Supercapacitor and photocatalytic performances of hydrothermally-derived Co3O4/CoO@carbon nanocomposite

Article abstract of DOI:10.1039/c7nj04638c

Cobalt oxide (Co₃O₄/CoO) nanoparticle-embedded carbon matrix (Co₃O₄/CoO@carbon) was synthesized by pyrolysis of cobalt-salen complex ([Co(salen)]) followed by hydrothermal treatment. The X-ray diffraction, Raman and Fourier transform infra-red spectroscopies confirmed the presence of Co₃O₄/CoO in the carbon matrix. The scanning electron microscopy observation showed highly agglomerated spike-like grains. The TEM observation confirmed that the Co₃O₄/CoO grains were embedded in the carbon matrix. The supercapacitor studies conducted on the Co₃O₄/CoO@carbon matrix revealed a specific capacity of 324 C g^-1 at 1 A g^-1 in 1 M KOH. The Co₃O₄/CoO@carbon electrode also exhibited long-term life cycle with a high Coulombic efficiency of 96%. It is believed that the carbon present in Co₃O₄/CoO acted as a conductive nano-network, leading to such a high supercapacitor performance. The Co₃O₄/CoO@carbon material was also tested for its catalytic property, and it was found that the prepared material exhibited excellent photocatalytic degradation of azure A dye.

Full text of DOI:10.1039/c7nj04638c

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The role of atropisomers on the photo-reactivity and fatigue of
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PleNaeswe dJoounrnoatlaodfjCuhsetmmiastrrgyins
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DOI: 10.1039/C7NJ04638C
New Journal of Chemistry
ARTICLE
Supercapacitor and Photocatalytic Performances of
Hydrothermally-derived Co3O4/CoO @ Carbon Nanocomposite
E. Duraisamy, Himadri T. Das, A. Selva Sharma and P. Elumalai*
Received 00th January 20xx,
Accepted 00th January 20xx
Cobalt oxide (Co3O4/CoO) nanoparticles-embedded carbon matrix (Co3O4/CoO @ carbon) was synthesized by pyrolysis of
cobalt-salen complex ([Co(salen)]) followed by hydrothermal treatment. The X-ray diffraction, Raman and Fourier
transformed infra-red spectroscopies confirmed the presence of Co3O4/CoO in the carbon matrix. The scanning electron
microscope observation showed highly agglomerated spike-like grains. The TEM observation confirmed that the Co3O4/CoO
grain are embedded in carbon matrix. The supercapacitor studies done on the Co3O4/CoO @ carbon matrix revealed a
specific capacity of 324 C g -1 at 1 A g -1 in 1 M KOH. The Co3O4/CoO @ carbon electrode also exhibited long-term life cycle
with high Coulombic efficiency of 96%. It is believed that the carbon present in the Co3O4/CoO acted as conductive nano
network leading to such a high supercapacitor performance. The Co3O4/CoO @ carbon material was also tested for its
catalytic property and it was found out that the prepared material exhibited excellent photocatalytic degradation of azure
A dye.
DOI: 10.1039/x0xx00000x
www.rsc.org/
different dimensions which exhibit long-term life span but they
failed in quantum of charge storage which limited their wide
use. The transition-metal oxides, hydroxides and some redox
polymers having fast reversible Faradaic reaction, are the most
explored classes of materials for supercapacitor which can store
maximum charge.10-17
1. Introduction
Development of clean and renewable-energy technologies is
indispensable for moving towards sustainable environment, as
the use of conventional fossil fuels leads to adverse effects on
the environmental ecosystem. The effective way of utilizing
clean and renewable energies is to store them when available
and use them when require. In this regard, much attention is
being currently devoted towards development of an efficient
energy-storage system. Electrochemical energy storage devices
such as batteries and supercapacitor are the reliable devices to
store energy and retrieve the same when required. Currently,
the batteries and supercapacitors are developed with advanced
electrode materials having robust energy as well as power
densities and life cycle.1-4 Being a portable storage-system, the
supercapacitor technology is advancing to meet the energy and
power demands of consumer electronics. Generally,
supercapacitors are of two types viz., electrical double layer
capacitors (EDLCs) and pseudocapacitors. The EDLCs store
energy by reversible charge separation at the
electrode/electrolyte interface (non-Faradaic), whereas the
pseudocapacitors store charge by means of redox (Faradaic)
reaction occurring on the surface of the electrode. Mostly, low
cost and good conducting carbon-derived materials (ex. carbon
nanofibers, carbon nanotube, reduced graphene-oxide etc.)
with high surface area fall under the EDLC materials category.5-
9 These materials are generated in fascinating morphologies in
The electron- and ion-transport efficiencies which are
directly linked to charge storage ability of material, depends on
its morphology, surface area, interaction with electrolyte etc. In
this regard, current efforts are being devoted for designing and
developing hybrid electrodes consisting of metal oxides
(Faradic) and carbon nano networks (non-Faradic).18-19 The
cobalt oxide is a semiconducting transition-metal oxide, one of
the most exploited materials for various applications such as
battery anode, catalysis, sensors etc. due to less band gap, ease
of synthesis and large abundance.4,20 Besides these admirable
features, the cobalt oxide has high theoretical specific capacity
(Co3O4: 404 C g-1, CoO: 1295 C g-1), adequate chemical and
thermal stabilities.21-23 One of the strategies to enhance the
supercapacitor performance is infusing CoO to other metal
oxides or conductive nano-network to generate high-energy
density material by reducing diffusion length which will
facilitate efficient charge transport.24-29 Recently, ultra-small Sn
nanoparticle embedded in nitrogen-doped porous carbon
obtained by pyrolysis of [Sn(salen)] complex was used as anode
material for lithium-ion battery (LIB). It was reported that the
carbon network facilitated high performance of Sn anode for
the LIB.30 Poly-[Ni(salen)] derived from [Ni(salen)] complex was
employed as supercapacitor electrode.31
Electrochemical Energy and Sensors Lab, Department of Green Energy
Technology, Madanjeet School Green Energy Technologies,
Pondicherry University, Puducherry-605014, India.
