THE ICE BILAYER ON PT(111) - NUCLEATION, STRUCTURE AND MELTING

The H2O-adsorption on Pt(111) at 120 K-240 K is investigated by temper ature-variable scanning tunneling microscopy. At 140 K the adsorption kinetics of the first bilayer is determined by heterogeneous nucleatio n at the upper side and the lower side of step edges as well as by hom ogeneous island nucleation on the terraces. Depending on the preparati on conditions the bilayer exhibits three different phases. Phase I can be characterized as an ideal ice bilayer rotated +/-7 degrees with re spect to the [112]-direction of the Pt(111)-surface. As a result the S TM-images show a Moiree pattern. The second phase, phase IIb, is less dense than phase I and appears to be a regular arrangement of regions of H2O-molecules with the root 3 X root 3 R30 degrees-distance of the Pt(111)-substrate and regions with higher density in between. This lea ds to two superstructure domains differing in orientation and periodic ity from the domains of phase I. Contrary to phase I, the super struct ure of phase IIb is not a Moiree pattern. The third phase, phase IIa, shows a super structure with the same periodicity as phase IIb, but wi th a different orientation. It has probably a similar atomic arrangeme nt as phase IIb and is also not a Moiree pattern. The three solid phas es can be transformed into each other either by changing the temperatu re and/or by applying a H2O-pressure at elevated temperatures. It turn s out that solid-solid-phase transformations are only possible. if the ice layer is partially molten. From the pressure dependence of the ph ase transitions an order of density of the different solid phases can be deduced. It is in agreement with the densities concluded from the s tructural analysis. All phase transformations can be described consist ently, if one assumes that the liquid like phase is denser than the th ree solid phases. This is analogous to the density anomaly of water in three dimensions. Below 135 K, second layer nucleation takes place. I t starts exclusively at special sites of the underlying superstructure . This heterogeneous nucleation leads to a regular array of small clus ters. The density of clusters in the second layer is nearly three orde rs of magnitude higher than the island density in the first bilayer. A n analysis of the contrast mechanism for STM-imaging of H2O on Pt(111) leads to the interesting conclusion that the H2O-layer next to the Pt (111)-surface has a metallic conductivity. This is in contrast to 3D-w ater and may have implications for the description of electron transfe r processes in electrochemical experiments.