Electrodeposition of Co-P films from alkaline electrolytes

Article abstract of DOI:10.1149/1.2988132

We study the influence of solution chemistry on the electrodeposition of Co-P from a citrate-glycinate alkaline electrolyte. The concentration of different cobalt complexes is calculated from chemical equilibria as a function of solution pH to examine the strength of the complexing agents, and compared with cyclic voltammetry (CV) data. CV and electrochemical quartz crystal microbalance are utilized to elucidate the effect of different P sources (NaH₂PO₂ or H₃PO₃) on the electrodeposition process. The influence of plating conditions as well as different P sources on the deposit composition, microstructure, and magnetic properties are investigated. Both citrate and glycine are good complexing agents for cobalt, the former having a stronger effect. The addition of NaH ₂PO₂ depolarizes cobalt reduction, while P codeposition from solutions containing H₃PO₃ is negligible. Pure cobalt and Co-P films grown from the H₃PO₃ source display a hexagonal close-packed (hcp) crystal structure and soft magnetic properties. The Co-P films grown from solutions containing NaH₂PO₂ display an amorphous structure with a P content above ～10 atom % in the deposit. Only when the plating current is small (<5 mA cm2) and the P content is low the films are crystalline with an hcp structure. These latter films display in-plane magnetic anisotropy and a relatively high coercivity.

Full text of DOI:10.1149/1.2988132

Electrodeposition of Co−P Films from Alkaline Electrolytes
X. Xu and G. Zangari
J. Electrochem. Soc. 2008, Volume 155, Issue 12, Pages D742-D749.
doi: 10.1149/1.2988132
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D742
Journal of The Electrochemical Society, 155 ͑12͒ D742-D749 ͑2008͒
0013-4651/2008/155͑12͒/D742/8/$23.00 © The Electrochemical Society
Electrodeposition of Co–P Films from Alkaline Electrolytes
X. Xua,b, and G. Zangari
*
a,c,**,z
aCenter for Electrochemical Science and Engineering, bDepartment of Chemical Engineering, and cDepartment
of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA
We study the influence of solution chemistry on the electrodeposition of Co–P from a citrate–glycinate alkaline electrolyte. The
concentration of different cobalt complexes is calculated from chemical equilibria as a function of solution pH to examine the
strength of the complexing agents, and compared with cyclic voltammetry ͑CV͒ data. CV and electrochemical quartz crystal
microbalance are utilized to elucidate the effect of different P sources ͑NaH2PO2 or H3PO3͒ on the electrodeposition process. The
influence of plating conditions as well as different P sources on the deposit composition, microstructure, and magnetic properties
are investigated. Both citrate and glycine are good complexing agents for cobalt, the former having a stronger effect. The addition
of NaH2PO2 depolarizes cobalt reduction, while P codeposition from solutions containing H3PO3 is negligible. Pure cobalt and
Co–P films grown from the H3PO3 source display a hexagonal close-packed ͑hcp͒ crystal structure and soft magnetic properties.
The Co–P films grown from solutions containing NaH2PO2 display an amorphous structure with a P content above ϳ10 atom %
in the deposit. Only when the plating current is small ͑Ͻ5 mA/cm2͒ and the P content is low the films are crystalline with an hcp
structure. These latter films display in-plane magnetic anisotropy and a relatively high coercivity.
© 2008 The Electrochemical Society. ͓DOI: 10.1149/1.2988132͔ All rights reserved.
Manuscript submitted July 17, 2008; revised manuscript received August 25, 2008. Published October 7, 2008.
Co–P alloy films have been extensively investigated due to their
composition, and crystal structure of the films were investigated by
scanning electron microscopy ͑SEM͒, energy-dispersive X-ray spec-
troscopy ͑EDS͒, and X-ray diffraction ͑XRD͒ techniques, respec-
tively.
interesting magnetic, electrical, and diffusion barrier properties. For
example, they have been studied as hard magnetic materials for
longitudinal recording,1-3 as soft magnetic materials,4 and as diffu-
sion barrier layers in microelectronic devices.5
Co–P alloys have mostly been grown by electroless deposition
from acidic or alkaline solutions.3,5-7 In these instances, NaH2PO2 is
used as the reducing agent as well as the phosphorus ͑P͒ source. On
a catalytic surface, such as Co or Ni, H2PO−2 is oxidized to H2PO−3
and releases electrons, which in turn reduce metal ions to form a
metallic film. This reduction process is accompanied by hydrogen
evolution and, in parallel, by phosphorus reduction and incorpora-
tion in the film. Amorphous films are obtained when P content is
Ͼϳ10 atom %.
