⇦Chinese Physics Letters, 2019, Vol. 36, No. 11, Article code 116801 Observation of Simplest Water Chains on Surface Stabilized by a Hydroxyl Group at One End * An-Ning Dong (董安宁)1,2,5, Li-Huan Sun (孙丽欢)1,2, Xiang-Qian Tang (唐向前)1,2, Yi-Kun Yao (姚一锟)1,2, Yang An (安旸)1,2, Dong Hao (郝东)1,2, Xin-Yan Shan (单欣岩)1**, Xing-Hua Lu (陆兴华)1,2,3,4** Affiliations 1Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 2School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190 3Collaborative Innovation Center of Quantum Matter, Beijing 100190 4Songshan Lake Materials Laboratory, Dongguan 523808 5Engineering Technology Department, Zolix Instruments Co. Ltd, Beijing 101102 Received 9 July 2019, online 21 October 2019 *Supported by the National Natural Science Foundation of China under Grant Nos 11774395 and 91753136, the Beijing Natural Science Foundation under Grant No 4181003, the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant Nos XDB30201000 and XDB28000000.
**Corresponding author. Email: shanxinyan@iphy.ac.cn; xhlu@iphy.ac.cn Citation Text: Dong A N, Sun L H, Tang X Q, Yao Y K and An Y et al 2019 Chin. Phys. Lett. 36 116801    Abstract The key to fully understanding water-solid interfaces relies on the microscopic nature of hydrogen bond networks, including their atomic structures, interfacial interactions, and dynamic behaviors. Here, we report the observation of two types of simplest water chains on Au(111) surface which is expected unstable according to the rules of hydrogen network on noble metal surfaces. A common feature at the end of chain structures is revealed in high resolution scanning tunneling microscopy images. To explain the stability in observed hydrogen bond networks, we propose a structure model of the water chains terminated with a hydroxyl group. The model is consistent with detailed image analysis and molecular manipulation. The observation of simplest water chains suggests a new platform for exploring fundamental physics in hydrogen bond networks on surfaces. DOI:10.1088/0256-307X/36/11/116801 PACS:68.37.Ef, 68.43.Fg, 82.65.+r © 2019 Chinese Physics Society Article Text The behavior of water molecules on solid surfaces at the microscopic scale has attracted significant research interest, due to the ubiquitous water-solid interfaces in our daily life, industrial manufactures, and biosystems.[1–3] Various water structures have been observed and interpreted by scanning tunneling microscopy (STM) combined with density functional theory (DFT), including cyclic water clusters,[4–12] quasi one-dimensional (1D) water chains[13–15] and two-dimensional (2D) water films.[16–19] It is worth noting that all water structures, except very small species, prefer a closed-loop structure on solid surfaces wherever water-substrate interaction is not playing a dominating role. This is because water molecules tend to build as many H-bonds as possible to reach the lowest-energy state, and it has been accepted as one of the rules for sub-monolayer ice growing on noble metal surfaces.[11,20] A water chain is an interesting type of water structure and it is believed to relate to the fundamental living activities in a cell.[21] For example, water molecules transport across the nanochannels on biological membranes in the form of chains.[22] It is thus of great interest to explore the characteristics of such water chains at the molecular scale. Following the rules of water structures on surfaces, the simplest water chains are unstable due to the unsaturated hydrogen bond at one of the chain ends. Water chains as reported in previous STM studies are either under strong constrained conditions or built with complex cyclic water clusters. Examples include water chains along the step edges on Pt surface[23] and crystal structure on Cu(110) surface,[8] and the quasi one-dimensional water structure constituted with face-sharing pentagonal water units on Cu(110) surface.[14] The question is whether it is possible to have the simplest water chains on a solid surface that are not constrained by linear surface reconstructions. Such nearly free water structures are better mimics of water chains in biological activities, and their properties and behaviors are expected to better reflect the nature of hydrogen bonds between water molecules in aqueous solutions. Here, we report the observation of the simplest water chains being stable and unconstrained on a noble metal surface using a low-temperature STM. Two types of water chains are imaged with molecular resolution in which sequential water molecules are linked by hydrogen bonds in zigzag and armchair patterns. To the best of our knowledge, there has not yet been any report of such water structures. Based on the observed topographic features, a structure model has been proposed to explain the stability in these water chains. Molecular manipulations have also been tried to validate the model and to explore the dynamic behavior in such linear hydrogen bond networks. An STM experiment was performed on Au(111) crystal surface in ultrahigh vacuum with pressure lower than 10$^{-10}$ Torr. The Au substrate was cleaned by cycles of sputter with Ar$^{+}$ and anneal around 900 K, then cooled down to about 17 K with liquid helium. Water vapors are then leaked into the vacuum chamber (with a pressure increase by about $0.5\times 10^{-11}$ Torr) and dosed onto the crystal surface. Figure 1(a) presents a typical STM topographic image where a variety of water structures are observed. Besides small water clusters, which have been reported and discussed in a previous report,[12] two new types of water chain structures are observed. We note here that such high-resolution images can be obtained only with carefully functionalized STM tips (see Method in the Supplementary Material).
