Direct Interaction of Water Ice with Hydrophobic Methyl-Terminated Si

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Direct Interaction of Water Ice with Hydrophobic Methyl-Terminated Si(111) Iradwikanari Waluyo,†,‡ Hirohito Ogasawara,‡ Maki Kawai,§,⊥ Anders Nilsson,‡ and Taro Yamada*,§ Department of Chemistry, Stanford UniVersity, Stanford, California 94305, United States, Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan, and Department of AdVanced Materials Science, The UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan ReceiVed: July 26, 2010; ReVised Manuscript ReceiVed: September 10, 2010

The effect of ice adsorption on the electronic structure of hydrophobic methyl-terminated Si(111) surface was studied by C 1s X-ray absorption spectroscopy. The transition to the LUMO orbital of the methyl group is symmetry-forbidden according to the dipole selection rule, which governs the X-ray absorption process. Upon ice adsorption, the LUMO peak intensity is enhanced dramatically, indicating that the ice layers directly perturb the excited state orbitals of the methyl groups, increasing the asymmetric character. This is unexpected since the hydrophobic nature of the surface is confirmed by the three-dimensional growth of ice layers. We propose that the interaction between the hydrophobic methyl surface and adsorbed ice results in the formation of weak hydrogen bonds. Introduction Hydrophobicity is often described when the interaction between water molecules is stronger than the interaction between water and the hydrophobic interface. Various experimental and theoretical techniques have been used to study the interaction between water and hydrophobic interfaces.1 On surfaces, temperature-programmed desorption and infrared studies suggested that ice layers have a tendency to form three-dimensional clusters on hydrophobic self-assembled monolayer (SAM) surfaces in order to minimize contact with the surface.2 X-ray reflectivity studies have shown the presence of a depletion layer of a few angstroms thickness between water and hydrophobic SAM surfaces.3,4 Additionally, vibrational sum frequency generation spectroscopy suggested that water molecules form weak but strongly oriented hydrogen (H) bonds at a hydrophobic interface.5 These studies agree with the traditional notion of hydrophobicity mentioned previously, where water molecules preferentially form strong interactions among themselves while minimizing interactions with the hydrophobic interface. In aqueous solutions containing small hydrophobic molecules, the hydrophobic effect is manifested in a different way. Frank and Evans proposed the iceberg model, in which water forms icelike cages around small hydrophobic solutes,6 which can be easily integrated in the H-bonding network of water. Experimentally, this has been observed from femtosecond vibrational spectroscopy where water molecules around hydrophobic solutes experience slower dynamics compared to bulk water and the number of these “immobilized” water molecules is linearly correlated to the number of methyl groups in the solute.7 Therefore, water molecules can be present around the hydrophobic moiety of a molecule although they only form weak interactions. * To whom correspondence should be addressed. E-mail: tayamada@ riken.jp. † Stanford University. ‡ SLAC National Accelerator Laboratory. § RIKEN. ⊥ The University of Tokyo.

Here we focus on the weak interaction of water molecules in its frozen form with a hydrophobic methyl-terminated Si surface. Methyl (-CH3)- terminated Si(111) surface [hereafter denoted CH3:Si(111)] was chosen as the substrate because it is the best defined organic monolayer deposited on Si surfaces.8-10 The -CH3 groups, terminating every outermost Si atom on Si(111) as the smallest organic moiety, form a (1 × 1) adlattice. The structure of this ideal monolayer has been investigated using STM8-10 and vibrational spectroscopy.9 The surface is hydrophobic with a contact angle of 78 ( 2°. In addition, alkylterminated Si(111) is generally an ideal substrate for water deposition due to its resistance to oxidation in water as well as in air.11 We used X-ray absorption spectroscopy (XAS) to investigate the electronic states of CH3:Si(111) both with and without adsorbed water. XAS is an element-sensitive technique for probing the unoccupied electronic states of a system through the excitation of a core electron to an unoccupied orbital. The sensitivity to the interface is maximized since all the carbon atoms are present only at the surface making full contact with the adsorbed ice film. In addition, surfacesensitive electron detection modes of XAS are particularly suited and have been used to extensively study various adsorbate-substrate systems such as hydrocarbons,12-14 glycine,15 and water16,17 on metal surfaces. The XAS process is governed by the dipole selection rule in which a core electron from a 1s orbital can only be excited to orbitals with p character. With the development of elliptically polarized undulator beamlines at synchrotron radiation facilities, the polarization of the incident beam can be easily controlled to probe electronic orbitals in different orientations. Experimental Methods CH3:Si(111) substrate was prepared by Grignard reaction between CH3MgBr and Cl:Si(111) surface. In short, hydrogenterminated Si(111) pieces were treated in Ar-diluted Cl2 in ambient light and then subjected to heating in CH3MgBr in tetrahydrofuran. The final samples were then stored in Ar-

