Synthesis of rare pure phase Ni3S4and Ni3S2nanoparticles in different primary amine coordinating solvents

Article abstract of DOI:10.1016/j.poly.2016.11.014

Two nickel(II) complexes, namely piperidine (1) and tetrahydroquinoline (2) dithiocarbamate complexes were synthesized and characterized by infra-red spectroscopy, elemental analysis and thermogravimetric analyses. The single crystal X-ray structure of complex (1) was also elucidated. The as-synthesized complexes have been utilized as single source precursors to afford nickel sulfide nanoparticles Ni₃S₄, Ni₃S₂and mixed phases via solvothermal decomposition in oleylamine (OLA), dodecylamine (DDA) and hexadecylamine (HDA) at different temperatures. Powder X-ray diffraction studies reveal that the temperature and capping agents play a significant role in determining the crystalline structure and chemical composition of the as-synthesized nanoparticles (NPs). Electron microscopy images showed formation of nanoparticles of various shapes ranging from spherical, tetrahedral and irregular shaped morphologies. Magnetization measurements indicated that Ni₃S₄nanoparticles prepared at 230 °C using DDA display ferromagnetic behaviour, while rhombohedral N₃S₂also prepared at 230 °C but using HDA, displayed paramagnetic property.

Full text of DOI:10.1016/j.poly.2016.11.014

Accepted Manuscript
Synthesis of rare pure phase Ni3S4 and Ni3S2 nanoparticles in different primary
amine coordinating solvents
Charles Gervas, Sixberth Mlowe, Matthew P. Akerman, Itegbeyogene Ezekiel,
Thomas Moyo, Neerish Revaprasadu
PII:
S0277-5387(16)30594-0
DOI:
http://dx.doi.org/10.1016/j.poly.2016.11.014
POLY 12321
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
28 September 2016
8 November 2016
13 November 2016
Please cite this article as: C. Gervas, S. Mlowe, M.P. Akerman, I. Ezekiel, T. Moyo, N. Revaprasadu, Synthesis of
rare pure phase Ni3S4 and Ni3S2 nanoparticles in different primary amine coordinating solvents, Polyhedron (2016),
doi: http://dx.doi.org/10.1016/j.poly.2016.11.014
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Synthesis of rare pure phase Ni3S4 and Ni3S2 nanoparticles in different primary
amine coordinating solvents
Charles Gervasa, Sixberth Mlowea, Matthew P. Akermanb, Itegbeyogene Ezekielc, Thomas
Moyoc, Neerish Revaprasadua*
aChemistry Department, University of Zululand, Private Bag X1001, KwaDlangezwa, 3886, South Africa. Email:
RevaprasaduN@unizulu.ac.za
bSchool of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01,
Scottsville, Pietermaritzburg, 3209, South Africa.
cSchool of Chemistry and Physics, Westville Campus, University of KwaZulu-Natal, Private Bag X54001,
Durban 4000, South Africa.
ABSTRACT
Two nickel(II) complexes, namely piperidine (1) and tetrahydroquinoline (2) dithiocarbamate
complexes were synthesized and characterized by infra-red spectroscopy, elemental analysis and
thermogravimetric analyses. The single crystal X-ray structure of complex (1) was also
elucidated. The as-synthesized complexes have been utilized as single source precursors to
afford nickel sulfide nanoparticles Ni3S4, Ni3S2 and mixed phases via solvothermal
decomposition in oleylamine (OLA), dodecylamine (DDA) and hexadecylamine (HDA) at
different temperatures. Powder X-ray diffraction studies reveal that the temperature and capping
agents play a significant role in determining the crystalline structure and chemical composition
of the as-synthesized nanoparticles (NPs). Electron microscopy images showed formation of
nanoparticles of various shapes ranging from spherical, tetrahedral and irregular shaped
morphologies. Magnetization measurements indicated that Ni3S4 nanoparticles prepared at 230
°C using DDA display ferromagnetic behaviour, while rhombohedral N3S2 also prepared at 230
°C but using HDA, displayed paramagnetic property.
Keywords:
Nickel sulfide, single source precursor, magnetic property, crystalline structure.

