Self-assembled Ni/NiO impregnated polyaniline nanoarchitectures: A robust bifunctional catalyst for nitrophenol reduction and epinephrine detection

Article abstract of DOI:10.1016/j.apcata.2021.118028

It is extremely significant to address an urgency toward the development of low-cost and efficient catalysts. Herein, for the first time, we synthesize a novel self-assembled Ni/NiO@PANI by a combustible crystallization followed polymerization method, constructed by impregnation of Ni/NiO in sulfonic acid-doped polyaniline, which can be used as a bifunctional catalyst for electrochemical sensing of epinephrine (EP) and reduction of nitro- to amino-phenol (4-NP to 4-AP). Due to the unique nanoarchitecture, ferromagnetic feature of Ni, anti-corrosion feature of PANI, and synergy between the system (metal/metaloxide@carbon) leads superior performance: it can rapidly reduce the 4-NP pollutant to environmentally friendly 4-AP within 8 min, and easy magnetic recycling; Also, it can be used as a sensor for EP neurotransmitters with a sensitivity, detection, and quantification limit of 0.117 nAμM^?1, 87.2, and 290.71 μM, respectively. This work provides a strategy to design low-cost and superior catalysts applied in catalysis and electrochemical sensor.

Full text of DOI:10.1016/j.apcata.2021.118028

Applied Catalysis A, General 613 (2021) 118028
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Self-assembled Ni/NiO impregnated polyaniline nanoarchitectures: A
robust bifunctional catalyst for nitrophenol reduction and
epinephrine detection
,
,
Manigandan Ramadossa b, Yuanfu Chena c,*, Suresh Ranganathanb, Krishnan Giribabub,
Dhanasekaran Thangavelub, Padmanaban Annamalaib, Narayanan Vengidusamyb **
,
a School of Electronic Science and Engineering and State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology
of China, Chengdu, 610054, PR China
b Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai, 600025, India
c School of Science and Institute of Oxygen Supply, Tibet University, Lhasa, 850000, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
It is extremely significant to address an urgency toward the development of low-cost and efficient catalysts.
Herein, for the first time, we synthesize a novel self-assembled Ni/NiO@PANI by a combustible crystallization
followed polymerization method, constructed by impregnation of Ni/NiO in sulfonic acid-doped polyaniline,
which can be used as a bifunctional catalyst for electrochemical sensing of epinephrine (EP) and reduction of
nitro- to amino-phenol (4-NP to 4-AP). Due to the unique nanoarchitecture, ferromagnetic feature of Ni, anti-
corrosion feature of PANI, and synergy between the system (metal/metaloxide@carbon) leads superior perfor-
mance: it can rapidly reduce the 4-NP pollutant to environmentally friendly 4-AP within 8 min, and easy
magnetic recycling; Also, it can be used as a sensor for EP neurotransmitters with a sensitivity, detection, and
Ni/NiO@polyaniline
Bifunctional catalyst
4-Nitrophenol
Epinephrine
Nanomolar detection.
quantification limit of 0.117 nA
μ
Mꢀ 1, 87.2, and 290.71
μM, respectively. This work provides a strategy to design
low-cost and superior catalysts applied in catalysis and electrochemical sensor.
1. Introduction
endured as the big-challenge due to its sluggish electrochemical kinetics,
poor structural stability, and low electrical conductivity. To improve the
existing properties of earth-abundant catalysts, doping [6,7], surface
functionalization [8], and composite/hybrids [9] are reported with
considerable attention. Among these, the fabrications of semi-
conductor/carbon catalysts with core-shell nanoarchitectured hybrids
have extensive attention and more favorable towards potential appli-
cations including, memory devices, sensors, electrochromic films,
cathode in a battery, catalysis, and magnetic materials [10–16]. For
instance, it is often demonstrated that the existence of carbon support
with transition-metal oxides could provide them superior d-band elec-
tronic DOS (density-of-states) at the Fermi level, which boosts physi-
cochemical features [17–19]. Specially, the addition of acid-doped
conducting polymers (polyaniline) or heteroatom-doped graphene oxide
is significantly enhanced the electrochemical sensing behaviour of
semiconductor materials [20,21]. Hence, we intended to synthesis
In general, noble metals (Au, Ag, and Pt) are being demonstrated as
one of the outstanding catalysts in various organic transformations and
sensing applications. However, they are highly expensive and every so
often suffers from a sudden fall in catalytic activity due to the aggre-
gation and leach of particles, which cause limits in large-scale applica-
tions [1–3]. Hence, more attention to constructing excellent earth
abundant or non-noble metal-based catalysts is required. Especially,
non-noble metal-based multicomponent hybrid systems also drawn wide
attention among scientists owing to its inexpensive, earth-abundant, low
toxic, adsorptive nature, magnetic-center, lattice order/disorder be-
haviours, fast ion/charge diffusivities, intrinsic corrosion resistance,
high surface area, and good conductivity [4,5]. However, the progress of
low-cost, recoverable, and highly efficient bifunctional catalysts for
pollutant reduction and detection using earth-abundant materials has
* Corresponding author at: School of Electronic Science and Engineering and State Key Laboratory of Electronic Thin Films and Integrated Devices, University of
Electronic Science and Technology of China, Chengdu, 610054, PR China.
