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On the Chemistry of Gold in Silicate Glasses Studies on a Nonthermally Activated Growth of Gold Nanoparticles.

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Angewandte
Chemie
Glasses
DOI: 10.1002/anie.200502174
On the Chemistry of Gold in Silicate Glasses:
Studies on a Nonthermally Activated Growth of
Gold Nanoparticles**
Maik Eichelbaum, Klaus Rademann,* Ralf Mller,
Martin Radtke, Heinrich Riesemeier, and Wolf Grner
The history of gold-ruby glasses goes back to the Roman
Empire. However, the first known written report about the
manufacturing of these glasses was published in 1689 by the
German pharmacist and alchemist Johannes Kunckel.[1]
Colorless glasses containing the “magic” noble metal gold
become characteristically ruby-colored after annealing
because of the formation of gold colloids. The absorption of
light is caused by an excitation of a collective oscillation of
gold valence electrons. The quantum of this plasma oscillation
is called the surface plasmon.[2] The surface plasmon resonance frequency mainly depends on the size, shape, topology,
and dielectric environment of metal clusters.[3] By changing
these parameters it should be possible to create materials that
can be used in nanophotonic devices. A large nonlinear
optical response with a fast response time is needed for
applications in the field of optical computing. Plasmonic
materials are exceptionally promising candidates for ultrafast
optical switches and modulators because of their large thirdorder nonlinear susceptibility c(3). A second advantage is a
response time in the range of picoseconds, which would
therefore enable processors with terahertz frequencies.[4]
Another challenging problem is the chemical and
mechanical stability of these materials. Gold clusters in glass
matrices exhibit the extended stability required for the
fabrication of nanodimensional optoelectronic circuits and
optical memory with ultrahigh recording speed and storage
density. With synchrotron radiation it is already possible to
create very small devices, circuits, and even miniaturized
lasers by UV and X-ray lithography. We have now succeeded
in the spatial synthesis of gold nanoparticles of defined size
and with a sharp size distribution by activation of gold silicate
glasses with synchrotron radiation and subsequent heating of
the samples. Understanding the chemical and physical
[*] Dipl.-Chem. M. Eichelbaum, Prof. Dr. K. Rademann
Institut f5r Chemie
Humboldt-Universit8t zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
Fax: (+ 49) 30-2093-5559
E-mail: klaus.rademann@chemie.hu-berlin.de
Dr. R. M5ller, Dr. M. Radtke, Dr. H. Riesemeier, Prof. Dr. W. GCrner
Bundesanstalt f5r Materialforschung und -pr5fung (BAM)
Unter den Eichen 87, 12205 Berlin (Germany)
[**] The authors thank Mr. Schadrack and Mrs. Koslowski (BAM) for
their help in preparing glass samples and Prof. Dr. SchlCgl, Dr. Su,
and Mr. Klein-Hoffmann (Fritz-Haber-Institut der Max-PlanckGesellschaft) for TEM analysis. This work was supported financially
by the Fonds der Chemischen Industrie.
Angew. Chem. Int. Ed. 2005, 44, 7905 –7909
processes of cluster formation is fundamental for the directed
modification of the size, shape, and topology of gold clusters
and hence for the creation of structures with a specific
functionality. Therefore we have studied the activation caused
by the reduction of gold ions to atoms as well as the
nucleation and the growth of the gold nanoparticles in gold
silicate glasses by UV/Vis spectroscopy, X-ray absorption
near-edge spectroscopy (XANES), transmission electron
microscopy (TEM), and high-resolution transmission electron
microscopy (HRTEM).
Soda–lime–silicate glasses were doped with 0.02 mol %
AuCl3 by melting the component materials at 1450 8C.
Selected samples were irradiated with a photon flux density
of 1012 photons mm2 s1 on a defined area with hard X-ray
radiation (32 keV) for 300 s in the Berlin electron storage ring
(BESSY II).[5] During this irradiation the activated area
turned brownish (Figure 1 a). Maxima at 315, 440, and
Figure 1. Light-microscopy images of a gold silicate glass after activation with 32-keV synchrotron radiation a) before thermal annealing,
b) after annealing at 550 8C for 30 min. The pattern consists of spots
with a size of 0.5 + 1.0 mm2. The distance between the lines of the blue
grid is 5 mm.
620 nm can be identified in the extinction spectrum of the
activated sample (Figure 2, curve b). These peaks can be
assigned to defect centers in the SiO2 framework.
In general, the absorption at 315 nm in soda–lime–silicate
glasses is caused by holes trapped in oxygen vacancies that
neighbor alkali-metal ions (E’ centers).[6] This extinction is
obviously much stronger in gold-doped samples than in
undoped ones (Figure 2, curve a). Antonietti et al. recently
reported TD-DFT (time-dependent density functional
theory) cluster model calculations of optical transitions in
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7905
Communications
Figure 2. Extinction spectra of silicate glasses activated with 32-keV
synchrotron radiation. a) Undoped glass sample; b) AuIII-doped
(0.02 mol %) glass sample.
