Heterocyclic lead(II) thioureato complexes as single-source precursors for the aerosol assisted chemical vapour deposition of PbS thin films

Article abstract of DOI:10.1016/j.ica.2018.04.023

The lead N’-benzoylthioureato complexes of N-morpholine (1) and N-pyrrolidine (2) were synthesized and characterized by elemental analysis, Fourier Transform Infra-Red (FT-IR) spectroscopy, proton nuclear magnetic resonance (¹H nmr) and thermogravimetric analyses (TGA). The X-ray single crystal structure of complex (1) was determined. The compounds were both used as single source precursors to deposit PbS films on glass substrates at 350, 400 and 450 °C using aerosol assisted chemical vapour deposition (AACVD) The surface morphology of PbS films were determined by scanning electron microscopy (SEM), the crystalline phases established by powder X-ray diffraction (p-XRD) analyses and composition by energy dispersive X-ray spectroscopy (EDX). The particle sizes were found to range between 82 and 85 nm from complex (1) and 70–105 nm for (2) The PbS films were studied by Near Infra-red (NIR) UV–Vis spectroscopy, band gap ranging from 1.46 to 1.55 eV were observed.

Full text of DOI:10.1016/j.ica.2018.04.023

Accepted Manuscript
Heterocyclic lead(II) thioureato complexes as single-source precursors for the
aerosol assisted chemical vapour deposition of PbS thin films
Kevin I.Y. Ketchemen, Sixberth Mlowe, Linda D. Nyamen, Ahmed A. Aboud,
Matthew P. Akerman, Peter T. Ndifon, Paul O'Brien, Neerish Revaprasadu
PII:
S0020-1693(18)30122-1
https://doi.org/10.1016/j.ica.2018.04.023
ICA 18214
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
23 January 2018
3 April 2018
14 April 2018
Please cite this article as: K.I.Y. Ketchemen, S. Mlowe, L.D. Nyamen, A.A. Aboud, M.P. Akerman, P.T. Ndifon,
P. O'Brien, N. Revaprasadu, Heterocyclic lead(II) thioureato complexes as single-source precursors for the aerosol
assisted chemical vapour deposition of PbS thin films, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/
j.ica.2018.04.023
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Heterocyclic lead(II) thioureato complexes as single-source precursors for the
aerosol assisted chemical vapour deposition of PbS thin films
Kevin I. Y. Ketchemena,b, Sixberth Mlowea, Linda D. Nyamenb, Ahmed A. Abouda,c,
Matthew P. Akermand, Peter T. Ndifonb*, Paul O’Briene* and Neerish Revaprasadua*
aDepartment of Chemistry, University of Zululand, Private Bag X1001, KwaDlangezwa
3880, South Africa. email: RevaprasaduN@unizulu.ac.za
bDepartment of Inorganic Chemistry, University of Yaoundé I, P.O. Box 812, Yaoundé,
Cameroon.
c Department of Physics, Faculty of Science, Beni-Suef University, Beni-Suef 62514, Egypt.
dSchool of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01, Scottsville,
Pietermaritzburg, 3209, South Africa.
eThe School of Chemistry and The School of Materials, The University of Manchester Oxford
Road, Manchester M13 9PL, UK.
ABSTRACT
The lead N’-benzoylthioureato complexes of N-morpholine (1) and N-pyrrolidine (2)
were synthesized and characterized by elemental analysis, Fourier Transform Infra-Red
(FT-IR) spectroscopy, proton nuclear magnetic resonance (1H nmr) and
thermogravimetric analyses (TGA). The X-ray single crystal structure of complex (1) was
determined. The compounds were both used as single source precursors to deposit PbS
films on glass substrates at 350, 400 and 450 °C using aerosol assisted chemical vapour
deposition (AACVD) The surface morphology of PbS films were determined by scanning
electron microscopy (SEM), the crystalline phases established by powder X-ray
diffraction (p-XRD) analyses and composition by energy dispersive X-ray spectroscopy
(EDX). The particle sizes were found to range between 82 – 85 nm from complex (1) and
70 – 105 nm for (2) The PbS films were studied by Near Infra-red (NIR) UV–Vis
spectroscopy, band gap ranging from 1.46 – 1.55 eV were observed.
Keywords: Single source precursors, lead sulfide, AACVD, thin films, optical properties.
1

