1D Hydrogen Bond Chain on Pt(211) Stepped Surface Observed by O

Jun 8, 2012 - (blue line) and the latter band in the NI spectrum (black line). The NI spectrum for a D2O multilayer obtained at 110 K is shown by a gr...
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1D Hydrogen Bond Chain on Pt(211) Stepped Surface Observed by O K-NEXAFS Spectroscopy Osamu Endo,*,† Masashi Nakamura,‡ Ryouhei Sumii,§ and Kenta Amemiya§ †

Department of Organic and Polymer Materials Chemistry, Faculty of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan ‡ Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba University, Inage-ku, Chiba 263-8522, Japan § KEK-PF, Tsukuba, Ibaraki 305-0801, Japan

ABSTRACT: 1D hydrogen bond chains of D2O molecules formed on a Pt(211) stepped surface are observed by polarization dependent near O K-edge X-ray absorption fine structure spectroscopy (O K-NEXAFS). The 1s→ σ*OD resonance at 535.0 eV in the normal incidence O K-NEXAFS spectrum corresponds to the free OD bond oriented perpendicularly to the step lines, and the resonance at 540.0 eV is related to the hydrogen bond extended in a zigzag manner parallel to the step lines. A structural model consistent with these observations is obtained from the DFT optimization, in which all of the molecules act as one hydrogen donor and acceptor in the 1D chain.

1. INTRODUCTION Hydrogen bond formation in water molecules on transition metal surfaces is a process that must be elucidated to understand many surface phenomena involving water including wetting and surface reactions such as dissociation, corrosion, and electrochemical reactions. Close-packed platinum surfaces can adsorb intact water molecules at low temperatures under ultrahigh vacuum (UHV). A very low coverage of these molecules leads to hydrogen-bonded clusters at temperatures above approximately 40 K, as the molecules gain enough thermal energy to move on the surface.1−4 With increasing coverage, water molecules wet the entire surface prior to 3D crystal growth when an ice bilayer is known to form on the Pt(111) terrace.5 These findings indicate that the water− surface interactions on the Pt(111) terrace are comparable to the interactions between water molecules. On the step of a Pt(111) surface, however, the molecules are adsorbed more tightly than on the terrace and they are therefore desorbed at higher temperatures.6 Although stability is considered to be increased primarily by a stronger interaction between water and a platinum step atom, the intermolecular interaction and the hydrogen bond formation may not be negligible. Because the step is a 1D template, the adsorbed water molecules may form 1D hydrogen bond chains on it. © 2012 American Chemical Society

The 1D hydrogen bond of water attracts interest because it can provide a fast proton transfer system.7,8 According to the Grotthuss mechanism, the rate determining process of proton transfer in water is the dissociation of the hydrogen bond of the water molecule that accepts a proton.9 The next step is a fast proton transfer that occurs as the potential barrier for the proton transfer vanishes as the intermolecular distance becomes less than 0.24 nm.10 Finally, the water molecule that donated the proton accepts a hydrogen bond from a surrounding molecule. This mechanism is affected by the molecular orientation and the number of hydrogen bonds per molecule. On the 1D water chain, in which one hydrogen atom donates (or accepts) within each water molecule, a proton can be simply transferred in sequence without the drastic molecular reorientation and extra hydrogen bond breaking, and, hence, the process is expected to be very fast. The molecules preferentially adsorbed on the step sites have been studied in detail on the single crystal stepped surface.11−13 It has been recently revealed by using surface X-ray scattering (SXS) that the water molecules are aligned 1D in a zigzag Received: March 15, 2012 Revised: June 6, 2012 Published: June 8, 2012 13980

