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Controlled facile synthesis, growth mechanism, and exothermic properties of large-
area Co3O4 nanowalls and nanowires on silicon substrates
Zhiqiang Qiao, Daguo Xu, Fude Nie, Guangcheng Yang, and Kaili Zhang
Citation: Journal of Applied Physics 112, 014310 (2012); doi: 10.1063/1.4731798
View online: http://dx.doi.org/10.1063/1.4731798
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/112/1?ver=pdfcov
Published by the AIP Publishing
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Controlled facile synthesis, growth mechanism, and exothermic properties
of large-area Co
3
O
4
nanowalls and nanowires on silicon substrates
Zhiqiang Qiao,
1
Daguo Xu,
2
Fude Nie,
1
Guangcheng Yang,
1,a)
and Kaili Zhang
2,a)
1
Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China
2
Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue,
Kowloon, Hong Kong
(Received 11 February 2012; accepted 30 May 2012; published online 3 July 2012)
Co
3
O
4
nanowalls and nanowires have been synthesized onto silicon substrates by low-temperature
thermal oxidation of sputtered Co thin films in static air. The synthesis method is very simple and
suitable for large-scale fabrication. The effects of the thermal oxidation temperature and duration on
the size, amount, and length of the nanowires and nanowalls are systematically investigated both by
scanning electron microscopy characterization and differential scanning calorimetry thermal analysis.
It is found that the Co/CoO oxidation and Co
3
O
4
decomposition are important factors contributing to
the growth of the Co
3
O
4
nanowalls and nanowires. The mechanical adhesion between the Co
3
O
4
nanowalls/nanowires/film and the silicon substrate is observed to be very strong, which is beneficial
for many practical applications. Based on the experimental observations, the detailed growth
mechanisms of the nanowalls and nanowires are presented. Finally, the promising novel exothermic
reaction properties of the Co
3
O
4
nanowalls and nanowires with Al are investigated by thermal
analysis. V
C2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.4731798]
I. INTRODUCTION
As an important anti-ferromagnetic p-type semiconduc-
tor, Co
3
O
4
is widely used as heterogeneous catalysts,
1,2
solid-
state sensors,
3
electro-chromic devices,
4,5
and absorbers of
solar energy.
6
Among various Co
3
O
4
structures, one-
dimensional (1D) Co
3
O
4
nanostructures including nanowires,
nanorods, and nanotubes have been extensively studied due to
their unique properties and promising applications.
1,7–13
Li
et al. synthesized Co
3
O
4
nanowires by an ammonia-
evaporation induced method and investigated their applica-
tions in lithium ion batteries.
7,10
Co
3
O
4
nanowires were also
synthesized by virus-enabled synthesis and assembly for use
as lithium ion battery electrodes.
8
Dong et al. prepared Co
3
O
4
nanowires by heating a cobalt foil and the nanowires exhibit
novel optical and magnetic properties.
9
Ag-coated Co
3
O
4
nanowire arrays were obtained and employed as electrodes for
supercapacitors and lithium ion batteries showing improved
specific capacity and enhanced rate capability.
11
Co
3
O
4
nano-
rods have been synthesized and they exhibit good field emis-
sion properties.
12
Li et al. prepared Co
3
O
4
nanotubes by a
porous-alumina-template method. The Co
3
O
4
nanotubes dis-
play high discharge capacity, superior cycling reversibility,
and excellent sensitivity to hydrogen and alcohol.
13
Although 1D Co
3
O
4
nanostructures have been widely
investigated, very few reports are found in the literature to
realize two-dimensional (2D) Co
3
O
4
nanostructures. It is
well-known that 2D nanostructures are important compo-
nents for nanoscale devices. They have found various prom-
ising applications due to their large surface areas that can
be exposed to gaseous environments and other unique
properties.
14–18
Therefore, it is desirable to also synthesize
2D Co
3
O
4
nanostructures in addition to 1D Co
3
O
4
nanostruc-
tures. In 2005, Yu et al. synthesized Co
3
O
4
nanowalls by
heating a Co foil in ambient air and studied their field emis-
sion properties.
18
Using a similar method, Dong et al.
achieved Co
3
O
4
nanowalls on the surface a Co foil and
Co
3
O
4
nanowires at the edge of the Co foil.
9
However, there are very few studies in the literature to re-
alize Co
3
O
4
nanowalls and nanowires onto silicon, a basic
material for microelectronics and microsystems. Synthesizing
nanomaterials onto silicon substrates is attracting much
attention.
