Synthesis and crystal structures of bis(dibenzyl dithiocarbamato)Cu(II) and Ag(I) complexes: Precursors for Cu1.8S and Ag2S nano-photocatalysts

Article abstract of DOI:10.1016/j.molstruc.2020.128791

We report the synthesis and crystal structures of bis(dibenzyl dithiocarbamato) copper(II) and silver(I) complexes and their use as precursors to prepare Cu_1.8S and Ag₂S nanoparticles. Single crystal analysis of bis(dibenzyl dithiocarbamato)Cu(II) complex consist of a monomeric entity where the two dibenzyl dithiocarbamate ligands form a distorted square planar geometry around the Cu(II) ion. The bis(dibenzyl dithiocarbamato)Ag(I) formed hexameric coordination complex consisting of two distorted hexagonal Ag₃S₃ rings. The complexes were thermolysed at 220 °C to prepare copper sulphide and silver sulphide nanoparticles. Powder X-ray diffraction (p-XRD) patterns of the copper sulphide nanoparticles were indexed to digenite Cu_1.8S, while silver sulphide nanoparticles were confirmed to be acathinite Ag₂S. High-resolution transmission electron microscopy (HRTEM) images showed Ag₂S nanoparticles with particle size in the range 11.4–27.4 nm while Cu_1.8S nanoparticles are semi-spherical in shape with slightly agglomerated particles in the range 6.9–24.0 nm. The as-prepared nanoparticles were used as nano-photocatalysts for the degradation of methylene blue dye (MB) under UV light irradiation with degradation efficiency of 42.52% and 48.39% for Cu_1.8S and Ag₂S nanoparticles respectively.

Full text of DOI:10.1016/j.molstruc.2020.128791

Journal of Molecular Structure 1221 (2020) 128791
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis and crystal structures of bis(dibenzyl dithiocarbamato)Cu(II)
and Ag(I) complexes: Precursors for Cu1.8S and Ag2S
nano-photocatalysts
Peter A. Ajibade*, Berlinda M. Sikakane, Nandipha L. Botha, Abimbola E. Oluwalana,
Bernard Omondi
School of Chemistry and Physics, University of KwaZulu Natal, Private Bag X01, Scottsville, Pietermaritzburg, 3209, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis and crystal structures of bis(dibenzyl dithiocarbamato) copper(II) and silver(I)
complexes and their use as precursors to prepare Cu1.8S and Ag2S nanoparticles. Single crystal analysis of
bis(dibenzyl dithiocarbamato)Cu(II) complex consist of a monomeric entity where the two dibenzyl
dithiocarbamate ligands form a distorted square planar geometry around the Cu(II) ion. The bis(dibenzyl
dithiocarbamato)Ag(I) formed hexameric coordination complex consisting of two distorted hexagonal
Ag3S3 rings. The complexes were thermolysed at 220 ꢀC to prepare copper sulphide and silver sulphide
nanoparticles. Powder X-ray diffraction (p-XRD) patterns of the copper sulphide nanoparticles were
indexed to digenite Cu1.8S, while silver sulphide nanoparticles were confirmed to be acathinite Ag2S.
High-resolution transmission electron microscopy (HRTEM) images showed Ag2S nanoparticles with
particle size in the range 11.4e27.4 nm while Cu1.8S nanoparticles are semi-spherical in shape with
slightly agglomerated particles in the range 6.9e24.0 nm. The as-prepared nanoparticles were used as
nano-photocatalysts for the degradation of methylene blue dye (MB) under UV light irradiation with
degradation efficiency of 42.52% and 48.39% for Cu1.8S and Ag2S nanoparticles respectively.
© 2020 Elsevier B.V. All rights reserved.
Received 16 May 2020
Received in revised form
18 June 2020
Accepted 27 June 2020
Available online 1 July 2020
Keywords:
Dibenzyldithiocarbamate
Crystal structures
Copper sulphide
Silver sulphide
Nano-photocatalyst.
