Hydroxyl Chain Formation on the Cu(110) Surface: Watching Water

Oct 21, 2008 - The formation of hydroxyl chains from water dissociation on the Cu(110) surface has been studied by using a combination of scanning tun...
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17672

J. Phys. Chem. C 2008, 112, 17672–17677

Hydroxyl Chain Formation on the Cu(110) Surface: Watching Water Dissociation Junseok Lee,†,§ Dan C. Sorescu,‡ Kenneth D. Jordan,† and John T. Yates, Jr.*,†,§ Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260, U.S. Department of Energy, National Energy Technology Laboratory, Pittsburgh, PennsylVania 15236, and Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22904 ReceiVed: August 20, 2008; ReVised Manuscript ReceiVed: September 9, 2008

The formation of hydroxyl chains from water dissociation on the Cu(110) surface has been studied by using a combination of scanning tunneling microscopy (STM), electron stimulated desorption ion angular distribution (ESDIAD), temperature programmed desorption (TPD), and density functional theory (DFT) calculations. Annealing the D2O-covered surface to a temperature of ∼200 K leads to desorption of D2O molecules and produces a zigzag structure due to adsorbed OD groups with a periodicity of 5 Å along the direction in the STM image. Coadsorption of O2 promotes the water dissociation reaction and produces hydroxyl chains with much higher coverage. ESDIAD measurements show a two-beam pattern consistent with OD(a) species inclined ∼40° with respect to the surface normal and orientated along the azimuth. The calculations reveal the existence of stable chain structures comprised solely of hydroxyl groups as well as of interacting water and hydroxyl groups that are consistent with the observed STM image. I. Introduction The structure of water, both intact and dissociated, on metal surfaces, has been the subject of numerous studies.1-11 The decomposition of water on metal surfaces leads to the production of surface hydroxyl groups, which can play a role in corrosion and catalytic reactions.1,2,7,8 In this work, we focus our attention on the arrangement of hydroxyl groups resulting from water dissociation on the Cu(110) surface.9-11 At low temperatures, water adsorbs intact on the Cu(110) surface, but for temperatures near 180 K hydroxyl formation occurs in competition with water desorption.11-15 A wide range of experimental techniques has been used to study hydroxyl groups on Cu(110). Vibrational spectroscopy, specifically, infrared reflection absorption spectroscopy (IRAS) and electron energy loss spectroscopy (EELS), have identified adsorbed hydroxyl species after heating the Cu(110) surface with adsorbed water to T > 200 K.16,17 More recent X-ray photoelectron measurements indicate that OH formation occurs at temperatures as low as 150 K, but the observation of OH groups at such low temperatures is likely due to the presence of oxygen atoms on the surface.10 Temperature programmed desorption (TPD) studies of H2O desorption from the water/Cu(110) surface show a single desorption peak around T ) 170 K, while on the oxygen-modified surface there are oxygen-induced H2O TPD peaks at T ) 200, 235, and 290 K.12 XPS studies indicate that for the oxygen-free surface the peak at T ) 170 K is due to water molecules adsorbed on the surface and that for the oxygenmodified surface the peaks at T ) 200 and 235 K result from hydroxyl-water complexes and that the peak at T ) 290 K corresponds to water production from hydroxyl-only species.9,10 ESDIAD studies of hydroxyl species on the Cu(110) surface reveal a two-beam pattern of H+ ions directed in a plane parallel to the azimuth indicating that the O-H bonds are tilted * To whom correspondence should be addressed. E-mail: johnt@ virginia.edu. † University of Pittsburgh. ‡ National Energy Technology Laboratory. § University of Virginia.