Figure 3. Extinction spectra of gold silicate glasses after activation with
synchrotron radiation and subsequent annealing for a) 45 min at
550 8C, b) 45 min at 590 8C, and c) 15 min at 630 8C. d) Spectrum after
only annealing for 240 min at 590 8C.
gold atoms bound to silicate defect centers.[7] The highest
calculated oscillator strength of possible gold–defect bonds is
predicted at 4.02 eV (308 nm) for gold atoms bound to E’
centers. Therefore the peak at 315 nm in the experimental
spectrum could be assigned to gold atoms that were generated
by activating the glass sample with X-rays.
Peaks at 440 and 620 nm can be attributed to the NBO
(nonbridging oxygen) centers HC1 and HC2 (holes trapped in
SiO4 tetrahedrons with three or two nonbridging oxygen
atoms, respectively).[6] These extinction maxima do not differ
very much in doped and undoped samples (Figure 2). The
irradiated glasses were subsequently annealed in a muffle
furnace. After 10 min of thermal treatment at 450 8C the
glasses became colorless again. Apparently the color centers
consisting of electrons and holes recombine and their
absorption vanishes. When the glass samples were annealed
at a higher temperature of 550 8C, the color of the activated
array changed to red within 30 min (Figure 1 b).
The red color corresponds to a peak at 540 nm in the
extinction spectrum. This extinction can be attributed to the
surface plasmon resonance (SPR) of gold nanoparticles.[2, 8]
When the activated gold silicate glass was annealed for 45 min
at 550 8C instead of for 30 min, the extinction peak shifted to
549 nm (Figure 3, curve a). A remarkable red shift of the SPR
with increasing duration of thermal treatment was also
observed when samples were annealed at 590 8C (SPR peak
after 10 min at 547 nm and after 45 min at 556 nm) and 630 8C
(SPR peak after 5 min at 547 nm and after 15 min at 554 nm)
(Figure 3, curves b and c, respectively).
The optical properties of spherical metal clusters can be
described with the Mie theory using the complex dielectric
function esphere = esphere
+ i esphere
of the metal and the refraction
1
2
index of the matrix material nm.[2, 9] For clusters with radii
between 1 and 10 nm the extinction Ext can be formulated
within a quasi-static approximation under the exclusive
consideration of dipolaric excitations [Eq. (1)].[2] Fitting to
Ext ¼ ðlg eÞ C d
7906
18pn3m
esphere
2
sphere
l ðe1
þ 2 n2mÞ2 þ ðesphere
Þ2
2
www.angewandte.org
ð1Þ
the height of the extinction maximum in the experimental
spectrum is achieved by varying the filling factor C. The
refraction index of the glass matrix nm is 1.52 and the thickness
of the sample d is 1.00 mm.
The dielectric function of a metallic sphere esphere can be
calculated from the dielectric function of the bulk material
ebulk, if one considers its dependence on the sphere radius R
caused by the effect of the limited mean free path of electrons
‘1 as described by Equations (2) and (3).[2] Values for ebulk
and
1
bulk
2
esphere
ðw,RÞ
¼
e
ðwÞ
þ
w
1
1
p
esphere
ðw,RÞ ¼ ebulk
2
2 ðwÞ þ
w2p
w
1
1
w2 þ G21 w2 þ GðRÞ2
GðRÞ
G1
w2 þ GðRÞ2 w2 þ G21
ð2Þ
ð3Þ
ebulk
were taken from ref. [10]. The Drude plasma frequency
2
wp of gold is 1.37 G 1016 s1 and the halfwidth G(R) is given by
Equation (4) with the Fermi velocity being uFermi = 1.40 G
GðRÞ ¼ G1 þ
uFermi uFermi uFermi
¼
þ
R
‘1
R
ð4Þ
106 m s1 and the mean free path of electrons being ‘1 =
42 nm.
Transmission electron microscopy (TEM) was used to
determine the mean cluster radius R of the gold nanoparticles
in two samples to be 3.2 0.9 nm (sample had been activated
and annealed for 45 min at 550 8C) and 6.2 1.5 nm (sample
had been activated and annealed for 15 min at 630 8C). The
shape of the curve and the halfwidth of SPR in the
experimental UV/Vis spectra are very well described by
calculating the extinction spectra for gold spheres with radii
of 3.2 and 6.2 nm, respectively (Figure 4). The red shift of SPR
with increasing cluster size cannot be explained with Mie
theory for radii between 1 and 10 nm. Only for remarkably
larger nanoparticles should red shifts and broadening of SPR
caused by retardation effects be expected.[2, 8a]
Equation (1) is valid only for spherical clusters. For the
investigation of the shape of the gold particles, HRTEM
images of the gold glass samples were made (Figure 5 a,b).
