Fabrication of nanoporous copper by dealloying of amorphous Ti-Cu-Ag alloys

Article abstract of DOI:10.1016/j.jallcom.2013.01.087

Ternary TiCuAg amorphous alloys were used as the starting materials for fabricating nanoporous copper (NPC) in HF solutions. NPC with a pore size of 8-55 nm was fabricated through dealloying from an amorphous Ti ₆₀Cu_40-xAg_x (x = 1, 2 at.%) ribbon alloy under free immersion conditions. A 3-dimensional bicontinuous NPC structure formed on Ti₆₀Cu₄₀, Ti₆₀Cu ₃₉Ag₁, and Ti₆₀Cu₃₈Ag₂ ribbon alloys with pore sizes of 130 nm, 48 nm, and 55 nm, respectively, after immersion in 0.13 M HF solution for 43.2 ks. The pore sizes of dealloyed ribbon alloys in 0.03 M HF solution were confirmed to be 71 nm, 41 nm and 39 nm, respectively. The pore size of NPCs dealloyed from Ag-added alloys was smaller than that of the Ti-Cu alloy. Smaller pores formed in the beginning of dealloying because even distributed Ag atoms in amorphous precursors suppressed the diffusion of Cu adatoms. The final characteristic pore size showed a weak dependence on the solution concentrations. Diffusivity decreased more than two orders due to the alloying of Ag. The dealloying residue was fcc Cu-Ag and Ag, with Ag adatoms concentrated at the grain boundary. The uneven distribution of Ag atoms caused the coarsening of nanoporous Cu after the prolonged dealloying.

Full text of DOI:10.1016/j.jallcom.2013.01.087

Journal of Alloys and Compounds xxx (2013) xxx–xxx
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Fabrication of nanoporous copper by dealloying of amorphous Ti–Cu–Ag alloys
b
b
a
a
a
Zhenhua Dan a,b, , Fengxiang Qin , Akihiro Makino , Yu Sugawara , Izumi Muto , Nobuyoshi Hara
⇑
a Department of Materials Science, Tohoku University, Sendai 9808579, Japan
b Institute for Materials Research, Tohoku University, Sendai 9808577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online xxxx
Ternary TiCuAg amorphous alloys were used as the starting materials for fabricating nanoporous copper
(NPC) in HF solutions. NPC with a pore size of 8–55 nm was fabricated through dealloying from an amor-
phous Ti60Cu40ꢀxAgx (x = 1, 2 at.%) ribbon alloy under free immersion conditions. A 3-dimensional bicon-
tinuous NPC structure formed on Ti60Cu40, Ti60Cu39Ag1, and Ti60Cu38Ag2 ribbon alloys with pore sizes of
130 nm, 48 nm, and 55 nm, respectively, after immersion in 0.13 M HF solution for 43.2 ks. The pore sizes
of dealloyed ribbon alloys in 0.03 M HF solution were confirmed to be 71 nm, 41 nm and 39 nm, respec-
tively. The pore size of NPCs dealloyed from Ag-added alloys was smaller than that of the Ti–Cu alloy.
Smaller pores formed in the beginning of dealloying because even distributed Ag atoms in amorphous
precursors suppressed the diffusion of Cu adatoms. The final characteristic pore size showed a weak
dependence on the solution concentrations. Diffusivity decreased more than two orders due to the alloy-
ing of Ag. The dealloying residue was fcc Cu–Ag and Ag, with Ag adatoms concentrated at the grain
boundary. The uneven distribution of Ag atoms caused the coarsening of nanoporous Cu after the pro-
longed dealloying.
Keywords:
Amorphous materials
Corrosion
Diffusion
Transmission electron microscopy
Dealloying
Nanoporous copper
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
of nanoporous Cu fabricated by the one-step dealloying of the
melt-spun Al–50 at.% Cu alloy was determined to be 86 10 MPa
Nanoporous metals, as novel functional materials, have great
potential in applications as diverse as catalysis, sensors, actuators,
fuel cells and microfluidic flow controllers, just to name a few [1–
4]. The microstructure of starting materials plays an important role
in the formation of the final nanoporous structure. The presence of
intermetallics or second phases has been shown to facilitate the
formation of a bimodal nanoporous structure from crystallized
Al–Cu alloys systems [5–8]. As has been reported, nanoporous cop-
per (NPC) was fabricated from amorphous Ti–Cu alloy systems in
HF solutions [9]. Because of the absence of multimodal microstruc-
tures in amorphous alloys, it has been suggested that amorphous
precursors may be better than crystalline alloys for use as starting
materials since they allow the formation of a uniform distribution
of nanoporosity [9].
