Dynamics of Silica Aerogel's Hydrophobic Groups: A Quasielastic

Article pubs.acs.org/JPCC

Dynamics of Silica Aerogel’s Hydrophobic Groups: A Quasielastic Neutron Scattering Study Wim J. Malfait,*,† Fanni Jurányi,*,‡ Shanyu Zhao,† Shelly A. Arreguin,† and Matthias M. Koebel† †

Laboratory for Building Energy Materials and Components, Empa, Swiss Federal Laboratories for Material Science and Technology, Ü berlandstrasse 129, CH 8600 Dübendorf, Switzerland ‡ Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland S Supporting Information *

ABSTRACT: Silica aerogels can be hydrophobized with a variety of functional groups, resulting in different material properties. Here we compare the mobility of hydrophobic groups using neutron spectroscopy. Data were collected between 1.5 and 300 K on the MARS backscattering spectrometer at SINQ (PSI) for three different silica aerogels and octakis(trimethylsiloxy)silsesquioxane (Q8M8) as a reference. The mobility persists to below 10 K for silica aerogels whose surfaces are decorated with trimethylsilyl (TMS) and ethoxy groups. The high low-temperature mobility of the hydrogen in these surface functional groups originates from the large rotational freedom in the aerogel’s open mesopores. In contrast, the mobility freezes in below 30 K for Q8M8, consistent with a reduced rotational freedom within the dense Q8M8 crystal structure. At room temperature, both the rotations of hydrogen within the methyl groups and the rotations of the entire TMS groups contribute to the mean square displacements for both the silylated silica aerogels and the Q8M8 reference. For the methyltrimethoxysilane (MTMS) based aerogel, the room temperature mean square displacements are limited to methyl rotations only. In addition, the mobility is completely frozen in below 30 K, presumably because ∼80% of the methyl groups are not on the surface but located inside the predominantly amorphous SiO1.5CH3 structure. No distinct methyl tunneling peaks were detected at 1.5 K for any of the samples, although a weak but statistically significant shoulder is present in the energy spectrum for the MTMS based aerogel.

1. INTRODUCTION Silica aerogels are sol−gel derived mesoporous solids with a range of exceptional properties, most notably an ultralow thermal conductivity,1 and are commercially available for thermal insulation applications. Other applications related to catalysis,2 oil−water separation,3 Knudsen pumps,4 etc., are under development. When left untreated, silica aerogels are hydrophilic with a large surface area (500−1000 m2/g) covered in silanol groups. As a consequence, the aerogel properties degrade dramatically upon exposure to water. Therefore, most silica aerogels undergo a hydrophobization treatment prior to drying. As an additional benefit, the hydrophobization of the silica surfaces results in a spring-back effect during ambient pressure drying, which obviates the need for supercritical drying.5,6 Hydrophobization is most commonly carried out by silylation, i.e., by covering the silica surface with trimethylsilyl (TMS) groups. After gelation and aging, the silica gel is placed inside a solvent mixture containing a silylating agent, e.g., trimethylchlorosilane (TMCS), hexamethyldisilazane (HMDZ), or hexamethyldisiloxane (HMDSO), and most of the hydrophilic surface silanol groups are replaced by hydrophobic TMS groups.6−9 Alternatively, difunctional © XXXX American Chemical Society

(dimethyldi(m)ethoxysilane, DMDMS/DMDES) or trifunctional (methyltri(m)ethoxysilane, MTMS/MTES) silanes can be added to the sol before gelation, resulting in inherently hydrophobic materials.5,10−14 On a qualitative level, the tools to characterize the surface chemistry have long been available, for example Fourier transform infrared (FTIR)15 and solid-state nuclear magnetic resonance (NMR) spectroscopy.16−18 More recently, we established solid-state NMR spectroscopy as a fully quantitative tool to study the surface chemistry of silica aerogels7 and determined the concentration, surface density, and connectivity of hydrophobic groups for a range of archetypal silica aerogel materials.8 However, the dynamic behavior of the hydrophobic groups, and the methyl group rotational dynamics in particular, have not been studied in detail, even though it may have important implications for nitrogen sorption measurements and the gas sorption capacity. Quasielastic neutron scattering (QENS) probes molecular motions and is particularly sensitive to hydrogen containing Received: June 19, 2017 Revised: August 21, 2017 Published: August 29, 2017 A

DOI: 10.1021/acs.jpcc.7b06011 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