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perpendicular to the C 2 axis of the D 2 O molecule, respectively.15,16 The origin of the band at 540.0 eV in the spectrum of the D2O molecule on the Pt(211) step is considered to be similar to that of the bands at 537.5 and 540.5 eV of the ice multilayer. Because this band is prominent in the NI spectrum and the surface parallel component of the electric field vector of the incident X-ray is parallel to the step lines for this spectrum, the band indicates hydrogen bond formation along the step line. In contrast, the bands observed at lower energies in the GI spectrum of the D2O molecule on the step are characteristic of D2O directly adsorbed on the Pt surface. Similar bands were observed for an ice bilayer on Pt(111),5 and the band at 533.0 eV is assigned to the resonance from the 1s orbital to the 2p lone pair orbital, which becomes partly unoccupied as a result of the bond formation between oxygen and platinum. The band at 538.5 eV is related to the ODPt three centered orbital for the molecule adsorbed with a configuration in which the deuterium is oriented to the surface platinum (D-down). The structural model is discussed in detail later. Parts a and b of Figure 2 show the O K-NEXAFS spectra for D2O on the stepped Pt(211) surface, which was obtained by

manner on the Pt(211) stepped surface.12 As SXS can only determine the position of the oxygen atom, the molecular orientation and the direction of the hydrogen bond remains unresolved. Although calculation results predict the 1D hydrogen bond formation on Pt(322)8 and Pt(533),11 the chain structure should be studied experimentally. Polarization dependent near O K-edge X-ray absorption fine structure spectroscopy (O K-NEXAFS) is a powerful technique used to observe the direction of the vacant orbital to which the 1s electron is excited with an intensity proportional to cos2θ.14 Here, θ denotes the angle between the direction of the transition dipole moment of the observed orbital and that of the electric field vector of the incident X-ray. The energy of the 1s → σ*OH transition is strongly affected by the formation of the hydrogen bond, especially when the molecule acts as the hydrogen donor.15,16 Therefore, this technique is useful to analyze the formation and direction of the hydrogen bond. In this study, we measured the O K-NEXAFS and O 1s X-ray photoelectron spectra (XPS) of deuterium water (D2O) adsorbed on a stepped Pt(211) surface under UHV. The deuterium water was used instead of water in order to minimize irradiation damage. The structural model is discussed with reference to the results of the DFT and ab initio spectral calculations.

2. RESULTS AND DISCUSSION 2.1. O K-NEXAFS and O 1s XPS. Figure 1 shows the O KNEXAFS spectra for D2O on a stepped Pt(211) surface, which

Figure 1. O K-NEXAFS spectra of D2O on a stepped Pt(211) surface heated to 160 K and observed at 110 K. The surface component of the electric field vector of the incident X-ray is parallel to the step lines. Black line, normal incidence (NI). Blue line, grazing incidence (GI). Green line, NI spectrum for an ice multilayer obtained at 110 K. Figure 2. O K-NEXAFS spectra of D2O on a stepped Pt(211) surface heated to 155 K(a) and 165 K (b) observed at 110 K. The surface component of the electric field vector of the incident X-ray is perpendicular to the step lines. Black line, NI. Blue line, GI.

was obtained by heating a multilayer to 160 K. At this temperature, water molecules on the terrace are desorbed and those adsorbed on the step remain.12 The surface parallel component of the electric field vector of the incident X-ray is parallel to the step lines in these spectra. The locations of three bands are marked with vertical lines at 533.0, 538.5, and 540.0 eV. The former two bands are prominent in the GI spectrum (blue line) and the latter band in the NI spectrum (black line). The NI spectrum for a D2O multilayer obtained at 110 K is shown by a green line for comparison. The multilayer spectrum reproduces that previously reported for bulk ice.15−17 The small feature at 535.0 eV is related to the 1s→ σ*OD resonance of the broken hydrogen bond,17 which may appear at the ice surface. The large bands located at 537.5 and 540.5 eV are assigned to the 1s→ σ*OD resonances of the OD forming hydrogen bonds.15,16 The resonances located at 537.5 and 540.5 eV contain the contribution of the moment parallel and

heating a multilayer to 155 K(a) and 165 K(b). The surface parallel component of the electric field vector of the incident Xray is perpendicular to the step lines. The spectra at 155 K (part a of Figure 2) are almost the same in shape as those obtained with parallel polarization (Figure 1). The 1s→ σ*OD resonance of the OD forming hydrogen bonds is observed at 540.0 eV in the NI spectrum (black line), and that of the ODPt for the Ddown molecule is observed at 538.5 eV as well as the OPt at 533.0 eV in the GI spectrum (blue line). This result suggests that the hydrogen bond network is formed in 2D across the steps when the molecules on the terrace remains. By further annealing at 165 K, the NI spectrum is considerably changed 13981

