Highly Proton-Ordered Water Structures on Oxygen Precovered Ru

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Highly Proton-Ordered Water Structures on Oxygen Precovered Ru{0001} N. Avidor,† H. Hedgeland,‡ G. Held,§ A. P. Jardine,‡ W. Allison,‡ J. Ellis,‡ T. Kravchuk,† and G. Alexandrowicz*,† †

Shulich Faculty of Chemistry, Technion, Haifa 32000, Israel Cavendish Laboratory, University of Cambridge, JJ Thomson Ave, Cambridge, CB3 0HE, United Kingdom § Department of Chemistry, University of Reading, Whiteknights, RG6 6AD, United Kingdom ‡

ABSTRACT: We present helium scattering measurements of a water ad-layer grown on a O(2  1)/Ru(0001) surface. The adsorbed water layer results in a well ordered helium diffraction pattern with systematic extinctions of diffraction spots due to glide line symmetries. The data reflects a well-defined surface structure that maintains proton order even at surprisingly high temperatures of 140 K. The diffraction data we measure is consistent with a structure recently derived from STM measurements performed at 6 K. Comparison with recent DFT calculation is in partial agreement, suggesting that these calculations might be underestimating the contribution of relative water molecule orientations to the binding energy.

’ INTRODUCTION The adsorption properties of water molecules on metal surfaces is a topic of fundamental importance and has attracted a huge range of experimental and theoretical research work.1 3 In particular, significant efforts have been devoted to understanding the adsorption of water on a ruthenium surface, a system that has produced significant controversy and debate for almost three decades.4 13 A common picture that emerges from various past studies is that H2O molecules can adsorb molecularly intact at low enough temperatures (105 K), however, partial dissociation of H2O takes place when the substrate is heated close to the desorption temperature.9,10,14 In addition, other perturbations such as trace amounts of coadsorbates and irradiation with electrons have also been shown to lead to partial dissociation.10,11,15 In recent years, several studies have focused on the influence of preadsorbed oxygen on the H2O/Ru(0001) system.5,16 18 On the one hand, small amounts of oxygen induce partial dissociation of the water molecules. On the other hand, water is adsorbed intact on top of surfaces with a higher coverage of oxygen. In fact, surfaces with high coverages of oxygen, above 0.25 ML, seem to provide a substrate for more stable adsorption of intact water than the clean Ru(0001) surface.17,19 r 2011 American Chemical Society

The ability to study the structure of water layers on metals using conventional surface science techniques is limited, in particular, LEED (low energy electron diffraction) measurements suffer from the sensitivity of water surfaces to electronic probes and the weak scattering from hydrogen atoms,11,20,21 whereas STM (scanning tunneling microscopy) experiments are restricted to extremely low temperatures where the mobility of the water molecules is sufficiently slow.22,23 On the other hand, thermal helium atom scattering, offers an extremely sensitive and gentle method for studying surfaces.24 The large cross section for helium scattering from small adsorbates20 combined with the truly nondestructive nature of the technique have resulted in many surface structure studies in general20 and of water surfaces in particular.25 31 The fact that this technique produces diffraction measurements complicates the analysis with respect to real space methods; however, it also allows structures to be determined at elevated temperatures where the thermal motion makes it Special Issue: J. Peter Toennies Festschrift Received: January 8, 2011 Revised: March 21, 2011 Published: April 19, 2011 7205

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impossible to use scanning probe techniques. While low temperature measurements can be used to reveal the lowest energy structure, measurements at higher temperatures are needed to assess the energy difference between the various possible surface structures. In this manuscript we present a helium diffraction study of water adsorbed on oxygen precovered ruthenium. The oxygen layer coverage for all the presented measurements was 0.5 ML (1 ML = 1 oxygen atom per surface atom), conditions known to initially produce an ordered (2  1) oxygen phase with three domains on this ruthenium surface. We present experimental evidence that the adsorption of water on this substrate leads to the formation of a proton-ordered water layer with a (4  2) periodicity, which maintains its structural order at temperatures as high as 140 K.