1. Introduction
synthetic routes, the thermolysis of singleesource precursors are
known to yield high quality nanoparticles due to the established
Interest in the synthesis of nanomaterials are due to their size-
dependent properties and potential applications in medicine [1e3]
and agriculture [4,5], among others. The chemical composition,
size, shape and other physical properties of nanomaterials have
been investigated to establish their potential applications [6e8].
The flexibility of metallic nanoparticles is ascribed to the synthetic
techniques through which their sizes, shapes, compositions, as well
as optical properties can be control [9,10]. Among metallic nano-
particles, great efforts have been dedicated to the synthesis and
morphological control of metal chalcogenides nanoparticles of
which copper sulphide and silver sulphide [11,12] are of paramount
importance due to their stoichiometry and potential applications.
Synthetic methods such as micro-emulsion [13], solvent-free [14],
enzyme treatment [15], colloidal [16], substrate template [17] and
single-source precursor [18] have been explored. Among these
metal-sulphur bond in the molecule through which the chemical
composition and physical properties of the as-prepared metal
sulphide can be control [19e23].
Copper sulphide (CuS) nanomaterials are being studied for ap-
plications in solar cells, optical filters, super ionic material, photo-
voltaic applications, and in combined photo acoustic imaging [24].
Silver sulphide (AgS) is the only semiconductor apart from mercury
sulphide with three polymorphous phase modifications (
Ag2S and -Ag2S) [25]. The phase modification is attributed to
transitions between superionic -Ag2S and semiconducting -Ag2S.
a-Ag2S, b-
g
b
a
This unique property makes them useful in infrared detectors in
resistance-switches and non-volatile memory devices and recently
quantum dots of silver sulphide are used as fluorescent labels [26].
In recent years, studies on photocatalytic activities of CuS and Ag2S
nanoparticles, are based on hybrids/composites [27,28]. Jiang et al.
reported the photocatalytic degradation of methyl orange by Janus
AgeAg2S coupled with TiO2 with efficiency of 80% after 50 min [29].
Yang et al. studied the photocatalytic activity of Ag2SeAg hybrid
* Corresponding author.
E-mail address: ajibadep@ukzn.ac.za (P.A. Ajibade).
https://doi.org/10.1016/j.molstruc.2020.128791
0022-2860/© 2020 Elsevier B.V. All rights reserved.

2
P.A. Ajibade et al. / Journal of Molecular Structure 1221 (2020) 128791
nanotubes for degradation of methyl orange and reduction of CrVI
under visible light irradiation and obtained a superior photo-
catalytic performance, which was ascribed to the enhanced charge
transfer from the conduction band of Ag2S to Ag in the hybrid [30].
Other studies have shown that the structural and optical properties
of metal sulphide nanoparticles can be controlled through the
precursor and synthetic routes [21,31e33]. In continuation of our
effort to develop nanoparticles with controlled structural,
morphological and optical properties using the single-source pre-
cursor approach, we present the syntheses and crystal structures of
dibenzyl dithiocarbamato Cu(II)) and Ag(I) complexes. The com-
plexes were thermolyzed at 220 ꢀC to prepare Cu1.8S and Ag2S
nanoparticles. The structural and optical properties of the as-
prepared nanoparticles were studied and their potential as nano-
photocatalysts were evaluated using methylene blue photo-
degradation under visible light irradiation.
diffraction (SAED) patterns were obtained using Japan electrical
optical labs (JEOL) HRTEM-2100. ZEISS scanning electron micro-
scope (SEM) equipped with an energy dispersive X-ray (EDX)
spectrometer was used for surface morphology and elemental
composition. Absorption and photoluminescence studies was done
on PerkinElmer Lambda UVeVis and PerkinElmer LS 45 fluores-
cence spectrometer respectively.
2.2. Synthesis of dibenzyl dithiocarbamate (dbdtc)
Dibenzyl dithiocarbamate (dbdtc) ligand was synthesized based
on
a modified literature method [22]. Cold dibenzyl amine
(0.005 mol) was added to cold solution of sodium hydroxide
(0.005 mol), followed by the dropwise addition of cold carbon
disulphide (0.005 mol). The reaction was stirred for 2 h at about
4
ꢀC. The white crystalline product was filtered, washed with
diethyl ether and dried in a desiccator.
