Efficient solar light Driven photocatalytic degradation of congo red dye on CdS nanostructures derived from single source precursor

Article abstract of DOI:10.1166/jnn.2018.16038

This article presents a facile synthesis of cadmium sulfide nanorods (CdS-NR) and nanospheres (CdS-NS) using cadmium(II) dithiocarbamate as a precursor in the presence of two different ther-malizing solvents {ethylenediamine (en) and octylamine (OA)}. The as-synthesized CdS-NR and CdS-NS were characterizedby powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), selected area electron diffraction technique (SAED), fourier transmission infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDS), UV-visible spectroscopy, steady-state and time resolved photoluminesce (PL). TEM analysis has confirmed the formation of nanorods and nanospheres in the presence of en and OA, respectively. The in situ generated different capping ligands, as confirmed by FT-IR analysis, may be responsible for variation in morphology. The XRD results revealed hexagonal (nanorods) and cubic mixed hexagonal phase (nanospheres). Owing to band gapes (UV-Vis) in the visible region, both these nanostructures (nanorods and nanospheres) were tested as a solar light drive photocatalyst for the degradation of Congo red (CR). The results indicated that CdS-NR exhibit better catalytic efficiency (K_app = 0.366 min^?1 toward CR degradation as compared to CdS-NS (K_app = 0.299 min^?1. The better photocatalytic activity of nanorods can be attributed to the anisotropically grown structure which infers longer electron hole recombination time as revealed by the time resolved PL.

Full text of DOI:10.1166/jnn.2018.16038

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Nanoscience and Nanotechnology
Vol. 18, 7405–7413, 2018
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Efficient Solar Light Driven Photocatalytic Degradation of
Congo Red Dye on CdS Nanostructures Derived from
Single Source Precursor
Jamal Abdul Nasir1, Shaheen Gul1, Azam Khan1, Zawar Hussain Shah1, Abrar Ahmad1, Zulfiqar2,
Rajwali Khan2, Zhongxin Liu3, Wei Chen4, Dan-Jae Lin5, and Zia-ur-Rehman1ꢀ∗
1Department of Chemistry, Quaid-i-Azam University Islamabad-45320, Pakistan
2Department of Physics, Abdul Wali Khan University Mardan 23200, KPK, Pakistan
3Key Laboratory of Advanced Materials of Tropical Island Resources, Ministry of Education; State Key Lab of
Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, China
4Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
5Department of Dental Hygiene, China Medical University, Taichung, Taiwan 40402, ROC
This article presents a facile synthesis of cadmium sulfide nanorods (CdS-NR) and nanospheres
(CdS-NS) using cadmium(II) dithiocarbamate as a precursor in the presence of two different ther-
malizing solvents {ethylenediamine (en) and octylamine (OA)}. The as-synthesized CdS-NR and
CdS-NS were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy
IP: 193.22.14.182 On: Mon, 20 Aug 2018 07:52:44
(TEM), selected area electron diffraction technique (SAED), fourier transmission infrared spec-
Copyright: American Scientific Publishers
troscopy (FTIR), energy dispersive X-ray spectroscopy (EDS), UV-visible spectroscopy, steady-state
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and time resolved photoluminesce (PL). TEM analysis has confirmed the formation of nanorods and
nanospheres in the presence of en and OA, respectively. The in situ generated different capping
ligands, as confirmed by FT-IR analysis, may be responsible for variation in morphology. The XRD
results revealed hexagonal (nanorods) and cubic mixed hexagonal phase (nanospheres). Owing to
band gapes (UV-Vis) in the visible region, both these nanostructures (nanorods and nanospheres)
were tested as a solar light drive photocatalyst for the degradation of Congo red (CR). The results
indicated that CdS-NR exhibit better catalytic efficiency (Kapp = 0.366 min−1ꢀ toward CR degradation
as compared to CdS-NS (Kapp = 0.299 min−1ꢀ. The better photocatalytic activity of nanorods can
be attributed to the anisotropically grown structure which infers longer electron hole recombination
time as revealed by the time resolved PL.
Keywords: CdS, Nanorods, Nanospheres, Mechanism, Photocatalysis.
