WaterâNO Complex Formation and Chain Growth on Cu(111) - The

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Water-NO Complex Formation and Chain Growth on Cu(111) Hiroyuki Koshida, Shinichiro Hatta, Hiroshi Okuyama, Akitoshi Shiotari, Yoshiaki Sugimoto, and Tetsuya Aruga J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12447 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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The Journal of Physical Chemistry

Water-NO Complex Formation and Chain Growth on Cu(111) Hiroyuki Koshida,† Shinichiro Hatta,† Hiroshi Okuyama,∗,† Akitoshi Shiotari,‡ Yoshiaki Sugimoto,‡ and Tetsuya Aruga† †Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan ‡Department of Advanced Materials Science, The University of Tokyo, Kashiwa 277-8561, Japan E-mail: [email protected]

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Abstract We report novel structure of a water-NO (nitric oxide) complex on Cu(111) by using scanning tunneling microscopy, electron energy loss spectroscopy, and noncontact atomic force microscopy. The fundamental motif of the complex is a triangular cluster, including four NO molecules and three water molecules. As coverage increases, the complex grows into a chain structure on the surface. The preferential formation of such unique complexes suggests that there is attractive interaction between water and NO that is strong enough to overcome NO-NO and water-water interactions. The origin of the interaction is argued in terms of electrostatics, where water donates a polar OH group to NO which is negatively charged via electron transfer from the surface.

Introduction Water is ubiquitous and has an essential role in many chemical and physical processes at solid surfaces. An atomic-scale understanding of how water impacts reactions at surfaces is indispensable for elucidating the mechanisms of processes such as corrosion and heterogeneous catalysis. This fact has motivated extensive studies of the coadsorption of water on metal surfaces under ultrahigh vacuum (UHV) conditions. 1–4 For example, coadsorption of water with alkali metal atoms has been studied intensively because alkali metal atoms are used as promoters in various heterogeneous catalytic systems, such as the water gas shift (WGS) reaction. At low coverage, alkali metal atoms usually donate their s-electrons to the substrate and act as positive ions. Thus, the configuration of a coadsorbed water molecule can be influenced by electrostatic interaction between positively charged alkali metal atoms and lone-pair electrons of water. Configuration changes of water have been reported in systems such as Na/Ru(001), 5 K/Pt(111), 6 and Cs/Cu(110), 7 mainly by using vibrational spectroscopy. As another example, the coadsorption of water with a hydroxyl (OH) group has also been the subject of fundamental research with regard to macroscopic surface properties such as wetting and friction. In a mixed overlayer on Pt(111), a H-bonded network with 2


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a composition of 1 OH to 1 H2 O was found to be favorable. 8–10 Li et al., who investigated this system by ab initio path-integral molecular dynamics, revealed that the shared H atom is delocalized between OH and H2 O, so that distinction between them is no longer possible, 11 indicating that a significant interaction exists between water and OH on the surface. These previous coadsorption studies suggest that the structure of water is readily influenced by coadsorbed species and vice versa, which is ascribed to the ability of water molecules to interact with other atoms/molecules via electrostatic interaction. In this study, we investigated the coadsorption of water with nitric oxide (NO) on a Cu(111) surface. Adsorption and molecular interaction are well defined for individual molecules. Water prefers island nucleation to monolayer wetting on Cu(111) owing to the dominance of water-water interaction over water-surface interaction, i.e., the hydrophobic nature of the surface. 4,12,13 This preference was confirmed by scanning tunneling microscopy (STM) observations that allowed direct imaging of individual ice clusters nucleated on the surface. 14,15 The adsorption of NO on Cu(111) has been studied by using various surfacesensitive spectroscopies, with monomeric adsorption reported to dominate at low coverage on Cu(111). 16–24 However, a recent study using STM revealed that NO prefers characteristic trimer formation, even at very low coverage, arising from covalent interaction between NO molecules possessing an unpaired 2π* electron. 25 Trimer formation was also verified by vibrational analysis using electron energy loss spectroscopy (EELS). 26 Thus, when adsorbed individually on Cu(111), water and NO prefer aggregation through H-bond and covalent interactions, respectively. When water and NO are coadsorbed, however, how they interact with each other and what structure they form on the surface remain open questions. In this study, we found attractive interaction between water and NO that produced mixed water-NO complexes. The smallest complex had the shape of a triangle, and as coverage increased, the complex developed into chains on the surface. The chemical information on the water-NO complexes was derived from vibrational spectroscopy using EELS. In addition, by using noncontact atomic force microscopy (NC-AFM), which has recently been developed to visualize

