Spiral adsorbate structures on monoatomic nanowire electrodes.

Monoatomic nanowires embedded in an electrolyte solution can be considered as one-dimensional electrodes. In order to investigate the interaction of the wire with the solution, classical molecular dynamics simulations are performed for a silver wire surrounded by an aqueous solution of NaCl. The required interaction potentials are obtained from ab initio calculations. An uncharged wire is covered by an ordered adsorbate structure, which consists of intertwined strands of Cl and Na ions that wind around the wire. This is in striking contrast to plane silver electrodes, at which Na ions do not adsorb at all. The difference in the adsorption behavior between monoatomic and plane electrodes is mainly caused by steric effects. Our findings suggest that the distinction between non-adsorbing and adsorbing ions, on which our understanding of the adsorption of bulk electrodes is based, does not hold for monoatomic wires. Nanowires are a highly popular area of research. This great interest is driven both by the new physical phenomena that occur at wires with diameters of a few atomic radii, and by their possible applications in nanotechnology. Naturally, the limit of monoatomic wires deserves special attention, and much research effort has been focused on their physical properties. Electrochemistry offers convenient methods for the generation of such monoatomic nanowires. 2] Experimental investigations of these wires have mainly dealt with their conductivity, and their elasticity—properties which are also investigated for wires in air or vacuum. However, monoatomic wires embedded in a solution can be considered as one-dimensional electrodes. Thus, the question if, and how, their properties differ from those of bulk electrodes is of obvious importance. Recently, Leiva et al. have shown that the capacity of these wires is substantially enhanced, and that the transport towards and away from these wires is faster by orders of magnitudes, compared to bulk electrodes. In addition, the potential of zero charge of gold and silver wires was shifted substantially towards more positive values. Herein, we focus on another fundamental property of electrochemical nanowires, namely their interaction with a surrounding electrolyte solution, especially with ions. At bulk electrodes, ions shed off a large part of their solvation shell during adsorption; therefore the adsorption of simple ions is governed by their solvation. Small, strongly solvated cations like Na or K do not adsorb at all. In contrast, practically all simple anions like Cl or Br adsorb at uncharged or positively charged electrodes, because their energy of solvation is substantially lower than that of cations with a similar size. For steric reasons, ions that are adsorbed on a monoatomic wire lose only a small part of their solvation shell. Therefore, we may expect that the energy of activation for adsorption is reduced. In order to explore the adorption of ions on nanowires quantitatively, we perform a theoretical study based on ab initio calculations combined with molecular dynamics. Our results indicate that, in contrast to flat electrodes, all ions adsorb on a nanowire. Furthermore, we propose the existence of a one-dimensional salt-like spiral if the nanowire is exposed to an aqueous solution, a structure that has never been observed or suggested on electrodes before. As our model system, we choose a monoatomic silver wire in contact with an aqueous solution of NaCl, and study its statistical mechanics by molecular dynamics simulation. The choice of the electrolyte is governed by the fact that the two kinds of ions exhibit contrasting adsorption behavior at bulk electrodes. Computer simulations require good interaction potentials, if the results are to be meaningful. There are well-tested potentials for bulk silver in contact with aqueous solutions, 7] but the interactions of particles with a monoatomic wire may differ from those with a flat electrode surface. Therefore, as a first step, we calculate the interaction potential of a silver wire with water, Na and Cl anew. For this purpose, we first determine the structure of a monoatomic silver wire itself, and perform DFT calculations for a ring of Ag atoms. The ring is chosen instead of a linear wire to avoid edge effects. We use the B3LYP hybrid exchange-correlation functional and the LANL2MB atom-centered basis set. 10] The ring consists of 30 atoms, as the energy per atom converges for this number of atoms. The optimized Ag Ag distance of 2.74 A shows a contraction of the Ag Ag bond compared to the metal crystal (2.89 A). This is in qualitative agreement with the results of Leiva et al. , though our interatomic distances are a little larger than theirs. This interatomic distance is kept constant in all other calculations, and in particular in the molecular dynamics simulations. The interaction potential between the wire and the ions is then obtained by first calculating the image potential, as this gives the interaction potential at large distances. The image potential is calculated from standard electrostatics, treating the wire as a conducting cylinder. At short distances, the ions are adsorbed and partially discharged. The energies of the adsorbed, partially charged atoms can be calculated by DFT; we [a] Prof. Dr. A. Groß, Prof. Dr. W. Schmickler Institut f r Theoretische Chemie Universit t Ulm, 89069 Ulm (Germany) Fax: (+49)731-502-2819 E-mail : [email protected] [b] Dr. S. Frank, Dr. C. Hartnig Zentrum f r Sonnenenergieund Wasserstoff-Forschung Baden-W rttemberg 89081 Ulm (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200700822.