Light Intensity Dependence of Photochemical Charge Separation in the BiVO₄/Ru-SrTiO₃:Rh Direct Contact Tandem Photocatalyst for Overall Water Splitting

Abstract

The energy conversion efficiency of tandem photocatalysts for the overall water splitting reaction (OWS) is currently limited by our understanding of carrier separation and recombination in such systems. What is the effect of the solid–solid and solid–liquid interfaces on the carrier dynamics, and how do the photovoltage and catalytic activity depend on the light intensity? In order to address these issues, we report here on the light intensity-dependent water splitting activity and open circuit potential (OCP) measurements for a core–shell tandem made from bismuth vanadate (BiVO4) microparticles and ruthenium-loaded rhodium-doped strontium titanate (Ru-SrTiO3:Rh) nanoparticles. The measurements identify three operational regimes of the tandem: a threshold intensity of 8–14 mW cm–2 below which no OWS occurs, a regime of strongly increasing apparent quantum efficiency (AQE) (17.7–70.2 mW cm–2), and a regime of nearly constant AQE (>171 mW cm–2). Open circuit potential measurements of the separate BiVO4 and Ru-SrTiO3:Rh absorbers in dilute H2SO4 solution (pH 3.5) confirm photoanodic behavior for BiVO4 and photocathodic behavior for Ru-SrTiO3:Rh and provide the light intensity dependent quasi-Fermi energies for the majority carriers in each material. The data allows modeling of the charge transfer dynamics in the tandem. In the dark, the materials form a weak p/n-junction which causes minority carrier recombination at the tandem contact and impedes the function of the photocatalyst under low light flux. At higher light intensity, charge separation of the tandem is increasingly controlled by minority carrier transfer at the solid/liquid contacts. As a result, majority carriers can flow to the SrTiO3:Rh/BiVO4 interface and recombine there and help equilibrate the majority carrier Fermi levels of both absorbers to a common value. The resulting shift of the band edges of both absorbers improves the rectifying character of the solid–liquid contacts and is the basis for the increase of the AQE from 0 to 1.11% (400 nm). Above 171 mW cm–2, the AQE of the tandem remains nearly constant and becomes limited by intrinsic lattice and interfacial recombination of the two absorbers, by the low absorption coefficient of SrTiO3:Rh, and by the slow water oxidation kinetics of the BiVO4 surface. Finally, under very strong illumination, the H2/O2 back reaction becomes rate-limiting. These insights will be useful for the optimization of OWS tandem photocatalysts, especially under light limiting conditions.

Charge carrier mapping for Z-scheme photocatalytic water-splitting sheet via categorization of microscopic time-resolved image sequences

“…A general understanding of the mechanism of the Z-scheme system is explained here based on the description provided by Osterloh et al 48 . Upon irradiation with visible light, both BVOM and STOR are photo-excited, and the photo-generated holes in BVOM and photo-generated electrons in STOR are used for water splitting to oxygen and hydrogen, respectively.…”

Fermi Level Pinning Controls Band Bending and Photochemical Charge Separation in Particles ofn-SrTiO3,n-SrTiO3:Al, andn-GaAs:Te

“…Electrochemical and impedance measurements provide the built-in potential and free carrier density, among other information. − Junctions in heterogeneous photocatalysts for the water-splitting reaction, − are more difficult to study with electrochemical methods because charge transfer in particle films is slowed by the high ohmic resistance of the contacts between the particles. − The popular Mott–Schottky analysis is no longer straightforward for particle films because the assumptions of a flat interface and of a negligible surface charge are no longer valid . Open-circuit potential (OCP) measurements provide a solution to these problems. ,,− At open circuit, there is no photocurrent, and transport-related potential losses can be minimized. For example, Hu’s group used open-circuit potential measurements on particle films to confirm that the band bending in SrTiO 3 photocatalyst particles is adaptive and controlled by the local kinetics and redox potentials of the reagents and products. , Similar adaptive junctions had been previously observed at TiO 2 /NiOOH photoelectrode interfaces using dual working electrode measurements .…”

Binary Transition Metal NiFeOx and CoFeOx Cocatalysts Boost the Photodriven Water Oxidation over Fe2TiO5 Nanoparticles

“…This assumption is reinforced by the fact that the O 2 evolution rate with NiFeO x /FeTO-S1 is superior to that with NiFeO x /FeTO-S0.5 up to 3 h and declines after it. [12,56,58] In the case of the nanomaterials modified with CoFeO x , we achieved an O 2 production enhancement from 7.0 μmol, for the pristine Fe 2 TiO 5 , to 43.6 μmol in 5 h, just with 10 min of MSD deposition with no production rate decline. This yield was improved to 56.0 μmol with the deposition time of 20 min (CoFeO x /FeTO-S20).…”

“…The larger cocatalyst particles tend to build up the resistance of the charge transfer to the liquid medium, as the path that the photoholes travel across the cocatalyst to reach the solid/liquid interface is longer. This assumption is reinforced by the fact that the O 2 evolution rate with NiFeO x /FeTO‐S1 is superior to that with NiFeO x /FeTO‐S0.5 up to 3 h and declines after it [12,56,58] …”