Electrochemical deposition of macroporous platinum, palladium and cobalt films using polystyrene latex sphere templates

Article abstract of DOI:10.1039/b004398m

Highly ordered macroporous films of platinum, palladium and cobalt with regular arrays of spherical pores with diameters of 0.40, 0.70 or 1 μm are prepared by electrochemical deposition into the interstitial spaces of a template formed by polystyrene latex spheres self-assembled on gold electrodes; after deposition of platinum, palladium or cobalt, the polystyrene spheres are fully removed by washing in toluene to leave a highly periodic, hexagonal close packed, interconnected network of monodisperse spherical pores within the metal film, the size of which is determined by the diameter of the polystyrene latex particles used to prepare the template.

Full text of DOI:10.1039/b004398m

Electrochemical deposition of macroporous platinum, palladium and cobalt
films using polystyrene latex sphere templates
Philip N. Bartlett,* Peter R. Birkin and Mohamed A. Ghanem
Chemistry Department, University of Southampton, Highfield, Southampton, UK SO17 1BJ.
E-mail: P.N.Bartlett@soton.ac.uk
Received (in Cambridge, UK) 1st June 2000, Accepted 27th July 2000
Highly ordered macroporous films of platinum, palladium
and cobalt with regular arrays of spherical pores with
diameters of 0.40, 0.70 or 1 mm are prepared by electro-
chemical deposition into the interstitial spaces of a template
formed by polystyrene latex spheres self-assembled on gold
electrodes; after deposition of platinum, palladium or cobalt,
the polystyrene spheres are fully removed by washing in
toluene to leave a highly periodic, hexagonal close packed,
interconnected network of monodisperse spherical pores
within the metal film, the size of which is determined by the
diameter of the polystyrene latex particles used to prepare
the template.
metal deposition is achieved in a few minutes rather than over
36 h; fifth, the template can be removed by dissolving the
polystyrene in toluene rather than requiring the use of HF.
Scanning electron micrographs of cobalt films prepared by
electrochemical deposition of cobalt at 20.90 V vs. SCE
through a template formed from 0.40 and 1.0 mm diameter
polystyrene latex spheres are shown in Figs. 1(a) and 1(b)
respectively. The quantities of charge passed were 20.80 and
21.15 C cm22 respectively. These values were chosen in order
to grow films whose thickness corresponded to the radius of the
template particles used in each case. Fig. 1(a) shows that the
cobalt film made using the 0.40 mm polystyrene latex sphere
template has a highly ordered two dimensional hexagonal
network of monodisperse submicron voids each with a
hemispherical shape. In the most ordered regions, the pores
have a diameter of 380 ± 10 nm (n = 10) and pore to pore centre
separation of 450 ± 10 nm (n = 30) The cobalt films made by
using the larger, 1.00 mm, polystyrene latex spheres [Fig. 1(b)]
have a pore diameter of 980 ± 10 nm (n = 10), however, this
film shows less two-dimensional order. We attribute this to the
fact that the larger latex spheres take a longer time to assemble
into a highly ordered structure.21
The production of materials with micron and submicron scale
structure in two and three dimensions is of importance in a range
of applications, such as photonic materials,1,2 high density
magnetic data storage devices,3 microchip reactors4 and
biosensors.5 Several new templating techniques are being
developed to achieve such macroporous solids with highly
ordered structure and high surface area.6–11 Highly ordered
three-dimensional macroporous oxides with pore diameters of a
few hundred nanometers have recently been prepared by the
chemical hydrolysis of the corresponding metal alkoxide in the
interstitial spaces of close packed arrays of polystyrene latex
spheres as the template.12–14 Macroporous titanium dioxide
photonic crystals synthesised by a similar procedure have been
shown to exhibit photonic band gaps.15,16 In related studies
other authors have described the synthesis of three-dimensional
microporous films of semiconducting cadmium selenium
(CdSe) by electrochemical deposition in the interstitial spaces in
a close packed array of polystyrene latex spheres assembled on
an indium tin oxide substrate surface.17 Van Duyne et al.18,19
have reported using monolayers or bilayers of polystyrene latex
particles as masks for the deposition by evaporation of two-
dimensional arrays of isolated silver nanoparticles on an
insulating substrate.
Fig. 1(c) shows a scanning electron micrograph of a platinum
film deposited into a template formed from 0.70 mm polystyrene
latex spheres at a deposition potential of 0.10 V vs. SCE (total
charge passed 21.20 C cm22). The image shows that the
electrochemically deposited platinum film consists of a highly
Here we report the preparation of highly ordered macro-
porous metal films with pores of predetermined sizes. The
preparation of the structured macroporous films was carried out
by the electrochemical reduction of [PtCl6]22, [PdCl4]22, or
[Co(Ac)2] complex ions in aqueous solution within the
interstitial spaces of a close packed polystyrene latex sphere
template, self-assembled on a gold surface. The thickness of the
resulting films can be many multiples of the diameter of the
polystyrene latex spheres used to prepare the template and is
readily controlled by the amount of charge passed during
deposition of the metal film.†
Most recently Xu et al. have described the electrochemical
deposition of nanoscale Ni and Au meshes through templates
made from close-packed silica sphere arrays (opal).20 Although
the two approaches give similar metal meshes there are several
important differences between the method described in our
work over that of Xu et al. First, the polystyrene latex particles
we use are commercially available in a range of sizes; second,
our close-packed templates can be prepared by evaporation in
2 d rather than by sedimentation over a period of several
months; third, there is no need to sinter the template; fourth, the
Fig. 1 Scanning electron micrographs of macroporous platinum and cobalt
films electrochemically deposited, at 0.10 and 20.90 V vs. SCE
respectively, through templates formed using polystyrene latex spheres pre-
assembled on gold electrode surfaces. (a) Cobalt film, deposition charge
20.85 C cm22, polystyrene latex sphere diameter 0.40 mm; (b) cobalt film,
deposition charge 21.15 C cm22, polystyrene latex sphere diameter 1.0 mm;
(c) platinum film, deposition charge 21.20 C cm22, polystyrene latex
sphere diameter 0.70 mm; (d) cross-sectional image of a platinum film,
deposition charge 21.00 C cm22, polystyrene latex sphere diameter
0.40 mm.
DOI: 10.1039/b004398m
Chem. Commun., 2000, 1671–1672
This journal is © The Royal Society of Chemistry 2000
1671

