Energy and environmental applications of ultrasonically sulfur doped copper-nickel hydroxides with heterostructures

Article abstract of DOI:10.1016/j.jallcom.2017.09.123

A series of sulfur doped copper-nickel hydroxides with heterojunctions were successfully fabricated on nickel foam by adjusting thiourea volume via a facile sonochemical pathway. The effect of volume of thiourea on the final morphology and chemical composition of the hybrids were also investigated by field-emission scanning electron microscopy, and X-ray photoelectron spectroscopy analyses. Furthermore, the electrochemical performance and catalytic activity of the as-obtained hybrids were also investigated. Among the tested electrode, the hybrid material fabricated using 6 ml of thiourea (TU-6) showed outstanding electrochemical properties comprising a high specific capacitance of about 2708 F g^?1 at 5 A g^?1. In addition, the TU-6 hybrid (catalyst) material displayed remarkable reductive degradation ability towards azo dyes viz., methyl orange (within 8 min) and congo red (within 20 min) in the presence of sodium borohydride (reducing agent) with fast kinetics and good reproducibility, respectively. The exceptional electrochemical performance and excellent catalytic activity of TU-6 hybrid electrode may be attributed to the formation of catalytically active sulfur doped copper-nickel hydroxides (CuS/Ni₃S₂/NiOOH) three-interface synergistic effect, and unique porous micro-rosette-like texture which increased the diffusion rate and adsorption capacity. The adopted strategy is a simple and generic way for material fabrication to solve the energy and environmental problems.

Full text of DOI:10.1016/j.jallcom.2017.09.123

Accepted Manuscript
Energy and environmental applications of ultrasonically sulfur doped copper-nickel
hydroxides with heterostructures
Namachivayam Karthik, Thomas Nesakumar Jebakumar Immanuel Edison, Raji
Atchudan, Mathur Gopalakrishnan Sethuraman
PII:
S0925-8388(17)33164-X
10.1016/j.jallcom.2017.09.123
JALCOM 43179
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 27 July 2017
Revised Date: 11 September 2017
Accepted Date: 12 September 2017
Please cite this article as: N. Karthik, T.N.J.I. Edison, R. Atchudan, M.G. Sethuraman, Energy and
environmental applications of ultrasonically sulfur doped copper-nickel hydroxides with heterostructures,
Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.09.123.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Effective method has been adopted to fabricate diverse textures of sulfur doped copper-nickel
hydroxides with surprisingly remarkable capacity and excellent catalytic performance.

ACCEPTED MANUSCRIPT
Energy and environmental applications of ultrasonically sulfur doped copper-
nickel hydroxides with heterostructures
Namachivayam Karthika, Thomas Nesakumar Jebakumar Immanuel Edisona,*, Raji Atchudana,
and Mathur Gopalakrishnan Sethuramanb,*
a School of Chemical Engineering, Yeungnam University, Gyeongsan 385-41,
Republic of Korea
b Department of Chemistry, Gandhigram Rural Institute – Deemed University, Gandhigram - 624
302, Tamil Nadu, India
*Authors for correspondence:
E-mail: jebakumar84@yu.ac.kr; mgsethu@gmail.com
Fax: +82-53-810-4631; Tel: +82-53-810-2529
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Abstract
A series of sulfur doped copper-nickel hydroxides with heterojunctions were successfully
fabricated on nickel foam by adjusting thiourea volume via a facile sonochemical pathway. The
effect of volume of thiourea on the final morphology and chemical composition of the hybrids
were also investigated by field-emission scanning electron microscopy, and X-ray photoelectron
spectroscopy analyses. Furthermore, the electrochemical performance and catalytic activity of
the as-obtained hybrids were also investigated. Among the tested electrode, the hybrid material
fabricated using 6 ml of thiourea (TU-6) showed outstanding electrochemical properties
comprising a high specific capacitance of about 2708 F.g-1 at 5 A.g-1. In addition, the TU-6
hybrid (catalyst) material displayed remarkable reductive degradation ability towards azo dyes
viz., methyl orange (within 8 min) and congo red (within 20 min) in the presence of sodium
borohydride (reducing agent) with fast kinetics and good reproducibility, respectively. The
exceptional electrochemical performance and excellent catalytic activity of TU-6 hybrid
electrode may be attributed to the formation of catalytically active sulfur doped copper-nickel
hydroxides (CuS/Ni3S2/NiOOH) three-interface synergistic effect, and unique porous micro-
rosette-like texture which increased the diffusion rate and adsorption capacity. The adopted
strategy is a simple and generic way for material fabrication to solve the energy and
environmental problems.
