Aqueous Interface Studied by

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Water Structure at the Buried Silica/Aqueous Interface Studied by Heterodyne-Detected Vibrational Sum-Frequency Generation Anton Myalitsin,† Shu-hei Urashima,† Satoshi Nihonyanagi,†,‡ Shoichi Yamaguchi,†,‡ and Tahei Tahara*,†,‡ †

Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Ultrafast Spectroscopy Research Team, RIKEN Center for Advanced Photonics (RAP), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

‡

S Supporting Information *

ABSTRACT: Complex χ(2) spectra of buried silica/isotopically diluted water (HODD2O) interfaces were measured using multiplex heterodyne-detected vibrational sum frequency generation spectroscopy to elucidate the hydrogen bond structure and up/down orientation of water at the silica/water interface at different pHs. The data show that vibrational coupling (inter- and/or intramolecular coupling) plays a significant role in determining the χ(2) spectral feature of silica/H2O interfaces and indicate that the doublet feature in the H2O spectra does not represent two distinct water structures (i.e., the iceand liquid-like structures) at the silica/water interface. The observed pH dependence of the imaginary χ(2) spectra is explained by (1) H-up oriented water donating a hydrogen bond to the oxygen atom of silanolate, which is accompanied by H-up water oriented by the electric field created by the negative charge of silanolate, (2) H-up oriented water which donates a hydrogen bond to the neutral silanol oxygen, and (3) H-down oriented water which accepts hydrogen bonds from the neutral silanol and donates hydrogen bonds to bulk water molecules. The broad continuum of the OH stretch band of HOD-D2O and a long tail in the low frequency region represent a wide distribution of strong hydrogen bonds at the silica/water interface, particularly at the low pH.

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INTRODUCTION Solid/liquid interfaces are central in many natural sciences and industrial processes ranging from photovoltaics1,2 to medical applications.3 However, it is difficult to characterize solid/liquid interfaces by conventional spectroscopic methods because they are “buried” between two bulk phases. In fact, the number of molecules at the interface is much less than that in the bulk liquid so that the weak signal from the interface is hidden by the signal from the bulk. Moreover, the buried interface is not readily accessible by most of spectroscopic and imaging methods. Vibrational sum frequency generation (VSFG) is one of the second-order nonlinear optical processes, and it is forbidden in centrosymmetric media under the dipole approximation.4 Therefore, VSFG has the inherent selectivity for the interface, where the symmetry is necessarily broken. This makes VSFG a unique tool for probing the interfaces. In the past two decades, the water structures at various aqueous interfaces have been extensively investigated by VSFG spectroscopy in the OH stretch region.5−11 In most of the past VSFG studies, conventional homodyne detection was employed, in which the absolute square of the second-order nonlinear susceptibility (|χ(2)|2) is measured. This type of VSFG measurement, however, has substantial drawbacks such as a loss of the sign information on χ(2) and spectral deformation due to the interference between resonances and nonresonant background. They often make the interpretation of the |χ(2)|2 spectra © 2016 American Chemical Society

difficult. To overcome these problems, interferometric techniques were developed, which allow us to measure the complex χ(2) spectra directly.12−23 In particular, multiplex heterodyne-detected VSFG (HD-VSFG)19−22 enables us to measure complex χ(2) spectra over a wide frequency range at once with a high signal-to-noise ratio. The imaginary part of the χ(2) (Im χ(2)) spectrum directly represents the vibrational spectrum at the interface and can be readily interpreted in the same manner as bulk IR and/or Raman spectra. Furthermore, the sign of the Im χ(2) spectrum indicates the net orientation of the molecular species at the interface.19,24 Furthermore, combining HD-VSFG spectroscopy with isotopic dilution method makes it possible to discuss the water structure by removing the effect of vibrational coupling from the spectra, which enables obtaining significantly new insights into the structure and dynamics of interfacial water at exposed air/ aqueous interfaces.20,21,23,25 By contrast, the application of these interferometric techniques to buried solid/water interfaces has been still very limited.12,13,26−29 The silica/aqueous interface is a prototypical system of the buried interface. Silica has a large number of hydroxyl groups (Si−OH) which can be deprotonated in contact with water at moderate and high pH, with the equilibrium reaction SiOH ⇄ Received: March 31, 2016 Revised: April 6, 2016 Published: April 7, 2016 9357

