**1. Introduction**

In the past several decades, with the booming of industry, the ever-increasing consumption of natural resources, especially fresh water and fossil resources, have caused alarming damage to the environment and seriously threaten the sustainability of human society [1–3]. As a worldwide concern, freshwater pollution drives people to seek for an effective approach to repair the polluted water environment. In general, the contaminants in water are mainly derived from the sewage effluent of industries (e.g., textile industry, paper industry, the pharmaceutical industry, etc.), and domestic contaminants (e.g., pharmaceuticals, pesticide, detergent, etc.) [4]. Until now, numerous contaminants have been detected and are classified as inorganic ions, organic chemicals, and pathogens; most of those contaminants are toxic to organisms [4–7]. Up to now, a variety of strategies including chemical or physical coagulation [8], sedimentation [9], adsorption [10], membrane filtration [11], and biological degradation method [12] have been invented to treat wastewater. However, due to the complex compositions and different physico-chemical properties of the contaminants, there are still several limitations of these traditional techniques, such as the low efficiency, high energy consumption, and the risk of secondary pollution [13–15]. Consequently, a promoted technique with high efficiency, low energy consumption, and being environmentally friendly is highly desired for the remediation of wastewater.

Nowadays, the advanced oxidation processes (AOPs) have been extensively explored to remove the non-biodegradable and highly stable compounds in water [16,17]. In fact, the AOPs are chemical processes that can generate highly reactive hydroxyl radicals (·OH) in situ. The ·OH in water exhibits

an extremely strong oxidizing property with a high oxidation potential of 2.80 V/SHE (·OH/H2O), such that it can non-selectively oxidize the contaminants and finally convert them to CO2, H2O, or small inorganic ions in a short time [17,18]. In most cases, the ·OH could be produced with the presence of one or more primary oxidants, and/or energy sources or catalysts. Therefore, the typical AOPs could be classified as Fenton reactions, the electrochemical advanced oxidation processes, and the heterogeneous photocatalysis [17]. Compared with the traditional water remediation techniques, the AOPs exhibit many advantages, which include: (1) the contaminants are directly destroyed or reduced in the water body, rather than simply coagulated or filtrated from the water, thus the secondary pollution could be avoided; (2) the AOPs are suitable for a wide range of contaminants including some inorganics and pathogens because of their robust non-selectively oxidizability; and (3) no hazardous byproducts will be generated due to the final reduction products of the AOPs being just CO2, H2O, or small inorganic ions. With the abovementioned merits, the AOPs have attracted significant attention from both scientific research and industrial processing [19].

Solar energy is a green, costless, and inexhaustible energy resource. Effective utilization of solar energy is of vital importance for enhancing the sustainability of industry, reducing pollution, and retarding global warming. Consequently, solar energy has been widely used in a range of applications, such as solar heating, photovoltaics, solar thermal energy, solar architecture, artificial photosynthesis, photocatalysis, etc. [20] Among which, photocatalysis is one of the most effective strategies for the AOPs, which just rely on the light radiation on the photocatalysts to drive the oxidization reaction at the ambient condition, and during the whole reaction process, no additional energy is needed and no toxic byproduct will be generated; therefore, it is a green chemical technique [21,22]. Actually, the core of photocatalytic AOPs are photocatalysts; semiconductors as the most employed heterogeneous photocatalysis for the AOPs have attained considerable development since Fujishima et al. [23] carried out the first photo-catalyzed AOP based on the titanium-oxide (TiO2) in 1972. Up to now, a myriad of photocatalytic AOPs have been designed for water treatment based on various semiconductors. In general, semiconductors are light-sensitive because of their unique electronic structure with a filled valence band (VB) and an empty conduction band (CB) [18,21]. Figure 1 and Equations (1)–(6) demonstrate the basic reaction process of a semiconductor to generate the photocatalytic radicals, which could be decomposed in the following steps: (i) photons with a certain energy are absorbed by the semiconductor; (ii) the absorbed photons with energy greater than the band gap energy (Eb) of semiconductors lead to the formation of electrons in the CB and corresponding holes in the VB; and (iii) the generated electron–hole pairs will migrate to the surface of semiconductors for redox reactions, and fast recombination in nanoseconds will happen at the same time (it should be mentioned that this process is negative for the AOPs, which shall be suppressed [21,22]).

**Figure 1.** Schematic illustration of the photocatalytic reaction process of a semiconductor. Adapted with permission from Reference [18]. Copyright (2012) Elsevier.

	- Recombination: e<sup>−</sup> + h+ <sup>→</sup> energy (2)

$$\text{Oxidation of }\mathrm{H\_2O} \colon \mathrm{H\_2O} + \mathrm{h^+}\_{\mathrm{VB}} \to \bullet \mathrm{OH} + \mathrm{H^+} \tag{3}$$

$$\text{Reduction of adsorbed O}\_2\text{: O}\_2 + \text{e}^- \rightarrow \text{O}\_2\text{:}\tag{4}$$

$$\text{Reaction with } \text{H}^+ \text{:} \text{O}\_2\text{\bullet}^- + \text{H}^+ \rightarrow \text{\bullet}\text{OH} \tag{5}$$

$$\text{Electrochemical reduction: } \bullet \text{OOH} + \bullet \text{OOH} \rightarrow \text{H}\_2\text{O}\_2 + \text{O}\_2 \tag{6}$$

However, it remains a significant challenge to fabricate a high-efficiency visible light photocatalyst solely based on an individual semiconductor photocatalyst. For example, the TiO2, as the most used photocatalyst, possesses various advantages with excellent chemical stability, large surface area, non-toxicity, and low cost [24]; however, its wide energy band gap (3.0–3.2 eV) means it can only be excited by the UV light (λ < 400 nm), such that less than 5% of the irradiated solar energy can be effectively used [25]. Moreover, the fast recombination speed of electron–hole pairs seriously limits the further improvement of its photocatalytic activity [18,22]. On the other hand, although the recently developed visible light response semiconductors have a lower energy band gap (<3 eV), such as BiOX (X = I or Br) [26], they still suffer from serious photo-corrosion problems in aqueous media via redox reactions and the fast recombination of electron–hole pairs during the reaction process. Therefore, it is highly urgent to find an effect strategy to further improve the performance of semiconductor photocatalysts.

From ancient times, people have recognized that the incorporation of two or more constituent materials could obtain various composite materials with intriguing properties superior to the individual components. Nowadays, a myriad of functional composite materials have been developed for different applications [27,28]. Actually, the enhanced performance of a composite material is mainly attributed to the synergistic effect of its individual constituent materials; meanwhile, this principle is also appropriate to the design of semiconductor photocatalysts. Up to now, there have been numerous pioneering studies reporting the design and fabrication of composite semiconductor photocatalysts via various methods, such as doping heteroatoms or constructing heterojunctions via directly compositing with individual semiconductors or carbonaceous nanomaterials, among others. Therefore, as shown in Scheme 1, in this review, we aim to provide a systematic appraisal of the recent development in the design and fabrication of various composite photocatalysts for the application of wastewater treatment. Meanwhile, some representative photocatalysts with composite structures and morphologies from the atomic scale to macroscopic scale are reviewed. Finally, the current developing status, challenges, and evolution trend of the composite semiconductor photocatalysts for wastewater remediation are briefly proposed.

**Scheme 1.** The schematic illustration demonstrating the design and synthesis strategies for composite photocatalysts.
