Anomalous electroless polyol deposition of FeNi powders and films

Article abstract of DOI:10.1149/1.1425794

Nanostructured FeNi powders and films were synthesized by reducing the metal acetates in refluxing ethylene glycol (EG). The chemistry of anomalous deposition of Fe and Ni was investigated for this polyol process. Unlike conventional polyol process in which the reaction solution was neutral, strong alkaline condition achieved by using NaOH was indispensable in the synthesis of FeNi. If NaOH was absent, the metal salts in EG solution tended to precipitate as a complex having FeNi(EG)₂ structure, and little reduction took place. The addition of NaOH caused the reduction of metal precursors to metals, resulting in film deposition on substrate and powder precipitation in solution. In solution the formation of Fe(OH)₂ led to the disproportionation of Fe(II) and increased the yield of Fe. However, Ni(OH)₂, another product of NaOH-induced reaction, inhibited the reduction of Ni(II) to Ni. In the vapor phase of this refluxing system, film deposition took place in an Fe-rich environment with the participation of FeNi(EG)₂. This generated complex was further reduced by EG vapor to preferentially dissociate metal Fe. As a result, high Fe concentration film could be obtained by vapor deposition.

Full text of DOI:10.1149/1.1425794

C68
Journal of The Electrochemical Society, 149 ͑1͒ C68-C73 ͑2002͒
0013-4651/2001/149͑1͒/C68/6/$7.00 © The Electrochemical Society, Inc.
Anomalous Electroless Polyol Deposition of FeNi Powders
and Films
H. Yin and G. M. Chowz
Department of Materials Science, National University of Singapore, Singapore 119260,
Republic of Singapore
Nanostructured FeNi powders and films were synthesized by reducing the metal acetates in refluxing ethylene glycol ͑EG͒. The
chemistry of anomalous deposition of Fe and Ni was investigated for this polyol process. Unlike conventional polyol process in
which the reaction solution was neutral, strong alkaline condition achieved by using NaOH was indispensable in the synthesis of
FeNi. If NaOH was absent, the metal salts in EG solution tended to precipitate as a complex having FeNi͑EG͒2 structure, and little
reduction took place. The addition of NaOH caused the reduction of metal precursors to metals, resulting in film deposition on
substrate and powder precipitation in solution. In solution the formation of Fe͑OH͒ led to the disproportionation of Fe͑II͒ and
2
increased the yield of Fe. However, Ni͑OH͒2, another product of NaOH-induced reaction, inhibited the reduction of Ni͑II͒ to Ni.
In the vapor phase of this refluxing system, film deposition took place in an Fe-rich environment with the participation of
FeNi͑EG͒2. This generated complex was further reduced by EG vapor to preferentially dissociate metal Fe. As a result, high Fe
concentration film could be obtained by vapor deposition.
© 2001 The Electrochemical Society. ͓DOI: 10.1149/1.1425794͔ All rights reserved.
Manuscript submitted March 13, 2001; revised manuscript received August 22, 2001. Available electronically December 11, 2001.
In the recent decade, a simple, single-step, and nonaqueous
disproportionation. When the initial concentration of Fe precursor
was not too high, the complexing properties of EG helped avoid the
oxidation reaction and the crystallization of magnetite (Fe3O4) was
a minor reaction.10 However, the yield of Fe disproportionation was
lower than that of reduced Ni͑II͒, resulting in Ni-rich powders. Fur-
thermore, the low-Fe concentration cannot be increased by increas-
ing the concentration of Fe precursor. The low yield of Fe was
explained by the competitive oxidation of Fe͑II͒ hydroxide by water
͑as shown in Reaction 2͒ because water is always present in the
liquid polyol and can also form in situ by dehydration at high
temperature8
polyol method has been applied to produce metal powders. This
method, involving the reduction of metal precursors in refluxing
ethylene glycol ͑EG͒ at 194°C, mitigates the problem of hydrolysis
and oxidation commonly encountered in aqueous electroless depo-
sition. Previous studies have shown that metal powders such as
silver,1 palladium,2 and copper3 and even less easily reducible metal
such as Ni4 were obtained by precipitation in the polyol. The reac-
tion in the solution involves the following steps: ͑i͒ suspension or
dissolution of the precursor; ͑ii͒ reduction of the suspended or dis-
solved species by the polyol; and ͑iii͒ nucleation and growth of the
metal particles from the solution. Besides powders, recently thin
films such as Cu,5 MoS2,6 and NiCo7 were also prepared success-
fully by this polyol method.
