Anna Ignaczak

A theoretical study of the interaction of water molecules with the Cu(100), Ag(100) and Au(100) surfaces

The interaction of a water molecule with copper, silver and gold surfaces has been studied using a cluster model approximation. The B3LYP method has been used to introduce a correlation effect into this type of calculation. Since this is a relatively new technique, its performance in conjunction with different basis sets has been tested using the Cu-H2O system as a test case. These tests were performed to select the basis sets for water and metal atoms to be used in studies on the water—metal cluster interaction. Additionally, a set of B3LYP calculations for the Cun-H2O (n = 2, 4, 5, 9, 12) systems has been performed to investigate the influence of the metal cluster size on the results. Significant variations in the quantities computed have been found for the smaller clusters, while for the larger ones, a degree of convergence seems to be achieved. Thus, the Cu12, Ag12 and Au12 clusters have been used to mimic the (100) crystallographic plane of noble metals. The comparative results of the water—metal interaction for three different positions (on-top, bridge and hollow) are given; for each site the results for three different orientations of H2O molecule are shown. Two preferred conformations, bridge-perpendicular and top-tilted, have been found to be effectively indistinguishable in terms of adsorption energy; in both, water adsorbs via its oxygen end. For the on-top site, the tilt angle between the H2O molecular plane and the normal to the metal surface has been found to be in the range 50–65°, depending on the metal. The copper has been found to be the most hydrophilic metal (ΔEads = −31.8 kJ mol−1) when compared with less attractive gold (ΔEads = −29.7 kJ mol−1) and silver (ΔEads= −26.6 kJ mol−1). The results obtained from our calculations are compared with some earlier theoretical results and with the experimental data available.

Interaction of halide ions with copper: the DFT approach

Abstract The applicability of density functional theory to the adsorbate-adsorbent interaction has been tested with the final goal of defining an appropriate combination of the DFT method with basis sets for metal cluster-halide ions studies. The Cu-X− (X = F, Cl, Br, I) systems have been taken as tests cases to probe the different DFT methods (SVWN, BP86 and B3LYP) together with different combinations of basis sets for systems of this type. For comparison, the standard HF and its MP2 and MP4 corrections have also been calculated. Additionally, the results of a test for the Cu5I− cluster are presented. The good quality of the DFT methods is recorded as they give results comparable to the MP2 and, for some cases, even to the MP4 level of standard calculations. The B3LYP method combined with a fairly inexpensive description of the metal atoms is proposed as an appropriate option for studies of the adsorption of halide ions at noble metal surfaces.

Theoretical study of a non-adiabatic dissociative electron transfer reaction

Abstract The kinetics of non-adiabatic bond-breaking electron transfer reactions have been investigated theoretically, taking tert -butyl chloride as a model system. The reaction rate was calculated from first-order perturbation theory. Since the model requires the overlaps between the initial and the final states, the potential-energy surfaces of (CH 3 ) 3 C-Cl and of the dissociating (CH 3 ) 3 C-Cl − system were obtained from quantum-chemical calculations. The calculated points could be represented by a simple analytical form whose parameters were fitted to the data, and from these the wavefunctions for both bound and unbound states were obtained numerically. The overlaps between the lower initial states and the final states appear to be very small for this system, and higher excited states are found to make a significant contribution to the rate, especially for low overpotentials. The activation energy is lower than that obtained from classical models, and corresponds to the excitation energy of levels that contribute most to the reaction rate.

Quantum and simulation studies of X−(H2O)n systems

This paper reports on studies of the interactions of halide ions with water. The standard Hartree‐Fock (HF) method was used to calculate the interaction between each of the four halide ions and the water monomer. The structural properties of the XH2O systems (XF, Cl, Br, I) are presented with a detailed comparison with experimental energies. A new ion‐water parameterised potential, derived from quantum calculations, is proposed for the description of the XH2O interactions in simulations. This potential was used in Monte Carlo (MC) studies of the gas-phase formation of X(H2O)n clusters (n1, …, 10) and of the solvation of the ions in dilute aqueous solutions. Thermodynamic properties, such as enthalpies, DHn 1,n, Gibbs free energies, DGn 1,n, and entropies, DSn 1,n, are presented for the gas-phase reactions: X(H2O)n H2O X X(H2O)n. The results follow the general experimental trends, but overestimate their absolute values for the smaller clusters. The structural properties of the small clusters were found to be in good agreement with the results of quantum calculations. For small n, the so-called surface (S) structure was found to be predominant, while for larger n the interior (I) structure is preferred. The transition from an (S) structure to an (I) structure was found to occur for n between 4 and 6, depending on the ion. In solution, the energy of solvation and the structural properties of each ion are reported and compared with the experimental data available. The energy values were found to be in good agreement with estimates reported for the three larger ions, while for fluoride they are slightly overestimated.

The potential of mean force on halide ions near the Cu(100) surface

Abstract This paper reports an investigation of the phenomenon of specific adsorption of halide ions on a Cu(100) surface using Monte Carlo simulations. The system was modeled by considering each ion in a water lamina placed between two copper walls. The potentials used in simulations were constructed by fitting to results of quantum calculations. The solvent contribution to the potential of mean force (pmf) was calculated for each of the four halide ions using the free energy perturbation method. Given the difficulty of finding a reliable ion–metal potential, several alternatives, representing extremal models, were tested in combination with the solvent mean force on the ions, F − , Cl − , Br − or I − . The results for the pmf on an ion near the metal surface are discussed in the light of the experimental data available. The sensitivity of the results to the type of ion–metal potential used in the simulations is stressed.