Keywords: Sonication; Sulfur doping; Micro-rosette; Copper-nickel hydroxides;
Supercapacitors; Dye-degradation.
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1. Introduction
The ever increasing energy crisis associated with environmental issues remains a severe
concern, have stimulated the development of new type of materials by exploring renewable
energy sources in our daily life [1,2]. In recent past, supercapacitors have fascinated as a new
class of energy storage devices, owing to their conspicuous features of high energy density, long
cyclic life, and superior safety [3,4]. On the basis of charge-storage mechanism, these
supercapacitors can be isolated into two groups: (1) electric double-layer capacitors (EDLCs,
normally composed of carbonaceous materials) storing charges via the electrolyte ionic
adsorption/desorption process between the active materials of the electrode and the electrolytes
(non-faradaic) and (2) pseudo-capacitors (such as in transition metal oxides/hydroxides/sulfides),
delivering energy through fast redox reactions on the electrode surface (faradaic). However, the
main obstacle impeding the development of these electrode materials such as low energy density
for carbonaceous materials and, low electrical conductivity for transition metal
oxides/hydroxides/sulfides have extremely mired their practical applications [5-7].
On the other hand, organic pollutants viz., dyes have been utilized in the textile, paper,
pulp, pharmaceutical, and tannery industries, but these have high chemical and photolytic
stability, hence they are hard to biodegrade. Besides, the dyes and their possible degraded
products are also carcinogenic, toxic, mutagenic, and teratogenic to living organisms. Therefore,
the purification or elimination of perilous pollutants from water is an urgent scenario in the
science community. In this corollary, azo dyes have been widely used in the textiles and other
industries are identified as potential genotoxic and carcinogenic agent as a result of their azo
functionalities. Among them azo dyes viz., methyl orange and congo red are the major
representative of around half of all of the dyes utilized in the industries [8]. A wide range of
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physico–chemical and biological methods, such as adsorption, ozonation, Fenton, photocatalysis,
reductive degradation, microbial process and two or more combination methods have been
successfully applied to the treatment of various dyes. Among these methods, reductive
degradation is a fast and low cost method, which is easy to implement for the industrial level dye
removal [9]. In relation to the above, there is a large extent rely on the development of novel
electrode materials with unique textures for enlightening the performance of supercapcitors and
dye degradation.
Recently, transition metal oxides/hydroxides/sulfides based materials have substantially
higher specific capacitance may therefore be potential candidates for supercapacitors due to the
satisfactory redox reaction of their metal ions [10]. Doping of heteroatoms into the transition
metal oxides/hydroxides network has proven not only the improvement of their performance, but
also better stability of the active catalytic center [11]. Particularly, transition metal oxides and
chalcogenides viz., NiS, NiO, Ni3S2, CuS@CNT, Cu2S. CuS/RGO/Ni3S2, Co9S8, Co3O4,
NiCo2O4, CoNi2S4, and NiCo2S4), possessing enhanced electrical conductivity, structural
stability, and reversible capacity compared with single component ones, have gained a
comprehensive attention for energy scarcity and environmental refinement [12-15]. Among the
family of metal sulfides, Ni3S2 is one of the most auspicious candidates as of its low cost,
environmentally-friendly nature and high theoretical capacitance. As mentioned earlier, Ni3S2
can also be used for photocatalytic degradation in addition to electrode material in energy storage
devices [16]. Furthermore, as one of the most important p-type semiconductors, copper sulfide
(CuxS) has fascinated substantial research attention because of their prospective applications in
photocatalysis, solar cell devices, biosensors and supercapacitor [15]. However, the composite of
two kinds of transition metal sulfides is rarely reported. These results suggest that fabrication of
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transition metal based bi-functional hybrid material with rational structure, and specific
composition by dissimilar methods is a breakthrough in the area of supercapacitors and catalysts.