1. Introduction
There is considerable interest in thin films of semiconductors for their novel and
potentially useful optoelectronic properties [1-4]. Amongst such semiconductors, PbS
has been extensively studied because of its cheapness and potential usefulness in solar
cells [5-7]. Lead sulfide (PbS) is a IV-VI semiconductor with a large Bohr exciton radius
of 18 nm and a small band gap of 0.41 eV (3100 nm) which is tunable with particle size
[8-10]. Furthermore, PbS has been found useful in several other applications such as
thermoelectric, sensors and photocatalysts [11-15]. The methods used to prepare or
deposit this material can profoundly affect phase composition, thermal stability,
crystallite size and morphology, which in turn can influence the functional behavior of
the material. Many methods including single source route [16-21], spray pyrolysis ,
chemical bath deposition [22], successive ionic layer absorption [23] and chemical
vapour deposition (CVD) [24-27], have been used.
Different single source precursors have been used in CVD for deposition of metal
chalcogenide thin films [28-31], and have the advantage of avoiding toxic precursors and
eliminating undesired effects of the anions of the metal salt [32]. Dithiocarbamates [33,
34], thiosemicarbazides [35], xanthate [36, 37], imidodiphosphinates , N-alkyl thiourea
[38], dichalcogenophosphinates and dichalcogenophosphates [39] lead complexes have
been investigated for the preparation of PbS
Herein we report the synthesis of N-morpholine (1) and N-pyrrolidine (2) lead (II) N’-
benzoylthioureato complexes. The X-ray single crystal structure of bis(N-morpholine-N’-
benzoylthioureato)lead(II) complex (1) is also elucidated. These complexes have been
used as single source precursors for the deposition of cubic, spherical and rod-shaped
PbS crystallites in thin films on glass substrate at various temperatures by AACVD.
2

2. Experimental
2.1. Chemicals
Benzoyl chloride 99%, morpholine 99%, pyrrolidine 99%, potassium thiocyanate 98.5%
(Sigma–Aldrich). Lead nitrate 99%, ethanol 99.5%, dichloromethane, chloroform and
acetone (Merck) were used as purchased without any further purification.
2.2. Instrumentation
Microanalysis was performed on a Perkin-Elmer automated model 2400 series II
1
CHNS/O analyzer. The H NMR spectra of both ligands were obtained using a Bruker
advance III 400 MHz spectrophotometer. Infrared spectra were recorded on a Perkin-
Elmer Spectrum Two UATR FT-IR spectrophotometer directly on small samples of the
compounds in the 500-4250 cm–1 range. Thermogravimetric analysis was carried out at
20 °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. Optical absorption measurements were
carried out using a Lambda 1050 Perkin-Elmer spectrophotometer. Measurements were
done at room temperature. The surface morphology and structure of the film was
determined using a Zeiss Sigma VP-03-67 field emission gun scanning electron
microscopy (FEGSEM), equipped with an Oxford instrument X-max 50 EDX detector.
Powder X-ray diffraction (pXRD) patterns of the nanoparticles were recorded at room
temperature in the angle 2θ range (20-80°) using an Advanced Bruker AXS D8
diffractometer, equipped with nickel-filtered Cu Kα radiation (λ = 1.542 Å) at 40 kV and
40 mA.
2.3. X-ray crystallographic analysis
Thin colourless, plate-like crystals of (1) were grown from a 1:1 mixture of
dichloromethane and ethanol. X-ray data was recorded on a Bruker Apex Duo equipped
with an Oxford Instruments Cryojet operating at 100(2) K and an Incoatec microsource
operating at 30 W power. The data were collected with Mo Kα (λ = 0.71073 Å) radiation
at a crystal-to-detector distance of 50 mm. Data were collected using omega and phi
scans with exposures taken at 30 W X-ray power and 0.50° frame widths using APEX2
[40] The data were reduced with the programme SAINT [40]using outlier rejection, scan
speed scaling, as well as standard Lorentz and polarization correction factors. A SADABS
semi-empirical multi-scan absorption correction [40] was applied to the data. Direct
methods, SHELX-2016 [41] and WinGX [42], were used to solve the structure. All non-
hydrogen atoms were located in the difference density map and refined anisotropically
with SHELX-2016 [41]. All hydrogen atoms were included as idealized contributors in
3

