Synthesis of cellular-like Co3O4 nanocrystals with controlled structural, electronic and catalytic properties

Article abstract of DOI:10.1016/j.jallcom.2012.10.164

This work reports on the preparation of cellular-like Co₃O ₄ nanocrystals with tailored physical properties via a seeded method by high temperature disposal. By increasing the disposal temperature, Co(OH)₂ seeds can be efficiently transformed to Co₃O ₄ nanocrystals. Systematic sample characterization using combined techniques of X-ray diffraction, scan electron microscope, Fourier transformed infrared spectra, UV-visible diffuse reflectance spectra, Barrett-Emmett-Teller technique and XPS spectra indicates that Co₃O₄ nanocrystals showed a monotonically lattice expansion with particle size reduction, which can be well-defined as a function of particle sizes as well as valence state of cobalt ions. As the consequence of lattice expansion of Co ₃O₄ nanocrystals, the ν₁ and ν₂ modes of Co₃O₄ in infrared spectra showed systematic red shift from 580, 669 to 568, 663 cm^-1, respectively. Co₃O₄ nanocrystals prepared at 300 °C displayed highest catalytic property for HCHO conversion, which can be explained by the balance of surface absorbed OH groups, BET surface area and charge state of cobalt ions.

Full text of DOI:10.1016/j.jallcom.2012.10.164

Journal of Alloys and Compounds 553 (2013) 163–166
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Synthesis of cellular-like Co3O4 nanocrystals with controlled structural,
electronic and catalytic properties
Li Lv a,b, Yiguo Su a, Xiaoqing Liu a, Haiyang Zheng a, Xiaojing Wang a,
⇑
a College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, Inner Mongolia 010021, PR China
b Chemical Engineering College of Inner Mongolia University of Technology, Hohhot, Inner Mongolia 010021, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2012
Accepted 28 October 2012
Available online 10 November 2012
This work reports on the preparation of cellular-like Co3O4 nanocrystals with tailored physical properties
via a seeded method by high temperature disposal. By increasing the disposal temperature, Co(OH)2
seeds can be efficiently transformed to Co3O4 nanocrystals. Systematic sample characterization using
combined techniques of X-ray diffraction, scan electron microscope, Fourier transformed infrared spectra,
UV–visible diffuse reflectance spectra, Barrett–Emmett–Teller technique and XPS spectra indicates that
Co3O4 nanocrystals showed a monotonically lattice expansion with particle size reduction, which can
be well-defined as a function of particle sizes as well as valence state of cobalt ions. As the consequence
Keywords:
Nanostructured materials
Catalysis
Optical spectroscopy
X-ray diffraction
of lattice expansion of Co3O4 nanocrystals, the m1 and m2 modes of Co3O4 in infrared spectra showed sys-
tematic red shift from 580, 669 to 568, 663 cmÀ1, respectively. Co3O4 nanocrystals prepared at 300 °C dis-
played highest catalytic property for HCHO conversion, which can be explained by the balance of surface
absorbed OH groups, BET surface area and charge state of cobalt ions.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
phologies [11–13]. Among which, the cobalt based intermediated
compounds, such as cobalt carbonates and beta-cobalt hydroxide
Over the past two decades, synthesis of nanoscale materials has
received considerable attention in the materials research commu-
nity, because electrical, optical, magnetic and catalytic properties
of the matters in this spatial scale are uniquely different from those
of their corresponding bulk counterparts [1–5]. All these properties
of nanocrystals are highly dependent on the lattice dimension and
band structures. For most metal oxide nanocrystals, variations in
lattice dimension and symmetry often accompany particle size
reductions. For instance, Tsunekawa and coworkers proposed that
the lattice expansion in metal oxide nanocrystals is due to the va-
lence reduction such as from Ce4+ to the larger Ce3+ in CeO2 nano-
crystals with particle size reduction [6]. Furthermore, in certain
transition-metal oxides, it has also been reported that there is an
ionicity increase as the particle reduction, which can be explained
on the basis of covalence increase [7]. Therefore, an experimental
identification of the lattice modifications of nanoscale metal oxides
is fundamentally important for tuning the band structure and
moreover physical properties.
