Preparation-Dependent Orientation of Crystalline Ice Islands on Ag(111)

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Preparation Dependent Orientation of Crystalline Ice Islands on Ag(111) Sarah-Charlotta Heidorn, Karsten Lucht, Cord Bertram, and Karina Morgenstern J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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The Journal of Physical Chemistry

Preparation dependent orientation of crystalline ice islands on 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

Ag(111) Sarah-Charlotta Heidorn,1 Karsten Lucht,2 Cord Bertram,2 and Karina Morgenstern2, ∗ 1

Leibniz Universität Hannover, Institut f¨ ur Festk¨ orperphysik, Appelstr. 2, D-30167 Hannover, Germany

2

Ruhr-Universität Bochum, Lehrstuhl f¨ ur physikalische Chemie I, Universit¨ atsstr. 150, D-44801 Bochum, Germany (Dated: May 23, 2017)

Abstract We observe the transformation of fractal ice islands grown at 96 K to compact ones annealed at 118 K and compare those to compact islands grown directly at 118 K. The low temperature grown islands form a four bilayer high wetting layer. The annealing causes a crystallization and reshaping of the islands and a substantial increase in height and roughness, in particular at higher coverage. Moreover, it leads to a dewetting of the ice film. The islands grown at the higher temperature show qualitative similarities to the annealed ones, at smaller nucleation density. However, their orientation with respect to the surface differs by 30◦ as compared to the annealed islands. PACS numbers: 68.37.Ef, 68.65.-k 68.55.A-, 68.43.Hn, 68.35.Ct

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INTRODUCTION 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

The interaction of water with solid surfaces is important in several scientific and industrial applications. In particular, the exact description of the water-metal interface on the atomic level is essential for understanding various phenomena in corrosion, electrochemistry, material science, and catalysis. For instance, the heterogeneously catalyzed nucleation of water in presence of an ice-nucleating agent has been reported intensely on the macroscopic level, but remains to be understood on a molecular level [1–3]. The key for understanding the role of water in the heterogeneous nucleation and in the other named disciplines is the description of the fundamental properties of the interaction of water with metal surfaces on a molecular level. On this level, water structures differ on different metallic surfaces because the binding strengths between water and metal surfaces are similar to the ones between water molecules [4]. Consequently, a rich variety of different crystalline structures of water were investigated by scanning probe microscopy in real space on a large variety of metallic surfaces, Au, Ag, Cu, Pd, Pt, Ru, Ni, and Rh, see [5–8], covering single molecules [9], dimers [10], and cluster [13] up to closed layers [11, 12]. The crystallization processes are still poorly understood. In many cases, it is not obvious whether the structures are kinetically limited or thermodynamically (meta- )stable phases. For instance, the distinction between cubic ice (Ic ), traditionally thought to nucleate at cryogenic temperatures, and hexagonal ice (Ih ), the most stable form under ambient conditions, has been removed, recently. It is now generally believed that stacking disordered ice Isd is the most prominent species [14]. Silver is classified as a hydrophobic surface, but water deposited at low temperature wets the surface [15]. So far, water has been investigated on Ag(111) at very low coverage, at which the most stable structures are hexamers to nonamers [4, 16]. The bonding of a monomer [17] and the most stable structure of a closed layer [18] were calculated. In contrast to the thermodynamically preferred structure, fractal structures were grown between 90 K and 118 K [19]. In the same coverage regime of around half a bilayer, the water wets the surface at around 70 K [20], but the structures are unordered, indicative of an amorphous layer, as usual for growth below 110 K [21, 22]. In this article, we investigate both, transformation of fractal islands grown far from the thermodynamic equilibrium by annealing and thermodynamically more stable structures directly grown at the annealing temperature. Our aim is to understand the formation of

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crystalline ice structure on a fundamental level. 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

