ORIGINAL PAPER

Fabrication and characterization of TEOS-based silica aerogelsprepared using rapid supercritical extraction

Suzanne K. Estok • Thomas A. Hughes IV •

Mary K. Carroll • Ann M. Anderson

Received: 22 August 2013 / Accepted: 26 January 2014

� Springer Science+Business Media New York 2014

Abstract Silica aerogels were prepared using the pre-

cursor tetraethylorthosilicate (TEOS) via a rapid super-

critical extraction (RSCE) method. Multiple consistent

batches of monolithic TEOS-based aerogels were fabri-

cated via an 8-h RSCE process. Fabricating TEOS-based

aerogels with an RSCE method offers some distinct

advantages. One advantage is the relative simplicity of the

RSCE approach: liquid precursors are mixed and poured

into a metal mold in a hydraulic hot-press, where gelation,

aging and extraction of liquid from the pores occur. The

precursor recipe employs TEOS, ethanol, water, oxalic acid

to catalyze hydrolysis, and ammonia to catalyze the sub-

sequent polycondensation reactions. Another advantage is

that reaction of TEOS to form sol gels yields ethanol as a

byproduct. A process that releases ethanol, rather than

methanol (as in tetramethylorthosilicate (TMOS)-based

aerogels) may be more appealing for commercial applica-

tions, involving scale-up of the process. The significantly

lower cost of TEOS, compared to TMOS, is a considerable

advantage. The TEOS-based RSCE aerogels are mesopor-

ous and optically translucent, have bulk densities of

0.099(±0.003) g/cm3 and surface areas of 460(±10) m2/g.

Signals observed in infrared and Raman spectra of the

aerogels are consistent with Si–O framework bonds. Using

scanning electron microscopy imaging, the surface mor-

phology of the aerogel samples was imaged at magnifica-

tions up to 150 kX.

Keywords Aerogel fabrication � Aerogel characterization �Silica aerogel � Rapid supercritical extraction (RSCE) �Tetraethylorthosilicate (TEOS)

1 Introduction

Silica aerogels have been the subject of extensive study. A

recent review by Pierre and Rigacci provides a survey of

the literature on synthesis and ageing of the sol–gel matrix,

extraction of solvent to yield aerogels, and properties of the

resulting materials [1]. Another recent review, by Gurav

et al. [2] highlights a variety of applications of silica

aerogels. The unusual physical properties of silica aerogels

render them attractive for use in applications as diverse as

insulating windows and chemical spill clean up. Silica

aerogels are generally very porous, made up of less than

10 % silicon dioxide [3], with low to ultralow densities

(0.003–0.500 g/cm3), high optical transmission in the vis-

ible region (*85–90 %) [4, 5] and large specific surface

area (*500–1,600 m2/g) [5, 6].

Silica sol gels can be processed in a number of ways to

yield aerogels, including freeze-drying, ambient-drying and

supercritical-drying approaches [1]. The approach taken in

this work is a high-temperature method, in which a con-

tained metal mold in a hydraulic hot press is employed for

rapid supercritical extraction (RSCE) of the solvent from

the pores of the sol–gel matrix [7]. Previously, use of this

RSCE method has focused on silica aerogels prepared

using a recipe that employs tetramethylorthosilicate

(TMOS) as the precursor, methanol as solvent, water to

achieve hydrolysis, and ammonia as the catalyst for both

hydrolysis and polycondensation reactions. The process for

making TMOS-based RSCE silica aerogels is robust, with

monolithic aerogels fabricated in as little as 3 h from

S. K. Estok � T. A. Hughes IV � M. K. Carroll (&)

Department of Chemistry, Union College, Schenectady,

NY 12308, USA

e-mail: carrollm@union.edu

A. M. Anderson

Department of Mechanical Engineering, Union College,

Schenectady, NY 12308, USA

123

J Sol-Gel Sci Technol

DOI 10.1007/s10971-014-3292-x

precursor preparation [8], although we more often employ

a 6–8 h process in order to allow for more gradual cooling

of the resulting aerogel monoliths. The lack of solvent

exchange steps in the RSCE process combined with the

translucency of the resulting TMOS-based RSCE silica

aerogels renders these aerogels suitable for applications in

chemical sensing [9, 10]. This RSCE process has been

shown to be suitable for a variety of precursor chemistries

[11], including hydrophobic and superhydrophobic silica

aerogels using TMOS and organically modified derivatives

of TMOS [12], titania- and titania-silica aerogels [13] and

alumina-based aerogels [14, 15].

