Effect of a redox-mediating ligand shell on photocatalysis by CdS quantum dots

Abstract

Semiconductor quantum dots (QDs) are efficient organic photoredox catalysts due to their high extinction coefficients and easily tunable band edge potentials. Despite the majority of the surface being covered by ligands, our understanding of the effect of the ligand shell on organic photocatalysis is limited to steric effects. We hypothesize that we can increase the activity of QD photocatalysts by designing a ligand shell with targeted electronic properties, namely, redox-mediating ligands. Herein, we functionalize our QDs with hole-mediating ferrocene (Fc) derivative ligands and perform a reaction where the slow step is hole transfer from QD to substrate. Surprisingly, we find that a hole-shuttling Fc inhibits catalysis, but confers much greater stability to the catalyst by preventing a build-up of destructive holes. We also find that dynamically bound Fc ligands can promote catalysis by surface exchange and creation of a more permeable ligand shell. Finally, we find that trapping the electron on a ligand dramatically increases the rate of reaction. These results have major implications for understanding the rate-limiting processes for charge transfer from QDs and the role of the ligand shell in modulating it.

Ligand Desorption and Fragmentation in Oleate-Capped CdSe Nanocrystals under High-Intensity Photoexcitation

“…Given that CW excitation results in a lower ⟨ N ⟩ than pulsed, we are in a regime where the vast majority of NCs have one or no excitons suggesting that Auger processes, multiphoton absorption, or ionization may be critical to ligand loss. It is important to note that higher intensity CW fluences have produced ligand loss and are accessed in LED and photocatalysis experiments. ,,, …”

“…It is important to note that higher intensity CW fluences have produced ligand loss and are accessed in LED and photocatalysis experiments. 31,32,44,45 Conversion of the collected data from Figure 3A to photons/NC provides a measure of the total absorbed dose across the four samples (Figure 3C). When plotted in this manner, ligand loss percentage appears to follow a trend between samples, with increasing desorption occurring as photon flux increases.…”

Pathways of CdS Quantum Dot Degradation during Photocatalysis: Implications for Enhancing Stability and Efficiency for Organic Synthesis

“…In terms of designing better catalysts, a thin shell that does not impede the extraction of either electrons or holes may be beneficial, but the shell may still be subject to the same degradation pathways as a core-only QD. A few methods of designing QDs that are potentially beneficial for catalysis and stability are the “ligand locking” method reported by several groups, , growth of a thin, oxide shell, or the use of appropriate redox mediators or redox-active ligands. , If the catalyst can withstand the flux of charge transfer and maintain its structure, then we can achieve a recyclable and scalable system.…”

Pathways of Quantum Dot Degradation during Photocatalysis

“…Alternatively, preventing charge accumulation in the QDs through the use of appropriate redox mediators or redox-active ligands may provide a path forward. 24,25 This work demonstrates QD photocatalyst degradation pathways that remain underreported. We show that a complete catalytic cycle leads to the etching of the CdS QD surface, loss of colloidal stability, and the formation of micro-crystalline cadmium metal that may serve as electron sinks for substrate reduction.…”