receives much research attention, because the inorganic salts have
long been used as solid precursors to prepare functional metal oxi-
des and advanced ceramic materials through cycles of thermal
annealing to deplete existing porosity and obtain high density
products. Zeng and coworkers have obtained porous Co3O4 from
CoCO3 submicrometer particles, demonstrating excellent perfor-
mance in gas sensor applications [14]. Lou et al. obtained Co3O4
nanotubes and mesoporous nanoneedles through appropriate
thermal treatments by using a beta-cobalt hydroxide precursor in
nanoneedle morphology [15,16].
In this work, we report on the preparation of cellular-like Co3O4
nanocrystals with tailored physical properties via a seeded method
by high temperature disposal. Systematic studies found that Co3O4
nanocrystals prepared at 300 °C displayed highest catalytic prop-
erty for HCHO conversion, which can be explained by variation of
lattice dimension and surface to volume ratio.
2. Materials and methods
Co3O4 is a very versatile oxide that is involved in many ad-
vanced physical applications and in heterogeneous catalysis, such
as NOx reduction, CO oxidation, and vapor-phase oxidation of or-
ganic molecules [8–10]. Till now, many efforts has been dedicated
to obtain Co3O4 nanocrystals with controlled dimensions and mor-
In a typical procedure, 36.01 of NaOH was dispersed into 300 ml distilled water.
47.142 g Co(NO3)2Á6H2O was dissolved into 100 ml distilled water, the solution of
3 mol LÀ1 NaOH was mixed with the solution of Co(NO3)2 under magnetically stir-
ring at room temperature for 2 h. When the reactions were complete, the products
were sufficiently washed and then dried at ambient conditions. Finally the samples
were synthesized by heating these precipitates at 200, 300, 400, 500, 600, 700 °C for
3 h, respectively. The obtained samples were coded as A–T, where T refers to the cal-
cination temperature.
⇑
Corresponding author. Tel.: +86 471 4994406; fax: +86 471 4992981.
E-mail address: wang_xiao_jing@hotmail.com (X. Wang).
0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.10.164

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L. Lv et al. / Journal of Alloys and Compounds 553 (2013) 163–166
Phase compositions of the samples were characterized by powder X-ray diffrac-
tion (XRD) at room temperature on X-ray diffractometer (D8 ADVANCE, Bruker)
with a copper target (Cu K , k = 0.15406 nm). The average grain sizes, D, were mea-
increasing the calcination temperature, the diffraction peaks be-
come sharper apparently, pronounced a particle size growth oc-
curred as is confirmed by XRD data (Fig. 1). As the temperature
rises to 700 °C, the particle size of Co3O4 nanocrystals was calculated
to be 75.6 nm, which is much larger than that of sample A-300.
Besides particle size variation, it should be noticed that the lat-
tice volume decreased monotonically with particle size reduction
of Co3O4 nanocrystals (inset of Fig. 1). Several reasons could be
responsible for the lattice variations, including particle size in-
duced lattice relaxation near the surface, lattice stress, as well as
variation of charge state. It is known that many metal oxides
showed lattice expansion with particle size reduction. Li et al. con-
sidered that the lattice variation in CaWO4 nanocrystals was prob-
ably due to the ‘‘negative pressures’’ that originated from strong
defect dipole interactions on surfaces [17]. Nevertheless, opposite
phenomena has been observed in the present work. To get insight
into the lattice volume variation, we should first investigate the
particle size effects. As documented in many cases, nanocrystals
with the diameter smaller than 30 nm often show apparent varia-
tions of lattice parameters [18,19]. However, for the present Co3O4
nanocrystals, the particle sizes of most samples were larger than
30 nm, which would pronounce little impact on the lattice volume.