19–23
One key reason is that once nanomaterials are
synthesized onto silicon substrates, it is straightforward to
integrate the nanomaterials with silicon based electronic cir-
cuits to achieve functional devices. Furthermore, the Co
3
O
4
nanowalls and nanowires fabricated onto Co foil substrates
are not firmly attached to the substrate (easy to be peeled
off).
9
This is not suitable for many applications, where the
devices need to be used under librating, rubbing, and impact-
ing environments. The firm adhesion between the Co
3
O
4
nanowalls and nanowires and the substrates is crucial for these
applications. Similar drawbacks exist for the CuO nanowires
synthesized by thermal oxidation of Cu foils and/or films. The
mechanical adhesion between the CuO nanowires/film and the
substrates is very weak. Cracking and flaking of the CuO
nanowires/film or even exfoliation from the substrates is still a
big challenge that is not well-resolved, which severely affects
their properties and practical applications.
24–26
Moreover, the
detailed growth mechanism of the Co
3
O
4
nanowalls and nano-
wires during the thermal oxidation remains unclear. Espe-
cially, the effects of the Co
3
O
4
decomposition reaction and
the local high temperature generated by the Co/CoO oxidation
on the nanostructures during the heating process are rarely
reported in the literature.
a)
Authors to whom correspondence should be addressed. Electronic addresses:
ygcheng@hotmail.com. Tel.: 86-816-2544436. Fax: 86-816-2495856 and
kaizhang@cityu.edu.hk. Tel.: 852-34427845. Fax: 852-34420172.
0021-8979/2012/112(1)/014310/9/$30.00 V
C2012 American Institute of Physics112, 014310-1
JOURNAL OF APPLIED PHYSICS 112, 014310 (2012)
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In this work, Co
3
O
4
nanowalls and nanowires are synthe-
sized onto silicon through thermal oxidation of Co thin films
deposited on the silicon substrates by magnetron sputtering.
Field emission scanning electron microscopy (FESEM), x-ray
diffraction (XRD), Raman spectrum, transmission electron
microscopy (TEM), high-resolution transmission electron mi-
croscopy (HRTEM), energy dispersive x-ray analysis (EDX),
and differential scanning calorimetry (DSC) are used to char-
acterize the nanowalls and nanowires. The Co
3
O
4
nanowalls
and nanowires are found to be strongly attached to the silicon
substrate. The detailed growth mechanisms of the nanostruc-
tures are proposed. Based on the DSC thermal analysis and
SEM characterizations, the optimal oxidation temperature and
duration for the growth of the nanostructures are determined.
Finally, the promising novel exothermic reaction properties of
the Co
3
O
4
nanowalls and nanowires with Al are presented.
II. EXPERIMENTS
The fabrication process starts with a 500-lm-thick
double-polished 4-in. p-type (100) silicon wafer. The wafer
is cleaned using acetone and chromic sulfuric acid mixture,
thoroughly rinsed by deionized water, and blow-dried by
nitrogen. Then, the wafer is placed into an oven at 200 C for
20 min for further drying. The clean dry wafer is put into a
magnetron sputtering system. After the vacuum level in the
sputter chamber arrives to 4.2 10
4
mTorr, Ar flow with a
flow rate of 10 sccm is introduced to the chamber. Co thin
films are sputtered onto the silicon wafer with an Ar pressure
of 10 mTorr and the wafer rotates at 12 rpm during sputter-
ing. The Co thin film on the silicon wafer is put onto a quartz
boat that is placed inside a horizontal muffle furnace. The Co
film is heated up to designed temperatures that are held for
0.5–24 h in static air and then the furnace is cooled down to
room temperature naturally.
The morphology and structure of the Co
3
O
4
nanowalls
and nanowires are examined with (Apollo 300 FESEM),
(Brukers D8 Advance XRD), Raman spectrum, and (Tecnai
F20 TEM), respectively. The Co
3
O
4
nanowalls and nanowires
are scraped off from the silicon substrate using a special very
sharp knife with large force and put into a Pt sample holder
for differential scanning calorimetry analysis (TA instruments
Q20 DSC). The DSC experiment is carried out from 20 to
700 Cataheatingrateof5
C/min under 99.99% Ar flow
with a flow rate of 40 ml/min. A second analysis is done on
the same sample and in the same conditions in order to help
the computation of the baseline correction of the DSC
analysis.
