Electroless deposition process has been applied to various industrial fields as a fundamental technique to fabricate metal thin films. To improve the controllability of this process, investigations of the reaction mechanism and the catalytic activity of metal surfaces for each reducing agent reaction are essentially important. For systematic analyses, theoretical calculation can be employed as a powerful tool. Our previous theoretical studies suggested that the dehydrogenation step of the reducing agents could be a rate-determining step in most cases [1]. This study investigates the reaction mechanisms of reducing agents with solvent molecules on Cu, Ni, and Pd, analyzing the electronic states during the reaction to identify the origin of the catalytic activity observed in each metal surface, by using density functional theory (DFT).

All calculations were performed by using Vienna Ab initio Simulation Package (VASP). The cutoff energy for the plane wave basis set was set at 400 eV. The core electrons were described by pseudopotentials constructed with the Projector Augmented Wave (PAW). The exchange-correlation energies were evaluated with the Perdew, Burke and Ernzerhof (PBE) functional. The k-point grids were taken as 3 × 3 × 1 for the geometry optimization and 15 × 15 × 1 for the density of states (DOS) calculations. Assuming that the process employs high pH solution, the molecular structures of formaldehyde and hypophosphite ions were modeled as H₂CO(OH)^- and H₂PO₂^-, respectively. The solvation structures were prepared by locating several explicit H₂O molecules, which were directly interacting with the molecules of the reducing agents.

Previous study of each elementary step, such as the adsorption and the dehydrogenation, indicated that H₂O played an important role to determine the geometrical structure or electronic state in those steps. To obtain a reasonable result of the solid-liquid interface system, the solvation structures of each reducing agent were investigated. H₂O molecules turned out to interact mainly with O atoms of formaldehyde via H atoms in the solvation structure (Fig. 1), which was almost the same as the case of hypophosphite. Due to the sterically bulky structure around O atoms, the two reducing agent molecules adsorbed on each metal surface via H atoms; H and center atoms locate on atop and bridge sites, respectively. The solvation structure prevented unreasonable dissociation of the molecules on metal surfaces, suggesting that the surrounding solvent molecules are necessary to build the practical adsorption model. The reaction energies of the dehydrogenation on each metal surface have good agreement with the catalytic activity shown in the experimental results [2]. DOS profiles demonstrate that the electronic state of the dissociated H atom of formaldehyde is sufficiently stabilized by the effective interaction with Cu surface atom, while that of hypophosphite is not. Such a detailed investigation for the electronic state of the reducing agents at each surface during the reaction is helpful to elucidate their reactivity.

[1] M. Kunimoto, T. Shimada, S. Odagiri, H. Nakai, T. Homma, J. Electrochem. Soc., 158, D585-D589 (2011).

[2] I. Ohno, O. Wakabayashi, S. Haruyama, J. Electrochem. Soc., 132, 2323-2330, (1985).