Article
pubs.acs.org/cm
The Kirkendall Effect in Binary Alloys: Trapping Gold in Copper Oxide
Nanoshells
Damien Thiry,† Leopoldo Molina-Luna,∥ Eric Gautron,† Nicolas Stephant,† Adrien Chauvin,† Ke Du,‡
Junjun Ding,‡ Chang-Hwan Choi,‡ Pierre-Yves Tessier,† and Abdel-Aziz El Mel*,†
†
Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière B.P. 32229, 44322 Nantes cedex 3, France
Department of Material- and Geosciences, Technische Universität Darmstadt, Alarich-Weiss-Strasse 2, 64287 Darmstadt, Hessen,
Germany
‡
Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States
∥
S Supporting Information
*
ABSTRACT: In this work, we report on the Kirkendallinduced hollowing process occurring upon thermal oxidation
of gold−copper (Au−Cu) alloy nanowires and nanodots.
Contrary to elemental metals, the oxidation reaction results in
the formation of gold nanostructures trapped inside hollow
copper oxide nanoshells. We particularly focus on the
thermally activated reshaping mechanism of the gold phase
forming the core. Using scanning transmission electron
microscopy coupled to energy dispersive X-ray spectroscopy
mapping, we show that such a reshaping is a consequence to
the reorganization of gold at the atomic level. The gold
nanostructures forming the core were found to be strongly
dependent on the chemical composition of the alloy and the
oxidation temperature. By selecting the appropriate annealing conditions (i.e., duration, temperature), one can easily synthesize
various heteronanostructures: wire-in-tube, yolk−shell, oxide nanotubes embedding or decorated by Au nanospheres. The
advanced understanding of the Kirkendall effect in binary alloy nanostructures that we have achieved in this work will open a new
door for the fabrication and the design of novel multifunctional heteronanostructures for potential applications in different
research fields including nano-optics/photonics, biomedicine, and catalysis.
Although the fundamental mechanisms of the Kirkendallinduced hollowing process occurring during thermal oxidation
of elemental metal nanostructures are very well documented in
the literature,3−15 the oxidation of binary alloys has received
little attention.46,47 In a recent study, Lewis et al. investigated
the oxidation of Ag−Au alloy nanospheres and have shown
how such an approach enables synthesizing Au/Ag2O nanoparticles with a core/shell or a yolk/shell structure.47 In terms
of application, such metal/metal oxide heteronanostructures,
consisting of metal oxide shells trapping noble metal
nanostructures, are of particular interest since they exhibit
enhanced chemical and physical properties compared to pure
hollow oxide nanostructures.48−54 They find applications in
many fields including catalysis,55−60 optics (e.g., wavelengthcontrolled optical nanoswitches,61 plasmonic waveguides62),
and biomedicine.63,64
It should be noted that, until today, the oxidation of binary
alloys involving the Kirkendall effect was limited on zero-
Discovered in 1942 by Kirkendall, the Kirkendall effect
describes the motion of the boundary between two metals
occurring during thermal annealing as a consequence to the
unbalanced ion diffusion rates of the two metals.1 Although the
Kirkendall effect was always considered as a nuisance in
soldering since it deteriorates the adhesion strength between
metals,2 when going down to the nanoscale it becomes
extremely useful for the synthesis of hollow nanostructures.
This has been demonstrated for the first time by Yin et al. in
2004 who investigated the transformation of nanoparticles from
solid to hollow upon thermal annealing under reactive
atmosphere such as O2.3 In the context of metal oxidation,
the Kirkendall effect occurs due to the net directional flow of
matter resulting from the unbalanced diffusion of the metal and
oxygen ions through the oxide skin formed on the metal surface
during the early stage of oxidation.3−15 As a consequence to
such unbalanced diffusion, a flow of vacancies will be generated
at the metal/metal oxide interface within the metal core to
balance the net flow of matter through the oxide shell. The
diffusion and coalescence of the injected vacancies result in the
formation of voids which expand in a further stage giving rise to
hollow nanostructures.3,8,11,13,14,16−45
© 2015 American Chemical Society
Received: June 22, 2015
Revised: August 25, 2015
Published: August 28, 2015
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dimensional (0D) nanostructures.46,47 Since the generation rate
and the confinement of vacancies in 0D and in one-dimensional
(1D) nanostructures differ,20 from a fundamental point of view,
it is of great importance to investigate the oxidation process of
binary alloy nanowires and evaluate how the 1D geometry may
impact the Kirkendall-induced hollowing process. In addition,
controlling the oxidation process of binary alloy nanowires may
allow designing novel 1D heteronanostructures with enhanced
properties that can be tailored according to the synthesis
conditions. In this context, here we report for the first time on
the Kirkendall effect in 1D binary alloys involving the oxidation
of gold−copper (Au−Cu) nanowires. The Au was selected as a
noble metal to limit the oxidation only to Cu, whereas Cu was
selected as a model system since the oxidation mechanisms of
pure Cu nanostructures are well understood.12,14,25,65 In
particular, we investigate the influence of the Au/Cu ratio
and the oxidation temperature on the Kirkendall-induced
hollowing process. The influence of the dimension/shape of the
nanostructures is also addressed by comparing the oxidation of
Au−Cu nanowires to the one of nanodots synthesized in
similar conditions.
