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Dalton
Transactions
PAPER
Cite this: Dalton Trans., 2013, 42, 7167
Received 27th November 2012,
Accepted 5th March 2013
DOI: 10.1039/c3dt32828g
www.rsc.org/dalton
Reaction temperature variations on the crystallographic
state of spinel cobalt aluminate†
Minori Taguchi,*
a,b
Takayuki Nakane,
b
Kenjiro Hashi,
b
Shinobu Ohki,
b
Tadashi Shimizu,
b
Yoshio Sakka,
b
Akiyuki Matsushita,
b
Hiroya Abe,
c
Toshitaka Funazukuri
a
and Takashi Naka
b
In this study, we report a rapid and simple technique for obtaining cobalt aluminate having a spinel struc-
ture. The products were prepared from a hydroxide precursor synthesized by coprecipitation of cobalt (Co
2+
)
and aluminum (Al
3+
) nitrates with an alkaline solution. The chosen precursor enabled low temperature fabri-
cation of cobalt aluminate with a spinel structure by sintering it for 2 hours at low temperatures (>400 °C).
Crystallographic and thermal analyses suggest that the low-temperature-sintered products contain Co
3+
ions
stabilized by chemisorbed water and/or hydroxide groups, which was not observed for products sintered at
temperatures higher than 1000 °C. The color of the products turned from clear blue (Thenard’sblue)to
dark green when sintering temperatures were below 1000 °C. Magnetic quantities, Curie constants, and
Weiss temperatures show a strong dependence on the sintering temperature. These findings suggest that
there are mixed valent states, i.e. Co
2+
and Co
3+
, and unique cation distributions at the different crystallo-
graphic sites in the spinel structure, especially in the products sintered at lower temperatures.
Introduction
Spinel-type transition metal oxides have attracted much atten-
tion in the inorganic chemistry, solid-state physics, and
materials science fields because of their interesting magnetic,
optical, electronic, and catalytic properties.
1–3
The (normal)
spinel structure is represented as AB
2
O
4
, where the A and B
sites indicate di- and trivalent metal cations, respectively, of
two different tetrahedral (T
d
) and octahedral (O
h
) sites in the
structure.
1–3
Among spinel materials, cobalt aluminate
(CoAl
2
O
4
), known as Thenard’s blue pigment, is widely used in
the ceramic industry as a coloring agent in glazes and porce-
lain stoneware because of its thermal and chemical stability.
4–6
Recently, CoAl
2
O
4
has also been found to be an effective cata-
lyst in methane reformation
7
and may be a useful catalyst in
the photoelectrochemical splitting of H
2
O.
8–10
The magnetic
properties are interesting because it is a magnetic spin system
with frustrated exchange interactions.
11–15
Additionally, the
physical properties of CoAl
2
O
4
depend on the crystallographic
parameters, such as the lattice constant, the crystallite size,
and the cation distribution,
16–18
and therefore tuning it will be
expected to result in not only improvement of the properties
but also discovery of novel properties.
To synthesize CoAl
2
O
4
, the conventional solid-phase reac-
tion (sintering) method of binary oxides of CoO (or Co
3
O
4
) and
α-Al
2
O
3
is generally employed. This process, however, requires
a high reaction temperature (>1000 °C) and a long reaction
time.
11–15
Sol–gel methods have become an important prep-
aration technique,
18–24
and although these methods enable
preparation of CoAl
2
O
4
nanoparticles, complicated procedures,
such as sequential addition of reagents, are required to pre-
cisely control the sol–gel conditions. Resultingly, the sol–gel
precursors were heat-treated at temperatures above 400 °C for
obtaining spinel CoAl
2
O
4
crystals.
Recently, we have developed a technique for the simple and
rapid synthesis of metal oxides and corresponding nanoparti-
cles through a hydrothermal method employing hydroxide
precursors.
25–32
The method can be used for the preparation
of not only simple metal oxides, but also surface-modified
metal oxides. In fact, CoAl
2
O
4
nanoparticles have also been
synthesized by this method using a Co–Al hydroxide as the pre-
cursor,
31
showing that the use of the hydroxide precursor can
lead to rapid crystallization of CoAl
2
O
4
under low temperature
(<400 °C) conditions. Although the nanoparticles almost had
single phase of a spinel structure, the molar ratio of Al/Co ions
was about 5 and lattice defects were present in the crystal
structure.
32
Since the amount of the Co ion that resulted in the
†Electronic supplementary information (ESI) available: A photograph and
27
Al-NMR spectra of the products are shown, as are the χ–Tplots of the products
and Co
3
O
4
synthesized from Co(OH)
2
. See DOI: 10.1039/c3dt32828g
a
Department of Applied Chemistry, Faculty of Science and Engineering,
Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan.
E-mail: mtaguchi@chuo-u.ac.jp
b
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
c
Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki,
Osaka 567-0047, Japan
This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 7167–7176 | 7167
blue color
19–24
was small, the color of the nanoparticles was
indeed dilute blue.
31
The non-stoichiometry in the CoAl
2
O
4
nanoparticles seemed to be caused by the different solubility
of the metal (Co and Al) ions under the hydrothermal con-
dition. The different solubility might affect the crystallization
process for spinel-type CoAl
2
O
4
. These results suggest that the
control of the stoichiometry, especially in the case of synthesis
of ternary metal oxide products like CoAl
2
O
4
, is very difficult
under the hydrothermal condition. Control of the solubility of
the metal ions on adding other reagents is necessary in order
to prepare the products with tuning the stoichiometry by using
hydrothermal methods. In contrast, the solid-phase reaction
(sintering) method can precisely control the stoichiometry of
the metal ions in the products. Additionally, the hydrothermal
method is not suitable for synthesis above ∼500 °C. However
the Co–Al hydroxide precursor utilized by the hydrothermal
method is indeed effective for rapid crystallization and lower
temperature preparation of CoAl
2
O
4
.