the least squares process. Their positions were calculated using a standard riding model
of SHELX with C-Haromatic distances of 0.93 Å and Uiso = 1.2 Ueq and C-Hmethylene distances
of 0.99 Å and Uiso = 1.2 Ueq.
2.4. Preparation of the ligands
The ligands were prepared using a procedure similar to that reported in the literature
[43, 44]. Benzoyl chloride (2.0 mL; 17 mmol) was dissolved in 50 mL of acetone and
added dropwise to a suspension of potassium thiocyanate (1.67 g; 17 mmol) in 30 mL of
acetone. The reaction mixture was heated under reflux for 30 min, then cooled to room
temperature. A solution of the corresponding amine (1.5 mL; 17 mmol) in 10 mL of
acetone was added and the resulting mixture stirred for 2 h. Hydrochloric acid (300 mL,
0.1 M) was added to the resultant solution and the solid product filtered off, washed
with water and purified by recrystallization in ethanol: dichloromethane (1:1) mixture.
N-morpholine-N’-benzoylthiourea HL1: White. Yield: 75%, m.p. 144-146 °C. Anal. Calcd
for C12H14N2O2S (%): C, 57.58; H, 5.64; N, 11.19. Found: C, 57.98; H, 5.62; N, 11.07. IR
(cm-1): ν(N-H) 3239 (br), ν(C = O) 1663 (s), ν(C = S) 1000 (s). 1H NMR: δ, 8.1 (s, 1H, NH),
7.80 (m, 2H, C6H5), 7.60 (d, 1H, C6H5), 7.54 (m, 2H, C6H5), 3.78 (s, 4H, CH2), 3.0 (s, 4H,
CH2).
N-pyrrolidine-N’-benzoylthiourea HL2: Yellow. Yield: 73%, m.p. 130-132.8 °C. Anal. Calcd
for C12H14N2OS (%): C, 61.51; H, 6.02; N, 11.96. Found: C, 61.92; H, 5.89; N, 11.08. IR (cm-
1
1): ν(N-H) 3148 (br), ν(C = O) 1643 (s), ν(C = S) 978.57 (s). H NMR: δ 8.0 (s, 1H, NH),
7.80 (m, 2H, C6H5), 7.60 (d, 1H, C6H5), 7.50 (m, 2H, C6H5), 3.0 (s, 4H, CH2), 2.0 (s, 4H, CH2).
2.5.
Preparation of the lead complexes
Lead nitrate (0.72 g; 2 mmol) was dissolved in 20 mL of water and added dropwise to a
solution of the corresponding ligand (4 mmol) in 50 mL of ethanol at room temperature.
The resulting mixture was stirred for 30 min and the product obtained was filtered off
and recrystallized in ethanol: dichloromethane (1: 1).
Bis(N-morpholine-N’-benzoylthioureato)lead(II) (1). White. Yield: 91%, m.p. 167-169
°C. Anal. Calcd for C24H26N4O4PbS2 (%): C, 40.84; H, 3.71; N, 7.94. Found: C, 39.85; H,
3.69; N, 7.93. IR (cm-1): ν(C = N) 1587 (s), ν(C = O) 1473 (s).
Bis(N-pyrrolidine-N’-benzoylthioureato)lead(II) (2). Yellow. Yield: 74%, m.p. 154-157
°C. Anal. Calcd for C24H26N4O2PbS2 (%): C, 42.78; H, 3.89; N, 8.31. Found: C, 41.14; H,
3.51; N, 7.43. IR (cm-1): ν(C = N) 1586 (s), ν(C = O) 1487 (s).
4