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although the GI spectrum keeps its shape (part b of Figure 2). The band centered at 540.0 eV reduces in intensity; however, the band at approximately 535.0 eV appears in the NI spectrum (black line). This new band is assigned to the 1s→ σ*OD resonance of a broken hydrogen bond or a free OD bond. From these observations, it can be concluded that hydrogen bonds across the steps are broken to leave the free OD oriented perpendicularly to the step lines as the D2O molecules are desorbed from the terrace at a temperature within a narrow region around 160 K. The hydrogen bonds oriented along the step lines with some component perpendicular to the step are unaffected during the heating. It should be noted that the D2O molecules on the step sites are desorbed and small amount of residual oxygen (probably atomic) occasionally remains on the surface above 170 K. Figure 3 shows the O 1s XPS spectrum of D2O on Pt(211), which was obtained by heating a multilayer to 165 K with an

Figure 4. Structural models for the water chains on a Pt(211) stepped surface. (a) The 1Dhydrogen bond chains with molecules nearly parallel to the surface. (b) The bilayer-like arrangement with an alternate parallel and H-down molecular plane.

hydrogen bond, and 0.285 nm for molecule 2, which accepts the hydrogen of the shorter hydrogen bond. The height of each molecule is slightly changed from its initial values (0.231 and 0.262 nm). The molecular plane of the former H2O molecule is almost parallel to the surface, whereas it is slightly (ca. 35°) inclined for the latter. The free OH bond is oriented perpendicularly to the step line in molecule 1 and is extended obliquely downward for molecule 2. In model (b), the lower molecule, which has a height of 0.225 nm and whose C2 axis is oriented perpendicularly to the step (molecule 3), donates two hydrogen atoms to the hydrogen bond with an intermolecular distance of about 0.303 nm. The upper molecule with a height of 0.314 nm (molecule 4), which accepts two hydrogen atoms, stands with one hydrogen directed to the surface (H-down) and the other free OH extended obliquely upward. The molecular height present in model (b) is considerably different from the initial values, especially for the upper molecule. The right panel of Figure 5 shows the results of an ab initio calculation that incorporates a core hole to simulate the NEXAFS spectra and that is performed using the GSCF3 code20,21 for a H2O molecule at the center of a 1D chain consisting of 9 molecules corresponding to the models shown in Figure 4 without platinum. The calculation results for an isolated molecule with the same orientation as that in the corresponding model are displayed in the left panel of Figure 5 for comparison. As the σ*OH resonance is strongly affected when the molecule donates hydrogen to the strong (or short) hydrogen bond, the resonance for molecule 1 within the model (a) (part a of Figure 5) is considerably altered from that of an isolated molecule. The resonance related to the free OH bond (along the x axis in this coordination) is generated at 534.0 eV, and that due to the hydrogen bond is observed at higher energies (537.5 and 540.5 eV) in the x and y directions for molecule 1. In contrast, the 4a1 and 2b2 resonances appear at positions similar to those of an isolated molecule for the H2O molecule 2 in model (a), which accepts hydrogen from the shorter hydrogen bond and donates to the longer bond (part b of Figure 5). Molecule 3 in model (b) weakly donates two hydrogen atoms so that the 2b2 orbital oriented along the chain axis (y direction) splits and shifts to higher energies (537.4 and 540.0 eV) (part c of Figure 5). The orbital delocalization is not prominent due to weak hydrogen bond interactions. The 4a1 orbital appears at a slightly higher energy (535.5 eV) in the direction perpendicular to the chain (x and z directions). Molecule 4 in model (b) only accepts hydrogen from the weak hydrogen bond with one hydrogen directed toward the

