NiCo2O4 nanorod: Synthesis and electrochemical sensing of carcinogenic hydrazine

Article abstract of DOI:10.1016/j.inoche.2020.107927

In this work, we explored a simple hydrothermal process to tune the NiCo₂O₄ morphology into well defined rod structure with nanoparticle on it. The as prepered NiCo₂O₄ was characterized by various characterization studies such as XRD, Raman, FT-IR, TGA, FESEM, TEM, XPS and HRTEM. The electrochemical sensing of hydrazine on NiCo₂O₄ nanorod was examined by cyclic voltammetry and differential pulse voltammetry techniques. The NiCo₂O₄ shows excellent electrocatalytic performance for the anodic oxidation of hydrazine. Interestingly, NiCo₂O₄ retain considerable current response after 50 cycles at 25 mV s^?1. The Ni and Co in bimetal oxides enhanced the current response due to their electroactive sites. The developed NiCo₂O₄ not only enhanced the current response of hydrazine but also exhibits good stability. NiCo₂O₄ demonstrates excellent detection limit and sensitivity of 260 nM and 48.25 μA cm^?2 mM^?1, respectively. For practical application, NiCo₂O₄/GCE was used for the sensing of hydrazine in real water samples.

Full text of DOI:10.1016/j.inoche.2020.107927

Journal Pre-proofs
NiCo O nanorod: Synthesis and electrochemical sensing of carcinogenic hy-
2
4
drazine
Velayutham Sudha, Sakkarapalayam Murugesan Senthil Kumar, Rangasamy
Thangamuthu
PII:
S1387-7003(20)30101-5
DOI:
https://doi.org/10.1016/j.inoche.2020.107927
INOCHE 107927
Reference:
To appear in:
Inorganic Chemistry Communications
Received Date:
Revised Date:
Accepted Date:
23 January 2020
20 March 2020
2 April 2020
Please cite this article as: V. Sudha, S. Murugesan Senthil Kumar, R. Thangamuthu, NiCo O nanorod: Synthesis
2
4
and electrochemical sensing of carcinogenic hydrazine, Inorganic Chemistry Communications (2020), doi:
https://doi.org/10.1016/j.inoche.2020.107927
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NiCo O Nanorod: Synthesis and Electrochemical Sensing of
2
4
Carcinogenic Hydrazine
Velayutham Sudha a, b, Sakkarapalayam Murugesan Senthil Kumar a, b, Rangasamy
Thangamuthu a, b*
a
Materials Electrochemistry Division (MED), CSIR-Central Electrochemical Research
Institute (CSIR-CECRI), Karaikudi-630 003, Tamil Nadu, India.
b
Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical
Research Institute, Karaikudi- 630 003, Tamil Nadu, India.
*
To whom correspondence should be addressed. E-mail: thangamuthu@cecri.res.in;
thangamuthu_r@yahoo.co.uk; Phone: +91-04565-241-350.
Abstract
In this work, we explored a simple hydrothermal process to tune the NiCo O
2
4
morphology into well defined rod structure with nanoparticle on it. The as prepered NiCo O
2
4
was characterized by various characterization studies such as XRD, Raman, FT-IR, TGA and
HRTEM. The electrochemical sensing of hydrazine on NiCo O nanorod was examined by
2
4
cyclic voltammetry and differential pulse voltammetry techniques. The NiCo O shows
2
4
excellent electrocatalytic performance for the anodic oxidation of hydrazine. Interestingly,
-1
NiCo O retain considerable current response after 50 cycles at 25 mV s . The Ni and Co in
2
4
bimetal oxides enhanced the current response due to their electroactive sites. The developed
NiCo O not only enhanced the current response of hydrazine but also exhibits good stability.
2
4
-2
NiCo O demonstrates excellent detection limit and sensitivity of 260 nM and 48.25 µA cm
2
4
1

-1
mM , respectively. For practical application, NiCo O /GCE was used for the sensing of
2
4
hydrazine in real water samples.
Key words: Hydrazine, NiCo O , Hydrothermal method, Electrochemical sensing.
2
4
Scheme 1. Schematic illustration of NiCo O characterization and electrochemical sensing of
2
4
hydrazine.
1
. Introduction
Hydrazine is widely used in fuel cell, food production, pharmaceutical intermediates,
rocket fuel, pesticides production, blowing agent, dye industries, oxygen scavengers and so
on [1–5]. The Environmental Protection Agency (EPA) identified hydrazine as a poisonous
chemical and the proposed level in the commercial effluents is 1 ppm. Due to volatile nature
of hydrazine, it is easily adsorbed on the human skin and damage the living organism
especially, liver, blood production system and kidney [6]. Vernot et al., found that exposure
to hydrazine for long time causes inhalation problem in different animals. Rats, harmsters,
mice and dog were analysed post exposure of 18 months, 1 year, 15 months, 38 months,
respectively. Male and female rats disclose dose-dependent prevalence of well-disposed nasal
adenomatous polyps and smaller numbers of malignant nasal epithelial tumours after 1 year
of exposure to hydrazine and 18 months post exposure holding [7], DNA damage [8] and
2

