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

Abstract

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