Rational design of marigold-shaped composite Ni3V2O8flowers: A promising catalyst for the oxygen evolution reaction

Article abstract of DOI:10.1039/d0nj01596b

Advancement of double spinel-type blended metal oxides and designing heterostructured nanomaterials with assorted shapes using two different metals remains an active area of research. In this work, we designed marigold flower-shaped Ni3V2O8 structures by simple and facile thermal decay of Ni(ii) 8-hydroxyquinoline and VO(ii) 8-hydroxyquinoline molecular precursors using a universal and green solvent, water. The marigold flower-shaped structure of Ni3V2O8 was characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Finally, we tested these marigold flower-shaped Ni3V2O8 structures for their electrochemical performance, such as the oxygen evolution reaction (OER). The results demonstrated that the marigold flower-shaped Ni3V2O8 structure has superior catalytic activity (overpotential of 328 mV at 10 mA cm-2 and a Tafel slope of 61 mV dec-1) compared with a physical mixture of V2O5 and NiO (overpotential of 496 mV at 10 mA cm-2 and a Tafel slope of 158 mV dec-1), pure NiO (overpotential of 553 mV at 10 mA cm-2 and a Tafel slope of 205 mV dec-1) or V2O5 (overpotential of 668 mV at 10 mA cm-2 and a Tafel slope of 314 mV dec-1) in alkaline medium. This higher OER activity of the marigold flower-shaped Ni3V2O8 structure strengthens its adoption as a potential candidate in the field of energy storage and conversion systems. This journal is

Full text of DOI:10.1039/d0nj01596b

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2020, DOI: 10.1039/D0NJ01596B.
Volume 42
Number
March 2018
Pages 3963-4776
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Yellowish and blue luminescent graphene oxide quantum dots
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Rational design of marigold shape composite Ni V O flower: a promising
catalyst for oxygen evolution reaction
3
2
8
a
a
b
c
a
Rathindranath Biswas , Avinava Kundu , Monochura Saha , Vishaldeep Kaur , Biplab Banerjee ,
Received 00th January 20xx,
Accepted 00th January 20xx
a
d
d
c
a
Rajendra S. Dhayal , Ranjit A. Patil , Yuan-Ron Ma , Tapasi Sen , and Krishna Kanta Haldar *
DOI: 10.1039/x0xx00000x
Advancement of double spinel-type blended metal oxide and designing assorted shaped heterostructured nanomaterials
of two different metals in course of action remains a wild zone of research. In this work, we designed marigold flower
shaped Ni
hydroxyquinoline molecular precursor using universal and green solvent, water. The marigold flower shaped structure of
Ni was characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Field-emission scanning
electron microscopy (FE-SEM) and Transmission electron microscopy (TEM). Finally, we have tested these marigold flower
shaped Ni structure for electrochemical performance, such as Oxygen Evolution Reaction (OER). The results
demonstrated that marigold flower shaped Ni structure has superior catalytic activity (overpotential 328 mV at 10
mA cm and Tafel slope of 61 mV dec ) compared with physical mixture of V and NiO(overpotential 496 mV at 10 mA
3 2 8
V O structure by a basic and effortless thermal decay of Ni(II) 8-hydroxyquinoline and VO(II) 8-
3 2 8
V O
3 2 8
V O
3 2 8
V O
−
2
−1
2
O
5
−
2
−1
−2
−1
cm and Tafel slope of 158 mV dec ), pure NiO (overpotential 553 mV at 10 mA cm and Tafel slope of 205 mV dec ) or
−
2
−1
2 5
V O (overpotential 668 mV at 10 mAcm and Tafel slope of 314 mV dec ) in alkaline medium. This higher OER activity of
marigold flower shaped Ni
and conversion systems.
V O
3 2 8
structure strengthens its adoption as a potential candidate in the field of energy storage
materials for OER. Of late, progress in transition metal oxides,
6, 22
23
5, 24
25, 26
27
1
. Introduction
for example MnO ,
MoO3, Co O ,
NiO,
Fe O ,
2 3
2
3
4
28
29
Fe O , CuO, etc. with high activity and ecological kind
3
4
In recent years, the Oxygen Evolution Reaction (OER) carry disposition show noticeable dominance in improving
incredible ramifications for the quest for substitutable energy electrochemical exhibitions of OER. Amongst above, either
conversion techniques, for example, water oxidation reactions, cobalt or nickel-based oxides have been extensively studied
1
2
rechargeable metal-air batteries and fuel cells etc. However, because of their enhanced safety, low cost, high chemical and
the presentation of this electrochemical procedure is thermal stability.
restricted by the inherently drowsy kinetics of the OER due to
the perplexing four-electron redox procedures, which in turn
Hence, low electrical conductivity, huge volume variety and
obstructs the potential application of substitutable energy ensuing molecule pummelling upon kinetics limit; make them
innovations. To overcome this inherent kinetics restriction, fairly hard to completely use OER active materials. To address
3, 4
discovery of an exceptionally dynamic electrocatalyst for OER this issue, one appealing methodology is to consolidate the
is urgently required. Be that as it may, the improvement of active materials with an electronic substrate, which can
OER electrocatalyst was found by utilizing RuO2 and IrO2, improve their electronic conductivity and structure soundness,
which is not feasible because of their highly expensive nature. prompting improved electrochemical execution. Moreover,
Consequently, a huge research exertion has been attempted the electrochemical execution of the readied composite
towards investigating transition metal based electrocatalysts materials displayed a cozy association with the electric
for their potential utilization towards OER. Along these lines, substrates. Another procedure is to plan two-fold metal oxides
various transition metal based electrocatalysts for example to improve their electrochemical exhibitions. The presentation
5-7
8-10
11-14
15-18
oxides,
hydroxides,
sulfides,
selenides,
of the second metal oxides can synergistically improve
19-21
phosphides
etc. have been widely demonstrated as active electrochemical exhibitions in comparison with the single-
stage metal oxides, containing synergistic effect, reversible
limit and mechanical durability. However, there are various
a.