2. Experimental
2.1. Chemicals and characterization techniques
2.3. Synthesis of bis(dibenzyldithiocarbamato)Cu(II) and Ag(I)
complexes
Dibenzyl amine, sodium hydroxide, carbon dioxide, silver(I)
nitrate, copper((II) chloride, oleic acid, oleylamine, diethyl ether,
chloroform, ethanol and toluene were purchased from Sigma-
Aldrich and used as received without further purification. Elec-
tronic absorption and Fourier-Infrared (FTIR) analysis for the ligand
and complexes were obtained using PerkinElmer lambda UVeVis
and Cary 630 FTIR (Agilent technologies) spectrometer. Different
analytical techniques were used for the characterization of Cu1.8S
and Ag2S nanoparticles. XRD patterns data for phase identification
The metal complexes were synthesized according to a modified
literature method [22]. (0.005 mol; 0.8494 g) CuCl2 or (0.01 mol;
0.4247 g) AgNO3 was dissolved in 50 mL of water in a beaker.
(0.01 mol; 2.95 g) dibenzyl dithiocarbamate (dbdtc) was dissolved
in 50 mL in a separate beaker. Then the ligand solution was added
into the respective metal salt solution and stirred for 4 h. The
products were filtered, washed with water and dried under vac-
uum. Copper(II) dibenzyl dithiocarbamate ([Cu(dbdtc)2]) was ob-
tained as a brown solid while silver(I) dibenzyl dithiocarbamate
([Ag3(dbdtc)6]) was yellowish in colour.
were collected on Bruker D8 Advance diffractometer with CuK
a
radiation (
l
¼ 1.5418 Å). HRTEM images and selected area electron
Table 1
Crystal data and structure refinement for [Ag3(dbdtc)6] and [Cu(dbdtc)2].
[Ag3(dbdtc)6]
[Cu(dbdtc)2]
Empirical formula
C92H86Ag6Cl6N6S12
C30H28CuN2S4
608.32
100(2) K
0.71073 Å
Triclinic
Formula weight
Temperature
Wavelength
Crystal system
Space group
2520.30
173(2) K
0.71073 Å
Triclinic
P1
P1
ꢀ
Unit cell dimensions (Å,
)
11.0126(6)
13.7098(9)
14.4254(8)
16.2955(10)
78.343(3)
14.3201(10)
16.2979(11)
64.916(4)
73.321(2)
72.302(3)
73.2140(10)
2354.0(2)
82.774(2)
2760.8(3)
V Å3
Z
1
4
rcalc/mg mꢁ3
1.778
1.709
1.464
1.117
m
/mmꢁ1
F(000)
1256
1260
Crystal size (mm3)
0.310 ꢂ 0.230 x 0.140
1.316 to 26.500
ꢁ13 ꢃ h ꢃ 13
ꢁ182 ꢃ k ꢃ 18
ꢁ20 ꢃ l ꢃ 19
35157
0.340 ꢂ 0.230 x 0.110
1.437 to 27.597
ꢁ17 ꢃ h ꢃ 17
ꢁ18 ꢃ k ꢃ 18
ꢁ20 ꢃ l ꢃ 21
39439
Theta range for data collection/ꢀ
Index ranges
Reflections collected
Independent reflections
Completeness to theta ¼ 25.242ꢀ
Absorption correction
9209 [R(int) ¼ 0.0326]
11444 [R(int) ¼ 0.0485]
94.7%
90.1%
Semi-empirical from equivalents
Full-matrix least-squares on F2
9209/0/550
1.044
Semi-empirical from equivalents
Full-matrix least-squares on F2
11444/0/667
1.070
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)] R1, wR2
R indices (all data) R1, wR2
Extinction coefficient
0.0388, 0.0937
0.0502, 0.1006
n/a
1.717 and ꢁ1.023
0.0446, 0.0914
0.0774, 0.1034
n/a
0.558 and ꢁ0.956
Largest diff. peak and hole (e.Åꢁ3
)

P.A. Ajibade et al. / Journal of Molecular Structure 1221 (2020) 128791
3
2.4. Synthesis of metal sulphide nanoparticles
a pseudo first-order kinetic model formula [34] (Eq (2)) which is
expressed as follows:
The metal sulphide nanoparticles were synthesized using
[Ag3(dbdtc)6] and [Cu(dbdtc)2], 0.01 mol of the respective complex
was added into an equimolar mixture of oleylamine and oleic acid
in a three-necked flask. The reaction mixture was refluxed for 1 h at
220 ꢀC under N2 atmosphere and cooled to 70 ꢀC. Cold ethanol was
added into the solution, the resulting precipitate was collected by
centrifugation.
