In situ gas bubble-assisted one-step synthesis of polymorphic Co 3O4 nanostructures with improved electrochemical performance for lithium ion batteries

Article abstract of DOI:10.1016/j.jallcom.2014.02.152

A novel and versatile gas bubble-assisted self-assembly technique was developed for the first-time preparation of Co₃O₄ nanobowl arrays by the rapid thermal decomposition of Co(NO₃) ₂×6H₂O on a flat substrate. The morphological modulation from novel nanobowl arrays, to nanotube arrays, to nanorods, and even to microspheres can be realized by only tuning decomposition temperature from 150 C to 700 C. The in situ generated (O₂, H₂O, NO ₂) bubbles guided the growth of Co₃O₄ nuclei, resulting in the final morphology of Co₃O₄ nanostructures. The Co₃O₄ nanostructures were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption-desorption isotherms. Analysis of electrochemical properties revealed that Co ₃O₄ nanobowl and nanotube arrays obtained at low temperatures displayed significant enhancement of electrochemical activity because of low crystallization, small grain size, high specific surface area, and hierarchically porous structure. This simple process was applicable to large-scale production and may be extended to other materials. The porous/hollow structure and high specific surface area of the as-obtained Co ₃O₄ nanobowl and nanotube arrays can enable their potential use in catalysis, chemical sensing, luminescence, energy storage, controlled release, and cellular applications.

Full text of DOI:10.1016/j.jallcom.2014.02.152

Journal of Alloys and Compounds 601 (2014) 167–174
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
In situ gas bubble-assisted one-step synthesis of polymorphic
Co3O4 nanostructures with improved electrochemical performance
for lithium ion batteries
a
b,
Guoxiu Tong a, , Yun Liu , Jianguo Guan
⇑
⇑
a College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, PR China
b State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel and versatile gas bubble-assisted self-assembly technique was developed for the first-time
preparation of Co3O4 nanobowl arrays by the rapid thermal decomposition of Co(NO3)2ꢀ6H2O on a flat
substrate. The morphological modulation from novel nanobowl arrays, to nanotube arrays, to nanorods,
and even to microspheres can be realized by only tuning decomposition temperature from 150 °C to
700 °C. The in situ generated (O2, H2O, NO2) bubbles guided the growth of Co3O4 nuclei, resulting in
the final morphology of Co3O4 nanostructures. The Co3O4 nanostructures were characterized by X-ray dif-
fraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron
microscopy, and nitrogen adsorption–desorption isotherms. Analysis of electrochemical properties
revealed that Co3O4 nanobowl and nanotube arrays obtained at low temperatures displayed significant
enhancement of electrochemical activity because of low crystallization, small grain size, high specific
surface area, and hierarchically porous structure. This simple process was applicable to large-scale
production and may be extended to other materials. The porous/hollow structure and high specific
surface area of the as-obtained Co3O4 nanobowl and nanotube arrays can enable their potential use in
catalysis, chemical sensing, luminescence, energy storage, controlled release, and cellular applications.
Ó 2014 Elsevier B.V. All rights reserved.
Received 17 November 2013
Received in revised form 21 February 2014
Accepted 24 February 2014
Available online 5 March 2014
Keywords:
Co3O4 nanobowl arrays
Nanotube arrays
Gas bubble-induced self assembly
Electrochemical activity
1. Introduction
involved. One is to synthesize such hard templates as polystyrene
nanosphere (PS), silica, poly-(vinyl alcohol) nanowell arrays, and
The preparation of bowl-shaped micro/nanostructures has been
the subject of many recent publications in the field of material sci-
ence. These bowl-shaped structures play a pivotal role in a wide
range of industrial and biological applications involving biomedical
and nanofluidic devices [1], nanosphere selection [2], coercivity
enhancement [3], and photocatalysis [4]. Strategies for synthesiz-
ing bowl-shaped containers, such as milling-etching method,
nanosphere lithography and sputtering technique, monolayer
self-assembly and electroforming approach, and chemical poly-
merization, have been extensively reported. Various bowl-shaped
micro/nanostructures, including polymer [5], metal oxide (ZnO
[6], TiO2 [7], CuO [8]), and metal (Pt [9], Ni [10], Co [1], Au [11])
have been successfully obtained. However, the above techniques
for preparing bowl-shaped microstructures are mainly based on
template strategy, that is, the following three steps are generally
PS-b-PEO diblock copolymer. Second is the formation of the nano-
shells by different coating techniques (e.g., reduction, evaporating/
sputtering deposition, electrochemical deposition, and solution/sol
dipping deposition). The last one is the removal of the templates
through annealing, dissolving, etching, or deep UV irradiation. In
addition, as for the milling–etching technique, ion milling was also
used to remove the top half of core–shell spheres. Apparently, the
aforesaid template-assisted procedures suffer a number of limita-
tions, such as complicated multi-step, especial experiment setup,
and high cost, which restrict their practical applications to some ex-
tent. The development of facile, low cost, and effective strategies for
synthesizing bowl-shaped structures, therefore, is urgently expected
in potential applications of shape-dependent property devices.