irreversible deterioration of nervous system [9]. Due to large consumption in industrial use,
hydrazine and derivatives are commonly detected in atmosphere and it creates health
hazardous particularly, in human. Nowadays, numerous methods are available to extensively
detect the concentration of hydrazine including spectrophotometry [9], potentiometry [8],
chemiluminscence [10], fluorimetry [11], gas chromatography-mass spectrometry [12],
columetry [13] and electrochemical method [14]. Compared with other method,
electrochemical technique possesses many advantages including high sensitivity, portable,
cost effective and simple operating procedure. Additionally, electrochemical method is a
good modest substitute for the detection of hydrazine. Bare electrode suffers the
disadvantages of high over potential and low sensing ability (signals) [15]. Commonly, the
electrode surface was modified with nanostructured metal oxides, bimetal oxides, material
possessing high surface area, catalytic activity and conductivity which improve the sensing
signals of the analytes. Metal oxides are one of the investigated nanomaterials in the case of
biotic and industrial applications due to their redox properties and catalytic activity. Mixed
MOs are single phase bi-MOs consist of nickel and cobalt cations which have excellent
electrocatalytic activity due to its synergetic effect in the case of mixed valence state and
complex chemical composition either than the combination of nickel oxide and cobalt oxide
[
16]. The spinel NiCo O is a transition metal oxide which has high theoretical capacity,
2 4
excellent electrical conductivity, easy preparation procedure, easily controllable
morphologies and environmental friendly. NiCo O adopts the pure spinel form in their all
2
4
the nickel (Ni) ions occupies octahedral (Oh) site and Co ions occupy both tetrahedral (Td)
and Oh sites [17,18].
Recently, researchers reported that the NiCo O modified electrode were used for the
2
4
sensing of various bioanalytes and environmental pollutants such as ascorbic acid, uric acid,
dopamine, glucose, lead and cadmium [19–21]. Due to their good electronic conductivity in
3

the form of mixed valances of Ni and Co cations is a beneficial for the fast electron transfer
and it has excellent redox activity than to NiO and Co O [16,22,23]. For the reason that,
3
4
NiCo O has atleast two order of higher magnitude than nickel oxide and cobalt oxide hence,
2
4
it possess potential applications in the field of lithium ion batteries, supercapacitor,
electrocatalyst optoelectronic devices and electrochemical sensor [24–30]. Prathap et al.,
already reported that NiCo O was used for the detection of lindane which provides excellent
2
4
sensing ability than single component oxides [30]. Recently, many methods are there for the
preparation of NiCo O nanomaterials with several structures which include nanoparticles,
2
4
nanowire, nanoneedle, nanosheet, nanoflake and nanoflower [31–36]. The spinel oxides
which has cobalt such as CuCo O MnCo O , NiCo O , ZnCo O MgCo O , etc. exhibit
2
4,
2
4
2
4
2
4,
2
4
great attention due to their physicochemical properties, technological applications, sensors to
electrode materials, catalysts and electrochemical devices [37]. In the present work, a
hydrazine sensor was successfully developed using NiCo O broken nanorod as nanoparticle
2
4
were situated on NR. NiCo O NR/GCE shows pronounced performance for the detection of
2
4
hydrazine in terms of sensitivity, selectivity, reproducibility and stability. There are no
reports available for the detection of hydrazine using NiCo O NR modified electrode.
2
4
2
2
. Experimental section
.1. Materials
CoCl .6H O, NiCl .6H O, urea were purchased from Merck. Disodium hydrogen
2
2
2
2
phosphate (Na HPO ) and monosodium hydrogen phosphate (NaH PO ) purchased from
2
4
2
4
Sigma Aldrich. These chemicals were used to prepare the phosphate buffer supporting
electrolyte solution (PBS) throughout the electrochemical measurements. All the chemicals
were used directly without any other purification. Throughout the study, Milli-Q water with a
resistivity of 18.2 MΩ collected from Mill-Q instrument. The collected Milli-Q water was
used for the experimental solution preparations.
4