Consequently, we focused on a hydroxide precursor utilized
by the hydrothermal method and the conventional sintering
method of maintaining a stoichiometry ratio. By sintering a
Co–Al hydroxide precursor, uniform CoAl
2
O
4
products while
maintaining the stoichiometry of the metal ions can be syn-
thesized at low temperature. By controlling the preparation
temperature, tunability of the lattice constant and the crystal-
lite size of spinel-type CoAl
2
O
4
can be achieved. As will be
shown herein, the heating temperature strongly affects the
crystallographic, optical, and magnetic characteristics of the
spinel CoAl
2
O
4
products. Therefore, in this study, we have sys-
tematically synthesized spinel CoAl
2
O
4
products via sintering
of a hydroxide precursor at various temperatures and evaluated
the properties of the products with respect to the structural,
optical, and magnetic characteristics.
Experimental
Sample preparation
The starting materials of Co(NO
3
)
2
·6H
2
O, Al(NO
3
)
3
·9H
2
O, and
NaOH were purchased from Wako Chemicals. The Co–Al
hydroxide precursor material was prepared by coprecipitation
with an alkaline solution. The two metal nitrates were dis-
solved in distilled water (100 ml) at room temperature with a
molar ratio of Al
3+
/Co
2+
= 2.00 and a total concentration of
0.10 M. In another container, NaOH was dissolved in distilled
water (100 ml) at a concentration of 0.30 M. The NaOH sol-
ution was slowly added to the solution of metal nitrates. This
process yielded a pink slurry, which was stirred at room temp-
erature for 24 hours in order to allow the reaction to take
place. After 24 hours, the unreacted material was removed
through a combination of repeated and alternating centrifu-
gation and decantation with distilled water several times.
Finally, the obtained pink product, the hydroxide precursor,
was dried in a vacuum for 24 hours. The hydroxide precursor
was sintered in a furnace at a selected temperature (from 400
to 1400 °C in increments of 200 °C) for 2 hours in a covered
alumina crucible under an air atmosphere. The heating rate to
the target temperature was fixed at 10 °C min
−1
. Hereafter, the
products obtained from the precursor are denoted as 400 (sin-
tering at 400 °C), 600 (at 600 °C), 800 (at 800 °C), 1000 (at
1000 °C), 1200 (at 1200 °C), and 1400 (at 1400 °C).
Reference compounds, Co(OH)
2
and Al(OH)
3
, were prepared
separately from Co(NO
3
)
2
·6H
2
O and Al(NO
3
)
3
·9H
2
O, respect-
ively, by precipitation with NaOH solution.
Characterization methods
Elemental analysis of the sintered products was monitored for
Al and Co by an inductively coupled plasma atomic emission
spectrometer (ICP-AES) and was conducted by Sumitomo Metal
Technology, Inc. The molar ratio of Al to Co in the precursor
was almost the same (2.05 ± 0.01) for all of the products.
X-ray diffraction (XRD) patterns of the products were
recorded using a RINT-2500 diffractometer (Rigaku) with
Cu Kα(λ=1.5416Å)radiationata2θscan speed of 2° min
−1
.
The lattice constant was calculated from the XRD patterns of the
products using a unit cell parameter refinement program (Cell
Calc).
33
The Scherrer equation was used to determine the crys-
tallite size of the spinel structure products from the full width
at half maximum (FWHM) of the typical peaks (220, 311, 400,
422, 511, and 440) and using a shape factor of 0.9. Addition-
ally, structural (lattice) defects of the products were verified
through analysis of the typical peaks in a Williamson–Hall
plot.
34
Note that the FWHM of the typical peaks as W(vertical
axis) was used in this plot instead of the integral width of the
peaks. For reference, the XRD pattern of normal spinel
CoAl
2
O
4
was simulated using CrystalMaker and CrystalDiffract
software (CrystalMaker Software Ltd). The parameters used in
the simulation were: X-ray wavelength (λ) = 1.5416 Å, crystallite
size = 100 nm, and lattice constant = 8.106 Å.
To analyze the crystal structure of the 800 product, a syn-
chrotron XRD (SXRD) measurement for it was carried out at
NIMS beamline BL15XU, SPring-8. A large Debye–Scherrer
camera and an imaging plate were used along with monochro-
matic synchrotron radiation with wavelength 0.65297 Å, which
was measured from the X-ray absorption edge of niobium, and
CeO
2
was selected as a standard material of SXRD measure-
ments. The fine powdered sample was packed into a Linden-
mann-glass capillary of 0.1 mm in diameter. The SXRD data
were recorded in a 2θrange of 3–60° at a step interval of 0.003°.
The linear absorption coefficient of each sample was estimated
by measurement of the transmittance of the incident X-ray
beam. The diffraction pattern was refined by the Rietveld
method using the RIETAN-FP program. In the refinement
process, the crystallographic parameters of the 800 product
(e.g. unit cell parameters, atomic positions, cationic distri-
bution, and isotropic temperature factors) were refined, and we
also considered an impurity γ-alumina with a spinel structure
35
and estimated the molar ratio of Al to Co from the ICP-AES.
Thermogravimetric and differential thermal analyses
(TG–DTA) of the precursor were performed using a DTG-60H
(Shimadzu) from room temperature to 1500 °C at a ramping
rate of 10 °C min
−1
without flowing gas. Fourier transform
Paper Dalton Transactions
7168 |Dalton Trans., 2013, 42, 7167–7176 This journal is © The Royal Society of Chemistry 2013
infrared (FT-IR) spectra were recorded using an FT/IR-6200
spectrometer (JASCO) with KBr pellets. UV–VIS powder diffuse
reflectance spectra were recorded using a V-650 spectrometer
(JASCO). The obtained spectra were converted to absorption
spectra using the Kubelka–Munk calculation.