1. INTRODUCTION
waste and required supporting materials are the problems
associated with aforesaid techniques.11 In recent years,
research has been focused on the development of effi-
cient photocatalyst to harvest the abundant solar energy
for generating environment friendly fuels (H2ꢁ and remove
organic pollutants to meet highly mounting energy crisis
and ensure safe and clean environment. In addition, envi-
ronmental friendliness photocatalytic dye degradation pro-
cesses is considered a cost-effective approach to degrade
organic pollutants.12
Synthetic dyes are widely used in various branches of
textile industry.1ꢀ2 Study survey estimated that 1–15%
of the synthetic dyes are lost during dying and finish-
ing processes i.e., 1–2% during production and 10–12%
during use. The lost and release of these dyes into
the wastewaters is a serious problem to aquatic bio-
logical processes.3–5 To mitigate this, a wide range
of methodologies have been developed such as ozone
treatment,6 adsorption,7 filtration,8 solvent extraction9
and biodegradation.10 However, formation of secondary
In this context various nanostructured semiconduc-
tors have been investigated as photocatalysts13 such as
nitrides,14 oxides,15–20 oxy-nitrides21–23 and sulfides24–27 to
address the mentioned problem of great concern. TiO2 is
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2018, Vol. 18, No. 11
1533-4880/2018/18/7405/009
doi:10.1166/jnn.2018.16038
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Efficient Solar Light Driven Photocatalytic Degradation of Congo Red Dye on CdS Nanostructures
Nasir et al.
considered the most durable photocatalyst for the remedia-
tion of environmental pollution.28 However, TiO2 has band
gap of about 3.0 to 3.2 eV and can only be activated by
UV light with a wavelength below 400 nm. This hamper its
uses as photocatalyst, because the solar spectrum contain
only 3–4% of UV light.29 So it is direly needed to develop
semiconductor that can harvest the bounteous visible light
(43%) of the solar spectrum. Among various type of the
semiconductors, metal sulfide nanocatalysts are promising
candidates for organic dyes degradation due to their pow-
erful oxidation capability.30ꢀ31 CdS, a visible light driven
photocatalyst is considered as a promising material for this
purpose because of its suitable band gap (2.42 eV), good
photochemical and photocatalytic properties.12 However,
the fast recombination process hinders its effective use. To
enhance the photocatalytic efficiency of CdS photocatalyst
some efforts have been made such as controlling the mor-
phology, particle size and crystallinity.32ꢀ33 These features
directly influence the position of band gap structure, trans-
port of the photogenerated carriers and their separation. So
far, numerous morphologies of CdS nanostructures such
as nanosheets, nanorods,34 micro/nano leaves,35 hollow
microrods,36 nanosteps,37 microtower,38 and multi-armed
nanorods39 have been synthesized by different techniques
and evaluated for photocatalysis.
2. EXPERIMENTAL DETAILS
2.1. Materials
Diethylamine and CS2 were purchased from Fluka. Congo
red (M.W = 696.67, C32H24N6O6S2Na2ꢁ was purchased
from Merck. Methanol, ethylenediamine, octylamine and
all other solvents and metal salts of analytical grade were
purchased from Sigma Aldrich and used without further
purification.
2.2. Synthesis of Ligand Salt, Cd(II) Dithiocarbamates
Complex and CdS Nanostructures
The
ligand,
Sodium
diethylcarbamodithioate
(C5H10NS2Na), was synthesized as described in our previ-
ous report.40 For the synthesis of Cd(II) complex, Sodium
diethylcarbamodithioate (171 g/mol, 1 g, 0.0058 moles),
was dissolved in methanol and added dropwise to
methanolic solution of cadmium nitrate tetrahydrate
(Cd(NO3ꢁ2 ·4H2O, 236 g/mol, 1.36 g, 0.0058 moles) with
constant stirring, which was continued for four hours. The
resulted cadmium complex [Cd(C5H10NS2ꢁ2] was filtered
off, washed with methanol and dried under IR lamp. The
as-synthesized complex was decomposed in two different
solvents i.e., ethylenediamine (en) and octylamine (OA)
to generate CdS NR and CdS-NS respectively. Appro-
priate amount of the precursor complex was mixed with
thermolysisng solvent (en, OA) in a two neck flask. The
In this work, Cd(II) dithiocarbamate was synthesized
and used as single source precursor for the synthesis of
cadmium sulfide NPs by thermolysing it in ethylenedi-
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mixture was slowly heated up to the boiling point of en
Copyright: American Scientific Publishers
ꢀ
ꢀ
(∼120 C) and OA (∼175 C) respectively, with constant
amine (en) and octylamine (OA) as schematically shown in
Scheme 1 and the photocatalytic efficiency of the obtained
nanostructures was tested towards CR degradation under
direct sunlight as a model reaction. Different parameters
of the photocatalytic reaction were investigated and their
kinetic behavior was assessed.