DOI: 10.1021/acs.jpcc.6b03275 J. Phys. Chem. C 2016, 120, 9357−9363

Article

The Journal of Physical Chemistry C SiO− + H+. The pH-dependent water structure at the silica/ aqueous interface has been extensively studied by conventional VSFG.5,12,30,31 In particular, Shen and co-workers have reported complex χ(2) spectra of the quartz/aqueous interface13 and discussed the existence of different types of water structures at the interface based on the χ(2) spectra measured at different pH in H2O.11,13,14 However, because it is known that H2O spectra are significantly affected by vibrational coupling, isotopic dilution is necessary to remove the vibrational coupling for clear discussion about the water structure based on the χ(2) spectra.20,32−35 Here, we present the Im χ(2) spectra of isotopically diluted water (HOD-D2O) at the buried silica/ aqueous interfaces.

between the LO SF and the sample SF. Both SF beams were introduced together into the spectrograph and detected by CCD. The SF, ω1, and ω2 beams were s-, s-, and p-polarized, respectively. The silica substrate (IR grade, Pier Optics, Co. Ltd.) was cleaned by soaking in concentrated H2SO4 and thoroughly rinsed with pure water. Ultrapure water (Millipore, 18.2 MΩ cm−1 resistivity) and deuterium oxide (NMR grade, 99.9 %, Wako) were mixed in a ratio of 1:4 to obtain predominantly HOD in D2O (H2O:HOD:D2O = 1:8:16). Phosphate buffer solutions were prepared by adding the necessary amount of phosphoric acid and phosphate salt to the HOD solution to keep the ionic strength constant at I = 10 mM, i.e., 0.022 M H3PO4 for pH 2.0, 0.0025 M HNa2PO4 and 0.0025 M H2NaPO4 for pH 6.7, and 0.001 M Na3PO4 and 0.0055 M NaOH for pH 11.7. For the 1:8:16 mixture of H2O:HOD:D2O, the same amount of salt was used as for pure H2O. The correction term between the pH meter reading and the actual value is +0.32 for the 1:8:16 mixture.36 This gives us the pH for the HOD solution as 2.1, 7.2, and 12.1, respectively (i.e., we added +0.3 to the pH meter reading.) This difference in pH between the H2O and HOD solutions is insignificant for our discussion. Since the application of HD-VSFG to the buried solid/liquid interface requires special care for the phase calibration, we describe our method in some detail below. For the interface exposed to the air, the χ(2) spectrum has been normalized with z-cut quartz which has a positive real χ(2)19,37,38 or the air/D2O interface which has a negative real χ(2).39,40 This normalization of the air/liquid signal is valid because the propagation medium (i.e., the air) and the light paths are the same for the sample and reference measurements, and hence the propagations of the incident lights in the air do not cause any relative phase difference. However, the normalization procedure of buried silica/water by air/quartz or air/D2O is obviously not viable. Therefore, for buried silica interfaces, we use the nonresonant signal of the silica/vapor interface as a reference due to the following reasons: First, silica is a common medium for the sample and reference measurements so that the propagation of the beams in silica with fixed thickness does not cause any phase shift. In fact, the variation of the thickness within one substrate was found to be around 1 μm and therefore negligible. Second, because bulk silica has no vibrational resonances in the CH and OH stretch regions, χ(2) of the silica/ air interface is considered purely real if the surface is properly treated. In fact, for eliminating adsorbed H2O which may make χ(2) not purely real, we washed the substrate with D2O and blew it dry with nitrogen. Then, we placed it on a Teflon cell which was half-filled with D2O, creating a D2O vapor saturated atmosphere. Since the silica surface after these treatments is terminated with Si-OD and any adsorbed water on the silica is D2O, we can consider χ(2) spectrum of the silica/(D2O saturated) air interface purely real, and hence we can safely use it as the reference. After measuring the reference spectrum, the cell was filled with an aqueous solution, and the χ(2) spectra of the silica/solution are measured. The quantity we show in this paper is the effective χ(2)

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EXPERIMENTAL SECTION Our HD-VSFG apparatus has been described elsewhere.19,25 Figure 1 depicts the schematic diagram of the experimental

