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1
Supporting Information
Surficial siloxane-to-silanol interconversion during room
temperature hydration/dehydration of amorphous silica films
observed by ATR-IR and TIR-Raman spectroscopies
Suzanne L. Warringa, David A. Beattie
b and A. James McQuillan
a*
a Department of Chemistry, University of Otago, P. O. Box 56, Dunedin 9054, New Zealand.
b Future Industries Institute, University of South Australia, Mawson Lakes, Adelaide SA
5095, Australia.
Contents
Central network force model
Energy dispersive spectroscopy (EDS), dynamic light scattering (DLS) and zeta-potential of colloidal
silica
Tables of silica IR spectral assignments
IR spectra of ~300 nm silica film at different RH relative to bare prism background
SEM images and IR spectrum of ~500 nm silica film
Variation of effective film thickness with refractive index, wavelength and with humidity
Band fitting of TIR Raman spectra
References
2
Central network force model
The central network force model was first derived by Galeener et al.4 This model assumes
local order consisting of two neighbouring SiO4 tetrahedra sharing a bridged oxygen within
the amorphous structure and correlates the vibrational frequency of the TO mode of the silica
lattice (ω) to the Si-O force constant (k) and Si-O-Si inter-tetrahedral bond angle (θ) through
Equation S1 where mO and mSi are the masses of oxygen and silicon atoms, respectively.
Previous studies have found that higher wavenumber TO modes indicate smaller
intertetrahedral bond angles and less porous structures.5,11
�� =�
��
�1 − �� � +4
3
�
���
(S1)
Energy dispersive spectroscopy (EDS), dynamic light scattering (DLS) and
zeta-potential of colloidal silica
Particle size distribution and zeta-potential measurements were performed on the Zetasizer
Nano ZS90, Malvern, UK with 173° back scatter. Samples were measured at 298 K and
allowed to equilibrate for 120 s. Zeta-potential and size measurements were performed
concurrently using a folded capillary cell. A refractive index of 1.48 from Khlebtsov et al8
was used in the Smoluchowski fitting of size data. For a 4.5 mg mL-1
Ludox suspension pH
~9 the average hydrodynamic diameter (3 measurements) of the particles was 105 nm and the
zetapotential was ~-51 mV.
3
Figure S1 – DLS size data for 4.5 mg mL-1
Ludox suspension pH ~9.
EDS analysis was performed concurrent to recording SEM images of the silica films (Figure
S6). This shows the presence of silicon, oxygen and sodium. The Ludox Syton HT-50 silica
nanoparticles are formed by cationic exchange between sodium silicate and silicic acid,
performed at high temperature in basic aqueous suspension.10
4
Figure S2 - EDS spectrum of silica particle films
Table S1 - IR band assignments for Figure 5 aqueous films and D2O films.
Wavenumber / cm-1 IR band assignment
1–7
3736 Isolated ν(SiO-H)
3647 H-bonded ν(SiO-H)
3400
2756
2688
ν(H2O)
Isolated ν(SiO-D)
H-bonded ν(SiO-D)
2480
1880
ν(D2O)
Overtone νas(Si-O-Si)TO
1870
1188 νas(Si-O)LO
1073
1064
νas(Si-O-Si)TO
966 ν(Si-OH)
5
800 νs(Si-O)
Table S2 - IR band assignments for Figure 6 aqueous film and Figure 7 D2O
film hydration spectra.
Wavenumber / cm-1 IR band assignment
3736 Isolated ν(SiO-H)
3624 H-bonded ν(SiO-H)
3400
3200
2754
2688
2500
ν(H2O)
Isolated ν(SiO-D)
H-bonded ν(SiO-D)
ν(D2O)
1630
1192
δ(H2O)
δ(D2O)
1112
1107
1060
νas(Si-O)TO
1033
1032
980
νas(Si-O)
ν(Si-OD)
956 ν(Si-OH)
888
876
νs(Si-O)
6
Figure S3 - Infrared spectra of silica film exposed initially to RH of 0% (dry argon) then
subsequently to RH of 8, 15, 30 and 40%. Spectral background was bare diamond prism.
