Facile synthesis, characterization and electrochemical properties of cuspate deltoid CoO crystallites

Article abstract of DOI:10.1016/j.jallcom.2008.03.074

Cuspate deltoid cobaltous oxide (CoO) crystallites with an average grain size of 3 μm were obtained via developing the Co₃O₄ nanoplatelets under the KCl/NaCl flux atmosphere. The as-synthesized products were characterized by powder X

Full text of DOI:10.1016/j.jallcom.2008.03.074

Journal of Alloys and Compounds 471 (2009) 268–271
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Facile synthesis, characterization and electrochemical
properties of cuspate deltoid CoO crystallites
Ying Yu, Guangbin Ji∗, Jieming Cao, Jinsong Liu, Mingbo Zheng
Nanomaterials Research Institute, College of Material Science and Technology, Nanjing University of Aeronautics
and Astronautics, Nanjing 210016, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Cuspate deltoid cobaltous oxide (CoO) crystallites with an average grain size of 3 ␮m were obtained
via developing the Co3O4 nanoplatelets under the KCl/NaCl flux atmosphere. The as-synthesized prod-
ucts were characterized by powder X-ray diffraction, transmission electron microscopy, field-emission
scanning electron microscope and selected area electron diffraction. The electrochemical properties of
the products were investigated by an Electrochemical Workstation device at room temperature. It has
been found that the final prepared cubic CoO crystallites are of uniform cuspate deltoid morphology. The
results of electrochemical tests indicate that the CoO shows acceptable capacitive behavior with specific
capacitance about 88 F/g.
Received 3 October 2007
Received in revised form 17 March 2008
Accepted 20 March 2008
Available online 2 May 2008
PACS:
81.16.Be
81.07.Vb
75.20.En
81.20.Fw
© 2008 Elsevier B.V. All rights reserved.
Keywords:
CoO
Electrochemical properties
Capacitance
1. Introduction
oleylamine was also applied to synthesize wurtzite CoO [12]. More-
over, different CoO nanocrystals, such as nanoparticles [13–16],
As one kind of transition-metal monoxides, cobaltous oxide
(CoO) has attracted much attention due to its interesting and
distinctive structures and properties [1–5], and thus has been
widely used for lithium-ion battery, cemented carbide, catalyst and
many electronic components [6–8]. Up to the present, considerable
researches have been devoted to study the preparation methods
of new shaped and structured CoO. Common methods for prepar-
ing the CoO powders include wet chemical process, combustion of
metal cobalt in air or vapor, and heating the hydroxide, carbonate
or oxalate of cobalt in isolated air at high temperature. For instance,
decomposition of cobalt acetate tetrahydrate in argon to produce
the rocksalt CoO was studied with time resolved neutron diffrac-
tion and thermo gravimetric analysis [9]. CoO in the zinc blende
structure was prepared by decomposing Co acetate in a nitrogen
atmosphere [10]. Risbud et al. [11] used the thermolytic decomposi-
tion of Co(acac)2 in refluxing benzyl ether to obtain CoO with fcc-Co
phase while decomposition of Co(acac)3 (acac = acetylacetonate) in
nanorod [17], nanoplatelet [18] have also been reported. However,
to our best knowledge, there is no report on the preparation of pure
cuspate deltoid CoO crystallites by decomposition of Co3O4 under
molten salt atmosphere.
The possibility of CoO being a candidate for capacitor application
was explored by Lin et al. [19], who prepared cobalt oxide xerogels
using a sol–gel technique, followed by a heating step to different
temperatures. The materials exhibit excellent cycle life with no sig-
nificant change in the cyclic voltammogram (CV). In this work, we
report the process of preparing CoO crystallites with new morphol-
ogy by means of developing Co3O4 nanoplatelets in the molten salt,
the capacitance of the prepared product was calculated and insight
was gained into the capacitance capacity of the sample. Meanwhile,
the factors affecting the growth morphology of the CoO crystallites
were also discussed, in order to enrich the research of crystallite
growth theory in molten salt.
