A novel method to synthesize cobalt oxide (Co3O4) nanowires from cobalt (Co) nanobowls

Article abstract of DOI:10.1002/pssa.200925503

A novel method suitable for the synthesis of the cobalt oxide (Co ³O⁴) nanowires at targeted regions is presented in this report. Cobalt (Co) nanobowls synthesized by colloidal crystal directed assembly were transformed into Co³O⁴ nanowiresby a simple heat treatment process. Co nanobowls exhibited a two phase (h.c.p. + f.c.c.) microstructure while single phase microstructure was observed for Co ³O⁴nanowires. Ferromagnetic Co nanobowls showed a dependence of coercivity on bowl size while Co³O⁴ exhibited weak ferromagnetic behavior.

Full text of DOI:10.1002/pssa.200925503

Phys. Status Solidi A 207, No. 4, 963–966 (2010) / DOI 10.1002/pssa.200925503
a
p s s
www.pss-a.com
applications and materials science
A novel method to synthesize cobalt
oxide (Co3O4) nanowires from cobalt (Co) nanobowls
,
A. K. Srivastava* 1,2, S. Madhavi1, and R. V. Ramanujan1
1 School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore
2 Fuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark,
Frederiksborgvej 399, 4000 Roskilde, Denmark
Received 14 September 2009, revised 21 October 2009, accepted 26 October 2009
Published online 25 November 2009
PACS 75.75.þa, 81.07.ꢀb, 81.07.Wx, 81.16.Be
* Corresponding author: e-mail akku@risoe.dtu.dk, Phone: þ45 4677 5838, Fax: þ45 4677 5858
A novel method suitable for the synthesis of the cobalt oxide phase (h.c.p. þ f.c.c.) microstructure while single phase micro-
(Co3O4) nanowires at targeted regions is presented in this structure was observed for Co3O4 nanowires. Ferromagnetic Co
report. Cobalt (Co) nanobowls synthesized by colloidal crystal nanobowls showed a dependence of coercivity on bowl size
directed assembly were transformed into Co3O4 nanowires by a while Co3O4 exhibited weak ferromagnetic behavior.
simple heat treatment process. Co nanobowls exhibited a two
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Cobalt (Co) and cobalt oxide (Co3O4) transforming Co nanostructures, which are well known to
nanostructures are of great interest due to various advan- exhibit sufficiently high magnetization to be processed in an
tages, e.g., Co nanobowls and nanowires exhibit interesting external magnetic field and can be targeted at desired
properties suitable for high density data storage and MRAM regions, into Co3O4 nanowires.
devices [1, 2], and can also be targeted at desired regions via
This paper reports a novel strategy to convert Co
an external magnetic field due to high magnetic moment nanobowls into Co3O4 nanowires by a simple heat treatment
while Co3O4 nanowires are extremely useful as anodes in process in which Co nanobowls of 1 mm diameter, prepared
batteries [3], catalysts, [4] and sensors [5]. Li et al. [3] have by chemical synthesis technique incorporated within
reported high capacity and rate capability of mesoporous colloidal crystal templated assembly, were turned into
Co3O4 nanowire arrays to be used as anodes in Li ion Co3O4 nanowires upon annealing at 600 8C for 3 h. The
batteries. Recently, Xie et al. [4] have shown surprisingly formation of Co3O4 at high temperature annealing was due to
high catalytic activity of Co3O4 nanorods. Co3O4 nanowires the formation of unstable f.c.c. phase of Co. Magnetic
can also be used as electrochemical sensor for hydrogen properties of these Co nanobowls and Co3O4 nanowires were
peroxide sensing [5]. Increasing application of Co3O4 might studied and presented. These nanobowls can also be used as
also require the targeting of nanowires at particular regions nanocontainers for targeted delivery of useful liquids. This
in a device or system but Co3O4 is an anti-ferromagnetic method has potential application for in situ synthesis of
material and exhibits zero net magnetization due to the Co3O4 nanowires for systems and devices which can
complete compensation of sub-lattice magnetizations [6]. withstand up to 600 8C.
