Favorable Carrier Dynamics in a Narrow-Gap Layered Oxynitride Enable Efficient Molecular Hybrid CO2 Photoreduction

Abstract

Efficient interfacial electron transfer from light-harvesting semiconductors to molecular catalysts remains a major challenge in molecular–semiconductor hybrid photocatalytic CO2 reduction, particularly for narrow-bandgap semiconductors, whose reduced thermodynamic driving force often requires assistance from metallic electron-transfer mediators such as Ag. Here, we show that, despite its narrow bandgap of 2.1 eV, the layered perovskite oxynitride K2LaTa2O6N (KLTON) functions as an effective semiconductor support for a supramolecular RuRu′ CO2-reduction catalyst without relying on Ag nanoparticles. The RuRu′/KLTON hybrid selectively converted CO2 to formate under visible-light irradiation with >99% selectivity, a turnover number of 691, and an apparent quantum yield of 6.1 ± 0.6% at 420 nm. Loading a small amount of Ag further increased the turnover number to 1077, but this 1.5-fold enhancement was modest compared with the larger Ag-induced activity enhancements reported for several conventional oxynitride-based hybrids, highlighting the weak Ag dependence of the KLTON-based system. Spatial photodeposition of Ag and MnOx revealed spatially differentiated surface redox reactivity on the terraced layered structure of KLTON, while time-resolved microwave conductivity and transient infrared absorption measurements showed favorable photogenerated-carrier dynamics and demonstrated that Ag loading modifies the electron dynamics of KLTON. Time-resolved emission measurements further revealed an additional nonradiative decay pathway at the Ru(II) photosensitizer/KLTON interface, consistent with interfacial electronic interaction without uniquely assigning its microscopic origin. These results identify favorable carrier dynamics and weak Ag dependence as key features of the KLTON-based molecular hybrid and highlight layered narrow-bandgap mixed-anion semiconductors as promising support materials for visible-light-driven molecular photocatalysis.