SEM images of silica particles of U1500 (a), C1500 (b) and FIB cross-sections (52° tilted view on cutting face, tilt correction enabled) for U1500 (c) and C1500 (d). Scale bars correspond to 1000 nm. Contrast in (c) is increased by 50% to enhance visibility of the internal ring structure (indicated by arrows). Vertical lines in (c) and (d) are milling artefacts (FIB curtaining effect).