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stirring. In each experiment the yellow precepitate of
CdS appeared at the boiling point of respective ther-
molysing solvent, which was filtered, washed two to three
ꢀ
times with methanol and then dried in oven at 60 C for
four hours.
Scheme 1. Synthesis of ligand, complex and CdS nanostructures.
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Nasir et al.
Efficient Solar Light Driven Photocatalytic Degradation of Congo Red Dye on CdS Nanostructures
2.3. Photocatalysis
(4H, q, J = 5.0 Hz, N–CH2ꢁ, 0.89–0.90 (6H, t, J =
5.1 Hz, CH3ꢁ, 13C-NMR (DMSO: ꢇ (ppm), 75 MHz):
The CdS nanorods and nanosphers mediated photocat-
alytic degradation of Congo red dye was investigated under
direct sunlight in sunny days in May 2017 between 9 to
11:30 am (GPS coordinates N = 33ꢀ43ꢁ5ꢁꢁ, E = 73ꢀ3ꢁ38ꢁꢁ).
Pyrex glass vessel of 100 mL capacity was used for per-
forming the photocatalytic experiments. Typically 20 mg
of CdS-NR and CdS-NS were uniformly and separately
dispersed in 100 mL of freshly prepared Congo Red (CR)
aqueous solution (10 mg/L). Before exposure to sunlight
the suspension was kept in dark with stirring for 30 min-
utes to establish the adsorption equilibrium between dye
and photocatalyst and then exposed to direct sunlight with
stirring. To monitor the degradation of dye, 2 mL of the
sample was taken out with the interval of three minutes
and centrifuged to separate the catalyst and then dye in
the supernatant was analyzed by UV-visible absorption
spectroscopy.
202.1 (CSS), 52.8 (–CH2ꢁ, 14.7 (–CH3ꢁ. The presence of
bands, in the FT-IR spectra of the complex, at 359 cm−1
is assignable to Cd–S stretch41ꢀ42 and a single peak for
CS at 1004 cm−1 indicated the bidentate dithiocarbamate-
Cd coordination.43 Moreover, the band at 1483 cm−1 due
to C–N stretch designates the partial double bond char-
acter, which arises due to resonance phenomena in the
CNSS moiety upon complexation.44ꢀ45 The assignment of
the proton resonances was made by their peak multi-
plicity, intensity pattern and comparison of the integra-
1
tion values with the expected composition. The H NMR
spectrum of the complex exhibited a quartet for methy-
lene protons (2.61–2.76 ppm) and triplet for methyl pro-
tons (0.89–0.90 ppm). The 13C NMR spectrum showed an
upfield shift of around 9 ppm in the SCS peak than the
free ligand, which confirms ligand-metal coordination.46
The remaining signals matched well with the expected val-
ues. Furthermore, the elemental composition (CHNS) of
the compound was found in close agreement with the the-
oretically predicted composition, thus confirmed the clean
synthesis of the precursor complex.
2.4. Characterization
The structure of ligand and complex was confirmed by
1
13C and H using Bruker AC 300 MHz-FT-NMR spec-
trometer. CHNS analyzer 932 LECO (USA) was used for
the determination of elemental analysis. The instrument
PAN analytical X’Pert powder was used for X-ray diffrac-
tion (PXRD) analysis. The instrument has a Cu Kꢂ´ radi-
ation wavelength (ꢃ = 1ꢄ5418 Å) and the values of 2ꢅ
3.2. Structural Analysis and Formation Mechanism of
CdS Nanostructures
To investigate the role of decomposing solvent and by-
ꢀ
ꢀ
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product as shape directing agents, FTIR analysis of both
Copyright: American Scientific Publishers
were recorded from 10 to 80 . To take the UV-visible
spectra of CdS-NR and CdS-NS, Shimadzu double beam
Spectrophotometer 1800 was used with UV-visible analyze
lamp SN: 500412 to provide UV source and its wavelength
was adjusted at 250 nm to 800 nm. Bio-Rad Excalibur FTS
model 3000 MX in the frequency range of 4000–400 cm−1
was used to record the infra-red absorption spectrum of
CdS-NR and CdS-NS. The determination of % compo-
sition of synthesized NPs was performed on the Energy
Dispersive X-ray (EDX) analyzer JED-2300/2300F. Trans-
mission Electron Microscopy (TEM) images were taken
on JEOL 2010 microscope with an accelerating voltage of
200 kV. Time-correlated single photon counting (TCSPC)
setup (FluoTime300, PicoQuant GmbH) was used for the
determination of time-resolved and Steady-state Photolu-
minescence (PL) measurements.
