Full text of DOI:10.1007/s003390100872

Appl. Phys. A 72, 729–733 (2001) / Digital Object Identifier (DOI) 10.1007/s003390100872
Applied Physics A
Materials
Science & Processing
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Growth of iron single crystals in the etched ion tracks of polymer foils
∗
D. Dobrev, J. Vetter, N. Angert, R. Neumann
Gesellschaft für Schwerionenforschung Darmstadt (GSI), Planckstrasse 1, 64291 Darmstadt, Germany
Received: 20 February 2001/Accepted: 21 February 2001/Published online: 3 May 2001 –  Springer-Verlag 2001
Abstract. Pulse reverse electrolysis in an ultrasonic field is
used to grow iron single crystals of micron size in templates
formed by etching the tracks of swift ions in polymer foils.
High-grade crystals are produced from high-temperature fer-
rous chloride baths. The crystals are oriented along their
deposition potential. Both factors sustain the single-crystal
growth as well.
Since iron is a ferromagnetic metal, its single crys-
tals are of particular interest. Their magnetic properties are
anisotropic, i.e. they depend on the crystallographic orienta-
tion. In the past, ferromagnetic metals have been deposited
in the pores of nuclear track membranes to realize verti-
cal magnetic recording media [12, 13]. For the same pur-
pose, electrodeposition of such materials has been applied
to nanoporous anodic alumina templates [14–16]. The dense
oxide barrier between porous aluminum oxide and metallic
aluminum restricts the use of porous alumina. Nevertheless,
due to the rectifying properties of the barrier layer, even AC
electrolysis can be applied. DC electrolysis in this case re-
quires very high voltages up to 60 V. At the initial stage in
this case, iron grew in the Al2O3 barrier layer as a fine glove-
like grain structure that later amalgamated in one needle, in
some cases forming a single crystal.
In this work, the reverse current electrolysis in an ultra-
sonic field was used to grow iron single crystals in etched
ion tracks of polymer foils. The electroplating of iron requires
high overvoltage and is normally accompanied by evolution
of hydrogen. In the case of anode polarization, oxidation
can also take place simultaneously with dissolution of the
metal, and this can result in the formation of oxide passi-
vation films. Therefore, non-stationary electrolysis is usually
brought about by a periodic interruption of the deposition
current. If an anode polarization is applied, the anode pulses
should be short and of low current density [17].
ꢀ
110ꢁ, ꢀ100ꢁ, and ꢀ111ꢁ crystallographic axes. Their orienta-
tion turns out to depend on supersaturation during the grow-
ing process. At low overvoltages of deposition, ꢀ110ꢁ and
ꢀ
100ꢁ orientations are observed. The crystals of ꢀ111ꢁ orien-
tation appear more frequently at higher cathode pulse current
density. The crystals possess prominent resistance to corro-
sion.
PACS: 81.10.-h; 81.15Pq; 61.82.-d
In the near past, it was realized that structures of micrometer
or nanometer dimensions exhibit novel physical and chem-
ical properties due mainly to their restricted size. This opened
a large new field of basic research as well as possible ap-
plications, in which electrochemical deposition became an
increasingly attractive method for the synthesis of new ma-
terials based on micro- and nanostructures. Possin [1] was
the first who deposited by this method tin, indium, and zinc
in the etched single-particle tracks in natural mica. Later, for
different purposes electrodeposition was applied with other
metals that are readily plated from aqueous solutions [2–6].
Recently, the method developed for replication of etched ion
tracks in polymer foils by electrodeposition [7, 8] was used
to grow single crystals of copper [9] and gold [10] using re-
verse pulse current in an ultrasonic field. Elaborate reverse
current pulses favored the growth of perfect crystals by re-
moving the defect sites from the growing front during the
anode polarization. The vigorous agitation of the electrolyte
led to a significant decrease of deposition overvoltage in the
ultrasonic field. As discussed in [9] and investigated in [11],
elevation of the electrolyte temperature also decreases the
1 Experimental
All experiments have been carried out with an electrolysis cell
described in detail in [7, 8]. In this work, an anode from low-
carbon steel was used. The anode-to-cathode area ratio was
about 20 : 1. The principal experimental procedure has been
illustrated in [9]. A diagram of the particular successive steps
of iron electrocladding is shown in Fig. 1. Polybisphenol-A
carbonate foils (Makrofol-N, Bayer Leverkusen) of different
thicknesses up to 60 µm and 50 mm in diameter were irra-
∗
Corresponding author.
