Changes in the shape and optical properties of gold nanoparticles contained within alumina membranes due to low-Temperature annealing

Article abstract of DOI:10.1021/jp971267w

The shape and optical properties of Au nanoparticles electrochemically deposited within the pores of porous alumina membranes have been studied as a function of the temperature to which the Au/alumina composite was heated. Low aspect ratio Au nanoparticles begin as thin skinned particles, essentially flakes of Au within the pores, but after heat treatment at low temperatures (<400?°C), they anneal to become dense, roughly spherical particles. Visually, the color of these membranes changes from blue to red after the thermal treatment. In contrast, the nanoparticles with an aspect ratio of ca. 3 exhibit essentially no change in either optical absorption characteristics or particle shape after exposure to the heating program. These membranes were initially red and did not change color with heating.

Full text of DOI:10.1021/jp971267w

J. Phys. Chem. B 1997, 101, 7727-7731
7727
Changes in the Shape and Optical Properties of Gold Nanoparticles Contained within
Alumina Membranes Due to Low-Temperature Annealing
John C. Hulteen, Charles J. Patrissi, David L. Miner, Erin R. Crosthwait,
Elizabeth B. Oberhauser, and Charles R. Martin*
Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523
ReceiVed: April 11, 1997; In Final Form: June 30, 1997X
The shape and optical properties of Au nanoparticles electrochemically deposited within the pores of porous
alumina membranes have been studied as a function of the temperature to which the Au/alumina composite
was heated. Low aspect ratio Au nanoparticles begin as thin skinned particles, essentially flakes of Au within
the pores, but after heat treatment at low temperatures (<400 °C), they anneal to become dense, roughly
spherical particles. Visually, the color of these membranes changes from blue to red after the thermal treatment.
In contrast, the nanoparticles with an aspect ratio of ca. 3 exhibit essentially no change in either optical
absorption characteristics or particle shape after exposure to the heating program. These membranes were
initially red and did not change color with heating.
Introduction
resonance band solely as a function of particle shape for island
films because the initial nanoparticles formed at room temper-
We1-6 and others7-10 have been studying the properties of
nanoscopic metal particles deposited within the pores of porous
alumina template membranes. Special interest has been devoted
to the optical properties of Au nanoparticle/alumina membrane
composites.2-6 The important feature of this template approach
for preparing Au nanoparticles is that both the diameter and
aspect ratio (length/diameter) of the particle can be varied at
will. In previous work, we demonstrated that when the diameter
of the Au nanoparticle was decreased, there was a blue shift in
the wavelength of maximum absorption intensity of the plasmon
resonance band, λmax, of the Au nanoparticle/alumina composite.
Also, when the aspect ratio of the nanoparticle was decreased,
a red shift in the λmax was observed. These effects have been
modeled using Maxwell-Garnett (MG) effective medium theory.3,4
More recently, we have developed a dynamic Maxwell-Garnett
(DMG) effective medium theory for the prediction of optical
properties of nanometal/alumina membrane composites.2,6 This
dynamic modification of the conventional MG theory allows
for more accurate modeling of the nanoparticle plasmon
resonance band when the diameter of the nanoparticle is not
infinitely small (i.e., when the nanoparticle diameter is ap-
proximately between 20 and 100 nm).
While investigating the thermal stability of these composite
membranes, we discovered that heating the Au nanoparticle/
alumina composite to temperatures well below the Au melting
point sometimes resulted in dramatic changes in the optical
properties (i.e., color) of the membranes. There has been a large
number of studies investigating the effects of annealing on the
structural and optical characteristics of nanosized metals,11-15
semiconductors,16 and inert gas crystalites17 at temperatures
significantly below their bulk melting point. Most of the prior
work on the metal nanostructures has been carried out on island
films.11-14 Studies of these metal island films have shown
diffusion and coalescence of the metal nanoparticles resulting
in changes in nanoparticle shape, size, and optical properties.
