The trend among the usage is like the reaction types (Figure ), with water splitting, and the related catalysts being the predominant ones. In addition to their usage directly as photocatalysts in powder or film form, photocatalysts are also used to fabricate devices or parts of the devices to study the properties of either the photocatalyst or the entire device. By analyzing the CAS Content Collection TM data, a list of the most reported devices or parts of devices was generated and are listed in Figure . Photocatalysts are fabricated as photocathodes, photoanodes, which are either part of photoelectrochemical cells used for water splitting to produce hydrogen or as solar cells. Photocatalysts are also part of various types of solar cells where the photogenerated electrons and holes are used to produce electricity instead of carrying out chemical reactions. Among these devices, patent to journal ratio is high among the solar cells, electroluminescent devices, photochemical reactors, and photoelectric devices.