Interfacial impedance of the boundary Ag/AgCl and its investigations by a novel method (original) (raw)
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Ab initio simulations on AgCl(111) surface and AgCl(111)/α-Al2O3(0001) interface
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The defect chemistry and ionic transport properties of the AgCl(1 1 1)/a-Al 2 O 3 (0 0 0 1) interface were considered by using ab initio slab calculations. These calculations were performed in the framework of plane-wave basis set combined with the density functional theory (DFT), as implemented into the VASP computer code, and Gaussian basis set combined with the Hartree-Fock method (CRYSTAL-98 code). We analyze the electron density distribution on the interface and the electrostatic potential distribution near the AgCl surface. The size of the silver ion is too great to enter the corundum surface layer and to create excess silver ions in this way. This is in agreement with the experiments on heterogeneous doping of AgCl revealing a-alumina to be inactive compared with c-alumina. The energy to thermally create a vacancy in the first layer at the expense of an interstitial ion is large compared with the bulk Frenkel energy. Despite the calculated low activation energy for vacancy transport in the first layer (DH # s ¼ 0:23 eV), the vacancy concentration will be too small to generate perceptible surface conductivity. The striking similarity of DH # s with the bulk value is due to the quite symmetrical arrangement of the considered interface.
The Adhesion Nature of Ag/MgO Interface: Hartree-Fock Study
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The atomic and electronic structure of the Ag/MgO interface are calculated using the ab initio Hartree-Fock approach and a supercell model. The electronic density distribution is analyzed in detail for isolated and interacting slabs of a metal and MgO. The energetically most favorable adsorption position for Ag atoms is found to be above the 0 atoms. The binding energy is 0.20 eV (0.41 eV) for one and three Ag layers atop MgO substrate, respectively. The relevant equilibrium Ag-O distance is 2.64 A(2.41 A). Neither appreciable charge transfer in the interfacial region, nor considerable population of bonds between the silver layer and the insulating substrate take place. The adhesion energy arises mainly due to the electrostatic interaction of substrate atoms with a complicated charge redistribution in the metal monolayer, characterized by large quadrupole moments and electron density redistribution towards gap position in the middle of nearest Ag atoms.
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Surface Science, 2001
Ab initio study of the Ag/MgO(0 0 1) interfaces based on a quantitative analysis of the bonding in the interfacial region is provided in the framework of Hartree±Fock approach. We are describing the way interfacial electronic and other properties evolve as a function of metal coverage. General conclusion that could be drawn from our calculations is that chemical bond formation is not important for the Ag/MgO(0 0 1) perfect interface. Physisorption of Ag atoms over surface O 2À ions associated with atomic polarization and charge redistribution in the metal planes are the dominant eects. The adhesion energy is enhanced by the interaction of the substrate Mg 2 ions with the surplus of electron density accumulated near the interatomic positions of the interfacial silver monolayer. Ó
Metamaterials IX, 2014
We report on measurements of optical, morphological and electrical properties of silver nanolayers. The Ag films of thickness from 10 to 500 nm are deposited in e-beam evaporator. Fused silica and sapphire substrates are used with nominal root-mean-square (RMS) roughness equal 0.3 and 0.2 nm, respectively. Silver is deposited either directly on substrates or on Ge, Ni, or Ti wetting interlayer. The refractive index n and the extinction coefficient of Ag films are derived from spectroscopic ellipsometry and reflectance measurements carried in air in the spectral range from 0.6 to 6.5 eV (2200 -193 nm) using a rotating analyzer ellipsometer (V-VASE, J.A. Woollam Co.). Surface roughness is measured using AFM (Ntegra NT-MDT) under tapping mode in air with sharp etalon probes and 5:1 aspect ratio. Ag layers of 10 and 30 nm thickness have nearly the same RMS roughness when deposited at temperatures from 180 to 350 K. The lowest RMS=0.2 nm is achieved for 10 nm film Ag/Ge evaporated at 295 K. The sheet resistance of the Ag films is measured using two methods: the van der Pauw method with the electrical contacts located on perimeters of the samples and four probes contacting the samples at points lying in a straight line. Specific resistivity of Ag films on fused silica change from >10 9 to 1.80 [μΩ•cm] when thickness increases from 10 to 500 nm. Specific resistivity of 10, 30 and 50 nm thick Ag films on 1 nm Ge wetting layer are equal 14. 01, 7.89, and 5.58 [μΩ•cm], respectively, and are about twice higher than those of Ag films on Ti or Ni interlayers.
