International Journal of Applied Chemistry.
ISSN 0973-1792 Volume 15, Number 2 (2019) pp. 81-97
© Research India Publications
http://www.ripublication.com
Electrochemical synthesis of 3D hierarchical
Co3S4/Co9S8 nanoparticles as photocatalysts for
degradation of Carboxylic acids
Gubran Alnaggar, Sanniaha Ananda*
Department of studies in chemistry, University of Mysore, Manasagangotri, Mysore- 570006, India.
*Corresponding author
Abstract
Three dimensional (3D) hierarchical Co3S4/Co9S8 nanoparticles were
synthesised by a facile electrochemical method. The structural, morphology
and optical properties of prepared Co3S4/Co9S8 nanoparticles are characterized
by X-ray diffraction (XRD), scanning electron microscope (SEM), UV-vis
spectroscopy, FTIR spectroscopy. The XRD pattern shows that the present of
spinel Co3S4 with hexagonal Co9S8 nanoparticles. The SEM images have been
confirmed the XRD results. The band gap of the prepared Co3S4/Co9S8
nanoparticles was found to be (2.88 eV) lesser than pure hexagonal Co9S8
which can be attributed to the present of a spinel Co3S4 with hexagonal Co9S8.
The degradation of carboxylic acids mainly acetic acid, formic acid and oxalic
acid as organic pollutants were used to estimate the catalytic activity of
prepared Co3S4/Co9S8 nanoparticles. The Taft linear free energy relationship
(LFER) was tested, isokinetic temperature β was calculated for oxidation of
carboxylic acids. The biological activity for prepared Co3S4/Co9S8
nanoparticles has been studied. The result indicates that the prepared
Co3S4/Co9S8 nanoparticles displayed high photocatalytic activity under
sunlight and the oxidation /degradation of carboxylic acids was enthalpy
controlled.
Keywords: Cobalt sulphide, electrochemical, photo catalyst, LFER and
biological activity
INTRODUCTION
Chalcogenides nanoparticles are extremely studied due to their significant
applications and size dependent properties. Metal chalcogenides like vanadium
82 Nur A. Limatahu, Nur Jannah Baturante, St. Hayatun Nur Abu, Nurul Aulia Rahman
sulphide, molybdenum sulphide, nickel sulphide and cobalt sulphide have been
studied for several kinds of applications [1-5].
Cobalt sulphide compounds with varied stoichiometric compositions, like Co9S8,
CoS2, CoS and Co1-xS, have attracted significant attention due to their electronic,
chemical, physical, and optical properties and their wide range of potential application
in Li-ion batteries, electrochemical supercapacitor and catalyst, owing to easy
oxidation of sulphide ion and easy reduction of cobalt ion which outcomes in the
hardness in controlling the stoichiometry of cobalt sulphide. Cobalt sulphide has been recorded as the most challenging metal chalcogenides to synthesise [1, 3-10].
Cobalt sulphides usually have many stable phases and intricate structure. However,
Co9S8 nanoparticles have attracted considerable attention. For instant, Co9S8 has
significant catalytic activity such as in hydrodesulphurization catalysts and magnetic
devices [2,7,10-12].
So far, cobalt sulphides have synthesised with different stiochiometry composition
and morphologies by several synthesis methods including hydrothermal, microwave-
assisted techniques, solvothermal method, wet-chemical methods, colloidal syntheses,
electrochemical method and catalytic chemical vapour deposition [3,7-10,12-14].
Colourless organic compounds are one of main class of environmental pollutants. The
organic acids are most widely used in the field of pharmaceutical, food, beverages and
also in chemical industries. Carboxylic Acids such as Oxalic acid, Formic acid and
Acidic acid are common Pollutants in industrial wastewater. Acetic acid is used in the
synthesis of plastics and acetyl cellulose and also in the dyeing and printing industry,
as well as in the food industries. Formic acid is used in the tanning, in rubber
processing, in textile industry and in the manufacture of pharmaceuticals [15-19].
Herein, cobalt sulphide nanoparticles were synthesised by a facile electrochemical
process which display a superior photocatalytic activity, under sunlight irradiation, for
different type of carboxylic acids mainly Acetic acid, Formic acid and Oxalic acid as
colourless pollutant organic compound. The volumetric titration methodology was
used to determine the degradation efficiency and results proved the complete
degradation of those pollutants.