’ EXPERIMENTAL METHODS The measurements were performed using the Cambridge 3He spin echo spectrometer.32 This instrument, which was built primarily to perform ultrahigh energy resolution measurements of surface dynamics,33 was used in this study as a fixed scattering geometry helium diffractometer (Θtotal = 44.4
). The probe is 3 He, with a mean beam energy of 7.5 meV. The sample temperature was controlled in the range of 120 1500 K using liquid nitrogen cooling and a UHV button heater (Heatwave Laboratories). The ruthenium crystal was cleaned using two methods, in both, cycles of sputtering (argon ions, 1 keV, 10 μA, sample temperature of 300 K) and annealing were used. In the first method, 10 min sputtering and annealing to 1450 K was used, followed by exposure to oxygen (1  10 7 mbar, 5 min, 750 K) to remove carbon contamination from the surface and a further flash anneal to 1400 K. In the second method, 20 min of sputtering was followed by a flash anneal to 1400 K. For both methods, surface helium reflectivity was 20%, indicating a clean and flat surface. The 0.5 ML oxygen layer was prepared by backfilling the chamber with molecular oxygen (2  10 8 mbar partial pressure, surface temperature of 400 K) while monitoring the helium signal up to saturation (5 min).34 The water layer was grown at 140 K, either by backfilling the chamber with water vapor or using a dosing pipe with a capillary array cap located approximately 5 cm from the crystal. No significant differences in the diffraction patterns were observed between the two different growth methods. 2D diffraction patterns were measured by rotating the surface normal within the scattering plane repeatedly for different crystal azimuths. ’ RESULTS AND DISCUSSION Considering first the oxygen growth, Figure 1a shows the intensity of the specularly scattered helium beam as a function of oxygen exposure, typically referred to in the literature as an uptake measurement. Two obvious features are a small peak at intermediate exposures followed by a second more pronounced increase in intensity which saturates for exposures larger than ∼9 L. Figure 1b shows the diffraction pattern obtained after dosing to saturation (11 L), measured along two high symmetry crystal azimuths. In both cases, clear half order peaks (ΔK = 2.37 Å 1 along Æ1120æ and ΔK = 1.3 Å 1 along Æ0110æ) can be seen, corresponding to (2  2) symmetry or alternatively to three domains of a (2  1) structure, in agreement with the O(2  1)/Ru(0001) surface we expect to obtain given our experimental conditions.35,36 We attribute the

Figure 1. (a) Helium specular scattering intensity, normalized with respect to the signal from the bare ruthenium surface, as a function of oxygen exposure (oxygen uptake measurement). The surface was maintained at 400 K during the measurement. (b) One-dimensional diffraction scans performed after oxygen exposure to saturation, along the two high symmetry azimuths. The continuous line is a guide to the eye. The half order diffraction peaks are consistent with the O(2  1)/ Ru(0001) structure determined in previous studies.

Figure 2. Helium specular scattering intensity relative to that of the oxygen precovered surface as function of water exposure (water uptake measurement). The oxygen precovered surface was maintained at 140 K during the measurement. The local maxima in the signal labled with the arrow marks the conditions under which the diffraction pattern shown in Figure 3a was obtained.

smaller peak at intermediate exposures to the formation of a O(2  2) structure, known to form at an oxygen coverage of 0.25 ML.37 39 A second diffraction measurement, measured at this intermediate coverage (data not shown), showed that this second ordered phase also produces half order peaks, consistent with our identification of the 0.25 ML phase as the O(2  2) phase. Following the growth of the O(2  1)/Ru(0001) overlayer, the crystal was cooled to 140 K and then exposed to H2O vapor, using the methods described above. Figure 2 shows the specular intensity during this exposure. Initially, the intensity reduces sharply; however, this decrease slows down and a small local 7206

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Figure 3. (a) 2D diffraction scan of H2O/O(2  1)/Ru(0001) at 140 K. The color plot shows the logarithm of the intensity after subtracting the slow varying background (second order polynomial function) from the data for clarity. (b) Identification of the diffraction peaks according to the standard notation. Note the two missing diffraction spots discussed in the text.