Influence of humidity on refractive index and effective sampling thickness
Ellipsometric studies have shown that as humidity increases from ~3 to 40 % the refractive
index of sol-derived silica films shows an increase from ~1.3 to 1.34, dependent on the sol
type.9 Due to the nature of total internal reflection this change causes an increase in
penetration depth (dp) and the effective sampling thickness (de) of the evanescent wave.15
Based on refractive index measurements performed by Rouse et al9 on various sol-derived
silica films the variation in dp and de for silica Sol A has been calculated as shown in Figure
S4.
7
Figure S4 – Variation of effective thickness of evanescent wave with refractive index of the
rarer medium.
At ~1100 cm-1
dp is ~900 nm for RH 3 % and ~925 nm for RH 40 %, giving a thickness
increase of ~3 %. Effective sampling thickness of the evanescent wave (de) at 1100 cm-1
increases from ~1.19 µm to ~1.28 µm, for RH change from 3 to 40 %, an increase of ~91 nm
(~8 %).
The strong absorption in the spectral range of 1100 – 1000 cm-1
could cause anomalous
dispersion9 in this spectral region. This effect causes absorption to be increased on the longer
wavelength side of the absorption envelope and decreased on the shorter wavelength side,
akin to spectral results shown in Figure 6. Films with thickness << dp give ATR-IR spectra
comparable to transmission sampling and the effect of anomalous dispersion is less evident.
In the present results dp is ~1 µm at 1100 cm-1
and the film thickness is ~300 nm, so
anomalous dispersion will have some effect on the 1100 cm-1
spectral envelope.
8
Figure S5 - Variation of effective field thickness with wavelength and with relative humidity.
SEM images and IR spectrum of ~500 nm silica film
Figure S6 - SEMs and IR spectrum of ~500 nm thick silica film formed from 8.4 mg mL-1
Ludox pH 2.5 suspension (a) film morphology, (b) film thickness and (c) IR spectrum.
9
Band Fitting of TIR Raman spectra12-14 in 1250-700 cm
-1 region and second
derivatives f′′(x)
Figure S7 – Deconvolution of TIR-Raman spectrum of a silica film formed from ~80 mg mL
-
1 pH 2.5 silica suspension at RH of ~40 %.
10
Figure S8 – Deconvolution of TIR-Raman spectrum of film formed from ~80 mg mL-1
pH
~9.5 silica suspension at RH of ~40 %.
Figure S9 – Deconvolution of TIR-Raman spectrum of a silica film formed from ~80 mg mL-
1 pH ~9.5 silica suspension at RH of ~0 % (dry argon).
11
Peak deconvolution was performed for the TIR-Raman, spectra were first smoothed using a
27-point Sav-Golvay function. Fitting utilized a combined Gaussian-Lorentzian function,
using the second derivative of the spectrum to determine peak positions. The deconvoluted
spectra show evidence of a minor absorption between the peaks at ~1060 and ~960 cm-1
.
From the deconvolution analysis for pH 10 films there is a small peak at ~1020 cm-1
whereas
for the pH 2.5 film this band has shifted down to 1009 cm-1
. These bands along with peaks at
970, 966 and 961 cm-1
(pH 2.5, RH 40 %; pH 10, RH 40 %; and pH 10, RH 0 % respectively)
are due to Q2 species. The bands at 1020 - 1009 cm-1
are indicative of the deprotonated form
of the Q2 species. The observation of the 970 cm-1
band at a higher wavenumber for the silica
film derived from a pH 2.5 suspension is in agreement with the IR results presented in Figure
10, indicative of decreasing surface charge density as SiO- groups become protonated
(discussed in the paper).
For the pH 2.5, RH 40 % and pH 10, RH 0 % spectra there are additional Q2 related
absorptions at 936 and 926 cm-1
, respectively. Bands 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 have previously
been related to Q1 species by Halasz and coworkers,16
who also assigned bands at ~780 cm-1
to Q0 species such as silicic acid (H4SiO4).
16 However, the intensity of the vibrational band
and its presence within all 3 spectra makes assignment of this band as a monomeric silicic
acid species highly unlikely. A 29
Si NMR and UV-Raman investigation of acid hydrolysis of
silicates and polycondensation of tetraethylorthosilicate by Depla et al17
indicated that such
species are not expected in bulk silica. It is far more likely that these bands between 870 and
730 cm-1
are due to TO-LO splitting of the νs(Si-O-Si) mode as first shown in Galeener and
Lucovsky's seminal work on vibrational spectroscopic assignment of tetrahedral glasses.18
References
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Dispersed Silica. Kolloid-Z. Z. Polym. 1964, 195 (1), 12–16.