Polymorphous Co3O4 nanostructures have attracted consider-
able attention because of their potential applications in many dif-
ferent scientific and technological fields [12–15]. As properties of
Co3O4 nanostructures greatly depend on their shape, Co3O4 nano-
structures of various morphologies have been prepared, including
nanocubes [16] hollow spheres [17], nanorods [18], nanowires
⇑
Corresponding authors. Tel.: +86 15167953171.
E-mail addresses: tonggx@zjnu.cn, wwhtgs@126.com (G. Tong), guanjg@whut.
edu.cn (J. Guan).
http://dx.doi.org/10.1016/j.jallcom.2014.02.152
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

168
G. Tong et al. / Journal of Alloys and Compounds 601 (2014) 167–174
2.4. Sample characterization
[19,20], nanotubes [21,22], nanowalls [23], and nanoflowers [24].
Among the rich nanostructural morphologies revealed by Co3O4,
the nanobowl is never seen, but such unusual structures represent
an interesting platform for various applications in optical,
electronic, and biomimetic fields.
The morphologies, chemical composition, and crystalline structure of the
as-prepared samples were studied by X-ray diffraction (XRD, D/MAX-IIIA, Cu K
radiation, k ¼ 0:15406 nm;10 deg =min), Fourier-transform infrared (FTIR, Nicolet
FTIR-170SX, 4000–400 cm–1, room temperature) absorption spectra, field-emission
scanning electron microscopy (FE-SEM, Hitachi S-4800, 10 kV), and transmission
electron microscopy (HRTEM, JEM-2100F, 200 kV).
Nitrogen adsorption–desorption isotherms were obtained on a nitrogen adsorp-
tion apparatus (ASAP 2020 M Micromeritics instruments, USA). The samples are
degassed at 150 °C prior to Brunauer–Emmett–Teller (BET) measurements. The
BET specific surface area (SBET) is determined by a multipoint BET method using
the adsorption data in the relative pressure P/P0 range of 0.05–0.25. The pore size
distribution derived from the desorption isotherm using the Barret–Joyner–Halender
method. The nitrogen adsorption volume at a P/P0 of 0.995 was used to determine
the pore volume and porosity.
a
The gaseous bubble/gas flow-assisted synthesis strategy, a sim-
ple template-free technique, has the potential to construct organic
and/or inorganic materials into micro/nanostructures with a range
of well-defined morphologies on a large scale [25–27]. The afore-
said synthesis strategy has aroused much popular interest for its
simplicity, high efficiency, and low cost, which can overcome the
disadvantages of the hard template. In particular, morphological
modulation can be easily achieved through kinetically controlling
the number, distribution, size, and self-assembly behavior of gas
bubbles in the dispersion system. Successful synthesis of some
particular micro/nanostructures had been achieved, including
polypyrrole microcontainers (bowls, cups, goblets, and bottles)
[28–31], polymer microstructure [32], Co hollow mesospheres
[33], and Fe nanocircle [34]. We have deeply studied the assembly
behavior of gas flow in CVD and gas bubbles in solution. We
reported that Fe nanofibers could form under the impact of the car-
rier gas flow and the spontaneous magnetization of magnetic
nanoparticles [35]. Moreover, using H2 gas bubbles in situ gener-
ated in the liquid reduction reaction to direct the assembly of Fe
nuclei can generate polymorphic nanostructures, including nano-
fibers, nanotubes, and hollow nanospheres [36]. Additionally, this
strategy was expanded to solid phase synthesis. We found that
the thermal decomposition of inorganic salt (e.g., nitrate salt) also
released a large number of gas bubbles or gas, which could serve as
template to induce the assembly and growth of flower-like
structures [37] and nanotube arrays [38].
2.5. Measurement of electrochemical properties
Electrochemical measurements were carried out using two-electrode cells with
lithium metal as the counter and reference electrodes. The working electrode was
fabricated by compressing a mixture of the active Co3O4 materials, conductive
material (acetylene black, ATB), and binder (polytetrafluoroethylene, PTFE) in a
weight ratio of Co3O4/ATB/PTFE = 70:30:10. The battery assembly was performed
in a glove-box filled with pure argon, in which the negative piece, Celgard2400
membrane, and lithium chip were sandwiched, and an appropriate amount of elec-
trolyte was added before sealing. The newly installed battery was measured by a
discharge–charge method on a model Land CT2001A multichannel battery tester
in the potential range of 0–3.0 V, at current densities of 35–70 mA gꢂ1 and at
20 °C after hold for 4 h.