By the way, among the various transition metal-oxides,
Co3O4 being a p-type semiconductor with efficient electronic
and magnetic properties is considered as the most versatile
oxide material for catalytic applications. Recently, Wang et al.,
*Corresponding
Author
E-mails:
drperumalelumalai@gmail.com;
elumalai.get@pondiuni.edu.in (P. Elumalai).
Electronic supplementary information (ESI) available: XRD, Raman and FT-IR spectra
of acid leached sample, Co3O4/CoO @ carbon and pyrolysed salen complexes.
This journal is © The Royal Society of Chemistry 20xx
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reported the synergistic effect of Co3O4 nanoparticles and electron microscope analysis of the prepared material was
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graphene as catalysts for degradation orange II dye.32 It was carried at an operating voltage of 200 kVD. OI: 10.1039/C7NJ04638C
reported that the combination of Co3O4 nanoparticles and
2.2. Electrochemical characterization
graphene sheets provided much higher catalytic activity than
that of pure Co3O4. In recent years, several efforts have been
The supercapacitor studies of the sample were done in a
adopted to improve the catalytic efficiency of the Co3O4 namely three-electrode cell consisting of the Co3O4/CoO @ carbon as
by preparing single, multi component and composite materials working electrode, platinum (Pt) as counter electrode and
etc.32-38 For example, Mohammad et al., have utilized Ag/AgCl as reference electrode. The working electrode was
nanostructured bicomponent cobalt metatitanate and cobalt fabricated using the mixture of the Co3O4/CoO @ carbon (80
oxides for effective degradation of triazo Direct Blue 71 dye wt.%), carbon black (10 wt.%) and polyvinylidene fluoride (10
under 250 W Hg light illumination.37 Orange II dye was wt.%). The carbon black was used to increase the conductivity
photocatalytically degraded to a large extent by using magnetic of the electrode material. The PVDF was used as binder in
cobalt nanoparticles anchored on graphene nanosheets. It was electrode fabrication to bind the electrode material on to the
reported that activation of graphene with peroxymonosulfate surface of nickel foam (NF) current collector (substrate). N-
had a profound effect in efficient removal of Orange II dye from methyl-2-pyrrolidone (NMP) was used as a solvent to obtain
water.35
slurry. The slurry thus obtained was pasted on the NF (1 cm x 1
In this work, Co3O4/CoO nanoparticles embedded carbon cm) and dried at 80 °C for overnight. The masses of the coated
matrix (hereafter known as, Co3O4/CoO @ carbon) was derived and uncoated electrodes were measured. The difference
by hydrothermal treatment of chemically synthesized between the mass of coated and uncoated electrode was the
cobaltsalen complex ([Co(salen)]) and its electrochemical active mass of the electrode. The active mass of the electrode
behavior is investigated with a target to achieve enhanced was 2 mg cm-2. All the electrochemical tests were performed by
supercapacitor performance. We have also evaluated the using Biologics electrochemical workstation (SP-150). The cyclic
photocatalytic ability of the Co3O4/CoO @ carbon matrix by voltammetry (CV), galvanostatic charge-discharge and
monitoring the degradation of Azure A (AZA) dye. The detailed electrochemical impedance spectroscopy (EIS) studies were
synthesis, characterization, electrochemical characteristics and carried out in a three electrode cell in 1M KOH. The EIS studies
photocatalytic property of the Co3O4/CoO @ carbon are were carried out at several charge-discharge cycles in the
reported here.
frequency range of 100 KHz-0.01 Hz.
2.3. Photocatalytic experiment
2. Experimental
The photocatalytic performance of the Co3O4/CoO @
carbon matrix was studied by examining the degradation of AZA
dye (AZA is a product of SD-Fine Chemicals, India). The dye was
purified and recrystallised as reported elsewhere.41 The AZA
solution was prepared by dissolving appropriate amount of the
AZA in double distilled water to get a final concentration of 2.80
× 10-5 mol dm-3. Then, the Co3O4/CoO @ carbon (10 mg) was
added to photocatalyst in 25 mL of aqueous AZA dye solution
(2.80 × 10-5 mol dm-3, pH=7). The solution mixture was then
stirred in dark for 1 h to achieve adsorption-desorption
equilibrium. The photocatalytic degradation study was carried
out in a Pyrex beaker under UV irradiation using a 250 W Philips
UV lamp. The wavelength of the UV lamp was 360 nm and it is
very much within the range of 250-400 nm. At specified time
intervals, 2.5 mL of solution mixture were taken out of the
beaker and filtered. The filtered solution was subjected to
absorption spectral measurements by JASCO V-630 UV-visible
spectrophotometer. Quartz cuvettes of path length 1 cm were
used to record the absorption spectra in all case. The entire
experiment was carried out in room temperature (24°C).
2.1. Material synthesis and characterization
Initially, (H2-salen) ligand was synthesized by reacting
known quantities of salicylaldehyde (Sigma-Aldrich, India) and
ethylene diamine (Sigma-Aldrich, India). Then, the [Co(salen)]
complex was prepared by reacting the H2-salen ligand and
cobalt acetate (Merck Pvt. Ltd., India) in a distilled ethanol in an
appropriate amount.30,39,40 The obtained [Co(salen)] complex
was subjected to one spot pyrolysis at 800°C under argon (Ar)
atmosphere for 5 h in a tubular furnace. Then, the pyrolyzed
sample (0.2 g) was dispersed in 40 ml of distilled water by
means of ultra-sonication. To this dispersed solution, 2 mM
urea was added and stirred for 2 h. Then, the above solution
was transferred into a Teflon-lined autoclave and kept in an air
oven at 180°C for 12 h (hydrothermal treatment). After the
treatment, the obtained sample was centrifuged, washed
several times with ethanol and dried to obtain final product.