3Fe͑OH͒2 → Fe3O4 ϩ 2H2O ϩ H2
͓2͔
FeNi alloys with different compositions exhibit great diversity in
their magnetic properties which are suitable for different applica-
tions, such as magnetic recording head and giant magnetoresistance
component. The study on FeNi alloys has constituted an interesting
field for many years. The polyol method has been used to synthesize
FeNi powders for microwave absorber.8
FeNi thin films find wide uses in magnetic recording heads due
to their low coercivity, high permeability, and near-zero
magnetostriction.11 During our initial work on FeNi film deposition
by the polyol method, we observed that black powders agglomerated
on the glassware surface above solution, and such powders were
found to be Fe rich. This phenomenon suggested that in the polyol
system, the Fe deposition was not confined only in the solution
phase, but it also occurred in the vapor phase above the refluxing
solution. In this paper, we report a detailed study of the anomalous
FeNi deposition in the strong alkaline solution and its vapor.
Although the polyol method can be used for Ni deposition, it is
not well suited for the synthesis of Fe, since Fe͑II͒ is more elec-
tronegative than Ni͑II͒. The half-wave potential of Fe͑II͒ and Ni͑II͒
cations in ethylene glycol were E0Fe͑II͒/Fe ϭ Ϫ1.240 V and EN0 i͑II͒/Ni
ϭ Ϫ0.926 V, respectively ͑25°C, with respect to Ag/AgCl refer-
ence electrode͒.9 Considering the reaction temperature of 194°C in
the polyol process, in order to achieve the simultaneous reduction of
Ni͑II͒ and Fe͑II͒ using EG as the reducing agent, the concentration
of Fe precursor should be 4.9 ϫ 106 times larger than that of Ni ͓as
calculated from Nernst equation E ϭ E0 Ϫ RT/nF ln Qth , equaliz-
ing the actual reduction potential ͑E͒ of Ni͑II͒ and Fe͑II͒, then,
Experimental
Two solutions were mixed in a 250 mL three-neck flask: the first
being 50 mL EG containing partly dissolved metal salts of 0.01 mol
nickel͑II͒ acetate tetrahydrate ͑purity Ͼ99%, Fluka Chemie AG͒ and
0.01 mol iron͑II͒ acetate ͑purity 95% Aldrich Chemical Company,
Inc.͒; the second being another 50 mL EG solution in which 0.2 mol
NaOH was dissolved by heating EG to boiling. Experiments were
carried out without NaOH under the same conditions for compari-
son. Samples numbered 1-4 differed from their starting materials
͑for details, see Table I͒. Polycrystalline, polished copper substrates
͑mechanically polished with 200, 320, 600, and 1200 grade silicon
paper followed by alumina powders with grain size of 12 and 3 ␮m͒
with dimensions of 25 ϫ 12 ϫ 2 mm were vertically suspended in
the flask. Three different processes were applied to the Cu sub-
strates, namely, above the solution, in the solution, and quenching.
Quenching was carried out such that during every 10 min deposi-
tion, the substrate was kept in the solution for 7 min and above the
solution for 3 min. The mixture was heated to the refluxing tempera-
ture ͑194°C͒ and the suspended nickel acetate and iron acetate pow-
ders were completely dissolved in EG alkaline solution. During
heating, nitrogen gas was introduced to the solution to prevent oxi-
Fe͑II͒ / Ni͑II͒ ϭ exp nF/RT(E0
Ϫ E0Fe͑II͒/Fe) , where n is
͓
͔ ͓
͔
͓
͔
Ni͑II͒/Ni
the electrons transferred in the reaction, R is gas constant, F is the
Faraday constant, and T is reaction temperature in kelvin͔. The cal-
culation showed that it was difficult to simultaneously produce Fe
and Ni by the same reduction mechanism.
