Surficial Siloxane-to-Silanol Interconversion during Room

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Surficial Siloxane-to-Silanol Interconversion during RoomTemperature Hydration/Dehydration of Amorphous Silica Films Observed by ATR-IR and TIR-Raman Spectroscopy Suzanne L. Warring,† David A. Beattie,‡ and A. James McQuillan*,† †

Department of Chemistry, University of Otago, P. O Box 56, Dunedin 9054, New Zealand Future Industries Institute, University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia

‡

S Supporting Information *

ABSTRACT: Silica has been frequently studied using infrared and Raman spectroscopy due to its importance in many practical contexts where its surface chemistry plays a vital role. The majority of these studies have utilized chemical-vapor-deposited films in vacuo after high-temperature calcination. However, room-temperature hydration and dehydration of thin silica particle films has not been well characterized in spite of the importance of such films as substrates for polymer and surfactant adsorption. The present study has utilized ATR-IR spectroscopy and thin silica particle films exposed to varying humidity to clearly show reversible conversion between surface siloxanes and hydrogen-bonded silanols without the need for semiempirical peak deconvolution. The IR spectra from corresponding hydration experiments on deuterated silica films has confirmed the vibrational mode assignments. The variation of humidity over silica films formed from silica suspensions of differing pH gave IR spectra consistent with the change in the relative populations of siloxide to silanol surface groups. In addition, total internal reflection Raman spectroscopy has been used to provide further evidence of roomtemperature dehydroxylation, with spectral evidence for the presence of three-membered siloxane rings when films are dehydrated under argon. The confirmation of room-temperature siloxane-to-silanol interconversion is expected to benefit understanding in many silica surface chemical contexts.

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INTRODUCTION Silica is an important technological material in fields such as chromatography, pharmaceuticals, fiber optics, semiconductors, and froth flotation in mineral ore processing.1−3 All of these areas depend on the surface chemistry of silica, which is determined by the relative populations of silanol (SiOH), siloxide (SiO−), and siloxane (Si−O−Si) surface groups. Calcined silica surfaces have greater siloxane relative populations due to the irreversible condensation of silanol groups and are more hydrophobic from the back bonding of oxygen lone pair electrons into the silicon d orbitals. Noncalcined surfaces and those exposed to humid environments have greater relative populations of silanol and siloxide groups, making them more hydrophilic. Furthermore, solution pH is an important variable with low pH giving more hydrophobic silica surfaces due mainly to the greater relative population of silanol to siloxide groups.4 For greater insight into the interfacial behavior of silica and reactions at silica surfaces, numerous infrared (IR) and Raman spectroscopic studies have provided comprehensive vibrational assignments and information regarding silica surface groups and microstructure.5−18 IR studies are particularly valuable because the high polarity of silicon−oxygen bonds gives intense absorption bands in the 1300−1000 cm−1 region, from which can be derived the intertetrahedral Si−O−Si bond angle.5 Absorption bands due to surficial silanol groups were identified © 2016 American Chemical Society

by McDonald in 1957 with the utilization of deuteration and differing inert atmospheres.19 Takamura et al. recorded transmission IR spectra of pressed KBr discs containing naturally occurring SiO2 in the forms of fused quartz, cristobalite, and silica gel.20 This work identified IR absorption bands at 950 and 870 cm−1 of functional groups which were present irrespective of crystallographic structure and which gave a good indication that the absorption bands were due to surface groups. The band assignments from that work were the Si−O stretching mode (ν(Si−O)) of surficial silanol groups (Si−OH) and the antisymmetric stretching mode (νas(Si−O−Si)) of surface siloxane bridges. More recently, Liu and Shen undertook a vibrational sum frequency generation (SFG) study of quartz surfaces21 which definitively supported the conclusions reached by Takamura et al.20 In his early work, Takamura posited that the hydroxylation of silica upon exposure to water vapor should cause decreased absorption in bands associated with bridging siloxanes and increases in bands associated with hydrogenbonded silanol groups (i.e., geminal and vicinal, Figure 1), due to the conversion of siloxane to silanol groups. Similar IR spectroscopic work has been performed on hydroxylated silica by Zhdanov22 with the surface reaction Received: December 10, 2015 Revised: January 23, 2016 Published: January 24, 2016 1568

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Langmuir

which are typical of four-coordinate silica. These transverse optical (TO) modes of the lattice essentially arise from antisymmetric and symmetric Si−O−Si stretch vibrations as illustrated in Figure 3.15 TO modes are split by Coulombic

Figure 1. Types of group present at colloidal silica surfaces.

exhibited clearly and later by Riegel with Raman spectroscopy.23 Riegel reasoned that the hydroxylation of silica surfaces occurred via the opening of three- and five-membered siloxane rings to form more stable six-membered rings as shown schematically in Figure 2.20 Such a mechanism is considered to be

Figure 3. Schematic representation of the transverse optical modes of silica having IR absorptions at ∼1080 and ∼800 cm−1.