2.6. Deposition of films by AACVD
In a typical deposition, 0.2 g (0.3 mmol) of the precursor was dissolved in 25 mL of
chloroform in a two-necked 100 mL round-bottom flask with a gas inlet that allowed the
carrier gas (argon) to pass into the solution to assist the transport of the aerosol. This
flask was connected to the reactor tube by a piece of reinforced tubing. The argon flow
rate was controlled by a Platon flow gauge. Seven glass substrates (approx. 1 х 2 cm)
were placed inside the reactor tube in a Carbolite furnace. The precursor solution in a
round-bottom flask was kept in a water bath above the piezoelectric modulator of a
Pifco ultrasonic humidifier (Model No. 1077). Aerosol droplets of the precursor were
transferred into the hot wall zone of the reactor by carrier gas. Both the solvent and the
precursor were evaporated and the precursor vapor reached the heated substrate
surface where thermally induced reactions and film deposition took place. The reaction
temperatures were varied between 350 and 450 °C.
3. Results and discussion
3.1. Characterization of the ligands and the complexes
The N-morpholine-N’-benzoylthiourea ligand and its corresponding Pb(II) complex (1)
were obtained as white powders whilst the N-pyrrolidine-N’-benzoylthiourea ligand and
its Pb(II) complex (2) were yellow. The compounds were obtained in good yields based
on the expected formulas as confirmed by microanalysis. IR results revealed that the
medium peak appearing at 3239 cm-1 for HL1 and 3148 cm-1 for HL2 is attributed to the
stretching of the N–H group adjacent to the carbonyl group. This N-H bond is replaced
by the C=N bond upon complexation of the ligand to the metal ion by action of a base.
The appearance of two new peaks in the IR spectra of the complexes is observed at 1587
cm-1 and 1586 cm-1 respectively for complexes (1) and (2) and attributed to the C=N
bond. The strong band at 1663 cm-1 for HL1 and 1643 cm-1 for HL2 is attributed to the
vibration of the C=O of carbonyl groups which shifts to lower wavenumbers upon
complexation. Deprotonation involved delocalization of C=O stretching vibration which
is in agreement with the literature [26, 45] thus confirming the coordination to the
central metal ion through oxygen atom from the carbonyl. The same observation was
made in complex (2). The proposed structure of bis-(N-pyrrolidine-N’-
(benzoyl)thioureato)lead (II) complex (2) is presented in ESI Fig S1.
The TGA profiles of both complexes showed a two-step decomposition pattern while the
DTA curves of both complexes exhibited a single endothermic peak centred at 230.61 °C
and 249.37 °C for complexes (1) and (2) respectively (ESI Fig. S2) which correspond to
5

the decomposition process. The first steps gave a loss of 58% (54.19%, 2MB =
2morpholinylbenzamide) (1) (Fig 1; step ii) and 52% (52.01%, 2PB
=
2pyrrolidinylbenzamide) (2) (Fig. 1; step iv) at 282 and 293 °C, respectively
corresponding to the loss of the organic moiety. Second decomposition steps at 485 and
493 °C with mass losses of a further 10% (calcd: 7.38%; 2CN, step i) (1) and 9% (7.73%;
2CN; step iii) (2) occurred, respectively [46]. The final residues of 33% (calc. 33.90%)
(1) and 39% (calc. 35.51%) (2) closely correspond to the theoretical values of PbS,
respectively. The higher experimental residues for (2) can be attributed to carbonaceous
contamination from aryl and heterocyclic groups.
100
90
(1)
80
70
(2)
60
50
(iv)
(iii)
(ii)
40
30
(i)
100
200
300
400
500
600
Temperature (oC)
Fig. 1. Thermogravimetric analysis (TGA) plots of complexes (1) and (2) at a heating
rate of 10 °C min-1 under nitrogen atmosphere.
3.2. Single crystal X-ray structure of N-morpholine-N’-benzoylthioureato
lead(II) (1)
The Pb(II) metal centre of complex (1) is four-coordinate with the coordination sphere
comprising two bidentate N-morpholine-derived benzoylthioureato ligands. The ligands
are coordinated through the carbonyl oxygen atom and a deprotonated thiol group
yielding a monoanionic ligand and neutral lead(II) chelate. The coordination geometry
of the lead(II) complex can be described as hemidirected i.e. the bonds between the
ligand and metal occupy only part of the encompassing globe of the metal ion, The
compound crystallised in the monoclinic space group P21/n with a single molecule in the
6

asymmetric unit (Z = 4). Crystal and structure refinement data are given in Table 1. The
contents of the asymmetric unit and the Difference Fourier map of (1) are shown in Fig.
2, the latter illustrates the likely position of the lone pair of electrons on the lead(II) ion.
Fig. 2. Displacement ellipsoid plot (50% probability) of (1) showing the distorted
trigonal bipyramidal geometry as well as the void in the coordination sphere, as
conventionally attributed to the electron density. Hydrogen atoms are rendered as
spheres of arbitrary radius. [Inset] Difference Fourier map showing the probable
location of the electron density residing on the lead (II) ion. The contour has ρ = -1.8 e,
the red and green grids indicate electron density maxima and minima, respectively. The
plot was rendered using OLEX2[47].
Table 1: Crystal data and structure refinement details for (1).
Crystal Data
Complex (1)
Chemical formula
Molar mass (g mol-1)
Crystal system, space group
Temperature (K)
a, b, c (Å)
C24H26N4O4PbS2
705.80
Monoclinic, P21/n
100(2)
13.430(2), 8.0145(13), 23.392(4)
α, β, γ (˚)
V (Å3)
Z
90, 92.456(5), 90
2515.6(7)
4
7

