Characterization of cobalt oxides studied by FT-IR, Raman, TPR and TG-MS

Article abstract of DOI:10.1016/j.tca.2008.04.015

The as-prepared cobalt oxide (assigned as CoO_x) was fabricated by precipitation-oxidation from aqueous cobalt nitrate solution using sodium hydroxide and oxidation with hydrogen peroxide. Another series of pure cobalt oxides was refined by the decomposition of CoO_x in a nitrogen environment at temperatures of 280, 450 and 950 °C (D-280, D-450 and D-950, respectively). Phase transformation, structural properties and red-ox properties were characterized by thermogravimetry-mass spectrometry (TG-MS), X-ray diffraction (XRD), infrared spectroscopy (IR), Raman spectroscopy and temperature-programmed decomposition/reduction (TPD/TPR). Analysis of the thermal behavior on CoO_x revealed that a series of pure cobalt oxide with particle sizes of 10-20 nm could be obtained easily. The results demonstrated that the refined samples D-280, D-450 and D-950 were CoO(OH), Co₃O₄ and CoO, respectively.

Full text of DOI:10.1016/j.tca.2008.04.015

Thermochimica Acta 473 (2008) 68–73
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Characterization of cobalt oxides studied by FT-IR, Raman, TPR and TG-MS
Chih-Wei Tanga, Chen-Bin Wangb,∗, Shu-Hua Chienc,d,∗∗
a Department of Chemistry, Chinese Military Academy, Kaohsiung 83059, Taiwan, ROC
b Department of Applied Chemistry and Materials Science, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan 33509, Taiwan, ROC
c Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, ROC
d Department of Chemistry, National Taiwan University, Taipei 10764, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
The as-prepared cobalt oxide (assigned as CoOx) was fabricated by precipitation–oxidation from aqueous
cobalt nitrate solution using sodium hydroxide and oxidation with hydrogen peroxide. Another series
of pure cobalt oxides was refined by the decomposition of CoOx in a nitrogen environment at temper-
atures of 280, 450 and 950 ◦C (D-280, D-450 and D-950, respectively). Phase transformation, structural
properties and red-ox properties were characterized by thermogravimetry-mass spectrometry (TG-MS),
X-ray diffraction (XRD), infrared spectroscopy (IR), Raman spectroscopy and temperature-programmed
decomposition/reduction (TPD/TPR). Analysis of the thermal behavior on CoOx revealed that a series of
pure cobalt oxide with particle sizes of 10–20 nm could be obtained easily. The results demonstrated that
the refined samples D-280, D-450 and D-950 were CoO(OH), Co3O4 and CoO, respectively.
© 2008 Elsevier B.V. All rights reserved.
Received 20 February 2008
Received in revised form 22 April 2008
Accepted 23 April 2008
Available online 29 April 2008
Keywords:
Cobalt oxides
Thermal analysis
TG-MS
1. Introduction
drally coordinated with oxygen ions. The complex is stable up to
800 ◦C and decomposes to CoO [14,20,21] above 900 ◦C.
Cobalt oxide has a wide range of applications in various indus-
trial sectors, including in rechargeable batteries [1], as a catalyst of
the abatement of CO [2], as a magnetic material [3] and in CO sen-
sors [4]. Five species of cobalt oxide [CoO2, Co2O3, CoO(OH), Co3O4
and CoO] have been reported [4–9]. However, cobalt oxide with a
valence of more than three is unstable in the natural environment.
Other cobalt oxides [Co3O4 and CoO] are more stable and useful
in industry. Cobalt oxyhydroxide, CoO(OH), has a hexagonal struc-
ture in which a divalent metal cation is located at an octahedral site
which is coordinated by six hydroxyl oxygen. Well-spread CoO(OH)
can be used as the conductive network in rechargeable alkaline bat-
teries [5]. CoO is an antiferromagnetic material whose magnetic
characteristics [10,11] and application of gas-sensors [12,13] have
been extensively studied. CoO nanocomposite film in gas-sensing.
Co3O4 exhibited high catalytic activity in CO oxidation [14–16].
The spinel tricobalt tetraoxide, Co3O4, has an energy band-gap of
1.4–1.8 eV [17] that can be used as a p-type semiconductor and
an antiferromagnetic material [18,19]. The distribution of cations
on spinel Co3O4 is shown to be Co2+ [Co23+] O42−: the cations
inside parentheses are octahedral and those outside are tetrahe-
A preview paper [9] reported that variation in the morphol-
ogy of cobalt oxides with calcination and reduction pretreatments.
