Victor Fung

Understanding seed-mediated growth of gold nanoclusters at molecular level

The continuous development of total synthesis chemistry has allowed many organic and biomolecules to be produced with known synthetic history–that is, a complete set of step reactions in their synthetic routes. Here, we extend such molecular-level precise reaction routes to nanochemistry, particularly to a seed-mediated synthesis of inorganic nanoparticles. By systematically investigating the time−dependent abundance of 35 intermediate species in total, we map out relevant step reactions in a model size growth reaction from molecularly pure Au25 to Au44 nanoparticles. The size growth of Au nanoparticles involves two different size−evolution processes (monotonic LaMer growth and volcano-shaped aggregative growth), which are driven by a sequential 2-electron boosting of the valence electron count of Au nanoparticles. Such fundamental findings not only provide guiding principles to produce other sizes of Au nanoparticles (e.g., Au38), but also represent molecular-level insights on long-standing puzzles in nanochemistry, including LaMer growth, aggregative growth, and digestive ripening.Synthetic nanochemistry currently lacks the molecular step-by-step routes afforded to organic chemistry by total synthesis. Here, the authors track the seeded growth of atom-precise gold nanoclusters using mass spectrometry, revealing that the clusters evolve through a series of intermediates in two-electron steps.

Precise control of alloying sites of bimetallic nanoclusters via surface motif exchange reaction

Precise control of alloying sites has long been a challenging pursuit, yet little has been achieved for the atomic-level manipulation of metallic nanomaterials. Here we describe utilization of a surface motif exchange (SME) reaction to selectively replace the surface motifs of parent [Ag44(SR)30]4− (SR = thiolate) nanoparticles (NPs), leading to bimetallic NPs with well-defined molecular formula and atomically-controlled alloying sites in protecting shell. A systematic mass (and tandem mass) spectrometry analysis suggests that the SME reaction is an atomically precise displacement of SR–Ag(I)–SR-protecting modules of Ag NPs by the incoming SR–Au(I)–SR modules, giving rise to a core-shell [Ag32@Au12(SR)30]4−. Theoretical calculation suggests that the thermodynamically less favorable core-shell Ag@Au nanostructure is kinetically stabilized by the intermediate Ag20 shell, preventing inward diffusion of the surface Au atoms. The delicate SME reaction opens a door to precisely control the alloying sites in the protecting shell of bimetallic NPs with broad utility.Doping metal nanoclusters at specific sites is a powerful strategy for tuning their properties. Here, the authors precisely control the alloying sites of bimetallic nanoclusters by replacing entire surface motifs with structurally similar heteroatom motifs, tuning the surface composition motif-by-motif rather than atom-by-atom.

Understanding oxidative dehydrogenation of ethane on Co3O4 nanorods from density functional theory

Co3O4 is a metal oxide catalyst with weak, tunable M–O bonds promising for catalysis. Here, density functional theory (DFT) is used to study the oxidative dehydrogenation (ODH) of ethane on Co3O4 nanorods based on the preferred surface orientation (111) from the experimental electron-microscopy image. The pathway and energetics of the full catalytic cycle including the first and second C–H bond cleavages, hydroxyl clustering, water formation, and oxygen-site regeneration are determined. We find that both lattice O and Co may participate as active sites in the dehydrogenation, with the lattice-O pathway being favored. We identify the best ethane ODH pathway based on the overall energy profiles of several routes. We identify that water formation from the lattice oxygen has the highest energy barrier and is likely a rate-determining step. This work of the complete catalytic cycle of ethane ODH will allow further study into tuning the surface chemistry of Co3O4 nanorods for high selectivity of alkane ODH reactions.

General Structure–Reactivity Relationship for Oxygen on Transition-Metal Oxides

Despite many recent developments in designing and screening catalysts for improved performance, transition-metal oxides continue to prove to be challenging due to the myriad inherent complexities of the systems and the possible morphologies that they can exhibit. Herein we propose a structural descriptor, the adjusted coordination number (ACN), which can generalize the reactivity of the oxygen sites over the many possible surface facets and defects of a transition-metal oxide. We demonstrate the strong correlation of this geometric descriptor with the electronic and energetic properties of the active sites on five facets of four transition-metal oxides. We then use the structural descriptor to predict C-H activation energies to show the great potential of using ACN for the prediction of catalytic performance. This study presents a first look into quantifying the relation between active site structure and reactivity of transition-metal-oxide catalysts.

Controlling Reaction Selectivity through the Surface Termination of Perovskite Catalysts

Although perovskites have been widely used in catalysis, tuning of their surface termination to control reaction selectivity has not been well established. In this study, we employed multiple surface-sensitive techniques to characterize the surface termination (one aspect of surface reconstruction) of SrTiO3 (STO) after thermal pretreatment (Sr enrichment) and chemical etching (Ti enrichment). We show, by using the conversion of 2-propanol as a probe reaction, that the surface termination of STO can be controlled to greatly tune catalytic acid/base properties and consequently the reaction selectivity over a wide range, which is not possible with single-metal oxides, either SrO or TiO2 . Density functional theory (DFT) calculations explain well the selectivity tuning and reaction mechanism on STO with different surface termination. Similar catalytic tunability was also observed on BaZrO3 , thus highlighting the generality of the findings of this study.