3. Results and discussion
3.1. Phase and structure characterizations
To clarify the structure of the products obtained at different Td,
XRD and IR tests were carried out. Fig. 1A presented the XRD
We further developed the above gas bubble-assisted assembly
method for the first time preparation of Co3O4 nanobowl arrays.
Decomposition temperature (Td), as a key kinetic factor, was used
to modulate the structure and morphology of the resultant Co3O4
nanostructures. The electrochemical properties of the obtained
polymorphous Co3O4 nanostructures were investigated in detail.
2. Experimental section
2.1. Chemicals
All reagents of analytical grade were obtained from Shanghai Chemical Reagent
Co. Ltd. in China and used without any further purification.
2.2. Preparation of nanobowl arrays
In a typical synthesis, a stainless steel substrate with 4.0 cm ꢁ 4.0 cm ꢁ 3.0 mm
in size was placed on a flat-heater (IKA C-MAG HS7) and was heated to 150 °C at a
heating-up rate of 10 °C minꢂ1. Then, 2.97 g Co(NO3)2ꢀ6H2O solid powders were
uniformly spread onto the substrate at once and kept at Td = 150 °C for 5 h. A black
film, which was easily peeled out of the substrate, was obtained by naturally cool-
ing to room temperature with the flat-heater, washing with dieonized water and
absolute ethanol for several times to remove unexpected impurities, and finally
drying in a vacuum oven at 80 °C for 5 h. The reaction process was carried out with-
in a mantle with an outlet to let the generated gases out to a 1.0 M sodium hydrox-
ide solution through a rubber pipe. Morphological modulation was achieved by
changing Td. For example, thermally decomposing Co(NO3)2ꢀ6H2O at Td = 200 °C
for 1 h can generate the contorted, tube-like nanostructures and at Td
300–400 °C for 1 h can form nanotube arrays.
=
2.3. Preparation of nanorods and microspheres
Unlike preparing nanotube arrays, Co(NO3)2ꢀ6H2O described above was uni-
formly spread into a ceramic vessel (1.2 cm ꢁ 9.0 cm ꢁ 1.0 cm) and immediately
placed in a horizontal tube furnace after the furnace temperature has been elevated
to the designated value (i.e., 500 °C for nanorods and 700 °C for microspheres). The
above vessel was kept at the designated temperature for 1 h. Other procedures were
the same with preparation of nanobowl arrays.
Fig. 1. XRD patterns (A) and IR spectra (B) of the products obtained at different Td.

G. Tong et al. / Journal of Alloys and Compounds 601 (2014) 167–174
169
patterns of the samples, in which all diffraction peaks possessed
the same position but different intensities. As for the samples
formed at 150 °C, these diffraction peaks at 10–90° can be indexed
to the face centered cubic Co3O4, space group Fd3m with the lattice
parameter a = 0.8056 nm (JCPDS Card No. 65-3103). Co3O4 then be-
gan to appear at the relatively low temperature of 150 °C for 5 h,
which is in good agreement with the literature [39]. Meanwhile,
low intensity peaks from the above samples suggest low crystalli-
zation and defect existence. Owing to the enhancement of crystal-
lization and crystallite size, a further increase in Td from 200 °C to
700 °C gradually strengthened the diffraction peaks. The mean
No nanobowls but contorted, tube-like Co3O4 nanostructures
were found at Td = 200 °C (Figs. 3A–C). When the Td varied from
300 and 400 °C, the products were a vertically aligned array of
nanotubes with an open end (Fig. 3E). All nanotubes had a rela-
tively uniform length up to approximately 300 nm, with one end
freestanding and the other end merged into a film (Fig. 3D). The
nanotubes were 50–60 nm in outer diameter, with a wall thickness
of 10 nm and a relatively rough surface feature (Fig. 3F).
By further elevating Td to 500 °C, only a mass of solid nanorods
30 5 nm in diameter and 50 nm to 200 nm in length could be ob-
served (Figs. 4A and B). More interestingly, numerous spherical
crystallite size (d) and strain (
e
) data of all the as-obtained samples
congregations 2.0 lm to 3.0 lm in size composed by the nanorods
were calculated by the Hall–Williamson equation and listed in
Table 1. The d values increased from 11.1 nm to 67.1 nm and the
were formed if the decomposition of Co(NO3)2ꢀ6H2O powders was
carried out at 700 °C (Figs. 4C and D). This result indicates that a
hollow/porous structure with low crystallization and small grain
size was easily formed at the relatively low Td, conversely, and
the solid rods and spheres with high crystallization and large grain
size were produced.
e
decreased from 0.510% to 0.108% when Td varied from 150 °C to
700 °C.
