## 3 Structural Design of Plasmon-Mediated Photocatalysts
### Generating Efficiency Improvement of HCs
#### 3.1.1 Design of Structures with \"Hot Spots\"
In the Au nanorod, the blue dot and red dot correspond to the EM field paralleled and perpendicular to the nanorods, respectively. In the Au nanostar, the different spike lengths, combined with a core of 25 nm, were calculated. (**c**) The theory and experiment results for the rates of the HCs’ generation in SiO\({}_{2}\) loaded Au@TiO\({}_{2}\). (**a–c**) Reproduced with permission. [63] Copyright 2016, American Chemical Society. (**d**) The morphology of the Au nanorods@TiO\({}_{2}\) dumbbells presented by transmission electron microscope (TEM). (**e**) The comparison of the rates of degradation of methyl blue (MB) in different Au rods/TiO\({}_{2}\) heterostructures. (**d,e**) Reproduced with permission. [64] Copyright 2016, American Chemical Society. (**f**) The scheme of Au nanocubes@Cu\({}_{2}\)O heterostructures. (**g**) The amounts of H\({}_{2}\) generation in four kinds of Au/Cu\({}_{2}\)O heterostructures. (**f,g**) Reproduced with permission. [65] Copyright 2016, American Chemical Society. are localized at the gap between the two or more units, and this globe field enhancement comes from coupling effects in the interaction of the modes of SPs, consequently resulting in a large absorption cross section [66; 67; 68]. It has been reported that the amplitudes of such EM enhancements are mainly related to the space of the gap and numbers of plasmonic metallic units. [PERSON] et al. [69] calculated the characters of optical absorption in metallic nanodimers with different gap spaces. Furthermore, they concluded that the absorption cross section could be increased with narrower gaps (Figure 4a), which then promoted higher rates for generating HCs (Figure 4b). Similarly, [PERSON] et al. [70] loaded Au rods-TiO\({}_{2}\) on the thermo-responsive polymer and controlled such gap \"hot spots\" structures dynamically by shrinking or expanding this substrate, mediated by variations of the temperature (Figure 4c). In an experiment of dye decomposition, they demonstrated the improvement of the rates for degrading RhB, which resulted from \"hot spots\" effects, in which the temperature, when increased to 40 \({}^{\circ}\)C, was more effective than in the case of 25 \({}^{\circ}\)C (red region for 40 \({}^{\circ}\)C and blue region for 25 \({}^{\circ}\)C in Figure 4d). [PERSON] et al. [71] investigated the factor of numbers of metallic units in Au@MOS\({}_{2}\) structures; both the results in the simulation and the absorption spectrum have shown that more units would bring the localization and enhancement of the intense EM field, followed by extended ranges of photons trapping (Figure 4e,f). Based on this, near 2.4-fold improvements were realized in the photocatalytic hydrogen production by embedding Au multimers in MoS\({}_{2}\), compared to pure MoS\({}_{2}\) (Figure 4g). Figure 4: “Gap hot spots” designed for an efficient photocatalytic process. (**a**) Calculated optical absorption cross section in a single Au sphere and Au nanodimers with different gap spaces. (**b**) The theoretical relationships between the rates of generation of HCs and the gap space. (**a**, **b**) Reproduced  
with permission. [69] Copyright 2016, American Chemical Society. (**c**) The scheme of \"hot spots\" structures built up by Au rods-TiO\({}_{2}\)/thermoresponse polymer and (**d**) the rates of degradation of RhB under 25 \({}^{\circ}\)C (blue regions) and 40 \({}^{\circ}\)C (red regions). (**c**,**d**) Reproduced with permission. [70] Copyright 2018, Wiley-VCH. (**e**) The absorption spectrum of Au@MOS\({}_{2}\) with different numbers of the Au core and (**f**) the corresponding distributions of the EM fields under a certain incident light (630 nm). Finally, (**g**) the rates of H\({}_{2}\) generation in these structures. (**e**-**g**) Reproduced with permission. [71] Copyright 2016, Elsevier Ltd.