Nature of hydrogen bonding in hydroxyl groups on a metal surface

We report a systematic study of small hydroxyl clusters on Cu(110), with scanning tunneling microscopy experiments and density functional theory calculations. Hydroxyl clusters are stabilized via hydrogen bonding, and their stability depends on the number of constituent hydroxyl groups. We reveal that there is a competition between hydroxyl groups in accepting hydrogen bonds, resulting in weak hydrogen bonding in odd-number clusters and stable dimer unit(s) in even-number clusters, as the hydroxyl on a metal surface is singly charged and has ability to form a strong hydrogen bond. Similar trends were found for other metal surfaces, suggesting a general feature of adsorbed hydroxyl groups.

Figure 1

Constant-current STM images and optimized structures with PBE-GGA for (OH)${}_2$ [(a), (b), (g), and (h)], (OH)${}_3$ [(c), (d), (i), and (j)], and (OH)${}_4$ [(e), (f), (k), and (l)]. The lattice of Cu(110) is superposed on the STM images (white grid lines). They can be reversibly switched between the two equivalent orientations by an injection of energetic electrons from the STM tip. The white dots in the images [(a)–(f)] represent the short-bridge sites to which the oxygen atoms are bound. It is noted that oxygen atoms are displaced from the exact short-bridge sites along [001], as shown in the optimized structures. The images were recorded at a sample bias voltage of $V=30$ mV and tunneling current of $I=0.5$ nA. The image sizes are $18\times 25$ Å${}^2$. The range of the height shown in the color bar is from $-0.56$ to 0.24 Å. Large, medium, and small spheres in (g)–(l) represent Cu, O, and H atoms, respectively.

Figure 2

$d^{2}I/d{V}^2$ spectra for (OH)${}_2$, (OH)${}_3$, and (OH)${}_4$. The latter two are vertically offset for clarity. The tip was fixed over the depression in the STM image (indicated by an asterisk in the inset image) at the set point giving $I=0.5$ nA with $V=24$ mV for (OH)${}_2$ and (OH)${}_4$, and $I=0.05$ nA with $V=24$ mV for (OH)${}_3$. The peak values were determined by fitting the spectra to the derivative of a Gaussian function.

Figure 3

Projection of the calculated charge density difference (CDD) onto the plane containing hydroxyl clusters for (a) (OH)${}_2$, (b) (OH)${}_3$, and (c) (OH)${}_4$. White, dark gray, and bright gray balls indicate H, O, and Cu atoms, respectively. Bright parts of the CDD in the vicinity of oxygen atoms indicate charge accumulation, whereas the black portion appearing at H sites means charge depletion, suggesting hydroxyl groups are polarized upon formation of H bonds.

Figure 4

(a) Density of states (PDOS) projected onto molecular orbital (MO) of OH radicals for Cu(110)-OH. Peaks at $\sim -4.4$ eV and at $\sim -1.2$ eV indicate bonding and antibonding characters of MOs with substrate states. (b) Molecular orbitals of the OH radical in the gas phase. Isosurface of charge density differences (c) $\Delta {\rho }^{\prime }={\rho }_{\text{OH/Cu}\left(110\right)}-{\rho }_{\text{OH}}-{\rho }_{\text{Cu(110)}}$ and (d) $\Delta \rho ={\rho }_{{\text{OH}}^{-}}-{\rho }_{\text{OH}}$, where ${\rho }_{{\text{OH}}^{-}}$, ${\rho }_{\text{OH}}$, ${\rho }_{\text{OH/Cu(110)}}$, and ${\rho }_{\text{Cu(110)}}$ are charge densities of isolated OH${}^{-}$, OH radical, Cu(110)-OH, and Cu(110), respectively. Red (light gray) surfaces correspond to charge accumulation and blue (dark gray) surfaces to charge depletion in units of $6.7\times {10}^{-2}$$e$Å${}^{-3}$.