■
RESULTS AND DISCUSSION
Oxidation of Au−Cu Nanowires at Low Temperature.
The Au−Cu alloy nanowires used in this study were grown by
the cosputtering process of Au and Cu targets in a cofocal
geometry on nanopatterned silicon substrates used as a physical
template. We have recently demonstrated the versatility of such
an approach to grow metal nanowire arrays with a length up to
several centimeters.12 Aiming to investigate the influence of the
initial chemical composition of the Au−Cu alloy on the
Kirkendall-induced hollowing process, the gold content within
the as-grown nanowires was varied (4, 8, and 16 at. %) while
fixing the nanowires diameter to about 140 nm. The choice of
these three Au contents was based on a preliminary study in
which we screened different conditions with Au contents up to
32 at. %. Then, the nanowires are transformed to oxide-based
hollow nanostructures by thermal oxidation at 300 °C in
ambient air. To monitor the various stages of the oxidation
process, the annealing time was varied between 5 and 150 min.
The shortest annealing time, 5 min, was selected based on our
previous work in which we had shown that such an annealing
time should be long enough to fully oxidize pure copper
nanowires and transform them into oxide nanotubes.12
The morphological evolution of the Au−Cu nanowires as a
function of the annealing time was monitored using ex situ
transmission electron microscopy (TEM) (Figure 1a-c). In all
cases, after 5 min of oxidation, a drastic change in the
morphology of the tube can be noticed, and an oxide shell
ascribed to CuO (see Figure S1 for the selected area electron
diffraction pattern) can be clearly identified; this result is in
agreement with our previous work reporting on the study of the
oxidation of pure copper nanowires.12 Contrary to pure copper
nanowires where fully hollow oxide nanotubes were found to
form after 5 min of oxidation,12 in the case of Au−Cu alloy
nanowires, a metal core remains present inside the tubes. This
core undergoes a gradual reorganization as further increasing
the annealing duration. By plotting the dimensions (i.e., the
total diameter of the tube and the oxide shell thickness) of the
tubes as a function of the annealing time, one can conclude
that, whatever is the Au content within the Au−Cu nanowires,
a noticeable increase is observed in both the oxide shell
thickness and the total diameter of the tube during the first 5
Figure 1. (a-c) Series of TEM micrographs showing the different
chronological events occurring during the oxidation process for an
annealing temperature of 300 °C of Au−Cu nanowires with various Au
contents: (a) 4 at. %, (b) 8 at. %, and (c) 16 at. %. Scale bar: 100 nm.
The numbers are referred to the annealing time (in min).
min before reaching a plateau for longer annealing time (Figure
S2). This indicates that the oxidation reaction reaches a steady
state after 5 min as previously reported for pure Cu
nanowires.12 Therefore, one can conclude that after 5 min of
oxidation, the Kirkendall-induced hollowing process ends up,
and the observed morphological evolution is only limited to the
reorganization of the interior metal core. Such reorganization
throughout the annealing time is found to be highly influenced
by the Au content within the as-grown Au−Cu nanowires.
For nanowires with 4 at. % of Au, after 5 min of oxidation,
the metal core exhibits a “spider’s web”-like structure consisting
of large discontinuous and periodic voids aligned along the
metal (core)/oxide(shell) interface and exhibiting a symmetric
distribution with respect to the tube axis (Figure 1a). At a
further stage of the conversion process (15 min), the thin paths
connecting the metal core to the oxide shell disappear resulting
in the formation of discontinuous metal nanochains located at
the center of the tube with the occurrence of metal nanograins
randomly distributed inside the nanotube. Increasing the
annealing time to 30 min results in the formation of isolated
nanoparticles with various shapes. When reaching 60 min,
isolated nanoparticles of a nearly spherical shape form inside
the tube. For annealing durations exceeding 60 min, no
significant variation in terms of size and shape of the metal
nanoparticles was noticed. In the case of nanowires with 8 at. %
of Au, 5 min of oxidation results in the formation of small voids
distributed along the metal(core)/oxide(shell) interface as well
as within the metal core (Figure 1b). It should be noted that in
comparison to the previous sample, the global porosity is
reduced since the formed voids are much smaller. Between 15
and 60 min of annealing, an increase in the voids size
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Figure 2. HAADF-STEM micrograph and the corresponding EDS mapping of Au−Cu nanotubes with 4 at. % of Au after thermal oxidation for an
annealing temperature of 300 °C for (a) 5 min, (b) 15 min, and (c) 150 min. (d) HAADF-STEM micrograph and the corresponding EDS mapping
of Au−Cu nanowire with 16 at. % of Au after thermal oxidation for 150 min. Scale bar: 200 nm.
accompanied by a coalescence of the metal core can be noticed.