For the polyol synthesis of Fe, the disproportionation of Fe͑II͒ as
shown in Reaction 1 has been used10
4Fe͑OH͒2 → Fe ϩ Fe3O4 ϩ 4H2O
͓1͔
Excess sodium hydroxide ͑NaOH͒ was added into the reaction
mixture to maintain a strong basic environment that favors Fe͑II͒
z E-mail:mascgm@nus.edu.sg
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Journal of The Electrochemical Society, 149 ͑1͒ C68-C73 ͑2002͒
Table I. FTIR analysis on condensed vapor obtained from different starting materials.a
C69
Sample no.
Starting materials
Specific peaks in FTIR
1
2
3
4
Fe acetate/Ni acetate ͑0.1 mol/0.1 mol͒
Fe acetate/Ni acetate ͑0.1 mol/0.1 mol) ϩ NaOH
Fe acetate ͑0.1 mol͒ ϩ NaOH
1724 cmϪ1
785 cmϪ1
958 cmϪ1
960 cmϪ1
Ni acetate ͑0.1 mol͒ ϩ NaOH
a The major peaks of FTIR spectra of condensed vapor were from EG, and the peaks listed here were not related to EG.
dation. After the refluxing started, the reaction was allowed to pro-
ceed for 60 min. The pH value of vapor phase was estimated with
pH indicator paper on the condensed liquid. Following deposition,
the substrate was removed from the mixture immediately, cooled
down to room temperature in air, and then was rinsed with acetone.
The precipitated powders were suction filtered, washed successively
with distilled water and acetone, and dried in air at 80°C.
The composition of films and powders was determined using
energy-dispersive X-ray spectroscopy ͑EDX͒ in a scanning electron
microscope. The postreaction solutions, synthesized powders, and
condensed vapor were analyzed using Fourier transform infrared
spectroscopy ͑FTIR͒. The structure of powders and films was stud-
ied using X-ray diffraction ͑XRD͒ in ␪-2␪ mode. The thermal sta-
bility of the powders was studied using thermogravimetric analysis
͑TGA͒. The magnetic properties of the powders were measured us-
ing vibrating sample magnetometry ͑VSM͒ at room temperature,
with a maximum applied field of Ϯ50 kOe.
To further understand the thermal instability present in both pow-
ders, XRD was used to determine the structure of Pal and Pne before
and after heating, and their results are shown in Fig. 2 and 3, re-
spectively. To avoid the oxidation of metal, argon was introduced to
the furnace chamber during heating. A high heating temperature
͑500°C͒ and long heating time ͑120 min͒ were set to ensure com-
plete decomposition of unstable substance. The XRD pattern of as-
synthesized powders of Pal ͑Fig. 2͒ showed polycrystalline FeNi
Results
Solution analysis.—The chemical composition of postreaction
solutions from different starting materials was studied using FTIR.
In Ni acetate/Fe acetate/EG system, the FTIR spectrum showed
great similarity with that of pure EG, and no new bonds were found.
However, with the addition of excess NaOH, two new peaks distin-
guished from EG were detected in the solution with wavenumbers of
833 and 926 cmϪ1, which were both characteristic of the stretching
vibration of the metal-OH bond. In the solution of only Fe deposi-
tion, the peak of 926 cmϪ1 was detected, whereas 833 cmϪ1 was
detected in the solution of only Ni deposition. This suggested that
after the addition of NaOH, Ni͑II͒ and Fe͑II͒ cations formed metal
hydroxides, Ni͑OH͒ and Fe͑OH͒2, respectively. At reflux tempera-
2
ture, some of the metal hydroxides could dissolve in EG solution
and showed specific absorption peaks in FTIR spectra.