Department of Chemistry, Central University of Punjab, Bathinda, 151001, India.
Indian Institute of Science Education and Research Kolkata, Nadia 741246, West
Bengal, India
Institute of Nano Science and Technology, Mohali, 160062, India.
Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan.
binary spinel-structured mixed metal oxide, for example
b.
30
7, 31, 32
36
33
34
35
CuCo O , NiCo O ,
NiFe O , CoFe O , ZnCo O ,
2 4 2 4 2 4
2
4
2
4
3
6
37, 38
c.
CuFe O , MnFe O , MnCo O
2 4
etc. are well documented
2
4
2
4
d.
†Electronic Supplementary Information (ESI) available: ATR-FTIR frequency data,
H NMR, HRMS, TGA plot, FE-SEM images, EDS, CV, and BET result, etc.
1
See DOI: 10.1039/x0xx00000x
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in literature. Which have indicated preferred electrochemical
performance over single-stage metal oxides due to their by previously reported procedure with smDaOlIl: t1u0.n10in3g9/(Ds0cNhJe0m15e96I)B.
upgraded electrical and ionic conductivity, reversible limit, and The synthesized compounds were characterized by various
mechanical soundness. Among existing blended oxides, metal spectroscopic technique such as 1H NMR, 13C NMR, ATR-FTIR,
vanadates have boundless applications in different fields and also with high-resolution mass spectrometry. Various
including photocatalytic water oxidation, oxygen evolution characterization techniques such as FE-SEM, HRTEM, SEAD,
reactions and LIB. Further, these materials are valuable for power XRD, XPS, TGA, etc. are utilization to explore the growth
vitality stockpiling applications since they can offer a lot more of Marigold flower shaped Ni V O structure. Finally, we have
Journal Name
The [Ni(Hq) ] and [VO(Hq) ] complexes were synthesized
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extensive potential window starting from the likelihood of the employed these marigold flower shape Ni V O structure
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progress metals, particularly, vanadium to exist in an based composite oxide as electrocatalyst active materials for
assortment of oxidation states. Vanadium-based blended Oxygen Evolution Reaction. Surprisingly, these marigold flower
metal oxides also have a few different focal points for energy shaped Ni V O structure based composite displays superior
capacity, viz. higher energy density, lower cost, and simple OER activity compared to commercial IrO2 and remarkable
accessibility. In addition, the introduction of nickel in the metal cycle strength. The as-fabricated marigold flower shaped
vanadates result vanadium-based mixed metal oxides Ni V O structure are cheap yet profoundly proficient OER
3
2 8
3
2 8
(
Ni V O )
which
show
astounding
electrochemical electrocatalyst.
3
2
8
performance originated from multivalence of nickel and
vanadium.
However, synthesis of such binary metal oxides cannot be
made directly, as it may be for settled single phase metal
oxides nanostructures. Liu et al. have synthesized M V O (M =
3
2 8
Ni or Co) nanoflake/nanoparticles via chemical co-precipitation
39
technique for high performance pseudocapacitive materials.
Lv et al. has fabricated Ni V O amorphous wire encapsulated
3
2
8
in crystalline tube nanostructure as anode materials for lithium
ion batteries by single spinneret electrospinning with
40
subsequent calcination. Soundharrajanetal. has reported the
one-pot synthesis of Ni V O electrodes by a simple metal
Scheme I. Synthesis of VO[Hq]
2 2
and Ni[Hq] complexes using dil.
3
2 8
NH OH (2M), EtOH, pH= 6.5-7.0 at room temperature.
organic framework-combustion technique for anode
4
41
applications in Li-ion batteries. Sambandam etal. has
synthesised Ni V O nanoparticles as an excellent anode
2
.1. Spectroscopic analysis
3
2 8
material for high-energy lithium-ion batteries through a
zeoliticimidazolate framework intermediate precipitation
As the ligand and both [Ni(Hq) ] and [VO(Hq) ] complexes
2
2
42
show the distinct characteristic absorption property, the UV-
Vis spectroscopy were used to identify complexation and
method using water as a solvent. Liu et al. has synthesised
Ni V O sensing electrode for ammonia (NH ) detection by sol-
3
2
8
3
2+
2+
4
3
understanding the coordinational behaviours of Ni and VO
metal ions with Hq ligands. The UV-Vis spectra of ligands and
both [Ni(Hq) ] and [VO(Hq) ] complexes were recorded at
gel method. Wang et al. has reported an efficient two-step
hydrothermal synthesis method to synthesize Ni V O
3
2 8
nanowires arrays organized by ultrathin nanosheets for
2
2
4
4
room temperature in chloroform solvent. Both the complex
formation was monitored by spectral changes during the UV-
Vis measurements were shown in Fig. 1. The absorbance peaks
appeared at 241 and 314 nm (Fig. 1a) corresponds to intra-
ligands π–π* transition of 8- hydroxyquinoline(Hq). However,
enhanced lithium storage properties. Briefing the results
from the literature, it has been observed that Ni V O mixed
3
2 8
oxide structures were mostly synthesized by multiple steps
technique. To the best of our knowledge, there is no report till
date on the preparation of marigold flower shaped Ni V O
2 8
3
in case of [Ni(Hq) ] complex (Fig. 1b), this intra-ligands π–π*
structure in one pot utilizing Ni(II) 8-hydroxyquinoline and
VO(II) 8-hydroxyquinoline complex atomic forerunner via a
hydrothermal process. Herein, we fabricated marigold flower
shaped Ni V O structure by a simple and facile thermal
2
type transition was shifted bathochromically from 241 and 314
nm to 284 and 345 nm, respectively and resulting the
formation of due [Ni(Hq) ] complex. Notably, a new broad
2
3
2 8
peak at 405 nm was observed in the UV-Vis spectrum of
decomposition of Ni(II) 8-hydroxyquinoline [Ni(Hq)2]
and
[
Ni(Hq) ] and this can be attributed by assigning the mixed
2
VO(II) 8-hydroxyquinoline [VO(Hq) ] complexes in presence of
2
2+
absorption modes caused by intra-ligandtransitions and Ni to
Hq ligands charge transfer which is consistent with the
a universal solvent, water. Finally, we have explored these
Marigold flower shaped Ni V O for OER and exhibit
3
2 8
4
5-47
previously reported result
. On the other hand, [VO(Hq)2]
magnificent OER activity compared to its constituent oxide as
the potential electrode materials.
