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Vibrational Sum Frequency Generation (SFG) Spectroscopy Study of Hydrous Species in Soda Lime Silica (SLS) Float Glass Jiawei Luo, Joy Banerjee, Carlo G. Pantano, and Seong H. Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00706 • Publication Date (Web): 02 Jun 2016 Downloaded from http://pubs.acs.org on June 6, 2016

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Vibrational Sum Frequency Generation (SFG) Spectroscopy Study of Hydrous Species in Soda Lime Silica (SLS) Float Glass Jiawei Luo,1,2 Joy Banerjee,2 Carlo G. Pantano,2,3 and Seong H. Kim1,2,3* 1. Department of Chemical Engineering, Pennsylvania State University, University Park, PA, 16802, USA. 2. Materials Research Institute, Pennsylvania State University, University Park, PA, 16802, USA. 3. Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA, 16802, USA. * Corresponding author: Seong H. Kim ([email protected]) submitted to: Langmuir

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Abstract It is generally accepted that the mechanical properties of soda lime silica (SLS) glass can be affected by the interaction between sodium ions and hydrous species (silanol groups and water molecules) in its surface region. While the amount of these hydrous species can be estimated from hydrogen profile and infrared spectroscopy, their chemical environment in the glass network is still not well understood. This work employed vibrational sum frequency generation (SFG) spectroscopy to investigate the chemical environment of hydrous species in the surface region of SLS float glass. SLS float glass shows sharp peaks in the OH stretching vibration region in SFG spectra, while the OH stretch peaks of glasses that do not have leachable sodium ions and the OH peaks of water molecules in condensed phase are normally broad due to fast hydrogen bonding dynamics. The hydrous species responsible for the sharp SFG peaks for the SLS float glass were found to be thermodynamically more stable than physisorbed water molecules, did not exchange with D2O, and were associated with the sodium concentration gradient in the dealkalized subsurface region. These results suggested that they reside in static solvation shells defined by the silicate network with relatively slow hydrogen bonding dynamics, compared to physisorbed water layers on top of the glass surface. A putative radial distribution of the hydrous species within the SLS glass network was estimated based on the OH SFG spectral features, which could be compared with theoretical distributions calculated from computational simulations.

Keywords: hydrous species; soda lime silica float glass; dealkalization, hydrogen bonding dynamics.


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1. Introduction Understanding the surface structure of soda lime silica (SLS) glass is of great importance to improve its chemical and mechanical properties, and to expand its applications beyond simple windows and bottles into high-tech industries such as electronic displays, photovoltaics, and so on.1–4 The surface properties of SLS glass are highly dependent on the manufacturing process, i.e., blowing, molding, polishing, or float processes.5 In the SLS float process, the SLS molten liquid is poured onto a molten tin bath; since the molten SLS and tin liquids are not miscible and tin has higher density than SLS, the SLS liquid floats and vitrifies on the liquid tin bath.6 This allows production of flat glass panels with a surface finish (roughness and flatness) as smooth as the molten liquid. In order to improve the durability of the produced glass panel, the commercial float glass process exposes the glass to SO2 gas before the glass is fully cooled.1 During this exposure, sodium ions (Na+) associated with non-bridging oxygen (NBO; Si-O-) in SLS glass react with SO2 and O2 and precipitate as Na2SO4 at the glass surface; the loss of Na+ ion is compensated by proton (H+) from water or H2, forming the silanol (Si-OH) group in the Na+-leached site:7

2 + + + → + 2

2 → +

(1) (2)

During this SO2 dealkalization process and post-manufacturing steps (such as rinsing with water or storage in humid air), water molecules can alter the surface chemistry of SLS glass, producing concentration gradients of Na+ and hydrous species in the subsurface region.8 Here, the term “hydrous species” means both silanol and molecular water; the combination of these species could be stoichiometrically equivalent to hydronium ions (H3O+) replacing sodium ions (Na+).9,10 The thickness of the dealkalization region varies with process conditions; it is typically about 60 – 100


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nm.11,12 Thus, the hydrogen bonding interactions between silanol groups and water molecules as well as bridging oxygen (BO; Si-O-Si) groups become far more complicated in the SLS float glass than pure silica and other silicate glasses without leachable ions.12–14 Figure 1 schematically illustrates the depth profiles of hydrogen and modifier ions (Na+, Ca2+, Mg2+) in the subsurface region of SLS float glass based on the literature8,12 and the probe depth of SFG and ATR-IR on the hydrous species.15–17 The exact nature of these hydrous species is not well understood yet and remains as one of the grand challenges in fundamental surface science of silicate glasses.9

Figure 1. Schematic illustration of depth profiles of H and modifier ions in the surface region of SLS float glass and the probe depth of SFG and ATR-IR. Note that the schematic is not drawn in scale.


