The stabilization of supported gold clusters by surface defects

Two-Dimensional Ultrathin Silica Films

Two-dimensional (2D) ultrathin silica films have the potential to reach technological importance in electronics and catalysis. Several well-defined 2D-silica structures have been synthesized so far. The silica bilayer represents a 2D material with SiO₂ stoichiometry. It consists of precisely two layers of tetrahedral [SiO₄] building blocks, corner connected via oxygen bridges, thus forming a self-saturated silicon dioxide sheet with a thickness of ∼0.5 nm. Inspired by recent successful preparations and characterizations of these 2D-silica model systems, scientists now can forge novel concepts for realistic systems, particularly by atomic-scale studies with the most powerful and advanced surface science techniques and density functional theory calculations. This Review provides a solid introduction to these recent developments, breakthroughs, and implications on ultrathin 2D-silica films, including their atomic/electronic structures, chemical modifications, atom/molecule adsorptions, and catalytic reactivity properties, which can help to stimulate further investigations and understandings of these fundamentally important 2D materials.

Au nanoparticles on Fe-modified rutile TiO2(110): Dispersion, thermal stability, and CO adsorption

Gold clusters on an iron-modified rutile TiO₂(110) surface have been characterized via scanning tunneling microscopy and x-ray photoelectron spectroscopy. This study is focused on the impact of submonolayer preadsorbed Fe on the morphologies, surface compositions, and thermal stabilities of bimetallic Au-Fe systems by comparing them to elemental Au and Fe adsorbates. We found that a submonolayer gold adsorbate followed the nucleation mode of the iron precursor, which considerably enhanced the dispersion of nano-gold while improving its thermal stability. Finally, the temperature-programmed CO desorption spectra of Au and Au-Fe nanoparticles on TiO₂(110) were compared.

Nanoclusters and nanolines: the effect of molybdenum oxide substrate stoichiometry on iron self-assembly

The growth of Fe nanostructures on the stoichiometric MoO₂/Mo(110) and oxygen-rich MoO_2+x /Mo(110) surfaces has been studied using low-temperature scanning tunnelling microscopy (STM) and density functional theory calculations. STM results indicate that at low coverage Fe nucleates on the MoO₂/Mo(110) surface, forming small, well-ordered nanoclusters of uniform size, each consisting of five Fe atoms. These five-atom clusters can agglomerate into larger nanostructures reflecting the substrate geometry, but they retain their individual character within the structure. Linear Fe nanocluster arrays are formed on the MoO₂/Mo(110) surface at room temperature when the surface coverage is greater than 0.6 monolayers. These nanocluster arrays follow the direction of the oxide rows of the strained MoO₂/Mo(110) surface. Slightly altering the preparation procedure of MoO₂/Mo(110) leads to the presence of oxygen adatoms on this surface. Fe deposition onto the oxygen-rich MoO_2+x /Mo(110) surface results in elongated nanostructures that reach up to 24 nm in length. These nanolines have a zigzag shape and are likely composed of partially oxidised Fe formed upon reaction with the oxygen-rich surface.

Structure-reactivity correlations in Pd-Au bimetallic nanoclusters

The effect of composition and morphology of bimetallic Pd-Au nanoclusters on their chemical reactivity has been studied with acetylene decomposition and conversion to ethylene and benzene as the chemical probe. High resolution transmission electron microscopy (HR-TEM) and CO-Temperature Programmed Desorption (TPD) measurements were employed for structure and chemical composition determination. Pd-Au clusters were prepared in ultrahigh vacuum (UHV) environment on SiO(2)/Si(100) by direct deposition (DD) to form 2D bimetallic nanostructures. Different bimetallic cluster morphology could be obtained by employing the buffer layer assisted growth (BLAG) procedure with amorphous solid water as buffer material. The BLAG bimetallic clusters were found to be more reactive than DD particles toward acetylene hydrogenation to ethylene and trimerization to benzene. The morphology and composition of DD clusters enabled the formation of both tilted (low adsorption energy) and flat laying (high adsorption energy) benzene, while mainly tilted benzene was detected upon adsorption of acetylene on BLAG clusters. Moreover, the reactivity of bimetallic clusters was compared to that of thin Pd film. Strong preference (100:1 ratio) toward acetylene hydrogenation to ethylene over trimerization to benzene has been correlated with the lack of extended Pd(111) facets on the bimetallic clusters that suppress the benzene formation.