EXPERIMENTAL METHODS

STM measurements are performed with a low-temperature STM under ultra-high vacuum (UHV) conditions (base pressure below 2 · 10−10 mbar). The Ag(111) surface is cleaned by sputter-anneal cycles. Ne+ ions at a pressure of 3 · 10−5 mbar are accelerated to 1.3 keV. The ion current of 1 to 2 μA is maintained for 45 to 60 min in the first, and for 20 to 30 min in the second cycle. The sample is annealed for 30 min at 900 K. After this procedure the terrace sizes are up to several 100 nm, some of them separated by step bundles. D2O of milli-Q quality is purchased from Sigma-Aldrich. Its isotopic purity is 99.96 %. The fluid is filled into a glass tube that is connected to the UHV chamber via a leak valve. Before deposition, the water is purified by freeze-thaw cycles. Its purity is checked by means of in-situ mass spectrometry of the vapor above the fluid. A pressure of 5 · 10−7 mbar or 1 · 10−6 mbar is then established in a small UHV chamber via the leak valve. This chamber is connected via a gate-through valve to the preparation chamber. Prior to opening that valve, the Ag(111) sample is turned such that its backside faces the valve. It is held above the desorption temperature of water, while the valve is opened for 2 min. By this procedure, some of the H2 O at the chamber walls is replaced by D2 O and the total amount of regular water in the rest gas is reduced. Subsequently, the sample is cooled to the deposition temperature of (96±3) K or (118±3) K and turned towards the tube that guides the water into the preparation chamber. After deposition, the sample is cooled to the lowest possible temperature and transferred into the STM. Measurements are performed at 5 to 6 K. For equilibration of structures grown at (96 ± 3) K, the sample is held for 20 min at (118 ± 3) K directly after growth and then quenched and transferred into the STM. The roughness on top of the water structures is measured as the root-mean-squared roughness

n 1 RMS = h2 n i=1 i

(1)

as implemented in WsxM 5.0 [23]. In order to avoid artifacts from noise on the surface, 3



which shifts the surface peak by up to 40 pm, the RMS is calculated only for those values 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

that are at least 50 pm above the surface layer. This reduces the absolute value of the roughness, but the relative values are better comparable than values including the surface. Note that the orientation of the Ag(111) surface with respect to the scanning direction is not identical for all measurements. Orientations given were always determined from images with atomic resolution.

RESULTS AND DISCUSSION Geometrical characterization

Fig. 1 shows large-scale and small-scale images of the investigated preparations. During growth at the lower temperature, branched islands are formed (Fig. 1a,b). The orientation of the arms of the islands do not show any preferred direction (Fig. 1c), the islands are thus fractal [19]. Fractal islands form, if molecules are not able to diffuse along the island border before other molecules reach the island [24]. The arm widths between 1.8 nm and 3.4 nm, at an uncertainty of 0.1 to 0.2 nm, have been recently attributed to mobility and attachment of clusters consisting of around 10 molecules [19]. Most islands, around 70 %, have an area of less than 200 nm2 , but some have areas up to 1400 nm2 . In the histogram (Fig. 1d), the mean of (229 ± 291) nm2 deviates considerably from the median, at 119 nm2 . Both observations are consistent with growth including the mobility of small islands [19, 25]. Annealing leads to compact islands (Fig. 1e,f) with the edges clearly following preferred directions along the [11¯1], [10¯1] and [1¯10] directions of the Ag(111) surface (Fig. 1g). The island area size distribution has a mean (81 ± 41) nm2 , a median at 79 nm2 , and a maximum at 170 nm2 , only (Fig. 1h); an order of magnitude smaller than the largest fractal islands. Though this size distribution does not result from growth, it resembles the size distribution expected from classical growth theory [26] more than the as grown structures. Furthermore, more substrate is visible after annealing (compare Fig. 1a to Fig. 1e). This suggests mass transport to higher layers during annealing, i.e. a dewetting of the surface. To confirm this mass transport, we now investigate the heights of the water structures at different coverages (Fig. 2). Note that STM measures so called apparent heights, because

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FIG. 1: Preparation dependence of water structures on Ag(111): (a-d) deposited at (96 ± 3) K, (0.47 ± 0.04) BL, 500 mV, 20 pA (e-h) deposited at (96 ± 3) K and annealed for 20 min at (118 ± 3) K, (0.4 ± 0.04) BL, 500 mV, 20 pA (i-l) deposited at (118 ± 3) K, (0.92 ± 0.09) BL, 109 mV, 7.3 pA and 100 mV, 8 pA (c,g,k) angular distribution measured at several images; green thick lines represent high symmetry 110 directions of Ag(111) determined from images with atomic resolution as exempified in the inset of panel l: (c) direction of arms (g) direction of edges (k) left plot: direction of edges for regions with rough top; right plot: directions of edges for regions with smooth top (d,h,l) area histograms; inset in (l): atomic resolution of Ag(111) for (i,j), 34 mV, 110 pA.