Co–P alloys can also be electrodeposited from acidic electro-
lytes; in this case, H3PO3 is used as a P source.8,9 Phosphorus can be
codeposited only in parallel with the reduction of a transition metal
͑Fe, Co, Ni͒; this is an example of induced deposition according to
Brenner. The detailed mechanism of this process is not yet under-
stood. The P content as well as the microstructure of the deposits
can be controlled by varying the pH value, concentration of H3PO3,
temperature, as well as plating current density.9 The codeposition of
P results in a decrease of the grain size of the film, film amorphiza-
tion ͑above 13 atom % P9͒, and changes in preferred orientation. For
example, Fukunaka reported a change in preferred orientation of the
hexagonal close-packed ͑hcp͒ structure from ͑10.0͒ to ͑00.2͒ with
increasing P content.9
Few reports are available on the electrodeposition of Co–P or
Co–Ni–P films from neutral or alkaline electrolytes, where
NaH2PO2 is chosen as the P source10-14 and complexing agents are
added to stabilize metal ions in the solution. We are not aware of any
studies on the role of different complexing agents, nor on the effect
of NaH2PO2 on the deposition process.
Electrolyte Solution Chemistry
Electrolytes and complexing agents.— Table I shows the com-
position of the electrolytes used for Co and Co–P electrodeposition.
Three solutions were used. Solution A, for Co electrodeposition,
contains 0.1 M Co͑SO3NH2͒2, 0.1 M ͑NH4͒2C6H6O7, and 0.1 M
NH2CH2COOH. Solutions B and C contain, in addition to the re-
agents in A, 0.1 M H3PO3, or 0.1 M NaH2PO2, respectively.
The pH of the solution was kept at 8 by the addition of NaOH,
unless pH effects were studied. The electrolyte is derived from an
electroless Co–P plating bath described in Ref. 15, with the pH
decreased from 10 to 8 to avoid electroless deposition. Ammonium
citrate ͓͑NH4͒2C6H6O7͔ is a derivative of citric acid, an organic
compound containing three carboxylic groups, capable of binding
transition metal ions, thus preventing their precipitation as hydrox-
ides in alkaline environments. Co in fact would precipitate as
Co͑OH͒ ͑solubility constant Ksp = 5.92 ϫ 10−15͒ in alkaline con-
2
ditions. The ammonium ion produced by dissociation of the citrate
salt may also contribute to Co2+ complexation. Finally, glycine
H2N–CH2COOH is an amino acid that complexes transition metals,
though less efficiently than citrate.
Chemical equilibria as a function of solution pH.— Relevant equi-
libria in the Co and Co–P electrolyte, and the corresponding equi-
librium constants ͑obtained from Ref. 16͒ are reported in Table II.
Co2+ tends to associate with citrate and ammonia according to the
equilibria Eqs. 1, 2, and 6. Such equilibria also depend on the dis-
sociation of citrate and ammonia ͑Eqs. 3–5, 7͒, which are a function
of solution pH. Co2+ associates with glycine as well ͑Eqs. 8–10͒, the
ionic form of which depends on solution pH ͑Eqs. 11, 12͒. Equilib-
ria related to the P-containing species have been neglected. The
equilibria listed in Table I were solved simultaneously and the con-
centration of the various Co species was calculated as a function of
pH. The results are shown in Fig. 1.
In this paper, the electrodeposition of Co–P alloys from alkaline
baths containing diammonium citrate and glycine as complexing
agents for cobalt ions was studied. The complexation strength of
each chemical compound was examined by cyclic voltammetry
͑CV͒ and the concentration of different complexes was calculated
according to the possible association equilibria. Two P sources, so-
dium hypophosphite ͑NaH2PO2͒ and phosphorous acid ͑H3PO3͒,
were used to study the influence of processing parameters, such as
pH and plating current, on the deposition process. Film morphology,
It is found that the main cobalt species present in solution is
CoC6H5O−, independent of pH. With increasing pH, the association
between c7obalt and glycine is enhanced and saturates around pH 9;
the concentration of free Co2+ continuously decreases and becomes
constant at about 2 ϫ 10−5 M above pH 9. The association between
NH3 and Co2+ strengthens drastically with increasing pH, yet the
2+
6
concentration of Co͑NH3͒ remains more than one order of mag-
*
Electrochemical Society Student Member.
Electrochemical Society Active Member.
nitude smaller than that of other complexes. Above pH 9 ͓Co2+͔ is
**
z E-mail: gz3e@virginia.edu
higher than its solubility limit and Co͑OH͒ will tend to precipitate.
2
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Journal of The Electrochemical Society, 155 ͑12͒ D742-D749 ͑2008͒
Table I. Composition of the electrolytes for Co–P electrodeposition.