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intramolecular structures with an atomic resolution, 27,28 we revealed the composition and configuration of the complexes. We postulate that the attractive interaction originates from a negatively charged NO molecule accepting a polar OH group from a water molecule.

Experimental STM, EELS, and NC-AFM experiments were carried out in separate UHV chambers at Kyoto University (STM and EELS) and the University of Tokyo (NC-AFM). The base pressure of all the chambers was below 1×10−10 Torr. Single-crystalline Cu(111) was cleaned by repeated cycles of Ar+ sputtering and annealing up to 780 K. The Cu(111) surface was exposed to NO and water gas via a tube doser through a variable-leak valve. Before the experiments, water was degassed and purified by performing freeze-pump-thaw cycles. For the STM measurements (USM-1200, Unisoku), an electrochemically etched tungsten tip was used as the probe. The clean surface was exposed to NO and water at 80 or 97 K, and subsequently cooled to 6 K for STM experiments. The STM images were acquired in constant current mode. The EEL spectra were obtained with a primary energy of 3.5–4.0 eV at incidence and reflection angles of 60◦ from the surface normal (LK-5000, LK Technologies, Inc.). The typical energy resolution was 1.5 meV. Exposures and measurements were conducted at 100 K. Exposures to NO (water), which were calculated by multiplying the background NO (water) pressure by time and by the magnification factor of the doser, are given in units of L (1 L = 1 × 10−6 Torr s). The magnification factor was estimated to be 600 for NO, 26 and 1000 for water by comparing the kinetics of adsorption from thermal desorption data. For the NC-AFM measurements (LT-STM/AFM, Omicron Nanotechnology), a tuning fork with an etched tungsten tip was used as the force sensor (resonance frequency f0 = 22.9 kHz, quality factor Q ≈ 2 × 104 ) with an oscillation amplitude A = 1 Å. The Cu(111) surface exposed to NO and water at ∼6 K was annealed at 110 K and subsequently cooled to

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4.8 K for STM/NC-AFM imaging. For NC-AFM, frequency shift (∆f ) images were obtained in constant-height mode at a sample bias Vs = 0 mV. The tip height was 50 pm higher than the set point determined by STM at I = 0.02 nA and Vs = 30 mV over bare Cu(111). A positive ∆z means that the tip was retracted against the surface (a high tip height), and vice versa. In order to highlight the edges of the images, the NC-AFM images in this paper were Laplacian-of-Gaussian-filtered, except that in Fig. 4b.

Results and discussion Figure 1a shows an STM image of Cu(111) exposed to NO and subsequently to water at 80 K. The exposure of Cu(111) to NO caused the formation of (NO)3 , imaged as a threefold symmetric ring (indicated by a dashed arrow). 25 Upon post-exposure to water, bright features appeared, as indicated by a solid arrow. The main product was imaged as a triangular shape, and larger structures were also observed. The triangle was aligned with the vertex pointing along the [¯1¯12] direction. Because these bright features appeared only after the exposure to NO and water, they were ascribed to mixed complexes of the two molecules. The triangular cluster was the smallest product observed, and thus, was a fundamental building block of the larger complexes. Figure 1b shows an STM image of the surface further exposed to water at 97 K. Because the tip was likely terminated by NO, the STM image (Fig. 1b) appeared much brighter than that obtained with a bare tip (Fig. 1a). 25 As the exposure to water increased, the complex developed into a chain structure. It was also found that the complexes formed faster as the temperature increased, indicating that the reaction involved an activation process. The (NO)3 trimers should be rearranged or even dispersed for the mixed water-NO complexes to be formed. This process requires the diffusion of (NO)3 on the surface and the rupture of NO-NO bonds and thus is thermally activated. The preferential formation of water-NO complexes indicates that attractive interaction exists between water and NO on the surface.

