Ag Surface Research Articles

Using an ionomer as a size regulator in γ-radiation induced synthesis of Ag nanocatalysts for oxygen reduction reaction in alkaline solution

Ag nanoparticles (Ag NPs) are among the most promising candidates to replace Pt as the catalyst for the oxygen reduction reaction (ORR) in anion exchange membrane fuel cells (AEMFCs). However, synthesizing size-controlled Ag NPs with efficient catalytic performance is still challenging. Herein, uniform Ag NPs are produced through a γ-radiation induced synthesis route in aqueous solutions, using the ionomer PTPipQ100 as both an efficient size regulator in the synthesis and a conductor of hydroxide ions during the ORR process. The origin of the size control is mainly attributed to the affinity of the ionomer to metallic silver. The resulting Ag NPs covered with ionomer layers can be applied as model catalysts for ORR. The nanoparticles that were prepared using 320 ppm ionomer in the reaction solution turned out to be coated with a ∼ 1 nm thick ionomer layer and exhibited superior ORR activity as compared to other Ag NPs of similar size studied here. The improved electrocatalytic performance can be attributed to the optimal ionomer coverage that enables fast oxygen diffusion, as well as interactions at the Ag-ionomer interface which promote the desorption of OH intermediates from the Ag surface. This work demonstrates the advantage of using an ionomer as the capping agent to produce efficient ORR catalysts.

Insight into the effect of alloying on the adsorption of benzene on Ag(100) surface: DFT calculations

Heterogeneous catalysis, molecular sensors, and photovoltaics all rely on the interfaces between organic molecules and metal surfaces. Here, we study the adsorption of benzene on Ag(100) surface doped by Aun, Pdn, Ptn(n= 1–4), Au2Pd2, Au2Pt2 and Pd2Pt2 using density-functional theory (DFT), with the inclusion of van der Waals interactions, and compare with available computational and experimental result. The calculated quantities reveal that the interaction strength of Benzene with the surface gradually increases when it is doped by Pd or Pt atoms, showing a change in the nature of the interaction, from physisorption to chemisorption. We found that with surface allowing, one can tune the interaction strength. The charge transfer was found to occur from the molecule to the clean and doped surfaces with Au atoms and from the doped surface with Pd to the molecule. The redistribution of charge density indicates modifications in the features of the interface between the doped Ag(100) surfaces and benzene. In addition, alloying the Ag(100) surface with reactive TM changes substantially its electronic properties, which impacts its interactions with benzene. Finally, mixing two dopants or more can tune the electronic structure in designing more effective interfaces for various applications.

Chemical Titration of Promoted Ag/α‐Al2O3 Ethylene Epoxidation Catalysts

AbstractWe enumerate the active site density for a promoted Ag/α‐Al2O3 ethylene epoxidation catalyst (35 wt.% Ag) by using trifluoroethanol as a chemical titrant. Trifluoroethanol, when introduced into the reactant stream, deprotonates to deposit stable trifluoroethoxy or trifluoromethyl groups on the Ag surface and inhibits ethylene oxide (EO) rates, thus enabling us to assess catalytic site densities. Active site densities vary between 52 and 69 μmolTFE gcat−1 across different chlorine coverages (0–0.4 monolayers) and different oxygen concentrations (2.5–7.7 mol%) in the reactor feed. Site densities measured by in situ trifluoroethanol chemical titration are distinct compared to those evaluated using ex‐situ N2O decomposition (12.3 μmolN2O gcat−1) suggesting that dynamic restructuring of the Ag surface under reaction conditions may render pre‐reaction chemisorption measurements insufficient to accurately enumerate the active sites relevant for ethylene epoxidation.