26
The coloration
of the products was numerically evaluated with the L*a*b*
color parameters, which were calculated by the CIE L*a*b* col-
orimetric method (JIS Z8701-1999) with standard lighting C
and a view angle of 10°.
The magnetic properties of the products were evaluated
using a superconducting quantum interference device magnet-
ometer (SQUID, MPMS-XL, Quantum Design). The temperature
dependencies of the magnetic susceptibilities, χ, and inverse
susceptibilities, χ
−1
, at a magnetic field of 100 Oe (after zero-
field-cooled (ZFC) and field-cooled (FC)) were measured for the
temperature, T, in the range from 2.0 to 300 K. The Weiss
temperature, θ, and the Curie constant, C, were determined
from the experimental data of the χ
−1
–Tcurves between 50 and
300 K using the Curie–Weiss law, χ=C/(T−θ). The effective
moment, p
eff
, was calculated using the equation p
eff
=(3k
B
/
N
A
μ
B
×C)
1/2
, where k
B
,N
A
, and μ
B
are the Boltzmann constant,
Avogadro’s number, and the Bohr magneton, respectively. The
theoretical effective moment (p
eff-theo
) of normal spinel
CoAl
2
O
4
was calculated using the equation p
eff-theo
=g×(S(S+
1))
1/2
, where gis the g-factor and Sis the spin quantum
number of the Co
2+
ion at the T
d
site in the spinel structure.
The g-factor value was assumed to be 2.4.
15
The spin quantum
number, S, of the Co
2+
ion was 3/2 at the T
d
site.
16
As for
normal spinel CoAl
2
O
4
, there is no coexistence of Co ions at
the O
h
site. We verified the consistency of the magnetic sus-
ceptibility for the 800 product. Based on the result of Rietveld
refinement, the p
eff-cal
of the product was calculated using the
equation p
eff-cal
=[nl(p
eff
T
d
)
2
+2nm(p
eff
O
h
)
2
]
1/2
, where land m
are the abundance ratios of the Co
2+
ion at the T
d
and O
h
sites,
respectively, p
eff
T
d
is the p
eff-cal
of the Co
2+
ion with a high spin
state at the T
d
site, and p
eff
O
h
is the p
eff-cal
of the Co
2+
ion with
a high spin state at the O
h
site. nis the molar ratio of the
spinel cobalt aluminate content in the 800 product, which is
estimated to be 0.56.
Solid-state NMR spectra of
27
Al (I= 5/2, γ/2π= 11.094 MHz
T
−1
) were recorded in a magnetic field of 9.4 T at room temp-
erature for the wide-band regions. Because some NMR spectra
were too broad to be uniformly excited with a single pulse, the
wide-band NMR spectra were obtained by summing a set of FT
spectra and shifting the transient frequency at an interval of
100 kHz. Each of the FT spectra was obtained by performing a
Fourier transform on a half of a spin echo signal excited with
aπ/2 pulse of 2 μs and a πpulse of 4 μs.
Results and discussion
1. Properties of the precursor
XRD patterns of Co(OH)
2
and Al(OH)
3
(Fig. 1) were consistent
with JCPDS cards (74-1057 and 83-2256, respectively). In
addition to the peaks from Al(OH)
3
, the XRD pattern of the
precursor (Fig. 1) showed pronounced peaks around 2θ=23
and 35°. These peaks were assigned to a Co–Al hydrotalcite
(Co–AlHtc) structure,
36
showing that the precursor was com-
posed of Al(OH)
3
and Co–AlHtc. Peaks from Co(OH)
2
were not
found in the pattern for the precursor.
To study the thermal stability of the precursor, TG–DTA
measurements were performed (Fig. 2). For Co(OH)
2
(Fig. 2a),
around 300 and 900 °C, significant weight loss and related
changes in the endothermicity were observed. The former
phenomenon was assigned to the decomposition and/or the
melting of Co(OH)
2
, and the latter was assigned to the
reduction of Co
3
O
4
to CoO.
37,38
The oxidation from Co
2+
to
Co
3+
became apparent around 200 °C,
37
and the reduction
from Co
3+
to Co
2+
occurred around 900 °C. For Al(OH)
3
(Fig. 2b), distinctive weight loss and a related endothermic
peak were seen around 300 °C in the TGA and DTA curves,
respectively. They were assigned to the decomposition and/or
the melting of Al(OH)
3
.
39
For the precursor (Fig. 2c), the main
weight loss was below 300 °C, and the TGA curve looked
similar to that of Al(OH)
3
. The DTA curve of the precursor,
however, differed from both the Co(OH)
2
and Al(OH)
3
curves
in that there were three distinct endothermic peaks at 220,
250, and 300 °C. The peaks at 220 and 250 °C were attributed
to the release of interlayer water and the intercalated anions in
the Co–AlHTc, respectively.
36
The peak at 300 °C was assigned
to the decomposition and/or the melting of the Co–AlHTc and
Al(OH)
3
.
36,39
The weight loss of the precursor occurred gradu-
ally over a broad range of temperature between 300 and 800 °C
and may possibly be the result of pyrolytic elimination of
residual hydroxide groups.
21
These results suggest that the pre-
cursor does not completely decompose and/or melt below
300 °C. Additionally, a subtle change was detected around
950 °C in the DTA curve of the precursor. This suggests the
presence of a small amount of Co
3+
ions below 1000 °C, and
the reduction from Co
3+
to Co
2+
ions between 900 and
1000 °C. However, the amount of pure Co
3
O
4
as a residual
product seems to be quite small in this sample, and the dis-
tinctive weight loss around 950 °C was not observed in the
DTA curve for Co(OH)
2
(Fig. 2a).
Fig. 1 XRD patterns of Co(OH)
2
, Al(OH)
3
, and the precursor. (■) The peaks
were assigned to a Co–AlHtc structure.
Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 7167–7176 | 7169
2. XRD analysis of the products
Typical sintering temperatures for the solid–solid reaction of
CoAl
2
O
4
to achieve a blue color are generally known to be over
1000 °C.
12–15
Products with a blue color were also obtained in
this study at temperatures above 1000 °C, denoted by 1000–
1400 (see Fig. S1†). However, the colors of the 400–800 pro-
ducts were dark green. The XRD patterns of the 1000–1400 pro-
ducts (Fig. 3) were clearly that of a spinel structure assigned to
face-centered-cubic (space group Fd3
ˉ
m) CoAl
2
O
4
. This is con-
sistent with the other studies.
4–24
Interestingly, XRD patterns
for the 400–800 products (Fig. 3) could be also assigned as
having a spinel structure. Although the peak widths in the
XRD patterns of the 400–800 products were broader than those
of the 1000–1400 products, they seem to have the same spinel
structure. Note that the signal of a spinel structure was also
found in the product sintered at 200 °C, although Al(OH)
3
peaks were still present. This result is consistent with the TG–
DTA result, which showed that the main reaction of the precur-
sor starts to occur around 200 °C. Therefore, the critical
process in the formation of all of the products, regardless of
the final color of the product, seems to be the synthesis of a
spinel cobalt aluminate above 200 °C. Pérez-Ramírez et al.
investigated the thermal decomposition of a Co–Al hydrotalcite
(HTc) product (Co/Al = 3) in air.
36
It decomposed above 150 °C
and crystallized as a spinel metal oxide (a solid solution of
Co
3
O
4
,Co
2
AlO
4
, and CoAl
2
O
4
denoted as Co(Co,Al)
2
O
4
) above
200 °C. This spinel structure was enhanced with increasing
heating temperature. The decomposition and crystallization of
the HTc product are similar to those of our precursor in this
study. So, low-temperature fabrication of spinel cobalt alumi-
nate products can be successfully executed using a hydroxide
precursor and the preparation process outlined herein. It
should be pointed out that while the main component rep-
resented in the XRD of the products is cobalt aluminate
having a spinel structure (Fig. 3a), additional weak peaks and
structures in the patterns were identified and assigned to
Fig. 2 TG (solid lines) and DTA (dashed lines) curves for (a) Co(OH)
2
, (b) Al-
(OH)
3
, and (c) the precursor. The measurements were conducted at a tempera-
ture ramp rate of 10 °C min
−1
without flowing gas.
Fig. 3 XRD patterns of the products (a) sintered at temperatures between 400
and 1400 °C and (the bottom pattern) the simulated XRD pattern of CoAl
2
O
4
and (b) the evolution of the characteristic 220 and 311 reflections. The peaks
were assigned to (●)γ-Al
2
O
3
(spinel structure) and (■)α-Al
2
O
3
(corundum
structure).
Paper Dalton Transactions
7170 |Dalton Trans., 2013, 42, 7167–7176 This journal is © The Royal Society of Chemistry 2013
γ-Al
2
O
3
(spinel structure) and α-Al
2
O
3
(corundum structure).
The phase transition from γ-toα-Al
2
O
3
occurred around 1000 °C.
40
Changes in the XRD peaks at 220 and 311 with sintering
temperature systematically shifted to high angle with decreas-
ing temperature (Fig. 3b), suggesting the reduction of the
lattice constants of the products. Peaks for the 400–800 pro-
ducts were broader than those of the 1000–1400 products. The
lattice constants and crystallite sizes of the products were
clearly smaller for the products formed at lower temperature
(Table 1). The lattice constants of the 1000–1400 products were
close to those of stoichiometric CoAl
2
O
4
.
13–18,24
The peak shift
of the XRD pattern is likely caused by local distortions in the
lattice and/or the cation distribution. In fact, previous reports
have shown that the randomness of the cation distribution
and the degree of lattice defects can influence the lattice
constant.
16–18
In order to evaluate the lattice defects in the pro-
ducts, an indexed Williamson–Hall plot
34
was created using
the crystallite sizes determined from the Scherrer equation
(Fig. 4). The gradients in the plot for the 1000–1400 products
were small. In fact, these plots can be approximated as a flat
line, suggesting that local distortions are small. Additionally,
deviations from the line at the respective indices imply that
the lattice defects are isotropic. In contrast, positive gradients
in the plots for the 400–800 products are very clear, even
though the deviations from the linear square line are large.
This suggests that there are anisotropic local distortions in the
400–800 products. Sintering temperature variations, therefore,
have a significant impact on the crystallite size, lattice con-
stants, and the degree of local distortion due to defects and/or
cation distribution in the spinel-type cobalt aluminate. Tsune-
kawa et al. investigated the dependence of the crystallite size
on the lattice constant for the metal oxide nanoparticles,
showing that the lattice constants of nanoparticles tended to
increase with decreasing crystallite size.
41
This is caused by
the lattice defects such as oxygen vacancies. That is, lattice
defects are easy to produce in nanosized crystals. The increase
of lattice constants was indeed seen in our previous studies for
metal oxide nanoparticles.
25–30
Therefore, the crystallite size
possibly correlates with the degree of lattice defects. This
evaluation suggested a transition for the crystallinity between
800 and 1000 °C, which is possibly due to the reduction from
Co
3+
to Co
2+
ions and the phase transition from γ-toα-Al
2
O
3
around 1000 °C. From the viewpoint of the crystallinity, low-
temperature products (400 to 800 °C) are distinguished from
high-temperature products (1000 to 1400 °C).