Figlarz et al. found that the hexagonal Co3O4 was obtained by
the decomposition of CoO(OH) at approximately 250 ◦C, based
on analysis by X-ray diffraction (XRD) and selected-area elec-
tron diffraction (SAED). To understand the phase transformation
and the composition of decomposed outlet gases, this work dis-
cusses the analysis of the thermogravimetry-mass spectrometry
(TG-MS). Temperature-programmed decomposition (TPD), X-ray
diffraction (XRD), infrared spectroscopy (IR), Raman spectroscopy
and temperature- programmed reduction (TPR) are employed to
characterize a series of cobalt oxides.
2. Experimental
2.1. Preparation of cobalt oxides
The as-prepared cobalt oxide (assigned as CoOx) with a high
valence state of cobalt was synthesized by the precipitation–
oxidation method in an aqueous solution. The precipitation process
was carried out at 50 ◦C with 50 ml of 0.6 M Co(NO3)2·H2O solution
added drop by drop to 100 ml of 3.2 M NaOH solution; 100 ml of
H2O2 (50 wt%) was then introduced dropwisely under constant stir-
ring. In order to avoid the contamination of chloride ion, the H2O2
was chosen as an oxidizing agent instead of NaOCl [22]. The precip-
itate was then filtered, washed with deionized water and dried in
∗
Corresponding author.
Corresponding author at: Institute of Chemistry, Academia Sinica, Taipei 11529,
∗∗
Taiwan, ROC.
E-mail addresses: chenbinwang@gmail.com (C.-B. Wang),
chiensh@gate.sinica.edu.tw (S.-H. Chien).
0040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2008.04.015

C.-W. Tang et al. / Thermochimica Acta 473 (2008) 68–73
69
an oven at 110 ◦C for 24 h. The dried CoOx was ground and preserved
in a desiccator as fresh samples. Further, other series of pure cobalt
oxide were refined from the decomposition of CoOx under nitro-
gen environment at different temperatures: 280, 450 and 950 ◦C,
respectively (assigned as D-280, D-450 and D-950, respectively).
2.2. Characterization techniques
X-ray diffraction (XRD) measurements were performed using a
˚
Siemens D5000 diffractometer with Cu K␣1 radiation (ꢀ = 1.5405 A)
at 40 kV and 30 mA with a scanning speed in 2ꢁ of 2◦ min−1. Using
the XRD diffraction data, the crystallite sizes of cobalt oxides were
able to be estimated using the Scherrer equation.
0.9ꢀ
B cos ꢁB
d =
(1)
Fig. 1. TPD profile of CoOx.
In this equation, d is the crystallite size (nm); ꢀ the X-ray wave-
length; B the full width at half maximum of the diffraction peak at
ꢁB; ꢁB is the diffraction angle.
Specific surface area measurements were carried out by using
the Brunauer–Emmett–Teller (BET) method on a Micromeritics
ASAP 2010 apparatus. Nitrogen adsorption isotherms at −196 ◦C
were determined volumetrically. The catalysts were pre-outgassed
at 5 × 10−5 Torr for 3 h at 110 ◦C. The surface area was determined
from the nitrogen adsorption isotherm.
units (amu). The measurement of outlet gas via mass spectromet-
ric intensities was plotted as a function of temperature that showed
two well separated regions of de-volatilization.
3. Results and discussion
3.1. Structural analysis of cobalt oxides
IR spectra of samples were obtained by a Bomen DA-8 spectrom-
Fig. 1 displays the TPD profile of the as-prepared CoOx sam-
ple. Three peaks of oxygen desorption at different temperatures
(Td) – D1 (Td = 260 ◦C), D2 (Td = 310 ◦C) and D3 (Td = 870 ◦C) –
are observed. The sequential desorption of oxygen from CoOx indi-
cates that cobalt oxide with different oxidation states may be
obtained by the appropriate thermal decomposition of CoOx in a
nitrogen environment.
eter with a resolution of 4 cm−1 in the range of 450–1000 cm−1
.