Exploring perovskites for methane activation from first principles

The diversity of perovskites offers many opportunities for catalysis, but an overall trend has been elusive. Using density functional theory, we studied a large set of perovskites in the ABO3 formula via descriptors of oxygen reactivity such as vacancy formation energy, hydrogen adsorption energy, and the first C–H activation energy of methane. It was found that changing the identity of B within a period increases the oxygen reactivity from the early to late transition metals, while changing A within a group has a much smaller effect on oxygen reactivity. Within the same group, B in the 3d period has the most reactive lattice oxygen compared to the 4d or 5d period. Some perovskites display large differences in reactivity for different terminations. Further examination of the second C–H bond breaking on these perovskites revealed that larger A cations and non-transition metal B cations have higher activation energies, which is conducive to the formation of coupling products instead of oxidation to CO or CO2. Balance between the first C–H bond breaking and methyl desorption suggests a just right oxygen reactivity as described by the hydrogen adsorption energy. These insights may help in designing better perovskite catalysts for methane activation.

Trends of Alkane Activation on Doped Cobalt (II, III) Oxide from First Principles

The surface doping of a metal oxide can tune its catalytic performance, but it remains unclear how the tuning depends on the dopant type and the surface facet. Herein we study doped Co3O4 (1 1 1) and (3 1 1) surface facets using first‐principles density functional theory (DFT) to obtain general descriptors for oxygen reactivity (which include vacancy formation energy and hydrogen adsorption energy) and correlate them to ethane C−H activation energy as a measure of the catalytic performance. The periodic trends of the dopants are investigated for a total of 20 dopants, namely, the elements from K to Ge. We find strong linear correlations between the oxygen reactivity descriptors and the computed energy barriers. We also discover a strong surface facet sensitivity among certain dopants such that different surface orientations and sites lead to different or even the opposite dopant performance. This work provides a useful guide for dopant performance in ethane activation on the two very different Co3O4 surfaces.

Golden single-atomic-site platinum electrocatalysts

Bimetallic nanoparticles with tailored structures constitute a desirable model system for catalysts, as crucial factors such as geometric and electronic effects can be readily controlled by tailoring the structure and alloy bonding of the catalytic site. Here we report a facile colloidal method to prepare a series of platinum–gold (PtAu) nanoparticles with tailored surface structures and particle diameters on the order of 7 nm. Samples with low Pt content, particularly Pt4Au96, exhibited unprecedented electrocatalytic activity for the oxidation of formic acid. A high forward current density of 3.77 A mgPt−1 was observed for Pt4Au96, a value two orders of magnitude greater than those observed for core–shell structured Pt78Au22 and a commercial Pt nanocatalyst. Extensive structural characterization and theoretical density functional theory simulations of the best-performing catalysts revealed densely packed single-atom Pt surface sites surrounded by Au atoms, which suggests that their superior catalytic activity and selectivity could be attributed to the unique structural and alloy-bonding properties of these single-atomic-site catalysts.Bimetallic nanoparticles with tailored structure constitute a desirable model system for catalysts. PtAu nanoparticles with Pt single-atom surface sites, prepared by a colloidal method, exhibit unprecedented electrocatalytic activity for formic acid oxidation.

Synthesis of Water-Soluble [Au25(SR)18]− Using a Stoichiometric Amount of NaBH4

Determination of the stoichiometry of reactions is a pivotal step for any chemical reactions toward a desirable product, which has been successfully achieved in organic synthesis. Here, we present the first precise determination of the stoichiometry for the reactions toward gold nanoparticle formation in the sodium borohydride reduction method. Leveraging on the real-time mass spectrometry technique, we have determined a precise balanced reaction, 32/ x [Au(SR)] x + 8 e- = [Au25(SR)18]- + 7 [Au(SR)2]- (here SR denotes a thiolate ligand), toward a stoichiometric synthesis of water-soluble [Au25(SR)18]-, where 8 electrons (from reducing agents) are sufficient to react with every 32 Au atoms, leading to the formation of high-purity [Au25(SR)18]-. More interestingly, by real-time monitoring of the growth process of thiolate-protected Au nanoclusters, we have successfully identified an important yet missing byproduct, [Au(SR)2]-. This study not only provides a new method for Au nanocluster synthesis using only a stoichiometric amount of reducing agent in aqueous solutions (although the synthesis of organic-soluble Au nanoclusters might require a more delicate design of synthetic chemistry) but also promotes the mechanistic understandings of the Au nanocluster growth process.

New Bonding Model of Radical Adsorbate on Lattice Oxygen of Perovskites

A new model of bonding between radical adsorbates and lattice oxygens is proposed that considers both the adsorbate-oxygen bonding and the weakening of the metal-oxygen bonds. Density functional calculations of SrMO3 perovskites for M being 3d, 4d, and 5d transition metals are used to correlate the bulk electronic structure with the surface-oxygen reactivity. Occupation of the metal-oxygen antibonding states, examined via the crystal orbital Hamilton population (COHP), is found to be a useful bulk descriptor that correlates with the vacancy formation energy of the lattice oxygen and its hydrogen adsorption energy. Analysis of density-of-states and COHP indicates that H adsorption energy is a combined result of formation of the O-H bond and the weakening of the surface metal-oxygen bond due to occupation of the metal-oxygen antibonding states by the electron from H. This insight will be useful in understanding the trends in surface reactivity of perovskites and transition-metal oxides in general.