Designed synthesis of cobalt and its alloys by polyol process

Article abstract of DOI:10.1016/j.jssc.2007.07.024

The role of polyol, precursor and reaction promoting agents in the synthesis of metal and alloy nanoparticles using polyol process has been investigated by analyzing the reaction steps involved in the synthesis of cobalt in Co ion-polyol-[OH^-] ion system in detail. The reducing potential of polyols and the easiness with which any metal salt can react to form reducible complexes has been evaluated using the orbital molecular theory and the results were experimentally verified. The reduction limit of polyol and their extension using reaction promoting agents such as [OH^-] ions is also explained. The reduction of cobalt is preceded by various reaction stages of complex/compound formation, which has been fully identified. Furthermore, the reducing form of cobalt has been identified as either cobalt alkoxide or cobalt hydroxide. The results confirmed that the complex forming reactions that take place prior to the formation of the precursor, which finally get reduced to metal, play a decisive role in determining the physical properties of the nanoparticles. The approach can be extended to reduce any metals or alloys using polyol process.

Full text of DOI:10.1016/j.jssc.2007.07.024

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Journal of Solid State Chemistry 180 (2007) 3008–3018
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Designed synthesis of cobalt and its alloys by polyol process
Ã
R.J. Joseyphusa, T. Matsumotob, H. Takahashia, D. Kodamaa, K. Tohjia, B. Jeyadevana,
aGraduate School of Environmental Studies, Tohoku University, Aramaki Aoba-ku, Sendai 980-8579, Japan
bDivision of Materials Control, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
Received 21 May 2007; received in revised form 10 July 2007; accepted 23 July 2007
Available online 10 August 2007
Abstract
The role of polyol, precursor and reaction promoting agents in the synthesis of metal and alloy nanoparticles using polyol process has
been investigated by analyzing the reaction steps involved in the synthesis of cobalt in Co ion-polyol-[OHꢀ] ion system in detail. The
reducing potential of polyols and the easiness with which any metal salt can react to form reducible complexes has been evaluated using
the orbital molecular theory and the results were experimentally verified. The reduction limit of polyol and their extension using reaction
promoting agents such as [OHꢀ] ions is also explained. The reduction of cobalt is preceded by various reaction stages of complex/
compound formation, which has been fully identified. Furthermore, the reducing form of cobalt has been identified as either cobalt
alkoxide or cobalt hydroxide. The results confirmed that the complex forming reactions that take place prior to the formation of the
precursor, which finally get reduced to metal, play a decisive role in determining the physical properties of the nanoparticles. The
approach can be extended to reduce any metals or alloys using polyol process.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Polyol process; Cobalt nanoparticles; Cobalt complex; Reduction
1. Introduction
nucleating agents like Pt, Ru have to be used along with
the polyols to accomplish enhanced reduction [14–16]. In
some cases, the mechanism for the formation of metal
nanoparticles proposed by the researchers has been found
to be controversial. For example, the formation of Fe
particles in polyols is claimed to be by disproportionation
reaction and the role of polyol is said to be limited to
solvent and not as a reducing agent [17]. But our recent
studies have indicated that the polyols do have a strong
role to play in reducing Fe at temperatures as low as 120 1C
and also the properties of the iron particles depended on
the type of polyol along with other experimental para-
meters such as reaction temperature and [OHꢀ] ion
concentration [18]. The control of reaction condition could
be utilized to obtain either Fe or Fe-oxides with various
morphologies from Fe-alkoxides [19]. Hence the reduci-
bility by polyol process and the ways to enhance the
reduction has to be understood inorder to extend the
process to synthesis any metal or alloy. Thus we provide
insight information on the entire reactions that take place
during the formation of cobalt metal in polyols from both
theoretical and experimental aspects, considering the
The synthesis of metal particles in polyols has been a
subject of active research [1–3] in recent years. However,
due to limited knowledge on the process itself, most of the
work up to now has been confined to the synthesis of
precious metals and their alloys [4–7] besides for a few
exceptions [8–10]. Although the physical properties such as
size, shape and crystal structure [11–13] of the particles
have been controlled by manipulating the synthesis
conditions that influence the nucleation and growth steps,
no attempts have been made to fully understand the
reaction mechanism that would lay the foundation for the
synthesis of transition metal alloys and other functional
alloy nanoparticles. As a consequence, particles with novel
properties have been realized only through trial and error
basis by carrying out large number of experiments varying
all the experimental parameters randomly. Also, secondary
reducing agents such as hydrazine or heterogeneous
Ã
Corresponding author.