coupling between TO and longitudinal optic (LO) modes, although only the LO shoulder at ∼1200 cm−1 on the intense ∼1080 cm−1 TO band is observable in the IR. There are also additional disorder-induced LO−TO splittings due to irregularities within the silica matrix.15,30 The TO modes at ∼800 and ∼1080 cm−1 are the strongest Raman bands of silica particles, with the ∼800 cm−1 mode arising from symmetric Si−O−Si stretches being the most intense. Of most interest for silica particles of small size with high specific surface area are the detectable vibrational modes of surficial groups. These include the O−H stretch and Si−O stretch IR absorptions of silanol (SiOH) groups at 3750−3650 and 960 cm−1, respectively, the Si−O stretch absorptions of siloxide (SiO−) groups at ∼1000 cm−1, and the antisymmetric stretch absorptions of surficial siloxane bridge groups ∼880 cm−1.19−21 Vibrational spectral studies in the 1200−800 cm−1 region have relied heavily on semiempirical peak fitting due to the complex nature of absorptions. Good peak fits require four to six Gaussian components and some fits require up to nine, and thus assignments in this spectral region have been controversial.14,15 The strong Si−O−Si antisymmetric stretch TO mode can cause the total absorption of infrared light in the ∼900−1300 cm−1 region even with the use of relatively thin samples, but this can be avoided with sufficiently thin films. Galeener notably related the wavenumber of the ∼1080 cm−1 TO absorption to the bond angle of intertetrahedral Si−O−Si bonds via the central network force model (Supporting Information).5 This model has been utilized extensively for providing insight into the porosity of silica samples, where a lower wavenumber of the TO mode indicates smaller bond angles and thus smaller ring structures and a more porous silica framework.14,15 In the past 20 years, however, computational methods such as density functional theory (DFT) have become favored for structural insights into amorphous silica frameworks, providing complementary analysis to Raman, IR, and 29 Si NMR spectroscopic experimental data.31,32 Pertinent to the present work from such computational studies is that uncalcined silica samples have a hydroxyl density of ∼5.5 OH nm−2 regardless of their synthetic origin and that room-temperature dehydroxylation occurs most likely via geminal silanol groups.24,25 In recent times, attenuated total internal infrared (ATR-IR) spectroscopy has been an invaluable tool for the in situ identification of a variety of hydrated siliceous species.33−35 Previous studies have utilized thermally grown oxides and hightemperature calcination before spectral analysis in vacuum of silica surface groups to provide insight into condensation

Figure 2. Schematic representation of the surficial hydroxylation of silica upon exposure to humid environments with three- and fivemembered rings opening to form a six-membered ring. Each ring member consists of a Si−O link with Si represented by red spheres and O represented by gray spheres. The red dashed circles enclose on the left a siloxane group which on hydration transforms to silanol groups on the right.

exothermic due to the high energy and bond angle strain of three-membered rings.24,25 The vibrational spectroscopic signature of siloxane rings was noted by Galeener in 1982, where it was seen that fourmembered ring formation requires less energy, with the activation energy of formation increasing for three-, five-, and sixmembered rings, although the larger rings are more stable due to larger intertetrahedral Si−O−Si bond angles.26 Uchino et al. studied fumed silica nanoparticles using Raman, IR, and X-ray diffraction and found a prevalence of three- and four-membered siloxane rings, considered to be high-energy defects, not found in great quantity in bulk silica.27 These specific groups were also found to be present in abundance at silica surfaces.28 Thus, when considering hydroxylation at silica nanoparticle film surfaces, as utilized in this study, it is important to consider a mechanism akin to the one proffered by Riegel.23 While there exists much IR spectroscopic data in the νO−H region for the analysis and detailed understanding of hydroxylation and dehydroxylation reactions at silica surfaces, studies lack an analysis of the mid-IR spectral region containing the intense νSi−O absorptions of the silica framework and also absorptions due to surficial groups involved in hydration reactions.22 It is of interest to probe this spectral region using a simple adsorbate such as water when IR absorption features can be more readily interpreted owing to the aforementioned bevy of in-depth spectroscopic studies on water adsorption. The results are expected to inform the interpretation of the spectral response of silica particle films with nonionic surfactant adsorbates, systems integral to the detergent, pharmaceutical, and cosmetics industries.29 The infrared spectra of silica particles in the 1300−600 cm−1 region contain principal absorptions about 1080 and 800 cm−1 1569

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Langmuir reactions.10,15,19 The present work employed both ATR-IR and Raman spectroscopy to address room-temperature hydroxylation/dihydroxylation of thin silica films exposed to argon atmospheres of varying humidity to aid the interpretation of surface functional group behavior. The vibrational spectra obtained under different humidity conditions have clearly confirmed siloxane-to-silanol interconversion at room temperature.