confining force of 4 N (corresponding to ∼35 kPa) to reduce excessive compression of the (weak) aerogel samples during the analysis. The specific surface area SBET was determined from the nitrogen sorption isotherms (Micromeritics, Triflex) using Brunauer−Emmet−Teller (BET) analysis.33 Because the nitrogen sorption data is affected by deformation during the nitrogen sorption measurements,34 the average pore diameter DPore was calculated from the density and surface area DPore = 4VPore/SBET, where VPore = 1/ρ − 1/ρ0, where the skeleton density ρ0 is approximated by 2.0 g/cm3. The concentration of TMS, methyl ,and (m)ethoxy groups of the aerogel samples was determined from quantitative 1H solidstate NMR spectra collected with a Bruker Avance III spectrometer equipped with a wide-bore 9.4 T magnet. The samples were loaded in 2.5 mm outer diameter zirconia rotors and magic angle spinning (MAS) with a frequency of 24 kHz was applied to maximize the spectral resolution. The pulse delay was set to at least 5 times T1 (as determined by saturation recovery) to ensure complete relaxation of the magnetization. The concentration of the various species was determined from the spectra through external calibration of the absolute signal intensities.7 Cross-polarization (CP) MAS NMR spectra (1H−13C and 1H−29Si) were collected with the same spectrometer. Samples were loaded in 7 mm outer diameter zirconia rotors to maximize sensitivity but limiting the MAS rate to 4 kHz. Note that, in contrast to the 1H spectra, these spectra are not quantitative because both absolute and relative peak intensities are related not only to the concentration of a specific species but also to the crosspolarization efficiency, which strongly depends on the hydrogen−carbon or hydrogen-silicon distance. In addition, the signal may be modulated by differential T1 relaxation for 1H as a pulse delay of two seconds (∼1.25T1) was applied to maximize sensitivity. The single pulse 1H, 13C, and 29Si spectra of Q8M8 were collected with a MAS rate of 4 kHz as part of a previous study.7 The microstructure of the aerogels was investigated by transmission electron microscopy (TEM) using a JEOL2200FS microscope operated at an acceleration voltage of 200 kV (spot size 1, α = 3). A drop of a dilute aerogel powder suspension in methanol was placed onto a thin carbon grid, after which the methanol was allowed to evaporate at ambient temperature, leaving behind submicrometer aerogel grains that can be imaged by TEM. Scanning electron microscopy (SEM) was carried out after coating with 15 nm of Pt with a FEI Nova NanoSem 230 microscope operating at 15 kV in immersion mode. 2.3. Neutron Scattering. The neutron scattering data were collected with the MARS backscattering spectrometer at the Swiss Spallation Neutrons Source (SINQ) operated by the Paul Scherer Institute (PSI). The measured spectra contain a resolution limited peak at zero energy transfer (elastic scattering), for which the intensity is proportional to the amount of static hydrogens in the sample for the studied materials. The wavelength of the elastically scattered neutrons was 6.65 Å (E = 1.85 meV). Mobile hydrogens exchange energy with the neutrons. Stochastic motions such as rotational diffusion of the functional groups cause broader peak(s), centered also at zero energy transfer (quasielastic scattering), whereas the energy of vibrations are so high that they fall outside the probed energy range. At low temperature, the thermal energy is insufficient to cross the energy barriers for rotation and quantum tunnelling transitions may be observed in

species because of hydrogen’s large incoherent scattering cross section. QENS has not been applied to determine the dynamics of hydrophobic groups grafted onto silica aerogel surfaces, but it has been applied to probe the dynamics of pore fluids in nanoporous silica materials, e.g., propane within silica aerogel’s mesopores,19 methane in carbon aerogel,20 toluene in mesoporous silicates,21 CH3I in silica xerogels22 or water in MCM-41-S silica,23 methyl group dynamics in polymers,24 polydimethylsiloxane (PDMS, silicone rubber),25,26 and selfdiffusion of water dissolved in high-pressure, high-temperature silica(te) melts.27 Particularly relevant is the work of Jalarvo et al.28,29 about the dynamics in polyhedral oligomeric silsesquioxanes (POSS) with different functional ligands, including methyl and TMS groups (i.e., the same material as our Q8M8 sample). The closely related method of inelastic neutron scattering (INS) has been applied to silica aerogels to determine the vibrational density of state,30−32 typically on samples that have been treated to remove as much hydrogen as possible to reduce incoherent scattering contributions. In this study, we investigate the mobility of the hydrophobic groups in three silica aerogel samples and a crystalline reference material (Q8M8) with quasielastic neutron scattering measurements between 1.5 and 300 K. Three silica aerogels with a surface chemistry consisting of (i) TMS and silanol groups, (ii) TMS and ethoxy groups, and (iii) surface and (predominantly) internal methyl groups were selected. The QENS data indicate a high mobility of the surface TMS and ethoxy groups that only freezes in at the lowermost temperatures. In contrast, the rotation of the methyl groups of sample (iii) is more restricted.

2. EXPERIMENTAL SECTION 2.1. Sample Synthesis. We investigate three silica aerogels with different surface chemistries. The first aerogel (“TMCS +HMDSO”) has abundant TMS groups but no ethoxy groups. For its synthesis, 7.5 mL of waterglass was mixed with 22.5 mL of distilled water, and the dilute silicate solution was passed through an ion exchanged column. The resulting silicic acid sol was pH 2.2 and 2.5 M ammonia was added to increase the pH to 5. After gelation and overnight aging at 55 °C (∼10 h), the hydrogel was immersed into a HMDSO/TMCS solution (30 mL of TMCS and 40 mL of HMDSO) for 48 h. The gel was finally dried at 150 °C for 3 h. The second aerogel (“Aero B”) was synthesized by acid−base catalysis of tetraethoxysilane (TEOS) and hydrophobized with HMDSO in acidic conditions. The sample was part of a previous solid-state NMR study,7 and a full description of its synthesis and surface chemistry can be found there. Its surface coverage contains numerous ethoxy groups in addition to TMS. The third aerogel sample (“MTMS”) is inherently hydrophobic with one methyl group per Si atom. A total of 5 mL of MTMS was diluted with 49.6 mL of methanol. The hydrolysis was initiated by the dropwise addition of 2.5 mL of 0.01 M oxalic acid in water and allowed to proceed for 24 h at room temperature. Gelation was initiated by the addition of 2.5 mL of 10 M NH4OH in water and the gels were aged at 30 °C for 13.5 h. Finally, the gel was dried at 65 °C for 24 h followed by 100, 150, and 200 °C for 1 h each. Octakis(trimethylsiloxy)silsesquioxane (Q8M8) was used as a crystalline reference material and sourced from Alfa Aesar; its composition and structure was confirmed by 29Si solid state MAS NMR spectroscopy.7 2.2. Basic Characterization. The envelope densities (ρ) were determined from the mass and volume measured by powder pycnometry (Micromeritics, GeoPyc 1360) with a low B


