Rapid and large-scale synthesis of Co3O4 octahedron particles with very high catalytic activity, good supercapacitance and unique magnetic properties

Article abstract of DOI:10.1039/c5ra20763k

The scarcity of rapid and large scale synthesis of functional materials hinders the progress from the laboratory scale to commercial applications. In this study, we report a rapid and large scale synthesis of micron size (1.3 μm) Co₃O₄ octahedron particles enclosed by (111) facets. The octahedron particles were composed of ±25 nm rectangular/cube shaped particles as seen from the TEM images. We have characterized and evaluated the catalytic, supercapacitance and magnetic properties of the as prepared material. The Co₃O₄ octahedron particles were highly active in heterogeneous PMS activation reaction. Formation of Co-OH bonding due to water molecule dissociation on the (111) surface of the particles was evident from the ELNEFS analysis. The as prepared octahedron materials showed >4 times higher pseudocapacitance properties (182 F g^-1) with good capacity retention ability (up to the 1000 cycles studied) compared to commercial microcrystalline Co₃O₄ powder (43 F g^-1). The material showed interesting magnetic properties at low temperature. The coexistence of superparamagnetic single domain and linear/quadratic behaviours was observed at low temperature for the as prepared Co₃O₄ octahedron particles.

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Rapid and large-scale synthesis of Co3O4 octahedron particles with
very high catalytic activity, good supercapacitance and unique
magnetic property
Received 00th January 20xx,
Accepted 00th January 20xx
Mahabubur Chowdhury1*, Oghenochuko Oputu1, Mesfin Kebede2, Franscious Cummings3, Oscar
Cespedes4, Aliwa Maelsand1, Veruscha Fester1
DOI: 10.1039/x0xx00000x
www.rsc.org/
Scarcity of rapid and large scale synthesis of functional materials, hinders the progress from laboratory scale to
commercial applications. In this study, we report a rapid and large scale synthesis of Co3O4 octahedron micron size (1.3
µm) particles enclosed by (111) facets. The octahedron particles were composed of ±25 nm rectangular/cube shaped
particles as seen from the TEM images. We have characterized and evaluated the catalytic, supercapacitance and magnetic
properties of the as prepared material. The Co3O4 octahedron particles were highly active in heterogeneous PMS
activation reaction. Formation of Co-OH bonding due to water molecule dissociation on the (111) surface of the particles
were evident from the ELNEFS analysis. The as prepared octahedron materials showed >4 times higher pseudocapacitance
properties (182 F/g) with good capacity retention ability (upto a 1000 cycle was studied) compared to commercial
microcrystalline Co3O4 powder (43 F/g). The material showed interesting magnetic properties at low temperature. A
coexistence of superparamagnetic single domain and linear/quadratic behaviours was observed at low temperature for
the as prepared Co3O4 octahedron particles.
generate SR in a homogeneous system, with Co(II) reported as
the best activator for PMS3. Pristine cobalt oxide (Co3O4), and
Co3O4 supported on other metal oxide supports has been
1. Introduction
Advanced oxidation technologies (AOTs) for environmental
remediation purposes are gathering attention these days.
Wide spread interest is based on their high theoretical
conversion of toxic compounds to CO2 and H2O. Efficiency of
the AOTs relies on the generation of strong oxidising species,
e.g. hydroxyl radicals (●OH, HRs), with standard reduction
potential in the range of 1.8-2.7 V1. Recently, sulphate radicals
(SR) have demonstrated high reduction potentials of 2.5-3.1 V
at neutral pH1. Sulphate radicals are usually generated by both
studied as potential heterogeneous catalyst for PMS
activations for degradation of organic pollutants4-11. However,
absence of large scale synthesis of highly active Co3O4 catalysts
restricts the application of Co3O4 in AOT applications.
The discovery of supercapacitance/supercapacitors in 1950s
have sparked off a growing research interest in their design
and testing. This is because supercapacitors are invaluable
energy storage units in domestic and military applications.
Supercapacitors are categorized into three major classes
depending on charge storage mode, i.e. electric double layer
based (EDLC), faradic pseudocapacitor and hybrid capacitor.
Pseudocapacitors rely on reversible surface redox reactions to
store charge12. Supercapacitors based on Faradic
pseudocapacitance materials are particularly important due to
their higher energy density and specific capacitance compared
to carbonaceous materials. Co3O4 is considered to be one of
the most attractive candidates among many other
pseudocapacitive materials owing to its high theoretical
specific capacitance values (up to 3560 F/g), good capacity
retention and high redox reactivity13, 14. However, slow kinetics
of ion and electron transport in electrodes and at the
electrode/electrolyte interface forces a compromise between
the power performance and reversibility in pseudocapacitive
materials13. Extensive research has been conducted to
homogeneous
and
heterogeneous
activation
of
peroxymonosulphate (PMS). Homogeneous catalysis is more
efficient compared to heterogeneous catalysis due to the fact
that every single catalytic entity can act as a unified active
site2. Transition metals such as Co(II), Ru(III), Fe(II), Ce(III),
Mn(II), Fe(III), Ni(II) etc. have been used to activate PMS to
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synthesize materials with interesting architecture to improve supercapacitance (~4 times higher than commercial Co3O4
redox kinetics in pseudocapacitive materials. Recently Co3O4 nanoparticles) and capacity retention ability and unique
material with; hollow sphere (346 F/g)15, nanowire array (754 magnetic properties.
