## 3. Sacrificial Anode CP
### Photoanodes  
In 1995, [PERSON] et al. [31] used the sol-gel method to coat a pure copper sheet with a TiO\({}_{2}\) film and found that the potential of TiO\({}_{2}\)/Cu in 0.3% NaCl solution under light was lower than that of the bare copper electrode, and the photoelectric effect of TiO\({}_{2}\) effectively reduced the potential of the copper sheet and provided CP to inhibit corrosion; thus, the concept of photoelectrochemical CP was proposed. The theoretical basis of photochemical CP is the photoelectric effect of semiconductor materials (mostly \(n\)-type semiconductor materials). The semiconductor energy band is usually composed of a low-energy valence band (VB) filled with electrons and an empty high-energy conduction band (CB). There is a region between the valence band and the conduction band called the forbidden band; the size of the region is called the forbidden bandwidth (\(Eg\)), as shown in Figure 1a. The general \(Eg\) of semiconductor material is less than 3 eV when the surface is irradiated with a suitable wavelength of light, that is when the light energy \(E=\text{hv}\geq Eg\). During semiconductor light absorption, VB electrons leap to the CB conduction band to produce negatively charged highly active electrons (e\(-\)). In the positively charged holes left by the movement of the VB electrons (h+) [32], when the n-type semiconductor material and the coupling of metal materials have a suitable potential match, the photogenerated electrons generated on the semiconductor material can be transferred to the coupled metal material. In this way, the semiconductor material plays the role of a sacrificial anode to provide electrons for the coupled metal, which is macroscopically expressed as a negative shift of the metal potential, thus playing the role of CP (Figure 1) [33]. Semiconductor materials are the core of photoelectric CP. The chemical materials used are mainly wideband semiconductor materials such as TiO\({}_{2}\), SrTiO\({}_{3}\)[34], ZnO, etc. TiO\({}_{2}\) is the most widely studied and applied photoelectric CP material, but a single TiO\({}_{2}\), because of its ECB close to \(-0.29\) V (vs. NHE), can only provide photoelectric CP for self-corrosion potential. It is more useful for some stainless steel and copper metals to provide photoelectrochemical CP; however, this CP effect is difficult to apply to some widely used metals (such as carbon steel and weathering steel) [35]. The single semiconductor photoanode made of a single semiconductor material often has weak light absorption performance in photocathode protection applications. It can only absorb ultraviolet light and a small amount of visible light (for example, SrTiO\({}_{3}\) can only be excited by ultraviolet light), and the disadvantages involving lack of effective separation and tendency of combination pathway of the electron-hole make its photoelectrochemical performance not ideal, and its photocathode protection capacity is too dependent on light [36]; however, through the modification of single semiconductor material, compounding can improve its photoelectrochemical performance [37]. [PERSON] et al. [38] coupled SnIn\({}_{4}\)S\({}_{8}\)/TiO\({}_{2}\) composite photoanode with Q253 carbon steel and found that the potential of Q235 carbon steel in 3.5 wt.% NaCl solution could reach \(-0.45\) V (v.s. SCE), and SnIn\({}_{4}\)S\({}_{8}\)/TiO\({}_{2}\) could provide efficient CP for Q235 carbon steel compared with pure TiO\({}_{2}\). In 2002, [PERSON] [39] found that SrTiO\({}_{3}\) doped with Sb and Cr showed strong absorption in the visible region, with \(Eg\) down to 2.4 eV. [PERSON] et al. [40] found that the M-S curves of the nano-TiO\({}_{2}\) layer showed a positive slope (n-type nature), and the Co(OH)\({}_{2}\) layer showed a negative slope (p-type nature) with positive and negative slopes, indicating the coexistence of n-type and p-type semiconductor properties.