The obtained cobalt sulphides nanoparticles and their structural, optical properties and
photocatalytic activity are studied.
EXPERIMENTAL PROCEDURE:
Materials:
Cobalt (II) Chloride hexahydrate (CoCl2 6H2O, Alfa Aesar, 99%), Sodium
sulphide (Na2S, Alfa Aesar, 99%) and Platinum electrode from Elico Pvt. Ltd were
used in the present work. All chemicals were used without further purification.
Deionised water produced by a PURELAB Ultra water purification system was used
for all the experiments.
Electrochemical synthesis of 3D hierarchical Co3S4/Co9S8 nanoparticles… 83
Synthesis:
Cobalt sulphide nanoparticals was synthesized by electrochemical process (E/C) using
cobalt coated Pt-electrodes in an aqueous system with sodium sulphide as a
conductive salt. Sodium sulphide was used as the sulphur source. The E/C synthesis
of nanoparticals of Cobalt sulphide involves two steps (Scheme 1), first,
electrochemically Cobalt is deposited on the surface of Pt-electrode from an aqueous
solution containing Cobalt ion, second, the Co2+ coated electrode is connected in
electrochemical cell as anode and gets oxidized and uncoated pt-electrode connected
as cathode in sodium sulphide solution (0.2M, 20 ml) to yield semiconducting Cobalt
sulphide nanoparticles. In the electrochemical cell, the electrodes were fixed in a
holder with a side distance of 1.5 cm, so that they were totally surrounded by the
electrolyte. DC source was used to apply 1.7 V as a potential difference between the
electrodes. The synthesis process was conducted galvanostatically at current density,
j, of 30.0 mA cm-2, without stirring, to simplify the formation of the crystal at room
temperature for 2 h (scheme 1).
In such synthesis, the size of the nanoparticle depends upon various parameters such
as current density, applied voltage, distance between 2 electrodes, concentration and
volume of Na2S solution so, synthesis was well optimized. As a result, yield of Cobalt
sulphide nanoparticle obtained is 0.6–1.5 g. obtaining a more amount of the
nanoparticles product, the repetition of the whole reaction is required at the standard
conditions. The obtained nanoparticles were washed several times with distilled
water, centrifuged and calcined for 1 h at 700 C0 to remove impurities which might be
formed due to atmospheric oxidation and electrolysis.
Scheme1. Electrochemical Synthesis of cobalt sulphide nanoparticles
Characterisation:
The structure of obtain cobalt sulphides nanoparticles were investigated by X-ray
diffractometer with a Cu Kα radiation source of λ = 1.5406 Å wavelength. The range
84 Nur A. Limatahu, Nur Jannah Baturante, St. Hayatun Nur Abu, Nurul Aulia Rahman
of scanning was 6–80° in steps of 0.02°. Field emission scanning electron microscope
(FE-SEM /EDX) was employed to study the morphology and the composition of the
prepared nanoparticles. Also, the optical properties was studied by Diffuse
Reflectance UV-vis spectrophotometer (UV-vis DRS, Shimadzu UV-3600 UV) and
Fourier transform infrared spectroscopy was recorded using (FT-IR, JASCO 460 plus,
using KBr wafers).
Photocatalytic study:
The sunlight photocatalytic performance for prepared cobalt sulphide nanoparticles
was investigated by decomposition of Carboxylic Acids (as colourless compounds)
mainly, Acetic Acid, Oxalic Acid and Formic Acid. In a typical experiment, 20 ml of
each Acid was taken to conduct the experiment. The effect of initial carboxylic acid
concentration, catalyst dosage and the Temperature in the degradation rate were
studied. In each case, to study the effect of photocatalyst weight in the degradation
rate 20, 30, 40 and 60 mg of the as-prepared photocatalyst were add to 20 ml of
carboxylic acid solution separately. In contrast, (0.0005, 0.001 and 0.002 Normality)
of each carboxylic acid were used to investigate the initial concentration effect of the
corresponding carboxylic acids, whereas the effect of Temperature was done at three
different temperature mainly 293, 313 and 333 Kelvin.