in this plane.15 Low-energy electron diffraction (LEED) studies of the hydroxyl species on Cu(110) show a streaky pattern along the azimuth indicating chain growth along the azimuth.9,12 Finally, several theoretical studies of hydroxyl groups on Cu(110) have appeared.18-20 In spite of this large body of research, the structure of the hydroxyl-covered surface has remained elusive. Here we report an investigation of the structure of hydroxyl (OD(a)) species on the Cu(110) surface using a combination of experimental (TPD, ESDIAD, STM) techniques and density functional theoretical (DFT) calculations. The experiments provide evidence for a hydroxyl chain structure different from that found on the Ag(110) surface, and the calculations permit us to narrow down the possible structures responsible for the observed spectra. II. Experimental and Theoretical Methods The ESDIAD and TPD experiments described in this paper were performed in an ultra high vacuum (UHV) chamber with a base pressure of about 1 × 10-11 mbar and an operating pressure of about 4 × 10-11 mbar. The Cu(110) single crystal could be cooled down to T ) 81 K using liquid nitrogen and heated up to 900 K by resistive heating. The crystal was cleaned by cycles of argon-ion bombardment followed by annealing to T ) 800 K. The cleanliness of the cleaned Cu surface was examined by Auger electron spectroscopy (AES); the amount of oxygen was below the detection limit of the CMA Auger spectrometer (Perkin-Elmer). The Cu(110) crystal was then exposed to D2O (99.5%, Aldrich) for TPD and ESDIAD experiments at T ) 81 K using a calibrated microcapillary array beam doser which produced uniform gas flux while maintaining low background pressure during adsorption (∆P ) 1-2 × 10-11 mbar).21 Even a very tiny amount of oxygen impurity showed evidence of the water dissociation reaction in both ESDIAD and STM experiments. Thus D2O sample was carefully degassed to remove impurity volatile gases, especially oxygen, by many freeze-pump-thaw cycles. The TPD measurements were carried out using a UTI 100C quadrupole mass spectrometer

10.1021/jp807467x CCC: $40.75  2008 American Chemical Society Published on Web 10/22/2008

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Figure 1. (a) TPD spectra of D2O (m/e ) 20) from the O2-free Cu(110) surface. Exposures of D2O (εD2O) are 0.44, 2.07, 4.78, 9.88, and 14.57 × 1014 molecules/cm2 from the bottom TPD trace. (b) TPD spectra of D2O from the oxygen-precovered Cu(110) surface. εD2O ) 4.78 × 1014 molecules/ cm2, εO2 ) 1.08 × 1014 molecules/cm2. Dotted TPD trace is from the oxygen-free Cu(110) surface (the third TPD trace in (a)). Heating rate ) 2 K/s. A bias of -90 V was applied to the crystal during the TPD experiment to prevent QMS filament electrons from reaching the surface.

(QMS) with a heating rate of dT/dt ) 2 K/s. The crystal was biased at -90 V to prevent possible water dissociation by the stray electrons from the QMS filament during TPD experiments. D+ ions were produced by electron stimulated desorption (ESD) with 40 kHz electron pulses which have 50 ns pulse width at an energy Ve ) 200 eV. The time-of-flight (TOF)-ESDIAD measurements (carried out at T ) 81 K) allow for the separation of ESDIAD patterns containing desorbing species having different masses. A positive accelerating voltage of +30 V was applied to the Cu crystal in the ESDIAD studies to compress the ESDIAD patterns, and several bias voltages were used to determine the initial D+ ion emission angle under field-free conditions. Electron-induced damage to the adsorbed D2O layer during the ESDIAD measurements was negligible in the pulsed mode of operation of the electron gun, with typical current pulses of less than 25 pA during data acquisition. The maximum total electron fluence for an ESDIAD measurement was about 1011 electrons/cm2 for 10 min of data acquisition time. Even for 1 h of irradiation (6 × 1011 electrons/cm2), there was no evidence of hydroxyl species formation (two-beam ESDIAD pattern) due to electron-induced dissociation of water. The STM experiments made use of a tungsten tip and were carried out in a separate commercial UHV STM chamber (Omicron LT-STM). The rectangular Cu(110) sample (MaTeck, 8 mm × 5 mm × 1.5 mm) was mounted on a Ta sample plate. The sample was cleaned by Ar+ sputtering and subsequent annealing to T ) 800 K in the preparation chamber. The sample temperature during STM imaging was maintained at 5 K using liquid He. The D2O and oxygen molecules were dosed using an in situ doser after opening a port in the radiation shield to the STM chamber. The temperature of the sample increased to ∼10 K during gas dosing due to radiative heating. Following each exposure, the crystal annealed to desired temperatures and immediately to 5 K for STM imaging. Annealing was accomplished by removing the sample holder from the liquid-He cooled region, allowing radiation and conductive warming in vacuum. The adsorption properties of OH species on the Cu(110) surface were investigated using plane-wave DFT calculations carried out with the Vienna ab initio simulation package (VASP).22-24 The electron-ion interactions were described using the projector augmented wave (PAW) method of Blo¨chl25 in