One milligram of each powder sample was diluted with 200 mg
of vacuum-dried IR-grade KBr powder and subjected to a pres-
sure of 10 tons. The measurements of Raman spectroscopy were
recorded using a Nicolet Almega XR dispersive Raman spectrome-
ter. The spectra were collected between 300 and 900 cm−1, using a
beam of diode laser (780 nm), with the sample exposed to the air
under ambient conditions.
D
D
D
3
1
2
CoOx−→ CoO(OH)−→ Co3O4−→ CoO
(2)
TPR of a series of cobalt oxides was performed using 10% H2 in
Ar as the reducing gas. The flow rate of H2/Ar was adjusted by mass
flow controller under 25 ml min−1. The cell was a quartz tube with
an inner diameter 8 mm and 80 mg of the catalyst was mounted
with quartz wool. The hydrogen consumption was monitored by a
thermal conductivity detector (TCD) on raising the sample temper-
ature from RT to 500 ◦C at a constant rate of 5 ◦C min−1. TPD profile
of CoOx sample was performed using He as carrier gas. The flow
rate of He was adjusted to 25 ml min−1. The oxygen desorption was
monitored by a TCD on raising the sample temperature from RT to
Cobalt ions with an oxidation state of +3 [CoO(OH)], +8/3
[Co3O4] and +2 [CoO] may be obtained from CoOx by thermal
decomposition at 280, 450 and 950 ◦C, respectively. Samples pre-
pared in this way are designated herein as D-280, D-450 and D-950.
Although D1 and D2 peaks in the spectrum are merged together, the
desired pure sample of CoO(OH) is easily obtained by the simple
thermal decomposition of CoOx at 280 ◦C in the TPD system (see
the characterizations below).
Fig. 2 shows XRD patterns of refined species of cobalt oxide.
Columns 2–4 of Table 1 presents the species, crystal phase and
particle size of theses oxides. The diffraction profile of the D-280
sample (Fig. 2(a)) matches the JCPDS (PDF-74-1057) [23] file, iden-
tifying cobalt oxyhydroxide, CoO(OH), with a hexagonal structure.
The average particle size is small (10 nm, according to the peak
width and the Scherrer equation). The D-450 sample (Fig. 2(b))
matches the JCPDS (PDF-76-1802) file identifying cobaltic oxide,
Co3O4, with a spinel structure. The D-950 sample (Fig. 2(c)) matches
the JCPDS (PDF-71-1178) file for cobaltous oxide, CoO, with a face-
centered cubic (fcc) structure. The peak width in the D-950 pattern
1000 ◦C at a constant rate of 10 ◦C min−1
.
On-line TG-MS analyses of the gases evolved from the sam-
ple were performed simultaneously using the STA-409CD with
Skimmer coupled to a quadruple mass spectrometer QMA 400
(maximum 512 amu). The TG experiments were operated from RT
to 1100 ◦C with heating rate of 10 ◦C min−1 under a continuous flow
of He (100 ml min−1). A transfer line, specially designed to connect
a vacuum pump in order to optimize the amount of evolved gas,
transferred from the TG to the MS. A mass analysis was performed
each 2 s recording mass fragments between 2 and 300 atomic mass
Table 1
A series of the species of cobalt oxide for characterization
Samples
XRD
SBET
FT-IR (m2 g−1
(cm−1
)
Raman shift (cm−1
)
TPR (C)
Co3O4
Structurea
d (nm)b
␯
)
CoO
Co
Co–O
CoOOH
Co3O4
CoO
Hexagonal
Spinel
fcc
10
11
16
59
55
5
584
570, 661
507
367, 482, 599, 809
482,519,621, 690
455, 675
240
–
–
295
340
–
500
500
550
a
According to the data base of JCPDS2001.
Particle size by Debye–Scherrer equation with CoOOH(1 1 1), Co3O4(3 1 1) and CoO(2 0 0)peak.
b

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C.-W. Tang et al. / Thermochimica Acta 473 (2008) 68–73
Fig. 2. XRD profiles of (a) D-280 (b) D-450 (c) D-950.
Fig. 3. FTIR spectra of (a) D-280 (b) D-450 (c) D-950.
indicates substantial sintering of primary crystallites (d increases to
16 nm). Nitrogen adsorption isotherms on a series of cobalt oxides
are obtained at −196 ◦C. They are similar across the adsorbents of
interest. They are type II by Brunauer’s classification. The decrease
in the surface area (column 5 in Table 1) is associated with the
sintering of D-950 sample.