complex in the chloroform solution shows almost similar
absorbance spectra to that of [Ni(Hq) ] complex where the
2
absorption maxima at 241 and 314 nm were shifted
bathochromically to the λmax 250 and 390 nm, respectively (Fig.
c). It has been reported that absorbance band appeared in
2
. Result and discussion
1
2
| J. Name., 2012, 00, 1-3
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the range of 450-600 nm can be attributed to the ligands to
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Finally, [Ni(Hq) ] and [VO(Hq) ] complexes were confirmed
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DOI: 10.1039/D0NJ01596B
metal charge transfer transitions where the transitions were by H NMR, C NMR and High resolution mass spectroscopy.
22, 48-50
1
most likely due to involving ligands orbital only.
The H NMR spectra of free Hq shows well intense signal
5
6, 57
1
within range 7.2 ppm to 8.7 ppm
.The H NMR of [Ni(Hq) ]
2
reveal broadened and shifted signals to high frequencies (Fig.
S1) and confirmed the presence and coordination of Hq with
Ni(II) centre. Additionally, the ESI-MS (Positive mode) spectra
Fig. S2 and S3) shows the most intense peak at m/z 347.0313
Calcd 347.0331) corresponding to molecular weight of
Ni(Hq) ], which is assumed to be a species with a 1:2 (metal to
ligand) stoichiometry. Similarly, the H NMR spectra of
VO(Hq) ] was to assign to Hq signals to coordination with
2
V(IV) metal (Fig. S4). Further, the mass spectra show peak at
m/z 356.0314 (356.0366) and cumulatively both spectra
(
(
[
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[
confirmed the molecular formula of [VO(Hq) ] (Fig. S5).
2
To study the thermal stability of [Ni(Hq) ] and [VO(Hq) ]
2
2
complexes and understand the fabrication of marigold shape
Ni V O flower, we have performed the Thermogravimetric
analysis (TGA) of [Ni(Hq) ] and [VO(Hq) ] complexes. Fig. S6
Fig. 1 UV-vis spectrum of (a) Hq, (b) [Ni(Hq)
complexes in chloroform at room temperature
2
] and (c) [VO(Hq)
.
2
]
3
2 8
2
2
represents the TGA curve of [Ni(Hq)2] and [VO(Hq)2]
To further know the formation of [Ni(Hq) ] and [VO(Hq) ] complexes. The thermogram (Fig. S6a) of [Ni(Hq) ] complex
2
2
2
complexes, we have studied ATR-FTIR spectroscopy. Formation exhibits two distinct cut stages. The first step shows the initial
of [Ni(Hq) ]was affirmed by the characteristic peaks at 297 and 11% weight loss in the temperature range of 100-170°C where
2
1
-
2
78 cm corresponding to asymmetric and symmetric in the second stage 71% weight loss is observed in the
stretching mode of Ni-O bond along with asymmetric and temperature range of 180-450°C. Thermal decomposition of
symmetric stretching mode of Ni-N bond appeared at 258 and the [VO(Hq) ] complex occurs in multiple stages (Fig. S6b). No
2
-
1
51, 52
2
31 cm , respectively(Fig. 2a).
In Fig. 2b the strong peak sharp weight change is observed. The thermogram of
-1
53, 54
centred at 950 cm due to V=O stretching.
This low [VO(Hq) ] complex shows 7% weight loss by two stage
2
stretching frequency imply substantial weakening of the V=O decomposition stage in the temperature range 35-285°C,
bond, consistent with strong σ and π electron donation by 8- where 70% weight loss is observed in the temperature range
hydroxyquinoline to antibonding orbitals of the V=O group. 290-558°C by multiple stage decomposition. A further increase
-1
Where the bands appeared at 373, 347 and 301 cm are in temperature above 450°C (490°C for [VO(Hq) ] complex)
2
attributed to V-O stretching modes and the peaks at 258 and reveals no loss in weight and this constant weight is due to
-1
51, 55
2
33 cm are assigned to V-N stretching modes
, which nickel oxide or vanadium oxide (confirmed by XRD studies),
confirm the formation of [VO(Hq) ] complex (details ATR-FTIR which appears as a final pyrolysis product of [Ni(Hq) ] and
2
2
stretching modes are given in Table S1).
[VO(Hq) ] complexes.
