There are some relevant effects of UV on these semiconductor sunscreens to consider. For example:
It is generally agreed that UV absorption excites an electron from the valence band to the conduction band of the semiconductor. The resulting excited electrons, in the otherwise empty conduction band, and the “positive holes” in the valence band allow charge transfer to the TiO2 surface which facilitates oxidation of surrounding molecules. Sometimes direct charge-transfer causes the oxidation.
https://www.mdpi.com/1420-3049/19/11/18192/htm
Some questions which stem from this finding include: what is being oxidized on the skin in its immediate vicinity when UV makes contact with the semiconductor sunscreen? What are the implications?
Is there any performance difference between nanoparticle sizes?