1. Introduction
The binary nickel sulfide system has raised interest because of its multiplicity of phases
and stoichiometries which include NiS (ɑ-NiS and ß-NiS), NiS2, Ni3S2, Ni3S4, Ni7S6, Ni6S5 and
Ni9S8 [1-4]. The stability fields of naturally occurring mineral nickel-sulfur systems have been
extensively studied and categorized as heazlewoodite (Ni3S2), polydymite (Ni3S4), millerite (Ni1-
xS) and vaesite (NiS2) [1]. The phase diagram of the Ni-S system reveals that temperature,
amount of nickel and/or sulfur present in the reaction system are key for the formation of a
unique pure phase of nickel sulfide. Whilst a few of these phases have been identified as
naturally occurring, others can be fabricated via laboratory protocols. The preparation of a
specific Ni-S system is a challenge due not only to the existence of many phases within a single
phase such as NiS (ɑ-NiS and ß-NiS) but also to the co-existence of different phases making it
difficult to extract a single pure phase [5].
The various phases of nickel sulfide nanomaterials are useful in many technological
applications. NiS2 has photocatalytic properties for hydrogen production and supercapacitors [6-
8]. NiS has shown potential in dye-sensitized solar cells as counter electrodes, IR detectors and
Li-ion storage applications [9-11]. Ni3S2 has been reported as a potent catalyst for water splitting
reactions [12-14] whereas Ni3S4 has been reported to have applications in high performance
supercapacitors [15, 16]. The low temperature heazlewoodite Ni3S2, NiS and Ni7S6 phases have
been reported to have common physical features of creamy bright yellow colour and are not
easily distinguished from each other [1]. Ni3S2 has two modifications namely ɑ-Ni3S2
(healzlewoodite) at low temperature (
°C) and ß- Ni S at higher temperature (wide range
T ≤ 838
3
2
of temperature) [17]. Reports have shown that at temperatures above 556 ºC, Ni3S2 changes into
Ni3+xS with an fcc crystal structure [18]. Both Ni3S2 and Ni7S6 are non-magnetic whereas NiS is
semi-metallic in nature, a p-type semiconductor whose paramagnetism is temperature
independent [3]. Ni3S4 polydymite has a cubic spinel crystal structure [19] which decomposes at
higher temperatures (400 ºC) to form NiS and NiS2.
Both nanocystalline NiS and Ni3S2 can be prepared by various methods. Keigo et al. [20]
reported the synthesis of NiS, Ni3S2 and Ni9S8 in high boiling point coordinating or non-
coordinating solvents. The formation of pure phases was influenced by time, temperature and

boiling solvents. Salavati-Niasari et al. [21] were able to isolate NiS nanocrystals by using
microwave irradiation from organometallic complexes. The same group reported the use of
nickel dithiosemicarbazide as a new precursor to obtain nickel sulfide nanoparticles [21, 22].
Karthikeyan et al. [23] used hydrothermal method to prepare nickel sulfide with EDTA as a
capping group. The obtained nanocrystals were mixed phases of Ni3S4, NiS2, NiS and Ni17S18.
Ni3S4 has been observed to co-exist with other forms of the Ni-S system i.e. NiS and NiS2. It is
also a challenge to prepare this particular phase of nickel sulfide in the laboratory by a solid state
reaction method, since at higher temperatures (360 ºC) it is unstable [19]. The phase diagram of
Ni-S system shows pure phase Ni3S4 to be stable up to 356 ºC [1, 14] and efforts to prepare it in
laboratory have resulted in mixed phases [18]. Various researchers have reported the synthesis of
Ni3S4 by the colloidal route [5, 16, 24]. The solvent-less synthetic route produced mixed phases
of NiS (millerite) and Ni3S4 [18, 25].
Single source precursors (SSPs) have been widely used in the synthesis of different metal
sulfides and various phases of nickel sulfide [25-28]. Dithiocarbamate complexes, in particular
have been extensively used for the preparation of nickel sulfide nanoparticles with different
phases [29-33]. The decomposition patterns of dithiocarbamates promote the formation of a
particular pure phase of nickel sulfide [29, 33]. For example, Hogarth and co-workers [33]
showed that temperature and solvents are key determinants of Ni-S phase obtained. Roffey et al.
[29] reported the use of dithiocarbamate complexes for the preparation of a wide range of phase-
pure nickel sulfide particles by the manipulation of different reaction conditions.
In this paper we report the use of bis(dipiperidinyldithiocarbamato) nickel(II) (1) and
bis(ditetrahydroquinolinyldithiocarbamato) nickel (II) (2) complexes for the synthesis of pure
phase of Ni3S4, Ni3S2 and mixed phase of nickel sulfide nanoparticles. Magnetic measurements
were showed to be phase dependent. An improved crystal structure of complex (1) is also
elucidated [34].