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The structural and chemical information on the water-NO complexes was deduced from vibrational spectroscopy. Figure 2a shows EEL spectra of Cu(111) after exposure to 0.50 L NO (bottom) and then to increasing amounts of water at 100 K. The spectra are shown as a function of exposure to water from the bottom to the top. Upon exposure to NO (bottom spectrum), the Cu(111) surface was covered with submonolayer (NO)3 , 25,26 and two main peaks were observed at 44 and 190 meV, which were assigned to the hindered-rotation (bending) and internal N-O stretching (νN−O ) modes of (NO)3 , respectively. 26 As the surface was subsequently exposed to water, the (NO)3 -derived peaks were attenuated and several new peaks appeared. Among them, two peaks at 157 and 175 meV were assigned to νN−O for NO involved in water-NO complexes. These peaks shifted to lower energies with increasing water exposure. The assignment was confirmed by isotope-labeling experiments using

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NO

and N18 O, which exhibited two peaks at 154 (152) and 173 (170) meV, respectively. 29 The presence of two νN−O modes suggests that two kinds of surface sites are occupied by NO in the complexes. It should be noted that So et al. observed the vibrational peak at 158 meV for the adsorption of NO (without water) on Cu(111), and assigned it to the N-O stretch mode for N2 O species produced by the reaction of NO. 19 The bottom spectrum in Fig. 2b shows the full-range spectrum of the water-NO complex. A small bump observed at ∼270 meV may correspond to the N-N stretch mode of N2 O. 30 However, we argue that N2 O formation is unlikely because of the following reasons. First, the bump at ∼270 meV is so small in intensity for the N-N stretch mode of N2 O; In general, the intensity of the N-N stretch mode is much larger than that of N-O stretch mode for N2 O on metal surfaces. 31,32 The bump is probably ascribed to the adsorption of residual CO from the background. 33 Second, Dumas et al. observed the vibrational peak for the Cu-ON2 stretch mode at 44 meV, 21 whereas no corresponding peak was observed in the spectrum for water-NO complex. Water related peaks were also observed, which were assigned based on isotope labeling experiments with D2 O. 29 The peaks at 32.0 and 199 meV shifted to 31.5 and 145 meV in the D2 O spectrum, leading to the assignment of these peaks to the (hindered) translation and

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scissoring mode of water, respectively. Several peaks in the range of 55–80 meV were assigned to the libration modes. The vibrational energies for the water-NO complex are summarized in Table 1. For comparison, the EEL spectrum of Cu(111) exposed only to 0.10 L water is shown in the top of Fig. 2b. The libration modes of water on Cu(111) exhibited a broad peak at ∼100 meV. A broad libration peak is commonly observed for water on hydrophobic surfaces, 1 where water molecules are aggregated to form 3D clusters, and this broadening is related to the vibrational coupling observed in proton-disordered ice. 34 In contrast, the libration modes for the water-NO complex showed distinct peaks (Fig. 2a), suggesting the absence of water nucleation in the complex. This finding was also supported by the vibrational energy of the scissoring mode, observed at 199 and 204 meV for the water-NO complex and water clusters, respectively; it is well known that the energy of the scissoring mode is blue-shifted upon the formation of H-bonds. 35 It is noted that the vibrational energy of the scissoring mode for a gas-phase water molecule is 198 meV. 34 Furthermore, the spectrum of water also exhibits a distinct O-H stretching (νO−H ) region (Fig. 2b). On bare Cu(111), water is aggregated to form 3D islands, 14,15 which manifest as an intense and broad structure 13 at 400 meV (top spectrum). The energy shift relative to free νO−H (453 meV) 34 results from the H-bond interaction between water molecules. In contrast, νO−H for the water-NO complex was not detected (bottom spectrum), in spite of the presence of water molecules on the surface. The absence of νO−H in the vibrational spectrum is characteristic of non-H-bonded water molecules, as reported for water monomers on Cu(100) and Pd(100). 36 The H-bond interaction between water molecules should cause polarization of the O-H bond and an increase of the dynamic dipole moment induced by νO−H excitation, giving rise to enhanced νO−H intensity. 37 Thus, the water-derived peaks in the vibrational spectra suggest that water molecules interact with NO but not with other water molecules in the complex. Next, we investigated the composition of the triangular cluster by using STM manipulation at 6 K. Figure 3a shows an STM image of a triangular cluster (circled in red). In order to obtain information on the composition of the cluster, we decomposed it by applying

