Using scanning tunneling microscopy we studied the adsorption and reactions of water on a Cu(110) surface partially covered with oxygen in the O(2 × 1) phase formed by Cu–O chains. The experiments were carried out from low (77 K) to higher temperatures to sequentially open reaction channels that require increased activation energy. At 77 K, water adsorbs at the edges and on top of the Cu–O chains. Heating to 155 K caused water molecules to react with the oxygen atoms in the chains to produce OH. These hydroxyl groups form elongated H-bonded structures mixed with excess unreacted water molecules. The Cu atoms freed in the reaction form small clusters at the location of the initial Cu–O chains. Heating to 180 K leads to a second reaction that desorbs all excess water (i.e., water not H-bonded with OH), leaving H2O–OH zigzag chains along [110] directions. At the low oxygen coverage studied here (<0.12 ML) the extent of the partial water dissociation reaction is stoichiometrically determined by the amount of preadsorbed oxygen (H2O + O → 2OH). A third annealing to 280 K resulted in desorption of all water and hydroxyl species and the reappearance of Cu–O chains. After heating to 280 K, the oxygen coverage returns to nearly the same value as that of the initially surface.

First-order phase transitions typically exhibit a significant hysteresis resulting for instance in boiling retardation and supercooling. The hysteresis arises, because nucleation of the new phase is activated. The free-energy change is positive until the nucleus reaches a critical size beyond which further growth is downhill. In practice, the barrier is often circumvented by the presence of heterogeneous nucleation centres, e.g. at vessel walls or seed crystals. Recently, it has been proposed that the homogeneous melting of ice proceeds via separation of defect pairs with a substantially smaller barrier as compared to the mere aggregation of defects. Here we report the observation of an analogous mechanism catalysing a two-dimensional homogeneous phase transition. A similar process is believed to occur in spin systems. This suggests that separation of defect pairs is a common trigger for phase transitions. Partially circumventing the activation barrier it reduces the hysteresis and may promote fluctuations within a temperature range increasing with decreasing dimensionality.

Halogens, including iodine, are commonly used as additives to enhance the selectivity of catalytic processes. In sustainable-energy applications, such as dye-sensitized solar cells or photocatalytic water splitting, iodine is often applied as redox shuttle. Since platinum is both a prominent catalyst as well as electrode material, we investigated the phases of iodine on Pt(110) at a coverage of 0.5 monolayers by scanning tunneling microscopy, low-energy electron diffraction, and by density functional theory (DFT) calculations. Three distinctly different phases occur at room temperature, involving occupation of two different binding sites. Preferred binding sites and phase stability ranges are different from the ones reported for Br on Pt(110), reflecting a different balance of adsorbate−substrate and adsorbate−adsorbate interactions. DFT results are in striking agreement with experimental results for the c(2 × 2) phase, where the substrate remains planar, but disagree with experiment for phases involving a surface buckling. This is attributed to a preference of DFT for local over nonlocal interactions, i.e., CDW/PLD correlations. Comparison with literature data reveals a trend for increasingly unspecific iodine bonding to the substrate in the sequence Pt(110), Pd(110), Au(110).