2. Experimental
All chemical reagents were AnalaR (AR) grade and used without further purifica-
tion. In a typical experiment, the procedure employed for preparing Co3O4 precursor
is as follows: 8.73 g Co(NO3)2·6H2O was dissolved in distilled water to form a 30 mL
solution ([Co2+] = 1.0 mol/L), precipitate immediately appeared as soon as excessive
∗
Corresponding author. Tel.: +86 25 52112902; fax: +86 25 84895289.
E-mail address: gbji@nuaa.edu.cn (G. Ji).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.03.074

Y. Yu et al. / Journal of Alloys and Compounds 471 (2009) 268–271
5 M NaOH solution was slowly added into the Co(NO3)2 solution, the pH value was
269
then modulated between 12 and 14 under constant stirring about 30 min, finally the
mixed solution together with 10 mL distilled water were transferred to a Teflon-lined
autoclave with about 50 mL capacity. The autoclave was sealed and maintained at
140 ◦C for 12 h and then cooled to room temperature naturally. The obtained Co3O4
precipitate was filtered off, washed with absolute ethanol and distilled water several
times, and then dried in a vacuum at 60 ◦C for 4 h. For the preparation of CoO crys-
tallites, the above-prepared Co3O4 precursor (1.0 g) was mixed with NaCl (2.0 g) and
KCl (2.0 g) in an agate mortar, the mixture was then ground for several minutes into
fine powders. The mixed sample was heated in an alumina crucible to 1000 ◦C and
kept at that temperature for 3 h in the air atmosphere. The heating rate of the muf-
fle furnace was maintained at 3 ◦C/min. Then the heat-treated sample was cooled
gradually to room temperature, washed several times with distilled water in order
to remove the NaCl/KCl flux, and finally filtered, dried in an oven at 90 ◦C for 3 h.
The obtained CoO powders were used to fabricate the electrode of elec-
trochemical capacitor according to the previous report [20]. The electrode was
prepared by mixing 80 wt% active CoO powders material, 15 wt% acetylene black
and 5 wt% polyvinylidene fluoride (PVDF) as binder before pressed onto nickel grid
(1.2 × 107 Pa) that serves as a current collector (surface is 1 cm2).
The power X-ray diffraction (XRD) patterns were acquired in a 2ꢀ range from 10◦
to 100◦ on a Bruker D8-ADVANCE X-ray powder diffractometer with Cu K␣ radia-
tion (ꢁ = 1.54178 A) at a scanning rate of 0.02◦ s−1. The scanning electron microscopy
˚
(SEM) images of the prepared product were obtained on a LEO1530VP field-emission
(FE) scanning electron microscope. Prior to the measurements, the sample was
coated with gold for 3 min in an Anatech hummer 6.2 sputtering system operat-
ing at 40 mTorr. The transmission electron microscopic (TEM) images, the energy
dispersive X-ray analyzer (EDX), and the selected-area electron diffraction (SAED)
patterns of the sample were recorded using a TECNAI G2 20 S-TWIN, electron micro-
scope, at an acceleration voltage of 200 kV. The sample was prepared by dispersing
the calcined material in water at room temperature. A few drops of this dispersion
were placed on a holey carbon-coated copper mesh and dried at room temperature.
Electrochemical characterization was carried out in a conventional three-electrode
electrochemical cell containing 2 M KOH aqueous solution as electrolyte. The freshly
prepared CoO film on Ni substrate was used as the working electrode, a platinum
foil (area 1 cm2) was used as the counter electrode, and saturated calomel elec-
trode (SCE) as the reference electrode. The cyclic voltammetry and galvanostatic
charge/discharge measurements were performed using a CHI660 model Electro-
chemical Workstation at room temperature.
3. Results and discussion
The typical XRD pattern of the Co3O4 precursor synthesized
with hydrothermal route is shown in Fig. 1(a). The diffraction peaks
˚
presented identify the sample as typical cubic Co3O4 [a = 8.094 A,
space group: Fd3m (2 2 7)], while no obvious peaks corresponding
to other cobalt oxides are detected, indicating the high purity of
the precursor. The well-resolved sharp diffraction peaks reveal the
good crystallization of the Co3O4 precursor. Fig. 1(b) shows the XRD
pattern of CoO crystallites sample prepared by developing Co3O4
precursor in KCl/NaCl flux at 1000 ◦C for 3 h. The well-resolved
Fig. 2. FE-SEM image (a) and TEM image (b) of the as-prepared Co3O4 precursor.