F/g)13, ultralayered highly porous (548 F/g)16, nanotubes (574
F/g)17, hollow boxes (278 F/g)18, nanoflake array (351 F/g)19
2. Experimental
etc. have been used as pseudocapacitive materials. Spinel
Co3O4 octahedron material enclosed by (111) facets has shown
2.1 Materials and methods for Co3O4 synthesis
excellent electrochemical properties20-23
.
Analytical grade CoCl2 and urea were purchased from Sigma
Aldrich South Africa and used without further purification. A
schematic diagram of the CHFS process used to synthesis
Co3O4 octahedron nanoparticles are presented in Fig. 1. A
description of the CHFS system is reported in our previous
publication elsewhere34. Briefly, two Isco Teledyne 260D
syringe pumps in series supplied deionised water (18 MΩ·cm)
through the two plate heater (1 kW) to the reactor. The
reactor temperature was 280°C. To prepare the precursor
solution 15g cobalt (II) chloride salt was dissolved in 250 mL of
water. In a separate beaker, 0,75g urea was dissolved in 250
mL water. The urea solution was introduced to the cobalt
solution drop wise. Precursor volume can be changed
according to the length of the run or the amount of product
needed. The precursor solution at ambient temperature was
pumped to the reactor by a Milton Roy metallic diaphragm
dosing pump where it was mixed with the heated water. This
resulted in rapid precipitation of pink, materials. The
precipitated pink powders were collected at the exit of the
back pressure regulator. The overall system pressure was
controlled at 20 MPa. All the stainless steel 316 tubing and
fittings used was provided by Swagelok. A typical sample of 1
litre nanoparticle laden slurry was collected after each run.
The slurry was centrifuged, washed and dried in an oven at
60°C and subsequently calcined at 300°C. After calcining the
pink powder transformed into black powder. This black
powder was used only for characterization and evaluation of
supercapacitance, magnetic and catalytic properties. Addition
of urea was essential for the precipitation of the materials. In
the absence of urea no precipitation occurred. The role of urea
was to release OH- ions at elevated temperature and pressure
in the reactor, which reacts with Co2+ to precipitate Co(OH)2.
Decomposition of urea affords NH3 and CO2 followed by the
release of OH- 35. For a typical run Co(OH)2 at a rate of 18gh-1
on average could be achieved in this study.
Antiferromagnetic (AFM) materials are very interesting
materials to study various quantum phenomena. Cobalt oxide,
Co3O4 has been reported to be an ideal candidate for studying
quantum phenomenon24. Recently Co3O4 has been used in
recording technology and medical science because of their
interesting magnetic properties25, 26. Many recent reports have
also discussed the application of Co3O4 nanomaterials as
electrochemical and gas sensors27-30
.
Various synthesis routes have been reported to produce Co3O4
nanoparticles of different shape and size to enhance catalytic,
supercapacitance, and magnetic properties of the materials.
Methods such as; hydrothermal, sol-gel, thermal
decomposition, solvothermal decomposition have been
reported31, 32. Process that favours shape control; i.e. hydro
and solvothermal routes and liquid phase precipitation are
33
considered the most powerful synthesis strategies
.
However, longer synthesis time and smaller product yield are
the two main factors that increase the manufacturing cost of
nanoparticles, which in turn restricts Co3O4 application as a
heterogeneous catalyst, supercapacitance and /or magnetic
material. Continuous hydrothermal flow synthesis (CHFS)
allows rapid and large scale synthesis of nanoparticles.
Synthesis of cubic shaped Co3O4 nanoparticles in CHFS system
have been reported before32. However, report on the
synthesis of octahedron Co3O4 particles enclosed by (111)
facets by CHFS technique is absent in literature to the very
best of our knowledge. Large scale synthesis of octahedron
particles enclosed by (111) facets is vital owing to their
excellent catalytic and electrochemical activity. Moreover,
report on the supercapacitance and catalytic properties of
Co3O4 particles of any morphology synthesized by CHFS
technique are scarce despite the dependence of nanomaterial
properties on factors, such as size, shape and preparative
route. Hence, evaluation of physical and chemical properties
of Co3O4 octahedron particles synthesized by CHFS technique
is important as it can allow the commercial scale production of
Co3O4 materials and spark practical applications, e.g.
heterogeneous catalysis; pseudocapacitive, magnetic material
etc.
In this contribution, we synthesized Co3O4 octahedron
particles enclosed by (111) facets in gram scale and evaluated
its catalytic, supercapacitance as well as magnetic properties.
