# Recent Advances and Applications of Semiconductor Photocatalytic Technology
## 5 Methods for Improving Photocatalytic Efficiency
### Metal or Non-Metal Ion Doping
#### 5.3.2 Non-Metal Ion Doping  
The doping of many metals such as V\({}^{3+}\), V\({}^{4+}\), Cr\({}^{3+}\), Mn\({}^{3+}\), Fe\({}^{3+}\), Co\({}^{3+}\), Ni\({}^{2+}\), Zn\({}^{2+}\), Ga\({}^{3+}\), Zr\({}^{4+}\), and Nb\({}^{5+}\) can expand the spectral response range of TiO\({}_{2}\) to the visible region [282; 283]. In addition, some metal-doped TiO\({}_{2}\) are not thermodynamically stable and easily cause an increase in recombination centers, thereby reducing the light absorption efficiency of TiO\({}_{2}\). Many researchers have therefore chosen to dope TiO\({}_{2}\) with non-metal atoms that form covalent bonds with titanium atoms [284]. In 2001, Japanese scientist [PERSON] et al. [285] prepared a TiO\({}_{2-x}\)N\({}_{x}\) photocatalyst using non-metallic N-doped TiO\({}_{2}\). The TiO\({}_{2-x}\)N\({}_{x}\) photocatalyst extends the optical excitation wavelength of TiO\({}_{2}\) to the visible region of 400-520 nm (Figure 7A) and maintains the photocatalytic activity of TiO\({}_{2}\) in the ultraviolet region. Methyl blue and acetaldehyde decomposition experiments showed that the TiO\({}_{2\times}\)N\({}_{x}\) photocatalyst shows high photocatalytic activity under both ultraviolet and visible light (Figure 7B). The research of TiO\({}_{2}\) has opened a new research area on a non-metal doped modified TiO\({}_{2}\) visible light catalyst, which is undoubtedly a pioneering work. Afterwards, scientists conducted systematic and detailed research on the doping modification of a TiO\({}_{2}\) photocatalyst with nonmetallic elements. Non-metals generally used for semiconductor doping are N [285; 286; 287; 288; 2890], C [291; 292; 293; 294; 295; 296; 297; 298; 299], S [101; 292; 294; 295; 296; 297; 298; 299], P [297; 300; 301], F [302; 303; 304; 305; 306; 307], Cl [308], I [309], B [310; 311; 312; 313], and Si [314; 315; 316], among others. [PERSON] et al. [286] reported the preparation of N-TiO\({}_{2}\) films by the radio frequency (RF) magnetron sputtering (RF-MS) deposition method, in which an N\({}_{2}\)/Ar mixture was used as a sputtering gas. It was found that the absorption band of the N-TiO\({}_{2}\) photocatalyst was shifted to the visible region, and the catalytic performance of the N-TiO\({}_{2}\) photocatalyst under visible light was remarkably improved. [PERSON] et al. [317] described the use of a hot solvent method to prepare a boron (B) ion doped TiO\({}_{2}\) catalyst, and they studied the effect of the boron doping amount and pH on the degradation of environmental pollutants. The experiment result shows the following: After UV irradiation for 300 min, the decomposition rates of pure TiO\({}_{2}\) and B-TiO\({}_{2}\) relative to dichlorophenol were 75.7% and 89.7%, respectively, and the decomposition rates of bisphenol A were 97.3% and 99.1%, respectively. After irradiation with visible light for 300 min, the decomposition rates of pure TiO\({}_{2}\) and B-TiO\({}_{2}\) relative to dichlorophenol were 20.2% and 75.1%, respectively, and the decomposition rates of bisphenol A were 46.0% and 97.7%, respectively. Since the forbidden band width of the B-doped TiO\({}_{2}\) semiconductor decreases, the modified material has an absorption response in the visible light region. In addition, two chemical bonds of B-O-Ti and B-O-B are formed during the doping process, and these two chemical bonds not only provide stable performance but also function as traps of photogenerated electrons. [PERSON] et al. [318] synthesized a C-doped TiO\({}_{2}\) photocatalyst by the hydrothermal method. As a carbon source, chitosan forms a porous morphology of the composite TiO\({}_{2}\), and a Ti-C chemical bond in the complex that promotes formation of surface oxygen vacancies. The porous morphology of the composite TiO\({}_{2}\) and the surface oxygen vacancies together improve the capture rate of organic molecules by the photocatalyst and improve the photocatalytic performance. The conversion rate of composite TiO\({}_{2}\) under visible light to NO gas can reach 71%.