## 2 Double Reductant Method
### Sodium Borohydride(NaBH\({}_{4}\)) and Ascorbic Acid (AsA)
[51] also found that hyperbranched silver nanostructures can be obtained by reduction of AgNO\({}_{3}\) by AsA with Na\({}_{3}\)CA. Based on these researches of hyperbranched silver nanostructures mentioned above, [PERSON] _et al_. [52] explored the growth mechanism of hyperbranched silver nanostructures in AsA. In their opinion, as Figure 5a shows, multifaceted particles with bulbous tips were formed from the reduction of AgNO\({}_{3}\) by AsA in the initial stage of reaction, which are easy to aggregate in AsA [53]. After 20-30 s, silver atoms were deposited on the bulbous ends of the seeds resulting in the growth of the branched particles (Figure 5b. Figure 5c,d illustrate that with longer reaction time, the size continues to increase and secondary branches began to grow from the first branches that originate from the seeds. Moreover, tips can be observed at the ends of branches. In addition to AsA which acts as a key factor in the formation of branched structures, stabilizers such as starch [54] and PEO-PPO-PEO tri-block copolymer [55] also play an important role in controlling the morphology of silver nanostructures. Besides, a method of using AsA to prepare a series of silver nanostructures by reducing AgCl in aqueous solution has been developed by [PERSON] _et al_. [56]. Their key idea for growing different morphologies silver nanoparticles is to adjust amount of sodium hydroxide (NaOH). Because the reduction power of AsA is related to pH which can be varied by adding NaOH, silver nanoparticles seeds of different sizes were prepared firstly in the presence of AsA and NaOH, and then AsA/AgCl/PVP/NaOH solution system was added to control the shape. Taking 50 nm silver seeds as an example, Figure 6 shows FE-SEM images of silver nanostructures obtained from as-prepared seeds in AsA with different dosages of NaOH. Furthermore, they follow the formula (1) to predict the size of the product:  
\[\mathrm{D}=\mathrm{d}\times\sqrt[3]{\frac{N+n}{n}} \tag{1}\]  
In the formula, D is the average size of product, d is the average size of seeds, and N and n are the molar amount of AgCl and seeds respectively. They fixed the amount of AgCl, and used 50 nm silver nanoparticles as seeds, so in theory if they want obtain the products with 100, 150, 200, 250 nm, the (N+n)/n should be 8, 27, 64, and 125 nm respectively. As shown in Figure 7, the experiment results are consistent with the prediction. Figure 6: Inhibition of the anisotropic growth of Ag crystals: (**a**) sketch of the twinned decahedron, rod- and wire-shaped crystals; (**b**) FE-SEM image, indicating the adsorption of PVP on the surface of a Ag particle; (**c**) TEM image of as-prepared 50 nm Ag particles; (**d–h**) HR-TEM images of as-prepared 50 nm Ag particles for the particles with the shape of decahedron, icosahedron, quasi-sphere, cube, and hexagon, respectively; insets are sketches of the corresponding structures (reprinted with permission from [56]. Copyright (2010) American Chemical Society). Figure 5: SEM images of the silver particles sampled at different reaction times (**A**) 3–8 s; (**B**) 20–30 s; (**C**) 1 min; and (**D**) 5 min. Arrows indicate examples of tip splits (reprinted with permission from [52]. Copyright (2008) American Chemical Society). Above all, we can get a conclusion which is that the products tend to be nanospheres of small sizes in NaBH\({}_{4}\) which are usually used as seeds for further growth of silver nanocrystals, while silver nanoparticles are easy to aggregate together in AsA. [PERSON] [57] presented a two-step method to produce silver nanodiscs and triangular nanoplates with both NaBH\({}_{4}\) and AsA as double reductants. They prepared Ag nanoparticles in NaBH\({}_{4}\) in the first step. When these nanoparticles used as seeds with different amounts were added into AgNO\({}_{3}\) solution in AsA, silver nanodiscs and triangular nanoplates formed, respectively. A high yield of triangular nanoplates can be obtained in lower amount of seed solution which provides a new path to synthesize triangular nanoplates.