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The formation and chemical environment of hydrous species in the surface region of glass have been investigated with theoretical calculations. Molecular dynamics (MD) simulations suggested that surface defect sites, including NBO, three-coordinated silicon, and two membered rings are prone to hydroxylation process when glass is exposed to water.18,19 Garofalini and colleagues have studied the interaction between silica surface and water using MD simulations with a dissociative water potential. The results indicated that the water molecules can penetrate the silica network up to 0.7 nm during their simulation time frame, forming silanol groups and hydronium ions.20 This process can be enhanced in the presence of surface stress and excessive amount of water molecules.21,22 The bond angle distribution of Si-O-Si is also modified with the formation of new SiOH on silica glass surface.23 Compared with silica glass, the chemical environment of hydrous species could be different in SLS glass since more NBO sites and Si-OH sites are available in the SLS surface region. Cormack and coworkers reported that sodium-rich and sodium-deficient sites can co-exist in MD simulations of silicate glass systems.24–26 When sodium ions are leached out of the surface region of SLS glass and hydrous species are introduced, it is likely that the chemical environments of these hydrous species in the SLS glass network could be very different from those found in condensed liquid or ice phases or physisorbed states on solid surfaces. It is well known that the OH stretching vibrational peak is very sensitive to the hydrogen bonding interactions. Thus, vibrational spectroscopy has been widely used to investigate the distribution of various hydrous species or hydrogen bonding interactions in the glass.12,14,16,27,28 For hydrous species in the surface region, attenuated total reflectance infrared (ATR-IR) spectroscopy is most extensively used.12,29 Since the information depth of ATR-IR for SLS glass is approximately one micron in the OH stretch vibration region (Figure 1) and its signal intensity is linearly proportional to the total concentration,16,17 it is suitable for quantification of the subsurface OH and H2O species in glass. The overtone or combination bands in the near-IR region could distinguish


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molecular water and silanol groups;29,30 but subtle differences in hydrogen bonding interactions of water molecules and silanol groups in the glass matrix are difficult to interpret in near-IR analyses. The fundamental peaks in the mid-IR range are more sensitive to small changes in hydrogen bonding interactions; but the spectral overlap of various species makes peak identification and interpretation difficult.31 One of the reasons for this difficulty is that ATR-IR detects all hydrous species within the information depth (~1 µm), so the structural information of the dealkalization region (within ~100 nm) is often masked by the spectral features of all hydrous species. This difficulty could be circumvented by employing vibrational sum frequency generation (SFG) spectroscopy.9 When a dielectric medium is irradiated with light, the medium is polarized in response to the electric field of the light ( ). The polarization () of the medium can be expressed as follows:32 = + + + ⋯

(3)

where is the permittivity in the vacuum, is the first-order or linear susceptibility, and are the second and third-order nonlinear susceptibilities, respectively. Note that is about eight orders of magnitude smaller than , and is even much smaller than .33 Thus, when the glass surface is probed with a relative weak field of light (i.e., the mid-IR beam in typical IR spectroscopy), only the contribution from in equation (3) is observed (Figure 1). The non-linear responses from and terms could become significant and measurable only when high power pulsed laser beams are used. In the case of SFG spectroscopy, the ps-laser pulses with visible ( and infrared frequencies (

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are spatially and temporally overlapped at the sample surface and the

polarization response at the sum frequency (

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+

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is detected. Since the SFG process requires

phase matching of three photons with different wavelengths (

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it can be

generated only within a coherence length defined by the phase mismatch condition. When the SFG 6 ACS Paragon Plus Environment