Based on the results, the XRD pattern of the 800 product
measured by a synchrotron source was evaluated using the
Rietveld refinement (Fig. 5). The lattice constants, a, the
oxygen positional parameters, x
O
, molar ratio, site occu-
pancies, g, isotropic thermal factors, B
iso
, and Bragg reliability
parameters, R
B
, for the respective phases in the product
(cobalt (Co) and aluminum (Al) cations on the tetrahedral
(T
d
or A-) and octahedral (O
h
or B-) sites) are listed in Table 2.
The composition of the 800 product was estimated at
(Co
0.87
Al
0.13
)[Al
0.60
Co
0.40
]
2
O
4
, where (…) and […] represent the
Table 1 Lattice constant (a) calculated from XRD patterns. Crystallite sizes from
XRD line broadening of the peaks
Products Lattice constant a(Å) Crystallite size (nm)
400 8.073(1) 20.2
600 8.087(1) 23.9
800 8.0953(5) 53.3
1000 8.1022(6) 78.3
1200 8.1089(4) 111
1400 8.1099(3) 132
JCPDS 82–2252 8.106 —
Ref. 13
a
8.10735(1) —
Ref. 13
a
8.09853(1) —
Ref. 14 8.1070(1) —
Ref. 15 8.1078(3) —
Ref. 16 (17)
b
8.092 —
Ref. 18 8.10452(5) —
Ref. 24
c
8.0995 —
Ref. 24
c
8.1051 —
a
The top and bottom values are polycrystalline bulk and single crystal
of CoAl
2
O
4
, respectively.
b
These references are from the same author
and include the same value.
c
The top and bottom values are the
values of the CoAl
2
O
4
products synthesized at 800 and 1100 °C,
respectively.
Fig. 4 Indexed Williamson–Hall plot of the products sintered at temperatures
between 400 and 1400 °C.
Fig. 5 SXRD pattern and Rietveld refinement for the 800 product. The
measured intensities (red dots) are compared with the calculated profile using
Rietveld refinement (black solid line). The Bragg positions of both the spinel
structures, (Co
1−x
Al
x
)[Al
1−y
Co
y
]
2
O
4
and γ-alumina, are indicated by the vertical
green bars and the residual of the refinement is shown by the bottom blue solid
line.
Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 7167–7176 | 7171
occupations at the T
d
(A-) and O
h
(B-) sites, respectively,
showing non-stoichiometry of the cations and excess of Co
ions. In the TG–DTA results, the weight loss of about 10% was
seen in Al(OH)
3
and the precursor between 300 and 800 °C
although there was no weight loss for Co(OH)
2
in the tempera-
ture range, suggesting that the Co content part in the precur-
sor seems to decompose and/or melt more easily than the
Al(OH)
3
part in it. It leads to some excess of Co in the final
product at low temperature below 800 °C, suggesting the for-
mation of a Co enriched spinel product. However, the product
had a cation distribution in the structure, supporting the
result of the Williamson–Hall plot. It suggests that the cation
distribution seems to correlate with the lattice defects in the
products. Additionally, from the viewpoint of the charge
balance in the spinel structure of the 800 product, about 80%
of Co
3+
ions occupied the O
h
site instead of Co
2+
ions. It
suggests that a large amount of Co
3+
ions is contained in the
products sintered at low temperatures, which is consistent
with the TG–DTA results. Other previous work also points to
the presence of Co
3+
ions in samples prepared at low tempera-
ture.
19,20
The Co
3+
ions seem to replace the Al
3+
ions at the O
h
site in the product. The evaluation of other products using the
Rietveld refinement is in progress now. Note that the spinel
cobalt aluminate products in this study are identified as
(Co
1−x
Al
x
)[Al
1−y
Co
y
]
2
O
4
(x≠y/2).
3. FT-IR spectroscopic analysis of the products
The FT-IR spectra of the products provide additional structural
information (Fig. 6). The spectra of the products were very
different from that of the precursor. The bands of the metal–
oxygen bonds in spinel structure appear in the region between
1000 and 400 cm
−1
.
20–23,37,42–44
In all of the products, distinc-
tive bands around 670 (ν2) and 570 (ν3) cm
−1
were observed
(Fig. 6b). These two bands were similar to the bands observed
for Co
3
O
4
.
37
A shoulder band is also present around 870 (ν1)
cm
−1
in the spectra from the 400–1000 products. The intensity
of this shoulder decreases for the products sintered at higher
temperatures. This trend is in contrast with the band around
510 cm
−1
(ν4), which increases in intensity as the sintering
temperatures rise above 800 °C. The observed three band struc-
ture (ν2, ν3, and ν4) in the FT-IR spectra is consistent with
those previously reported for CoAl
2
O
4
.
20–23
Inspection of the
product spectra reveals that the ν2 and ν3 bands shift to lower
wavenumbers and the intensity of the ν2 band increases with
increasing sintering temperature, suggesting a change in the
bonding between the metal and oxygen ions in the spinel
structure of the products ((Co
1−x
Al
x
)[Al
1−y
Co
y
]
2
O
4
). Considering
some of the results from the XRD study, such as the sintering
temperature variation on lattice constants and local distortion,
observed changes in the FT-IR spectra may be the result of the
strong dependence of the distribution of Co and Al ions on the
sintering temperature.