E-mail address: jeya@mail.kankyo.tohoku.ac.jp (B. Jeyadevan).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.07.024

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reactions undergone both by the reactants and polyols, to
choose the reducibility of any metal in polyol.
complexes has been monitored by UV–visible spectro-
scopy, which is a useful tool employed to study the
nanoparticle formation in noble metals and alloys [25,26]
and also the reaction processes in aqueous systems.
Larcher and Patrice [20] presented a theoretical ap-
proach based on the estimation of the Gibbs free energy of
the metal-polyol system to determine the reducing ability
of oxides and hydroxides to obtain their respective metal
particles. They studied the effect of type of polyol and
starting material in the reduction of various metal ions and
discussed their reducibility. However, it should be noted
that this analysis will only provide us with the information
about the possibility of any metal ion being reduced and
will not provide any clue for the selection of metal salts or
the parameters influencing the reduction of the same.
Furthermore, the experimental observations have proved
that the calculations are not consistent enough as some of
the oxides and hydroxides with negative Gibbs free energy
were not reduced. This anomalous behavior was explained
on the basis of the formation of intermediate compound/
complexes, which possess higher Gibbs free energy than the
starting material. On the other hand, when the synthesis of
cobalt metal or other cobalt based compound using polyol
is considered, the emphasis has been on obtaining the end
products of metal alone or metal oxides/alkoxides and not
on the reaction scheme leading to the formation of the
above [21–24]. Thus, an understanding of the reaction
scheme during the reduction of metal ions in polyol is
necessary to design the process for the synthesis of metals
and alloys. Consequently, this may lead to the establish-
ment of a generalized scheme for the reduction of metal
ions in polyol. Hence, a detailed study has been undertaken
to understand the influence of the type of reducing agent
(polyol), type of metal salts, additives such as [OHꢀ], and
reaction temperature to determine the rate determining
steps involved prior to the formation of metal nanoparti-
cles taking cobalt-[OHꢀ]-polyol system.
The precipitates formed at various temperatures in
polyols are also sampled out, centrifuged, washed in
alcohol and dried for crystal phase and morphology
analysis using X-ray diffractometer (XRD) (Rigaku) with
a Cu target and a scanning electron microscope (SEM)
(Hitachi S-4100). The morphology and electron diffraction
patterns were also analyzed using a transmission electron
microscope (TEM) (Hitachi HF2000). The UV–visible
spectrum is recorded in a Hitachi UV–visible spectrometer
by loading the sample in a quartz cell. The samples were
scanned in the wavelength range of 300–800 nm with a scan
speed of 2 nm/s in 1 nm step. The Fourier transform
infrared spectrum (FTIR) is recorded in a Thermo Nicolet
FTIR spectrometer (Avatar 360) fitted with a Duroscope in
the scan range 800–4000 cmꢀ1. The theoretical estimation
of the reaction rate of metal salts, and the reduction
potential of polyols were determined from the ab initio
calculations using Gaussian 98 [27] performed on a
Compaq Alpha XP1000 computer. All geometrical opti-
mizations were carried out by using the HF/LanL2MB
basis set [28] to reduce calculation time and make them
applicable to large molecules. The stability of the Hartree-
Fock wave function was tested using stable option [29].
