Journal: Nanoscale

Article Title: Competing oxidation mechanisms in Cu nanoparticles and their plasmonic signatures.

doi: 10.1039/d2nr01054b

Figure Lengend Snippet: Fig. 1 ADF STEM of single Cu particle oxidation at different temperatures. (a) ADF STEM images acquired during the oxidation of Cu nanoparticles at 50, 100, 150 and 200 °C, in 3 mbar O2, after different O2 exposure times (indicated in the images). Scale bars are 50 nm. Specifically relevant fea- tures are highlighted: (b) the nucleation of oxide islands on the metallic surface at the initial stage of oxidation, (c) the formation of a homogeneous outer oxide shell after oxide island coalescence, and the vacancy gap layer between the outer shell and the metal core, (d) the typical NK void nucleation site at the metal – gap layer interface, (e) the inner oxide shell formed inside the particle and separated from the outer oxide shell by the vacancy gap layer, and (f) a comparison of the oxide morphology for oxidation at 50 °C (left) and 200 °C (right), revealing larger grain size and increased surface roughness at 200 °C.

Article Snippet: The ADF STEM images were segmented using the image processing toolbox in MATLAB.

Techniques: Comparison

Journal: Nanoscale

Article Title: Competing oxidation mechanisms in Cu nanoparticles and their plasmonic signatures.

doi: 10.1039/d2nr01054b

Figure Lengend Snippet: Fig. 2 In situ EELS of single Cu nanoparticle oxidation at 50 °C. (a) Selected ADF STEM images of a Cu nanoparticle oxidised at 50 °C (see Fig. 1 for additional images). (b and c) EELS signals from the particle recorded during oxidation with the electron beam positioned as specified in (a), revealing the LSPR signal of the particle. (d) The shift in the peak energy, ΔEpeak, and (e) the relative peak intensity, I/I0, of the LSPR response for two perpendicular polarisations as functions of volume oxidation fraction, δ, (squares and circles) plotted together with the results of corresponding FDTD simulations (triangles). The error bars in ΔEpeak, I/I0 and δ correspond to the energy dispersion per pixel in the EELS measurements, the average uncertainty of the fitted peak intensity, and the average uncertainty of the image segmentation process, respectively. The dashed lines indicate transition from oxide island to homogeneous oxide shell growth.

Article Snippet: The ADF STEM images were segmented using the image processing toolbox in MATLAB.

Techniques: In Situ, Dispersion

Journal: Nanoscale

Article Title: Competing oxidation mechanisms in Cu nanoparticles and their plasmonic signatures.

doi: 10.1039/d2nr01054b

Figure Lengend Snippet: Fig. 3 In situ EELS of Cu nanoparticle oxidation at 100 °C. (a) Selected ADF STEM images of a Cu nanoparticle oxidised at 100 °C in 3 mbar O2, showing the expansion of the NK void following first a linear and later angular growth (see Fig. 1 for additional images and Fig. S2.7† for ana- lysis of oxidation at 150 and 200 °C). The accumulated O2 exposure time is indicated for each image. All scale bars are 20 nm. (b and c) EELS LSPR signals acquired during oxidation from the set of positions indicated by the markers in (a). (d) Peak energy shift, ΔEpeak, and (e) relative peak intensity, I/I0, of the LSPR in (b and c) plotted as functions of the oxi- dised particle volume, δ. The markers (circles and squares) correspond to the positions in (a). Error bars are measured according to the same description as in Fig. 2. The experimentally obtained data is plotted together with corresponding FDTD simulations carried out for equi- valent excitation polarisations (grey markers). The dashed lines indicate transition between different stages indicated by the schematics: oxide island nucleation, homogeneous oxide shell growth, Kirkendall void for- mation and transition from linear to angular void expansion.

Article Snippet: The ADF STEM images were segmented using the image processing toolbox in MATLAB.