At this stage, the voids become mainly located at the metal/
metal oxide interface. After 150 min, the metal core transforms
into a discontinuous nanowire (around 45 nm in diameter)
located at the center and oriented along the tube axis. In this
case, the metal filaments connecting the metal core to the oxide
shell almost disappear. For the nanowire with 16 at. % of Au
(Figure 1c), after 5 min of oxidation, the interior morphology is
found to be similar to the one identified for the previous sample
(i.e., Au content of 8 at. %), namely the formation of small
voids distributed along the metal(core)/oxide(shell) interface
as well as at the interior of the metal core. The density of the
voids is reduced in comparison with the nanowires containing 8
at. % of Au. An increase in the annealing time from 15 to 150
min leads to the progressive increase in voids density in a
continuous manner accompanied by a reorganization of the
metal core. In such a case, the reorganization of the metal core
leads to the formation of heterogeneous metal/metal oxide
nanostructures exhibiting a wire-in-tube structure with an inner
wire diameter of about 80 nm.
To probe the Au and Cu distribution within the synthesized
heteronanostructures, high angle annular dark field (HAADF)
imaging coupled to spatially resolved energy-dispersive X-ray
spectroscopy (EDS) was carried out using scanning transmission electron microscopy (STEM). At first, we focused
mainly on the sample with 4 at. % of Au since the metal cores
formed inside the nanotubes have shown the most notable
morphological evolution during the oxidation process (Figure
2a-c). The STEM-EDS mapping demonstrates that for 5 min of
oxidation, the metal core, exhibiting the “spider’s web”-like
structure, contains both Au and Cu, whereas the shell is
constituted of Cu and oxygen (Figure 2a). For the sample
oxidized for 15 min, although copper is also identified within
the metal core formed at the center of the tube, the extremities
of the metal filaments bridging the metal core to the oxide shell
are found to be gold-rich (Figure 2b). For 150 min of
annealing, the metal nanograins formed on the inner surface of
the oxide shell are found to be richer in Au compared to the
ones at the center of the nanotube (Figure 2c). In order to
verify if the initial copper content impacts the chemical
composition of the metal core, the same experiment has been
performed on the wire-in-tube structure obtained by oxidizing
for 150 min a set of Au−Cu nanowires containing 16 at. % of
Au (Figure 2d). Similarly to the previous sample (i.e., Au
content of 4 at. %), the metal core is found to contain Au and
some copper residue. It should be noted that, from these
measurements, one cannot conclude about the amount of
copper present in the inner metallic particles; this is related to
the fact that it is not straightforward to accurately estimate the
Au/Cu ratio of the metal core using EDS-mapping as the
contribution of the Cu signal originating from the copper oxide
shell cannot be excluded. This error is amplified due to the
spatial variation in thickness originating from the tubular
geometry of the analyzed nano-objects. However, assuming that
the metal core inside the oxide nanotubes is entirely constituted
of gold, the atomic gold content can be calculated and then
compared to the real gold content in the as-grown Au−Cu
nanowires measured experimentally. For simplicity, in the case
of the samples with 8 and 16 at. %, we assume that the metal
core formed after 150 min of annealing represents a continuous
cylinder of pure Au located inside a CuO tube. This approach
has not been applied to the sample containing 4 at. % of Au
since the presence of metal nanoparticles with different shapes
randomly dispersed inside the nanotube complicates the
calculation and would lead to an erroneous conclusion. Based
on this methodology, the estimated atomic gold content (atC. %
Au) can be calculated as follows (Supporting Information,
section 3)
1
atc. % Au =
(
1+
ρCuO
ρAu
.
2
2
− d inn.
MAu (d tot.
)
.
2
MCuO
dAu
)
·100 (%)
(1)
where ρ (g/cm ) and M (g/mol) represent the density and the
molar mass of the considered material (Au or CuO),
3
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respectively. dAu (cm) represents the diameter of the gold wire,
while dtot. (cm) and dinn. (cm) correspond to the diameter of
the entire (or total) and inner tube, respectively. They can be
deduced from the TEM micrographs shown in Figure 1b-c.