** Corresponding author.
E-mail addresses: yfchen@uestc.edu.cn (Y. Chen), vnnara@yahoo.co.in (N. Vengidusamy).
https://doi.org/10.1016/j.apcata.2021.118028
Received 23 November 2020; Received in revised form 28 January 2021; Accepted 31 January 2021
Available online 8 February 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

M. Ramadoss et al.
Applied Catalysis A, General 613 (2021) 118028
ultra-small Ni/NiO nanoparticles embedded within conducting poly-
aniline (DBSA-PANI) to attain core-shell nanoarchitectured hybrids for
fast electron-transfer ability with maximal current density.
solution was turned to purple and then bluish during the slow addition
of hydrazinocarbonic acid mixture (aqueous 0.01 M ammonium car-
bonate mixture and 1 mM hydrazine (N2H4) with 1:0.1 ratio). These
intermediate precursors were stirred for half an hour at 60 ◦C in a
magnetic stirrer and then centrifuged. Excessive addition of N2H4 in
hydrazinocarbonic acid mixture might result in a dark blue solution, due
to the excessive solubility of an intermediate precursor. Finally, the
precipitates were washed well with the water-ethanolic mixture, and the
resulting powders were combusted and kept in a muffle furnace in an air
atmosphere at 300 ◦C for 30 min to increase the polycrystalline nature of
In general, all the nitro-aromatics are known to be certainly
poisonous than amino-aromatic compounds. As an utmost refractory
pollutant, 4-NP is regularly coexistent in industrial effluents, whereas its
reduced compound (4-AP) is utilized as
a photo developer,
anticorrosion-lubricant, hair-dyeing compound, analgesic, and antipy-
retic drugs [22]. Hence, the catalytic chemical conversion of 4-NP
hazards into environment-friendly 4-AP is crucial. Likewise, epineph-
rine (EP) is another important neurotransmitter complicated in dynamic
biological processes, in which the detection of EP at physiological con-
ditions is a crucial requirement. EP is commonly known as adrenaline
and this one stimulates glucose production in the biological systems and
changes in the concentration or abnormal levels of EP inside the
digestive scheme leads to neurological disorders and quite a lot of ill-
nesses including in a digestive, mood/sleep, emesis, sexuality, and
appetite [23–25]. Also, epinephrine is an electrochemically active
compound and its electro-kinetics has been measured widely. Hence, the
◦
Ni/NiO. The same precipitates are treated at 500 C for 2 h in an air
atmosphere and argon atmosphere at 300 ◦C for 90 min to obtain NiO
and Ni0 samples, respectively.
Caution: The obtained precipitates are found to be explosive and too
vigorous with nickel nitrate in its place of nickel acetate.
2.3. Synthesis of the Ni/NiO@PANI nanohybrid
The Ni/NiO@PANI, NiO@PANI, and DBSA-PANI nanohybrids were
synthesized by a self-assembled oxidative polymerization method in
presence of DBSA. The experimental procedure is given as follows: 0.02
mL of aniline monomer was mixed with the solution of DBSA, dissolved
in 150 mL DD water to form an emulsion of aniline-DBSA complex. The
reaction is maintained at 5 ◦C under stirring for 45 min. Then aqueous
ammonium persulphate (APS) as an oxidant to the above reaction
mixture was mixed well before the addition of the quantitative amount
of Ni, NiO, or Ni/NiO nanoparticles (0.2 g) in aniline-DBSA/APS (the
ratio of aniline, DBSA, and APS is 1:0.2:2 w/w), which was left overnight
to obtain blackish-green colloids of Ni@PANI, NiO@PANI or Ni/
NiO@PANI nanohybrids, respectively. The products were washed with
DI water three to five times and then dried under a vacuum for a day.