D743
Solution
Co͑SO3NH2͒ ͑M͒
͑NH4͒2C6H6O7 ͑M͒
NH2CH2COOH ͑M͒
H3PO3 ͑M͒
NaH2PO2 ͑M͒
2
A
B
C
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0
0.1
0
0
0
0.1
Experimental
dm/dt
MCo
iCo
=
ϫ 2 ϫ 96,500
Electrochemical measurements.— The experiments were per-
formed in a three-electrode cell ͑300 mL volume͒, with the elec-
trodes placed in a vertical configuration. No stirring of the solution
was used. Polycrystalline gold films ͑100 nm thickness͒ sputtered on
glass using a first Cr adhesion layer ͑5 nm͒ were used as working
electrodes. A Pt mesh was used as the counter electrode. A saturated
calomel electrode ͑SCE͒, separated from the main compartment
through a Luggin capillary, was used as the reference electrode; all
potentials herein are referred to the SCE ͑E0SCE = 0.2412 VSHE͒. All
electrochemical measurements were carried out on an EG&G
Princeton Applied Research potentiostat-galvanostat model 283 at
room temperature.
Electrodeposition of Co and Co–P films was carried out at 65°C
from the electrolytes reported in Table I; the solution pH was kept at
8. A constant current density ͑cd͒ ranging from 5 to 100 mA/cm2
was applied for a predetermined time to study the effect of current
density on the P content, the structure, morphology, and magnetic
properties of the deposits. Film thickness was estimated by the
charge passed using Faraday’s laws, assuming pure Co and 100%
plating efficiency. CV was carried out to investigate the effect of the
various reagents present in the electrolyte.
where MCo = 58.9 g/mol, 2 is the number of electrons transferred
for the reduction of one cobalt ion ͑Co2+͒, and 96,500 is the Fara-
day’s constant. When little P is codeposited, this calculation is ac-
curate. With the largest P codeposition observed ͑ϳ15 atom %͒, the
Co partial current is overestimated by 9%.
Film characterization: Crystal structure, morphology, and mag-
netic properties.— To study the crystal structure of the deposits, a
XDS2000 Scintag instrument was used to collect XRD patterns in
the Bregg–Brentano geometry. The morphology and composition of
the deposited films were investigated by SEM and EDS using a
JEOL JSM-6700F instrument. The magnetic properties were mea-
sured by a vibrating sample magnetometer ͑ADE Technologies,
VSM model 886͒ at room temperature with the magnetic field ap-
plied parallel to the film plane, up to a maximum field of 5000 Oe.
Results and Discussion
Effect of complexing agents.— Figure 2 shows the CV curves
for Co deposition from solutions containing different complexing
agents. In the presence of glycine, a well-defined cobalt reduction
Electrochemical quartz crystal microbalance ͑EQCM, model
QCM922, Princeton͒ was utilized to deconvolute the partial current
for metal deposition from that for hydrogen evolution, thus deter-
mining the instantaneous efficiency of the deposition process. The
working electrode for EQCM was a 0.5 cm diameter, 9 MHz Au
resonator with an area of 0.196 cm2. The output from the EQCM is
the change in resonant frequency ⌬f of the Au resonator, which is
directly proportional to the mass change ⌬m
peak is present; deposition starts at a potential of about −0.8 VSCE
,
whereas hydrogen evolution reaction ͑HER͒ starts below about
−1 V. When either sodium citrate or ammonium citrate is added to
the Co salt, Co reduction is polarized to potentials similar to or
lower than those for HER onset, and the corresponding reduction
peak is hidden. The CVs observed in the presence of these two
reagents are similar, indicating that the main ligand associating with
cobalt is citrate, not ammonia.
− ⌬f ϫ A ϫ ͱ
The observed CVs are qualitatively in agreement with the chemi-
cal equilibria calculations. Citrate is a stronger complexing agent
than glycine and thus polarizes Co reduction to a more cathodic
potential. The effect of the cation ͑Na+ or NH4+͒ is to slightly change
the current density at a given potential; this is probably a conse-
quence of the larger grain size of films deposited in the presence of
ammonia,11 which increases the film roughness and effective elec-
trode area.
␮ ϫ ␳
q
2 ϫ F2q
q
⌬m =
Here, A is the quartz crystal surface area, ␮ is the AT-cut quartz
q
crystal constant ͑2.947 ϫ 1011 dyne/cm2͒, ␳ is the quartz crystal
q
density ͑2.648 g/cm3͒, and Fq is the reference frequency ͑9.00
MHz͒. The mass change is thus given directly by the following
equation
Figure 3 shows the effect of various combinations and concen-
tration of complexing agents on the Co reduction process. The curve
͑upright triangles͒ is from an electrolyte containing 0.1 M cobalt
sulphamate and 0.1 M glycine. When the concentration of glycine is
⌬m͑ng͒ = 1.608 ϫ ⌬f͑Hz͒
Partial currents were calculated by neglecting the contribution to the
sample weight from P codeposited using the equation
Table II. Equilibria and association constants (or deionization constants for acids). All the constants are from Ref. 16.
Complexing
agent
Association or
deionization
Equilibrium
constants
Equation
no.