Effect of Water on the Rearrangement of Hydrogen-Bonded Structures of 4-Aminobenzoic Acid on Au(111) and Ag(111) Surfaces

Water plays a very important role in promoting the biological self-assembly process, while our understanding is limited about how water affects and reshapes the assembly structure. Herein, the water-induced assembly structures of 4-aminobenzoic acid (PABA) on Au(111) and Ag(111) are investigated by scanning tunneling microscopy and density functional theory calculations. Water molecules could break the weak-hydrogen-bonded Kagomé structure on Au(111) and the consequent four-leaf clover structure formed by intermediate hydrogen bonds between the functional groups of PABA molecules and spontaneous release of water molecules. Upon water exposure, the four-leaf clover structure remains intact on the Au(111) surface, while the water molecules can break selectively the weak hydrogen bonds connecting the four-leaf clovers on Ag(111) and form preferential hydrogen bonds with the functional groups of PABA molecules, resulting in the generation of the water-involved structure. The adsorption energy of this hydrated structure on Ag(111) is significantly lower than that on Au(111), which may be the reason that the water-involved structure only appears on Ag(111). The result provides the real-space evidence of the interaction between water and organofunctional groups related to the type of the metal surface.

Atomic structure and peculiarities of the electronic properties of Br layers on Ag(111) at different coverages

The structure and electronic properties of Br layers on Ag(111) at different coverage were studied by low-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy (STS) and density functional theory (DFT) calculations. The Br layers are found to form a hexagonal (3×3)R30∘ structure for coverage ⁓ 0.3 monolayer (ML) and a striped (61−12) structure for coverage ⁓ 0.4 ML. STS measurements reveal suppressed differential conductance for both structures at voltages ranging between − 1.2 and 1.4 V, and surprisingly intense peaks at 1.5 V and 2.0 V. The position of the high-intensity peak in the spectra for the striped structure shows lateral variations within the unit cell and is determined by the Br coverage. In our structural model for the striped structure, supported by DFT calculations, the hexagonal (3×3)R30∘ structure transforms to the striped structure due to uniaxial inhomogeneous compression of Br atomic rows. The local work function for the Br/Ag(111) surface, estimated from dz/dV(V) measurements, is 5.0 ± 0.2 eV, which is higher than for the Ag(111) surface (4.4 ± 0.1 eV). This suggests an increase in the electrostatic dipole barrier on Ag(111) due to Br adsorption. Combined with the suppressed local density of states around the Fermi level observed by STS, these results clarify the usefulness of Br layers in reducing the coupling between Ag(111) electrons and adsorbates.

Ag nanoparticle-loaded to MnO2 with rich oxygen vacancies and Mn3+ for the synergistically enhanced oxygen reduction reaction

MnO2 is considered to be one of the most promising electrocatalysts for oxygen reduction reactions (ORR) in alkaline media and can be applied to various electrochemical energy conversion and storage devices. However, it is limited by the relatively slow kinetics of the cathodic electrochemical reactions. In addition, it is difficult to control the presence state of Ag during the modification of MnO2. To this end, an efficient ORR electrocatalyst of Ag nanoparticles supported by MnO2 nanorods was successfully synthesized by using NH3·H2O as a complexing agent to inhibit the Ag+ intercalating into the tunnels of MnO2. The half-wave potential (E1/2) and limiting current density (Jlim) of the obtained Ag/MnO2 electrocatalysts are 0.81 V and −5.6 mA cm−2, respectively, showing comparable ORR catalytic activity to commercial Pt/C catalysts. The excellent catalytic performances can be attributed to the presence of abundant oxygen vacancies and Mn3+ species on the MnO2 surface, as well as the synergistic effect between MnO2 substrates and Ag nanoparticles. Among them, oxygen vacancies enhances the adsorption of O2, Mn3+ facilitates the displacement of O22−/OH−, MnO2 inhibits the accumulation of peroxide species to improving the oxygen environment on the Ag surface and Ag accelerates the electron transfer in the whole process. This work provides a useful guide for the design of efficient Mn-based ORR electrocatalysts.

Self-Limited Embedding Alternating 585-Ringed Divacancies and Metal Atoms into Graphene Nanoribbons.