the implementation of Kresse and Joubert.26 The calculations employed the PW91 generalized gradient approximation (GGA) of Perdew et al.,27,28 together with periodic boundary conditions, with the one-electron pseudo-orbitals being expanded over a plane-wave basis set with a cutoff energy of 500 eV. Calculations on bulk Cu using a Monkhorst-Pack29 set of k-points with a 11 × 11 × 11 mesh gave a lattice constant and bulk modulus of 3.6365 Å and 136.56 GPa, respectively, in excellent agreement with the corresponding experimental values of 3.6150 Å and 137 GPa.30,31 The adsorption of OH on the Cu(110) surface was studied using a (4 × 4) supercell model along the [001] × [11j0] crystallographic directions of the Cu(110) surface. The supercell model employed a vacuum region of 12 Å and five layers of Cu atoms, with the top two layers being relaxed during geometry optimizations. For the surface calculations a (2 × 2 × 1) k-point grid was used, and dipole corrections in the direction perpendicular to the surface were employed. The binding energies of the adsorption configurations identified in this work were determined using the expression Eads ) 1/n[n*Emolec + Eslab E(n*molec+slab)], where n is the number of adsorbed OH groups, Emolec is the energy of an isolated OH molecule at its calculated equilibrium bond length, Eslab is the total energy of the slab, and E(n*molec+slab) is the total energy of the adsorbate/slab system. A positive Eads is taken to correspond to a stable adsorbate/slab system. III. Results 1. Thermal Desorption of D2O. The results of our TPD measurements are consistent with those of earlier studies13,15,32 and have been used to calibrate the coverage in the ESDIAD experiments. Figure 1a shows five representative TPD spectra of D2O on the clean Cu(110) surface. At the lowest exposure of D2O (εD2O ) 4.4 × 1013 molecules/cm2), a single TPD peak appears at T ) 168 K. As the coverage of D2O increases, the desorption peak gradually increases to T ) 178 K and saturates. For exposures higher than 9.88 × 1014 molecules/cm2 (slightly over a full monolayer of D2O), a second desorption peak due to multilayer formation develops at lower temperature (∼160 K) without saturation. Figure 1b illustrates the effect of O2 coadsorption on the D2O TPD spectra. In these experiments, an O2 exposure of 1.08 ×

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Figure 2. ESDIAD measurements of D+ ions. (a) After annealing the surface covered with a full monolayer of D2O to temperatures up to T ) 200 K. The two D+ ESDIAD beams are aligned along the azimuth. (b) After annealing the coadsorbed D2O (εD2O ) 4.4 × 1013 molecules/cm2) + O2 (εO2 ) 2.7 × 1013 molecules/cm2) layer to T ) 160 K.