Figure 3. O 1s XPS spectra of D2O on a stepped Pt(211) surface heated to 165 K and observed at 110 K with an excitation energy of 650 eV. The binding energy was calibrated by referring to the Pt 4f7/2 peak at 71.2 eV.

excitation energy 650 eV. The main peak located at 532.9 eV is assigned to the oxygen of intact D2O molecules. The O 1s binding energies reported in the literature for D2O molecules are 532.3 eV on Pt(111)18 and 533.0 eV on Ru(0001).19 The value of the 1s binding energy 532.9 eV suggests that the absorption step can exist at this energy (Fermi level step)14 in the O K-NEXAFS spectra and it may contribute to the feature around 533.0 eV. A small shoulder at a lower binding energy is fitted with a peak located at 530.9 eV, and this is attributed to atomic oxygen or OD. The binding energies of O and OD on Pt(111) are reported to be 529.9 and 530.1 eV18 respectively and that of OD on Ru(0001) 530.8 eV.19 The area intensity ratio of the two peaks in Figure 3 is about 8:1. The lower binding energy peak is occasionally observed and can be ascribed to residual oxygen species on the surface. It should be noted that the irradiation effect seems negligible because no spectral shape change occurs during the repeated measurements. 2.2. Structural Model. Figure 4 shows the structural models of water on a Pt(211) stepped surface obtained using a DFT optimization for two initial configurations: the oxygen position was adopted from the SXS results12 for both the models and the direction of the molecular plane was set to be parallel to the surface (a) and alternately perpendicular (b). The model (b) used the 1D chain contained in the ice-bilayer model of water on Pt(111) as a reference.5 The total energy for these two models is almost equivalent. In the model (a), unequally spaced hydrogen bonds are formed in 1D with bond lengths of 0.276 and 0.322 nm. The molecular height is 0.235 nm for molecule 1, which donates the hydrogen to the shorter 13982

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Figure 6. Sum of GSCF3 calculation results of two H2O molecules in the 1D chains in Figure 5 to simulate the O K-NEXAFS spectra. Two error function steps with the inflection point at 532.9 eV referring to the Fermi level and that at 537.9 eV to the vacuum level are added to model the contribution of the continuum states. (a) The results relating to the 1D hydrogen bond chains (part a of Figure 4). (b) Results relating to the bilayer-like arrangement (part b of Figure 4). (c) Experimental spectra.

Figure 5. GSCF3 calculation results of H2O in 1D chains (right panel) and of isolated molecules which adopt the corresponding orientations (left panel). (a) The results relating to the water that donates hydrogen atoms to the shorter hydrogen bond of the 1D hydrogen bond chains (molecules 1 in part a of Figure 4). (b) Results relating to the water that donates hydrogen atoms to the longer hydrogen bond (molecules 2 in part b of Figure 4). (c) Results relating to the water that is lying on the surface in the lower layer of the bilayer-like arrangement (molecules 3 in part b of Figure 4). (d) Results related to water with one hydrogen atom directed to the surface (H-down) in the upper layer of the bilayer-like arrangement (molecules 4 in part b of Figure 4).

with the experimental spectra (c), that is the resonance attributed to the free OD bond is directed perpendicularly to the step lines and the hydrogen bond is formed along the step line in a zigzag manner. In comparison, the hydrogen bond and the free OH perpendicular to the step are not effective in model (b). Although the ODPt resonance in the GI spectra might suggest that the D-down molecule is the upper molecule in model (b), the resonance intensity in the present experimental spectra does not seem to be as strong as that of Pt(111);5 also, the height of this molecule deviates from the SXS results.12 Therefore, model (a) is more plausible for the structure of water on a stepped Pt(211) surface. Model (a) indicates that all water molecules may donate or accept one hydrogen atom and that the molecular reorientation seems to occur readily when a proton is transferred along this 1D hydrogen bond chain. Further effort of the molecular orbital calculation with a core hole should be required for a more detailed description of the NEXAFS spectra of water on platinum.