The obtained black powder was characterized by using X-ray
diffraction (Bruker D8 advance Da vinci) employing Cu Kα
radiation source at a scan rate of 1 per minute, Raman
spectroscopy (Witec Confocal Raman instrument with Ar ion
laser 514.5 nm CRM200), Fourier transformed infra-red (Nicolet
6700) spectroscopy by means of KBr pellet method and Field
emission scanning electron microscope (FE-SEM carl Zeiss
SUPRA55VP) equipped with energy dispersive X-ray analysis
(EDAX). High resolution transmission (HR-TEM, JEOL JEM 2100)
3. Results and Discussions
3.1. Crystal structure and morphology
Fig. 1a shows the XRD pattern recorded for the Co3O4/CoO
@ carbon matrix sample obtained from pyrolysis of the
[Co(salen)] complex followed by hydrothermal treatment. The
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set of Bragg peaks located at diffraction angles (2θ), 46.5, 62.3, scattering of graphitic carbon.44 It is well knoVwienw AtrthicaletOntlhinee
DOI: 10.1039/C7NJ04638C
66.2 and 70.5° could be indexed to the cubic phase of CoO as disorder in the structure is measured by the intensity ratio of
per the JCPDS PDF # 89-2803. The set of diffraction peaks the D and G bands (ID/IG). The observed (ID/IG) of the Co3O4/CoO
present at 32.8, 38.5, 51.6 and 54.2° are assigned to Co3O4 as @ carbon sample is 0.95 which is very close to 1. Such a high
per the JCPDS PDF# 65-3103. In addition, the XRD pattern also (ID/IG) ratio indicates the presence of large amount of defects in
has peaks at 25.1, 26.4 and 44.2° which confirm presence of the graphitic carbon. Additionally, a distinct peak at 2204 cm-1
graphitic carbon as per JCPDS PDF# 89-8487.42-44 The presence is seen. This peak is attributed to the laser scribed graphene
of asymmetric peak at 44.2° confirmed the turbostatic nature of (LSG). The appearance of this 2D peak indicates the presence of
the disordered carbon layer in the graphitic carbon. Thus, the lower structural defect in the sp3 carbon of the carbon matrix.
XRD data indicate that the hydrothermally-derived sample The peak at 517 cm-1 corresponds to Co-O bond. The Raman
consists of CoO, Co3O4 and carbon. The XRD results have been spectroscopy data have been substantiated by the FT-IR data
substantiated by the Raman and IR spectral data.
shown in Fig. 1c. The IR spectrum of the sample has low intense
two sharp peaks at 3625 and 3737 cm-1 which are assigned to
N-H bonds. The peaks at 1565 and 1345 cm-1 are ascribed to C=C
and C=N bonds, respectively. The strong band observed at 1095
cm-1 is due to C-N-C bond. The peak present at 950 cm-1 is
corresponding to C-H bond. In addition, sharp peak at 665 and
intense peak at 510 cm-1 are also present which are ascribed to
Co3+-O and Co2+-O bonds, respectively. The appearance of Co3+-
O and Co2+-O bonds confirms that the hydrothermally-derived
sample consists of CoO and Co3O4. It is well known that in the
Co3O4 spinel, Co3+ is in the octahedral environment, while Co2+
is in the tetrahedral environment. 45,46 The detailed description
of FT-IR of the Co3O4/CoO @ carbon matrix and pyrolyzed salen
complex has been given in Supplementary materials (Fig. S2).
: CoO (89-2803)
: Co3O4 (65-3103)
: C (89-8487)



(a)













10
20
30
40
50
60
70
2 (Degree)
G
(b)
D
(ID/IG = 0.95)
Fig.
2
(a-f) shows the surface morphology of the
hydrothermally-derived Co3O4/CoO @ carbon sample obtained
from FE-SEM taken at different magnifications. The morphology
consists of largely agglomerated spike-like grains with the
thickness about 80 nm. In addition, the matrix also consists of
different shape and size grains. Usually, a flat-smooth surface
will result in for the carbon material. In the present case, the
smooth surface has been lost, this could be due to the presence
of the Co3O4/CoO grains. To identify the elements present in the
sample, EDX profile was recorded. The Fig. 2g shows the
obtained EDX profile. The presence of carbon, oxygen and
cobalt is clearly visible. From the intensities of the peaks, the
estimated amounts of C, O and Co are 64, 12 and 24 wt.% ,
respectively. The rich carbon content would have resulted from
the decomposition of [Co(salen)] complex during the adopted
synthetic route. The EDX data have been supported by the
elemental mappings shown in Fig. 2(h-j). It can be seen that all
the three elements are homogeneously distributed throughout
the matrix. The high density of the elemental mapping is
observed for ‘C’, indicating its presence in the sample. To
substantiate the EDX results and to quantify the amount of
carbon and oxide composition, known quantity of the prepared
sample (50 mg) was subjected to acid leaching in conc.HNO3, 24
h to dissolve the cobalt oxide. It was seen that the solution was
magenta color and the residue was black in color. After
complete dissolution, the formed magenta color solution was
decanted and the residue was washed with the distilled water
2D
Co-O
500
1000
1500
2000
2500
3000
3500
Raman shift (cm-1)
N-H
(c)
4000
C-N-C
Co2+-O
3000
2000
1000
Wavenumber (cm-1)
Fig. 1 (a) XRD pattern, (b) Raman spectrum and (c) FT-IR spectrum recorded for the
hydrothermally-derived Co3O4/CoO @ carbon matrix.
Fig. 1b shows the Raman spectrum obtained for the
hydrothermally-derived Co3O4/CoO @ carbon sample. Distinct
four bands located at Raman shifts 513, 1330, 1603 and 2204
cm-1 are clearly seen. The Raman shifts observed at 1330 and
1603 cm-1 correspond to the D and G bands of graphitic carbon.
The appearance of the D band originates from the edges of
defected carbon layers due to breakdown of translational
symmetry, while the G band is attributed to the second order
several times, and finally dried at 110
the mass of obtained residue was weighed using electronic
balance. It was found that 30 mg of the residue was obtained.