Microstructure, surface chemistry and electrochemical response of Ag|AgCl sensors in alkaline media
Journal of Materials Science
Characterization of the Ag/AgCl electrode is a necessary step toward its application as a chloride sensor in a highly alkaline medium, such as concrete. The nucleation and growth of AgCl on Ag in 0.1 M HCl was verified through cyclic voltammetry. Ag anodization was performed at current densities, determined by potentiodynamic polarization in the same (0.1 M HCl) medium. The morphology and microstructure of the AgCl layers were evaluated via electron microscopy, while surface chemistry was studied through energy-dispersive spectroscopy and X-ray photoelectron spectroscopy. At current density above 2 mA/cm 2 , the thickness and heterogeneity of the AgCl layer increased. In this condition, small AgCl particles formed in the immediate vicinity of the Ag substrate, subsequently weakening the bond strength of the Ag/AgCl interface. Silver oxide-based or carbon-based impurities were present on the surface of the sensor in amounts proportional to the thickness and heterogeneity of the AgCl layer. It is concluded that a well-defined link exists between the properties of the AgCl layer, the applied current density and the recorded overpotential during Ag anodization. The results can be used as a recommendation for preparation of chloride sensors with stable performance in cementitious materials.
Journal of the Electrochemical Society, 2014
Electrochemical impedance spectroscopy was used to study the degradation behavior of thin multilayer stacks; Si 3 N 4 /Ag/Si 3 N 4. Measurements were carried out in 0.5 M Na 2 SO 4 adjusted at pH 10. The impedance data collected at the open circuit potential were analyzed with a physical model where all elements are clearly defined. The results suggest a constant phase element behavior at the electrolyte dielectric interface assigned to an in-depth distribution of local resistivity through the dielectric layer induced by the electrolyte penetration. To explain the electrochemical behavior of the thin silver layer, a localized corrosion in a delaminated zone between the upper dielectric and the silver layers was adopted. The localized corrosion was found to be related to the granular structure of the silver layer in which pores are formed between the grains of silver. This model is equivalent to a recently developed double porosity or pore-in-pore model.
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A systematic study of silver ion electrodeposition at the water/air and water/dichloromethane interfaces, under potentiostatic control, is presented. We study the morphology of the deposits and their growth rate, as a function of several important physical and chemical parameters, namely, the electric voltage, the silver ion concentration, the conductivity, the viscosity, and the presence of a surfactant and an organic anionic additive known to associate with silver. The results are discussed in terms of a generalized Wagner number, which is shown to correlate well with the observations and to be a highly reliable predictive tool for the morphologies.
Defect chemistry and transport characteristics of β-AgI
Journal of Physics and Chemistry of Solids, 2000
The defect chemistry and d.c. transport characteristics of b-AgI are reconsidered by taking into account (i) two structurally different interstitial positions, (ii) short-range interactions via associations, (iii) anisotropy of the wurtzite structure, (iv) longrange defect-defect interactions via Coulomb forces, and (v) formation of highly conducting layers perpendicular to the c-axis via a disordered interface structure with stacking faults. Besides microstructural characterization the analysis relies on the ionic conductivity data by impedance spectroscopy with conventional as well as micro-electrodes, and utilizes recent reports on the defect concentration and defect energies of b-AgI by molecular dynamics simulations and on AgI:Al 2 O 3 composites. Static valence sum calculations were performed to elucidate the ion conduction pathways and related migration barriers.
Comparative theoretical study of the Ag–MgO (100) and (110) interfaces
Surface Science, 1999
We have calculated the atomic and electronic structures of Ag-MgO(100) and (110) interfaces using a periodic (slab) model and an ab initio Hartree-Fock approach with a posteriori electron correlation corrections. The electronic structure information includes interatomic bond populations, effective charges, and multipole moments of ions. This information is analyzed in conjunction with the interface binding energy and the equilibrium distances for both interfaces for various coverages. There are significant differences between partly covered surfaces and surfaces with several layers of metal, and these can be understood in terms of electrostatics and the electron density changes. For complete monolayer (1:1) coverage of the perfect MgO(100) surface, the most favorable adsorption site energetically for the Ag atom is above the surface oxygen. However, for partial (1:4) coverage of the same surface, the binding energies are very close for all the three likely adsorption positions (Ag over O, Ag over Mg, Ag over a gap position). For a complete (1:1) Ag monolayer coverage of the perfect MgO(110) interface, the preferable Ag adsorption site is over the interatomic gap position, whereas for an Ag bilayer coverage the preferred Ag site is above the subsurface Mg2+ ion (the bridge site between two nearest surface O2− ions). In the case of 1:2 layer coverage, both sites are energetically equivalent. These two adhesion energies for the (110) substrate are by a factor of two to three larger than over other possible adsorption sites on perfect (110) or (100) surfaces. We compare our atomistic calculations for one to three Ag planes with those obtained by the shell model for 10 Ag planes and the Image Interaction Model addressing the case of thick metal layers. Qualitatively, our ab initio results agree well with many features of these models. The main charge redistributions are well in line with those expected from the Image Model. There is also broad agreement in regard to orders of magnitude of energies.