Another important reason is the variation of charge state.
a
sured from the most intense XRD peak (311) using the Scherrer formula, D = 0.9k/
(bcosh), where k is the X-ray wavelength employed, h is the diffraction angle of the
most intense peak (311), and b is defined as the half width after subtracting the
instrumental broadening. The lattice parameters were obtained by structural
refinement using Retica Rietveld program. Nickel powders were used as the internal
standard for peak position calibrations. The morphologies of the samples were
investigated by a field-emission scanning electron microscopy (SEM) using an
instrument under an acceleration voltage of 110 kV. The specific surface areas of
the samples were determined from the nitrogen absorption data at liquid nitrogen
temperature by using the Barrett–Emmett–Teller (BET) technique on a Micromeri-
tics ASAP 2000 surface area and porosity analyzer. X-ray photoelectron spectra
(XPS) were recorded with a Amicus spectrometer using Mg K
at 12 kV and 10 mA. The binding energies were calibrated using C 1s peak of con-
taminant carbon (BE = 284.8 eV) as standard, and quoted with precision of
a radiation operated
a
0.5 eV. The surface composition of the samples in terms of atomic ratios was cal-
culated, and Gaussian–Lorentzian and Shirley background were applied for peak
analysis. The infrared optical properties were measured using a Fourier Transform
Infrared Spectrophotometer (FTIR-8300) at resolution of 4 cmÀ1, using a KBr pellet
technique. The activity of the catalysts for HCHO oxidation was evaluated in a con-
tinuous flow U-shaped glass rector (8 mm i.d.). The gas mixture of 3.0% HCHO bal-
anced with air was passed through 200 mg catalyst at the space velocity of
40,000 ml gÀ1 h–1. The produced CO2 was detected using FQ-B CO2 IR analysis
instrument in online.
3. Results and discussion
Identification of the composition and chemical states of the
samples is done by determining the line energies in X-ray photo-
electron spectra (XPS). Fig. 2 shows the XPS spectra of (a) Co 2p
and (b) O 1s positions of Co3O4 nanocrystals. As indicated in
Fig. 2a, two strong peaks centered on 780.9 and 795.9 eV for all
samples, which are in agreement with the binding energies of Co
2p3/2 and Co 2p1/2, respectively [20]. According to previous litera-
ture, the Co 2p3/2–Co 2p1/2 splitting of approximately 15 eV and
the shake-up satellite structure of the Co 2p3/2 at ꢀ9 eV higher
than the main peaks are associated with the Co3O4 [21,22]. The
typical satellite peak of CoO, at about 2.1 eV, higher than the posi-
tion of the main peak of Co 2p3/2, is not found in the spectra [23].
From the results of XPS, it also demonstrates that the as-prepared
samples are Co3O4. The asymmetric O 1s peak in the surface was
coherently fitted by two nearly Gaussian components, centered
at 530.5 and 532.7 eV, respectively, as shown in Fig. 2b [24]. The
first peak on the low binding energy side of the O 1s spectra can
be attributed to the O2À ions in Co3O4 host lattice that are
surrounded by Co atoms with their full complement of nearest-
neighbor O2À ions. Moreover, XPS has also been used to quantita-
tively estimate the concentration of metal or non-metal ions.
As indicated in Fig. 2a, the strong peaks located at 780.9 eV can
XRD patterns of the samples prepared at different temperature
are shown in Fig. 1. It is found that the XRD pattern of as-prepared
precursor can be readily indexed to a hexagonal structure, which
is accordance to Co(OH)2. However, as the temperature rises to
300 °C, Co(OH)2 exhibits a phase transformation from a hexagonal
phase to a cubic Co3O4 structure. As indicated in Fig. 1, the XRD pat-
terns ofsamples A-300 and A-700 show diffractionpeaks (Fig. 1) that
are essentially identical to the standard data for bulk Co3O4 (Fd3m)
(JCPDS 42-1467). No distinct diffraction peak for other cobalt oxides
was observed, indicating the formation of single-phase cubic Co3O4.