Such changes can occur at extremely low temperatures ranging
from 400 °C down to room temperature. However, it is very
difficult, if not impossible, to follow changes in the plasmon
ature coalesce into significantly larger nanoparticles after heat
treatment.
In this paper, we will show low-temperature-induced Au
nanoparticle size and shape changes and the effect of these
changes on the optical properties without interference from
nanoparticle coalescence. This is possible with the template-
synthesized Au nanoparticle composite membranes because each
Au nanoparticle is confined to its own pore. The temperature-
induced changes in size and shape were determined using
transmission electron microscopy. The corresponding change
in optical properties were investigated using UV/Visible absorp-
tion spectroscopy.
Experimental Section
Preparation of Nanoporous Alumina Template Mem-
branes. Nanoporous aluminum oxide template membranes
were prepared in-house as described previously.2-6 Briefly,
99.999% purity aluminum was electropolished to a mirror finish
in 2:3 (v/v) phosphoric/sulfuric acid solution at 70 °C for 10
min at a current density of 75 mA cm-2. The aluminum was
then anodized at a constant potential (30 V) in 15% oxalic acid
at a temperature between 0 and 5 °C at a current density between
1 and 2 mA cm-2. This process results in the growth of a porous
aluminum oxide film on the surface of the aluminum. Detach-
ment of this alumina film from the aluminum substrate is
accomplished via voltage reduction and subsequent immersion
of the alumina/aluminum composite in 25% phosphoric acid.3
The approximately 25 µm thick nanoporous alumina template
was then washed thoroughly with ultrapure water (obtained by
passing distilled water through a Millipore water purification
system) and dried at room temperature in air. These alumina
template membranes contained ca. 45 nm diameter pores, as
determined using transmission electron microscopy.2-6
Gold Nanoparticle Electrodeposition. The Au nanoparticle/
alumina composites were fabricated via the electrodeposition
of gold within the pores of the alumina template. This procedure
is described in detail elsewhere.2,3 Briefly, Ag is first thermally
evaporated onto one face of the alumina template membrane.
This silver layer functions as a cathode for the constant current
electrodeposition of silver “nanoposts” which extend several
* Corresponding author. E-Mail: crmartin@lamar.colostate.edu.
X Abstract published in AdVance ACS Abstracts, August 15, 1997.
S1089-5647(97)01267-4 CCC: $14.00 © 1997 American Chemical Society

7728 J. Phys. Chem. B, Vol. 101, No. 39, 1997
Hulteen et al.
Figure 1. Transmission electron micrographs of Au nanoparticles before (A, C, E) and after heating (B, D, F) the membrane containing the
particles to 400 °C. Particles having three different aspect ratios are shown. The aspect ratio was varied by varying the number of Coulombs used
to electrochemically deposit the Au within the template membrane. The number of coulombs used was 0.40 C (A, B), 0.20 C (C, D) and 0.08 C
(E, F).
microns into the pores of the template. Gold nanoparticles are
subsequently electrodeposited on top of these silver posts using
a commercial Au plating solution (Orotemp 24, Technic, Inc.).
The silver posts ensure that the gold particles are formed well
into the template pores where pore diameter is more uniform.
The silver is then dissolved away by immersion in concentrated
nitric acid, leaving an Au nanoparticle within each pore of the
alumina membrane.
sition.2,6 In this work, Au nanoparticles were prepared by
electrodepositing 0.40, 0.20, and 0.08 Coulombs of Au over a
geometrical template surface area of 3.54 cm2. An EG&G
Princeton Applied Research Model 173 potentiostat equipped
with a PAR Model 176 digital coulometer was used for the
electrodeposition.