Figure 1. Schematic of the experimental HD-VSFG setup.

setup used in the present study. Briefly, the output from a regenerative amplifier (Spectra-Physics, SpitfireProXP, 3.5 W, 1 kHz) was split in two. A part of the output was narrowed by a band-pass filter (Optoscience, center wavelength 795 nm, bandwidth 1.5 nm) and was used as the visible (ω1) light for the HD-VSFG measurement. The major part of the regenerative amplifier output was used for excitation of a commercial optical parametric amplifier that is combined with a difference frequency generator (Spectra-Physics, TOPAS C and DFG1) for generating broadband IR (ω2). For buried interface measurements, the “LO first configuration” (LO: local oscillator) was employed to avoid strong frequency dependence of reflectivity of ω2 at the silica/water interface.25 The ω1 and ω2 beams are spatially and temporarily overlapped on the gold mirror to generate sum frequency (SF) (ω1 + ω2), which acts as LO. Subsequently, the reflected beams were refocused on the lower surface of a 2 mm thick silica glass (IR grade, Pier Optics). The incident angles for ω1 and ω2 beams at the air/ silica upper surface are 38° and 46°, which correspond to the incident angle at the silica/water bottom surface being 24° and 31°, respectively. In this geometry, ω2 light is more delayed by passing through the silica substrate with respect to ω1 due to the different path length. Therefore, a “delay compensator” (0.2 mm thick silica plate) is inserted into the beam path of ω1 to compensate for this effect and obtain better temporal overlap at the buried silica surface. The LO passed through a 2 mm thick silica plate, which delayed the LO pulse with respect to ω1 and ω2 pulses by around 3 ps. This generated a time difference

(2) χeff,silica/water

=

(2) asilica/water χyyz ,silica/water (2) asilica/vapor χyyz ,silica/vapor

(1) 24

where a includes Fresnel factors and other prefactors and asilica/vapor is a positive real constant. The Fresnel factors for the 9358

DOI: 10.1021/acs.jpcc.6b03275 J. Phys. Chem. C 2016, 120, 9357−9363

Article

The Journal of Physical Chemistry C

Figure 2. (a) Complex χ(2) spectrum of the buried silica/H2O interface for neat water (pH ∼ 5.6). Black: real part of χ(2); red: imaginary part of χ(2). (b) Comparison of |χ(2)|2 spectra measured by the homodyne and heterodyne measurements: blue: magnitude square of the heterodyne signal; green: homodyne signal. The homodyne spectrum of silica/H2O was normalized by silica/gold and scaled for comparison. (c) Complex χ(2) spectrum of the silica/HOD-D2O interface. The color codes are the same as in (a). (d) Corresponding |χ(2)|2 spectrum of the silica/HOD-D2O shown in (c).

H2O.20,23,33,35 We measured the Im χ(2) spectrum of silica/ water interface using isotopically diluted water (HOD-D2O) for examining the effect of vibrational coupling on the Im χ(2) spectrum of the silica/H2O interface. Actually, the data of the isotopic dilution is indispensable for unambiguous discussion about the water structure at the interface and its pH dependence based on the Im χ(2) spectra.23,35 The Im χ(2) spectra of the silica/HOD-D2O interface (H2O:HOD:D2O = 1:8:16) are shown in Figure 2c, and the corresponding |χ(2)|2 spectra are shown in Figure 2d. At this isotope concentration, 80% of the OH stretch signal comes from HOD and only 20% from H2O. (Note that H2O has two OH while HOD has one OH so that the ratio of OH bonds is 2:8.) As clearly seen, upon isotopic dilution, the two positive peaks at 3200 and 3450 cm−1 become one peak, as it was observed in previous homodyne VSFG studies of the highly charged silica/aqueous interface.33 This clearly indicates that the doublet feature in the H2O spectrum does not indicate distinct two structures as previously assigned to the ice-like (3200 cm−1) and liquid-like (3400 cm−1) bands11,13 because the water structures at the silica/H2O and silica/HOD-D2O interfaces should be essentially the same. The doublet features observed for H2O solutions can be attributed to the strong inter- and intramolecular vibrational couplings (e.g., Fermi resonance) as in the case of the air/ charged aqueous interfaces.20,25,32,35 This result demonstrates that the vibrational coupling largely affects the spectral feature of not only the monolayer/water interface but also the silica/ water interface. Because the vibrational coupling significantly affects the Im χ(2) spectrum of the silica/H2O interface, we discuss pHdependent water structure at the silica/aqueous interface based on the HOD-D2O spectra hereafter. Figure 3 shows the Im χ(2) spectra (a) and |χ(2)|2 spectra (b) of the silica/HOD-D2O interface at different bulk “pH”. Here pH refers to both

