Low Temperature Growth of Amorphous Water Ice on Au(111)

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Low Temperature Growth of Amorphous Water Ice on Au(111) Sarah-Charlotta Heidorn, Cord Bertram, and Karina Morgenstern J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02187 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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The Journal of Physical Chemistry

Low Temperature Growth of Amorphous Water Ice on Ag(111) S.-C. Heidorn#, C. Bertram#+, K. Morgenstern+* #

Leibniz Universität Hannover, Institut für Festkörperphysik, Appelstr. 2, D-30167 Hannover,

Germany +

Ruhr-Universität Bochum, Lehrstuhl für physikalische Chemie I, Universitätsstr. 150, D-44801

Bochum, Germany, email: [email protected]

ABSTRACT While the wetting of hydrophilic surfaces on the macroscale is a well-known phenomenon, we here develop a microscopic understanding of the more recently discovered complete covering of hydrophobic surfaces by a uniform water layer. For this aim, we deposit D2O on Ag(111) at two different temperatures, 20 K and 96 K, and investigate the geometry of the layers at coverages up to four bilayers by low-temperature scanning tunneling microscopy. In the coverage range up to 0.5 BL, the ice grows in the form of islands that differ largely in size, shape, and density, but surprisingly not in their height. Moreover, the water fills the layer with islands of the same thickness of three to four bilayers at both temperatures. The different island shapes and densities in the coverage range before coalescence are attributed to details in the interaction between water nanoclusters and activated cluster diffusion at the higher growth temperature of 96 K, as visualized in time-lapsed series of STM images.

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Introduction The interaction of water with solid surfaces is important in several scientific and industrial applications. In particular, the exact description at the water-metal interface is essential for understanding diverse phenomena in corrosion, electrochemistry, material science, and catalysis.1–3 The key for understanding the role of water in these disciplines is the description of the fundamental properties of nucleation and growth of water on metal surfaces on a molecular level. While it is well understood why water wets hydrophilic surfaces under ambient conditions, wetting phenomena on hydrophobic organic surfaces were only recently explained by surface defects, at which small droplets nucleate.4,5 Such nanodroplets are also at the origin of small contact angles, indicative of wetting, on several anorganic hydrophobic surface as H-Si(111), graphite and mica.6 On these surfaces, water even adsorbs amounts typical for hydrophilic surfaces. Unfortunately, molecular scale resolution is not achievable under the ambient conditions used in these and other studies that investigate wetting. Unexpectedly, also silver is not only covered completely by water deposited at low temperature;7,8 but also more uniformly than expected for a Poisson distribution of layer filling in kinetically limited growth. We here investigate this phenomenon for the Ag(111) surface. So far, the bonding of a monomer9 and a closed crystalline layer10 were calculated on Ag(111). Experimentally, it was shown that water forms clusters consisting of six to nine molecules at low coverage and temperature.11,12 Fractal structures grow in a temperature range between 90 and 120 K at approximately bilayer/min growth rates.13


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In this article, we investigate water structures grown on Ag(111) at two temperatures, 20 K and 96 K, both below the wetting-dewetting transition on Ag(111).8 Our low temperature scanning tunneling microscopy study reveals that a three to four bilayer high water layer gradually fills the surface at both temperatures despite a considerable difference in shape of the growing structures. We discuss further similarities and differences between the nanoclusters formed at these two temperatures and relate the difference to the mobility of ice clusters.