C for overnight. Then,

This accounts for 60% of the residue. Further, the residue was
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DOI: 10.1039/C7NJ04638C
(a)
(b)
(c)
(c)
(a)
Carbon
Co3O4/CoO
(440)
Co3O4
10 m
1 m
5 m
(d)
(e)
(f)
200 nm
Carbon
200 nm
(b)
10 nm
(d)
Co3O4(440)
4 m
CoO(200)
CoO(111)
Co3O4/CoO
200 nm
(j)
(g)
(h)
(i)
Element
C K
O K
Percentage
64.51
11.56
Co3O4(111)
Co L
Total
23.93
100
O - K
C - K
Co - L
Carbon
Fig. 2 (a-f) SEM images at various magnifications, (f, g) EDX profile and (h-j) elemental
mappings taken on the surface of the hydrothermally-derived Co3O4/CoO @ carbon
sample.
Fig. 3 (a-b) TEM images, (c) Lattice fringes and (d) SAED pattern recorded for the
hydrothermally-derived Co3O4/CoO @ carbon matrix
subjected to XRD and Raman analysis. The obtained XRD and
Raman data of the residue are given in supplementary material
(Fig. S1). It can be seen that the XRD pattern consists of Bragg
It is believed that during the hydrothermal process, urea
angles, at 26.4 and 44.2. These Bragg peaks correspond to decomposes to give hydroxyl ion which can increase pH of the
carbon as per the standard XRD pattern (PDF # 89-8487). Thus, content in the Teflon autoclave leading to form Co(OH)2 which
the Co3O4/CoO has gone in to the solution when the sample is on dehydration lead to formation of CoO and Co3O4
dissolved in conc. HNO3 and carbon remained as residue in the nanoparticles as shown below. It is also believed that the
leached sample. The XRD results have been corroborated by the Co3O4/CoO nanoparticles are embedded in the carbon matrix
Raman data (supplementary materials, Fig. S1b). It is seen that during the grain growth. Prior to the hydrothermal treatment,
the Raman spectrum of the acid-leached sample has the pyrolysis of [Co(salen)] complex was done to generate
characteristic carbon bands at 1312, 1586 and 2613 cm-1 which carbon matrix.
corresponds to D, G and 2D bands of graphene. It is noteworthy
that the characteristic Raman shift of the Co-O at 675 cm-1 is
absent. This clearly confirms that cobalt oxide is completely
leached out from the carbon matrix upon acid treatment
followed by washing with water. Thus, based on the
(NH2)2CO + H2O → 2NH3 + CO2
NH3 + H2O → NH4+ + OH-
Co2+ + OH → Co(OH)2
Co(OH)2 →CoO + H2O
3 Co(OH)2 + ½O2 → Co3O4 + 3H2O
(1)
(2)
(3)
(4)
(5)
aforementioned XRD, Raman, FT-IR and SEM/EDX data, it is Thus, the salen complex having abundance of carbon source,
confirmed that the hydrothermally-derived sample is composed yielded as high as 60% carbon in the Co3O4/CoO @ carbon
of Co3O4/CoO and carbon in 40% and 60% proportions, sample. The formed carbon nanonetwork is expected to
respectively.
increase conductivity of the sample and decrease the diffusion
Fig. 3a and 3b show the TEM images of the Co3O4/CoO @ path. Such an enhanced conductivity as well as reduced
carbon matrix. It is confirmed that the Co3O4/CoO grains are
embedded in the carbon matrix. Boundary between the oxide
grains and carbon is seen. Fig. 3c shows the lattice fringes
observed for the Co3O4/CoO @ carbon sample. The presence of
Co3O4 (440) peaks has been confirmed (Fig. 3c). The selected
area electron diffraction pattern (SAED) of the hydrothermally-
derived Co3O4/CoO @ carbon is shown in Fig. 3d. It can be seen
that the SAED pattern shows well-defined spots and rings. The
appearance of well-defined spots and rings in the SAED pattern
reveals that the hydrothermally-derived Co3O4/CoO @ carbon
sample is highly crystalline in nature. The d-spacing values
estimated from the rings match well with the d spacing values
of the Co3O4 [PDF # 65-3103] and the CoO [PDF # 89-2803],
Scheme 1. Schematic representation of the hydrothermally-derived Co3O4/CoO @
carbon matrix showing Co3O4/CoO grains are anchored on to the carbon.
Substantiating the above discussed XRD data.
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diffusion path is expected to deliver excellent supercapacitor
and photocatalytic performances. Based on the results
obtained here, schematically the material can be best described
as shown in Scheme 1.
View Article Online
DOI: 10.1039/C7NJ04638C
5
60
40
20
0
1 : 2 mV s-1
2 : 5
(a)
3 : 10
3.2. Supercapacitor performances
4 : 20
5 : 50 mV s-1
The electrochemical activity of the hydrothermally-derived
Co3O4/CoO @ carbon was investigated using cyclic voltammetry
and galvanostatic charge-discharge studies in 1 M KOH in the
potential range of 0–0.5 V vs. Ag/AgCl for application in
supercapacitors. Fig. 4a. shows the CV curves recorded for the
Co3O4/CoO @ carbon sample in the potential range of 0-0.5 V
at various scan rate of 2, 5, 10, 20, 50 mV s-1 in 1 M KOH. A pair
of redox peaks is clearly seen in all the cases suggesting the
electrochemically active nature of the prepared material and
demonstrating the conventional behaviour of a battery-type
electrode. For comparison, the CV curve recorded on the bare
nickel foam (NF) substrate is also given. It is seen that the bare
substrate did not show any significant current in 1 M KOH,
indicating it is electrochemically inert and provides only support
for the active material. Thus, the charge storage is dominated
by the Faradaic reaction. The anodic oxidation peak appears at
0.33 V and the cathodic reduction peak occurs at 0.25 V with
the peak potential separation of about 80 mV at 2 mV s-1 scan
rate. The peak potential separation observed here is slightly
higher value than the value of an ideal reversible reaction (59
mV), implying the redox reaction is quasi reversible. The
following electrochemical reactions are responsible for the
appearance of current peaks in KOH:23, 47
1
NF-bare
substrate
-20
-40
0.0
0.1
0.2
0.3
0.4
0.5
Potential (V) vs. Ag/AgCl
(b)
20
10
0
Ia
-10
-20
Ic
2
3
4
5
6
7
2 CoO + 2OH- ↔ H2O + Co2O3 + 2 e-
CoO + OH- ↔ CoOOH + e-
Co3O4 + OH- + H2O ↔ 3 CoOOH + e-
(6)
(7)
(8)
√ (V s-1)1/2
Fig. 4 Cyclic voltammetry curves at different scan rates and (b) Randles plot obtained for
the hydrothermally-derived Co3O4/CoO @ carbon electrode in 1 M KOH.