silica/water interface are complex in general, but the effect of the Fresnel factors on the shape of χ(2) is negligible under the present experimental condition (see Figure S1 in Supporting Information). The subscripts of χ(2) yyz in eq 1 refer to the laboratory axes. The z-axis is parallel to the surface normal, and the y-axis is parallel to the s polarization. χ(2) yyz,silica/vapor is positive because χ(2) of the geometrically opposite interface, i.e., χ(2) yyz,air/silica is negative (see Figure S2).

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RESULTS AND DISCUSSION The complex χ(2) spectra of the silica/H2O interface are shown in Figure 2a. The Im χ(2) spectrum (red line) shows two broad positive OH bands around 3200 and 3450 cm−1. Figure 2b shows a comparison of |χ(2)|2 spectra, one of which (blue line) is obtained as the magnitude square of the complex χ(2) spectrum shown in Figure 2a, and the other (green line) is obtained with conventional homodyne-detected VSFG measurements. (The homodyne SFG signal was normalized by the SFG from the silica/gold interface.) The square and homodyne-detected spectra consistently show the doubleband shape with comparable intensity and highly resemble the homodyne spectra that were reported previously.5,12 This confirms the validity of the buried silica/water interface measurement for the two aspects: First, the silica/vapor and silica/gold reference interfaces have no vibrational resonances because these references give consistent results for |χ(2)|2 spectra. Second, the heterodyne scheme does not create any artificial spectral features for the measurements of this buried interface. Similar double-band shapes are seen for the spectra of the air/water interface and air/monolayer/water interfaces.11,17,19,41 These features were first assigned to two distinct water structures in early VSFG studies,11 but recently it has been shown that they arises from the vibrational coupling due to 9359

DOI: 10.1021/acs.jpcc.6b03275 J. Phys. Chem. C 2016, 120, 9357−9363

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The Journal of Physical Chemistry C

At the neutral pH (pH 7.2), the positive OH band in the lower frequency region loses the intensity, and the signal in the lowest frequency region becomes negative (Figure 3a, red curve). The negative peak can be assigned to water molecules orientated with their hydrogen atoms pointing down toward the bulk water (H-down orientation). At neutral pH, some of the negatively charged silanolate is protonated to generate neutral silanol, and therefore the surface charge of the silica surface decreases. This leads to a decrease of H-up oriented water molecules and appearance of H-down oriented water which accepts H-bonds from the neutral silanol and donates Hbonds to bulk water molecules. At acidic pH (pH 2.1), the low-frequency negative OH band grows in intensity and becomes very broad, but the OH band in the higher frequency region remains positive (Figure 3a, black curve). This indicates that at pH 2.1, although the majority of water are in H-down orientation, yet some water molecules keep the H-up orientation. At this low pH, the silica surface is expected to be completely neutral.46 The neutral silanol facilitates H-down oriented water by providing H-bonds donor sites, resulting in the increase of the negative OH band.47 Because the lower frequency region changes the sign of the OH band by changing bulk pH, i.e., protonating/ deprotonating the surface silanol, it can be considered that the OH band in the low-frequency side includes more contribution from the water H-bonded with the silanolate/ silanol group (Si−O−/Si−OH), even though some contribution from the silanol OH may be present. In contrast, the OH band in the high-frequency region remains positive even at pH 2.1, independently from the bulk pH. The high-frequency positive band does not change even at more acidic condition (see Supporting Information), suggesting the presence of a weakly bound water species that is largely independent of the deprotonation of the silanol. Previously, it was proposed that the positive band at low pH is a result of negatively charged surface sites that are deprotonated even at very low pH.13 However, the surface charge is not the only factor that determines the water orientation at the interface in particular at low pH where the charge density is known to be very low.42 In fact, molecular dynamics simulations have suggested that neutral silanol can induce both H-up and H-down orientations of water via hydrogen bond with the oxygen atom of silanol being H-bond acceptor and with hydrogen atom of silanol being H-bond donor, respectively.47−50 In addition to the oxygen atom of the silanol group, it is also possible that siloxane bridges (Si−O−Si) acts as a weak H-bond acceptor.51 Because the bridge site provides only a weak interaction52,53 and it does not respond to the bulk pH, it can explain why the OH band appears in the higher frequency side and does not change the sign with changing pH. Both mechanisms can explain the H-up orientation of water at very low pH, without assuming the presence of negative charge.13 This interpretation is consistent with both the isoelectric point measured by electrophoresis and the point of zero charge by titration, which are consistently reported around pH 2 (see ref 42 and references therein). The pH-dependent water orientations that are indicated by the present study are graphically summarized in Figure 4. At high pH, interfacial water molecules mostly take H-up orientation due to the negative Si−O− (Ia). At low pH, Hdown orientation is induced by the neutral Si−OH (II). Some water molecules remain in H-up orientation because they are hydrogen-bonded to the oxygen atom of the bridge siloxane