(Fax: +49-6159/71-3266, E-mail: r.neumann@gsi.de)

730
Fig. 1. Diagram of iron electrocladding in the etched nuclear pores of
a polymer membrane
diated at the UNILAC linear accelerator of GSI. Heavy ions
accelerated to energies in the range of 11.4–14 MeV/u were
5
6
2
used at fluences between 10 and 10 ions/cm . The latent
ion tracks were etched in a 6-M/l solution of NaOH contain-
◦
ing 10% of methanol at 40 C, until the diameter of the holes
Fig. 2. Reverse voltage and current pulses used for iron electrodeposition
reached the desired size. A thin gold film was sputtered onto
one side of the porous membrane. Subsequently, the mem-
brane was inserted into the plating cell, the conductive layer
facing the anode. The bright copper bath Cupatierbad (Riedel
Company) was used as electrolyte. A 20-µm-thick copper
2 Results
Only iron single crystals that were grown from the second
electrolyte are considered in the following, since the growth
from the ferrous ammonium sulfate was not satisfactory. Fig-
ure 3 shows the tops of differently oriented crystals embedded
in the etched nuclear pores of the polycarbonate foil. The
layer was deposited on the gold film at a current density of
2
5
0 mA/cm and at room temperature. Thus, the pore orifices
were completely closed from this side. Afterwards, the mem-
brane was turned upside down, and iron was deposited in the
pores from the open sides by pulse reverse plating in the ul-
trasonic field. Two classical electrolyte baths were tried [18]:
(
1) Ferrous ammonium sulfate (Moor salt): FeSO4 ·
(
NH4)2 ·SO4 ·6H2O – 250 g/l, operating at pH = 2.8, bath
◦
2
temperature 55 C, and optimal current density 20 mA/cm .
2) Iron (II) and calcium chloride (Fischer–Langbein) so-
(
lution: FeCl2 – 300 g/l, CaCl2 – 300 g/l, operating within
◦
pH = 1−1.5, at bath temperature 90 C, and optimal current
2
density 60 mA/cm .
For the alternating current deposition, rectangular reverse
pulses were applied to the plating cell from a function gen-
erator, type Voltcraft MS 9140, through an amplifier. The
cathode/anode potential ratio was selected in order to ensure
a 2 : 1 current ratio keeping the cathode pulse current density
2
between 3 and 5 mA/cm . The negative/positive polarization
was maintained over 4/1 s. A schematic diagram of the volt-
age pulses and the corresponding electric current through the
cell is depicted in Fig. 2. The galvanic cell was sunk into
an ultrasonic bath filled with water. The Bandelin Sonorex
bath of the type RK255H used here was operated at 35 kHz
with a high-frequency power of 160/320 W. After comple-
tion of the iron deposition, the host polycarbonate membranes
were dissolved in CH2Cl2. In this way, freestanding single
crystals were prepared on the metal substrate. The structures
produced as described above were imaged with a scanning
electron microscope, type Philips XL30. The iron crystals
were also characterized by energy-dispersive X-ray (EDX)
analysis using the EDX facility of this microscope.
Fig. 3. Iron single crystals embedded in the polymer foil near the orifices of
the nuclear pores

731
samples were covered with a thin gold layer by sputtering, to
avoid electrostatic charging under the incident electron beam
while imaging them in the microscope.
Bare iron single crystals as appearing after dis-
solving the foil are presented in Figs. 4–6. The crystals
in Figs. 4 and 5 were grown by applying a voltage of
2
1
10 mV and a current density of 3 mA/cm during the cath-
◦
ode pulse. The bath temperature was kept constant at 90 C.
The needle-shaped crystals deposited in these conditions are
oriented with their symmetry axes predominantly along the
ꢀ
110ꢁ or ꢀ100ꢁ crystallographic directions. Crystals having
only ꢀ110ꢁ orientation are collected in Fig. 4, while exam-
ples representative of the ꢀ100ꢁ orientation are shown in
Fig. 5. In Fig. 6, all crystals are ꢀ111ꢁ-oriented. Crystals of
this orientation are more frequently observed in the sam-
ples when the cathode pulse current density is increased to
2
5
mA/cm .
EDX spectra, taken from different places, which are
Fig. 6. Iron single crystals oriented along the ꢀ111ꢁ crystallographic axis
marked with crosses on the single crystal in Fig. 7a, are iden-
tical. One such spectrum is shown in Fig. 7b. No oxygen
was observed on different crystal planes after long-time stor-
Fig. 7a,b. EDX analysis of iron single crystal. a The sites of EDX analysis
on different facets are marked with crosses. b An EDX spectrogram typical
of all three analyzed sites. No oxygen line was observed in the spectra after
long-time storage. The small Cu peak originates from the sample substrate
Fig. 4. Iron single crystals oriented along the ꢀ110ꢁ crystallographic axis
age of the samples under ambient conditions in a chemical
laboratory.
