## 2 Double Reductant Method
### Sodium Borohydride(NaBH\({}_{4}\)) and Ascorbic Acid (AsA)  
Aside from EG and DMF, NaBH\({}_{4}\) is also frequently used as a reductant to synthesize nanosized Ag colloids. Many groups have studied the reduction of Ag\({}^{+}\) ion in NaBH\({}_{4}\) which is usually used with some stabilizer, such as citrate and PVP. [PERSON] _et al_. [44] explored a new way to control the size and shape of Ag nanospheres by changing the rate of nucleation and growth in the reduction of Ag\({}^{+}\) ion by NaBH\({}_{4}\) together with trisodium citrate (Na\({}_{3}\)CA) and PVP. In the absence of Na\({}_{3}\)CA, nanospheres cannot be prepared because Ag nucleuses are immediately protected by PVP as soon as they form leading to the anisotropic growth of Ag nanoparticles due to selective adsorption of PVP on the (100) facets. When PVP was absent, the products were also nanospheres with larger size. The results indicate that the role of Na\({}_{3}\)CA is to promote the reduction of Ag\({}^{+}\) ion into nanospheres. On the other hand, PVP can promote nucleation and prevent the aggregation of nanoparticles. In our previous work [45], we prepared branched silver nanowires and nanomeshworks by using both NaBH\({}_{4}\) and Na\({}_{3}\)CA in the presence of PVP. Silver seeds were prepared firstly, and then exposed under light leading to aggregation and were thus welded together, so the final morphology was decided by the concentration of PVP. [PERSON] [46] also studied the important effect of citrate on the reduction of Ag\({}^{+}\) ions by NaBH\({}_{4}\). Stable Ag nanoclusters only appeared when citrate was added, otherwise reduction of Ag\({}^{+}\) ion by NaBH\({}_{4}\) under the conditions with citrate led to a precipitate at the bottom of solution. Citrate is a stabilizer as well as a reductant. In this regard, [PERSON] _et al_. [47] used NaBH\({}_{4}\) and Na\({}_{3}\)CA as a double reductant system to prepare Ag nanoplates via a multistage procedure. In the first stage, Ag nanoplates were prepared as seeds with NaBH\({}_{4}\) and Na\({}_{3}\)CA. Then these as-prepared seeds evolved into larger nanoplates with tunable sizes of 40 nm to 260 nm by adding different volumes of Na\({}_{3}\)CA solution. Each addition of Na\({}_{3}\)CA solution and seed solution was named one stage, the size of the nanoplate increased without change of shape, and the SPR peak shifted red. To get triangular nanoplates with satisfactory size distribution and yield, [PERSON] _et al_. [48] applied a novel double reductant method which consists of three steps. In the first step, silver nanospheres  
Figure 4: Growth mechanisms of silver nanoflags from silver nanorods in DMF (reprinted with permission from [43]. Copyright (2010) American Chemical Society). were prepared through reduction of AgNO\({}_{3}\) by NaBH\({}_{4}\) in the presence of Na\({}_{3}\)CA. In the second step, sodium dodecyl sulfate (SDS) was added as stabilizer into the as-prepared silver nanoparticle colloid and then citrate-stabilized silver nanospheres were converted into SDS-stabilized silver nanospheres. In the last step, silver nanoplates were formed under the reduction of SDS-stabilized silver nanospheres by citrate and aged in NaCl solution. When SDS-stabilized silver nanospheres aged in NaCl solution for one week, they began to transformed into nanoplates. Three weeks later, high-yield silver nanoplates with larger size were formed. In their further experiments, if only NaBH\({}_{4}\) was used as reductant in the presence of SDS, no silver nanoplates appeared, which indicated citrate is essential to reduce SDS-stabilized silver nanospheres in the third step. More recently, a method of using AsA to prepare unique silver nanostructures such as flower-like and string forms has been developed [49]. [PERSON] _et al_. [50] got silver dendrites when they reduced AgNO\({}_{3}\) by AsA in the presence of cetyltrimethylammonium bromide (CTAB) and SDS. [PERSON] _et al_.