Targeting a material at a desired region can be carried out
using an external magnetic field provided the material
exhibits sufficiently high magnetic moment. Ferromagnetic
2 Experimental
2.1 Formation of colloidal templates Monodis-
behavior with a very small magnetic moment of Co3O4 persed polystyrene (PS) latex microsphere suspensions
nanostructures has been reported previously [7]; however it (particle density 1.05 g/cm3 and a refractive index of 1.59
is impractical to process the nanostructures with such a weak at 589 nm) with a mean diameter of 1.0 mm and a size
magnetization using an external magnetic field. An alterna- distribution of 3% (Duke Scientific Corporation, USA) were
tive method of delivering Co3O4 nanowires is in situ used as received. The latex sphere suspensions were spun for
synthesis at the desired regions. This can be achieved by 48 h at 800 rpm followed by drying in a centrifuge tube to
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a
p s s
964
A. K. Srivastava et al.: Synthesis of cobalt oxide (Co3O4) nanowires from cobalt (Co) nanobowls
obtain centimeter-scale well-ordered close packed polymer Since the synthesis of the material takes place within the
templates. The template was strengthened by annealing at interstitials, it acquires the shape of a bowl. Removal of PS
100 8C for 10–15 min before infiltration of the void space.
template was then carried out by annealing at 400 8C for 3 h
in air which leaves Co nanobowls. Figures 1a and b presents
the SEM micrograph of a close packed template of PS
spheres and the Co nanobowls of 1 mm diameter, respec-
tively. The average wall size of the bowls was found to be
70 nm. Heat treatment of the nanobowls at 400 8C for longer
duration (6 h) revealed a disrupted nanobowl structure
(Fig. 1c) while heat treatment of the nanobowls at 600 8C
for 3 h resulted in nanowires (Fig. 1d). High resolution SEM
micrograph of a single nanowire is presented in the inset of
Fig. 1d. The average diameter of nanowires was found to be
53 nm.
X-ray diffraction patterns collected on Co nanobowls are
presented in Fig. 2. Interestingly, no oxide formation of Co
was observed for the nanobowls samples obtained by
annealing at 400 8C for 3 h in air, instead, a two-phase
(h.c.p. þ f.c.c.) microstructure of metallic Co was observed
(Fig. 2a). The chemistry of borohydride reduction of
CoCl2.6H2O leading to the formation of Co is as follows
[8–10]; Co2B and Co (BO2)2 are formed as the primary
product (Eqs. (1)–(3)).
2.2 Formation of nanobowls and nanowires In
a typical experiment to form Co nanobowls [8], 0.3 M cobalt
chloride (CoCl2.6H2O) (LR/Resource, APS Ajax Finechem,
Australia) and 0.6 M sodium borohydride (NaBH4) (Fluka,
Milwaukee, WI) solutions were prepared in DI water.
Vacuum-assisted infiltration was used to impregnate
CoCl2.6H2O solution inside the interstitials of closed packed
templates of PS spheres of 1 mm diameter. This was followed
by impregnation of NaBH4 solution leading to the formation
of Co particles within the interstitial spaces of the PS
colloidal template. All these steps were performed inside a
N2 filled dry box (Plas Labs, Lansing, MI). The template
removal was carried out by annealing at 400 8C for 3 h in air
followed by toluene etching. Co3O4 nanowires were
prepared by the heat treatment of nanobowls at 600 8C for
3 h. The morphologies of the nanostructures were investi-
gated using scanning electron microscopes (SEM) (JSM
5410LV and JSM-6360, JEOL, Japan) and a transmission
electron microscope (TEM) (JEM-2010; JEOL, Japan).
Powder X-ray diffraction (XRD) patterns were collected
using a Shimadzu 6000 Lab X diffractometer (Japan) with
CuKa radiation. XRD peaks were matched using Inorganic
Crystal Structure Database (ICSD), Find It program.
Magnetic measurements were carried out at room tempera-
ture using a Lakeshore 7404 (Westerville, OH) VSM.
2CoCl2 þ 4NaBH4 þ 9H2O ! Co2B þ 4NaCl
(1)
þ 12:5H2 þ 3BðOHÞ3;
NaBH4 þ 2H2O ! NaBO2 þ 4H2O;
Co2þ þ 2NaBO2 ! 2Naþ þ 2CoðBO2Þ2:
(2)
(3)
3 Results and discussion In order to synthesize Co
nanobowls, CoCl2.6H2O was chemically reduced within the
interstitial spaces of the PS colloidal template by NaBH4.
