Scanning and transmission electron microscopy (SEM and TEM) images of the powder test materials. A: micron-scale ZnO, B: BaSO4, C: nano-ZrO2, D: coated nano-TiO2, E: coated nano-ZnO, F: nano-CeO2, G: Al-doped nano-CeO2. A- C: SEM images. D- G: TEM images. Note slightly different sets of scales in order to point to the individual characteristics of the respective test materials.