To further verify the results, the products formed at Td = 150 °C
to 700 °C underwent FT-IR analysis. The IR absorption peaks cen-
tered at ca. 666 and 575 cmꢂ1 (Fig. 1B) confirmed the formation
of spinel Co3O4 [40–42]. The aforementioned peak intensity in-
creased with Td, hinting Co3O4 crystallization enhancement
[36,43]. The peaks at 3431 and 1640 cmꢂ1 were assigned to the
OꢂH stretching and bending modes of physical-absorption water
or hydroxide [44], respectively. The sharp peak at 1384 cmꢂ1 came
3.3. Formation mechanism
Formation of bowl-shaped micro/nanostructures is mainly
dependent on template strategy. Obviously, no templates or sur-
factants were introduced to the current reaction system. Based
on the aforementioned data, therefore, we proposed a simple gas
bubble-induced self-assembly technique for the novel one-step
preparation of polymorphous Co3O4 nanostructures, as depicted
in Fig. 5. Such nanostructures were synthesized by uniformly
spreading Co(NO3)2ꢀ6H2O solid powders on a flat stainless steel
substrate and heating at various Td. The pink powders were first
melted and then turned into a black film covering the substrate.
During the process, several major steps were involved: (1)
Co(NO3)2ꢀ6H2O grains (Fig. 5A) quickly melted into a thin liquid
membrane, which covers the substrate, because of a low melting
point of only 55 °C (Fig. 5B); and (2) Thermal decomposition of
Co(NO3)2ꢀ6H2O occurred in terms of the following chemical
reaction:
from m
3 vibration of NOꢂ3 [45], indicating the existence of minor
intermediate products. Owing to the thorough conversion of minor
intermediate products to Co3O4, the peaks located at 3431, 1640,
and 1384 cmꢂ1 completely disappeared when Td was further ele-
vated. The above data demonstrate that the crystallization and
grain size of the products can easily be modulated by changing
the Td. Apparently, the products formed at the relatively low Td
had low crystallization and small grain size, exhibiting distinct
and novel electrochemical properties because of a quantum and
surface effect.
3.2. Morphology observation
Further insight into the morphology and microstructure of the
Co3O4 products was gained using SEM and TEM. Fig. 2A showed
that many Co3O4 nanobowls were uniformly arrayed on the sub-
strate in high densities of 5 ꢁ 105 units mmꢂ2 to 6 ꢁ 105
units mmꢂ2 while decomposing Co(NO3)2ꢀ6H2O at 150 °C. Gradu-
ally magnified SEM images in Figs. 2B and C revealed that these
D
3CoðNO3Þ2 ꢀ 6H2O ꢂꢂꢂ! Co3O4 þ O2 " þ6NO2 " þ18H2O "
ð1Þ
Obviously, the products were Co3O4 nuclei in combination with
a large amount of gas bubbles, including O2, NO2, and water vapor
(Fig. 5C).
nanobowls were a square with sides of 1.5–2.0 lm and wall thick-
(3) Gas bubbles induced the self assembly and growth of Co3O4
nanostructures. According to our previous work, some important
factors, including the viscosity of the carrier solution, number, size,
and transport rate of the gas bubbles, and the growth rate of the
nuclei, all strongly influence the gas bubble-induced assembly
behavior of nuclei. These factors can be tuned by changing the
reaction time, temperature, and even air pressure [37,38]. In the
current study, Td affected the gas bubble-induced self assembly
of Co3O4 nanostructures in the following two aspects. One, Td pro-
vided a driving force for the decomposition of Co(NO3)2ꢀ6H2O and
the nucleation and growth of Co3O4. The Td determined the decom-
position velocity of Co(NO3)2ꢀ6H2O. The higher Td, the more quickly
Co(NO3)2ꢀ6H2O decomposed and the more Co3O4 nuclei and bub-
bles in situ generated at per unit time. Therefore, higher Td favored
the nucleation and growth of Co3O4, as well as the generation of
gas bubbles. The other, Td adjusted the viscosity of molten
Co(NO3)2ꢀ6H2O and the diffusion rate/assembly behavior of the
gas bubbles, as well as the evaporation rate of crystal water. In gen-
eral, the higher Td, the higher evaporation rate of crystal water, and
the lower viscosity of Co(NO3)2ꢀ6H2O solution phase [46,47]. In
such cases, bubbles can carry Co3O4 nuclei to rapidly move up in
the solution and then were easily separated from Co3O4 nuclei
nesses of 100–200 nm, as well as smooth surface. Otherwise, circu-
lar nanobowls with 80 20 nm in diameter, 20 5 nm in wall
thickness, and 25 5 nm in depth can also be observed (Figs. 2D–
2F). The enlarged TEM images from the edge of a nanobowl
(Fig. 2G) showed that the nanobowls consisted of uniform and tiny
nanoparticles of 10–20 nm in size. The FFT of the entire region in
Fig. 2G was carried out and the pattern was shown in Fig. 2H.