cpl-36-11-116801-fig1.png
Fig. 1. Water chains on Au(111) surface. (a) Typical high resolution STM image of water structures on surface. Most water molecules constitute closed-loop structures, while water chains are occasionally observed as well. Imaging set point: sample bias $V=-200$ mV, tunneling current $I=50$ pA. [(b), (c)] STM image of water chains with molecular resolution, illustrating armchair and zigzag patterns, respectively. Both chains present a common feature at one of the ends, as indicated by the dotted line. Imaging set point: $V=-100$ mV, $I=50$ pA.
The water molecules in the chains are linked in a zigzag or armchair pattern. The chain length as observed is in the range of 3–16 molecules. The distance between adjacent water molecules in the chains is about 2.95 Å, close to the substrate lattice constant (2.90 Å). Careful examination, however, reveals that the centers of each water molecule are not located at the same position of the substrate atomic lattice. The O–O–O angle between adjacent water molecules is 108$^{\circ}\pm10^{\circ}$ for the armchair and 94$^{\circ}\pm10^{\circ}$ for the zigzag patterns (see Fig. S1 in the Supplementary Material). Some water chains present a mixture of zigzag and armchair patterns. These observations indicate that the hydrogen bonding in these water chains is stronger than or at least comparable to the strength of water-substrate interaction. It is thus a valuable platform in exploring nature of hydrogen bonds in a nearly free two-dimensional environment. The height of water molecules along the water chain is around 0.90 Å, which is close to the height of water molecules in a cyclic hexamer. It is very interesting that each water chain has two distinct ends. At one end of the chains, the first two water molecules (especially the second molecule) are slightly higher than the other molecules in the chain. The last water molecule at the other end, on the other hand, appears lowest along the chain. The difference in heights between the highest one and the lowest one reaches about 0.2 Å (see Fig. S1 in the Supplementary Material). The other distinct feature is the depression in topography near the lowest molecule at the end. The depression appears as an arc around the molecule at a distance of about 5 Å, and it is about 0.06 Å deep refer to the surface nearby, as shown in Fig. 2.
cpl-36-11-116801-fig2.png
Fig. 2. Feature of depression at one end of the water chain. (a) STM image of an armchair water chain. Imaging set point: $V=-100$ mV, $I=50$ pA. (b) Height profile along the line-cut as indicated in (a). The depression is marked by the arrow.
The existence of such a water chain on a flat surface is surprising because according to the rules for ice growing on surfaces,[20] water molecules tend to build closed-loop structures with as many hydrogen bonds as possible to reach the lowest energy state. Based on the observed topographic features, we propose a hydroxyl-terminated structure model to explain the stability of such nearly free water chains on noble metal surfaces. Figure 3 shows the atomic model for the proposed hydroxyl-terminated water chain. Adjacent water molecules are linked through hydrogen bonds, the molecule at one end acts as a pure hydrogen 'donor' and the species at the other end being a hydroxyl group. This hydroxyl group makes the formation of close-loop unnecessary and thus stabilizes the chain structure.
cpl-36-11-116801-fig3.png
Fig. 3. Atomic model of hydroxyl-terminated 1D water chain. (a) Armchair water chain, (b) zigzag water chain. The black dotted line corresponds to the depression position in STM topographs.