Light-induced selective deposition of Au nanoparticles on single-wall carbon nanotubes

Novel applications of single-walled carbon nanotubes (SWNT) rely on the development of new strategies to make them easier to handle without affecting their structural properties. In this work, we have selectively deposited Au nanoparticles (Au NP) on SWNT assisted by UV light irradiation. XPS analysis and UV-vis spectroscopy indicate that the deposition occurs at the defects generated after oxidation of the SWNT. By addition of n-dodecylthiol, the separation of oxidized tubes with Au NP (Au-ox-SWNT) from tubes devoid of Au NP (bare tubes, b-SWNT) was achieved. Raman and UV-vis-NIR spectra indicate that UV irradiation induces a faster nucleation of Au NP on metallic SWNT. This new technique can be useful for the preparation of nanohybrid composites with enhanced properties, as increased thermal stability, and to obtain purified SWNT.

Gold promoted S,N-doped TiO(2): an efficient catalyst for CO adsorption and oxidation

Mesoporous S,N-TiO(2) nanocomposite was prepared by a one-pot template free homogeneous coprecipitation technique using titanium oxysulfate sulfuric acid complex hydrate, thiourea, ethanol, and water. Nano gold deposition on mesoporous S,N-TiO(2) was preformed by a borohydrate reduction method. To evaluate the structural and electronic properties, these catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HR-TEM), UV-vis DRS, photoluminescent (PL) spectra, Fourier transform infrared (FTIR), and TPO/TPD. CO adsorption and CO + O(2) interaction over these catalysts were investigated by in situ FTIR. Sulfur and nitrogen doping enhances the catalytic activity of Au/TiO(2.) Higher catalytic activity of Au/S,N-TiO(2) compared to Au/TiO(2) was attributed to the presence of oxygen vacancy and creation of new adsorption sites at Au/TiO(2) interfaces for the adsorption and activation of O(2) molecules.

Electrocrystallization of Phase I, CuTCNQ (TCNQ = 7,7,8,8-Tetracyanoquinodimethane), on indium tin oxide and boron-doped diamond electrodes

The electrochemical reduction of TCNQ to TCNQ*- in acetonitrile in the presence of [Cu(MeCN)4]+ has been undertaken at boron-doped diamond (BDD) and indium tin oxide (ITO) electrodes. The nucleation and growth process at BDD is similar to that reported previously at metal electrodes. At an ITO electrode, the electrocrystallization of more strongly adhered, larger, branched, needle-shaped phase I CuTCNQ crystals is detected under potential step conditions and also when the potential is cycled over the potential range of 0.7 to -0.1 V versus Ag/AgCl (3 M KCl). Video imaging can be used at optically transparent ITO electrodes to monitor the growth stage of the very large branched crystals formed during the course of electrochemical experiments. Both in situ video imaging and ex situ X-ray diffraction and scanning electron microscopy (SEM) data are consistent with the nucleation of CuTCNQ taking place at a discrete number of preferred sites on the ITO surface. At BDD electrodes, ex situ optical images show that the preferential growth of CuTCNQ occurs at the more highly conducting boron-rich areas of the electrode, within which there are preferred sites for CuTCNQ formation.

Dynamic Electronic Structure of a Au/TiO2 Catalyst under Reaction Conditions

The electronic structure of a highly active Au/TiO2 powder catalyst was probed in situ by synchrotron X-ray photoelectron spectroscopy (XPS) in the 10-1 mbar range. The electronic structure of the Au component was found to respond sensitively to changes in temperature and indicated the absence of bulklike metallic Au under the conditions of highest catalytic activity. Concurrent modification of interfacial sites adjacent to Au on the TiO2 support was not evident from the Ti photoemission, but may have been below the detection limit of XPS.