platinum surface and the other obliquely upward. The latter can be a free OH bond, and the former should give rise to a three centered OHPt bond. As the latter bonding effect is not included in the present GSCF3 calculation without platinum, the spectra in part d of Figure 5 seems almost unaffected by the chain formation and resembles that of an isolated molecule. Figure 6 shows the sum of the calculation results for the two molecules of each model. The Gaussian functions centered at the resonance energy having fwhm of 1.5 eV (6.0 eV) below (above) the ionization potential are adopted taking the experimental resolution and the vibrational and lifetime broadening effects into consideration. Two error function steps with the inflection points at 532.9 eV for the Fermi level and at 537.9 eV for the vacuum level are added to model the contribution of the continuum states.14 The work function of the Pt(211) surface was estimated to be 5.0 eV.22 The simulation results for model (a) seem to be in good agreement

3. CONCLUSIONS D2O molecules adsorbed on the step of a Pt(211) surface form 1D hydrogen bond chains. The NI O K-NEXAFS spectra show resonances corresponding to the free OD bond oriented perpendicularly to the step lines at 535.0 eV and to the 13983

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hydrogen bond extended in a zigzag manner along the step lines at 540.0 eV. In the GI O K-NEXAFS spectra, the resonances related to OPt and ODPt bond formation were observed at 533.0 and 538.5 eV, respectively. The structural model obtained using DFT optimization and consistent with the O K-NEXAFS spectra suggests a chain structure with one hydrogen donation and acceptance per molecule for two different hydrogen bond lengths.

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REFERENCES

(1) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211−385. (2) A., M.; Henderson. Surf. Sci. Rep. 2002, 46, 1−308. (3) Michaelides, A.; Ranea, V. A.; de Andres, P. L.; King, D. A. Phys. Rev. Lett. 2003, 90 (216102), 1−4. (4) Hodgson, A.; Haq, S. Surf. Sci. Rep. 2009, 64, 381−451. (5) Ogasawara, H.; Brena, B.; Nordlund, D.; Nyberg, M.; Pelmenschikov, A.; Pettersson, L. G. M.; Nilsson, A. Phys. Rev. Lett. 2002, 89 (276102), 1−4. (6) Morgenstern, M.; Michely, T.; Comsa, G. Phys. Rev. Lett. 1996, 77, 703−706. (7) Scipioni, R.; Donadio, D.; Ghiringhelli, L. M.; Site, L. D. J. Chem. Theory Comput. 2011, 7, 2681−2684. (8) Meng, S.; Wang, E. G.; Gao, S. Phys. Rev. B 2004, 69 (195404), 1−13. (9) Agmon, N. Chem. Phys. Lett. 1995, 244, 456−462. (10) Komatsuzaki, T.; Ohmine, I. Chem. Phys. 1994, 180, 239−269. (11) Grecea, M. L.; Backus, E. H. G.; Riedmu1ller, B.; Eichler, A.; Kleyn, A. W.; Bonn, M. J. Phys. Chem. B 2004, 108, 12575−12582. (12) Nakamura, M.; Sato, N.; Hoshi, N.; Soon, J. M.; Sakata, O. J. Phys. Chem. C 2009, 113, 4538−4542. (13) van der Niet, M. J. T. C.; den Dunnen, A.; Juurlink, L. B. F.; Koper, M. T. M. J. Chem. Phys. 2010, 132 (174705), 1−8. (14) Stö hr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin Heidelberg New York Tokyo, 1992. (15) Cavalleri, M.; Ogasawara, H.; Pettersson, L.; Nilsson, A. Chem. Phys. Lett. 2002, 364, 363−370. (16) Nilssona, A.; Ogasawara, H.; Cavalleri, M.; Nordlund, D.; Nyberg, M.; Wernet, P.; Pettersson, L. G. M. J. Chem. Phys. 2005, 122 (154505), 1−9. (17) Nordlund, D.; Ogasawara, H.; Wernet, P.; Nyberg, M.; Odelius, M.; Pettersson, L.; Nilsson. Chem. Phys. Lett. 2004, 395, 161−165. (18) Schiros, T.; Näslund, L.-øA.; Andersson, K.; Gyllenpalm, J.; Karlberg, G. S.; Odelius, M.; Ogasawara, H.; Pettersson, L. G. M.; Nilsson, A. J. Phys. Chem. C 2007, 111, 15003−15012. (19) Andersson, K.; Nikitin, A.; Pettersson, L. G. M.; Nilsson, A.; Ogasawara, H. Phys. Rev. Lett. 2004, 93 (196101), 1−4. (20) Kosugi, N.; Kuroda, H. Chem. Phys. Lett. 1980, 74, 490−493. (21) Kosugi, N.; Shigemasa, E.; Yagishita, A. Chem. Phys. Lett. 1992, 190, 481−488. (22) Besocke, K.; Krahl-Urban, B.; Wagner, H. Surf. Sci. 1977, 68, 39−46. (23) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169−11186. (24) Kresse, G.; Furthm, J. Comput. Mater. Sci. 1996, 6, 15−50. (25) http://www.vasp.at/. (26) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758−1775. (27) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671− 6687.