2
2
.2 Morphology Investigation
Architecture of marigold flower shaped Ni V O structure was
3
2 8
optimised after performing a number of controlled reactions
to changing several reaction parameters, for example, reaction
time, variation of precursor concentration ratios, growth
temperature etc. It was indicated in the experimental section
that the marigold flower shaped Ni V O structure were
fabricated by mixing [Ni(Hq) ] and [VO(Hq) ] complexes in 3:2
molar ratio under hydrothermal methods in one pot. We have
also conducted the same reaction under identical conditions in
absence of [VO(Hq) ] complexes and got anisotropic NiO micro
rod shape structure (Fig. S7a-b). On the other hand, without
Ni(Hq)2] complexes, the same reaction produced V2O
5
structure having mixture of spheres and rods shape(Fig. S7c-d)
and the corresponding energy dispersive x-ray spectroscopy
3
2 8
2
2
2
[
2 2
Fig. 2 ATR-FTIR spectra of (a) [Ni(Hq) ] and (b) [VO(Hq) ] complexes.
(
EDS) of V O and NiO micro rods are shown in the Fig. S8.
2 5
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We have tested the similar reaction under the same conditions However, the Ni V O flower in whole is polycrystalline in
at different time intervals (like 8, 16 and 24h) but didn’t nature as indicated by SAED pattern shoDwOnI:i1n0.i1n0s3e9t/Do0fNFJ0ig1.5946dB.
produce desired shape of Ni V O structure (Fig. S9a-c). No Both of high magnification FE-SEM image and the TEM image
products were found at temperature below 190 C, even the clearlyshows that the curly and folded thin nanosheets of
hydrothermal reaction continued for 72h (Fig. S9d). Therefore, thickness of 7.5 .3 nm are branched out from the central
the use of a molar ratio 3:2 of [Ni(Hq) ] and [VO(Hq) ] zone of the flower. These marigold shape Ni V O structures
3
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3
2 8
o

2
2
3 2 8
complexes is critical to achieve marigold flower shaped were further analysed to get detailed compositional
Ni V O structure. information by elemental mapping as shown in Fig. 5.
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Fig. 3 FE-SEM images of marigold shape Ni V O flower: (a), (b) and(c)
panoramic FE-SEM images and (d) FE-SEM image of a single Ni
flower
3 2 8
V O
The morphology of Ni V O structure was observed using field
emission SEM equipped with EDS and high resolution TEM. Fig.
3a-d shows representative panoramic FE-SEM images of
3
2 8
Ni V O structures. The FE-SEM images clearly reveal the Fig. 5(a) FE-SEM image of marigold shape Ni V O flower and
3
2
8
3
2
8
formation of marigold flower like Ni V O structures with corresponding elemental mapping: (b) Ni, (c) V, (d) O of marigold
diameter of 66
3
2 8

3 μm.
3 2 8 3 2 8
shape Ni V O flower and (e) EDS of marigold shape Ni V O flower.
These marigold flower shaped Ni V O structures were further The elemental mapping study clearly revealed that Ni, V and O
investigated by TEM to get detailed structural information. are the fundamental elements exist (Fig. 5b-d) in Ni V O
2 8
TEM images shown in Fig. 4a-c clearly illustrate the fabrication flower and it was also observed that Ni, V and O elements are
of flowers-shaped structure having a sheet-like within the well-dispersed throughout Ni V O flower. The EDS (Fig. 5e)
flower. The sheet thickness of the Ni V O flower was around data show no other contaminations were detected in the
3
2 8
3
3
2 8
3
2 8
~7.5
.3 nm (Fig. 4b). The presence of clear lattice fringes Ni V O flower.
3 2 8
i
showing the (131) planes in the inset images (Fig. 4c ) in high
resolution TEM (HRTEM) indicates that the individual 2.3. Structural Analysis
nanosheets are single crystalline in nature as shown in Fig. 4c.
With the aim of getting of crystalline structures and identify
the crystal phases of Ni V O flowers and pure V O and NiO
3
2
8
2 5
structures, we have analyzed the powder X-ray diffraction
(XRD) study. Fig. 6a shows the XRD pattern of as synthesized
V2O5 micro rods. The narrow and intense diffraction peaks
centered at 2θ = 15.4°, 20.3°, 21.7°, 26.1°, 31.1°, 32.4°, 34.3°,
41.3°, 42.0°, 45.4°, 47.3°, 48.8°, 51.2°, 52.0°, 55.6°, 61.1°, and
62.1° corresponded to (020), (001), (011), (110), (040), (101),
(130), (002), (012), (141), (060), (102), (200), (061), (201),
(240), and (170) planes, respectively indicate that V O
3 2 8
Fig. 4 TEM images of marigold shapeNi V O flower at different
magnifications, (a)TEM image where area is marked to achieve the
i
HRTEM. The inset (c ) is the lattice spaces obtaining from HRTEM
2
5
image of (b) showing the (131) planes of the Ni
Represents the typical SAED pattern of Ni flower.
3
V
2
O
8
flower, (d)
58
structures (JCPDS No. 60-0767) were highly crystalline. No
diffraction peak for other contaminations was detected,
indicating the high purity of as synthesized V O structure. Fig.
3 2 8
V O
2
5
6b presents the XRD pattern of as synthesized NiO
4
| J. Name., 2012, 00, 1-3
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nanostructures. Peaks appeared at 2θ = 36.98° (110), 43.0° of Ni, V, and
O
and the nonappearance of other
View Article Online
(
200), 62.6° (220), 75.1° (311) and 79.1° (222) are attributed to contamination. The C peak arises from thDeOrIe: f1e0r.1e0n3c9e/D.0TNhJe01h5i9g6hB-
cubic NiO (PDF 01-078-4359) and clearly conform the resolution scan for Ni 2p is shown in Fig. 7b. There are two
formation of crystalline NiO nanostructures.
the XRD pattern of as synthesized marigold shape Ni V O
flowers. The strong peaks appeared at 2θ = 35.8° (122), 43.2° addition, “shake up” satellite peaks of Ni 2p are also observed
5
9, 60
Fig. 6c shows major peaks at 873.4 and 855.7 eV, which are assigned to the
Ni 2p1/2 and 2p3/2 spin-orbit peaks of Ni V O , respectively. In
3
2
8
3 2 8
4
0, 62
(151), and 62.8° (442) along with weak peaks at 18.9° (120), at the binding energies 879.5 and 861.6 eV.