Methods STM measurements are performed under ultra-high vacuum (UHV, base pressure < 2  10-10 mbar) conditions at low temperature. 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 A is maintained for 45 to 60 min in a first, and for 20 to 30 min in a second cycle. The sample is annealed for 30 min at 900 K after each cycle. After this procedure, the terrace sizes are up to several 100 nm, some of them separated by step bundles. The D2O of milli-Q quality, purchased from Sigma-Aldrich, has an isotopic purity of 99.96 %. The fluid is filled into a glass tube that is connected to a UHV chamber via a leak valve and purified by freeze-pump-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 deposition, the H2O at the chamber walls and in the rest gas is reduced by flushing the preparation chamber with D2O for 2 min. During that time, the sample is in a separate chamber. Subsequently, the sample is transferred to the preparation chamber and cooled to the deposition temperature of (20 ± 3) K or of (96 ± 3) K and its face is turned towards



the gate-through valve. After deposition, the sample is cooled to the lowest possible temperature and transferred into the STM, where the measurements are performed at 5 K. The coverages given are calculated based on a calibration of the source at crystalline ice structures. Coverages are given in bilayers (BL). One bilayer corresponds to a hexagonally closed-packed crystalline ice layer, which is buckled with the molecules alternating by ≈ 48 pm above and below the plane of the layer.9 As compared to metal-on-metal growth terminology, a bilayer equals a single layer at a molecule density of 2/3 of the atom density of a fcc(111) face. The roughness of the water structures is measured as the root-mean-squared roughness RMS, as usually used for roughness analysis in scanning probe microscopy to minimize the contribution of noise to the roughness signal: RMS 

1 n   hi , j  h n i 1



2

with hi,j the height values of pixels in an STM image above the average height and n the total number of pixels, as implemented in WSxM 5.0.14 The RMS is calculated only for those values that are at least 50 pm above the surface layer maximum such that only the roughness of the water structures contributes to the value and not the surface roughness.


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Results & Discussion

Figure 1. Water structures on Ag(111): (a-c) (0.37 ± 0.04) BL deposited at (20 ± 3) K, (d-f) (0.47±0.04) BL deposited at (96±3) K, (a,b,d,e) STM images, 500 mV, 20 to 28 pA (c,f) area histograms; (f) reproduced from [13] . The islands grown at 20 K and 96 K differ in island density and island shape (Fig. 1). The island density is much higher at the lower temperature, as expected (Fig. 1a vs. Fig. 1d). At the low temperature, small clusters of varying shapes are formed (Fig. 1b). Their mean area is (3.8 ± 0.4) nm2 ; the largest clusters have an area below 12 nm2 (Fig. 1c). Note that the STM tip broadens the clusters at each border by 0.5 to 1 nm and thus the values stated are upper bounds of the real area. We showed before that hexamers to nonamers are imaged at approx. 1 nm2,11 considerably larger than the typical width of a monomer. 15 90% of the clusters observed at ~0.4 BL are larger



than these structures.16 Most clusters here contain thus 10 molecules or more. The size of the clusters is in the same order of magnitude as clusters observed under ambient conditions. 6 The cluster size distribution in Fig. 1c shows a typical behavior as described in the theory of nucleation and growth by Venables with one maximum, two asymmetric flanks, and the mean at approximately the median.17 Note that Venables theory assumes that nuclei are stable and immobile during growth. The non-uniform shapes suggest that these clusters are amorphous. We call these structures amorphous clusters. At the higher growth temperature, branched islands with arm widths between (1.8 ± 0.1) nm and (3.4 ± 0.2) nm are formed (Fig. 1d and e). We characterized similar branched islands grown between 89 and 119 K, before.13,18 We recapitulate the relevant results for comparison to the low-temperature islands. Most of the high temperature islands have an area of less than 200 nm2, though occasionally islands with areas up to 1000 nm2 are observed (Fig. 1f). In contrast to low temperature growth, the mean of (208 ± 263) nm2 deviates considerably from the median, at 106 nm2. Such an island size distribution is consistent with growth including the mobility of small islands.13,19 These branched islands are fractal with a fractal dimension that varies between D = 1.63 to D = 1.89 ( 0.03) between 90 and 119 K.13 We call them fractal islands in the following.