Based on eq 1, for the nanowires containing an Au content of
16 at. % as measured by EDS on the nonoxidized sample, a
value of about 20% has been obtained. On the other hand, for
the nanowire with 8 at. % of Au (measured experimentally on
the nonoxidized sample), one can estimate a gold content of
about 7 at. %. Since the calculated values of gold contents agree
well with the experimental ones measured on the as-grown
Au−Cu nanowires, it is reasonable to conclude that the metal
core present in the oxide nanotubes is mainly composed of
gold. This result is in agreement with the work of Mattei et al.
where 3 to 7 at. % of copper residue was found after the
complete thermal oxidation of Au−Cu nanoparticles.66
Oxidation of Au−Cu Nanowires at High Temperature.
In order to evaluate the influence of the oxidation temperature
on the Kirkendall induced hollowing process of Au−Cu
nanowires, a similar methodology has been followed for an
oxidation temperature of 600 °C. The chronological evolution
of the nanowires upon annealing is investigated using ex situ
TEM analysis (Figure 3). Similarly to the samples annealed at
300 °C, a noticeable morphological evolution can be remarked
after 5 min of oxidation with the occurrence of hollow
nanostructures constituted of an oxide shell embedding Au
nanospheres. As for the previous case, the hollowing process is
complete after 5 min of oxidation as supported by the evolution
of the diameter of the nanotube as a function of the annealing
time (Figure S3). Nevertheless, in this case, the expansion in
size of the tube is less significant in comparison to the low
temperature regime at 300 °C. Oxidizing the sample for longer
than 5 min results in the reorganization of the metal core; such
reorganization is found to be dependent on the Au content in
the as-grown Au−Cu nanowires.
For an Au content of 4 at. %, after 5 min of oxidation,
dispersed metal rich-Au particles with a spherical shape form
inside the tubes (Figure 3a). The nanospheres are periodically
separated by a gap of ∼100 nm, and their diameter is within the
range 40−70 nm. Longer annealing times do not induce any
variation in size and shape of the particles. The only difference
lies in the presence of several gold nanoparticles inside the
oxide phase forming the shell. On the other hand, the
morphology of the oxide nanotubes is found to evolve as the
annealing process progresses in time. Contrary to the low
temperature regime, for a long annealing time at 600 °C, the
nanotube walls are found to deform, and “nanocracks” can be
seen in the oxide shell.
For Au content of 8 at. %, after 5 min of oxidation, elongated
and isolated particles (width about 60 nm and a length ranging
from 100 to 250 nm) form inside the nanotubes (Figure 3b).
The global porosity of the nanowires is found to be reduced in
comparison with the previous case since a more important
volume inside the shell is occupied by gold. For 15 min of
annealing, the elongated nanoparticles tend to break into
smaller nanospheres. Furthermore, it can be noticed that a
portion of the particles are embedded within the oxide phase
forming the shell. Longer annealing time does not lead to a
significant morphological variation. Regarding the oxide shell,
similarly to the nanotube containing 4 at. % of Au, the wall
thickness is found to be less uniform compared to the low
temperature regime.
Figure 3. (a-c) Series of TEM micrographs showing the different
chronological events occurring during the oxidation process for an
annealing temperature of 600 °C of Au−Cu nanowires with various Au
contents: (a) 4 at. %, (b) 8 at. %, and (c) 16 at. %. Scale bar: 200 nm.
The numbers are referred to the annealing time (in min). The
deterioration of the oxide shell can be noticed in the case of the
objects with 16 at. % of Au annealed for longer than 30 min.
For higher gold content (16 at. % of Au), similar to the
previous condition, after 5 min of oxidation, elongated and
separated Au nanostructures form inside the oxide shell (Figure
3c). The mean length of these elongated particles range from
300 to 600 nm which is longer than the structures observed in
the case of wires with 8 at. % of Au. For the annealing durations
not exceeding 30 min, no significant morphological change can
be noticed. When exceeding 60 min of annealing, the elongated
particles undergo a reshaping process leading to the formation
of smaller Au-rich nanoparticles with a nearly spherical shape;
such transformation is accompanied by the formation of
“nanocracks” in the oxide shell. Furthermore, from the TEM
images, it can be seen that the Au nanoparticles are not fully
embedded within the oxide shell. In this context, in
complement to the TEM analysis, scanning electron microscopy (SEM) experiments have been carried out on this sample
(Figure 4). For an annealing duration of 5 min, the SEM
micrograph reveals the presence of numerous nanocracks in the
oxide shell which indicate that the nanotubes are mechanically
unstable at such high temperature regime (Figure 4a). When
switching to the backscattering mode, the SEM micrographs are
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Figure 4. (a) SEM micrographs showing the cracks formed in oxide shells obtained by oxidation of Au−Cu nanowires with 16 at. % of Au at 600 °C
for 5 min and the associated back scattered electron SEM micrographs recorded with an acceleration voltage of (b) 5 kV and (c) 10 kV. (d-e)
Backscattered SEM micrographs recorded on Au−Cu nanowires with 16 at. % of Au oxidized at 600 °C for 150 min; the acceleration voltage used
for imaging was 5 kV. Scale bar: 100 nm.
condense forming a single hole and giving rise to a hollow
nano-object.3,9,18,20 Nevertheless, in our case, gold, inert toward
oxidation, remains trapped inside the oxide shell revealing that
the process is different in the case of binary alloys containing a
noble metal.67−69 The shape adopted by the gold nanostructures was found to be dependent on the atomic composition of
the alloy as well as the selected annealing regime (300 °C vs
600 °C).
