Direct observation of the water dimer adsorbed on graphite

Journal Pre-proofs Full Length Article Direct observation of the water dimer adsorbed on graphite Min-Long Tao, Kai Sun, Xin Zhang, Li-Juan Zhao, Da-Xiao Yang, Zi-Long Wang, Jun-Zhong Wang PII: DOI: Reference:

S0169-4332(19)33416-6 https://doi.org/10.1016/j.apsusc.2019.144600 APSUSC 144600

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Applied Surface Science

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Please cite this article as: M-L. Tao, K. Sun, X. Zhang, L-J. Zhao, D-X. Yang, Z-L. Wang, J-Z. Wang, Direct observation of the water dimer adsorbed on graphite, Applied Surface Science (2019), doi: https://doi.org/ 10.1016/j.apsusc.2019.144600

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Direct observation of the water dimer adsorbed on

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graphite

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Min-Long Tao, Kai Sun, Xin Zhang, Li-Juan Zhao, Da-Xiao Yang, Zi-Long Wang,

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Jun-Zhong Wang*,[email protected]

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School of Physical Science and Technology, Southwest University, Chongqing, China

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Abstract

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Water/solid interfaces are closely related to our daily lives and play an important role in

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nature and technology. In this paper, we investigate the water molecules adsorbed on highly

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oriented pyrolytic graphite (HOPG) surface. The frontier molecular orbitals of adsorbed water

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dimer are directly observed by means of the low-temperature scanning tunneling microscopy

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(LT-STM). Furthermore, we find there are many water dimers and few small water clusters

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on HOPG, since the average adsorption energy per molecule of water dimer are higher than

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that of others. These results shed important light on the water/graphite interactions.

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Keywords: water dimer, graphite, STM

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PACS: 68.35.B-, 68.37.Ef, 68.55.Nq

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1. Introduction

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Water/solid interfaces are important to an incredibly broad range of everyday

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phenomena, including corrosion, lubricants, electrochemistry, and so forth [1-3]. Due to the

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ubiquitous presence of water on surfaces, many issues such as water pollution, energy

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shortage, and climate change are relevant to the water/solid interfaces, stimulating a large

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number of scientists devoting themselves to the research. Many achievements are obtained

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about the adsorption geometries and electronic structures of water molecules, ranging from

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isolated monomers to two-dimensional layers, especially on metal surfaces [4-13]. Jing Guo

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et al reported the quantitative assessment of nuclear quantum effects on the strength of a

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single hydrogen bond formed at a water-salt interface [4]. Splitting patterns were measured in

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rotational transitions of the water hexamer prism, resulting from geared and antigeared

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rotations of a pair of water molecules [11]. A novel monolayer ice was observed in the real

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space which was built exclusively from water hexamers but without shared edges[12].

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Since the graphite is a good surface science model for the structure of dust grains, the

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adsorptions of water molecules on graphite and graphene have been investigated extensively

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[14-22]. Ice desorption from a nanostructured graphite surface and the growth of ice were

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studied in the previous experiments [20, 21]. Simultaneously, in the past decades, theoretical

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studies presented the possible low-energy adsorption geometries on graphite and graphene,

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such as one-leg and two-leg configurations of isolated water molecule, chain configuration of

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water dimer, and cyclic configurations from trimer to hexamer. Many novel phenomena about

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water/graphene interface were predicted -- a single water cluster on graphene has a very small

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average dipole moment which is in contrast with an ice layer that exhibits a strong dipole

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moment[17], and water molecules undergo ultra-fast diffusion when deposited on graphene at

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low temperatures[18]. However, these investigations about water clusters adsorbed on

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graphite or graphene are in theoretical researches, and the experimental work is far from

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complete. For instance, it still remains challenging to obtain the real-space imaging and

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determine in experiments the structure of water clusters on graphite or graphene, which is

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helpful in understanding the fundamental aspects of water-graphite interaction.

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Here we deposit water molecules on HOPG and obtain the STM images of water clusters

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at liquid nitrogen temperature. Furthermore, we find the water dimer is most stable in small

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clusters adsorbed on HOPG.

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2. Experiments and calculations

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STM Measurements

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The experiments were performed in a Unisoku ultra-high vacuum LT-STM system with

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the base pressure better than 1.2×10-10 Torr. The ultrapure water (Sigma-Aldrich, 99.9%) was

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further purified by freeze-thaw cycles under vacuum, removing the residual impurities. Via a

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leak valve, water molecules were deposited in situ onto a HOPG surface attached to a thick 2

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Cu plate, while the substrate cooled by liquid nitrogen maintained at 78 K. A polycrystalline

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tungsten tip, prepared by electrochemical etching and heated by electron beam, was used for

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STM imaging. The STM topographic images were acquired in constant-current mode at 78 K.

