In Situ-Raman Spectroscopy and Comparison with DFT Computational and Experimental Study of Electroless Nickel Deposition

Abstract

The kinetic mechanism of the electroless deposition of NiP from hypophosphite solutions was investigated from first principles with the intent of determining a set of elementary reactions that could effectively identify the most relevant steps of this complex process. The outcome of the computational approach is discussed in terms of experimental results obtained by electrochemical methods and surface analysis techniques [1,2]. Simulations were performed with Density Functional Theory DFT using the B3LYP exchange and correlation functionals [3], the 6-311+G(d,p) basis set for P,C,H and O atoms [4], and the Stuttgart effective core potential basis sets for Ni [5]). The interaction of Ni++ and H2PO2- with the Ni surface was studied using Ni clusters of different sizes (from 4 to 12 atoms). Solvation in water was modeled using the implicit polarizable continuum model [6]; calculations were performed using the G09 computational suite [7]. Simulations were initially devoted to the identification of the first reaction steps that lead to the adsorption of Ni++ on the Ni surface and to its successive reduction by means of the hypophosphite anion. Though several theories were proposed to describe this key step in the literature, our search was guided by the original insight published several years ago [8], according to which the reduction of Ni++ requires the formation of an intermediate species containing the NiOH group. Such species would then take active part in the transfer of the OH group to H2PO2-, thus favoring its oxidation to H2PO3-. In addition, it was imposed that the searched elementary mechanism satisfies the experimental observation, obtained with marked deuterium species, that each molecule of hydrogen that is evolved contextually to the deposition of NiP contains two H atoms coming from two H2PO2- anions. The possibility that NiOH+ is formed in solution prior to deposition was investigated determining computationally the pKa of Ni++. Results revealed that the formation of NiOH+ is not favored in the pH ranges in which NiP is usually deposited. A similar result was obtained when considering the possibility that NiOH+, after being formed in solution, is stabilized through the formation of a complex with H2PO2-. A thermodynamically favored set of reactions was instead found when it was investigated the possibility that NiOH is formed at the surface contextually with a Ni-H2PO2 complex. It was in fact found that the following set of reactions has a globally slightly negative (≈ -5 ± 5 kcal/mol) free energy change: R1) Ni ++ + surf ® Niads ++ R2) Niads ++ + 2 H2O + H2PO2 - ® NiOH(H2PO2)ads + H3O+ The minimum energy structure of the surface NiOH(H2PO2)ads species optimized at the B3LYP level on a Ni8 cluster in implicit water. The two reactions set R1-R2 provides a mechanism for the initial step of reduction of Ni++, thermodynamically viable and consistent with the hypotheses formulated above. The results of this analysis suggest therefore that the initial step of Ni++ reduction during NiP deposition involves a concerted mechanism in which adsorbed Ni++ contextually reacts with H2PO2- and a water molecule. This mechanism can be also supported by electrochemical measurements, indicating the need of OH funtionalization of the catalytic surface in order to have the reaction of hyposphite. In order to understand completely the adsorption phenomena on the electrode surface during the electroless deposition process, in situ-Raman analyses were performed. The purpose of this experimental section is to discern the different complexes adsorbed on the electrode surface while the reaction proceeds. These tests were carried out at different temperature, starting from the ambient temperature to the effective process temperature. The effect of each complexing agent dissolved into electroless NiP bath on complexes adsorbed on the electrode surface and the comparison with the DFT computational analysis will be presented.