The validity of this model is based on following arguments. First, the depression at one end of the chain suggests the presence of a hydroxyl group. The species at the chain end could be a complete water molecule, a hydroxyl group, or an oxygen atom. Obviously, one can exclude the possibility of being a complete water molecule. An oxygen atom on noble metal surface will present as a round depression in STM topography, as reported previously,[24] which is different from the end feature as observed. An hydroxyl group on noble metal surface, on the other hand, appears as a symmetric double depression,[24] the half of which presents the feature at the end of the water chain. The reason for the appearance of single depression instead of a symmetric doublet at water chain end is due to the interaction with linked water molecules, which prohibits the wobbling of the hydrogen atom. The depression is not observed in small water clusters imaged with the same STM tip, which excludes the tip effect as well. Secondly, the depression is not related to any defect on surface. The water chain can be manipulated to a different site, exposing intact flat terrace without any surface defect. The feature can even be switched from one end to the other end of the chain by applying a voltage pulse with tip set at the middle of the chain, as shown in Fig. 4. Lastly, a hydroxyl group at water chain end establishes an energy-favorable configuration. Due to the unsaturated coordination in hydroxyl group, the lone pair orbital in the O atom leads to an enhancement in O-metal interactions and renders a local nadir of potential energy. Consequently, the hydroxyl group bonds to the underlying metal atom and stabilizes the 1D water chains on the surface.
cpl-36-11-116801-fig4.png
Fig. 4. Manipulation of water chain. (a) Configuration change between armchair and zigzag in a water chain consists of six molecules, stimulated by a voltage pulse through STM tip. (b) Stimulated proton transfer along an armchair water chain. The feature of depression at one end disappears and emerges at the other end of the chain after a voltage pulse. Imaging set point: $V=-100$ mV, $I=50$ pA. The marks indicate the positions where the voltage pulses ($-0.5$ V with 200 ms duration) were applied.
The zigzag and armchair configurations can be mixed in a single water chain, indicating that they are close in energy. This can be further confirmed by the fact that a water chain consist of one pattern can be manipulated into the other pattern. As shown in Fig. 4(a), following a voltage pulse stimulation, a water chain composed of six water molecules in armchair configuration is converted to the zigzag configuration. The basic features, including slight slope in height along the chain and the depression at one end of the chain, remain unaltered though. Further manipulation experiments show that the hydroxyl group can transfer from one end of the water chain to the other end, when stimulated by a voltage pulse from STM tip, as demonstrated in Fig. 4(b). One possible method for such transfer process is the actual move of hydroxyl group along the surface. Considering the location of stimulation and the topographic positions of the water chain, the other way is more reasonable in which the transfer of hydroxyl group is achieved by reconfiguring the hydrogen bonds between adjacent water molecules. The hydroxyl group accepts one hydrogen atom from its neighbor and turns into a water molecule, and the last water molecule donates one hydrogen atom and becomes a hydroxyl group. As a result, a hydrogen atom is transferred through the whole molecular chain, which is also referred to as proton transfer process.[25–27] The interesting point is whether the hydrogen atom or the proton is transferred through each molecule sequentially or, all water molecules act simultaneously (co-tunneling). The fact that the stimulation is performed at the middle of the chain suggests that a co-tunneling scenario is more plausible. Further controlled experiments, of course, are needed to confirm such hypothesis. Nevertheless, the manipulation experiments demonstrate that these simplest water chains are of significant advantage in exploring dynamic behaviors in hydrogen bond networks. A hydroxyl-terminated water chain on a noble metal surface has not been predicted by any density function theory calculation. Recently, Peköz and Donadio investigated the partial dissociation of water chains at (221) stepped metallic surface by density functional theory and molecular dynamics simulations.[28] Their results predicted that the favorable site for a hydroxyl group is in the middle of the water chains instead of the ends, which maximizes the water-metal and water-water interactions and leads to a low energy state. The discrepancy, of course, can be due to the difference in substrate activities (active Pt, Ir, Rh, and Pd surfaces, versus nonactive Au surface) and the constraint of step edges. Further attempts with first-principle calculations, based on PBE functional[29] with modified parameters (optB86b-vdW[30] scheme), still cannot explain the experimental observations.[31] The inconsistency between theoretical calculation and experimental observation, again, illustrates the long-standing challenges in exploring fundamental physical principles in nature of water where the subtle balance between various interactions renders exuberant phenomena. In summary, the simplest water chains on noble metal surfaces consisting of armchair and zigzag configurations have been observed with molecular resolution using a low-temperature STM. A hydroxyl-terminated model has been proposed to explain the stability in these water chains, based on common features in high-resolution STM images. The model is further validated by manipulation with STM tip in which both chain reconfiguration and proton transfer are observed. These simplest water chains on noble metal surfaces provides a desired platform in studying hydrogen bond networks on nearly free two-dimensional environment, and the discrepancy between theoretical simulation and experimental observation may stimulate new insights in nature of hydrogen bond networks, as well as their dynamic behaviors. 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