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voltage pulses (1 V, 1000 ms) with the tip fixed over the cluster. The sequential decomposition process of the cluster is shown in Figs. 3a–3c. The first voltage pulse induced partial decomposition of the cluster; one vertex of the triangular unit was lost and a ring-shaped protrusion appeared in the vicinity (Fig. 3b). In addition, another bright spot appeared and was attached to the adjacent (NO)3 at the bottom right. The ring-shaped protrusion has been previously assigned to a NO monomer; the ring-shaped appearance of was ascribed to the degeneracy of the two 2π* orbital of upright NO on Cu(111). 25 The bright spot was ascribed to a water molecule attached to (NO)3 because we have observed the same species after exposing (NO)3 /Cu(111) to water at 10 K. 29 Therefore, the first voltage pulse partially decomposed the cluster, yielding a NO monomer and a water molecule. The remaining cluster in Fig. 3b was imaged as an aggregate of three rings, and was transformed into (NO)3 by the second voltage pulse (Fig. 3c). From these observations, it is suggested that the remaining cluster in Fig. 3b was composed of three NO molecules, and hence the original triangular cluster in Fig. 3a involved four NO molecules. It is noted that the water molecule attached to (NO)3 in Fig. 3b jumped to another (NO)3 at the bottom left (Fig. 3c) with the second pulse. Thus, water molecules were readily diffused or even desorbed by the voltage pulses, which hindered the determination of the number of water molecules involved in the cluster. As shown later, however, it is possible to image individual molecules in the cluster by NC-AFM, allowing us to definitely determine the number of water molecules and composition of the cluster. From the relative position to (NO)3 , we determined the registry of a triangular cluster to the surface. Figure 3d shows an enlarged STM image of the clusters with the Cu(111) lattice superimposed. Previous work has revealed that the three NO molecules in (NO)3 are located at the face-centered-cubic (fcc) hollow sites (red dots) around the Cu atom. 25 Therefore, the lattice is depicted in such a way that individual NO trimers are centered at the on-top sites of Cu(111). The relative position indicates that the center of the triangular cluster is located at the hexagonal-close-packed (hcp) threefold site (green dot). Based on

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this registry and the image of the partially decomposed cluster in Fig. 3b, we propose that the four NO molecules in the cluster are arranged in such a way that one is positioned at the hcp threefold site and the other three are positioned at the neighboring fcc threefold sites, as shown by green and red dots in Fig. 3d. The composition of the complex and the location of water molecules in the complex remain to be clarified. We investigated the triangular cluster by simultaneous imaging with NC-AFM and STM. Figure 4a shows an STM image of (NO)3 and water-NO triangular clusters (indicated by red arrows). The tip was likely terminated by NO, as in Fig. 1b. Figure 4b shows an NC-AFM image of the same area as the STM image in Fig. 4a. The NC-AFM image of (NO)3 shows three bright spots, which s how the position of NO molecules in the trimer (Fig. 3d). The NC-AFM image of the triangular clusters in Fig. 4b exhibits seven hexagonally arranged spots, which likely indicate molecular positions in the cluster. Figure 4c shows an enlarged AFM image of a triangular cluster with the Cu(111) lattice superimposed. Among the seven spots, the four spots indicated by red and green dots are associated with NO molecules, based on the structure proposed in Fig. 3d. On the other hand, we assign the rest of the spots (blue dots) in the cluster to water molecules adsorbed at the on-top sites. We note that the location of water in the cluster was clarified by an NC-AFM image obtained at a lower tip height. 29 From the STM/NC-AFM results, we conclude that the composition of the water-NO triangular cluster is (NO)4 -(H2 O)3 , as schematically illustrated in Fig.4 d, where one NO molecule occupies the hcp hollow site at the center, the other three NO molecules occupy √ the fcc hollow sites, which are 2/ 3 a0 (2.96 Å) away from each other, and the three water molecules are located at on-top sites between two NO molecules. This structure is consistent with the EELS result that the NO molecules in the complex occupy two kinds of sites on the surface. According to the recently modified structure-frequency relationship, 17 it has been proposed that the adsorption at the hollow sites gives a νN−O energy in the 181–201 meV range. The observed vibrational energies at 157 and 175 meV for NO bonded to the two