2. Experimental details
2.1. Materials
Hexadecyalamine (HDA) 98%, oleyamine (OLA) 99%, dodecylamine (DDA), tri-n-
octylphosphine (TOP) 90 %, piperidine 99 %, tetrahydroisoquinoline 99 %, carbon disulfide 99
%, nickel acetate tetrahydrate (Sigma Aldrich), sodium hydroxide 98 %, methanol 99.5 %,
chloroform, acetone and hexane were used as purchased without any further purification.
2.2. Instrumentation
Microanalysis was performed on a Perkin-Elmer automated model 2400 series II CHNS/O
analyser. Infrared spectra were recorded on a Bruker FT-IR Tensor 27 spectrophotometer
directly on small samples of the compounds in the 200-4000 cm–1 range Thermogravimetric
analysis was carried out at 10 °C min−1 heating rate using a Perkin Elmer Pyris 6 TGA up to 600
°C in a closed perforated aluminium pan under N2 gas flow. The 1H NMR spectra were recorded
with a Bruker 400 spectrometer, using CDCl3 as a solvent. Single crystal X-ray structure
analyses: A green needle-like single crystal measuring about 0.21 × 0.04 × 0.02 mm for (1),
mounted on the goniometer of a Bruker Apex II Duo CCD diffractometer using a 500 µm-long
needle cryoloop (MiTeGen®) mount. After initial suspension and selection in Paratone® oil.
Intensity data were collected using Mo Kα radiation from an Incoatec® microsource (IuS,
Quazar mirror optics) at a temperature of 100(2) K. The structures were solved using direct
methods with SHELXS [35] running in WinGX [36]. The structures were refined using least-
squares methods (SHELXL) [35]. All non-H-atoms were refined anisotropically; hydrogen
atoms were included in calculated positions, assigned isotropic thermal parameters (Uiso for H
atoms = 1.2Uiso for the attached C atom) and allowed to ride on their parent carbon atoms using
the standard HFIX parameters in SHELXL. Crystal data and structure refinement parameters are
presented in ESI Table S1.
Optical absorption measurements were carried out using a Perkin-Elmer Lambda 1050
UV-Visible NIR spectrophotometer. The samples were placed in silica cuvettes (1 cm path
length), using hexane as a reference solvent. A Perkin-Elmer, LS 55 Luminescence
spectrophotometer was used to measure the photoluminescence of the particles. Samples were