In addition to the peak at 0.25 V, it is noted that a shoulder
peak appears at 0.05 V. The appearance of such a shoulder peak
implies that more than one redox reaction is proceeding on the
electrode. The appearance of anodic peak is due to the
oxidation of Co2+ to Co3+ and the cathodic peak corresponds to
the reduction to lower oxidation state of Co3+. Thus, it is
believed that both CoO and Co3O4 are undergoing redox
reaction with the KOH. It is seen that as the scan rate increased,
the peak current also increased, as it has been usually observed
for a battery-type electrode.48 Fig. 4b shows the variation of
anodic and cathodic peak currents with the square root of scan
rate (known as Randles-Sevcik plot). In both cases, the peak
current varies linearly with the square root of the scan rate
which is characteristics of a diffusion controlled fast reversible
Faradaic reaction.49
range of 0.25–0.4
aforementioned CV data, which clearly confirms that the
Faradaic process is a typical battery-type electrode. In addition,
V vs. Ag/AgCl substantiating the
the Co3O4/CoO
@
carbon electrode exhibits high
charge/discharge times; this validate the role of carbon matrix
in the sample. The carbon matrix facilitates the charge transport
providing an efficient conductive pathway leading to enhance
the battery-type characteristics of the Co3O4/CoO @ carbon
electrode. The specific capacity of each electrode was
estimated from the charge-discharge curves using the following
formula:50
Es = I t/m C g -1
(9)
where, Es is the specific capacity (C g-1), t is the discharge time
(s), I is the constant discharge current (A), m is the active mass
of the electrodes (g). The uniformity in shape of all the charge-
discharge profiles confirms that the redox reactions are highly
reversible.
Fig. 5a shows the galvanostatic charge
recorded for the Co3O4/CoO @ carbon electrode at current
densities of 1, 2, 3, 4, 5 and 10 A g-1 in a potential window of 0
to 0.45 V. It can be seen that the charge-discharge profiles are
nearly mirror images revealing that the Faradaic reactions are
reversible. Interestingly, it is noted that for the Co3O4/CoO @
carbon electrode, clear plateau region is seen in the potential
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Journal Name
Fig. 5b shows the dependence of specific capacity on the electrode exhibits specific capacity of about 207 C g-1. The
View Article Online
-1
current density. The specific capacity decreases with increase of specific capacity in the range of 75-300 CDgOI:h10a.s10b3e9e/Cn7NreJp04o6r3te8Cd
current rate. At lower current, high specific capacity and at high for the CoO or Co3O4 as shown in Table 1. Interestingly, the
Co3O4/CoO @ carbon electrode exhibits much better specific
capacity than several available reports, implying that the
0.5
0.4
0.3
0.2
0.1
0.0
(a)
Co3O4/CoO@ carbon can be
supercapacitor application.
a potential electrode for
High stability is one of the vital parameters of
a
supercapacitor for an actual application. Thus, the glavanostatic
charge-discharge cycles were done for 1000 cycles at a current
of 10 A g-1. In each cycle, the specific capacity and Coulombic
efficiency were calculated by using the following formula:
1 A g-1
2 A g-1
3
4
5

= QD / QC
(10)
10
where, QD is the discharge capacity and QC is the charge
capacity. Fig. 5c shows the cycling stability and Coulombic
efficiency obtained for the Co3O4/CoO @ carbon electrode. The
Co3O4/CoO @ carbon electrode exhibits specific capacity of 207
C g-1 at the first cycle and 192 C g-1 at the end of 1000th cycle.
Thus, the Co3O4/CoO @ carbon electrode retained over 91% of
charge even at 1000th cycle. Interestingly, the Co3O4/CoO @
carbon matrix also exhibited high Columbic efficiency of 96 %
even at 1000th charge-discharge cycle. It can be seen that the
charge-discharge profiles at the first cycle and the 1000th cycle
are nearly similar, exhibiting excellent stability (inset of Fig. 5c).
It is believed that such an excellent stability and high Coulombic
efficiency could be due to the presence of conductive carbon
nano-network which would have facilitated the charge storage
efficiently by reducing diffusion path. The merits of the
Co3O4/CoO @ carbon over other oxides are its excellent battery-
type behaviour and stability due to non-dissolution of metal
ions in the aqueous electrolyte which are very important for
developing a practical hybrid energy-storage device.
200
400
Time (s)
600
800
500
400
300
200
100
0
1 M KOH
(b)
0
1
2
3
4
5
6
Current density (A g-1)
250
200
150
100
50
(c)
100
80
60
40
20
Table 1 Specific capacity reported for cobalt oxide-based materials
Specific
capacity
(C g-1)
S.
No.