1014 molecules/cm2 was preadsorbed on the clean Cu(110) surface at T ) 81 K. Subsequently (solid line) at the same temperature, D2O was coadsorbed at an exposure of 4.78 × 1014 molecules/cm2 (the same exposure as used for the third TPD curve in Figure 1a). As a result of the O2 exposure, the water TPD peak observed at T ) 175 K for the Cu(110) surface without oxygen coadsorption is completely washed out and three higher temperature D2O features appear at 200, 239, and 284 K, close to those reported previously.13 As noted in the Introduction, the T ) 200 and 239 K features are associated with water interacting with hydroxyl species and the T ) 284 K peak to the disproportionation reaction of hydroxyl species (OD).12,13,15 2. D+ Ion Angular Distribution by ESDIAD from OD(a) Species. At a full monolayer coverage of D2O at 81 K and in the absence of coadsorbed oxygen, only one normal D+ beam (results not shown) appears in the ESDIAD measurements, indicating that the D+ ions originate from OD moieties in undissociated adsorbed D2O directed perpendicular to the surface plane.15 After annealing this surface by heating to T ) 200 K, the one beam D+ ESDIAD pattern is replaced by a two-beam D+ ESDIAD (Figure 2a), with the tilted D+ beams observed along the azimuth, indicating O-D bonds inclined along a plane parallel to the azimuth. The one-beam to twobeam conversion process takes place only when the surface is annealed to temperatures higher than the D2O desorption temperature (i.e., T J 180 K). The effect of oxygen preadsorption was investigated by coadsorbing O2 and D2O molecules. The Cu(110) surface was exposed to O2 (εO2 ) 8.2 × 1012 molecules/cm2) and then to D2O molecules at an exposure of 4.4 × 1013 molecules/cm2 which is about 8% of the full monolayer coverage. The twobeam D+ ESDIAD pattern is now observed (Figure 2b) after annealing to T ) 160 K. Thus, as reported previously,13 water dissociation producing the two-beam D+ ESDIAD pattern occurs at a lower temperature in the presence of chemisorbed oxygen. 3. Polar Angles of Ejected D+ Ions from ESDIAD Measurements. In principle, the angular distribution of the D+ ions from the ESDIAD measurements reflects the angle of the OD groups relative to the Cu(110) crystal lattice and to the surface normal. It is clear that the OD groups are inclined parallel to the azimuth. Figure 3a shows for the hydroxyl-covered surface the measurements of the polar angle, φOD, of D+ ions relative to the surface normal as a function of the crystal bias voltage. These results have been obtained for hydroxyl species produced from the surface with coadsorbed D2O and O2 to achieve higher intensities. Ion optics simulations of the polar angle vs. crystal bias (dashed lines) produces fieldfree polar ejection angles of φ0OD ) 47° and φ0OD ) 45° for

Lee et al. D+ ions following annealing at T ) 220 and 250 K, respectively. Applying corrections for image-charge interaction and final state ion neutralization33-35 gives polar angles of 44° and 42° for the two annealing temperatures. Similar results were obtained, but with lower ESDIAD signal intensities, during D2O dissociation on the Cu(110) surface in the absence of adsorbed oxygen. Measuring the D+ ESDIAD yield as a function of the annealing temperature provides insight into the surface processes involved as the D2O layer desorbs and dissociates to form OD(a) species. Figure 3b shows these results for the pure D2O layer and for a D2O layer deposited on top of an oxygen-covered surface. For the O2-free surface, the D+ yield decreases as Tanneal is increased from 81 to about 180 K, with a rapid fall off between Tanneal ) 150 and 180 K due to the desorption of most of the adsorbed water molecules over that temperature range (The ESDIAD measurements were carried out at T ) 81 K in each case). Interestingly, there is a small peak in the D+ yield near Tanneal ) 200 K due to the formation of OD groups (likely involved in hydrogen bonding with water molecules) on the surface. The D+ yield goes to zero for Tanneal ≈ 275 K. Similar trends are seen in the case of a D2O layer deposited on top of an oxygen-covered surface. Namely, the D+ yield decreases as Tanneal is increased from 81 to 180 K (but much more gradually than for the oxygen-free surface), then increases as the annealing temperature is further raised to 200 K, and decreases to zero at a temperature near 275 K. The decrease in the D+ yield as Tanneal increases from 81 K to about 160 K, which is below the D2O desorption temperature, must be due to rearrangements in the structure of the adsorbed water layer as the temperature is increased. The increase in D+ yield upon heating from 180 to 200 K is postulated to be due to the formation of OD species as D2O dissociates. 4. Hydroxyl Chain Formation via Water Dissociation Observed by STM. Figure 4a shows the STM image of the intact D2O layer adsorbed at submonolayer coverage on the Cu(110) surface at T ) 10 K, and subsequently cooled to T ) 5 K. Intact D2O chain structures parallel to the azimuth and 2D island features are observed as previously reported.36 Upon annealing to T ) 150-180 K, the 2D islands disappear, producing a surface exclusively covered with chains of water molecules aligned along the direction. Figure 4b displays the STM structure obtained by annealing the D2O + O2 surface to T ≈ 200 K followed by cooling to T ) 5 K. The STM measurements reveal chain-like structures running now along the direction. The density of chain structures is greater for the oxygen-precovered surface than for the surface without O2 exposure, consistent with their being due to OD groups, either ”free” or associated with water molecules. Figure 4c shows a high-resolution STM image of one of the hydroxyl chain structures produced on the surface after annealing to ∼200 K. A zigzag structure with a repeat distance of 5 Å is clearly observed which is about two lattice spacings (a0 ) 2.56 Å). The apparent height of the bright features is about 0.15 Å when measured relative to the clean surface regions. The width of the chain is about 5.5 Å. Annealing at 300 K for 30 min removes these features from the surface, leaving few STM features on the surface. 5. Theoretical Investigation of Possible Structures for Hydroxyl Chains on Cu(110). The information obtained for the structure of the hydroxyl species so far includes the O-D bond angle, the formation of zigzag chains with two-lattice spacing periodicity, and the chain propagation direction. With these key elements in mind, DFT calculations have been