Figure 1. TGA curves of powders obtained in neutral solution (Pne) and
alkaline solution (Pal).
Powder analysis.—It has been reported that in the system of Fe
chloride, Ni chloride ͑with nominal molar ratio 50:50͒, NaOH, and
EG, anomalous deposition could be observed which led to a large
composition deviation between the starting materials and the
products.8,10 The Fe concentration of the precipitated powders was
below 30%. The same result was observed in our experiments using
metal acetates as precursors. However, it was very interesting to find
that the composition of the powders obtained using acetate precur-
sors without NaOH ͑in neutral reaction condition͒ was close to the
nominal ratio of 50:50. The powders were gray-green colored in-
stead of the black FeNi powders obtained in the alkaline solution. To
investigate the reactions in the alkaline and neutral polyol process,
the two types of powders were studied in detail. For discussion,
powders precipitated in alkaline and neutral solutions are denoted as
Pal and Pne , respectively.
The TGA results ͑as shown in Fig. 1͒ indicated that both Pal and
Pne had two weight losses in the whole heating procedure ͑from
room temperature to 500°C, with heating rate 10°C/min͒. The first
one ͑below 200°C͒ was attributed to the volatile portion such as
residual EG or water. The weight loss at higher temperature indi-
cated the existence of thermally unstable component. In Pal , this
weight loss was only 10%, whereas the weight loss in Pne was 42%.
Such difference showed that the powders precipitated in neutral con-
ditions consisted of a larger amount of thermally unstable substance.
Figure 2. XRD patterns of ͑a͒ Pal as-synthesized powders and ͑b͒ after heat-
ing in Ar at 500°C for 120 min.
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Journal of The Electrochemical Society, 149 ͑1͒ C68-C73 ͑2002͒
Figure 4. Hysteresis loops of Pne as-synthesized powders and after heating
in Ar at 500°C for 120 min. ͑ ͒ Paramagnetic curve.
Figure 3. XRD patterns of ͑a͒ Pne as-synthesized powders and ͑b͒ after
heating in Ar at 500°C for 120 min.
Ms of as-synthesized Pal was 42 emu/g, and the Ms of heated sample
increased to 86 emu/g. Compared with the Ms of bulk FeNi alloy
with 25 atom % Fe (Ms ϭ 109 emu/g͒,13 lower Ms was also caused
by the existence of paramagnetic portion. The increase of Ms after
heating confirmed that the Pal included thermally unstable compo-
nent. However, the quantity of this unstable phase was relatively
phase, with an additional small peak at 2␪ ϭ 60.2°. After heating,
the grain size ͑obtained from the line broadening of ͑111͒ peak using
Scherrer formula͒ increased from 10 ͑as-synthesized͒ to 15 nm. At
the same time, the peak at 60.2° disappeared. This suggested that
this peak was related to the thermally unstable component in the Pal
powders. Similar XRD peak located at 60° had been observed when
the polyol process was applied to prepare Ni particles using Ni͑OH͒
lower than in Pne
.
2
as precursor. This peak was assumed to correspond to the structure
The TGA curves, assuming that 200°C was the temperature be-
tween the first and second weight loss, indicated that at this tem-
perature, the absorbed water and EG had been almost removed and
the thermally unstable component still remained. To study the com-
position of this component, Pne powders were heated in Ar at 200°C
for 30 min. Figure 5 showed the FTIR spectra of ͑a͒ Pne powders
as-synthesized, ͑b͒ after heating at 200°C for 30 min, and ͑c͒ heating
at 500°C for 120 min. The as-synthesized powder had apparent EG
peaks located at 1056 and 881 cmϪ1. Comparing the three FTIR
curves, the relative intensity of EG peaks diminished after heating at
of intercalation of EG molecule in Ni͑OH͒ sheets, but it was not
2
clear whether the inserted EG molecule was bonded or nonbonded
with the surrounding Ni͑OH͒2 .12 In contrast, Pne as-synthesized
powders showed a nearly amorphous structure in XRD patterns ͑Fig.
