Cobalt-bilayer catalyst decorated Ta3N5 nanorod arrays as integrated electrodes for photoelectrochemical water oxidation

Abstract

Ta₃N₅ nanorod arrays were fabricated by nitridation of fluorine-containing tantalum oxide (F–Ta₂O₅) nanorod arrays grown in situ on Ta substrates by a one-pot vapour-phase hydrothermal induced self-assembly technique. In this protocol, the in situ generation and the morphology of arrays elaborately adjusted by reaction time, play a vital role in the formation of the F–Ta₂O₅ nanorod arrays and a highly conductive interlayer between the nanorods and the substrate. Due to the shape anisotropy, ordered hierarchical structure and high surface area, a high photoelectrochemical activity was achieved by the optimum Ta₃N₅ nanorod photoelectrode with a photocurrent density of 1.22 mA cm⁻² under AM 1.5G irradiation at 1.23 V vs. RHE (reversible hydrogen electrode). Furthermore, a higher and more stable photocurrent was demonstrated by combining the highly active Ta₃N₅ nanorods with stable Co₃O₄/Co(OH)₂ (Co₃O₄/Co(II)) bilayer catalysts when compared with that demonstrated for Co(II)/Ta₃N₅ and Co₃O₄/Ta₃N₅ photoelectrodes, exhibiting that not only is the onset potential negatively shifted but also the photocurrent and the stability are significantly improved, which is correlated to an order of magnitude reduction in the resistance to charge transfer at the Ta₃N₅/H₂O interface. Specifically, about 92% of the initial stable photocurrent remains after long-term irradiation at 1.23 V vs. RHE. At 1.23 V vs. RHE, the photocurrent density of Co₃O₄/Co(II)/Ta₃N₅ arrays reached 3.64 mA cm⁻² under AM 1.5G simulated sunlight at 100 mW cm⁻², and a maximum IPCE of 39.5% was achieved at 440 nm. This combination of catalytic activity, stability, and conformal decoration makes this a promising approach to improve the photoelectrochemical performance of photoanodes in the general field of energy conversion.