Spectral Characterization. UV/visible absorption spectra
were acquired using a Hitachi U-3501 spectrophotometer. The
template membranes were oriented perpendicular to the incident
beam such that the diameter of the Au nanoparticles was aligned
The aspect ratio of the Au nanoparticles is determined by
the number of coulombs of charge used during the electrodepo-

Au Nanoparticles in Alumina Membranes
J. Phys. Chem. B, Vol. 101, No. 39, 1997 7729
parallel with the electric field vector of the light. To study the
effects of temperature on the optical properties of the Au/
alumina composites, the membranes were heated to the desired
temperature, held at this temperature for 1 h, and then cooled
to room temperature. An absorption spectrum of each Au
nanoparticle/alumina composite was measured before and after
each heat treatment. The heating temperatures used ranged from
40 to 400 °C.
Transmission Electron Microscopy (TEM). The dimen-
sions of the Au nanoparticles were characterized using a JEOL
2000 transmission electron microscope. This was accomplished
by dissolving the alumina template and collecting the Au
nanoparticles on a carbon-coated copper TEM grid (Ted Pella,
Inc.). This procedure involved placing a section of composite
on the grid in a 1 mL plastic container. The container was
filled with 1 M NaOH. After a 2 h immersion period, the NaOH
was removed carefully so as not to disturb the gold particles.
The container was then refilled with ultrapure water and drained
again. After two additional rinse cycles, the grid was dried in
air at room temperature and imaged with the TEM.
Results and Discussion
All of the Au nanoparticles investigated here were obtained
by electrodepositing Au within a nanoporous alumina membrane
with 45 nm diameter pores. Au nanoparticles with varying
aspect ratios were obtained by electroplating three different
amounts of Au within the pores of the template membrane. TEM
images of these Au nanoparticles before and after heating the
Au/alumina composite to a temperature of 400 °C are shown
in Figure 1.
Figure 2. Absorption spectra obtained before and after heating the
0.40 C Au nanoparticle/alumina composite. The temperatures used are
as indicated.
Figure 1A,B shows images of the Au nanoparticles obtained
after passing the largest amount of charge (0.40 C) during the
Au electrodeposition. As would be expected, these nanoparticles
have the highest aspect ratio (ca. 3) of any of the nanoparticles
investigated in this study. A comparison of the images in parts
A and B of Figures 1A and 1B shows that neither the size nor
the shape of these Au nanoparticles changes upon heating to
400 °C. Electron diffraction patterns before and after heating
showed no discernible changes, indicating that there are no
major changes in crystallinity of these Au nanoparticles.
This lack of temperature-induced change in the size and shape
of these aspect ratio ) 3 Au nanoparticles is reflected in the
optical absorption spectra. Membranes containing these Au
nanoparticles were heated at various temperatures up to 400
°C, and the optical absorption spectra were obtained after each
heat treatment, Figure 2. Note that the position of the plasma
resonance band does not change with temperature. This is
further illustrated in Figure 3 (curve A), which shows a plot of
the λmax for the plasmon resonance band versus the temperature
at which the membrane was heated. These data clearly show
that the λmax is insensitive to applied temperature for the
membranes containing these highest aspect ratio particles.
The Au nanoparticles in Figure 1C,D were obtained by
depositing 0.20 C of Au within the pores of the alumina
membrane. Looking first at the TEM image obtained after
heating to 400 °C, Figure 1D, these Au nanoparticles have, as
expected, a lower aspect ratio than the 0.40 C nanoparticles
discussed previously. More interesting, however, there is now
a change in the shape of the Au nanoparticles before versus
after heating to 400 °C. After heating, the Au nanoparticles
have a uniform, almost spherical shape with the expected ca.
45 nm diameter. In contrast, before heating, the Au nanopar-
ticles have very irregular shapes and sizes, Figure 1C. Some
of the particles appear like thin skins, as if some of the Au
deposited preferentially along the pore wall. This skin-like
Figure 3. Plot of λmax for the plasmon resonance band of the three
different Au/alumina composites versus the temperature at which the
Au nanoparticle/alumina composite was heated: (A) 0.40 C, (B) 0.20
C, and (C) 0.08 C.
interpretation is reinforced by the electron transparency of some
of the particles in this image. However, after heating, the
nanoparticles are electron opaque.
This change in shape of the 0.20 C Au nanoparticles upon
heating is reflected in the optical absorption spectra, Figure 4.