Article

The Journal of Physical Chemistry C the form of inelastic excitations. Ground state splitting for methyl tunnelling should be observed below 655 μeV, which is the rotational constant of the methyl group and therefore the upper energy limit for a barrier-free rotor. Typical values for methyl tunneling were collected by Prager and Heidemann.35 At higher temperatures a crossover to classical rotational diffusion should be visible: tunnelling lines shift to lower energy and broaden and simultaneously quasielastic peak appears.35 In our study, an overview about the dynamics was obtained by doing a temperature scan, where the data in a certain energy range are integrated. For the so-called fixed window scan (FWS), the spectra were integrated within the fwhm of the elastic peak (±7 μeV). For the inelastic fixed window scan (IFWS), the spectra were integrated between 20 and 60 μeV, where the contribution from the elastic peak is negligible. Depending on the tunnelling energy and parameters of the rotational diffusion relative to the instrument parameters, the FWS and IFWS curves may have different shapes. In neutron spectroscopy, not only the energy but also the momentum transfer (q) is detected, which provides information about the geometry of the motion(s), although at MARS, only five spectra can be measured (q1 = 0.19−0.51 Å−1; q2 = 0.75−1.05 Å−1; q3 = 1.2−1.45 Å−1; q4 = 1.55−1.72 Å−1; q5 = 1.8−1.88 Å−1;). Last, but not least, if the scattering is incoherent as for our samples, the vibrational mean square displacement (MSD) can be obtained via:26 2

Table 1. Aerogel Synthesis, Properties, and Surface Chemistrya sample name silica precursor hydrophobization agent drying method drying temperature [°C] density, ρ [g/cm3] total pore volume,b VPore [cm3/g] BJH mesopore volume [cm3/g] porosity, ϕ [%] SBET [m2/g] pore diameter,c DPore [nm] TMS content [mmol/g] methyl groups [mmol/g] silanold [mmol/g] ethoxy [mmol/g] methoxyd [mmol/g]

Aero B

MTMS

waterglass HMDSO +TMCS APD 150 0.106 8.9

TEOS HMDSO

MTMS

APD 150 0.306 2.8

APD 65/100/150/200 0.207 4.3

3.7

2.4

0.2

95 610 59

85 931 12

90 396 44

2.9 ± 0.3

2.0 ± 0.2 15.5 ± 1.6

2.1 ± 1.0 3.0 ± 0.5 0.24 ± 0.10

a

APD: ambient pressure drying; TEOS: tetraethoxysilane, Si(OC2H5)4; TMCS: trimethylchlorosilane, Si(CH3)3Cl; HMDSO: hexamethyldisiloxane, O(Si(CH3)3)2; MTMS: methyltrimethoxysilane, Si(CH3)3(OCH3); TMS: trimethylsilyl group, −Si(CH3)3 bVPore = 1/ρ − 1/ρ0, where the skeleton density ρ0 is approximated by 2.0 g/cm3. c Approximated by DPore = 4VPore/SBET. dThe silanol (Si−OH) and methoxy (−OCH3) contents display large uncertainties because they are derived from low intensity, broad peaks.

2

S(Q , ω ≈ 0)T /S(Q , ω ≈ 0)T = 0 ∝ e−(Q ⟨u⟩ /3)

TMCS +HMDSO

(1)

This equation is often applied in cases where quasielastic scattering occurs, but for such cases, the obtained values have to be considered with caution: although diffusion also causes a reduction of the elastic intensity, its q dependence is different from the vibrations. Therefore, the calculated MSDs can be used at most qualitatively to compare the mobility in similar systems. However, for such cases one can consider to worsen the instrument resolution so that the quasielastic scattering is not any more resolvable to get a more correct MSD for vibrations. Here we mimic this situation and extract a value for the MSD′ by using the total scattering from the entire probed energy window (up to 60 μeV) instead of the more narrow elastic scattering window. For the measurements, between 0.6 and 1.2 g of crushed aerogel powder was placed inside a cylindrical aluminum container with a diameter of 12 mm and height of 5 cm.

organic groups); therefore, the adsorbed H2O will not contribute significantly to the neutron scattering spectra. The nitrogen sorption isotherms are displayed in Figure 1. The BET surface area33 was derived from the sorption curves at low partial pressures (P/P0 < ∼0.25) and is highest for the Aero B aerogel (931 m2/g), intermediate for the TMCS+HMDSO aerogel (610 m2/g), and lowest for the MTMS based aerogel (396 m2/g). The quantity of absorbed N2 at high partial pressure (P/P0 ≈ 1) is, at least qualitatively, indicative of the volume fraction of mesopores, although the BJH analysis37 of silica aerogels is known to be affected by sample deformations during the analysis.34 The waterglass (TMCS+HMDSO) and TEOS based aerogels (Aero B) display type IV isotherms typical for silica aerogel materials.34 The MTMS sample also displays a type IV isotherm, but the quantity of absorbed nitrogen at P/P0 ≈ 1 is much lower, indicating that the material is predominantly macroporous, with only a small volume fraction of mesopores (0.2 cm3/g according to BJH analysis; Table 1). 3.2. Surface Chemistry and Microstructure. The concentration of the organic groups can be quantified from the single pulse 1H solid-state NMR spectra7,8 (Figure 2a) and is listed in Table 1. In contrast, the 1H−13C (Figure 2b) and 1 H−29Si (Figure 2c) CP NMR spectra are not quantitative, but they do confirm and complement the results from the proton spectra. As expected, the surface chemistry of the waterglass based aerogel (TMCS+HMDSO) consists primarily of TMS and silanol groups, whereas the TEOS based aerogel (Aero B) is covered predominantly with TMS and ethoxy groups (1H and 1H−13C CP spectra). The silica backbone of both these aerogels consists of Q4 and Q3 species, where Qn is a Si atom bonded to n bridging oxygen atoms, i.e., oxygen atoms that link