Here, we demonstrate that this RSCE fabrication

method is suitable for preparation of silica aerogel mono-

liths, adapting a TEOS-based recipe from Rao and Bhagat

[5]. In addition to taking advantage of the relatively short

processing time, lack of solvent exchanges, and potential

for scale-up that are characteristic of this RSCE process,

there are specific advantages to employing TEOS-based

recipes for the preparation of silica aerogels. TEOS is

significantly less expensive than TMOS (for example, Nov.

2013 prices for TEOS from Sigma-Aldrich are\20 % that

of TMOS). Moreover, the hydrolysis and polycondensation

reactions involving TEOS generate ethanol rather than

methanol as a byproduct. A process that releases ethanol,

rather than methanol (as in TMOS-based aerogels) may be

more appealing for commercial applications, involving

scale-up of the process. The cumulative advantages of the

RSCE process and the selection of TEOS as the silica

precursor render this approach attractive for future com-

mercial applications.

2 Methods and materials

2.1 Materials and equipment

Reagent grade tetraethylorthosilicate (TEOS, 98 %) and

oxalic acid dihydrate were procured from Sigma-Aldrich

Chemical Co., and were used without further purification.

Reagent grade ethanol and ammonia were procured from

Fisher Scientific, and were used without further purifica-

tion. Solutions of oxalic acid and ammonia were prepared

in deionized water.

Aerogels were fabricated using a rapid supercritical

extraction method in a contained mold within a hydraulic hot

press [7]. Here, a 24-ton hydraulic hot press from Tetrahedron

Associates was employed. An 11 cm 9 11 cm 9 1.9 cm

stainless-steel mold was machined with 16 wells, each 1.9 cm

in diameter and 1.9 cm deep. Prior to use, the mold was coated

with a non-stick spray (CRC Industrial Dry PTFE Lube: Dry

Film Lubricant). To seal the mold prior to processing, gasket

material was used (as described below).

2.2 Aerogel fabrication

A TEOS-based precursor recipe (Table 1) was adapted from

the literature [5]. The appropriate volumes of TEOS, ethanol,

deionized water and aqueous oxalic acid catalyst solution

(0.0071 M) were mixed together, then sonicated for 15 min.

The mixture was covered and left to rest at room temperature

for 45 min to allow hydrolysis of TEOS to proceed. Then an

aqueous solution of ammonia (0.375 M) was added and the

mixture was stirred with a glass stirring rod.

The mixture was then processed by RSCE as follows.

First, a 1.6-mm-thick graphite sheet was placed on top of

the lower hot-press platen, followed by a layer of 0.013-

mm-thick stainless steel foil, then the mold, followed by

another layer of stainless steel foil and, finally, another

layer of graphite. The mold was sealed in the hot press

using a 89 kN restraining force for 10 min in order to

adhere the gasket material to the bottom of the mold to

prevent leakage. After sealing, the top layers of gasket

material were removed, and the liquid precursor solution

was poured into the 16 wells of the mold. Fresh layers of

stainless steel foil and graphite were placed on top of the

mold, and the following hot press processing program was

employed. In the first step of the process, a 178-kN

restraining force was applied to re-seal the mold (now fil-

led) within the hot press. During the second step, the

temperature was gradually increased to 288 �C (at a rate of

1 �C/min). The sol gel formed within the mold and the

temperature of the sol–gel matrix and the solvent within the

pores of the matrix increased. The mold was held at 288 �C

for 30 min, then the restraining force was gradually

decreased to 4.4 kN; the solvent escaped the mold as a

supercritical fluid and was vented. Finally, the mold con-

taining the resulting aerogel monoliths was cooled gradu-

ally at a rate of 1.7 �C/min. The aerogel monoliths formed

were observed to completely fill the wells within the mold.