morphologies was undertaken (Fig. 1(c)). FT-IR spectra of
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both morphologies are closely similar and showing absorp-
tion peaks at 2918, 2929 cm−1 and 2854, 2845 cm−1
due to symmetric and asymmetric stretches of methy-
lene group, while the stretching bands around 3430 and
3442 cm−1 are assigned to –NH2 group and at 1591
and 1578 cm−1 to N–H bending vibrations. Moreover,
the observed peaks at 1361, 1372, 1008, 996, 716 and
730 cm−1 is assignable to mixed stretching of N–C
S
moiety41 (Scheme 2). The FT-IR results indicate the pres-
ence of organic moiety with similar functional groups on
the surface of both morphologies. On the basis of FT-IR
results and molecular structure of the thermolysing sol-
vents (en and OA) it can be assumed that the surface of
CdS-NR is capped by in situ generated 3-(2-aminoethyl)-
1,1-dibutylethiourea, while that of CdS-NS is capped by
1,1-diethyl-3-octylthiourea as both of these compounds
contain similar functional groups. Thus, it can be inferred
that different capping molecules (which have been formed
by using different thermolysing solvent) have played a sig-
nificant role in tuning the morphology of CdS. Previous
reports showed that en as a thermolysing solvent assist
in anisotropic growth resulting in nanorods.47 In contrast,
long chain organic amines have been found to be the most
effective surfactant usually leads to the formation of small
nanoparticles with spherical shape.48
3. RESULTS AND DISCUSSION
3.1. Spectroscopic Characterization of the
Precursor Complex
The synthesis of the precursor complex was easily accom-
plished at ambient conditions by simply mixing the
metal salt and ligand and was found soluble only in
DMSO. Actual. Yield: (1.45 g, 79%); White solid; m.p.:
ꢀ
194 C; Anal. Cal. (Found), for C10H20CdN2S4: C, 29.37
(29.32), H, 4.93 (4.99), N, 6.85 (6.82), S, 31.36 (31.33)%.
FT-IR (neat, cm−1ꢁ: 1483 (ꢆC−Nꢁ, 1004 (ꢆSCSꢁ, 359
Thermolysing the precursor complex in en resulted in
the formation of nanorods with length between 23–44 nm
1
(ꢆCd−Sꢁ. H NMR (CDCl3: ꢇ (ppm), 300 MHz): 2.61–2.76
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Efficient Solar Light Driven Photocatalytic Degradation of Congo Red Dye on CdS Nanostructures
Nasir et al.
c)
d)
CdS-NR
CdS-NS
CdS-NS
CdS-NR
(002)
(002)
(101)
(013)
(103)
(121)
(112)
(110)
(110)
(100)
(102)
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4000 3500 3000 2500 2000
1000 500
20
25
30
35
40
45
50
55
60
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cm–1
2θ
Figure 1. TEM images of (a) CdS-NR and its inset SAED pattern (b) CdS-NS and its inset SAED pattern (c) FTIR analysis and (d) PXRD diffraction
of as-prepared CdS-NR and CdS-NS nanostructures.
and the diameter lie between 7–12 nm, however, in
OA, it afforded nanospheres with size in the range of
2.89–5.30 nm as can be evidenced in the TEM images
(Figs. 1(a, b)). The SAED pattern of CdS-NR and CdS-NS
are shown in inset of Figures 1(a) and (b), respectively.