2
.2. Synthesis of nanostructured NiCo O
2 4
NiCo O was prepared by hydrothermal method. Initially, 1 g of CoCl .6H O and 0.5
2
4
2
2
g of NiCl .6H O and 1 g of urea were dissolved in 30 mL of deionized (DI) water and stirred
2
2
continuously until pink colour was observed. The transparent pink solution was transferred to
the Teflon lined stainless autoclave. The sample was heated at 130 ºC for 6 hrs. Then, the
solution was cool down to room temperature followed by washing with DI water and ethanol
for more than five times. The sample was dried in vacuum over at 80 ºC for 6 hrs. To end, the
powdered sample was calcined 400 ºC under air atmosphere at a heating rate of 5 ºC/min for
3
hrs. The absolute products were examined and used for electrochemical studies. Similar
procedure was followed for the synthesis of NiO and Co O .
3
4
2
.3. Characterizations
The sample phase purity of the NiCo O material was examined by X-ray diffraction
2
4
(XRD) studies by Cu Kα radiation (λ = 0.15 nm using Bruker D8 ADVANCE X-ray
diffractometer within the range of 10 – 80°. Thermogravimetric analysis (TGA), by
TGA/DTA system (Model of SDT Q600), was performed to explore the material
decomposition temperature in the heating range of 0 °C to 590 °C in an open airspace.
Fourier transform infra-red (FT-IR) spectroscopy for powder sample was accomplished by
KBr pellet method using Bruker Optik GmbH, Germany Model No: TENSOR 27 in the scale
-1
from 400 to 4000 cm . Raman spectroscopic study was examined by He−Ne laser
(wavelength λ = 633 nm) using RENISHAW I laser Raman microscope in order to
understand the chemical behaviour of the material. The chemical composition (EDAX) of the
prepared samples was characterized by scanning electron microscope using TESCAN (Supra
5
5VP) operating at an accelerating voltage of 30 kV. NiCo O was studied by Field emission
2 4
scanning electron microscopy (FESEM) (Oxford instrument). The structural features were
5

2
studied by transmission electron microscopy (TEM; JEOL-JEM 2010 and TecnaiTM G F20,
FEI which operate with an accelerating voltage of 200 kV) The structural features of the
material were observed using high resolution transmission electron microscopy (HRTEM;
2
TecnaiTM G TF20 working at an accelerating voltage of 200 kV). Elemental compositions
and survey scan of the prepared NiCo O material was probed using Mg Kα (1253.6 eV) as
2
4
X-ray source (Thermo Scientific, MULTILAB 2000) via X-ray photoelectron spectroscopy
XPS) using Theta Probe AR-XPS system.
(
2
.4. Electrochemical measurement
The hydrazine sensing performance of NiCo O was studied using differential pulse
2
4
voltammetry (DPV) and cyclic voltammetry (CV) techniques by AUTOLAB PGSTAT302N
using three electrodes system with cleaned glassy carbon electrode (GCE; 3 mm diameter
2
electrode geometric area=0.07 cm ) as working electrode, platinum electrode as counter
electrode and Hg/HgCl /KCl
(saturated calomel electrode (SCE)) was used as reference
(sat)
2
electrode. Highly-pure N gas used to purge into the experimental suspension before
2
initiation of each experiments in order to remove the dissolved O in the PBS (0.1 M, pH =
2
7
). The electrocatalytic behaviour of NiCo O NR/GCE towards oxidation of hydrazine was
2 4
examined at room temperature.
2
.5. Preparation of NiCo O NR/GCE
2 4
Prior to the modification, bare GCE was polished with alumina slurry subsequently
rinsed thoroughly in Mill-Q water and ultra-sonicate the solution with Milli-Q water for the
removal of adsorbed piece on the GCE. For the preparation of NiCo O suspension, 3 mg of
2
4
NiCo O , 0.1 ml Mill-Q water and 0.9 ml of N, N-Dimethyl formamide (DMF) were mixed.
2
4
The prepared mixture was ultra-sonicated for 30 min in order to get the uniform suspension. 3
6

µl of resultant suspension was taken out using micro pipette and drop casted on the finely
polished GCE surface and dried at room temperature.
3
. Results and Discussion
Scheme 2: The electrocatalytic oxidation of hydrazine on the NiCo O NR modified
2
4
electrode.
3
.1 XRD studies
The sample phase purity and crystal structure were confirmed by XRD. Figure 1A
shows the XRD pattern of NiCo O . The calcined NiCo O diffraction peaks are found at 2θ
2
4
2
4
values of ~18.9 º, ~31.1 º, ~36.7 º, ~38.4 º, ~44.6 º, ~55.4 º, ~59.1 º, ~64.9 º and ~77.0 º
which can be indexed with (111), (220), (311), (222), (400), (422), (511), (440) and (533)
planes and all the diffraction peaks are assigned in the cubic phase of NiCo O . The observed
2
4
diffraction pattern of NiCo O matches with ICDD card data with a reference number of 01-
2
4
0
73-1702. The observed XRD pattern was also in good agreement with the earlier reports
38]. No other impurity phase peak was observed which further substantiated the purity of
sample. Thermal stability of NiCo O was studied by TG analysis and the result is shown in
[
2
4
Figure 1B. The first weight loss of about 3 % was observed below 150 ºC which indicating
7