Moreover, sample A-300 showed relatively broadened diffraction
peaks, demonstrating fine nature and small crystal size of Co3O4.
The internal strains were very small and ignored here. The average
particle sizes calculated using Scherrer formula, according to the
(311) diffraction peak of Co3O4 nanocrystals. The average particle
size was about 11.8 nm for the sample of A-300. SEM image of sam-
ple A-300 is shown inset of Fig. 1. It is found that Co3O4 nanocrystals
exhibit the morphology of cellular-like spheres with particle sizes in
the range of 100–200 nm. This observation clearly illustrates
these spheres are composed with tiny Co3O4 nanocrystals. With
Fig. 1. XRD patterns of the precursor and Co3O4 nanocrystals. Inset are the variation
of lattice as a function of particle sizes and SEM image of A-300 sample.
Fig. 2. XPS spectra of typical Co 2p and O 1s for Co3O4 nanocrystals.

L. Lv et al. / Journal of Alloys and Compounds 553 (2013) 163–166
165
systematic red shift toward lower frequencies with particle size
reduction. Wang et al. reported that the two bands shift to lower
wavenumber due to a large number of defects at the surface of the
smaller nanocrystals to weaken the Co–O band strength [27]. With
a size reduction of Co3O4, the surface water content (see Fig. 3a) is
largely increased by surface stabilization and reconstruction. From
the kinetic viewpoint, the strains are generally compressive such
as
c-Fe2O3 [28], which due to the decrease of the lattice parameters.
Lattice contraction has been suggested for
c
-Fe2O3 nanocrystals
upon surface hydration because of the reduced surface dipole mo-
ments. Consequently, the force between the pairs of Co(II)–O and
Co(III)–O for the intrinsic lattice vibrations could be smaller, since
the corresponding lattice contraction in Co3O4 nanoparticle become
significant with a size reduction.(Fig. 3b). Therefore, the red shift at
about 668 and 557 cmÀ1 of Co3O4 can be predominantly ascribed to
the size-induced lattice variations.
Fig. 3. IR spectra of the as-prepared Co3O4 nanocrystals.
The catalytic activity of Co3O4 nanocrystals were studied by de-
tected the temperature of the complete conversion for the catalytic
HCHO combustion. The catalytic activities of the samples for the
combustion of formaldehyde as a function of the reaction temper-
ature are depicted in Fig. 4a. From Fig. 4a, it can be seen that sam-
ple A-700 can catalyze the oxidization of nearly 100% of methanal
to CO2 and H2O at 220 °C. However, the conversion temperature for
A-300 was just about 168 °C. Thus, it can be concluded that the
corresponding conversion temperature of Co3O4 decreases with
particle size reduction. Several reasons are responsible for this
observation, such as BET surface area, active oxygen species, and
surface OH groups. It is well-known that the catalytic reaction oc-
curred on the surface of Co3O4 nanocrystals. For HCHO oxidation,
O2 as the oxidant, O2 activation is the key step [29]. Therefore, it
is essential to know the activation process of molecular O2 on
the catalyst surfaces. Because of extensive studies on the photocat-
alytic water-splitting reaction on TiO2 surfaces, it is well known
that surface OH groups on TiO2 can facilitate adsorption and acti-
vation of molecular oxygen [30]. As shown in Fig. 4b, the chemical
absorbed water was greatly reduced by increasing temperature,
which would certainly depress the oxidation activity. As for BET
surface area, it also decreased as the temperature increased, which
can also compress the catalytic activities. Similar results have been
observed by Wang and coworkers [31]. On the other hand, it is rea-
sonable to the charge state for cobalt ions is crucial to the catalytic
performance for low temperature oxidation reactions. For example,
Lin et al. found that the catalytic activities toward to CO oxidation
were decreased significantly with the oxidation state of cobalt,
i.e. CoO(+2) P Co3O4(+8/3) P CoO(OH)(+3) P CoOx(>+3) [32]. For
Co3O4 nanocrystals, the molar ratio of Co3+ and Co2+ was highly
be divided into two components by Gaussion function fitting. The
two components located at 780 and 782 eV are the characteristic
binding energy for Co3+ and Co2+, respectively. Interestingly, the
integrated ratio of Co3+ and Co2+ was gradually reduced from
2.06 to 0.963 with increasing the temperature from 300 °C to
700 °C. For Co3O4, the molar ratio of Co3+ and Co2+ is 2. However,
for the present Co3O4 nanocrystals, the molar ratio of Co3+ and
Co2+ is greatly reduced with increasing temperature. It is well
accepted that the ionic radius for Co2+ is larger than that of Co3+
.