The mineralization of carboxylic acids was measured by the drop down value of
Chemical Oxygen Demand COD of the solution. Standard dichromate titration
method was used to determine COD before and after the degradation process. The
COD value and the degradation efficiency were calculated [20][21][22] by the
following equation
COD = 8000 (vol. of FAS in blank – vol. of FAS in acid soln.) normality of FAS ∕
sample volume
Photodegradation kinetics by volumetric method:
Volumetric titration is widely-used in quantitative analytical method. In the present
work, the volumetric titration methodology was used to determine the degradation
efficiency. The concentrations of the different carboxylic acids used in this work were
measured by the titration against standard sodium hydroxide solution in the present of
phenolphthalein indicator. The volume of sodium hydroxide solution required to react
at different interval of time with the acid before and after the degradation was
compared and recorded.
RESULTS AND DISCUSSION:
Morphology and structural properties:
Fig.1. shows the XRD Pattern of prepared cobalt sulphide nanoparticles. The analysis
of the obtained peaks indicated the presence of mixed phases of cobalt sulphide. The
Electrochemical synthesis of 3D hierarchical Co3S4/Co9S8 nanoparticles… 85
high intensity peaks corresponded to (200), (220), (311), (400), (422), (511), (440),
(531) and (533) crystal planes are coherent with the standard pattern of hexagonal
Co9S8 (JCPDS card No. 75-2023).[10] The other six diffraction peaks, mainly (220),
(311), (400), (511), (440) and (620) crystal planes are well matched with standard
pattern of spinel Co3S4 (JCPDS card no. 19-0367)[23]. The high intensity peaks
indicate the high crystalline product. The average crystalline size of the prepared
particles (D) was calculated by using Scherrer’s formula is 44nm.
Figure1. Powder XRD Pattern of Cobalt sulphide nanoparticles
The morphology and surface properties of the as-prepared nanoparticles were studied
by using Scanning Electron Microscopy (SEM). The SEM Images of the as-prepared
nanoparticles were shown in Fig.2. It is clear that the presence of two phases in the
prepared cobalt sulphide nanoparticles. Two different morphologies are available
which one can notice that easily. The first and major one is hexagonal structure
(surrounded by black circles) which beautifully decorated by the second one with
flower-like nanoparticles (pointed by arrows), it is in good agreement to XRD
Results, as XRD explanation, the hexagonal structure representing Co9S8 nanparticles
and the flower-like structure (spinel structure ) representing Co3S4 nanoparticles.
FTIR spectroscopy study:
The FT-IR spectrum of prepared cobalt sulphide nanoparticles was shown in Fig.3,
bands at 400, 422, 449, 484, 550, 600, 620 and 720 (symmetrical stretch) and 1100
86 Nur A. Limatahu, Nur Jannah Baturante, St. Hayatun Nur Abu, Nurul Aulia Rahman
cm-1 (asymmetrical stretch) were assigned to sulphides. The band at 484 cm-1
frequency could be attributed to S-S disulphide group which indicates about
polysulfide formation [3][6], which is in agreement to XRD results.
Figure 2. SEM images of Cobalt sulphide nanoparticles
Figure 3. FTIR spectra of Cobalt sulphide nanoparticles
Optical properties
To understand the optical behaviour of prepared cobalt sulphide nanoparticles, the
optical absorption at room temperature was investigated by using UV-Vis
spectroscopy. Figure (4) shows the absorption spectrum of prepared cobalt sulphide
nanoparticles. It showed that the maximum absorption peak at (415nm) which
indicate the prepared cobalt sulphide nanoparticles visible light active. Based in the
direct transition, the band gap was calculated from a Tauc plot (inset, fig.4) and found
Electrochemical synthesis of 3D hierarchical Co3S4/Co9S8 nanoparticles… 87
to be (2.88 eV). According to the literature reports, the band gap value of pure Co8S9
is 3.2 eV [2]. The decrease in band gap value could be attributed to the formation of
composite from two different phases of cobalt sulphide where the flower-like
structure (spinel structure) represented Co3S4 nanoparticles acts as doping material
with the hexagonal structure of Co8S9.