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kinds of threefold sites are lower than the energy range proposed for threefold sites. The reduced energies are possibly associated with the interaction of NO with water molecules, as described below. Then, what kind of molecular interaction is responsible for the formation of the waterNO complex? From the results discussed above, we ascribe the formation of the water-NO complex to water-NO electrostatic interaction. As depicted in Fig. 4d, we suggest that NO accepts a polar OH group from a water molecule, forming attractive interaction with each other via a hydrogen atom (NO· · · H–OH). This interaction, which is electrostatic, is enhanced if NO is negatively charged via electron transfer from the surface (back-donation). We confirmed back-donation using EELS by observing the νN−O peaks for water-NO complexes with reduced energies (Fig. 2); back-donation to the half-filled antibonding 2π* orbital of NO decreases the νN−O energies. 16,17 Hence, we propose that surface-enhanced electrostatic interaction exists between NO and water molecules. This electrostatic interaction between surface-bound NO and water was nominally called “H-bond” interaction, and a similar mechanism has been previously proposed to rationalize the attractive interaction between NO and water on Cu(110), 38 NO and NH3 on Pt(111), 20,39–41 and O2 and NH3 on Pt(111). 42,43 Typical H-bond interaction involves O-H or N-H groups, where the H-atom of the polar group is donated to the nonbonding orbital (lone pair electrons) of another group. 44,45 The main mechanism involved in the formation of a H-bond is polarization of the nonbonding orbital induced by the donated polar group and partial electron transfer of the lone pair electrons (overlap of the orbitals) between the molecules. A nonbonding orbital is essential for this mechanism to work, which gives rise to the characteristic intensity and broadening of the νO−H peak in infrared or EEL spectra, as described above. In the present case, the NO molecule has no lone-pair electrons, and thus, the water-NO interaction is different from the typical H-bond interaction explained above. Apart from this typical H-bond interaction, π-bonded pairs, such as those of aromatic molecules, can be polarized by H-atom donation from a polar group, and thus act as a H-bond acceptor, the mechanism of which is known

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as “π H-bond” interaction. 44 We argue that the water-NO attraction is possibly ascribable to this kind of interaction; the hydrogen atom of a water molecule is donated to NO and polarizes the 2π* electrons. Furthermore, electron transfer from the surface causes NO to be negatively charged and enhances the electrostatic interaction. 38 As a consequence, the water-NO interaction is stronger than the water-water and NO-NO interactions, leading to the formation of mixed complexes. In addition to the electrostatic interaction, there is possibly NO-NO interaction in the water-NO complex because the nearest NO-NO distance (∼3 Å) is short enough for the 2π* orbitals to be coupled through the substrate. 46 We note that theoretical study would be helpful in elucidating the mechanism of water-NO interaction on Cu(111) and verifying the composition and structure of the complex. We also found water-NO complexes that were larger than the triangular cluster. Figure 5a shows an example cluster (indicated by a red arrow), which has a “starfish” shape. Figure 5b shows an NC-AFM image of the same area as Fig. 5a, and Fig. 5c shows an enlarged image of the cluster. As shown in the NC-AFM image, the “starfish” cluster consists of 13 bright spots. In analogy to the triangular cluster (Figs. 4c and 4d), we proposed a composition for the “starfish” cluster, as shown by red (NO in fcc hollow sites), green (NO in hcp hollow sites), and blue (water in on-top sites) dots in Fig. 5c. We note that the component water molecules were distinguishable from NO molecules in the cluster by modifying the tip height in the same manner as for the triangular complex. 29 The structure of “starfish” cluster is schematically illustrated in Fig. 5d, where NO molecules are arranged alternately at the fcc and hcp hollow sites, and connected together via water molecules. Such a large cluster may be considered as a precursor to the chain structure shown in Fig. 1b.

