Unravelling the Influence of Major Seawater Salt Ions on the Photogenerated Charge Carriers in a Sr-Doped NaTaO3 Photocatalyst via ATR-FTIR

The branching of research on photocatalytic water splitting into seawater signifies an important breakthrough that advances the potential for sustainable green energy harvesting in the future. Despite efforts to assess and enhance the performance of different photocatalytic materials in seawater, the mechanistic and kinetic aspects involved in seawater splitting remain largely unexplored. Herein, attenuated total reflectance IR spectroscopy was employed to monitor the decay kinetics of photoexcited electrons in Sr-doped NaTaO3 (Sr-NTO) when immersed in solutions containing different salt ions that are found in seawater. Between the two anions examined, Cl– ions were found to suppress electron decay, whereby SO42– ions did not exert a notable effect. It is suspected that Cl– functioned as a hole scavenger during photoirradiation, thus quenching the electron–hole recombination and inhibiting the decay. As for the cations, acceleration of electron decay was observed only in solutions containing Mg2+ and Ca2+ ions, with Mg2+ imposing the most substantial effect. The shared divalent nature suggests the adsorption of these ions onto the surface of Sr-NTO due to their high ionic strength, which results in the formation of an ionic barrier that hindered the holes from migrating toward the surface. This leads to an accumulation of holes within the photocatalyst, in turn spurring electron decay through recombination. The same interpretation is also applicable to the comparison between Mg2+ and Ca2+ ions, where the considerably smaller ionic radius of Mg2+ gives it a higher charge density, consequently increasing its tendency to absorb onto the photocatalyst as compared to Ca2+.