The same synthetic procedure was followed to obtain DBSA-PANI (the
ratio of aniline, DBSA, and APS is 1:0.2:2 w/w) in the absence of NiO
and Ni/NiO.
catalytic
chemical
conversion
of
4-NP
hazards
into
environment-friendly 4-AP and detection of EP in clinic medicine at
practical conditions is a crucial requirement. A key challenge in a
fabrication of metal/oxide/carbon core-shell hybrids/composite mate-
rials is particle size and homogenous distribution, which requires
multi-step profound complex procedures. Herein, among several
possible fabrication pathways, we chose a relatively facile combustible
redox crystallization reaction and subsequent oxidative polymerization
technique for Ni/NiO@PANI hybrids synthesis with uniform size dis-
tribution; besides, embedding within polymer may arrest the post
oxidation of nickel, thus advances the physicochemical properties and
excellent stability of as-synthesised catalysts. Herein, for the first time,
by using a combustible crystallization followed by the oxidative poly-
merization method, a novel sulfonic acid-rich PANI covered Ni/NiO
core-shell nanohybrid is synthesized for the catalytic conversion of 4-NP
and electrocatalytic detection of EP. The most intriguing feature could
be microstructure and homogeneity in Ni/NiO@PANI core-shell struc-
ture and the synergy arises between acid-base sites, which boosts the
catalytic performances. Due to its unique core-shell nanoarchitecture,
the ferromagnetic feature of Ni, and the anti-corrosion feature of PANI,
the Ni/NiO@PANI catalyst show superior performance. The physico-
chemical properties of Ni/NiO@PANI, the catalytic reduction of nitro- to
aminophenol, and electrocatalytic detection of EP have been systemat-
ically investigated.
2.4. Characterization techniques
The crystalline and structural data of the nanohybrids were deter-
mined with Rich-Siefert 3000 diffractometer - Cu Kα (λ =1.5406 Å)
1
radiation. HR-XPS measurements were done using GMBH, Omicron
nanotechnology, 1483 eV monochromatic Al Kα (XM-1000) source,
Germany, operated at 0.3 kW, 15 kV, emission current-20 mA, and a 50
nbar base pressure. The survey measurements were accomplished with a
step size - 0.5 eV along with pass energy - 50 eV. The high-resolution
emission scans were made with 30 meV as the step size and pass en-
ergy - 20 eV with 3 sweep segments. The DRS-UV–vis absorbance was
recorded by the Perkin-Elmer spectrophotometer and FTIR spectra were
recorded with the Perkin-Elmer Infrared spectrometer. Magnetic spectra
were executed at RT using a Lakeshore-7404 vibrating-sample-magne-
tometer. The morphologies of DBSA-PANI, Ni/NiO, and Ni/NiO@PANI
nanohybrids were analyzed by FE-SEM, HITACHI SU6600 field emission
EDAX coupled SEM, and TEM, FEI TECNAI using T-30, G2 model with
200 kV accelerating voltage.
2. Experimental section
2.1. Materials and agents
Nickel (II) acetate, aniline (C₆H₇N), ammonium carbonate
(CH₈N₂O₃), ethylene glycol (EG, C₂H₆O₂), hydrazine (N2H4), 4-nitrophe-
nol (4-NP), dodecylbenzene sulfonic acid (DBSA, C18H30O3S) and
borohydride (NaBH4) were bought from SRL India Ltd. Epinephrine (EP)
was purchased from Sigma-Aldrich. For the making of buffer (pH (1–9)),
sodium dihydrogen phosphate (NaH2PO4), disodium hydrogen phos-
phate (Na2HPO4), sodium acetate (C₂H₃NaO₂), acetic acid (C₂H4O₂),
hydrochloric acid (HCl), sodium hydroxide (NaOH), potassium chloride
(KCl), and ethanol were used (Merck) and deionized (DI) water was used
as a solvent for all the experiments. Before the preparation of polyaniline
(PANI), aniline was freshly distilled.
2.5. Electrochemical sensing of epinephrine (EP)
The electrochemical sensing measurements were implemented at RT
using a CHI1103A three-electrode cell electrochemical-workstation, in
which a sat. A calomel electrode (SCE) was a reference, a platinum
electrode was a counter and a glassy-carbon electrode (GCE) was a
working electrode throughout the experiments. The CV’s were achieved
at a fixed potential window (various scan rates of .01 to 0.5 Vsꢀ 1) in N2
saturated 0.1 M buffer solutions. Our previous drop-cast assisted elec-
trode fabrication methods were used for the catalyst loading process on
the electrode. Briefly, the ultrasonicated catalyst suspensions were made
by dispersing 2 mg of electrocatalysts in 1.5 mL of ethanol and 0.5 mL
2.2. Ni/NiO nanomaterial
The Ni/NiO nanomaterial was prepared by a facile combustible
redox crystallization strategy. Briefly, the aqueous 0.01 M Ni2+ and 10
% ethylene glycol solutions were gradually added into 1 mM hydrazine:
0.01 M ammonium carbonate. The initial colour of the pale-green
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M. Ramadoss et al.
