Role of Sodium Ion on TiO2 Photocatalyst: Influencing Crystallographic Properties or Serving as the Recombination Center of Charge Carriers?

There have been continuing debates about the role of Na + on TiO 2 photocatalyst in the past decades. Most researchers accepted that Na + served as the recombination center of photogenerated electrons and holes. Nevertheless, other opinions also existed, such as Na + increased the crystallite size of TiO 2, Na + hampered the crystallization of anatase TiO 2, and Na + promoted the formation of brookite TiO 2 or titanate sodium. In this research, we have systematically investigated the role of Na + during the fabrication of TiO 2 film and powder through the sol–gel method and studied the influences of crystallinity and the content of Na + on the photocatalytic activities of TiO 2 film and powder. It has been found that the existence of Na + in TiO 2 film and powder should influence their crystallographic properties, in detail, inhibiting the crystallization and growth of anatase phase in TiO 2 film and powder, promoting the formation of brookite phase in TiO 2 film, and increasing the transformation temperature of anatase to rutile phase in TiO 2 powder. Even though the existence of Na + forms the Ti–O–Na bond on the surface of TiO 2, however, the widely adopted hypothesis of Na + serving as the recombination center of photogenerated electrons and holes is not correct.