10.1021/jp106944d  2010 American Chemical Society Published on Web 10/15/2010

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Figure 1. C 1s XAS spectra of (top) condensed CH4 (from ref 19) and (bottom) gas-phase CH4 (from ref 18). The 3a1 peaks are indicated by the dotted line. Inset: the orbital plot of the symmetric LUMO 3a1 orbital of CH4 (from ref 19).

charged containers for transportation. Further details of sample preparation can be found elsewhere.9 Experiments were performed using the ultrahigh vacuum (UHV) surface science end station at elliptical undulator beamlines 5-1 and 13-2 at Stanford Synchrotron Radiation Lightsource (SSRL), which was equipped with an electron analyzer for X-ray photoelectron spectroscopy (XPS) and a partial electron yield detector for XAS.17 The sample was heated to 350 K for 15 min to remove carbonaceous contaminants, as confirmed by XPS measurements. D2O ice films were deposited at varying thicknesses at a deposition rate of ∼0.1 layers/s at a temperature of 100 K. We minimized beam damage by using D2O and scanning the sample continuously in front of the spectrometer during the measurements. C 1s XPS and C 1s XAS spectra measured before the deposition and after the removal of ice films were identical, indicating that neither beam damage nor water-induced oxidation had occurred. Results and Discussion First, we describe the orbitals of CH4, a simpler but closely related system that has been extensively characterized. Figure 1 shows the C 1s XAS spectra of gas-phase (bottom, from ref 18) and condensed-phase CH4 (top, from ref 19). The strongest absorption peak of gas-phase CH4 at 288 eV arises from the 2t2 LUMO+1 orbital. Features at higher energies originate from transitions to Rydberg orbitals. The spectrum of condensed CH4 only shows small changes from the gasphase spectrum; specifically, peaks due to excitations to LUMO and LUMO+1 orbitals are preserved, although they are slightly broadened, and the Rydberg fine structures are replaced with broad resonances. The C-C distance in condensed CH4 is large (∼4.16 Å20); therefore, only higher energy orbitals such as Rydberg orbitals overlap with neighboring molecules, leading to the broad resonances observed in Figure 1 (top), while the 3a1 and 2t2 orbitals remain unperturbed. The 3a1 LUMO orbital has a totally symmetric character (shown in the inset of Figure 1); therefore, the excitation of a 1s electron to this orbital is symmetry-forbidden according to the dipole selection rule. However, a small peak corresponding to a transition to this orbital can be observed at 286.9 eV, which is explained by asymmetrical C-H bond vibrations resulting in dynamic

Figure 2. C 1s XAS spectra of (top) bare CH3:Si(111) with in-plane (parallel to the surface) E-vector and (bottom) bare CH3:Si(111) with out-of-plane (perpendicular to the surface) E-vector. The A1 peaks are indicated by the dotted line. Both spectra were normalized at the maximum intensity.