Applied Catalysis A, General 613 (2021) 118028
–
water. From this ultrasonicated catalyst suspension, 5
μ
L was dropped
aliphatic C H starching/bending modes, respectively [2,26]. The peaks
on the surface of highly-polished GCE and dried in an oven. Various N2
saturated pH (1–9) solutions were adjusted with the aid of pH meter
using the below-prepared stock solutions, 0.1 M KCl and HCl for pH (1–2
buffered-saline), 0.1 M CH3COOH, CH3COONa (ABS, pH 3–5 acetic acid
buffered-saline), and 0.1 M Na2HPO4, NaH2PO4, and 0.1 M NaOH (PBS,
phosphate-buffered saline pH > 6) into1 L of DI water.
owing to PANI are clearly described, but the peak corresponds to νNi
–
stretching vibration of NiO are less clear (Fig. S2), in contrast, thOe
presence of Ni/NiO into a Ni/NiO@PANI is well-identified through XRD
studies. Following this, the FT-IR bands of NiO@PANI and Ni/NiO@-
PANI composites are almost similar with significant peak suppression
with a slight change in peak position. Hence these corresponding data
confirm the formation of Ni/NiO impregnated sulphonic acid-rich pol-
yaniline nanocomposites, which are in good-agreement with PXRD, and
XPS (Figs. 1 and 2).
2.6. Catalytic conversion of 4-NP
The XPS survey spectrum of Ni/NiO@PANI composite nano-
structures (Fig. 2a) shows that the composite is composed of Ni, C, N, O,
and S elements merely. It can be noted that the Ni/NiO@PANI (Fig. 2b)
shows multiple split peaks (Ni 2p3/2) at 853.1, 855.3, and 860.4 eV,
which corresponds to a metallic Ni◦, oxidized form of Ni-O and shakeup
satellite peaks [29]. The deconvoluted Ni 2p3/2 core level emission peak
is centered at about 855.3 eV with corresponding superimposed multiple
splits, Ni 2p1/2 at about 873.1 eV, and a spin-energy separation of ΔE =
17.8 eV, as shown in Fig. 2b. The XPS core level spectrum of the O 1s
(Fig. 2c) shows a peak at 528.6 V related to adsorbed oxygen (O2),
530.35 eV related to the NiO lattice oxygen (Ni-O), while a broad peak is
observed at 532.6 eV can be associated to a surface hydroxyl group
(-O-H) [30]. The deconvoluted C 1s core-level spectrum shows three
An aqueous dispersion of ultrasonicated catalyst (10 mL containing 4
mg) was added with aqueous 0.3 M NaBH4, then the suspensions were
stirred for a few minutes at RT. 4-nitrophenol (3 mM) was mixed to the
suspensions, which was agitated until a bright-yellow solution colour
progressively turn to colourless (Figs. S1, S2). The reduction reaction
progress had been monitored with
spectrophotometer.
a
UV–vis absorption
3. Results and discussions
3.1. Structural studies
The diffraction patterns recorded for PANI, NiO@PANI, and Ni/
NiO@PANI are shown in Fig. 1a. NiO@PANI exhibits diffraction peaks
at 37.3◦, 43.3◦, and 62.9◦ corresponds to (111), (200), and (220) planes,
respectively of cubic NiO (JCPDS No. 065-5745). The Ni/NiO@PANI
shows additional diffraction peaks at 44.7◦ and 51.8◦ due to (111), and
(200) crystal planes, respectively of cubic structured Ni0 (JCPDS No;
065-0380) along with peaks of NiO. In the PXRD patterns of PANI,
NiO@PANI, and Ni/NiO@PANI, broad diffraction peaks (20ꢀ 30◦) has
risen due to parallel polymer chain, but diffraction peaks correspond to
the polymer at Ni/NiO@PANI are less clearly observed [26]. This may
be due to the dominant polycrystalline Ni/NiO diffraction peaks. How-
ever, the diffraction peak intensities of Ni/NiO@PANI are look sup-
pressed could be due to the shell-like formation of the PANI layer on the
core inorganic particles.
–
peaks (Fig. 2d). The peak at 284.6 eV is related to the C C, the peak at
–
285.9 eV is related to the CN group and the peak at 287.6 eV can be
–
–
–
recognized as the O CO group [29,31]. The deconvoluted XPS core
level spectrum of N 1s exhibits 3 peaks (Fig. 2e). The peak, 399.3 eV is
–
–
associated with amine (
N ) and a 401.9 eV peak is associated with
–
-N•+- (both polarons and bipolarons) at the binding energy range of N1
s. The 403.8 eV peak is related to the protonated amine, which is
observed at higher binding energy because of the strong electron
localization associated with sp3 bonded poor-conjugated sites. The
high-resolution XPS spectra of the SO24ꢀ core level peak is centered at
about 169.65 eV (Fig. 2f), which is observed due to the associated DBSA
within the PANI composite [32].