Some distinctive bands were observed in the spectra of the
products besides the metal–oxygen bands. A broad band
between 3600 and 3200 cm
−1
and a band at around 1650 cm
−1
were seen in the spectra for the 400–1000 products. These
Table 2 Lattice constants, oxygen positional parameters, molar ratio, site occupancies, isotropic thermal factors, and Bragg reliability parameters for the respective
phases of the 800 product. The weighted profile reliable parameter for the 2-phase refinement is R
wp
= 2.517
Composition a(Å) x
O
Molar ratio gB
iso
(Å
2
)R
B
Cobalt aluminate ((Co
0.87
Al
0.13
)[Al
0.60
Co
0.40
]
2
O
4
) 8.09348(5) 0.26342(7) 0.56(1) 0.8710(37) (Co, 8a) 4.317
0.1290(4) (Al, 8a) 0.5 (8a)
0.6000(31) (Al, 16d) 0.5 (16d)
0.4000(26) (Co, 16d) 0.32 (Oxy)
1 (Oxy)
0.58 (Al, 8a) 0.5 (8a)
γ-Alumina (Al
8/3
O
4
) (Ref. 35) 7.96669 0.2547(05) 0.44(1) 0.84 (Al, 16d) 0.5 (16d) 2.414
0.17 (Al, 32e) 0.5 (32e)
1 (Oxy) 0.5 (Oxy)
Fig. 6 (a) FT-IR spectra of the precursor and products sintered at the tempera-
tures between 400 and 1400 °C. (b) Evolution of the spectra for the products
enlarged in the range of 1100 to 400 cm
−1
.
Paper Dalton Transactions
7172 |Dalton Trans., 2013, 42, 7167–7176 This journal is © The Royal Society of Chemistry 2013
bands were attributed to the O–H mode of chemisorbed water
and/or terminated hydroxides,
25–27,45,46
suggesting that water
and/or hydroxide groups are chemisorbed on the surface and/
or incorporated into the product crystals.
46
Chemisorbed water
and/or terminated hydroxides are believed to coordinate to the
metal ions and possibly cause lattice defects in the
products.
26–28
The O–H bands disappear in the spectra of the
products prepared at high temperatures above 1200 °C, which
is particularly noticeable in the bands between 3600 and
3200 cm
−1
. The weakened intensity of these bands indicates a
decrease in the amount of the chemisorbed water and/or the
terminated hydroxides. From the Williamson–Hall plot of the
products (Fig. 4), the degree of the lattice defects in the 400–
800 products was higher than that in the 1000–1400 products.
The amount of chemisorption, therefore, seems to correlate to
the quantity of lattice defects in the products. Notably, the che-
misorption seems to strongly affect the crystallinity of the pro-
ducts with small crystallite size since the surface-to-volume
ratio is generally large for nanosized crystals. Although the
chemisorption was detected in the 1000 product, the degree of
lattice defects was the same as those of the 1200 and 1400 pro-
ducts. The chemisorption possibly does not affect the crystalli-
nity of the 1000 product.
4. UV–VIS spectroscopic analysis of the products
The color of the spinel structure products varied with the sin-
tering temperature (Fig. S1†). As mentioned previously, the
1000–1400 products had a blue color, while the color of the
low-temperature 400–800 products was dark green. The product
color comes from the Co ion because pure aluminum oxides
and hydroxides show little absorption in the visible region. No
distinctive absorption bands were observed in the region for
the spectra of Al(OH)
3
and Al
2
O
3
(not shown). The color change
of the products suggests a change in the crystal (ligand) field,
such as the coordination environment and/or a valence change
of the Co ion.
47,48
The absorption spectrum for the Co ion is
known to contain contributions from a spin-allowed, d–dtran-
sition with Jahn–Teller distortion of the coordination structure
and ligand-to-metal charge transfer (LMCT).
47,48
The UV–VIS spectrum of the precursor shows bands around
530 and 450 nm, which are assigned to the d–d transition of
the Co
2+
ion with the octahedral (O
h
) structure of Co–AlHtc in
the precursor (Fig. 7).
36,47,48
These transition bands are from
the
4
T
1g
ground state of the d
7
electrons.
47,48
The spectra of
the 1000–1400 products are similar to the well-studied spec-
trum of the CoAl
2
O
4
blue pigment.
19–24
The pronounced
bands around 620, 580, and 550 nm were assigned to the
[
4
A
2
(F) →
4
T
1
(P)] of the d–d transitions of the Co
2+
ion at the
T
d
site.
19–24,47,48
The distinctive blue color originates from
these absorption bands, which are increasingly prominent for
the products formed under a higher sintering temperature,
suggesting that the amount of Co
2+
ion at the T
d
site increases
in the spinel structure. In the 1200 and 1400 products, small
bands around 480 and 410 nm appeared in the spectra and
were assigned to the spin-forbidden d–d transition of the Co
2+
ion at the T
d
site.
20,48
In contrast, the spectra for the 400–800 products were notice-
ably different from those of the 1000–1400 products. The spectra
of the 400 and 600 products, with a dark green color, show two
broad absorption bands, one between 800 and 550 nm and the
other between 550 and 300 nm. These spectra were similar to
that of Co
3
O
4
,
38,49
and according to the band assignment for
Co
3
O
4
, the two aforementioned bands were assigned as LMCT
bands. The band between 800 and 550 nm arises from LMCT
from O
2−
to Co
3+
ions at the O
h
site, and the band between 550
and 300 nm is from LMCT from O
2−
to Co
2+
ions at the T
d
site in
the products with spinel structures ((Co
1−x
Al
x
)[Al
1−y
Co
y
]
2
O
4
),
respectively. The color of the 800 product was dark aqua, and
there was a broad band between 550 and 300 nm in the spec-
trum. However, the broad band between 800 and 550 nm split
into several bands around 620, 580, and 550 nm, representing
the d–d transitions of the Co
2+
ion at the T
d
site. Additionally, a
broadbandaround720nmwasobservedandassignedasa
LMCT band from O
2−
to Co
3+
ions at the O
h
site. Note that the
bandbetween550and300nmandthebandaround720nm
decreases in the spectra of the 1000–1400 products. The band
below 400 nm remains a shoulder, and therefore can be assigned
to the LMCT band from O
2−
to Co
2+
ions at the T
d
site in
CoAl
2
O
4
with blue color. These optical results are consistent with
the TG–DTA and XRD analyses showing the presence of the Co
3+
ion. The drastic change in the spectra between the 800 and 1000
products may be attributed to the reduction of the Co
3+
and a
characteristic configuration of the cations in the metallic sites of
the spinel structure. However, since the shoulder band around
720 nm was seen in the spectrum of the 1000 product, the
reduction from Co
3+
to Co
2+
ions does not seem to finish at
1000 °C for 2 hours. In other words, almost all of the Co ions
come to reside on the T
d
(A-) site as Co
2+
.