3. RESULTS AND DISCUSSION 3.1. Aerogel Properties and Surface Chemistry. The aerogel densities are plotted in Table 1. The density of the Aero B aerogel (0.306 g/cm3) is three times higher than that of the TMCS+HMDSO aerogel (0.106 g/cm3) because the insufficient silylation of the former resulted in an incomplete springback during ambient pressure drying. In fact, this incomplete silylation is why this sample was selected for QENS measurements as it displays a surface chemistry of both TMS and ethoxy groups (see below).7 The MTMS sample has a measured density of 0.207 g/cm3 although the powder pycnometry may overestimate the density because of the high compressibility of the MTMS-based aerogel. Based on dynamic vapor sorption measurements on similar materials,36 the studied aerogels are expected to contain at most 2 but more likely ca. 1 wt % adsorbed H2O, corresponding to 0.2 and 0.1 wt % H respectively (compared to ca. 3 wt % H from the C


Article


Figure 1. Nitrogen sorption isotherms of the three aerogel samples.

Figure 2. 1H (a), 1H−13C (b), and 1H−29Si (c) solid-state NMR spectra of the three different aerogel samples; band assignments7,8 are noted next to the peaks, and SSB denotes spinning side bands. All spectra are normalized to a maximum intensity of 1 and offset for clarity.

area of these narrow bands is estimated at 3 ± 1% of the total intensity. On SEM length scales, the TMCS+HMDSO and Aero B aerogels display a typical aerogel microstructure with secondary particles (20−50 nm diameter) that enclose the mesopores (Figure 3a,e). The higher density of the Aero B aerogel compared to the TMCS+HMDSO aerogel can also be discerned. The MTMS based aerogel displays a much coarser structure with secondary particle sizes of 1 to 2 μm in diameter (Figure 3i). At TEM length scales, the TMCS+HMDSO and Aero B aerogels display the typical pearl necklace microstructure associated with silica aerogels (Figure 3b,c,f−g). Primary silica particles ∼5 nm in diameter are connected by thinner interparticle necks to form a three-dimensional mesoporous network. The MTMS based aerogel (Figure 3j) displays a coarser microstructure consistent with the lower surface area (Table 1). Note that a fraction of the material displays various crystalline morphologies (Figure 3k), including sheet-like structures, and this is consistent with the presence of a minor narrow component to the 29Si NMR band of T3

two Si atoms, and 4-n nonbridging oxygen atoms. Note that the Q3-OH peak is shifted further from the Q4 peak compared to the Q3-OCH2CH3 peak, resulting in a better spectral resolution for the TMCS+HMDSO sample compared to the Aero B sample (Figure 2c). The MTMS based aerogel consists primarily of T3 sites, with a minor contribution from T2, where Tn is a Si atom bonded to 1 carbon, n bridging oxygen atoms, and 3-n nonbridging oxygen atoms (Figure 2c). The 1H and 1H−13C CP spectra are dominated by the methyl signal, with a minor peak for methoxy groups (linked to T2). The nonbridging oxygens are connected to methoxy (Figure 2a,b) and, possibly, silanol groups. Note that the methyl content estimated from the proton NMR (15.5 ± 1.6 mmol/g) is the same as that predicted from the composition of the MTMS precursor assuming full hydrolysis and condensation into a material with a SiO1.5CH3 stoichiometry (14.4 mmol/g), validating the quantification procedure. Note that the T3 band includes two narrow peaks on top of the major, broader band (Figure S1). Although difficult to quantify accurately due to the limited resolution and signal-to-noise ratio, the combined D


Article


Figure 3. SEM images (a,e,i), TEM images (b,c,f,g,j,k), and schematic illustrations (d,h,l) of the local environments of the various hydrophobic groups (Si, purple; O, red; C, gray; H, white).

POSS are cube-like structures with SiOSi sides and eight tetrahedral Si atoms as vertices, bonded to an organic ligand (Figure 4). Chemically, a POSS with TMS ligands (Q8M8 or TMS-POSS, Figure 4a) resembles a miniature silylated silica nanoparticle with 8 Q4 units connected to 8 TMS groups through a siloxane bond, and the NMR spectra of Q8M8 and the silylated silica aerogels (TMCS+HMDSO and Aero B) are indeed similar, apart from the narrower line widths for the crystalline Q8M8 (Figure 2). A POSS with methyl groups on the vertices (M-POSS) contains 8 T3 units with one methyl ligand each and thus has a speciation that is expected to be similar to MTMS based aerogels (Figure 2) as trifunctional silanes such as MTMS are typical precursors for both M-POSS and silica aerogels.39 Although Q8M8 closely resembles the silylated aerogels from a speciation perspective, its structure is very different. Aerogels are (predominantly) amorphous and display a high surface area and large mesoporosity (Table 1 and Figures 1 and 3). In contrast, Q8M8 is a dense, nonporous, crystalline material with negligible surface area with a very

(Figure S1) and the XRD pattern (Figure S2) which strongly resembles that of organic−inorganic phyllosilicates.38 In a previous study, we demonstrated (within uncertainty) that, for silylated silica aerogels, all TMS and ethoxy groups are present on the outside of the silica particles.8 Thus, the TMS and ethoxy groups grafted onto the silica surface point out into the open pore space for the TMCS+HMDSO and Aero B aerogels (Figure 3c,f). In contrast, the methyl groups from the MTMS based aerogel are present both internally, inside the predominantly amorphous SiO1.5CH3 structure, and externally, pointing out into the pore space (Figure 3i). Based on the surface area of 396 m2/g (Table 1) and assuming a surface density of 4.5 methyl groups per nm2 of surface area,7 approximately 3 mmol from the 15.5 mmol/g of methyl groups (∼20%) are estimated to be on surfaces accessible to nitrogen sorption analysis, whereas the rest (∼80%) are located inside the SiO1.5CH3 structure, which may constrain their rotational freedom (see below). E