For low Au content (i.e., 4 at. % of Au) considering the low
temperature regime, after the complete oxidation of the
nanotube, the gold phase adopts a “spider’s web”-like structure
(Figure 5A-(a), stage 1). It is most likely that the existence of
such a unique structure is a result of a possible “freezing” of the
formed voids by Au after the complete extraction of Cu out of
the core. The presence of these metallic filaments had likely
improved the extraction of copper outside the metal core
during the oxidation reaction through a surface diffusion
mechanism as the corresponding diffusion coefficient is several
orders of magnitude higher than its bulk counterpart.9,12,18,20 In
view of reducing the surface free energy of the system, in a
further stage, the gold-rich core is driven to transform from a
“spider’s web”-like structure into isolated gold nanoclusters.17,21
As Au may undergo a self-diffusion mechanism at temperatures
not exceeding 300 °C,70 it can be supposed that the observed
reorganization process originates from the random diffusion of
Au inside the nanotube (Figure 5A-(a), stage 2). The transport
of gold during the reorganization is likely to occur through a
surface self-diffusion mechanism since the activation energy is
quite low (62 kJ mol−1) in comparison to the one for bulk selfdiffusion (212 kJ mol−1).71 Since at 300 °C the Au atoms do
not exhibit enough energy to diffuse in/on the oxide shell,50 the
isolated nanochains transform to nanospheres by dewetting on
the inner surface of the oxide shell in order to minimize the
total surface energy and achieve a more stable configuration
(Figure 5A-(a), stage 3). For nanowires with a similar gold
content but annealed in the high temperature regime (600 °C),
spherical and isolated gold particles directly form during the
first 5 min (Figure 5A-(b), stage 1). This behavior could be
ascribed to an enhanced kinetic of the Au reorganization
found to be dependent on the acceleration voltage (Figure 4b
and c). This is related to the fact that in such a mode, the
contrast is directly related to the atomic number of the probed
element (heavy elements appear brighter on the image than the
lighter ones). At 5 kV, elongated bright domains (representing
the Au core) surrounded by dark areas (representing the oxide
shell) can be hardly seen (Figure 4b). Increasing the
acceleration voltage to 10 kV allows enhancing the contrast
between the bright (gold) and the dark (CuO) domains which
is an evidence of the presence of elongated Au nanoparticles
inside the nanotubes and not on their outer surface (Figure 4c);
this result reinforces the TEM experiments presented and
discussed previously and proves that gold has not left the
interior of the tube. Nevertheless, the backscattered SEM
images of nanowires annealed for 150 min unambiguously
show that gold undergoes an outward diffusion resulting in the
formation of Au nanospheres on the outer surface of the oxide
shell (Figure 4d and e).
Hollowing Process Mechanism. Based on our experimental data, a scenario can be proposed to explain the different
behaviors regarding the reshaping of the gold core during the
annealing process of Au−Cu alloy nanowires using a low (300
°C) and a high (600 °C) temperature regime (Figure 5). In all
cases, at the early stage of oxidation, a thin copper oxide shell
forms around the Au−Cu nanowire. In a further stage, at the
metal/oxide interface, the copper and oxygen ions undergo an
outward and inward diffusion across the formed oxide shell,
respectively. Since the outward (i.e., outside of the tube)
diffusion coefficient of copper ions across the oxide shell is
higher than the inward (i.e., toward the metal core) diffusion
coefficient of the oxygen ions,8,12 vacancies will be injected
within the metal core.14,25 As the oxidation process proceeds in
time, copper continues to be progressively extracted from the
Au−Cu alloy and then reacts with the oxygen ions resulting in
the expansion of the copper oxide shell. In a further stage, the
generated vacancies condense together and form multiple voids
which become larger as the reaction proceeds in time. In the
case of the conversion of elemental metals nanostructures, once
all the material is consumed by the shell, the formed voids
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Figure 5. Scenarios explaining the reorganization of the gold core with (A) low and (B) high gold content for the (a) low and (b) high temperature
regime. The orange and the yellow solid spheres represent the copper and the gold atoms, respectively. The columnar copper oxide shell is
represented in bright-gray.