Equilibria equations
Co2+ + C6H5O73− ⇔ CoC6H5O7−
Co2+ + C6H6O72− ⇔ CoC6H6O7
C6H8O7 ⇔ C6H7O−7 + H+
C6H8O7
Association
with Co2+
K = 6.76 ϫ 104
K = 1.55 ϫ 103
K1 = 7.42 ϫ 10−4
K2 = 1.75 ϫ 10−5
K3 = 3.99 ϫ 10−6
K = 7.94 ϫ 104
K = 5.6 ϫ 10−10
K = 4.36 ϫ 104
K = 2.88 ϫ 108
K = 6.46 ϫ 1010
K = 1.67 ϫ 10−10
K = 4.47 ϫ 10−3
1
2
3
4
5
6
7
8
9
Deionization
C6H7O−7 ⇔ C6H6O72− + H+
C6H6O27− ⇔ C6H5O37− + H+
2+
Co2+ + 6NH3 ⇔ Co͑NH3͒
NH3
Association
Deionization
Association
6
NH+4 ⇔ NH3 + H+
+
NH2CH2COOH
Co2+ + NH2CH2COO− ⇔ Co͑NH2CH2COO͒
Co2+ + 2NH2CH2COO− ⇔ Co͑NH2CH2COO͒
2
−
3
Co2+ + 3NH2CH2COOH ⇔ Co͑NH2CH2COO͒
NH2CH2COOH ⇔ NH2CH2COO− + H+
NH3CH2COOH+ ⇔ NH2CH2COOH + H+
10
11
12
Deionization
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D744
Journal of The Electrochemical Society, 155 ͑12͒ D742-D749 ͑2008͒
0.010
1
0.1M glycine
0.2M glycine
0.1M di-ammonium citrate
0.005
1E-5
0.1M glycine + 0.1M di-ammonium citrate
0.000
-0.005
-0.010
-0.015
Co2+
CoC6H6O7
1E-10
1E-15
1E-20
-
CoC6H5O7
Co-Gly+
Co(Gly)2
Co(Gly)3
2+
Co(NH3)6
Hydrogen
Evolution
Co Reduction
-0.9
-1.1
-1.0
-0.8
-0.7
-0.6
3
4
5
6
7
8
9
10 11 12 13
pH
E, vs SCE(V)
Figure 1. ͑Color online͒ Equilibrium concentration of different cobalt com-
plexes as a function of solution pH.
Figure 3. ͑Color online͒ CV at a Au electrode from solutions containing
0.1 M Co sulphamate and different combinations of complexing agents as
shown; scan rate 10 mV/s.
doubled ͑downward triangles͒, cobalt reduction is polarized further
and the peak current is smaller. The addition of 1:1 molar ratio of
citrate ͑solid circles͒ to the Co salt solution polarizes cobalt reduc-
tion into the HER region and the cobalt reduction peak correspond-
ingly disappears. Addition of glycine to the citrate-complexed bath
does not result in further polarization, although the concentration of
free Co2+ should be more than one order of magnitude smaller ac-
cording to the equilibria calculation. This is an indication that the
HER dominates in this potential region.
In the case of glycine, it is tempting to assign the decrease in the
Co reduction peak amplitude to a decrease in free Co2+ concentra-
tion. However, the peak amplitude is too large to be assigned to
reduction of the free cobalt ion. In fact, with the addition of 0.1 M
glycine, the concentration of the free cobalt ion is calculated to be
about 0.024 M. Assuming a reversible reduction process, the peak
current ip from a CV curve can be calculated by the fol-
lowing equation17
mately 10−5 cm2 s−1͒, C is the concentration of free Co2+ in the
*
bulk solution, and is the scan rate. The reduction peak current is
v
thus calculated to be 5.8 mA/cm2, about half the experimental value
from the CV curve ͑12 mA/cm2͒. Increasing the concentration of
glycine to 0.2 M, the calculated ip is 0.28 mA/cm2, more than an
order of magnitude smaller than the value from the CV curve
͑8 mA/cm2͒.
If we assume an irreversible reaction, the peak current can be
calculated with the equation17
1/2
ip = ͑2.99 ϫ 105͒n3/2
␣
1/2D1/2
C
*
v
where ␣ is the symmetry factor in the Butler–Volmer equation. This
is about 80% of the value calculated from the reversible case, as-
suming the symmetry factor is 0.5. The discrepancy between the
calculated result and the experimental data is thus even larger.
This discrepancy can be accounted for by assuming that HER
occurs in parallel in the potential range considered, particularly at
more cathodic potentials. In addition, Co reduction may occur di-
rectly from the Co glycine complex as well. The direct reduction
from metal complexes has been suggested for example by Chassaing
in the case of copper electrodeposition from citrate baths.18
1/2
ip = ͑2.69 ϫ 105͒n3/2D1/2
C
*
v
where n is the number of electrons transferred in the reduction pro-
cess ͑2 for Co2+͒, D is the diffusion coefficient of Co2+ ͑approxi-
Effect of electrolyte pH and P sources.— Figure 4 shows the
CV at a Au electrode from solution A at different pH values. With
increasing pH, the onset of current is polarized to more negative
potentials. In addition, the Co reduction peak can be better discerned
from the hydrogen evolution current and its amplitude decreases.
