## 3 Structural Design of Plasmon-Mediated Photocatalysts
### Generating Efficiency Improvement of HCs
#### 3.1.3 Hollow Heterostructure Designing  
In conventional PMH, both the plasmonic metal and semiconductor are usually solid, and the chemical reactions only take place on the outside surface. In such typical structures, most of the photons are missed due to the small interactional area between the light and heterostructures. In addition, the effects of scatter and reflections brought by heterostructures themselves also inevitably waste photons. The proposed hollow structures can solve this problem, and they have been attracting much attention for their high utilization of photons [80; 81]. [PERSON] et al. [82] reported an Au@TiO\({}_{2}\) hollow heterostructure and proposed that the Au core in hollow TiO\({}_{2}\) served as an effective scattering center, increasing the degrees of reflection of the light and optical path (Figure 6a,b). Therefore, the scattered or reflected photons are re-utilized by the catalyst and promote the generation of HCs. This special effect is called the slow-photon effect, with which the absorption of photons are improved dramatically at the absorption range of the catalyst [83]. Furthermore, they also found that the ordered 3D arrays  
Figure 5: The design of the plasmonic cavity for broad band photons trapping. (**a**) The schematic of Au NPs-TiO\({}_{2}\) layers-Au films “sandwich structures” (the inset is the transportation of incident photons between Au NPs and Au films). (**b**) The comparison of the absorption spectrum in absorbers-spacer-mirror (ASM) structures with control groups. (**a,b**) Reproduced with permission. [74] Copyright 2016, American Chemical Society. (**c**) The band diagram and flow of HCs in Au nanorods-TiO\({}_{2}\) dumbbells/Au films under visible light illumination. (**d**) The comparison of rates of decomposing MB in Au/TiO\({}_{2}\)/ITO and Au/TiO\({}_{2}\)/ITO under different wavelengths of the incident light, respectively. (**c,d**) Reproduced with permission. [75] Copyright 2018, Wiley-VCH. (**e**) The scheme of surface plasmon polaritons (SPPs)-mediated broadband light absorption structures, which was built up by patterned Ag gratings, TiO\({}_{2}\) layers and Al films (from top to bottom). (**f**) The total absorption of rectangular gratings (RG) and trapezoidal gratings (TGs) on top, respectively. (**e,f**) Reproduced with permission. [78] Copyright 2018, Springer.  
constructed by this heterostructure are much more efficient for the H\({}_{2}\) generation when compared to random ones (Figure 6c,d). The enhancement of the activity in ordered 3D arrayed structures was ascribed as being \"cavity\" effects that were constructed by the rational arrangement of the unit of Au-TiO\({}_{2}\). In this special cavity, light was regulated by building up oriented photons transporting channels, in which the recombination of HCs that occurred in adjoining units is suppressed via the migration of HCs [84]. Apart from the re-utilization of photons, the fast separation of HCs is another superior advantage; the generated HCs are separated and transported to the inner and outer surface simultaneously [85]. In this way, the recombination of HCs resulting from the long transporting distance can be avoided, which thus speeds up the reaction process. Besides, the process of photocatalysis only takes place at the surface of the catalysts, which entails that the enhancement of the activity with a higher specific surface area [86]. Therefore, the addition of an inside surface from hollow structures means that there is a higher opportunity for active atoms to come into contact with more reactants, thus improving the efficiency of catalytic reactions.