Thus, it can be interpreted that Co2+ incorporated in Co3O4 host
lattice and substituted Co3+, which would certainly induced the
lattice expansion with increasing temperature.
The surface properties of Co3O4 nanocrystals were investigated
by infrared spectroscopy. As shown in Fig. 3a, two broad bands
observed at 3440 cmÀ1 and 1630 cmÀ1 can be attributed to the
vibrations of O–H bonds for the water absorbed on Co3O4 surfaces.
Co3O4 has a cubic spinel structure with Co2+ and Co3+ located
tetrahedral and octahedral sites, respectively, which belongs to
Fd3m space group. According to the group theory analysis, there
are eight vibration modes in spinel [22,25]:
C = A1g(R) + Eg(R) +
F
1g(in) + 3F2g(R) + 2A2u(in) + 2Eu(in) + 4F1u(IR) + 2F2u(in), where (R),
(IR) and (in) represent Raman-active vibrations, infrared-active
vibration and inactive modes, respectively. Fig. 3b shows that a set
of bands in the range of 450–800 cmÀ1 is characteristic of the Co–
O band vibrations of the samples treated at different temperatures.
Two peaks at 668 (m (m2) can be observed in the sam-
1) and 557 cmÀ1
ple of A-300, corresponding to absorptions of Co(II)–O and Co(III)–O
stretching vibrations, respectively [26]. As indicated in Fig. 3b, it is
noted that the peaks located at about 668 and 557 cmÀ1 showed a
Fig. 4. (a) HCHO conversion efficiencies curve using as-prepared Co3O4 nanocrystals as catalysts, (b) BET surface area and TG weight loss before 600 °C of the as prepared
Co3O4 nanocrystals.

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L. Lv et al. / Journal of Alloys and Compounds 553 (2013) 163–166
reduced, which would promote the catalytic performance. There-
fore, the balance of these reasons led to the regular of this catalytic
reaction.
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Cellular-like Co3O4 nanocrystals were successfully prepared via
a seeded method by high temperature disposal for tailored physi-
cal properties. By increasing the disposal temperature, Co(OH)2
seeds can be efficiently transformed to Co3O4 nanocrystals. Co3O4
nanocrystals showed a monotonically lattice expansion with parti-
cle size reduction, which can be well-defined as a function of par-
ticle sizes as well as valence state of cobalt ions. As the
consequence of lattice expansion of Co3O4 nanocrystals, the m1
and m2 modes of Co3O4 in infrared spectra showed systematic red
shift. Co3O4 nanocrystals prepared at 300 °C displayed highest cat-
alytic property for HCHO conversion, which can be explained by
the balance of surface absorbed OH groups, BET surface area and
charge state of cobalt ions. This work reported here may give a can-
didate for further studies of low dimensional physics as well as for
applications in various fields of nanotechnology.
Acknowledgements
This work is financially supported by National Natural Science
Foundation of China (No. 20863004 and 20967004), the Chunhui
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Mongolia National Science Foundation (2010Zd08 and
2011MS0209), the Project of Scientific and Technological
Innovation Team of Inner Mongolia University (12110614), Inner
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