## 2 Double Reductant Method
### N,N-dimethylformamide (DMF) and EG  
DMF is a well-known organic solvent as well as an active reductant under suitable condition which has been demonstrated [31]. [PERSON] \(\mathbf{\hat{m}}\)'s group first employed DMF to reduce AgNO\({}_{3}\) for the preparation of silver nanostructures which paved a new way for shape control [32]. In their later works,they successfully synthesized nanospheres [33], nanoprisms [34,35] and nanowires [36] via reduction of AgNO\({}_{3}\) by DMF in the presence of PVP. In addition, [PERSON] _et al_. [37] prepared silver decahedrons in high yield with PVP as stabilizer in DMF. [PERSON] _et al_. [38] provided new information on the growth of decahedrons and icosahedrons in DMF through a stepwise route. [PERSON] _et al_. [39] realized the finely tuned size of nanoplates from 20 to 50 nm by varying the molar ratio of PVP/DMF. In DMF the main products are triangular and hexagonal plates, decahedra and icosahedra having (111) facets, as shown in Figure 1, while in EG the typical products are cubes, right-triangular, bipyramids and pentagonal rods and wires [9, 10, 11,14, 15, 16, 20, 40, 41] having (100) facets, as shown in Figure 2.  
Figure 1: SEM and TEM images of silver nanoparticles with different morphologies, prepared by reduction of Ag\({}^{+}\) ions by DMF, in the presence of PVP: (**A**) decahedrons (reprinted with permission from [37]. Copyright (2006), Elsevier); (**B**) prisms (reprinted with permission from [35]. Copyright (2009) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany). Figure 2: Electron microscopy images of single-crystal Ag nanocrystals: (**A**) nanocubes prepared in ethylene glycol with PVP as a capping agent; (**B**) nanobars prepared in ethylene glycol in the presence of PVP and Br\({}^{-}\); (**C**) nanowires prepared in ethylene glycol in the presence of PVP; (**D**) bipyramids prepared in ethylene glycol in the presence of PVP. (reprinted with permission from [41]. Copyright (2007) American Chemical Society). Based on this, [PERSON] _et al._[42] opened a new way to synthesize octahedral and triangular having (111) facets which were hard to obtain via previous methods. The production process was to prepare cubic and right bipyramids having (100) facets by reducing AgNO\({}_{3}\) in EG first, then the shape of these nanoparticles were transformed into octahedral and triangular shapes in DMF. The shape evolution from cubic and right bipyramidal to octahedral and triangular is assigned to the different facets of Ag nanocrystals in EG and DMF. Sequently, they fabricated Ag nanoflags using a similar two-step method by preparing Ag nanorods as seeds in EG and growing nanoflags in DMF [43]. The following [AgNO\({}_{3}\)]\({}_{1}\) and [AgNO\({}_{3}\)]\({}_{2}\) stand for the concentration of AgNO\({}_{3}\) in the first and second step respectively. In the first step silver nanorod as shown Figure 3a was prepared by reduction of AgNO\({}_{3}\) in EG in the presence of PVP, and then was dispersed in DMF as a seed solution. In the second step, seed solution was added into a DMF solution containing AgNO\({}_{3}\) and PVP. With the increase of the ratio of [AgNO\({}_{3}\)]\({}_{2}\)/[AgNO\({}_{3}\)]\({}_{1}\), different structures were observed (Figure 3b-f). They explained the growth mechanism of silver nanoflags from nanorods in detail as shown in Figure 4. In the second step, a trapezoid plate formed from the side surface of silver nanorods, and then grew into a triangular twin plate. Next the triangular flags transformed into tetrahedral flags via two routes. As route 1a in Figure 4 shows, another triangular flag formed in another side of the silver nanorod, and then grew to the other one until the two triangular flags connected together leading to tetrahedral flags. The other route (route 1b in Figure 4) is that two triangular pyramidal structures grew from the former triangular flags and finally evolved into tetrahedral flags.