The process by which Co is obtained is considered to be
the pyrolysis of a mixture of Co2B and Co (BO2)2 in air
yielding Co and B2O3 (Eq. (4)). The other possibility is the
Figure 1 SEM micrographs of (a) colloidal template of 1 mm PS
spheres, (b) Co nanobowls of 1 mm diameter, (c) Co nanobowls heat
treated at 400 8C for 6 h, and (d) Co3O4 nanowires obtained by heat Figure 2 (online color at: www.pss-a.com) XRD pattern of Co
treatment of Co nanobowls of 1 mm diameter at 600 8C for 3 h. Inset nanobowlsof1 mmdiameterobtainedbyheattreatmentat(a)400 8C
of Fig. 1d presents the high resolution micrograph of a single Co3O4 for 3 h, (b) 400 8C for 6 h. TEM micrograph of a single Co nano-
nanowire.
particle is presented in the inset.
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com

Original
Paper
Phys. Status Solidi A 207, No. 4 (2010)
965
sacrificial oxidation of Co2B again yielding Co and B2O3 as (without using PS template) is provided in the inset of Fig. 2
the final product (Eq. (5)).
which clearly reveals the presence of stacking faults. These
stacking faults act as nuclei for this h.c.p. to f.c.c. phase
transformation as the h.c.p. and f.c.c. structures differ only
by the stacking sequence of close packed atomic layers
(ABCABCABC for f.c.c. and ABABAB for h.c.p.). The
f.c.c. structure of Co is less stable at room temperature and is
more likely to oxidize in air and form Co3O4 [12]. Jahn–
Teller distortion renders the h.c.p. structure more stable at
room temperature than the f.c.c. structure [13]. This f.c.c.
phase formed at higher temperature does not transform back
to h.c.p. phase even after cooling the sample to room
temperature. Oxide formation at high temperature annealing
was responsible for nanowire formation as observed in SEM
micrographs (Fig. 1d). The density of Co3O4 (6110 kg/m3,
equivalent to a molar volume of 39.4 ꢁ 10ꢀ6 m3/mol) is
significantly less than the density of Co (8900 kg/m3,
equivalent to a molar volume of 6.6 ꢁ 10ꢀ6 m3/mol). The
increase in molar volume of Co3O4 compared to Co
(approximately sixfold) gives rise to large compressive
stresses. In order to relieve these compressive stresses,
protrusion of nanowires takes place as observed in Fig. 1d. It
is noteworthy here that the nanobowls heated for 6 h were
disrupted by diffusional processes [14] and no nanowires
were observed (Fig. 1c) which confirms that the density
change due to oxide formation is responsible for nanowire
formation. A similar observation was made in the case of
intermetallic formation in relation to Sn whiskers for
microelectronics applications [15].
2Co2B þ CoðBO2Þ2 ! 5Co þ 2B2O3;
(4)
(5)
4Co2B þ 3O2 ! 8Co þ 2B2O3
The removal of B2O3 (Eq. (4) and (5)) was carried out by
washing the sample in DI water. The nanobowls heated
at 400 8C for 6 h also shows the presence of same phases
(h.c.p. and f.c.c.); no Co3O4 peaks were observed
(Fig. 2b).
X-ray diffraction pattern of the Co nanobowls of 1 mm
diameter annealed at 600 8C for 3 h reveals the formation of
Co3O4 nanowires (Fig. 3). The peaks observed in the XRD
pattern can be indexed as (111), (220), (311), (400), (422),
(511), (440), (620), and (533) planes of a cubic unit cell. The
formation of f.c.c. phase at higher temperature is responsible
for the oxidation of Co into Co3O4. It is well known that Co
exhibits h.c.p. crystal structure (e-phase) at room tempera-
ture and f.c.c. crystal structure (a-phase) at higher tempera-
tures (>420 8C). The thermodynamics of Co at room
temperature can be used to explain this phenomenon [11].