Among the various synthetic strategies, microwave method and sonochemical synthesis share an
advantage of reduced reaction time. Apart from these, sonochemical method has been
successfully applied to the preparation of numerous nanostructured metal sulfides [17].
Conversely, these metal sulfides prepared by conventional methods usually require expensive,
toxic organic agents and high reaction temperatures, and produce pungent H2S from the sulfur
sources used [18]. While, thiourea is used as a reactant, structure-directing agent and also as the
flux to replace the toxic and expensive salts in the conventional synthesis process. In the
sonochemical process, thiourea undergoes hydrolysis reaction resulting in the production of S2−
ions the reaction is also accompanied by reduction of metal ions and being able to complex
effectively with metal ions as well as the source of sulfur [19].
Taking into account of all the aforementioned issues together, herein we have attempted
to prepare sulfur doped copper-nickel hydroxides with different morphologies via a simple
sonochemical way by fine-tuning thiourea (sulfur source) volumetric concentration.
Subsequently, the resulting hybrid foams were investigated for pseudo-capacitive performance,
and the resulting hybrid foam with superior specific capacitance was applied for the reductive
degradation of methyl orange (MO) and congo red (CR) in the presence of sodium borohydride.
Thus, the TU-6 hybrid foam shows excellent specific capacitive behavior, and catalytic activity
realistically confirms that there is a synergistic effect between the hydroxide shell and the sulfur
core might play an important role in the collective application of sulfur doped copper-nickel
hydroxides are promising solution to solve the energy and environment related problems.
2. Experimental
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2.1. Materials
Copper (II) chloride dihydrate (CuCl2·2H2O – 99 %), thiourea (N2H4CS, TU – 99 %),
sodium borohydride (NaBH4 – 98 %), methyl orange (C14H14N3NaO3S, MO – dye content 98 %),
congo red (C32H22N6Na2O6S2, CR – dye content 85 %), and anhydrous potassium hydroxide
(KOH – 99 %) were acquired from Sigma Aldrich, South Korea. Deionized water was used
throughout the experiments. All the chemicals were of analytical grade and used as received
without further purification. Nickel foam with a thickness of ~1.6 mm and 95% of porosity
(Heze Jiaotong Group Corp., South Korea) was used as the substrate and also employed as the
current collector for energy storage application and utilized as a catalyst for degradation analyses.
2.2. Electrode Fabrication
Ni foam (size: 1 cm2) was immersed into a mixture of solution consisting of 1% aq.
CuCl2 (5 ml) and 1 % aq. TU (2 mL, 4 mL, 6 mL, 8 mL, and 10 mL, respectively), then the total
volume of the solution was adjusted to 15 mL using deionized water. Subsequently, the precursor
solution was sealed in an auto-clavable bottle (50 mL) and irradiated in an ultrasonic bath
operating at 40 kHz ultrasonic waves (Branson-3800, China) for 30 min at 60 °C. After stewing,
the modified foam was cleaned with deionized water for several times to remove residuals and
dried in a vacuum at 70 °C for 1 h. For comparative study, the experiment was repeated without
the addition of TU. The tailored foams without and with TU were marked as TU-0, TU-2, TU-4,
TU-6, TU-8, and TU-10, respectively. The mass of the active material loaded on the modified
foams along with the standard deviations were 2.21±0.02 (TU-0), 1.29±0.04 (TU-2), 1.53±0.07
(TU-4), 2.01±0.03 (TU-6), 2.26±0.05 (TU-8), and 2.48±0.01 (TU-10) mg/cm2. The experimental
terms are listed in Table 1. The modified electrode shows good reproducibility for five
successive measurements on the same electrode.
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2.3. Characterization
The surface morphologies of as-obtained hybrids were acquired using Hitachi S-4800
scanning electron microscope (SEM). X-ray photoelectron spectroscopy (XPS) spectra of
resultant hybrids were recorded using an ESCALAB 250Xi photoelectron spectrometer (Thermo
Fisher Scientific, USA) with Al (Kα) excitation source.