the removal of physically and chemically adsorbed water (H O) content. A major weight loss
2
was observed at the temperature range of ~300 ºC to 370 ºC which is attributed to the
decomposition of Ni2+ and Co . Therefore, 400 ºC was chosen as the calcination
temperature. Figure S1A shows Raman spectroscopy of NiCo O which gives the
3+
2
4
information related to changes in structure and composition. The peak observed 691 cm-1
A ) vibration corresponds to Oh oxygen ions in Co 3p present in Co O , and other two
(
1
g
3
4
-1
-1
peaks were located at 484 cm and 523 cm combined vibration of Td and Oh oxygen atoms
in the lattice. The observed three peaks were more sensitive to Ni 2p ion substituted in Co O
3
4
-1
spinel lattice. Hence, the peak at 691 cm come to be weak and shifted to lower frequencies,
when Ni 2p exchanged in Co 3p in the Oh sites [39]. From the above vibrational peaks
confirms the formation of NiCo O because the peak were observed at 660, 503, 460 and 187
2
4
-1
cm corresponding to A F , E , F and modes of NiCo O [40,41]. Figure S1B shows FT-
1
g, 2g
g
2g
2
4
-1
IR studies of NiCo O . The peaks were observed at 555, 642 cm correspond to M-O
2
4
-1
vibration of NiCo O . The peak at 1366 cm may be associated with physically adsorbed
2
4
-1
CO . The peaks at 1636 and 3484 cm corresponding to vibration mode of absorbed water
2
molecules [42].
Figure 1 (A) XRD and (B) TGA of NiCo O NR.
2
4
3
.2 Morphological studies
8

Figure 2 (A) FESEM; (B) TEM; (C) HRTEM images and (D) SAED pattern of NiCo O
2
4
NR.
The FESEM, TEM and HRTEM analysis were used as a tool to study the surface
morphology, size of the nanostructure and SAED pattern of NiCo O NR. Figure 2A gives
2
4
the low magnification (FESEM) micrographs of the NiCo O NR which shows that the as-
2
4
synthesized material has broken nanorod as nanoparticle were situated on nanorod like
structure. The TEM micrograph of the material was shown in Figure 2B where the NiCo O
2
4
NR micrograph was seen clearly. The observed morphological study, confirmed that the
broken NR were situated on the NR. HRTEM morphology of NiCo O NR was displayed in
2
4
Figure 2C. Figure 2D displays a selected area electron diffraction (SAED) pattern of
NiCo O NR which clearly indicates that the NiCo O NR was formed. The overall HRTEM
2
4
2
4
and SAED results have clearly exposed the information about morphological, size and
structure of the material.
9

Elemental mapping of NiCo O NR is shown in Figure 3A-D. HRTEM micrograph of
2
4
NiCo O NR is shown in Figure 3A and insert of Figure 3A shows elemental mapping of all
2
4
the three elements such as Ni, Co and O. Figure 3 (B-D) clearly display the elemental
mapping of Ni, Co and O, respectively. These figures evidently confirm the formation of
NiCo O NR with well distribution of all three elements. The EDAX analysis also carried out
2
4
using SEM techniques the result is shown in Figure S2. EDAX study also confirms the
presence of Ni, Co and O elements without any other impurities.
Figure 3 (A) HRTEM image; Elemental mapping of Ni (B), Co (C) and O (D) of NiCo O
2
4
NR.
3
.3 X-ray photoelectron spectroscopic studies
1
0

The chemical composition, surface oxidation state and nature of the each component
exist in the NiCo O NR was analysed by XPS study. Figure 4A depicts the XPS survey
2
4
spectrum of NiCo O NR. Survey spectrum clearly identified the existence of Ni, Co and O
2
4
without other impurities. Gaussian fitting method was used for the high magnification XPS
studies of Co 2p, Ni 2p and O 1s spectrum which were shown in Figures 4B-D. Figure 4B
shows that the fitted Ni 2p has two spin orbit doublets (Ni 2p3/2 and Ni 2p1/2 electronic
2
+
3+
configuration) two kind of Ni species has been found which are assigned to Ni and Ni .
Particularly, the peaks at the binding energies of 857.1 eV and 874.7 eV are characteristic of
3
+
2+
Ni and the peaks observed at 855.3 eV and 872.9 eV correspond to Ni [43].
Figure 4 XPS study of NiCo O NR (A) survey scan; (B) Ni 2p; (C) Co 2p and (D) O 1s.
2
4
The fitted Co 2p spectrum was displayed in Figure 4C. The Co 2p has two spin orbit
doublets Co 2p3/2 and Co 2p1/2 electronic configuration at the binding energies of 779.4 eV
2
+
3+
and 794.3 eV corresponding to characteristic of Co and Co , respectively. In this spectrum
two shakeup satellite peaks (identified as “satellite”) were observed along with the two spin
1
1

