Dual-Sensitizer Photoanode for Bromide Oxidation

A dual-sensitizer mesoporous thin-film photoanode has been characterized for visible light-driven bromide oxidation in an aqueous pH 5.6 solution. The thin film is composed of interconnected nanoparticles with a rutile SnO2 core, a TiO2 shell sensitized to visible light with (1-cyano-2-(4-(di-p-tolylamino)phenyl)vinyl)phosphonic acid (Org), and an Al2O3 overlayer to which [Ru(bpz)2(4,4′-(PO3H2)2-2,2-bipyridine]2+ (Ru) was anchored, where bpz is 2,2′-bipyrazine. This material herein referred to as CS|Org|Al2O3|Ru is composed of two spatially isolated sensitizers: Org, a potent photoreductant that facilitates quantitative excited-state electron injection into the core/shell nanoparticle (ϕ = 1), and Ru that regenerates Orgox and catalyzes bromide oxidation. The Ruox product was found to react with bromide with a rate constant kreg = 2 × 107 M–1 s–1. In an operational HBr splitting cell, the dual-sensitizer photoanode sustained 200 μA/cm2 of photocurrent, significantly outperforming photoanodes with either Org (40 μA/cm2) or Ru (2 μA/cm2) alone. The photocurrent enhancement was achieved in spite of a nonproductive reductive quenching pathway (Org + Ru* → Orgox + Rured) that was identified through transient absorption spectroscopy. The thickness of the insulating Al2O3 layer between the two sensitizers was found to impact the yield of the reductive quenching pathway. Time-resolved anisotropy measurements with Monte Carlo simulations provided the rate constant for the lateral intermolecular Ru* + Ru ↔ Ru + Ru* energy transfer across an insulating oxide surface, a behavior expected to enhance the probability of encounters with Orgox in CS|Org|Al2O3|Ru. The data indicate that a dual-sensitizer photoanode approach could be utilized for a mediated water oxidation that exploits conditions where catalysis is more favorable.