The free energy of formation of the cubic phase, starting
from the hexagonal phase, is given by
DGe!a ¼ DHe!a ꢀ TDSe!a
(6)
where DG, DH, and DS, respectively, represents the free
enthalpy, energy, and entropy of transformation from e- to
a-phase, and T is the temperature. For Co, DHe!a and
DSe!a are positive, thus the enthalpy term favors the
hexagonal phase while the entropy term opposes it. At room
temperature, the enthalpy term dominates and the phase is
hexagonal, while at higher temperatures (>420 8C) the
entropy term dominates yielding the cubic phase. TEM
micrograph of a single Co particle synthesized by similar
borohydride reduction technique as used in this work
The magnetic hysteresis loops measured on Co nano-
bowls and Co3O4 nanowires are presented in Fig. 4. The
saturation magnetization (Ms) and coercivity (Hc) values of
Co nanobowls were found to be 57.6 emu/g and 507.5 Oe,
respectively. The Co3O4 nanowires obtained by heat
treatment of nanobowls showed a very small magnetic
moment (Ms ꢂ 0.3 emu/g). Similar weak ferromagnetic
Figure 4 (online color at: www.pss-a.com) Magnetic hysteresis
Figure 3 XRD pattern of Co3O4 nanowires obtained by heat treat- loop measured on Co nanobowls and Co3O4 nanowires. Inset shows
ment of Co nanobowls of 1 mm diameter at 600 8C for 3 h.
the magnified view of magnetization curve of Co3O4 nanowires.
www.pss-a.com
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a
p s s
966
A. K. Srivastava et al.: Synthesis of cobalt oxide (Co3O4) nanowires from cobalt (Co) nanobowls
behavior of Co3O4 has also been observed previously which
was attributed to uncompensated surface spins and/or finite
size effects or the partly inverted spinel structure [6].
[3] Y. Li, B. Tan, and Y. Wu, Nano Lett. 8, 265 (2008).
[4] X. Xie, Y. Li, Z. Q. Liu, M. Haruta, and W. Shen, Nature 458,
746 (2009).
[5] Y. Li, B. Tan, and Y. Wu, J. Am. Chem. Soc. 128, 14258
(2006).
4 Conclusions A technique to synthesize Co3O4
nanowires from ferromagnetic Co nanobowls was devel-
oped. This technique is useful for the synthesis of Co3O4
nanowires at desired regions. Co nanobowls were synthes-
ized using colloidal crystal directed assembly process. These
nanobowls were transformed into Co nanowires by high
temperature annealing process. Co nanobowls exhibited
the two phase (h.c.p. þ f.c.c.) microstructure and strong
ferromagnetic behavior while a single phase microstructure
with weak ferromagnetic signal was obtained for Co3O4
nanowires.
[6] T. Ozkaya, A. Baykal, M. S. Toprak, Y. Koseoglu, and
Z. Durmus, J. Magn. Magn. Mater. 321, 2145 (2009).
[7] Q. Yuanchun, Z. Yanbao, and W. Zhishen, Mater. Chem.
Phys. 110, 457 (2008).
[8] A. K. Srivastava, S. Madhavi, T. J. White, and R. V.
Ramanujan, J. Mater. Chem. 15, 4424 (2005).
[9] G. N. Glavee, K. J. Klabunde, C. M. Sorensen, and
G. C. Hadjapanayis, Langmuir 8, 771 (1992).
[10] G. N. Glavee, K. J. Klabunde, C. M. Sorensen, and
G. C. Hadjipanayis, Langmuir 9, 162 (1993).
[11] J. L. Maurice, D. Imhoff, P. Etienne, O. Durand, S. Dubois,
L. Piraux, J. M. George, P. Galtier, and A. Fert, J. Magn.
Magn. Mater. 184, 1 (1998).
[12] T. Li, S. Yang, L. Huang, and B. Gu, Nanotechnology 15,
1479 (2004).
[13] M. E. Eberhart, R. C. O’Handley, and K. H. Johnson, Phys.
Rev. B 29, 1097 (1984).
[14] A. K. Srivastava, S. Madhavi, T. J. White, and R. V.
Ramanujan, J. Mater. Res. 22, 1250 (2007).
[15] P. J. T. L. Oberndorff, M. Dittes, and L. Petit, in: Proceedings
of the IPC/Soldertec International Conference, 2003,
pp. 170–178.
References
[1] A. K. Srivastava and R. V. Ramanujan, in: Magnetic Nanos-
tructures, edited by H. S. Nalwa, 2nd ed. (American Scientific
Publishers, USA, 2009) pp. 641–655.
[2] A. K. Srivastava, R. S. Singh, K. E. Sampson, V. P. Singh,
and R. V. Ramanujan, Metall. Mater. Trans. A 38, 717–724
(2007).
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com