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MATERIALS AND METHODS

Materials. Water was distilled and then deionized (Millipore, MilliQ RG) to a resistivity of 18 MΩ cm. An aqueous silica suspension (Syton HT-50 Ludox, 50 wt % SiO2, pH 9.9, Na+ counterion, density 1.4 g mL−1, Sigma-Aldrich) which has been prepared from the hydrolysis of sodium silicate was diluted for thin film formation. Silica particles had a mean hydrodynamic diameter of ∼100 nm as measured by dynamic light scattering (DLS), a primary particle size of 7−50 nm, and a BET surface area of ∼140 m2 g−1 as indicated by the supplier. DLS data and an energy-dispersive spectrum (EDS) of a thin silica particle film, which detected only sodium, oxygen, and silicon, are shown in the Supporting Information. Argon (>99.995%, 99.999%, 99.9% D, NMR grade, Cambridge Isotope Laboratories) was used to prepare deuterated samples. Infrared Spectroscopy. Attenuated total reflection infrared (ATR-IR) spectra were recorded using a 3-mm-diameter diamondfaced ZnSe triple-reflection prism (ASI DuraSamplIR) accessory in a Digilab FTS4000 FTIR spectrometer. For D2O experiments, the spectrometer was purged with dried air which had varying CO2 levels, giving fluctuating absorption changes at 2350, 3615, and 3715 cm−1. For aqueous film experiments, the spectrometer was purged with nitrogen (BOC, oxygen-free) to prevent silanol-related OH stretching bands at ∼3740 cm−1 from being obscured. All spectral analysis was performed in Digilab Win IR Pro version 4.0 and Origin version 8.1. Strong diamond IR absorptions from 2300 to 1900 cm−1 precluded spectral data analysis in this range. Spectra were recorded with 64 coadded scans, at 4 cm−1 resolution, with an acquisition time of 75 s. The refractive index of ∼2.42 for diamond and the ATR accessory internal reflection angle of 45° result in an evanescent wave penetration depth into the sample of ∼1 μm at 1000 cm−1.36 Prior to use, the diamond surface was cleaned with a γ-alumina (Alfa, 0.015 μm) aqueous slurry on a polishing microcloth (Buehler) and then thoroughly rinsed with water. Thin silica films were formed from aqueous suspensions diluted from the stock suspension by water or dilute HCl. A range of suspension concentrations was used to obtain different film thicknesses. Typically, a droplet of the chosen suspension was placed on the diamond prism and slowly dried under an argon flow of ∼0.5 L min−1. The resulting silica thin film was covered with a custom-built Kel-F cell as shown in Figure 4. Humidity was determined by the rate of flow of argon through Teflon tubing over ∼1 mL of either water or deuterium oxide into the Kel-F cell, as previously employed by Miranda et al.37 The relative humidity of the effluent argon gas was measured to ±1% using a humidity probe (Vaisala, HMP45A). Scanning Electron Microscopy (SEM). A scanning electron microscope (FE-SEM 6700, JEOL, Tokyo, Japan) was used to determine the dried silica film morphology and thickness. Particle suspension samples were placed on thin glass coverslips, dried under an argon atmosphere, and sputter-coated with a gold−palladium mixture. In general, the film thickness from the drying process was somewhat variable on diamond or glass surfaces. Film surface topographic images were recorded normal to the surface. Cross-sectional SEM images were obtained by scoring the glass with a diamond-tipped pen, fracturing the glass, and collecting images at −2° with respect to the coated glass surface. Total Internal Reflection (TIR) Raman Spectroscopy. For the collection of Raman scattered light, a Renishaw Raman microscope

Figure 4. Schematic of the apparatus for recording thin film IR spectra under controlled humidity. was used (Ramascope System 1000), which has a single spectrograph fitted with holographic notch filters and a Peltier-cooled CCD detector. Two Leica microscopes were fixed in seriesone a DMLM for calibration with a computer-controlled stage and the other with a free space and stage to allow for the total internal reflection (TIR)Raman assembly to fit underneath the objective lens (in this case, a Leica long working distance objective, ×50, 0.5 NA). The TIR liquid cell (custom-built stainless steel) has been described previously (Beattie et al.38) and is mounted on an xy motion stage assembly with a micrometer controlled z-axis platform for accurate positioning in the plane of focus/collection of the spectrometer. The light source for TIR-Raman measurements was a SpectraPhysics Millennia V continuum YVO4 laser at 532 nm. The system was calibrated using a silicon wafer, giving a peak centered at 520 cm−1. Laser light delivery was controlled by a fiber optic coupler (Oz Optics, HPUC-23AF) from the laser head to an output coupler/beam expander (Oz optics, HPUCO-23AF) which produced a beam of ∼1 cm diameter. The fiber optic cable was not polarization-maintaining; therefore, the output had mixed perpendicular and parallel polarization (relative to the plane of the prism surface), although an inspection of the output intensity using a polarizing cube indicated a predominance of parallel polarization. The output coupler and associated focusing optics (15 cm focal length UV silica lens, CVI) and filter (Semrock Maxline laser line filter LL01-532-25 to block Raman scattering from the fiber core and allow transmission of the laser line) were mounted on an optical railing attached to a second xyz translation stage (Newport UMR series) with a rotation mount for control of the incident angle of the laser beam. The laser spot size was ∼40 μm in diameter. For all TIR-Raman measurements, a trapezoidal CaF2 prism was used, and most experiments pertained to the CaF2/water interface which has a critical angle θc of ∼69°. The CaF2 prism was cleaned with 2% Helmanex and then polished with a silica slurry and Buehler cloth, copiously rinsed with water, and then plasma cleaned (Harrick, PDC-OD2). The silica film was prepared as indicated in the ATR-IR section with the exception that the concentration of Ludox was increased to ∼80 mg mL−1 to produce slightly thicker silica films due to the lower signal levels of Raman compared to those of IR. Films were dried in air or under a flow of argon while the prism was mounted in the flow cell. TIR-Raman measurements used a laser power of ∼500 mW at the sample, an acquisition time of 60 s for each spectrum, and the coaddition of five spectra to produce the final spectrum. Spectral acquisition was performed with WiRE (Renishaw), while analysis was performed with Digilab Win IR Pro 4.0 and Origin 8.1.

