## 3 Classification and Synthesis of Bimetallic Nanoparticles
### Simultaneous Methods
#### 3.3.3 Radiolytic Synthesis  
The radiation-induced generation of metal clusters has proven to be a powerful method in the synthesis of bimetallic nanoparticles. In particular cases of the radiolytic reduction of metal ions in aqueous solution, the reducing radicals produced during the radiolysis of the solvents are used to reduce metal ions [124]. The radiolytic approach can offer certain advantages because of the fine control over the rate of generation of the growing species afforded by the control over the dose delivered to the sample. However, the radiolytic approach can suffer from difficulty in controlling the shape of the nanoparticles. It has been found that \(\gamma\) irradiation of mixed solutions of the metal ions of two elements can yield perfectly ordered bimetallic clusters. The structure of the bimetallic nanoparticles depends on the irradiation dose rate because of the competition between the radiolytic reduction and the electron-transfer reaction from less noble metal atoms to other metal cations [124]. Alloy clusters are formed by radiolytic reduction at high dose rates because the reduction is sudden and faster than a possible inter-metal electron transfer. On the other hand, the radiation-induced reduction of mixed ions gives rise to bimetallic core/shell structures at low dose rates. For example, the production of AuAg bimetallic nanoparticles at different dose rates was studied by [PERSON] et al. [64]. These researchers found that the reduced silver atoms transfer electrons to gold ions at low dosage rates. When the AuIII ions are totally reduced, the reduction of silver ions occurs in the second step at the surface of the gold clusters to afford silver-coated gold core/shell nanoparticles. In contrast, bimetallic alloyed AgAu clusters are formed at high dosage rates. Other bimetallic structures generated by \(\gamma\) irradiation include Ag-Pt nanoparticles [99], Au-coated Ag nanoparticles [125], PdAu [126], and trimetallic PdAuAg particles [126].  
Similarly, [PERSON] et al. reported the synthesis of subclustered Au and Co bimetallic nanoparticles consisting of nanoclusters fabricated by pulsed laser processes [81]. Femtosecond laser fragmentation offers the possibility to fabricate non-equilibrium nanostructures due to the rapid quenching that occurs during the synthesis. For this type of process, the nanoparticles are reheated and broken down into nanoclusters, which subsequently coalesce and aggregate with their neighbors. These metastable structures can also be transformed into more stable core-shell states using pulsed laser annealing [81]. In a separate study, [PERSON] proposed a method for preparing submicrometer-sized spheres by selectively pulse-heating colloidal nanoparticles in solution [127]. Recently, these researchers reported the generation of AuCo alloy particles by following the same laser ablation-in-liquid technique and using an unfocused laser to irradiate a solution mixture of colloidal gold and cobalt oxides [82]. Furthermore, this technique was found to be applicable for other nanoparticles with combinations of metals that are not thermodynamically stable under equilibrium, such as AuFe and AuNi [82]. For example, [PERSON] et al. were able to overcome the thermodynamic limitations of the room-temperature synthesis of AuFe nanoparticles by using their laser synthesis in solution (LASiS) method, illustrated in Figure 8[78]. This environmentally-friendly, low-cost, laser-assisted preparation method shows promise as a synthetic method for the preparation of bioimaging agents [79].