Note that in contrast to the higher aspect ratio particles, there
is now a small blue shift in the λmax of the plasmon band with
increasing temperature. This is illustrated in Figure 3 (curve
B), which shows a plot of λmax for the plasmon resonance band
versus temperature at which the membrane was heated. This
change in λmax is more easily seen with the eye as the
membranes go from blue before heating to pink after heating.
In addition to the shift in λmax for the plasmon resonance
band, the band also becomes significantly more narrow after
heating, Figure 4. This narrowing reflects the uniformity in
nanoparticle size and shape after thermal treatment, as indicated
in the TEM images. The unheated 0.20 C particles begin as an
ensemble of irregular sizes, shapes, and aspect ratios, and as a
result, the plasmon absorption is very broad. However, after

7730 J. Phys. Chem. B, Vol. 101, No. 39, 1997
Hulteen et al.
Figure 4. Absorption spectra obtained before and after heating the
0.20 C Au nanoparticle/alumina composite. The temperatures used are
as indicated.
Figure 5. Absorption spectra obtained before and after heating the
0.08 C Au nanoparticle/alumina composite. The temperatures used are
as indicated.
thermal treatment, the particles adopt a uniform size and shape,
and the plasmon band narrows.
With respect to the changes in the optical properties of these
membranes, we have recently reported on the optical properties
of Au nanoparticles with aspect ratios less than unity (i.e., oblate
or pancake-like particles).6 We found that as the aspect ratio
went from below unity (pancake-like) to unity (sphere-like),
Finally, the Au nanoparticles in Figure 1E,F were obtained
from membranes that were plated with a much lower amount
of Au, 0.08 C. Even more evident than with the 0.20 C
particles, the unheated form of these 0.08 C particles is irregu-
larly shaped and is apparently quite thin, as indicated by the
electron transparency of the particles, Figure 1E. After heating,
these particles appear to have a uniform, mostly spherical shape
with a diameter of ca. 35 nm, Figure 1F. With less Au electro-
deposited within the pores, the final annealed nanoparticles are
definitely smaller than the previously discussed particles.
This change in shape of the 0.08 C Au nanoparticles upon
heating is reflected in the optical absorption spectra, Figure 5.
There is now a dramatic blue shift in the position of the plasmon
band with increasing temperature. This is clearly illustrated in
the plot of λmax versus membrane annealing temperature, Figure
3 (curve C). Once again, the plasmon resonance band of the
0.08 C particles becomes significantly more narrow after
heating, Figure 5. As before, this narrowing reflects the
uniformity in nanoparticle size and shape after thermal treatment,
as indicated in the TEM images.
there was a blue shift in the λmax
. These experimental
observations were found to be in agreement with the predictions
made via the DMG effective medium theory.6 These results
provide an explanation for the spectral changes observed in this
report for the 0.20 and 0.08 C Au nanoparticles. As shown in
the TEM images, the as-deposited particles are thin skins and
flakes (pancake-like). Our prior studies6 indicate that such
particles with aspect ratios from 0.7 to 0.3 would have a λmax
between 560 and 670 nm (i.e., the membrane would be blue in
appearance). The observed absorption maxima for these as-
deposited 0.20 and 0.08 C nanoparticles fall within this range,
Figure 3. After heating, all these irregularly shaped, flat
particles become significantly more spherical, resulting in the
aspect ratio of the nanoparticles increasing to ca. unity. Our
previous studies2,6 show that these annealed nanoparticles should
have a λmax of ca. 540 nm (i.e, the membrane would appear
red), which is observed, Figure 3.