Radiation type
μ (mm-1)
Mo Kα
6.91
Crystal size (mm)
Data Collection
Diffractometer
0.6 × 0.05 × 0.02
Bruker APEXII CCD diffractometer
Absorption correction
Tmin, Tmax
Multi-scan, SADABS
0.645, 0.746
No. of measured, independent and observed
[I > 2σ(I)] reflections
23490, 6186, 5398
Rint
0.035
Refinement
R[F2 > 2σ(F2)], wR(F2), S
No. of reflections
No. of parameters
H-atom treatment
0.024, 0.052, 1.05
6186
316
H-atom parameters constrained
Δρmax, Δρmin (e Å−3)
1.00, –0.64
Table 2: Selected geometric parameters describing complex (1).
Bond
Length (Å)
2.448(2)
2.455(2)
2.687(1)
2.689(1)
1.262(4)
1.275(4)
Bond
Angle (°)
79.63(5)
76.81(6)
76.24(5)
85.11(5)
96.07(2)
147.99(7)
118.3(3)
117.1(3)
Pb1–O1
Pb1–O3
Pb1–S1
Pb1–S2
C1–O1
O1–Pb1–S1
S1–Pb1–O3
O3–Pb1–S2
S2–Pb1–O1
S1–Pb1–S2
O1–Pb1–O3
O1–C1–C7
O3–C14–C15
C14–O3
8

The data in Table 2 illustrate the bond parameters associated with the metal ion
coordination sphere, thus making it difficult to formally categorize the coordination
geometry. The bond parameters are comparable to those of related four-coordinate
hemi-directed lead(II) chelates with pseudo trigonal bipyramidal geometry [48-50]. The
carbonyl groups have a mean C=O bond length of 1.268(6) Å, highlighting their double
bond character. The O3–C14–C15 bond angle which measures 117.1(3)° further
highlights the sp2 hybridised nature of C14.
The structure packs as inversion dimers (Fig. 3) supported by complementary Pb–S
interactions between the lead ion and sulfur atom of an adjacent molecule with an
interaction distance of 3.6157(9) Å. This motif leads to the formation of a four-
membered ring. Interactions of this type are common, but not ubiquitous in comparable
O2S2 lead(II) complexes [48, 49]. And also seen in many related cadmium complexes. The
dimeric structures are linked into a one-dimensional supramolecular structure (Fig. 3)
through C–H···O interactions with a C···O interaction distance of 3.399(4) Å. This
structure propagates collinear to the b-axis. Bond length cannot be definitively linked to
bond strength due to various packing constraints in the crystal lattice. However, both
the Pb···S and C–H···O interactions are significantly shorter than the sum of the van der
Waals radii of the interacting atoms, 0.304 and 0.204 Å, respectively. This short bond
length suggests a genuine and indeed moderately strong interaction. In the case of the
C–H···O interaction, this statement is further supported by the fact that the bond angle
(171.4°) does not deviate significantly from the ideal angle of 180°.
The dimeric structure indicates weak stabilizing Pb···Pb interactions. The separation of
the metal ions comprising the dimer measures 3.9777(7) Å. In the literature there is
debate as to whether Pb–Pb bonds exist or would be better described as van der Waals
interactions [51]. The van der Waals parameter for a bond of this nature is 4.04 Å, thus
the distance noted in the present compound is only modestly shorter and may indicate
weak stabilizing van der Waals interactions between the metal ions. If the Pb···Pb
contact and lone pair are considered part of the coordination sphere then the chelate
could be loosely categorized as a distorted octahedron.
9

