| Abstract
| - Copper nanoparticles with a diameter of 3.0 ± 0.3 nm were generated under vacuum, using a sputtering−aggregation source, and deposited onto glass or highly ordered pyrolytic graphite substrates. Upon exposure to room-temperature air, core−shell particles formed, as determined by surface plasmon resonance spectroscopy, high resolution X-ray photoelectron spectroscopy, and quartz crystal microgravimetry. At 160 °C in air, the core−shell particles converted predominantly to Cu2O. At 220 °C, they converted to CuO almost exclusively. The maximum oxide shell thickness attainable at room temperature was 0.56 nm, as determined by quartz crystal microgravimetry and scanning tunnelling microscope imaging. The shell thicknesses formed are compatible with a simple charge-transfer-based model akin to Mott and Fromhold−Cook theories of metal oxidation processes. The model, which yields the change in the height of the energy barrier to diffusion as a function of shell thickness, is found to be consistent with many adsorbate-induced diffusion processes involving core−shell nanoparticles.
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