placed in a quartz cuvette (1 cm path length). TEM and HRTEM analyses were performed using
a JEOL 1010 TEM and JEOL 2100 HRTEM. Samples were prepared by placing a drop of dilute
solution of nanoparticles on Formvar-coated Cu grids (150 mesh) for TEM and holey carbon
grids for HRTEM. The samples were allowed to dry completely at room temperature and viewed
at an accelerating voltage of 100 kV (TEM) and 200 kV (HRTEM), and images captured
digitally using a Megaview III camera; stored and measured using Soft Imaging Systems iTEM
software (TEM) and Gatan camera and Gatan software (HRTEM). Particle sizes were measured
using ImageJ Tool 1.4.6r software (a total number of >10 particles). Powder X-ray diffraction (p-
XRD) patterns were recorded in the high angle 2θ range of 20 - 80° using a Bruker AXS D8
Advance X-ray diffractometer, equipped with nickel filtered Cu Kα radiation (λ = 1.5406 Å) at
40 kV, 40 mA and at room temperature. The variation of room temperature hysteresis loops was
investigated by using a Lakeshore model 735 vibrating sample magnetometer (VSM). In this
case, the hysteresis loops were obtained in external applied magnetic fields of only up to 14 kOe.
2.3. Synthesis of ligands and nickel complexes
The piperidine and tetrahydroquinoline dithiocarbamate ligands and the corresponding
nickel complexes were synthesized using the procedures described in our previous work [26].
The synthesis of complexes is detailed as follows; nickel acetate (4.1 g, 16.4 mmol) was
dissolved in deionized water (25.0 mL), and then added drop-wise to the corresponding solution
of the dithiocarbamate ligand (6.0 g, 32.8 mmol). The reaction mixture was stirred for 45 min,
and the precipitate formed was vacuum filtered, washed with deionized water and dried in an
oven at 70 ºC overnight. Two complexes were prepared as follows:
1
[Ni(pip-dtc)2] (1), (where pip = piperidinyl and dtc = dithiocarbamate) yield: 79%. H
NMR (400 MHz, CDCl3): δ 1.29 (m, 8H, 3-CH2), 1.68 (t, 4H, 4-CH2), 3.75 (t, 8H, 2-CH2).
Microanalysis: Calculated for C12H20N2NiS4: C, 38.00; H, 5.32; N, 7.39. Found: C, 38.12; H,
5.78; N, 7.24. IR (cm-1, ATR): 998, v (C═S); 1438, v (C═N); 1507, v (C-N); 385-365, v (Ni–S).
[Ni(thq-dtc)2] (2), (where thq = tetrahydroquinyl and dtc = dithiocarbamate) yield: 67%.
1H NMR (400 MHz, CDCl3): δ 2.99 (m, 4H, 3-CH2), 3.99 (t, 4H, 4-CH2), 4.99 (t, 4H, 2-CH2),
7.20–7.50 (m, 8H, Ar–H). Microanalysis; Calculated for C20H20N2NiS4: C, 50.54; H, 4.24; N,

5.89. Found: C, 50.34; H, 4.52; N, 5.56. IR (cm-1, ATR): 985, v (C═S); 1441, v (C═N); 1521,
v(C-N); 385-365, v (Ni–S).
2.4. Preparation of nickel sulfide nanoparticles
In a typical procedure, 0.3 g of complex (1) or (2) was dissolved in 3.0 mL of tri-n-
octylphosphine (TOP) and injected in hot DDA in a three-necked flask at 190 ºC. A drop in
temperature of about 20 ºC was observed and the solution turned to a blackish colour. The
reaction temperature was maintained for 2 hours, after which the heating was stopped. Ethanol
was then added to flocculate the nickel sulfide nanoparticles. The DDA capped nickel sulfide
nanoparticles were separated by centrifugation followed by washing with ethanol and acetone.
The reaction was repeated at a temperature 230 ºC. The above reaction procedures were also
repeated at temperatures 190 ºC, 230 ºC and 270 ºC, using HDA and OLA as the capping groups.
3. Results and discussion
3.1. Characterization of the ligands and complexes
The nickel dithiocarbamato complexes used in this work were synthesized from cost-
effective starting materials using relatively simple reaction protocols. They are stable in air
similar to other complexes of dithiolates [32]. The ligands and the complexes were obtained in
good yields and microanalysis confirmed their purity. The complexes were green
microcrystalline powders soluble in most organic solvents. IR spectroscopy revealed a very
broad band around 3324-3367 cm-1 in the spectra of the ligands due to the ν (O-H) of water. This
band was absent in the spectra of the complexes. Both the ligands and complexes showed bands
in the 1466–1485 cm-1 region, corresponding to ν (C=N) and ν (C-N). The ν (C=S) band was
observed at 999-975 cm-1 region in the spectra of the ligands. The upward shift of this band in
the complexes compared to the free ligand, together with a strong band (or very close bands)
attributed to ν (C-S) was indicative of a bidentate or slightly anisobidentate dithiocarbamate. The
TGA plots of both complexes displayed a single step decomposition pattern. The TGA plots
showed a weight loss of 74.7% and 71.0% (calc = 76.0% and 80.1%) at 353.5 and 315.4 °C for
complexes (1) and (2), respectively (Figure 1), corresponding to the loss of the organic moiety
and sulfur atoms. Dithiocarbamate metal complexes decompose in solid state either by

