Real-time plasmon spectroscopy study of the solid-state oxidation and Kirkendall void formation in copper nanoparticles

Abstract

Oxidation and corrosion reactions have a major effect on the application of non-noble metals. Kinetic information and simple theoretical models are often insufficient for describing such processes in metals at the nanoscale, particularly in cases involving formation of internal voids (nano Kirkendall effect, NKE) during oxidation. Here we study the kinetics of solid-state oxidation of chemically-grown copper nanoparticles (NPs) by in situ localized surface plasmon resonance (LSPR) spectroscopy during isothermal annealing in the range 110-170 °C. We show that LSPR spectroscopy is highly effective in kinetic studies of such systems, enabling convenient in situ real-time measurements during oxidation. Change of the LSPR spectra throughout the oxidation follows a common pattern, observed for different temperatures, NP sizes and substrates. The well-defined initial Cu NP surface plasmon (SP) band red-shifts continuously with oxidation, while the extinction intensity initially increases to reach a maximum value at a characteristic oxidation time τ, after which the SP intensity continuously drops. The characteristic time τ is used as a scaling parameter for the kinetic analysis. Evolution of the SP wavelength and extinction intensity during oxidation at different temperatures follows the same kinetics when the oxidation time is normalized to τ, thus pointing to a general oxidation mechanism. The characteristic time τ is used to estimate the activation energy of the process, determined to be 144 ± 6 kJ mol, similar to previously reported values for high-temperature Cu thermal oxidation. The central role of the NKE in the solid-state oxidation process is revealed by electron microscopy, while formation of CuO as the major oxidation product is established by X-ray diffraction, XPS, and electrochemical measurements. The results indicate a transition of the oxidation mechanism from a Valensi-Carter (VC) to NKE mechanism with the degree of oxidation. To interpret the optical evolution during oxidation, Mie scattering solutions for metal core-oxide shell spherical particles are computed, considering formation of Kirkendall voids. The model calculations are in agreement with the experimental results, showing that the large red-shift of the LSPR band during oxidation is the result of Kirkendall voiding, thus establishing the major role of the NKE in determining the optical behavior of such systems.

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

“…Subsequently, we gently oxidized three samples at 150°C in 1% O 2 in Ar carrier gas at atmospheric pressure for 30, 45 or 60 minutes, respectively. At these conditions, predominantly Cu 2 O is formed, while the growth of CuO is suppressed, 4,5,17,18,26 as we also have confirmed by XPS analysis in a previous study carried out at these conditions. 19 Simultaneous recording of the dark-field scattering response of a representative single Cu nanoparticle on each sample yields the temporal evolution of the recorded single particle spectra summarized in Fig.…”

“…This is in good qualitative agreement with LSPR-based measurements on ensembles of Cu nanoparticles. 4 Interestingly, upon further oxidation, the scattering intensity suddenly starts to decrease, which has been ascribed to the Cu core volume starting to shrink upon Kirkendall void growth. 4 Simultaneously, as the decrease in scattering intensity occurs, we see the resonance split into two modes, an effect not resolved in ensemble measurements due to ensemble averaging.…”

“…4 Interestingly, upon further oxidation, the scattering intensity suddenly starts to decrease, which has been ascribed to the Cu core volume starting to shrink upon Kirkendall void growth. 4 Simultaneously, as the decrease in scattering intensity occurs, we see the resonance split into two modes, an effect not resolved in ensemble measurements due to ensemble averaging. To capture this behavior in a more quantitative fashion we fit two Lorentzians to the scattering spectra to deconvolute the contribution from the two modes.…”

“…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).…”

“…Among non-invasive in situ experimental techniques used to study metal nanoparticle oxidation, [15][16][17][18] visible light optical spectroscopy based on localized surface plasmon resonance (LSPR) is attractive because of its remote readout compatible with ambient pressures and elevated temperatures, and it has proven to be very useful in studies of the oxidation of Cu nanoparticles and the NKE. 4,5,19 However, to date such studies have been carried out on large ensembles of Cu nanoparticles (with one exception that did not focus on the oxidation process as such 20 ), which makes direct correlations between particle nanostructure, defects and oxidation mechanism difficult. To this end, we have recently introduced single particle plasmonic nanospectroscopy, which enables in situ real time characterization of multiple individual nanoparticles at identical experimental conditions and the direct correlation of the obtained response with the particle nanostructure obtained from post mortem electron microscopy analysis.…”