III. RESULTS AND DISCUSSION
A. Morphology and structure characterizations
Figure 1shows the 1000 nm-thick Co film after being
thermally oxidized at 380 C for 8 h in static air. Both Co
3
O
4
nanowalls and nanowires grow from the surface of the Co
film. The average thickness of the nanowalls is around
40–100 nm and the length of the nanowalls is about
500–1000 nm. The average length of the nanowires is around
3–5 lm and the diameter of nanowires is about 10–20 nm.
Most of the Co
3
O
4
nanowalls and nanowires are aligned per-
pendicularly to the substrate surface with longer nanowires
tending to fall down on the substrate. In our experiments,
only nanowalls (no nanowires) are found at the edge of the
film (see inset of Figure 1(b)), and interlaced nanowalls and
nanowires are observed on the surface of the film. This is
very different from what Dong et al. observed while the Co
foil was heated under a water vapor flow in a tube furnace.
9
Only nanowires (no nanowalls) were found at the edge of the
Co foil in their experiments.
The XRD pattern of the sample is shown in Figure 2,
where the strong Si (100) peak is from the well-crystallized
single-crystal silicon substrate. Both Co
3
O
4
and CoO are
found from the pattern. When the Co is thermally oxidized,
FIG. 1. SEM images of the 1000 nm-thick
Co film on Si substrate heated at 380 C for
8 h: (a) top view and (b) cross-section view.
FIG. 2. XRD pattern of the 1000 nm-thick Co film on Si substrate heated at
380 C for 8 h.
014310-2 Qiao et al. J. Appl. Phys. 112, 014310 (2012)
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CoO is first formed as shown in Eq. (1). The CoO serves as
the precursor for the growth of Co
3
O
4
nanowalls and nano-
wires though future oxidation as indicated in Eq. (2).
9,18
Co þO!CoO;(1)
6CoO þO2!2Co3O4:(2)
This oxidation process starts from the surface of the Co thin
film to the deeper part of the film. Therefore, we believe that
CoO exits in the deeper region of the film that is close to the
substrate. To further identify the composition of the nano-
walls and nanowires, Raman spectrum has been measured at
room temperature as shown in Figure 3. There are four peaks
at 483, 522, 621, and 693 cm
1
in the pattern, which is like
the Raman spectra of the nanowalls shown in Ref. 27. The
four peaks correspond to one E
g
, two F
2g
, and A
1g
models of
Co
3
O
4
, respectively. As aforementioned, CoO peaks are
found in the XRD pattern. However, they are not observed in
the Raman spectrum. The main reason is that XRD can
achieve deeper penetration depth to detect the CoO com-
pared to Raman spectrum. From the results of the XRD and
Raman spectrum, it is reasonable to believe that the nano-
walls and nanowires are made of Co
3
O
4
.
The nanowires and nanowalls are also characterized by
TEM, HRTEM, and EDX. The nanowalls and nanowires are
manually scratched from the silicon substrate using a special
very sharp knife, mixed with ethanol, and deposited onto
carbon-coated copper girds. Figure 4(a) is the TEM image of
the nanowires indicating that the diameter of nanowire
ranges from 10 nm to 20 nm. Figure 4(b) shows a HRTEM
image of a nanowire and inset is a corresponding Fourier
transformation of the HRTEM image. The image shows clear
fringes with an interplanar spacing of 0.202 nm, which corre-
sponds to the separation between the (400) lattice planes.
Figure 4(c) shows the EDX spectrum, where the peaks of Co
and O can be clearly seen. The Co/O ratio of the nanowire is
difficult to determine since the intensity of the EDX detec-
tion for lighter element of oxygen is uncertain. The peaks of
Cu and C are from the copper grid covered by a carbon film.
B. Effect of the thermal oxidation temperature
To achieve a better control of the synthesized Co
3
O
4
nanowalls and nanowires, the effect of the thermal oxidation
temperature on the synthesis is investigated. Both DSC ther-
mal analysis and FESEM characterization are used to study
the effect of the thermal oxidation temperature. DSC thermal
analysis is first performed for the Co
3
O
4
sample after thermal
oxidation at 380 C for 8 h. The Co
3
O
4
nanowalls and nano-
wires are scraped off from the silicon substrate using a spe-
cial very sharp knife. It needs to be mentioned that large
FIG. 3. Raman spectrum of the 1000nm-thick Co film on Si substrate
heated at 380 C for 8 h.