The blueness (b* value) of the products increased with
increasing sintering temperature (Table 3). Notably, the blue-
ness of the 1200 and 1400 products was remarkably high com-
pared to measurements reported previously.
24,31,50
This simple
and rapid synthesis method for preparing spinel cobalt alumi-
nate products with a bright blue color could thus be useful for
the synthesis of industrial pigment.
Fig. 7 UV–VIS powder diffuse reflectance spectra of the precursor and the pro-
ducts sintered at the temperatures between 400 and 1400 °C.
Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 7167–7176 | 7173
5. Magnetic analysis of the products
According to the structural and optical analyses described
above, the coordination environment and the electronic state
of the Co ions in the spinel structure ((Co
1−x
Al
x
)[Al
1−y
Co
y
]
2
O
4
)
strongly depend on the sintering temperature. These variations
can be further verified by magnetic analysis since changes in
the coordination environment of the Co ions affect the mag-
netic moment. Theoretical calculations have shown that the
magnetic state of a Co
2+
ion located on the T
d
or O
h
site is dis-
tinguishable because the number of unpaired d-electrons on
the Co
2+
ion is different.
16
Additionally, the valence change of
the Co ions can also affect the magnetic moment.
Temperature, T, dependencies of the molar magnetic sus-
ceptibilities, χ, and inverse susceptibilities, χ
−1
, for the pro-
ducts were investigated (Fig. 8). Both the χ–Tand χ
−1
–Tcurves
of the products changed systematically. In the low temperature
region, the χ–Tcurves of the 400–1000 products were quite
different from those of the 1200 and 1400 products (Fig. 8a). A
magnetic anomaly around 10 K was observed for the 1200 and
1400 products and is similar to what was seen in previous
studies.
11–15
In the 400–1000 products, the behavior of the χ–T
curves is similar, and the magnetic transition is indistinguish-
able. The χ
−1
–Tcurves of all the products followed the Curie–
Weiss (CW) law in the higher temperature regions (Fig. 8b). It
should be noted that, even though the Co
3+
ion was contained
in the 400–1000 products, the magnetic behaviors were quite
different from that of Co
3
O
4
that shows an antiferromagnetic
transition at 33 K.
14,30,38,51
The magnetic behavior of Co
3
O
4
prepared from the reference Co(OH)
2
was thus different from
that of all the products (Fig. S2†). As expected, the prepared
Co
3
O
4
products showed an antiferromagnetic transition
around 30 K. The difference in the magnetic properties
between the products and Co
3
O
4
also proves that the spectral
data obtained for the products contained no Co
3
O
4
impurities.
Tristan et al. synthesized spinel Co[Al
1−x
Co
x
]
2
O
4
(x> 0), and
the magnetic properties of them were measured.
14
The
maximum point in the susceptibility increases with increasing
xin Co[Al
1−x
Co
x
]
2
O
4
, that is, the antiferromagnetic behavior is
stabilized with increasing x. The magnetic susceptibilities of
Co[Al
1−x
Co
x
]
2
O
4
with various xvalues are different from those
of our products sintered at low temperatures in this study.
Therefore, the magnetic behaviors of the products sintered at
low temperatures seem to be an intrinsic phenomenon,
although the products are nonstoichiometric. It suggests that
undiscovered spinel products were formed at low temperatures
that is possibly induced by various cation distributions in the
spinel structure ((Co
1−x
Al
x
)[Al
1−y
Co
y
]
2
O
4
).
The Weiss temperature (θ), the Curie constant (C), and the
effective moment (p
eff
) for all products were calculated
(Table 4). The values of p
eff
for the 1200 and 1400 products
were close to the values for high-temperature synthesized
CoAl
2
O
4
,
11–15
suggesting that both the magnetic moment and
magnetic interactions systematically decreased by lowering the
sintering temperature. Unpaired d-electrons of the Co
2+
ions
decreased through changing the coordination environment of
the Co
2+
ions from the T
d
to the O
h
sites and the formation of
Co
3+
ions at the O
h
sites in the spinel structure. The p
eff-cal
of
the 800 product, 3.45μ
B
, was estimated from the result of the
Rietveld refinement, (Co
0.87
Al
0.13
)[Al
0.60
Co
0.40
]
2
O
4
and n= 0.56.
Remarkably, as indicated in Table 4, the p
eff-cal
is close to the
observed p
eff
for the 800 product. Note that the mixed valent
Fig. 8 (a) Zero-field-cooled and field-cooled magnetic susceptibility ( χ)versus
temperature (T) plots measured at 100 Oe. The inset shows enlarged χ–Tplots.
(b) Inverse susceptibility ( χ
−1
)versus temperature (T) plots of the products sin-
tered at temperatures between 400 and 1400 °C.
Table 3 CIE L*a*b* colorimetric parameters of the precursor, the products sin-
tered at the temperatures between 400 and 1400 °C, and values reported in the
literature
Products L*a*b*
Precursor 83.3 9.91 −2.59
400 41.8 −1.95 4.04
600 40 −2.74 3.92
800 41.2 −4.32 1.21
1000 47.2 −7.74 −9.57
1200 50.3 −6.47 −26
1400 49.9 −0.13 −35.4
Ref. 24
a
32 —−16
Ref. 24
a
26 —−26
Ref. 31 81.99 −3.9 −15.62
Ref. 50 38.63 −3.28 −14.39
a
The top and bottom values are the values of the CoAl
2
O
4
products
synthesized at 800 and 1100 °C, respectively.