volatilization, leaving behind almost no residue or decomposition via a number of steps to yield
respective metal sulfides [37]. Our metal complexes are in the latter category as the final residues
25.3% and 21.0 % were close to the calculated values of 24.0% and 19.1%, for NiS from
complexes (1) and (2), respectively.
100
80
60
40
(2)
(1)
20
100
200
300
400
500
600
Temperature (οC)
Fig. 1. Thermogravimetric analysis (TGA) plots of complexes (1) and (2).
3.2. Single-crystal X-ray structure
The X-ray data for complex (1) was previously recorded at ambient temperature by
Radha et al. [34]. The data presented herein shows improvement in data/parameter ratios as well
as significant improvements in the R1, wR2 and accuracy in the C–C bond lengths. Complex (1)
crystallised in the monoclinic space group P21/c with a half molecule in the asymmetric unit and
Z = 2. The molecule has crystallographically imposed inversion symmetry with a centre of
inversion located at the Ni(II) ion (Figure 2).

Fig. 2. Displacement ellipsoid plot of complex (1) showing 50% probability surfaces. Hydrogen
atoms have been rendered as spheres of arbitrary radius. The molecule has inversion symmetry
with the centre of inversion located at the Ni(II) ion. Symmetry code: (i) -x, -y, -z (CCDC
1502999).
The d8 Ni(II) ion has adopted a nominally square planar coordination geometry. The bond
parameters are summarized in Table 1. The most notable deviation from an ideal square planar
geometry lies in the intraligand S1–Ni1–S2 bond angle which measures 79.49(1)°, significantly
more acute than the ideal bond angle of 90°. This acute bond angle is attributed to the small bite
of the dithiocarbamate ligand and the associated 4-membered chelate ring. Consequently, the
interligand S1–Ni1–S2 bond angle is obtuse, measuring 100.51(1)°. The S1-C1 and S2-C1 bond
lengths measure 1.727(1) Å and 1.722(1) Å, respectively. These bond lengths are intermediate to
those of a formal single or double C–S bond, highlighting the delocalisation of the
dithiocarbamate ligand. Further, the C1-N1 bond length measures 1.321(2) Å. This is shorter
than a typical sp2 C–N single bond (1.377 Å) [38] suggesting a portion of double bond character
and illustrating further delocalisation.

Table 1
Selected bond parameters of (1).
Bond
Length
(Å)
Bond
Angle (°)
79.49(1)
100.51(1)
Ni1–S1
Ni1–S2
2.2010(3) S2–Ni1–S1
(intraligand)
2.2117(4) S2–Ni1–S1
(interligand)
S1–C1
S2–C1
C1–N1
1.727(1)
1.722(1)
1.321(2)
S1–C1–N1
S2–C1–N1
125.4(1)
124.8(1)
The four coordinating atoms and the metal ion are planar by definition; however, this
planarity does not extend to the C–N bond which shows a significant out-of-plane distortion. The
atoms C1 and N1 are displaced from the 5-atom mean plane defined by the coordination sphere
by 0.132(1) Å and 0.346(1) Å, respectively. The piperidine rings are approximately
perpendicular to the coordination sphere and are in a trans-configuration. This geometry leads to
a herringbone pattern in the crystal lattice as shown in the packing diagram in Figure 3.
The structure does not show any signs of axial coordination by the sulfur atoms of
adjacent dithiocarbamate ligands as is noted in previously reported copper (II) dithiocarbamate
structures [39], seemingly the steric bulk of the piperidine rings preclude the formation of these
interactions. The lattice is stabilized by weak C–H···S interactions between the C6 methylene
CH and S1 of an adjacent molecule. The C–H···S distance measures 2.935(2) Å, only
fractionally shorter than the sum of the van der Waals radii suggesting a weak interaction. The
X-ray data for complex (2) were recorded at ambient temperature by Hogarth et al. [40].

