Origin of the overall water splitting activity of Ta₃N₅ revealed by ultrafast transient absorption spectroscopy

Tantalum nitride (Ta₃N₅) is one of the few visible light absorbing photocatalysts capable of overall water splitting (OWS), by which the evolution of both H₂ and O₂ is possible. Despite favourable energetics, realizing the OWS or efficient H₂ evolution in Ta₃N₅ prepared by the nitridation of tantalum oxide (Ta₂O₅) or Ta foil remains a challenge even after 15 years of intensive research. Recently our group demonstrated OWS in Ta₃N₅ when prepared by the short time nitridation of potassium tantalate (KTaO₃). To obtain a mechanistic insight on the role of Ta precursor and nitridation time in realizing OWS, ultrafast dynamics of electrons (3435 nm probe) and holes (545 nm probe) is investigated using transient absorption spectroscopy. Electrons decay majorly by trapping in Ta₃N₅ prepared by the nitridation of Ta₂O₅, which do not show OWS. However, OWS activity in Ta₃N₅ prepared by 0.25 hour nitridation of KTaO₃ is particularly favoured by the virtually absent electron and hole trapping. On further increasing the nitridation time of KTaO₃ from 0.25 to 10 hour, trapping of both electron and hole is enhanced which concurrently results in a reduction of the OWS activity. Insights from correlating the synthesis conditions-structural defects-carrier dynamics-photocatalytic activity is of importance in designing novel photocatalysts to enhance solar fuel production.