symmetry distortions, thereby allowing the orbital to gain a small p character making it dipole selection rule active.18 It has been shown that when a CH4 molecule is adsorbed on a Pt surface, the LUMO symmetry is broken through orbital mixing with the metal substrate, thereby increasing the p character in the direction toward the surface and enhancing the intensity of the LUMO peak in the XAS spectrum with the E-vector polarization perpendicular to the surface.12,21 A similar phenomenon is observed in polarization-dependent C 1s XAS spectra of bare CH3:Si(111) surface displayed in Figure 2. The out-of-plane E-vector (perpendicular to the sample surface) mainly probes the Si-C bond with some contributions from the C-H bonds of the -CH3 groups. The shoulder at 285.5 eV corresponds to the LUMO orbital, analogous to the gas-phase CH4 3a1 orbital, whose symmetry has been broken through the formation of the Si-C bond. This orbital is, therefore, denoted A1 orbital. Since in a -CH3 adsorbate the Si-C bond is strongly covalent, this orbital involves more p character as evidenced by the relatively strong intensity compared to the physisorbed CH4 on Pt where all C-H bonds are still intact.12,21 In addition, the A1 peak is shifted by 1.4 eV compared to the 3a1 peak of gas-phase and condensed CH4 while the analogous peak on the spectrum of CH4/Pt(111) is unshifted.12,21 The rest of the out-of-plane spectrum is also broadened compared to gas-phase and condensed CH4. Therefore, it is reasonable to conclude that the energy shift and broadening is caused by the strong interaction between -CH3 groups and Si atoms. The in-plane spectrum (Figure 2, top) is of particular interest in investigating the electronic effect of adsorbed D2O ice, since the p character is very weak and any orbital modifications of the surface -CH3 groups could cause an increase in the p character to the A1 orbital and enhance the intensity of the peak. A weak peak at 285.0 eV corresponding to LUMO excitation is also present in the in-plane spectrum (E-vector parallel to the surface plane), although the orbital should be symmetry-forbidden since this polarization only probes the symmetric orbitals along C-H bonds. Similar to gas-phase CH4, asymmetrical vibrations of C-H bonds can

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Figure 4. O 1s XAS of ice films grown on CH3:Si(111) substrate at different coverages. The solid lines represent in-plane polarization while the dotted lines represent out-of-plane polarization.

Figure 3. C 1s XAS spectra of CH3:Si(111) with adsorbed D2O ice layers (red 3 layers, green 5 layers, blue 8 layers) compared with the bare surface (black) measured with in-plane (top) and out-of-plane (bottom) E-vector polarizations. The A1 peak intensity is dramatically enhanced by ice adsorption for both polarizations. The dotted line indicates the A1 peak for the bare surface, while the dashed lines indicate the A1 energy shift. All spectra were normalized at the maximum intensity.

explain the appearance of the LUMO A1 peak. The intensity is much lower compared to the out-of-plane spectrum due to the inherently weaker nature of asymmetrical distortions through vibrational motions compared to covalent bonding. Also, different chemical bonding environments account for the slightly different energy positions of the A1 peak (285.5 eV for out-of-plane and 285.0 eV for in-plane). The broad appearance of the spectral features is likely caused by a wide distribution of structures across the substrate and/or shorter C-C distance between -CH3 groups (3.8 Å, the lattice constant of Si(111)-(1 × 1) surface) compared to the C-C distance in condensed CH4 (4.16 Å). The adsorption of D2O ice on CH3:Si(111) induces quite dramatic changes to the C 1s XAS spectra, as displayed in Figure 3, specifically in the A1 peak, which is enhanced in both in-plane and out-of-plane spectra. The changes in the in-plane spectra (Figure 3, top) are especially interesting since the hydrophobic -CH3 groups are not expected to electronically interact with water. However, the dramatic increase in the intensity of the A1 peak as the coverage of water increases suggests that the presence of the ice overlayer enhances the p character of the -CH3 orbitals on the surface. Further evidence of the orbital modification is seen in a shift of ∼0.3 and ∼0.55 eV toward lower excitation energies from 0 to 8 layers ice for the in-plane and out-of-plane polarizations, respectively. Both these observations are a clear indication that there is an orbital interaction between water and -CH3 groups despite the hydrophobicity of the surface. The changes in the A1 peak of the out-of-plane spectra (Figure 3, bottom) are less dramatic than the in-plane spectra, but the same trend is observed. The