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 7905 –7909
Angewandte
Chemie
Figure 4. Experimental (solid lines) and calculated extinction spectra
(dashed lines) of gold clusters in silicate glasses. a) Spectrum measured for a gold silicate glass that was activated with synchrotron
radiation activated and subsequently annealed for 45 min at 550 8C;
b) spectrum calculated for gold spheres with R = 3.2 nm; c) spectrum
measured for a gold silicate glass that was activated with synchrotron
radiation and subsequently annealed for 15 min at 630 8C; d) spectrum
calculated for gold spheres with R = 6.2 nm.
Figure 5. HRTEM images of characteristic gold clusters in gold silicate
glasses. Samples were activated with synchrotron radiation and
subsequently annealed for a) 45 min at 550 8C and b) for 15 min at
630 8. c) Sample was only annealed (240 min at 590 8C).
Thus it appears that the shape of nanoparticles is approximately spherical. When the glasses were not activated by
synchrotron radiation, a violet stain could be observed only
after they had been annealed for 60 min at 590 8C. The
maximum of the observed SPR in the extinction spectrum was
shifted from 574 nm (after annealing for 60 min) to 580 nm
(after annealing for 240 min) (Figure 3, curve d). TEM images
give a mean cluster radius of 27 3 nm, and HRTEM images
indicate a distinct deviation of particle shape from an ideal
spherical symmetry (Figure 5 c).
In 1951 Weyl tried to explain the violet stain of gold-ruby
glasses after defined thermal treatment by a nonspherical
shape of gold colloids. Thus the typical ruby color would be
observed only for spherical particles.[1b] Experiments conducted by Doremus et al. confirm that the effect of particle
size on the SPR frequency is marginal.[8c] They examined the
growth of gold nanoparticles with radii between 3 and 6 nm in
borosilicate glass and found a SPR wavelength of 525 nm.
This value, which was independent of cluster size, is also in
agreement with our calculations.
As a result of long annealing times of up to 150 h and the
low diffusion constant in borosilicate glasses one can assume
that the spherical nanoparticles are formed by a very slow
Angew. Chem. Int. Ed. 2005, 44, 7905 –7909
growth process. Therefore red shifts of SPR observed in our
samples could be explained by an increasing deviation of gold
clusters from spherical shape. Studies on noble-metal ellipsoids in glass matrices would also confirm this thesis because
of the observation of a red shift of SPR with increasing aspect
ratio (and consequently an increasing deviation of spherical
shape).[3] This property of our gold silicate glasses is very
interesting because a shift of SPR can be achieved in
unexpectedly small clusters within a very short period of
time only by changing the annealing temperature and thus the
optical properties can be modified very easily.
In activated glasses cluster growth occurs at a lower
temperature and within a shorter period of time than in
nonactivated samples. Light microscopy and TEM images
show that the distribution of gold nanoparticles is more
homogenous and cluster density is much higher than in
exclusively annealed gold glasses. An explanation therefore
could be that the activation with synchrotron radiation leads
through a reductive process to a very high density of gold
atoms, which can diffuse much easier through the glass matrix
than bound, oxidized gold species. Thereby an effective
cluster growth is facilitated when the samples are annealed at
a temperature near the glass transformation temperature.
The chemistry of gold in glass is so far a mystery. The only
certainty is that the glass compounds are doped with gold(iii)
before the melting process starts. Assumptions of the
oxidation state in the resulting glass are contradictory.
However, recent experiments come to the conclusion that
the noble metal is mainly in cationic form before either
annealing or activation.[11] Furthermore, one must assume
that the gold cations undergo reduction to produce the
coloring gold colloids.[2, 8b–d]
We used Au LIII-edge XANES spectroscopy to study the
effect of synchrotron radiation on the oxidation state.
XANES spectroscopy is a reliable method for determining
oxidation states and is also an established method for the
qualitative analysis of gold nanoparticles.[12] In our studies a
gold film with a thickness of 40 nm on silica glass, gold(i)
cyanide, and gold(iii) oxide were used as reference materials.
Cationic gold standards show distinct white lines (caused by
transitions of 2p electrons into vacant 5d states) at 11 922 eV
(AuIII) and 11 926 eV (AuI) and can be clearly distinguished
from Au0 samples, whose spectra do not show any white lines.
A XANES spectrum was recorded for the activated area of a
gold silicate glass. If the oxidation values o, + i, and/or
+ iii of gold exist in the sample, its spectrum should be
composed additively of the reference spectra. Therefore the
spectra of the gold film and gold(iii) oxide were added in
different proportions (Figure 6), and the LIII edges of the gold
film and gold silicate glass were found to match up almost
exactly. The white line at 11 922 eV in the glass spectrum also
indicates a minor fraction of gold(iii).