2.4. Pseudo-capacitor performance quantification
The electrochemical behavior of as-obtained hybrid foams were performed using a
Corrtest-CS350 electrochemical workstation coupled with Cview and Zview software in a three-
electrode setup: modified Ni foam (working electrode), Pt plate (counter electrode), Ag/AgCl
(reference electrode), and 2 M KOH solution (electrolyte). The cyclic voltammetric (CV)
analysis was conducted between 0 and 0.5 V vs. Ag/AgCl at scan rates ranging from 2 to 75 mV
s−1. The specific capacitance was calculated using the following equation (1) [20];
IdV
∫
(1)
CS =
mVS
Cs (F.g-1) represents the specific capacitance; where ∫ IdV is the area between the scanned
potential (A.V); V (V) is the potential window; S (V s-1) is the scan rate and m (g) represents the
active mass.
The galvanostatic charge-discharge (GCD) analysis was carried out in a potential window
(0 to 0.5 V) at different current densities from 6 to 50 A.g−1. Generally, the specific capacitance
can also be determined from the discharge profile of the GCD curves. However, the discharge
curves exhibit a non-linear pattern, therefore, the capacitances were evaluated as per the equation
(2) using the integral area under the discharge curve [21];
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2im Vdt
∫
(2)
CS =
V 2
Cs (F.g-1) represents the specific capacitance; im (A.g-1) is the current density (where i =
current and m = active mass), ∫Vdt is the integral area of the discharge curve, where V is the
potential window. The electrochemical impedance study was done by attaining Nyquist plots by
applying an AC voltage of 5 mV in the frequency range from 0.01 Hz to 100 kHz at open circuit
potential.
2.5. Catalytic activity evaluation
MO and CR are effective chemicals used to evaluate the reductive-catalytic performance.
Typically, a piece of modified foam (0.5 cm width × 1.0 cm height) used as catalyst was blended
with 3 mL of MO or CR dye solution (1 mmol) taken in a 4 mL quartz cuvette. The reaction was
commenced by adding 0.5 mL of NaBH4 (0.1 M). The time dependent UV-vis spectra were
obtained in the range from 200 to 800 nm (visible region). The control experiments were also
carried out without modified foam. The degradation kinetics of CR and MO were also studied.
3. Results and Discussion
3.1. Formation mechanism
The plausible formation mechanism of hybrid foams with diverse architectures was
illustrated in Fig. 1. Sonochemistry is an acoustic phenomenon results from the constant
formation, growth and implosive collapse of bubbles in aqueous solution, which can achieve
terribly high temperature (>5000 K), high pressure (>20 MPa) and very high cooling rates (1010
K s−1) upon the collapse of bubbles. The radical species can originate from the water molecules
which decompose in the inner gas phase of the collapsing bubbles [22]. On the hole, thiourea
(TU) is used as a sulfur source for the synthesis of transition metal chalcogenides in aqueous
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medium. Lone pair of electrons on the nitrogen and sulfur atoms of TU, enables exceptional
n+
coordinating ability with metals (Mn+) to form M (TU)x complexes [23]. So, under the
ultrasound environments, TU molecule disintegration can be enhanced to generate CO2 gas, OH−
and S2− anions, and simultaneously the crystal nucleation rate of sulfur doper copper-nickel
hydroxides can be accelerated further. The main chemical reactions can be expressed as
following equations (3-7) [24].
2+
x M2+ + x TU
x TU + H2O
(MTU)x
(3)
(4)
(5)
(6)
(7)
(M = Cu, Ni)
+
2-
CO2 + 2NH4 + (S)x
(MS)x
x M2+ + x S2-
M2+ + OH-
M(OH)2
M(OH)2 + OH-
MOOH + H2O + e-
3.2. XPS analysis
In order to probe the surface composition and valence state of each element present in the
as-fabricated hybrids without and with TU, X-ray photoelectron spectroscopy (XPS) was
conducted and the respective survey scan was depicted in Fig. 2. The XPS survey spectrum of
TU-0 mainly consists of Cu, Ni, O, and Cl which is well agreed with our earlier report [25].