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Frequency Spectroscopy. Phys. Rev. Lett. 2008, 101 (1), 016101.
(3) Davydov, V. Y.; Kiselev, A. V.; Zhuravlev, L. T. Study of the Surface and Bulk
Hydroxyl Groups of Silica by Infra-Red Spectra and D2O-Exchange. Trans. Faraday
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12
(4) Galeener, F. Band Limits and the Vibrational Spectra of Tetrahedral Glasses. Phys.
Rev. B 1979, 19 (8), 4292–4297.
(5) Innocenzi, P. Infrared Spectroscopy of Sol–gel Derived Silica-Based Films: A Spectra-
Microstructure Overview. J. Non. Cryst. Solids 2003, 316 (2-3), 309–319.
(6) Almeida, R. M.; Pantano, C. G. Structural Investigation of Silica Gel Films by Infrared
Spectroscopy. J. Appl. Phys. 1990, 68 (8), 4225.
(7) Patis, A.; Dracopoulos, V.; Nikolakis, V. Investigation of Silicalite-1 Crystallization
Using Attenuated Total Reflection/Fourier Transform Infrared Spectroscopy. J. Phys.
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Concentration, and Refractive Index of Silica Nanoparticles from Turbidity Spectra.
Langmuir 2008, 24 (16), 8964–8970.
(9) Rouse, J. H.; Ferguson, G. S. Preparation of Thin Silica Films with Controlled
Thickness and Tunable Refractive Index. J. Am. Chem. Soc. 2003, 125 (50), 15529–
15536.
(10) Sasaki, S. Method for Preventing Agglomeration of Colloidal Silica and Silicon Wafer
Polishing Composition Using the Same. US Pat. 5,226,930 1993, 1–5.
(11) Fidalgo, A.; Ilharco, L. M. The Defect Structure of Sol–gel-Derived
Silica/polytetrahydrofuran Hybrid Films by FTIR. J. Non. Cryst. Solids 2001, 283 (1-
3), 144–154.
(12) Halasz, I.; Kierys, A.; Goworek, J.; Liu, H.; Patterson, R. E. 29 Si NMR and Raman
Glimpses into the Molecular Structures of Acid and Base Set Silica Gels Obtained
from TEOS and Na-Silicate. J. Phys. Chem. C 2011, 115 (50), 24788–24799.
(13) Allwardt, J. R.; Schmidt, B. C.; Stebbins, J. F. Structural Mechanisms of Compression
and Decompression in High-Pressure K2Si4O9 Glasses: An Investigation Utilizing
Raman and NMR Spectroscopy of Glasses and Crystalline Materials. Chem. Geol.
2004, 213 (1-3), 137–151.
(14) Brown, M. a.; Arrigoni, M.; Héroguel, F.; Beloqui Redondo, A.; Giordano, L.; van
Bokhoven, J. a.; Pacchioni, G. pH Dependent Electronic and Geometric Structures at
the Water–Silica Nanoparticle Interface. J. Phys. Chem. C 2014, 118 (50), 29007–
29016.
(15) Harrick, N. J. Internal Reflection Spectroscopy, 3rd ed.; John Wiley & Sons, Inc.: New
York, USA, 1987; p 30.
(16) Halasz, I.; Kierys, A.; Goworek, J.; Liu, H.; Patterson, R. E. 29
Si NMR and Raman
Glimpses into the Molecular Structures of Acid and Base Set Silica Gels Obtained
from TEOS and Na-Silicate. J. Phys. Chem. C 2011, 115 (50), 24788–24799.
(17) Depla, A.; Verheyen, E.; Veyfeyken, A.; Van Houteghem, M.; Houthoofd, K.;
13
Van Speybroeck, K.; Waroquier, M.; Kirschhock, C. E. A. and Martens, J. A. UV-
Raman and 29
Si NMR SpectroscopyInvestigation of the Nature of Silicate Oligomers
Formed by Acid Catalyzed Hydrolysis and Polycondensation of
Tetramethylorthosilicate. J. Phys. Chem. C 2011, 115, 11077-11088.
(18) Lucovsky,G . Spectroscopic evidence for valence-alternation
pair defect states in vitreous SiO2. Phil. Mag.1979, 39, 513-530.