# Recent Advances and Applications of Semiconductor Photocatalytic Technology
## 5 Methods for Improving Photocatalytic Efficiency
### Metal or Non-Metal Ion Doping
#### 5.3.1 Metal Ion Doping  
Metal ion doping is the most commonly used method for photocatalyst modification. The metal ion doping method is to dope transition metal ions in the catalyst by high-temperature firing or auxiliary deposition. The influence of metal ion doping on the photocatalyst mainly includes the suppression of recombination of photogenerated electrons and holes, as well as the expansion of the response spectrum range of the photocatalyst. By doping TiO\({}_{2}\) with chromium and vanadium ions, the excitation wavelength range of TiO\({}_{2}\) can be expanded to the visible light region (to the vicinity of 600 nm). However, the incorporation of metal ions sometimes becomes the recombination center of electrons and holes, affecting the photocatalytic effect. The type and concentration of doping ions are different, and the influence on the photocatalysis experiment also differs. Experimentally doped metal ions include transition metal ions and rare earth metal ions. Transition metal ions mainly include cobalt ions (Co\({}^{2+}\)), iron ions (Fe\({}^{3+}\)), and copper ions (Cu\({}^{2+}\)). Different metal ion doping shows different effects on the performance of a photocatalyst. Doping some specific metal ions can improve the photon efficiency, such as Fe\({}^{3+}\)[255], while doping some other metal ions, such as Cr\({}^{3+}\), is harmful [253]. [PERSON] et al. [276] studied more than 20 metal ion doped TiO\({}_{2}\) nanocrystals and found that doping 0.5 wt% of Fe\({}^{3+}\), Ru\({}^{2+}\), Mo\({}^{5+}\), Re\({}^{2+}\), Os\({}^{2+}\), V\({}^{5+}\), and Rh\({}^{2+}\) in the crystal lattice can increases the photocatalytic activity of photocatalyst. [PERSON] et al. [277] applied a simple hydrothermal method to prepare Fe\({}^{3+}\) doped TiO\({}_{2}\) nanotube array catalysts. The semiconductor catalyst prepared at a concentration of Fe\({}^{3+}\) of 1 mmol/L showed good catalytic performance under irradiation with a solar lamp. The time required for the Fe\({}^{3+}\) doped semiconductor catalyst to decompose 98.79% methyl blue is only 120 min. The reduction ratio of Cr\({}^{6+}\) by the Fe\({}^{3+}\)-doped semiconductor catalyst is as high as 83.79%, which is much higher than the reduction ratio of pure TiO\({}_{2}\) to Cr\({}^{6+}\) (39.6%). [PERSON] et al. [278] produced Cr\({}^{3+}\) doped TiO\({}_{2}\) nanoparticle photocatalysts by three preparation methods and found that the doping samples prepared by flame spraying method have higher catalytic performance than other preparation methods. The sample prepared by the flame spray method degraded chlorophenol under visible light, and the decomposition rate reached 61% in just 5 h. Nanoparticles prepared by the flame spray method have a high specific surface area and increase the active site of the complex, thereby improving its photocatalytic efficiency. [PERSON] et al. [279] used a co-precipitation method to prepare Mg, Zn, and Cr--three kinds of ion co-doped TiO\({}_{2}\) nano-catalysts--and used them to degrade Congo red dye. Experimental results showed that the MgZnCr-TiO\({}_{2}\) catalyst has good catalytic performance in the visible range. The MgZnCr-TiO\({}_{2}\) sample has recyclability and maintains a stable photocatalytic performance after five cycles. Rare earth elements were originally used as surface treatment agents for TiO\({}_{2}\) to improve the vividness of TiO\({}_{2}\). With the rapid development of photocatalysts, the role of rare earth elements becomes more important. Rare earth metal ions have a special electronic layer structure compared to transition metal ions. In addition to the benefits of doped transition metals, the doping of rare earth metal ions can also cause semiconductor lattice distortion and impurity defects, form traps for electron holes, and enhance the quantum effect of the semiconductor catalyst. [PERSON] et al. [280] analyzed the catalytic properties of Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Samarium (Sm), Europium (Eu), and Gadolinium (Gd) rare earth doped TiO\({}_{2}\).