Symmetric hydrogen bond in a water-hydroxyl complex on Cu(110)

Water-hydroxyl complexes were produced on Cu(110) and characterized by a scanning tunneling microscope (STM) and first-principles calculations. A water molecule was brought to a fixed hydroxyl (OH) group in a controlled manner with the STM, and two kinds of hydrogen-bonded complexes were produced selectively. A side-on complex, in which a water molecule is bonded to an OH group along the atomic row, is metastable with relatively weak hydrogen bond (0.13 eV). On the other hand, a bridge complex, in which a water molecule is bonded to an OH group across the atomic trough, is most stable and characterized by the strong hydrogen bond (0.44 eV) and the short distance between oxygen atoms $\left(2.5\text{Å}\right)$. The distance is in the range of the “low-barrier hydrogen bond,” and we show that a symmetric hydrogen bond (HO-H-OH) is formed in the bridge complex, wherein zero-point nuclear motion plays a crucial role.

Figure 1

(Color online) (a) STM images for a water molecule and an OH group located on the same row (dashed line) on Cu(110). (b) A water-OH complex produced by dragging the water molecule to the OH group along the atomic row. The images appear quite large as compared to their actual size, which may arise from extended perturbation of the substrate electronic states due to adsorption. (c) The complex superimposed with the lattice of the substrate. (d) The counterpart of (c). (e) Temporal evolution of tunneling current during a voltage pulse of 179 mV. The tip was fixed over the complex at the height corresponding to 24 mV and 0.5 nA. The thick lines result from the fast flip motion between (c) and (d), and the abrupt decreases correspond to the detachment of the water molecule. Note that the complex was formed again immediately after the first dissociation. The sample bias voltage and tunneling current during the image acquisition were 24 mV and 0.5 nA, respectively, and the size is $47\times 22{\text{Å}}^2$ for (a) and (b) and $15\times 14{\text{Å}}^2$ for (c) and (d). The color palette represents the tip height in the constant current mode. Under this high gap resistance $\left(48\text{M}\Omega \right)$, the adsorbates were observed without perturbation and the substrate Cu atoms were not resolved.

Figure 2

(Color online) (a) STM images for a water molecule and an OH group located on adjacent rows (dashed lines). (b) The complex produced under this geometry shows an oval shape in contrast to the case of Fig. 1. Thus two kinds of the complexes were produced selectively depending on the reaction geometries. No interconversion between the two complexes was induced by voltage pulses. (c) An STM image of the complex on which the lattice of Cu(110) is superimposed. (d) The height profiles along the solid ([001]) and dashed $\left(\left[1\overline{1}0\right]\right)$ lines in (c). The sample bias voltage and tunneling current during the image acquisition were 24 mV and 0.5 nA, respectively, and the size is $47\times 22{\text{Å}}^2$ for (a) and (b) and $17.5\times 17.5{\text{Å}}^2$ for (c).

Figure 3

(Color online) (a) Typical trace of the tip when it tracks an oval complex $\left({\text{H}}_2\text{O-OH}\right)$ at 453 mV and 0.5 nA. The red and blue lines indicate the displacements in the $\left[1\overline{1}0\right]$ and [001] directions, respectively. The hopping rates were determined from the inverse of average residence time $t$. (b) The hopping rates of the oval complexes as a function of bias voltage. The circles and squares represent the rates for ${\text{H}}_2\text{O-OH}$ and ${\text{D}}_2\text{O-OD}$, respectively. The tunneling current was kept at 0.5 nA. The inset shows the result for ${\text{D}}_2\text{O-OD}$ with the tunneling current of 20 nA. (c) The hopping rates as a function of tunneling current with the bias voltage kept at 443 and 343 mV for ${\text{H}}_2\text{O-OH}$ and ${\text{D}}_2\text{O-OD}$, respectively. The corresponding slopes in the logarithmic scale are $1.1\pm 0.1$ and $2.0\pm 0.1$, indicating single- and double-electron processes, respectively.

Figure 4

(Color online) The calculated structures for ${\text{H}}_2\text{O-OH}$ complexes. (a) Top and (b) side views of the side-on complex. (c) Side and (d) top views of the bridge complex. (e) The corresponding simulated image with the lattice representing the positions of substrate Cu atoms. The potential energies with respect to water and the OH group isolated on the surface are 0.13 and 0.44 eV for the former (metastable) and latter (stable), respectively. In both cases, the OH groups are slightly negatively charged and water molecules act as hydrogen-bond donors. (f) Side and (g) top views of the bridge complex in the symmetric configuration. The potential energy is 16 meV higher than that for the asymmetric one [(c) and (d)]. (h) The corresponding simulated image. The STM simulations were conducted without considering zero-point effect.

Figure 5

(Color online) Adiabatic potential energy for the bridge complex along the minimum-energy path of proton transfer between two oxygen atoms (open circles). The two minima and peak correspond to the asymmetric and symmetric configurations, respectively, as shown in the inset. Static potential energies for symmetric (squares) and asymmetric (triangles) configurations were calculated by moving proton along the minimum-energy paths with other degrees of freedom fixed at their optimized geometries. The curves were obtained by a cubic spline fit to the calculated data. Using the obtained potential data, we solved one-dimensional Schrödinger equations numerically. The dashed lines indicate the levels of total energies for each configuration.

Figure 6

(Color online) Adiabatic potential energy for the side-on complex along the minimum-energy path of proton transfer between oxygen atoms (open circles). The curve was obtained by a cubic spline fit to the calculated data. As the proton transfers, the binding site of the original OH group changes gradually from the bridge site to the top site, and that of the water molecule in the opposite way. The transition state lies in the off-centered position along the path, resulting in the potential curvature of asymmetric shape.