FIG. 4. TEM characterization of the
1000 nm-thick Co film on Si substrate
heated at 380 C for 8 h: (a) TEM image
of the nanowires and nanowalls, (b)
HRTEM image of a nanowire in (a), inset
is a corresponding Fourier transformation
of the HRTEM image, and (c) EDX spec-
trum of the nanowire in (a).
014310-3 Qiao et al. J. Appl. Phys. 112, 014310 (2012)
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force is necessary to scratch the sample from the silicon sub-
strate due to the very strong adhesion between the nano-
walls/nanowires and the silicon substrate. The very firm
mechanical adhesion should be caused by the formation of
cobalt silicide at the interface between the Co film and the
silicon substrate during thermal oxidation at 380 C in this
work. It is known that cobalt silicides are formed by the
solid-state reaction when cobalt and silicon are annealed at
temperatures higher than 350 C. The first phase to grow at
the Co/Si interface is Co
2
Si. If silicon is in excess, the CoSi
is then formed.
28,29
The DSC thermal analysis is performed in 99.99% argon
flow with a flow rate of 40 ml/min. Figure 5shows the DSC
curve. Exothermic reactions are found in the temperature
range of 200–400 C. This is caused by the oxidation of CoO
into Co
3
O
4
by the trace oxygen (about 50 ppm) contained in
the Ar. It is known that the CoO existing between the Co
3
O
4
and silicon substrate is exposed for oxidation when the film
is scraped off from the substrate. When the temperature is
above 420 C, an endothermic process occurs corresponding
to the decomposition of Co
3
O
4
as shown in Eq. (3). Accord-
ing to the DSC result, the optimized temperature for the
growth of Co
3
O
4
nanostructures should be around 400 C.
2Co3O4!6CoO þO2:(3)
To further investigate the effect of temperature on the
nanostructures, Co films on the silicon substrates are heated
from 300 C to 500 C for 6 h and the results are shown in
Figures 6and 7, respectively. As shown in Figures 6(a) and
6(b), at 300 C very tiny nanowalls and nanowires are
observed on the Co film. When the temperatures are
increased to 350 C, 380 C, and 400 C, the nanowalls
become longer and higher, and the diameter of nanowires
become larger as indicated in Figures 6(c)–6(h), respectively.
However, when the thermal oxidation temperatures are
further increased to 420 C and 450 C, the nanowalls
become fewer, and the nanowires get shorter and sparser as
can be seen in Figures 7(a)–7(d), respectively. Finally, when
the temperature is increased to 500 C, the nanowalls and
nanowires disappear completely (images not shown here).
The above results agree well with the DSC curve in Figure 5.
It also demonstrates that the decomposition temperature of
the nanostructured Co
3
O
4
(nanowalls and nanowires here) is
much lower than that for bulk Co
3
O
4
materials. It is known
that bulk Co
3
O
4
will decompose at around 850 C in nitro-
gen
30
and around 900 C in air.
31
The decomposition temper-
ature (450 500 C) for the very tiny Co
3
O
4
nanowalls and
nanowires is around 300 400 C lower than that for bulk
Co
3
O
4
. The much lower decomposition temperature can be
caused by a high density of defects, such as dislocations and/
or grain boundaries existing in the nanoscale Co
3
O
4
walls
and wires. The similar reason has been employed to explain
the lower decomposition temperature (700 C) of NbSe
2
nanowires compared to that (840 C) of bulk NbSe
2
.
32
C. Effects of the thermal oxidation duration and Co
film thickness
The effect of the thermal oxidation duration is studied as
shown in Figures 8(a)–8(f), where the Co films on the silicon
substrates are heated at 380 C in static air for 1 h, 2 h, 6 h, 8 h,
FIG. 5. DSC curve of Co
3
O
4
powder scraped off from the substrate, where
the Co film is oxidized at 380 C for 8 h.
FIG. 6. SEM images of Co film on Si substrate heated under different tem-
peratures for 6 h: (a) 300 C, top view (b) 300 C, cross-section view, (c)
350 C, top view, (d) 350 C, cross-section view, (e) 380 C, top view, (f)
380 C, cross-section view, (g) 400 C, top view, and (h) 400 C, cross-
section view.