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to leave a regular array of interconnected spherical voids. The
size of these voids is determined by the size of the polystyrene
latex particles used. Since polystyrene latex particles of tightly
controlled size distribution are readily commercially available
in sizes from 0.05 to 90 mm the size of the voids can be readily
controlled. Control over the quantity of charge passed in the
electrochemical deposition of the film allows control over the
final film thickness. Thus this method represents a simple route
to the production of ordered macroporous films of metals, metal
alloys, and polymers with potentially interesting and useful
photonic, catalytic, magnetic, or other properties.
We thank the Embassy of the Arab Republic of Egypt,
Educational and Cultural Bureau, London for their support of
Mohamed A. Ghanem. We also thank Alastair Clark for
assistance in obtaining the SEM images and manufacture of the
gold substrates.
Notes and references
† Experimental: Cobalt acetate [Co(Ac)2·4H2O, 99.5%] and potassium
acetate (KAc, 99.4%) were obtained from Fluka. Hexachloroplatinic acid
and diammonium tetrachloropalladate were obtained from Aldrich (purity
99.99%). 0.5 wt% suspensions of 0.40 and 1.00 mm diameter polystyrene
latex spheres in water were obtained from Agar Scientific, 2.5 wt%
suspensions of 0.70 mm diameter polystyrene latex spheres in water were
obtained from Alfa Asear. The working electrodes were prepared by
evaporating 10 nm of a chromium adhesion layer followed by 200 nm of
gold onto thin glass microscope slides. A large area platinum gauze was
used as the counter electrode and home made saturated calomel (SCE) as the
reference electrode. Before use, the gold working electrodes were sonicated
in propanol for 1 h and then rinsed with deionised water. A volume of ca.
0.3 cm3 of the suspension of polystyrene latex spheres diluted with water to
0.5 wt% was spread over ca. 1 cm2 area of the gold electrode and allowed
to dry slowly over 2 d in a controlled humidity chamber. The working
electrode, together with the adherent, dry, polystyrene latex bead template,
was dipped in to the appropriate electrolyte solution to deposit the metal
film. The deposition solutions were either 50 mmol dm23 H2PtCl6, 40 mol
dm23 (NH4)2PdCl4, or 0.1 mol dm23 Co(Ac)2 with 0.1 mol dm23 KAc. The
deposition was carried out potentiostatically at potentials of 0.10, 0.25 or
20.90 V vs. SCE for the electrochemical deposition of platinum, palladium,
or cobalt. After deposition the polystyrene latex spheres were dissolved out
of the metal films by soaking in toluene for 24 h.
Fig. 2 Scanning electron micrographs of macroporous films of platinum,
palladium and cobalt electrochemically deposited, at potentials of 0.10, 0.25
and 20.90 V vs. SCE respectively, through templates formed using
polystyrene latex spheres pre-assembled on gold electrode surfaces. (a)
Platinum film, deposition charge 22.00 C cm22, polystyrene latex sphere
diameter 0.40 mm; (b) cobalt film, deposition charge 21.40 C cm22
,
polystyrene latex sphere diameter 0.40 mm; (c) cross-sectional image of a
platinum film, deposition charge 21.50 C cm22, polystyrene latex sphere
diameter 0.40 mm; (d) palladium film, deposition charge 21.15 C cm22
polystyrene latex sphere diameter 0.70 mm.
,
periodic hexagonal array of monodisperse pores. The pore to
pore centre separation in this case was 770 ± 10 nm (n = 30).
Fig. 1(d) shows a cross sectional image of a platinum film
prepared by deposition of 21.00 C cm22 through a template of
0.40 mm polystyrene latex spheres. The formation of a
monolayer of spherical pores embedded in the platinum film
(which in this case was grown to a thickness of approximately
0.4 mm) can be clearly seen.
Figs. 2(a) and 2(b) show scanning electron micrographs of
thicker films of platinum and cobalt where sufficient charge has
been passed to deposit films several multiples of the polystyrene
latex sphere diameter in thickness. The micrographs show that
the spherical pores left in the platinum or cobalt films after
removal of the polystyrene latex spheres are arranged in a well
ordered three dimensional hexagonal close packed structure. In
addition, the connections between the spherical voids within the
film, formed where the polystyrene latex spheres in one layer
were in contact with those in the underlayer, can be seen as the
three dark areas within each pore. In Fig. 2(a) the mouths of the
pores have a rounded triangular shape. This appears to be a
regular feature of films which are grown to a thickness
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1, 2, …. Figs. 2(c) and 2(d) show cross-sectional images of
platinum and palladium films prepared using templates formed
from 0.40 and 0.70 mm polystyrene latex spheres. These
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