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Calculations

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The theoretical calculations were performed by using the ab initio simulation.[23, 24]

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The electron-ion interactions were described with the projector augmented wave (PAW)

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potentials[25, 26] and the electronic exchange-correlation energy was treated by

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generalized-gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE).[27] The

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optB86b-vdW was employed to consider the nonlocal dispersion forces.[28] The kinetic

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energy cutoff for the plane-wave expansion was set to 500 eV. The k-point sampling in the

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Brillouin zone was implemented by the Monkhorst-Pack scheme with the grids of 5×5×1. For

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the geometry optimization, the convergence criteria of electronic and ionic iterations were

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1.0×10-5 eV and 5.0×10-2 eV/Å, respectively. The adsorption calculations were conducted on

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5×5 graphene with a 20 Å gap. The graphite is replaced by graphene in calculations to reduce

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the computational effort because of the weak interaction between the adjacent layers of

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graphite. All atoms were relaxed during the optimization and the periodic boundary condition

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was considered for all the systems. The simulated STM images were obtained using the

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constant current mode based on calculated charge densities.

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3. Results and discussion

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When water molecules deposited on clean HOPG surface, most of them gathered into

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disordered domains with different size because of the high mobility.[18] Between the

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domains, there are visibly small clusters in some places. We find that the binding of water

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clusters on graphite has no relation with defects and impurities on substrate (see Fig.S1). The

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images of water clusters obtained at different bias voltage show different molecular orbitals.

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In Figure 1(a-c), we choose the typical images of water monomer, dimer and trimer measured

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at different bias voltage. Water monomer [Fig. 1(a)] presents a bright spot with a slit,

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corresponding to the calculated HOMO of adsorbed water monomer [Fig. 1(d)]. Water dimer

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[Fig. 1(b)] and trimer [Fig. 1(c)] show two and three bright spots, similar to the simulative

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STM images [Fig. 1(e-f)]. The simulative STM images are based on the optimized structures

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of water clusters (see later).

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Surprisingly, via a large number of searches, we found that water dimers were

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commonly observed on HOPG besides the disordered water domains. There are few other

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clusters, such as monomer and trimer, which is different from the various water clusters with

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broad size distribution on NaCl(001) [5] and Cu(111) [13] as well as the monodisperse water

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hexamers on Bi(111) [29]. Based on our statistical analysis, the ratio of the number of water

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molecules in dimer, monomer and other small clusters is 45:2:3. Furthermore, STM images

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show monomer and trimer are always surrounded by a wide range of clean graphite without 3

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any other water molecules. We assume that their existence is because there is no other water

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molecule nearby to form dimers or domain.

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As shown in Figure 2(a), there are monodisperse water dimers with the same size and

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brightness. At high bias, water dimer appears a bright spot. When decreasing the bias, the spot

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splits into two bright spots [Fig. 1(b)], and then into four lobes or two lobes. Figure 2(b)

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shows the high-resolution STM image of a water dimer obtained at low positive bias,

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revealing a four-lobe structure. Relying on the density functional theory calculations, it is

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found that the structure of four lobs agrees perfectly with the calculated HOMO of adsorbed

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water dimer [Fig. 2(d)]. By the way, the calculated image is based on the optimized

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adsorption structure inserted in Figure 2 (d) and (e), corresponding to the reportedly

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optimized structure of water dimer on graphene.[14, 17] The asymmetric lobes with different

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brightness implicate that the two molecules in water dimer are tilted. When the bias changes

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to a low negative bias, the four-lobe structure fades out and two lobes appear with

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significantly different brightness [Fig. 2(c)], very closely resembling the calculated LUMO of

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water dimer [Fig. 2(e)].

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Figure 3 shows the STM images of water dimers acquired by two typical types of STM

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tips. In the previous report, it is demonstrated that different tip apexes not only lead to the

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variation of the energy scale of LUMO/HOMO, but also selectively enhance the HOMO

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states or the LUMO states around EF.[5] Our result is similar to it. The tip in Figure 3(a) is

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sensitive both to the LUMO and HOMO states. At high bias 1V, the STM image appears a

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bright spot because of low resolution. When the bias decreases, it splits into two spots

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standing close together representing two water molecules, and then into four lobes which are

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consistent to HOMO. As we continue to lower the bias to - 0.1 V, one lobe brightens and

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others darken, due to the superposition of LUMO and HOMO. The tip in Figure 3(b)

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selectively enhances the LUMO. STM images reveal a bright spot at high bias and two lobes

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with a far different brightness at low bias, consistent to HOMO.

