High-Temperature Ammonolysis of Thin Film Ta₂O₅ Photoanodes: Evolution of Structural, Optical, and Photoelectrochemical Properties

Abstract

Tantalum nitride (Ta3N5) and oxynitride (TaON) are promising materials for photoelectrochemical (PEC) water splitting due to near-ideal band gaps and band edge positions. However, the high-temperature ammonolysis process that is usually used to make these materials depends sensitively on the process parameters and specific design of the annealing system, and reproducing highly efficient (oxy)­nitride photoanodes from recipes reported by other laboratories has proven to be challenging. To understand and monitor the nitridation process in more detail, we employ an optical absorption spectroscopy technique that allows us to follow the transformation of thin Ta2O5 films in situ at temperatures up to 800 °C. Our results show that the incorporation of nitrogen in a dry ammonia atmosphere starts at 575 °C and is accompanied by a gradual red-shift of the Ta2O5 absorption edge and an expansion of the lattice due to the larger ionic radius of N3– relative to O2–. Although coloration of the material due to an N-2p → Ta-3d transition occurs readily, the films do not show any visible-light PEC activity until the nitrogen concentration is high enough to form a continuous N-2p impurity band. Ta3N5 is found to be the only thermodynamically stable phase between 575 and 800 °C, with no traces of TaON. Longer nitridation times result in lower defect concentrations, larger grain sizes, and improved PEC performance. The photocurrent of well-crystallized films is limited by slow water oxidation kinetics. This can be effectively remedied by depositing IrO2 nanoparticles as a water oxidation cocatalyst, which results in external quantum efficiencies of up to 45%. The smaller enhancement of the PEC performance at longer wavelengths reveals that hole transport in Ta3N5 limits the water splitting performance of IrO2-catalyzed Ta3N5 photoanodes.

Tantalum nitride films integrated with transparent conductive oxide substrates via atomic layer deposition for photoelectrochemical water splitting

“…This performance falls short of the recent report by Li and coworkers, 24 who reported a photocurrent density of ∼12.1 mA cm –2 at 1.23 V vs. RHE with a photocurrent onset potential of ∼0.7 V vs. RHE for the electrode prepared on a Ta foil which was nitridized under ammonia at 950 °C for 6 h. Van de Krol et al also recently studied the formation of Ta 3 N 5 as a function of ammonolysis conditions on Pt foil. 14 The maximum photocurrent density of ∼1.1 mA cm –2 at 1.23 V vs. RHE with an onset photocurrent potential of ∼0.9 V vs. RHE was found for the Ta 3 N 5 film prepared at 800 °C for 10 h with the addition of IrO 2 cocatalysts. To the best of our knowledge, however, this is the first report of PEC water oxidation of Ta 3 N 5 on any TCO (Fig.…”

Oxynitrogenography: Controlled Synthesis of Single-Phase Tantalum Oxynitride Photoabsorbers

“…The gradual red shift of the absorption occurs for about 6 hours, until the solubility limit for nitrogen into the Ta 2 O 5 phase is reached. 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 goes a transformation, presumably into an oxynitride phase. Based on the change in optical absorption, it can be inferred that most of the phase transformation occurs within 2 hours.…”

“…For example, in our previous work we were unable to form single phase β-TaON thin films in either wet or dry ammonia. 27,36 Moreover, while a significant amount of literature is available on its crystal and electronic structure, 28,29,30,31 and photoelectrochemical performance, 8,10 to date a quantitative analysis of the carrier mobility and lifetime of β-TaON is missing.…”

Tantalum nitride for photocatalytic water splitting: concept and applications

“…The Ta 3 N 5 samples synthesized at different temperatures with similar NH 3 flow rates and samples synthesized at 800°C with different NH 3 flow rates did not exhibit any significant change in lattice parameters. Hence, the incorporation of N into Ta 2 O 5 (i.e., the formation of doped-Ta 2 O 5 ) is unlikely, as observed for the synthesis of Ta 3 N 5 films [52]. In addition, the lattice parameters are also compared with those reported in the literature.…”