While, in the as-fabricated hybrids with TU showing all the anticipated elemental peaks,
including Cu, Ni and S. The intense O 1s peak at 532.4 eV of the tailored hybrids, demonstrated
the large degree of surface oxidation [26].The high-resolution XPS spectra and fitted result in Cu
2p, Ni 2p and S 2p regions of tailored hybrids were recorded and the corresponding binding
energies were represented in Table 2.
3.2.1. TU-2 hybrid
The high-resolution spectra of main elements present in TU-2 hybrid were shown in Fig.
3. As inferred from Fig. 3a, Cu 2p3/2 spin-orbit peaks were located at 932.7 and 952.8 eV. The
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corresponding shakeup satellite peak (Cu 2p1/2) was observed at 933.9 eV, which indicate the
presence of Cu2+ in the sample [27]. In the spectrum of Ni 2p (Fig. 3b), two main peaks at 874.2
and 856.5 eV can be attributed to Ni 2p1/2 and Ni 2p3/2 core level of Ni2+. The wide peaks around
861.4 and 879.7 eV were ascribed to the satellite peaks of main peaks, indicated the presence of
nickel–oxygen species on the surface. The spin-energy separation between main peaks of about
17.7 eV, which reiterates the formation of Ni(OH)2. Fig. 3c, exhibits an intensive peak at 162.9
eV belongs to S 2p3/2, confirmed the formation of CuS bonds [28-31].
3.2.2. TU-4 hybrid
Two main binding energy peaks located at 932.4 and 952.2 eV in Cu 2p region (Fig. S1a),
were assigned to Cu 2p3/2 and Cu 2p1/2, respectively, are ascribed to Cu2+ [32]. In Figure S1b,
two major peaks appeared at 856.4 and 874.1 eV, corresponds to Ni 2p1/2 and 2p3/2, respectively.
The spin-orbit energy difference between these two main peaks is approximately equal to the
standard value of Ni(OH)2 (17.7 eV) [33]. The peak at 162.5 eV can be attributed to the sulphur
2-
ion (S2-) in NiS2. The shoulder around 168-169 eV implies the presence of SO4 [26]. These
values subordinate with previously reported data [34].
3.2.3. TU-6 hybrid
The Cu 2p spectrum (Fig. 4a) exhibits two main peaks at 933.1 and 952.6 eV, resembles
to Cu 2p3/2 and Cu 2p1/2 spin-orbit peaks of the Cu2+, respectively [35]. In Fig. 4b, the two
prominent shake-up satellites peaks (862.4 and 880.6 eV), are the typical characteristic peaks of
Ni3+ species, distinctly corroborating the formation of NiOOH, was further supported from the
spin energy separation of about 18.3 eV [36]. Besides, the binding energies of Ni 2p1/2 and Ni
2p3/2 fitting peaks at 874.6 and 856.3 eV are characteristic Ni+ and Ni2+ species in Ni3S2 [37].
The binding energies at 161.6 and 162.8 eV correspond to S 2p3/2 and S 2p1/2 (Fig. 4c),
respectively, were accorded to the reported Ni3S2 data [35,38].
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3.2.4. TU-8 hybrid
The XPS spectra and fitted result in Cu 2p, Ni 2p and S 2p regions were shown in Fig. S2
(a–c). Two main binding energy peaks located at 934.0 eV and 952.8 eV are assigned to Cu 2p3/2
and Cu 2p1/2, respectively, these two peaks are ascribed to Cu2+ and peak at 932.4 eV is
analogous to Cu+ [39]. While, two main peaks at 875.5 and 857.0 eV were consigned with Ni
2p1/2 and Ni 2p3/2, respectively, accompanied by the satellite peaks (880.9 and 862.5 eV). The
energy difference between main peaks is 18.5 eV, suggesting the coexistence of Ni2+ and Ni3+
[40]. In S 2p region the peak located at 162.9 eV could be designated to S 2p1/2 in Ni3S4, and
2-
another peak at 168.4 eV corresponds to S 2p of SO4 reveals the oxidation of sulfur with
residual oxygen [39].