Paper Dalton Transactions
7174 |Dalton Trans., 2013, 42, 7167–7176 This journal is © The Royal Society of Chemistry 2013
state of the Co ion at the O
h
site is expected to establish due to
the charge neutrality in the determined crystallographic state,
that is, 16% Co
2+
and 84% Co
3+
ions at the O
h
site. As shown
in Fig. S3,†broadening of the
27
Al-NMR spectra can be attribu-
ted to changes in the internal magnetic field resulting from
the magnetic spin on the Co
2+
ions around the Al ions,
21
most
likely via the dipole–dipole or the transferred hyperfine inter-
actions. These magnetic properties are also consistent with the
structural and optical analyses, and are reflected by the cation
distribution. The spectra of the 400 and 800 products can be
assigned to γ-Al
2
O
3
as the impurity phase. It should be empha-
sized that besides the sharp signal weak broad signals were
seen in the spectra which can be assigned to
27
Al nuclei sur-
rounded by the electronic spins on Co ions dominating in the
cation sites of the spinel structure. Note that the sharp com-
ponent in the spectrum is apparently intensified compared
with the broad component, since the excitation frequency
region and power are limited in this
27
Al-NMR measurement.
The experimental data show the presence of lattice distor-
tions and/or Al
2
O
3
as impurities in the products, and the non-
stoichiometry of the cations. The changes in the crystallo-
graphic and the physical properties are not only attributed to
the changes in the valence state of the Co ions but also to the
cation distributions in the spinel structure ((Co
1−x
Al
x
)-
[Al
1−y
Co
y
]
2
O
4
). The physical properties of the products sintered
at low temperatures were undiscovered. Since the physical pro-
perties of the products sintered at high temperatures were
similar to that of conventional spinel CoAl
2
O
4
, the stoichi-
ometry of them seems to be close to the conventional one.
These results suggest that the cation distribution strongly
affects the physical properties. These unique properties con-
tribute to the crystallographic degree of freedom in the spinel-
type CoAl
2
O
4
.
Conclusion
A simple and rapid method for the synthesis of spinel cobalt
aluminate products ((Co
1−x
Al
x
)[Al
1−y
Co
y
]
2
O
4
with x≠y/2) via
sintering of a hydroxide precursor was developed and
described. Products prepared at temperatures over 1000 °C
were essentially the commonly described form of CoAl
2
O
4
known as blue pigment. Interestingly, however, low tempera-
ture preparation (>400 °C) of the products with a spinel struc-
ture as the main phase was also achieved. Lattice constants
and crystallite sizes of the products systematically increased
with increasing sintering temperature. The color of the pro-
ducts turned from dark green at low temperatures to blue
between 800 and 1000 °C. The products prepared at tempera-
tures over 1000 °C were bright blue. This blue color was attrib-
uted to the d–d transition of the Co
2+
ion at the T
d
site in the
spinel structure. Therefore, the degradation of the blue color
in the low-temperature formation of the products resulted
from the decrease of the Co
2+
ion at the T
d
site. The degree of
lattice defects in the products prepared at temperatures below
1000 °C tended to be high compared to the high-temperature
products. The low-temperature products also showed evidence
of chemisorbed water and/or hydroxide groups. These results
are possibly due to the reduction of the Co
2+
ion at the T
d
site
and subsequent increase of the amount of Co
3+
ion at the O
h
site. The Weiss temperature and the effective moment of the
products increased with increasing sintering temperature. The
magnetic analysis suggested that an extreme change in the
cation distribution occurred for the low-temperature products.
The products from the low-temperature preparation showed a
change in both the valence of the Co ions and the cation distri-
bution in the spinel structure.
The described preparation technique for synthesizing
spinel cobalt aluminate products is simpler and faster than
the conventional solid-phase reaction and can thus have direct
industrial application. This study revealed that the physical
properties of the products prepared at low temperature were
very different from that of conventional CoAl
2
O
4
known as
blue pigment. The obtained properties are also important for
the development of novel functional materials for CoAl
2
O
4
.
Treatment at low temperature seems to be important for con-
trolling the cation distribution of spinel CoAl
2
O
4
. While this
low-temperature sintering method using a hydroxide precursor
is very effective for the preparation of spinel cobalt aluminate
products, the presence of Co
3+
ions and Al
2
O
3
should be elimi-
nated as much as possible, and the stoichiometry of the
cations in the spinel products should be the same as that of
conventional CoAl
2
O
4
and will be the next issue of this study.
Acknowledgements
This work was partially supported by Grant-in-Aids for Scienti-
fic Research (KAKENHI) (A) No. 21241023 and (S) No.
20226015.
Table 4 Curie constants (C), Weiss temperature (θ), and effective moment
(p
eff
) determined from the CW fits to the magnetic susceptibility of the pro-
ducts and the inversion parameter, α, estimated from the magnetic quantities
Products Curie constant: C
(emuK mol
−1
)Curie–Weiss
temperature : θ(K)
Effective
moment: p
eff
(μ
B
)
400 1 −79.9 2.83
600 1.27 −80.2 3.19
800 1.68 −83.6 3.67
1000 2.18 −90.7 4.18
1200 2.7 −109 4.65
1400 2.71 −102 4.66
Ref. 11 —−96(8) 4.46(5)
Ref. 12 —−89(6) 4.45(8)
Ref. 13
a
—−99(1) 4.62(2)
Ref. 13
a
—−94(1) 4.63(2)
Ref. 14 —−119 4.88
Ref. 15 —−104(2) 4.65(9)
Np
eff-
theob
—— 4.65
a
The top and bottom values are polycrystalline bulk and single crystal
of CoAl
2
O
4
, respectively.
b
The theoretical effective moment (Np
eff-theo
)
of the normal spinel structure was shown.
Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 7167–7176 | 7175
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