During ab initio calculations, all internal coordinates were
optimized by means of the Berny algorithm, and conver-
gence was tested against the criteria for the maximum force
component, root-mean-square force, maximum displace-
ment component, and root-mean-square displacement.
3. Results and discussion
For a given metal salt, the properties of nanoparticles
such as size and its distribution, crystal structure, etc.
synthesized by reducing the metal ions in polyol depends
on the reaction rate of the process, which is a function of
various experimental parameters described by the equation
given below [30]
2. Experimental
In a typical experiment, either cobalt acetate tetrahy-
drate [Co(OAc)2 ꢁ 4H2O] or cobalt chloride hexahydrate
[CoCl2 ꢁ 6H2O] of molar concentration 0.01 M was intro-
duced directly into 100 mL of ethylene glycol (EG) or
trimethylene glycol (TMEG) at room temperature (RT).
Then, the metal salts-polyol system was heated at a rate of
15 1C/min to the boiling point of the polyol under constant
mechanical stirring. For reactions involving [OHꢀ] ions,
NaOH of 0.1 M is introduced along with the metal
precursors at RT. The solution was sampled out for
UV–visible spectroscopic studies at various reaction
temperatures and duration. The selection of cobalt ion
for this study was based on the fact that they enable the
formation of various metal complexes and could function
as a model element to extract information that will be
useful to understand the behavior of other metals in polyol.
Furthermore, the cobalt complexes formed prior to the
generation of the solid precipitate are associated with color
changes in the visible region. The formation of the
r ¼ f ðPredox; Mconc:; Hconc:; Treac:Þ,
(10)
where Predox.; the reduction potential of polyol, Mconc.; the
metal ion concentration, Hconc.; the concentration of
hydroxyl ions and Treac.; the reaction temperature.
When the Predox. of the polyol is high, the metal ions are
reduced rapidly forming an avalanche of nuclei consuming
a large portion of the metal species and the particle
diameter is consequently small due to the limited supply of
metal ions for growth. Furthermore, the reaction rate is
enhanced when the Hconc. in metal ion—polyol system is
increased. The concentration of hydroxyl ions [OHꢀ]
necessary for the reduction of metal ions in polyol is also
a function of the type of polyol. Highly reducing polyols
need lesser amounts of [OHꢀ] to synthesize particles of
comparable diameters [13]. The presence of [OHꢀ] in the

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metal ion-polyol system is believed to act as a catalyst in
accelerating the formation of precursor complexes. The
effects of temperature (Treac.) and molar concentration
(Mconc.) on metal forming reaction in polyol are independent
of other factors such as the type of metal salts, polyol and
additives and the outcome could be predicted to a reasonable
extent. Thus, we shall focus our discussions to the influence
of experimental parameters like metal salts, additives and
polyols on the reaction steps involved in polyol process.
wavelengths 489 and 520 nm. However, the spectrum of
cobalt acetate began to vary with reaction duration
exhibiting additional absorption lines at wavelengths 462
and 575 nm. The absorption peak intensity at 575 nm
increases when the reaction duration is increased from 5 to
10 min whereas the lower wavelength absorption intensity
decreases suggesting the transformation of acetates to
complex of the type CoX2(OAc)2. However, in the case of
cobalt chloride hexahydrate, the absorption spectra does
not change significantly even after 1 h at 200 1C. This
indicates that cobalt ion derived from cobalt acetate easily
forms complexes compared to cobalt chloride. The chloride
anion coordinates with the cation whose chemical bond is
comparatively stable [23] and hence it is possible for the
cobalt derived from acetates to form a complex more easily
than chlorides. For example, the formation of CoX2(OAc)2
complex is a necessary condition for the reaction to
proceed to form Co metals.
