Copper is a metal at the borderline between molecular and dissociative adsorption of water. Its wettability responds sensitively to the subtle changes of crystal orientation of the surface. While the Cu(111) surface is hydrophobic and inert at water vapor pressures of up to 1Torr, the Cu(110) surface is hydrophilic and dissociates water at measurable rates even at cryogenic temperature. Molecular level understanding of the adsorption and reactions of water on the Cu(110) surface is pivotal for deciphering its peculiar wetting behavior, and has important implications in several technological systems such as electrochemistry and catalysis. The author of this work utilized scanning tunneling microscopy (STM) and investigated the low coverage water on Cu(110) at temperature range from 77K to 340K. The results of this work unveil key aspects of the water adsorption and dissociation mechanism on Cu(110).
This work studies with STM the structure of one dimensional (1D) chains of water molecules running along [001] direction of the Cu(110) surface formed upon adsorption below 120K. The chains have a zigzag morphology which was reported before and proposed to be built from side-sharing pentagonal units. Using high resolution STM, this author finds that the pentagon based model does not fit the experimentally measured dimension of the 1D zigzag chains. Instead we propose a model that restores the archetypical water hexagon ring motif as the best matching structure model. Density functional theory (DFT) calculation shows that between the pentagon and hexagon based models there is less than a few tens of mini-electron-volt difference in water adsorption energy; we argue that such small difference does not provide justification of one model over another, and that only a better experimental match of the measured dimensions and symmetry of the STM images makes the hexagon based model a better fit than the pentagon model.
This work explores also the structural transformation of the 1D water chains induced by elevating the sample temperature above 120K. Small growth in lateral direction by around 3Å from the original 7.8Å width of the 1D zigzag chains was found above 120K, which changes the morphology and long range order from zigzag to rectangular at 130K. We also found that the zigzag geometry is restored at 140K but the periodicity along [001] doubles. The observation that STM manipulation by means of voltage pulses applied to the tip changes the 1D zigzag chains into a rectangular structure, only observed above 130K, indicates that the lateral growth of the 1D chains is driven by energy lowering but needs to overcome certain barrier. DFT calculations find that when they are intact, the side molecules of the 1D zigzag chain are not bound to the Cu substrate atoms via O orbitals but instead have a H atom pointing to the surface, which makes them a bit more elevated above the surface than the O-bound molecules. These high-lying molecules are energetically less favorable to accept additional molecules to bind to them in the lateral direction. Although the lateral growth with intact molecules is possible, as occurs in other (111) surfaces of many metals, we believe that the partially dissociated layer formed by a mixture of H2O and OH is the most stable. This author believes that high-lying molecule dissociation induced by either elevated temperature or artificial energy activation such as voltage pulse provides a pathway for lateral growth of the width-confined 1D zigzag chains, suggesting the key role of water dissociation in expansion of sub-monolayer water structures on Cu(110). The 2D ice rules developed based on water structures on (111) type surfaces are examined and we conclude that they still hold true for the case of Cu(110) surface.
This work also reports that a substrate temperature above 180K completely transforms the 1D water/hydroxyl chains running along [001] into another type of 1D water/hydroxyl chains running along [110]. More importantly, as more water dissociation occurs above 180K, the products of water dissociation, i.e., OH, O and H, forms a rich variety of surface structures. The most dramatic one is the formation of monoatomic copper wires along [110] formed between 180K and 220K. It is believed that H atoms produced by water dissociation plays a role in assisting and stabilizing the growth of the monoatomic copper wires. This observation reveals the strong effect of water adsorption and reactions on Cu(110).
In addition to 1D water/hydroxyl structures formed by low water dosage, this work also touches upon the complete overlayer of water/hydroxyl structures at high water dosage. A c(2 × 2) overlayer is observed here for the first time on the clean Cu(110) surface, with a clear honeycomb structures resolved by the STM below 120K. When the fully covered sample was heated to 180K, superstructures with a periodicity of ~5× the lattice spacing along [001] were discovered. Analysis of the STM images of the superstructures lead to the conclusion that the repeating stripes along [110] are terminated by OH groups, each of which is bound through one hydrogen bond to the inner stripe that retains the c(2 × 2) structure. The neighboring stripes have uncoordinated OH groups facing each other, making the stripe interface an array of Bjerrum defects. The observation of Bjerrum defect arrays indicates that they might play a role in stabilizing the c(2 × 2) overlayer as increasing amount of OH groups are produced by water dissociation near 180K.