orbit doublets [44]. The high resolution O 1s showed in Figure 4D exhibits four peaks at
5
5
29.3 eV (denoted as “O 1”), 530.7 eV (denoted as “O 2”), 531.8 eV (denoted as “O 3”) and
32.6 eV (denoted as “O 4”) was consigned to lattice oxygen in NiCo O NR, oxygen in –
2
4
-
OH , the defect sites with low oxygen coordination and chemisorbed/physisorbed water
molecule on the surface respectively [43].
4
4
. Electrochemical studies
.1 Cyclic voltammetry comparison study
Cyclic voltammetric (CV) study was performed to assess the electrocatalytic
behaviour of NiO/GCE, Co O GCE and NiCo O NR/GCE towards electro-oxidation of
3
4/
2
4
hydrazine as shown in Figure S3. This figure clearly displays that the NiCo O NR
2
4
demonstrates higher catalytic activity compared with NiO and Co O Figure 5 illustrates the
3
4.
CV of bare GCE and NiCo O NR/GCE in the absence of hydrazine (curve ‘a’ and ‘b’) and
2
4
the NiCo O NR/GCE in the presence of 3 mM of hydrazine (curve ‘c’) in 0.1 M supporting
2
4
-1
electrolyte at a scan rate of 25 mV s . The modified electrode shows abrupt increase in
oxidation peak current value at much lower potential due to excellent electrocatalytic activity
and high conductive nature of NiCo O NR [16]. These observations intensely spotlight and
2
4
indicate the electrocatalytic behaviour of modified electrode for hydrazine oxidation.
1
2

Figure 5 CV of bare GCE (a); NiCo O NR/GCE in the absence of hydrazine (b) and
2
4
-1
NiCo O NR/GCE in the presence of 3 mM hydrazine (c) at 25 mV s .
2
4
4
.2 pH effect
Figure 6 Effect of pH on peak current and peak potential for the electrooxidation of 2 mM of
hydrazine at NiCo O NR/GCE
2
4
Electrochemical oxidation of hydrazine depends on the solution pH. Due to this
factor, optimisation of pH is essential for the efficient electrocatalytic detection of hydrazine.
Figure 6 shows as the pH of solution increases from 6.0, the peak current increases and peak
potential shifts towards negative direction. The maximum peak current was observed at
pH=7. Hence, pH 7 was chosen as optimum pH in the present study for the detection of
hydrazine.
4
.3 Effect of scan rate
In order to get the information on kinetics of electrochemical oxidation of hydrazine
oxidation on NiCo O NR/GCE, CV measurements were performed in 0.1 M PBS containing
2
4
3
mM of hydrazine at different scan rates as shown in Figure S4A. As shown in Figure S4B,
1
3

the anodic oxidation current increased linearly with square root of scan rate. Also, the slope
of log(peak current) vs log(scan rate) plot (Figure S4C) was found to be nearly 0.5. These
results explicitly show that the electrooxidation of hydrazine was controlled by the diffusion
of electroactive species (i.e., hydrazine) from bulk to the electrode/electrolyte interface and
the rate of electron transfer between the modified electrode and hydrazine is fast [45].
4
.4 Effect of concentration
Figure 7A displays the catalytic performance of NiCo O NR/GCE in various
2
4
concentration of hydrazine. The oxidation peak was observed at 0.36 V and the oxidation
peak current value also increased linearly with increase in the concentration of hydrazine
from 0.07 to 6.0 mM. The corresponding current density vs. concentration plot is shown in
Figure 7B. From the calibration plot, the linear range and sensitivity with correlation
2
-2
-1
coefficient R =0.998 were found to be 0.07 to 1.78 mM, 290 µA cm mM , respectively.
The DPV of NiCo O NR/GCE is shown in Figure 7C which showed that the anodic
2
4
oxidation of hydrazine was observed at 0.25 V with respect to NiCo O NR/GCE. Increasing
2
4
the concentration of hydrazine the oxidation current value increases correspondingly. Figure
D shows a plot of current density vs. the hydrazine concentration, which depicts linear
7
correlation in the range of 0.01 to 2.25 mM. The best linear fitted equation j (µA) =
p
2
4
8.25±1.6 (mM) + 2.56±1.8 with a correlation coefficient R = 0.989. The sensitivity and
-2
-1
detection limit (LOD) were 48.25 µA cm mM and 0.26 µM, respectively. Limit of
qualification (LOQ) (10 × standard deviation/slope) is 0.874 µM. As shown in Table 1, the
analytical factors such as sensitivity, linear range and detection limit concerning the sensing
of hydrazine using NiCo O NR/GCE which is compared with previous reports [46-54] based
2
4
on various sensors. From the above table we can conclude that NiCo O NR is a suitable
2
4
catalyst for the hydrazine sensing.
1
4

jF
Figure 7 (A) CV response of NiCo O NR/GCE in different concentration (0.07, 0.14, 0.21,
2
4
0
.28, 0.35, 0.42, 0.49, 0.56, 0.63, 0.70, 0.77, 0.945, 1.12, 1.67, 1.78, 3.0, 4.0, 5.0 and 6.0
-1
mM) of hydrazine; (B) A plot of j vs. [hydrazine]; Scan rate at 5 mV s ; (C) DPV of
pa
NiCo O NR modified electrode in hydrazine concentration range from 0.01 mM to 2.25 mM
2
4
at the NiCo O NR/GCE and (D) corresponding calibration curve.
2
4
Table 1 Comparison of the analytical performance of NiCo O NR modified electrode
2
4
with other sensors for the electrochemical hydrazine detection.
Sensor techniques
Linear range
µM)
.0-40.0
Sensitivity
Limit of
Reference
(
detection (µM)
6
-
1.1
0.4
20
[46]
[47]
[48]
[49]
Colorimetric sensor
Chromatography
0.5-20.0
500-10000
-
-
0
.17-50
0.17
1
5

