ИСТИНА

One of the basic problems in working with shape-controlled platinum surface is the characterization of its crystallographic structure. The commonly employed surface analysis based on TEM or SEM data fails to account for the difference in the surface structure in air and in the solution and the complexity of the actual surface microprofile that often represents a distribution of various single crystal orientations. Other methods of nanoparticle (NP) surface characterization are based on structure–sensitive reactions, such as nitrate electroreduction on Pt(100) terraces or irreversible adsorption of metal adatoms (Bi, Te, Ge) on Pt manifesting surface redox reactions typical for the corresponding surface domains. Pt NPs with the (100) surface orientation (“cubic NPs”) have been lately increasingly used for various electrocatalytic applications, as they have a high reaction surface area and their oxidation in air is much slower than that of single crystals. However, wide large terraces may be crucial for the optimal target electrocatalytic reaction rate. Thus, there is a drastic demand for a method characterizing the width of (100) terraces on NPs and not just yielding the general area of the so-called (100) “domains”. Such a method can be provided by analyzing the evolution of cyclic voltammograms (CVs) of Ge redox transitions on the Pt surface in the presence of Ge in the working solution. The better understanding of processes occurring on a faceted Pt surface requires studying single crystals with a known surface structure (terrace width, presence of steps). The commonly used procedure involves irreversible Ge adsorption on the Pt surface under OCP conditions from a solution with a high Ge concentration. This involves a risk of Pt surface oxidation in the time space prior to Ge adsorption, as well as formation of compressed overlayers (with the Ge coverage above 0.25). Gradual adsorption from a Ge–containing solution eliminates the first risk and allows analyzing the adsorbed layer structure on the basis of the redox peak distribution and CV evolution in time. Even in the case of wide (100) terraces, Ge adsorption results in appearance of more than a single peak (Ge(0)↔Ge(II)). Decomposition into Lorentz peaks shows that there is indeed the main peak (I) with saturation at the Ge:Pt atomic ratio of 1:4 and an additional prepeak (II) at lower potentials that corresponds to an additional Ge overlayer. 1b). The height of peak (I) diminishes upon Ge adsorption, while prepeak (II) grows and shifts towards lower potentials. Brief electrode conditioning at 0.68 V leads to desorption of more weakly adsorbed Ge adatoms and restoration of the height of peak (I). In the case of stepped single crystal surfaces, the analysis of the surface structure is complicated by the presence of additional peaks and redistribution of peak areas/heights, as compared to CVs obtained for Ge adsorbed under OCP conditions. As even the cubic NP surface commonly represents an array of terraces of various dimensions, adsorption of Ge on such NPs both under OCP conditions and at potentials below 0.6 V leads to a halo feature in the CVs that cannot be resolved into individual peaks. The correct conditioning procedure with minute analysis of CV evolution for both single crystal and NP surfaces brings out the contribution of the wider terraces and provides a more accurate assessment of the NP quality.