The indexed, clear, diffused rings in the SAED pattern suggest that
the nanobowls were polycrystalline.
Table 1
Effect of decomposing temperature (Td) on mean crystallite size (d), strain (e), BET
specific surface area (SBET), pore volume (V), and pore size (D) of the Co3O4
nanostructures.
T °C Shape
d nm
e
%
Textural characteristics
SBET m2 gꢂ1 V cm3 gꢂ1 D nm
150 Nanobowl
200 Contorted nanotube 13.7
300 Nanotube array
500 Nanorod
700 Microsphere
11.1
0.510 66.31
0.429 63.01
0.253 41.72
0.061
0.18
0.12
0.00076
/
3.70
11.20
11.32
4.65
/
15.9
48.2
67.1
0.148
0.108
0.65
/

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G. Tong et al. / Journal of Alloys and Compounds 601 (2014) 167–174
Fig. 2. Low- and high-magnification SEM images of (A–C) quadrate and (D–F) circular Co3O4 nanobowl arrays obtained via decomposing Co(NO3)2ꢀ6H2O at 150 °C. (G) HRTEM
image of the region highlighted by the square in (F). (H) Corresponding fast Fourier translation (FFT) pattern of the (G).
Fig. 3. Tilted side view (A), top view (B), and high-magnification FE-SEM images of the top (C) of contorted, tube-like Co3O4 nanostructures formed via thermal decomposition
of Co(NO3)2ꢀ6H2O on Fe foil at 200 °C. Bottom view (D), top view (E), and magnified SEM image of the arrayed Co3O4 nanotubes formed at 300 °C.
and the reaction system. All these details play critical roles in the
morphological evolution of the Co3O4 nanostructures.
decomposition velocity and high viscosity hindered the transport
and separation of the bubbles in/from the solution, therefore form-
ing the intermittent column-like gas flow. These ruleless gas flows
acted as the template and assisted the Co3O4 nuclei to assemble
into the contorted tube-like nanostructures.
At the low Td (i.e., 150 °C to 400 °C), these bubbles carry the
Co3O4 nuclei to the solution surface to form a Co3O4 thin film
(Fig. 5C1). Herein, the formation of nanobowls at Td = 150 °C may
be caused by the low decomposition velocity and the high viscos-
ity. The former caused the decreased number of gas bubbles, and
the latter hindered the transport and separation of gas bubbles
in/from the solution. In this case, small amounts of gas bubbles
were slowly produced and suspended on the surface of the film.
These bubbles acted as template to guide the self assembly of
Co3O4 nuclei and finally broke down (Fig. 5D1) to form nanobowls
(Fig. 5E1).
As the Td increased to 200 °C, the increased number of gener-
ated gas bubbles resulted in the higher pressure inside the film
than outside. Afterward, the gas bubbles carried Co3O4 nuclei to
slowly move up and broke through the Co3O4 thin film, forming
protrusion crevasses or holes (Fig. 5D2). The relatively low
By contrast, the growth of nanotube arrays at Td = 300–400 °C
may be ascribed to the increased decomposition velocity and de-
creased viscosity. In this case, a lot of gas bubbles were generated
at a high diffusion rate. These bubbles rapidly moved up and
passed through the Co3O4 thin film. As a result, protrusion
crevasses or holes appeared because of the increased pressure
(Fig. 5D2). Subsequently, nanotubes grew at the edge of the holes
via the assembly of Co3O4 nuclei carried by gas bubbles, eventually
forming vertically aligned arrays of Co3O4 nanotubes (Fig. 5E2).
At the relatively high Td (e.g., 500 and 700 °C), the high reaction
rate of molten Co(NO3)2ꢀ6H2O increased the diffusion/aggregation
rate of gas bubbles. In particular, the high evaporation rate of
crystal water led to the rapid conversion of the reaction system

G. Tong et al. / Journal of Alloys and Compounds 601 (2014) 167–174
171
Fig. 4. Low- and high-solution SEM images of (A and B) Co3O4 nanorods formed at 500 °C and (C and D) Co3O4 microspheres produced at 700 °C.