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RESULTS AND DISCUSSION Thin Silica Film Characterization by SEM and IR Spectroscopy. Thin silica films were formed by identical means on glass surfaces for SEM images and on the diamond ATR-IR prism from 4.5 mg mL−1 silica suspensions of pH ∼9. SEM images of film morphology in Figure 5a show particles ∼100 nm in diameter, in agreement with DLS data recorded for 1570

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Langmuir aqueous suspensions (Supporting Information), and a film thickness of ∼300 nm as shown in Figure 5b. The infrared spectra for films prepared from both aqueous and D2O suspensions are shown in Figure 5c.

structure at the surface of the film. Porosity changes have been found to affect the observed wavenumber of the TO band.10 Previous studies have found that the wavenumbers of the TO and LO modes are indicative of the intertetrahedral bond angle and ring size within the silica and thus the porosity of the silica particle film.7,14,15 A shift to higher wavenumber is typically indicative of decreased porosity, increased Si−O−Si bond angle, and longer Si−O−Si bonds.13,15 IR Spectral Changes of Thin Silica Films in Argon with the Variation in Humidity. ATR-IR spectroscopy facilitates probing the molecular nature of hydration/dehydration processes at the surfaces of very thin silica films in contact with atmospheres of varied humidity. Figure 6 shows the

Figure 5. Thin silica film characteristics. (a) SEM image indicating film morphology, (b) SEM image showing film thickness, and (c) ATR-IR spectra of films formed from aqueous and D2O 4.5 mg mL−1 suspensions of pH ∼9 dried under argon. Minor adventitious hydrocarbon peaks are at ∼2920 and 2860 cm−1.

Figure 6. Absorbance changes in IR spectra of an ∼300 nm silica film initially exposed to RH of 0% (dry argon) and then subsequently to RH values of 8, 15, 20, 30, and 40% . Zero absorbance is shown by a dashed line.

The most intense IR absorption peaks at 1073 cm−1 for aqueous films and 1064 cm−1 for D2O films are due to the TO phonon mode.10,13−15 The shoulder centered at ∼1188 cm−1 is due to the LO mode.17 Both spectra also showed the presence of the siloxane Si−O−Si symmetric stretch vibration at 800 cm−1. The film from an aqueous suspension shows residual water with the characteristic bulk water O−H stretch peak at 3400 cm−1. The vicinal silanol OH stretch mode is observed at 3647 cm−1, and that of isolated silanols, at 3743 cm−1.19 The film prepared with deuterium oxide shows O−D stretches at 2480 cm−1 for bulk D2O, at 2688 cm−1 for H-bonded silanols, and at 2756 cm−1 for isolated silanol groups.19 Both spectra have weak Si−O−Si stretching overtone modes, at 1870 cm−1 for the aqueous film and at 1880 cm−1 for the D2O film.39 These overtone absorptions have been useful in indicating the amount of bulk SiO2 present within a film.39 In the deuterated SiO2 spectra, there is a separation of ∼66 cm−1 between vibrational bands of isolated and vicinal silanols while in the aqueous spectra the separation is ∼84 cm−1. This implies that hydrogen bonds are weaker between SiOD species, which is expected due to isotope effects.40,41 Assignments of the vibrational bands presented in the Figure 5c spectra are found in the Supporting Information, Table S1. Notably, the wavenumber of the νas(Si−O−Si)LO mode is identical in both film spectra, while both the overtone and TO wavenumbers have decreased by ∼10 cm−1 in D2O. The presence of D2O within the film is likely to affect the pore

absorbance changes in IR spectra (difference spectra) of an ∼300 nm silica film, formed from 4.5 mg mL−1 suspension of pH ∼9, with the variation of relative humidity from a reference condition RH of 0% (dry argon) to 8, 15, 20, 30, and 40%. Humidity increases of up to 40% within the cell gave clear gains in bulk and interfacial water absorption at ∼3400, 3200, and 1630 cm−1, with the absorption maximum at ∼3400 cm−1 most pronounced. During this RH increase, an absorption loss occurs at 3736 cm−1 in the O−H stretching band of isolated silanols. This correlates with absorption gains in a shoulder at ∼3636 cm−1 due to O−H stretching and a band at 956 cm−1 from the Si−O stretching of vicinal silanols, suggesting an increasing surface concentration of vicinal silanols with surface hydration. As interfacial and bulk water absorptions increase with increasing humidity, it is expected that isolated silanols will cooperate in hydrogen bonding and therefore these species (Figure 1) will be eliminated at the surface. Significantly, the absorption loss in the band associated with isolated silanols at 3736 cm−1 is accompanied by an absorption loss in a band at 876 cm−1 due to the symmetric stretching of surficial siloxane bonds. In the Figure 5 spectrum of the thin film, the surface group absorptions in the ∼1000 cm−1 region are buried beneath the much more intense bulk silica absorptions. The most prominent absorption changes in the 1100−1020 cm−1 Si−O stretch region are an absorption gain peak at 1033 cm−1 and an absorption loss peak at 1107 with an apparent isosbestic point at 1060 cm−1. This absorption gain/loss spectral feature arises 1571

