Plasmon imaging: An efficient TEM-based method for locating noble metal particles dispersed on oxide catalysts at very low densities
We have used transmission electron microscopy to study catalysts comprising nanoparticulate gold dispersed on a highly porous nanoparticulate TiO2 (anatase) support. The similarity of the morphology of the two phases, and the low number density of gold particles (1 in 65,000) makes this challenging. Diffraction contrast imaging could not differentiate the two phases, since TiO2 oriented at strong Bragg conditions, produced similar contrast to the Bragg/mass-thickness contrast of the gold. Mass-thickness contrast imaging allowed gold to be differentiated from TiO2 only in the thinnest regions, where the mass-thickness of TiO2 was low. Plasmon imaging, using an energy loss of 24 eV and an energy window width of 5 eV, was very effective at locating gold. Both the TiO2 and impregnating resin produced a strong plasmon signal, while the much weaker signal from the gold made it appear dark. This permitted the gold particles to be readily located, irrespective of whether they were located in the thin or thick regions of the TiO2 support.