CHFS technique was used to precipitate cobalt hydroxide
octahedron material and subsequently annealed to transform
into Co3O4 octahedron particles. The as prepared material
exhibited very high PMS activation ability, good
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Fig 1: Schematic of the CHFS system for the synthesis of Co3O4 material
2.2 Electrode fabrications and supercapacitance testing
2.3 Magnetic properties measurement
Electrochemical measurements were performed in a three- Magnetic properties were measured using two vibrating
electrode cell configuration utilizing Co3O4 nanoparticles with sample magnetometers: a MagLab from Oxford Instruments
carbon black and polyvinylidene difluoride (PVdF) coated on and an MPMS3 from Quantum Design. The first one operates
nickel foam current collector as the working electrode. Prior to using conventional electric pick-up coils, it has a sensitivity of
use, the nickel foam was cleaned by heat treatment under 10 ꢀemu and was used to measure hysteresis loops up to 9 T
Ar/H2 (95:5 ratio) environment at a temperature of 1000°C for from 2.5 to 100 K. The second uses a superconducting
about 30 minutes. A beaker-type three-electrode cell was quantum interference device (SQUID) to detect the magnetic
fabricated utilizing Co3O4 nanoparticles with carbon black and signal, and it was also used to measure hysteresis loops up to 5
polyvinylidene difluoride (PVdF) as the working electrode. T, AC/DC susceptibility and magnetisation at temperatures
Co3O4 nanoparticles (60 wt %), carbon black (30 wt %), and from 2 to 350 K with a sensitivity better than 10-8 emu. Both
PVdF binder (10 wt %) were ground thoroughly and dispersed instruments showed the same results, including a non-linear
in N-methyl-2-pyrrolidinone (NMP) solvent to form a slurry. susceptibility term at low temperatures as described in the
Then the slurry was spread onto a piece of nickel foam and later section.
maintained at 120 °C in a vacuum oven overnight. Platinum
mesh and a Ag/AgCl were used as the counter electrode and
2.4 Evaluation of catalytic activity
the reference electrode, respectively. The electrochemical
Catalytic activity of the octahedral Co3O4 particles was
properties were evaluated in 2 M KOH electrolyte at room
conducted in a 200 mL glass vessel. Methyl orange (MO) was
temperature by the cyclic voltammetry (CV) method. CV
used as an organic probe to evaluate the catalytic activity of
measurements were conducted over the voltage range from -
the material. In a typical reaction 0,03 g/L of the octahedron
0.25 to 0.55 V at various scan rates (5, 10, and 20 mV s-1).
Co3O4 particles were dispersed in 40 mg/L of MO. The solution
Galvanostatic charge−discharge cycling tests were performed
in the same voltage range, with a current density of 0.5 A g-1
was stirred for 30 min to reach adsorption equilibrium.
Commercial
oxone
can
be
used
to
generate
for 1000 cycles.
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peroxymonosulfate, PMS. Small amount of 0,9 mM of oxone Co3O4 octahedron particles are enclosed with (111) faces as
was added to the solution. Sample was withdrawn at regular shown in Fig. 2c. The colour of the calcined material changed
interval using a membrane syringe filter. Adsorption of dye on from pink (Co(OH)2) to black (Co3O4) indicating oxidation of
the membrane filter was negligible as was measured via Co(II) to Co(III) in the presence of atmospheric oxygen. TEM
control experiments. The MO concentration was measured image of a single Co3O4 particle bounded by face normal
using UV/Vis technique at a wavelength of 464 nm. The projection of an octahedron (the rectangular red lines) is
solution pH was not controlled to minimise the number of shown in Fig. 2d. Average size of the rectangular particles
processing parameters involved in the catalytic reaction. which acted as a building block (supporting document, Fig. S-1)
Intermediate products during catalytic degradation of MO for Co3O4 particles was measured to be ±25 nm. The d-spacing
were identified using Liquid chromatography mass (Fig. 2e) was measured and found to be 0.468 nm
spectroscopy (LCMS). An Agilent 6530 LCMS-TOF (Time of corresponding to the (111) plane of Co3O4 material. The d-
flight) running on mass hunter software, using a Symmetry C8 spacing of hexagonal (001) lattice plane of (Co(OH)2) is also
150 mm column preceded by a 2mm guard column was used 0.46 nm (JCPDS card No: 30-0443). This closed orientation
for intermediate identification. Elution was based on the relationship between Co(OH)2 and Co3O4 has been determined
gradient; 2mM ammonium acetate (A): methanol (B); A 85 % previously as (the hexagonal (001) lattice plane parallel to the
(t=0mins): A 0 % (t=30 minutes) at a flow of 0.3 mL/min. cubic (111) plane) the direct result of phase transformation of
36,
Ionization was by electron spray (ESI source temperature 350 CO(OH)2 to Co3O4 37. Selected area electron diffraction
°C) and ions were acquired in negative mode. Filtered samples (SAED) pattern of the cubic/rectangular shaped Co3O4 sample
(GHP Acrodisc filters) were analyzed without prior treatment.
is shown in Fig. 2f. The SAED spot pattern represents a
rectangular dot pattern which compliments the TEM results
and is also indicative of the monocrystalline character of the
material, with a Co3O4 d111= 0.469 nm. X-ray diffraction pattern
of the calcined material is presented in Fig. 2g. XRD pattern of
the calcined material matches the diffraction pattern of pure
spinel Co3O4 structure (JCPDS card no: 42-1467). No impurity
peaks or Co(OH)2 peaks were observed in the diffraction
pattern. This indicates complete transformation of Co(OH)2 to
Co3O4. The growth rate ratio, R, along the (001) and (111)
directions are responsible for controlling the shape of a face
centred crystal38. Xiao and co-authors reported that enhanced
growth along the (001) direction due to the presence of high
NaOH was responsible for Co3O4 octahedron particle
formation21. Hence, in our case the release of OH- from urea
successfully enhanced the growth rate along the (001)
direction for the formation of octahedron particles. The BET
surface area of the octahedron particles was measured to be
12 m2/g.
