CMDITR Review of Undergraduate Research - Pluto - University of ...

Synthesis and Self-Assembly of Au@SiO 2 Core-Shell Colloids
Adam Hubbard, Walla Walla College
Joe McLellan and Younan Xia
Xia Lab, Dept. of Chemistry, University of Washington
Introduction
Photonic crystals have a broad range of
applications that are just beginning to be
realized. These applications include lasers, light
emitting diodes, thin film photovoltaics, and
various telecommunication devices. 1 These
potential uses are largely due to the periodic
modulation of refractive index that enables a
photonic crystal to the control propagation of
photons in ways analogous to a semiconductor’s
control of electrons. One method for fabrication
of photonic crystals is self-assembly of
monodispersed spherical colloids. 1 A particularly
useful variety of colloid is the core-shell type
where one substance, the core, is coated with
another material, the shell. This design allows
one to tailor the properties of a photonic crystal
in several ways, including the distance between
adjacent cores by varying shell thickness. The
goal of this work was to synthesize gold-silica
core-shell colloids and self-assemble them into
colloidal crystals for future study of their linear
and nonlinear optical properties.
Theory
Gold-silica core-shell colloids were prepared
using a modified Stöber method, a variant of solgel
processing. Sol-gel chemistry consists of two
steps, hydrolysis of an alkoxide precursor into a
sol and the subsequent condensation of that sol
into a gel network. Both steps can be catalyzed
with either an acid or a base. More specifically,
the modified Stöber method generates an
amorphous silica network at room temperature
using liquid silicon precursors known as silicon
alkoxides of the general molecular formula
Si(OR) 4 . First, the readily hydrolysable alkoxy
groups (OR where R = C n H 2n+1 ) undergo basecatalyzed
hydrolysis resulting in a hydrolyzed
precursor or sol.
Base Catalyzed Hydrolysis 4
Si(OR) 4 + nOH- → Si(OR) 4-n (OH) n + nOR-
The second step in the modified Stöber process
is the condensation of highly cross-linked sol
particles into a gel network. In this basecatalyzed
step, the silanols (Si-OH) react with
other silanols and/or non-hydrolyzed alcoxy
groups to form a siloxane bond (Si-O-Si), the
beginning of a silica network. Subsequent
condensation results in a network of silicon and
oxygen commonly known as silica.
Base Catalyzed Condensation 4
X 3 SiOH + HOSiX 3 → X 3 Si-O-SiX 3 + H 2 O or
X 3 SiOR + HOSiX 3 → X 3 Si-O-SiX 3 + ROH
Condensation can result in homonucleation
where the silica networks nucleate independently
or heteronucleation where the gold nanoparticles
serve as sites for the silica to nucleate around.
Research Method
Materials
The citrate-stabilized gold particles,
purchased from Ted Pella, had an average
diameter of 50nm with a coefficient of variation
of