## 3 Classification and Synthesis of Bimetallic Nanoparticles  
### Classification of Bimetallic Nanoparticles  
It has been reported that heterogeneous bimetallic nanoparticles have more intriguing properties than the corresponding single component metal nanoparticles owing to their tunable chemical and physical properties [43]. The properties of bimetallic nanoparticles can be tuned by adjusting composition, atomic ordering, morphology, and size. Although bimetallic nanoparticles consist of only two different metals, there are still many types of possible structures. In terms of structure, bimetallic nanoparticles can be broadly classified into two types, as shown in Figure 4: mixed structures (Figure 4a,b) and segregated structures (Figure 4c-f). In terms of atomic ordering, the bimetallic nanoparticles can be further categorized into four types: alloy, intermetallic, subclusters, and core-shells [44]. Figure 3: Schematic illustration of a magnetization hysteresis loop (blue) which shows the change in magnetization as a function of an applied magnetic field. A non-hysteretic magnetic curve (red) is representative of superparamagnetic behavior. The nature of the atomic ordering in the two types of structures, with similar atomic ratios and compositions, plays an important role on the overall properties of the materials. The atomic ordering of mixed structures can be random (Figure 4a) or have an ordered configuration (Figure 4b). An example of a mixed structure with random arrangement is the alloyed structure, while an example of an ordered arrangement is the intermetallic structure. Alloyed nanoparticles are composed of nanocrystals in which the two metals are randomly mixed; while in intermetallic structures, the two metals are mixed in an ordered way. Solid solution structures occur when one metal dissolves in a different metal in the solid state. Solid solution structures can be further divided into substitutional and interstitial. Substitutional structures form when the size of the two metal atoms is similar, allowing metal atoms of one material to replace metal atoms of the second material in regular lattice sites. Whereas, if there is a dramatic size difference between the metal atoms, the smaller atom will position itself between the larger atoms to form an interstitial structure [46]. On the other hand, segregated structures are composed of two separate components with a shared interface, subcluster structure (Figure 4c), or a metal core surrounded with a second metal, core-shell structure (Figure 4d) [47]. Subcluster structures, in which two metals are exposed to the nanoparticle's environment, have also been defined as having a Janus configuration [48]. Unlike mixed structures, segregated structures are achieved in multi-step reactions in which the second metal is added once the first metal has formed an initial structure. Examples of segregated structures include the core-shell structure (Figure 4d), the multi-shell core-shell structure, in which the shells are in an alternating arrangement, similar to onion rings (Figure 4e), or the multiple small cores coated by a single shell (Figure 4f) [45,49,50]. Various synthetic strategies used for the formation of all types of atomically ordered structures are discussed in the following section. Similar to organic chemical reactions, the concept of thermodynamic and kinetic control can be applied to the synthesis of metal nanoparticles [51]. For example, thermodynamic and kinetic control for the shape-controlled synthesis of nanoparticles is illustrated in Figure 5a [51]. The thermodynamically most stable product (i.e., the structure with the lowest Gibbs free energy, shape II) is considered the thermodynamic product, while the structure formed from the reaction pathway with the lowest activation energy (E\({}_{\text{a}}\)) is considered the kinetic product. Changing the reaction conditions, such as the temperature, can lead to thermodynamic or kinetic control.