3
Discussion
Iron belongs to a group of transition metals that possess a low
exchange rate with solutions containing their ions. The large
energy needed for the ion discharge and possibly a slow crys-
tallization process are responsible for the high overvoltage
of iron deposition [19]. The latter brings about a simultan-
eous hydrogen evolution that provokes complications in the
electroplating process. The rates of iron deposition and disso-
lution strongly depend on pH, temperature of the electrolyte,
and current density. An elevation of the bath temperature
◦
above 60–70 C results in a steeper decrease of the overvolt-
age of iron deposition rather than in a reduction of the hydro-
gen discharge. This increases the iron current yield [20]. The
Fig. 5. Iron single crystals oriented along the ꢀ100ꢁ crystallographic axis

732
+
discharge of H3O accompanying iron deposition elevates
the pH value at the electrode surface. With certainty, hydrol-
ysis of iron salts occurs even in relatively acid media and
rhombododecahedral habitus. Near to the entrance orifices,
low-index singular crystallographic planes confine the free
growing front. In the bcc lattice of iron, these are the planes
(111), (100), and (110) in an order of increasing reticular
density of the atoms. Vicinal higher-index planes have oc-
casionally also been observed, but singular planes prevail in
total. This permitted us to use the habitus of the crystals to
determine their orientation. The predominant orientation of
the crystals depends on the deposition conditions, as is ob-
vious from the experiments. At low overvoltage, tantamount
to low supersaturation of deposition, iron crystals develop in
directions normal to the two most densely packed (110) and
(100) planes. They are ꢀ110ꢁ and ꢀ100ꢁ directions having the
lowest growth rates. The increase of current density, i.e. over-
voltage of deposition, leads to an orientation along the most
loosely packed ꢀ111ꢁ direction with the highest growth rate
due to the crystal anisotropy. This result is in accord with
earlier findings for the texture formation in electrolytically
plated cobalt [23], iron [24, 25], copper [26], silver [27–30],
and tin [31] as well as in vapor-deposited samples of sil-
ver [32, 33], iron [34], and tin [35] on indifferent substrates.
The remarkable corrosive stability of the iron single crys-
tals produced in this work should be specially emphasized.
+
FeOH is formed in significant concentrations. It can be ad-
sorbed on the cathode and contribute to the delayed discharge
of the iron ions. Also, at pH < 6, corrosion processes occur
at potentials close to the stationary ones [21]. A spontaneous
metal oxidation also takes place due to the oxidants present
in the solution. The formation of oxide barriers as well as ad-
sorption of hydrolysis products can passivate the cathode. If
applying a reverse anode potential, the oxide passivation be-
comes of major importance. Primarily, on the surface of most
metals, oxygen is adsorbed creating chemical bonds with the
metal. Chemisorbed oxygen serves as an intermediate layer
for the formation of a new oxide phase. When the formation
of metal ions in the solution breaks down, the potential shifts
to positive values and a new anode process starts in accord
with the more positive potential. During anode polarization
when the potentials are not sufficient to produce free oxygen,
−
the OH ions react directly with metal and form either oxides
or other adsorbed layers. In the presence of Cl , MeClm or
−
−
Me|Clm (absorbed Cl ) layers are created at less positive po-
tentials and prevent the formation of the oxygen barriers. The
metal chlorides themselves do not possess protective proper-
ties because of their high solubility [13]. Thus, the successful
deposition of iron single crystals from the FeCl2-CaCl2 elec-
trolyte can be explained by the depassivation properties of
4 Conclusion
−
Cl ions. Such a depassivation effect does not take place dur-
Iron single crystals can be grown by electrochemical deposi-
tion in etched ion tracks of polymer foils used as templates.
High-grade crystals are produced from concentrated chloride
baths containing ferrous ions. The single-crystal growth is
favored by vigorous ultrasonic agitation, elaborated reverse
current pulses, and an elevated temperature. Near the pore
orifices, the growing forms are confined by singular crystal-
lographic planes.
The predominant orientation of the crystals appears to
depend on the applied electric potential during the cathode
current pulses. At low potentials, the crystals are oriented
along their ꢀ110ꢁ and ꢀ100ꢁ crystallographic axes. The crys-
tals with ꢀ111ꢁ orientation appear more frequently at higher
current densities, i.e. at higher potential. Thus, the preferred
orientation of the crystals can be controlled by the choice of
proper overvoltage of electrodeposition. The bare crystals are
resistant to corrosion in ambient conditions.
ing the deposition from the Moor salt electrolyte, which does
−
not contain any Cl ions.
On the other hand, iron deposition or dissolution rates
are strongly influenced by the stability of the metal surface,
which also depends on different adsorbed species. Princi-
pally, the acceleration of electrochemical processes in an ul-
trasonic field is reached mainly by a decrease of the overvolt-
age of the metal deposition. The concentration polarization
is diminished when homogenizing the electrolyte concentra-
tion by vigorous agitation. The diffusion layer thins dramati-
cally and ensures a high transport rate of ions. The chemical
polarization can be affected as well [22]. Surfactants and im-
purities caught on the cathode are removed, thus increasing
the active surface suitable for deposition. Also degassing of
the electrolyte increases quickly, removing the gas bubbles
produced by the hydrogen evolution.
The short inversion of the current from cathode to an-
ode polarity preferentially removes the metal from areas that
tend to become overplated during the cathode cycle. It is thus
possible to considerably retard the dendrite formation. Co-
deposited hydrogen is also removed during the inversion of
the current. Within the off-time period recrystallization can
also take place.
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