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In order to verify the reasons for the most frequent occurrence of water dimers,

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theoretical calculations were performed by using DFT. We first address the most stable

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geometries of water clusters adsorbed on graphene, containing up to six molecules. In

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accordance with the earlier studies [17, 22], we find that the water substructure is often very

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similar to that in the corresponding global minimum, because of the hydrophobic nature of the

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water-graphite interaction. The optimized clusters, from trimer to hexamer (Fig.4), consist of

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rings of water molecules. One hydrogen atom in every water molecules forms a hydrogen

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bond with its neighbors, and the others point out of the ring plane alternating their direction as

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much as possible in order to minimize the interaction between hydrogen atoms of neighboring

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molecules. The water molecules in clusters prefer the adsorption site locating at the center of

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the carbon hexagon. In the image with the bias 0.05 V in Fig.3(b), we can clearly distinguish

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not only the HOMO of dimer, but also the carbon atoms on the substrate. Two oxygen atoms 4

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in water dimer are located near the top of the lattice points of graphite, which is very

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consistent with the optimized structure of water dimer on graphene in Fig.4.

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After obtaining the relaxed geometries, we calculate the average adsorption energy per

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molecule, which is computed as 𝐸𝑎𝑑𝑠 = (𝐸𝐻2𝑂/𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒 ― 𝐸𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒 ― 𝐸(𝐻2𝑂)𝑛)/𝑛

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Where 𝐸𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒 and 𝐸𝐻2𝑂/𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒 are the energies of a perfect graphene layer and the

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relaxed cluster-graphene system;

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the number of water molecules in the cluster.

𝐸(𝐻2𝑂)𝑛 is the energy of an adsorbed water cluster, and n is

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The calculated adsorption energies per molecule are presented in Figure 5. We find all of

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these water clusters are weakly bound to the graphene surface with adsorption energy per

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molecule. Except water dimer, they have small difference ranging from -140 emV to -134

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emV. While, for the dimer, the average adsorption energy per molecule (-167 emV) increases

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significantly, up to 1.25 times that of others. The obvious distinction is also showed in the

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results calculated at different levels of DFT.[17, 22] B. S. Gonza´lez et al.[14] calculated that

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the adsorption energy of small water cluster (n ≤ 6) on graphite correspond to the sum of

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the dispersion-repulsion energy between the oxygen and carbon atoms and the energy

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associated with the polarization. The former is proportional to the number of oxygen atoms,

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that is, the number of water molecules n (n ≤ 6), so the dispersion-repulsion energy per

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molecule has the same value in small clusters. However, the latter has the maximum value at

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n = 2 because water dimer has the largest dipole moment in small clusters,[17] resulting the

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average adsorption energy of water dimer is larger than others. It is a common knowledge that

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lower adsorption energy leads to higher mobility. Water dimer with higher adsorption energy

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obviously has lower mobility, resulting that it becomes the most stable cluster on graphite.

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The higher adsorption energy makes it possible for dimer to adsorb on graphite, rather than to

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migrate and gathered into disordered domains.

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4. Conclusion

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Water molecules are deposited on HOPG at liquid nitrogen temperature and observed by

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LT-STM. The frontier molecular orbitals of water dimer have been identified from the high

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resolution STM images. On the surface, water dimers were the most common besides the

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disordered water domains. The results of the first-principle calculations show that water

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dimers have the much higher adsorption energy per molecule on the substrate than other small

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clusters, resulting in the stabilization of water dimers on HOPG.

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Acknowledgments

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This work was supported by the National Natural Science Foundation of China (Grant Nos.

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11804282, 11574253, 11874304, 11604269) and the Fundamental Research Funds for the Central

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Universities (XDJK2018C081). 5

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Fig. 1 Experimental STM images (a-c) and Simulative STM images (d-f) of water monomer,

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dimers and trimer adsorbed on graphite, respectively. (a-c) The tunneling currents are 30 pA, and

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the bias voltages are -0.5 V (a), 0.8 V (b) and 1.0 V (c).

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Fig. 2 (a) The STM image of water dimers absorbed on graphite, 3.00V, 30 pA. The insert is

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high-resolution image of graphite, 200 mV, 32 pA. (b, c) The high-resolusion STM images of the

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HOMO (b) and LUMO (c) of water dimer. (b) 100 mV, 30 pA. (c) -60 mV, 30 pA. (d, e) The

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calculated HOMO (d) and LUMO (e) of the water dimer adsorbed on graphene. O and H atoms of

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H2O are denoted by red and white spheres, respectively.

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Fig. 3 Bias-dependent orbital imaging of water dimers obtained with two different types of tips (a)

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and (b). Tunneling current is 30 pA. The bias voltage is inserted in the upper left corner of each

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image. The black dots are graphite lattices. Tip (a) is sensitive to the HOMO and HOMO. Tip (b)

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is sensitive to LUMO.

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Fig. 4 The optimized structures of the water clusters adsorbed on graphene (view from the top and

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the side).

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Fig. 5 The average adsorption energy per molecule versus the number of water molecules in

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clusters.

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