Figures 5 and 6 display similar CVs collected upon addition of
0.1 M H3PO3 ͑solution B͒ or 0.1 M NaH2PO2 ͑solution C͒, respec-
tively, to the Co electrolyte. Also in these solutions pH plays a
similar role, its increase polarizing the reduction of cobalt to more
negative potentials.
0.005
0.1M glycine
0.1M di-ammonium citrate
0.1M di-sodium citrate
0.000
-0.005
Hydrogen
Evolution
In the presence of H3PO3, as pH increases, the cobalt reduction
peak is better defined and the onset of cobalt reduction is depolar-
ized with respect to solution A ͓at pH 8 the current onset for solution
B occurs at −0.94 V, while in solution A it occurs at −0.97 V ͑Fig.
4͔͒. This is probably a consequence of the buffering effect due to the
-0.010
Co reduction
16
-0.015
dissociation equilibria of H3PO3
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
H3PO3 ⇔ H2PO−3 + H+ K1 = 3.7 ϫ 10−2
͓13͔
͓14͔
E, vs SCE(V)
H2PO−3 ⇔ HPO23− + H+ K2 = 2.1 ϫ 10−7
Figure 2. ͑Color online͒ CV at a Au electrode of Co reduction from solu-
tions containing 0.1 M Co sulphamate and different complexing agents as
shown; scan rate 10 mV/s.
The pH increase occurring at the electrode surface due to hydro-
gen evolution
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Journal of The Electrochemical Society, 155 ͑12͒ D742-D749 ͑2008͒
D745
0.010
0.005
0.01
0.00
0.000
-0.01
-0.005
-0.010
-0.015
-0.020
-0.025
-0.030
pH=4
pH=6
pH=8
pH=9
-0.02
pH=4
pH=6
pH=8
pH=9
-0.03
-0.04
-0.05
-0.06
Co reduction peak
-1.2
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-1.2
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
E, V vs SCE
E, vs SCE(V)
Figure 4. ͑Color online͒ CV curves of Co deposition from an electrolyte
containing 0.1 M Co sulphamate, 0.1 M diammonium citrate, and 0.1 M
glycine ͑solution A͒ at various pH values; scan rate 10 mV/s.
Figure 6. ͑Color online͒ CV at a Au electrode of Co–P deposition from
NaH2PO2 source ͑solution C͒, at various pH values; scan rate 10 mV/s.
͑1͒ Adsorption and dissociation of H2PO−2 at a catalytic surface
2H2O + 2e → H2 + 2OH−
͓15͔
H2PO− → ͑HPO−͒ + ͑H͒
͓16͔
ads
2
ads
is thus balanced by the association equilibrium of Eq. 14. This buff-
ering effect is particularly pronounced at pH 7–8 because, at this
pH, K2 is similar to the proton concentration. A decrease in the
surface pH results in a higher concentration of free Co2+ and the
depolarization of cobalt reduction.
2
͑2͒ Oxidation of the reducing agent and electron release
͑HPO−͒ + OH− → H2PO− + e
͓17͔
ads
2
3
͑3͒ The electrons made available by hypophosphite oxidation,
and, in our system, also by the external power supply, can be used
up for
The onset of a reduction current from solution C occurs at simi-
lar potentials as in solution B except at pH 4. The rate of hydrogen
evolution is highest in solution C, as shown for example by com-
parison of the current densities at −1.2 V and pH 8. In addition, the
Co reduction peak is not as well defined as in the other two solu-
tions. As discussed below, the above effects can be ascribed to the
catalytic effect of NaH2PO2 for Co reduction and hydrogen evolu-
tion.
͑a͒ Co reduction
Co2+ + 2e → Co
͓18͔
͑b͒ Phosphorus reduction, possible only onto a catalytic surface
H2PO−2 + 2H+ + e → P + 2H2O
͑c͒ Hydrogen evolution
͓19͔
The reaction mechanism for the autocatalytic reduction of tran-
sition metals by hypophosphite is a matter of debate; however, a
likely framework for
hypophosphite system is the following, derived from the van den
Meerakker mechanism as discussed in Ref. 5 and 6
a reaction mechanism in the metal–
2H+ + 2e → ͑2H͒ → H2
͓20͔
ads
The overall autocatalytic reaction, neglecting P reduction, can
thus be approximately written as follows
Co2+ + 2H2PO2− + 2OH− → Co + 2H2PO−3 + H2
͓21͔
0.01
0.00
The above framework suggests that: ͑i͒ the presence of adsorbed
hypophosphite may induce catalytic Co reduction in parallel with
electrochemical reduction; under an applied potential, this may oc-
cur even on otherwise noncatalytic surfaces and at potentials where
the latter does not occur, and ͑ii͒ the catalytic process may increase
the amount of Co deposited and accelerate the hydrogen reduction
process.
