Hydrogen-Bonded Cyclic Water Clusters Nucleated on an Oxide Surface Coleman X. Kronawitter, † Christoph Riplinger, ‡ Xiaobo He, † Percy Zahl, ∥ Emily A. Carter, ‡,§ Peter Sutter, ∥ and Bruce E. Koel* ,† † Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States ‡ Department of Mechanical and Aerospace Engineering, Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey 08544, United States § Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States ∥ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States * S Supporting Information ABSTRACT: We report the observation and molecular-scale scanning probe electronic structure (dI/dV) mapping of hydrogen-bonded cyclic water clusters nucleated on an oxide surface. The measurements are made on a new type of cyclic water cluster that is characterized by simultaneous and cooperative bonding interactions among molecules as well as with both metal and oxygen sites of an oxide surface. Density functional theory + U + D calculations confirm the stability of these clusters and are used to discuss other potential water-oxide bonding scenarios. The calculations show that the spatial distributions of electronic states in the system are similar in character to those of the lowest unoccupied molecular orbitals of hydrogen-bonded water molecules. On the partially oxidized Cu(111) investigated here, experiment and theory together suggest that Cu vacancies in the growing islands of cuprous oxide inhibit water adsorption in the centers of the islands (which have reached thermodynamic equilibrium). A stoichiometric, less stable cuprous oxide likely exists at island edges (the growth front) and selectively binds these water clusters. ■ INTRODUCTION When water interacts with a solid, an energetic competition exists between hydrogen bonding among adjacent water molecules and bonding with the surface. The outcome of this competition yields phenomena at the water−surface interface that are integral to a number of technologically critical processes: water dissociation, heterogeneous ice nucleation, wetting, as well as the development of the double layer in aqueous electrochemical systems. 1,2 At low coverage, water clusters form when lateral hydrogen bonding between water molecules is favored over individual molecules bonding to the surface (associated with wetting). Clustering is often observed on close-packed metal surfaces, such as Cu(111), 3,4 Ag(111), 4,5 and Pd(111), 6 where it is possible for highly organized structures resembling natural ice to develop (e.g., the highly stable cyclic hexamer). Water is less likely to cluster on pristine nonmetallic (including oxide) surfaces, 7 since these can bind water strongly and often are associated with a potential energy landscape that inhibits molecule diffusion. 2 Here using scanning probe techniques we report the first observations of a new type of hydrogen-bonded cyclic water cluster that is characterized by a simultaneous and cooperative bonding interaction with both metal and oxygen sites of an oxide surface. Detailed electronic structure measurements are performed to map the spatial and energetic distributions of electronic states associated with adsorbed water. Density functional theory + U calculations confirm the stability of these clusters and are used to discuss other potential water-oxide bonding scenarios. The observations are made on highly defective Cu 2 O(111), an oxide semiconductor whose aqueous electrochemistry and photoelectrochemistry are of interest for enabling solar fuel synthesis in energy conversion devices. 8,9 The observation of room temperature-stable cyclic water clusters on an oxide surface is relevant to this application, since our mechanistic understanding of electron transfer across oxide-H 2 O interfaces involves the electric double layer and therefore the structure of surface-bound H 2 O molecules. 10 Our calculations differentiate between strongly bound clusters consisting of H 2 O molecules coordinatively bonded to metal sites and those clusters of H 2 O molecules only hydrogen-bonded to oxygen sites, a distinction important for reactions whose activation energies are tied to the displacement of surface-bound H 2 O. Clusters of water molecules are prototypical systems for understanding the interactions that govern hydrogen bond- ing. 11,12 The bonding rules governing cluster formation increase in complexity when nucleation occurs on a surface. 1 In the context of water adsorption on metals, the competition between the ability of H 2 O molecules to bond to a substrate and to accept H bonds originates from the bonds’ common Received: June 14, 2014 Published: September 2, 2014 Article pubs.acs.org/JACS © 2014 American Chemical Society 13283 dx.doi.org/10.1021/ja5056214 | J. Am. Chem. Soc. 2014, 136, 13283−13288