Besides the experimental evidence, we made an attempt to
understand the behavior of these salts using molecular
orbital theory, which also suggested that cobalt acetate
transforms faster than cobalt chloride. On comparing the
cobalt acetate with cobalt chloride, the values from
molecular orbital calculations are given in Table 1. When
we consider the values of cobalt acetate and cobalt chloride,
it could be inferred from the table that the alpha occupied
value for cobalt acetate is ꢀ8.716 eV whereas for cobalt
chloride it is ꢀ11.878 eV that is lower than that for cobalt
acetate. Similarly the other values of alpha and beta virtual
and occupied of cobalt acetate are higher than chloride.
Alpha represents the energy where orbital is at, without
overlap of molecular orbital and beta corresponds to the
overlap integral. The higher occupied and virtual alpha and
beta values of cobalt acetate than their corresponding value
for cobalt chloride, implies a faster reaction process for
cobalt acetate in polyols (longer time taken for the
formation of reducible form). From the experimental and
theoretical considerations, we believe that the formation of
reducible forms of metal complexes is the prime criteria and
it is achieved rapidly when the metal salt is cobalt acetate.
Hence, the synthesis of cobalt particles using cobalt acetate
and chloride was attempted in the presence of [OHꢀ] ions.
3.1. Effect of metal salts
The synthesis of cobalt metal particles by polyol process
is realized only with specific metal salts in polyols, which
undergoes the next step of complex formation. In an
octahedral and tetrahedral cobalt (II) complex, the
4
4
transitions T1g (P) ’4T1g and T1(P) ’4A2, respectively,
can be observed in the visible region. In cobalt–chloride
complexes, the absorption peaks at 575 nm corresponds to
CoX2Cl2 where X represents pyridine, quinoline and
alcohols whereas complex of the type CoXClꢀ3 or CoCl4ꢀ
could be identified from the absorption peak at 595 nm
[31,32]. Similar behavior can be expected for cobalt acetate.
The absorption at wavelengths 489 and 520 nm are
characteristics of [Co(H2O)6]2+ octahedral complex [31].
The absorption peak at 575 nm correspond to CoX2(L)2
(where X and L are EG and OAc here, respectively) as in
the case of chloride systems, whereas other types of
complexes like CoX4Cl2 show absorption around
525–540 nm and CoXClꢀ3 shows primary peak position at
595 nm [32]. Fig. 1 shows the UV–visible spectrum of
cobalt (II) acetate tetrahydrate (continuous line) and cobalt
chloride (broken line) in EG at a reaction temperature of
200 1C for various reaction times in the absence of [OHꢀ]
ions. The UV–visible spectra at RT and 200 1C were similar
for both the cobalt salts exhibiting absorption lines at
3.2. Effect of hydroxyl ions
In polyols, the reduction of metals is accompanied by the
dehydration and oxidation of the polyol. Thus the polyols
are found to take part in the reaction process through the
Table 1
The result of the molecular orbital calculation for cobalt acetate and
cobalt chloride. (unit: eV)
Alpha
occupied
Alpha
virtual
Beta
occupied
Beta
virtual
Fig. 1. The UV–visible spectrum of cobalt (II) acetate tetrahydrate in EG
at various temperatures and reaction duration. The dotted line shows the
UV–visible spectra of cobalt chloride hexahydrate in EG.
Co(OAc)2
CoCl2
ꢀ8.716
2.470
0.942
ꢀ8.800
2.731
ꢀ2.177
ꢀ11.878
ꢀ11.383

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equation mentioned below [1]:
(1)
In the above equation, EG dehydrates to acetaldehyde
followed by the formation of diacetyl [33]. It is believed that
the [OHꢀ] ions help in the acceleration of the reaction
through the enhancement in the formation of acetaldehyde.
