Interaction of Ag with MgO(100)

An amphiprotic paper-based electrode for glucose detection based on layered carbon nanotubes with silver and polystyrene particles

In this work, a flexible amphiprotic amino-bonded carbon nanotube-Ag nanoparticle/polystyrene (CNT-NH₂-Ag/PS) paper electrode was fabricated to measure glucose in human body fluids by a combination of vacuum filtration and high temperature baking. The front side of the fabricated paper electrode was hydrophobic and conductive, whereas its back side was hydrophilic and nonconductive. In the fabrication process, the coating sequence of CNT-NH₂, Ag and PS was critical to determine the performance of the resulting CNT-NH₂-Ag/PS electrode besides other parameters (e.g., amount of soluble starch, PS and Ag nanoparticles, type and amount of CNT-NH₂, and electrode sensing area). Based on a series of experimental observations, the possible mechanism of glucose detection on the paper electrode was proposed, in which glucose was more favorable to migrate to the hydrophilic back side of the paper and interact with the active species (e.g., O₂^-) on the electrode surface. The electrochemical results showed that the CNT-NH₂-Ag/PS paper electrode maintained stable electrochemical properties even after five cycles of use and 60 days of storage in air. The amphiprotic paper electrode demonstrated excellent sensing performance for glucose with a linear range of 1 μM to 1000 μM, a low detection limit of 0.2 μM, and a sensitivity of 31 333.0 μA mM^-1 cm^-2. The fabricated paper electrode was also successfully applied to detect different levels of glucose in complex human body fluids such as saliva, urine, and serum. These features make this type of paper electrode promising for glucose measurement.

Size-dependent surface reactions of Ag nanoparticles supported on highly oriented pyrolytic graphite

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.

Ultrathin, ordered oxide films on metal surfaces

Metal oxides and oxide thin films are extensively used as active catalysts and catalytic supports, as well as in many other important technical applications. Unlike TiO(2), which is a semiconductor and can be investigated using a variety of surface science techniques, most metal oxides are insulators, which seriously restricts their use as model surfaces with modern surface science techniques. This difficulty can be circumvented by synthesizing ultrathin oxide films a few nanometers in thickness with well-defined structures, that mimic the corresponding bulk oxides yet are thin enough to be sufficiently conducting. In this review, preparations, structures, electronic and chemical properties of four representative oxides, alumina, magnesium oxide, silica, and titania, are addressed. Of these MgO is found to grow in a layer-by-layer fashion, allowing preparation of crystalline thin film structures with varying thicknesses. Crystalline TiO(2) and Ti(2)O(3) can also be synthesized, whereas SiO(2) and Al(2)O(3), although amenable to synthesis as well-defined monolayer structures, have only been grown to date as amorphous multilayers.

Structure of Ag Clusters Grown on Fs-Defect Sites of an MgO(1 0 0) Surface

The structure of AgN clusters (N=1-4, 6, 8, 10), both in the gas phase and grown on the MgO(1 0 0) surface containing Fs-defects, has been investigated by a density functional basin-hopping (DF-BH) approach. In analogy with what observed in the case of gold clusters, it is found that the presence of the defect implies a double frustration and a cylindrical invariance of the metal-surface interaction, causing small Ag clusters growing around the Fs defect to be highly fluxional. Nevertheless, two different structural crossovers are found to be induced by the metal-defect interaction for the adsorbed clusters such that: 1) planar structures prevail for N<or=4 (as in the gas phase); 2) noncrystalline (fivefold symmetric) structures, which are the lowest energy ones in the gas phase for medium sized AgN clusters (N>or=7), prevail for N=6 and N=8; 3) distorted face-centered cubic (fcc) structures grown pseudomorphically on the defected surface prevail for N=10. The transition from fivefold to fcc motifs is rationalized in terms of the double-frustration effect, which increases the bond strain of the noncrystalline structures. Detrapping energies from the defect were also calculated; the lowest energy pathway corresponds to the detachment of a dimer.

CO Adsorption on Ag(100) and Ag/MgO(100)

Theoretical studies of CO adsorption on a two-layer Ag(100) film and on a two-layer Ag film on a MgO(100) support are reported. Ab initio calculations are carried at the configuration interaction level of theory using embedding methods to treat the metal-adsorbate region and the extended ionic solid. The metal overlayer is considered in two different structures: where Ag-Ag distances are equal to the value in the bulk solid, and for a slightly expanded lattice in which the Ag-Ag distances are equal to the O-O distance on the MgO(100) surface. The calculated adsorption energy of Ag(100) on MgO(100) is 0.58 eV per Ag interfacial atom; the Ag-O distance is 2.28 A. A small transfer of electrons from MgO to Ag occurs on deposition of the silver overlayer. CO adsorption at an atop Ag site is found to be the most stable for adsorption on the two-layer Ag film and also for adsorption on Ag deposited on the oxide; CO adsorption energies range from 0.12 to 0.19 eV. The CO adsorption energy is reduced for the Ag/MgO system compared to adsorption on the unsupported metal film thereby providing evidence for a direct electronic effect of the oxide support at the metal overlayer surface. Expansion of the Ag-Ag distance in the two-layer system also reduces the adsorption energy.