Formation of nanoporous noble metal thin films by electrochemical dealloying of PtxSi1-x

Article abstract of DOI:10.1063/1.2161939

We demonstrate the synthesis of nanoporous Pt thin films on Si by electrochemical dealloying. Amorphous Ptx Si1-x films (～100-250 nm thick) are formed by electron beam codeposition and dealloyed in aqueous HF solutions at an electrochemical potential sufficient to selectively remove Si while allowing self-assembly of Pt into a nanoporous structure. The Pt nanoporous layers have a pore size of 5-20 nm, ligament thickness ～5 nm, a surface area enhancements >20 times, and an ultrafine grain polycrystalline microstructure.

Full text of DOI:10.1063/1.2161939

Formation of nanoporous noble metal thin films by electrochemical dealloying of Pt x
Si 1 − x
J. C. Thorp, K. Sieradzki, Lei Tang, P. A. Crozier, Amit Misra, Michael Nastasi, David Mitlin, and S. T. Picraux
Citation: Applied Physics Letters 88, 033110 (2006); doi: 10.1063/1.2161939
View online: http://dx.doi.org/10.1063/1.2161939
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APPLIED PHYSICS LETTERS 88, 033110 ͑2006͒
Formation of nanoporous noble metal thin films
by electrochemical dealloying of PtxSi1−x
J. C. Thorp
Department of Chemical and Materials Engineering, Arizona State University, Tempe, Arizona 85287
K. Sieradzki and Lei Tang
Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, Arizona 85287
P. A. Crozier
Center for Solid State Science, Arizona State University, Tempe, Arizona 85287
Amit Misra, Michael Nastasi, and David Mitlina͒
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
S. T. Picrauxb͒
Department of Chemical and Materials Engineering, Arizona State University, Tempe, Arizona 85287
and Los Alamos National Laboratory, Los Alamos, New Mexico 87545
͑Received 23 September 2005; accepted 6 December 2005; published online 19 January 2006͒
We demonstrate the synthesis of nanoporous Pt thin films on Si by electrochemical dealloying.
Amorphous PtxSi1−x films ͑ϳ100–250 nm thick͒ are formed by electron beam codeposition and
dealloyed in aqueous HF solutions at an electrochemical potential sufficient to selectively remove Si
while allowing self-assembly of Pt into a nanoporous structure. The Pt nanoporous layers have a
pore size of 5–20 nm, ligament thickness ϳ5 nm, a surface area enhancements Ͼ20 times, and an
ultrafine grain polycrystalline microstructure. © 2006 American Institute of Physics.
͓DOI: 10.1063/1.2161939͔
Electrochemical dealloying is a well known process for
the selective removal of the most active species from an
alloy and the resulting formation of an interconnected porous
metal structure. It has been studied for a wide range of cor-
rosion reactions where removal of the more active compo-
nent occurs under open circuit conditions, e.g., Cu–Zn
͑brass͒, Cu–Au.1 More recently dealloying of bulk metal-
metal binary alloys has been studied in detail for the Ag–Au
system.2 The dissolution of the more active species ͑Ag͒
from a homogeneous alloy results in the self-assembly of the
remaining species ͑Au͒ by surface diffusion into a high sur-
face area bicontinuous network with pore sizes as small as
2–3 nm. High surface area catalytic electrode materials play
a central role in fuel cells, electrochemical sensors, and hy-
drogen generation in photocatalytic cells and there is increas-
ing interest in methods to form nanoporous architectures on
surfaces, membranes, and in microsystems.3–6
Based on the dealloying approach we have developed a
method to synthesize nanoporous layers of noble metals
͑such as Pt and Au͒ integrated onto silicon substrates. Our
synthesis approach is based on the electrochemical dealloy-
ing of vapor deposited homogeneous metal silicon films. We
demonstrate that dealloying of amorphous PtxSi1−x films re-
sults in the formation of Pt nanoporous structures with pore
sizes between 5 nm and 20 nm. While dealloying has been
studied extensively in binary metal alloy systems, there are
no reported studies of nanoporous structures formed by deal-
loying metal silicide systems. This approach enables the in-
tegration of a “bottom up” dealloying self-assembly process
with “top down” fabrication for Si-based microsystems.
PtxSi1−x films of uniform composition with xϳ0.2, 0.35,
and 0.5 were deposited at room temperature on n-doped Si
͑100͒ substrates by coevaporation in a multiple source
electron-beam system with a base pressure of 5ϫ10−8 Torr.