The
3
1.2° (131), 58.5° (162) conform the formation of Ni V O
3 2 8
deconvoluted Ni 2p3/2 spectrum into two distinct peaks at the
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structures (PDF 01-074-1485). Additionally, EDS analysis binding energy of 855.5 and 856.7 eV , indicating the
justifies the presence of the elements Ni, V, and O in the occurrence of both Ni (II) and Ni (III) state in Ni V O flower. It
6
3
3
2 8
Ni V O flowers as shown in Fig. 5e. EDS study also revels that has been reported that both V (IV) and V (V) state may be
3
2
8
there are no other impurities as well Ni, V and O elements and observed in some vanadates, with heavily close binding energy
supports the XRD analysis of Ni V O structures.
5
+
4+
of 516.4-517.4 eV (V , 2p3/2) and 515.4-515.75 eV (V
,
3
2 8
6
4, 65
2
p3/2).
Thus, the peaks fitted in the XPS spectrum of V 2p
area at binding energies of 517.17 eV and 515.62 eV can be
ascribed to V 2p3/2 and V 2p3/2, respectively (Fig. 7c).
5
+
4+
Fig. 7 XPS spectra: (a) survey spectra, (b) Ni 2p, (c) V 2p, (d) O 1s in
3 2 8
as-synthesized marigold shape Ni V O flowers.
3+
2+
5+ 4+
Therefore, the coupling of Ni /Ni and V /V can be
confirmed in the as-synthesized Ni V O flowers, and the
interaction between these mixed valences ions may attribute
to the improvement of the overall electrochemical
3
2 8
Fig. 6 Upper panel: XRDpattern of as synthesized (a) V
(b) NiO micro rods, and (c) Marigold shape Ni
panel:(e) and (d) display a coordinating octahedron representation of
crystal structure of Ni
2
O
5
micro rods,
V O flowers.Lower
3 2 8
2
-
performance. There appear two types of oxygen (O ) ions in O
I
II
I
XPS profile (denoted as O and O ) (Fig. 7d). The O ions at
30.8 eV corresponds to lattice oxygen atoms which are in a
3 2 8
V O .
5
II
fully-coordinated environment, and the other O ions at 532.8
eV are O , suggesting the oxygen-deficient regions within the
Ni V O phase.
The majority of the diffraction peaks were all around recorded
to the standard orthorhombic crystal system of Ni V O (JCPDS
No. 074-1484) having a place with the Acam space group with
unit cell parameters a, b, and c determined to be 8.2400,
11.3800, and 5.9060 Å individually. In this spinel Ni ions
structure an edge shared NiO (octahedral) layers, which are
2-
3
2 8
3
2 8
2
+
3. Electrochemical Studies (OER Activity)
6
5
+
Apart from the morphology and composition study, the
electrochemical OER activity of the as synthesised Marigold
flower shaped Ni V O samples was compared withphysical
isolated by VO (V ions, tetrahedral), bringing about a curious
Kagome'- staircase geometry.
4
6
1
3
2 8
mixture of V O and NiO,pure NiO and V O , using linear
2
5
2 5
To further confirm the chemical composition and electronic
structure, for example valence state, of each element in the
Ni V O flowers, XPS investigation was performed and the
sweep voltammetry (LSV) technique measurements performed
in 1 M KOH solution. In correlation, commercial IrO2 was
picked as reference. Fig. 8a depicts the LSV curves of Ni V O ,
3
2
8
3
2 8
corresponding results are revealed in Fig. 7. The survey scan of
the Ni V O flowers presented in Fig. 7a, suggests the presence
physical mixture of V O and NiO, pure NiO and V O catalysts.
2
5
2 5
3
2 8
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ARTICLE
Marigold flower shaped Ni V O achieved the current density analysis and suggests that Marigold flower shaped Ni V O
View Article Online
3 2 8
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DOI: 10.1039/D0NJ01596B
of 10 mA cm at an overpotential of only 328 mV whereas displays the quickest charge-transfer process among all the
commercial IrO2 shows similar current density at an three electrocatalyst, which might be due to the rough surface
overpotential of 360 mV. Similarly, Ni V O shows premier OER of the catalyst resulting in the extraordinary OER activity. Thus,
3
2 8
activity over the physical mixture of V O and NiO, pure NiO this Marigold flower shaped Ni V O flower 3D structures
2
5
3 2 8
-2
and V O for attaining current density of 10 mA cm at improve the overall surface area reasonable for better
2
5
overpotential of 496, 553 and 668 mV, respectively. In order to catalysis. Hence, to study the relationship between the
figure out the kinetics of OER processes, Tafel polarization plot catalytic activities and the surface accessibility of
is an important tool that can be used to determine the Ni V O flower along with physical mixture of V O and NiO,
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mechanistic pathway of the OER activity. From the linear pure NiO and V O catalyst, the electrochemically active
2
5
fitting of the Tafel plot, the Tafel slope derived from the surface area (ECSA) was measured in terms of double layer
corresponding LSV curves were shown in Fig. 8b and was capacitance (C ). Both the anodic and cathodic double-layer
dl
-
1
found to be 61 mV dec for Ni V O flower composite charging currents (J and J , respectively) were measured from
3
2
8
a
c
structure, which is much lesser than that of physical mixture of the CV curves (Fig. 9a) at a fixed potential of 0.2 V and are
-
1
V O and NiO, (158 mV dec ) and its constituent NiO (205 mV plotted against corresponding scan rates (Fig. 9b).The
2
5
-
1
-1
-2
dec ) and V O (314 mV dec ) catalyst. Notably, Ni V O
flower exhibits lower Tafel slope than commercial IrO (65 mV corresponding ECSA is found to be 68 cm , which remained
dec ). These observations also underpin the superior OER much higher than that of physical mixture of V O and NiO or
calculated Cdl for Ni V O flower is 2.72 mF cm and the
3 2 8
2
5
3
2
8
2
2
-1
2
5
activity of the Marigold shape Ni V O flower catalyst.