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Figure 3. D+ ion ejection direction and temperature-dependent D+ ion yield change. (a) Bias dependence of two-beam D+ ESDIAD patterns produced by annealing to T ) 200 and 250 K. The ion trajectories were simulated using SIMION 3D (Bechtel BWXT Idaho, LLC) program (dashed lines) which yielded the D+ ejection direction corrected for ion optical effects in the ESDIAD apparatus. (b) Temperature-dependent D+ ESD yield change for O2-free and O2 coadsorbed surface (εD2O ) 8.8 × 1013 molecules/cm2, εO2 ) 5.5 × 1012 molecules/cm2). The sample was annealed to the desired temperature and the ESD yield measurement was done for 10 min after cooling to T ) 81 K. The shaded region represents the D2O desorption temperature range. A small increase in D+ ESD yield was found between T ) 180 and 200 K annealing temperature on both surfaces (inset), and this is due to continued OD production during D2O dissociation.

Figure 4. STM topographic images at different conditions on the Cu(110) surface. (a) Submonolayer coverage of D2O on Cu(110). The chains are aligned along the azimuth. (Vsample ) 0.3 V, I ) 50 pA, 46.1 × 46.1 nm2) A high resolution image of water chain is shown in the inset. (b) After annealing the oxygen coadsorbed D2O layer to T ≈ 200 K, the linear chains become aligned along the azimuth. (0.3 V, 30 pA, 100 × 100 nm2) (c) High-resolution image of one of the linear features in (b). The distance between the repeating units is ∼5 Å. (0.015 V, 100 pA, 49.5 × 49.5 Å2). The width of the chain is ∼5.5 Å.

performed on several candidate structures for the hydroxyl (OH) chains. Five of the most stable arrangements considered are depicted in Figure 5. For each structure, the calculated energy of adsorption and the polar angle of the O-H bonds from the surface normal are indicated. The structures in Figure 5a and b have the OH species bonded to the short-bridge sites on the Cu(110) surface. These configurations are calculated to be 0.4 kcal/mol less stable and 0.3 kcal/mol more stable, respectively, than an isolated OH group adsorbed on the surface and bound by 78.7 kcal/mol. Additionally, both of these structures are consistent with the periodicity seen in the STM. However, the calculated tilt angles of the OH groups with respect to the surface normal ranges are 60.5-62.7° and 69.6-70.5° for panels a and b of Figure 5, respectively. These values are appreciably larger than the 42-44° value deduced from analysis of the D+ ESDIAD pattern. Ab initio MD simulations on Figure 5b carried out at T ) 100 K, close to the temperature of the ESDIAD experiments, give a broad distribution of the polar angles ranging from about 35° to 80°, with an average polar angle of 50° rather than the 70° value for the potential energy minimum, greatly reducing the disparity in bond angle between theory and experiment. The preferential thermal (81 K) broadening of the D+ ESDIAD beam in the direction (Figure 2) is consistent with the molecular dynamics behavior calculated here.