After removal of the gasket materials, the monoliths were

pushed out of the wells of the mold.

Table 1 Recipe for preparation of 100 mL of the TEOS-based sol–

gel precursor recipe

Reagent Volume

(mL)

Overall molar

ratio

TEOS (tetraethylorthosilicate) 35.8 1.00

EtOH (ethanol) 46.1 4.93

H2O (deionized water) 1.15a 6.27b

H2C2O4 (oxalic acid): 0.0071 M 13.8 6.1 9 10-4

NH3 (ammonia): 0.375 M 3.09 7.2 9 10-3

a This volume of water in the recipe does not include the volumes of

the aqueous solutions of oxalic acid and ammoniab Molar ratio takes into account the water introduced to the mixture

from the aqueous solutions of oxalic acid and ammonia

J Sol-Gel Sci Technol

123

2.3 Experimental methods of characterization

Aerogel monolith samples were characterized using bulk

and skeletal density, infrared (IR) and visible transmission

spectroscopy and Raman spectroscopy, nitrogen gas

adsorption for surface area and porosimetry measurements,

and scanning electron microscope (SEM) imaging.

Bulk densities for TEOS aerogels were calculated from

the mass of each sample, obtained using an Ohaus Explorer

Pro (Model EP64C) analytical balance, and the volume of

the monolith, using the volume of the well as an estimation

of the monolith. Skeletal density measurements were per-

formed on samples through the use of a Micromeritics

helium pycnometry system, with each sample (ca.

0.15–0.2 g) crushed and placed in a sample vial prior to

analysis. The sample was first purged 99 times to remove

any adsorbed species, then the density measurement was

repeated 100 times, and the last 10 measurements were

averaged to estimate the density. (It was noted that initial

measurements overestimated the skeletal density, and also

that the final mass of the sample was somewhat lower,

indicating that adsorbed species accounted for some of the

initial mass. The final mass ±0.005 g was used in the

calculations.)

IR spectra were taken with a Nicolet Avatar 330 FT-IR

with a Smart Orbit Diamond ATR attachment using a

resolution of 4 cm-1 and 32 scans. Optical transmission

measurements of 13 monolithic aerogels were taken with a

HP/Aglient 8453 Diode Array Spectrophotometer over the

range of 400–1,100 nm. The measurements were made

down the length of the cylindrical monolith so that the path

length was the height of the monolith, *1.9 cm. Raman

spectroscopy was taken of crushed samples from four

aerogel batches with a DeltaNu Advantage 200A Raman

Spectrometer. An average of 64 scans at low resolution

with baseline correction was taken.

Surface areas were acquired with a Micromeritics Tri-

star 3000 and a Micromeritics Smartprep degasser. The

samples were degassed for 2 h at 90 �C and then for 10 h

at 200 �C. Surface areas were estimated using the Bru-

nauer, Emmett, Teller (BET) method [16] using 5 partial

pressure points from 0.05 to 0.3 and a 20 s equilibration

time. A Micromeritics ASAP 2010 nitrogen gas adsorption

system was used to measure the pore distribution of the

aerogel powders. Measurements were again made using a

20 s equilibration time at each partial pressure, in order to

minimize the effects of compression during testing [17].

The pore distribution was calculated using the Barrett-

Joyner-Halenda (BJH) mode [18] applied to the desorption

isotherms.

The surface morphology of crushed samples of TEOS

aerogels was observed with a Zeiss Evo 50 Scanning

Electron Microscope. The aerogel samples were sputter-

coated, using a Denton Vacuum Desk IV Sputter Coater

with a gold/palladium target, prior to SEM analysis.

3 Results

3.1 Aerogel appearance

We initially employed a recipe modified from Rao and

Bhagat [5]: a precursor mixture of TEOS, ethanol, and

water (molar ratio of 1.0:4.9:6.3), with oxalic acid to cat-

alyze hydrolysis, followed by aqueous ammonia to catalyze

the polycondensation reactions. The result was opaque

monolithic RSCE aerogels (Fig. 1a) with average bulk

density *0.09 g/cm3, average BET surface areas up to

320 m2/g, and skeletal density of 2.04 g/cm3. The con-

centrations of the acid and base catalysts used in the recipe,

and the time between addition of acid and base catalysts to

the mixture were varied during fabrication of several bat-

ches, in order to refine the process. The final recipe

(Table 1) consistently resulted in optically translucent

aerogels, as shown in Fig. 1b,c.