Fig. 3. Packing diagram of (1) viewed down the a -axis. The packing diagram illustrates the
trans-configuration of the piperidine rings and the resultant herringbone pattern. The lattice is
stabilized by C-H…S interactions, shown as dashed purple tubes.
3.3. Nickel sulfide nanoparticles
The thermolysis of precursors in high boiling point primary amine solvents is a well
reported route to high quality nanoparticles [3]. The size of the nanoparticles depends on the
reaction time, temperature, precursors and capping agent. In particular, long chain amines are
found to be suitable surfactants for II–VI semiconductor nanomaterials. In this work we decided
to explore hexadecylamine (DDA), dodecylamine (HDA) and oleylamine (OLA) as capping
groups for nickel sulfide nanoparticles. Both complexes (1) and (2) were thermolysed in
coordinating solvents at temperatures of 190 °C, 230 °C for DDA and 190 °C, 230 °C and 270
°C for HDA and OLA for 2 hrs.
3.4. Structural and morphological characterization
The powder X-ray diffraction (p-XRD) pattern for both precursors using capping groups
at different temperatures show a change of phases as the capping group is varied. DDA which is
a shorter carbon chain (12 carbon) capping group gave pure phase of Ni3S4 (Card No. 00-043-
1469) when complex (1) and (2) were used as single source precursors at 190 and 230 °C (Figure
4 and ESI Fig. S1). All diffraction peaks with hkl values (022), (113), (004), (115), (044) and

(335) can be indexed to the sulfur rich pure phase of cubic Ni3S4 (Card No. 00-043-1469). This
is in good agreement with literature [4], where at temperatures lower than 250 °C the cubic phase
Ni3S4 is obtained. The results are in agreement with the decomposition pattern of other
dithiocarbamate complexes reported elsewhere [33].
1.8
1.6
1.4
230 °C
1.2
1.0
0.8
0.6
0.4
0.2
0.0
190
80
°C
20
30
40
50
60
70
2θ (deg)
Fig. 4. Powder X-ray diffraction (p-XRD) pattern for DDA capped nickel sulfide nanoparticles
from complex (1).
The primary amine with 16 carbon chain (HDA) gave pure phase of Ni3S2 at
temperatures of 190 ºC, 230 ºC and 270 ºC (card no.00-002-0772) (Figure 5 and ESI Fig. S2).
The XRD peaks were indexed to the (101), (012), (003), (202), (211), (122), (303), (214) and
(401) mirror planes of rhombohedral ɑ-Ni3S2. The diffraction patterns show an increase in
crystallinity as the temperature is raised from 190 ºC to 270 ºC. Particle sizes as estimated by the
Debye-Scherrer equation show average crystallite sizes of 3.1 nm, 7.1 nm and 11.9 nm for
nanoparticles prepared at 190 ºC, 230 ºC and 290 ºC respectively. These sizes were corroborated
by the TEM studies.

When OLA was used as a capping agent, both complex (1) and (2) gave nanoparticles
with mixed phases (Figure 6 and ESI Fig S3). The diffraction peaks show a dominant phase of
hexagonal NiS at 270 °C (card number 03-065-3419) which is indexed to (100), (101), (102),
and (110) planes. Unlabeled peaks could also be matched to a rhombohedral phase of nickel
sulfide (card #; 00-012-0041). At lower temperatures (190 °C and 230 °C), a mixture of cubic
phase of polydymite (Ni3S4) was identified labelled # (card number: 00-047-1739) which fits
well in what has been reported on the formation of this phase at lower temperatures [33]. Phase
transition in nickel sulfide due to an increase in temperature has been reported previously [30].
The results of the parameters and properties of the as-synthesized nickel sulfide nanoparticles are
presented in Table 2.
3.0
270
230
°
C
2.5
2.0
1.5
1.0
0.5
0.0
°
C
190
80
°
C
20
30
40
50
60
70
2θ
(deg)
Fig. 5. X-ray diffraction (p-XRD) pattern for HDA capped nickel sulfide nanoparticles from
complex (1).