The phase transformation revealed by XRD is accompanied by
simultaneous variation in both IR and Raman spectra. Fig. 3 shows
the IR absorption spectra of D-280, D-450 and D-950 samples. The
IR spectra of D-280 and D-950 samples (Fig. 3(a) and (c)) display sin-
gle bands (584 and 507 cm−1) that are likely to be associated with
the cobalt ion in octahedral holes, meaning in an oxygen octahedral
environment [24].
The variation may be caused by the difference between their
structures (hexagonal vs. face-centered cubic). The IR spectrum
of D-450 sample (Fig. 3(b)) displays two distinct bands that orig-
inate from the stretching vibrations of the metal-oxygen bonds
[22,23,25]. The first band (ꢂ1) at 570 cm−1 is associated with the
OB3 vibration in the spinel lattice, where B denotes Co3+ in an octa-
hedral hole. The second band (ꢂ2) at 661 cm−1 is attributed to the
ABO3 vibration, where A denotes the Co2+ in a tetrahedral hole.
Fig. 4 displays the Raman spectra of a series of the cobalt oxides
formed by thermal decomposition. The D-280 sample has bands
at 367, 482, 599 and 809 cm−1 (Fig. 4(a)), which are assigned to
CoO(OH). Following the higher-temperature treatments, different
bands were observed at 482, 519, 621, 690 cm−1 for D-450 (Fig. 4(b))
and 455, 675 cm−1 for D-950 (Fig. 4(c)), probably because of the
phase transformation under heat treatment. The prominent Raman
peaks correspond to the Eg (482 cm−1), F2g (519 and 621 cm−1), A1g
(690 cm−1) modes of the Co3O4 crystalline phase (D-450) and are
consistent previous investigations [26,27]. Upon heat treatment at
950 ◦C, the 519 and 621 cm−1 peak disappeared and broad bands
at 455 and 675 cm−1 appeared, because of the formation of cubic
CoO. These are shifted from the positions of the main peaks, 672 and
468 cm−1, in previous studies of bulk CoO [28–30], shifted because
of the nano-sized effect. This result confirms that the crystalline
structure of the series of cobalt oxides can be identified by Raman
spectroscopy.
3.2. Reductive behavior of cobalt oxides
Fig. 5 compares the reduction behavior of the series of cobalt
oxides. The D-280 sample (Fig. 5(a)) yields three sequent signals
at 215, 260 and 350 ◦C. According to our previous study [2], we
have stopped each TPR step (under three reduction temperature) to
record the XRD pattern of the sample and then to have a definitive
knowledge of the phase formed after each reduction peak (from
CoO(OH) to Co3O4, then to CoO). So, we suggests that the CoO(OH)
(D-280) is initially reduced to Co3O4, and then further reduced to
CoO and Co metal. The following equations are designated.
6CoO(OH) + H2 → 2Co3O4 + 4H2O
Co3O4 + H2 → 3CoO + H2O
CoO + H2 → Co + H2O
(3)
(4)
(5)
Fig. 5(b) shows the TPR profile of D-450 sample; it includes only
two reductive singles at 300 and 367 ◦C. Sexton et al. [31] found
that the reduction profile of Co3O4 includes a low-temperature
peak below 300 ◦C and a high-temperature peak at approximately

C.-W. Tang et al. / Thermochimica Acta 473 (2008) 68–73
71
Fig. 5. TPR profiles of (a) D-280 (b) D-450 (c) D-950.
Fig. 4. Raman spectra of (a) D-280 (b) D-450 (c) D-950.
D-280 sample can be decomposed into Co3O4 and then CoO under
heat treatment.
500 ◦C. According to the literature [32–35], the low-temperature
peak is associated with the reduction of Co3+ ions to Co2+ (Eq. (4))
with the subsequent structural change to CoO. The second higher
temperature peak is due to the reduction of CoO to metallic cobalt
(Eq. (5)). A comparison with the D-280 sample, and in particular,
the disappearance of the peak at around the lowest temperature
indicates that the reductive behavior is in good agreement with
the proposed Co3O4. The TPR profile of the D-950 sample (Fig. 5(c))
includes only a single peak, which is assigned unambiguously to
the direct reduction of CoO to cobalt metal.
