Electrochemical fabrication of nanoporous gold decorated with manganese oxide nanowires from eutectic urea/choline chloride ionic liquid. Part III − Electrodeposition of Au–Mn: a study based on in situ Sum-Frequency Generation and Raman spectroscopies

In two previous papers (C. Mele, M. Catalano, A. Taurino, B. Bozzini. Electrochim. Acta 87 (2013) 918; B. Bozzini, A. Gianoncelli, C. Mele, M. Kiskinova. Electrochim. Acta 114 (2013) 889), we have: (i) fabricated high-capacitance materials consisting of nanoporous gold (NPG)-supported MnO2 nanowires (NW), by electrochemical etching of single-phase Au-Mn alloys electrodeposited from a deep eutectic solvent (DES) electrolyte and (ii) investigated some aspects of the precursor Au-Mn alloy electrodeposition process by following it in situ with space-resolved soft X-ray fluorescence (XRF) and absorption (XAS) microspectroscopies: this study has allowed to single-out the peculiarities of elemental and chemical-state distribution that contribute to the nanostructure fabrication. The present paper completes the electrodeposition study by investigating the potential-dependent interfacial composition of the growing Au-Mn alloys by complementary in situ linear (Raman) and non-linear (Sum-Frequency Generation - SFG) vibrational spectroscopies. The results regarding alloy electrodeposition are compared to those obtained with a pure Au bath. In situ spectroscopy during electrodeposition reveals that both choline cation and urea are present at the growing metal/DES interface, coadsorbed with CN resulting from the decomposition of the Au complex. Electrostatic adsorption controls the surface coverage scenario at the Au/DES interface, while Mn favours the relative surface coverage with urea. Moreover, the interaction of urea with the metal film is modified by the addition of Mn, switching from solid-like to liquid-like at the Mn-alloying potential threshold. Also the CN adsorption scenario is sensitive to surface alloying: the Mn-containing interface shows two adsorption sites with lower degree of metal-adsorbate charge transfer. Finally, the degree of surface enhancement correlates well on the one hand with the applied potential and the interfacial chemistry, and on the other hand with the crystallite morphology induced by alloying Au with Mn. The correlation among the spectroelectrochemical scenario, the potential-dependent alloy composition and the crystallite shape expressed by this investigation fits within the framework set by recent modelling of dynamic electrodeposition morphochemistry and opens up novel opportunities for improving the control over the functional properties of net-shape electrodeposited materials