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Langmuir primarily from a hydration-induced peak shift of up to 2 cm−1 to lower wavenumber of the intense TO band, measured at 1073 cm−1 (Figure 5c) in the dried silica film. See Figure S3 in the Supporting Information for the spectra of the film at different RH values relative to that of the bare prism surface, where it can be seen that this peak shift is due to a broadening of the peak to lower wavenumber. An advantage of the humidity change spectral data of Figure 6 is that changes in surface group absorptions become clearly evident. There appears to be an isosbestic point between the peaks of opposite polarity at 876 and 956 cm−1, although the point is not absolute. Isosbestic points occur in spectra when one absorbing species is being converted to another and at a wavenumber where the molar absorption coefficients of these species are equal.42 The loss of surface siloxane absorption with the concomitant gain of silanol absorption in this spectral region with increasing humidity must arise from the interaction with adsorbed water (further discussed in a later section). These spectral changes correspond to siloxane-tosilanol interconversion as described by Takamura and revealed spectroscopically by Zhdanov (Figure 1).20,22 In fact, spectral results for the ν(O−H) spectral region (3000−3800 cm−1) presented in this work bear a striking resemblance to those shown by Zhdanov for silica films under evacuation.22 However, the present silica films were formed and studied at room temperature while the silica films of Zhadanov had been precalcined at elevated temperatures. The present work illustrates that the hydration of these silica films under an inert atmosphere not only alters the amount of interfacial and bulk water adsorbed but also causes surficial hydroxylation at room temperature, as indicated by the isosbestic behavior of the 956 cm−1 absorption increase and the 876 cm−1 absorption decrease. Silica films ∼500 nm thick were prepared from 8.0 mg mL−1 silica suspension with the pH adjusted to 2.5. Such preparations arose from solution adsorption experiments where silica thin films prepared from lower-pH suspensions were found to be much more stable compared to those prepared from the water dilution of high-pH stock silica suspensions which tended to lose some particles under flow. SEM images and an IR spectrum of such a film are shown in Figure S6 of the Supporting Information. An ∼500-nm-thick film approaches the thickness giving total absorption of the IR beam at the intense TO band, but the ∼300 nm film is clear of this difficulty. The bands shown in the IR spectrum of this thicker film are identical in wavenumber to those of the ∼300 nm film in Figure 5. The absorbance changes in IR spectra (difference spectra) from exposing a ∼ 500 nm thick film to increasing humidity from RH of 0% as performed for the ∼300 nm film are presented in Figure 7. The spectral changes from the hydration changes are similar to those found for the ∼300 nm thick film with peaks at 1024 and 970 cm−1 compared to those in Figure 6 at 1033 and 956 cm−1, and there is increased spectral noise in the TO mode region due to nearly total absorption. Subsequent to the hydration of the film, the reverse process was carried out using flowing dry argon. The spectral changes from the dehydration of the silica film using dry argon are also shown in Figure 7. The dehydration results mirror those found for the hydration of the films with humidified argon and illustrate that the hydroxylation/dehydroxylation of silica at room temperature is completely reversible. The shift of the ν(Si−OH) peak to 970 cm−1 in Figure 7 compared to 956 cm−1 in Figure 6 is due to the ∼500 nm film

Figure 7. Absorbance changes in IR spectra of ∼500 nm silica films, formed from a pH 2.5 suspension, during hydration and dehydration. Spectra showing increasing absorption from hydration were recorded during the RH change from 0 to 40% with a RH of 0% as the background. Spectra showing decreasing absorption from dehydration were recorded during the RH change from 40 to 0% with a RH of 40% as the background. Zero absorbance is shown by the dashed line.

being formed from a pH 2.5 silica suspension. The decreased pH of the silica suspension affects the hydroxylation of the silica surface both in the suspension and in films formed from these suspensions. An upward shift of the associated silanol νSi−OH absorption occurs, and this effect will be discussed more fully in a later section. In the absorption spectra presented in Figure 7 compared to those in Figure 6, the band due to νs(Si−O−Si) of surficial siloxanes has shifted to 888 cm−1 from 876 and the 970 cm−1 absorption band is also more intense than the 1024 cm−1 absorption band, whereas the opposite occurs for the bands with the corresponding hydration change in Figure 6 for the thinner film spectra. These spectral changes are directly correlated to changes in the surface charge density of the silica surface with hydroxylation. Lagström et al.42 have recently shown that the νSi−OH wavenumber from colloidal silica aqueous suspensions shifts from 962 to 983 cm−1 with the pH change from 10 to 1.4. The wavenumber shift from 956 to 970 cm−1 in the present νSi−OH data with the decrease in pH of the film-forming silica suspension is consistent with this observation. Lagström et al.43 have also suggested (as have others20) that νSi−O absorptions of siloxide (Si−O−) groups occur at ∼1040 cm−1 and are thus expected to make a contribution to absorption in this region when siloxide concentrations are significant. Thus, the apparent isosbestic point at 1060 cm−1 is not real because it arises from a shift in the major absorption to lower wavenumber due to an increased siloxide contribution. The greater relative intensity in Figure 6 of the 1033 cm−1 peak (siloxide-related) with respect to the 956 cm−1 peak (silanol) for films from alkaline suspensions compared to that of the corresponding 1024 cm−1 peak with respect to the 970 cm−1 peak in Figure 7 for films from pH 2.5 suspensions is consistent with this νSi−O− assignment and expected siloxide-to-silanol relative concentrations for films formed from different-pH suspensions. The absorbance loss at 1111 cm−1 coupled with an absorbance gain at 1024 cm−1 could be attributed to the changing refractive index and film thickness as humidity is increased. Simulated and recorded IR spectra have shown that increases in the refractive index of silica films correlate to the increased wavenumber of the 1572

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Langmuir TO modes.44,45 This reported trend is contrary to the present results which show a distinct decrease in the wavenumber of the intense TO mode with increasing humidity. Ellipsometric studies have shown that as the humidity increases from ∼3 to 40% the refractive index of sol-derived silica films increases from ∼1.3 to 1.34, depending on the sol type.46 Because of the nature of total internal reflection, this change causes an increase in the penetration depth (dp) and the effective sampling thickness (de) of the evanescent wave.36 (See Supporting Information and Figure S5.) However, any such influence on spectral absorbance would be general throughout the spectral range, which is not observed in the present data and can therefore be discounted. The present work would benefit from determining the influence of changing refractive index on the main νSi−O absorption envelope at ∼1180−1000 cm−1 due to relative humidity changes. Thereby, it could be identified definitively if the differential band at 1200−1000 cm−1 is due to a speciation change at the silica surface or an anomalous dispersion.36 This could be achieved by the variation of the internal reflection angle θi and the observation of how this affects the main absorption envelope. Additionally, Kramer−Kronig analysis of ellipsometric measurements could be used to determine the absolute change in refractive index.1 IR Spectra of Thin Silica Films in Argon with the Variation in D2O Humidity. The assignments of silanol spectral bands were tested with ∼300 nm silica films formed from a silica suspension in D2O (pH ∼9) and then exposed to argon humidified with D2O as for the aqueous systems. Spectral results are shown in Figure 8, with the bulk D2O stretch and

