**2. Principle of the Semiconductor Photocatalysts for Wastewater Remediation**

As mentioned above, the trace contaminants (e.g., phenol, chlorophenol, oxalic acid) derived from the dyeing industry, petrochemical industry, and the agricultural chemicals are quite difficult to remove from the water due to the low concentration and complex compositions [4]. A photocatalytic degradation method is considered as the most promising strategy to deal with this problem. According to the previous studies [18,29,30], as shown in Figure 2, the basic mechanism of the photocatalytic degradation process of a contaminant could be characterized as the following steps: (i) the target contaminants transfer from the water body to the surface of the photocatalysts, in which the migration rate of corresponding contaminants may be influenced by the morphology and surface properties of the catalysts (e.g., surface area, porosity, and surface charges); (ii) the contaminants are adsorbed on the surface of catalysts with photon excited reaction sites, therefore a high surface area of the catalysts can provide more active sites for the reaction; (iii) the redox reactions of the photon activated sites with the adsorbed contaminants and the degraded intermediates are produced, which are finally degraded to CO2 and H2O; (iv) part of the generated intermediates and the resultant mineralization products (CO2 and H2O) desorb from the surface of catalysts to expose the active sites for the subsequent reactions; and (v) the desorbed intermediates transfer from the interface of catalysts and water to the bulk liquid, and part of the intermediates will repeat the procedure i–v until they are completely degraded to CO2 and H2O. Based on the abovementioned principles of the semiconductor photocatalysts for water contaminants degradation, five main criteria for the design of an effective photocatalyst could be proposed as follow: (1) a semiconductor with a lower Eg is preferred so that the electron–hole pair could be excited easier; (2) the photon absorption capacity of the catalysts shall be as high as possible to generate more electron–hole pairs; (3) the recombination process of electron–hole pairs must be prevented as much as possible to enhance the quantum efficiency of the photo-generated electron–hole pairs; (4) the surface area of the catalysts shall be large to provide more reaction sites; and (5) the chemical and physical structures of photocatalysts must be stable and be beneficial for the mass transfer in water. To meet the abovementioned requirements, a variety of strategies have been developed for the design, some of the most-used strategies will be summarized in this review.

**Figure 2.** Schematic diagram demonstrating the removal of contaminants in water with the presence of photocatalysts [18,29,30].

### **3. Heteroatoms Doping**

Recently, the strategy of introducing heteroatoms into the lattice of corresponding semiconductors has been widely employed to regulate the band gap of the semiconductor photocatalysts so as to improve their absorption capacity for visible lights, which takes up almost 45% in the solar light spectrum [31]. In general, the most commonly used dopants in semiconductors (e.g., TiO2) could be classified as the metal cations and the non-metallic elements [32,33].

#### *3.1. Metal Cations Doping*

The most-used metal cation dopants for semiconductors mainly involve transition metal ions, such as Fe3+, Co3+, Mo5+, Ru3+, Ag+, Cu2+, Rb+, Cr3+, V4+, etc. [32,34–37]. In most cases, the redox energy states of those employed metal cations lie within the band gap states of corresponding semiconductors (e.g., TiO2); therefore, the introduction of those metal ions will result in an intraband state near the CB or VB edge of a semiconductor. Consequently, the red shift in band gap absorption of a metal-cation-doped semiconductor is mainly contributed by the charge migration between the d electrons of the doped cations and the CB (or VB) of the corresponding semiconductors. In addition, the doped metal cations could act as an electron–hole trap, regulating the charge carrier equilibrium concentration [38–40]. Although some transition metal cations could provide new energy levels as electron donors or acceptors, and virtually improved the visible light absorption capacity of corresponding semiconductors, this approach is also known to suffer from many disadvantages, such as bad thermal stability, significant increase in the carrier-recombination centers, and the high cost for an expensive facility, which are critical limitations for the generalization of this strategy.
