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Journal of Non-Crystalline Solids 315 (2003) 166–179

Chemical reaction kinetics leading to the first Stober silica
nanoparticles – NMR and SAXS investigation
D.L. Green a, S. Jayasundara b, Yui-Fai Lam c, M.T. Harris


Department of Chemical Engineering and Institute of Physical Science and Technology, University of Maryland,
College Park, MD20742, USA
Department of Civil and Environmental Engineering, University of Maryland, College Park, MD 20742, USA
NMR Lab, Department of Chemistry, University of Maryland, College Park, MD 20742, USA
Received 20 November 2001

Si-NMR and 13 C-NMR were used in methanol and ethanol to monitor the intermediates or hydrolyzed monomers
that lead to the formation of the firstprimary particles as detected by small angle X-ray scattering. This identification
was facilitated by using initial NH3 and H2 O levels at the lower end of those experienced in Stober synthesis to slow the
reaction kinetics. We found that [NH3 ] and [H2 O] control the balance between hydrolysis of tetraethylorthosilicate
(TEOS) and the condensation of its hydrolyzed monomers. Transesterificationbetween methanol and TEOS did occur;
however, it was negligible compared to the production of hydrolyzed intermediates. The first nanostructures appear at a
hydrolyzed monomer concentration around 0.1 M, indicating that formation of the primary structures is thermodynamically controlled by a supersaturation of the intermediate species. Differences in particle size between methanol and
ethanol areattributed to thermodynamic interactions between the solvent and the hydrolyzed intermediates.
Ó 2003 Elsevier Science B.V. All rights reserved.

1. Introduction
Stober synthesis [1], the ammonia-catalyzed
reactions of tetraethylorthosilicate [Si(OR)4 or
TEOS; R ¼ C3 H5 ] with water in low molecular
weight (MW) alcohols, produce highly monodisperse, spherical silica nanoparticles that range insize from 5 to 2000 nm. These particles have ap-


Corresponding author. Present address: School of Chemical Engineering, Purdue University, West Lafayette, IN
479907-1283, USA. Tel.: +1-765 494 0963; fax: +1-765 494
E-mail address: (M.T. Harris).

plications in the polishing of wafers to improve the
storage capacity of high tech devices such as harddrives and as nanosupport materials for biotechnology (pharmaceuticals) [2]. This wide usage of
silica warrants further investigation to elucidate
the early stages of Stober particle formation.
Specifically, we seek an understanding of the intermediate species produced from the base-catalyzed hydrolysis of TEOS, since these species lead
to the formation of the first nanostructures, nuclei,
orprimary particles.
In this paper, 29 Si- and 13 C-NMR are used to
monitor the production of these intermediate ÔsilicaÕ species. This information was combined with
the results from small-angle X-ray scattering

0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 5 7 7 - 6

D.L. Green et al. / Journal of Non-CrystallineSolids 315 (2003) 166–179

(SAXS) to determine if there is a correlation between the soluble silica concentrations and the
detection of the primary particles. To our knowledge, this research represents the first time NMR
and SAXS, which probes the transition between

the molecular (1–2 A) and colloidal length (1–40
nm) scales, have been used to link the reactions in
the liquid-phase to thefirst detection of colloids in
the solid phase. Our findings are consistent with the
supposition presented by Boukari et al. [18–20],
explaining that the induction period before the
formation of primary particles represents the
build-up of the species produced from the hydrolysis of TEOS.

2. General reactions and literature review
The reactant concentrations that produce
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