D1) 150 oC
E1)
E2)
C1) Low Td
A
B
D2) 200–400 oC
D3) 500 oC
E3)
C2) High Td
D4) ≥700 oC
E4)
.
.
Solid Co(NO3)2 6H2O grain
molten Co(NO3)2 6H2O
Co3O4 nuclei
gas bubble ( O2, NO2 and/or vapor)
Fig. 5. Schematic illustration of the experimental procedure that generates Co3O4 nanobowls, nanotubes, nanorods, and microspheres: (A) Co(NO3)2ꢀ6H2O grains added to the
base were heated and formed molten state (B); (C1 and C2) Co3O4 nucleus generation through decomposing Co(NO3)2ꢀ6H2O at various temperatures; (D1–D4) Co3O4 nucleus
assembly into the polymorphous nanostructures guided by the in situ generated gas bubbles (including O2, NO2, and/or vapor) under different temperatures; (E1–E4)
Formation of Co3O4 nanostructure arrays with final morphologies ranging from microspheres, to nanotubes, nanorods, and nanobowls.
from liquid phase to solid phase. Obviously, the reaction system of
liquid phase is fit for the gas bubble-induced self-assembly strat-
egy, so this strategy will lose effectiveness in the high Td. Under
this condition, the continuous gas flow was formed at Td = 500 °C,
which directed the assembly of Co3O4 nuclei into solid nanorods.
Meanwhile, the relatively high Td also provided a powerful driving
force for the nanocrystal growth. These nanocrystals grew up by
the fusion of Co3O4 nuclei or nanoparticles, causing the shrinkage
and even disappearance of the pore size with increased Td. There-
fore, the growth of microspheres produced at Td = 700 °C was
ascribed to the high nucleation and growth rate rather than the
induction role of gas bubbles. The above results indicate that
changing Td can effectively tailor the shape and structure of the
as-made products.
In addition, heating rate is also a key factor for forming nano-
structures. We found a high heating rate favored the formation
of nanobowls, nanotubes, and nanorods. On the contrary, if the
samples were slowly heated up to higher temperature, only spher-
ical aggregates were obtained. This may be because the slow heat-
ing process leads to the evaporation of a mass of crystal water. In
the absence of crystal water, the gas bubble-induced self-assembly
technique no longer worked. This further verified the hypothesis of
the gas bubble-induced self-assembly strategy proposed above. So
we selected to directly spread Co(NO3)2ꢀ6H2O onto the heated
substrate.
Our method has several important advantages over other tech-
nologies. Our approach is more simple, convenient, and efficient;
the differences involved are as follows: Above all, a rapid thermal

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G. Tong et al. / Journal of Alloys and Compounds 601 (2014) 167–174
decomposition route based on only a common cobalt nitrate salt,
without the addition of any chemical reagents (including the sur-
factant, even the solvent), is used instead of the solution phase
reaction. Moreover, the gas bubbles with high speed are generated
in situ in the thermal decomposition reaction other than those re-
leased from the additional reaction (e.g., the electrolysis process of
H2O or the decomposition of hydrogen peroxide [21]). Last but not
the least, various kinetic factors, such as thermal decomposition
temperature and pressure, were applied to modulate the size,
number, and release speed of the gas bubbles and therefore easily
tuned the final morphology of the resulting products. Our novel ap-
proach is hopefully extended to synthesize hollow metal and/or
metal oxide nanotubes or nanotube arrays on various substrates.
bimodal pore-size distribution was further confirmed by the corre-
sponding pore-size distribution of the Co3O4 products (Fig. 6B). As
shown in the pore distribution curves of Fig. 6B, the Co3O4 nano-
bowl and nanotube arrays produced at the low Td of 150 °C to
300 °C had a bimodal pore distribution using the Barrett–Joyner–
Halenda (BJH) method, that is, smaller pores in regions of 1 nm
to 8 nm and larger pores ranging from 10 nm to 120 nm. Indeed,
the formation of hierarchical nanopore structures on a multi-
length scale was highly conducive to lithium ion insertion and
extraction reactions, as well as the electrolyte transportation.
However, the nanorods obtained at 500 °C just showed a small hys-
teresis loop with flat and low single-modal pore-size distribution
curve. As Td increased, the pore peak located at the small mesopore
region shifted gradually to the larger mesopore region and disap-
peared at 500 °C; the one located at the large mesopore region
strengthened first and then weakened. These variations accorded
with the structure evolution of the products.
