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Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

Gold octamers (Au8) bound to oxygen-vacancy F-center defects on Mg(001) are the smallest clusters to catalyze the low-temperature oxidation of CO to CO2, whereas clusters deposited on close-to-perfect magnesia surfaces remain chemically inert. Charging of the supported clusters plays a key role in promoting their chemical activity. Infrared measurements of the stretch vibration of CO adsorbed on mass-selected gold octamers soft-landed on MgO(001) with coadsorbed O2 show a red shift on an F-center-rich surface with respect to the perfect surface. The experiments agree with quantum ab initio calculations that predict that a red shift of the C-O vibration should arise via electron back-donation to the CO antibonding orbital.

https://science.sciencemag.org/content/sci/307/5708/403.full.pdf

Charging Effects on Bonding and Catalyzed Oxidation of CO on Au8 Clusters on MgO

Highly fluorescent, water-soluble, few-atom noble-metal quantum dots have been created that behave as multielectron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near infrared. These molecular metals exhibit highly polarizable transitions and scale in size according to the simple relation EFermi/N1/3, predicted by the free-electron model of metallic behavior. This simple scaling indicates that fluorescence arises from intraband transitions of free electrons, and these conduction-electron transitions are the low-number limit of the plasmon—the collective dipole oscillations occurring when a continuous density of states is reached. Providing the missing link between atomic and nanoparticle behavior in noble metals, these emissive, water-soluble Au nanoclusters open new opportunities for biological labels, energy-transfer pairs, and light-emitting sources in nanoscale optoelectronics.

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Highly fluorescent noble-metal quantum dots.

Reducing carbon dioxide to hydrocarbon fuel with solar energy is significant for high-density solar energy storage and carbon balance. In this work, single atoms of palladium and platinum supported on graphitic carbon nitride (g-C3N4), i.e., Pd/g-C3N4 and Pt/g-C3N4, respectively, acting as photocatalysts for CO2 reduction were investigated by density functional theory calculations for the first time. During CO2 reduction, the individual metal atoms function as the active sites, while g-C3N4 provides the source of hydrogen (H*) from the hydrogen evolution reaction. The complete, as-designed photocatalysts exhibit excellent activity in CO2 reduction. HCOOH is the preferred product of CO2 reduction on the Pd/g-C3N4 catalyst with a rate-determining barrier of 0.66 eV, while the Pt/g-C3N4 catalyst prefers to reduce CO2 to CH4 with a rate-determining barrier of 1.16 eV. In addition, deposition of atom catalysts on g-C3N4 significantly enhances the visible-light absorption, rendering them ideal for visible-light reduction of CO2. Our findings open a new avenue of CO2 reduction for renewable energy supply.

/pdf/single-atom-pd-pt-supported-on-graphitic-carbon-nitride-as-1o5n8h0lte.pdf

Single Atom (Pd/Pt) Supported on Graphitic Carbon Nitride as an Efficient Photocatalyst for Visible-Light Reduction of Carbon Dioxide.

Temperature-dependent rf-ion trap mass spectrometry and first-principles simulations reveal the detailed reaction mechanism of the catalytic gas-phase oxidation of CO by free Au2- ions in the presence of O2. A metastable intermediate with a mass of Au2CO3- was observed at low temperatures. Two alternative structures corresponding to digold carbonate or peroxyformate are predicted for this intermediate. Both structures are characterized by low activation barriers for the formation of CO2. These combined experimental and theoretical investigations provide a comprehensive understanding of the kinetics, energetics, and atomic arrangements along the reaction path, thus allowing a formulation of the catalytic cycle for the oxidation reaction.

Catalytic CO oxidation by free Au2-: experiment and theory.

Since the advent of intense cluster sources, physical and chemical properties of isolated metal clusters are an active field of research. In particular, gas phase metal clusters represent ideal model systems to gain molecular level insight into the energetics and kinetics of metal-mediated catalytic reactions. Here we summarize experimental reactivity studies as well as investigations of thermal catalytic reaction cycles on small gas phase metal clusters, mostly in relation to the surprising catalytic activity of nanoscale gold particles. A particular emphasis is put on the importance of conceptual insights gained through the study of gas phase model systems. Based on these concepts future perspectives are formulated in terms of variation and optimization of catalytic materials e.g. by utilization of bimetals and metal oxides. Furthermore, the future potential of bio-inspired catalytic material systems are highlighted and technical developments are discussed.

Gas phase metal cluster model systems for heterogeneous catalysis.

Gas-phase kinetics and catalytic reactions of small silver and gold clusters

The cluster ions Aun(CO)m(O2)u−
(n = 2,3; m = 1, u = 1,2) are successfully synthesized via coadsorption of oxygen and carbon monoxide onto mass-selected gold clusters inside a temperature controlled ion trap at 100 K. The investigated gold clusters Au1–3− show a significant size dependence in the reactivity toward oxygen and carbon monoxide. Whereas no reaction products are detected for the monomer ion, simultaneous adsorption of CO and O2 onto the dimer leads to the formation of Au2(CO)(O2)− which is regarded as a model complex to study the mechanism of catalytic CO oxidation on gold clusters. In the case of Au3− the conditioning of
the cluster by CO preadsorption is found essential to enable O2 coadsorption.

Thorsten M. Bernhardt

Papers

Catalytic CO oxidation by free Au2-: experiment and theory.

Gas phase metal cluster model systems for heterogeneous catalysis.

Gas-phase kinetics and catalytic reactions of small silver and gold clusters

Coadsorption of CO and O2 on small free gold cluster anions at cryogenic temperatures: Model complexes for catalytic CO oxidation

Methane Activation and Catalytic Ethylene Formation on Free Au2