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

Au9 nanocluster adsorption and agglomeration control through sulfur modification of mesoporous TiO2

In the present work phenyl phosphine-protected Au9 nanoclusters were deposited onto (3-mercaptopropyl) trimethoxysilane (MPTMS) modified and unmodified mesoporous screen printed TiO2. The removal of the cluster ligands by annealing was applied to enhance the interaction between Au cluster cores and semiconductor surfaces in the creation of efficient photocatalytic systems. The heat treatment could lead to undesired agglomeration of Au clusters, affecting their unique properties as size specific clusters. To address this challenge, the semiconductor surfaces were modified by MPTMS. Characterization techniques confirm the effectiveness of the modification processes, and XPS demonstrates that S functionalized MTiO2 is more efficient than MTiO2 in increasing Au9 NCs adsorption by a factor of 10 and preventing Au cluster agglomeration even after annealing. Overall, this work contributes valuable insights into photocatalytic systems through controlled modification of semiconductor surfaces and Au nanocluster deposition.

Effect of TiO2 Film Thickness on the Stability of Au9 Clusters with a CrOx Layer

Radio frequency (RF) magnetron sputtering allows the fabrication of TiO₂ films with high purity, reliable control of film thickness, and uniform morphology. In the present study, the change in surface roughness upon heating two different thicknesses of RF sputter-deposited TiO₂ films was investigated. As a measure of the process of the change in surface morphology, chemically -synthesised phosphine-protected Au₉ clusters covered by a photodeposited CrO_x layer were used as a probe. Subsequent to the deposition of the Au₉ clusters and the CrO_x layer, samples were heated to 200 ℃ to remove the triphenylphosphine ligands from the Au₉ cluster. After heating, the thick TiO₂ film was found to be mobile, in contrast to the thin TiO₂ film. The influence of the mobility of the TiO₂ films on the Au₉ clusters was investigated with X-ray photoelectron spectroscopy. It was found that the high mobility of the thick TiO₂ film after heating leads to a significant agglomeration of the Au₉ clusters, even when protected by the CrO_x layer. The thin TiO₂ film has a much lower mobility when being heated, resulting in only minor agglomeration of the Au₉ clusters covered with the CrO_x layer.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

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.

Non-negative matrix factorization for mining big data obtained using four-dimensional scanning transmission electron microscopy

Scientific instruments for material characterization have recently been improved to yield big data. For instance, scanning transmission electron microscopy (STEM) allows us to acquire many diffraction patterns from a scanning area, which is referred to as four-dimensional (4D) STEM. Here we study a combination of 4D-STEM and a statistical technique called non-negative matrix factorization (NMF) to deduce sparse diffraction patterns from a 4D-STEM data consisting of 10,000 diffraction patterns. Titanium oxide nanosheets are analyzed using this combined technique, and we discriminate the two diffraction patterns from pristine TiO₂ and reduced Ti₂O₃ areas, where the latter is due to topotactic reduction induced by electron irradiation. The combination of NMF and 4D-STEM is expected to become a standard characterization technique for a wide range materials.

Sub-monolayer Au9 cluster formation via pulsed nozzle cluster deposition

Submonolayer coverages of chemically synthesised triphenylphosphine-protected Au₉ clusters on mica and TiO₂ substrates were achieved through the development of a Pulsed Nozzle Cluster Deposition (PNCD) technique under high vacuum conditions. This method offers the deposition of pre-prepared, solvated clusters directly onto substrates in a vacuum without the potential for contamination from the atmosphere. AFM and TEM were used to investigate the rate of gold cluster deposition as a function of cluster solution concentration and the number of pulses, with pulse number showing the most effective control of the final deposition conditions. TEM and XPS were used to determine that the clusters retained their unique properties through the deposition process. Methanol solvent deposited in the PNCD process has been shown to be removable through post-deposition treatments. A physical model describing the vapour behaviour and solvent evaporation in a vacuum is also developed and presented.

Metal Clusters on Semiconductor Surfaces and Application in Catalysis with a Focus on Au and Ru

Metal clusters typically consist of two to a few hundred atoms and have unique properties that change with the type and number of atoms that form the cluster. Metal clusters can be generated with a precise number of atoms, and therefore have specific size, shape, and electronic structures. When metal clusters are deposited onto a substrate, their shape and electronic structure depend on the interaction with the substrate surface and thus depend on the properties of both the clusters and those of the substrate. Deposited metal clusters have discrete, individual electron energy levels that differ from the electron energy levels in the constituting individual atoms, isolated clusters, and the respective bulk material. The properties of clusters with a focus on Au and Ru, the methods to generate metal clusters, and the methods of deposition of clusters onto substrate surfaces are covered. The properties of cluster-modified surfaces are important for their application. The main application covered here is catalysis, and the methods for characterization of the cluster-modified surfaces are described.

Towards the geometric structure of small supported Au9 clusters on Si

Ultra-small clusters containing few atoms are of high interest in both fundamental research and applications due to their specific functional, magnetic or chemical properties which depend on size and composition. The experimental results of the morphology of the size-selected clusters, consisting of few atoms can be an ideal benchmark for sophisticated theoretical models. With this motivation we have investigated the geometrical structure of mass-selected Au₉ clusters deposited on a silicon substrate prepared by soft-landing conditions. We present results obtained experimentally by Grazing-Incidence Small-Angle X-ray Scattering (GISAXS). Considering the ultra-small size of the clusters and small quantities of material on the surface, we combined advanced techniques which allowed us to investigate the surface structure of the sample. The resulting structural sizes are in concordance with cluster theory. Using a model-based approach, the advanced X-ray techniques allow for understanding how to resolve the possible cluster structure, identify optimal experimental conditions and obtain the probable morphological information which is challenging to be obtained otherwise.

A comprehensive study of catalytic, morphological and electronic properties of ligand-protected gold nanoclusters using XPS, STM, XAFS, and TPD techniques

A combination of microscopy and spectroscopy techniques comprehensively elucidates the unique properties of distinct ligand-protected gold nano clusters.

Density functional theory study on the metal–support interaction between a Au9 cluster and an anatase TiO2(001) surface

The microstructure and electronic structure of three different Au₉ isomers loaded on anatase TiO₂(001) surface were studied. The adsorption energy of Au₉ 2D configuration is larger than that of 3D configuration, owing to the stronger interactions with more adsorption sites.

Grouping and aggregation of ligand protected Au9 clusters on TiO2 nanosheets

Au₉ clusters forming groups of clusters on titania nanosheets at least partially consist of individual clusters both before and after annealing. Au₉ clusters also can attach as individual clusters.