Name of sample
Method
Reference
First 3 cycles
Last 3 cycles
0.5
0.4
0.3
0.2
0.1
0.5
0.4
0.3
0.2
0.1
0.0
1
α-Co(OH)2 and CoO
Radiation
induced
195 in
1M KOH
[12]
method
2
Co3O4/carbon
microspheres
Low
200 in
2M KOH
[18]
temperature
solution route
Solution
combustion
Ball milling
0
40
80
120
38200
38120 Ti3m816e0 (s)
Time (s)
200 400
Cycle number
3
4
5
6
7
8
Co3O4 and CoO
CoO
160 in
6M KOH
300 in
1M KOH
180 in 2
M KOH
140 in 3
M NaOH
220 in
[23]
[48]
0
600
800 1000
Fig. 5 (a) Galvanostatic charge-discharge profiles at different currents, (b) Dependence
of specific capacity on the current densities and (c) Cycle-life data as well as Coulombic
efficiency at 10 A g-1 recorded for the hydrothermally-derived Co3O4/CoO @ carbon
sample in 1 M KOH. Inset: first three and last three cycles of charge-discharge profiles
recorded at a current density of 10 A g-1
CoO/RGO
Microwave
synthesis
Hydrothermal
[49]
CoO/graphene foam
CoO on grapheme
CoO/Co3O4@carbon
[51]
current lower specific capacity have been resulted which is a
characteristic of a battery-type material. The fast charging-
discharging process at high current density is always associated
with poor utilization of the active mass resulting in low capacity.
The specific capacity of the Co3O4/CoO @ carbon electrode
estimated at 1 A g-1 1 M KOH was as high as 324 C g-1. It is noted
that even at a high current of 10 A g-1, the Co3O4 /CoO @ carbon
Hydrothermal
Hydrothermal
[52]
3M KOH
324 C g-1
in 1 M
This work
KOH
6 | New J. Chem., 2017, 00, 1-3
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at different cycle numbers obtained for the Co3O4/CoO @ carbon matrix electrode in 1
To understand the kinetics of ion and electron transports in
the Co3O4/CoO @ carbon matrix, the EIS studies were done for
freshly assembled cell as well as at the end of several charge-
discharge cycles. Fig. 6a shows the Nyquist plots obtained for
the representative charge-discharge cycles. Each plot consists
of a depressed semicircle in the high-frequency region and an
extended spike (Warburg element) in the low-frequency region.
Similar Nyquist plot has been reported elsewhere.50,51 It is noted
that as the number of charge-discharge cycle increases, the
diameter of the semicircle increases and the spike shifts to
higher resistance value. The presence of Warburg element
implies the dominance of diffusion controlled Faradaic reaction,
especially at higher number of charge-discharge cycles. The
obtained Nyquist plots were fit into an equivalent circuit as
shown in inset of Fig. 6b. The equivalent circuit consists of
solution resistance (Rs), charge-transfer resistance (Rct) of a
Faradaic reaction occurring on the electroactive surface,
constant phase element (Cpe) and Warburg component (W).49-51
From the best fit, the Rs and Rct values were obtained for the
representative charge-discharge cycles. It was found that both
Rs and Rct increased as the number of charge-discharge cycle
increased. This means the increase of intrinsic resistance of the
electrode material due to interaction with the OH ions. Thus,
the electrode exhibited decreased specific capacity at higher
charge-discharge cycle. This has been substantiated by the
conductivity of the electrode matrix estimated from the Rs value
as shown in Fig. 6c. The conductivity of the electrode was
estimated using the following equation:52
View Article Online
M KOH.
DOI: 10.1039/C7NJ04638C
It is noted that the phase angle at high-frequency region is low
due to high electrode/electrolyte resistance, supporting lower
specific capacity at higher charge-discharge cycles.
3.3. Photocatalytic degradation of AZA dye
The photocatalytic property of the Co3O4/CoO @ carbon
material was analyzed in terms of the decoloration of the AZA
dye in aqueous solution under UV irradiation. The aqueous
solution of AZA exhibits absorption maxima at 625 nm owing to
its monomeric form.53 The degradation of the dye was studied
by monitoring the changes in the absorption maxima of the AZA
with respect to the irradiation time.
Absorption profiles of the AZA solution in the presence of
catalyst was recorded under UV radiation exposed from 0 to 70
min. The obtained absorption profiles are displayed in Fig. 7a.
For comparison, the absorption profile of the AZA dye solution
in the absence of the catalyst was also recorded and presented
in Fig. 7a. It is seen that in all cases, the absorption maximum is
centred at 625 nm which is characteristic of the AZA absorption.
It is noteworthy that the absorbance recorded for the AZA dye
solution in the absence of catalyst is about 1.2. Upon addition
of the catalyst, the absorbance of the AZA dye decreased to
nearly half (0.6 at 0 min). The decreases in intensity soon after
the addition of the catalyst could be due to the adsorption of
the AZA dye molecules on the surface of the catalyst. Such an
adsorption can facilitate effective degradation.54 It can be
noticed from Fig. 7a that the absorbance of the AZA dye steadily
decreases with increasing UV irradiation time in the presence of
the Co3O4/CoO @ carbon catalyst. The absorption spectral
change is accompanied by simultaneous colour fading of the
AZA dye from blue to colourless and such colour fading is often
associated with gradual decomposition of chromophoric group
of the dye molecule.34,53 To confirm the degradation of the AZA
dye in the presence of the catalyst, the absorption profile of the
AZA dye solution in the absence of the catalyst was recorded
under UV radiation exposure 0 to 70 min. The obtained
absorption profiles are shown in Fig.7b. It is noted that the
absorption maximum is centred at 625 nm in all examined
period with the absorbance value of in the range of 1 - 1.2 even
after 1h of UV exposure. The formation of free radicals and self-
aggregation of the AZA dye have resulted in the decrease of
absorption intensity upon UV irradiation. Similar observation
has been reported for photoactive phenothiazine dye
elsewhere.38 The data obtained from absorption spectral
studies were utilized to calculate the decomposition rate (Ct/C0)
of the AZA dye as a function of UV irradiation time and the
obtained plot is displayed in Fig. 7c. The C0 and Ct are the
concentrations of AZA dye before and after UV light irradiation,
respectively. The degradation (%) of the AZA was found to be
95% and it is obvious from Fig. 7 (b and c). that in the absence
of the Co3O4/CoO @ carbon catalyst, the decomposition of the
AZA dye under the same UV light irradiation conditions was
found to be insignificant, confirming the excellent photo
σs= l /Rs x A S cm-1
(11)
where, σs is electrode conductivity (S cm-1), l is the distance
between the two electrodes (cm), Rs is the solution resistance
(Ohm), A is the area of electrode interacting with electrolyte
(cm2). It is seen that as we move from 1st cycle to 1000th cycle,
the conductivity decreases significantly. Fig. 6d shows the Bode
plot recorded at the representative charge-discharge cycles.