2.5 Characterisations
X-ray diffraction patterns of the synthesized products were
determined using a Phillips PW 3830/40 Generator with Cu-Kα
radiation. The surface morphology and the electron energy
loss spectroscopy (EELS) of the sample was studied using a FEI
Tecnai F20 field emission gun TEM operated at 200kV,
equipped with a Gatan GIF-2001 energy filter. EELS was
utilized to study the electron energy loss near edge fine
structure (ELNEFS) signals of Co and Oxygen present within the
specimen. Each spectrum was collected in normal parallel
beam, bright-field TEM mode for 10 seconds. Infrared spectra
were collected with
a Perkin Elmer 1000 series FTIR
spectrometer in the range of 4000-200 cm-1 in a KBr matrix.
FTIR spectra were recorded using a Perkin Elmer spectrum
1000. Photoluminescence was measured using a Perkin Elmer
LS55 luminescence spectrometer. Electrochemical properties
were measured using a Biologic VMP3 analyser. Micromeritics
Gemini VII 2390 surface area analyser was used to measure
the BET surface area.
3. Results and discussion
3.1 Characterisation of the Co3O4 particles
Fig. 2a presents the SEM micrograph of the uncalcined
material. It is clearly visible from the SEM image that the
uncalcined material possesses octahedron morphology. Fig. 2b
presents the SEM image of the calcined (300°C) sample. It can
be seen from Fig. 2b that the morphology of the calcined,
Co3O4, and uncalcined sample are similar (inset Fig. 2b), i.e.
octahedron. The size of the calcined materials was found to be
±1.3 µm, similar size of a bacterium. All eight faces of the
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Fig. 2: a & b) SEM micrograph of the uncalcined and calcined sample respectively, c) schematic of the octahedral Co3O4 particles, d) TEM image of a single Co3O4 particle, e) lattice
fringe of the rectangular particle, f) SAED of the Co3O4 particles and g) XRD of the Co3O4 octahedron particles
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The ratio between Co2+ and Co3+ on the predominantly
exposed planes of Co3O4 plays a role on its catalytic property39.
For catalytic oxidation reactions Co3+ is the more active species
than the Co2+ 40. The Co valence states of the as synthesized
octahedral particles were determined by energy loss near edge
fine structures (ELNEFS) analysis. ELNEFS of the cobalt L2,3
-
edge at 779 eV is sensitive to the chemical environment of the
respective elements, thus offers a valuable means for selective
identification of Co valence states. From Fig. 3 (a) an average
L3/L2 ratio of 2.7 was calculated for the synthesized Co3O4
octahedron particles. The L3/L2 ratio of 2.7 for the Co3O4
particles is indicative of a 0.33Co2+ + 0.67Co3+ configuration
present within the structure41. To further investigate the L2,3
line-shapes of the Co3O4 samples, the L3 peak of the sample
was deconvoluted and is presented in Fig. 3b. The line-shape
was filtered with
a Savitzky-Golay algorithm and the
background was subtracted using a simple least squares model
to get accurate quantitative results from the deconvolution
procedure. It can be observed from Fig. 3b that the L3 line-
shape comprises of peaks of Co2+ and Co3+ also of the surface
hydroxyl, Co-OH bond. The hydroxylated Co-OH peaks forms
because hydroxyl groups are ubiquitous in Co3O4 because of
air42, 43 and chemisorbed water. From the deconvoluted data
of Fig. 3b, a ratio 1:2.03 for Co2+:Co3+ was obtained from the
integrated areas under peaks. This is in good agreement with
the L3/L2 line-shape ratios of Co3O4 material.
Fig. 3: a) ELNEFS spectra comparing the cobalt - L2,3 edges of the Co3O4 octahedral
particles & b) deconvolution of the L3 line-shape of the Co3O4 octahedral particles
3.2 Evaluation of catalytic properties
Catalytic activity of the Co3O4 octahedron particles are
presented in Fig. 4. Control experiments were conducted to
distinguish between catalytic activity, adsorption and PMS self-
oxidation. It can be seen from Fig. 4a that the degradation of
MO due to adsorption is negligible. Only 2% MO was removed
due to adsorption process. After 33 minute of PMS self-
oxidation, 81% of MO was degraded, implying that absence of
catalyst was not effective to produce enough reactive species
for significant MO degradation. In the presence of the
synthesized Co3O4 octahedron particles and 0.9 mM oxone,
100% of MO was degraded just after 2 minutes of reaction.