experimental studies of solvated H4SiO4 and D4SiO4, where the observed shift was 12 cm−1.48 This upshift in wavenumber is due to a reduction in hydrogen bond strength for Si−OD groups, causing a reduction in the Si−O bond length of the deuterated silanols and thereby an increase in the observed vibrational frequency.48 The symmetric Si−O−Si stretching mode of surficial siloxane bonds in D2O is observed at 890 cm−1 and is 14 cm−1 higher in wavenumber than that in the corresponding aqueous spectra. This higher wavenumber can be explained by less interference from the D2O librational absorption which is found at lower wavenumbers compared to the aqueous film spectra and the significant wavenumber increase in the Si−OD shoulder absorption. The 980 cm−1 ν(Si−OD) mode is diminished in intensity in comparison to the observed ν(Si−OH) mode observed for IR hydration studies in Figures 6 and 7. This may be due to the deuteration of silanol groups being selective of surficial silanol groups owing to heavy water not being able to penetrate beyond the silica surface.47 The 1060 cm−1 absorption loss is more pronounced in the D2O spectra than in the corresponding aqueous spectra. Differences between the amorphous silica film structure and the nonuniformity in thickness will render some vibrational bands more active than others. However, the bands at 1112, 1060, and 1032 cm−1, due to νas(Si−O−Si) modes, have not shifted significantly in wavenumber and have similar absorption gains and losses in comparison to those in the aqueous film spectra (Figure 6). IR Spectral Study of the Hydration of Silica Thin Films Prepared from Different pH Suspensions. An important consideration for the spectral response of the silica film with varying humidity is how the pH of the silica suspension used in film preparation affects the film spectra, particularly the bands of surficial silanol functional groups. Dilution of the 50 wt % Ludox suspension with water results in the pH of the suspension being reduced from the original value of 9.9. To analyze this influence on the spectra of the silica films utilized in this study, silica suspensions with pH decreased by HCl addition were dried as previously. The changes in the IR spectra of silica films at equilibrium with argon at RH ∼40% in comparison with RH 0% and prepared from pH 8.5, 6.5, and 4.5 suspensions are shown in Figure 9.

Figure 8. Absorbance changes in the IR spectra of an ∼300 nm silica film with D2O humidity changes from an initial RH of 0% to RH values of 11, 24, 33, and 40%. Zero absorbance is shown by the dashed line.

bend maxima at 2500 and 1192 cm−1, respectively. The band at 2754 cm−1 corresponds to the free non-hydrogen-bonded νSiO−D mode.47 Increasing humidity causes an absorption loss in this band, as observed for the corresponding aqueous spectra due to siloxane-to-silanol interconversion, and an increase in vicinal silanol stretching observed at 2688 cm−1. The Si−OD stretching is found at 980 cm−1, which, compared to the corresponding aqueous spectra, has increased in wavenumber by 26 cm−1. Assignments are found in Table S2 of the Supporting Information. A frequency increase in deuteration is unusual and has been noted by McIntosh et al. in computational and

Figure 9. Absorbance changes in IR spectra of ∼500 nm films prepared from 8.6 mg mL−1 colloidal silica suspensions at pH 8.5, 6.5, and 4.5 exposed to ambient humidity after being dried in argon (RH 0%). Zero absorbance is shown by the dashed line. 1573

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Langmuir There are increases in absorption associated with bulk and interfacial water at 3400 and 1630 cm−1 and losses in bands at 3740 and ∼880 cm−1 due to isolated silanol groups and siloxane bridges, respectively. The small shift in the strong TO mode at ∼1100 cm−1 is not very clear in the humidity change spectra due to almost total IR absorption with this film thickness. Significantly, as the pH decreases, an increase in the wavenumber of the Si−OH absorption band in the 955−976 cm−1 region is observed, as previously noted by Lagström et al.43 The isoelectric point of silica is found at pH ∼249−52 so that as the pH drops, siloxide (SiO−) groups decrease in population on the surface as they become hydroxylated.53,54 Lagström et al. also attributed the decreased absorption at 1040 cm−1 with decreasing pH to the loss of siloxide group absorption.43 In the present results, a similar correlation is observed where the peak at 1020 cm−1 has a decrease in absorbance from pH 8.5 to 6.5 as the Si−OH bands shift upward in frequency. However, at pH 6.5 the 1020 cm−1 band has a slightly smaller absorbance than that at pH 4.5. This trend may be due to the Si−O− group population remaining similar between these pH values or the band being affected by the total absorption at higher wavenumber. The changes in relative population of silanol to siloxide groups with pH may explain the lack of a clear isosbestic point between the absorption bands at 876 and 956 cm−1 in Figure 6. As the film prepared from an alkaline suspension is exposed to a humidified atmosphere derived from circumneutral water, the charge density of the surface will decrease. Such a pH change would induce a silanol νSi−O wavenumber increase from 952 to 956 cm−1 and a concurrent shift to higher wavenumber of the 876 cm−1 absorption decrease, resulting in the loss of a clear isosbestic point between the relevant peaks. This may explain why an isosbestic point is more readily observed in the thicker film prepared from a pH 2.5 silica suspension (Figure 8), where it is expected that the surface is almost fully hydroxylated and exposure to a humidified atmosphere would not significantly alter the surface charge density. TIR-Raman of Silica Films. Raman spectroscopy has been used to examine silica films formed from ∼80 mg mL−1 silica suspensions primarily to seek evidence of changes in surface ring structure as previously noted by Brown.32 Figure 9 shows TIR-Raman spectral data obtained from such silica films prepared from suspensions with pH 2.5 and 9.5 at RH ∼40% as well as from films prepared from pH ∼9.5 suspensions at RH 0 and ∼40%. Residual water, which is noted in the 1628 cm−1 bending mode for both of the RH 40% spectra, is most intense in the pH 2.5 spectrum while in the 0% spectrum it is undetectable. Also observed in all of the TIR-Raman spectra is a small CH2 bending band at 1454 cm−1 due to an adventitious hydrocarbon impurity. Denoted in Figure 10 are Q n connectivities (also depicted as Qn) where n signifies the number of bridging oxygen atoms for that Si atom.11 Previous studies of condensed SiO4 structures have utilized this nomenclature in the assignment of Raman, XPS, and 29Si NMR spectra, and a good correlation exists between the different Qn structures by the three techniques.11,32,55 The νas(Si−O−Si) peak at 1187 cm−1 observed in the pH 2.5 and ∼9.5, RH 40% spectra is due to Q 4 from an SiO4 bound to four other tetrahedra as highly linked silica units and is thus indicative of bulk vibrations of the silica matrix. The Q 4 signal is quite diminished and almost unobservable in the pH ∼9.5 and RH 0% Raman spectrum. All three spectra show a mode at 1060 cm−1 due to Q 3 SiO4 species with nonbonded oxygens which can have varying degrees of

