text: The earliest mention came in 1911, when German chemist Dr. Alexander Eibner integrated the concept in his research of the illumination of zinc oxide (ZnO) on the bleaching of the dark blue pigment, Prussian blue. Around this time, Bruner and Kozak published an article discussing the deterioration of oxalic acid in the presence of uranyl salts under illumination, while in 1913, Landau published an article explaining the phenomenon of photocatalysis. Their contributions led to the development of actinometric measurements, measurements that provide the basis of determining photon flux in photochemical reactions. After a hiatus, in 1921, Baly et al. used ferric hydroxides and colloidal uranium salts as catalysts for the creation of formaldehyde under visible light.
  In 1938 Doodeve and Kitchener discovered that  , a highly-stable and non-toxic oxide, in the presence of oxygen could act as a photosensitizer for bleaching dyes, as ultraviolet light absorbed by  led to the production of active oxygen species on its surface, resulting in the blotching of organic chemicals via photooxidation. This was the first observation of the fundamental characteristics of heterogeneous photocatalysis.