3͒. After heating, polycrystalline FeNi peaks emerged. These inves-
tigations suggested that in alkaline conditions FeNi powders could
precipitate from the solution with a small amount of crystalline,
thermally unstable component. However, in neutral conditions the
powder was amorphous and decomposed to FeNi alloy after heating.
Since Ni and Fe are ferromagnetic materials, the unstable impu-
rity must have great influence on its magnetic properties. To esti-
mate the composition of Pne , its saturation magnetization (Ms) be-
fore and after heating ͑in Ar, 500°C, 120 min͒ was measured by
VSM. The hysteresis loops are shown in Fig. 4. As-synthesized
powders showed no hysteresis, indicating that they were paramag-
netic. After heating, an Ms value as high as 108 emu/g was obtained.
It was lower than the bulk FeNi alloy with 50 atom % Fe (Ms
ϭ 154 emu/g͒.13 The Mossbauer spectroscopy of Pne ͑data not
¨
shown here͒ indicated that the lower Ms resulted from the paramag-
netic portion in the heated powders. Because the unstable substance
would completely decompose when heated at 500°C for 120 min
͑see the TGA results͒, the paramagnetic portion should not be re-
¨
lated to the unstable impurity. According to a Mossbauer study on
FeNi particles,14 only Fe-rich particles (Fe% ϭ 93%) can have
single ferromagnetic spectra; particles with other compositions had a
combination of ferromagnetic and paramagnetic subspectra, the
fraction of each spectrum varying with the Fe content. Therefore, in
our case as the Fe:Ni ratio was 50:50, the appearance of paramag-
netic portion in the heated sample was reasonable. The difference of
magnetism of Pne before and after heat-treatment suggested that the
as-synthesized Pne was not FeNi alloy. After heating the thermal
unstable Pne decomposed to FeNi alloy and showed strong
magnetism.
The magnetic properties of Pal powders were also studied using
VSM, and their hysteresis loops showed magnetic saturation. The
Figure 5. FTIR spectra of ͑a͒ Pne as-synthesized powders, ͑b͒ after heating
in Ar at 200°C for 30 min, and ͑c͒ after heating in Ar at 500°C for 120 min.
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Journal of The Electrochemical Society, 149 ͑1͒ C68-C73 ͑2002͒
C71
Table II. Composition of powders and films deposited from different starting materials.
Composition
of powders
Sample no.
1
Starting materials
͑atomic ratio͒
Composition of film ͑atomic ratio͒
In solution
Fe acetate/Ni acetate
͑0.1 mol/0.1 mol͒
Fe/Ni ϭ 50/50
Fe/Ni ϭ 25/75
Fe/Ni ϭ 10/90
Quenching
Above solution
In solution
Fe/Ni ϭ 10/90
Fe/Ni ϭ 10/90
Fe/Ni ϭ 25/75
Fe/Ni ϭ 40/60
Fe/Ni ϭ 90/10
2
Fe acetate/Ni acetate
͑0.1 mol/0.1 mol͒
ϩ NaOH
Quenching
Above solution
3
4
Fe acetate ͑0.1 mol͒
ϩ NaOH
Ni acetate ͑0.1 mol͒
No powder
No film
Pure Ni powder
Pure Ni film
ϩ NaOH
200°C, and completely disappeared after heating at 500°C, which
can be explained by the evaporation of absorbed EG. More impor-
tantly, the intensity of the peak at 785 cmϪ1 remained unchanged
after heating at 200°C, and disappeared after heating at 500°C. This
peak might be attributed to asymmetric stretching of the metal-
oxygen bond.15 The appearance of such a bond should not be related
to metal oxide, because this absorption peak could not be detected
after heating at a higher temperature, as the metal oxide should
remain stable up to this temperature. Thus, the oxygen component in
only subjected to deposition in the vapor phase, above the refluxing
solution, the Fe concentration of the film increased to about 90%.
