## 3 Structural Design of Plasmon-Mediated Photocatalysts
### Extraction of HCs
#### 3.2.1 Surface and Interfacial Quality Engineering  
In quite a few works, it has been found that the abundant defects and interfacial lattice mismatches in heterostructures were difficult to thoroughly avoid, limited by the poor control in the process of chemical synthesis. Furthermore, these defects and mismatches usually served as electron capture bands that trapped the HCs when they were moving in an orientated way [87; 88; 89]. This process will decrease the amounts of electrons that are injected into the adjacent semiconductor, and their final utilization efficiency is further reduced. To optimize the channel for transferring HCs, it is necessary to  
Figure 6: The scheme of the pathway of light in (**a**) 3D-array Au-TiO\({}_{2}\) hollow structures and random distributions of Au-TiO\({}_{2}\) under visible light illumination (>420 nm), and (**b**) in single Au-TiO\({}_{2}\) hollow and TiO\({}_{2}\) hollow structures under simulated daylight. The comparison of the amounts of H\({}_{2}\) production in three typical structures under (**c**) simulated daylight and (**d**) visible light illumination (>420 nm). Reproduced with permission. [82] Copyright 2016, American Chemical Society. elaborately engineer the interfacial states by achieving high quality monocrystalline nanostructures (both for the metal and semiconductor) and smooth interfacial regions [90; 91]. [PERSON] et al. [92] proposed experimentally an aqueous cation exchange method for preparing Au@MS (M = Cd, Zn) with atomically organized interfaces (Figure 7a). A transient absorption spectroscopy measurement showed the strongest signals of these high quality structures compared to Au and Au@MS, based on conventional methods (Figure 7b). In such a process, the avoidance of non-conduction binding ligands and the mild interfacial transition made them a \"soft\" contact, where the interfacial defects were reduced dramatically by reorganizing the lattice structure; the calculated efficiency of the injection of HCs was raised to \(\sim\)48%. Similarly, [PERSON] et al. [93] compared the thickness of shell matters for the decomposition of MB by using this kind of method (Figure 7c). Additionally, they demonstrated that these core-shell type heterostructures with a thinner shell were efficient in degrading the MB due to the limited diffusion length of the HCs (Figure 7d). In addition, by constituting a chemical bond, the insulating organic ligands used for binding the metal and semiconductor is another hindrance for electron transfers [94; 95]. Therefore, a post-treatment process for efficiently removing non-conducting macromolecular materials, such as polyvinyl pyrrolidone (PVP), hexadecyl trimethyl ammonium bromide (CTAB), etc., is of considerable importance for the effective transfer and injection of HCs moving to the interface [96; 97; 98; 99; 100]. [PERSON] et al. [96] reported a ligand-exchange method combined with a mild thermal post-treatment for effectively removing insulating ligands (PVP) in Au-TiO\({}_{2}\) heterostructures (Figure 8a). In their work, the original PVP was replaced by mercaptopropionic acid (MPA) for their higher affinity to Au. Then, they removed MPA completely by low-temperature annealing (200 \({}^{\circ}\)C). The efficiency of dye decompositions was improved significantly due to the tight formation of the Schottky contact and the maintenance of the original morphology compared to conventional deposition-decomposition methods and samples without any treatment (Figure 8b). An alternative way, the solvent extraction procedures, was brought into focus as well. [PERSON] et al. [97] proposed a water extraction method to remove the excess  
Figure 7: The synthesis of high quality heterostructures. (**a**) The scheme of the method of aqueous cation exchange for Au@CdS heterostructures and (**b**) the transient absorption spectrum of three types of structures, Au35@CdS10 (the sizes of the Au core and CdS shell are 35 nm and 10 nm, respectively.