ꢀ
ꢁ
A0
At
ln
¼ ꢁ kt
(2)
where A0 is the initial maximum absorbance at time 0, At is the
maximum absorbance at time t and k is the rate constant.
2.6. X-ray crystallography
2.5. Photocatalytic studies
Crystals of [Ag3(dbdtc)6] and [Cu(dbdtc)2] were obtained by
slow evaporation of their respective chloroform solution. Crystal
evaluation and data collection were performed on a Bruker Smart
Photocatalytic performance of the as-prepared nanoparticles
was evaluated by measuring the degradation of methylene blue dye
(10 mg/L) under visible light. In five 10 mL vials containing 5 mL of
methylene blue solution, 3 mg of nanoparticles was added in each.
The mixtures were sonicated for 5 min to form a homogenous
suspension and stirred in a dark place magnetically for 30 min to
attain adsorption-desorption equilibrium. The solutions were
exposed to OSRAM VIALOX 70W incandescent mercury lamp (light
source) 5 cm away at different time intervals (30, 60, 90 and
120 min). The samples were centrifuged to remove residual catalyst
and dye degradation was analysed using UVeVis spectrophotom-
etry. Methylene blue dye absorbs at wavelength of 664 nm. The
percentage (%) degradation was calculated using formula (1).
APEXII diffractometer with Mo Ka radiation (
l
¼ 0.71073 Å). The
diffractometer to crystal distance was set at 4.00 cm. We obtained
the initial cell-matrix from three series of scans at different starting
angles. Each series consisted of 12 frames collected at intervals of
0.5ꢀ in a 6ꢀ range with an exposure time of 10 s per frame. Suc-
cessful indexing of the reflections was done by an automated
routine built in the APEXII program suite [35]. The final cell con-
stants were calculated from a set of 6460 strong reflections from
the actual data collection. Data collection method involved v scans
of width 0.5ꢀ. The program SAINT plus [35] was used for data
reduction and structure solved by direct methods using SHELXS
[36] and refined using SHELXL [37]. All structures were checked for
solvent-accessible cavities using PLATON [38] and the graphics
were performed with the DIAMOND [39] visual crystal structure
information system software as well as MERCURY. Non-H atoms
were first refined isotropically and then by anisotropic refinement
A0 ꢁ At
D ð%Þ ¼
ꢂ 100
(1)
A0
A plot of ln(A0/At) vs irradiation time was obtained according to
Fig. 1. (a) The hexagonal core consisting of Ag3S3, (b) the molecular structure of [Ag3(dbdtc)6], (c) the full molecular structure of [Cu(dbdtc)2], (d) an overlay of [Cu(dbdtc)2]
molecules 1 and 2.

4
P.A. Ajibade et al. / Journal of Molecular Structure 1221 (2020) 128791
with full-matrix least-squares calculations based on F2 using
SHELXS. We positioned all H atoms geometrically and allowed to
ride on their respective parent atoms and refined isotropically.
Absorption corrections were based on fitting a function to the
empirical transmission surface as sampled by multiple equivalent
measurements [39]. Crystal data and structure refinement infor-
mation for [Ag3(dbdtc)6] and [Cu(dbdtc)2] are summarized in
Table 1.