rest of the spectral features in both out-of-plane and in-plane spectra do not seem to be significantly influenced by the ice overlayer. It should be noted that for both in-plane and out-of-plane polarizations, the increase of the A1 peak intensity is not linearly correlated with the number of ice layers; i.e., the change from 5 to 8 layers is greater than 0 to 3 or 3 to 5 layers. This is likely caused by the morphology of the ice, which forms threedimensional amorphous ice clusters on the hydrophobic surface instead of flat layers that cover the surface. We expect that as more water was dosed and the size of the clusters increased not only in height but also laterally, more of the -CH3 groups would become covered with water. This is also observed in the growth of ice on the hydrophobic first water monolayer on Pt(111).22,23 Eventually at high enough coverage, the whole surface would be covered with water and the C 1s XAS spectra would stop changing. Unfortunately, at much higher water coverages, measurement becomes more difficult since fewer electrons can escape through the thick layer, resulting in a lowquality spectrum. As shown in Figure 4, the polarization dependent O 1s XAS spectra are isotropic at all coverages, indicating that three-dimensional nonwetting ice clusters were formed instead of flat wetting layers. This is similar to the case of ice monolayer grown on Cu(111) surface.24 If flat layers were formed, the polarization dependent spectra would show anisotropy as observed for ice monolayer on Pt(111).24 In addition, we can determine that amorphous ice was formed from the absence of the fine structure beyond the post-edge that is present in crystalline ice due to multiple scattering.19 The thickness stated here is the equivalent thickness if the ice were assumed to form flat uniform layers. Since this is not the case, some areas of the surface are not covered with ice at low coverages, but the fraction of the area exposed to vacuum decreases with increasing coverage. This is distinctly different from the ice grown on other hydrophobic surfaces such as Au(111)25 or graphene/Pt(111),26 which was shown to form two layer crystalline films that maximize the number of water-water H-bonding. However, the hydrophobic nature of Au(111) and graphene/Pt(111) is different from that of CH3:Si(111) since no free C-H groups are available to form C-H---O H-bonds with water.

Interaction between Water and CH3:Si(111) The electronic effect of adsorbed ice on the excited state orbital of -CH3 group is surprising considering the hydrophobic nature of the substrate. The interaction between -CH3 groups and the water molecules perturbs the -CH3 A1 orbital in such a way that it gains p character in both in-plane and out-of-plane directions so that the orbital shape is no longer spherically symmetric in the excited state. However, the observation of a direct interaction between a hydrophobic system and water is not unprecedented. For example, experimental vibrational Raman spectra of methane hydrates, a rigid high-pressure system in which CH4 molecules are trapped in icelike cages, show a red shift of the C-H stretching band from gas-phase CH4.27,28 Furthermore, we speculate that the direct interaction between CH3:Si(111) surface and adsorbed D2O ice results in the formation of weak H-bonding. Although X-H---O H-bonds are more often observed with highly electronegative atom X (i.e., X ) N, O, or F), C-H---O H-bonding has been observed numerous times in the literature.29 For instance, DFT simulations of adsorbed glycinate on Cu(110) showed that a model in which neighboring molecules form C-H---O H-bond is the best model to reproduce experimental XAS data.30 C-H---O H-bonds were also observed by nuclear magnetic resonance and infrared studies of 1,4-dioxane and water mixtures.31 However, in our study, C-H---O H-bonding likely occurred between water and the -CH3 groups despite the hydrophobic nature of the surface. The formation of threedimensional ice clusters as shown in Figure 4 results from the strong water-water H-bonding in order to minimize contact with the hydrophobic surface. Nevertheless, at low temperatures where the mobility of water molecules is restricted, a direct interaction in the form of weak H-bonding is formed between water molecules in ice and methyl C-H bonds. We estimated the surface density of -CH3 groups 9 and of water in ice films to be 7.7 nm-2 and 5.6 nm-2 at a maximum (derived from the lattice constant of 4.5 Å for the (0001) plane of Ih ice32), respectively. Therefore, one H2O molecule can be weakly H-bonded to at least one -CH3 group at the interface covered by ice islands. The strength of C-H---O H-bonds is generally in the range of 0.3-3.5 kcal/ mol depending on the hybridization of the C atom,29 compared to 5 kcal/mol for O-H---O H-bond in water.33 For CH3:Si(111), it is expected to be close to the value of 0.3 kcal/mol for the C-H---O H-bond between CH4 and H2O as obtained from ab initio calculations.34 Though obviously much weaker than the H-bonds between water molecules, it was shown that the C-H---O H-bond between CH4 and H2O shows linearity,34 which, indeed, is one of the defining characteristics of a hydrogen bond. As mentioned previously, the condensation of gas-phase CH4 does not significantly change the XAS spectra due to the large C-C distance. A similar sample such as methanol (CH3OH) also displays only small changes in the C 1s XAS spectra between gas and liquid phases, and the overall spectra resemble the CH4 spectrum despite the different symmetry.35 Therefore, the electronic interaction between -CH3 groups and water implies that the C-H---O distance between a water molecule and a surface -CH3 group is likely shorter than the C-C distance in condensed CH4 (∼4.16 Å20) or liquid CH3OH (∼3.8 Å36). In addition, for glycinate ions on Cu(110), the C-H---O distance was determined to be less than 2.68 Å.30 However, since the -CH2- group in a glycinate ion is a more acidic H-donor than the -CH3 groups on Si(111) surface, the C-H---O H-bond between two glycinate ions is likely stronger than the