Figure4. UV-vis spectra and Tauc’s plot
Photocatalytic activity and COD Measurements:
The kinetic reaction constants have been obtained in the presence of sun light, by
taking the slope of ln (V/V0) versus time as showed in Fig.6a, 6b where V0 is the
volume before solar light irradiation and the V is the volume at different interval of
time. The kinetics indicates that the photocatalysis obey first order reaction
mechanism. The COD measurement of the carboxylic acids solution was carried out
before and after degradation by Potassium dichromate methods as shown in Fig. 6b,
6d and Table-1. The results showed the disappearance of carboxylic which indicates
that it has undergone decomposition [24]. The rate of degradation of carboxylic acids
follows the trend.
Oxalic acid > Formic acid > Acetic acid
This could be attributed to the fact that the electron donating capacity decreases the
rate of degradation reaction, which is supported by Taft LEFR.
88 Nur A. Limatahu, Nur Jannah Baturante, St. Hayatun Nur Abu, Nurul Aulia Rahman
Effect of initial acid concentration:
The effect of initial acids concentration on the degradation efficiency was tested by
using three different concentrations mainly (0.0005, 0.001 and 0.002 N), as shown in
Fig.5a. The photodegradation efficiency was decreased with increased the initial
carboxylic acids concentration. Noticeably, with the increase of carboxylic acids
concentration the kinetics rate constant decrease. This can be ascribed to that, higher
carboxylic acids concentration leads to higher acid molecules around the catalyst
which prevents the absorbance of sunlight by the surface of the catalyst. As a result,
the formation of •O2-and •OH decreases and hence the rate of the reaction decrease, as
a result, the photocatalytic efficiency decreases [21, 25, 26] Table-1.
Effect of catalyst dosage:
The effect of catalyst dosage on the degradation efficiency for acids (0.001 N)
different amount of catalyst has used 0.02, 0.03, 0.04 and 0.06 gm. As shown in
Fig.5c as the catalyst dosage increase from 0.02g to 0.04 the photodegradation
efficiency increase, further increasing in catalyst dosage above 0.04g, the
photodegradation efficiency decreases. Initially, the numbers of the active sites in the
surface of the catalyst increase so the efficiency of the degradation increase, but the
present of more particles of catalyst causes light scattering which effect on the light
absorption by the catalyst and thus the number of active sites species [20, 25, 26]. The
kinetics rate constant for the degradation reaction and degradation efficiency of
carboxylic acids were the highest at 0.03g catalyst dosage Table-1.
Effect of Temperature:
The decomposition of the carboxylic acids solution carried out at three different
temperatures Fig.6a. It has been found that with increase of the temperature the
degradation rate also increase [21, 26] Table-1. The thermodynamic parameters are
calculated Table-3.
Reusability of the photocatalyst:
The possibility of using the photocatalyst again for degradation reaction was
examined Fig.6b. The used photocatalyst was filtered from the first solution and used
for fresh acid solution. The result shows that the photocatalyst still active but with
lesser degradation efficiency Table-1.
Linear free energy relation (LFER):
Attempts were made to arrive at a linear free energy relation for the
oxidation/degradation of carboxylic acids by using cobalt sulphide nanoparticles [27].
Test of Taft equation was obtained for plot of logk v/s σ* (σ* = Polar substituent
Electrochemical synthesis of 3D hierarchical Co3S4/Co9S8 nanoparticles… 89
constant) Fig.7b. The following Regression was obtained [28]
log k = 0.27 σ* ─ 3.13 (r = 0.993)
The positive slope value of polar constant ρ* although small it indicates that electron
donating capacity decreases the rate of degradation [27-28] as it is seen from Table-2.
The rate of oxidation/ degradation of carboxylic acids by using cobalt sulphide
nanoparticles decrease in the order
Oxalic acid > Formic acid >Acetic acid
The activation energy value is highest for the slowest reaction and vice-versa,
indicating that the reaction is enthalpy controlled Table-3. The activation enthalpies
(∆H#) and entropy (∆S#) for the degradation of carboxylic acids through oxidation are
linearly related Table-4. From the slope of ∆H# v/s ∆S# Fig.7c the isokinetic
temperature β was calculated and found to be 381 K0. [29] This is further verified by
employing the Exner criterion with a plot of logk1 293 K0 v/s log K2 333K0 which is
linear Fig.7d. From the Exner’s slope β was calculated by using following expression
[30] and found to be 388.5 K0.
