## 3 Classification and Synthesis of Bimetallic Nanoparticles
### Simultaneous Methods
#### 3.3.4 Sonochemical Synthesis  
The sonochemical synthesis of colloidal particles involves a rapid reaction rate and the ability to form unusually small metal particles. When solutions are exposed to strong ultrasound, micro bubbles in the solution implusively collapse and grow via acoustic fields where high-temperatures and high-pressures are produced, an effect known as acoustic cavitation [97]. The temperature generated through this method is sufficiently high to decompose molecules in the bubbles. In addition, sonication of a liquid medium also leads to the generation of oxidizing and reducing radicals [128]. These radicals play important roles in the synthesis of colloidal particles. Particularly for aqueous systems, primary radicals are generated within the bubbles through sonication, which consequently generate secondary radicals through the reaction with the bulk solution. A more detailed discussion of the sonochemical process to produce nanomaterials is found in the literature [129]  
Sonochemical methods have been used for the synthesis of bimetallic nanoparticles. [PERSON] et al. have applied the sonochemical method for the preparation of bimetallic AuPd nanoparticles with a narrow size distribution [59; 86]. The particles were obtained by the simultaneous reductions of Au(III) and Pd(II) ions by ultrasonic treatment of an aqueous solution of sodium tetrachlorourate(III) dihydrate and sodium tetrachloropalladate(II) in the presence of sodium dodecyl  
Figure 8: Two-step synthesis of PEG–AuFe NPs. (**a**) LASiS was used to synthesize AuFeNPs in ethanol from a bulk Au\({}_{72}\)Fe\({}_{28}\) alloy. (**b**) EDTA and thiolated PEG was used to obtain water stable PEG–AuFe NPs. TEM images of (**c**) the nanoparticles obtained after LASiS and (**d**) after coating with the thiolated PEG. Reproduced with permission from ref. [78]. Copyright 2013 Royal Society of Chemistry.  
sulfate. Subsequent studies found that these bimetallic nanoparticles showed higher activity for the hydrogenation of 4-pentenoic acid than an analogous mixture of monometallic nanoparticles. In separate studies, [PERSON] et al. prepared bimetallic AuPd nanoparticles using sonochemical reduction and a sol-gel processes at a relatively low temperature, less than 200 \({}^{\circ}\)C [87]. Separately, [PERSON] et al. reported the sonochemical preparation of bimetallic CoCu nanoparticles in aqueous solution [130]. The synthesis method involved ultrasonic treatment of a hydrazine solution containing a mixture of copper chloride and cobalt chloride. The resultant bimetallic CoCu nanoparticles were shown to have an average diameter of 50 nm and a core-shell structure in which a copper core was surrounded by a thin cobalt shell. [PERSON] et al. deposited FePt nanoparticles on silica microspheres modified with polyelectrolytes by a sonochemical process, using iron and platinum acetylacetonates as metal precursors [108]. Moreover, after the nucleation of FePt nanoparticles onto silica microspheres, they were coated with ZnS. The ZnS/FePt/SiO\({}_{2}\) nanospheres displayed both luminescent and magnetic properties after annealing, which renders them promising candidates for photocatalytic applications [108].