-0.01
-0.02
The occurrence of these phenomena is supported by CV data
obtained with different complexing agents in the solution. Figure 7
shows the CV curves resulting from the addition of various com-
plexing agents in the plating bath, with and without the addition of
NaH2PO2. When only glycine is added, both the onset potential for
cobalt reduction ͑ϳ − 0.81 V͒ and the cobalt reduction peak
͑−0.87 V͒ are unaffected by the addition of NaH2PO2. However, as
the potential sweeps to values more negative than −0.90 V, an en-
hancement of the HER is observed, as indicated by the increase in
current ͑dotted arrow͒. We ascribe this phenomenon to the catalytic
effect of NaH2PO2, and we hypothesize that the processes described
by Eq. 16-20 occur in parallel with electrochemical reduction, and
are enabled by the applied potential. Adsorption and oxidation of
H2PO−2 onto Ni surfaces has in fact been observed during cathodic
-0.03
pH=4
pH=6
pH=8
pH=9
-0.04
-0.05
-0.06
-1.2
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
E, vs SCE(V)
Figure 5. ͑Color online͒ CV at a Au electrode of Co–P deposition from
H3PO3 source ͑solution B͒, at various pH values; scan rate 10 mV/s.
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D746
Journal of The Electrochemical Society, 155 ͑12͒ D742-D749 ͑2008͒
-0.81V
-0.91V
-0.97V
0
-5
0.00
-0.01
-0.02
-0.03
-0.867V
-0.882V
itotal
iCo
-10
-15
-20
-25
-0.87V
Co reduction
Co+Glycin
Co+Glycin+NaH2PO2
Co+AmoCitr
Co+AmoCitr+NaH2PO2
H2 evolution
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-1.2
-1.0
-0.8
-0.6
E, vs SCE(V)
E, vs SCE(V)
Figure 9. ͑Color online͒ Total current and calculated partial current of Co–P
reduction obtained during a cathodic scan in solution B ͑containing 0.1 M
Co sulphamate, 0.1 M glycine, 0.1 M diammonium citrate, and 0.1 M phos-
phorus acid͒ at pH 8; scan rate 1 mV/s.
Figure 7. ͑Color online͒ CV curves showing the effect of complexing agents
on the catalytic effect of NaH2PO2; scan rate 10 mV/s.
polarization by performing CV in electrolytes containing KPF6 and
NaH2PO2.19 The mechanism by which process would occur is un-
clear at this point and is the subject of ongoing investigations.
The presence of ammonium citrate results in cobalt reduction
being polarized to a more negative potential range, at which the
catalytic effect of NaH2PO2 is already triggered; as a consequence,
both cobalt reduction and HER are depolarized: the onset potential
for cobalt reduction is shifted from −0.97 to − 0.91 V, and the
HER current is increased ͑dashed arrow͒. As a result, a vaguely
discernible shoulder which could be assigned to cobalt reduction is
observed.
deposition from H3PO3-containing solutions, and in an overestimate
of iCo up to 9% when dealing with NaH2PO2-containing solutions.
In the case of pure Co ͑solution A, Fig. 8͒, reduction of cobalt
starts at −0.9 V; HER occurs in parallel starting at the same poten-
tial, as shown by the immediate deviation of iCo from itotal. iCo seems
to approach a limiting current at high cathodic potentials.
In the presence of H3PO3 ͑solution B, Fig. 9͒, buffering results in
a lower surface pH value and the potential for the onset of cobalt
reduction is depolarized to −0.87 V, at which HER does not occur.
HER starts at −0.88 V, where iCo shows a well-defined reduction
peak. At more cathodic potentials iCo reaches its limiting current,
which later increases again due to the enhanced stirring from hydro-
gen evolution.
EQCM study of the effect of different P sources.— Figures 8-10
plot the cd itotal and the partial cd for Co reduction iCo as a function
of electrode potential E. The potential was scanned at 1 mV/s to
minimize delays originated by the instrument response time. iCo is
calculated from the weight increase of the electrode measured by the
EQCM, assuming that only Co is deposited. As discussed in the
Experimental section, this results in a small error when considering
In the presence of NaH2PO2 ͑solution C, Fig. 10͒, cobalt reduc-
tion starts at −0.837 V, a potential more positive than Co reduction
from the other two solutions. As discussed above, this is probably
induced by the catalytic effect of NaH2PO2, and may be enabled by
0
0
-0.854V
-0.905V
-5
-10
-15
-20
-25
itotal
iCo
-5
itotal
-10
iCo
-15
-20
-25
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
E, vs SCE(V)
E, vs SCE(V)
Figure 8. ͑Color online͒ Total current and calculated partial current of Co
reduction obtained during a cathodic scan in solution A ͑containing 0.1 M Co
sulphamate, 0.1 M glycine, and 0.1 M diammonium citrate͒ at pH 8; scan
rate 1 mV/s.