Article


Figure 4. Structure of (a) octakis(trimethylsiloxy)silsesquioxane (Q8M8 or TMS-POSS) and (b) octakis(methyl)silsesquioxane (MPOSS).

different, more constrained local environment for the TMS groups. 3.3. Neutron Scattering. 3.3.1. Low Temperature Energy Spectra. At the lowest temperature (1.5 K), we have collected data over an extended energy range by shifting the energy window of the incoming neutrons. The energy spectrum of the Q8M8 sample was very similar to that from a vanadium reference sample, measured with a somewhat different geometry, and was therefore considered to represent the instrument resolution (Figure 5a). No resolved methyl rotational tunneling peaks were detected in the energy spectra collected at 1.5 K for any of the samples (Figure 5b−d). This is not unexpected as the instrument resolution only enables the detection of tunneling signals at energy transfers in excess of 0.020 meV, which would require very high energy barriers.35 In addition, methyl tunneling is expressed as a broadening of the elastic peak for amorphous materials, similar in appearance to a quasielastic scattering signal, because the structural disorder results in a distribution of rotational barriers.24,40,41 For the TMCS+HMDSO and Aero B aerogels, no broadening of the elastic peak can be observed compared to the Q8M8 sample or instrument resolution (Figure 5a−c), indicative of relatively modest rotational energy barriers for the methyl groups from the surface TMS and ethoxy groups. The elastic peak for the MTMS based sample does display a small (∼0.2% of total intensity) but statistically significant shoulder compared to the instrument resolution (Figure 5a,d), indicating higher rotational energy barriers than for the other aerogels and consistent with the interpretation of the FWS and MSD data at higher temperatures (see below). 3.3.2. Fixed Window Scans. Since hydrogen has an exceptionally high neutron scattering cross section, the contribution of other elements to the neutron scattering spectra can be neglected. This means that the neutron scattering spectra of TMCS+HMDSO, Aero B, MTMS, and Q8M8 samples can be attributed to the TMS, nearly equal amounts of TMS and ethoxy, methyl, and TMS groups, respectively. Contributions from the silanol and methoxy groups can be neglected. The FWS and IFWS data are plotted in Figure 6. Both are normalized to the FWS value at the lowest temperature. In the absence of a clear tunnelling spectrum at the lowest temperature (see section 3.3.1), the FWS intensity can be interpreted as the relative amount of immobile hydrogens or those hydrogens that move slowly compared to

Figure 5. Energy spectra collected at 1.5 K, normalized to the intensity of the elastic line. (a) All four samples on a logarithmic scale; the discontinuities at high energy transfers correspond to energies with zero counts and the logarithm of zero is not defined. (b−d) Zoom on the region of interest (linear scale) for the aerogel samples (b−d), where the thick line corresponds to the measurement and the thin lines to the 95% confidence intervals. The spectrum of the Q8M8 sample represents the instrument resolution (see text).

the instrument time scale. As expected, the FWS intensity monotonically decreases due to the increasing mobility at higher temperatures. Simultaneously, the IWFS intensity increases. The apparent decrease of the quasielastic scattering above 140−170 K is not related to a decreased mobility upon heating but is due to the broadening of the quasielastic signal that results in a decreasing quasielastic intensity inside our selected energy window, even though the total quasielastic intensity is either constant or increases. The FWS and IFWS spectra vary smoothly as a function of temperature for the three aerogel samples (Figure 6a−c), consistent with their (predominantly) amorphous nature and the expected broad distribution of local environments of the hydrogen-containing functional groups. In contrast, the quasielastic intensity (IFWS) for the (crystalline) Q8M8 sample (Figure 5d) increases up to 62 K, decreases slightly between 62 and 72 K, and increases again above 72 K, which reflects two distinct dynamical processes. Indeed, a change in the shape of the energy spectra is evident in the same temperature range (Figure S3). For the TMCS +HMDSO and Aero B aerogel samples, with TMS and TMS plus ethoxy groups, respectively (Figure 2), quasielastic scattering appears already at the lowermost temperatures. In F


Article


Figure 6. FWS and IFWS intensities derived from the total scattered intensities (sum of data over the entire q range), normalized to the FWS intensity at the lowermost temperature (1.5 K).

Figure 7. Temperature and q dependence of the normalized elastic incoherent structure factor.

Figure 8. Mean square displacement ⟨u2⟩ as a function of temperature, derived from the q dependence of the elastic scattering intensity (Figure 6), calculated from the elastic (MSD) and total (MSD′) scattered intensities; note that the plotted error bars represent the statistical uncertainty only.

number of q values and a limited energy range, do not allow us to derive a reliable and meaningful elastic incoherent structure factor (EISF) and to analyze the geometry of the motions in this manner. On the other hand, the mean-square displacement (MSD and MSD′, see section 2.3) can be derived from the q dependence of the (nearly) elastic scattering intensities with eq 1 (Figure 7). For the Q8M8 sample, it is clearly visible that eq 1 will provide only a poor fit to the data. It is less obvious from Figure 7 but eq 1 also fits the data poorly for the other samples.

contrast, the functional groups remain static for the MTMS based aerogel and for the Q8M8 reference sample until about 40 K (Figure 6c,d). Last, but not least we can conclude that the methyl groups are mobile not only at the surface but also in the interior of the MTMS sample, because the FWS curve would not drop below the fraction of interior methyl molecules (0.8) otherwise. 3.3.3. Mean Square Displacement. The experimental conditions in the used setup, with data collected over a limited G