0.5012 which is relatively high and is due to the difference in the
orientation of the benzyl moieties of the molecules. The CeS bond
length of copper complex ranges between 1.7178(13)-1.7272(13) Å;
these values are less than the typical CeS bond (1.8 Å), hence
showing a partial double bond character typical to dithiocarbamate
complexes [45]. The trans S4eCu1eS1 and S2eCu1eS3 angles are
179.26ꢀ(15) and 179.73ꢀ(16), respectively, which are slightly equal
3. Results and discussion
3.1. Molecular structures of Ag(I) and Cu(II) dibenzyl
dithiocarbamate complexes
The molecular structure of [Ag3(dbdtc)6] is shown in Fig. 1a and
b, selected bond distance (Å) and bond angles (ꢀ) are summarized
ꢀ
in Table 2. [Ag3(dbdtc)6] crystallizes in the triclinic space group Pı.
The complex has a core consisting of two distorted hexagonal Ag3S3
rings indicated by bold black bonds (see Fig. 1a) from which
SeCeSeAg moieties protrude to give a distorted hexagonal paddle
wheel. Two Ag/Ag interactions (dashed red bonds) with distances
of 2.9198(5) and 3.0374(5) Å, which further supports the two Ag3S3
rings, as indicated in Fig. 1a. In this arrangement, two Ag atoms are
coordinated by three S atoms from dibenzyl dithiocarbamate and
an Ag/Ag interaction completing a distorted tetrahedral coordi-
nation geometry around the Ag atom. The third Ag centre in the
asymmetric unit is coordinated by three S atoms and two Ag/Ag
interactions resulting in a distorted five-coordinate Ag center [40].
In both cases the AgS3 bite angle is essentially trigonal planar
with SeAgeS bond angles of 135.35(4), 129.72(4) and 91.95(3) for
one Ag(I) centre, 117.67(4), 114.64(4) and 113.79(4) for the second
Ag(I) centre and 137.30(4), 114.29(4) and 104.99(4) for the third
Ag(I) centre, close to ideal trigonal planar arrangement. The Ag/Ag
interactions point out of this plane and are not orthogonal. The
other two Ag/Ag distances are long but are only interactions
(3.3210(5) and 3.3465(5) Å, which is shorter than the sum of two
silver atoms Van der Waals (3.44 Å) signifies the presence of AgeAg
metallophlic bond [41].
Fig. 2. Electronic spectra of the ligand and the metal complexes.
The AgeS distances range between 2.4421(11) to 2.5476(12) Å
and are comparable to those of related structures [40,42e44]. The
CeS bond length of silver ranges between 1.710(2)-1.714(2) and
these values are less than the typical CeS bond (1.8 Å), hence
showing a partial double bond character typical to dithiocarbamate
complexes [45].
Copper(II) complexes of dithiocarbamate have three types of
coordination geometry: they are five-coordinate dimers, infinite
polymeric chain and square planar monomer [46]. Fig. 1c shows the
molecular structure of [Cu(dbdtc)2] with atom numbering scheme.
[Cu(dbdtc)2] has two non-symmetrically related molecules in the
asymmetric unit. The Cu(II) center in the molecular structure of
[Cu(dbdtc)2] has a distorted square planar geometry. In this
arrangement, the Cu(II) center accommodates two ligands each
with CueS distances ranging from 2.2924(11) to 2.3231(8) Å
correlating with reported copper(II) dithiocarbamate complexes
with four coordinate [46]. Fig. 1d shows an overlay of the two
molecules of [Cu(dbdtc)2], having a root mean square deviation of
Table 2
Important bond distances (Å) and angles (ꢀ).
Parameter
[Ag3(dbdtc)6]
[Cu(dbdtc)2]
MdS ranges
2.4421(11) e 2.5476(12)
2.9198(5)d3.3465(5)
91.95(3)d135.35(4)
2.2924(9) e 2.3231(8)
e
…
M
M ranges
SdMdS ranges
77.48(3)d179.73(3)
Fig. 3. Powder X-ray diffraction patterns of the metal sulphide nanoparticles.

