*3.2. Non-Metallic Anions Doping*

Alternatively, doping the semiconductors with appropriate non-metallic anions has been proven to be a facile way to regulate the intrinsic electronic structure of semiconductors and could construct various heteroatomic surface structures such that the resultant non-metallic-anion-doped semiconductors exhibit improved photocatalytic performances under solar light irradiation [33,41]. In general, the chemical states and locations are key factors for the regulation of the electronic state of the dopant and the corresponding heteroatomic surface structures of the composite semiconductor catalysts. According to a previous study [18], three requirements needed to be satisfied for the doping of a semiconductor: (i) the doping process should construct states in the band gap of corresponding semiconductors with an enhanced visible light absorption capacity, (ii) the CB minimum including the doped states should be equal to that of the semiconductor's or higher than that of the H2/H2O level such that the photoreduction can be conducted, and (iii) the states in the gap should sufficiently overlap with the band states of semiconductors to ensure that the photoexcited carriers could migrate to the surface of catalysts within their lifetime. Based on the abovementioned principles, various elements, including C, N, F, P, and S, were employed to substitute for the O in TiO2 [42], and the results showed that N was the most effective dopant for the improvement of visible-light photocatalysis of TiO2 because the *p* states of N can narrow the band gap of N-doped TiO2 via mixing with the O *2p* states [43]. Moreover, owing to the comparable atomic size with oxygen, small ionization energy, and high stability, the nitrogen has been one of the most promising elements for promoting the photocatalysis performance of the semiconductors. In general, the doped N in the TiO2 could be classified as the substitutional type and interstitial type (Figure 3), the substitutional type N-doped TiO2 is attributed to the oxygen replacement, while the interstitial type is attributed to the additional N element in the lattice of TiO2 [41]. Up to now, the N-doping of semiconductors can be realized via several strategies, and the most-used techniques with certain industrial application prospects could be mentioned as the magnetron sputtering, ion implantation, chemical vapor deposition, atomic layer deposition, and sol-gel and combustion method, which will be discussed as follows.

**Figure 3.** Schematic diagram demonstrating the N doping in the lattice of TiO2. Adapted with permission from Reference [41]. Copyright (2011) Royal Society of Chemistry.

#### 3.2.1. Magnetron Sputtering Method

The magnetron sputtering method is widely used for the preparation of various hybrid semiconductors. For example, Kitano et al. [44] fabricated nitrogen-substituted TiO2 thin films by using a radio frequency magnetron sputtering (RF-MS) method. The N2/Ar gas mixtures with different concentration of N2 was used as the sputtering gas. They systematically investigated the influence of nitrogen content on the properties of the obtained N-TiO2 thin films via regulating the concentration of N2 in the sputtering gases. Meanwhile, they proved that the extent of substitution of oxygen positions with N in the lattice of TiO2 as well as the surface morphologies of TiO2 could be controlled well. As a result, the visible light absorption capacity of the obtained N-TiO2 was obviously enhanced with bands up to 550 nm, and it was found that the band red shift extent was closely related to the content of the substituted N element in the TiO2 lattice. Moreover, they found that the as-prepared N-TiO2 photocatalyst exhibited an optimized photocatalysis reactivity with the N content of 6%. This result was because of the excessive substituted N, which causes the formation of undesirable Ti3+ species and acts as the recombination centers to decrease the photocatalytic activity [44]. Apart from the TiO2, some other N-doped semiconductors could also be prepared based on the RF-MS method. Recently, Salah et al. [45] fabricated a series of N-doped ZnO nanoparticles films by employing the RF-MS method. As shown in Figure 4, the obtained N-doped ZnO films exhibited an improved response to the visible light, and possessed significantly enhanced degradation/mineralization performance for 2-chlorophenol (2-CP), 4-chlorophenol (4-CP), and 2,4-dichlorophenoxyaceticacid (2,4-D) solely under the drive of natural sunlight.

**Figure 4.** (**a**) The degradation and (**b**) mineralization of N-doped ZnO films for 2-CP, 4-CP, and 2,4-D. (**c**) The stability of pristine ZnO and N-doped ZnO film. (**d**) The stability and reusability of an N-doped ZnO film for the degradation of 2-CP. Adapted with permission from Reference [45]. Copyright (2016) Elsevier.