014310-4 Qiao et al. J. Appl. Phys. 112, 014310 (2012)
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FIG. 7. SEM images of Co film on Si sub-
strate heated under different temperatures
for 6 h: (a) 420 C, top view, (b) 420 C,
cross-section view, (c) 450 C, top view, and
(d) 450 C, cross-section view.
FIG. 8. Top-view SEM images of Co films
heated at 380 C in static air for (a) 1 h, (b)
2 h, (c) 6 h, (d) 8 h, (e) 12 h, and (f) 24 h,
respectively.
014310-5 Qiao et al. J. Appl. Phys. 112, 014310 (2012)
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12 h, and 24 h, respectively. When the duration of the thermal
oxidation is 1 h, very tiny nanowalls and nanowires start to
grow out of the thin film. After the anneal durations are
increased to 2 h, 6 h, 8 h, and 12 h, respectively, the nanowalls
become longer and higher, and the nanowires are getting
thicker and longer gradually. However, when the duration is
too long (24h here in our experiments), the nanowalls and
nanowires become sparser. This is due to the complete oxida-
tion of the Co film and a slow decomposition of Co
3
O
4
.Con-
sequently, thermal oxidation duration is also an important
parameter that needs to be considered while synthesizing
Co
3
O
4
nanowalls and nanowires by thermal oxidation of Co
films.
The thickness effect of the pre-deposited Co film on the
resulted nanostructures is also investigated as shown in
Figure 9. Figures 9(a) and 9(c) show the top view and cross-
section view SEM images of the 500 nm-thick Co film
heated at 380 C in static air for 12 h, respectively. Figures
9(b) and 9(d) are the top view and cross-section view SEM
images of the 1000 nm-thick Co film heated at 380 Cin
static air for 12 h, respectively. Basically, the thickness of
the Co film has little effect on the resulted nanostructures in
accordance with on our experiments.
D. Growth mechanism
The growth mechanism has been discussed for the
Co
3
O
4
nanowalls and nanowires that are synthesized by ther-
mal oxidation of Co foils.
9,18
However, the detailed mecha-
nism about the growth of Co
3
O
4
nanowalls and nanowires in
atmosphere is still unclear.
9
Based on our experimental
results, we propose the detailed mechanism here to explain
the growth of Co
3
O
4
nanowalls and nanowires that are syn-
thesized by thermal oxidation of sputtered Co films in static
air as shown in Figure 10. For the growth of Co
3
O
4
nano-
walls, when the temperature is close to 300 C, the surface of
the Co film starts to melt to form the liquid Co. The oxygen
in static air diffuses into the liquid Co and oxidizes it to
CoO. Consequently, a thin layer of CoO is formed on top of
the Co film. Further oxidation of the surface of the thin CoO
layer leads to the formation of Co
3
O
4
. The Co
3
O
4
will
FIG. 9. Top view and cross-section view
SEM images of Co films heated at 380 Cin
static air for 12 h: (a) and (c) 500 nm-thick
Co film, (b) and (d) 1000 nm-thick Co film.
FIG. 10. Sketches of the nanowires growth in different conditions: (a) nano-
wires formed in the central part of the Co film in static air in this study, (b)
nanowires formed at the edge of the Co foil with a water vapor flow in Ref. 9.
014310-6 Qiao et al. J. Appl. Phys. 112, 014310 (2012)
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precipitate after saturation in liquid state to start the growth
of the nanowalls. The nanowalls become longer with increas-
ing the oxidation duration, which will stop upon cooling and
the liquid condenses into solid state Co
3
O
4
nanowalls. This
two-step oxidation process proceeds further inside the Co
film with increasing oxidation time. That is the reason why
CoO exits between the Co
3
O
4
and the silicon substrate as
confirmed by the XRD and Raman characterization in
Sec. III A. The growth mechanism of Co
3
O
4
nanowalls is
similar to the solid-liquid-solid (SLS) process as described in
Refs. 18 and 33.
However, the SLS process should not be the dominant
growth mechanism of Co
3
O
4
nanowires. In our experiments,
the Co film is heated in static air, Co
3
O
4
nanowires are found
in the central part of the film, but not at the edge of the film.
Nevertheless, in the experiments performed by Dong et al.