180
15
(b)
(a)
6
150
120
90
60
30
0
Rs
Rct
5
12
9
4
3
CPE
1: Cycle 1
2: Cycle 10
2
1
3: Cycle 100
4: Cycle 500
5: Cycle 800
6: Cycle 1000
6
6
5
3
4
3
1, 2
0
10
20
30
Z’ ()
40
50
(c)
3
6
9
12
15
Z’ ()
20
0.56
0.54
0.52
0.50
(d)
0
4
-20
-40
-60
-80
1
1: Cycle 10
2: Cycle 100
3: Cycle 500
4: Cycle 1000
-3 -2 -1
200 400 600 800 1000
Cycle number
0
1
2
3
4
5
0
log (frequency/ Hz)
Fig. 6 Nyquist plots (a) full range, (b) high frequency region at representative charge-
discharge cycles, (c) variation of conductivity with the cycle number and (d) Bode plots
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degradation performance of the dye on the Co3O4/CoO @ performance of the Co3O4/CoO
carbon material. The change in absorption spectra of the AZA comparable with that of the previously reported TiO2 catalyst
dye upon equilibrating with the catalyst in dark is given in Fig. for the degradation of phenothiazine dyes.38
Journal Name
@
carbon material is
View Article Online
DOI: 10.1039/C7NJ04638C
7d. It is obvious from Fig. 7d that upon
3.4. Product analysis of degraded AZA
The analysis of the photodegraded AZA dye was done by using
UV-Vis and FT-IR spectroscopies. Fig. 8a shows the obtained UV-
Vis spectra of the AZA dye before and after the photocatalytic
treatment. It is evident that before photocatalytic degradation,
the AZA exhibits two well-resolved maxima at 245 nm and 289
nm in the UV region and an intense absorption maximum at 625
nm in the visible region.57 The presence of conjugated system
arising out of the dimethylamine substituted aromatic rings
through the sulfur and nitrogen is responsible for the
appearance of intense peak at 625 nm, whereas the small bands
in the UV region (centered at 245 nm and 289 nm) are
attributed to the substituted benzene rings.57 As is seen, after
the photocatalytic degradation, all the three characteristic
absorption peaks of the AZA dye are completely disappeared.
No significant absorption peak was observed. Thus, it is evident
that the AZA dye has undergone complete mineralization owing
to the photodegradation in the presence of the Co3O4/CoO @
carbon catalyst under UV radaition. The degraded product did
not have any significant functional group.
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(b)
(a)
0 min
0 min
0
1
2
3
4
5
6
7
Azure A
Azure A
without
Catalyst
10 min
20 min
30 min
40 min
50 min
60 min
70 min
10 min
20 min
30 min
40 min
50 min
60 min
70 min
Azure A
+
0
Catalyst
7
400 450 500 550 600 650 700 750 800
400
500
600
700
800
0.7
Wavelength (nm)
Wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
(c)
AZA in the
(d)
absence of catalyst
0.6
0 min
70 min
0.0
0.5
0.4
0.3
0.2
0.1
0.0
-0.5
-1.0
-1.5
0
15 30 45 60
Time (min)
AZA in the presence of catalyst
0
10 20 30 40 50 60 70 80
Time (min)
400 450 500 550 600 650 700 750 800
Wavelength (nm)
Fig. 7 (a) Absorption spectral profiles of the AZA dye in the presence and (b) absence of
Co3O4/CoO @ carbon catalyst (c) under dark condition at 0 and 70 min.
To substantiate the UV-Vis spectral data as well as to get better
insights into the final degradation pattern of the AZA dye, FT-IR
analysis of the AZA dye before and after photodegradation
under catalyst, was performed. It is obvious from Fig. 8b that
the FT-IR spectra of the AZA before degradation exhibits band
incubation in the dark even upto 70 min, the spectral profile of
the AZA dye remains unperturbed which clearly rules out the
possibility of any dark reactions. It is worth mentioning that the
amount of added catalyst and the irradiation time showed a
prominent effect on the degradation rate. However, we have
optimized the photoreaction in such a way that the amount of
the Co3O4/CoO @ carbon is kept minimal for cost-effective
performance.
To understand the reaction kinetics involved in the
degradation reaction of the AZA dye, we analyzed the
absorption spectral data of degraded dyes over a specified
period of time by the pseudo-first order kinetic model using
equation,55
−1
at 3421 cm corresponding to N-H stretching modes. The
asymmetric and symmetric bending vibrations of
dimethylamino are observed at 1483 and 1392 cm−1,
respectively. The most intense band in the IR spectrum at 1609
cm−1 is caused by the tangential C=C stretching vibration. The
strong band at 1484 cm−1 is attributed to the C–N–C asymmetric
stretching vibration of the central ring and phenyl ring. The ring-
stretching mode of C–N bond vibration is observed as a strong
band at 1336 cm−1. The strong and medium bands at 897 and
872 cm−1 are assigned to C–S–C stretching vibrations of the
central ring coupled with phenyl ring deformation modes and
the wagging mode of the amino group, respectively.57
Interestingly, the FT-IR spectrum of the photodegraded AZA dye
exhibited bands corresponding to OH stretching at 3280 cm-1,
OH bending vibrations at 1632 cm-1 and the band at 1051 cm-1
is ascribed to C-C bond arising out of trace carbon elements,
respectively. The absence of characteristic N-H stretching, C–S–
C stretching, phenyl ring-N (C–N) stretching and dimethylamino
bending stretching vibrations in the FT-IR data suggests that the
aromatic ring of the AZA dye is completely degraded.
ln(Ct) = ln(C0) − Krt
(12)
where, C0 is the initial concentration of the AZA and Ct
represents the concentration of the AZA at a time t and kr is the
rate constant of the pseudo-first order reaction. The kinetic plot
for the photocatalytic degradation of the AZA is displayed in the
inset of Fig. 7c. As shown in the inset of Fig. 7c, the plot of
ln(Ct/C0) vs t obeys a linear regression with a slope value equal
to rate constant kr. It is well known that a linear plot is indicative
of the existence of first order reaction kinetics.
