The document summarizes research on the mechanical properties of nanoporous gold produced via de-alloying of gold-silver alloys. Key findings include: 1) Nanoporous gold can have a Young's modulus in the GPa range when dry, but as low as 60 MPa when wet, far below predictions from foam mechanics. 2) The mechanical properties, including modulus and strength, depend strongly on the alloy composition, de-alloying conditions, and whether the material is dry or wet. 3) When kept wet, the material maintains integrity and flexibility, but drying can induce bending, curling, or cracking depending on the preparation process.

1. 212th ECS Meeting, Abstract #1438, © The Electrochemical Society
Mechanics of De-alloyed Au-Ag
F. Pop, S. Nunnari, M.T. Kortschot,
A.G. Carcea and R.C. Newman
University of Toronto
Department of Chemical Engineering
and Applied Chemistry
200 College Street, Toronto, ON, M5S 3E5
Canada
De-alloying (selective dissolution from a homogeneous
alloy) produces a nanoporous metal residue with a
number of curious properties, some of them dependent on
the size scale of the nanoporosity. The ligament size in
the porous structure can be varied by ambient coarsening
in acid or by varying the formation conditions (potential,
anion, etc.). Recently Volkert et al. [1] and Mathur and
Erlebacher [2] studied the elastic properties of
nanoporous gold made by de-alloying Au-Ag. They
concluded, respectively, that the material shows near-
theoretical strength in compression, and has a scale-
dependent modulus. The moduli measured by both groups
were in the GPa range: 7-12 GPa for unloading of
compressed micro-pillars [1] and 5-40 GPa depending on
the size scale of the porosity, for thin-film buckling
experiments [2]. A sharp increase from ~10 to 40 GPa
occurred as the ligament size was reduced from ~15 to ~5
nm. All these experiments were done in the dry state;
Volkert et al. used annealed Ag-25at.%Au, and de-alloyed
it in aqueous perchloric acid under potential control,
while Mathur and Erlebacher used cold-rolled ‘white
gold’ leaf (Ag-35at.%Au) and de-alloyed it in strong
nitric acid, either by simple immersion or at a controlled
potential.
The details of alloy composition and de-alloying
conditions are even more influential than is apparent in
the literature cited above. Senior and Newman [3]
believed they had optimized the mechanical integrity of
nanoporous Au by selecting a 23at.% Au alloy and de-
alloying it in aqueous perchloric acid under carefully
controlled conditions that allowed limited but significant
coarsening of the porosity during de-alloying. They
showed that above a particular potential, extensive tensile
stress and cracking developed during de-alloying (see
Figure 1), and attributed this to hindrance of surface
diffusion by formation of a monolayer of gold hydroxide.
Almost as an aside, it was noted that 100 micron foils of
de-alloyed material prepared by this procedure could be
bent elastically through huge angles. No formal
measurements of modulus were made, but clearly this was
a very compliant material. It was claimed, on the basis of
preliminary wet and dry bending experiments, that both
the modulus and the strength of the de-alloyed material
were dependent on the environment.
We have pursued the preliminary observations of Senior
and Newman using compression testing of wet and dry,
de-alloyed Ag-23at.% Au foils, 100 and 200 microns in
thickness. A compression tester designed to study paper
was used. The deflection of cantilevered foil strips under
their own weight in de-ionized water was used to confirm
the modulus of the wet material. This could not be done
with the dry material because it curled slightly during
drying, increasing its structural stiffness. In fact, a variety
of bizarre behaviour was observed during drying of
differently prepared samples, including bending, curling
and spontaneous fracture. The reversibility of this
embrittlement noted by Senior and Newman is not always
reproduced, and apparently requires a very high degree of
uniformity in the material prior to de-alloying. Every
detail of sample preparation, de-alloying procedure and
post-treatment affects the mechanical response of the
dried material, whereas when kept in de-ionized water it
behaves perfectly and stays flat.
The main finding of the study so far is that the Young’s
modulus of our wet material is extremely low, so low that
it is hard to credit that it can have mechanical integrity,
yet it can be manipulated quite confidently without
fracturing. Values as low as 60 MPa have been obtained,
and confirmed qualitatively by cantilever bending. The
question of whether there is a difference in modulus
between the wet and dry material is still under
investigation, and results will be reported at the meeting.
The difference in fracture strength is clear.
The explanation for the low modulus is that, far from
obeying foam mechanics, the de-alloyed 23Au material is
close enough to the continuum percolation threshold or
critical volume fraction (though not that close – this
threshold is ca. 0.16) that its stiffness is somehow
dominated by relatively few connected paths. How this
can happen and yet maintain such impressive
macroscopic integrity is the subject of further work.
Figure 1 - Pattern of cracks formed when de-alloying is
performed at too high a potential, allowing monolayer Au
oxidation and hindering the relief of induced tension by
surface diffusion of Au.
References
[1] C.A. Volkert, E.T. Lilleodden, D. Kramer and J.
Weissmueller, Appl. Phys. Lett., 89, 061920 (2006).
[2] A. Mathur and J. Erlebacher, Appl. Phys. Lett., 90,
061910 (2007).
[3] N.A. Senior and R.C. Newman, Nanotechnology, 17,
2311-2316 (2006).