We also examined adsorption of OH species on rows of Cu adatoms located along the short bridge direction at the 4-fold hollow sites within the troughs of the (110) surface. Although an isolated OH group binds quite strongly (Eads ) 83.2 kcal/ mol) across two adjacent Cu adatoms, OH bonding arrangements involving Cu adatoms and consistent with the pattern in the observed STM image are appreciably less stable. Figure 5c depicts one such structure with a binding energy of 72.5 kcal/ mol per OH group, a significantly smaller energy than for the configurations in Figure 5a and b analyzed above. Additionally, the polar angle of the O-H bond in this configuration is close to 26°, considerably smaller than the observed experimental values. Hence, it seems unlikely that structures involving Cu adatoms are responsible for the observed chain structures. The most stable hydroxyl structure identified for the surface without Cu adatoms and in the absence of adsorbed water molecules involves H-bonded dimers as shown in Figure 5d. In this arrangement, the adsorption energy per OH group, including the stabilization resulting from the OH · · · OH hydrogen bond, is 80.8 kcal/mol, and the polar angle of the free O-H group is calculated to be 39.7°, which is close to that observed experimentally. However, this structure has a periodicity along the chain direction twice as large as that observed in the STM measurements. Thus, although the structure in Figure 5d is calculated to be 1.8 kcal/mol more stable per OH group than in

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Figure 5. Top and side views of possible structures of the zigzag hydroxyl chain. The theoretical adsorption energy and the polar angle of the O-H bond direction (φ(n-O-H)) with respect to the surface normal are also indicated. (Here n refers to the surface normal.) For increased clarity the surface atoms in the top layer are indicated in green while the other Cu atoms in deeper levels are colored in brown. (a) Hydroxyls on the short-bridge sites with the O-H bond pointing outward from the chain. (b) Hydroxyls on the short-bridge sites with O-H bond pointing inward. (c) Cu adatoms on the hollow sites in the troughs between Cu rows. The OH species adsorb on the 3-fold sites made by two Cu adatoms and one underlying Cu row atom. (d) Hydrogen-bonded hydroxyl chain made by units composed of an H-donor and an H-acceptor OH species arranged in a zigzag way. (e) Mixed water-hydroxyl chain with a H2O/OH stoichiometry of 2:1. (f) The calculated empty state STM image of the chain structure given in panel (e) at a voltage of +0.3 V.

Figure 5b, on the basis of the observed periodicity we rule out this species as a candidate for the experimentally observed hydroxyl chain. Finally, we considered several structures with adsorbed OH groups in the presence of adsorbed water molecules. One such arrangement is shown in Figure 5e. This is a 2:1 H2O/OH complex, related to Figure 5a, but with each OH group on one side of the Figure 5a chain being replaced by two water molecules, adsorbed approximately on top of the Cu atoms and connected among themselves by hydrogen bonding; every other water molecule is hydrogen bonded to one of the OH groups. In this structure, the water molecules are bound by 17.0 kcal/ mol, which is appreciably greater than the binding energy of an isolated water molecule on the surface (∼7.9 kcal/mol). The OH bond direction in the coadsorbed water lying closely parallel to the surface will not contribute ions to the ESDIAD pattern. From the geometrical parameters indicated in Figure 5e it is seen that both the width and the periodicity of the chain are consistent with the observed STM data. Additionally, the calculated tilt angle of the OH group relative to the surface normal is 44.4°, consistent with the observed 42-44° tilt angles deduced from ESDIAD measurements. Further insight into the electronic properties of various configurations can be obtained from the analysis of the calculated STM images. Figure 5f shows the calculated STM image corresponding to the mixed H2O-OH chain represented in Figure 5e. The STM image was calculated using the Tersoff-Hamann approach37 with a bias of 0.3 V above the Fermi level, similar to that used experimentally. The calculated STM image of Figure 5e is in good overall agreement with the experimentally observed STM image given in Figure 4c. Based on the theoretical results presented in this section we conclude that the best agreement with the experimental data is obtained for chains containing both H2O and OH species in a 2:1 ratio.