9
where the Co foil is heated under a water vapor flow, Co
3
O
4
nanowires are not observed in the central part of the foil, but
at the edge of the foil. It is difficult for the SLS mechanism
to explain the big difference. In this study, we propose a so-
called solid-liquid-vapor-solid (SLVS) mechanism to explain
the growth of the Co
3
O
4
nanowires as follows. Upon heating,
the surface of the Co film melts to form the liquid Co, which
is oxidized into CoO and then Co
3
O
4
by the oxygen con-
tained in static air. The large amount of heat that is released
by the oxidation of Co and CoO may induce local high tem-
perature
34
and boils the liquid media into micro-vapor,
although the temperature inside the furnace is far below the
boiling point of bulk Co. When the vapor arises from the sur-
face of the film, it is oxidized into Co
3
O
4
and also changed
into solid state to form nanowires by self-assembly. The rea-
son for the vapor to change into solid is that the temperature
away from the film surface (local high temperature exits) is
low due to the temperature gradient. With a greater cooling
space and oxidization of smaller amount of Co and CoO, a
low temperature area may form near the edge of the Co film
as shown in Figure 10(a). As a result, the Co liquid cannot
be boiled to micro-vapor at the edge. Consequently, nano-
wires cannot form near the edge of the film.
This SLVS mechanism can also be used to explain the
observations in the experiments performed by Dong et al.
9
where the Co foil is heated under a water vapor flow and
Co
3
O
4
nanowires are only found at the edge of the coil. When
the water vapor flow passes the surface of Co foil in a tube
furnace, the Co vapor generated in the central area could be
carried away by the gas flow. Some of the vapor condenses at
the edge of the Co foil, where the temperature is lower than
that in the central area due to the local high temperature gen-
erated by the oxidation of Co and CoO. Consequently, the
Co
3
O
4
nanowires are formed through the vapor-solid process
at the edge of the Co foil as shown Figure 10(b).
IV. IMPROVED EXOTHERMIC REACTION PROPERTIES
WITH AL
For the applications of Co
3
O
4
nanowalls and nanowires,
previous studies mainly focus on those in lithium ion bat-
teries,
7,8,10,11
catalysis,
1,13
and field-emission.
12,18
Here, we
demonstrate a new promising application of Co
3
O
4
nanowalls
and nanowires in energetic materials (EMs) field, where the
Co
3
O
4
nanowalls and nanowires show improved exothermic
properties when reacting with Al. EMs including explosives,
pyrotechnics, and propellants are being widely used in airbag
igniters, automobile belt tentioners, mining, de-construction,
fuses, joining, soldering, brazing, and also in many defense-
rated areas.
35
Nanoenergetic materials (nEMs) have shown
improved performance in energy release, ignition, and other
properties compared to conventional EMs. This makes nEMs
have promising applications in actuation, ignition, propulsion,
power, welding, fluidic, and electro-explosive devices at the
micro and nanoscale.
36–39
The combination of Al and Co
3
O
4
is a kind of promising EMs because the exothermic reaction
between Al and Co
3
O
4
shown in Eq. (4) has a high theoretical
heat of reaction of 4232 J/g and adiabatic reaction temperature
of 3201 K.
40
8Al þ3Co3O4!4Al2O3þ9Co:(4)
In this study, Al/Co
3
O
4
based nEMs are achieved by deposit-
ing Al on the Co
3
O
4
nanowalls and nanowires by conven-
tional evaporation as shown in Figure 11. Figure 11(a) is a
top view SEM image of the Co
3
O
4
nanowalls and nanowires
after Al deposition. Al is uniformly integrated with the nano-
walls and nanowires, and most of the nanowires fall on the
nanowalls because they get thicker and heavier after Al inte-
gration. Figure 11(b) shows the differential scanning calo-
rimetry curve of the Al/Co
3
O
4
based nEMs. A strong
exothermic reaction with the heat release of 1770 J/g is
observed with an onset temperature of around 560 C, and
the total heat of reaction of the Al/Co
3
O
4
nEMs is estimated
to be about 3100 J/g (see supplementary material
43
). The
onset temperature of the nanoscale Al/Co
3
O
4
based energetic
materials studied here is about 395 C lower than that
FIG. 11. (a) SEM image of the Al/Co
3
O
4
based nEMs and (b) DSC curve of the Al/
Co
3
O
4
based nEMs.
014310-7 Qiao et al. J. Appl. Phys. 112, 014310 (2012)
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115.200.72.75 On: Mon, 09 Dec 2013 07:45:49
(955 C) of the microscale Al/MoO
3
based energetic materi-
als,
41
and around 480 C lower than that (1040 C) of the
microscale Al/CuO based energetic materials.
