MP2 Study on Water Adsorption on Cluster Models of Cu(111) Henna Ruuska and Tapani A. Pakkanen* Department of Chemistry, UniVersity of Joensuu, FIN-80101, Joensuu, Finland Richard L. Rowley Department of Chemical Engineering, Brigham Young UniVersity, ProVo, Utah 84602 ReceiVed: August 27, 2003; In Final Form: October 29, 2003 Interaction energies of a water molecule with a model Cu(111) surface were investigated using the cluster model approach and high-level ab initio methods. Potential energy scans were calculated for a Cu 10 model cluster and a water molecule with a second-order Møller-Plesset (MP2) method using a RECP/6-31+G* basis set and counterpoise correction (CP) for the basis set superposition error (BSSE). Different adsorption sites and water orientations were considered. A full geometry optimization for a water molecule at the on-top adsorption site of the Cu 10 cluster gave 77 degrees for the optimal tilt angle between the water plane and the surface normal, and non-CP interaction energy of -41 kJ/mol. Interaction energies between a Cu 18 model cluster and H 2 O were also calculated at selected points. Without the CP correction, the orientation with hydrogens parallel to the Cu 18 surface was the most favorable with interaction energy of -48 kJ/mol. After CP correction, the interaction was significantly weaker and the orientation with hydrogens toward the surface was the most favorable energetically for the chosen method with a CP-corrected minimum of -16 kJ/mol. 1. Introduction Copper is an important transition metal, with high electrical and thermal conductivity. It is used in many industrial processes, in the pure state as sheets, tubes, rods, and wires, but also as an alloy with other metals. Electrodeposition, a process where metal ions are adsorbed on the metal surface, is used to improve material properties such as electrical conductivity or corrosion protection. Electrodeposition has applications also in the electronics industry, and can be used to form device electrical contacts and connectors. 1,2 A detailed understanding at the atomic level of the electrodeposition mechanism is essential for design, optimization, and control of such processes. We are currently using molecular simulations to study the dynamics and equilibrium properties of aqueous copper solutions near a solid copper surface. Key to such modeling is the interaction energy between the metal surface and the molecules and ions in aqueous media. Here we report the results of a quantum mechanical study on the interaction energy between water molecules with copper clusters representing the metal surface. Much experimental and theoretical work has been performed on the interaction of water with transition metal surfaces. Metal-molecule interactions are usually viewed as arising from electronic σ donation from the ligand to the metal as well as π back-donation of electrons from the metal to the ligand. The balance between these two effects differs for different types of ligands, and water is an example of a pure σ donor molecule, without the π back-binding. 3-6 Water is also a prototypical hydrogen-bonded system. On the metal surface, water tends to form hydrogen-bonded clusters. 7-9 In the present study, only one water molecule is considered. Quantum chemical methods have their advantages in providing atomic-level understanding of the adsorption mechanism. There have been many studies on the interaction between a single Cu atom 3-4,10-14 or Cu + ion 4,15-22 with a water molecule. When complexes include metallic positive ions, association arises from electrostatic interactions, whereas in complexes with neutral metal atoms the attraction is due primarily to dispersion forces 4,10-11 and nearly all of the interaction comes at the correlation level. A computational method that includes electron correlation is thus necessary for accurate modeling of the Cu-H 2 O system. 4,10,13,21 In addition, the basis set superposition error (BSSE) correction has a strong influence on the interaction energies. 3-4,13 These studies give valuable information about the nature of the Cu- H 2 O interaction, but the interaction of a water molecule with a metallic surface may be quite different. The surface is commonly described using a cluster model, but the cluster size has a strong influence on the adsorption energies obtained for a metal/copper surface. 23-26 Hu and Boyd 23 showed in their DFT study that adsorption energies converge very well when the cluster has more than 18 atoms. They conclude that the atom at the adsorption site should have as many coordination atoms as there are in the real surface environment, and a sufficiently large surface area. Small clusters can give larger or smaller interaction energies than those obtained from larger cluster calculations or experiment, and they may give the wrong order for the cleavage planes. 23 According to Balbuena et al. 24 the ends of the cluster, or low-coordinated atoms tend to concentrate the spin density; atoms having larger coordination numbers tend to have the highest negative charge. Many properties (dissociation energy, HOMO-LUMO gap, ionization potential) have differences between the end atoms and central ones. 24 The adsorption sites and adsorbate orientations for water on a copper surface have been studied with various experimental and theoretical methods. However, studies using high-level computational methods and a realistic surface model have been very limited. The preferred water geometry and adsorption site calculated with angular-dependent Lennard-Jones potentials by * Corresponding author. E-mail: Tapani.Pakkanen@joensuu.fi. 2614 J. Phys. Chem. B 2004, 108, 2614-2619 10.1021/jp031022l CCC: $27.50 © 2004 American Chemical Society Published on Web 01/31/2004