3.2 Spectroscopic analysis

Optical transmission spectra were collected for thirteen

monolithic TEOS-based RSCE aerogels. All are smooth,

featureless curves. The aerogels are opaque (0 %T) below

420 nm. A typical 1.9-cm monolith has\10 %T at 500 nm,

\25 %T at 600 nm, \45 % T at 700 nm, and \55 %T at

800 nm, with higher transmittance in the near-infrared

region. (When adjusted for a conventional 1-cm pathlength,

these values correspond to\30 %T at 500 nm,\48 %T at

600 nm,\66 % T at 700 nm, and\73 %T at 800 nm.) For

the 900–1,100 nm range the optical transmission of the

aerogel monoliths was typically greater than 60 %.

Every IR spectrum obtained for the RSCE TEOS-based

silica aerogels contained silica peaks around 1,073, 796

Fig. 1 Photographs of TEOS-based RSCE aerogel monoliths:

a opaque monolith from initial recipe; b translucent aerogel monolith,

from recipe shown in Table 1; c translucent aerogel monolith (same

recipe, different batch from view b) shown in optical transmission

holder

J Sol-Gel Sci Technol

123

and 452 cm-1. A representative IR spectrum for a TEOS

RSCE aerogel is shown in Fig. 2 (the spectrum for a

TMOS RSCE aerogel is provided for comparison). The

peak at 1,073 cm-1 had a shoulder *1,200 cm-1. There

were no significant –OH stretching or –CH stretching peaks

observed in the spectra. In the Raman spectra, all of the

aerogel samples studied contained two silica peaks at

*496 and 1,260 cm-1.

3.3 Surface area and porosimetry analysis

The average surface area of four batches of TEOS-based

RSCE aerogels was 460 m2/g, with a standard deviation of

10 m2/g. The uncertainty in the BET surface area mea-

surements for individual aerogel samples ranges from 10 to

20 m2/g. This surface area value is smaller than that of the

TMOS-based RSCE aerogels (prepared using the recipe

described in Anderson et al. [8] under similar processing

conditions) whose average surface area ranges from 517 to

590 m2/g.

When the BJH pore distribution of TEOS-based RSCE

aerogel is investigated, the aerogels are found to be mes-

oporous. These materials have pore diameters in the

10–100 nm range, with peak distribution in the 30–40 nm

range. Figure 3 plots the pore distribution based on the

desorption isotherm for the TEOS-based RSCE aerogel and

compares it to a TMOS-based RSCE aerogel.

3.4 Bulk and skeletal density

Bulk densities of 48 monolithic TEOS-based RSCE aero-

gels were measured. The mean density was 0.099 g/cm3,

with a standard deviation of 0.003 g/cm3. All the bulk

densities measured fell within the range of 0.094–0.104 g/

cm3.

Skeletal densities for samples from three different

aerogel batches were measured. The skeletal density values

ranged from 1.97(±0.07) to 2.06(±0.05) g/cm3. These

results are larger than the TMOS-based RSCE aerogels

whose bulk density is 0.065 g/cm3 and skeletal density is

1.9 g/cm3.

3.5 Scanning electron microscopy (SEM)

Using SEM, the surface morphology of the aerogel samples

was imaged. Representative images are shown in Fig. 4.

The images show typical silica aerogel morphology with

clear evidence of pores in the tens of nanometer (meso-

porous) range.

