First-principles modelling of the new generation of subnanometric metal clusters: Recent case studies

The very recent development of highly selective techniques making possible the synthesis and experimental characterization of subnanometric (subnanometer-sized) metal clusters (even single atoms) is pushing our understanding far beyond the present knowledge in materials science, driving these clusters as a new generation of quantum materials at the lower bounds of nanotechnology. When the size of the metal cluster is reduced to a small number of atoms, the d-band of the metal splits into a subnanometric d-type molecular orbitals network in which all metal atoms are inter-connected, with the inter-connections having the length of a chemical bond (1-2 Å). These molecular characteristics are at the very core of the high stability and novel properties of the smallest metal clusters, with their integration into colloidal materials interacting with the environment having the potential to further boost their performance in applications such as luminescence, sensing, bioimaging, theranostics, energy conversion, catalysis, and photocatalysis. Through the presentation of very recent case studies, this Feature Article is aimed to illustrate how first-principles modelling, including methods beyond the state-of-the-art and an interplay with cutting-edge experiments, is helping to understand the special properties of these clusters at the most fundamental level. Moreover, it will be discussed how superfluid helium droplets can act both as nano-reactors and carriers to achieve the synthesis and surface deposition of metal clusters. This concept will be illustrated with the quantum simulation of the helium droplet-assisted soft-landing of a single Au atom onto a titanium dioxide (TiO₂) surface. Next, it will be shown how the application of first-principles methods have disclosed the fundamental reasons why subnanometric Cu₅ clusters are resistant to irreversible oxidation, and capable of increasing and extending into the visible region the solar absorption of TiO₂, of augmenting its efficiency for photo-catalysis beyond a factor of four, also considering the decomposition and photo-activation of CO₂ as a prototypical (photo-) catalytic reaction. Finally, I will discuss how the modification of the same material with subnanometric Ag₅ clusters has converted it into a "reporter" of a surface polaron property as well as a novel two-dimensional polaronic material.

Cr2O3 layer inhibits agglomeration of phosphine-protected Au9 clusters on TiO2 films

The properties of semiconductor surfaces can be modified by the deposition of metal clusters consisting of a few atoms. The properties of metal clusters and of cluster-modified surfaces depend on the number of atoms forming the clusters. Deposition of clusters with a monodisperse size distribution thus allows tailoring of the surface properties for technical applications. However, it is a challenge to retain the size of the clusters after their deposition due to the tendency of the clusters to agglomerate. The agglomeration can be inhibited by covering the metal cluster modified surface with a thin metal oxide overlayer. In the present work, phosphine-protected Au clusters, Au₉(PPh₃)₈(NO₃)₃, were deposited onto RF-sputter deposited TiO₂ films and subsequently covered with a Cr₂O₃ film only a few monolayers thick. The samples were then heated to 200 °C to remove the phosphine ligands, which is a lower temperature than that required to remove thiolate ligands from Au clusters. It was found that the Cr₂O₃ covering layer inhibited cluster agglomeration at an Au cluster coverage of 0.6% of a monolayer. When no protecting Cr₂O₃ layer was present, the clusters were found to agglomerate to a large degree on the TiO₂ surface.

The interaction of size-selected Ru3 clusters with RF-deposited TiO2: probing Ru-CO binding sites with CO-temperature programmed desorption

Small Ru clusters are efficient catalysts for chemical reactions such as CO hydrogenation. In this study 3-atom Ru₃ clusters were deposited onto radio frequency (RF)-deposited TiO₂ which is an inexpensive, nanoparticulate form of TiO₂. TiO₂ substrates are notable in that they form strong metal-substrate interactions with clusters. Using temperature programmed desorption to probe Ru-CO binding sites, and X-ray photoelectron spectroscopy to provide chemical information on clusters, differences in cluster-support interactions were studied for Ru₃ deposited using both an ultra-high vacuum cluster source and chemical vapour deposition of Ru₃(CO)₁₂. The TiO₂ was treated with different Ar⁺ sputter doses prior to cluster depositions, and SiO₂ was also used as a comparison substrate. For cluster source-deposited Ru₃, heating to 800 K caused cluster agglomeration on SiO₂ and oxidation on non-sputtered TiO₂. For cluster source-deposited Ru₃ on sputtered TiO₂ substrates, all Ru-CO binding sites were blocked as-deposited and it was concluded that for the binding sites to be preserved for potential catalytic benefit, sputtering of TiO₂ before cluster deposition cannot be applied. Conversely, for Ru₃(CO)₁₂ on sputtered TiO₂ the clusters were protected by their ligands and Ru-CO binding sites were only blocked once the sample was heated to 723 K. The mechanism for complete blocking of CO sites on sputtered TiO₂ could not be directly determined; however, comparisons to the literature indicate that the likely reasons for blocking of the CO adsorption sites are encapsulation into the TiO _x layer reduced through sputtering and also partial oxidation of the Ru clusters.

