Xiang Shao

Tailoring the Shape of Metal Ad-Particles by Doping the Oxide Support†

Doping is a versatile yet little examined approach to tailor the physical and chemical properties of oxide thin films. By means of scanning tunneling microscopy (STM), we demonstrate how tiny amounts of Mo embedded in a CaO matrix change the growth behavior of gold. Whereas 3D deposits are formed on the pristine oxide surface, strictly 2D growth prevails on the doped material. The crossover in particle shape is driven by charge-transfer processes from the Mo dopants into the Au islands, as elucidated with density-functional theory (DFT) calculations.

Oxidation of Au by Surface OH: Nucleation and Electronic Structure of Gold on Hydroxylated MgO(001)

The nucleation and electronic structure of vapor-deposited Au on hydroxylated MgO(001) surfaces has been investigated under ultrahigh vacuum conditions. Hydroxylated MgO(001) surfaces with two different hydroxyl coverages, 0.4 and 1 monolayer, respectively, were prepared by exposure to water (D(2)O) at room temperature. Scanning tunneling microscopy experiments show significantly higher gold particle densities and smaller particle sizes on the hydroxylated MgO surface as compared to gold deposited on clean MgO(001). Infrared spectroscopy and X-ray photoelectron spectroscopy experiments were performed to reveal details about the initial nucleation of gold. Gold atoms are found to chemically interact with a specific type of hydroxyl groups on the MgO surface, leading to the formation of oxidized gold particles. The enhanced adhesion of Au particles, which is due to the formation of strong Au-O interfacial bonds, is responsible for the observed higher stability of small Au clusters toward thermal sintering on hydroxylated MgO surfaces. The results are compared to similar studies on Au/TiO(2)(110) model systems and powder samples prepared by the deposition-precipitation route.

Lattice-Directed Formation of Covalent and Organometallic Molecular Wires by Terminal Alkynes on Ag Surfaces

Surface reactions of 2,5-diethynyl-1,4-bis(phenylethynyl)benzene on Ag(111), Ag(110), and Ag(100) were systematically explored and scrutinized by scanning tunneling microscopy, molecular mechanics simulations, and density functional theory calculations. On Ag(111), Glaser coupling reaction became dominant, yielding one-dimensional molecular wires formed by covalent bonds. On Ag(110) and Ag(100), however, the terminal alkynes reacted with surface metal atoms, leading to one-dimensional organometallic nanostructures. Detailed experimental and theoretical analyses revealed that such a lattice dependence of the terminal alkyne reaction at surfaces originated from the matching degree between the periodicities of the produced molecular wires and the substrate lattice structures.

Adsorption, Activation, and Dissociation of Oxygen on Doped Oxides†

Charge transfer in the presence of dopants is relevant for the adorption and activation of small molecules, such as O2 . Scanning tunneling microscopy and DFT calculations provide evidence for the formation of strongly bound superoxo species on chemically inert, Mo-doped CaO films. This oxygen surface species shows a high propensity to dissociate. Dopants could also be important for the activation of hydrocarbons on inert oxides.

Formation of one-dimensional electronic states along the step edges of CeO2(111)

Scanning tunneling microscopy (STM) combined with density functional theory (DFT) are used to analyze the structural and electronic properties of step edges on the surface of CeO(2)(111) films grown on Ru(0001). Depending on the preparation conditions, 211 or 110-oriented steps develop on the surface, which results in the formation of ceria ad-islands with hexagonal or triangular shapes. STM conductance spectroscopy reveals pronounced differences in the electronic properties of the step edges, as reflected in different onset positions of the ceria conduction band. The band shifts are related to the development of distinct edge electronic states that split-off from the ceria conduction band, as shown with DFT calculations. The separation of the edge states from the main band is governed by the atom-coordination and local charge-distribution along the edge, the latter giving rise to the development of electrostatic dipoles. We expect that the observed edge morphologies determine not only the electronic properties but also the adsorption behavior of step edges on the CeO(2)(111) surface.