4 Discussion

The goal of this work was to prepare monolithic, TEOS-

based RSCE aerogels with reasonably high surface areas

that were sufficiently transparent for use in the development

of aerogel-platform chemical sensors. Multiple, consistent

batches of monolithic TEOS-based aerogels were produced

via the recipe and 8-h RSCE process described here. The

total time required from beginning to prepare the precursor

solution to removing aerogel monoliths from the mold was

\9.5 h. Active solution preparation time was\20 min and

processing time in the hot press was 8 h 7 min. These times

are comparable to those for the method used routinely in our

lab for TMOS-based monolithic silica aerogel preparation

[20]; however, the TEOS-based aerogel process is, overall,

about 1 h longer because of the longer sonication time and

0

5

10

15

20

25

1 10 100 1000dV

/dlo

g(D

) P

ore

Vo

lum

e (c

m³/

g·n

m)

Pore Diameter (nm)

TEOS-based

TMOS-based

Fig. 3 BJH pore distributions from desorption isotherms show peaks

*20 nm for the TMOS-based RSCE aerogels (squares) and *40 nm

for the TEOS-based RSCE aerogel (diamonds)

400

800

1,20

0

1,60

0

2,00

0

2,40

0

2,80

0

3,20

0

3,60

0

4,00

0%

Ref

lect

ance

Wavenumbers (cm-1)

TEOS

TMOS

Fig. 2 FTIR spectra of TEOS-based (top line) and TMOS-based

(bottom line) RSCE aerogels. The spectra have been offset for clarity,

so y-axis values are omitted to avoid direct comparison

J Sol-Gel Sci Technol

123

the 45-min rest period to allow for hydrolysis prior to

adding base catalyst. Based on prior work fabricating silica

aerogels from a TMOS-based recipe and the RSCE process

[8], it is likely that a shorter processing time could be

achieved without negatively impacting the monolithicity,

transparency or surface area of the aerogels. That earlier

work demonstrated that changing the restraining force,

heating and cooling rates, pressure release rate and mold

volume fill ratio had no significant effect on the physical

properties of the resulting aerogels; the only processing

condition found to impact the aerogel properties was the

maximum temperature employed [8]. On the other hand,

variations in sol–gel precursor recipe, including the con-

centrations of acid and base catalysts, and the time interval

before addition of base to the acidic sol, are anticipated to

affect significantly the surface area and transparency of the

resulting aerogels, as described by Rao and Bhagat [5] and

observed in the work presented here.

The TEOS-based materials have a low bulk density

(0.099 ± 0.003 g/cm3) and their skeletal density is indic-

ative of silica. Bulk densities achievable for a TMOS-based

recipe with this RSCE processing method are lower

(*0.065 g/cm3) [8]; however, in the present work no

attempt was made to alter the recipe to produce lower-

density aerogels. The skeletal density results are compa-

rable to our TMOS-based RSCE aerogels [8].

These TEOS-based RSCE aerogels have surface areas of

460 m2/g, which is lower than TEOS aerogels prepared via

certain other methods. When a CO2 supercritical extraction

method is employed for preparation of TEOS-based silica

aerogels, surface areas can be routinely as high as 700 to

[1,100 m2/g [19]. In prior work from our laboratory, it

was noted that RSCE TMOS-based aerogels typically have

surface areas in the mid-500 m2/g range [8], at or below

literature values for TMOS aerogels prepared via other

methods [21]. When a maximum temperature above

288 �C was employed in RSCE processing of a TMOS-

based precursor mixture, the aerogels produced had lower

surface areas [8]. Significantly lowering the maximum

temperature employed is not an option; it is necessary to

exceed the supercritical point of the solvent.

The BJH porosimetry data indicate that the TEOS-based

RSCE aerogels are mesoporous, with peak pore diameters

in the 10–100 nm range and higher peak distribution than

those obtained using the same processing conditions with a

TMOS-based recipe (as shown in Fig. 3). Reichenauer and

Scherer have shown that underestimation of pore sizes can

occur due to compression of compliant materials during

nitrogen adsorption [17]. In the experimental work pre-

sented here, powdered aerogel samples were analyzed, and

relatively long equilibrium times employed in order to

minimize this problem. Use of a higher maximum tem-

perature in the RSCE process might yield higher peak pore

distributions, as has been observed for TMOS-based RSCE

aerogels [8].

SEM images of these materials show the mesoporous

morphology typical of silica aerogels; however, the authors

do not have access to a higher-resolution SEM or TEM, so

are unable to make more specific claims about the aerogels’

nano-structure.