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experiment is carried out in a reflection geometry on an optically flat surface, then the SFG coherence length is less than 100 nm from the surface (Figure 1).15 One unique feature of SFG is that the second harmonic response of χ(2) vanishes in a random or centrosymmetric medium.32 Since the glass network is amorphous, it cannot meet the noncentrosymmetry requirement of χ(2). Thus, there will be no SFG signal from the bulk. However, the surface of glass breaks the randomness of two bulk phases (glass and surrounding gas). Thus, the glass surface naturally provides the noncentrosymmetry, allowing detection of molecules at the surface.9,34 The third harmonic response of χ(3) in equation (3) does not require noncentrosymmetry; but when the molecules are under a strong electric field (%& ), then the %& term becomes noncentrosymmetric (because %& has polarity) and can be detected in SFG.28,35 Such local electric field could originate from the subsurface concentration gradient of Na+ ions in the glass.28 Therefore, both hydrous species at the glass/air interface ( response) and in the subsurface ( %& response) can be probed using SFG spectroscopy (Figure 1). The overall SFG intensity ('()*+ (*, ) of the glass material could be expressed as follows:

'()*+ (*, ∝ 0 + %& '()*+ '(*, ./ ∝

(4)

where 0 ./ is the nonlinear part of the polarization from equation (1), '()*+ and '(*, are the intensity of input visible and IR laser beams, respectively. In this paper, the subsurface hydrous species in glass were analyzed with in-situ SFG measurements during heating to obtain thermodynamic information relevant to the interfacial structures. For glasses without leachable alkali ions, the main hydrous species detected with SFG are water molecules adsorbed on the glass surface. In the case of SLS float glass, the spectral features in the OH stretch region of the SFG spectrum are very different – multiple sharp peaks are observed


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which have not been reported in vibrational spectroscopy of other glass systems or condensed water phases.9,36 The hydrous species responsible for these sharp SFG peaks of the SLS float glass are found to have higher stabilities than the physisorbed molecules and do not readily exchange with the water molecules in the gas phase. Also, they disappear when the subsurface Na+ concentration gradient is suppressed by annealing at a temperature higher than glass transition temperature (Tg). These findings suggest that the multiple OH SFG features with narrow peak widths of SLS float glass originate from the structural differences of hydrous species within the subsurface region where the Na+ ion profile is modified during the SO2 dealkalization process. Using the known empirical relationship of the OH peak position and the hydrogen bond distance, a putative radial distribution of the O−H···O distance for subsurface hydrous species in the SLS float glass is proposed, which could provide an experimental basis for verification of computational predictions.

2. Experimental Procedures The SLS float glasses were provided by Asahi Glass Co (Tokyo, Japan). These samples were retrieved from the float glass manufacturing line before they were stacked for commercial packing and shipping and there was no coating applied to the surface. The bulk composition of the glass (weight%) was measured by X-ray fluorescence and found to be 72.3% SiO2, 13.3% Na2O, 7.7% CaO, 1.9% Al2O3, 4.4% MgO, 0.3% K2O, and 0.1% Fe2O3. Two different thicknesses (0.7mm and 1mm) of the SLS float glass were used in this study. In the float glass manufacturing, the glass with different thickness undergoes different degree of stretching and SO2 treatment time.6 Because of this, the SLS float glasses with different thicknesses have different chemical composition, silicate network structure, and amount of hydrous species in the subsurface region (see the Supporting Information). The glass transition temperature of the SLS float glass used in this study was around 550°C. During the float process, two sides of the glass were produced in different chemical conditions; one side was 8 ACS Paragon Plus Environment

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in contact with air and the other with the molten tin. Since the tin-side contains a small amount of tin which alters the glass structure, this study focused on the air-side only. For a comparative study, fused quartz and borosilicate glasses were also analyzed. Fused quartz (SiO2) slides with a thickness of 1 mm were obtained from Technical Glass Products. Borosilicate (BOROFLOAT® 33; BF33) glass slides with a thickness of 1 mm were obtained from Schott Glass. The mass composition of BF33 was 81% SiO2, 13%B2O3, 2%Al2O3 and 4% Na2O/K2O. All glasses were cut with a diamond cutter to a size of 1 cm × 1 cm. All samples were pre-cleaned by rinsing with MilliQ water and pure ethanol, then UV-ozone cleaning for 20 minutes.37 The surface composition of glass was analyzed with x-ray photoelectron spectroscopy (XPS). A Kratos Analytical Axis Ultra spectrometer (Chestnut, NY) fitted with a monochromatic Al Kα (1486.6 eV) source and a low-energy electron beam charge-neutralizing flood gun was used for elemental analysis of the top 10 nm region of the SLS float glass. In the survey spectra (80 eV pass energy and 0.3 eV step size), O 1s, Na KLL, Ca 2p, Mg KLL, K 2p, C 1s, Si 2p and Al 2p peaks were used to calculate the atomic concentrations. The relative sensitivity factors (RSF) used for quantification were calibrated using a vacuum-fractured SLS float glass surface and the bulk composition acquired via inductively coupled plasma atomic emission spectroscopy (ICP-AES).38 In addition, O 1s and C 1s high-resolution, narrow energy peaks (20 eV pass energy and 0.1 eV step size) were captured for peak fitting. The binding energies of all elemental peaks were corrected with the adventitious alkyl peak (high-resolution) adjusted to 285.0 eV. The true glass composition was determined by a mathematical correction for the attenuation caused by adventitious hydrocarbon contamination on the glass surface.38,39 The total amount of hydrous species in the glass was analyzed with ATR-IR spectroscopy with diamond and Ge ATR crystals.16 The IR beam incident angles for Ge ATR (VariGATR; Harrick Scientific Products, Inc. Pleasantville, NY) and diamond ATR (MVPPro; Harrick Scientific Prodcusts, Inc., Pleasantville, NY) were 60o and 45o, respectively. For ATR-