Film thicknesses were varied from 100 to 250 nm. A 1 nm
layer of Cr or Ti was deposited just prior to the PtSi film
deposition; without this adhesive layer stress-induced
delamination occurred during the electrochemical dealloy-
ing. Also in most cases a thin ͑20–40 nm͒ Pt underlayer was
deposited between the adhesive layer and the PtSi film,
which served as a convenient electrochemical marker for the
end of the dealloying process. The room temperature depos-
ited silicon rich PtxSi1−x alloy films are expected to be amor-
phous and the pure Pt deposited underlayers polycrystalline,
and this was confirmed by x-ray diffraction. Rutherford
backscattering spectrometry ͑RBS͒ with 2 MeV He together
with RUMP simulations was used to determine the film com-
positions and thicknesses. Some oxygen was found to be
present in the as-deposited films using He resonance scatter-
ing, with the amount quantitatively determined by RBS
analysis. The dealloying characteristics of the films were
studied using a standard three-electrode cell configuration
with an electrolyte of 3% hydrofluoric acid in 18 M⍀ dis-
tilled and deionized water ͑Nanopure system͒. The electrodes
consisted of a Pt counter electrode, a saturated calomel ref-
erence electrode ͑SCE͒, and the deposited PtxSi1−x films
͑ϳ1 cm2 area͒ as the working electrode.
The potential sweep vs SCE for a codeposited film con-
taining 21% Pt in Si on top of a thin Pt underlayer is shown
in comparison to that for pure Si in Fig. 1. The clear change
in morphology from an initially smooth to a highly porous
surface after the potential sweep indicates the formation of
the nanoporous structure with observed pore sizes ranging
between ϳ5 nm and 20 nm and Pt ligament widths of
ϳ5 nm. The increase in current density in Fig. 1 near 0.2 V
a͒
Current address: Department of Chemical and Materials Engineering, Uni-
versity of Alberta, Edmonton, Alberta, Canada T6G 2E1.
b͒
Author to whom correspondence should be addressed; electronic mail:
picraux@lanl.gov
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033110-2
Thorp et al.
Appl. Phys. Lett. 88, 033110 ͑2006͒
FIG. 1. Linear potential sweeps at 10 mV/s for pure silicon and for a
130 nm Pt0.21Si0.70O0.09 film with a 20 nm Pt underlayer on Si. Insets show
schematic cross sections and plan-view SEMs of the as-deposited ͑left side͒
and dealloyed ͑right side͒ samples.
is thus inferred to be due to the dealloying of Si ions from
the PtSi alloy and is consistent with the rapid increase in Si
dissolution for a bulk Si ͑100͒ surface. The subsequent drop
in current density for the PtSi film above 0.7 V is attributed
to exhausting the Si from the PtSi alloy. Similar dealloying
results were obtained for four different PtxSi1−x codeposited
films with a Pt composition between x=0.18 and 0.21 and
thicknesses between 100 and 250 nm. The oxygen back-
ground concentrations ranged from 2 to 9 at.%, in these
films, suggesting these levels of O do not critically affect the
dealloying process. The critical potential for dealloying
͑ϳ0.2 V in Fig. 1͒ is a rate dependent parameter which de-
creases as sweep rate is increased. Based on our conditions
the subsequent film dealloying studies were carried out at a
constant potential of 0.6–0.8 V.
FIG. 2. RBS spectra using 2 MeV He for PtSi thin film samples as depos-
ited and after dealloying for ͑a͒ Pt0.21Si0.70O0.09 film with underlying 20 nm
Pt barrier layer ͑same deposition as for Fig. 1͒, and ͑b͒ Pt0.20Si0.75O0.05 with
no Pt barrier layer. Backscattering level near channel 300 for pure Pt ͑as
indicated by horizontal line in upper right corner͒ corresponds to the scat-
tering level for pure Pt and is consistent with the observed level after deal-
loying, indicating the complete removal of Si ͑and O͒ atoms by dealloying.
Dealloying was also investigated for PtxSi1−x codepos-
ited films with higher Pt concentrations corresponding to x
=0.35 and 0.5. In addition polycrystalline PtSi films were
formed by Pt deposition and annealing to form the most Si
rich silicide phase, PtSi. It was found that for x=0.35 the Si
dissolution and nanoporous Pt film formation occurred with
the average pore sizes somewhat larger than for xϳ0.2.
However, for x=0.5 dealloying was not observed for either
the codeposited amorphous or the polycrystalline PtSi sili-
cide films, consistent with the typically 15%–40% concen-
trations required for the more noble species in binary metal-
lic alloys for dealloying to occur.
The dealloyed nanoporous Pt films were heated in a
flowing inert gas atmosphere tube furnace for 10 min at 800,
850, and 900 °C. As shown in Fig. 4, SEM micrographs of
the surface morphology show a pore size increasing with
anneal temperature from approximately 10 nm to 40 nm at
the highest anneal temperature. At these temperatures, which
correspond to homologous temperatures of 0.52 to 0.57, sur-
face diffusion is expected to become appreciable, thus en-
abling the observed coarsening.
In addition to SEM analysis, depth dependent composi-
tional analysis was performed by RBS before and after deal-
loying to verify the removal of Si. Figure 2 contains spectra
for two different samples containing 20% Pt with and with-
out an underlying pure Pt barrier layer. The RBS spectra
verify complete removal of silicon atoms from the PtxSi1−x
alloys while retaining all the Pt atoms in the original films.