Figure 3. (a) Imaginary part of the χ(2) spectra of the buried silica/ HOD-D2O interface for different pH in phosphate buffer (I = 10 mM, ratio of H2O:HOD:D2O = 1:8:16). (b) Corresponding magnitude square of the χ(2) spectra (|χ(2)|2 spectra) of the buried silica/HODD2O interface for different pH in phosphate buffer. The pH of the solution for each spectrum is indicated in the Figure.

hydrogen and deuterium atoms as described in the Experimental Section. At basic pH (pH 12.1), the entire OH band in the Im χ(2) spectrum is positive (Figure 3a, blue curve), indicating that water molecules at the silica/water interface are net orientated with the hydrogen atoms pointing up toward the silica surface (H-up orientation). The peak frequency of the OH band appears around 3400 cm−1 which is similar to that in the bulk IR spectrum, indicating that the strength of hydrogen bonding at the interface is comparable to the bulk water. However, the OH band is asymmetric in shape, exhibiting a long tail toward the low-frequency region even in the HOD mixtures. This indicates that some strongly hydrogen-bonded water molecules exist at the silica/aqueous interface in a very broad distribution. At pH > 10, the silica surface is known to be completely deprotonated.42 At such high pH, the H-up orientation is induced for two reasons: First, water can form hydrogen bonds directly to the Si−O− groups. Second, water dipoles are aligned due to the electric field created by the negative charge on the silica surface, as suggested in many previous studies.5,13,30,43,44 This electric field induces not only the orientation of water molecules in direct contact with the silica surface but also that inside the electric double layer (EDL). According to classical Gouy−Chapman theory, the EDL is estimated to have a thickness of 3 nm at the ionic strength in the present experiment (I = 10 mM), and the SFG signal can be generated by this thickness. This results in the higher signal intensity of the OH band than that at lower pHs. (Note that the phase mismatch of the signal generated in the EDL is estimated to be about 2° as shown in the Supporting Information and therefore negligible.45) For a more detailed analysis of EDL contributions, rigorous determination of ionic strength dependence is required, but it is beyond the scope of this study. 9360

DOI: 10.1021/acs.jpcc.6b03275 J. Phys. Chem. C 2016, 120, 9357−9363

Article

The Journal of Physical Chemistry C

Finally, we mention the effect of the charge on the hydrogen bond strength which has been discussed in previous homodyne VSFG studies. The |χ(2)|2 spectra of the silica/aqueous solutions (Figure S3b) agree quite well with the previously reported homodyne VSFG spectra for H2O5,12,13,56,57 as well as those for HOD-D2O.58 In these homodyne VSFG studies, it was concluded that charge of the deprotonated silica surface facilitates the hydrogen bonding at high pH on the basis of the change in the intensity ratio between the 3200 and 3450 cm−1 bands in the pH-dependent H2O spectra5,12,13,56,57 or the red-shift of the OH stretch peak in |χ(2)|2 spectra of HOD.58 However, the Im χ(2) spectra of HOD-D2O clearly show that the negative OH band observed in the very low-frequency region (