Pore size had a significant effect on the physico-chemical prop-
erties of the nanoporous metals. The yield strength of nanometer-
sized ligaments of nanoporous Au increased from ꢁ880 MPa to
4.6 GPa as the pore size decreased from 50 nm to 10 nm [10].
Hayes et al. [11] reported a yield strength of 128 37 MPa for
NPC with a ligament diameter of 135 31 nm. The yield strength
with ligaments of 300–500 nm [6]. The surface enhanced Raman
scattering effects, surface plasma properties and the catalysis per-
formance of various nanoporous metals was enhanced by the finer
nanoporous structures [1,12–16]. It is therefore important to fabri-
cate nanoporous metals with ever finer nanostructures, with smal-
ler pore sizes.
The evolution of nanoporosity is affected by the composition
elements in the alloy systems with different properties in surface
diffusivities. The surface diffusivity of Ag is considerably lower
than Cu by as much as several orders in magnitude [6,13,17]. The
dealloying process is evoked at the metal/electrolyte interface,
and the diffusivity of the more noble metals played an important
role in the rearrangement of the atoms [18,19]. In our former re-
search, the addition of minor Au to Ti–Cu amorphous alloys did
not change their amorphous properties and the final nanoporous
Cu obtained by immersing in various HF solution for 43.2 ks had
a characteristic pore size of 9–19 nm for Ti60Cu39Au1 alloys, and
7–12 nm for Ti60Cu38Au2 amorphous alloys. Compared to the
nanopores formed on Ti–Cu amorphous alloys, those on these
Ti60Cu39Au1 and Ti60Cu38Au2 alloys were more than one order in
size smaller due to the drastic decrease in the surface diffusivity
[20,21]. The dealloying of Ti–Cu–Ag amorphous alloys is expected
to achieve finer, cost-effective NPCs.
⇑
Corresponding author. Address: Institute for Materials Research, Tohoku
University, 2-1-1, Katahira, Aoba-ku, Sendai 9808577, Japan. Tel.: +81 22 215
2718; fax: +81 22 215 2714.
The present research is part of a series of efforts aimed to exam-
ine the range of the characteristic length scales of the pores of NPCs
E-mail address: zhenhuadan@imr.tohoku.ac.jp (Z. Dan).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.01.087
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Z. Dan et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx
obtained through dealloying Ti60Cu40ꢀxAgx (x = 0, 1, 2 at.%) in
0.03 M HF and 0.13 M HF solutions for various times. The distribu-
tion of the residue of Ag and Cu was analyzed by transmission elec-
tron microscope in order to determine the key effects of the third
alloying elements.
the case of Ti60Cu40 alloy, the lattice constants of NPC were
0.3613 nm, larger than that standard value of powder Cu of
0.3608 nm. Those of residual Cu–Ag alloys were less than 1% larger
than those of nanoporous Cu, indicating that most of the Ag atoms
precipitated to form an Ag phase separate from the NPCs and small
portion of the Ag atoms formed a nanoporous Cu–Ag solid solution.
The residual phases of the as-dealloyed Ti–Cu–Ag ribbon alloys
were crystalline fcc Cu–Ag and Ag.
2. Experimental
Ternary alloys with nominal compositions of Ti60Cu40ꢀxAgx (x = 0, 1, 2 at.% Ag)
were prepared by arc melting
(99.9 wt.%) and Ag (99.9 wt.%) in an Ar atmosphere. The melt spinning method
a mixture of pure Ti (99.99 wt.%), pure Cu
3.2. Porosity evolution on Ag-added TiCu alloys
was used to prepare amorphous Ti–Cu–Ag ribbons 20
in width.
Dealloying was performed in HF solutions with a concentration of 0.03 M and
0.13 M in a free corrosion condition at 298 K. After various immersion times, the
dealloyed ribbons were taken out and well rinsed with alcohol to remove any resid-
ual HF solution.
The amorphocity of the as-spun Ti–Cu–Ag samples was confirmed by X-ray dif-
fractometer (Rigaku RINT-4200) with Cu Ka radiation (k = 0.15418 nm). The micro-
structures of as-prepared and as-dealloyed Ti–Cu–Ag ribbons were investigated by
a high-resolution transmission electron microscope (JEOL, JEM-ARM210F). The
morphology of the as-dealloyed samples was observed by a scanning electron
microscope (JEOL, JIB-4610F).
lm in thickness and 2 mm
Fig. 2 shows the formation process of nanopores on Ti60Cu38Ag2
ribbons after various immersion times in HF solutions. The mean
pore sizes after immersion of 1.8 ks in 0.03 M HF and 0.13 M HF
solutions were 8 nm and 13 nm, respectively. The mean pore sizes
after 10.8 ks were 23 nm and 32 nm, and after 43.2 ks, they were
39 nm and 55 nm. Wider pores formed in the concentrated HF
solutions. The ligament size gradually increased with immersion
time. As shown in Fig. 3, the pores on the Ti60Cu39Ag1 ribbon alloy
increased to 41 nm and 48 nm in size after 43.2 ks of immersion in
0.03 M HF solution and 0.13 M HF solution, respectively. The mean
pore size of the nanoporous structure of as-dealloyed Ti60Cu40 rib-
bons was about 71 nm and 130 nm after dealloying in 0.03 M and
0.13 M HF solution after an immersion time of 43.2 ks, respec-
tively. The pore sizes drastically decreased in 0.13 M HF solution.