Summary We found that water molecules have attractive interaction with NO and water-NO complexes are formed on a Cu(111) surface. By using STM manipulation and atomically resolved NC-

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AFM imaging, we revealed that the fundamental unit of the water-NO complexes consists of four NO and three water molecules [(NO)4 -(H2 O)3 ] arranged in a hexagonal configuration. From the experimental results, we proposed a mechanism for water-NO complex formation based on electrostatics, where negatively charged NO molecules accept polar OH groups from water molecules. This study suggested the importance of water-NO electrostatic interaction on the surfaces, which surpasses water-water H-bond interaction and NO-NO covalent interaction, and thus induces the formation of water-NO complexes.

Acknowledgement This work was supported by JSPS KAKENHI Grant Numbers JP16H00966, JP16H00959, JP17K19024, and JP16H00933.

Supporting Information Available Supporting Information. Isotope dependence of the EEL spectra, STM image of the (NO)3 /Cu(111) exposed to water at 10 K, and tip height dependence of the NC-AFM image of water-NO complexes. This material is available free of charge via the Internet at http: //pubs.acs.org/.

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with Cu(100) and Cu(111) surfaces using low energy electron diffraction and electron spectroscopy. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2143–2159. (24) Kinoshita, I.; Misu, A.; Munakata, T. Electronic excited state of NO adsorbed on Cu(111): A two-photon photoemission study. J. Chem. Phys. 1995, 102, 2970–2976. (25) Shiotari, A.; Hatta, S.; Okuyama, H.; Aruga, T. Formation of unique trimer of nitric oxide on Cu(111). J. Chem. Phys. 2014, 141, 134705. (26) Koshida, H.; Okuyama, H.; Hatta, S.; Aruga, T. Vibrational spectroscopic evidence for (NO)3 formation on Cu(111). J. Chem. Phys. 2016, 145, 054705. (27) Gross, L.; Mohn, F.; Moll, N.; Liljeroth, P.; Meyer, G. The chemical structure of a molecule resolved by atomic force microscopy. Science 2009, 325, 1110–1114. (28) Shiotari, A.; Sugimoto, Y. Ultrahigh-resolution imaging of water networks by atomic force microscopy. Nat. Commun. 2017, 8, 14313. (29) See supporting information at http://dx.doi.org/XXX for isotope dependence of the EEL spectra, STM image of the (NO)3 /Cu(111) exposed to water at 10 K, and tipheight dependence of the NC-AFM image of water-NO complexes. (30) Nakamoto, K. Infrared and Raman spectra of inorganic and coordination compounds; Wiley, 1977. (31) Brown, W.; Sharma, R.; King, D.; Haq, S. Adsorption and Reactivity of NO and N2 O on Cu{110}: Combined RAIRS and Molecular Beam Studies. J. Phys. Chem. 1996, 100, 12559–12568. (32) Brown, W.; Gardner, P.; King, D. Very Low Temperature Surface Reaction: N2 O Formation from NO Dimers at 70 to 90 K on Ag{111}. J. Phys. Chem. 1995, 99, 7065–7074.