β = T2T1 (b-1) / (b T2 – T1)
Where, k1 and k2 are the rate constants at the temperature T2 and T1 respectively and
T2>T1, b is the slope of logk2 against logk1.
The value of β is higher than the temperature range employed in the present work,
supporting the fact that the oxidation of carboxylic acids is enthalpy controlled. In this
region, the reaction with the lowest activation energy will react the fastest.
The fairly high negative values of entropy of activation point towards the formation of
fairly rigid activated state [29] Table-3. The constancy of ∆G# values indicates that the
carboxylic acids undergo oxidation/degradation via an identical mechanism [29-31].
Antibacterial assay:
The antibacterial susceptibility of synthesized Cobalt sulphide nanoparticles was
evaluated by using the disc diffusion Kirby-Bauer method in Mueller Hinton Agar
Plate [32]. All reference bacterial strains were obtained from Microbial Typing
Culture Collection (MTCC), Chandigarh, India. Gram-positive Bacillus subtilis (MTCC 2763) and gram-negative Escherichia coli (MTCC 40) were cultured as per
the protocol prescribed by MTCC.
Antimicrobial activity
To test disc diffusion assay, 20 mL of sterilized and molten Mueller Hinton Agar
90 Nur A. Limatahu, Nur Jannah Baturante, St. Hayatun Nur Abu, Nurul Aulia Rahman
media was poured in to the sterilized petri plates. The reference bacterial strains were
cultured overnight at 37 °C in Mueller Hinton broth and adjusted to a final density of
107 CFU/mL by 0.5 McFarland standards. 100 μL of the pathogenic bacteria cultures
were transferred onto plate and made culture lawn. The comparative stability of discs
containing Gentamycin was made. Test synthesized Cobalt sulphide nanoparticles
were loaded into 6 mm sterile discs and placed on the culture plates and incubated at
370C for 24 hours. By measuring the diameter of the ZOI formed around the disc, the
antibacterial efficacy of synthesized Cobalt sulphide nanoparticles was determined
[33]. All assays were performed in triplicates.
Figure8. Shows the growth inhibition Compound inhibited bacterial growth by the
clear inhibition zone (a concentration of30 μg/ml). The diameter of inhibition zones
(in millimetres) around the Compound against test strain are shown in Table 4.
Table 4 Antibacterial effect of synthesized Cobalt sulphide nanoparticles by Zone of
Inhibition (mm) against test strains
Test Bacteria Co3S4/Co9S8 Positive control Gentamycin (30
μg/ml)
Bacillus subtilis MTCC 2763
12.07 ± 0.12 30.05 ± 0.07
Escherichia coli MTCC 40 11.91 ± 0.04 26.14 ± 0.12
Note: Values are the mean ± SE of triplicate experiments
Figurer8. Zone of inhibition against (A) Bacillus subtilis and
(B) Escherichia coli bacteria
Electrochemical synthesis of 3D hierarchical Co3S4/Co9S8 nanoparticles… 91
Table (1) Effect of different variation on the rate of photo degradation of carboxylic
acids (Oxalic acid, Formic acid and Acetic acid)
Carboxylic
Acid
Concentration
Of the acids
(N, 20 ml)
Dosage
in g
Rate
constant
*104 S-1
Time (min)
For complete
degradation
COD (mg/L) Deg.
Efficiency
(%) Before
degradation
After
degradation
Oxalic
acid
0.001
0.02 13.2 60
816
60 92.6
0.03 23 40 15 98.2
0.04 4.6 20 15 98.2
0.06 1.1 60 100 87.7
0.0005
0.03
32 20 610 8 98.7
0.001 23 40 816 15 98.2
0.002 9.3 60 991 85 91.4
Reuse(0.001) 17 40 816 30 96.3
Effect of Temp.