Self-assembly of highly luminescent bi-1,3,4-oxadiazole derivatives through electron donor–acceptor interactions in three-dimensional crystals, two-dimensional layers and mesophases

We report on the synthesis and self-assembly of a new series of highly luminescent bi-1,3,4-oxadiazole derivatives [2,2′-bis(4-alkoyphenyl)-bi-1,3,4-oxadiazole, BOXD-n, n = 1, 3, 4, 5, 6, 7, 10, 16]. Fully conjugated conformations were demonstrated either by computer simulation or in a single-crystal state. Well-defined 3D donor–acceptor (DA) architectures with strong face-to-face and edge-to-edge donor–acceptor interactions were observed in the single-crystal structure of BOXD-1. Highly oriented face-on two-dimensional DA layered structures due to edge-to-edge donor–acceptor interactions were observed on highly oriented pyrolytic graphite (HOPG) in BOXD-7 and BOXD-16. Nematic phase, smectic C phase with large tilted angle (θ ≈ 50°) and relatively large transition enthalpic values and a highly ordered smectic X phase were demonstrated in BOXD-n (n = 5, 6, 7, 8, 10, 16) through tailing the terminal chains and relatively large scale monodomains were prepared in the smectic X phase of BOXD-5 even without any surface treatment. Strong blue fluorescent emissions were observed in BOXD-6 either in cyclohexane (ФF ≈ 92%) or in solid state (ФF ≈ 57%). The donor–acceptor interactions between alkyoxyphenylene rings and 1,3,4-oxadiazole rings were thought to be the driving force for the molecules to self-assemble into a large angle tilted layered structure.

Li/Mo codoping of CaO films: A means to tailor the equilibrium shape of Au deposits

Using scanning tunneling microscopy, we have investigated how the doping of CaO thin films affects the growth behavior of gold. Whereas 3D deposits develop on pristine films, 2D islands form after inserting 4% of Mo into the CaO lattice. Adding small amounts of Li to the Mo-doped CaO reinstalls the initial 3D growth regime. We assign this morphology crossover to charge transfer processes between the dopants and the ad-metal. Whereas Mo acts as an electron donor and provides excess charges to be transferred into the gold, Li creates electron traps in the oxide lattice that interrupt the charge flow toward the metal. The different Au charge states in the presence of the dopants are derived from different growth morphologies with anionic gold favoring a 2D mode due to an enhanced interface adhesion. Our work demonstrates how oxide doping can be exploited to tailor the equilibrium geometry of ad-particles on supported metal catalysts.

Probing the 4f states of ceria by tunneling spectroscopy

Low-temperature scanning tunneling microscopy and spectroscopy have been employed to analyze the local electronic structure of the (111) surface of a ceria thin film grown on Ru(0001). On pristine, defect-free oxide terraces, the empty 4f states of Ce(4+) ions appear as the only spectral feature inside the 6 eV oxide band gap. In contrast, occupied states are detected between -1.0 and -1.5 eV below E(Fermi) in conductance spectra of different point and line defects, such as surface oxygen vacancies, grain boundaries and step edges. They are assigned to partially filled 4f states localized at the Ce(3+) ions. The presence of excess electrons indicates the oxygen-deficient nature of the direct oxide environment. The f state spectroscopy with the STM allows us to probe the spatial distribution of Ce(3+) ions in the ceria surface, providing unique insight into the local reduction state of this chemically important material system.

Formation of Water Chains on CaO(001): What Drives the 1D Growth?

Formation of partly dissociated water chains is observed on CaO(001) films upon water exposure at 300 K. While morphology and orientation of the 1D assemblies are revealed from scanning tunneling microscopy, their atomic structure is identified with infrared absorption spectroscopy combined with density functional theory calculations. The latter exploit an ab initio genetic algorithm linked to atomistic thermodynamics to determine low-energy H2O configurations on the oxide surface. The development of 1D structures on the C4v symmetric CaO(001) is triggered by symmetry-broken water tetramers and a favorable balance between adsorbate-adsorbate versus adsorbate-surface interactions at the constraint of the CaO lattice parameter.

Unraveling Charge State of Supported Au Single-Atoms during CO Oxidation

Thermally stable Au single-atoms supported by monolayered CuO grown at Cu(110) have been successfully prepared. The charge transfer from the CuO support to single Au atoms is confirmed to play a key role in tuning the activity for CO oxidation. Initially, the negatively charged Au single-atom is active for CO oxidation with its adjacent lattice O atom depleted to generate an O vacancy in the CuO monolayer. Afterward, the Au single-atom is neutralized, preventing further CO reaction. The produced O vacancy can be healed by exposure to O2 at 400 K and accordingly the reaction activity is restored.