Fig. 1b shows the FTIR spectra of PANI, NiO@PANI, and Ni/
NiO@PANI composite structures. The FT-IR spectrum of hybrid shows
characteristic bands as shown in Fig. 1b corresponds to DBSA-doped
PANI. In particular, the band recognized at ≈ 1320 cmꢀ 1, and the
band at ≈ 1580 cmꢀ 1 attribute to the presence of benzenoid-quinoid
3.2. The formation mechanism of Ni/NiO@PANI composite
nanostructures
The FE-SEM and TEM images are presented in Fig. 3a, b and c, d,
respectively correspond to PANI (Fig. 3a, c) and Ni/NiO@PANI (Fig. 3b,
d) nanostructures. It can be established that the three-dimensional
structures are constructed with tube-shaped polymers with inorganic
particles. The homogenous distribution of a particle tells a possibility of
extraordinary interaction occurs between Ni/NiO particles and PANI.
Similarly, the FE-SEM and TEM images of Ni/NiO nanostructures are
shown in Figs. S3a and S3b-c, respectively. The FESEM image, EDAX
elemental analysis, and composition of as-synthesized PANI and
NiO@PANI are shown in Fig. S4 and S5, which confirm the presence of
–
–
–
–
(aromatic Q B) rings (stretching C N, and CC vibrations), and the
–
peak at ≈ 1265 cmꢀ 1 is recognized associated to the emeraldine state
bipolaron polymer structure [26,27]. Additionally, high-intensity peaks
between ≈ 1005ꢀ 1150 cm-1 are closely related to the respective sym-
–
–
metric sulphonic (S O)/asymmetric SO
stretch vibrations, which
–
clearly confirm the presence of sulphonic acid-rich polyaniline [2,28]. In
addition, The FTIR spectrum of Ni/NiO@PANI shows broad peaks at
3380, and 2980ꢀ 2870/870 cmꢀ 1 agree with NH stretching mode, and
–
Fig. 1. a) XRD patterns and b) FTIR spectra of PANI, NiO@PANI, and Ni/NiO@PANI.
3

M. Ramadoss et al.
Applied Catalysis A, General 613 (2021) 118028
Fig. 2. (a) XPS survey spectrum, (b-f) Ni 2p, O 1s, C 1s, N 1s, and S 2p core-level spectra of Ni/NiO@PANI, respectively.
N, C, S, Ni, O, and the existing elements quantitative weight percentages,
respectively. These obtained results are in good agreement with FTIR
(Fig. 1b), and XPS (Fig. 2) studies, which clearly confirm the formation
of hierarchical sulphonic acid-rich polyaniline nanostructures.
experimental results detailed above.
3.3. Physicochemical behaviours of Ni/NiO@PANI
We can see the carbon tubes are decorated with spherical particles. It
confirms that the Ni/NiO attached to the surface of the PANI layer
nanoparticles, and the PANI layer is relatively thicker. As per attained
results and discussion, the formation of Ni/NiO@PANI composite
nanostructures can be explained as follows: initially, based on reports
[23,26], the possible formation of Ni/NiO (combustible redox synthe-
sized) could be explained through the thermal growth of N-N-rich
high-energy nickel hydrazinocarboxylate precursor, which is explosive
at high temperature; secondly, Ni/NiO core and PANI outer shell are
designed as Fig. S3. In brief, when the DBSA/ANI/APS is added into the
dispersion containing Ni/NiO nanoparticles, the dodecyl benzene
sulphonates are adsorbed uniformly on the surface of the nanoparticles.
Moreover, the aniline monomer is reacted in the above DBSA solution,
which becomes protonated anilinium cation (ANI+), thus uniformly
adsorbed on a Ni/NiO nanoparticles (Ni/NiO-DBSAδꢀ -ANIδ+) owing to
prevalent electrostatic attraction. Herein, the reaction of ammonium
persulphate (APS) as a strong oxidant (Ni/NiO-DBSA-ANI/APS), the
polymerization is initiated, which outcomes the conductive sulfonic
acid-rich polymer layer on the Ni/NiO particles thus results in
Ni/NiO@PANI nanostructures. Following this, the combustible redox
crystallized followed by oxidative polymerization mechanism (self--
assembly solution-solid-solid growth) is proposed based on our
The UV–vis absorption spectra of NiO@PANI, and Ni/NiO@PANI
nanostructures are shown in Fig. 4a. The absorption spectrum shows two
absorption bands at approx. 390 and 690 nm can be assigned to carbon
–
support (PANI) and (Ni O), respectively [23,27,33]. The absorption
band approx. 390 nm corresponds to π-π* transition and band at approx.