3.2.5. TU-10 hybrid
It can be seen from Fig. 5a, that Cu 2p3/2 peaks at 932.6 eV and 952.3 eV were
comparable to Cu+ [39]. The Ni 2p3/2 and Ni 2p1/2 emission spectra (Fig. 5b) were fitted with two
spin-orbit doublet characteristics of Ni2+ and Ni3+. In details, the Ni 2p3/2 spectrum shows a peak
at 856.3 eV that corresponds to the Ni2+ ions located in the octahedral sites and a peak at 873.4
eV that corresponds to Ni3+ ions located in the tetrahedral sites. Fig. 5c shows the high-resolution
S 2p spectrum, in which the two major peaks about 163.7 and 162.4 eV coincide with S2- in NiS
bonds on the surface [41,42].
3.3. Morphological analysis
Typically, the morphology, structure, and size of a material play an important role in its
final physical and chemical properties. Thus it is important for us to investigate the influence of
the morphology, structure, and dimensions of the hybrids to better understand why their
electrochemical properties vary. Thus, the thiourea concentration has an impact on the texture of
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our samples and the micro-structures of the tailored hybrids are shown in Fig. 6. It is observed
from Fig. 6a that a micro-wire like texture was obtained with 2 mL of TU. Irregular micro-
crystal like morphology was acquired by increasing the TU volume to 4 mL (Fig. 6b). When the
concentration of TU was 6 mL, the as-fabricated hybrid exhibits hierarchical micro-rosette like
texture (Fig. 6c). Upon further increase of TU volume to 8 mL and 10 mL, the hybrid products
displays micro-flakes (Fig. 6d) and micro-spheres (Fig. 6e) like structures, respectively. It
reveals that TU used in the present study plays an important role, which may alter the reaction
rate and final morphologies.
3.4. Electrochemical properties
The electrochemical properties of as-fabricated sulfur doped copper-nickel hydroxides
were evaluated in 2 M KOH aqueous solution. Fig. 7a shows a comparative CV curve of TU-2 to
TU-10 hybrids measured at a scan rate of 2 mV.s−1. It can be observed that all of the CV curves
exhibits a pair of well-defined redox peaks, revealing the typical pseudo-capacitive
characteristics [43]. The existence of redox peaks suggest that the capacitive ability is
predominantly aroused from the Faradaic reaction (similar to that of metal oxides and sulfides)
could be expressed as follows [44].
CuS + OH-
CuSOH + e-
(8)
NiS + OH-
NiSOH + e-
(9)
Ni3S2 + 3OH-
Ni(OH)2 + OH-
NiOOH + H2O + e-
Ni3S2(OH)3 + 3e-
NiOOH + H2O + e-
Ni(OH)2 + OH-
(10)
(11)
(12)
The area under the CV curve was used to estimate the capacitance of the system via
equation (1). In view of the above, the integral area of TU-6 is larger than those of other tailored
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hybrids, affirming that TU-6 hybrid possesses higher specific capacitance, may be attributed to
the enhanced electrical conductivity, fast electron transport and rapid ion diffusion process.
2−
Conversely, the as-obtained hybrids with SO4 anion possess smaller integral area, thus
displayed a limited specific capacitance. Moreover, the CV curves of TU-6 hybrid (Fig. 7b) at
different scan rates ranging from 5 to 75 mV.s−1 [45]. The calculated specific capacitance, as a
function of scan rate was plotted in Fig. 7c, the specific capacitance decreased from 1636 to 297
F.g−1, which can be imputed to the restricted diffusion of electrolytes at higher scan rate [35].