P.A. Ajibade et al. / Journal of Molecular Structure 1221 (2020) 128791
5
and consistent with reported dithiocarbamate complexes with
square planar environment [45,47].
ligand (dbdtc) at 261 nm, 291 nm and a little shoulder at 367 nm
which are attributed to ( *) from the NeC]S system, (
*) from the SeC]S group and (n/ *) due to lone pair of elec-
tron on sulphur atoms respectively [21]. The absorption of
[Ag3(dbdtc)6] has only one band at 280 nm, which is assigned to
* intra ligand charge-transfer transitions. [Cu(dbdtc)2] showed
three bands, the bands observed at 277 and 294 nm are assigned to
* intraligand charge-transfer transitions. The third band is
very broad and intense at 438 nm, which could be ascribed to
p
/
p
p /
p
p
3.2. Spectroscopic studies
p
/
p
The FTIR spectrum of the ligand shows two stretching vibra-
tional bands at 972 cmꢁ1 and 1032 cmꢁ1 assigned to symmetric and
asymmetric CeS vibrations, respectively [22]. These vibrations
were replaced by one vibration in the complexes spectra at
985 cmꢁ1 for [Ag3(dbdtc)6] and 976 cmꢁ1 for [Cu(dbdtc)2]. This
indicates that the ligand coordinated to the metal ions in sym-
metric bidentate chelating mode [45]. The CeN vibration was
observed in the ligand at 1448 cmꢁ1, which shifted to higher
wavenumbers in the complexes at 1479 cmꢁ1 and 1478 cmꢁ1 for
[Ag3(dbdtc)6] and [Cu(dbdtc)2] respectively. This shift could be
attributed to electron flow from the (NdCS2) moiety to the metal
centre, thereby increasing the CeN double bond character after
coordination [46].
p/ p
d
xy/dxz transitions of the Cu(II) ion in a square planar geometry.
However, the band corresponding to dxy/d2z transitions was not
observed [48].
3.3. Structural studies of the nanoparticles
3.3.1. Powder X-ray diffraction of the nanoparticles
Fig. 3 show the powder X-ray diffraction patterns of the copper
sulphide and silver sulphide nanoparticles. pXRD patterns of cop-
The overlay electronic spectra of the ligand and the complexes is
displayed in Fig. 2. Three bands characterize the absorption of the
per sulphide nanoparticles showed peaks at 2
q
¼ 38.09, 43.75,
47.40 and 54.90ꢀ, indexed to (105), (01117), (110) and (1112) planes
Fig. 4. HRTEM and SAED images of the metal sulphide nanoparticles.

6
P.A. Ajibade et al. / Journal of Molecular Structure 1221 (2020) 128791
of the digenite Cu1.8S (JCPDS 95-005-3331) [49]. The p-XRD pat-
terns of the silver sulphide nanoparticles showed diffraction peaks
range 6.9e24.0 nm. This might be due to the high themolysis
temperature (220 ꢀC), which favours the Oswald ripening process
[53] in Cu1.8S. The thermolysis of the Ag(I) complex precursor fa-
vours the formation of mono-dispersed spherically shaped Ag2S
nanoparticles [54]. Also, from Fig. 4, the Ag2S selected area electron
diffraction (SAED) patterns showed distinct rings confirming the
crystallinity of the particles. However, SAED patterns of the Cu1.8S
showed two circular rings patterns that are blurred by amorphous
layers deposited on the surface ascribed to the capping agent. The
results obtained confirm that the capping agent (oleylamine) and
the reducing agent (oleic acid) used worked well with [Ag(dbdtc)]
than [Cu(dbdtc)2] as single-source precursor [55].