For nanowires containing higher gold contents (i.e., 8 and 16
at. %), in the low temperature regime, a complete oxidation
reaction results in the formation of a gold core with a
nanoporous structure (Figure 5B-a, stage 1). In this case, the
formed voids are relatively small in comparison to the ones
formed in the case of nanowires with low gold content (i.e., 4
at. %). In contrast with the previous case (i.e., low Au content),
due to the presence of a high amount of Au, the progressive
surface diffusion of Au allows transforming the porous structure
into a dense single nanowire oriented along the oxide nanotube
axis in view of reducing the surface free energy of the system
(Figure 5B-(a), stages 2 and 3). It should be noted that the gold
nanowires may reach higher diameters by simply increasing the
gold content within the as-grown Au−Cu nanowires to values
above 16 at. % (data not presented here). Concerning the high
temperature regime, there are two noticeable differences
compared to the low temperature regime:
i) The first difference is related to the reorganization
mechanism of the gold core (Figure 5B-(b)) inside the
nanotube. The morphology of the core obtained at the
beginning of the reshaping process consists of separated
elongated and undulated rich-gold nanoparticles (Figure 5B-
process owing to the increase in gold self-diffusion coefficient
with temperature.72 In a further stage, the Au nanoparticles
tend to agglomerate and reorganize on the inner surface of the
oxide shell (Figure 5A-(b), stage 2). This organization is
accompanied by the cracking of the oxide shell. The cracks play
the role of diffusion channels for gold through the oxide shell
which is driven to progressively form Au nanoclusters within
the oxide phase (Figure 5A-(b), stage 3). In addition to the
formation of nanocracks in the oxide shell, a collapsing effect of
the nanotubes can be also remarked resulting in a nonuniform
wall thickness. Such instability has been also reported for pure
CuO nanotubes for an annealing temperature around 500 °C.14
In our case, an additional potential factor contributing to this
behavior is the strain generated due to the lattice mismatch
between the metal core and the metal oxide shell during the
oxidation process.40 The diffusion of gold inside the oxide shell
could also bring additional stress contributing to the roughening of the nanotube as reported by Yang et al. who
investigated the diffusion of gold in the Zn2SiO4 shell.50
However, it should be pointed out that the exact mechanism
responsible for the nonuniformity of the walls thickness
observed at high temperature is still open to question.
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(b), stage 1). With further annealing durations, these particles
undergo a reshaping process at the nanoscale resulting in the
formation of nearly spherical particles (Figure 5B-b, stage 2).
Actually, as experimentally reported, a gold nanowire confined
in oxide nanotube tends to undulate followed by its
fragmentation into smaller and nearly spherical nanoparticles.49,72,73 The driving force of such transformation arises
from the decrease in the surface free energy according to the
well-known Rayleigh instability.49,73−77 It should be noted that
such reorganization can be prevented by increasing the
diameter of the gold nanowire as reported by Qin et al.49 To
trigger the transformation of the material, a certain temperature
is required which allows activating the atomic diffusion of gold
enabling the transformation of the nanowire into a row of
nanospheres.73 Based on these considerations, the wire-in-tube
structure obtained at the low temperature regime is likely
present in a metastable state. At 600 °C, one can overcome the
kinetic activation barrier which allows reaching a more
energetically favorable configuration. In our case, once formed,
the metal particles tend to progressively migrate through the
oxide shell resulting in the formation of a nanotube decorated
with gold nanoparticles (Figure 5B-b, stage 3). As previously
observed by SEM (Figure 4a), numerous cracks are present in
the oxide shell. These defects likely act as channels promoting
the outward diffusion of gold through the oxide shell. It is
important to note here that the stress applied to the inner gold
cores as a consequence to the collapsing of the oxide shell is an
additional driving force promoting the extraction of gold to the
outer surface of the oxide shell.
ii) The second difference is the observed shrinkage of the
oxide shell taking place during the conversion process at high
temperature (Figure 5B(b), stage 3). This is most probably
related to the fact that hollow nanostructures are thermodynamically unstable at such a high temperature. As a
consequence, they tend to shrink into an energetically more
favorable solid structure (a solid nanostructure exhibits a lower
surface energy with respect to a hollow one).78 Nakamura et al.
studied the particular case of copper oxide and have
demonstrated that the shrinkage temperature of a hollow
copper oxide nanospheres into a solid one is about 400 °C
which is well below the temperature used in our work (i.e., 600
°C). It is important to point out here that since in our case the
nanotubes are filled with gold, the shrinkage process is expected
to be altered compared to the case of pure copper oxide. The
nanocracks observed in the oxide shell in such conditions are
most probably among the main consequences to the alteration
of the shrinkage process.