Figure 10. ͑Color online͒ Total current and calculated partial current of
Co–P reduction obtained during a cathodic scan in solution C ͑containing
0.1 M Co sulphamate, 0.1 M glycine, 0.1 M diammonium citrate, and 0.1 M
sodium hypophosphite͒ at pH 8; scan rate 1 mV/s.
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Journal of The Electrochemical Society, 155 ͑12͒ D742-D749 ͑2008͒
D747
14
12
10
8
6
Figure 13. SEM images of 1 ␮m thick Co films ͑no P source in the elec-
trolyte, solution A͒. From left to right: 5, 10, and 20 mA/cm2.
4
0
10
20
30
40
50
diffraction peaks decreases, indicating a decrease in the crystallinity
of the films. This is confirmed by the decreasing feature sizes ob-
served in the SEM images ͑Fig. 13͒. Faster nucleation at higher
overpotential and growth inhibition due to the stronger HER may
both lead to the observed smaller grain size.
i, mA/cm2
Figure 11. ͑Color online͒ Atomic fraction of P in Co–P films deposited from
NaH2PO2-containing electrolyte ͑solution C͒.
P codeposition with Co causes important changes in film micro-
structure. For the Co–P films deposited from solution C, strong hcp
͑10.0͒ and ͑11.0͒ peaks are observed only at a very small cd, i.e.,
2 mA/cm2 ͑Fig. 14͒. As the cd increases to 5 mA/cm2, the grain
size gets smaller ͑Fig. 15͒ and a tendency to film amorphization is
observed from the XRD pattern. For example, a wide feature due to
the applied potential through an as yet undetermined mechanism.
Plating efficiency remains almost 100% until the potential reaches
−0.88 V, at which HER occurs.
Characterization of Co and Co–P films Composition.— Com-
position.— Figure 11 plots the EDS compositional data of Co–P
films deposited from solution C. The P content approximately in-
creases with cd and finally saturates at almost 14 atom % P.
The P content in the Co–P films deposited from solution B is
negligible ͑Ͻ0.4% from EDS measurement͒. This is in agreement
with the deposition mechanism outlined by Fukunaka,9 whereby a
necessary condition for P reduction is H3PO3 adsorption on the elec-
trode surface. Considering the H3PO3 dissociation equilibria ͑Eq. 13
and 14͒, it can be calculated that the relative concentration of the
undissociated acid decreases with increasing pH, from 2.7
ϫ 10−3 M at pH 4 to 2.7 ϫ 10−6 M at pH 8. Reduction of H3PO3
resulting in P incorporation is thus expected to be negligible at al-
kaline pH values.
*
*
*
2mA/cm2
*
*
5mA/cm2
20mA/cm2
50mA/cm2
Microstructure and morphology.— Figure 12 shows the XRD pat-
terns of pure cobalt films deposited from solution A on Au. Peaks
indicated by asterisks belong to the Au substrate. The films exhibit
an unoriented hcp structure, where the hcp ͑00.2͒ reflection may be
hidden by the Au ͑200͒ peak. As cd increases, the intensity of the
30
40
50
70
80
90
100
60 2ꢀ
Figure 14. ͑Color online͒ XRD patterns of 1 ␮m thick Co–P films deposited
from NaH2PO2-containing electrolyte ͑solution C͒. From top to bottom: 2, 5,
20, and 50 mA/cm2. Peaks indexed by are reflections from the Au sub-
strate.
*
*
*
*
*
*
5mA/cm2
10mA/cm2
20mA/cm2
50mA/cm2
30
40
50
70
80
90
100
602ꢀ
Figure 12. ͑Color online͒ XRD patterns of 1 ␮m thick pure Co films depos-
ited at various current densities ͑solution A͒. From top to bottom: 5, 10, 20,
Figure 15. SEM images of Co–P films deposited from NaH2PO2-containing
electrolyte ͑solution C͒: from left to right: 2 mA/cm2 100 nm; 2 mA/cm2
1000 nm; 5 mA/cm2 100 nm.
and 50 mA/cm2. Peaks indexed by are reflections from the Au substrate.
*
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D748
Journal of The Electrochemical Society, 155 ͑12͒ D742-D749 ͑2008͒
1000000
*
*
*
*
*
100000
10000
1000
5mA/cm2
Figure 16. ͑Color online͒ XRD patterns
of 1 ␮m thick Co–P films deposited from
H3PO3-containing electrolyte ͑solution B͒.