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This fast degradation of MO highlights the very good catalytic 0.5 – 2.3 mM (Fig. 4d). It can be seen from Fig. 4d that the
PMS activation property of the synthesized Co3O4 octahedron reaction rate constant increases with increasing PMS
particles. Catalyst reproducibility and catalytic activity concentration from 0.5 to 1.14 mM. A catalyst should be active
repeatability is an issue in catalyst science. In this study the with minimum external input. Hence 0.9 mM oxone
Co3O4 octahedron materials were produced five times in concentration was used for all experiments in this study. The
different batches and tested for their catalytic activity. It can reaction rate constant decreases with further increase of PMS
be seen from Fig. 4a that the catalysts show excellent concentration to 2.3 mM. It is possible that a higher oxone
reproducibility and catalytic activity repeatability (data for concentration has a radical quenching effect. Table 1 presents
three repeated experiments are presented in Fig 4a). The MO a comparison of catalytic activity between the as prepared
degradation followed a first order reaction kinetics (R2= 0.93). material and the literature. The as prepared pristine material
The MO degradation rate constant for catalysis is significantly exhibits a very high reaction rate constant for 0.12 mM MO
higher than the PMS self-oxidation (Fig. 4b). From the degradation at very low [PMS] and catalyst load compared to
environmental point of view, conversion of any organic the literature.
pollutant, as MO, is only the first step in the ultimate
objective, i.e. to attain the mineralization of the corresponding
solutions. It can be seen from Fig. 4c that 68% of COD was
removed after 60 minutes of catalytic reaction compared to
32% of PMS self-oxidation. This highlights the potential of the
as prepared material in practical waste water treatment
application. The role of PMS concentration on the Co3O4
catalytic activity was evaluated in the concentration range of
Fig. 4: a) MO degradation via various route, b) MO degradation kinetics, c) COD removal via catalysis and PMS self-oxidation and d) role of [PMS] on reaction rate constants
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Table 1: Comparison of reaction rate constant between literature and this study
Ref
Catalyst
Organic compound
PMS
Catalyst
Reaction rate constant
(K, min-1)
[mM]
load (g/L)
5
Co3O4
Acid orange 7 (0.2 mM)
Methyl blue (0.05 mM)
Acid orange 7 (0.6 mM)
2
0.5
6
0.5
0.19
1.08
0.97
11
43
Co3O4/MgO
Co3O4/GO
0.05
0.05
44
45
46
47
48
49
Co3O4
CoCl2.6H2O
Co3O4/GO
Phenol (0.2 mM)
6.5
6
0.2
0.07
0.06
0.5
0.039
0.07
0.10
0.29
0.01
0.4
2,4-dichlorophenol (0.307 mM)
Phenol (0.2 mM)
6.5
0.5
0.92
2.67
Co3O4/Titanate
Co3O4/TiO2
Methyl orange (0.03 mM)
2,4-dichlorophenol (0.307 mM)
2,4-dichlorophenol (0.12 mM)
0,1
Commercial
Co3O4
0.01
Co3O4
Methyl orange (0.12 mM)
0.9
0.15
3.45
This
Study
Note: In all cases, it was reported that the composite material showed higher catalytic activity and lower Co leaching
compared to the pristine material.
Generation of sulphate radicals (SR) by catalytic decomposition luminescent 7-hydroxycoumarin (7-HC). The concentration of
of PMS in homogeneous and/or heterogeneous system in the 7-HC is proportional to the OH• concentration. Though this
presence of Co3O4 was considered to be the primary reactive method has been used previously to measure OH• in
species for oxidation of organic pollutant. Wang and co- photocatalysis50, 51 and catalytic ozonation52, it has never been
worker44 used electron paramagnetic resonance (EPR) used in catalytic decomposition of PMS where SR and OH• are
technique to show that the generation of OH• also takes place present simultaneously. It can be seen from Fig. 5a that in the
in Co3O4 mediated catalytic activation of PMS. However, absence of Co3O4 no 7-HC was detected highlighting that PMS
measurement of OH• using EPR technique is costly. In this self-oxidation doesn’t generate OH•. In the case of Co3O4
study, we used a fluorescence based technique to confirm and catalysed PMS activation reaction significant amount of 7-HC
show that photoluminescence (PL) based technique can also was detected. Hence, the Co3O4 octahedral particle catalysed
be used to verify the generation of OH• in catalytic PMS PMS activation reaction not only produce SR but also OH•.
activation. Coumarin (COU) was used (10 mM) as
a
Moreover, COU based fluorescence method can also be used
fluorescence probe. COU reacts with OH• to convert into highly to measure OH• in catalytic PMS activation reaction. Equation
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showing formation of OH• from Co3O4 catalysed PMS particles are enclosed by (111) facets. Zasada and co-workers
activation reaction is given below44:
showed previously that water molecules dissociates when
adsorbed on Co3O4 (111) surfaces and forms Co-OH groups,
where as molecular adsorption is favoured on <110>
surfaces53. Evidence of Co-OH bonding was found from the
ELNEFS analysis of the octahedron particles as it was discussed
in earlier section (Fig. 3b). Formation of surface hydroxyl
groups or Co-OH bonds are critical for the catalytic activation
of PMS, as the radical generation is dependent on the CoOH+
formation as shown in the equations (2-4)43.