Article


because the orientation of the Si−C bond is fixed by the sp3 hybridization of the tetrahedral Si and the three SiO Si bonds that incorporate it into the rigid siloxane network. Overall, the room temperature dynamics for all three aerogels are similar to those of the equivalent POSS, and this similarity also holds at low temperature for M-POSS and the MTMS aerogel, where most of the methyl groups are located inside the SiO1.5CH3 structure (see section 3.1). In contrast, the low temperature dynamics of the silylated silica aerogels (TMCS +HMDSO and Aero B) are very different to those of Q8M8, with nonzero MSDs even below 20 K. Presumably, these differences arise from the different local environments: the mobility of the TMS groups is more restricted inside the Q8M8 crystal structure compared to TMS and ethoxy groups that point into the open pore space of the silylated silica aerogels. In summary, the hydrogen atoms in the MTMS aerogel display a reduced mobility compared to the silylated silica aerogels. At low temperatures, the reduced mobility is related to the location of the methyl groups inside the SiO1.5CH3 structure in contrast to the free-standing TMS and ethoxy groups on the surfaces of the silylated aerogels. At higher temperature, the rotation of the entire TMS groups increases the MSDs for the silylated aerogels (and Q8M8), but a similar rotation is not possible in the MTMS based aerogel.

However, as mentioned in section 2.3, we actually do not expect eq 1 to hold because hydrogen participates also in the rotational motion of the functional groups and not only performs vibrations. Furthermore, other effects like coherent and/or multiple scattering or non-Gaussian vibrations could play a role. Based on the present data, it is not possible to identify the reason(s) for this. Despite these limitations, we can still use the derived MSD-s and MSD′-s for qualitative data analysis. For all samples, the amplitude of MSD increases with increasing temperature (Figure 8). For the MTMS and Q8M8 samples, MSD is close to zero for temperatures below ∼40 K, whereas it increases already from the lowermost temperatures for the TMCS+HMDSO and Aero B aerogels, as indicated already by the FWS and IFWS data. At room temperature, the MSD is significantly lower for the MTMS sample compared to the samples with TMS groups (TMCS+HMDSO, Aero B, and Q8M8). Since we have seen from the FWS data that interior methyl groups also perform rotational motions, this must be due to a smaller volume/area of the rotational motion: either because less space is available inside the MTMS structure or because not only methyl group rotations but also the rotation of the entire TMS group is detected in the TMS bearing samples. The room temperature MSD is higher for the Aero B sample with a surface coverage of TMS and ethoxy groups in near equal amounts (based on a H content; Figure 2a,b), compared to the TMCS+HMDSO sample covered with mostly TMS and minor silanol groups (Figure 2a). The data do not permit us to conclude if this is related to a higher mobility of the ethoxy than the TMS groups or rather due to secondary effects of a mixed surface coverage on the overall mobility, e.g., increased rotational freedom of TMS groups due to ethoxy− TMS interactions in a mixed monolayer, compared to TMSTMS interactions in a (nearly) TMS-only monolayer. Comparison of the temperature dependence of MSD and MSD′ enables us to determine the onset of the rotational diffusion as the temperature where the two curves deviate from each other: the QENS signal at low temperature is so narrow that it influences only the value of MSD but not the MSD′, where it is integrated together with the elastic line. For the MTMS aerogel and the Q8M8 sample, MSD′ and MSD are identical below ca. 50 K, whereas they deviate from each other essentially in the whole temperature range for the TMS modified aerogels (TMCS+HMDS and Aero B). The comparison of the aerogel MSDs with those reported for POSS28,29 is highly informative. Jalarvo et al.28,29 characterized POSS with a variety of ligands, including M-POSS and TMSPOSS. Although their data were collected on a different spectrometer with different instrument settings, our MSD for Q8M8 is in general agreement with the Jalarvo et al. data, with near-zero displacements below ∼40 K and a room temperature MSD on the order of 3 Å2. They conclude that, below ca. 140 K, only the methyl group rotation is visible, whereas above 140 K, the MSD of Q8M8 (i.e., TMS-POSS) increases additionally because of the rotation of the three methyl groups around the axis passing through the O−Si (siloxy) bond in the TMS groups.29 At room temperature, the silylated silica aerogels (TMCS+HMDSO and Aero B) display MSDs of similar magnitude as Q8M8, consistent with the rotation of the hydrogens around the axis passing through the Si−C bond as well as the rotation of the three methyl groups around the axis passing through the O−Si bond. The room temperature MSDs of M-POSS and the MTMS based aerogel are markedly lower,

4. CONCLUSIONS The neutron scattering data highlights the strongly dynamic behavior of the organic functional groups tethered on the silica surface. Methyl groups inside the aerogel prepared from an MTMS precursor experience relatively high rotational energy barriers, as evidenced from the mean square displacements as well as from the tunneling spectra. In contrast, the functional groups (TMS and/or ethoxy, plus minor silanol) tethered onto the surfaces of classical silica aerogels prepared from waterglass or TEOS precursors and hydrophobized through silylation, point into the open pore space, and experience relatively low rotational energy barriers. As a result, the hydrogens in these functional groups are characterized by significant mean square displacements, even at temperatures well below liquid nitrogen temperature. Thus, a classical hydrophobic silica aerogel’s surface is highly dynamic, and this may affect for example the deposition of N2 monolayers during nitrogen sorption experiments (BET)33 or the sorption of CO2 on aminefunctionalized aerogels.42,43

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06011. Additional characterization of the MTMS based aerogel (XRD and solid-state NMR) and QENS spectra of Q8M8. (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +41587654983. *E-mail: [email protected]. Phone: +41563103176. ORCID