Fluorescence
sensor
0.2-9.3
-
0.08
[50]
2
.4-820
-
1.0
40
[51]
[52]
[53]
[54]
0
-1200
75 µA mM-1
-
0
.07-500
0.04
0.07
0
.2-50
136.2 µA mM-
1
cm-2
3
.0-300
-
1.0
[6]
Electrochemical
sensor
270.0 µA mM-
0
0
.25-3400
0.06
[55]
1
cm-2
.05-1600
1.95 µA mM-1
cm-2
0.028
1.4
[56]
[57]
5
.0-1300
449.7 µA mM-
1
cm-2
1
0-2250
48.25 µA mM-
1cm-2
0.26
Present
work
As shown in Table 1, the analytical factors including sensitivity, linear range and
limit of detection of NiCo O NR/GCE are comparable with previous reports [46-54] based
2
4
on various techniques. From the above table we can conclude that NiCo O NR is a suitable
2
4
catalyst for the electrochemical sensing of hydrazine.
4
.5 Real sample analysis
In order to check the practical use of the proposed sensor (NiCo O NR/GCE),
2
4
hydrazine detection was carried out with different water samples. The as-collected water
1
6

samples from Karaikudi city, Tamil Nadu were tested for hydrazine concentration level in
water without further pretreatment by proposed sensor technique. As collected water samples
are found to be free from hydrazine (no peak response/signal) therefore, externally particular
amount of hydrazine was spiked into the collected water samples to make the water
contaminated. Following standard addition method the recoveries were examined by the
catalytic current response corresponding results were displayed in Table 2. The observed
results present reliable recovery with acceptable RSD values which indicates that our
NiCo O NR/GCE could be a potential candidate for the detection of hydrazine in practical
2
4
purpose.
Table 2 Real sample analysis of hydrazine in real water samples by NiCo O
2
4
NR/GCE.
Samples
Detected
Added
Found$
(µM)
147
Recovery RSD
(
µM)
150
120
(%)
98.0
98.6
(%)
4.1
Sample 1
Sample 2
-
-
118
2.6
$
-Standard addition method
4
.6 Reproducibility and stability studies
Figure 8A shows the differential pulse voltammogram response recorded for the
2+
selectivity of hydrazine sensor and several common coexisting interfering substances (Cd ,
2
+
Pb , CaCl , NaNO , KCl, NaSO and MgCl ). The oxidation response was recorded for
2
3
4
2
hydrazine and 20 times higher concentration of interferents. The outcome result depicts that
no obvious current changes were observed for the interference. Hence, the interferents are not
affecting the hydrazine oxidation. From this result we can conclude that our proposed sensor
has excellent selectivity towards the detection of hydrazine. The stability of the sensor was
1
7

examined by cyclic voltammetry study. The NiCo O NR shows good stability for the
2
4
detection of hydrazine by retaining 89 % of the initial response after 50 cycles.
Figure 8 (A) Differential pulse voltammogram current response of NiCo O NR for 0.02 M
2
4
2
+
2+
of hydrazine, 0.4 M of Cd , Pb , CaCl , NaNO , KCl, NaSO and MgCl interferents into
2
3
4
2
0
0
.1 M PBS; (B) Cyclic voltammogram responses of the NiCo O NR to 2 mM hydrazine in
2 4
-1
.1 M PBS (pH=7) for 50 cycles. Scan rate: 25 mV s ; (C) and (D) Differential pulse
voltammogram of repeatability and fabrication reproducibility of NiCo O NR modified
2
4
electrode.
Reproducibility of the NiCo O NR modified electrode for hydrazine determination
2
4
was investigated using DPV. As shown in Figure 8B, the stability (50 cycles) of the modified
-1
electrode was examined in the presence of 2 mM of hydrazine at the scan rate of 25 mV s .
The relative standard deviation RSD was 4 %. Repeatability of the sensor was examined by
repeating the measurement ten times with 5 mins interval of time and the results are
displayed in Figure 8C. The peak current value and peak potential occurred at similar
1
8