Oxidation of Au−Cu Nanospheres. In order to
investigate if the geometries/shapes of the nano-objects alter
the oxidation process of Au−Cu alloys, a similar study was
carried out on nanodots. In this case, a nanopillared silicon
substrate was used, instead of the nanograted silicon ones, as a
physical template to site-specifically grow the Au−Cu nanodots
on the pillars tips by cosputtering, similar to the approach
employed in our previous work to grow pure Cu nanodots.12
As for the nanowires, nanodots with various gold contents (4,
8, and 16 at. %) were prepared by tuning the deposition
conditions and then oxidized for different durations for an
annealing temperature of 300 °C (Figure 6a-c). A typical TEM
micrograph of an as-deposited nanodot is presented in Figure
S4. For all the samples, after 5 min of oxidation, the nanodots
transform into hollow CuO shells containing metal nanospheres (Figure 6 a-c). Similar to the case of nanowires, based
Figure 6. Series of TEM micrographs showing the different
chronological events occurring during the oxidation process of Au−
Cu nanodots with various Au contents: (a) 4 at. %, (b) 8 at. %, and (c)
16 at. %. Scale bar: 50 nm. The numbers are referred to the annealing
time (in min).
on the evolution of both the total diameter of the nanodot and
the oxide shell thickness as a function of the annealing time,
one can conclude that the oxidation reaction reaches a steady
state upon 5 min of annealing (Figure S5). Longer annealing
time results only in the reorganization of the metal core as
shown in Figure 6a-c. After 5 min of oxidation, a metal core is
found to be connected to the oxide shell through metal
filaments. The global porosity is found to increase as decreasing
the Au content within the Au−Cu nanodots. Further annealing
(30 min) results in the gradual aggregation of the metal core
accompanied by the disappearance of the metal filaments
connecting the metal core to the oxide shell. When reaching
150 min of annealing, Au/CuO nanoparticles with a yolk−shell
structure are obtained. The size of the metal core was found to
increase from ∼63 to ∼148 nm as increasing the Au content
from 4 to 16 at. %. From the HAADF micrographs coupled to
the STEM-EDS mapping, one can conclude that, for all the
samples, the metal core observed after 150 min is composed
mainly of gold with some copper residue. Similar to nanotubes,
the oxide shell forming the skeleton of the nanodots is
constituted of copper and oxygen, and no traces of Au were
detected (Figure 7). Based on the corresponding TEM
micrographs, the gold content has been calculated assuming
(i) the oxide nanodots and the metal cores have a spherical
geometry and (ii) the metal core is entirely composed of Au.
On this basis, the estimated atomic concentration of Au (atC. %
Au) is calculated according to eq 2 (Supporting Information,
section 7)
1
atc. % Au =
·100 (%)
3
3
ρCuO MAu (d tot.
)
− d inn.
1+ ρ . M .
3
(
Au
CuO
dAu
)
(2)
where dAu (cm) stands for the diameter of the spherical gold
core. dtot. (cm) and dinn. (cm) represent the total and the inner
diameter of the oxide nanodot, respectively, as deduced from
the TEM micrographs shown in Figure 4 a-c.
Based on eq 2, the calculated Au contents are 2, 5, and 16 at.
%. These values are consistent with the Au content within the
Au−Cu nanodots determined experimentally before oxidation:
6380
DOI: 10.1021/acs.chemmater.5b02391
Chem. Mater. 2015, 27, 6374−6384
Article
Chemistry of Materials
Figure 7. HAADF-STEM micrograph and the corresponding EDS mapping of Au−Cu nanodots with different Au contents after thermal oxidation
for 150 min: (a) 4 at. %, (b) 8 at. %, and (c) 16 at. % of Au. Scale bar: 100 nm.
Growth of Au−Cu Nanostructures. The Au−Cu nanostructures
were grown by DC cosputtering in pure argon plasma of Au
(diameter: 50.8 mm; purity: 99.99%) and Cu (diameter: 76.2 mm;
purity: 99.99) targets in a cofocal geometry. The distance between the
targets and the substrate was 130 mm. To ensure a homogeneous
deposition over a large surface, the samples were rotated at a speed of
5 turns/min. The deposition of the Au−Cu films was then carried out
at a pressure of 0.5 Pa without any intentional heating of the substrate.
For all depositions, the base pressure was less than 4·10−5 Pa. To
control the composition of the films, the electrical power applied to
the Cu target was fixed to 300 W, whereas the one applied to the Au
target was tuned. Three electrical powers applied to the Au target were
selected: 10, 25, and 50 W allowing growing nanostructures with 4, 8,
and 16 at. % of Au as found by EDS, respectively.
Oxidation Process. The oxidation of the nanostructures was
performed by thermal annealing in air using a conventional oven. In
order to precisely control the annealing time, gradual heating and
cooling were avoided. When the temperature inside the oven gets
stabilized at 300 or 600 °C, the samples were introduced and then
immediately taken out after achieving the required annealing time.