Because of their theoretically predicted intriguing properties, it is interesting to embed periodic 585-ringed divacancies into graphene nanoribbons (GNRs), but it remains a great challenge. Here, we develop an on-surface cascade reaction from periodic hydrogenated divacancies to alternating 585-ringed divacancies and Ag atoms via intramolecular cyclodehydrogenation in a seven-carbon-wide armchair GNR on the Ag(111) surface. Combining scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy combined with first-principles calculations, we in-situ-monitor the evolution of the distinct structural and electronic properties in the reaction intermediates. The observation of embedded Ag atoms and further nudged elastic band calculations provide unambiguous evidence for Ag adatom-mediated C-H activation in the intramolecular cyclodehydrogenation pathway, where the strain-induced self-limiting effect contributes to the formation of the GNR superlattice with alternating 585-ringed divacancies and Ag atoms, which shows a band gap of about 1.4 eV. Our findings open an avenue to introducing periodic impurities of single metal atoms and nonhexagonal rings in on-surface synthesis, which may provide a novel route for multifunctional graphene nanostructures.

Metal Atoms Participate in the Self-Assembly and On-Surface Reaction Behaviors of 1,4-DBN on Ag(111) Surface.

Herein, using 1,4-dibromonaphthalene (1,4-DBN) as the precursor molecule and Ag(111) surface as the substrate, we have characterized the various coordination and covalent structures formed by 1,4-DBN by low-temperature scanning tunnelling microscopy. We observed that there are three ordered structures (phase I, II, III) and one metal-organic short-chain structure (phase IV) at high coverage, meanwhile a new type of chiral structure (phase V) is observed coexisting with phase II, III, IV at low coverage. Surprisingly, all these structures have surface Ag adatoms incorporated. In addition, the phase III should be formed by a dissymmetric dehalogenation reaction of 1,4-DBN. Furthermore, we showed that the Ullmann coupling and cyclodehydrogenation of 1,4-DBN to form the armchair-shaped graphene nanoribbons will occur after thermal annealing. Combining the experiment data and density functional theory simulations, our results show that the surface Ag adatoms play a critical role in both the self-assembly and the on-surface reaction.

Self-assembly and tiling of a prochiral hydrogen-bonded network: bi-isonicotinic acid on coinage metal surfaces

Submolecular resolution scanning tunnelling microscopy and qPlus atomic force microscopy reveal that, close to thermal equilibrium, bi-isonicotinic acid (4,4'-COOH-2,2'-bpy) assembles into extended molecular rows on both Au(111) and Ag(100) surfaces, driven primarily by the formation of OH··· N hydrogen bonds. Both the intermolecular separation and inter-row spacing for Au(111) and Ag(100) are identical within experimental uncertainty, highlighting that the assembly of bi-isonicotinic acid networks on both metal surfaces is predominantly driven by intermolecular hydrogen-bonding and that the potential energy variation due to the substrate has relatively little influence. Nonetheless, the surface plays a key role in molecular organisation: symmetry-breaking induces prochiral behaviour, which drives the molecular enantiomers to form a racemic mixture of rows of different handedness. We adapt a tiling model previously introduced to model the formation of 2D networks of tetracarboxylic derivatives [Blunt et al. Science 322, 1077 (2008)] to the bi-isonicotinic acid system, providing key insights into the growth kinetics and attaining good agreement with the molecular morphologies observed in experiment.

Interaction between bilayer borophene and metal or inert substrates

Bilayer borophene have gradually come into our horizons in experimental and theoretical studies due to the superior antioxidation and advantageous to the exfoliation compared with monolayer borophene. Using systematical first-principles calculations, we explore the interactions between bilayer borophene and various metal surfaces of Ag (111), Au (111), Al (111), Cu (111) and inert substrates of monolayer hexagonal boron nitride (h-BN), monolayer MoS2 and GaN (0001) surface. The electronic band structures, partial density of states, total electron distribution and Bader charge analysis have been systematically calculated to explore for the interaction strength. The orbital hybridization and chemical bonding occur in the interface of four metals and GaN heterostructures attributed to the active surficial atoms, differ from Van Der Waals interaction of borophene-MoS2 and h-BN inert substrates. In addition, we find that metal Al (111) with smaller work function and inert substrates of monolayer h-BN and MoS2 have tiny tunneling barrier at the interface of bilayer borophene-substrates, and might be appropriate to the synthesis of bilayer borophene.

