Nanoscale Kirkendall Effect and Oxidation Kinetics in Copper Nanocrystals Characterized by Real‐Time, In Situ Optical Spectroscopy

Abstract

The low‐temperature oxidation of ≈10 nm diameter copper nanocrystals is characterized using in situ UV–vis absorbance spectroscopy and observed to lead to hollow copper oxide shells. The kinetics of the oxidation of solid Cu nanocrystals to hollow Cu2O nanoparticles is monitored in real‐time via the localized surface plasmon resonance response of the copper. A reaction‐diffusion model for the formation of hollow nanoparticles is fit to the measured time for complete Cu nanocrystal oxidation, and is used to quantify the diffusion coefficient of Cu in Cu2O and the activation energy of the oxidation process. The diffusivity measured here in single‐crystalline nanoscale systems is 1–5 orders of magnitude greater than in comparable systems in the bulk, and have an Arrhenius dependence on temperature with an activation energy for diffusion of 37.5 kJ mol−1 for 85 °C ≤ T ≤ 205 °C. These diffusion parameters are measured in some of the smallest metal systems and at the lowest oxidation temperatures yet reported, and are enabled by the unique nanoscale single‐crystalline material and the in situ characterization technique.

Resolving single Cu nanoparticle oxidation and Kirkendall void formation with in situ plasmonic nanospectroscopy and electrodynamic simulations

“…The nanoscale Kirkendall effect (NKE) 1,2 occurs during Cu nanoparticle oxidation as the consequence of O-ions diffusing more slowly in the oxide compared to the Cu-ions. [3][4][5][6] As the end result, this leads to the conversion of the metal particle into a characteristic hollow oxide shell, with the amount of hollowing depending on the ratio between the diffusion rates of the Cu-and O-ions, as well as on the nanostructure itself and on the abundance of defects 7 (Fig. 1a).…”

Kinetic description of metal nanocrystal oxidation: a combined theoretical and experimental approach for determining morphology and diffusion parameters in hollow nanoparticles by the nanoscale Kirkendall effect

“…Copper nanocrystals were synthesized by the thermal decomposition of metal precursors as detailed in the literature [21,22]. The standard synthesis involved degassing Cu (I) acetate (0.245 g), oleic acid (1.78 g, 2 mL) and trioctylamine (6.06 g, 7.5 mL) at room temperature, followed by heating at 180°C for 1 h and then at 270°C for 1 h under air-free conditions.…”

“…The resulting monodisperse Cu nanocrystals were spherical with diameters of 11 nm as characterized by transmission electron microscopy (TEM, Phillips CM100 operated at 80 kV). The oxidation of the nanocrystals from solid Cu to hollow Cu 2 O was performed in air at temperatures ranging from 80 to 205°C and monitored by in situ UV-Vis absorbance spectroscopy [21]. Briefly, thin films of Cu nanocrystals were prepared in a glovebox by dropcasting the concentrated colloidal suspension of Cu nanocrystals in hexanes onto a clean glass slide.…”

“…The in situ UV-vis absorbance spectroscopy measurements provide experimental data for the time for full oxidation of the Cu nanoparticles: t full = 335 s at 100°C, 79.5 s at 150°C, and 37.5 s at 200°C as in Figure 7(a) [21]. Recall from the model derivation that A = t full /τ MX and s MX R g TR 2 o =D MX Dg Cu 2 O j j.…”

“…Neglecting surface energies is achieved by specifying γ a = γ ρ = γ R = 0 and numerically solving Equations (20)- (21). Figure 2 shows a plot of the radii vs. oxidation time for the Cu/Cu 2 O system (i.e.…”