550ꢀ 750 nm corresponds to benzenoid rings into quinoid ring transi-
tions [32]. The absorption spectrum of Ni/NiO@PANI compared to that
of NiO@PANI shows that the corresponding peak positions of
benzenoid-quinoid ring transition are shifted to lower wavelengths. The
NiO@PANI and Ni/NiO@PANI composites result in narrowed transi-
tions with wavelength shift, where the electron transitions become
easier, thus suppresses the energy gap [34]. The absorption band of
Ni/NiO@PANI composite nanostructures resembles the NiO@PANI ab-
sorption band. The bandgap, Eg is calculated from Tauc’s (Tauc’s
equation:
α
h
υ
= A(h
υ
-Eg)2) plot. The value of h
ν extrapolated to zero
absorbance (
α
= 0) gives the direct allowed transition (bandgap) be-
tween valence to conduction band excitation. The order of Eg value is
NiO@PANI > Ni//NiO@PANI, as shown in Fig. 4b. The synthesized
Ni//NiO@PANI results in a spectral shift to its nanoscale particles of
Ni//NiO corresponds to its quantum confinement effect.
The Mꢀ H curves of NiO@PANI and Ni/NiO@PANI measured at 298
K are shown in Fig. 4c. As shown in Fig. 4d, the Ni/NiO@PANI
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M. Ramadoss et al.
Applied Catalysis A, General 613 (2021) 118028
Fig. 3. a, b) SEM and c, d) TEM images of PANI, Ni/NiO@PANI, respectively.
Fig. 4. a) DRS UV–vis absorption spectra b) Tauc’s plot, c) Mꢀ H curves and d) magnified illustration of NiO@PANI and Ni/NiO@PANI.
5

M. Ramadoss et al.
Applied Catalysis A, General 613 (2021) 118028
composite shows a ferromagnetic behavior due to the presence of
magnetic Ni metal sites along with NiO. However, the NiO@PANI shows
more or less a paramagnetic behavior, which is mainly contributed by
the coreless NiO and PANI. The cyclic voltammogram of Ni/NiO@PANI
in 0.1 M H2SO4 is presented in Fig. 5a. The voltammograms are char-
acterized by three anodic and corresponding three cathodic peaks,
indicating electroactive regions with three well-defined redox electro-
chemical systems [35]. The initial redox-reaction (first redox peak at
approx. 0.49 V) is associated with the transition between fully-reduced
leucoemeraldine-semiconductor results in half-oxidized emer-
aldine-conductor, whereas the second redox peak at approx. 0.38 V is
associated with oxidized products free in the polymeric-matrix. A third
transition peaks at approx. 0.09 V indicates the redox reaction of
emeraldine-pernigraniline species as an oxidized form of PANI. EIS
spectra circuit fit endorses the higher charge/ion transport behaviour of
Ni/NiO@PANI/GCE compared to bare GCE, as shown in Fig. 5b.
The pH effect on a voltammogram of Ni/NiO@PANI is studied be-
tween pH 1–9. As seen in Fig. 5c, the respective peak potentials of the
redox couple are shifted more negatively with respect to increasing pH,
which is triggered by the initial protonation of a sulfonic group func-
tionalized amines in a polymer chain. These charge transfer redox
couples are not detected at pH values higher than pH 6.0. The following
conclusions (the lack of electrochemical activity and being of electro-
chemical behaviour can be found at more-negative potential while
moving toward basicity) may be incidental based on behaviour of PANI
peak couple at higher pH solutions The CV’s of sulfonic acid-rich PANI in
0.1 M H2SO4 at various scan rates (10ꢀ 500 mV/s) are displayed in
Fig. 5d. As expected, it can be seen that the respective peak potentials
and its corresponding current values are varied with a change in scan
rates. These results indicate that the synthesized PANI nanotubes are
electroactive and the charge/ion transfer process is coupled to the
diffusion processes, to be precise, the hopping assisted electron trans-
portations are along the sulfonic group functionalized conjugated
polymeric nanostructures.
3.4. Catalytic reduction of 4-NP to 4-AP transformation
To evaluate the excellence of Ni/NiO@PANI catalyst, the potential
catalytic reduction reaction of 4-Nitrophenol (4-NP) is confirmed. The
UV–vis absorption spectra of the 4-NP reduction reaction are displayed
in Fig. 6. The schematic catalytic reaction of 4-NP reduction is explained
in Fig. S2 and the proposed mechanistic paths are almost similar to
previous mechanisms [36]. In general, 4-NP favorably interacts with the
metal surface in the presence of excess NaBH4 and undergoes a reduc-
tion of nitro groups. Here, NiO, Ni, and Ni/NiO along with
polymer-supported materials are used as the catalyst for 4-NP reduction.
Catalytic reduction of 4-NP starts with color intensification as yellow to
dark yellowish, in a presence of catalyst and NaBH4, which notifies a
formation of an intermediate system (4-nitrophenolate-BHꢀ4 ) as shown
in Fig. 6a. As evidenced, in presence of Ni/NiO@PANI, while reacted
with an aqueous NaBH4, an absorption band (~400 nm) associated with
4-nitrophenolate-BHꢀ4 , the system is decreased with the increase in
absorbance at 298 nm (Fig. 6b), which infers that desorption of 4-AP as a
reaction product from a catalyst surface [20,36]. In accordance with the
results, the adsorption-desorption followed catalytic reduction reaction
proceeds rapidly with the time of about 8 min as presented in Fig. 6c.