Analysis and experiment of polarization characteristics of Off-axis freeform optical system

As freeform surfaces are widely used in off-axis optical systems with large apertures, large fields of view, and long focal lengths, the polarization effect caused by the system cannot be ignored. For this in mind, a polarization aberration analysis method for freeform optical system with fringe Zernike polynomial is proposed. An analytical model for the polarization aberration in this case is constructed. The influence of the surface shape on the polarization aberration is investigated. Our analysis shows that the distribution of phase aberration, diattenuation and retardance at the exit pupil introduced by the freeform surface in the system is related to the entrance pupil, incident field angle, and also closely dependent on the surface sag of the freeform surface. The introduction of freeform surfaces causes the system polarization aberration distribution to lose rotational symmetry. Finally, a freeform surface multi-angle polarization characteristic test platform was built to test the validity of the theoretical model. The polarization aberration characteristic data of the incident mirror at different angles of view at different positions were obtained and analyzed. The measured results show that the relative errors of phase aberration, diattenuation and retardance are all less than 7.2%, which is basically consistent with the theoretical simulation analysis results. The research in this paper provides a theoretical basis for guiding the design of an off-axis freeform surface polarization imaging optical system, and provides theoretical guidance for improving the polarization aberration calibration of a freeform surface optical system.

Developing highly reliable SERS substrates based on Ag grown on alumina nanomeshes anodized under 1 V for efficiently sensing Raman-active molecules

We have developed silver-nanostructures grown on anodic alumina nanomesh (AAN) films to create new-type substrates for surface-enhanced Raman scattering (SERS). AAN with uniform thin sidewall of ∼ 5 nm was fabricated by anodizing Al sheets at 1 V in 6% H3PO4 solution. Subsequent AC electrochemical deposition of silver created an array of nanoparticles or nano-islands depending on growth time. The particle-island transition is non-monotonic evolution, since metal growth and dissolution compete in AC electrodeposition process. Systematic SERS study on various Ag-AAN films with trial probes of adenine solutions reveals collective contribution of electron-plasma oscillation and surface area of silver nanostructures in Raman enhancements. SERS signals are primarily contributed by surface area under excitation wavelength of 532 nm (away from plasmonic resonance). The average correlation coefficient between the SERS intensity and surface area was 0.85, indicating robust correlation. This value was reduced to 0.61 under excitation wavelength of 633 nm (closer to plasmonic resonance). Furthermore, increased Ag-deposition reduced the relative standard deviation of SERS intensities and thus improved both the uniformity and quality consistency of SERS substrates. Therefore, fabrication of SERS substrate with larger Ag surface area under similar SERS enhancement factors is suggested for high throughput in commercial sectors.

Step-guided epitaxial growth of blue phosphorene on vicinal Ag(111)

Blue phosphorene (blue-P) is a promising candidate for developing next-generation nanoelectronics devices due to its unique properties. Therefore, the controllable growth of a two-dimensional (2D) wafer-scale blue-P monolayer on substrates is a fundamental issue. However, the direct growth of blue-P on substrates currently remains a daunting challenge. In this paper, by using first-principles calculations, we propose a feasible route for fabricating blue-P via epitaxial growth. The growth mechanism of one-dimensional (1D) phosphorene chains to finite-sized blue-P clusters on Ag(111) is elucidated, and the reason why blue-P clusters cannot grow into a large-area monolayer on the Ag(111) surface is revealed. Moreover, the growth of blue-P on vicinal Ag(111) is explored and the energetic benefit of 1D zigzag blue-P nanoribbon growth along the $\mathrm{Ag}\ensuremath{\langle}110\ensuremath{\rangle}$ step edge is confirmed. More importantly, we propose that the unidirectionally orientated blue-P nanoribbons can merge into large-area blue-P nanosheets through a step-guide growth mode. The feasibility of this strategy is validated by using molecular dynamics simulations, and a series of candidate substrates is selected. This study not only provides a promising substrate for epitaxial growth of blue-P but also sheds light on the preparation of more 2D materials.