Material Characterization. Transmission electron microscopy
(TEM) and selected area electron diffraction (SAED) were performed
on a Hitachi H9000-NAR microscope (LaB6 filament, 300 kV,
Scherzer resolution: 0.18 nm). For these TEM analyses, the nanowires
or nanodots were dispersed on a carbon coated TEM grid.
Energy dispersive X-ray spectroscopy (EDS) was carried out with a
JEOL EDS system (JED-2300T) equipped with a 50 mm2 lightelement-sensitive X-ray detector, a digital pulse processor for highspeed accumulation, and a Be double tilt holder inserted in a JEOL
JEM ARM 200-F (S)TEM operated at 200 kV. STEM-EDS mapping
was performed with JEOL JED-2300 series digital mapping software,
and spatial drift correction was applied. A hard X-ray aperture of 200
μm was inserted to avoid stray radiation. Elemental maps were
obtained for O−K, Au−M, and Cu−K signals with a resolution of 512
× 512, dwell time of 0.5 ms, and a sweep count of 450.
Secondary electron and backscattered imaging was carried out on a
field emission gun JEOL JSM 7600 F scanning electron microscope.
For the secondary electron imaging mode, the SEM operated at 5 kV
whereas for the backscattered mode at 5 and 10 kV.
4, 8, and 16 at. %. Similar to the nanowires, this consistency
between the calculated and the experimental values proves that
the metal core formed after oxidation is mainly constituted of
gold.
■
CONCLUSION
In summary, we have investigated the oxidation mechanisms of
Au−Cu alloy nanowires and nanodots. Contrary to pure
elemental metal, when the Kirkendall effect comes into play in
such a binary system, it results in the formation of porous
nanostructures constituted of copper oxide shells containing
gold-rich cores. When the Cu phase is fully oxidized, further
increasing the annealing time leads to a reorganization at the
atomic scale of the thermodynamically unstable Au-rich cores.
Such reorganization is found to be dependent on the Au
content within the as-grown Au−Cu alloy nanostructures. In
the case of nanowires, for a given chemical composition, an
increase in the oxidation temperature enables the overcoming
of the diffusion kinetic activation barrier giving rise to
additional reshaping mechanisms of gold taking place during
the annealing process.
The versatility of the approach developed in this work is
located in the fact that it enables fabricating Au/CuO
heteronanomaterials with various nanoarchitectures as for
instance a wire-in-tube or a yolk−shell structure by simply
tuning the synthesis conditions (e.g., Au contents within the
Au−Cu nanowires or nanodots, annealing time, annealing
temperature, etc.). The approach developed herein is not only
limited to the gold/copper system but can also be extended to
other metal alloys allowing designing multifunctional heterogeneous buildings blocks for numerous applications including
catalysis, biomedicine, and optics/photonics.
■
METHODS
■
Nanopatterned Substrates. The nanopatterned silicon substrates, used to site-specifically grow the gold/copper nanowires and
nanodots, were prepared by laser interference lithography followed by
deep reactive ion etching.79 More details about the nanofabrication
approach can be found elsewhere.12 Two types of high-aspect-ratio
silicon nanostructures were prepared:
(i) Nanogratings of 220 nm in periodicity and an aspect ratio
around 6 (160 nm in width and 1000 nm in height).
(ii) Nanopillars of 250 nm in periodicity and an aspect ratio of 3.5
(130 nm in diameter and 450 nm in height).
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemmater.5b02391.
TEM micrographs and the associated SAED patterns;
Diameter and oxide shell thickness of Au−Cu nanowires
after oxidation at 300 °C; Description of the method
6381
DOI: 10.1021/acs.chemmater.5b02391
Chem. Mater. 2015, 27, 6374−6384
Article
Chemistry of Materials
■
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employed to estimate the gold content within the
nanotubes; Diameter and oxide shell thickness of
Au−Cu nanowires after oxidation at 600 °C; Diameter
and oxide shell thickness of Au−Cu nanodots after
oxidation at 300 °C; Description of the method
employed to estimate the gold content within the
nanodots (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors would like to thank la Région des Pays de la Loire
- France for financially assisting this research project through
the Postdoctorats internationaux program. The JEOL JEM
ARM 200-F (S)TEM used in this work was partially funded by
the German Research Fundation (DFG/INST163/2951).
■
ABBREVIATIONS
TEM, transmission electron microscopy; SEM, scanning
electron microscopy; STEM, scanning transmission electron
microscopy; EDS, energy-dispersive X-ray spectroscopy;
HAADF, high-angle annular dark-field
■
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Chem. Mater. 2015, 27, 6374−6384