XANES spectra of glass samples were furthermore
compared with spectra obtained by combination of oxidation
values o and + i (Figure 7). It is remarkable that the white
line of AuCN at 11 926 eV differs clearly from the white line
of the activated glass sample. Indeed one cannot exclude a
significant influence of ligands in the vicinity of AuI on the
XANES spectrum. This effect should be clarified by inves-
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7907
Communications
be explained by the increasing deviation from spherical
symmetry with the increase in gold cluster size. Au LIII-edge
XANES spectra of activated gold silicate glasses can be
described best by a combination of the spectra of a gold film
and an Au2O3 reference. This indicates that cationic gold
species were found at least as minor compounds in the glass
samples investigated. Because of the short wavelength used it
should be possible to create nanostructures in glasses with Xray lithography. This could provide an opportunity for
developing nanophotonic devices in glass matrices for optoelectronic circuits, whose surface plasmon resonance could be
tuned over a wide range of wavelengths by changing the size
and shape of the gold nanoparticles.[13]
Figure 6. Normalized Au LIII-edge XANES spectra of different gold
samples (solid lines) and spectra composed additively of reference
spectra (dashed lines) obtained from a) gold silicate glass activated
with synchrotron radiation; b) Au film; c) 0.2 Au2O3 + 0.8 Au film;
d) 0.6 Au2O3 + 0.4 Au film; e) Au2O3.
Figure 7. Normalized Au LIII-edge XANES spectra of different gold
samples (solid lines) and spectra composed additively of reference
spectra (dashed lines) obtained from a) gold silicate glass activated
with synchrotron radiation; b) Au film; c) 0.2 AuCN + 0.8 Au film;
d) 0.6 AuCN + 0.4 Au film; e) AuCN.
tigating further reference materials especially compounds
with linearly coordinated oxygen ligands (e.g. CsAuO) as they
would be expected in glass.
Although XANES measurements were conducted with
photon flux densities (1010 photons mm2 s1) well below
activating densities (1012 photons mm2 s1), occurrence of
color centers could be observed in nonactivated samples, too,
as a result of long measurement times. Hence it is not possible
to get information about the oxidation state before synchrotron irradiation. In future work fast XANES spectroscopy
must be used, because this method would provide a determination of oxidation values in a very short period.
In conclusion, activation of gold silicate glasses with
synchrotron radiation allows spatial reduction of cationic
gold. By subsequent annealing, gold clusters with a sharp size
distribution can be synthesized within a short period of time.
With increasing cluster size a red shift of surface plasmon
resonance is observed in the extinction spectra. This effect can
7908
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Experimental Section
Glasses composed of 70 SiO2·20 Na2O·10 CaO (values in mol %) were
doped with 0.02 mol % AuCl3·2 H2O. Reagent-grade SiO2, Na2CO3,
CaCO3, and AuCl3·2 H2O were used as starting materials. Approximately 35-g batches were mixed and melted at 1450 8C for 90 min in
an electric furnace. The glass melt was then slowly cooled down to
room temperature and cut, and samples were polished to sizes of 10 G
10 G 1.00 mm3.
Selected samples were irradiated (“activated”) on an area of
approximately 2.0 G 1.5 mm2 with synchrotron radiation from BESSY II with 32 keV for 300 s. In the subsequent thermal treatment the
glass samples were placed into a platinum crucible and annealed in a
muffle furnace. XANES spectroscopy was also recorded at BESSY II.
Au LIII-edge XANES spectra were acquired in fluorescence mode
(detection at the Au La line at 9707 eV) between 11 900 eV and
11 960 eV with an increment of 1 eV. Peak integration was determined with the program QXAS 3.5. The background spectra was
corrected by subtracting the absorption value at 11 900 eV and
normalizing the value at 11 960 eV to unity. Extinction spectra were
acquired with a StellarNet EPP2000C-50 spectrometer and a
Mikropack DH-2000 deuterium/halogen light source. For the activated samples a nonactivated area of the same glass was defined as
the reference, and its spectrum was subtracted from the extinction
spectrum of the activated area. For exclusively thermally treated
samples an untreated glass with the same composition was defined as
the reference, and its spectrum was subtracted from the extinction
spectrum.
TEM measurements were made at Fritz-Haber-Institut der MaxPlanck-Gesellschaft in Berlin with the microscope Philips CM 200
LaB6 operating at an acceleration voltage of 200 kV.
Received: June 21, 2005
Revised: August 2, 2005
Published online: October 31, 2005
.
Keywords: glasses · gold · nanostructures · surface plasmon
resonance · X-ray absorption spectroscopy
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 7905 –7909
Angewandte
Chemie
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