When the substrate experienced repeated vapor and solution depo-
sition via a quenching process, a medium range of Fe concentration
͑about 40%͒ was obtained.
When Ni only was deposited ͓Ni͑II͒ acetate starting concentra-
tion was 0.1 mol/L͔ in the presence of NaOH, both pure Ni powders
and pure Ni films were obtained within 60 min ͑on all three sub-
strate positions͒. Compared with the reaction without NaOH in
which film could be deposited on the substrate within 5 min,17 our
reaction time was much longer. The Ni film could not form on the
substrate until 20 min after reflux started and the density of such
film was lower. With increasing deposition time, the film density
increased ͑data not shown here͒.
When Fe only was deposited ͓Fe͑II͒ acetate starting concentra-
tion was 0.1 mol/L͔ in the presence of NaOH, neither powder, nor
film was obtained. However, when the Fe͑II͒ concentration in-
creased to 0.2 mol/L, Fe powders were synthesized without any film
formation. The increase of starting Fe concentration from 0.1 to 0.2
mol/L to synthesize Fe powders has been reported.10 A suitable Fe
precursor concentration was needed in order to prevent the crystal-
lization of magnetite (Fe3O4) and to retain unreacted Fe͑II͒ species
in solution.10
Pne was supposedly related to the EG molecule, which indicated that
the existence of EG was not solely due to absorption. Some EG
molecules interacted with metal, forming metal-EG complex, which
was a thermally unstable substance and decomposed before 500°C.
The second weight loss in TGA curves might be caused by decom-
position of such complex, and the thermally unstable substance with
paramagnetic properties should also be related to this complex.
Since the mass ratio of metal to EG in the complex was close to 1:1
͑from the TGA curve of Pne͒ and the element ratio of Fe to Ni was
also 1:1 ͑from EDX measurement͒, it was estimated that the com-
plex had a molecular formula of Fe͑II͒Ni͑II͔͒͑EG͒ .
͓
2
Vapor analysis.—The FTIR spectra of the condensed vapor from
sample 1 to 4 showed that the main portion of the vapor was EG.
There were some differences between the four samples ͑see Table I͒.
A strong peak at 1724 cmϪ1, corresponding to the stretching vibra-
tion of carbonyl group, was found in sample 1. This could be caused
by dehydrolysis of EG and further oxidation by metal cations16
Discussion
Fe deposition.—In our study, the Fe film could not be synthe-
sized even with higher starting Fe concentrations. To explain the
failure in depositing Fe films, the catalytic activities of the Fe, Ni,
and Cu surfaces were evaluated from the anodic polarization curves
in EG as shown in Fig. 6. It was found that Ni was slightly more
active than Cu substrate. However, Fe was much less active than
both Ni and Cu. These results suggested that the precipitation of Ni
particles on Cu substrate could provide a slightly active interface for
further deposition. While the precipitation of Fe particles could dra-
matically decrease the substrate activity, further deposition would
continue with greater difficulty. As a result, Fe deposition was dif-
ficult to occur in such polyol process. Therefore, Fe film formation
must be accompanied by the codeposition of Ni, which was surface-
active and provided suitable interface for further film growth.
Ϫ2H O
2
2CH OH-CH OH —— 2CH3CHO
→
2
2
M͑II͒
—— CH3-C-C-CH3 ϩ H2O ϩ M
→
ʈ
ʈ
͓3͔
O O
In the FTIR spectrum of sample 2, a peak related to the
metal-EG complex at 785 cmϪ1 was observed. Its appearance sug-
gested that the vapor deposition in the presence of NaOH might also
involve the formation of metal-EG complex. In the FTIR spectra of
samples 3 and 4, a peak characteristic of acetate at about 960 cmϪ1
was detected. ͑FTIR of Fe acetate and Ni acetate show strong peaks
at 958 and 960 cmϪ1, respectively.͒ It could be simply explained by
the evaporation of metal precursors.
