Carrier dynamics of a visible-light-responsive Ta3N5 photoanode for water oxidation

Abstract

The physicochemical properties of a tantalum nitride (Ta₃N₅) photoanode were investigated in detail to understand the fundamental aspects associated with the photoelectrochemical (PEC) water oxidation. The Ta₃N₅ thin films were synthesized using DC magnetron sputtering followed by annealing in air and nitridation under ammonia (NH₃). A polycrystalline structure with a dense morphology of the monoclinic Ta₃N₅ films was obtained. A relatively low absorption coefficient (10⁴ to 10⁵ cm⁻¹) in the visible light range was measured for Ta₃N₅, consistent with the nature of the indirect band-gap. Ultra-fast spectroscopic measurements revealed that the Ta₃N₅ with different thicknesses films possess low transport properties and fast carrier recombination (<10 ps). These critical kinetic properties of Ta₃N₅ as a photoanode may necessitate high overpotentials to achieve appreciable photocurrents for water oxidation (onset ∼0.6 V vs. RHE).