Gold Nanoclusters

Electronic structure, catalysis, and energy transfer in metallic nanoparticles

Colloidal gold nanoparticles (AuNPs) have garnered immense interest due to the unique effects quantum confinement introduces over their electronic structure, including the localized surface plasmon resonance, superparamagnetism, oxidative/reductive catalytic activity, and the photothermal effect. Several spectroscopic techniques have been used to probe how the ligand sphere, chemical environment, average size, and shape control their chemical and physical properties, however, generalizations are often necessary for interpretation due to the ∼10% or greater size dispersity inherent to many preparation methods. Our group is able to overcome this obstacle by using cryogenic ion vibrational predissociation (CIVP) action spectroscopy to measure mass-selective vibrational or electronic spectra. This technique affords us the opportunity to spectroscopically investigate how the chemical/physical parameter space of AuNPs influences nanocrystalline growth, metal-ligand electronic coupling, and interparticle electronic coupling – phenomena fundamental to synthetic methodology, chemosensing, molecular electronics, and photocatalysis.

Atomically-tailored nanocluster catalysts for CO₂  reduction

Nanoclusters have generated much excitement for their potential to serve as designer catalysts, essentially nanoclusters with optimized, reproducible active sites for efficient generation of products.  However, the necessity to separate and isolate nanoclusters from polydisperse synthetic mixtures makes it difficult to perform classic catalysis optimization studies, producing "volcano plots," and other systematic studies.  We use the mass spectrometric nature of our experiments as a shortcut: we can select specific cluster compositions out of a polydisperse mixture and probe elementary reactions critical for their mechanisms.  We hope to identify particularly promising compositions that are worth the effort to isolate and use as catalysis, as well as understand the underlying synthetic principles that can be used to tune their performance.

A major part of this effort is understanding the nature of species very transiently bound to the surface of clusters, most importantly, hydrogens.  These surface-bound hydrogens often formally appear as hydrides, but they strongly couple with the molecular orbitals delocalized across the gold cluster core.  However, their transient nature and the difficulty of observing them in x-ray crystal structures makes them hard to study.  We can straightforwardly isolate these species by mass spectrometry to probe their chemical nature and reactivity.

Intermediates involved in AuNP crystal growth

Several syntheses of colloidal AuNPs follow a common recipe: (1) Introduce a gold(III) complex to a capping agent (or ligand), and (2) reduce the metal center to induce metal-metal bonding to produce nanoparticles (Figure 1). Mass spectra reveal that, during synthesis, many intermediates are formed which fall outside the so-called “magic cluster” size regime. Understanding the evolution of sub-nm clusters into the final product will shed light onto the growth mechanism at play, and will allow us to understand how to better control size dispersion and particle chemistry. Furthermore, spectroscopic studies of non-magic clusters promise to reveal how clusters on the order of 1 – 2 atoms above or below the magic cluster shell closing impact the electronic stability, influence the growth mechanism, and perturb the metallic core geometry.

Ligand-metal core electronic coupling 

Electron delocalization within inorganic molecules is often achieved by complexing aromatic ligands to metal centers, and adjusting the ligand binding group or ancillary functional groups to improve metal-to-ligand electron transfer. Surprisingly, recent work by several groups has shown that ligand-protected nanoparticles can be thought of as an oversized inorganic complex; the extension of classical inorganic reasoning to electron delocalization in metallic nanoparticles has seldom been applied to harvesting electrons. We plan to study the effects of ligand-metal core electronic coupling by studying the evolution of vibrational and electronic absorption spectra as a function of the number of methylene groups between the binding head group and an aromatic tail group. Such studies will reveal the extent to which surface electrons are capable of tunneling within theligand environment, and how this may be controlled chemically (Figure 2). Additionally, changing the size of the metal core will elucidate how the electronic wave function of the metal partitions between surface and subsurface states as a function of diameter. 

Interparticle electron transfer 

As a natural extension to examining the effect of ligand-metal core electronic coupling, it is critical to be able to separate the electron from the originating core over long distances. Using molecular inorganic analogues as a hypothetical basis, tethering two particles between a conjugated bridge is expected to increase charge mobility, while tethering cores of two distinct sizes will provide a thermodynamic driving force for charge separation (Figure 3). Such supramolecular “mixed valence” nanomaterials have yet to be realized due to the difficulty associated with their preparation, along with simultaneously disentangling the effects of a dielectric continuum, ligand, and particle polydispersity from nearly featureless optical absorption spectra. Mass selection and variable temperature control from 3 K up to 310 K in the gas phase puts our method in a unique position to isolate and examine the spectral signatures of electron transfer in bridged nanomaterials, and will allow us to establish the rules governing interparticle electron transfer.

This project is supported by: