The Ligand Shell as an Energy Barrier in Surface Reactions on Transition Metal Nanoparticles

Abstract

Transition metal nanoparticles, including those employed in catalytic, electrocatalytic, and photocatalytic conversions, have surfaces that are typically coated with a layer of short or long-chain ligands. There is little systematic understanding of how much this ligand layer affects the reactivity of the underlying surface. We show for Ag nanoparticles that a surface-adsorbed thiol layer greatly impedes the kinetics of an ionic chemical reaction taking place on the Ag surface. The model reaction studied is the galvanic exchange of Ag with Au(3+) ions, the kinetics of which is measured on individual thiol-coated nanoparticles using in situ optical scattering spectroscopy. We observe a systematic lowering of the reactivity of the nanoparticle as the chain length of the thiol is increased, from which we deduce that the ligand layer serves as an energy barrier to the transport of incoming/outgoing reactive ions. This barrier effect can be decreased by light irradiation, resulting from weakened binding of the thiol layer to the metal surface. We find that the influence of the surface ligand layer on reactivity is much stronger than factors such as nanoparticle size, shape, or crystallinity. These findings provide improved understanding of the role of ligand or adsorbates in colloidal catalysis and photocatalysis and have important implications for the transport of reactants and ions to surfaces and for engineering the reactivity of nanoparticles using surface passivation.

Tuning the Accessibility and Activity of Au25(SR)18 Nanocluster Catalysts through Ligand Engineering

“…[21] This is consistentw ith ap revious observation, where as hort thiolate ligand length would form at hin protecting layer around the gold core, whereas al onger thiolate ligand might be self-organized into bundles owing to the strong hydrophobic interaction between theh ydrophobic alkyl chain, thus increasing the packing density of the ligand shell that could create at hicker barrierw ith more steric hindrancea nd highere nergy barrier for the diffusion of the reactants. [13,35] This could readily explain the catalytic trend observed in the presents tudy.I np articular,a ni ncreasei nt he ligand chain length would provide ah igher density barrier for the reactants to diffuse inside the ligand shell to approacht he goldc ore of Au 25 (SR) 18 NCs, leading to al ongeri nductionp eriod and al ower catalytic activity.…”

“…For example, several studies suggested negative effects of ligands on the catalytica ctivity of Au NCsb ecause the ligand shell on the NC surfacec ould createaphysicalb arrier,w hich would hindert he diffusion of substrates to the catalytic actives ites. [1,13,[27][28][29] On the other hand,s ome studies also highlighted positivee ffects of the ligandso nt he catalytic performance of Au NCs. [30,31] For example, Katz et al demonstratedt hat different-sized calixarene ligandsc ould be used to facilitate the accessibility of substrates towardst he metal core of Au NCs.…”

“…For example,o rganic ligands like phosphine, [3] thiol, [4,5] selenolate, [6] alkyne, [7,8] carbene, [9] and amine, [10] have been successfully used to modulate the accessibility,a ctivity,s electivity, ands tability of the homogeneous catalyst, based on their steric hindrance and electronic effects on the metal catalytic active sites. [3,4,[11][12][13] As imilar type of ligand engineering could also be realized in an emerging class of catalysts in the field, so called ligand-protected gold nanoclusters (Au NCs) or more specifically thiolate-protectedAu NCs. [2,14] Similar to the homogeneous catalyst, thiolate-protectedA uN Cs also feature with well-definedm olecular size and structure, andt heir physicochemical properties could also be readily controlled by the thiolate ligands.…”

Colloidally Stable CdS Quantum Dots in Water with Electrostatically Stabilized Weak‐Binding, Sulfur‐Free Ligands

“…Access of molecular substrates to the inorganic surface of the QD is necessary for applications in photo-redox catalysis, in order to provide sufficient electronic coupling for electron and hole extraction, [15][16][17] so water-solubilization strategies that involve replacing the hydrophobic shell of the QD with as ilica shell or encapsulatingt he hydrophobic QD in an amphiphilic polymer are not suited for QD-basedc atalysis. [18,19] Exchange of native hydrophobicligandsfor very polar or charged ligands is av ersatile strategy for creatingw ater-solublep articles, since not only the chemicals tructure but also the density of ligands can be controlled. In addition to providing access to the QD surface and, ideally,e ncouraging catalytic substrates to adsorb in activated geometries,s uch ligandsn eed to make the particles colloidally stable in water for the lifetimeo ft he illumination (or longer)b yi nhibiting agglomeration,w hichd ecreases catalytically actives urfacea rea and eventually leads to precipitation.T he most basic requirement for preventing agglomeration is that ligands mustn ot desorbf rom the QD surface at the catalytically relevant pH, ionic strength,a nd reagent/product concentrations.…”

Effect of Protective Agents upon the Catalytic Property of Platinum Nanocrystals

“…Nevertheless, protective agents, such as polymers, ligands, and surfactants, are usually required during preparation to obtain stable monodisperse nanocrystals with specific morphologies and structures. For better or worse, protective agents adsorbed on the surface of nanocrystals will extremely affect the catalytic property of the nanocatalyst . Consequently, research on the effect of protective agents upon the catalytic property of nanocrystals is significant for understanding the intrinsic reaction behaviors of reactant molecules over nanocrystal catalysts, and helpful for further design and application of highly efficient catalysts prepared with various nanocrystals as building blocks.…”