Inset is the SAED pattern, showing single crystal property for the product and being
indexed onto the corresponding oxide crystal structure.
sharp diffraction peaks assigned to (1 1 1), (2 0 0), (2 2 0), (3 1 1),
and (2 2 2) planes can be indexed to a typical cubic rock salt CoO
phase (space group: Fm3hm), in good accordance with the results
reported in the literature [21].
The morphology and microstructure of the obtained Co3O4 pre-
cursor can be observed in FE-SEM and TEM images. Fig. 2a shows
FE-SEM image of the precursor obtained using hydrothermal syn-
thesis route at 140 ◦C for 12 h. It can be seen that the precursor
displays wafery sheets with an average thickness of ∼30 nm and
diameter up to about 400 nm. TEM measurement of the Co3O4 pre-
cursor was performed as shown in Fig. 2b. The lamellar morphology
observed is consistent with the results presented in the FE-SEM
image. SAED pattern of the Co3O4 powders was obtained by focus-
ing the electron bean on an individual crystal, typical single crystal
pattern can be seen in the inset of Fig. 2b.
Fig. 3a shows typical FE-SEM image of the cubic CoO samples, it
can be seen that nearly all of the growth planes of the crystallites
display equilateral triangle with cuspate morphology, and crystal
stocks with an average size about 3 ␮m are of the main crystal
shape for the collision and connected growth of the crystallites in
molten salt atmosphere. The partial magnified image with higher
Fig. 1. XRD patterns of (a) the as-prepared Co3O4 precursor and (b) final developed
CoO crystallites.

270
Y. Yu et al. / Journal of Alloys and Compounds 471 (2009) 268–271
Fig. 3. FE-SEM images (a and b), TEM image (c), and EDX spectra (d) of the final developed CoO crystallites (inset in (c) is the SAED pattern indexed).
resolution is shown in Fig. 3b. It is clear that the crystallites are
cuspate deltoid with quite uniform layered appearance, just like
the usually triangular alluvial deposit at the mouth of a river. As
is known to all, when the temperature is over 900 ◦C, Co3O4 will
be decomposed into CoO, which is attributed to the cobalt oxida-
of O atoms escaped from the lattices of the Co3O4 single crystal
precursor, resulting in the form of plentiful screw dislocations. By
combining the Ostwald ripening mechanism and the classic layer
growth theory of single crystal, we can find that large numbers of
screw dislocations will enhance the density of step kink on the sur-
face of crystal particles, the surface morphology and structure of
single crystal will vitally be affected by the density of step kink,
because it is regarded as the determinant for the crystal surface’s
screw patterns as roundness or polygon. In this paper, by means of
Co3O4-to-CoO transformation, adequate quantities of screw dislo-
cations on the CoO crystal surface appeared, and after treat with
molten salt at high temperature, CoO fine particles aggregated
along the step kink. In this experiment the density of the step kink
determined the screw veins appeared as triangular. TEM measure-
ments of the final CoO single crystals were performed as shown in
Fig. 3c, the marked diffraction spots in the inset revealed the final
CoO single crystal nature. The EDS pattern of the CoO crystallites
prepared by molten salt growth route is presented in Fig. 3d. It is
clear that the prepared sample consist of Co, C, Cu and O elements
while the C and Cu elements are impurities introduced during the
preparation of the sample. The relative atom number rate of Co-
to-O demonstrates that the CoO prepared is almost pure while
tion reaction as follows: 2Co3O4
−→
deoxidization6CoO + O2 [22]. Reaction
proceeds until 950 ◦C, but it can be inferred from the XRD patterns
of the Co3O4 precursor and the final CoO sample that there was no
change in the lattice structure during the Co3O4-to-CoO transfor-
mation [23]. According to the Ostwald ripening mechanism [24,25],
the dissolution degree of the primary transformed CoO particles
under molten salt atmosphere was affected by the character of the
starting material, such as the particle size and chemical activity
because the dissolution rate of the material depends on such char-
acters. Meanwhile some of the CoO fine particles aggregated to form
larger crystallites thus reduced the surface area of the material,
the aggregating rate was also affected by the viscosity of the flux
during heating the system. Since adding NaCl/KCl can significantly
decrease the viscosity of the flux, this makes the mobility of com-
ponents in the flux to become easier. Thus, adding NaCl/KCl can
provide a favorable environment for the growth of CoO particles.