As previously mentioned, studies of metal island films have
shown diffusion and coalescence of metal nanoparticles resulting
in changes in nanoparticle shape, size, and optical properties at
low temperatures ranging from 400 °C down to room
temperature.11-14 In addition, it has been well established that
the activation energy for diffusion of Au surface atoms (versus
bulk Au) is sufficiently low that these Au atoms have high
mobilities at temperatures well below the bulk melting point.18-21
Such surface diffusion processes are most likely responsible for
the shape changes of the low aspect ratio Au nanoparticles
investigated here. It is of interest to note, however, that the
aspect ratio ) 3 nanoparticles did not change shape upon
heating. This is undoubtedly due to the lower surface area of
these particles relative to the flake-like particles obtained from
the 0.20 and 0.08 C depositions. Furthermore, since these aspect
ratio ) 3 nanoparticles fill the entire diameter of the pore, there
is no free space within the pore for rearrangement of the particle
shape.
Conclusions
We have found that low aspect ratio Au nanoparticles (0.08
and 0.2 C used in the electrodeposition) start out as irregularly
shaped skins or flakes of Au within the pores, but after heat
treatment at surprisingly low temperatures, they anneal to
become dense, roughly spherical particles. The visual color of
these membranes started out blue but turned pink after this
thermal treatment. In contrast, the 0.4 C nanoparticles (aspect
ratio ) 3) exhibited no change in optical absorption character-
istics, gold particle shape, or electron opacity after exposure to
the prescribed heating treatment. This membrane was initially
red and did not change color with heating.
Acknowledgment. This work was supported by the Office
of Naval Research. We also acknowledge the CSU Electron
Microscopy Center for access to equipment.

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J. Phys. Chem. B, Vol. 101, No. 39, 1997 7731
(10) Blondel, A.; Meir, J. P.; Doudin, B.; Ansermet, J.-Ph. Appl. Phys.
Lett. 1994, 65, 3019.
(11) Schimmel, T.; Bingler, H.-G.; Franzke, D.; Wokaum, A. AdV. Mater.
1994, 6, 303.
(12) Semin, D. J.; Rowlen, K. L. Anal. Chem. 1994, 66, 4324.
(13) Van Duyne, R. P. Hulteen, J. C.; Treichel, D. A. J. Chem. Phys.
1993, 99, 2101.
References and Notes
(1) Martin, C. R. Science 1994, 266, 1961.
(2) Hornyak, G. L.; Patrissi, C. J.; Martin, C. R. J. Phys. Chem. 1997,
101, 1548.
(3) Foss, C. A., Jr.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J.
Phys. Chem. 1994, 98, 2963.
(4) Foss, C. A., Jr., Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J.
Phys. Chem. 1992, 96, 7497.
(5) Foss, C. A., Jr.; Tierney, M. J.; Martin, C. R. J. Phys. Chem. 1992,
96, 9001.
(6) Hornyak, G. L.; Martin, C. R. Thin Solid Films, in press.
(7) Moskovits, M. J.; Routkevitch, D.; Bigioni, T. J. Phys. Chem. 1996,
100, 14037.
(14) Schlegel, V. L.; Cotton, T. M. Anal. Chem. 1991, 63, 241.
(15) Buffat, Ph.; Borel, J.-P. Phys. ReV. A 1976, 13, 2287.
(16) Goldstein, A. N.; Echer, C. M.; Alivisatos, A. P. Science 1992,
256, 1425.
(17) Briant, C. L.; Burton, J. J. J. Chem. Phys. 1975, 63, 2045.
(18) Ercolessi, F.; Andreoni, W.; Tosatti, E. Phys. ReV. Lett. 1991, 66,
911.
(8) Huber, C. A.; Huber, T. E.; Sadoqi, M.; Lubin, J. A.; Manelis, S.;
Prater, C. B. Science 1994, 263, 800.
(9) Masuda, H.; Kazuyuki, N.; Noboyushi, B. Thin Solid Films 1993,
223, 1.
(19) Yu, X.; Duxbury, P. M. Phys. ReV. B 1995, 52, 2102.
(20) Gobel, H.; von Blanckenhagen, P. Surf. Sci. 1995, 331/333, 885.
(21) Bardotti, L.; Jensen, P.; Cadete Santos Aires, F. Surf. Sci. 1996,
367, 276.