Understanding charge separation in CdS/Ce-UiO66-NH2 heterojunctions for enhanced photocatalytic hydrogen evolution†‡

Abstract

UiO metal–organic frameworks (MOFs) are regarded as promising photocatalysts due to their unique stability and band designability. We recently demonstrated that the cerium-based Ce-UiO-NH₂ exhibited an enhanced hydrogen evolution relative to zirconium (Zr)-UiOs, when loaded with cadmium sulfide (CdS). However, the underlying charge separation dynamics of this system is unclear. In this work, we optimised the CdS loading and used transient absorption and electrochemical spectroscopy to investigate the charge separation dynamics and energetics in the CdS/Ce-UiO-NH₂ heterojunction. The optimised heterojunction showed improved stability and achieved an external quantum efficiency (EQE) of 2.2% under 420 nm LED illumination whilst using methanol as a sacrificial agent. The heterojunction facilitates charge separation, generating long-lived (ms) holes on Ce-UiO-NH₂ and electrons on the CdS. In contrast with electron-accepting Zr-UiOs, this study reveals a reversed charge separation direction in CdS/UiO heterojunctions with Ce-UiO-NH₂ acting as the electron donor.