Interface engineering of Ta3N5 thin film photoanode for highly efficient photoelectrochemical water splitting

Abstract

Interface engineering is a proven strategy to improve the efficiency of thin film semiconductor based solar energy conversion devices. Ta₃N₅ thin film photoanode is a promising candidate for photoelectrochemical (PEC) water splitting. Yet, a concerted effort to engineer both the bottom and top interfaces of Ta₃N₅ thin film photoanode is still lacking. Here, we employ n-type In:GaN and p-type Mg:GaN to modify the bottom and top interfaces of Ta₃N₅ thin film photoanode, respectively. The obtained In:GaN/Ta₃N₅/Mg:GaN heterojunction photoanode shows enhanced bulk carrier separation capability and better injection efficiency at photoanode/electrolyte interface, which lead to a record-high applied bias photon-to-current efficiency of 3.46% for Ta₃N₅-based photoanode. Furthermore, the roles of the In:GaN and Mg:GaN layers are distinguished through mechanistic studies. While the In:GaN layer contributes mainly to the enhanced bulk charge separation efficiency, the Mg:GaN layer improves the surface charge inject efficiency. This work demonstrates the crucial role of proper interface engineering for thin film-based photoanode in achieving efficient PEC water splitting.

Fig. 1. Schematic diagram for the preparation of In:GaN/Ta₃N₅/Mg:GaN heterostructure thin film.

1 EB evaporation of InO_x thin layer on Nb substrate. 2 ALD of GaO_x layer using TEG as precursor. 3 EB evaporation of TaO_x layer. 4 ALD of Mg:GaO_x layer using MgCp2 and TEG as precursors. 5 One-step thermal nitridation in NH₃ atmosphere at 1000 °C for 6 h.

Fig. 2. Structural properties of In:GaN/Ta₃N₅/Mg:GaN heterostructure thin film on Nb substrate.

a Cross-sectional STEM-EDS elemental mappings of Ta, N, Nb, Ga, Mg, and In. b Annular dark-field STEM image of the cross section of the sample. c Overlapping the AFD STEM image with the EDS mappings of the metallic elements. d Bright field TEM image of the film. e HRTEM image showing the lattice fringes of Ta₃N₅ with a spacing of 0.256 nm.

Fig. 3. Spectroscopic characterizations of GaN and Ta₃N₅-based thin films.

a XRD patterns of different Ta₃N₅-based films deposited on quartz glass substrate. The narrow scan XRD patterns on the right side shows the (002) diffraction peaks of GaN. b PL spectra of Mg:GaN film deposited on quartz glass substrate under 270 nm LED excitation. and c PL spectra of In:GaN film deposited on quartz glass substrate under 375 nm laser excitation. d UPS spectrum of Mg:GaN deposited on Nb substrate. e UPS spectrum of In:GaN deposited on Nb substrate. f Schematic diagram of band structure for In:GaN/Ta₃N₅/Mg:GaN film determined from UPS and UV–vis absorption measurements. g PL spectra and h TRPL spectra of four Ta₃N₅-based films on quartz glass substrate measured at 10 K under 375 nm laser excitation. i PL spectra of four Ta₃N₅-based films measured at 10 K under 510 nm laser excitation.

Fig. 4. PEC performance of the In:GaN/Ta₃N₅/Mg:GaN photoanode on Nb substrate.

a J–V curves of Ta₃N₅-based photoanodes with different layered structures. All the photoanodes were modified with NiCoFe-B_i co-catalyst and tested in 1 M KOH electrolyte under AM 1.5 G illumination. b The steady-state photocurrent of In:GaN/Ta₃N₅/Mg:GaN photoanode under low-bias conditions. c ABPE curves calculated from the J-V curves in a. d Stability of the pristine Ta₃N₅ and In:GaN/Ta₃N₅/Mg:GaN photoanodes measured at an applied potential of 1.0 V vs. RHE. e IPCE spectrum of the In:GaN/Ta₃N₅/Mg:GaN photoanode at 1.0 V vs. RHE and the corresponding solar photocurrent and integrated photocurrent calculated using the standard AM 1.5 G solar spectrum (ASTM G173-03). f Amount of O₂ evolved from the In:GaN/Ta₃N₅/Mg:GaN photoanode under an applied potential of 1.0 V vs. RHE.

Fig. 5. Electrochemical characterizations of the Ta₃N₅-based thin films with different layered structures on Nb substrate.

a Bulk charge separation efficiency (η_bulk). b Surface injection efficiency (η_inj). c PEIS of Ta₃N₅ and In:GaN/Ta₃N₅/Mg:GaN photoanodes measured in 1 M KOH electrolyte at 1.0 V vs. RHE under AM 1.5 G illumination. Red lines show the fitting of the PEIS data. M-S plots of d, Ta₃N₅ photoanode, and e In:GaN/Ta₃N₅/Mg:GaN photoanode. The M-S plots were measured in the dark without co-catalyst. f Carrier lifetimes derived from OCP-decay curves at the light on-off transient for Ta₃N₅ and In:GaN/Ta₃N₅/Mg:GaN photoanodes.