Besides, the Ni/NiO@PANI facilitates outstanding catalytic performance
than the other obtained catalysts (PANI, Ni@PANI, and NiO@PANI).
The kinetic plot (ln (At/A0) = ꢀ kt, where k - constant, A - relative ab-
sorption intensity and t - time) of 4-NP reduction has showed the
pseudo-first-order reaction kinetics with an order of k value: PANI
(0.002 s-1) NiO@PANI (0.024 s-1) < Ni@PANI (0.236 s-1) < Ni/NiO@-
PANI (0.31 s-1), as displayed in Fig. 6d. The greater k value suggests the
excellent catalytic activity of materials, which suggests the significant
role of Ni particles at NiO@PANI towards the 4-NP reduction, and also
these results are comparable with reports. The observed excellent cat-
alytic reduction efficiency (Fig. 6e) is owing to its
synergetic/interfacial-coupling effect between Ni/NiO particles and
PANI backbone. More interestingly, the change in color (yellowish to
colorless) has been noticed due to the conversion of 4-nitrophenol to its
Fig. 5. a) CV and b) EIS of bare and Ni/NiO@PANI. c) Effect of pH (1-9) on peak potentials and peak currents. d) Cyclic voltammograms of Ni/NiO@PANI in 0.1 M
H2SO4 at various scan rates.
6

M. Ramadoss et al.
Applied Catalysis A, General 613 (2021) 118028
Fig. 6. a-c) Time-dependent UV–vis absorption spectra of before, during, and after the 4-NP to 4-AP catalytic reduction, d) Ln (At/A0) versus time kinetic plot, e)
reduction efficiency, f) before catalysis g) after catalysis and h) catalytic reusability test of 4-NP reduction.
corresponding aminophenol as observed in Fig. 6f, g. After catalysis, the
magnetic recovery highlights the advantages of Ni/NiO@PANI to
determine the catalytic reusability and long-term behaviors. The
magnetically recovered catalysts are washed well using water followed
by acetone, while applied for reusability test; the catalyst is performing
excellent until five cycles as shown in Fig. 6h. Herein, excellent
long-term reusability of Ni/NiO@PANI could be achieved by a PANI
surface, which prevents the surface fouling and agglomeration of
Ni/NiO particles (the catalytic centers for reduction reaction).
3.5. Electrocatalytic sensing of epinephrine
Fig. 7 depicts the CVs of 1 × 10ꢀ 4 M EP (0.1 M acetate buffer saline,
ABS, pH = 5) at bare GCE, NiO@PANI/GCE, and Ni/NiO@PANI/GCE
with a scan rate of 50 mVsꢀ 1. Ni/NiO@PANI/GCE represents well-
defined redox peaks correspond to the dissolved neurotransmitter (EP)
with the maximal current than NiO@PANI/GCE, while bare GCE (Fig. 7)
shows negligible current with a broad oxidation peak current. Based on
the above discussion and the previous report [37–39], the electro-
catalytic redox mechanism of EP at Ni/NiO@PANI can be explained as
shown in Fig. 7.
Figs. 8a, and S6a show an influence of the scan rate on an
Fig. 7. CVs of 1 × 10ꢀ 4 M EP in 0.1 M ABS at 50 mVsꢀ 1 and pictorial view of redox behaviour of EP at Ni/NiO@PANI/GCE.
7

M. Ramadoss et al.
Applied Catalysis A, General 613 (2021) 118028
Fig. 8. a) CVs with different scan rates and b) DPV’s with different concentrations vs. current of EP at Ni/NiO@PANI/GCE.