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FIG. 2: Height determination: all STM images recorded at 500 mV: (a-e) as grown fractal islands; (a-c) STM images: (a) (0.15 ± 0.01) BL, 29 pA (b) (0.47 ± 0.04) BL, 20 pA (c) (0.92 ± 0.07) BL, 20 pA (f-j) annealed compact islands; (f-h) STM images: (f) (0.09 ± 0.01) BL, 20 pA (g) (0.4 ± 0.04) BL, 20 pA (h) (0.84 ± 0.06) BL, 14 pA (d,i) height profiles along the lines marked in the STM images (e,j) pixel histograms of islands height at the three coverages in lighter color, hatched, and darker color for low, medium and high coverage, respectively.. the real heights of insulators are underestimated. However, relative apparent heights of the structures can be used to identify mass transport into higher layers. The STM images in Fig. 2a to c and f to h show grown and ripened islands, respectively, at different coverages after deposition with (1.9 ± 0.3) ·10−3 BL/s. The height of the fractal islands does not depend markedly on coverage (Fig. 2d) [29]. The largest apparent height is at (0.41 ± 0.04) nm approximately constant (Fig. 2d). In contrast, the annealed structures have different heights at different coverages (Fig. 2i), almost doubling their apparent height at the largest coverage. This confirms the material transport to higher layers during annealing. To quantify the layer transport, we analyze height histograms as shown before [30]. Discrete maxima are found for all preparations and all coverages in such height histograms (Fig. 2e,j). The maxima are attributed to tabulated geometric heights (Tab. 1). The smallest apparent height of (0.16 ± 0.03) nm corresponds to the apparent height of the first bilayer [19]. All types of islands show maxima at (0.25 ± 0.03) nm, (0.33 ± 0.03) nm, and (0.41 ± 0.03) nm. The compact islands after annealing have in addition maxima at (0.50 ± 0.03) nm, (0.59 ± 0.03), and (0.69 ± 0.03) nm. Further layers have thus apparent heights of 0.08 6


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apparent height apparent height geometric height geometric height BL 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

per layer

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TABLE 1: Comparison of measured apparent heights to literature values of geometric heights from [21] and assigned to the number of bilayers (BL). nm to 0.09 nm, at half the apparent height of the first layer. We conclude that fractal islands have heights of up to 4 BL and annealed ones up to 7 BL; the number of bilayers almost doubles. This massive mass transport to higher layers during annealing reflects the larger interaction energy for the molecule-molecule interface than for the molecule-metal interface [6].

Island density and roughness

The images in Fig. 2c,h suggest that fractal islands wet the surface, but compact ones don’t. We quantify the changes to the islands by measuring the island density and island roughness in dependence on coverage. The island density of the fractal islands increases up to ≈ 0.7 BL with an exponent of ≈ 0.34 (Fig. 3a). It then decreases exponentially due to coalescence. At around 3 BL the whole surface is covered by ice, the molecules wet the surface. The island density of the annealed islands follow the same trend initially, at a similar exponent of 0.37±0.04 (Fig. 3b). However, the island density increases further for all investigated coverages (Fig. 3a). Thus, we do not observe coarsening at this temperature, i.e. most molecules stay attached to their original island at lower coverage. At higher coverage the fractal islands even break apart into smaller units, the more the larger the coverage. 7


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FIG. 3: Coverage dependence: Island density and substrate roughness for as grown islands (red open circles) and annealed islands (black filled squares) (a) Island density ρ vs. coverage θ with fits (see text) (b) Double logarithmic plot of island density ρ after annealing with linear fit (c) RMS roughness. The roughness analysis confirms this dewetting (Fig. 3c). Up to 0.55 BL the roughness increases by only ≈ 20% upon annealing. At higher coverage, the roughness decreases for the as-grown islands indicative of the closure of the layers, but it increases for the annealed islands reflecting the increase in apparent height shown above. More over, islands of different heights are found in this coverage range (c.f. Fig. 2j) enforcing an increase in roughness. The edges of the annealed water structures (Fig. 1g) follow the close-packed directions of the Ag(111) surface. This is surprising in view of growth studies on other surfaces. The edges of water structures ripened or grown at higher temperatures on Cu(111) [27] and Pt(111) [28], respectively, are oriented along the 112 directions, at 30◦ to the islands observed here. Different reasons for this difference are possible. First, the surfaces might enforce a different orientation because of the different lattice mismatch with the ice structure. However, the difference in lattice constant between platinum and silver is a mere 4%. Second, the stable orientation might be height dependent, but the large scale rearrangement necessary to reach the thermodynamically stable orientation is not feasible at the annealing temperature. For √ √ instance, DFT calculations showed that both a double and a triple bilayer form 3x 3 overlayers on Cu(111) [35]. However, in the thermodynamically most stable structure of a triple bilayer the lower most bilayer is shifted by one lattice spacing with respect to the double bilayer. A large rearrangement at the interface is thus needed to reach thermodynamic

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equilibrium. To test this prediction, we deposit the water directly at the annealing temperature (Fig. 1i to l). The island density is much smaller at the higher temperature, as expected (Fig. 1i). Correspondingly the island areas are around an order of magnitude larger (Fig. 1l). The mean is at (750 ± 230) nm2 . Note that the island size distribution is qualitatively similar to the one of the annealed structures, which in turn was discussed above to be consistent with the classical growth scenario [26].