Effect of NaOH.—Due to the presence of NaOH in EG solution,
two competitive reactions occurred for Ni͑II͒ and Fe͑II͒: the reac-
tion between the metal cations and NaOH resulting in the formation
of Ni͑OH͒ and Fe͑OH͒2, and a complexation between the metal
cations and EG. From our experimental data, the first reaction was
2
Film analysis.—The composition of films obtained from differ-
ent starting materials is shown in Table II. Films with very low Fe
concentration were produced in both solution and vapor deposition
when NaOH was absent, regardless of the substrate position. With
the addition of NaOH, Fe concentration of deposited films greatly
depended on the substrate position. When the substrate was kept in
the reaction mixture, i.e., deposition only occurred in the solution
phase, the Fe content was still low ͑25%͒. When the substrate was
favorable in alkaline solution, whereas FeNi͑EG͒ complex precipi-
2
tated out as powders ͑as shown in Pne͒ in the pH neutral condition.
Only a small amount of complex was obtained in Pal powders.
Because of the formation of Ni͑OH͒2, Ni could be reduced by
two paths, i.e., reduced from hydroxide, or reduced from Ni͑II͒ di-
rectly. The two kinds of reactions are
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C72
Journal of The Electrochemical Society, 149 ͑1͒ C68-C73 ͑2002͒
tion, which favored the disproportionation reaction. Thus, the Fe
concentration in both powders and film obtained in solution in-
creased from 10 to 25% when NaOH was used. The EG solvent
helped prevent undesirable oxidation reaction. Further, the complex-
ing characteristics of EG prevented crystallization of magnetite
͑Fe3O4 produced in Reaction 1͒ and increased the yield of
disproportionation.8
Vapor deposition.—From the FTIR spectra of condensed vapor
of sample 3 and 4, the metal acetates and EG were found in vapor
phase. In sample 1, without the presence of NaOH, the films depos-
ited in solution and in vapor had similar composition. Based on this
observation, it could be assumed that Fe and Ni acetates had similar
vapor pressure in neutral EG solution and experienced similar reac-
tions ͑reduction by EG͒ in both solution and vapor phases.
Of the reactions in the presence of NaOH, Fe deposition was
apparently favored in the vapor phase because Fe-rich films were
synthesized when the substrate was suspended above solution. In the
solution phase, the yield of Ni reduction was much higher than that
of Fe disproportionation, which resulted in high Ni concentration in
both powders and films. However, in the vapor phase, elemental
analysis on the condensed vapor of sample 2 showed a higher Fe
concentration. The FTIR of condensed vapor of this sample showed
an evident oxygen-metal-oxygen bond located at 785 cmϪ1, whereas
this peak was absent in the spectra of the solution of sample 2. It
therefore suggested that the metal-EG complex could not precipitate
from strong alkaline solution because of the competitive reaction
between metal cations and NaOH. Because no hydroxide was de-
tected in the vapor phase ͑pH 7͒ even with the addition of NaOH in
solution, the disproportionation of Fe could not be realized in the
vapor. Assuming that a simple reduction-oxidation reaction took
place in the vapor ͑EG as reducing agent, and metal acetate as
oxidizer͒, the yield of Fe could not be high due to the weak oxida-
tion power of EG, as in the case of sample 1. To explain the Fe-rich
vapor deposition, Reactions 6 and 7 are suggested based on the
formation of metal-EG complex as intermediate. A similar reaction
scheme has been previously postulated in studying anomalous elec-
trodeposition of iron group metals.19 In electrodeposition, the com-
plex was reduced by applied electrons, while in our vapor deposi-
tion, the reducing agent was EG
Figure 6. Anodic polarization curves of different metals in EG solution
containing 2 mol/L NaOH.
Ni͑OH͒2 ϩ 2e ϭ Ni ϩ 2 OHϪ
Ni͑II͒ ϩ 2e ϭ Ni
͓4͔
͓5͔
To estimate whether Reaction 4 or 5 predominates, it is essential
to know the reduction potentials of Ni͑OH͒2 and Ni͑II͒ cation in EG.
