Insight into the Rate-Determining Step in Photocatalytic Z-Scheme Overall Water Splitting by Employing A Series of Perovskite RTaON2 (R = Pr, Nd, Sm, and Gd) as Model Photocatalysts

Abstract

Photocatalysis is an intricate process that involves a multitude of physical and chemical factors operating across diverse temporal and spatial scales. Identifying the dominant factors that influence photocatalyst performance is one of the central challenges in the field. Here, we synthesized a series of perovskite RTaON₂ semiconductors with different A-site rare earth atoms (R = Pr, Nd, Sm, and Gd) as model photocatalysts to discuss the influence of the A-site modulation on their local structures as well as both physical and chemical properties and to get insight into the rate-determining step in photocatalytic Z-scheme overall water splitting (OWS). It is interesting to find that, with a decreasing ionic radius of the A-site cations, the RTaON₂ compounds exhibit continuous blue shift of light absorption and a concomitant reduction in the lifetime of photogenerated carriers, revealing a significant influence of A-site atoms on the light absorption and charge separation processes. On the other hand, the A-site atomic substitution was revealed to significantly modulate the valence band positions as well as surface oxidation kinetics. By employing the Pt-modified RTaON₂ as H₂-evolving photocatalysts, the activity of photocatalytic Z-scheme OWS for hydrogen production on them is found to be determined by its surface oxidation process instead of light absorption or charge separation. Our results give the first experimental demonstration of the rate-determining step during the photocatalytic Z-scheme OWS processes, as should be instructive for the design and development of other efficient solar-to-chemical energy conversion systems.

Figure 1

Structural and morphological characterizations. (a) XRD and the enlarged XRD patterns for the series of RTaON₂ compounds with R representing Pr, Nd, Sm, and Gd, including enlarged views to highlight subtle variations. (b) Schematic representation of the crystal structure for GdTaON₂. (c–g) SEM images of (c) PrTaON₂, (d) NdTaON₂, (e) SmTaON₂, and (f, g) GdTaON₂. (h–k) EDS elemental mapping images of Gd, Ta, O, and N for GdTaON₂.

Figure 2

Band structure characteristics. (a) UV–vis DRS. (b) Partial DOS of GdTaON₂. (c) Band structure of GdTaON₂. (d) Tauc plots. (e) Mott–Schottky plots. (f) Diagram of the band structure.

Figure 3

(a) TRPL. (b) TAS recorded at 20,000 cm^–1. (c) Fourier transform-extended X-ray absorption fine structure spectra of Ta L-edge for RTaON₂ (R = Pr, Nd, Sm, and Gd).

Figure 4

Photocatalytic and photoelectrochemical performance. (a) Rate of H₂ evolution on 1.5 wt % Pt/RTaON₂. (b) Current–potential curves of the RTaON₂ electrode in 10 vol % CH₃OH 0.5 M Na₂SO₄ solution under chipped irradiation. (c) Rate of H₂ and O₂ evolution on a Z-scheme system for OWS with 1.5 wt % Pt/RTaON₂ as a H₂-evolving photocatalyst, 0.5 wt % PtO_x/WO₃ as an O₂-evolving photocatalyst, and IO₃^–/I^– as shuttle ions. (d) Current–potential curves of the RTaON₂ electrode in 0.1 M KI under chipped irradiation.