The structure of a dealloyed nanoporous Pt film was
studied in more detail using field-emission transmission elec-
tron microscopy ͑JEOL 2010F͒. In Fig. 3͑a͒ the light regions
of the Z-contrast scanning transmission electron micrograph
͑STEM͒ indicate a well defined bicontinuous “honeycomb”
structure with areas of high and low density conforming to
irregularly shaped pore sizes of 10–25 nm. The ligaments
connecting the pores in Fig. 3͑a͒ have a width of about
5–7 nm. Electron energy-loss spectroscopy ͑EELS͒ verified
the presence of Pt for the nanoporous structure from the Pt
3d ionization edge at 2122 eV ͓Fig. 3͑a͒ inset͔, as well as no
detectable Si at 1839 eV. Both high resolution transmission
electron microscopy ͑HREM͒ and selected area electron dif-
fraction ͓Fig. 3͑b͒ and inset͔ indicate the nanoporous con-
necting ligament microstructure is composed of fine-grained,
randomly oriented fcc polycrystalline Pt with an average
nanocrystalline grain size of 3–5 nm. We note that for nano-
porous structures with about 5 nm wide ligaments the frac-
Since the formation of nanoporous structures have, to
our knowledge, not been reported previously for noble
metal-silicon alloys,7 it is illustrative to compare this system
to the characteristics typically possessed by binary alloys
that form nanoporous structures by dealloying.1,2,8 First, one
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tion of atoms at a surface in the structure approaches 10%.
species is typically more noble than the other by several
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033110-3
Thorp et al.
Appl. Phys. Lett. 88, 033110 ͑2006͒
FIG. 4. SEM images of the dealloyed Pt surface ͑a͒ as-deposited, and after
10 min anneals at ͑b͒ 800 °C, ͑c͒ 850 °C, and ͑d͒ 900 °C showing an
increase in pore size due to coarsening with increasing temperature.
nanoporous films, a value comparable to that for Pt black
catalysts. This UDP determined surface area is also consis-
tent with simple geometric estimates based on our measured
mean pore size, surface density, and film thickness for the
nanoporous layers.
To explore the catalytic reactivity of these nanoporous Pt
electrodes preliminary measurements were carried out for
hydrogen production in a photoelectrochemical cell.10 The Pt
electrode served as the cathode of a hybrid cell that couples
a dye-sensitized nanoparticulate wide band gap semiconduc-
tor photoanode to the enzyme-based oxidation of glucose at
the cathode. For the nanoporous Pt electrode the hydrogen
production rate, while below that observed for commercial Pt
impregnated fuel cell electrodes, was found to be increased a
factor of 10 over that for a smooth Pt foil electrode.
In summary electrochemical dealloying of amorphous
platinum-silicon codeposited alloys results in the self-
assembly of ultrafine grain polycrystalline nanoporous noble
metal structures, providing a new approach to forming high
surface area electrodes on Si for emerging applications such
as microfuel cells, biosensors, microbatteries, and superca-
pacitors.
FIG. 3. ͑a͒ Z-contrast STEM image after dealloying for the Pt nanoporous
structure revealing pores sizes on order of 10–25 nm ͑the lighter areas
correspond to the nanoporous Pt ligaments͒. The sample was prepared by
floating the nanoporous film in the electrolyte onto a Cu grid after dealloy-
ing for a Si substrate without a Cr adhesive layer. Insert shows the electron
energy-loss spectrum near the Pt M45 edge. ͑b͒ HREM showing randomly
oriented nanoscale grains with lattice fringes corresponding to the ͑111͒ and
͑200͒ plane spacing of Pt metal. Insert shows selected area electron diffrac-
tion pattern which indexes to Pt metal.
hundred millivolts such that the less noble species is electro-
chemically extracted upon dealloying at an intermediate po-
tential, and this is the case for our PtxSi1−x system. Second,
the composition is usually richer in the less noble species
and this is consistent with our observations of dealloying for
20 and 35 at. % Pt in Si, but not for 50% Pt. Third, in order
for a homogenous dealloyed microstructure to evolve, the
alloy must be single phase. Our method of codeposition of
metal-silicon films near room temperature enables the ready
formation of homogeneous single phase alloys, which in
most cases are expected to be amorphous, and we note that
the dealloying process proceeds in our amorphous PtSi al-
loys similar to that for the previously studied crystalline bi-
nary metallic systems.1–6,8
The authors gratefully acknowledge support by the Na-
tional Science Foundation DMR-0301007 ͑K.S.͒ and the
AVS Undergraduate Student Research Awards ͑J.T.͒ and also
thank D. Gervasio, D. Gust, T. Clement, and M.
Hambourger.
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was measured by Cu underpotential deposition ͑UPD͒. At
potentials slightly higher than the thermodynamically revers-
ible potential UPD can results in the deposition of a single
pseudomorphic Cu monolayer onto polycrystalline Pt
surfaces.9 The nanoporous Pt was swept and held at the po-
tential where one full Cu monolayer forms for an extended
time to allow Cu ions to diffuse into the porous structure and
then swept back to strip the Cu2+ ions at a slow sweep rate
͑1 mV/s͒ to measure the deposited Cu monolayer which is
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On: Mon, 01 Dec 2014 08:22:15