Fig. 3. [Top] Dimeric structure of (1) stabilised by Pb···S and weak Pb…Pb interactions.
[Bottom] One dimensional polymer of (1) comprising dimeric molecules cross-linked by
short, 3.399(4) Å, C, C-H…O interactions. Respective symmetry codes: 1-x, -y, 2-z and x, -
1+y, z. Interactions are shown as dashed purple tubes, all atoms are rendered as spheres
of arbitrary radius.
3.3. Physical and morphological studies of PbS thin films
The films deposited by AACVD were investigated by powder X-ray diffraction. The
recorded patterns for films obtained using complexes (1) and (2) are summarized in the
ESI (Figs. S3 and S4). The films deposited are all of well-defined galena PbS with the
halite structure (ICDD 05-0592). The deposited films all gave very sharp peaks in the p-
XRD and indeed at values greater than ca 60⁰ of the 2Ѳ the peaks for 1.5405 Å and
1.54434 Å of Cu K alpha radiation are seen. These results indicate good crystallinity.
There are many literature studies of PbS: the original mineralogical study in 1925 [52]
gave 5.3 Å lattice parameter, whilst Wyckoff in 1963 gives 5.9363 Å [53] and one at 298
K by Noda 1987 [54] gives 5.9315 Å.
Table 3. Calculated Lattice Parameter/Å (Calculated for all peaks below 2ϴ = 60⁰).
10

Temp./°C
350
Complex (1)
5.930(5)
5.931(6)
5.923(6)
5.928(4)
Complex (2)
5.933(4)
5.932(4)
5.932(4)
5.932(5)
400
450
Average
500
Average
400
300
200
100
6000
4000
2000
0
70.5
70.7
70.9
71.1
71.3
71.5
Average
20
40
60
2ϴ (deg)
80
Figure 4. Highly resolved p-XRD pattern. The lattice constant a value can be measured
using the average Cu Kα value of 1.54184 Å for all values of 2ϴ less than ca 60⁰. The
lattice parameters are remarkable consistent and summarized in Table 3. At higher 2ϴ
angles the peaks for both the 1.5405 Å and 1.54434 Å of Cu K alpha wavelengths are
seen, and values for lattice constants can be measured for all 6 samples twice, again
accurate and consistent values are obtained summarized in Table 4. Histograms of
spectra are quite consistent for all samples with some evidence for slight rod like
orientation (Fig 5). Inset: average XRD patterns for all depositions, with of course little
or no difference in peak position.
The lattice constant determined for our samples from all peaks below 63⁰ gave
consistent values for the lattice constant from each deposition (Table 3) and an overall
average of 5.930(5) Å consistent with the literature. An average plot of all samples is
shown in Figure 4, a small non-indexed peak appearing at 2θ = 22.71° is seen for the
11

films deposited at 400 °C. At the higher angle diffraction close to 71⁰ is weak and
somewhat noisy and the average of all peak is shown (inset Fig 4). However the two
component peaks for the Cu Kα components can be seen and beautifully corresponding
values for the a value of 5.934 Å and 5.935 Å are seen from the (420) reflection. These
results show that the thin films are well crystallised.
Table 4. The values of the lattice parameter calculated twice for each temperature from
the ca 71⁰ (420) peak.
Sample
2Ѳ
2Ѳ
λ = 1.5405 Å
5.936
λ = 1.54434 Å
5.934
C(1) 350 ⁰C
C(1) 400 ⁰C
C(1) 450 ⁰C
C(2) 350 ⁰C
C(2) 400 ⁰C
C(2) 450 ⁰C
70.940
70.990
70.960
70.950
70.970
70.960
71.180
71.180
71.170
71.160
71.160
71.160
Av
5.933
5.934
5.935
5.934
5.936
5.935
5.934
5.935
5.935
5.935
5.935(1)
5.934(1)
The relative orientations are summarized in Fig. 5 and slightly rod like SEM images were
observed. The coverage area is largely close to 100% at the substrate temperature at
several temperatures (Fig. 5 and always >80–90). Various particle sizes and
morphologies were obtained and the average particle size was estimated using the
intercept technique where a random straight line is drawn through the micrograph, the
number of grain boundaries intersecting the line are counted. The average grain size is
found by dividing the number of intersections by the actual line length.
Average grain size =1/(number of intersections/actual length of the line)
At 350 °C when complex (1) was used, cubic-shaped PbS particles were formed with an
average size of 85 nm (Fig. 5a). A rise in temperature to 400 °C resulted in the formation
of a mixture of spherical and cubic shaped particles with an average size of 84 nm (Fig.
5b). At higher deposition temperature of 450 °C, predominantly rod-shaped PbS
particles were obtained with an average size of 83 nm (Fig. 5c). A slight decrease in the
particle grain size with an increase of temperature was observed. When complex (2)
was used as precursor, a remarkable difference in grain sizes and uniformity was
observed as the temperature was varied from 350 to 450 °C. At 350 °C, spherical
particles aggregates were formed with an average size of 71 nm (Fig. 5d). As the
12