### 3.2.2. Ion Implantation Method

The ion implantation method as a typical materials engineering strategy that can effectively regulate the physical, chemical, and electronic properties of semiconductors, and the operation process does not involve any other elements except the selected element, which ensures the purity of the dopant [46]. Moreover, owing to the controllable parameters of ion beam implantation, such as ion element, ion energy, ion density, uniformity of ion beam, and the doping efficiency, ion beam implantation is a powerful approach for the heteroatom doping of semiconductors. For example, Tang et al. [47] fabricated an N-doped TiO2 layer with macrospores on a titanium substrate by using the plasma-based ion implantation method. The fabrication process involves four steps: (i) a helium plasma was employed to generate He bubbles in the substrate, (ii) an oxygen plasma treatment and a followed annealing in air were used to obtain rutile and anatase phases of TiO2, (iii) an Ar ion sputtering method was used to exposure the He bubbles on the surface; and (iv) the pre-treated samples were doped by nitrogen though the nitrogen beam ion implantation method. Moreover, co-doping of two or more non-metallic anions into a semiconductor photocatalyst (e.g., TiO2) could also be realized using the ion implantation method. For example, Song et al. [48] prepared C/N-implanted single-crystalline rutile TiO2 nanowire arrays by using carbon and nitrogen ions beam to treat the as-prepared TiO2 nanowire arrays. After an annealing treatment, the obtained C/N-doped TiO2 nanowire arrays exhibited a superior visible light response activity, which was attributed to the synergistic effect between the doped C and N atoms. Their work proved that the co-doped C and N in the lattice of TiO2 not only greatly improves the visible light absorption capability, but also enhances the separating and transferring property of photo-generated electron–hole pairs (Figure 5).

**Figure 5.** (**a**) UV–vis absorption spectra of TiO2 and various doped TiO2. (**b**) Linear sweep voltammograms of C/N-TiO2 and TiO2. (**c**) Photo-response of TiO2 and the various doped TiO2 samples under visible light. (**d**) Incident photon-to-current conversion efficiency spectra of TiO2 and various doped TiO2. Adapted with permission from Reference [48]. Copyright (2018) Wiley.

#### 3.2.3. Chemical Vapor Deposition Method

Chemical vapor deposition (CVD) is a low-cost and scalable technique, which can directly grow a solid-phase material from a gas phase containing specific precursors. The CVD method has been widely used for the fabrication of semiconductors and the corresponding composite of oxides, sulfides, nitrides, and other mixed anion materials [49]. For example, Lee et al. [50] prepared TiO2 composite materials doped by C (TiOC) and N (TiON) with the titanium tetraisopropoxide (TTIP), oxygen, and NH3 as the precursors via combing the CVD method with a fluidized bed. The results demonstrated that the visible light photocatalysis performance of the composite TiO2 (e.g., TiON) was significantly improved compared to the commercial TiO2 catalyst (P25, Degussa). Similarly, Kafizas et al. [51] employed a combinatorial atmospheric pressure chemical vapor deposition (cAPCVD) method to prepare an anatase TiO2 film with a gradating N content. The obtained TiO2 film exhibited a gradating substitutional (Ns) and interstitial (Ni) nitrogen concentration, and the transition process from predominantly Ns-doped TiO2 to Ns/Ni mixtures, and finally to purely Ni-doped TiO2 was precisely characterized. In addition, the UV and visible light photocatalytic activities of the obtained N-doped TiO2 were evaluated. As a result, this work demonstrated that Ni-doped anatase TiO2 results in a better visible light photocatalytic activity than that of predominantly Ns-doping. They proved that the different influences of substitutional and interstitial nitrogen doping on the photocatalytic activity of TiO2 were due to that the greater stability of electron–holes in Ni-doped TiO2 compare with that of Ns-doped TiO2, while the propensity of the Ns-doped TiO2 for recombination is greater. This result indicated that the doped structures is well-deigned to improve the photocatalytic activity of a semiconductor. Additionally, the CVD could also be combined with other materials

synthesis strategy; for example, as shown in Figure 6, Youssef et al. [52] prepared the N-doped anatase films via a one-step low-frequency plasma enhanced chemical vapor deposition (PECVD) process. Furthermore, they demonstrated that this method did not need the subsequential annealing step or post-incorporation of the doping agent, and the as–prepared N-TiO2 film exhibited good visible-light-induced photocatalytic performance.

**Figure 6.** Schematic view of the capacitively-coupled low frequency PECVD reactor. Adapted with permission from Reference [52]. Copyright (2017) Elsevier.