Figure 10. TIR-Raman spectra of silica films prepared from pH 9.5 and pH 2.5−80 mg mL−1 silica suspensions with spectra recorded at RH values of ∼40 and 0% (argon atmosphere). Spectra are offset on the absorbance scale for clarity. Q n SiO4 structures are shown schematically, Raman bands are assigned to their respective structures, and the Raman band of the three-membered D2 ring is also shown.

protonation. Because of the greater intensity of the Q 3 band compared to that of the Q4 band, it can be inferred that the silica sampled by Raman scattering has a higher proportion of Q 3 sites.55 The ∼1030−700 cm−1 spectral region pertains to Q 2, Q 1, and Q 0 bands which have greater intensity than Q 3 and Q 4 bands due to stronger Raman scattering.55 Previous studies employing 29Si NMR have shown the expected predominance of Q 3 and Q 4 sites in silica nanoparticles.32,55 Thus, the Raman band intensity cannot be accurately correlated to the population of the differing surficial groups. Assignments in this region were aided by spectral deconvolution of which full details are given in the Supporting Information. Q 2 bands have been identified at 970, 966, and 961 cm−1 at pH 2.5, RH 40%; pH 10, RH 40%; and pH 10, RH 0% spectra with additional Q 2 bands at 936 and 926 cm−1 in pH 2.5, RH 40% and pH 10, RH ∼ 0% spectra. Bands in the 1020−1009 cm−1 range are from the deprotonated Q 2 species. Q 1 bands have been identified at 870 and 804 cm−1 in the pH 2.5, RH ∼40% spectrum and at 868 and 821 cm−1 in the pH 10, RH 0% spectrum. The most significant finding of the Raman spectra presented here is the observation of a defect mode at 618 cm−1 in only the pH 10, RH ∼0% spectrum which has been assigned previously as a D2 mode due to breathing-type vibrations of 3-fold siloxane rings.31,56,57 This is indicative that these high-energy defects are present with the removal of interfacial water by dry argon, and the mechanism as presented by Riegel et al.23 with three- and five-membered ring openings with increased humidity to form a larger six-membered ring is the likely contributor to the spectral changes in the infrared and Raman data presented here. The general consensus in the literature is that the evacuation of particulate silica at room temperature causes the removal of sorbed water, with perhaps a monolayer of adsorbed water remaining until the silica is calcined above 200 °C.4 A combination of experimental spectroscopic and quantum mechanical studies have shown that the dehydroxylation of a silica surface will not occur until the temperature is >170 °C.25,58 1574

