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 TiO2 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 TiO2, Na+ hampered the crystallization of anatase TiO2, and Na+ promoted the formation of brookite TiO2 or titanate sodium. In this research, we have systematically investigated the role of Na+ during the fabrication of TiO2 film and powder through the sol gel method and studied the influences of crystallinity and the content of Na+ on the photocatalytic activities of TiO2 film and powder. It has been found that the existence of Na+ in TiO2 film and powder should influence their crystallographic properties, in detail, inhibiting the crystallization and growth of anatase phase in TiO2 film and powder, promoting the formation of brookite phase in TiO2 film, and increasing the transformation temperature of anatase to rutile phase in TiO2 powder. Even though the existence of Na+ forms the Ti-O-Na bond on the surface of TiO2, however, the widely adopted hypothesis of Na+ serving as the recombination center of photogenerated electrons and holes is not correct.