## 3 Classification and Synthesis of Bimetallic Nanoparticles
### Synthesis of Bimetallic Nanoparticles  
There are two general approaches to the synthesis of metal nanomaterials: top-down and bottom-up. The top-down approach is based on a solid-state physics strategy used to make nanostructures. This method begins with macroscopic-sized materials that are then made into structures having nanoscale sizes [53]. The top-down approach presents challenges in producing large quantities of uniformly shaped particles; however, to circumvent these issues, a bottom-up approach can be used. The bottom-up approach relies on synthetic pathways to prepare the nanomaterials of interest. In the bottom-up method, nanostructures are formed from a buildup of atoms found in solution or in the gas phase. Wet chemical syntheses, especially the reduction of metal salts, are an example of a bottom-up method that is reliable and frequently used [54]. The formation of metal nanoparticles by wet chemical methods can be divided into two main pathways: (1) the reduction of metal ions or (2) the thermal decomposition of metal precursors, as illustrated in Figure 6. Both initial pathways then lead to the formation of atoms, which will aggregate into clusters and grow to form the desired nanostructures [22]. Figure 5: Schematic illustrations of thermodynamic and kinetic control for (**a**) two reactions, and (**b**) a series of reactions that involve product formation. Reproduced with permission from ref. [51]. Copyright 2015 American Chemical Society. The preparation methods reported thus far can be divided into two categories: simultaneous and successive [55]. The simultaneous methods involve precursors of the two metals that are present in the same reaction system with or without protective agents; examples include chemical synthesis by co-reduction (citrate reduction [56], alcohol reduction [57], and polyol process [58]), sonochemical synthesis [59,60], thermal decomposition [61,62], microwave irradiation [63], and radiolytic synthesis [64]. The successive methods involve reducing metal ions over the surface of other nuclei followed by particle growth over time. Mild reducing agents used in these types of methods include formaldehyde [65], hydroxylamine hydrochloride [66], sodium citrate [67], and ascorbic acid [68]. As highlighted in the following sections and Table 1, certain methods are better suited and more commonly used for the synthesis of certain nanomaterials (i.e., most noble metal bimetallic nanoparticles use reduction methods, while most transition metal bimetallic nanoparticles are synthesized using thermal methods). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Synthetic Method** & **Size** & **Reference** \\ \hline \multirow{8}{*}{AuAg} & Polyol reduction & 50 nm & [69] \\  & Galvanic replacement & 10.2 nm & [70] \\ \cline{2-3}  & Co-reduction & 5 nm & [71] \\ \cline{2-3}  & Chemical reduction & 32 and 45 nm & [72] \\ \cline{2-3}  & Radiolysis & 1.5–12 nm & [64] \\ \cline{2-3}  & Seed-mediated growth & 10–16 nm & [73] \\ \cline{2-3}  & Seed-mediated polyol process & 52 nm & [74] \\ \cline{2-3}  & In situ co-reduction method & 2.6 nm & [75] \\ \cline{2-3}  & Successive ionic layer absorption and reaction (SILAR) & 10–20 nm & [76] \\ \hline \multirow{2}{*}{AuFe} & Chemical reduction & 15–30 nm & [61,77] \\ \cline{2-3}  & Laser ablation of a bulk alloy & 30–60 nm & [78,79] \\ \hline \multirow{4}{*}{AuCo} & Chemical reduction & 3–5 nm & [80] \\ \cline{2-3}  & Chemical reduction & 14.2 nm & [26] \\ \cline{2-3}  & & 15–30 nm & [61] \\ \cline{2-3}  & Pulsed laser processes & 11 nm, 230 nm & [81,82] \\ \hline \multirow{2}{*}{AuNi} & Chemical reduction & 15–30 nm & [61] \\ \cline{2-3}  & Electrodeposition & 100 nm and 200 nm & [83] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Methods used for the synthesis of bimetallic nanoparticles. Figure 6: Reduction pathway (**a**) and decomposition pathway (**b**) for the formation of metal nanoparticles by chemical methods.