IV. Discussion 1. Hydroxyl Chain Formation on the Cu(110) Surface. LEED studies show that after annealing the Cu(110)/H2O surface to T ≈ 200 K, stripes parallel to the direction intersecting the p(2 × 1) diffraction beam appear, indicating the formation of chains parallel to the direction.9,12 XPS measurements show that at T ) 250 K only a hydroxyl-related O1s XPS signal is observed for Cu(110) surfaces exposed to H2O,9 indicating that intact H2O molecules do not persist on the surface at this temperature, whereas at T ) 200 K, both hydroxyl and water XPS signals are observed. It was previously suggested that under conditions where water remains on the surface, the two-beam ESDIAD pattern originates from 1:1 hydrogen-bonded water-hydroxyl complexes.15 A hydroxyl only structure consisting of alternating bridging OD groups on adjacent copper rows is shown in Figure 5b. The STM image calculated for this structure is consistent with that observed experimentally, although the calculated tilt angle of the OH groups (∼70°) is appreciably greater than that observed experimentally. We do not believe that this discrepancy rules out this structure as thermal averaging reduces the average tilt angle and the D+ beam may be broadened in the direction by thermal motion. The second possible structure is the 2:1 D2O:OD complex in Figure 5e. The calculated STM image for Figure 5e is close to that observed experimentally as is the calculated tilt angle. We did examine theoretically several 1:1 H2O/OH structures, but none of these gave STM images or OH tilt angles consistent with experiment. For the annealing temperatures used in the STM studies, it is expected, based on the XPS11 and TPD measurements, that water still remains on the surface coupled with OH groups. Upon heating to above 250 K, these water molecules desorb which would leave the hydroxyl chain structure in Figure 5b. In other words, the

Hydroxyl Chain Formation on the Cu(110) Surface formation of OH chain structures such as in Figure 5a and b is likely to proceed via mixed water: OH complexes such as in Figure 5e. Thus the ESDIAD/STM studies have observed an adsorbed water layer in the process of dissociation. 2. Comparison with the Hydroxyl/Ag(110) System. ESDIAD studies of hydroxyl species on the Ni and Ag fcc(110) surfaces also display two-beam ESDIAD patterns consistent with inclined hydroxyl species oriented along the azimuth as found for hydroxyl species on Cu(110).13,15,38-40 STM measurements of hydroxyl groups on Ag(110) reveal the existence of chains along the direction as for Cu(110), with all OH groups in a chain on Ag(110) apparently pointing in the same direction.41,42 The hydroxyl groups have been proposed to be located between the Ag rows, slightly displaced toward the 3-fold hollow sites.38 The distances between the close-packed rows are 4.09 and 3.61 Å for Ag(110) and Cu(110), respectively, and we speculate that the shorter interrow distance of Cu(110) makes 3-fold hollow sites less favorable than bridging sites for hydroxyl groups for this surface. V. Conclusions Water (D2O) adsorbed by itself or coadsorbed with oxygen on the Cu(110) surface produces hydroxyl (OD(a)) species upon annealing to temperatures greater than 180 or 160 K, respectively. ESDIAD measurements of the hydroxyl-covered surface reveal a two-beam D+ pattern with the deduced O-D bond angles relative to the surface normal being between 44° and 42°. Low-temperature STM measurements reveal a zigzag chain structure, with a periodicity of 5 Å, which corresponds to two lattice spacings along the close-packed direction of the Cu(110) surface. DFT calculations suggest two different types of structures for the hydroxyl chain species on Cu(110) involving D2O-OD interactions with 2:1 water/hydroxyl species being the most stable structure. Based on the calculated energetics of the various water and hydroxyl species on the Cu(110) surface, it appears likely that pure OH chain structures, if they do form, originate from water/hydroxyl chains. Acknowledgment. The authors (J.L. and J.T.Y.) thank the Department of Energy, Office of Basic Energy Sciences and the National Science Foundation (K.D.J.) for supporting this work. We also thank H. Liu for useful discussions about structures of hydroxyl on Cu(110). A grant of computer time at the Pittsburgh Supercomputer Center is gratefully acknowledged. References and Notes (1) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (2) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 5. (3) Feibelman, P. J. Science 2002, 295, 99.

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