electrocatalytic response of 0.1 × 10ꢀ 3 M EP at Ni/NiO@PANI, with
various scan rates of a) 25, b) 50, c) 75, d) 100, e) 125, f) 150, g) 175, h)
200, i) 250, j) 300, k) 350, l) 400, m) 450, and n) 500 mV/s, respectively
in 0.1 M ABS solutions. The peak current corresponds to anodic oxida-
tion potential is moved positively and cathodic reduction is moved
negatively. Figs. 8a and S6 demonstrate that the peak currents are lin-
4. Conclusions
In this research work, to construct a catalytically active Ni/NiO ho-
mogeneously impregnated PANI nanoarchitecture as a robust bifunc-
tional catalyst, a combustible redox crystallization followed by oxidative
polymerization approach is followed. This obtained potentially desir-
able bifunctional Ni/NiO@PANI catalyst is showing nanomolar detec-
early proportional to a square root value of the scan rates (
ν
1/2) between
25ꢀ 500 mV sꢀ 1. Collectively, all these results recommend that the Ni/
NiO@PANI electrocatalyst has efficient electrocatalytic and fast charge
transfer activity towards the detection of EP and these findings are
consistent with the reports [40–45]. Likewise, it is also suggesting that
the electrode processes relate to a diffusion-controlled reaction
(Fig. S6a). The number of charge transfer involved in these processes can
tion ability toward EP with a nanomolar sensitivity of 0.117 nA
μ
Mꢀ 1
,
detection limit of 0.087 nM and quantification limit values of 0.29 nM,
similarly and showing excellent catalytic reduction toward nitro- to
amino-phenol (4-NP to 4-AP) catalysis within 8 min, which is only
composed of nonprecious abundant elements. Hence, the Ni/NiO@PANI
hybrid active site with unique core-shell nanoarchitecture, tuned elec-
tronic properties, ferromagnetic feature of Ni, anti-corrosion feature of
PANI, and synergy between core-shell systems is yielded owing to
rational construction of metal/metal oxide/carbon hybrid interfaces
(Ni/NiO@PANI), which overcomes significant aggregation, enhanced
ion/charge transport and magnetically recovery during catalysis, which
holds promise as an excellent bifunctional catalyst. Further functional-
ization or changes would be essential and responsible for the multi-
functional activity, including sensor platform, magnetically recoverable
catalyst, and storage devices.
be obtained by this equation: Number of electrons = 4IRT/FQυ,
accordingly, the total number of e- transfer involved in this EP redox
process is 2, whereas, Ip is a peak current, R is a gas constant, Q is a
charge, F is a Faraday, and
υ is scan rate).
The linear regression equation for an EP redox process: Ia = 1.121 ν1/2
(m Vs
)
ꢀ 1 1/2 - 3.720 (R2 = 0.983) (Fig. S6a).
The linear equation: I(
(Fig. S6b).
μA) = 0.0117 [EP] (
μ
M) + 0.2533 (R2 = 0.9987)
Fig. 8b displays the DPV plot of current concerning the EP concen-
tration at Ni/NiO@PANI/GCE in 0.1 M ABS. The peak current corre-
sponds to the EP in between the range of 0 to 0.6 V potential windows
are increased gradually with an EP gradual addition. The calibration
plot for the determination of EP is plotted as shown in Fig. S6b, the
linear range is found between 1–100 × 10ꢀ 6 M with nanomolar sensi-
CRediT authorship contribution statement
Manigandan Ramadoss: Conceptualization, Methodology, Investi-
gation, Software, Validation, Writing - original draft. Yuanfu Chen:
Supervision, Project administration, Funding acquisition, Writing - re-
view & editing. Suresh Ranganathan: Writing - review & editing,
Software, Validation. Krishnan Giribabu: Formal analysis, Validation.
Dhanasekaran Thangavelu: Resources, Software. Padmanaban
Annamalai: Investigation, Resources. Narayanan Vengidusamy:
Conceptualization, Supervision, Writing - review & editing.
tivity of 0.117 nA
μ
M-1. The obtained detection limit and quantification
limit values are 0.087 nM and 0.29 nM, respectively. The results suggest
that the Ni/NiO@PANI electrocatalyst on GCE possesses an ultrahigh
nanomolar sensitivity towards oxidation of EP. Overall, the significant
shift in the overall anodic oxidation and cathodic reduction peaks of Ni/
NiO@PANI/GCE than NiO@PANI/GCE, toward the less positive region
with respect to 1 × 10ꢀ 4 M epinephrine, might be attributed to its multi-
component synergetic/coupling effect between semiconductor (Ni/NiO)
and conductive carbon (PANI). This electrocatalytic sensing property is
mainly attributed to the available abundant Ni/NiO electro-active cat-
alytic sites within PANI composite nanostructures. Finally, to check the
stability of Ni/NiO@PANI/GCE, the electrode is stored for a month at 5
◦C at after the detection test, since the current only reduced by approx. 5
%, which could be attributed to the excellent protection effect of PANI
against the desorption or decomposition of Ni/NiO electroactive cata-
lytic sites on the GCE surface. Hence, these results suggest that the
proposed Ni/NiO@PANI electrochemical sensor is a robust and stable
electrode for the nanomolar detection of Ep, which identifies a new class
of attractive modified electrodes for chemical sensors.
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
The research was supported by the National Natural Science Foun-
dation of China (Grant No. 21773024), Sichuan Science and Technology
program (Grant Nos. 2020YJ0324, 2020YJ0262), Reformation and
Development Funds for Local Region Universities from China Govern-
ment in 2020 (Grant Nos. ZCKJ 2020-11) and China Postdoctoral Sci-
ence Foundation (Grant No. 2019M653376).
8

M. Ramadoss et al.
Applied Catalysis A, General 613 (2021) 118028
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Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the
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