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Figure 2. Coverage dependence of water growth at a deposition rate of (1.9 ± 0.3) ·10 −3 BL/s at (a-d) 20 K and (e-h) 96 K: (a-c, e-g) STM images: (a-c) amorphous clusters at coverages of (a) (0.08 ± 0.01) BL (b) (0.37 ± 0.04) BL (c) (0.69 ± 0.06) BL (e-g) fractal islands at coverages of (e) (0.15 ± 0.01) BL (f) (0.47 ± 0.04) BL (g) (0.92 ± 0.07) BL (d) pixel histogram of STM images of amorphous clusters recorded at different coverage, cyan for 0.1 BL, blue for 0.4 BL (h) pixel histogram of images of both, amorphous clusters and fractal islands and at the different coverages presented in panels a to c and e to f; (a-c,g) 500 mV (e) 600 mV (f) 806 mV, (a,b,f) 20 pA (c,e) 29 pA (g) 21pA. The shape of the amorphous clusters is preserved throughout a large coverage range (Fig. 2a to c) as described before for the fractal islands and Fig. 2e to g.13,18 At both temperatures, the amount of uncovered surface decreases continuously. However, for amorphous clusters their number increases with coverage, while for fractal islands the sizes increase.



Figure 3. (a) Coverage dependent RMS roughness for amorphous clusters (green open circles) and fractal islands (black filled squares, reproduced from [18]) (b) rising region of (a) on doublelogarithmic scale; fit has slope of (0.06 ± 0.01) (c) falling region of (a) on half-logarithmic scale; fit has slope of (-0.27 ± 0.03). Surprisingly, the amorphous clusters and fractal islands have the same roughness at all investigated coverages (Fig. 3a). For both structures the roughness increases to a maximum of (95 ± 8) pm at (0.53 ± 0.03) BL before decreasing exponentially. The linear dependence of the first region on double logarithmic scale (Fig. 3b) gives a potential dependence with a rather small exponent of 0.06. A potential increase of roughness with coverage indicates a reduced mass transport of particles from higher to lower layers. 20 The rather weak dependence here indicates that interlayer mass transport is only slightly reduced. The linear dependence in the second region on half-logarithmic scale reveals an exponential decay in roughness typical for closure of a layer (Fig. 3c). As shown above, the lateral dimension of the structures formed at the two temperatures differs largely. Surprisingly, this is not the case for the height of the structures. Note that STM measures so called apparent heights, because insulators contribute much less to the tunneling current than metals for tunneling bias voltages within their band gap. However, for layered structures, real heights can be estimated from apparent heights as outlined in [21]. In this study, the apparent


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height of the layered structures was correlated to the heights measured with tunneling bias voltages above the conduction band minimum of ice on Cu(111), which are much closer to real heights. In order to relate the here measured apparent heights to real heights, we analyze pixel histograms of STM images. The pixel histogram of the amorphous clusters shows distinct maxima (Fig. 2d). The fractal island height's maxima occur at the same values.13 A common histogram for both preparations and several coverages show the same maxima even more pronounced (Fig. 2h). We relate these maxima to individual layers as in [21]. The smallest height is at (0.16 ± 0.03) nm, attributed to the first bilayer. Further maxima are at (0.25 ± 0.03) nm, the second bilayer, (0.33 ± 0.03) nm, the third bilayer and (0.41 ± 0.03) nm, the fourth bilayer, indicating a maximum height of 4 BL.23 This is consistent with the closure of the layer at above 3 BL (Fig. 3a). While subsequent bilayers of ice are usually at identical distance, equivalent to the distance of bilayers in bulk ice, 0.37 nm, the first layer was calculated to be at a height of 0.31 nm above the Ag(111) only.24 A structure of 4 BL, imaged here at an apparent height of (0.41 ± 0.03) nm, thus has a real height of 1.4 nm. This kind of simultaneous multilayer growth is very different from growth of fractals in metalon-metal epitaxy. For instance for Ag grown on Ru(0001) at room temperature, the second layer nucleates only after almost completion of the first layer. 22