3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
270 oC
#
#
#
#
230 oC
190 oC
#
20
30
40
50
60
70
80
2θ
(deg)
Fig. 6. X-ray diffraction (p-XRD) pattern of OLA capped Nickel sulfide nanoparticles
synthesized using complex (1).
Figure 7 shows TEM images for DDA capped nickel sulfide nanoparticles from both
complex (1) and (2) at various temperatures. The morphology and average particle sizes of the as
synthesized nickel sulfide nanoparticles varied with temperature and precursor type. Nanoplates
are observed at lower reaction temperature of 190 °C when complex (1) was used (Figure 7 (a)).
These nanoplates with unusual cross-like shape have been reported previously by Wu et al. [41].
The nanoplates in our case have four seemingly equal wings, with each lateral wing having a
dimension of approximately 65 nm. At 230 °C particles without a regular shape are observed
(Figure 7(b)). Complex (2) at 190 °C gave spherical nanoparticles with a relatively smaller
average size of 8.63 nm (Figure 7 (c)) while at 230 °C, short rods with average width of 8.68 nm
and length of 53.89 nm were observed (Figure 7(d)). At higher magnification, the HRTEM
image for nickel sulfide particles in Figure 7(d) shows lattice fringes with a d-spacing of 2.87 Å,
assigned to the (311) plane of Ni3S4 (ESI Fig.S.4 (c) ). A summary of the morphology, particle
size and phase formation of nickel sulfide synthesized using different reaction parameters is
shown in Table 2.

TEM images for HDA capped nickel sulfide nanoparticles from complex (1) and (2) are
represented in Figure 8. Complex (2) gave slightly larger nanoparticles as compared to complex
(1). The nickel sulfide synthesized from complex (1) were spherical with an average size of 2.9
nm, 7.4 nm and 12.1 nm at 190 °C, 230 °C and 270 °C, respectively. For complex (2), the
estimated particle size for the spherical nanoparticles was 5.9 nm (190 °C), 9.8 nm (230 °C)
whereas at 270 °C, irregular shaped particles were formed. The high resolution TEM image for
Figure 8 (f) represented in ESI Fig S4 (a) and its corresponding SAED ESI Fig S4 (b) revealed
the formation of crystalline Ni3S2 nanoparticles.
Table 2 Summary of the reaction conditions, sizes and shapes of the obtained nickel sulfide
nanoparticles.
Complex Capping group Temp (°C) Morphology
Particle
size/aspect
ratio (nm)
67.71
Phase
(1)
DDA
HDA
190
230
Nanoplates
Ni3S4
Ni3S4
Irregular shaped rods
7.67
190
230
270
190
230
270
190
230
190
230
270
190
230
270
Spherical
5.93
9.80
-
Ni3S2
Ni3S2
Ni3S2
Mixed
Spherical
Irregular shape
Close to sphere
OLA
5.17
Irregular tetrahedral shape 8.96 length Mixed
Irregular shape
Spherical
Short rods
Spherical
Spherical
Spherical
Spherical
9.67 length Hex. NiS
(2)
DDA
HDA
8.63
6.21
2.93
7.39
12.06
4.21
Ni3S4
Ni3S4
Ni3S2
Ni3S2
Ni3S2
OLA
Mixed
Mixed
Hex. NiS
Irregular tetrahedral shape 8.31
Spherical 9.83

Fig. 7. TEM images for DDA capped nickel sulfide nanoparticles from complex (1) synthesized
at (a) 190 °C (b) 230 °C and complex (2) at (c) 190 °C and (d) 230 °C.

Fig. 8. TEM images for HDA capped nickel sulfide nanoparticles at (a) 190 °C, (b) 230 °C and
(c) 270 °C from complex (1) and (d) 190 °C, (e) 230 °C and (f) 270 °C from complex (2).
TEM images for OLA capped nickel sulfide nanoparticles synthesized from precursor (1)
and (2) are presented in Figure 9. The TEM images (Figure 9) for OLA capped nickel sulfide
nanoparticles synthesized from precursor (1) and (2) show the formation of close to spherical
nanoparticles and irregular shaped nanoparticles depending on the reaction parameters as
presented in Table 2. The particle size increase with an increase in temperature for particles
synthesized from both complexes. High resolution TEM for Figure 9 (c) shows fringes with a d-
spacing of 2.86 Å which fit well with the data obtained from p-XRD results for the same in
Figure 4 (ESI Fig. S4 (d)).