It has been reported that oxidative mineralisation of
thiazine dyes (Note: AZA is a thiazine dye) involves a series of
pathway involving demethylation (formation of azure C and
thionine) and ring opening products (amino-diol, sulfonic acid
and aniline).58 The absence of characteristic peaks of the
aforementioned intermediates in the absorption and FT-IR
spectral data of the degraded AZA dye, clearly implies that the
AZA dye got completely mineralised. The process of complete
If the order of the reaction is anything other than one
(second (2) or third (3) order) then the graph of ln(Ct/C0) vs t will
be a curve (not a straight line).56 Thus, from the above
observation it is inferred that the photocatalytic degradation of
the AZA in the presence of the Co3O4/CoO @ carbon catalyst
follows first order rate kinetics and the rate constants for the
photocatalytic degradation of the AZA was found to be 0.023
min-1. It is worth mentioning that the photocatalyic
8 | New J. Chem., 2017, 00, 1-3
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3.5. Explanation for Photocatalytic degradation of AZVAiewdyAerticle Online
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
The ability of semiconductor nanopDaOrtIi:c1l0e.s103t9o/C7gNeJ0n4e6r3a8tCe
electron-hole pairs play a vital role in the photocatalytic
degradation of dye molecules.32,38 Upon the interaction of the
UV light with the Co3O4/CoO @ carbon catalyst, an electron in
the valence band of the catalyst material is excited to the
conduction band with the simultaneous generation of a hole in
the valence band. The holes that are accumulated on oxide
surface will oxidize water or surface hydroxyls to form reactive
hydroxide radicals (OH). Whereas, the photogenerated
electrons will react with oxygen to produce super oxide radicals
(a)
1 : AZA-Before Degradation
2 : AZA-After Degradation

1
(O2 ), which then reacts with the adsorbed AZA dye molecules
to produce oxidized species or decomposed products. In
addition to the above factors, the synergistic effect of the
Co3O4/CoO @ carbon material is expected to have a significant
effect on the photocatalytic degradation of the AZA. For
instance, the decrease in the recombination of the
photogenerated electrons and holes allows them to take part in
the photocatalytic reaction. The presence of the carbon matrix
around the Co3O4/CoO acts as a good supporting material for
the catalyst by reducing diffusion path and ensures the
adsorption of more amount of the AZA dye molecules over the
catalyst surfaces for efficient catalytic performance. Thus, the
Co3O4/CoO @ carbon can be a potential dye degradation
material for practical application. Owing to the wide spread use
of the AZA in pigments, insecticides and staining agent, the AZA
dye is extensively discharged to the nearby vicinity, heavily
polluting water and soil on the environmental eco-system. The
developed catalyst can be utilized for degradation of the AZA in
these places to mitigate any adverse effects caused due to the
discharge of the dye in the environment.
2
200
300
400
500
600
700
800
Wavelength (nm)
(b)
AZA after degradation
AZA before degradation
3600
3000
2400
1800
1200
600
Wavenumber (cm-1)
Fig. 8 (a) Absorption profiles and (b) FT-IR spectra recorded for the AZA dye before and
after photodegradation in the presence of Co3O4/CoO @ carbon catalyst.
Conclusions
mineralisation of thiazine dyes in the presence of photocatalyst
is driven by oxidative process involving both the hydroxide and
super oxide radicals generated at the interface of the
Co3O4/CoO @ carbon catalyst.58 Thus, from the UV-Vis
absorption and FT-IR spectral analysis, it is confirmed that the
AZA dye gets mineralised under photodegradation in the
presence of the Co3O4/CoO @ carbon catalyst. The degradation
pattern of the AZA can be depicted as shown in the Scheme 2
below:
The Co3O4/CoO @ carbon electrode exhibited a specific capacity
of 324 C g-1 at 1 A g-1 with high Coulombic efficiency as well as
excellent long-term stability. It is believed that the carbon
present in the Co3O4/CoO acted as conductive nanonetwork by
reducing diffusion path which lead to such a high supercapacitor
performance. The Co3O4/CoO @ carbon material also exhibited
high photocatalytic activity for the degradation of the AZA dye
under UV illumination and the degradation percentage of AZA
dye was found to be as high as 95% with short duration. Thus,
the hydrothermally-derived Co3O4/CoO @ carbon can be a
potential material for supercapacitor application and can
function as efficient photocatalyst.
Conflicts of interest
There are no conflicts to declare.
Scheme 2 Plausible degradation mechanism of the AZA dye under UV radiation in the
presence of Co3O4/CoO @ carbon catalyst.
Acknowledgements
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Journal Name
28 J. Wu, C. Ouyang, S. Dou and W.S. Shuangyin, Nanotech.,
The authors acknowledge the central instrumentation facility
(CIF) of Pondicherry University. PE thanks SERB, New Delhi,
Govt. of India for the research scheme (EMR/2016/001305).ED
thanks MNRE for National Renewable Energy Fellow. AS thanks
SERB, New Delhi, Govt. of India for National Post-Doctoral
Fellowship (PDF/2016/002815).
View Article Online
2015, 26, 1-8.
DOI: 10.1039/C7NJ04638C
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New Journal of Chemistry
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DOI: 10.1039/C7NJ04638C
Supercapacitor and Photocatalytic Performances of Hydrothermally-derived
Co3O4 / CoO @ Carbon Nanocomposite
E. Duraisamy, Himadri T. Das, A. Selva Sharma and P. Elumalai*
Department of Green Energy Technology, Madanjeet School Green Energy Technologies
Pondicherry University, Puducherry-605014, India.
Table of Contents
Hydrothermally-derived Co3O4/CoO @ Carbon Nanocomposite with spike-like cobalt oxide
anchored on conductive carbon network showed excellent supercapacitor and photocatalytic
performance
1