Water grown at annealing temperature

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FIG. 4: Characterization of growth at (118 ± 3) K, (0.92 ± 0.09) BL at 0.42 BL/min: (a) STM image, 109 mV, 8.2 pA (b) Height profile across island along line shown in (a). (c) Pixel height histogram of island height. Note that most islands grown at 118 K have regions with rough and smooth surfaces (Fig. 1i). Interestingly, the angular distribution differs largely for the two regions (Fig. 1k). The orientation of the edges along certain preferred directions for the smooth parts is already clearly visible in the STM images (Fig. 4a). For the rough regions, the edges follow no preferred direction, indicative of amorphous growth. For the regions with smooth tops, the edges follow the 112 directions of the surface. These are at 30◦ to the preferred directions of the annealed structures, but the same directions as in the case of Cu(111) and Pt(111). However, the mean apparent height of the smooth parts is, at (0.36 ± 0.04), rather in the range of the fractal than of the annealed islands, while the one of the rough parts is closer to the height of the annealed structures (Fig. 4b,c). Thus, it is not the height that determines 9



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Our data is thus consistent with the following scenario: The orientation of the initial nuclei of the fractal growth direct the orientation of the annealed water structures on Ag(111). Indeed, small nuclei of up to 10 molecules show a preferred orientation along the closedpacked rows of Ag(111) [4], the orientation found for the annealed water structures. During growth at the higher temperature, a reorientation happens as soon as the nuclei surpass a critical size, a size much smaller than the final size of the fractal islands before annealing. This reorientation during the early stages of growth lead to a different orientation of the ice islands at 30◦ of the preferred orientation of the annealed islands. During post-growth annealing, the number of molecules that have to rearrange simultaneously to change to the thermodynamically preferred orientation of a larger island is substantially larger and thus the structure is trapped in a kinetically limited low-temperature phase. The rearrangement might be possible at higher temperature as a rearrangement of the interface layer was observed during layer-by-layer growth on Ru(0001) and Pt(111) at 145 K [36]. At the lower temperature used here, the orientation of the islands can be enforced in dependence of their preparation history.

CONCLUSION

We observed the transition of kinetically limited fractal ice islands to compact ones by annealing and compared those to islands grown at the annealing temperature. The orientation of the islands depends on their preparation history. This is an important ingredient with respect to the large variety of ice structures observed and controversially discussed. As the structure of water determines its behaviour with respect to, e.g., electron solvation or molecular adsorbance, our results could also pave a way for influencing the reactivity of water layers by preparation pathway.

ACKNOWLEDGMENTS

We acknowledge financial support from the Deutsche Forschungsgemeinschaft within the framework of the Cluster of Excellence RESOLV (EXC 1069) and the project MO960/18-1.

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∗

Electronic address: [email protected]

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imaged with STM. Phys. Rev. B 2008, 77, 195425. 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

[29] All images in Fig. 2 are recorded at similar tunneling conditions, 14 to 29 pA and 500 mV. The change in tunneling current changes the apparent height thereby by less than 3 %. [30] Mehlhorn, M.; Morgenstern, K. Height analysis of amorphous and crystalline ice structures on Cu(111) in scanning tunneling microscopy. New J. Phys. 2009, 11, 093015. [31] Mandelbrot, B.B. The Fractal Geometry of Nature, 1983, Spektrum Akademischer Verlag, Weinheim. [32] Witten, T.A.; Sander, L.M. Diffusion-Limited Aggregation, a Kinetic Critical Phenomenon. Phys. Rev. Lett. 1981, 47, 1400-1403. [33] Witten, T.A.; Meakin, P. Diffusion-limited aggregation at multiple growth sites. Phys. Rev. B 1983, 28, 5632-5642. [34] Röder, H.; Bromann, K.; Brune, H.; Kern, K. Diffusion-Limited Aggregation with Active Edge Diffusion. Phys. Rev. Lett. 1995, 74, 3217-3220. [35] Mehlhorn, M.; Schnur, S.; Groß, A.; Morgenstern, K. Molecular-Scale Imaging of Water Near Charged Surfaces. ChemElectroChem 2014, 2, 431-435. [36] Maier, S.; Lechner, B.A.J.; Somorjai, G.A.; Salmeron, M. Growth and Structure of the First Layers of Ice on Ru(0001) and Pt(111) J. Am. Chem. Soc. 2016, 138, 3145-3151.

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TOC: 200 12 fractalislands annealedstructures

150

RMS (pm)

10 8

N

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

6 4 2

100 50

0 0

50

100

150

A (nm2)

0 0.0

200

50 nm

50 nm

14


0.2

0.4

0.6

q (BL)

0.8

1.0

100 nm

Preparation-Dependent Orientation of Crystalline Ice Islands on Ag(111)

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