Phylogenic study of toxoplasmosis in aborted women in Al- Diwnaniya province

The current study included the detection of Toxoplasma gondii from aborted women attented to the Maternity and Childhood Teaching Hospital to detected the rate of abortion due to toxoplasmosis using PCR technique, the samples which was used placenta and fetus, this study also identify the differentiation between it by phylogenic with SAG3 gene.
 A total of 60 ( placenta and fetus) samples were collected randomly from aborted women who received to the Maternity and Childhood Teaching Hospital.
 The present study include molecular method to detected small subunit ribosomal RNA ( ssRNA) using Nested Polymerase Chain Reaction ( nPCR) ,the results of the nPCR for ssRNA gene, the result showed that the rate of toxoplasmosis in the aborted women was Positive 60%. (There is a statistically significant between species in P <0.05).
 The present study also detected for Surface Antigen (SAG3) gene to conform the nPCP result and to send the samples for sequences.
 The SAGA gene showed 100% positive results according to the ssRNA gene .
 T.gondii isolates surface Ag SAG3 gene with NCBI BLAST Toxoplasma gondii isolates, and the DNA nucleotides sequencing analysis of surface Ag SAG3 complete gene was show clear gentic variation between isolates from different host. 

Influence of the Deposition Rate and Substrate Temperature on the Morphology of Thermally Evaporated Ionic Liquids

The wetting behavior of ionic liquids (ILs) on the mesoscopic scale considerably impacts a wide range of scientific fields and technologies. Particularly under vacuum conditions, these materials exhibit unique characteristics. This work explores the effect of the deposition rate and substrate temperature on the nucleation, droplet formation, and droplet spreading of ILs films obtained by thermal evaporation. Four ILs were studied, encompassing an alkylimidazolium cation (CnC1im) and either bis(trifluoromethylsulfonyl)imide (NTf2) or the triflate (OTf) as the anion. Each IL sample was simultaneously deposited on surfaces of indium tin oxide (ITO) and silver (Ag). The mass flow rate was reproducibly controlled using a Knudsen cell as an evaporation source, and the film morphology (micro- and nanodroplets) was evaluated by scanning electron microscopy (SEM). The wettability of the substrates by the ILs was notably affected by changes in mass flow rate and substrate temperature. Specifically, the results indicated that an increase in the deposition rate and/or substrate temperature intensified the droplet coalescence of [C2C1im][NTf2] and [C2C1im][OTf] on ITO surfaces. Conversely, a smaller impact was observed on the Ag surface due to the strong adhesion between the ILs and the metallic film. Furthermore, modifying the deposition parameters resulted in a noticeable differentiation in the droplet morphology obtained for [C8C1im][NTf2] and [C8C1im][OTf]. Nevertheless, droplets from long-chain ILs deposited on ITO surfaces showed intensified coalescence, regardless of the deposition rate or substrate temperature.

Biological Response Evaluation of Human Fetal Osteoblast Cells and Bacterial Cells on Fractal Silver Dendrites for Bone Tissue Engineering.

Prosthetic joint replacement is the most widely used surgical approach to repair large bone defects, although it is often associated with prosthetic joint infection (PJI), caused by biofilm formation. To solve the PJI problem, various approaches have been proposed, including the coating of implantable devices with nanomaterials that exhibit antibacterial activity. Among these, silver nanoparticles (AgNPs) are the most used for biomedical applications, even though their use has been limited by their cytotoxicity. Therefore, several studies have been performed to evaluate the most appropriate AgNPs concentration, size, and shape to avoid cytotoxic effects. Great attention has been focused on Ag nanodendrites, due to their interesting chemical, optical, and biological properties. In this study, we evaluated the biological response of human fetal osteoblastic cells (hFOB) and P. aeruginosa and S. aureus bacteria on fractal silver dendrite substrates produced by silicon-based technology (Si_Ag). In vitro results indicated that hFOB cells cultured for 72 h on the Si_Ag surface display a good cytocompatibility. Investigations using both Gram-positive (S. aureus) and Gram-negative (P. aeruginosa) bacterial strains incubated on Si_Ag for 24 h show a significant decrease in pathogen viability, more evident for P. aeruginosa than for S. aureus. These findings taken together suggest that fractal silver dendrite could represent an eligible nanomaterial for the coating of implantable medical devices.