To quantify the capacitance of the electrode, galvanostatic charge–discharge (GCD)
profiles at different current densities were recorded. Fig. 8a presents the GCD curves of TU-2 to
TU-10 hybrids measured at a current density of 5 A.g−1. As inferred from Fig. 8a that, the
capacitance is mainly imports from pseudo-capacitance, which is in accordance with the CV
results. The specific capacitance (Cs) was calculated by equation (2) as a function of the current
density, by using the discharge curves. Compared with other hybrids, TU-6 hybrid accomplishes
highest specific capacitance (2708 F.g−1) at the same current density (5 A.g−1) and the best rate
capability. The massive difference in their structures is responsible for this dissimilarity. The
specific capacitances of TU-6 hybrid from a series of lower (6 to 10 A.g−1) to higher current
densities (20 to 50 A.g−1) were evaluated from the discharge curves and are plotted in Figs. 8b
and 8c. It should be noted that the specific capacitances decreases as the discharge current
density increases from 6 to 50 A.g−1 because of the increment of voltage drop and the
insufficient active material involved in the redox reaction at higher current densities. Notably,
when the current density increases from 6 to 10 A.g−1 (Fig. 8d), 86 % capacitance was retained
(2449 F.g−1 vs 2107 F.g−1). However, at high current density (20 to 50 A.g-1), preserve
capacitance retention as 37 % (1229 F.g−1 vs 796 F.g−1) which indicates that the device exhibits
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excellent rate capability and superior electrochemical capacitance (Fig. 8e). To the best of our
knowledge, this specific capacitance value is higher than those of metal/sulfide/oxide hybrid as
supercapacitor electrodes [46,47]. Even it is higher than our previously reported copper/nickel
oxide composite specific capacitance value 1773 Fg-1 (data not shown) fabricated without the
addition of TU (TU-0) [25]. The enhancement of supercapacitor performance of sulfur doped
copper-nickel hydroxides owing to the short ion/electron diffusion path and large contact area
between active materials and electrolyte [18]. This high specific pseudo-capacitance as well as
outstanding rate capability of TU-6 hybrid can be attributed to: (i) the synergistic effect
originating from the CuS2, provides excellent electrical conductivity due to direct growth on the
surface of nickel backbone, facilitating charge transport. (ii) Ni3S2/Ni(OH)2/NiOOH with unique
rosette-like texture with void spaces between the neighboring rosette offers a short route and
sufficient diffusion channels for good accessibility of the solution, ensured adequate surface-
dependent Faradic reactions even at high scan/current rates. [48,49].
The fundamental behavior of supercapcitors was further examined by electrochemical
impedance spectroscopy (EIS) measurements. The Nyquist plots of all the hybrids are illustrated
in Fig. 8f, with an enlarged view at high frequency of TU-2, TU-8 and TU-10 (inset) hybrids. All
the electrodes exhibit analogous spectra with only a straight line at low frequency, since no clear
semicircles can be observed at high frequency. Besides, the slope of TU-6 hybrid curve is the
largest in the low frequency regions than other studied composites, indicating better pseudo-
capacitive behavior with lower diffusion resistance [50,51]. In summary, TU-6 hybrid is a
promising electrode material for energy storage applications.
3.5. Evaluation of catalytic activity
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In the dyeing industry, azo dyes viz., methyl orange (MO) and congo red (CR) are widely
used as commercial coloring agents, have been preferred as the model substrate for analyzing the
catalytic activity [52]. The efficacy of TU-6 hybrid as a catalyst towards the degradation of MO
and CR using NaBH4 as a reducing agent was investigated. As inferred from Figs. 9a and 9b, that
MO and CR were found to exhibit absorption maxima of 465 and 495 nm in the visible region
due to the presence of azo groups and displayed corresponding red and orange color in water,
respectively. The catalytic degradation of these dyes was monitored by observing the changes in
absorbance of these peaks using UV-visible spectrometer at room temperature [53]. The
degradation of MO/CR did not proceed in the absence of either reducing agent (NaBH4) or the
catalyst (TU-6 hybrid). Moreover, after the addition of TU-6 composite to the reaction mixtures
containing MO (or) CR and NaBH4, instant degradation takes place, which can be noticed from
the decolorizing of both MO and CR at various time intervals via UV-vis spectra, respectively.
The gradual decrease of absorbance intensities with reaction time was due to the initiation of
degradation of MO and CR, which were completed within 8 min and 20 min in the presence of
TU-6 hybrid, respectively. Figs. S3a and S3b represent the reduction percentages of MO and CR
as a function of time. MO and CR degradation was negligibly small in the absence, whereas it
was completely reduced in the presence of investigated catalyst (TU-6 hybrid). The
corresponding ln(At/A0) vs. time plot is presented in Fig. 9c and the calculated rate constant
values for the reductive-degradation of MO and CR are 0.3765 and 0.0688 min-1, respectively.