During the Co3O4-to-CoO transformation process, large numbers

Y. Yu et al. / Journal of Alloys and Compounds 471 (2009) 268–271
271
the supercapacitive performance is inherent in the material itself.
There is no absolute morphological impact on supercapacitor mate-
rials’ supercapacitive behavior. So as a whole, we cannot say in this
article, that the favorable supercapacitive performance has direct
and confirmed link to the morphologic character of CoO.
4. Conclusion
Cuspate deltoid CoO crystallites were prepared by developing
the Co3O4 nanoplatelets precursor in NaCl/KCl flux. The resulted
morphology and structure of the CoO crystallites indicate that the
cubic Co3O4 spontaneously decomposed during the deoxidation
process, and there was no change in the lattice structure dur-
ing Co3O4-to-CoO transformation. The preliminary electrochemical
test results show that the prepared CoO sample behaves as super-
capacitor, for the galvanostatic charge/discharge peaks are clearly
visible, and the overlapping reactions lead to the current–voltage
signatures similar to that of a supercapacitor, this results persuade
us to accept the CoO as promising supercapacitor materials. Addi-
tionally, the growth of the crystalline CoO in KCl/NaCl flux is in
accordance with the Ostwald ripening mechanism and the layer
growth theory, starting from the step kink of the screw disloca-
tions on the crystal surface and resulted in the layered and cuspate
deltoid surface appearance of CoO particle. This synthetic method
may extend to prepare other transition-metal oxides for different
applications in chemical sensors, high-density electronic devices
and lithium-ion batteries.
Acknowledgements
Financial support from the National Natural Science Foundation
of China (50502020 and 50701024) is gratefully acknowledged.
Fig. 4. (a) CVs of the as-prepared CoO electrode at a sweep rate of 10 mV s−1 using
2 M KOH as the electrolyte at room temperature; (b) galvanostatic charge/discharge
of the electrochemical capacitor built from the CoO powders.
References
[1] S. Massidda, M. Posternak, A. Baldereschi, R. Resta, Phys. Rev. Lett. 82 (1999)
430–435.
[2] N. Koshizaki, K. Yasumoto, T. Sasaki, Sens. Actuat. B. 66 (2000) 122–124.
[3] J.L. Mackay, V.E. Henrich, Phys. Rev. B 39 (1989) 6156–6160.
[4] H.K. Lin, H.C. Chiu, H.C. Tsai, S.H. Chien, C.B. Wang, Catal. Lett. 88 (2003) 169–174.
[5] S. Sarangapani, B.V. Tilak, C.P. Chen, J. Electrochem. Soc. 143 (1996) 3791–3795.
[6] E. Antolini, E. Zhecheva, Mater. Lett. 35 (1998) 380–382.
[7] D.N. Srivastava, N. Perkas, G.A. Seisenbaeva, Y. Koltypin, Ultrason. Sonochem.
10 (2003) 1–3.
[8] V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, Nature 423
(2003) 850–853.
[9] R.W. Grimes, A.N. Fitch, J. Mater. Chem. 1 (1991) 461–465.
[10] M.J. Redman, E.G. Steward, Nature 193 (1962) 867–870.
[11] A.S. Risbud, L.P. Snedeker, M.M. Elcombe, A.K. Cheetham, Chem. Mater. 17
(2005) 834–839.
