Synthesis of Ta3N5 Nanotube Arrays Modified with Electrocatalysts for Photoelectrochemical Water Oxidation

Tantalum nitride (Ta3N5) is a promising material for photoelectrochemical (PEC) water oxidation with a narrow band gap (2.1 eV) that can effectively utilize visible light in the solar spectrum. Ta3N5 nanotube (NT) arrays were synthesized on a Ta foil by electrochemical anodization followed by an ammonia treatment at 800 °C. The photocurrent of nanostructured Ta3N5 was over 3 times higher than that of a dense regular Ta3N5 film in 0.1 M Na2SO4 aqueous solution at pH 11. Several electrocatalysts (IrO2 nanoparticles (NPs), Co3O4 NPs, cobalt phosphate, and Pt NPs) were used to modify Ta3N5 NTs for PEC water oxidation. The photocurrent of Ta3N5 NTs modified with IrO2 and Co3O4 was ca. four times higher than that of unmodified NTs. Cobalt phosphate also showed a positive improvement for PEC water oxidation on Ta3N5 NTs, whereas Pt was ineffective. Scanning electrochemical microscopy was used to measure the faradaic efficiency of the Ta3N5 photoanodes for water oxidation, which can reach as high as 88% for a Co3O4–Ta3N5 NTs photoanode, but is less than 15% at best, for Ta3N5 without the electrocatalyst. The results indicate that cobalt oxide and cobalt phosphate are promising candidates as electrocatalysts on Ta3N5 for water oxidation because Co is an earth-abundant material.