Figure 4. Height determination for amorphous clusters: (a,b) STM images, 500 mV, 20 pA (a) (0.08 ± 0.01) BL (b) (0.69 ± 0.06) BL (c) height profiles along lines indicated in panels a,b. Heights consistent with the maxima in the pixel histograms (Fig. 2d,h) are observed for individual amorphous clusters (Fig. 4) consistent with the heights of individual fractal island’s parts presented before.13,18 We conclude that the water structures on Ag(111) are up to four layers high in the investigated range between 0.1 and 3 BL. Why should amorphous ice show a layering at all? It is well established that fluid water shows a pronounced layering close to metal surfaces due to ordering effects at surfaces and the absence of neighbors as compared to bulk water. 25 Furthermore, amorphous water formed at low temperatures was classified as a frozen equivalent of fluid water. 26 This suggests that the layering is induced by the metal surface in a similar way as in fluid water. In order to understand the striking similarities in height and roughness at different temperatures, we now investigate the island density in dependence of coverage and compare it to predictions of growth theories. Before discussing our experimental data, we recapitulate those elements of classical growth theories that are relevant here. We consider the classical growth theory by Venables,17 which leads to compact islands and should hold for the amorphous


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clusters. For the fractal islands, we consider a refined model by Amar et al.,27 which describes the growth of fractal islands. The Venables model identifies three coverage regions: nucleation, growth, and coalescence. The model by Amar identifies a fourth region, an intermediate region between nucleation and growth. In both theories, the adsorbed particles initially form a two-dimensional gas on the surface.17 With increasing coverage, the density of the gas increases up to a threshold value, at which nuclei are formed, i.e. small agglomerations of particles, which are assumed to be static.28 In this nucleation region the number of islands increases as θp with θ the coverage and p a fractional exponent that depends on the size of the critical nucleus, the nucleus that is stable against dissolution. The exponent varies between 0.5 and 1 at high temperature and is smaller than 0.29 at low temperature for three-dimensional island growth.17 In the intermediate region of the refined model the particle concentration decreases, while the island density increases at a lower rate than during the nucleation regions proportional to θ1/3. In the growth region, the island density is approximately constant in both models. This region shortens at higher temperature. In the final coalescence region, the island density decreases rapidly with coverage. In the Venables model, the maximum number of islands is observed at approx. 0.15 ML coverage, with 1 ML as full coverage.29 In the refined model, the nucleation turns to the intermediate region at approx. 3 · 10−4 ML, from the intermediate to the growth region at approximately 0.08 ML, and from the growth to coalescence region at approximately 0.4 ML.



Figure 5. Coverage dependent island density in bilayers (BL) for amorphous clusters at two deposition rates, (1.9 ± 0.3)·10−3 BL/s (black rectangles) and (1.0 ± 0.2) ·10−3 BL/s (green down triangle); fractal islands for a deposition rate of (1.9 ± 0.3)·10−3 BL (red circles); (a) on double logarithmic scale; vertical line marks highest density; fit in raising region yields an exponent of 1 for the amorphous clusters with a prefactor of (0.23 ± 0.03) nm2/BL for the higher deposition rate and of (0.15 ± 0.03) nm2/BL for the lower deposition rate and an exponent of 0.34 for fractal islands; fit in falling region from half-logarithmic plot in (b) (b) on half-logarithmic scale for coverages above 0.45 BL with exponential fits; fits yield exponents of –(0.60 ± 0.02) and –(0.36 ± 0.02). Having recapitulated the main ingredients of nucleation theory, we now compare these predictions to our experimental data starting with the amorphous clusters for two deposition rates of (1.0 ± 0.2) ·10−3 BL/s and (1.9 ± 0.3) ·10−3 BL/s. For the higher deposition rate, the island density increases linearly up to (0.44 ± 0.03) BL (Fig. 5a, black squares, cf. Fig. 2a-c) without a marked increase in average island size. The maximum in island density is at (0.11 ± 0.01)/nm2. A separate growth region with constant island density, expected from Venables nucleation theory, 12 Environment ACS Paragon Plus