The details about the specific surface area (SBET), pore volume
(V), and pore size (D) of the samples derived from the nitrogen
adsorption/desorption isotherm are listed in Table 1. These data
clearly show that the SBET decreased from 66.31 m2 gꢂ1 to
0.65 m2 gꢂ1 with Td increasing from 150 °C to 500 °C, whereas V
and D showed the reversed U-shape variations. The SBET values of
66.31 m2 gꢂ1 for nanobowls and 63.01 m2 gꢂ1 for nanotube arrays
were significantly larger than 24.8 m2 gꢂ1 of macroporous Co3O4
platelets and 7.6 m2 gꢂ1 of nanotubes [23]. A possible reason is that
Td provided different driving forces for the growth of nanocrystals
and the mass transport rate of the gas bubbles. At low Td, the
growth and nucleation rates of nuclei were small, but the increased
mass transport rate of the gas with Td facilitated the enlargement
of pore volume and pore size. Therefore, the high growth and
nucleation rates of nuclei at relatively high Td are the key factors
for the formation of the solid structures. Similar phenomenon
can also be reported in our previous work [48,49].
3.4. Textural characteristics
Aside from the modulation of crystallinity, grain size, and
morphology, the Td also contributed significantly to the textural
properties of the products. The textural properties of the Co3O4
products were further characterized by measuring nitrogen physi-
cal adsorption/desorption isotherms, as shown in Fig. 6. The prod-
ucts formed at 150 °C to 300 °C showed the type IV isotherms
(Fig. 6A). With Td gradually increased to 300 °C, the curves were
distinctly split into two separate hysteresis loops, indicating the
appearance of the bimodal pore-size distributions. The hysteresis
features of Co3O4 products at a low relative pressure (0.4 < P/
P0 < 0.75) should be classified as type H2 loop, suggesting the pres-
ence of ink-bottle pore. In contrast, these hysteresis loops at a high
relative pressure (0.75 < P/P0 < 1.0) were of type H3, corresponding
to slit-shaped pores from aggregates of plate-like particles. The
3.5. Electrochemical properties
The electrochemical performance of as-prepared Co3O4 samples
was evaluated by a standard method based on Co3O4/Li haft cell, in
which Co3O4 and lithium foil worked as positive and negative elec-
trode, respectively. Fig. 7A shows the voltage/capacity curves for
the Co3O4 nanobowls/Li cell, measured between 3.00 and 0.01 V,
which was similar to those of Co3O4 nanomaterials with other
structures prepared by other methods. During the first discharge,
the voltage decreased steeply to approximately 1.0 V, whereupon
a plateau set in and continued until a capacity of approximately
1000 mA gꢂ1, followed by a gradual drop in voltage until the end of
discharge. As for the Co3O4 nanobowl arrays, only one well-defined
voltage plateau at approximately 1.0 V was observed, which is gener-
ally attributed to the reduction processes from Co3+ to Co0 [23] No
high voltage plateau was observed at ca. 1.3 eV, which was previ-
ously caused by low surface area or very low current density of
nanotextured Co3O4. According to the reaction mechanism
proposed in the earlier studies [15,21,38], the following chemical
reactions were involved in the discharging and charging process.
Co3O4 þ 8Li þ 8eꢂ $ 4Li2O þ 3Co0
8Li $ 8Liþ þ 8eꢂ
ð2Þ
ð3Þ
Co3O4 þ 8Li disꢂcꢂha!rge 4Li2O þ 3Co0
ð4Þ
charge
Fig. 7B shows the curves of specific capacities versus cycle num-
ber for the Co3O4 nanobowl electrodes at various current densities.
The Co3O4 nanobowl electrodes had an initial discharge capacity
Fig. 6. Nitrogen adsorption/desorption isotherms (A) and the corresponding pore
size distribution curves (B) of the products obtained at various Td.

G. Tong et al. / Journal of Alloys and Compounds 601 (2014) 167–174
173
of passivation layers [55], electrolyte degradation, dissolution of
the active material within the organic electrolyte occurring during
first discharge [50], or concomitant formation of the decomposi-
tion layer on the surface of the reduced particles [56]. However,
these nanobowl arrays still maintained a capacity as high as
992.3 mAh gꢂ1 at 35 mA gꢂ1 and 774.8 mAh gꢂ1 at 100 mA gꢂ1
after the first cycle. Moreover, these high capacities could almost
be completely retained even after 10 cycles (Fig. 7B) with
843.2 mAh gꢂ1 at 35 mA gꢂ1 and 680.1 mAh gꢂ1 at 100 mA gꢂ1
,
corresponding to a fading rate of 0.6% per cycle. Additionally,
843.2 mAh gꢂ1 was higher than that (750 mAh gꢂ1) of nanowires
and that (ca. 800 mAh gꢂ1) of mesoporous nanowire arrays [20].
