Nanoparticle Capping Ligands
Capping ligands are molecules bound to nanoparticle surfaces, such as thiols or phosphines, that control growth and keep particles colloidally stable through steric and electrostatic repulsion. In catalysis they are often regarded as poisons that block active sites, but they can also determine whether discrete, active particles exist at all, and can modify activity and selectivity.
Ligands as enablers of sustained photocatalytic activity (2022)
Simon et al. found that strongly binding thiolated ligands (PEG thiols from about 271 Da to 5 kDa, and 8-mercaptooctanoic acid) were needed to keep in situ formed Au, Au/Cu, Ni, Co and Pd nanoparticles discrete and active for photocatalytic hydrogen evolution over three irradiation rounds (Simon et al. 2021). The more labile phosphine BSPP stabilized pure Au but not Au/Cu, which aggregated and lost activity. The authors conclude that the role of ligands is more complex and potentially more beneficial than usually assumed, and that ligand presence and identity should be considered in both experiment and computational prediction. Summary: simon2022.
Optimal ligand loading for in situ cocatalysts (2022)
Building on the finding that PEGSH keeps in situ formed NPs active, Bhat and coworkers mapped activity against PEGSH and metal concentration for Au, Cu, Ni and Fe (Bhat et al. 2022). Au and Cu activity was less sensitive to ligand than to metal concentration, while Ni and Fe were more sensitive to ligand concentration; optima lay near 0.8 to 1.1 mM PEGSH for 1.2 to 1.6 mM metal. TEM after 45 hours of illumination showed discrete, unaggregated particles for all four metals, and the authors recommend about 1 mM metal with 1 mM PEGSH as a high-activity region. Summary: bhat2022.