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is not observed. In the second region the amorphous clusters are so close that they merge and thus the island density decreases. For the lower deposition rate, the initial linear rise has the same slope and reaches a maximum island density of (0.06 ± 0.01) /nm2 at the same coverage of (0.44 ± 0.03) BL (Fig. 5a, green triangles). Thus half of the deposition rate leads to half the number of islands. The directly following decrease in island density is smaller for the lower deposition rate, such that the layer is filled at approximately the same coverage. The linear fits in the halflogarithmic plot yield -(0.60 ± 0.02) and -(0.36 ± 0.02) for the higher and lower deposition rate, respectively (Fig. 5b). The thus confirmed exponential decrease of the island density corroborates layer filling. Also during formation of fractal islands at 96 K, the island density shows two regions only, here separated at a coverage of (0.72 ± 0.03) BL (Fig. 5a, circles). The increase in island density on coverage in the first region is, at θ0.34±0.04, in the range expected for the intermediate region in the growth model of Amar et al. (θ0.33). In the second region, the island density decreases exponentially due to coalescence. Again, a pure growth region with constant island density, as suggested by theory, is not resolved here, consistent with an observation of newly nucleated islands up to rather large coverage (e.g. Fig. 2e, upper left). The maximum island density of the amorphous clusters is at more than three times higher coverage than in classical nucleation theory. As this theory assumes growth of monolayer high islands, while the water islands here grow three-dimensional, this is consistent with a growth of islands that are not one but three to four layers high, as deduced in the height and roughness analysis above.



In the theory of fractal islands, the intermediate region is expected up to a coverage of 0.08 ML only. The region should thus continue only till 0.32 BL for four-layer high islands. However, we observe this region up to 0.72 ML, more than twice the expected value from theory. We summarize the main differences between our data and the theories discussed above. 17,27 First, nucleation is observed up to coalescence at both growth temperatures. Second, coalescence is delayed in the case of fractal island growth. All theories are necessarily based on assumptions, some of which might not be valid here. In particular, growth theories were developed for metal-on-metal growth, i.e. for atoms. The interaction between atoms is angle independent without an attachment barrier for additional atoms to existing nuclei. However, a highly directional hydrogen bond has to be formed between an approaching water molecule and an existing water cluster. We propose that this causes a barrier for attachment, because the molecules have to reorient to form the hydrogen bond. Indeed, the orientation of a water monomer on the surface with its hydrogen atoms pointing in parallel to the surface11 is not always favorable for forming bonds to the cluster. The energy difference between different orientations is considerable; already 20 meV between a planar geometry and one with an angle of 20◦ between the molecule and the surface plane. 30 Another difference between atoms and water molecules is their dipole moment. This dipole moment causes electrostatic repulsion for certain molecular orientations. Reorientation and dipole-dipole effects alter the sticking coefficient of the water molecules to the existing nuclei. Note that this effect leads to a locally varying sticking coefficient, in dependence of the arrangement of border molecules. Overall, a reduced sticking coefficient promotes further nucleation throughout the growth region. We thus explain the first deviation from classical growth theories by a reduced sticking coefficient of water molecules/clusters to existing islands.


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To understand the second deviation, we recall the surprising similarities of the growth scenarios at the largely differing temperatures in height, roughness, and layer filling. In addition, circular islands with diameters of the arm width of fractal islands would have apparent areas between ≈ 2.6 nm2 and ≈ 11.6 nm2 covering approximately the area range of clusters formed at low temperature (cf. Fig. 1c). Finally, we noted above that the cluster size distribution points to a mobility of nuclei in the fractal case. We thus propose that clusters of the size of the fractal arm width, or slightly smaller, diffuse during the growth of the fractal islands. Attachment of clusters to existing islands without much coagulation eventually leads to the fractal islands. Such a cluster mobility would reduce the number of newly formed nuclei and thus explains the smaller exponent during nucleation of fractal island growth than the one of amorphous cluster growth and the larger coverage for the onset of coalescence than in the model.27 Moreover, ramified islands of similar appearance as observed here were found after deposition of Au clusters on HOPG (highly oriented pyrolytic graphite), on which the particles easily diffuse due to a large lattice mismatch between adsorbate and substrate, but without complete coagulation. 31 In this case, the arms of the islands were significantly broader than the typical cluster size, pointing to a considerable coagulation. Also in this study, the heights of the clusters in [31] indicated an upward mass transport as observed here.



