Understanding the Oxidation Behavior of Fe/Ni/Cr and Fe/Cr/Ni Core/Alloy Nanoparticles

Abstract

Values related to thin film oxidation for Fe, Cr, and Ni (Table S1) and for the bulk diffusion couples shown in Figure 6 (Table S2-Table S6) were taken from the literature. Note that in compiling values from multiple references for the diffusion data we have been mindful of not reporting the same data twice, as sometimes values for the prefactor (D 0 ) and the activation energy (E a ) were within very close agreement but with slight differences in temperature or one of the variables.The validity of the application of CM to the oxidation of metallic NPs can be further explored by considering the limiting oxide thickness experimentally observed in previous studies. Reported limiting oxide layer thicknesses for Fe, Ni, and Cr nano-interfaces have been reported to be 2-3 nm, 1-3 1-3 nm, 4-6 and ~ 3nm, 7 respectively. Both Cr and Ni approach this limiting rate at 3 nm, but Fe does approach a limiting thickness implying that the values of W, N, and V M need re-assessment. Interestingly, all of these W's are significantly less than those calculated from diffusion studies of bulk materials; for example, an average W for Fe selfdiffusion from a compilation of bulk experiments was found to be 2.54 eV, while Fehlner's fitting of thin film oxidation found a value of 0.8 eV. 8 Encouragingly, other researchers looking at the oxidation of Fe 3 O 4 NPs to the corresponding Fe 2 O 3 products obtained W in the range of 0.85-0.91 eV, 9,10 implying the use of values lower than bulk measurements is appropriate. Using 3 nm as the limiting thickness when the oxidation rate reaches 10 -5 Å/s, one can simulate the range of reasonable values for W with varying V M (Fig S6 ): taking Fe as our example, if we use V M = 1.6 V, the corresponding range of activation energies would be 1.2 -1.6 eV. However, if we fix W = 0.8 eV, as Fehlner did, the corresponding V M would be < 0.1 V, which is exceedingly small and difficult to reconcile with the high potentials in the Cr and Ni system. It is likely, then, that in our approximation the activation energy for Fe 2+ transport is larger than the 0.8 eV value.

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“…According to Figure 2c, the Ni peak around 852.8 eV that is assigned to the zero oxidation state of Ni (i.e., Ni 0 ) is negligible in SSM but prominent in those of SSM-Ni and SSM-Ni-P, confirming that both samples are Ni-rich. Furthermore, the peak around the binding energy of 856.1 eV in SSM-Ni suggests the presence of the Ni peak of Ni 2+ from NiCl 2 , 38,52 while the intensities of these peaks reduced in SSM-Ni-P prescribing the binding of Ni with phosphorus. Moreover, XPS results further justify that Ni exists in both Ni 0 and Ni 2+ forms.…”

“…The peak around 573.2 eV corresponding to Cr 0 disappears in SSM-Ni-P, suggesting the removal of elemental Cr in the phosphorized sample. Moreover, the peak around 576.2 eV corresponding to Cr in the FeCr alloy 51,52 in the SSM sample also shifted to the higher binding energy in both SSM-Ni and SSM-Ni-P, indicating oxidation or modification in the electronic structures of both SSM-Ni and SSM-Ni-P. According to Figure 2c, the Ni peak around 852.8 eV that is assigned to the zero oxidation state of Ni (i.e., Ni 0 ) is negligible in SSM but prominent in those of SSM-Ni and SSM-Ni-P, confirming that both samples are Ni-rich.…”

“…The peak around 707.0 eV is assigned to the Fe 0 peak, 38 and the peak around 710.8 eV is assigned to the Fe peak in the FeCr alloy. 51,52 It can be observed that the Fe 0 peak of SSM shifted to the higher binding energy, suggesting oxidation of the Fe 0 and changes in the electronic states of SSM-Ni and SSM-Ni-P. In addition, the Cr 2p XPS spectra of the three samples are compared in Figure 2b.…”

Alloying and Phase Transformation of Fe/FeNi Core/Alloy Nanoparticles at High Temperatures

“…30,31 We have been developed a core/alloy (CA) approach towards crafting alloy nanoparticle interfaces. 2,[32][33][34] By focusing on depositing or forming a nanometer thin alloy at a pre-formed nanoparticle interface, we attempt to overcome challenges often faced when mixing metal salts followed by co-reduction, 6,[35][36][37] electrochemical reduction, 38,39 performing galvanic displacement, 34 or thermal treatment, 40,41 where differences in redox potentials, decompositions, and precursor reactivity makes controlling the final alloy composition, distribution, and phase homogeneity, difficult.…”

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“…Yet, following recent advances in nanotechnology, interest in efficient T C control has been rejuvenated in the context of fascinating memory, sensing, and drug delivery properties of magnetic nanoalloys. M-Cr nanoalloys (where M = Fe, Co, and Ni), in particular, have been investigated in a large number of experiments on nanoparticles, thin films, core-shell structures, and artificial nanocomposites [3][4][5][6]. A major hurdle toward the implementation of such materials, however, is the development of mature technologies for the production of magnetic nanoalloys with well-defined crystalline phase, size, shape, and composition, concurrent with an appropriate magnetic ordering [7,8].…”