Surface characterization of the modified bimetallic electrodes, as well as base substrate electrodes, were performed ex situ by atomic force microscopy Simultaneously recorded topographic and phase images provided insight into different characteristics of the bimetallic surfaces, such as the dimensions of the deposited islands, occurrence of the preferential deposition along specific surface sites and the substrate coverage with deposited islands. Phase AFM images display an obvious chemical contrast between two materials, independent of the surface topography enabling thus quite accurate estimation of the surface coverage, particularly important in the cases when the conventional ways are not accurate enough or not applicable. X-ray photoelectron spectroscopy has shown the oxidation state of the surface constituents of the as prepared bimetallic electrodes.
Pronounced catalytic activity for hydrogen evolution reaction (HER) of gold single crystals decorated by palladium nanoislands with submonolayer coverage has been reported so far for different Pd/Au(111) nanostructures in acid [1] and alkaline [2] media. Rh/Au(111) bimetallic electrodes, obtained by the same spontaneous deposition method have shown even better catalytic activity for HER in acid solution [3]. The activity was in agreement to the one theoretically predicted for Au(111) surface covered with Rh overlayers, which was comparable to that of Pt(111) [4]. It was shown that among different Rh/Au(111) nanostructures, the one with 50% Rh coverage has shown the highest activity for HER in acid media, starting at approx. 100 mV more positive potential than the most active Pd/Au(111) [1], and very close to the activity of Pt(111), although not exceeding it. Similar order of activity has been reported for HER in alkaline solution [2,5]. Comparison of the catalytic activity of the most active Pd/Au(111) and Rh/Au(111) in acid and alkaline media is shown in Fig. 1. The research is extended to the use of Au(100) electrode as the substrate for the same foreign metals deposition. The comparison of the catalytic activity of bimetallic surfaces consisting of Pd or Rh islands spontaneously deposited with the same coverage on differently oriented gold single crystals have shown that those bimetallic surfaces having Au(111) as substrate is more active than the ones deposited on Au(100) as substrate.
The improved catalytic activity of the hereby explored bimetallic electrodes was interpreted by means of geometric and electronic effects of the substrates, which are responsible for different electrocatalytic properties with respect to both constitutive metals. We belive that these results contribute to the current search for the efficient catalyst for HER.
[1] M.Smiljanić, I. Srejić, B. Grgur, Z. Rakočević, and S. Štrbac, Electrocatal. 3, 369 (2012).
[2] M.Smiljanić, I. Srejić, B. Grgur, Z. Rakočević, and S. Štrbac, Electrochim. Acta 88, 589 (2013).
[3] M.Smiljanić, I. Srejić, B. Grgur, Z. Rakočević, and S. Štrbac, Electrochem. Commun. 28, 37 (2013).
[4] G. Soldano, E.N. Schulz, D.R. Salinas, E. Santos, and W. Schmickler, Phys. Chem. Chem.
Phys. 13, 16437 (2011).
[5] S. Štrbac, M. Smiljanić, and Z. Rakočević, J. Electrochem. Soc. 163, D3027 (2016).