DOI: 10.1021/acs.langmuir.5b04506 Langmuir 2016, 32, 1568−1576

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Langmuir

(3) Misra, P. K.; Mishra, B. K.; Somasundaran, P. Organization of Amphiphiles. Colloids Surf., A 2005, 252 (2−3), 169−174. (4) Zhuravlev, L. T. The Surface Chemistry of Amorphous Silica. Zhuravlev Model. Colloids Surf., A 2000, 173 (1−3), 1−38. (5) Galeener, F. Band Limits and the Vibrational Spectra of Tetrahedral Glasses. Phys. Rev. B: Condens. Matter Mater. Phys. 1979, 19 (8), 4292−4297. (6) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (7) Galeener, F.; Leadbetter, A.; Stringfellow, M. Comparison of the Neutron, Raman, and Infrared Vibrational Spectra of Vitreous SiO2, GeO2, and BeF2. Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 27 (2), 1052−1078. (8) Boyd, I. W. A Study of Thin Silicon Dioxide Films Using Infrared Absorption Techniques. J. Appl. Phys. 1982, 53 (6), 4166. (9) Olsen, J. E.; Shimura, F. Infrared Reflection Spectroscopy of the SiO2-Silicon Interface. J. Appl. Phys. 1989, 66 (3), 1353. (10) Almeida, R. M.; Pantano, C. G. Structural Investigation of Silica Gel Films by Infrared Spectroscopy. J. Appl. Phys. 1990, 68 (8), 4225. (11) Xue, X.; Stebbins, J.; Kanzaki, M. Pressure-Induced Silicon Coordination and Tetrahedral Structural Changes in Alkali OxideSilica Melts up to 12 GPa - NMR, Raman, and Infrared-Spectroscopy. Am. Miner. 1991, 76, 8−26. (12) Greeley, J. N.; Meeuwenberg, L. M.; Banaszak Holl, M. M. Surface Infrared Studies of Silicon/Silicon Oxide Interfaces Derived from Hydridosilsesquioxane Clusters. J. Am. Chem. Soc. 1998, 120 (31), 7776−7782. (13) Fidalgo, A.; Ilharco, L. M. The Defect Structure of Sol−gelDerived Silica/polytetrahydrofuran Hybrid Films by FTIR. J. NonCryst. Solids 2001, 283 (1−3), 144−154. (14) Gallardo, J. Structure of Inorganic and Hybrid SiO2 Sol−gel Coatings Studied by Variable Incidence Infrared Spectroscopy. J. NonCryst. Solids 2002, 298 (2−3), 219−225. (15) Innocenzi, P. Infrared Spectroscopy of Sol−gel Derived SilicaBased Films: A Spectra-Microstructure Overview. J. Non-Cryst. Solids 2003, 316 (2−3), 309−319. (16) Takada, S.; Hata, N.; Seino, Y.; Yamada, K.; Oku, Y.; Kikkawa, T. Mechanical Property and Network Structure of Porous Silica Films. Jpn. J. Appl. Phys. 2004, 43 (5A), 2453−2456. (17) Jung, H. Y.; Gupta, R. K.; Oh, E. O.; Kim, Y. H.; Whang, C. M. Vibrational Spectroscopic Studies of Sol−gel Derived Physical and Chemical Bonded ORMOSILs. J. Non-Cryst. Solids 2005, 351 (5), 372−379. (18) Arnoldbik, W.; Tomozeiu, N.; van Hattum, E.; Lof, R.; Vredenberg, A.; Habraken, F. High-Energy Ion-Beam-Induced Phase Separation in SiOx Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71 (12), 1−7. (19) McDonald, R. S. Study of the Interaction between Hydroxyl Groups of Aerosil Silica and Adsorbed Non-Polar Molecules by Infrared Spectrometry 1. J. Am. Chem. Soc. 1957, 79 (4), 850−854. (20) Takamura, T.; Yoshida, H.; Inazuka, K. Infrared Characteristic Bands of Highly Dispersed Silica. Colloid Polym. Sci. 1964, 195 (1), 12−16. (21) Liu, W.-T.; Shen, Y. Surface Vibrational Modes of AQuartz(0001) Probed by Sum-Frequency Spectroscopy. Phys. Rev. Lett. 2008, 101 (1), 016101. (22) Zhdanov, S. P.; Kosheleva, L. S.; Titova, T. I. IR Study of Hydroxylated Silica. Langmuir 1987, 3 (6), 960−967. (23) Riegel, B.; Hartmann, I.; Kiefer, W.; Groβ, J.; Fricke, J. Raman Spectroscopy on Silica Aerogels. J. Non-Cryst. Solids 1997, 211 (3), 294−298. (24) Ugliengo, P.; Saunders, V.; Garrone, E. Silanol as a Model for the Free Hydroxyl of Amorphous Silica: Ab-Initio Calculations of the Interaction with Water. J. Phys. Chem. 1990, 94 (6), 2260−2267. (25) Ewing, C. S.; Bhavsar, S.; Veser, G.; McCarthy, J. J.; Johnson, J. K. Accurate Amorphous Silica Surface Models from First-Principles Thermodynamics of Surface Dehydroxylation. Langmuir 2014, 30 (18), 5133−5141.

However, the spectral results presented here are indicative of the reversible opening and closing of three- and five-membered siloxane rings at room temperature via a hydroxylation/ dehydroxlation mechanism involving geminal silanol groups, as has been previously proposed.25

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CONCLUSIONS The infrared and Raman spectral data from this work is clearly indicative of siloxane-to-silanol interconversion with roomtemperature hydration/dehydration. The presented data illustrates not only bulk and interfacial water adsorption at thin silica films but also surficial siloxane-to-silanol interconversion, as evidenced by the isosbestic point between the absorption loss at ∼880 cm−1 due to νs(Si−O−Si) of surface siloxane bridges and the absorption increase at ∼960 cm−1 due to ν(Si−OH) of hydrogen-bonded silanol groups. Cycles of dehydration and hydration have shown that this process is completely reversible. In addition, TIR-Raman data has shown that three-membered rings exist on the surface of the silica nanoparticle film at 0% humidity, leading to the conclusion that room-temperature interconversion likely occurs via hydroxylation and subsequent ring opening of these strained siloxane rings as proposed by Zhdanov.22 These spectral results will facilitate the interpretation of room-temperature vibrational spectroscopic data from polymers/surfactants adsorbed to silica particle films from aqueous and nonpolar solutions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04506.

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Silica infrared spectral assignments, ATR-IR theory, EDS data, DLS data, Raman peak fitting, and the central network force model (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 64 3479 7906. Tel: 64 3479 7924. Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

Ministry of Business, Innovation and Employment, New Zealand and the University of Otago. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge David Savory for his contribution to the graphics and Liz Girvan for her assistance with the acquisition of SEM images.

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REFERENCES

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