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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12

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13

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Si NMR SpectroscopyInvestigation of the Nature of Silicate Oligomers

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