pure NiO and V O catalyst (Fig. S10) (details are summarized
2 5
3
2
8
in Table 1).
Table 1. Comparative results of OER performance of IrO
2
, physical
mixture of V O and NiO,
V
2
O
5
, NiO, and Ni catalysts.
3 2 8
V O
2
5
Further, the Brunauer-Emmett-Teller (BET) estimation results
proved this pattern, where the Ni V O flower has the largest
3
2 8
2
−1
2
−1
surface area of 11.47 m g , trailed by NiO (8.642 m g ) and
Fig. 8The parameters of evaluating the catalytic performance:(a) V O (4.543 m g ) (Fig. S11). Again, the LSV curves of Ni V O
2
−1
2
5
3 2 8
polarization curves (LSV) plot and (b) corresponding Tafel plots of IrO
Ni ,physical mixture of V and NiO, NiO and V catalyst, (c) electrodes were normalized to explain the OER activity. As
electrochemical impedance spectra at 350 mV (Ag/AgCl), and (d) presented in Fig. S12, the Ni V O flower electrodes exhibit
2
,
flower, physical mixture of V O and NiO, NiO, and V O5
2 5 2
3
V
2
O
8
2
O
5
2 5
O
3
2 8
variation of R
s
and Rct of Ni
3
V
2
O
8
,physical mixture of V
2
O
5
and NiO, NiO superior OER activity (current standardized by ECSA) compared
to its constituent oxide due to higher specific OER activity of
Ni V O flower electrodes. The specific OER correlation affirms
and V catalyst.
2
O
5
3
2 8
To further get insight into the superior catalytic activity of that the formation ofNi V O spinel flower results in larger
3
2 8
Marigold shape Ni V O flower, electrochemical impedance intrinsic activity.
3
2 8
spectroscopy (EIS) was studied and displayed in Fig. 8c. From
the EIS analysis, it has been found that Ni V O flower exhibits Catalytic durability is another important parameter for an
3
2 8
lowest charge transfer resistance (R ) of 3.4 Ω, whereas efficient electrocatalyst. Here, we examined the long-term
ct
physical mixture of V O and NiO, pure NiO and V O possess 2000 continuous LSV cycles using 1M KOH solution an
2
5
2 5
Rct of 19.2 Ω, 23.9 Ω and 195.1 Ω, respectively (Table 1). The electrolyte. Fig. 9c shows the LSV curve of initial and after
th
solution resistance (R ) of Ni V O , physical mixture of V O
2000 cycle of Ni V O electrode. After 2000 LSV cycles, the
s
3
2
8
2
5
3 2 8
-2
and NiO, NiO, and V O was found to be 1.11, 1.38, 1.5, and change in the overpotential at current density of 10 mA cm
2
5
0
.82 Ω, respectively (Table 1).The lowest Rct value of Ni V O
was found to be only 8 mV (from 328 mV to 336 mV) which
3
2 8
indicates that Marigold flower shaped Ni V O
is demonstrates the long-term superior stability of the
electrocatalytically more active towards OER compared to Ni V O flower toward OER activity.
3
2 8
3
2 8
physical mixture of V O and NiO, pure NiO and V O catalyst.
2
5
2 5
This impedance result is in good agreement with the LSV
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The electrocatalytic OER activity of Ni V O , physical mixture of
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View Article Online
DOI: 10.1039/D0NJ01596B
V2O and NiO, pure NiO, and V O materials belongs to
5
2 5
heterogeneous catalytic reaction where the M–O interaction
within the intermediates of MOH, MO and MOOHare very
essential for the general electrocatalytic performance of the
catalysts. The high OER activity of Ni V O can be attributed to
3
2 8
5+
the synergetic effect between variable oxidation state of V (V
and V ) and Ni (Ni and Ni ) metal ions which provide more
active metal site for the adsorption of the OH anion (stage
4+
3+
2+
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−
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)on the surfaceof Ni V O and consequently facilitates the
3 2 8
other steps for OER. Therefore, it can be concluded that as
Marigold shape Ni V O flowerpossesses highest
3
2 8
electrochemical active surface area and active metal sites over
physical mixture of V O and NiO,pure NiO, and V O5
2
5
2
catalyst,henceNi V O exhibits highest OER activity in alkaline
3
2 5
medium.
3 2 8 a c
Fig. 9 (a) Cyclic voltammetry curves of Ni V O , (b) plot of J and J
4
. Experimental Section
against scan rate for the determination of double layer capacitance
st
(Cdl). Stability test: (c) Polarization curves of the Ni
3
V
2
O
8
after 1 and
th
4.1 Materials
2,000 cycles of continuous operation and (d) Chronoamperometric
3 2 8
study of Ni V O at an overpotential of 369 mV for 10 h.
