Rutile and anatase are polymorphs of TiO2, which are both naturally occurring. Rutile is a source for titanium smelting and is used in the semiconductor industry to produce dye-based solar cells (Grätzel cells). Anatase shows a high photocatalytic activity, which limits the usability as a pigment in polymers, but also opens certain applications as a catalyst. Anatase has a higher photocatalytic activity than rutile but is thermodynamically less stable.