12CoO(OH) → 4Co3O4 + O2 + 6H2O
2Co3O4 → 6CoO + O2
(6)
(7)
Fig. 7 shows the TG-MS profile of the D-450 sample. At under
300 ◦C, the tardy weight loss is associated with desorption of
adsorbed water on the surface of Co3O4. The red curve is assigned
3.3. Analysis of evolved gas of cobalt oxides
To confirm the phase transformation of CoO(OH) to Co3O4 and
CoO under heat treatment, coupled quantitative and qualitative
analysis by TG-MS was performed to prove the structural identifi-
cations in Section 3.1. Fig. 6 presents the TG-MS profile of the D-280
sample. The thermogravimetric plot presents two marked weight
losses at about 280 and 850 ◦C. Column 4 in Table 2 shows that these
are 12% and 20%, respectively. The coupled TG-MS analysis reveals
that the evolved gases are oxygen (O2, m/z = 32) and steam (H2O,
m/z = 18). The red curve assigned to O2 has a small first peak and a
large second peak. The blue curve assigned to H2O is related to the
crystal structure. Comparing the theoretical weight loss of the pro-
posed reactions (columns 2 and 3 in Table 2) with the experimental
results demonstrates the simultaneous desorption of O2 and H2O
from CoO(OH) into Co3O4 at around 280 ◦C.
Fig. 6. Analysis of the outlet gases from the decomposition of D-280 by TG-MS
(heating rate of 10 ◦C min−1 under He): H2O (blue) and O2 (red). (For interpretation
of the references to color in this figure legend, the reader is referred to the web
version of the article.)
The second weight loss, confirmed by MS analysis, shows the
decomposition of Co3O4 to CoO at around 850 ◦C. Therefore, the

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C.-W. Tang et al. / Thermochimica Acta 473 (2008) 68–73
Table 2
Experimental and theorization of a series of cobalt oxide for TG-MS
Sample
CoOOHc
Reaction process
Weight loss (%)a
Theoretical
Outlet gas (C)b
Experimental
H2O
300
O2
12CoO(OH) → 4Co3O4 + O2 + 6H2O
2Co3O4 → 6CoO + O2
13
19
7
12
20
8
920
c
Co3O4
2Co3O4 → 6CoO + O2
–
–
820
920
A
B
CoOd
6CoO + O2−→ 2Co3O4−→ 6CoO + O2
A:7
B:-7
A:7
B:-8
a
Measurement of TG.
Measurement of mass analyzer.
By TG-MS flow He.
By TG-MS flow air.
b
c
d
4. Conclusions
A convenient procedure for preparing a series of pure cobalt
oxides is presented; it involves the decomposition of CoOx in a
nitrogen environment at different temperatures.
TG-MS, XRD, Raman, FTIR and TPR analyses are demonstrated to
be useful in identifying the series of cobalt oxides. The D-280 sam-
ple [CoO(OH)] has a hexagonal structure that gives rise to Co3O4
and CoO in helium at 280 and 850 ◦C, respectively. The D-450 sam-
ple [Co3O4] has a spinel structure that gives rise to CoO in helium at
820 ◦C. The D-950 sample [CoO] has a faced-centered cubic struc-
ture that can be oxidized to Co3O4 at 200–600 ◦C and decomposed
to CoO at 850–1000 ◦C in air, respectively. The preparation of mate-
rials and the analysis of their thermal behavior is a convenient way
to control and determining the purity of cobalt oxide.
Fig. 7. Analysis of the outlet gases from the decomposition of D-450 by TG-MS (heat-
ing rate of 10 ◦C min−1 under He): O2 (red). (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of the article.)
Acknowledgements
We are pleased to acknowledge financial supports for this study
from Academia Sinica and the National Science Council of the
Republic of China.
to O2 at 820 ◦C. The weight loss of 8% is caused mainly by the
decomposition of Co3O4 to CoO, according to Eq. (7).
Fig. 8 shows the TG profile of the D-950 sample in air. The weight
increase of 7% involves mainly the oxidation of CoO to Co3O4 as
the temperature is increased from 250 to 900 ◦C, followed by the
desorption of oxygen to CoO at above 900 ◦C. Comparing the theo-
retical weight variations of the proposed reaction (columns 2 and
3 in Table 2) with the experimental results demonstrated that CoO
can undergo oxidation–reduction in air, according to Eq. (8).
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