Interfacial Design of a Ta₃N₅ Thin-Film Photoanode for Highly Stable Oxygen Evolution over a Wide pH Range

Abstract

Ta 3 N 5 -based visible-light-absorbing semitransparent photoanodes for the oxygen evolution reaction (OER) are one of the most promising candidates for the construction of photoelectrochemical (PEC) water-splitting devices with a tandem configuration. Although the PEC properties of Ta 3 N 5 have been evaluated in alkaline aqueous solutions, those in acidic or near-neutral pH aqueous solutions are still unclear. An efficient OER by Ta 3 N 5 in acidic and near-neutral pH aqueous solutions is still challenging because it requires surface modifiers with chemical durability and efficient electrocatalytic properties for the OER. This study focuses on the interfacial design of Ta 3 N 5 semitransparent photoanodes to achieve an efficient and stable OER over a wide pH range. The results of PEC measurements and X-ray photoelectron spectroscopy reveal that self-photooxidation occurs on the bare Ta 3 N 5 surface regardless of the pH values, resulting in photocurrent decrease. A uniform coating of IrO x applied using spin-coating successfully protected Ta 3 N 5 , and the IrO x /Ta 3 N 5 photoanodes maintained ∼80% of their initial OER photocurrent even after 4 h. The pH dependence of the photocurrent value of IrO x /Ta 3 N 5 was found to follow that of the electrocatalytic OER rate of IrO x . Furthermore, the onset potential for the OER on IrO x /Ta 3 N 5 was independent of the pH value and was fixed at 0.7 V versus a reversible hydrogen electrode.

Physicochemical insights into semiconductor properties of a semitransparent tantalum nitride photoanode for solar water splitting

“…The peaks centered at 529.8 and 531.9 eV can be assigned to lattice oxygen (O 2À ) and chemisorbed H 2 O (or hydroxy groups), respectively. 24,48,60 Based on a previous report concerning Ta 3 N 5 , the XPS peak derived from O 2À is attributed to the formation of TaO x , in agreement with the Ta 4f peak fitting results (Fig. 5a).…”

“…3a), the onset potential ( E on ) was defined as the photoelectrode potential required to generate an anodic photocurrent of 0.1 mA cm −2 . 45–48 The onset potential, E on , for these NiFeO x /Ta 3 N 5 /SiO 2 photoanodes was 0.65 V RHE except in the case of the Ta 3 N 5 /SiO 2 processed at 850 °C, which had an E on value of 0.72 V RHE . The difference between the E on values for these specimens (with a value of 0.65 V RHE ) and the standard redox potential for the OER for water (1.23 V RHE ) is 0.58 V. This potential difference indicates that photogenerated carriers inside the Ta 3 N 5 /SiO 2 reduced the applied bias potential required to drive the OER.…”

“…8 Interestingly, E on values previously reported for Ta 3 N 5 -based photoanodes have been in the range of 0.5-0.8 V RHE regardless of the preparation procedure, the type of substrate and the loaded cocatalysts. 35,40,46,48 The Fermi-level pinning effect caused by surface defect states (meaning oxygen impurities, nitrogen vacancies and/or reduced Ta center (Ta 3+ )) may inhibit the shift of E on to a potential lower than 0.5 V RHE . 7,24,34,47 The J values were found to increase with increasing electrode potential from E on , irrespective of the Ta 3 N 5 /SiO 2 sample being assessed (Fig.…”

“…The higher N d value obtained from the MS plot than those obtained from Hall effect measurements suggests that the surface states related to the defect and the oxidized layer of Ta 3 N 5 likely affect the results of MS analysis. 47,48 Therefore, quantitative surface analysis of Ta 3 N 5 is further needed, which will be discussed in detail later.…”

“…44 Our group previously demonstrated that Ta 3 N 5 thin films prepared on quartz insulating substrates (Ta 3 N 5 /SiO 2 ) can promote the PEC OER as a consequence of the self-conductivity of Ta 3 N 5 , even without the insertion of a transparent conductive layer. [45][46][47][48] Following loading of NiFeO x as an electrocatalyst on Ta 3 N 5 /SiO 2 , variations in photocurrent density were observed between specific samples but these photoanodes were able to generate photocurrent densities from 2 to 6 mA cm À2 at 1.23 V RHE under AM 1.5G solar illumination. [45][46][47] The effects of surface modification with an electrocatalyst layer comprising NiFeO x and/or IrO x on the performance of semitransparent Ta 3 N 5 /SiO 2 were also examined with the aim of improving the durability of this material during the OER.…”