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(33) Raval, R.; Parker, S.; Pemble, M.; Hollins, P.; Pritchard, J.; Chesters, M. FT-RAIRS, EELS and LEED studies of the adsorption of carbon monoxide on Cu(111). Surf. Sci. 1988, 203, 353–377. (34) Petrenko, V. F.; Whitworth, R. W. Physics of ice; OUP Oxford, 1999. (35) Falk, M. The frequency of the H–O–H bending fundamental in solids and liquids. Spectrochim. Acta, Part A 1984, 40, 43–48. (36) Nyberg, C.; Tengstål, C.; Uvdal, P.; Andersson, S. Adsorption of water on Cu(100) and Pd(100) at low temperatures: observation of monomeric water. J. Electron. Spectrosc. Relat. Phenom. 1986, 38, 299–307. (37) Iogansen, A. Direct proportionality of the hydrogen bonding energy and the intensification of the stretching ν(XH) vibration in infrared spectra. Spectrochim. Acta, Part A 1999, 55, 1585–1612. (38) Shiotari, A.; Hatta, S.; Okuyama, H.; Aruga, T. Role of hydrogen bonding in the catalytic reduction of nitric oxide. Chem. Sci. 2014, 5, 922–926. (39) Esch, F.; Greber, T.; Kennou, S.; Siokou, A.; Ladas, S.; Imbihl, R. The formation of a NO–NH3 coadsorption complex on a Pt(111) surface: a NEXAFS study. Catal. Lett. 1996, 38, 165–170. (40) Peronio, A.; Cepellotti, A.; Marchini, S.; Abdurakhmanova, N.; Dri, C.; Africh, C.; Esch, F.; Peressi, M.; Comelli, G. NH3 –NO Coadsorption System on Pt(111). I. Structure of the Mixed Layer. J. Phys. Chem. C 2013, 117, 21186–21195. (41) Cepellotti, A.; Peronio, A.; Marchini, S.; Abdurakhmanova, N.; Dri, C.; Africh, C.; Esch, F.; Comelli, G.; Peressi, M. NH3 –NO Coadsorption System on Pt(111). II. Intermolecular Interaction. J. Phys. Chem. C 2013, 117, 21196–21202.

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(42) Herceg, E.; Jones, J.; Mudiyanselage, K.; Trenary, M. Formation and hydrogenation of p(2×2)-N on Pt(111). Surf. Sci. 2006, 600, 4563–4571. (43) Liang, Z.; Kim, H.; Kim, Y.; Trenary, M. Molecular oxygen network as a template for adsorption of ammonia on Pt(111). J. Phys. Chem. Lett. 2013, 4, 2900–2905. (44) Maréchal, Y. The hydrogen bond and the water molecule: The physics and chemistry of water, aqueous and bio-media; Elsevier, 2006. (45) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P. et al. Definition of the hydrogen bond (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83, 1637– 1641. (46) Shiotari, A.; Kitaguchi, Y.; Okuyama, H.; Hatta, S.; Aruga, T. Imaging covalent bonding between two NO molecules on Cu(110). Phys. Rev. Lett. 2011, 106, 156104.

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(b)

(a)

[112]

[112]

[110]

[110] High

Low

High

Low

Figure 1: (a) STM image of the Cu(111) surface exposed to NO and subsequently to water at 80 K. The adsorption of NO resulted in the preferential formation of (NO)3 , which was observed as a threefold symmetric ring (indicated by a dashed arrow). Upon the postexposure to water, bright islands were newly observed and ascribed to water-NO complexes. The smallest motif of complexes is of triangular shape (indicated by a solid arrow). (b) STM image of the surface exposed further to water at 97 K. At elevated temperature, the complexes formed faster and developed in chains. The building block of the chain is the triangular unit (indicated by a solid arrow). The image was obtained at Vs = -125 mV and I = 0.5 nA for (a), and Vs = 100 mV and I = 0.9 nA for (b). White scale bars represent 10 Å.

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Table 1: Vibrational energies of NO+H2 O and NO on Cu(111). NO+H2 O (meV) 17 32

Assignment NO HT a H2 O HT a

59 62 73 77

H2 O libration H2 O libration H2 O libration H2 O libration

157 175

N-O stretch N-O stretch

199 NO (meV) 15 44

H-O-H scissors

190

N-O stretch

a b

Assignment NO HT a NO HR b

Hindered Translation Hindered Rotation

19



(a)

NO + H2O / Cu(111)

7377 62 17 32 59

199

175

×3

Normalized Intensity

157

+H2O 190

44

15

NO

0

Normalized Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50

100 150 Energy Loss / meV

(b)

