22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma-vapour-deposition synthesis of Au, Ag and AuAg core-shell nanoparticle
on metal oxide semiconductors
S. Peglow1, M.-M. Pohl2, A. Kruth1 and V. Brüser1
1
Leibniz-Institute for Plasma Science and Technology e.V., Felix-Hausdorff-Strasse 2, DE-17489 Greifswald, Germany
2
Leibniz-Institute for Catalysis e.V., Albert-Einstein-Strasse 29a, DE-18059 Rostock, Germany
Abstract: Due to their exceptional optical properties noble metal nanoparticles are of
interest for various applications. A novel synthesis method for the production of
homogenous nanoparticles using a combination of plasma vapour deposition and a
subsequent thermal annealing process is presented. The optical and morphological
properties of gold, silver and gold-silver core-shell particles on titania semiconductor are
investigated.
Keywords: core-shell nanoparticles, plasma-vapour deposition, solid state dewetting
1. Introduction
Noble metal nanoparticles are used in a variety of fields
such as photocatalytic applications [1], surface plasmon
resonance spectroscopy [2], information technology [3],
environmental protection [4], biochemical sensing [5] and
cancer treatment [6]. Commonly, gold nanoparticles are
produced via chemical methods (Turkevich [7], Brust
[8]). Similar proceedings are taken for silver nanoparticle
synthesis [9]. Core-shell particles consist of a core and a
differently composed outer shell. The material
combination sequence and elemental ratio determine the
properties of these hybrid structures and their applications
in catalysis [10], photoluminescence [11,12], biomedicine
(drug delivery [13] and controlled release [14],
bioimaging [11]), pharmacy [15] and photonic crystals
[16]. Because of its biocompatibility, preferred optical
properties and protection of the core material from
corrosion and oxidation, gold is favoured as shell
material. Choosing silver as an outer layer results in
antibacterial properties [17]. Combining two metals that
are known for their plasmonic activity and tuning their
elemental ratios enables to shift the resulting plasmon
resonance frequency between the resonance frequencies
of both materials involved [6].
This work presents a novel and easy synthesis method
for monometallic and bimetallic core-shell nanoparticle
production using conventional magnetron sputtering and
subsequent thermal annealing. This method combination
takes advantage of a process referred to as solid state
dewetting describing nanoparticle formation through
annealing of thin metal films [18]. Radio frequency (RF)
magnetron sputtering produces homogeneous layers [19]
ensuring high control over nominal metal layer thickness
preparation and thus a direct influence on nanoparticle
size after annealing [20]. Since research in metalsemiconductor photocatalysis using Au, Ag and AuAg
core-shell particles emerged in recent years, the
investigated particles were deposited on titanium dioxide
semiconductor substrates.
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2. Experimental
As a substrate a 270 nm thick TiO 2 layer was deposited
by a reactive direct current (DC) magnetron sputtering
process onto rough fluorine doped tin oxide (FTO, TCO
22-7, Solaronix) on soda lime glass. For the deposition of
gold and silver metals, two 2 inch sputtering targets (both
99,999%, MaTeck) were used. To create nanoparticles,
the metal/TiO 2 samples were put straight into a quartz
tube furnace for thermal annealing (no temperature ramp
was applied; all samples were annealed for 30 min in an
oxygen atmosphere). For pure Au and AuAg/TiO 2
samples 400°C was applied, whereas the silver layer was
treated at 200°C. To decrease their size distribution the
gold nanoparticles synthesis was divided into two
repeated deposition/annealing steps leading to particles of
10-30 nm in diameter. For AuAg core-shell structures, a
gold layer was deposited on top of a silver layer before
the whole sample was thermally treated. The AuAu ratio
was varied by tuning the initial layer thickness of gold
and silver films. For transmission electron microscopy
(TEM) measurements, a 30 nm thick TiO 2 layer was
sputter-deposited onto silicon nitride grids applying the
same PVD method as described above.
3. Results and Discussion
Due to the rough morphology of the titania substrate,
the deposited gold film did not form a fully continuous
layer. After application of the annealing treatment, the
semi-continuous gold film reshaped into discrete Au
nanoparticles, see scanning electron microscopy (SEM)
image in Fig. 1. Their particle size seemed to depend on
their three dimensional location on the titania surface.
Large particles formed on top of the TiO 2 columns
whereas smaller particles are located in the intercolumnar space.
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nanoparticles at 520 nm is originated in the interaction
with the titania environment. Additionally, a shift of the
absorption band to higher wavelength can be observed at
repetitions of the deposition-annealing steps.
Due to the optical cut-off of the substrate, the LSPR of
silver nanoparticles could not be observed in this system.
Regarding core-shell nanoparticles and considering the
limitations of the before mentioned optical cut-off at the
silver LSPR frequency, an increase in silver amount led to
a decreases in absorption at higher wavelength. This
proved that a change in the elemental ratio of gold silver
core-shell nanoparticles effected their optical properties.
Fig. 1. SEM image of Au nanoparticles on titan dioxide
after two cycles of subsequent RF-magnetron sputtering
and thermal annealing.
In contrast, the deposition of silver at similar loadings
as for gold immediately resulted in the formation of
separate islands. Thermal annealing at 200°C led to a
reduction in the nanoparticle size from 10-40nm to 510nm, as well as a decrease in size distribution and
concentration of Ag nanoparticles on top of the titania
nanocolumns. The last phenomenon can be explained
with Ag diffusion commencing at 200°C and occurring
more pronounced at 400°C.
AuAg core-shell nanoparticles were prepared and
compared for three different elemental ratios (3:1; 1:1 and
1:3, respectively).The observed particles can be roughly
divided into larger particles with a main diameter of 1020 nm and multiple small nanoparticles around and
below 3 nm (both derived from SEM images not shown
here) in size. An increase of the gold amount in the coreshell nanoparticles led to an increase in number of large
particles. TEM images confirmed the increase in size for
large particles from Ag/Au (1:1) to Ag/Au(1:3). However,
at the lowest Au amount, Ag/Au (3:1), large irregularly
shaped particles are observed. The number of small
particles seems to increase with increasing silver content.
Energy dispersive x-ray spectroscopy (EDX)
measurements confirmed that the large particles consist of
a gold core surrounded by a silver shell. The small
nanoparticles appeared to be almost entirely silver
containing. A higher silver ratio at the preparation
resulted in an increase in silver shell thickness and in a
decrease in the gold core diameter. Additionally,
multicore nanoparticles consisting of several small gold
cores encapsulated in one silver shell were observed.
The absorption properties of the gold, silver and goldsilver core-shell particles can be explained by localized
plasmon resonance. A broadening of the absorption peak
at 620 nm for gold nanoparticles was observed due to a
significant particle size distribution. The red-shift of the
absorption peak compared to the localized surface
plasmon resonance (LSPR) frequency of isolated Au
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4. Conclusion
A synthesis method for the preparation of monometallic
and bimetallic core-shell nanoparticles combining a
conventional magnetron sputtering process with a thermal
annealing procedure was presented. Tuning of the process
parameter allows for adjusting the optical and structural
properties of the nanoparticles. Due to alternative
sputtering of silver and gold the formation of core-shell
nanoparticles with adjustable shell thickness could be
realized.
5. Acknowledgement
This work was funded by the German Research
Foundation (DFG). We would like to thank Daniel Köpp
for PVD of the TiO 2 substrate, Anja Albrecht for XRD
measurements, and Dr. Harm Wulff from University of
Greifswald for the crystallite size calculation.
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