The final nanopores were a similar characteristic size, but the liga-
ments were slightly different in size due to the difference of their
porosity evolution behavior from Ti–Cu alloys. As shown in
Fig. 4, the pore sizes of dealloyed Ag-added alloys were less than
13 nm even in 0.13 M HF solution in the beginning immersion of
3.6 ks. After Ti60Cu39Ag1 ribbon alloy was immersed in 0.03 M HF
solution for 10.8 ks, the pore size was about 16 nm, however, the
final pore size was 41 nm. The increase in pore sizes over longer
immersion times appeared to be more rapid than during the first
10.8 ks. The natural logarithm plot of pore size, ln[d(t)], versus
dealloying time, lnt, showed linear dependency. The slope of the
fitted lines was located at between 0.34 and 0.52. This slope related
to the coarsening exponent, n, which symbolizes the kinetic param-
eter for the surface relaxation of roughened metals in solution, com-
monly has a value of 0.25 [22]. The deviation between the
experimental and the theoretical values had a factor of more than
2 for the dealloyed Ti60Cu39Ag1 ribbon alloy in 0.03 M HF solution.
3. Results and discussion
3.1. Characterization of TiCu alloy and nanoporous Cu–Ag alloys by
XRD
Fig. 1 shows the XRD patterns of as-spun and as-dealloyed
Ti60Cu40, Ti60Cu39Ag1 and Ti60Cu38Ag2 ribbons. The single diffrac-
tion peak appeared around 41° in the XRD patterns of as-spun
Ti–Cu–Ag ribbons indicated that the Ag-added TiCu alloys were
amorphous in structure. The XRD pattern of as-dealloyed Ti60Cu40
ribbon alloys exhibited the diffraction peaks attributed to fcc Cu
(111), (200), (220) (JCPDS card No.: 02-1225) and Cu2O (111)
(JCPDS card No.: 74-1230). The as-dealloyed Ti60Cu39Ag1 and Ti60-
Cu38Ag2 ribbon alloys showed diffraction peaks assigned to fcc Ag
(111), (220) and (311) (JCPDS card No.: 04-0783) in addition to
the Cu peaks. The ratios of the diffraction peak of Ag (111) to that
of Cu (111) increased from 0.03 to 0.34 as the amount of Ag added
was increased from 1 to 2 at.%. The diffraction peaks of Cu
appeared at almost the same diffraction angles, and the lattice con-
stants of nanoporous Cu–Ag alloys were estimated to be 0.3615 nm
for that obtained from the Ti60Cu39Ag1 alloy and 0.3627 nm for that
obtained from the Ti60Cu38Ag2 alloys on the basis of XRD data. In
3.3. Characterization of element distribution and crystalline properties
of as-dealloyed nanoporous structures
Fig. 5 shows the bright-field (BFI) and high resolution TEM
(HRTEM) images, selected area diffraction pattern (SADP), twin
boundary and its FFT pattern. The BFI indicated the formation of
a bicontinuous nanoporous structure. As measured from the TEM
image, the mean pore size was 28 nm and the characteristic liga-
ment size was 55 nm, which is consistent with SEM observed data.
The SADP shows that the nanoporous structure consisted of crys-
talline fcc Cu and Ag (Fig. 5b). The HRTEM image shows the forma-
tion of crystalline Ag grains. The interplanar distance from the
adjacent lattice fringes marked in the HRTEM images was
0.237 nm, corresponding to the {111} planes of fcc Ag (Fig. 5c).
The TEM-EDX data shows the Ag content in binary Cu–Ag nanopor-
ous alloys was 2.3 at.% from Ti60Cu39Ag1 ribbons and 6.3 at.% from
Ti60Cu38Ag2 ribbons in 0.13 M HF solution. Defects, including grain
boundary and twin boundary defects, were formed after dealloying
(Fig. 5d), and the FFT pattern on the twin boundary, a type of stack-
ing fault structure, was similar to those commonly observed in the
fcc metals during plastic straining [23], indicating that the twin
boundary was metastable. Fig. 6 shows BFI image and high-angular
dark-field TEM image and the corresponding elemental distribution
Fig. 1. XRD patterns of Ti60Cu40, Ti60Cu39Ag1 and Ti60Cu38Ag2 amorphous alloys
before (a–c) and after (d–f) dealloying in 0.13 M HF solution for 43.2 ks at 298 K.