The concentric circular fringes in the patterns revealed the
polycrystalline nature of both morphologies. The calcu-
lated “d” values are 0.355, 0.334, 0.315, 0.244, 0.206,
0.188, 0.177 nm, which is in agreement with (100), (002),
(102), (103), (112), (203) and (114) planes of CdS-NR
and 0.3363, 0.2118, 0.1738 and 0.1419 nm agree to (002),
(110), (013) and (121) “(hkl)” planes in case of CdS-
NS. These results are in firm agreement with XRD pat-
terns (Fig. 1(d)) of both morphologies. The diffraction
peaks of CdS-NR can be indexed to the hexagonal CdS
with lattice constants a = b = 4.12 Å and c = 6.63 Å,
which correspond to ICSD file [01-080-0006] hexago-
nal structure having space group (P63mc). These promi-
nent peaks correspond to the reflections at 2ꢅ = 25ꢄ04ꢀ
(100), 26.68ꢀ (002), 28.30ꢀ (101), 36.83ꢀ (102), 43.94
(110), 48.17ꢀ (103) and 52.11ꢀ (112). Compared to the
standard ICSD file the highly intense peak at 26.68ꢀ
with a d-spacing value of 0.334 nm can be assigned
to the (002) plane which reveals the oriented growth
of the hexagonal CdS along c-axis, a clear indication
of nanorod morphology.39ꢀ49 The absence of extra peaks
beside the reflections from CdS, ruled out the possibili-
ties of any other impurity. Similarly, the diffraction peaks
of CdS-NS are indexed as the Cubic mixed hexagonal
CdS which correspond to ICSD file [00-001-0647] hav-
ing space group (F-43m). The XRD spectrum of CdS-NS
(Fig. 1(d)), apparently exhibited only four broad peaks,
centered at 2ꢅ = 27ꢄ15ꢀ (002), 44.50ꢀ (110), 48.16ꢀ (013)
and 52.31ꢀ (121). On close observation, the broad peak
at 27.15ꢀ was found to have been originated by the over-
lap of shoulders on both the sides at 2ꢅ = 24ꢀ and 28ꢀ
(resulting from the overlap of reflections from (100), (002)
and (101) planes) of the hexagonal W-type structure.50
The peaks broadening and overlapping can be attributed
to the smaller size of the CdS-NS, which can also be
evidenced in the TEM results. Moreover, the three most
prominent peaks for cubic CdS with Z-type structure occur
at 27.15ꢀ (111), 44.50ꢀ (220) and 52.31ꢀ (311) reveal the
existence of cubic CdS and could not be ruled out based
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Nasir et al.
Efficient Solar Light Driven Photocatalytic Degradation of Congo Red Dye on CdS Nanostructures
IP: 193.22.14.182 On: Mon, 20 Aug 2018 07:52:44
Copyright: American Scientific Publishers
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Scheme 2. Mechanism for the preparation of CdS-NR and CdS-NS in the presence of en (mechanism 1) and OA (mechanism 2), respectively.
on XRD information, which suggest that CdS-NS pos-
sesses prominent features of both phases and had a dis-
torted structure resulting due to the fractional contents of
both the phases.51ꢀ52 The Energy-dispersive X-ray Spec-
troscopy (EDS) spectra of the synthesized CdS-NR and
CdS-NS measurements confirmed the presence of Cd and
S in 1:1 stoichiometric ratio. The peaks due to carbon,
aluminum, oxygen, zirconium and bromine were because
of sputter coating of the glass substrate on the EDS stage
and were not taken into consideration.
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Efficient Solar Light Driven Photocatalytic Degradation of Congo Red Dye on CdS Nanostructures
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3.3. Optical Property
activity than CdS-NS (kapp = 0.299 min−1ꢁ. The degrada-
tion percentile for CdS-NR and CdS-NS were found more
than 98% and 96%, respectively (Fig. 3(d)).
To assess the optical properties of the synthesized mate-
rials, UV-visible and PL spectroscopy was undertaken.
As shown in Figure 2(a) the absorption maxima of CdS-
NR appears at 480 nm and CdS-NS at 470 nm which
corresponds to the band gap of 2.58 eV and 2.63 eV,
respectively. The band gap energy of both morphologies
suggests their photocatalytic potential in the visible region.
The photoluminesce (PL) spectrum of CdS-NR and CdS-
NS is shown in Figure 2(b). The decrease in PL intensity
in the case of CdS-NR can be attributed to the spatial
anisotropic effect of the optical dipole emission of NR.
The effect of such emission from nanocrystals is expected
to be maximum in the plane perpendicular to the dipole
and minimum when it is along the direction of nanorod
axis. Moreover, the time decay curve in the case of CdS-
NR clearly indicated enough long lifetime for electron–
hole pairs and suppression of charge recombination rate
(Fig. 2(c)).
