# Recent Advances and Applications of Semiconductor Photocatalytic Technology
## 5 Methods for Improving Photocatalytic Efficiency
### Surface Dye Photosensitization  
Surface dye photosensitization refers to the process of using a photosensitizer to broaden the wavelength response range of the photochemical reaction. Photosensitizers are usually some inorganic or organic chromophores. Photosensitizers have strong absorption under visible light and can extend the spectral response range of the photocatalytic system to the visible region. Therefore, the sensitized semiconductor catalyst can be excited with visible light of a much lower energy than ultraviolet light, which can broaden the application of a photocatalyst in the visible light region. Previous work on the photosensitization of semiconductor dyes mainly involved solar cells and photocatalytic hydrogen production, and in recent years the use of dye-sensitized semiconductor catalysts for decomposing organic pollutants has attracted the intense attention of researchers. Surface dye photosensitization has become one of the common methods for modifying semiconductor catalysts. Common photosensizers include various organic dyes and transition metal complexes such as Ru and Pt chloride. [PERSON] et al. [324] used a modified hydrothermal reflux composite preparation method to prepare porphyrin-sensitized TiO\({}_{2}\)/reduced graphene oxide (TiO\({}_{2}\)/RGO) composite nanorod catalysts. Under visible light irradiation, the decomposition rate of methylene blue by TiO\({}_{2}\)/RGO composite nanorods is very high at 92%, which is 4.3 times the decomposition rate of pure TiO\({}_{2}\) nanorods to methylene blue. [PERSON] et al. [325] used three asymmetric sensitizers 5-(p-Nitrophenyl)-10,15,20-triphenylporphyrin, Cu(II)-porphyrin, and Zn(II)-porphyrin for the sensitization modification of polycrystalline TiO\({}_{2}\) powder. The experimental results showed that the degradation rate of Congo red by the Zn(II)-porphyrin sensitized TiO\({}_{2}\) semiconductor catalyst is higher than that of other photocatalysts. [PERSON] et al. [326] attached the carboxylate porphyrins of Cu, Co, and Zn to the surface of TiO\({}_{2}\) by a hydrothermal method. In the degradation study of nitrophenol, the three sensitized semiconductor catalysts had a higher catalytic activity than pure TiO\({}_{2}\). Among the three sensitized semiconductor catalysts, Cu-porphyrin-TiO\({}_{2}\) has the best decomposition ability of nitrophenol. This is because Cu ions are excellent electron acceptors, and it is easier to attract electrons to reach a stable state, thereby achievingthe effect of separating electron-hole pairs. [PERSON] et al. [327] used a two-step method of hydrothermal and reflux to prepare a Cu(II)-tetrakis(4-carboxyphenyl)porphyrin and RGO co-sensitized TiO\({}_{2}\) nanorod photocatalyst. After irradiation for 120 min under visible light, the decomposition rate of methylene blue by the composite TiO\({}_{2}\) photocatalyst was 95%, which was five times that of pure TiO\({}_{2}\) to methylene blue. Furthermore, the composite TiO\({}_{2}\) photocatalyst still maintained stable catalytic performance after being used six times in photocatalyst experiments. [PERSON] et al. [328] reported the use of eosin dye to photosensize TiO\({}_{2}\) and successfully reduced the band gap of a TiO\({}_{2}\) semiconductor by 1 eV. After sensitization, the catalyst showed a strong photo response in the visible region. Eosin dyes can inject excited electrons into the TiO\({}_{2}\) conduction band to achieve the same function as photogenerated electrons. [PERSON] et al. [329] photosensitized Co-TiO\({}_{2}\) nano-doped particles with a metal-free complex-free phthalocyanine dye derivative by the sol-gel method. The prepared semiconductor catalyst exhibited an obviously red shift in the absorption wavelength, a shift which improved the application of the catalyst in the visible region.