## 3 Etching Technique
### Hydrogen Peroxide (H\({}_{2}\)O\({}_{2}\))  
Early in the 1950s, H\({}_{2}\)O\({}_{2}\) is used to etch pits in metals, which plays an important role of dislocations in determining or affecting the mechanical properties of crystalline materials [58]. The standard potential in the H\({}_{2}\)O\({}_{2}\)--water couple is dependent on the pH value of the solution. In acidic solutions:  
\[H_{2}O_{2}+2H^{+}+2\mathrm{e}^{-}\to 2H_{2}O\quad E^{0}=1.763V \tag{2}\]  
In alkaline solutions:  
\[H_{2}O_{2}+2\mathrm{e}^{-}\to 2 OH^{-}\quad E^{0}=0.867V \tag{3}\]  
Because the potentials are higher than that of \(\ A\mathrm{g}^{+}\)/\(\ A\mathrm{g}\) ( \(E^{0}=0.7996V\)) [59], H\({}_{2}\)O\({}_{2}\) can be used as an effective etchant to dissolve metallic silver. Very recently, etching techniques are used for controlling the shapes of Ag nanoparticles because some nanostructures are not easily prepared in high yield or mono-dispersed sizes. Among these various shapes, Ag nanoprisms and triangulars have attracted intense interest due to their unique optical properties and related applications. Generally, two main methods have been developed to prepare Ag nanoprisms, including the photochemical method and chemical reduction method. Compared with the photochemical method [60, 61, 62, 63], the chemical reduction method is considered a popular and a simple route, however, this method cannot provide relative uniform size distributions. [PERSON] and [PERSON] [64] reported a new chemical reduction route to prepare Ag nanoprisms which is a great breakthrough in the size control of silver nanostructures. In the process, in the absence of H\({}_{2}\)O\({}_{2}\), the products were nanospheres, which indicated that H\({}_{2}\)O\({}_{2}\) plays a critical role of Ag nanoprism formation. Sequentially, [PERSON] _et al_. [65] studied in detail the role of H\({}_{2}\)O\({}_{2}\) in the same reaction system. They proposed that H\({}_{2}\)O\({}_{2}\) can remove the relatively unstable nanoparticles in the nucleation stage and promote the formation of anisotropic structures, finally all metallic silver particles can be directly transformed into silver nanoplates regardless of size and shape, but if the concentration of H\({}_{2}\)O\({}_{2}\) was too high, the obtained nanoprisms also would be etched and disappear. To further study the mechanism, [PERSON] _et al_. [66] examined the relative etch rate of prism to sphere by monitoring the time-dependent SPR band of the mixture of spheres and prisms. Similarly, they used a flag type of Ag nanostructure which contained triangular plates and a pentagonal rod to examine the relative etch rate of prism to rod. The results were: \(\ V_{\mathrm{prism}}<V_{sphere}\) and \(\ V_{\mathrm{prism}}<V_{rod}\). If either H\({}_{2}\)O\({}_{2}\) or Na\({}_{3}\)CA is absent, the transformation from spheres to prisms will not happen and Na\({}_{3}\)CA must be added before H\({}_{2}\)O\({}_{2}\). The major difference between the method and [PERSON]'s is that the etching of metallic Ag nanostructures and reduction of Ag\({}^{+}\) ion occur at the same time in the latter reaction process, but the former argued that the first step is complete dissolution of Ag nanowire to Ag\(+\) ion and then reduction of Ag\({}^{+}\) ion to Ag\({}^{0}\). They also successfully prepared Ag nanoprisms from nanocubes and nanobipyramids. This essay made a great contribution to preparing high-yield Ag nanoprisms from different metallic Ag nanostructures. Figure 8 presents the growth mechanisms of silver nanoprisms from the mixture of spherical nanoparticles and prisms and nanorods, cubes, and bipyramids. Recently, metallic hollow nanostructures such as cages [67, 68, 69, 70] and frames [71, 72] which have bigger surface area have aroused intense interest because they can broadly tune localized surface plasmon resonance (LSPR) and exhibit excellent optical, electronic and catalytic properties. [PERSON] _et al_. [73] explored a new method based on H\({}_{2}\)O\({}_{2}\) etching to prepare Au nanoboxes.