Nature of Point Defects on SiO2/Mo(112) Thin Films and Their Interaction with Au Atoms

We have studied by means of periodic DFT calculations the structure and properties of point defects at the surface of ultrathin silica films epitaxially grown on Mo(112) and their interaction with adsorbed Au atoms. For comparison, the same defects have been generated on an unsupported silica film with the same structure. Four defects have been considered: nonbridging oxygen (NBO, [triple bond]Si-O(*)), Si dangling bond (E' center, [triple bond]Si(*)), oxygen vacancy (V(O), [triple bond]Si-Si[triple bond]), and peroxo group ([triple bond]Si-O-O-Si[triple bond]), but only the NBO and the V(O) centers are likely to form on the SiO(2)/Mo(112) films under normal experimental conditions. The [triple bond]Si-O(*) center captures one electron from Mo forming a silanolate group, [triple bond]Si-O(-), sign of a direct interaction with the metal substrate. Apart from the peroxo group, which is unreactive, the other defects bind strongly the Au atom forming stable surface complexes, but their behavior may differ from that of the same centers generated on an unsupported silica film. This is true in particular for the two most likely defects considered, the nonbridging oxygen, [triple bond]Si-O(*), and the oxygen vacancy, [triple bond]Si-Si[triple bond].

Physical Properties of γ Alumina Surface Hydroxyls Revisited through a Large Scale Periodic Quantum-Chemistry Approach

We have studied surface hydroxyls adsorbed onto (001), (011), and (111) gamma alumina surfaces using a quantum-chemistry approach in order to compare with empirical models proposed in the literature. Local electronic structures and geometries in the low OH coverage limit have been evaluated for both ideal and relaxed surfaces with the help of a large scale periodic quantum-chemical code. Hydroxyl groups are adsorbed onto surfaces, and a study of their local electronic structure, vibrational frequencies, charges, and adsorption energies is performed and analyzed as a function of their adsorption site geometry. Our results show that, even on ideal (nonrelaxed) surfaces, OH local environments are more complicated than those stated by empirical models and strongly influence the hydroxyl stretching vibrational mode. Large scale simulation shows that disorder takes place even at 0 K, and the analysis of the vibrational frequencies leads to a revision of Knözinger's empirical model. Cationic vacancies in the first surface layers have also been taken into account; they have a significant influence on the surface atomic and electronic structures, modifying the physical properties of adsorbed OH entities. This work emphasizes the necessity to perform an electronic structure calculation to better understand adsorbed OH properties on gamma alumina surfaces and reveals the difficulty to make a one-to-one correspondence between OH stretching frequencies and their other physical properties. Finally, we show that these results agree with some available experimental studies.

Au and Pd atoms adsorbed on pure and Ti-doped SiO2∕Mo(112) films

The interaction of Pd and Au atoms with a silica surface and SiO2Mo(112) ultrathin films has been studied with periodic density-functional theory-generalized gradient approximation calculations. On both unsupported and supported silica, Pd and Au are weakly bound. No charge transfer occurs to the empty Pd and Au orbitals. Differently from Au, Pd can easily penetrate with virtually no barrier into the hexagonal rings of the supported silica film and binds strongly at the SiO2-Mo interface. The same process for Au implies overcoming a barrier of 0.9 eV. Completely different is the behavior of Ti-doped silica films. Au forms a direct covalent bond with substitutional Ti at the expense of the Ti...O-Mo interface bond which breaks. The global process is exothermic by 1 eV and nonactivated, showing that Ti doping results in solid anchoring points for the adsorbed Au atoms and for nucleation and growth of small gold particles. The effect of Ti doping is less pronounced for Pd but still visible with substantial enhancement of the Pd adsorption strength.