Figure 6. Cluster diffusion: (a,b,d) Snapshots from movies recorded at (a) 5 K (25 mV, 12 pA) (b) 15 K (25 mV, 10 pA) and (d) 19 K (25 mV, 8 pA); time in h:min:s; large circles mark positions of the water clusters (Cl) in the first image; small circle marks a monomer (M); impurities/depressions serve as markers (c) relative change in position for the two water clusters in (b). In order to proof this point, we performed diffusivity studies of water clusters, similar to our studies of CO on Cu(111) described in [32,33]. To this end, we deposited a small amount of


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water onto the surface at 12 K and imaged the same clusters for several hours. Though we are not able to unequivocally identify the exact sizes of small clusters on Ag(111) as monomers are already mobile at 5 K (Fig. 6 a), the width of the clusters suggest, at 1 to 1.5 nm, that they are slightly larger than hexamers.11 This corresponds to the main size of the amorphous clusters as well as the branch width of the fractal islands. The clusters perform a random motion on Ag(111) at 15 K (Fig. 6b,c) and are more mobile at 19 K as expected for activated diffusion (Fig. 6d). Though, the attachment of these clusters to larger ones leads to immobile clusters in the accessible temperature range up to 20 K, our diffusion experiments corroborate that clusters of the size of the branch width are highly mobile at the growth temperature of the fractal islands of 96 K.

Conclusions In conclusion, water structures formed at 20 K and at 96 K show remarkable similarities in growth despite their distinctly different shapes, compact versus fractal. The differences were traced back to different growth kinetics, with limited and high mobility of nanoclusters. Despite the different shapes, the ice layer fills the surface with islands of 3 to 4 bilayers height at both temperatures. Such a layer filling is different from the Poisson-like distribution due to limited kinetics in low-temperature growth. It rather reflects the layering of fluid water near metallic surfaces due to ordering effects at surfaces and the absence of neighbors as compared to bulk water. On a more general footing, we identify assumptions of growth models developed for metal-onmetal growth, which are not met in growth based on the formation of hydrogen bonds. Refined models for molecules with directional bonds will have to be developed in future.



AUTHOR INFORMATION Corresponding Author * [email protected] NOTES The authors declare no competing financial interests. ACKNOWLEDGMENT We thank Christopher Zaum for experimental assistance. 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. The diffusion part was supported by the German-Israeli foundation. REFERENCE LIST (1) Sharp, A. A Peek at Ice Binding by Antifreeze Proteins. Proc. Natl. Acad. Sci. USA 2011, 108, 7281 - 7282. (2) Liou, Y.; Tocilj, A.; Davies, P.; Jia, Z. Mimicry of Ice Structure by Surface Hydroxyls and Water of a Beta-Helix Antifreeze pPotein. Nature 2000, 406, 322 - 324. (3) Abbatt, J.P.D. Interactions of Atmospheric Trace Gases with Ice Surfaces: Adsorption and Reaction. Chem. Rev. 2003, 103, 4783 - 4800. (4) Rudich, Y; Benjamin, I.; Naaman, R.; Thomas, E.; Trakhtenberg, S.;. Ussyshkin, R. Wetting of Hydrophobic Organic Surfaces and Its Implications to Organic Aerosols in the Atmosphere. J. Phys. Chem. A 2000, 104, 5238 - 5245.


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TOC Graphic


Low Temperature Growth of Amorphous Water Ice on Au(111)

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