An intense source for cold cluster ions of a specific composition

The demand for nanoscale materials of ultra-high purity and narrow size distribution is addressed. Clusters of Au, C₆₀, H₂O, and serine are produced inside helium nanodroplets using a combination of ionization, mass filtering, collisions with atomic or molecular vapor, and electrostatic extraction, in a specific and novel sequence. The helium droplets are produced in an expansion of cold helium gas through a nozzle into vacuum. The droplets are ionized by electron bombardment and subjected to a mass filter. The ionic and mass-selected helium droplets are then guided through a vacuum chamber filled with atomic or molecular vapor where they collide and "pick up" the vapor. The dopants then agglomerate inside the helium droplets around charge centers to singly charged clusters. Evaporation of the helium droplets is induced by collisions in a helium-filled radio frequency (RF)-hexapole, which liberates the cluster ions from the host droplets. The clusters are analyzed with a time-of-flight mass spectrometer. It is demonstrated that using this sequence, the size distribution of the dopant cluster ions is distinctly narrower compared to ionization after pickup. Likewise, the ion cluster beam is more intense. The mass spectra show, as well, that ion clusters of the dopants can be produced with only few helium atoms attached, which will be important for messenger spectroscopy. All these findings are important for the scientific research of clusters and nanoscale materials in general.

Soft- and reactive landing of ions onto surfaces: Concepts and applications

Soft- and reactive landing of mass-selected ions is gaining attention as a promising approach for the precisely-controlled preparation of materials on surfaces that are not amenable to deposition using conventional methods. A broad range of ionization sources and mass filters are available that make ion soft-landing a versatile tool for surface modification using beams of hyperthermal (<100 eV) ions. The ability to select the mass-to-charge ratio of the ion, its kinetic energy and charge state, along with precise control of the size, shape, and position of the ion beam on the deposition target distinguishes ion soft landing from other surface modification techniques. Soft- and reactive landing have been used to prepare interfaces for practical applications as well as precisely-defined model surfaces for fundamental investigations in chemistry, physics, and materials science. For instance, soft- and reactive landing have been applied to study the surface chemistry of ions isolated in the gas-phase, prepare arrays of proteins for high-throughput biological screening, produce novel carbon-based and polymer materials, enrich the secondary structure of peptides and the chirality of organic molecules, immobilize electrochemically-active proteins and organometallics on electrodes, create thin films of complex molecules, and immobilize catalytically active organometallics as well as ligated metal clusters. In addition, soft landing has enabled investigation of the size-dependent behavior of bare metal clusters in the critical subnanometer size regime where chemical and physical properties do not scale predictably with size. The morphology, aggregation, and immobilization of larger bare metal nanoparticles, which are directly relevant to the design of catalysts as well as improved memory and electronic devices, have also been studied using ion soft landing. This review article begins in section 1 with a brief introduction to the existing applications of ion soft- and reactive landing. Section 2 provides an overview of the ionization sources and mass filters that have been used to date for soft landing of mass-selected ions. A discussion of the competing processes that occur during ion deposition as well as the types of ions and surfaces that have been investigated follows in section 3. Section 4 discusses the physical phenomena that occur during and after ion soft landing, including retention and reduction of ionic charge along with factors that impact the efficiency of ion deposition. The influence of soft landing on the secondary structure and biological activity of complex ions is addressed in section 5. Lastly, an overview of the structure and mobility as well as the catalytic, optical, magnetic, and redox properties of bare ionic clusters and nanoparticles deposited onto surfaces is presented in section 6.

Atomically resolved structure of ligand-protected Au9 clusters on TiO2 nanosheets using aberration-corrected STEM

Triphenylphosphine ligand-protected Au9 clusters deposited onto titania nanosheets show three different atomic configurations as observed by scanning transmission electron microscopy. The configurations observed are a 3-dimensional structure, corresponding to the previously proposed Au9 core of the clusters, and two pseudo-2-dimensional (pseudo-2D) structures, newly found by this work. With the help of density functional theory (DFT) calculations, the observed pseudo-2D structures are attributed to the low energy, de-ligated structures formed through interaction with the substrate. The combination of scanning transmission electron microscopy with DFT calculations thus allows identifying whether or not the deposited Au9 clusters have been de-ligated in the deposition process.

Catalysis by clusters with precise numbers of atoms

Clusters that contain only a small number of atoms can exhibit unique and often unexpected properties. The clusters are of particular interest in catalysis because they can act as individual active sites, and minor changes in size and composition--such as the addition or removal of a single atom--can have a substantial influence on the activity and selectivity of a reaction. Here, we review recent progress in the synthesis and characterization of well-defined subnanometre clusters, and the understanding and exploitation of their catalytic properties. We examine work on size-selected supported clusters in ultrahigh-vacuum environments and under realistic reaction conditions, and explore the use of computational methods to provide a mechanistic understanding of their catalytic properties. We also highlight the potential of size-selected clusters to provide insights into important catalytic processes and their use in the development of novel catalytic systems.