Vanadyl sulphate (VOSO .5H O, 95%, Molychem, AR), nickel
4
2
nitrate hexahydrate ([Ni(NO ) .6H O] (Loba Chemie), 8-
Further, we have also checked the stability of the Ni V O
2 8
catalyst by chronoamperometric method and found a current
3 3
2
3
hydroxyquinoline (Loba Chemie, AR/ACS 99%), ammonium
-
2
hydroxide solution (NH OH, 25%, AR), ethanol (AR),
density of 20 mA cm for 10 hours constantly (Fig. 9d). Thus,
overall electrochemical observations conclude that
Ni V O flowers are promising catalyst for the OER activity in
4
chloroform (Loba Chemie), potassium hydroxide (KOH, Loba
Chemie), Iridium oxide (IrO , Sigma-Aldrich ), 5 % Nafion™ 117
2
3
2
8
solution (Sigma-Aldrich). The chemicals used in this work were
of analytical grade and utilized directly after purchase without
further purification. Double distilled water (DDW) (18.2
millipores) was used in all respect.
alkaline medium. And from OER investigations, we believe that
nickel oxide unit in Ni V O as a catalyst precursor and
vanadium oxide in the Ni V O flowers serve as the actual
reactant site to produce oxygen with the persistent activation
of the catalyst.
3
2 8
3
2 8
4
.2 Synthesis of Ni(II) 8-Hydroxyquinoline Complex [Ni(Hq)2]
3.1 OER Pathway
[
Ni(Hq) ] was synthesised here by adopting a reported
2
68
method with little modification, In a typical synthesis
The overall OER pathways under basic condition start with the
adsorption and release of the OH anion at the catalyst surface
of a functioning metal site (M) to make adsorbed OH species
(stage 1). The adsorbed OH species further react with OH to
−
procedure, 1.5 mmol (436.20 mg) of Ni(NO ) was dissolved in
3 3
−
1
0 mL ethanol. 3.075 mmol (446.43 mg) of 8-Hydroxyquinoline
−
was dissolved in 30 mL ethanol in a 100 mL round bottom
flask. Separate solution of Ni(NO3)3 and 8-hydroxyquinoline
generate H O and adsorbed atomic O* and discharge an
2
−
were prepared in a 1: 2.05 molar ratio. Then the Ni(NO3)3
electron (stage 2). After that, an OH anion reacts with an
adsorbed O* atom to produce adsorbed OOH (stage 3).
Moreover, further reaction with extra OH anions generates
solution was added drop wise into the solution of 8-
hydroxyquinoline under stirring condition at room
temperature (Scheme I). The pH of each solution was adjusted
−
adsorbed O and H O (stage 4). Adsorbed O then discharge in
2
2
2
to 6.5-7.0 by adding 2M NH OH solution. The reaction was
the stage 5. All the steps associated in the reaction mechanism
have been portrayed as follows:
4
7
, 66,67
continued for 150 minutes. Thereafter the reaction mixture
was filtered out and yellow-green precipitate was collected,
washed with DDW followed by ethanol to remove excess 8-
hydroxyquinoline for several times, and dried at 90°C.
M + OH− → M − OH + e−
(stage 1)
(stage 2)
(stage 3)
−
∗
−
M − OH + OH → M − O + e + H O
2
1
H NMR and TOF-MS of [Ni(Hq) ] Complex
2
∗
−
−
M − O + OH → M − OOH + e
1
H NMR (DMSO-d )δ 15.30 (b, 1H), 17.02 (b, 1H), 18.30 (b, 2H),
0.49 (b, 2H) (Fig. S1). TOF-MS: m/z [M+H] calculated for
6
−
−
M − OOH + OH → M − 푂 + e + H O (stage 4)
2
+
2
2
C H N O Ni is 347.0331, found 347.0313 (Fig. S2) and calc.
18 13
2 2
M − 푂 → M + 푂2
2
(stage 5)
+
for C H N O NiNa [M+Na] is 360.0150 found 369.0068 (Fig.
18 12
2 2
S3).
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.3Synthesis of VO(II) 8-Hydroxyquinoline Complex [VO(Hq)2]
techniques. The NMR spectra were recorded with 500 MHz
View Article Online
DOI: 10.1039/D0NJ01596B
Bruker instruments. Chemical shifts are reported in δ values
[
VO(Hq) ] was synthesised by the same methods as Ni(II) 8- relative to the solvent peaks. The solvents used for the
2
Hydroxyquinoline complex was synthesised. 1.5 mmol (379.62 spectroscopy experiments were brought from Cambridge
mg) of VOSO was dissolved in a solution of 10 mL ethanol and isotope laboratories. The mass spectrometric data were
4
1
5 mL double distilled water. 3.075 mmol (446.43 mg) of 8- obtained from an Acquity TM Ultra Performance LC-
Hydroxyquinoline was dissolved in 30 mL ethanol in a 100 mL ESI/quadruple-TOF MS. UV-Vis measurements were performed
round bottom flask. Separate solution of VOSO4 and 8- with a UV-Vis spectrophotometer (Shimadzu), ATR-FTIR was
Hydroxyquinoline were prepared in a 1: 2.05 molar ratio. Then carried out using Bruker (Model: Vertex 70v) ATR
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the VOSO solution was added drop wish in to the solution of spectrometer. The Field-emission scanning electron
4
8
-Hydroxyquinoline under stirring condition at room microscopy (FE-SEM) images were taken using Carl Zeiss
temperature. The pH of each solution was adjusted to 6.5-7.0 (Merlin compact, 2014) FE-SEM. The transmission electron
by adding 2M NH OH solution. The reaction was continued for microscopy (TEM) images were taken using a JEOL-2100
4
1
50 minutes. Thereafter the reaction mixture was filtered out transmission electron microscope with an operating voltage of
and yellow-green precipitate was collected, washed with DDW 200 kV. The crystalline structures and phases were studied by
followed by ethanol to remove excess 8-Hydroxyquinoline for X-ray diffraction pattern measurements which were done with
several times, and dried at 90°C.