Techniques: In Situ, Lysis

Journal: Nanoscale

Article Title: Competing oxidation mechanisms in Cu nanoparticles and their plasmonic signatures.

doi: 10.1039/d2nr01054b

Figure Lengend Snippet: Fig. 4 Self-limited oxide growth at 50 °C. (a) A schematic drawing of the oxidation process at 50 °C including the formation of oxide islands followed by homogenous outer oxide shell growth separated from the metal core by a vacancy gap layer. (b) ADF STEM image of a particle oxi- dised at 50 °C in 3 mbar O2 (same as in Fig. 2a). The outer oxide shell and the vacancy gap are marked. (c) The effective oxide shell thickness measured from ADF STEM images vs. the accumulated O2 exposure time of 7 particles oxidised at 50 °C. (d) Time evolution of the effective oxide shell thickness of the particle in (b), with the error bars showing the uncertainty of the image segmentation process. The dashed line marks the transition from the oxide nucleation phase to a homogeneous oxide shell. The Cabrera–Mott model (solid line) is fitted to the data after the oxide nucleation phase (from 10 min) that yields the values for the Mott potential ϕMott = −3.2 eV and the barrier W = 1.1 eV for the particle in (b).

Article Snippet: The ADF STEM images were segmented using the image processing toolbox in MATLAB.

Techniques:

Journal: Nanoscale

Article Title: Competing oxidation mechanisms in Cu nanoparticles and their plasmonic signatures.

doi: 10.1039/d2nr01054b

Figure Lengend Snippet: Fig. 5 Outer and inner oxide shell growth. (a) Schematics of the stages of oxide growth; the oxide island nucleation, homogenous oxide shell growth – both inner and outer oxide shells, and the formation of the NK void. (b) A particle before (top left) and after (bottom left) oxidation at 200 °C. To the right, the pre and post-oxidation images are overlayed, with the red dotted line marking the approximate inside edge of the inner oxide shell, showing the positions of the inner and outer oxide shells relative to the perimeter of the pre-oxidation particle. (c) Top row: EELS maps of the O and Cu signals from a section of an oxidised particle. Bottom row: ADF STEM image of the mapped region together with a map of the rela- tive composition of Cu and O, confirming the lack of Cu at the vacancy gap between the outer and inner oxide shells. The effective inner (red tri- angles) and outer (blue squares) oxide shell thicknesses vs. O2 exposure time as extracted from ADF STEM images for a number of single Cu particles oxidised at (d) 50 °C, (e) 100 °C, (f) 150 °C and (g) 200 °C.

Article Snippet: The ADF STEM images were segmented using the image processing toolbox in MATLAB.

Techniques:

Journal: Nanoscale

Article Title: Competing oxidation mechanisms in Cu nanoparticles and their plasmonic signatures.

doi: 10.1039/d2nr01054b

Figure Lengend Snippet: Fig. 6 Kinetic analysis of Cu nanoparticle oxide growth. (a) The volume oxidation fractions vs. O2 exposure times for three Cu particles oxidised at 100, 150 and 200 °C, respectively, obtained from ADF STEM images (error bars show the uncertainty of the image segmentation). The solid lines are fits to the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, with the extracted numbers for parameter n and rate constant k indi- cated next to each fit. (b) Corresponding Arrhenius plot of log(k) extracted from JMAK model fits, yielding an overall oxidation Ea = 0.37 ± 0.1 eV. 21 particles were analysed here and correspond to the individual data points. (c and d) Arrhenius plots for the characteristic times to grow 5 nm thick outer oxide shell (c) and 0.5 nm inner oxide shell (d), from which the respective apparent activation energies are obtained, Eout = 0.30 ± 0.1 eV and Ein = 0.45 ± 0.1 eV. The shaded areas show the 95% confidence interval of each fit.

Article Snippet: The ADF STEM images were segmented using the image processing toolbox in MATLAB.

Techniques: Activation Assay