[12] W.S. Seo, J.H. Shim, S.J. Oh, E.K. Lee, N.H. Hur, J. Am. Chem. Soc. 127 (2005)
6188–6190.
[13] J.F. Liu, Y. He, W. Chen, G.Q. Zhang, Y.W. Zeng, J. Phys. Chem. C 111 (2007) 2–5.
[14] J.F. Liu, S. Yin, H.P. Wu, Y.W. Zeng, X.R. Hu, J. Phys. Chem. B 110 (2006)
21588–21592.
[15] M. Ghosh, E.V. Sampathkumaran, C. Rao, Chem. Mater. 17 (2005) 2348–2351.
[16] M. Verelst, T.O. Ely, C. Amiens, E. Snoeck, P. Lecante, A. Mosset, M. Respaud, J.M.
Broto, B. Chaudret, Chem. Mater. 11 (1999) 2702–2705.
[17] K. An, N. Lee, J. Park, S.C. Kim, Y. Hwang, J.G. Park, J.Y. Kim, J.H. Park, M.J. Han,
J.J. Yu, T. Hyeon, J. Am. Chem. Soc. 128 (2006) 9753–9756.
[18] Y.L. Hou, H. Kondoh, M. Shimojo, T. Kogure, T. Ohta, J. Phys. Chem. B 109 (2005)
19094–19096.
[19] C. Lin, J.A. Ritter, B.N. Popov, J. Electrochem. Soc. 145 (1998) 4097–4099.
[20] F.B. Zhang, Y.K. Zhou, H.L. Li, Mater. Chem. Phys. 83 (2004) 260–263.
[21] Q.Z. Guo, H.K. Mao, J.Z. Hu, J.F. Shu, J. Phys: Condens. Matter 14 (2002)
11369–11372.
taking into account the excessive O element introduction during
the testing process.
Fig. 4a illustrates the CVs of the prepared CoO electrode
cycled in negative potentials at a sweep rate of 10 mV s−1 in
the voltage range from 0.0 to 0.5 V. The electrode was stable for
>1000 cycles. The CV curves (0.0–0.5 V) reveal a pair of current
peaks corresponding to the surface Faradic reactions according
to: CoO + OH− ⇔ CoOOH + e−, which is comparable to the redox
reaction seen in nickel oxide material [26]. The reason for non-
rectangular profile of CV could be due to the increase in the particle
size of CoO owing to the aggregation of electrolyte ions. The specific
capacitance of the CoO was estimated from the CVs by integrating
the area under the current–potential curve and then dividing by
the sweep rate v, the mass of the film m and the potential win-
ꢀ
dow according to the equation: 1/(mv(Va − Vc)) Vc I(V) dV, where
Va
(Va − Vc) represents the potential window. The specific capaci-
tance value of the CoO in this experiment was calculated as 88 F/g
using KOH as the electrolyte at room temperature according to
the above equation. The curve in Fig. 4a showed a rapid current
response on voltage reversal at each end potential, indicating the
high-electrochemical reversibility of the CoO. The performances of
the electrochemical capacitor were further confirmed by the gal-
vanostatic charge/discharge characteristics at a current density of
10 mA/cm2. Fig. 4b shows the typical curves (four cycles) of the CoO
sample electrode in potential range between 0 and 0.5 V in 2 M KOH
electrolyte. The almost constant slopes of these discharge curves
reveal that the CoO electrode has high-electrochemical reversibil-
ity and excellent capacitive characteristics. It should be noted that
[22] V. Srinivasan, J.W. Weidner, J. Power Sources 108 (2002) 15–19.
[23] R.S. Young, Cobalt, Its Chemistry Mutallurgy and Users, Reinhold Publ. Corp.,
New York, 1960.
[24] W.Z. Wang, C.K. Xu, G.H. Wang, Y.K. Liu, C.L. Zheng, Adv. Mater. 14 (2002)
837–841.
[25] P.W. Vorhees, Annu. Rev. Mater. Sci. 22 (1992) 197–199.
[26] Y.G. Wang, Y.Y. Xia, Electrochim. Acta 51 (2006) 3223–3224.