position. The RSD value = 3.9 %. It indicates that our NiCo O NR/GCE sensor has excellent
2
4
repeatability for the electrooxidation of hydrazine. Finally, fabrication reproducibility
Figure 8D) (inter electrode) was examined by preparing five modified electrodes with same
(
fabrication condition. All the electrodes gave excellent fabrication reproducibility with the
RSD value of 1.6 %. From the above results we can conclude that our fabricated sensor has
excellent reproducibility, stability, repeatability and selectivity.
5
. Conclusion
In conclusion, NiCo O NR has been synthesized by a simple hydrothermal approach.
2
4
It has been characterized by several physiochemical and electrochemical techniques. XPS
study confirms the formation of NiCo O NR and the oxidation state of the material. The
2
4
HRTEM study reveals that our synthesized material was NR like structure. In view of their
unique structural advantage and electrochemical performance, the possibility of employing
NiCo O NR for the detection of hydrazine was carefully investigated. NiCo O NR shows
2
4
2
4
efficient catalytic performance for the detection of hydrazine with wide linear range and low
limit of detection. The NiCo O NR shows higher electrocatalytic activity for the detection of
2
4
hydrazine oxidation compared with bare electrode. The proposed sensor depicts good
sensitivity (48.25 µA cm-2 mM ), low LOD (260 nm), stability, reproducibility and
selectivity. As-synthesised NiCo O NR was used as a modified electrode in the field of
-1
2
4
analytical uses for the sensing of hydrazine in the analysed water samples. Hence, this work
provides simple and efficient method for the sensing of hydrazine.
Acknowlegment
We thank the CSIR and UGC, New Delhi for the award of a UGC-Senior Research
Fellowship for Mrs.V. Sudha. We wish to acknowledge the Central Instrumentation Facility
(CIF), CSIR-CECRI, Karaikudi for characterization studies.
1
9

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2
4

NiCo O Nanorod: Synthesis and Electrochemical Sensing of
2
4
Carcinogenic Hydrazine
Velayutham Sudha a, b, Sakkarapalayam Murugesan Senthil Kumar a, b, Rangasamy
Thangamuthu a, b*
a
Materials Electrochemistry Division (MED), CSIR-Central Electrochemical Research
Institute (CSIR-CECRI), Karaikudi-630 003, Tamil Nadu, India.
b
Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical
Research Institute, Karaikudi- 630 003, Tamil Nadu, India.
*
To whom correspondence should be addressed. E-mail: thangamuthu@cecri.res.in;
thangamuthu_r@yahoo.co.uk; Phone: +91-04565-241-350.
Abstract
In this work, we explored a simple hydrothermal process to tune the NiCo O
2
4
morphology into well defined rod structure with nanoparticle on it. The as prepered NiCo O
2
4
was characterized by various characterization studies such as XRD, Raman, FT-IR, TGA and
HRTEM. The electrochemical sensing of hydrazine on NiCo O nanorod was examined by
2
4
cyclic voltammetry and differential pulse voltammetry techniques. The NiCo O shows
2
4
excellent electrocatalytic performance for the anodic oxidation of hydrazine. Interestingly,
-1
NiCo O retain considerable current response after 50 cycles at 25 mV s . The Ni and Co in
2
4
bimetal oxides enhanced the current response due to their electroactive sites. The developed
NiCo O not only enhanced the current response of hydrazine but also exhibits good stability.
2
4
-2
NiCo O demonstrates excellent detection limit and sensitivity of 260 nM and 48.25 µA cm
2
4
2
5

-1
mM , respectively. For practical application, NiCo O /GCE was used for the sensing of
2
4
hydrazine in real water samples.
Key words: Hydrazine, NiCo O , Hydrothermal method, Electrochemical sensing.
2
4
Scheme 1. Schematic illustration of NiCo O characterization and electrochemical sensing of
2
4
hydrazine.
1
. Introduction
Hydrazine is widely used in fuel cell, food production, pharmaceutical intermediates,
rocket fuel, pesticides production, blowing agent, dye industries, oxygen scavengers and so
on [1–5]. The Environmental Protection Agency (EPA) identified hydrazine as a poisonous
chemical and the proposed level in the commercial effluents is 1 ppm. Due to volatile nature
of hydrazine, it is easily adsorbed on the human skin and damage the living organism
especially, liver, blood production system and kidney [6]. Vernot et al., found that exposure
to hydrazine for long time causes inhalation problem in different animals. Rats, harmsters,
mice and dog were analysed post exposure of 18 months, 1 year, 15 months, 38 months,
respectively. Male and female rats disclose dose-dependent prevalence of well-disposed nasal
adenomatous polyps and smaller numbers of malignant nasal epithelial tumours after 1 year
of exposure to hydrazine and 18 months post exposure holding [7], DNA damage [8] and
2
6

irreversible deterioration of nervous system [9]. Due to large consumption in industrial use,
hydrazine and derivatives are commonly detected in atmosphere and it creates health
hazardous particularly, in human. Nowadays, numerous methods are available to extensively
detect the concentration of hydrazine including spectrophotometry [9], potentiometry [8],
chemiluminscence [10], fluorimetry [11], gas chromatography-mass spectrometry [12],
columetry [13] and electrochemical method [14]. Compared with other method,
electrochemical technique possesses many advantages including high sensitivity, portable,
cost effective and simple operating procedure. Additionally, electrochemical method is a
good modest substitute for the detection of hydrazine. Bare electrode suffers the
disadvantages of high over potential and low sensing ability (signals) [15]. Commonly, the
electrode surface was modified with nanostructured metal oxides, bimetal oxides, material
possessing high surface area, catalytic activity and conductivity which improve the sensing
signals of the analytes. Metal oxides are one of the investigated nanomaterials in the case of
biotic and industrial applications due to their redox properties and catalytic activity. Mixed
MOs are single phase bi-MOs consist of nickel and cobalt cations which have excellent
electrocatalytic activity due to its synergetic effect in the case of mixed valence state and
complex chemical composition either than the combination of nickel oxide and cobalt oxide
[
16]. The spinel NiCo O is a transition metal oxide which has high theoretical capacity,
2 4
excellent electrical conductivity, easy preparation procedure, easily controllable
morphologies and environmental friendly. NiCo O adopts the pure spinel form in their all
2
4
the nickel (Ni) ions occupies octahedral (Oh) site and Co ions occupy both tetrahedral (Td)
and Oh sites [17,18].
Recently, researchers reported that the NiCo O modified electrode were used for the
2
4
sensing of various bioanalytes and environmental pollutants such as ascorbic acid, uric acid,
dopamine, glucose, lead and cadmium [19–21]. Due to their good electronic conductivity in
2
7