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Z. Dan et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx
3
Fig. 2. SEM morphology of Ti60Cu38Ag2 amorphous alloys after dealloying in 0.03 M and 0.13 M HF solutions for 1.8 ks (a and d), 10.8 ks (b and e) and 43.2 ks (c and f) at
298 K.
Fig. 4. Natural logarithm plot of the pore size of Ti60Cu39Ag1 and Ti60Cu38Ag2
amorphous alloys, ln[d(t)], after dealloying in 0.03 M HF (a and b) and 0.13 M HF
solutions (c and d) verse dealloying time, lnt, at 298 K.
3.4. Estimation of surface diffusivity
Surface diffusivity was estimated by the following equation
[24]:
4
dðtÞ kT
Ds ¼
ð1Þ
32ct
a4
Fig. 3. SEM morphology of Ti60Cu39Ag1 amorphous alloys after dealloying in 0.03 M
(a) and 0.13 M HF (b) solutions for 43.2 ks at 298 K.
where k is the Boltzmann constant (1.3806 ꢂ 10ꢀ23 J Kꢀ1),
c
is sur-
face energy, t is the dealloying time (43,200 s), d(t) is the pore size
at t, T is the temperature (298 K), and
a is the lattice constant. The
surface energy was chosen to be 1.79 J mꢀ2 for Cu [13] since minor
additions had a very limited influence on the surface energy.
The calculated data of surface diffusivities is listed in Table 1.
The addition of Ag resulted in a decrease in the surface diffusivity
by approximately two orders, with changes from 2.80 ꢂ 10ꢀ17 to
8.17 ꢂ 10ꢀ19 m2 sꢀ1 in 0.13 M HF solution. Larger amounts of Ag
addition did not result in a marked decrease in surface diffusivity.
It should be acknowledged that these results differed from those
of Cu and Ag. Large grain boundaries were clearly detected. The grain
sizes were confirmed to be about 20 nm. The Ag distribution prefer-
entially occurred in the grain boundary except the uniform distribu-
tion in the grains, as shown in Fig. 6c and d. This indicates that a Cu–
Ag solid solution formed inside the grains and some Ag particles
preferentially precipitated on the grain boundary. The TEM and
XRD analysis proved the existence of an Ag phase and Cu–Ag solid
solution.
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Z. Dan et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx
Fig. 5. BFI image (a), SADP (b), HRTEM images (c) and HRTEM on twin boundary (d) and corresponding FFT pattern (e) of Ti60Cu39Ag1 amorphous alloys after dealloying in
0.03 M HF solution for 43.2 ks at 298 K.
Fig. 6. TEM image (a), ADF TEM image (b) and elemental distribution of Cu (c) and Ag (d) at the corresponding area on Ti60Cu39Ag1 amorphous alloys after dealloying in
0.03 M HF solution for 43.2 ks at 298 K.
Table 1
Lattice constants and surface diffusivity of as-dealloyed Ti60Cu40, Ti60Cu39Ag1 and Ti60Cu38Ag2 amorphous alloys in 0.03 M and 0.13 M HF solution after immersion of 43.2 ks at
298 K.
Alloy system
Ti60Cu40
HF concentration (mol/l)
Lattice constant* a (nm)
Surface diffusivity Ds (m2 sꢀ1
)
0.03
0.13
0.3615
0.3613
2.47 ꢂ 10ꢀ18
2.80 ꢂ 10ꢀ17
Ti60Cu39Ag1
Ti60Cu38Ag2
0.03
0.13
0.3613
0.3615
2.26 ꢂ 10ꢀ19
1.46 ꢂ 10ꢀ19
0.03
0.13
0.3623
0.3627
1.81 ꢂ 10ꢀ19
8.17 ꢂ 10ꢀ19
*
Calculated from XRD data.
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Z. Dan et al. / Journal of Alloys and Compounds xxx (2013) xxx–xxx
5
obtained in our earlier research on the refinement of nanoporous
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4. Conclusions
Cost-effective NPCs with a pore size of 8–55 nm was achieved
by modifying the surface diffusivity by minor Ag addition. Amor-
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0.03 M and 0.13 M HF solutions under free immersion conditions.
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This research is partially supported by the Ministry of Educa-
tion, Culture, Sports, Science, and Technology (MEXT) through a
Grant-in-Aid for Young Scientists (B) under Grant No. 24760567.
Please cite this article in press as: Z. Dan et al., J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2013.01.087