maximum of the signal can be observed at ∼2.1 L, suggesting the onset of a relatively ordered structure at this intermediate exposure. Further exposure leads to a further signal reduction and eventually what seems like an asymptotic decay leveling off at approximately 3% of the intensity from the oxygen precovered surface. In Figure 3a we present 2D diffraction measurements of the surface after stopping the water dose at “Point A” (as marked in Figure 2), corresponding to the local maximum point seen in the uptake curve. The pattern contains 10 different spots that can be seen above the noise level, verifying that indeed the local maximum in the water uptake curve corresponds to the creation of some well ordered surface structure. In Figure 3b these spots are schematically redrawn and labeled according to standard notation. Note that Figure 3a shows the intensity on a logarithmic scale and that the data was processed as described in the caption of Figure 3; the intensities of the different observed spots vary by up to a factor of 150. All spots seen in Figure 3a are part of a (4  4) diffraction pattern, indicating that the real space structure has either a (4  4) translational symmetry or is composed of three domains of coexisting (2  4) superstructures rotated by 120
with respect to each other. The specular peak is located at the apex of the triangular region ΔK = [0,0]. (To not saturate the detector, the center of the specular peak was not included within the 2D diffraction scan.) Note that along the Æ0110æ direction two spots are missing, which would be expected in the complete (4  4) diffraction pattern, namely, (0,1/4) and (0,3/4). From the signal-to-noise ratio we estimate that, if these peaks exist, they should be at least 400 times smaller than the strongest feature. The systematic absence of every second diffraction spot is a sign for glide-line symmetry with the glide line pointing along this Æ0110æ direction, which requires a rectangular unit cell, that is, (2  4). The diffraction pattern together with the systematic absences indicates that the H2O on a 0.5 ML of oxygen precovered ruthenium, after dosing at 140 K, is characterized by a superstructure with three domains of (2  4) periodicity, rotated by 120
, and a glide line along the Æ0110æ direction. Figure 4a shows a model proposed by Maier et al.40 based on a DFT interpretation of 6 K STM data, which satisfies both of the conditions mentioned above and correspondingly gives rise to the experimental diffraction pattern we measured. (The comparison is performed only in terms of symmetry, i.e spot positions and missing spots. The relative intensity of the diffraction pattern requires a detailed scattering calculation which is beyond the

Figure 4. Different structure models for the H2O/O(2  1)/Ru(0001) system. Large-empty (white) and medium-gray circles represent Ru atoms and preadsorbed oxygen atoms correspondingly. Water molecules can be identified by their shape. (a) Proposed water structure on O(2  1)/Ru(0001). The oxygen atoms reconstruct to form a honeycomb structure with two alternating orientations of the water molecules (“two-orientation configuration”). The hydrogen orientation forms a glide line (dashed line) and a total (2  4) symmetry of a PG plane group (unit cell: red full line). Gladys et al. have suggested this structure before for the H2O(2) species,17 as discussed in the text. (b) A different structure for water adsorbed on O(2  1)/Ru(0001), where the water molecules are all aligned (“single-orientation configuration”). A recent DFT study of this configuration40 calculated the energy as identical to that of the “two-orientation configuration” shown to the left.

scope of this work.) In this model, the oxygen atoms (atomic O and H2O) alone constitute a honeycomb structure with a (2  2) periodicity, as proposed earlier by Gladys et al.17 The honeycomb structure in Figure 4a requires a site change of every second oxygen atom from the original (2  1)-O structure, thus, freeing one in four ruthenium atoms to interact directly with a water molecule, similar honeycomb reconstructions have been previously reported for coadsorption of 0.5 ML oxygen with CO and NO on the same Ru(0001) surface.41,42 The water molecules within the honeycomb cells in the present structure have two distinct orientations with their hydrogen atoms pointing toward two of the three neighboring oxygen atoms on fcc adsorption sites. We refer to this model as the “two-orientation” model. Note that the existence of these two distinct orientation increases the size of the original (2  2) unit cell to a (2  4) cell, whereas the relative angle and distance between the two water molecules leads to the glide line symmetry. The honeycomb model was first proposed by Gladys et al.17 to explain their photoelectron spectroscopy data from the watercovered (2  1) oxygen layer. Gladys et al. suggested this particular model for an adsorbed species of water they referred to as 7207

