Nucleation and Growth of Gold
Nanoparticles Studied via in situ Small
Angle X-ray Scattering at Millisecond
Time Resolution
Jo ¨ rg Polte,
†
Robert Erler,
†
Andreas F. Thu ¨ nemann,
†
Sergey Sokolov,
‡
T. Torsten Ahner,
‡
Klaus Rademann,
§
Franziska Emmerling,
†,
* and Ralph Kraehnert
‡
†
BAM Federal Institute of Materials Research and Testing, Richard-Willsta ¨tter-Strasse 11, D-12489 Berlin, Germany,
‡
Technische Universita ¨t Berlin, Technische Chemie,
Strasse des 17, Juni 124, D-10623 Berlin, Germany, and
§
Humboldt-Universita ¨t zu Berlin, Department of Chemistry, Brook-Taylor-Strasse 2, D-12489 Berlin, Germany
M
etallic nanoparticles have at-
tracted much attention owing to
their unique properties and nu-
merous promising applications. In particu-
lar gold nanoparticles were investigated
with regard to potential applications in bio-
technology,
1
catalysis,
2
and optoelectron-
ics.
3
In this context a profound understand-
ing of the mechanisms and kinetics of
particle formation is essential for tuning
their size and morphology.
Recently we demonstrated the possibili-
ties of combined in situ small-angle X-ray-
scattering/X-ray near edge structure (SAXS/
XANES) measurements in a solution of
levitated sample droplets, where SAXS de-
livers information on size and shape of the
formed particles. XANES can be applied to
monitor the progress of the reaction by
means of the oxidation state.
4
Because of
experimental limitations the above-
mentioned method did not reach a time
resolution sufficient for one of the typical
fast gold nanoparticle synthesis, that is, the
reduction via sodium borohydride (NaBH
4
)
or ascorbic acid (C
6
H
8
O
6
), which occurs in
the time span of milliseconds to a few sec-
onds. Since the acquisition time of an X-ray
scattering curve is typically in the range of
several minutes, such fast particle formation
processes cannot be investigated by com-
mon laboratory SAXS setups using a con-
ventional Roentgen tube. Even when using
synchrotron radiation only few beamlines
provide the needed time resolution in the
range of 200 ms.
5
For the investigation of
such rapidly proceeding liquid-phase syn-
theses that require a time resolution of
100-200 ms a rapid mixing of the reactant
solutions is essential.
These fast syntheses can be carried out
in microstructured mixers that ensure rapid
local mixing. The synthesis of different ma-
terials (chalcogenides,
6,7
oxides,
8
metals
9-11
) in such mixing devices has
been described in the literature. The
materials produced using microstructured
mixers typically show a more homogeneous
constitution.
12,13
For time-resolved SAXS analysis
stopped-flow and continuous-flow tech-
niques have been establishedOboth in-
cluding a rapid mixing device.
14-19
The
stopped-flow devices usually demand syn-
chrotron facilities for sufficiently fast data
acquisition, where the achieved time resolu-
tion is limited by the characteristics of the
beamline.
*Address correspondence to
franziska.emmerling@bam.de
Received for review October 27, 2009
and accepted January 11, 2010.
Published online January 20, 2010.
10.1021/nn901499c
© 2010 American Chemical Society
ABSTRACT Gold nanoparticles (AuNP) were prepared by the homogeneous mixing of continuous flows of an
aqueous tetrachloroauric acid solution and a sodium borohydride solution applying a microstructured static mixer.
The online characterization and screening of this fast process (2 s) was enabled by coupling a micromixer
operating in continuous-flow mode with a conventional in-house small angle X-ray scattering (SAXS) setup. This
online characterization technique enables the time-resolved investigation of the growth process of the
nanoparticles from an average radius of ca. 0.8 nm to about 2 nm. To the best of our knowledge, this is the first
demonstration of a continuous-flow SAXS setup for time-resolved studies of nanoparticle formation mechanisms
that does not require the use of synchrotron facilities. In combination with X-ray absorption near edge structure
microscopy, scanning electron microscopy, and UVvis spectroscopy the obtained data allow the deduction of a
two-step mechanism of gold nanoparticle formation. The first step is a rapid conversion of the ionic gold precursor
into metallic gold nuclei, followed by particle growth via coalescence of smaller entities. Consequently it could
be shown that the studied synthesis serves as a model system for growth driven only by coalescence processes.
KEYWORDS: nanoparticle formation mechanism · SAXS · microstructured static
mixer · continuous flow
ARTICLE
VOL. 4 ▪ NO. 2 ▪ POLTE ET AL. www.acsnano.org 1076