All experiments were conducted at the soft X-ray beamline BL7A of the Photon Factory at the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK-PF). A mechanically polished single crystal of Pt(211)(SPL) was cleaned under UHV by Ar+ sputtering (1 kV), O2 annealed (O2 pressure: 2 × 10−7 Torr, 700 K), and flush annealed at 1100 K. The formation of the (1 × 1) surface was checked using LEED and the cleanliness was verified via the C and O K-edge absorption intensity. After cooling the surface to 110 K with liquid nitrogen, 10 L (5 × 10−8 Torr ×200 s) of D2O was dosed from a variable leak valve. The sample was heated to 155−165 K for 5 min for the extra molecules to be desorbed. The O K-NEXAFS spectra were obtained at 110 K using the partial electron yield method with a retarding voltage of 400 V. The incidence angle with respect to the surface normal was set to 0° (normal incidence, NI) and 75° (grazing incidence, GI). The azimuth dependence was checked by turning the surface. The electron yield was normalized with the incident intensity of the X-ray source monitored via the Au mesh current, subtracting a linear background, and renormalizing using the value at the excitation energy of 550 eV. The O 1s XPS spectra was measured at 110 K with an excitation energy of 650 eV. The binding energy was calibrated by referring to the Pt 4f7/2 peak at 71.2 eV. DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP).23−26 The system was modeled assuming two water molecules in a unit cell with an 11 layer slab as a platinum substrate, sampled by a grid of 10 × 10 × 1 k-points. A relatively large number of platinum layers was needed for appropriately describing the stepped surface. The lower 3 layers were fixed during the optimization. A cutoff energy of 400 eV and generalized gradient corrections (GGA) PW91 were applied.27 The ab initio calculations incorporating a core hole structure were carried out using the GSCF3 code20,21 for nine H2O molecules with a core hole introduced at the site of the oxygen atom of the center H2O molecule.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The O K-NEXAFS measurements were performed under the approval of the Photon Factory Program Advisory Committee (Proposal No.2009G102 and No. 2008G074). This work was supported by Grant-in-Aid for Young Scientists (B) (No. 22760027). The authors are grateful for the theoretical support of the GSCF3 calculation and the fruitful discussion to Prof. Kosugi (UVSOR). 13984

dx.doi.org/10.1021/jp302509k | J. Phys. Chem. C 2012, 116, 13980−13984