However there are no convincing reports on the enhance-
ment in acetaldehyde. Hence, in order to verify the presence
of acetaldehyde, and also to understand the role of [OHꢀ]
ions in the reduction reaction, we performed 2,4 dinitrophe-
nyl hydrazine test which is an indicator test for trace amount
of carbonyls [34,35]. Here, the presence of a carbonyl is
detected by the formation of 2,4 dinitrophenyl hydrazone on
reacting the test solution of 2,4 dinitrophenyl hydrazine with
acetaldehyde according to the following equation:
Fig. 2. The UV–visible spectrum of (a) EG solution, and vapors of EG (b)
without [OHꢀ] and (c) with [OHꢀ] introduced into 2,4 dinitrophenyl
hydrazine test solution. The absorption peaks indicate a positive test for
acetaldehyde through the formation of 2,4 dinitrophenyl hydrozones.
R
R'
NH2
of diacetyl by using FTIR. However, we were not successful
due to insufficient concentration of diacetyl. Thus, the
enhancement in the formation of acetaldehyde, which
subsequently gets oxidized, may also help in the formation
of hardly reducible metals such as Fe from ferrous ions.
Fig. 3 illustrates the experimentally verified potential limit of
polyol process in reducing various metal elements studied up
to now. In this figure, the standard reduction potential (E1)
of ions for reduction from and to various oxidation states in
aqueous solutions at 25 1C is represented [38,39]. A higher
standard reduction potential value represents easy formation
of metals from their corresponding oxidation states.
Although, the standard reduction potential of various ions
in aqueous solution is compared at RT due to non-
availability of sufficient data in polyols, the oxidation
potentials of the polyols are lower than the reduction
potential of metals at lower temperatures. However, the
oxidation potential of polyol at higher temperatures, often at
the boiling temperature of the polyol, reduces than that of
the reduction potential of the metal as in the case of EG [40].
Hence the figure is an illustrative description of the reducing
ability of polyols for various metal ions. The thick horizontal
line in the figure represents the experimentally derived
reduction potential limit of –0.403V derived from the
reduction of Cd by using polyol process. The dotted
horizontal line represents the extension of the above limit
to –0.447V by introducing [OHꢀ] ions. This was demon-
strated by the synthesis of pure Fe particles using EG–OHꢀ
mixture, which was considered impossible [18]. In some cases
like Mn, although the Eo value for reduction of Mn from 3+
oxidation state is more than the other reducible ions, Mn
could not be reduced in polyols. This is due to the fact that
the standard potential for the Mn3+ to Mn2+ transition is
positive whereas Mn3+ or Mn2+ to Mn0 is more negative
than the reduction potential limit of polyol. In contrast to
N
NH
NH
R
O
C
+
NO2
NO2
R'
NO2
NO2
Carbonyl
2, 4 Dinitrophenyl hydrazine 2, 4 Dinitrophenyl hydrazone
(2)
The test solution was prepared by adding 20ml of ethanol
with 0.5 mM of 2,4 dinitrophenyl hydrazine and 2 drops of
concentrated HCl. A positive test is detected by the
formation of yellow or orange precipitate, whose presence
is detected by appearance of absorption peaks in the
UV–visible spectrum at 435 [36] and 358 nm [37]. We
performed the experiments by directing the vapor from
the EG at boiling point into the test solution of 2,4
dinitrophenyl hydrazine. The presence of acetaldehyde in the
vapor of EG was confirmed as shown in Fig. 2. Thus, we
carried out the above test in the presence of high
concentration of [OHꢀ] and confirmed the enhancement in
the peak intensity corresponding to acetaldehyde as shown
in Fig. 2(c). Similar test was performed on EG solution
heated to boiling temperature in the presence and absence of
[OHꢀ] ions. However, the absorption peak indicating the
presence of acetaldehyde was absent in either case that may
be due to the lower boiling point of acetaldehyde (21 1C),
which escapes out of the system. The confirmation of
acetaldehyde from FTIR analysis is difficult for the same
reason. Similar analysis was carried out using TMEG
instead of EG and positive results were obtained. On the
other hand, an attempt was made to study the influence of
[OHꢀ] on the reduction reaction by analyzing the vapor
generated during the reduction of cobalt ion for the presence