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IR analysis on the elevated temperatures, diamond ATR was used. Specular reflectance infrared (SRIR) spectroscopy was carried out with a Bruker Hyperion 3000 Microscope (Bruker, Co.) with a 15x objective lens. A gold mirror was used as the reference background. The detailed set-up of the vibrational SFG spectroscopy system was described elsewhere.40,41 In brief, frequency-doubled laser pulses (532 nm) from a 27 ps Nd:YAG laser and tunable IR pulses generated from an optical parameter amplifier and generator system were used as

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and

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incident beams, respectively. The incident angles of IR and visible pulses were 56° and 60°, respectively. These two laser pulses were overlapped spatially and temporally on the glass surface to generate SFG signal. The SFG signal was detected at the phase matching direction. The polarization combination used in this study was s for SFG signal, s for 532nm laser pulses, and p for IR laser pulses (ssp). The visible and IR pulse energies before reaching the sample were 174 µJ and 136-220 µJ, respectively. The IR wavenumber was calibrated with SFG spectra of DMSO/air interface in ssp polarization.42 For thermodynamic analysis of hydrous species, the SFG signal intensities were recorded as a function of temperature while the glass sample was heated at a 2 K/min rate (β) using a Linkam Scientific heater (model No. TS1500, UK) in a controlled vapor gas environment, as shown in Figure 2. Similar set-up was applied in the ATR-IR analysis.


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Figure 2. (a) Schematics of ssp-polarization SFG analysis of a glass surface as a function of temperature in a controlled vapor environment; (b) Schematics of monitoring the intensity of two SFG peak intensities at the same time during the temperature ramping at a constant rate (β)

3. Results and Discussion Figure 3 compares the OH stretch region of the SFG spectra of 0.7 mm and 1 mm thick SLS float glasses with those of fused quartz and BF33. For each sample, spectra were taken in air (with 30% relative humidity) (i) at room temperature, (ii) at 200°C, and (iii) after cooling back to room temperature for the same spot. These measurements were repeated 3 or more times at different locations; the spectra shown in Figure 3 are the representative ones. In order to rule out the possibility of interferences from the signals reflected from the back surface, especially in the case of multiple peaks of the SLS float glass samples, SFG analysis was performed with a refractive index matching liquid (for example, CCl4) in the back surface; no differences were observed (see Figure S3 in the Supporting Information), indicating that the multiple peaks are not artifacts due to interferences of signals from two surfaces. The 0.7mm thick SLS float glass (Figure 3a) shows three


