Abstract:This paper presents a detailed study of a water adlayer adsorbed on Pt(111) and Rh(111) surfaces using periodic density functional theory methods. The interaction between the metal surface and the water molecules is assessed from molecular dynamics simulation data and single point electronic structure calculations of selected configurations. It is argued that the electron bands around the Fermi level of the metal substrate extend over the water adlayer. As a consequence in the presence of the water layer the s… Show more
“…58 This was attributed to the hydrogen-down configuration being energetically more stable than any other orientation of molecule. 59,60 Similarly, a relatively small positive peak follows a negative dip located near h = d (near the tip). This suggests that water molecules near the tip orient preferentially so that the hydrogen atoms are closer to the tip (here, the hydrogen up orientation is preferred).…”
The water meniscus condensed between a nanoscale tip and an atomically flat gold surface was examined under humid conditions using grand canonical Monte Carlo simulations. The molecular structure of the meniscus was investigated with particular focus on its width and stability. The capillary force due to the meniscus showed a dampened oscillation with increasing separation between the tip and surface because of the formation and destruction of water layers. The layering of water between the tip and the surface was different from that of the water confined between two plates. The humidity dependence of the capillary force exhibited a crossover behavior with increasing humidity, which is in agreement with the typical atomic force microscopy experiment on a hydrophilic surface.
“…58 This was attributed to the hydrogen-down configuration being energetically more stable than any other orientation of molecule. 59,60 Similarly, a relatively small positive peak follows a negative dip located near h = d (near the tip). This suggests that water molecules near the tip orient preferentially so that the hydrogen atoms are closer to the tip (here, the hydrogen up orientation is preferred).…”
The water meniscus condensed between a nanoscale tip and an atomically flat gold surface was examined under humid conditions using grand canonical Monte Carlo simulations. The molecular structure of the meniscus was investigated with particular focus on its width and stability. The capillary force due to the meniscus showed a dampened oscillation with increasing separation between the tip and surface because of the formation and destruction of water layers. The layering of water between the tip and the surface was different from that of the water confined between two plates. The humidity dependence of the capillary force exhibited a crossover behavior with increasing humidity, which is in agreement with the typical atomic force microscopy experiment on a hydrophilic surface.
“…42,43 The theoretical modelling of electrochemical reactions is equally complex, as it needs to account for the effect of the solvent on the adsorbed intermediates, the highly charged electric field in the double layer, the free energy of the electrons in the solid and the free energy of the solvated reactants as a function of potential. [44][45][46][47][48][49][50][51][52][53][54] However, it turns out that the overall trends can be 7 ‡ We note that a fuel cell would probably be operated at potentials lower than 0.9 V, to maximise the power output. However, catalysts are typically benchmarked at 0.9 V to minimise artefacts from the measurements.…”
Section: Theoretical Trends In Activity For Pt and Its Alloysmentioning
The high cost of low temperature fuel cells is to a large part dictated by the high loading of Pt required to catalyse the oxygen reduction reaction (ORR). Arguably the most viable route to decrease the Pt loading, and to hence commercialise these devices, is to improve the ORR activity of Pt by alloying it with other metals. In this perspective paper we provide an overview of the fundamentals underlying the reduction of oxygen on platinum and its alloys. We also report the ORR activity of Pt 5 La for the first time, which shows a 3.5-to 4.5-fold improvement in activity over Pt in the range 0.9 to 0.87 V, respectively. We employ angle resolved X-ray photoelectron spectroscopy and density functional theory calculations to understand the activity of Pt 5 La.
“…For water adsorbing on undeuterated Pt, STM studies have shown that on (100) step edges, water initially forms one-dimensional chains along the steps, whereas for (110) steps these are not clearly adsorbed [16]. On deuterium-covered Pt, the interaction of water with such surfaces will be driven by the (in)ability of water to form relatively stable two-dimensional hydrogen-bonded networks extending onto the terraces, versus the tendency of water to be structured into solvation shells of the adsorbed deuterium.…”
mentioning
confidence: 99%
“…On deuterium-covered Pt, the interaction of water with such surfaces will be driven by the (in)ability of water to form relatively stable two-dimensional hydrogen-bonded networks extending onto the terraces, versus the tendency of water to be structured into solvation shells of the adsorbed deuterium. We note that from density-functional theory (DFT) calculations, the interaction of water layers with Pt(111), i.e., the water-Pt bonding, is known to be very weak and its stability on such surfaces is mainly determined by the two-dimensional hydrogen bonding network [16]. Therefore, we expect that the fact that adsorbed deuterium inhibits the direct interaction of water with the platinum surface atoms, will have only a minor effect on its stability.…”
Stepped platinum surfaces can become hydrophobic when they are hydrogenated. Even though the Pt(533) and Pt(553) surfaces have similar geometries, the hydrophobicity on the deuterated surface is surprisingly different: on Pt(533) the surface is hydrophobic with water clustering at steps, whereas the entire surface is wet on Pt(553). DOI: 10.1103/PhysRevLett.107.146103 PACS numbers: 68.08.Bc, 68.43.Hn, 82.45.Jn, 82.65.+r Groups of water molecules at an interface have the choice between interacting mostly with the substrate or with each other. This competition lies at the heart of a large variety of phenomena with wetting behavior of surfaces likely being the most well-known example. Contact angle studies date at least as far back as Thomas Young's work from the start of the 19th century [1]. Other important areas where such competition governs physical behavior are, e.g., protein folding and micelle formation [2].For solid surfaces, hydrophilic vs hydrophobic behavior appears difficult to predict. A recent study of water interacting with carbon nanotubes shows that in such confined spaces small changes in temperature may cause a switch between hydrophilic and hydrophobic interaction [3]. Also, minute details of the substrate appear to be of great relevance. A single molecular layer of amorphous solid water is hydrophilic [4], whereas the same layer of crystalline ice is hydrophobic [5].In this Letter, we show that a small change at the atomic level in substrate morphology without changing chemical identity or confinement size may also affect how water molecules adsorb to a surface. A switch from hydrophobic to hydrophilic behavior is not only apparent from drastic changes in H 2 O's desorption characteristics, but also in the chemical reactivity toward H-D exchange at well-ordered platinum surfaces. Our results impact on general thinking on long-range ordering of water molecules at interfaces and pose opportunities in tailoring chemistry occurring on nanoparticles as used in, e.g., heterogeneous catalysis and electrochemistry.As a substrate, we use single crystalline platinum discs, cut and polished to expose either the (533) or (553) surface. Schematic representations of these surfaces for top and side views are shown in Fig. 1 with every circle representing a Pt atom. The only difference between these surfaces is the step type that separates the 4-atom wide (111) terraces. The (533) surface contains the steeper (100) step type, whereas the (553) surface has the more gently sloped (110) step type. The angles that the singleatom high steps make with the terraces are, respectively, 116.6 and 125.3 . Our platinum surfaces are cleaned and studied under ultrahigh vacuum (UHV) conditions. Details on experimental procedures can be found in the Supplemental Material [6]. Low energy electron diffraction (LEED) confirms the atomic ordering of the surface as depicted in Fig. 1.The cleaned platinum surfaces are first exposed to D 2 by background dosing until no more dissociation occurs. To minimize contamination by H, t...
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