Influence of cationic surfactants on the formation and surface oxidation states of gold nanoparticles produced via laser ablation

We report on the time evolution of gold nanoparticles produced by laser ablation in the presence of the cationic surfactants cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) in aqueous solution. The broader applicability of a laser-induced nanoparticle formation kinetic model previously developed by us for the case of anionic surfactants in aqueous solution [ J. Phys. Chem. C 2010 , 114 , 15931 - 15940 ] is shown to also apply in the presence of cationic surfactants. We explore the surface properties of the nanoparticles produced in the presence of the cationic surfactants via synchrotron X-ray photoelectron spectroscopy (XPS). The XPS data indicate that at CTA(+) concentrations approximating the aqueous critical micelle concentration Au(III) is present on the nanoparticle surface. Such oxidation is not observed at (i) lower CTA(+) concentrations, (ii) in the presence of an anionic surfactant, or (iii) in the case of pure water as a solvent.

Transition from molecule to solid state: reactivity of supported metal clusters

The evolution of the adsorption energy of carbon monoxide (CO) molecules on palladium (Pd) clusters as function of Pd particle size from the molecular regime (less than ~100 atoms per particle) to the bulk regime has been revealed. This adsorption energy is retrieved from the residence time of CO molecules on the Pd clusters, measured by a pulsed molecular beam technique, versus temperature. Unprecedented accuracy on the determination of the particle size has been achieved here by using a regular array of metal clusters exhibiting a size dispersion down to the ultimate limit of a Poisson distribution. This allows getting rid of the convolution effects that generally occur when considering particles grown through other techniques.

Interaction of Au with thin ZrO2 films: influence of ZrO2 morphology on the adsorption and thermal stability of Au nanoparticles

The model catalysts of ZrO(2)-supported Au nanoparticles have been prepared by deposition of Au atoms onto the surfaces of thin ZrO(2) films with different morphologies. The adsorption and thermal stability of Au nanoparticles on thin ZrO(2) films have been investigated using synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS). The thin ZrO(2) films were prepared by two different methods, giving rise to different morphologies. The first method utilized wet chemical impregnation to synthesize the thin ZrO(2) film through the procedure of first spin-coating a zirconium ethoxide (Zr(OC(2)H(5))(4)) precursor onto a SiO(2)/Si(100) substrate at room temperature followed by calcination at 773 K for 12 h. Scanning electron microscopy (SEM) investigations indicate that highly porous "sponge-like nanostructures" were obtained in this case. The second method was epitaxial growth of a ZrO(2)(111) film through vacuum evaporation of Zr metal onto Pt(111) in 1 × 10(-6) Torr of oxygen at 550 K followed by annealing at 1000 K. The structural analysis with low energy electron diffraction (LEED) of this film exhibits good long-range ordering. It has been found that Au forms smaller particles on the porous ZrO(2) film as compared to those on the ordered ZrO(2)(111) film at a given coverage. Thermal annealing experiments demonstrate that Au particles are more thermally stable on the porous ZrO(2) surface than on the ZrO(2)(111) surface, although on both surfaces, Au particles experience significant sintering at elevated temperatures. In addition, by annealing the surfaces to 1100 K, Au particles desorb completely from ZrO(2)(111) but not from porous ZrO(2). The enhanced thermal stability for Au on porous ZrO(2) can be attributed to the stronger interaction of the adsorbed Au with the defects and the hindered migration or coalescence resulting from the porous structures.

Systematic study of Au6 to Au12 gold clusters on MgO(100) F centers using density-functional theory

We present an optimized genetic algorithm used in conjunction with density-functional theory in the search for stable gold clusters and O2 adsorption ensembles in F centers at MgO(100). For Au8 the method recovers known structures and identifies several more stable ones. When O2 adsorption is investigated, the genetic algorithm is used to imitate structural fluxionality, increasing the O2 bond strength by up to 1 eV. Extending the method to Au(6,10,12), strong O2 adsorption configurations are found for all sizes. However, the effect of fluxionality appears to wear off with increasing cluster size.

Photoreduction kinetics of sodium tetrachloroaurate under synchrotron soft X-ray exposure

We report on the time evolution of the sodium tetrachloroaurate (NaAuCl(4)) chemical properties as a function of soft X-ray exposure in a dried sample on a silicon surface using X-ray photoelectron spectroscopy (XPS). Our investigations provide mechanistic insight into the photoreduction kinetics from Au(III) to Au(I) and then Au(I) to Au(0). We unambiguously show that XPS photoreduction occurs in stepwise fashion via the Au(I) state. Both photoreduction steps undergo first-order kinetics.