200

H 2O NO + H2O

×0.2 204

157

0

100

×10

200 300 400 Energy Loss / meV

500

Figure 2: (a) EEL spectra of the Cu(111) surface exposed to 0.50 L NO (bottom) and subsequently to increasing amounts of water. The exposure to water was 0, 0.01, 0.02, 0.03, 0.05, 0.06 L from the bottom to the top. The bottom spectrum shows a peak at 190 meV which is assigned to the N-O stretching mode (νN−O ) of (NO)3 . As the surface was exposed to water, the 190 meV peak was replaced by two peaks at 157 and 175 meV. These peaks were assigned to νN−O of the water-NO complex. The peaks observed for (NO)3 and water-NO complex are tagged with red and black bars, respectively. (b) The full-range spectra for the water-NO complex (bottom) and water clusters (top) on Cu(111). The latter was recorded for the surface exposed to 0.10 L water. The spectra in (a) and (b) were normalized to the intensities of elastic peaks.

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(a) [112] [110]

(b)

monomer

High

water monomer

(c)

trimer

Low

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


water

(d)

[112] [110]

hcp fcc

Figure 3: (a)–(c) STM images obtained before and after sequential voltage pulses (1 V, 1000 ms) were applied to a water-NO complex. The first pulse was applied to a triangular cluster (dotted circle) in (a), causing a NO monomer to detach from the cluster (red arrow in (b)), leaving a partially decomposed cluster. At the same time, a water molecule appeared and attached to the (NO)3 at the bottom right. The second pulse was applied to the partially decomposed cluster in (b), causing the cluster to convert into (NO)3 , as shown in (c). The second pulse also caused the NO monomer to diffuse, as illustrated by a red arrow in (c). (d) The registry of the triangular cluster to the Cu(111) lattice. The approximate positions of NO in the complex as well as in the trimer are shown by red (fcc hollow) and green (hcp hollow) dots. The images were obtained at Vs = -125 mV and I = 0.5 nA for (a), Vs = -50 mV and I = 0.4 nA for (b) and (c), and Vs = -125 mV and I = 0.4 nA for (d). The image size are 34 × 51 Å for (a), (b), (c), and 25 × 30 Å 21 for (d). ACS Paragon Plus Environment


(b)

-4.3 Hz

(a)

High

[112]

[112]

(c)

-7.9 Hz

[110]

Low

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

[110]

hcp fcc

(d)

[112]

NO [112]

H2O [110]

Figure 4: Simultaneously acquired (a) STM and (b) NC-AFM images of (NO)3 and waterNO triangular cluster (indicated by red arrows) on Cu(111) with a NO-terminated tip. NO and water molecules in the cluster were observed as hexagonally-arranged seven spots in (b). (c) NC-AFM image of the cluster superimposed by the Cu(111) lattice. The approximate positions of molecules in the complex are shown by red (fcc hollow), green (hcp hollow), and blue (on-top) dots. (d) Schematic structure of the water-NO triangular cluster, (NO)4 (H2 O)3 . The STM image in (a) was obtained at Vs = 30 mV and I = 0.02 nA. The image size are 31 × 31 Å for (a), (b) and 12 × 13 Å for (c).

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(b)

(a)

[112]

[112]

[110]

(c)

[110]

(d)

hcp fcc

[112]

[112]

[110]

Figure 5: Simultaneously acquired (a) STM and (b) NC-AFM images of (NO)3 and waterNO "starfish" cluster (indicated by a red arrow) on Cu(111) with a NO-terminated tip. (c) Enlarged NC-AFM image of the "starfish" cluster in (b). The approximate positions of individual molecules in the cluster are shown by red (fcc hollow), green (hcp hollow), and blue (on-top) dots. (d) Schematic structure of the water-NO “starfish” cluster, (NO)8 (H2 O)5 . The STM image in (a) was obtained at Vs = 30 mV and I = 0.02 nA. The image size are 27 × 27 Å for (a), (b) and 19 × 19 Å for (c).

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STM Water-NO The Journal of Physical Chemistry Page Cu(111) 24 of 25 Complex (NO)3 ACS Paragon Plus Environment

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Cu(111) (NO)3

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