## 3 Results and Discussion
560-570 nm was observed; [PERSON] _et al._ reported that decreased absorbance accompanied the oxidation of cob(I)alamin to cob(II)alamin [29]. Cob(I)alamin is one of the intermediates of vitamin B12 produced by the reduction of Co(III)-based cobalamins or cob(II)alamin [33]. Based on this observation, it may indicate that the released electron from the cobalamin oxidation can be used for the reduction of Pd ions. In order to investigate morphological properties of the synthesized Ag, Au, and Pd samples, SEM and TEM analyses were performed. Figures 4-6 show the SEM and TEM images of each sample of Ag, Au, and Pd. As shown in Figures 4a and4b, for Ag samples, large aggregates with irregular shapes and diameters ranging from 70 to 600 nm were synthesized using vitamin B12. TEM images show the formation of nanoparticles and large aggregates synthesized using vitamin B12. Nanoparticles with diameters less than 30 nm were observed in the sample treated with MW irradiation for 6 min (see red rectangles in Figure 4d). Figure 5a-d for Au samples reveal the formation of particles of irregular shapes and sizes; small nanoparticles with an average diameter of \(40\pm 11.7\) nm were obtained under MW irradiation for 3 min, whereas much larger particles with a size over 500 nm were formed after 6 min of irradiation. Spherical-, triangular-, and decahedron-shaped microparticles were observed. The TEM images reaffirmed the formation of nanoparticles and microparticles for different durations of MW irradiation. For the MW-heated 6 min sample, nanoparticles of an average diameter of \(9.9\pm 2.2\) nm formed with the irregular, large micro-sized particles (see red rectangle in Figure 4d); the increase in particle size is presumably due to longer MW exposure time and crystal growth of the particles. Figure 4: SEM (**a**,**b**), and TEM (**c**,**d**) images of synthesized Ag; samples with the present method: (**a**,**c**) for Ag sample MW-heated for 3 min, and (**b**,**d**) for Ag sample irradiated for 6 min. The SEM and TEM images of synthesized Pd samples are shown in Figure 6a-d. In keeping with the general trend, larger aggregates of an average size of 220 \(\pm\) 45 nm were formed for the samples treated under longer durations of MW irradiation (_i.e._, 6 min). In contrast, nanoparticles with an average diameter of 40.2 \(\pm\) 7.3 nm were discerned in the sample irradiated by MW for 3 min (Figure 6a,c). Moreover, the TEM images show that the sample irradiated by MW for 6 min contained primary particles with two different average diameters of 43.9 \(\pm\) 7.1 and 6.6 \(\pm\) 2.1 nm (Figure 6b,d). As shown in Figures 4 and 5, micro-sized and nano-sized particles coexist, especially for Ag and Au samples irradiated with MW for 6 min. ; the growth of particles is possibly due to a longer reaction time since no surfactants were used to control their structures. In order to investigate the crystal phase in detail, HR-TEM analysis was performed on the synthesized samples. For the Ag sample, the lattice spacing of 0.145 nm was measured, which corresponds to the (220) plane of Ag (see Figure 7a) [34,35]. As shown in Figure 7b, the measured lattice spacing of 0.236 nm, corresponding to the (111) plane of Au, indicated that Au cations were well reduced via this method [36]. Additionally, as shown in Figure 7c, the lattice spacing of 0.137 nm was in good agreement with the lattice spacing of the (220) plane of Pd. [37] These HR-TEM analysis results corroborated the XRD analysis, thus reaffirming the prowess of this vitamin B\({}_{12}\) synthesis method to produce noble nanometals under MW irradiation conditions. Based on the results, we postulate a plausible mechanism for the synthesis of metal nanoparticles. Metal ions could be reduced using vitamin B\({}_{12}\) due to the reduction and oxidation of cobalamin from cob(I)alamin to cob(II)alamin under MW irradiation.