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

Tantalum nitride (TaN) is one of the few visible light absorbing photocatalysts capable of overall water splitting (OWS), by which the evolution of both H and O is possible. Despite favourable energetics, realizing the OWS or efficient H evolution in TaN prepared by the nitridation of tantalum oxide (TaO) or Ta foil remains a challenge even after 15 years of intensive research. Recently our group demonstrated OWS in TaN when prepared by the short time nitridation of potassium tantalate (KTaO). 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 TaN prepared by the nitridation of TaO, which do not show OWS. However, OWS activity in TaN prepared by 0.25 hour nitridation of KTaO is particularly favoured by the virtually absent electron and hole trapping. On further increasing the nitridation time of KTaO 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.