Simulations of adiabatic bond-breaking electron transfer reactions on metal electrodes

Abstract Adiabatic electron transfer at metal electrodes accompanied by bond breaking is studied by molecular dynamic simulations in a two-dimensional model potential. In the simulations the redox system is coupled to a thermal bath that ensures proper statistical mechanics and provides the energy fluctuations required for the system to cross the activation barrier. The rate constants and transmission factors obtained from the simulations are analysed for various system parameters such as the applied overpotential and the strength of the interaction between the reactants and the electrode. A dependency of the rate constant on the friction parameter is also tested considering both isotropy and anisotropy effects. The Kramers turnover region is found in both cases, but the rate constant is much lower than predicted by the Kramers theory. In all simulations a strong saddle-point avoidance was observed.

Simulation of water solutions of Ni2+ at infinite dilution

Abstract A new ab initio pair potential is developed to describe the nickel—water interactions in Ni(II) aqueous solutions. Results of Monte Carlo simulations for the Ni(II)(H2O)200 system are presented for this pair potential with and without three-body classical polarization terms (the water—water interaction is described by the ab initio MCY potential). The structure of the solution around Ni(II) is discussed in terms of radial distribution functions, coordination numbers and thermal ellipsoids. The results show that the three-body terms have a non-negligible effect on the simulated solution. In fact, the experimental coordination number of six is reproduced with the full potential while a higher value is predicted when the simple pairwise-additive potential is used. The equilibrium NiO distance for the first hydration shell is also dependent on the use of the three-body terms. Comparison of our distribution functions with those obtained by neutron-diffraction experiments shows a reasonable quantitative agreement. Statistical pattern recognition analysis has also been applied to our simulations in order to better understand the local thermal motion of the water molecules around the metal ion. In this way, thermal ellipsoids have been computed (and graphically displayed) for each atom of the water molecules belonging to the Ni(II) first hydration shell. This analysis revealed that the twisting and bending motions are greater than the radial motion, and that the hydrogens have a higher mobility than the oxygens. In addition, a thermodynamic perturbation method has been incorporated in our Monte Carlo procedure in order to compute the free energy of hydration for the Ni(II) ion. Agreement between these results and the experimental ones is also sufficiently reasonable to demonstrate the feasibility of this new potential for the nickel—water interactions.

A theoretical approach to the adsorption of ions on metal surfaces

Abstract This paper gives a short review of the current status of quantum and simulation studies of the specific adsorption phenomenon. In the light of results of some recent investigations on the adsorption of halide ions on noble metals, the difficult problem of the interfacial interactions is discussed. Several difficulties associated with the modeling of the adsorption process in the simulations are pointed out. Some of them are shown to be related to the cluster model calculations commonly used in the fitting of the analytical potentials for the ion–metal interaction. The adequacy of the additive analytical form of this potential which is usually applied, is also discussed. The problem of the description of the interaction of the ion with the electrode in the intermediate region, when the ion is still surrounded by water molecules, is also addressed.

The role of tannic acid and sodium citrate in the synthesis of silver nanoparticles

AbstractWe describe herein the significance of a sodium citrate and tannic acid mixture in the synthesis of spherical silver nanoparticles (AgNPs). Monodisperse AgNPs were synthesized via reduction of silver nitrate using a mixture of two chemical agents: sodium citrate and tannic acid. The shape, size and size distribution of silver particles were determined by UV–Vis spectroscopy, dynamic light scattering (DLS) and scanning transmission electron microscopy (STEM). Special attention is given to understanding and experimentally confirming the exact role of the reagents (sodium citrate and tannic acid present in the reaction mixture) in AgNP synthesis. The oxidation and reduction potentials of silver, tannic acid and sodium citrate in their mixtures were determined using cyclic voltammetry. Possible structures of tannic acid and its adducts with citric acid were investigated in aqueous solution by performing computer simulations in conjunction with the semi-empirical PM7 method. The lowest energy structures found from the preliminary conformational search are shown, and the strength of the interaction between the two molecules was calculated. The compounds present on the surface of the AgNPs were identified using FT-IR spectroscopy, and the results are compared with the IR spectrum of tannic acid theoretically calculated using PM6 and PM7 methods. The obtained results clearly indicate that the combined use of sodium citrate and tannic acid produces monodisperse spherical AgNPs, as it allows control of the nucleation, growth and stabilization of the synthesis process. Graphical abstractᅟ

DFT calculations of the interaction of alkali ions with copper and silver

Abstract The interaction of alkali ions with the Cu(100) and Ag(100) surfaces is studied using the DFT method. The results of the B3LYP calculations performed for five cations, Li+, Na+, K+, Rb+ and Cs+, adsorbed on the surface of the M12(6,6) cluster (M=Cu, Ag) are presented. On both metals the interaction is found to be strongest for the Li+ ion and weakest for the Cs+ ion. Three sites were tested for the adsorption of ions on the (100) surface: top, bridge and hollow. For the two smaller ions, Li+ and Na+, the hollow site is found to be the most stable. For the three larger ions the top position is more attractive. Nevertheless, the energy value at the different sites for a given ion in most cases, differs by less than 5 kJ mol−1. For all ions the interaction with silver is stronger than the interaction with copper.