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The Journal of Physical Chemistry A H2O(2), which appeared in the spectrum after annealing to temperatures between 185 and 210 K. More recently, Maier et al.40 have performed a combined STM and DFT study of water adsorbed on a O(2  1)/Ru(0001) surface. In their experiment they prepared the water ad-layer at 140 K, followed by cooling the surface to 6 K, where the STM measurements were performed. With the help of the accompanying DFT calculations they interpreted the STM images of the 6 K surface as corresponding to the honeycomb “two-orientation” water structure shown in Figure 4a.40 Maier et al. also performed STM measurements of the surface after annealing to above 180 K and cooling to 6 K, annealing temperatures which are closer to the conditions under which the H2O(2) peak was originally observed by Gladys et al.17 Maier et al. found that most of the water had desorbed after annealing to 180 K, whereas the remaining water molecules created linear structures in the 6 K STM images. The water molecules completely disappeared from the STM image after annealing to 185 K and cooling back to 6 K. In the present experiments, we found that all the quarter-order diffraction spots disappeared after annealing the surface to 185 K for 3 min and cooling back to 140 K, leaving us with a (2  2) diffraction pattern. This result is consistent with the desorption of most of the water molecules and a surface that is predominantly covered by a pure oxygen over layer. Our observations are in general agreement with the observation made by Maier et al.; nevertheless, our data is also consistent with partial water desorption at these higher temperatures, where the remaining water has a (2  2) structure. The periodicity and glide-line symmetry of the He diffraction pattern in Figure 3 strongly suggests that the same structure extracted from the 6 K STM measurements remains intact to a temperature of at least 140 K. Thus, the proton order of the water molecules on top of the oxygen preadsorbed ruthenium surface is maintained up to temperatures close to where desorption and dissociation would have normally taken place on the bare ruthenium surface. It is remarkable that in LEED only a (2  2) pattern is observed.4 Considering that the scattering cross section of hydrogen is much higher for He atoms than for electrons, the extra spots of the apparent (4  4) pattern observed in Helium atom diffraction can therefore be assigned predominantly to proton order with larger periodicity. Maier et al.40 performed DFT calculations on relatively large unit cells (up to 2  12 Ru unit cells) examining six different configurations with up to six distinct orientations of the water molecules on top of the honeycomb oxygen structure. The two energetically most favorable structures are the “two-orientations” structure, shown in Figure 4a, and a single orientation structure, where all water molecules are aligned, as shown in Figure 4b, which does not have a glide line. According to the DFT calculations, these two structures are energetically equivalent, whereas the adsorption energies are only slightly lower (ranging between 17 and 35 meV) for the other four calculated configurations. The comparison of the different structures lead the authors to conclude that the factor determining the stability of the layer is the number of water molecules sharing one oxygen atom and not the relative orientation of the molecules.40 As we mentioned earlier, the systematic absence of spots in the diffraction pattern in Figure 3 is consistent with the “twoorientation” structure, which has a (2  4) periodicity and the glide line symmetry. While the single orientation model (Figure 4b) is clearly incapable of producing the quarter order spots we measured, we cannot categorically rule out coexistence

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of this structure with the two-orientation structure without a nontrivial scattering intensity analysis, because the half-order spots within the diffraction pattern that we measure are common to both structures. However, because the calculated energy differences between these different structures are comparable to both the entropy contribution to the free energy43 and to the thermal energies at the elevated temperatures of the experiment (140 K), we would expect considerable proton disorder. Correspondingly, this should result in the absence of a helium diffraction pattern, in analogy with the recent explanations for the diffraction of helium from water structures on the bare ruthenium surface.31 Therefore, the well-defined diffraction pattern that we observe at 140 K suggests that the DFT calculations are possibly underestimating the energy differences between the “two-orientation” structure and alternative ad-layer structures with different water molecule orientations.

’ CONCLUSION Using He atoms diffraction, we observe an apparent (4  4) diffraction pattern for water coadsorbed with 0.5 ML oxygen on Ru(0001). Systematic absences of spots indicate a glide line symmetry and a superstructure, which consists of three distinct domains of (2  4) periodicity rotated by 120
. The diffraction data are compatible with a (2  2) honeycomb arrangement of oxygen atoms in which the embedded water molecules show hydrogen-bond order with a larger (2  4) periodicity. The fact that in LEED only a (2  2) superstructure is observed indicates that the larger superstructure is only due to proton order, which is observed in HAS because of the higher scattering cross section between He atoms and protons. The diffraction pattern we measured suggests that the structure inferred recently from 6 K STM experiments still exists at 140 K. The observation of proton order which persists up to temperatures close to the desorption temperature is in contrast with the case of water on a bare ruthenium surface, where molecularly intact molecules are believed to be proton disordered.3 The persistence of proton order at these elevated temperatures also indicates that the energy difference associated with the relative orientation of water molecules on the surface is probably higher than predicted by recent DFT calculations of this system. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors would like to thank Prof. Andrew Hodgson and Prof. Micha Asher for useful discussions and to thank Dr. Pepa Cabrera-Sanfelix for making the manuscript of ref 40 available to us prior to publication. This work was funded by the German Israeli foundation for scientific research and development (GIF). ’ REFERENCES (1) Thiel, P.; Madey, T. Surf. Sci. Rep. 1987, 7, 211. (2) Henderson, M. Surf. Sci. Rep. 2002, 46, 1. (3) Hodgson, A.; Haq, S. Surf. Sci. Rep. 2009, 64, 381. (4) Doering, D.; Madey, T. Surf. Sci. 1982, 123, 305. (5) Thiel, P.; Hoffmann, F.; Weinberg, W. Phys. Rev. Lett. 1982, 49, 501. (6) Feibelman, P. Science 2002, 295, 99. 7208

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