Ag/Ag3VO4 (under visible light λ ≥ 550 nm). (**d**) Photocatalytic degradation of tetracycline with different samples under visible light irradiation. Adapted with permission from Reference [95] Copyright (2016) Elsevier.

#### *5.2. Nanofibers/Nanorods (1D)*

Recently, nanofibrous photocatalysts have been intensively studied owing to their unique long aspect ratio, large surface area, and being easily functionalized. Up to now, various strategies had been developed to synthesize the 1D materials with different morphology like: wires, belts, rods, tubes, and rings [61,89,96], among which, the hydrothermal method and electrospinning are the most-used techniques. Consequently, in this part, we present the development of composite semiconductors with nanofibrous morphology derived from the electrospinning method and hydrothermal method for the treatment of wastewater.