To study the effect of catalyst dose, the catalyst amount
was varied for the constant amount of CR dye. It was
observed that rate constant was gradually increased with
increasing catalyst dose (4 mg to 24 mg) (Fig. 3(e)). This
is due to an increase in the number of available catalyst
particles, which in turns increases the number of active
sites, thus enhances the adsorption of CR molecules and
high photons trapping required for degradation.54ꢀ55 The
photocatalytic behavior of CdS-NR and CdS-NS was con-
firmed by no CR degradation in the absence of NPs,
though solar light was available. Moreover, no significant
activity was observed in the dark despite the presence of
catalysts for almost 8 hours, which further confirm that
degradation process is only photocatalytically driven i.e.,
both light and catalyst are required.
3.5. Stability and Reusability of CdS Nanostructures
To check the stability and reusability of CdS-NR and
CdS-NS, the five-time recycling process was carried out
for both catalysts under the similar experimental condi-
tions (Fig. 3(f)). The stability constant Kapp = 0.261 min−1
(CdS-NR) and 0.229 min−1 (CdS-NS) was calculated in
the fifth cycle. Furthermore, the catalyst materials col-
lected after fifth cycle were dispersed in aqueous media
3.4. Photocatalytic Activity
To assess the solar light assisted photocatalytic potential
of both morphologies, a CR dye degradation was carried
out as a model reaction. The progress of degradation was
inspected with time interval of 3 minutes by observing
the decrease in absorption intensity of azo (497 nm) and
naphthalene (338 nm) groups. As shown in Figures 3(a)
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and (b), the degradation of CR dye on CdS-NR is rela-
tively fast (21 min.) than CdS-NS (27 min.). On CdS-NR,
and subjected to UV-visible absorption analysis to exam-
Copyright: American Scientific Publishers
ine any shift in the absorption maximum (ꢃmaxꢁ. The red
shift in absorption spectrum of CdS-NS was observed to be
more significant, an indication of particles aggregation dur-
ing photo degradation.54 Commensurate with this analysis,
we have observed a slight decrease in the catalytic activity
after each cycle. This might be due to run off the cata-
lyst material during filtering and washing. Importantly, we
have not detected any significant concentration of leached
Cd2+ ion {detection limit ≥ 3 ꢉg/L (0.003 ppm)} when
titrated the degraded dye solution containing CdS particles
(200 mg/L) against EDTA (0.1 M) as titrant and erichrome
black-t (0.2 g in 15 mL) as indicator at pH of 6.5–7.56
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both azo and aromatic ring were degraded more efficiently
(98% and 94%) than that of CdS-NS (95% and 91%). The
linear relationship of lnꢈCi/C0) versus time (min) indi-
cated that both photocatalysts followed pseudo-fist-order
kinetics (Fig. 3(c)). The obtained rate constant expression
is represented by the following equation (Eq. (1))53
lnꢈCi/C0ꢁ = kapp
Where kapp is the rate constant and Ci and C0 are the
initial and final concentrations of CR, respectively. The
CdS-NR (kapp = 0.366 min−1) shows high photocatalytic
t
(1)
Figure 2. (a) UV-visible absorption spectra (b) steady-state photoluminescence (PL) and (c) time-resolved of as-prepared CdS nanostructures.
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Efficient Solar Light Driven Photocatalytic Degradation of Congo Red Dye on CdS Nanostructures
1.5
1.0
0.5
0.0
(a)
1.5
(b)
(c)
dye
CdS-NR
CdS-NS
dye
0 min
3 min
6 min
9 min
12 min
15 min
18 min
21 min
24 min
27 min
0 min
3 min
6 min
9 min
12 min
15 min
18 min
21 min
0
–2
–4
1.0
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0.0
200
300
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nm.
500
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200
300
400
nm.
500
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0
5
10 15 20 25 30
Time (min)
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
(d)
(e)
(f)
CdS-NR
CdS-NS
CdS-NR
CdS-NS
CdS-NR
CdS-NS
100
0.35
0.30
0.25
0.20
0.15
0.10
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0.00
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60
40
20
0
0
5
10 15
20
25
30
5
10
15
20
25
1
2
3
4
5
Time (min)
Catalyst loading/mg
Cycles
Figure 3.
