Robustness of Bilayer Hexagonal Ice against Surface Symmetry and Corrugation

Two-dimensional (2D) bilayer hexagonal ice (BHI) is regarded as the first intrinsic 2D ice crystal. However, the robustness of such a structure or its derivatives against surface symmetry and corrugation is still unclear. Here, we report the formation of 2D BHI on gold surfaces with 1D corrugation, using noncontact atomic force microscopy. The hexagonal arrangement of the first wetting layer was visualized on the Au(110)-$1\times 2$ surface. Upon depositing more water molecules, the first layer would rearrange and shrink, resulting in the formation of buckled BHI. Such a buckled BHI is hydrophobic despite the appearance of dangling OH, due to the strong interlayer bonding. Furthermore, the BHI is also stable on the Au(100)-$5\times 28$ surface. This work reveals the unexpected generality of the BHI on corrugated surfaces with nonhexagonal symmetry, thus shedding new light on the microscopic understandings of the low-dimensional ice formation on solid surfaces or under confinement.

Figure 1

(a) Schematic model of the reconstructed Au(110)-$1\times 2$ surface. (b) Atomic-resolution STM image of Au(110)-$1\times 2$ surface (set point: 10 mV, 1 nA; size: $2.3\mathrm{nm}\times 1.8\mathrm{nm}$). Dash rectangles represent the $1\times 2$ unit cell. (c),(d) Overview (c) and close-up STM (d) images of monolayer ice (set point: 100 mV, 5 pA). (e) Constant height AFM image of the same region as in (d), recorded at a tip height of $-70\mathrm{pm}$. The red arrows indicate the scratching noises in the AFM image (e) induced by the disturbance of the CO-terminated tip on the higher-lying water molecules (d). The tip height of the experimental AFM image is defined in the Materials and Methods. The oscillation amplitude of the AFM image is 100 pm.

Figure 2

(a) Close-up STM image (set point: 100 mV, 5 pA) of a monolayer ice island on Au(110)-$1\times 2$ surface. (b)–(d) Height dependent AFM imaging at tip heights of 100 pm (b), $-50\mathrm{pm}$ (c), $-130\mathrm{pm}$ (d). The isolated H-up water molecule on the topmost gold rows and flat-lying water molecule in the trench is highlighted by yellow circles and red arrow, respectively. The yellow arrow in (b) indicates the jump of the flat-lying water molecule from near-top to near-bridge sites. (e) Top and side views of the atomic model of monolayer ice. Au, H, and O atoms are denoted as yellow, white, and red spheres, while water molecules at the bridge site are highlighted by blue spheres. (f)–(h) Simulated AFM images acquired at tip heights of 11.2 Å (f), 10.1 Å (g), 9.6 Å (h). The dash rectangles indicate the $5\times 2$ unit cell. The tip heights of experimental and simulated AFM images are defined in the Materials and Methods.

Figure 3

(a) Overview STM image (set point: 300 mV, 5 pA) of buckled BHI on the Au(110)-$1\times 2$ surface. The yellow rhombus and rectangle represent the hexagonal and rectangle arrangements of bilayer ice, respectively. (b) Height dependent AFM imaging at tip heights of 320 pm (left), 270 pm (middle), 200 pm (right). (c) Simulated AFM images acquired at tip heights of 14.2 Å (left), 13.6 Å (middle), 13.1 Å (right). (d),(e) Top and side views of the calculated structure of buckled BHI. (f),(g) First layer (f) and second layer (g) structure. The black dash circles in (d),(e),(g) represent the H-up water molecules in the second layer. The yellow and black rectangles in (b)–(g) indicate the $3\times 2$ unit cell.

Figure 4

(a) Schematic model of the reconstructed hexagonal array of the topmost layer (yellow spheres) on the bulk Au(100) surface with square symmetry (green spheres). (b) STM topography of reconstructed Au(100) surface, showing $5\times 28$ reconstruction (set point: 100 mV, 50 pA). (c) Overview STM image of 2D ice on reconstructed Au(100) surface (set point: 1 V, 5 pA). (d)–(f) High resolution STM image (set point: 100 mV, 4 pA) (d) and height dependent AFM imaging at tip heights of 320 pm (e) and 290 pm (f). Inset of (e): close-up AFM image of a H-up water in second layer (size: $0.8\mathrm{nm}\times 0.8\mathrm{nm}$). (g),(h) Top and side views of the flat BHI model with isolated dangling OH (blue spheres).