However, the value of reduction potential of Ni͑OH͒ in EG is not
2
available in the literature. It is noted that the order of the potentials
of the same electrode reaction as a rule does not strongly differ
between different media.17 Moreover, the half-wave potentials of
metal cations in EG are also in agreement with their active order in
aqueous solution.9 Here the potential values in aqueous solution
were taken to estimate the case occurring in EG. Considering the
influence of concentration and reaction temperature on reduction
potential, Ni͑OH͒ (ENi͑OH͒
ϭ Ϫ0.70 V͒ is still more electrone-
2
/Ni
2
gative than Ni͑II͒ (ENi2ϩ ϭ Ϫ0.31 V͒. Ni would easily precipitate
Fe0 ϩ Ni͑II͒ ϩ 2EG
Ni0 ϩ Fe͑II͒ ϩ 2EG
͓6͔
͓7͔
/Ni
͓Fe͑II͒Ni͑II͔͒EG2 ϩ 2e
͓Fe͑II͒Ni͑II͔͒EG2 ϩ 2e
ꢀ
ꢀ
from the reduction of Ni͑II͒. Previous study18 reported that without
NaOH, Ni particles and films could also be obtained from EG solu-
tion. In our study, the addition of NaOH did not favor the reduction
of Ni. The longer deposition time which was needed for Ni deposi-
tion in alkaline condition was perhaps due to the formation of
Ni͑OH͒2, which consequently decreased the concentration of Ni͑II͒
and inhibited Reaction 5.
The smaller Ni͑II͒ concentration in the vapor would push the
forward reaction in Reaction 6 and increased the Fe yield in the
product. At the same time, Reaction 7, which caused Ni deposition,
was inhibited by the higher concentration of Fe͑II͒ in vapor.
In our study, the Fe concentration in deposited FeNi film without
NaOH was only 10% and was much lower than the starting Fe
content, indicating that the reduction of metallic Fe by EG was
difficult. The addition of NaOH led to the formation of Fe͑OH͒2,
and the disproportionation of Fe͑OH͒2 became an alternative path to
synthesize Fe. The excess of NaOH provided strong alkaline condi-
Conclusion
The polyol deposition of FeNi powders and films has been in-
vestigated. In the Fe-only deposition, the initial precipitation of Fe
dramatically decreased the activity of Cu substrate. Further growth
became difficult. With the codeposition of Ni that is the more
Table III. Summary of the main reactions under neutral and alkaline conditions.
Alkaline solution
Neutral solution
Fe͑II͒ ϩ 2 OHϪ ϭ Fe͑OH͒
Solution
2
Ni͑II͒ ϩ 2 OHϪ ϭ Ni͑OH͒
Fe͑II͒ ϩ Ni͑II͒ ϩ 2EG ϭ ͓Fe͑II͒Ni͑II͔͒EG2
2
4Fe͑OH͒2 ϭ Fe ϩ Fe3O4 ϩ 4H2O
Ϫ2H O
Ni͑OH͒2 ϩ 2e ϭ Ni ϩ 2 OHϪ
2
2CH OH-CH OH —— 2CH CHO
→
2
2
3
Ni͑II͒
Vapor
Fe͑II͒ ϩ Ni͑II͒ ϩ 2EG ϭ Fe͑II͒Ni͑II͔͒EG
͓
2
—— CH -C-C-CH ϩ H O ϩ FeNi
→
3
3
2
Fe͑II͒
ʈ
ʈ
O O
͓Fe͑II͒Ni͑II͔͒EG2 ϩ 2e
ꢀ
Fe0 ϩ Ni͑II͒ ϩ 2EG
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Journal of The Electrochemical Society, 149 ͑1͒ C68-C73 ͑2002͒
surface-active metal, FeNi films can be synthesized by the polyol
C73
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¨
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