Transmission electron microscopy characterization of colloidal copper nanoparticles and their chemical reactivity
Guangjun Cheng, Angela R. Hight Walker
A colloidal synthesis route was developed to produce face-centered cubic (fcc) copper (Cu) nanoparticles in the presence of the surfactants in an organic solvent under an argon environment. Various synthetic conditions have been explored to control the size and size distribution of the as-prepared nanoparticles by changing precursor, varying the amount of surfactant mixture, and tuning the reaction temperature. Transmission electron microscopy (TEM), selected-area electron diffraction, and high-resolution TEM have been used as the main tools to characterize these nanoparticles. Upon the exposure to air, these nanoparticles are oxidized at different levels dependent on their sizes: (1) inhomogeneous fcc copper (I) oxide (Cu2O) layer forms at the surface of Cu nanoparticles (~ 30 nm); (2) Cu nanoparticles (~ 5 nm) are immediately oxidized into fcc Cu2O nanoparticles (~ 6 nm). The occurrence of these different levels of oxidization suggests the reactive nature of Cu nanoparticles and the size effect on their reactivity. Utilizing their chemical reactivity and converting spherical Cu nanoparticles into copper (II) sulfide (CuS) nanoplates through the nanoscale Kirkendall effect have been demonstrated. The oxidization and sulfidation of Cu nanoparticles have been compared. Different atom diffusion and growth behaviors were involved in these two chemical transformations, resulting in the formations of isotropic Cu2O nanoparticles during the oxidization and anisotropic CuS nanoplates during sulfidation.