From top to bottom: 5, 10, 20, and
*
10mA/cm2
50 mA/cm2. Peaks indexed by
flections from the Au substrate.
are re-
20mA/cm2
50mA/cm2
100
40
50
60
70
80
90
2ꢀ
Figure 17. SEM of 1 ␮m thick Co–P
films deposited from H3PO3-containing
electrolyte ͑solution B͒. from left to right:
5, 10, 20, and 50 mA/cm2.
the hcp ͑00.2͒ peak superposed to the ͑200͒Au reflection can be
observed at 5 mA/cm2, which weakens further at a higher cd. Cor-
respondingly, SEM images ͑Fig. 15͒ indicate a decrease in apparent
grain size and no acicular features. Film amorphization approxi-
mately occurs at a P content higher than 10 atom %, in agreement
with other observations. Increasing plating time at 2 mA/cm2 is ob-
served to lead to coarsening in the grain size, as shown in Fig. 15.
Co–P films deposited from solution B show the formation of an
hcp crystal structure within a wide range of current densities ͑Fig.
16͒. The intensity of the reflections is lower than in the case of Co,
indicating a lower degree of crystallinity. SEM images show a fila-
mentary morphology at a low cd and acicular structures similar to
those of Co at higher cd ͑Fig. 17͒. This could be due to the adsorp-
tion of H2PO−3 at low cd, which tends to desorb when electrode
potential is brought to more negative values.
increases and the films become amorphous, the hysteresis loops
show a two-step magnetization process, typical of a soft material
with perpendicular anisotropy which exhibit stripe domains.20,21 The
perpendicular anisotropy could be due to the magnetoelastic energy
1.0
0.5
0.0
Magnetic properties.— Figures 18 and 19 show the in-plane hyster-
esis loops of Co ͑grown from solution A͒ and Co–P films grown
from solution B, respectively. The films exhibit in-plane magnetic
anisotropy due to the random dispersion of crystallographic orienta-
tion and dominant shape anisotropy. Co–P films deposited from so-
lution C display very different magnetic properties. Figure 20 shows
the in-plane hysteresis loops of 1 ␮m thick Co–P films grown at
different cd’s. Crystalline Co–P films grown at 2 mA/cm2 exhibit
in-plane anisotropy. Their coercivity is however higher ͑ϳ330 Oe͒
than Co or Co–P obtained from solution B. This is probably due to
the phosphorus present in the deposit, which tends to precipitate at
grain boundaries and form nonmagnetic phases that may act as do-
main wall pinning sites, thus enhancing coercivity. As the P content
5mA/cm2
10mA/cm2
-0.5
20mA/cm2
50mA/cm2
-1.0
-1500
-1000
-500
0
500
1000
1500
H, Oe
Figure 18. ͑Color online͒ Normalized in-plane hysteresis loops of 1 ␮m
thick pure Co films.
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Journal of The Electrochemical Society, 155 ͑12͒ D742-D749 ͑2008͒
D749
NaH2PO2 or H3PO3 added as the P source, where diammonium
citrate and glycine serve as complexing agents for cobalt ions. Cal-
culation of chemical equilibria and electrochemical measurements
showed that citrate and glycine associate with Co2+, citrate being a
stronger complexing agent. pH plays an important role in determin-
ing the concentration of free Co ions and the polarization of the
reduction process. NaH2PO2 induces catalytic reduction of Co, su-
perposed to electrochemical reduction, and codeposition of P. P con-
tent increases with cd, resulting in amorphous deposits for cd higher
than 5 mA/cm2. The codeposition of P from H3PO3 solutions is
very limited ͑P less than 0.4%͒, resulting in cobalt films with a
hexagonal structure. The minimal P incorporation is due to the dis-
sociation of H3PO3 in alkaline solution, which hinders the adsorp-
tion of P-containing species on the electrode. Films grown from the
H3PO3 solution show soft magnetic properties and in-plane magne-
tization, while Co–P films deposited from a NaH2PO2 source show
a relatively large coercivity and in-plane anisotropy at a small plat-
ing current. Amorphous Co–P films deposited at a larger cd display
soft magnetic properties and perpendicular magnetic anisotropy.
1.0
0.5
0.0
2mA/cm2
5mA/cm2
10mA/cm2
20mA/cm2
50mA/cm2
-0.5
-1.0
-1000
-750
-500
-250
0
250
500
750
1000
H, Oe
Figure 19. ͑Color online͒ Normalized in-plane hysteresis loops of 1 ␮m
thick Co–P film deposited from H3PO3-containing electrolyte ͑solution B͒.
Acknowledgments
The financial support through grant no. NSF-DMR 0705042 is
gratefully acknowledged.
2mA/cm2
1.0
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induced by stresses during the growth process. A detailed discussion
on the magnetic properties of Co–P deposited from NaH2PO2 source
was published elsewhere.22
Conclusions
We synthesized Co and Co–P films onto polycrystalline Au sub-
strates using a Co–sulphamate-based alkaline electrolyte with
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