To further reveal the effect of morphology and structure on
electrochemical properties, the electrochemical performance of
polymorphous Co3O4 samples were also investigated under the
same conditions (Fig. 7C). The initial discharge capacities of nano-
tube array, nanorods, and microspheres were 1293.05, 1250.77,
and 1008.62 mAh gꢂ1 at 35 mA gꢂ1, respectively. After the first cy-
cle, the above initial capacities decreased by 73.87% to
955.19 mAh gꢂ1 for nanotube arrays, 74.86% to 936.28 mAh gꢂ1
for nanorods, and by 57.80% to 582.98 mAh gꢂ1 for microspheres.
After 10 cycles, Co3O4 nanotube arrays still took on a high capacity
of 895.2 mAh gꢂ1
, whereas nanorods and microspheres only
showed charge capacities of 709.8 mAh gꢂ1 and 538.2 mAh gꢂ1
respectively.
,
The data clearly indicate that nanobowl and nanotube arrays
formed at low Td exhibited higher capacities and better cyclic life
than those solid products obtained at the high Td. The superior
electrochemical performances were caused by the aforesaid nano-
scaled materials with low crystallization, small grain size, high
SBET, hierarchically porous structure, and orderly arrangements.
First, quantum and surface effects from nanobowls and nanotubes
formed at low Td strongly affected electrochemical activities. As we
know, nanomaterials have more exposed high-energy crystal
planes and dangling bonds on the surface, which reduce the oxida-
tion–reduction gaps and enhance chemical reactivity, causing the
acceleration of reaction rates. In the current study, the lower Td
helps the formation of the products with smaller grain size and
lower crystallization (see Table 1). In this case, with decreased
Td, an increasing proportion of the total number of atoms lay on
the surface, improving the structural strains and being more active
for lithium electrochemical reaction. This event led to the capacity
ordered as Co3O4 nanobowl arrays > Co3O4 nanotube arrays >
Co3O4 nanorods > Co3O4 microspheres. Second, large specific
surface areas of bowl-shaped/tube-like hollow structures provided
more grain boundaries for lithium insertion and facilitated the
delivery of lithium ions and electrons to the surface and therefore
provided more active sites. Third, the hierarchically porous struc-
tures could enhance the contact area between the electrode and
electrolyte, shorten diffusion length for lithium ions, allow better
penetration of the electrolyte, and accommodate the large volume
changes induced by lithium insertion/extraction, thereby enhanc-
ing the rate cycling stability [57].
Fig. 7. (A) Discharging and charging curves of the electrodes made by the as-
prepared Co3O4 nanobowl arrays obtained at 150 °C cycled between 3.00 and 0.01 V
at 35 mA gꢂ1 and 20 °C. Specific capacities versus cycle number for the electrodes
made by (B) Co3O4 nanobowl arrays and (C) other Co3O4 nanostructures obtained at
different temperatures of 300 °C to 700 °C at 35 mA gꢂ1. For each set of data, the
upper dot is for discharge and the lower dot is for charge.
The structures suffer from poor capacity retention upon cycling
and/or poor rate capability, which remains the major challenge for
use in practical cells. However, still nearly 100% capacity retention
may be obtained by using additive or prelithiation for compensation
or the active modification of the tubular wall [48,58,59]. Thus, the
Co3O4 nanobowl and nanotube arrays will be under consideration
as electrode materials or advanced lithium batteries in the future.
as high as 1467.9 mAh gꢂ1 at the current density of 35 mA gꢂ1 and
1376.8 mAh gꢂ1 at the current density of 100 mA gꢂ1. The above val-
ues are larger than their theoretical values (890 mAh gꢂ1), owing to
irreversible reactions (e.g., decomposition of electrolyte) to form a
solid electrolyte interphase layer or possibly interfacial lithium stor-
age [50,51]. Note that the capacities were higher than those of Co3O4
nanoparticles (830 mAh gꢂ1), nanobelts (1204 mAh gꢂ1) [52], Co3O4
nanotubes (850 mAh gꢂ1) [53], and mesoporous nanowire arrays
(1124 mAh gꢂ1) [20].
4. Conclusions
Just as many electrode materials, Co3O4 nanobowl arrays dis-
played a large irreversible loss of 26–30% in the first cycle. This
inevitable loss is normally ascribed to agglomeration [54], growth
In summary, Co3O4 nanobowl arrays, nanotube arrays, nano-
rods, and microspheres have been successfully synthesized via a

174
G. Tong et al. / Journal of Alloys and Compounds 601 (2014) 167–174
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