at 2
q
¼ 33.76, 36.67, 38.97 40.12, 42.85, 44.02, 47.40, 50.79, 53.93,
55.01, 56.97, 68.88, 70.81 and 76.59 ꢀcorresponding to (ꢁ121),
(013), (ꢁ014), (031), (ꢁ202), (ꢁ212), (ꢁ214), (014), (ꢁ224), (ꢁ142),
(042), (015), (133), (ꢁ212) and (ꢁ227) miller indices respectively
[50]. The patterns matched with the literature XRD patterns of
acanthite Ag1.93S crystalline phase [51]. The average particle size
was calculated using the Debye-Scherrer equation (3) [52]:
0:9
l
D ¼
(3)
bcos
q
Silver sulphide shows clustered fibre layers in the SEM micro-
graph while copper sulphide nanoparticles shows plain or smooth
surface morphology. The EDX spectra confirmed the presence of the
respective metals and sulphur in the particles. The spectra also
showed the carbon and oxygen peaks which are attributed to the
capping agent coordinated to the surface of the particles.
where D is the average particle size,
l
is the wavelength (1.5406 Å),
b
is the half-width peak at maximum, 0.9 is a constant and q is the
diffraction angle. The average particle size was 15.53 nm for Ag2S
and 26.42 nm for Cu1.8S.
3.3.2. HRTEM and SEM images of the nanoparticles
The structural and morphological properties of the metal sul-
phide nanoparticles were studied using HRTEM and SEM. Fig. 4
shows monodispersed spherical Ag2S nanoparticles with particle
size in the range 11.4e27.4 nm. For Cu1.8S, the nanoparticles are
agglomerated semi-spherically shaped particles with size in the
3.3.3. Optical studies of metal sulphide nanoparticles
Absorption and emission spectra of the as-prepared nano-
particles are presented in Fig.
5
were obtained using
UltravioleteVisible absorption analysis and photoluminescence,
The absorption band edge of Ag2S appeared at 286 nm, while that of
Fig. 5. Absorption (a), Tauc's plot (b) and emission spectra (c,d) and of the metal sulphide nanoparticles.

P.A. Ajibade et al. / Journal of Molecular Structure 1221 (2020) 128791
7
Cu1.8S appeared at 284 nm. This indicates that the nanoparticles
will absorb ultraviolet radiation and excites its atoms to upper
levels [56]. The bandgap energies of the as-prepared nanoparticles
obtained from the Tauc plots (Fig. 5b) are 3.44 eV and 3.56 eV for
Ag2S and Cu1.8S nanoparticles, respectively. These values are
greater than the bandgap energies of bulk Ag2S (0.9e1.11 eV) and
Cu1.8S (1.21 eV) [25,57].
The results revealed that the energy bandgap of the as-prepared
nanoparticles is blue-shifted, showing the effect of quantum
confinement which is due to the decrease in the size of the nano-
particles [58]. The emission spectra of the nanoparticles (Fig. 5c)
showed emission peaks observed at 382 nm and 379 nm for Ag2S
and Cu1.8S respectively, which is attributed to trapped electron-
hole recombination on the surface of the nanoparticle [22]. The
presented in Fig. 6a-b, which shows that the intensity of absor-
bance peak at 664 nm for MB dye decreases with an increase in
irradiation time. Cu1.8S and Ag2S nanoparticles were able to
degrade 42.52% and 48.39% of MB dye after 120 min, respectively.
Ag2S shows higher photocatalytic activity compared to Cu1.8
S
nanoparticles which may be attributed to the better separation of
the electron-hole pairs, while the less activity of the Cu1.8S nano-
particles can be attributed to rapid recombination of electron-hole
pairs [59]. Most Ag2S nanoparticles used for photocatalysis are
coupled with other metal sulphides or metal oxides nanoparticles.
To the best of our knowledge, the only Ag2S nanoparticles used for
photocatalysis with high efficiency have porous worm-like
morphology and low bandgap energy (~1.01e1.03 eV) [60]. Also,
the reduced efficiency of Cu1.8S nanoparticles might be due to its
smooth surface morphology and surface interaction with the dye
molecule as Cu2S nanoparticles reported with high efficiency has
flower-like morphology [61e63].
Langmuir-Hinshelwood kinetic model [64] was used to describe
the photocatalytic degradation of methylene blue dye by the
nanoparticles. Fig. 6d shows a plot of ln(A0/At) vs irradiation time.