Methionine Capped Nanoparticles as Acetylcholinesterase Inhibitors

The silver and gold L-methionine capped nanoparticles (Ag and Au @LM NPs) were analyzed as prospective acetylcholinesterase (AChE) inhibitors to test their potential in the treatment of cognitive impairment in depression and Alzheimer's disease. The stability of NPs, and their ability to inhibit AChE were studied by UV-Vis and FTIR spectrophotometry. At the same time, TEM and SEM measurements, DLS, and zeta potential measurements were employed in the structural characterization of NPs. Nearly spherical, negatively charged Ag and Au @LM NPs, with 17 nm and 31 nm in diameter, respectively, showed moderate inhibitory potential toward AChE in the given frame of investigated concentrations. For both NPs IC50 is not reached. Furthermore, the adsorption of enzyme molecules on the surface of Ag and Au @LM NPs was demonstrated. Hence, our assumption is that inhibition of AChE is caused by blockage of the enzyme‘s active site due to the steric hindrance of NPs.

On-surface synthesis of enetriynes

Belonging to the enyne family, enetriynes comprise a distinct electron-rich all-carbon bonding scheme. However, the lack of convenient synthesis protocols limits the associated application potential within, e.g., biochemistry and materials science. Herein we introduce a pathway for highly selective enetriyne formation via tetramerization of terminal alkynes on a Ag(100) surface. Taking advantage of a directing hydroxyl group, we steer molecular assembly and reaction processes on square lattices. Induced by O2 exposure the terminal alkyne moieties deprotonate and organometallic bis-acetylide dimer arrays evolve. Upon subsequent thermal annealing tetrameric enetriyne-bridged compounds are generated in high yield, readily self-assembling into regular networks. We combine high-resolution scanning probe microscopy, X-ray photoelectron spectroscopy and density functional theory calculations to examine the structural features, bonding characteristics and the underlying reaction mechanism. Our study introduces an integrated strategy for the precise fabrication of functional enetriyne species, thus providing access to a distinct class of highly conjugated π-system compounds.

Flexible Au@AgNRs/CMC/qPCR film with enhanced sensitivity, homogeneity and stability for in-situ extraction and SERS detection of thiabendazole on fruits

In this study, a high-performance, stable and homogeneous Au@AgNRs/CMC/qPCR flexible film surface-enhanced Raman scattering (SERS) substrate was constructed by synergistically stabilizing and protecting bimetallic core–shell Au@Ag nanorods (Au@AgNRs) with carboxymethylcellulose (CMC) and fluorescent-quantitative-polymerase-chain-reaction (qPCR) film. The network structure of CMC immobilized and aligned Au@AgNRs through coordination of carboxyl groups with surface Ag atoms to provide intensive and stable ‘hot spots’, and the qPCR bilayer film performed as carrier and barrier to protect Au@AgNRs from oxidation, humidity and optical damage and improved the robustness and stability. The Au@AgNRs/CMC/qPCR film was used for in-situ extraction and SERS detection of thiabendazole on nectarine (0.24 ppm) and lemon (0.27 ppm) with low detection of limits. Furthermore, it retained 98.6% SERS performance after storage for 90 days under ambient conditions, revealing the great potential in promoting the commercialization of the SERS technique for sensitive contaminants sensing with simple fabrication procedures, homogeneity, reproducibility and long-term stability.

Development of low-temperature and ultrahigh-vacuum photoinduced force microscopy.

In this paper, we develop optical and electronic systems for photoinduced force microscopy (PiFM) that can measure photoinduced forces under low temperature and ultrahigh vacuum (LT-UHV) without artifacts. For our LT-UHV PiFM, light is irradiated from the side on the tip-sample junction, which can be adjusted through the combination of an objective lens inside the vacuum chamber and a 90° mirror outside the vacuum chamber. We measured photoinduced forces due to the electric field enhancement between the tip and the Ag surface, and confirmed that photoinduced force mapping and measurement of photoinduced force curves were possible using the PiFM that we developed. The Ag surface was used to measure the photoinduced force with high sensitivity, and it is effective in enhancing the electric field using the plasmon gap mode between the metal tip and the metal surface. Additionally, we confirmed the necessity of Kelvin feedback during the measurement of photoinduced forces, to avoid artifacts due to electrostatic forces, by measuring photoinduced forces on organic thin films. The PiFM, operating under low temperature and ultrahigh vacuum developed here, is a promising tool to investigate the optical properties of various materials with very high spatial resolution.