Wim J. Malfait: 0000-0002-1668-8749 Notes

The authors declare no competing financial interest. H


Article


■

Carbon Aerogel: A Combined Quasi-Elastic and Small-Angle Neutron Scattering Study. Microporous Mesoporous Mater. 2010, 132, 148−153. (21) Moreno, A. J.; Colmenero, J.; Alegría, A.; Alba-Simionesco, C.; Dosseh, G.; Morineau, D.; Frick, B. Methyl Group Dynamics in a Confined Glass. Eur. Phys. J. E: Soft Matter Biol. Phys. 2003, 12, 43. (22) Dimeo, R.; Neumann, D.; Glanville, Y.; Minor, D. Pore-Size Dependence of Rotational Tunneling in Confined Methyl Iodide. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 104201. (23) Liu, L.; Chen, S.-H.; Faraone, A.; Yen, C.-W.; Mou, C.-Y.; Kolesnikov, A. I.; Mamontov, E.; Leao, J. Quasielastic and Inelastic Neutron Scattering Investigation of Fragile-to-Strong Crossover in Deeply Supercooled Water Confined in Nanoporous Silica Matrices. J. Phys.: Condens. Matter 2006, 18, S2261. (24) Colmenero, J.; Moreno, A. J.; Alegría, A. Neutron Scattering Investigations on Methyl Group Dynamics in Polymers. Prog. Polym. Sci. 2005, 30, 1147−1184. (25) Arrighi, V.; Ganazzoli, F.; Zhang, C.; Gagliardi, S. New Interpretation of Local Dynamics of Poly(dimethyl siloxane) Observed by Quasielastic Neutron Scattering. Phys. Rev. Lett. 2003, 90, 058301. (26) Arrighi, V.; Gagliardi, S.; Zhang, C.; Ganazzoli, F.; Higgins, J. S.; Ocone, R.; Telling, M. T. F. A Unified Picture of the Local Dynamics of Poly(dimethylsiloxane) Accross the Melting Point. Macromolecules 2003, 36, 8738−8778. (27) Yang, F.; Hess, K. U.; Unruh, T.; Mamontov, E.; Dingwell, D. B.; Meyer, A. Intrinsic proton Dynamics in Hydrous Silicate Melts as Seen by Quasielastic Neutron Scattering at Elevated Temperature and Pressure. Chem. Geol. 2017, 461, 152. (28) Jalarvo, N.; Gourdon, O.; Ehlers, G.; Tyagi, M.; Kumar, S. K.; Dobbs, K. D.; Smalley, R. J.; Guise, W. E.; Ramirez-Cuesta, A.; Wildgruber, C.; et al. Structure and Dynamics of Octamethyl-POSS Nanoparticles. J. Phys. Chem. C 2014, 118, 5579−5592. (29) Jalarvo, N.; Tyagi, M.; Crawford, M. K. Quasielastic Neutron Scattering Study of POSS Ligand Dynamics. EPJ Web Conf. 2015, 83, 02007. (30) Conrad, H.; Buchenau, U.; Schätzler, R.; Reichenauer, G.; Fricke, J. Crossover in the Vibrational Density of States of Silica Aerogels Studied by High-Resolution Neutron Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 41, 2573. (31) Reichenauer, G.; Fricke, J.; Buchenau, U. Neutron Scattering Study of Low-Frequency Vibrations in Silica Aerogels. EPL (Europhysics Letters) 1989, 8, 415. (32) Copley, J. R.; Udovic, T. J. Neutron Time-of-Flight Spectroscopy. J. Res. Natl. Inst. Stand. Technol. 1993, 98, 71. (33) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309−319. (34) Reichenauer, G. Structural Characterization of Aerogels. In Aerogels Handbook; Aegerter, M. A., Leventis, N., Koebel, M. M., Eds.; Springer: New York, 2011; pp 449−498. (35) Prager, M.; Heidemann, A. Rotational Tunneling and Neutron Spectroscopy: A Compilation. Chem. Rev. 1997, 97, 2933−2966. (36) Huber, L.; Zhao, S.; Malfait, W. J.; Vares, S.; Koebel, M. M. Fast and Minimal-Solvent Production of Superinsulating Silica Aerogel Granulate. Angew. Chem., Int. Ed. 2017, 56, 4753−4756. (37) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms. J. Am. Chem. Soc. 1951, 73, 373−380. (38) Burkett, S. L.; Press, A.; Mann, S. Synthesis, Characterization, and Reactivity of Layered Inorganic-Organic Nanocomposites Based on 2:1 Trioctahedral Phyllosilicates. Chem. Mater. 1997, 9, 1071− 1073. (39) Shimizu, T.; Kanamori, K.; Nakanishi, K. Silicone-Based Organic−Inorganic Hybrid Aerogels and Xerogels. Chem. - Eur. J. 2017, 23, 5176. (40) Moreno, A. J.; Alegría, A.; Colmenero, J.; Frick, B. MethylGroup Dynamics from Tunneling to Hopping in NaCH3CO2.H2O: Comparison Between a Crystal and its Glassy Counterpart. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 134202.

ACKNOWLEDGMENTS This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland and was funded by Empa and PSI primary funding.