the form of mixed valances of Ni and Co cations is a beneficial for the fast electron transfer
and it has excellent redox activity than to NiO and Co O [16,22,23]. For the reason that,
3
4
NiCo O has atleast two order of higher magnitude than nickel oxide and cobalt oxide hence,
2
4
it possess potential applications in the field of lithium ion batteries, supercapacitor,
electrocatalyst optoelectronic devices and electrochemical sensor [24–30]. Prathap et al.,
already reported that NiCo O was used for the detection of lindane which provides excellent
2
4
sensing ability than single component oxides [30]. Recently, many methods are there for the
preparation of NiCo O nanomaterials with several structures which include nanoparticles,
2
4
nanowire, nanoneedle, nanosheet, nanoflake and nanoflower [31–36]. The spinel oxides
which has cobalt such as CuCo O MnCo O , NiCo O , ZnCo O MgCo O , etc. exhibit
2
4,
2
4
2
4
2
4,
2
4
great attention due to their physicochemical properties, technological applications, sensors to
electrode materials, catalysts and electrochemical devices [37]. In the present work, a
hydrazine sensor was successfully developed using NiCo O broken nanorod as nanoparticle
2
4
were situated on NR. NiCo O NR/GCE shows pronounced performance for the detection of
2
4
hydrazine in terms of sensitivity, selectivity, reproducibility and stability. There are no
reports available for the detection of hydrazine using NiCo O NR modified electrode.
2
4
2
2
. Experimental section
.1. Materials
CoCl .6H O, NiCl .6H O, urea were purchased from Merck. Disodium hydrogen
2
2
2
2
phosphate (Na HPO ) and monosodium hydrogen phosphate (NaH PO ) purchased from
2
4
2
4
Sigma Aldrich. These chemicals were used to prepare the phosphate buffer supporting
electrolyte solution (PBS) throughout the electrochemical measurements. All the chemicals
were used directly without any other purification. Throughout the study, Milli-Q water with a
resistivity of 18.2 MΩ collected from Mill-Q instrument. The collected Milli-Q water was
used for the experimental solution preparations.
2
8

2
.2. Synthesis of nanostructured NiCo O
2 4
NiCo O was prepared by hydrothermal method. Initially, 1 g of CoCl .6H O and 0.5
2
4
2
2
g of NiCl .6H O and 1 g of urea were dissolved in 30 mL of deionized (DI) water and stirred
2
2
continuously until pink colour was observed. The transparent pink solution was transferred to
the Teflon lined stainless autoclave. The sample was heated at 130 ºC for 6 hrs. Then, the
solution was cool down to room temperature followed by washing with DI water and ethanol
for more than five times. The sample was dried in vacuum over at 80 ºC for 6 hrs. To end, the
powdered sample was calcined 400 ºC under air atmosphere at a heating rate of 5 ºC/min for
3
hrs. The absolute products were examined and used for electrochemical studies. Similar
procedure was followed for the synthesis of NiO and Co O .
3
4
2
.3. Characterizations
The sample phase purity of the NiCo O material was examined by X-ray diffraction
2
4
(XRD) studies by Cu Kα radiation (λ = 0.15 nm using Bruker D8 ADVANCE X-ray
diffractometer within the range of 10 – 80°. Thermogravimetric analysis (TGA), by
TGA/DTA system (Model of SDT Q600), was performed to explore the material
decomposition temperature in the heating range of 0 °C to 590 °C in an open airspace.
Fourier transform infra-red (FT-IR) spectroscopy for powder sample was accomplished by
KBr pellet method using Bruker Optik GmbH, Germany Model No: TENSOR 27 in the scale
-1
from 400 to 4000 cm . Raman spectroscopic study was examined by He−Ne laser
(wavelength λ = 633 nm) using RENISHAW I laser Raman microscope in order to
understand the chemical behaviour of the material. The chemical composition (EDAX) of the
prepared samples was characterized by scanning electron microscope using TESCAN (Supra
5
5VP) operating at an accelerating voltage of 30 kV. NiCo O was studied by Field emission
2 4
scanning electron microscopy (FESEM) (Oxford instrument). The structural features were
2
9