UV-visible absorption spectraIPof:s1uc9c3es.s2iv2e.1C4R.1de8g2radOatino:nMinothne,p2re0seAncuegof2(0a)1C8dS0-7N:R52(b:)4a4nd CdS-NS (c) the plot lnꢈC /C ꢁ versus
i
0
reaction time (min−1
ꢁ (d) % degradation plot (eC) eoffpecytroigf hcatt:aAlysmtseloraicdianng aSndci(ef)ntthieficrecPyuclbinlgisshtaebrilsity of the synthesized CdS-NR and CdS-NS.
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In view of above results and some previous reports, the
nanorod morphology exhibited better photocatalytic activ-
ity as compared to other morphologies. A relatively longer
life time of electron–hole pair and suppression of recombi-
nation phenomenon was observed in case of rod-like mor-
phology, as evident from the PL results. This is because
the charge carriers transport in case of the nanorod is well
channelized which in turn suppresses the recombination
process.40 Furthermore, low photocatalytic efficiency of
CdS-NS despite of its smaller NPs size than the Cd-NR,
may be due to high particles merging of the fomer that
may in turn decrease the quantum efficiency.
H2O. On the other hand, a photoinduced electron reduces
the adsorbed molecular oxygen (O2ꢁ to the oxygen radical
(superoxide radical anion/ꢂO2−ꢁ (Step 4)54 which is then
further oxidized by hole and generate HOꢂ2 radical (5). The
highly reactive HOꢂ2 radicals combine and produce H2O2
molecule (Step 6) which is further reduced by electron and
3.6. Photocatalytic Mechanism
A proposed mechanism on the basis of above-mentioned
results is explained as follow: Irradiation of light on the
surface of photocatalyst leads to the formation of electron–
hole pairs when the photon energy equal to or exceeds the
band gap energy of semiconductor (Fig. 4) (step 1). The
main protagonist behind the degradation of CR dye are
ꢂ
ꢂ
O−2 ion and OH radicals. The O−2 ion is formed when
photoinduced electron reduces the molecular oxygen.54
While the highly reactive ꢂOH radical is generated by two
different pathways. In one of the path way, photogenerated
holes (h+ꢁ is captured by a hydroxyl group and produce
highly reactive hydroxyl radicals (ꢂOH) (Step 2 and 3) and
Figure 4. The proposed mechanism for the degradation of Congo Red
(CR) dye on CdS nanostructures.
J. Nanosci. Nanotechnol. 18, 7405–7413, 2018
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Efficient Solar Light Driven Photocatalytic Degradation of Congo Red Dye on CdS Nanostructures
Nasir et al.
ꢂ
generate OH and −OH (step 7). Furthermore, the molec-
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ꢂ
the presence of oxygen (step 8). These two highly reactive
species O−2 and OH are the main driven forces behind
the efꢂfective decomposition of CR dye (Step 9). Moreover,
ꢂO−2 , OH and HOꢂ2 are the three main reactive intermedi-
ates in the whole process.
ꢂ
ꢂ
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4. CONCLUSION
In summary, synthesis and conversion of the precursor
complex to CdS nanostructures were easily achieved under
mild conditions using en and OA as thermolysing solvents.
Two different morphologies nanorods (en) and nanosphers
(OA) were obtained as confirmed by TEM analysis. The
morphological effect of solvent can be attributed to the
role of different in situ generated capping ligands, as con-
firmed by FT-IR analysis. Furthermore, the XRD results
revealed the growth of nanorods into hexagonal while the
nanospheres into cubic mixed hexagonal phase. Both mor-
phologies absorb in the visible region (UV/visible absorp-
tion spectroscopy), thus have the ability to be used as solar
light driven photocatalysts. However, the decrease in PL
and time decay curve have confirmed the low electron–
hole recombination of NR than NS, thus signified the bet-
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further confirmed by efficient solar light assisted degrada-
tion of CR on CdS-NR (kapp = 0.366 min−1ꢁ than CdS-NS
(kapp = 0.299 min−1ꢁ. Furthermore, both CdS-NR and CdS-
NS are stable enough to be used in multiple cycle without
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Disclosure Statement
We have no potential conflict of interest.
Acknowledgments: We acknowledge the financial sup-
port from the Higher education Commission (HEC) of
Pakistan and International Joint Project of Hainan Province
(grant no. ZDYF2016212).
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Received: 1 February 2018. Accepted: 26 March 2018.
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