■

REFERENCES

(1) Aegerter, M. A.; Leventis, N.; Koebel, M. M. Aerogels Handbook; Spriner-Verlag: New York, 2011. (2) Pajonk, G. M. Aerogel Catalysts. Appl. Catal. 1991, 72, 217−266. (3) Reynolds, J. G.; Coronado, P. R.; Hrubesh, L. W. Hydrophobic Aerogels for Oil-Spill Cleanup - Synthesis and Characterization. J. NonCryst. Solids 2001, 292, 127−137. (4) Zhao, S.; Jiang, B.; Maeder, T.; Muralt, P.; Kim, N.; Matam, S. K.; Jeong, E.; Han, Y.-L.; Koebel, M. M. Dimensional and Structural Control of Silica Aerogel Membranes for Miniaturized Motionless Gas Pumps. ACS Appl. Mater. Interfaces 2015, 7, 18803−18814. (5) Bhagat, S. D.; Oh, C.-S.; Kim, Y.-H.; Ahn, Y.-S.; Yeo, J.-G. Methyltrimethoxysilane Based Monolithic Silica Aerogels via Ambient Pressure Drying. Microporous Mesoporous Mater. 2007, 100, 350−355. (6) Prakash, S. S.; Brinker, C. J.; Hurd, A. J.; Rao, S. M. Silica Aerogel Films Prepared at Ambient Pressure by Using Surface Derivatization to Induce Reversible Drying Shrinkage. Nature 1995, 374, 439−443. (7) Malfait, W. J.; Verel, R.; Koebel, M. M. Hydrophobization of Silica Aerogels: Insigths from Quantitative Solid-State NMR Spectroscopy. J. Phys. Chem. C 2014, 118, 25545−25554. (8) Malfait, W. J.; Zhao, S.; Verel, R.; Iswar, S.; Rentsch, D.; Fener, R.; Zhang, Y.; Milow, B.; Koebel, M. M. The Surface Chemistry of Hydrophobic Silica Aerogels. Chem. Mater. 2015, 27, 6737−6747. (9) Rao, A. V.; Nilsen, E.; Einarsrud, M.-A. Effect of Precursors, Methylation Agents and Solvents on the Physicochemical Properties of Silica Aerogels Prepared by Atmospheric Drying Method. J. Non-Cryst. Solids 2001, 296, 165−171. (10) Hayase, G.; Kanamori, K.; Nakanishi, K. New Flexible Aerogels and Xerogels Derived from Methyltrimethoxysilane/Dimethyldimethoxysilane Co-precursors. J. Mater. Chem. 2011, 21, 17077. (11) Nadargi, D. Y.; Rao, A. V. Methyltriethoxysilane: New Precursor for Synthesizing Silica Aerogels. J. Alloys Compd. 2009, 467, 397−404. (12) Rao, A. V.; Bhagat, S. D.; Hirashima, H.; Pajonk, G. M. Synthesis of Flexible Silica Aerogels Using Methyltrimethoxysilane (MTMS) Precursor. J. Colloid Interface Sci. 2006, 300, 279−285. (13) Kanamori, K.; Aizawa, M.; Nakanishi, K.; Hanada, T. New Transparent Methylsilsesquioxane Aerogels and Xerogels with Improved Mechanical Properties. Adv. Mater. 2007, 19, 1589−1593. (14) Hayase, G.; Kugimiya, K.; Ogawa, M.; Kodera, Y.; Kanamori, K.; Nakanishi, K. The Thermal Conductivity of Polymethylsilsesquioxane Aerogels and Xerogels with Varied Pore Sizes for Practical Application as Thermal Superinsulators. J. Mater. Chem. A 2014, 2, 6525−6531. (15) Al-Oweini, R.; El-Rassy, H. Synthesis and Characterization by FTIR Spectroscopy of Silica Aerogels Prepared Using Several Si(OR)4 and R″Si(OR′)3 Precursors. J. Mol. Struct. 2009, 919, 140−145. (16) Sindorf, D. W.; Maciel, G. E. 29Si CP/MAS NMR Studies of Methylchlorosilane Reactions on Silica Gel. J. Am. Chem. Soc. 1981, 103, 4263−4265. (17) Sindorf, D. W.; Maciel, G. E. Cross-Polarization/Magic-AngleSpinning Silicon-29 Nuclear Magnetic Resonance Study of Silica Gel using Trimethylsilane Bonding as a Probe of Surface Geometry and Reactivity. J. Phys. Chem. 1982, 86, 5208−5219. (18) Sindorf, D. W.; Maciel, G. E. Solid-State NMR Studies of the Reactions of Silica Surfaces with Polyfunctional Chloromethylsilanes and Ethoxymethylsilanes. J. Am. Chem. Soc. 1983, 105, 3767−3776. (19) Gautam, S.; Liu, T.; Rother, G.; Jalarvo, N.; Mamontov, E.; Welch, S.; Sheets, J.; Droege, M.; Cole, D. R. Dynamics of Propane in Nanoporous Silica Aerogel: A Quasielastic Neutron Scattering Study. J. Phys. Chem. C 2015, 119, 18188−18195. (20) Chathoth, S. M.; Mamontov, E.; Melnichenko, Y. B.; Zamponi, M. Diffusion and Adsorption of Methane Confined in Nano-Porous I


Article

The Journal of Physical Chemistry C (41) Moreno, A. J.; Alegría, A.; Colmenero, J.; Frick, B. Methyl Group Rotational Tunneling in Glasses: a Direct Comparison with the Crystal. Phys. B 2000, 276−278, 361−362. (42) Wörmeyer, K.; Alnaief, M.; Smirnova, I. Amino Functionalised Silica-Aerogels for CO2 Adsorption at Low Partial Pressure. Adsorption 2012, 18, 163−171. (43) Cui, S.; Cheng, W.; Shen, X.; Fan, M.; Russell, A.; Wu, Z.; Yi, X. Mesoporous Amine-Modified SiO2 Aerogel: a Potential CO2 Sorbent. Energy Environ. Sci. 2011, 4, 2070−2074.

J


Dynamics of Silica Aerogel's Hydrophobic Groups: A Quasielastic

Recommend Documents