J. Phys. Chem. C, Vol. 114, No. 44, 2010 19007 similar H-bond between H2O and -CH3. Therefore, we deduce that the C-H---O distance in our sample is between 2.68 Å (glycinate H-bonding30) and 3.8 Å (C-C distance in liquid methanol36). This is also in agreement with the expected value for C-O distance of 3.1-3.5 Å in C-H---O H-bonded systems.20 However, at higher temperatures, the entropic contributions most likely will lead to a substantial increase in C-H---O distance, resulting in the formation of a depletion layer. Conclusion In conclusion, we present clear evidence from experimental XAS that adsorbed D2O ice directly interacts with hydrophobic -CH3 groups on Si(111). The ground state of the LUMO A1 orbital of the -CH3 groups is spherically symmetric and, therefore, is symmetry-forbidden for XAS excitation. Upon D2O ice adsorption, the excited state of the A1 orbital is asymmetrically distorted by the ice overlayer, thereby increasing the p character and enhancing the absorption intensity in both in-plane and out-of-plane directions. We propose that weak H-bonds are formed between C-H bonds of methyl groups and water molecules in the first layer of the ice films. This occurred despite the obvious hydrophobic nature of the surface, as evidenced by the formation of three-dimensional ice clusters observed from the O 1s XAS spectra. This is an indication that macroscopic observations of hydrophobicity (e.g., contact angle, formation of threedimensional ice clusters) do not necessarily preclude a molecular orbital interaction at low temperatures. Acknowledgment. This work was supported by the National Science Foundation (U.S.) CHE-0809324 and CHE-0431425 and by KAKENHI (Grant No. 19360024), Grant-in-Aid for Scientific Research on Innovative Areas (B) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. We thank Mr. K. Inouye, Mr. S. Sodezawa, and Dr. T. Yamamoto of Japan Carlit Co., Ltd., and Silicon Technology, Inc., for providing untreated Si wafers, CH3:Si(111) wafers, and contact angle measurement. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. References and Notes (1) Ball, P. Chem. ReV. 2008, 108, 74–108. (2) Engquist, I.; Lundstro¨m, I.; Liedberg, B. J. Phys. Chem. 1995, 99, 12257–12267. (3) Poynor, A.; Hong, L.; Robinson, I. K.; Granick, S.; Zhang, Z.; Fenter, P. A. Phys. ReV. Lett. 2006, 97, 266101. (4) Mezger, M.; Reichert, H.; Schoeder, S.; Okasinski, J.; Schroeder, H.; Dosch, H.; Palms, D.; Ralston, J.; Honkimaki, V. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 18401–18404. (5) Scatena, L. F.; Brown, M. G.; Richmond, G. L. Science 2001, 292, 908–912. (6) Frank, H. S.; Evans, M. W. J. Chem. Phys. 1945, 13, 507–532. (7) Rezus, Y. L. A.; Bakker, H. J. Phys. ReV. Lett. 2007, 99, 148301. (8) Niwa, D.; Inoue, T.; Fukunaga, H.; Akasaka, T.; Yamada, T.; Homma, T.; Osaka, T. Chem. Lett. 2004, 33, 284–285. (9) Yamada, T.; Kawai, M.; Wawro, A.; Suto, S.; Kasuya, A. J. Chem. Phys. 2004, 121, 10660–10667. (10) Yu, H. B.; Webb, L. J.; Ries, R. S.; Solares, S. D.; Goddard, W. A.; Heath, J. R.; Lewis, N. S. J. Phys. Chem. B 2005, 109, 671–674. (11) Yamada, T.; Takano, N.; Yamada, K.; Yoshitomi, S.; Inoue, T.; Osaka, T. J. Electroanal. Chem. 2002, 532, 247–254. ¨ stro¨m, H.; Ogasawara, H.; Na¨slund, L. Å.; Pettersson, L. G. M.; (12) O Nilsson, A. Phys. ReV. Lett. 2006, 96, 146104. ¨ stro¨m, H.; Triguero, L.; Nyberg, M.; Ogasawara, H.; Pettersson, (13) O L. G. M.; Nilsson, A. Phys. ReV. Lett. 2003, 91, 046102.

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