−
2−
Co(II) + HSO5 → Co(III) + SO4 +• OH
(1)
Ethanol (EtOH) and tert-butanol (TBA) was added as a SR and
OH• scavenger respectively. It can be seen from Fig. 5b that
the addition of TBA did not decrease the reaction rate
constant significantly. However, addition of EtOH decreased
the rate constant considerably due to the quenching of
generated SR. So, in Co3O4 catalysed PMS activation reaction,
SR is the dominant reactive species produced. Leaching of Co
from unsupported pristine Co3O4 is a common phenomenon in
Co3O4 catalysed PMS activation reaction. Any leached Co from
the solution can be extracted using classical resin column or
ion exchange process before discharge if required. The amount
of leached cobalt in the solution was found to be 3.8 ppm (Fig.
5c) in this study. Homogeneous activation of PMS for MO
degradation was studied, using concentration higher (8 mg/L
was studied compared to 3.8 mg/L leached in solution) than
the leached Co amount to evaluate the effect of homogeneous
and heterogeneous catalysis. It can be seen from Fig. 5d that
the heterogeneous catalysis is more effective than the
homogeneous catalysis. This highlights the excellent catalytic
properties of the as prepared octahedron material. From the
TEM analysis it was shown earlier that the octahedron
Co2+ + H2O ↔ CoOH+ + H+
(2)
−
−•
CoOH+ + HSO5 → CoO+ + SO4 + H2O
(3)
CoO+ + 2H+ → Co3+ + H2O
(4)
The effect of reaction temperature was investigated to
calculate the activation energy. Reactions were carried at 20,
35 and 40 degree C (data not shown). Using classical Arrhenius
equation the activation energy was found to be 49 kJ/mol.
Fig. 5: a) Generation of 7-HC as a reaction between COU and OH•, b) effect of quenching reagents on the MO degradation reaction rate constants, c) Co leaching during PMS
activation reaction and d) comparison between homogeneous and heterogeneous PMS activation
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Intermediate compounds were sought based on their the fact that the PMS self oxidation and catalytic PMS
molecular features and masses were assigned formula based activation might follow a separate reaction path way. The
on algorithm restriction C ≤ 20, H ≤ 20, N ≤ 5, O ≤ 10 and S ≤ 2. cleavage compounds III and IV may well possibly be why the
Allowances were therefore given to accommodate cobaltous route presented better degradation rates compared
intermediates formed from reaction of MO with SO4•-, OH• and to the PMS route. Further time based intermediates observed
•
CH3 . Fig. 6 represents a time based degradation scheme for during PMS and Co3O4/PMS degradation are summarized in
MO as followed by LCMS using PMS alone and the Co3O4 supporting document (Table S1). Compound III and IV were
catalysed breakdown of PMS respectively. At t = 0.5 minutes, not observed after 5 minutes of reaction for both reaction
intermediate identified in both systems (PMS self-oxidation systems. The remaining signals (Table S1) M+H=282, M+H=332
and Co3O4 /PMS system) had an M+H = 321 (C14H15N3O4S) and and M+H 382 could not be simply assigned to any
is depicted by structure (II) (structure I and II is given in Fig 6). simultaneous introduction and/or detachment of any OH, CH2,
This is based on the compelling argument presented by CH3, SO2, SO3 groups and may probably be compounds formed
54
Baiocchi and co-workers
. This suggests that mono- from rearrangement reactions or reaction of specific transient
hydroxylation of MO was the starting mechanism leading to its intermediates. Compound M+H= 332 (m/z=321) has also been
subsequent breakdown. It also supports our experimental previously identified, however, no probable structure for the
results from PL study that activation of PMS also produce OH• compound was provided56. The absence of assigned formula
(Fig. 5a). This compound has also been confirmed as catalytic for several compounds in Table S1 highlights the point that a
breakdown product of MO degradation in earlier research54-56
.
search based on the molecular feature of the parent
Chen et. al.,56 had reported that the mono-hydroxylation compound is inadequate in identifying products of this type of
could occur on any of five positions on the MO molecule . reaction; a limitation based on the understanding of the
However, only one compound with M+H=321 was observed in reaction process itself and would require further in depth
this study. After 2 minutes of oxidation, there is a marked study of the reaction mechanism. Chromatogram at different
difference in (supporting document (Table S1)), the time interval is given in Fig S2.
intermediates identified by LCMS for both reaction systems.
Two new intermediates were identified: compound III In addition, to the obvious environmental benefits, good
(M+H=158) and compound IV (M+H= 291) (Fig. 6). Compound heterogeneous PMS activation ability of the Co3O4 octahedron
III is the product of cleavage of the azo link to the particles discussed here synthesized using a reproducible rapid
benzenesulfonate group, while compound IV is the product of and large scale synthesis technique can be considered as an
cleavage of one of the methyl groups on MO. Compound III advancement of the catalysis field of research.
and IV were not detected in all LCMS runs for the PMS self-
oxidation system and have been identified as catalytic
cleavage products of MO in earlier studies 54-56. This highlights
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Fig. 6: Time based degradation products observed within the first 2 minutes of reaction
Vc
1
3.3 Supercapacitance properties of the material
(7)
Cs =
IVdV
∫
)
Va
vw
(
va − vc
Cyclic voltammograms recorded for the Co3O4 nanoparticles
supercapacitance electrode at a potential sweep rates of 5, 10
and 20 mV s-1are presented in Fig. 7a. An increase in scan rate
results in increased peak current was observed in CV profile.