Bruker Eco D8 advance X-ray powder diffractometer using Ni-
filtered with Cu Kα radiation λ=1.54056 Å. The data was
collected from 10˚< 2θ < 80˚ with an increment of 0.019˚. The
high-resolution X-ray photoelectron spectroscopy (XPS)
measurements were performed using the Thermo Scientific
1
H NMR and TOF-MS of [VO(Hq) ] Complex
2
1
H NMR (400 MHz, MeOH-d ) δ 7.14 (d, 2H, 7.96), 7.27 (d, 2H, J Inc. system equipped with a microfocus monochromatic Al Kα
4
=
5.68), 7.58 (mt, 2H), 7.33 (d, 6.4 Hz), 8.17 (d, 1H, J = 4.4 Hz), X-ray source of energy ~1450 eV. Brunauer-Emmett-Teller
8
3
3
.27 (d, 1H, J = 6.4 Hz), 8.35 (d, 1H, J = 2.92 Hz), 8.56 (d, 1H, J = (BET) measurement was carried out with Make-QuantaChrome
.2 Hz) (Fig. S4). TOFMS: m/z [M+H] calc. for C H N O V is instruments, Model: AutosorbiQ and ASiQwin. All the
56.0366, found 356.0314 (Fig. S5).
+
18 13
2 3
electrochemical measurements were performed using
Metrohm Autolab (Multichannel-204) connected to a standard
three-electrode system using Nova 2.1.4 software.
4
.4 Synthesis of marigold flower shaped Ni V O structure
3
2
8
VO(II) 8-Hydroxyquinoline and Ni(II) 8-Hydroxyquinoline 4.7 Fabrication of working electrode
complexwere first dissolved in 2:3 molar ratio in 16 mL
methanol which was further diluted to 36 mL by the addition Catalyst inks were prepared by dissolving 3 mg of the catalyst
of 20 mL double distilled water. The obtained solution was in 0.68 mL of double distilled water and 0.4 mL of ethanol in a
transferred into a Teflon-lined stainless-steel autoclave and 1.5 mL micro centrifuge tube. This mixture was sonicated for
heated at 220°C for 32h. The temperature was raised to 220°C 30 minutes prior to addition of 20 μL 5% Nafion solution, then
with 10°C min heating rate. After the completion of the further sonicated for another 15 min. The as-prepared 5 μL
reaction, the heating was stopped and the hydrothermal catalyst ink was drop casted on to the glassy carbon (GC)
autoclave was allowed to cool down naturally to room electrode and left overnight in vacuum desiccator for drying.
temperature. The precipitate was washed by centrifugation The dried electrode is the used as working electrode (WE) for
with methanol and DDW several times to remove the the study of oxygen evolution reaction (OER) activity and the
-
1
-2
unreactive species. The Ni V O flower product thus obtained mass loading of catalyst kept as 0.19 mg cm . All the
was dried in a hot air oven at 70 C for 12h and finally stored electrochemical experiments are performed at the room
3
2 8
o
for further studies.
.5 Synthesis of V O and NiO structure
temperature.
For the electrochemical characterizations Ag/AgCl (3M KCl)
and platinum (Pt) electrode were used as reference and
counter electrode, respectively. Here, GC (3 mm diameter)
4
2
5
For the preparation of pure V O or NiO, a process similar to electrode loaded with catalyst served as working electrode. All
that followed for the synthesis of Ni V O was carried out, the experiments were carried out in 1M KOH (pH 13.8)
except that [VO(Hq) ] or [Ni(Hq) ] complex was not subsist in solution prepared using DDW. The linear sweep voltammetry
the reactions medium. We have also prepared a physical (LSV) was carried out in potential range of 0 V to 1 V
2
5
3
2 8
2
2
-1
mixture of V2O5 and NiO in equimolar ratio for OER vs.Ag/AgCl with
a
scan rate of 10 mV
s
and cyclic
performance.
.6. Instrumentation
voltammetry (CV) experiments were carried in the potential
range of 0.1V to 0.3 V (vs. Ag/AgCl) with scan rate varying from
20 mV s to 100 mV s . The long-term stability of the catalyst
was tested using chronoamperometric method by applying a
-1
-1
4
The structures of the compounds were determined by nuclear constant overpotential of 369 mV for 10h. Further, stability of
magnetic resonance spectroscopy and other spectroscopic the Ni V O catalyst was also studied by observing the change
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in overpotential after contentious running of 2000 LSV cycles
at a constant scan rate of 50 mV s . Electrochemical
impedance spectroscopy (EIS) was recorded at frequencies 1. F. Cheng and J. Chen, Chem. Soc. Rev., 2012, 41, 2172-2192.
from 100 kHz to 0.1 Hz at 350 mV (vs. Ag/AgCl) in 1 M KOH 2. Y. Bing, H. Liu, L. Zhang, D. Ghosh and J. Zhang, Chem. Soc.
solution. All the electrochemical measurement data, the
potential with respect to RHE were converted using the
following Nernst equation:
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DOI: 10.1039/D0NJ01596B
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Ni V O flowers designed by a thermal decay of molecular
precursors show excellent OER activity with an
overpotential of 328 mV.
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