Characterization of two- and one-dimensional water networks on Ni(111) via atomic force microscopy

Determination of the adsorption structure of water molecules on metal surfaces is an imperative challenge to understanding the mechanisms of the wetting process and water-related heterogeneous catalysis. We identify water monolayers formed on Ni(111) via low-temperature atomic force microscopy, which enables the visualization of individual water molecules in monolayers with higher spatial resolution than scanning tunneling microscopy. On the terraces of Ni(111) at 150 K, water forms monolayers comprising fused pentagonal, hexagonal, and heptagonal rings. Water adsorbates on step sites assemble in a different manner, forming a hydrogen-bonding network with fused pentagonal and octagonal rings aligned in the step direction. Because similar water networks with pentagonal rings have been proposed in monolayers or their defect sites on other metal surfaces, our structural characterization of ${\mathrm{H}}_2\mathrm{O}/\mathrm{Ni}\left(111\right)$ provides an insight into water adsorption structures on metals.

Figure 1

(a) STM image of ${\mathrm{H}}_2\mathrm{O}/\mathrm{Ni}\left(111\right)$ at 78 K. (b),(c) STM image of the same sample as in (a) but after annealing at 145 and 150 K for 5 min, respectively. The red and green lines indicate the orientations of $\pm {19}^{\circ }$ relative to the Ni atomic lattice (white allows). (d) Magnified STM image of (c). Images in (a)–(d) were obtained with a metal tip [(a),(b) $V=-500$ mV, $I=20$ pA, 78 K and (c),(d) $V=-200$ mV, $I=20$ pA, 4.8 K]. The inset shows an atomically resolved STM image of the bare Ni surface obtained with a CO tip ($V=10$ mV, $I=2$ nA, 4.8 K). The red and green rhombuses represent the unit cells for domains i and ii of the $(\sqrt{28}\times \sqrt{28})R{19}^{\circ }$ superstructure, respectively. (e),(f) STM ($V=30$ mV, $I=20$ pA) and AFM ($z=-135$ pm) images of an island of domain ii, respectively, obtained with a CO tip at 4.8 K. (g) Laplacian-filtered image in (f). The blue, red, and green flames indicate the pentagonal, hexagonal, and heptagonal rings, respectively.

Figure 2

(a) STM image of an island of domain ii ($V=30$ mV, $I=20$ pA, 4.8 K, CO tip). (b)–(e) AFM images of the same area as in (a) with $z=-50$, $-100$, $-150$, and $-200$ pm, respectively. (f) Schematic of the DFT-optimized “$\sqrt{28}$ di-vacancy structure” reported in Ref. [11]. The gray, cyan, red, yellow, and green spheres represent Ni, H, bottommost O, middle O, and topmost O atoms, respectively. The gray rhombus represents the unit cell of $(\sqrt{28}\times \sqrt{28})R{19}^{\circ }$. (g) Schematic structure of the water monolayer proposed according to the AFM appearances. The bottom panels of (f) and (g) show side-view illustrations of the structures (H atoms are not shown for clarity). The vertical distance between the bottommost ${\mathrm{H}}_2\mathrm{O}$ and the substrate in (g) is tentatively assumed to be the same as that in (f). (h) $\mathrm{\Delta }f\left(z\right)$ curves recorded over the markers in the inset (4.8 K, CO tip). The inset shows an AFM image of domain i ($z=-110$ pm). (i) Short-range force curves calculated from (h) after subtraction of the contribution of the bare Ni surface [black curve in (h)]. The inset schematically shows the water network for the inset image in (h).

Figure 3

(a) STM image of a monoatomic step of ${\mathrm{H}}_2\mathrm{O}/\mathrm{Ni}\left(111\right)$ ($V=30$ mV, $I=20$ pA, 4.8 K, CO tip). The green arrow indicates a characteristic defect in the water networks at the step edge. (b) Schematic structure of the step and the tip. The gray, cyan, red, and black spheres represent Ni, H, O, and C atoms, respectively. The scanning trajectory for AFM imaging are also shown schematically. (c),(d) Constant-height AFM images of the step edge in (a), obtained with $z=-60$ and $-135$ pm, respectively. (e) AFM image of the same area as in (c) and (d) but with the tip trajectory aligned to the STM topography as shown by the blue solid curve in (b) ($V=30$ mV and $I=20$ pA for STM; $V=0$ mV and $\mathrm{\Delta }z=-210$ pm for AFM). The red dots represent the O-atom positions based on (e). (f) Schematic structure of the water network at the step edge proposed according to the AFM appearances.