Fig. 9d shows the re-usability of catalyst for the successive degradation of MO and CR. The
degradation time increases as the number of cycle increases, which indicated the lowering of
catalytic performance. Such a decrease in catalytic performance might be due to their oxidation
and slight release of hybrid particles into the solution [54].
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The superior degradation of TU-6 hybrid catalyst may be ascribed to the following
reasons; (i) NaBH4 act as electron donor and hydride source, TU-6 hybrid as the electron transfer
mediator, probably activated the azo nitrogen results in the weakening of azo bonds via
conjugation, (ii) mechanistically, adsorption occurs through the chemical interaction between M+
−
and SO3 /NH2 of MO/CR, (iii) rosette-like texture containing more number of active sites are
expected to display hydrophilic interactions with the azo dyes, which carry the dye molecules
near the catalytic sites, (iv) might be due to the fast generation of electron-hole pair in the
presence of S2-/Sn2- redox couples to maintain its stability [55,56]. The brief description for the
degradation of MO and CR has been presented in Fig. 10.
4. Conclusions
The fabrication, structure analysis, pseudo-capacitive behavior and degradation features
of sulfur doped copper-nickel hydroxides with hetero-structures were effectively accomplished
via a facile sonochemical route. It is demonstrated that the volume of thiourea has a significant
effect on the morphology of hybrid materials, which results in the growth of diverse textures
over nickel foam. This difference in morphology also accounts for the huge difference in their
final electrochemical performance. Among these materials, TU-6 hybrid was found to exhibit
high pseudo-capacitance (2708 F.g-1 at 5 A.g-1) along with excellent catalytic activity towards
the effective degradation of methyl orange (8 min) and congo red (20 min) in the presence of
NaBH4. The notable specific capacitance and catalytic activity of TU-6 hybrid, is ascribed to the
exceptional electrical conductivity of ternary metal sulfides/oxy hydroxides interfaces enables
fast electron transfer and mass transport, as well as the rosette-like texture affords large
electrochemically active sites for uniform dissemination of active materials. Thus, the
hydrophilic nature facilitates high concentration of organic compound adsorption and surface
16

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reaction. These results validate that the shape-controlled fabrication with the appropriate
composition could be a promising route for the energy storage and environmental applications.
†Electronic
Supplementary
Information
(ESI)
available.
See
DOI:
10.1039/x0xx00000x. Fig. S1 (a) Cu 2p, (b) Ni 2p and (c) S 2p XPS of TU-4 hybrid. Fig. S2 (a)
Cu 2p, (b) Ni 2p and (c) S 2p XPS of TU-8 hybrid. Fig. S3 (a) % of degradation efficiency of
MO and (b) CR with and without TU-6 hybrid, respectively.
.
17

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Fig. 1. Plausible schematic illustration of formation of diverse architecture of sulfur doped
copper-nickel hydroxides.
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(a) TU-0
(b) TU-2
(c) TU-4
(d) TU-6
(e) TU-8
(f) TU-10
1400000
1200000
1000000
800000
600000
400000
200000
0
Cu 2p
Cu 2p
O KLL
(a)
(b)
(c)
(d)
(e)
(f)
0
200
400
600
800
1000
1200
Binding energy (eV)
Fig. 2. XPS survey spectrum of as-fabricated different hybrids without and with TU.
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(a) 110000
100000
90000
45000
44000
43000
42000
41000
40000
39000
38000
37000
(b)
Cu 2p
Ni 2p
2p
3/2
856.5
CuS 2p
932.7
3/2
CuS 2p
952.8
Sat. 2
861.4
1/2
2p
1/2
874.2
Sat. 2p
80000
1/2
933.9
Sat.
1
2p
3/2
855.3
879.7
70000
60000
50000
17.7 eV
40000
30000
965
960
955
950
945
940
935
930
925
880
875
870
865
860
855
850
845
Binding energy (eV)
Binding energy (eV)
22000
20000
18000
16000
14000
12000
10000
8000
(c)
S 2p
S-S 2p
1/2
164.1
CuS 2p
162.9
3/2
6000
170
168
166
164
162
160
158
Binding energy (eV)
Fig. 3. (a) Cu 2p, (b) Ni 2p and (c) S 2p XPS of TU-2 hybrid.
28