42
The reduced
onset temperature (and also high heat release) of the
Al/Co
3
O
4
-nanowalls-nanowires based nEMs is caused by the
enhanced solid-solid diffusion process that is promoted by
the higher surface energy associated with the nanoscale
Co
3
O
4
structures (nanowalls and nanowires). The low onset
temperature (thus low reaction temperature) is very attractive
for many applications that need energetic materials with low
ignition temperatures.
Since the Al/Co
3
O
4
based nEMs are synthesized onto
silicon, it is likely to obtain patterned nEMs on top of the
microheaters and microelectronic circuitry fabricated on the
silicon substrate by standard silicon-based microfabrication
technology. This makes it possible to achieve the addressing
and controlled ignition of individual nEMs patterns in a
nEMs matrix on the substrate, which will open the door to
achieve the nEMs based functional devices that have many
promising applications in actuation, ignition, propulsion,
power, and fluidics at the micro and nanoscale.
Compared to previous investigations on Co
3
O
4
nano-
walls and nanowires, the main contributions of this work are
summarized as follows: (1) the synthesis process in static air
is very simple, only a conventional furnace (or even a hot-
plate) is needed; (2) through systematical investigations by
both SEM characterizations and DSC thermal analysis, the
optimal synthesis temperature range and oxidation duration
are determined as 380–400 C and 8–12 h, respectively,
where the Co
3
O
4
decomposition and the local high tempera-
ture generated by the Co/CoO oxidation during the heating
process are found to have great effects on the resulting nano-
structures; (3) the Co
3
O
4
nanowalls and nanowires are syn-
thesized onto silicon, a basic material for microelectronics
and microsystems. Consequently, it is straightforward to
integrate the nanowalls and nanowires with silicon-based
electronic circuits to achieve functional devices; (4) the pres-
ent back-end CMOS technology allows a maximum tempera-
ture of 400–450 C, the limit being set by the mechanical
integrity of low dielectric constant inter-metal dielectrics
and metallization layers. Therefore, the synthesis in this
work is CMOS-compatible because the optimal synthesis
temperature range is 380–400 C; (5) the adhesion between
the Co
3
O
4
nanowalls and nanowires and the silicon sub-
strates is very firm, which is beneficial for many applications
where the devices need to be used under librating, rubbing,
and impacting environments; (6) the detailed growth mecha-
nisms for the nanowalls and nanowires are proposed. Espe-
cially, a novel so-called SLVS mechanism is suggested for
the growth of Co
3
O
4
nanowires; (7) the promising novel exo-
thermic reaction properties of the Co
3
O
4
nanowalls and
nanowires with Al are demonstrated.
V. CONCLUSION
Through simple low-temperature thermal oxidation of
sputtered Co thin films in static air, large-area Co
3
O
4
nano-
walls and nanowires are synthesized on the silicon sub-
strates. The thermal oxidation temperature and duration have
great effects on the size, amount, and length of the nanowalls
and nanowires via the balance between the reactions of CoO
oxidation and Co
3
O
4
decomposition. The optimal synthesis
temperature range and oxidation duration are determined as
380–400 C and 8–12 h, respectively, based on the systemati-
cal studies through both DSC thermal analysis and SEM
characterizations. The adhesion between the Co
3
O
4
nano-
walls and nanowires and the silicon substrates is found to be
very strong, which is beneficial for many practical applica-
tions of the Co
3
O
4
nanowalls and nanowires. From our ex-
perimental observations, the detailed growth mechanisms of
the nanowalls and nanowires are proposed. The SLS process
is believed to be the main mechanism to control the growth
of Co
3
O
4
nanowalls. A SLVS mechanism is suggested for
the growth of Co
3
O
4
nanowires, where the local high tem-
perature generated by the large amount of heat released by
the oxidation of Co and CoO plays the key role for boiling
the liquid media into micro-vapor. Finally, the promising
exothermic reaction properties with much lower onset tem-
perature of the Co
3
O
4
nanowalls and nanowires with Al are
demonstrated.
ACKNOWLEDGMENTS
This work was supported by the Foundation of China
Academy of Engineering Physics (Grant Nos. 2010B0101013
and 2009A0302017) and the National Natural Science
Foundation of China (Grant Nos. 11002128, 11172276, and
11172275).
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See supplementary material at http://dx.doi.org/10.1063/1.4731798 for
the details of differential thermal analysis and differential scanning calo-
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