All the CV profile exhibits asymmetry of the CV scans which
essentially indicate Faradic reactions and a consequence of
ohmic resistance as a result of electrolyte diffusion within the
electrode, which are the typical characteristics of Faradaic
pseudocapacitance behaviour. The anodic peak at 0.37 V and
cathodic peak at 0.267 V in the CV curves are due to the
oxidation and reduction processes, respectively. The redox
reactions which are responsible for pseudocapacitance of
Co3O4 in the alkaline solutions are given by equations 5 and 6.
v
w
is the mass of the Co3O4
Where,
and ( va
is sweep rate (Vs-1),
- vc ) is the potential window. The maximum specific
capacitance of 182 F g-1 was obtained at a 5 mV s-1 scan rate in
2M KOH solution. Our result is higher as compared with
reported commercial Co3O4 powders giving
a maximum
specific capacitance value of 43 Fg-1 as reported by Wang and
co-workers31. The applicability of the supercapacitor can also
be evaluated by means of the galvanostatic charge−discharge
studies. Typical charge-discharge curves are presented in Fig
S3 (supporting document). The supercapacitive performance
properties of Co3O4 nanoparticles sample were evidenced by
the galvanostatic charge–discharge experiments, as shown in
Fig. 7b. We have examined the charge-discharge cycles of the
sample at 0.5 Ag-1 current density for 1000 cycles with
absolute operating voltage windows in agreement with the CV
data. It is noted that the specific capacitance of the Co3O4
nanoparticles decreases in the first 50 cycles and then starts to
increase. In the first 50 cycles the capacity of the sample
decreases by 50%, then the capacity of the sample starts to
increase. The sample retains higher capacity after 1000 cycles
CoOOH+ OH− ↔ CoO2 + H2O + e− (5)
Co3O4 + H2O + OH− ↔ 3CoOOH + e− (6)
The specific capacitance (Cs) of Co3O4 was calculated using the
CV curves shown in Fig. 7, using the equation below:
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(128 mA h g-1) than its 2nd cycle capacity (123 mA h g-1). This particles outweighs the loss in specific capacitance when
highlights the long term stability and potential application of compared to other reported Co3O4 material in literature. The
the Co3O4 materials as supercapacitor material. A possible reported synthesis technique will allow the commercial scale
reason for the decrease in capacity during the first 50 cycles production of Co3O4 material with good specific capacitance
may be the incomplete activation of all the reversible redox and capacity retention ability for supercapacitor application,
couples (Co2+/Co3+) prior to the second cycle after which their which is absent in the other reported method in literature. It is
activation yields and increased capacitance. Pseudocapacitors worthy to mention that the reported Co3O4 material shows ~4
rely on their reversible surface redox reactions to store charge. times higher specific capacitance than the commercially
The sample starts to deliver a continuously increasing capacity available Co3O4 powder (43 Fg-1)31.
upon repetitive cycling for remaining 1000 cycles as more
redox couples were being activated. A comparison of the
different Co3O4 synthesis technique used and subsequent
supercapacitance properties reported between the literature
and this study is presented in Table 2. The advantage of rapid
and large scale synthesis of the as prepared Co3O4 octahedron
Fig. 7: a) Cyclic voltammograms (CVs) of as-synthesized Co3O4 particle electrode films & b) discharge capacity of as-synthesized Co3O4 particle electrode films at a current density of
0.5 A g-1 up to 1000 cycle. [Potentials are presented as V VS Ag/AgCl]
12 | J. Name., 2012, 00, 1-3
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Table 2: Comparison of synthesis technique used and reported specific capacitance of Co3O4 between the literature and this study
Ref
Morphology
Synthesis time
(h)
Technique
Quantity
produced
Calcination
specific
capacitance
Fg-1
Reported
stability
(Cycle)
temperature °C
13
15
Nanowire array
9h
Hydrothermal (120
°C)
Small
Small
350 (2h)
n/a
754
346
4000
50
Hollow sphere on
reduced graphene
oxide
24h
Hydrothermal
(180°C)
16
Ultralayered
>24h
Hydrothermal (120
°C)
Small
300 (not
548
4000
reported)
17
18
19
31
Nanotube
Hollow boxes
Nanoflake array
Nanorods
5h
Days
5h
Templated growth
Soft chemistry
Small
Small
Small
Small
500 (4 h)
300 (10 min)
250 (2h)
574
278
289
281
1000
500
Electrpdeposition
4000
6
Hydrothermal (105
°C)
300 (for 3h)
Not
reported
37
57
Porous
1
Soft chemistry
Small
Small
200 (for 1h)
185
192
181
>1000
1200
1000
hierarchical
morphologies
Hollow octahedra
>4h
Soft chemistry +
hydrothermal (200
°C)
n/a
Octahedra
Instantaneous
Continuous
hydrothermal flow
(280°C)
Large scale
production
300 (3h)
This
study
3.4 Evaluation of magnetic properties
10-15 K (Fig. 8b). If the linear and quadratic components of the
susceptibility are subtracted, the particles show a typical
superparamagnetic behaviour, with a sigma-shaped loop at
300 K and a ferromagnetic characteristic at low temperatures
(see Fig.s 8c-d). The ferromagnetic loop shows a different
initial susceptibility, which could be attributed to a training
effect seen in coexistent antiferromagnetic-ferromagnetic
hybrids. The coexistent superparamagnetic single domain and
linear/quadratic behaviours could correspond to particles of
different size/composition, or to different magnetic properties
in the core and the surface of the nanoparticle. It was shown
from the TEM images that the octahedral particles are
composed of nano size rectangular/ cube shaped particles.
