Optimal growth of sodium titanate nanoflower on TiO2 thin film for the fabrication of a novel Ti/TiO2/Na2Ti3O7 photoanode with excellent stability

This study reports the effect of the Ti source (Ti foil or TiO2 thin layer) and NaOH concentration on the growth of sodium titanate nanostructures via hydrothermal treatment to improve the photoelectrochemical (PEC) water splitting performance. An optimal condition to grow sodium titanate nanoflower composed of ultrathin nanosheets on the surface of the thin TiO2 film led to the formation of a novel nanocomposite TiO2/Na2Ti3O7 photoelectrode with great adhesion of the two phases. The Ti/TiO2/Na2Ti3O7 photoanode demonstrated a significant enhancement in photocurrent density and shifted the onset potential to a more negative value compared to the pure Ti/TiO2 and Ti/Na2Ti3O7 samples. This noticeable improvement is attributed to the suitable band alignment between the TiO2 and Na2Ti3O7 phases and their direct contact with the Ti foil, as well as the large surface area of nanoflower sodium titanate, which significantly facilitate the generation, separation, and transfer of photogenerated electrons from the active phases to the substrate. More interestingly, the prepared photoanode retained its initial photocurrent density even after 16 h. An ion-exchange method and calcination treatment were used at different temperatures to convert the Na2Ti3O7 phase in the Ti/TiO2/Na2Ti3O7 photoanode to TiO2 to investigate the role of type II heterojunction for efficient photogenerated charge carrier separation in this photoanode. It was concluded that converting Na2Ti3O7 into TiO2 in the Ti/TiO2/Na2Ti3O7 system would reduce its PEC performance due to the formation of a pure phase of TiO2 in the Ti/TiO2/TiO2 photoanode.