Rate constants of 0.0051 minꢁ1 and 0.0065 minꢁ1 were obtained
for Cu1.8S and Ag2S respectively which indicate pseudo-first-order
kinetics. The rate constant obtained further supports that Ag2S
nanoparticles exhibit more photocatalytic activity when compared
emission peaks are red shifted in comparison to Ag2S and Cu1.8
S
absorption band edges at 286 nm and 284 nm. In addition, Ag2S has
a lower emission intensity compared to Cu1.8S, which means it has a
low electron-hole pair recombination rate which will help in
photocatalytic efficiency [59].
3.4. Photocatalytic studies
The time-dependent photocatalytic degradation graphs of
methylene blue (MB) dye in the presence of Cu1.8S and Ag2S are
Fig. 6. MB UVeVis absorption degradation spectra (a,b), degradation efficiency bar chart (c) kinetic plot of ln(A0/At) versus irradiation time (d) in the presence of Ag2S and Cu1.8
nanoparticles.
S

8
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4. Conclusion
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Dibenzyl dithiocarbamato copper(II) and silver(I) complexes
were synthesized and characterized using single crystal X-ray
crystallography. Molecular structure of the Ag(I) complex reveals
hexameric configuration in which three Ag atoms are bonded to six
molecules of didenzyldithiocarbamato ligands leading to the for-
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Cu1.8S were evaluated under UV irradiation. The degradation effi-
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nanoparticles respectively. Ag2S was found to be more efficient,
which is an indication that it may find potential application as
nano-photocatalyst.
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CCDC 1864305 and 1864306 contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html or from the
Cambridge crystallographic data centre, 12 Union Road, Cambridge,
CB2 1EZ, UK; fax: (þ44)-1223-336-033 or email: deposit@ccdc.
cam.ac.uk.
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nanomaterials via thermal decomposition of single-source precursors,
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sulfide nanocrystals prepared from copper(II) dithiocarbamate single source
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Author statement
This research work was conducted as part of the postgraduate
training of BMS, NLB and AEO under the mentorship of PAA. BMS,
NLB and AEO each did part of the experiments, data analyses and
draft manuscript while BO wrote the description of the crystal
structures. PAA was responsible for conceptualization, funding
acquisition, project administration, supervision, writing of draft,
review and final editing.
[23] Y. Ma, H. Wan, Y. Ye, L. Chen, H. Li, H. Zhou, J. Chen, In-situ synthesis of size-
tunable silver sulfide nanoparticles to improve tribological properties of the
polytetrafluoroethylene-based nanocomposite lubricating coatings, Tribol. Int.
148 (2020) 106324.
[24] P. Roy, S.K. Srivastava, Nanostructured copper sulfides: synthesis, properties
and applications, CrystEngComm 17 (2015) 7801e7815.
[25] S.I. Sadovnikov, A.I. Gusev, Recent progress in nanostructured silver sulfide:
from synthesis and nonstoichiometry to properties, J. Mater. Chem. A 5 (2017)
17676e17704.
[26] S.I. Sadovnikov, A.I. Gusev, A.A. Rempel, Nanocrystalline silver sulfide Ag2S,
Rev. Adv. Mater. Sci. 41 (2015) 7e19.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
[27] S.L. Lee, C.J. Chang, Recent Progress on metal sulfide composite nanomaterials
for photocatalytic hydrogen production, Catalysts 9 (2019) 457.
[28] E. Shi, Z. Xu, W. Wang, Y. Xu, Y. Zhang, X. Yang, Q. Liu, T. Zeng, S. Song, Y. Jiang,
L. Li, Ag2S-doped core-shell nanostructures of Fe3O4@Ag3PO4 ultrathin film:
major role of hole in rapid degradation of pollutants under visible light irra-
diation, Chem. Eng. J. 366 (2019) 123e132.
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scaled Janus AgeAg2S nanoparticles and their photocatalytic properties,
CrystEngComm 13 (2011) 7189e7193.
The authors gratefully acknowledged NRF financial support.
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