In the IR spectra, the strong silica peak (1,073 cm-1

with shoulder at 1,200 cm-1) can be assigned as a mas(Si–

O–Si) vibration, with contributions from siloxane (SiO)x

rings, where x = 4 or 6 [22]. The peak in the Raman

spectrum at 496 cm-1 can be attributed to small three- or

four-member siloxane rings, and the peak at 1,260 cm-1

attributed to to fused silica (Si–O stretching) [22]. Since

there was no (SiO)3 Raman peak observed at about

600 cm-1, it appears likely that the structure contains

primarily four-member siloxane rings [23].

Fig. 4 SEM images of coated TEOS-based RSCE silica aerogel at 150 kX (left) and 50 kX (right) magnification

J Sol-Gel Sci Technol

123

The lack of –OH and –CH stretching peaks in the IR

spectra indicates that few unreacted Si–O–H and Si–O–

CH2CH3 groups remain in the aerogels and that no sig-

nificant amount of water has been adsorbed by the aerogel

under ambient conditions. One might expect to find evi-

dence of leftover Si–O–H and Si–O–CH2CH3 groups from

incomplete polycondensation reactions in the aerogel

framework, which would result in –OH and –CH stretching

peaks and would render the aerogel hydrophilic. However,

the lack of observed –OH and –CH peaks is fully consistent

with prior results for silica aerogels prepared from a

TMOS-based recipe using the same RSCE process [12].

Note that the TEOS-based RSCE silica aerogels are not

hydrophobic. Attempts to measure contact angles were

unsuccessful because water droplets placed directly on the

surface of the TEOS-based RSCE aerogel were taken up by

the aerogel.

The RSCE TEOS aerogels are optically translucent,

with good but not excellent transmittance in the red and

near-IR ([50 % for 1-cm pathlength). Considerable sam-

ple-to-sample variation in %T values was observed even

for aerogels from the same batch. This is likely due to

small variations in the pathlength of the individual mono-

liths. A number of factors impact transparency of silica

aerogels, including the precursors and the drying method

used [3, 24]. For example, silica aerogels prepared by

Hegde and Rao from a TEOS-based recipe using a two-step

acid–base sol–gel process with methanol supercritical

extraction in an autoclave had optical transmission of 75 %

T at 700 nm (1-cm pathlength) in the visible region [4]. In

preliminary work, we have determined that more translu-

cent aerogels could be obtained using a TEOS-based recipe

modified from Bhagat et al. [25] with RSCE processing;

however, that recipe includes methanol rather than ethanol

as a solvent. The use of methanol is a disadvantage for

future scale-up of the RSCE process, but does yield aero-

gels of improved optical quality, which would likely be of

interest in certain applications, including chemical sensing.

5 Conclusions

Monolithic, translucent TEOS-based RSCE aerogels can be

fabricated rapidly and reproducibly using an RSCE

method. These aerogels are mesoporous with high surface

areas, low bulk densities, skeletal densities consistent with

silica, moderate optical transmission in the visible and

good optical transmission in the near-infrared. FTIR and

Raman spectra show peaks consistent with the silica

backbone, with little evidence that unreacted Si–OH or Si–

OCH2CH3 groups remain in the aerogel matrix. The RSCE

process does not involve any solvent exchanges, so it is

faster and generates less solvent waste than does the more

conventional supercritical CO2 extraction method for pre-

paring silica aerogels.

Acknowledgments This material is based upon work supported by

the National Science Foundation (NSF) under Grant No. CHE-

0847901. The authors thank Stephen J. Juhl for collecting the pyc-

nometry data, Lutao Xie for her work with the TMOS-based RSCE

aerogels, Ryan M. Bouck for acquiring SEM images of the aerogels

and Thomas M. Kolb for preliminary work on the use of methanol in

TEOS-based precursor recipes. SKE is grateful for summer funding

from the NASA New York Space Grant and additional support of this

project through the Union College Internal Education Fund program.

The Union College Aerogel Laboratory has been funded by instru-

mentation and research grants from the NSF (CTS-0216153, CHE-

0514527, CMMI-0722842, and CHE-0847901). The SEM instrument

was funded through grants from the NSF (MRI 0619578) and New

York State Assembly RESTORE-NY.

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