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sharp peaks at ~3272 cm-1, ~3544 cm-1, and ~3824 cm-1, while the 1mm thick float glass (Figure 3b) shows five peaks at ~3180 cm-1, ~3392 cm-1, ~3552 cm-1, ~3728 cm-1, and ~3920 cm-1. In contrast, the glasses without leachable sodium ions show broad features in SFG. In the SFG spectrum of fused quartz (Figure 3c), a broad peak centered around 3300 cm-1 (spanning from 3000 to 3600 cm-1) and a sharp peak at 3760 cm-1 are observed, which is consistent with the previous report.9 The broad peak below 3600 cm-1 is generally assigned to the hydrogen-bonded OH groups and the sharp peak at 3760 cm-1 is assigned to the free OH groups without hydrogen-bonding interactions.13,31,43,44 The BF33 glass also gives a broad SFG peak centered around 3500 cm-1 (spanning from 3000 to 3700 cm-1) and a relatively sharp peak centered around 3850 cm-1, as shown in Figure 3d. The broadness of the hydrogen-bonded OH peaks in the SFG spectra of fused quartz and BF33 is related to the dynamics of hydrogen bonding interactions. In the condensed liquid phase or adsorbate layers, the OH groups of water molecule undergo very fast formation and dissociation processes of hydrogen bonds with surrounding water molecules, resulting in an ultra-short transient lifetime of the hydrogen bonds with solvating molecules.45,46 Thus, the vibrational relaxation time of the excited OH group is extremely short, which leads to the homogeneous line-broadening of the vibrational absorption band.47–49 Applying this relationship between the OH peak width and the hydrogen bonding dynamics, one could interpret the sharpness of the OH SFG peak (Fig. 3a and 3b), compared to the fused quartz and BF33 spectra (Fig. 3c and 3d), as a longer life-time or slower hydrogen-bonding dynamics of hydrous species in the SLS float glass. In other words, the hydrous species detected with SFG for SLS float glass are in a more static (or less dynamic) solvation shell. Such solvation shells could be formed by the solid silicate networks surrounding silanol groups or water molecules.50 Then, the distinct positions of the multiple peaks could indicate subtle differences in the hydrogen bond


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distance distribution among the species involved in hydrogen bonding interactions (for example, NBO, BO, Si-OH, and H2O) within the SLS float glass structure.

Figure 3. SFG spectra taken at room temperature, at 200°C, and after cooling back to room temperature for (a) 0.7 mm thick SLS float, (b) 1mm thick SLS float, (c) fused quartz, and (d) BF33 glasses.


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In Figure 3, it is noted that the SFG intensities of all glasses decrease to zero at 200 °C. As the samples are cooled back down to room temperature, all peaks fully recover to the same peak positions and intensities, implying the observed changes are reversible. Since 200°C is well below the Tg of the glasses studied in this work, the silicate network does not change during the heating to 200 oC and cooling.5 Also, 200 °C is not high enough for total dehydroxylation of silanol groups.51 Thus, the observed changes upon heating in Figure 3 must be due to physical changes in the state of molecular water at or within the glass. For fused quartz and BF33 which would not have the %& contribution since there is no concentration gradient of leachable sodium ions, this reversible process must be solely due to desorption and adsorption of water molecules at the air/glass interface; i.e. responses (Figure 1).52 The broadness of the SFG features for these glasses is also consistent with the physisorbed water molecules which are subject to fast relaxation dynamics of the OH excitation through hydrogen bonding interactions with surrounding water molecules.53 In the case of SLS float glass, %& could contribute to SFG spectra because the concentration gradient of Na+ in the dealkalization region could induce substantial %& (Figure 1).54 In the dealkalization region, polar water molecules could be aligned if strong %& field exists. In the thermal poling experiment, we have observed that the OH signals of water molecules have opposite phases for the anode-poled and cathode-poled surfaces.28 The phase of OH signals can be related to the direction of OH bonds with respect to the surface.55–57 The subsurface %& field or Na+ concentration gradient could also stabilize certain binding sites where the dipoles of water molecules align favorably with the electric field. As the temperature of the sample is increased, thermal energy would counteract this gradient-induced ordering and desorb water from the stabilized binding site. If thermal energy of the system is high enough, then the degree of gradient-induced alignment of water


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molecules in the dealkalization region would decrease and so does the SFG intensity. This could cause the disappearance of the sharp OH SFG peaks for the SLS float glass upon heating and reappearance upon cooling. If these interpretations are correct, then we expect different thermodynamic properties for the water molecules adsorbed on fused quartz and BF33 glass surfaces and the hydrous species (water molecules interacting with NBO, BO, and Si-OH) in the silicate network of SLS float glass. In order to test this hypothesis, we measured the SFG signal intensity of each peak as a function of temperature (Figure 2b). This method can be called temperature-programmed SFG (TP-SFG) in analogy with temperature-programmed desorption (TPD) in typical surface science studies of adsorbed molecules.58–60 In a TPD experiment, the concentration of surface species is calculated by integrating the intensity of the desorbing species.60 In TP-SFG experiments, the SFG signal intensity is related to the concentration of surface species.32,61 In both TPD and TP-SFG, the rate of desorption from unit surface area could be modeled by the Polarnyi-Wagner equation:60 −

23 24

= 56 7 6 89−

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Vibrational Sum Frequency Generation Spectroscopy Study of

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