Fig. 9. TEM images for OLA capped nickel sulfide nanoparticles synthesized from complex (1)
at (a) 190 °C, (b) 230 °C and (c) 270 °C and complex (2) at (d) 190 °C, (e) 230 °C, (f) 270 °C.
3.5. Magnetic properties
The magnetic properties of the samples annealed at 230 °C were recorded at 17 °C by a vibrating
sample magnetometer. The hysteresis loops are presented in Figure 10. The sample (DDA
PIPDTC) prepared in DDA using complex (1) presents the cubic-Ni3S4 sample with
magnetization at 2000 Oe of about 1.6 emu/g and a coercive field of 224 Oe. Similarly, the
sample (DDA THQDTC) prepared in DDA using complex (2) showed cubic-Ni3S4 exhibiting
magnetization of about 3.2 emu/g and a coercive field of 93 Oe, with a saturation magnetization
that is higher than that from complex (1) prepared under similar conditions. The coercive field of
the sample prepared using complex (1) is higher than that from complex (2) both in DDA. Both
samples prepared in DDA showed ferromagnetic behaviour, typical of Ni3S4 phase [42]. It can be
seen that the rhombohedral (ɑ-Ni3S2) sample (HDA THQDTC) prepared from complex (2) using

HDA shows an almost linear plot of magnetization against applied field with almost zero
coercive field which corresponds to a paramagnetic phase. The magnetic properties of
nanomaterials are believed to be highly dependent on the structure and particle sizes. The
coercive fields tend to correlate well with particle sizes. From Figure 10 and Table 2, the DDA
Ni3S4 sample with higher coercive field has a relatively larger particle size. These results appear
consistent with transformation from a single to multi-domain structure with increase in particle
size [43]. The coercive fields HC and particle sizes D are expected to vary as HC = a1 − b / D2
1
and HC = a2 + b2 / D for single domain and for multi-domain regions respectively where ai and
b are constants.
i
Fig. 10. Magnetic hysteresis curves of the nickel sulfide samples synthesized by the hot injection
thermal decomposition of complexes at 230 °C. The hysteresis loops were recorded at 17 °C.

3.6. Optical properties
The absorption spectra of HDA capped nickel sulfide nanoparticles synthesized 270 °C
displays an absorption onset at 327 nm (ESI Fig S5). Absorption (UV-Vis) spectra for nickel
sulfide nanoparticles synthesized at various reaction conditions are presented in ESI Figs. S5 –
S10. All spectra show absorption in the higher energy region ranging from 3.5 eV to 4.1 eV
which is a blue shift in relation to bulk nickel sulfide (2.7 eV) affirming the size quantization
effect of the nanoparticles.
4. Conclusion
Piperidine (1) and tetrahydroquinoline (2) nickel(II) dithiocarbamate complexes have
been synthesized and characterized. An improved low temperature single X-ray crystal structure
of complex (1) has also been elucidated. These complexes have been used as single source
precursors to synthesize high quality nickel sulfide nanoparticles via a solvothermal route using
dodecylamine, hexadecylamine and oleylamine as the coordinating solvents. Nickel sulfides of
different mineral phases, namely NiS, Ni3S4 and Ni3S2 have been produced. The rare pure phases
produced are in agreement with previous reports, where the formation of phase pure Ni-S
systems is temperature dependent. In addition, the formation of mixed phases when oleylamine
is used and the formation of Ni3S4 for dodecylamine (at all temperatures) suggest further studies
in the field. Nanoparticles can display a variety of magnetic properties near room temperature
which are dependent on the crystalline phases. The coercive field data can therefore be
investigated on the basis of transformation from single to multi-domain structure. This study
provides a relatively reliable single source precursor route for preparing stoichiometric nickel
sulfide nanocrystals with well-defined crystalline structures, compositions and morphologies.
Appendix A.
Supplementary data CCDC 1502999 contains the supplementary crystallographic data for
bis(dipiperidinyldithiocarbamato) nickel(II) (1). These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.

Acknowledgements
Authors acknowledge the National Research Foundation (NRF), South Africa
through the South African Research Chair Initiative (SARChI) and also Royal Society
DFID Africa Capacity Building Initiative for financial support. The authors are also grateful to
the University of Zululand Research Office for financial support.
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High temperature solution based thermolyis of single source precursors has been employed to
synthesize phase pure magnetic nickel sulfide nanoparticles.