Given the magnetic moment at 2 K after polynomial correction
The magnetisation versus temperature (M vs. T) of our
samples is very similar to that reported previously for particles
synthesized by a hydrothermal method31, with a magnetisation
peak at 35 K (Fig. 8a). At high temperatures, the samples show
-1
a large linear susceptibility and a (T+
magnetisation with
θ
)
dependence of the
θ~110 K. However, interestingly below 35
K, in addition to the linear susceptibility there is a remarkable
quadratic dependence of the magnetisation with the applied
field,
M α
H2. This has previously been observed in
antiferromagnetic nanoparticles below the Néel temperature58
and has been associated to multiferroic systems with weak
magnetisation59, 60. However, quadratic dependence of the
magnetisation with the applied field is scarcely reported for
pristine Co3O4 material. The quadratic component is anti-
correlated to the linear susceptibility and it has a maximum at
(150
be roughly estimated at just ~0.1% of the total.
ꢀ
emu in a 3 mm3 sample), the single domain phase can
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b)
a)
4.5
4.0
3.5
3.0
2.5
2.0
1.5
40
M ∝ H
M ∝ H
25
24
23
22
21
20
100 Oe
ZFC
2
FC
30
20
10
0
0
50
100
150
200
250
300
0
20
40
60
80
100
Temperature (K)
Temperature (K)
c) 1.0
d)
12
8
2
1
0
40
20
0
300 K
0.5
0.0
2 K
4
0
-4
-8
-12
-1
-2
-20
-0.5
-1.0
-40
-60
-40
-20
0
20
40
60
-60
-40
-20
0
20
40
60
H (kOe)
H (kOe)
Fig. 8: a. Zero field cooled – field cooled (ZFC-FC) magnetisation measured at 100 Oe. The dashed red line is a high temperature fit to a (T+θ)-1 dependence with θ ≈ 110 K. b.
Variation with temperature of the linear and quadratic dependence of the susceptibility with the applied field as calculated from hysteresis loops measured up to 9 T. The unusual
quadratic component emerges below the Néel temperature (~35 K), it is anti-correlated to the linear susceptibility and has a maximum at ~10-15 K, the temperature for the
magnetisation minimum. c. Magnetisation at 300 K. The blue dots show the remanent magnetisation after the linear component has been subtracted. d. Magnetisation at 2 K. The
blue dots show the magnetisation after both the linear and quadratic components have been subtracted – the red line is the initial magnetisation in the virgin state.
octahedron particles exhibited very high catalytic PMS
Conclusions
activation properties. Dissociation of adsorbed water
molecules on (111) surfaces resulting in Co-OH bonding was
evident from the ELNEFS analysis. The Co–OH complexes,
which efficiently activate PMS to oxidise MO, were formed on
In this study, we demonstrated a scalable large scale synthesis
of octahedral Co3O4 microparticles (1.3 µm) enclosed by (111)
facets using continuous hydrothermal flow process. The
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the surface via interaction of Co2+ with hydroxyl groups. This
22. X. Wang, L. Yu, X.-L. Wu, F. Yuan, Y.-G. Guo, Y. Ma and J. Yao,
J. Phys. Chem. C., 2009, 113, 15553-15558.
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G. Fernig and N. T. K. Thanh, Contrast Media Mol. Imaging, 2008,
3, 150-156.
process facilitates the heterogeneous activation of PMS.
Presence of both hydroxyl and sulphate radicals were found
during PMS activation reaction. The prepared material showed
~4 times higher pseudocapacitance properties when compared
to commercial Co3O4 microcrystalline material. The as
prepared material showed very interesting magnetic
properties at low temperature. The material exhibited a
coexistence of superparamagnetic single domain and
linear/quadratic behaviours at low temperature.
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Acknowledgements
This study was supported by the National Research Foundation
of South Africa (Grant no. 88220; UID: 78697).
29. T. Chen, X. Li, C. Qiu, W. Zhu, H. Ma, S. Chen and O. Meng,
Biosens. Bioelectron., 2014, 53, 200-206.
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297x209mm (150 x 150 DPI)