(K0)
293
0.03
11.5 60
816
15 98.2
313 23 40 15 98.2
333 60 10 15 98.2
Formic
acid
0.001
0.02 4.9 50
672
55 91.8
0.03 12 40 13 98
0.04 18 30 13 98
0.06 1.88 60 80 88.1
0.0005
0.03
17 30 499 7 98.6
0.001 12 40 672 13 98
0.002 3.8 60 792 77 88.5
Reuse(0.001) 9.5 40 672 20 97
Effect of Temp.
(K0)
293
0.03
3.8 60
672
13 98
313 12 40 13 98
333 39 30 13 98
Acetic acid
0.001
0.02 3.35 60
544
60 88.9
0.03 6.6 50 9 98.3
0.04 7.22 30 9 98.3
0.06 1.78 60 100 81.6
0.0005
0.03
13.7 20 443 5 98.9
0.001 6.6 50 544 9 98.3
0.002 4.6 60 692 85 87.7
Reuse(0.001) 4.5 50 544 15 97.2
Effect of Temp.
(K0)
293
0.03
2.25 60
544
12 97.8
313 6.6 50 9 98.3
333 25 40 8 98.5
92 Nur A. Limatahu, Nur Jannah Baturante, St. Hayatun Nur Abu, Nurul Aulia Rahman
Table (2) Rate constants of Carboxylic acid
(T=313 K0, [Acid] = 0.001 N, Catalyst dosage = 0.03g)
Carboxylic Acid Rate constant
(k, S-1) 4+log k σ*
Oxalic Acid 23 x 10-4 1.4 2.08
Formic Acid 12 x 10-4 1.08 0.49
Acetic Acid 6.6 x 10-4 0.81 0
Table (3) Thermodynamics parameters
Substrates Temperature
(Kelvin)
∆H#
(KJ/mol)
∆ S#
(J/mol.K)
∆ G#
(KJ/mol)
Ea
(kcal/mol)
Oxalic acid
293 35.82 -178.57 88.15
9.15 313 35.69 -181.71 92.59
333 35.48 -181.58 95.98
Formic acid
293 45.44 -155.06 90.92
11.44 313 45.27 -156.52 94.43
333 45.10 -156.44 97.24
Acetic acid
293 58.58 -113.76 92.17
14.64 313 58.65 -118.74 95.85
333 58.49 -119.95 98.45
Electrochemical synthesis of 3D hierarchical Co3S4/Co9S8 nanoparticles… 93
Figure5. (a) Effect of initial acid concentration in the Photodegradation Kinetic of
Acids (b) COD value before and after the degradation at different concentration of
carboxylic acid(c) Effect of dosage in the Photodegradation Kinetic of carboxylic
acids, (d) COD value before and after the degradation at different catalyst dosage
Figure6. (a) Effect of Temperature in the Photodegradation Kinetic of carboxylic
acids (b) Degradation efficiency of the Reusability of the catalyst
94 Nur A. Limatahu, Nur Jannah Baturante, St. Hayatun Nur Abu, Nurul Aulia Rahman
Fig.7. (a) Arrhenius plot for carboxylic acids (b) Taft plot LFER for carboxylic acids
(c) Isokinetic Temperature relationship for carboxylic acids (d) Exner criterion for
carboxylic acids
CONCLUSION:
In the current work, 3D hierarchical Co3S4/Co9S8 nanoparticles are synthesized by a
facile electrochemical method. The prepared Co3S4/Co9S8 nanoparticles exhibit lesser
band gap value than pure Co9S8 which can be attributed to the present of Co3S4 as
doped material. The obtained Co3S4/Co9S8 nanoparticles exhibit high activity in the
photodegradation of carboxylic acid mainly acetic acid, formic acid and oxalic acid
and exhibit moderate biological activity. LFER indicate similar mechanism involves
in the degradation of carboxylic acids.
Acknowledgements:
The authors Acknowledge UGC (BSR) DST-PURSE Programme, IOE, UPE CPEPA
and University of Mysore, Mysuru, India
Conflict of interest
The authors declare that there is no conflict of interests regarding the publication of
this article.
Electrochemical synthesis of 3D hierarchical Co3S4/Co9S8 nanoparticles… 95
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