Characterization of Ternary CuNiCo Metallic Nanoparticles Produced by Hydrogen Reduction

Abstract

Different methods of producing nanostructured materials at the laboratory scale have been studied using a variety of physical and chemical techniques, though the challenge here is the homogeneous distribution of the elements which also depends on the precursor elements. This work thus focused on the micro-analytical characterization of Cu-Ni-Co metallic nanoparticles produced by an alternative chemical route aiming to produce solid solution nanoparticles. This method was based on two steps: the co-formation of oxides by nitrates' decomposition followed by their hydrogen reduction. Based on the initial composition of precursor nitrates, three homogeneous ternaries of the Ni, Cu and Co final alloy products were pre-established. Thus, the compositions in %wt of the synthesized alloy particles studied in this work are 24Cu-64Ni-12Co, 12Cu-64Ni-24Co and 10Cu-80Ni-10Co. Both precursor oxides and metallic powders were characterized by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM/EDS) and transmission electron microscopy (TEM). The results show that the synthesis procedure was successful since it produced a homogeneous material distributed in different particle sizes depending on the temperature applied in the reducing process. The final composition of the metallic product was consistent with what was theoretically expected. Resulting from reduction at the lower temperature of 300 °C, the main powder product consisted of particles with a spheroidal and eventually facetted morphology of 50 nm on average, which shared the same FCC crystal structure. Particles smaller than 100 nm in the Cu-Ni-Co alloy agglomerates were also observed. At a higher reduction temperature, the ternary powder developed robust particles of 1 micron in size, which are, in fact, the result of the coarsening of several nanoparticles.

Keywords: Cu–Ni–Co alloy; characterization; hydrogen reduction; nanoparticles.

Figure 1

Thermodynamics diagram for (a) the thermal decomposition of nitrates; and (b) the hydrogen reduction of Cu, Ni and Co oxides.

Figure 2

General schematic vision of the experimental procedure.

Figure 3

XRD of the co-formed oxides thermally decomposed at 500 $${}^{°}$$C: (a) 24Cu–oxide; (b) 12Cu–oxide; (c) 10Cu–oxide.

Figure 4

SEM analysis of the 10Cu–oxide sample thermally decomposed at 500 $${}^{°}$$C.

Figure 5

TEM bright-field image and elemental mapping of 10Cu–oxide nanoparticles (thermal decomposition at 500 $${}^{°}$$C) (a) metallic nanoparticles aggregate with the corresponding elemental EDS mapping shown at the bottom. The boxed area enlarged in (b) marks the positions of four-point analysis.

Figure 6

TEM bright-field image of a 10Cu–oxide nanoparticle aggregate and the dark field image of an isolated single crystal nanoparticle and the corresponding electro diffraction pattern.

Figure 7

X-ray diffraction pattern of the ternary Cu–Ni–Co alloy powder in three different compositions (reduction at 900 $${}^{°}$$C).

Figure 8

Secondary electrons SEM images under two magnifications of the co-formed ternary powder: 12Cu64Ni24Co for the different reduction temperatures: (a) 300 $${}^{°}$$C; (b) 600 $${}^{°}$$C; and (c) 900 $${}^{°}$$C.

Figure 9

TEM bright-field image and elemental mapping of the co-formed ternary nanoparticle aggregate of metallic powder: 10Cu–80Ni–10Co (reduction at 300 $${}^{°}$$C/30 min).

Figure 10

Two TEM bright/dark-field images of a co-formed and coalesced ternary nanoparticle aggregate: 10Cu–80Ni–10Co (reduction at 600 $${}^{°}$$C/10 min) obtained at a slightly different incident beam/sample orientation (a), the corresponding diffraction pattern (b) marking the operating reflection for the dark field images (c) revealing different portions of the nanocrystalline aggregate.

Figure 11

(a) TEM bright-field image of the co-formed ternary aggregate of three nanoparticles: 10Cu–80Ni–10Co and the corresponding selected area diffraction (b). The single reflection marked as 1 in this pattern generated the dark field image of the single nanoparticle of (c) and the double reflection marked as 2 generated the dark field mage of the two nanoparticles image of (d) (reduction at 300 $${}^{°}$$C/10 min).

Figure 11

(a) TEM bright-field image of the co-formed ternary aggregate of three nanoparticles: 10Cu–80Ni–10Co and the corresponding selected area diffraction (b). The single reflection marked as 1 in this pattern generated the dark field image of the single nanoparticle of (c) and the double reflection marked as 2 generated the dark field mage of the two nanoparticles image of (d) (reduction at 300 $${}^{°}$$C/10 min).