temperature was increased to 400 °C, the shape of crystallites changed from spherical to
cubic particles with an average size of 90 nm (Fig. 5e) and the deposition at 450 °C
resulted in the growth of large cube-shaped PbS particles with a average size of 105 nm
(Fig. 5f). An increase in the particle size with an increase of temperature was observed.
EDX analysis results showed that the films are sulfur deficient (ESI Table S1). This could
be due to the large atomic size of lead as compared to sulfur.
(a)
(b)
(c)
100 nm
200 nm
200 nm
100
80
60
40
20
0
100
80
60
40
20
0
% Lit [x]
% Lit [x]
(1) 350 ⁰C
(1) 400 ⁰C
(1) 450 C⁰
(2) 350
(2) 400
(2) 450
(d)
(e)
(f)
200 nm
200 nm
1 µm
Fig. 5. SEM images of PbS films deposited from complex (1) onto glass substrates at (a)
350, (b) 400 and 450 °C and from complex (2) at (d) 350, (e) 400 and (f) 450 °C. The
orientations of the powder patterns for both depositions are shown in the middle
compared to the literature pattern of PbS. There is a small tendency for films to extend
along one axis as seen in SEM and by XRD.
3.4. Optical properties of PbS thin films
The room temperature UV–Vis absorption spectra of the PbS samples recorded in the
wavelength range of 250 –2000 nm, uncoated glass substrate as a reference are shown
in the ESI Figs. S5 and S6. The energy band gap (Eg) was determined from the Tauc plot
13

equation [55] using their transmission and reflectance curves as well as the following
formula:
is the energy of the incident light, the band gap of the sample and is the
absorption coefficient.
A strong blue shift as compared to the bulk PbS was observed in the absorption band
edges of all the PbS films deposited at the different temperatures. The observation is in
line with the literature where the reported PbS band gap varying in a wide range up to
2.3 eV from the bulk value of 0.41 eV [56-59]. This work reports PbS thin films with
band gap in the range of 1.46 – 1.55 eV. These band gap values are larger than expected
for particles with sizes in the range of 70 – 105 nm, greater than PbS Bohr exciton radii
(18 nm) [2, 59]. The results further reveal that the band gap of the as-deposited PbS
films gradually decreases (from 1.49 eV to 1.46 eV – complex (1) and 1.55 eV to 1.46 eV
– complex (2)) as temperatures were increased from 350 °C to 450 °C, respectively [57].
4. Conclusion
N-morpholine (1) and N-pyrrolidine (2) lead(II) N’-benzoylthioureato complexes have
been synthesized, characterized and the single crystal X-ray structure of N-morpholine-
N’-benzoylthioureatoPb(II) complex (1) elucidated. The lead(II) complexes were
succefully used to deposit PbS thin films by AACVD at various temperatures. The films
showed blue shift in their absorption band edge as compared to bulk PbS and the
particle size was tunable with the deposition temperature. Analyses confirmed the
formation of crystalline PbS in the galena form (halite structure) of excellent
crystallinity compatible with PbS. There may be some sulfur deficiency in the PbS films
may make the PbS thin films and study their potential application as gas sensors.
Acknowledgements
The authors thank the Royal Society/Department For International Development (RS-
DFID) Africa Capacity Building Initiative and the National Research Foundation (NRF)
South Africa through the South African Research Chair Initiative (SARChi) for their
financial support.
Appendix A. Supplementary data
CCDC 1545392 contains the supplementary crystallographic data for (1). These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
14

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.
References
[1] M. A. Malik, M. Afzaal, P. O’Brien, Chem. rev. 110 (2010) 4417.
[2] I. Kang, F. W. Wise, J. Opt. Soc. Am. B 14 (1997) 1632.
[3] N. O. Boadi, M. A. Malik, P. O'Brien, J. A. Awudza, Dalton Trans. 41 (2012) 10497.
[4] J. M. Skelton, S. C. Parker, A. Togo, I. Tanaka, A. Walsh, Phys. Rev. B 89 (2014) 205203.
[5] X. Yao, S. Liu, Y. Chang, G. Li, L. Mi, X. Wang, Y. Jiang, ACS Appl. Mater. Interfaces 7
(2015) 23117.
[6] C. E. Pérez-García, R. Ramírez-Bon, Y. Vorobiev, MATEC Web of Conferences: EDP
Sciences (2015).
[7] J. Akhtar, M. Sher, M. Dilshad, W. Khalid, N. Revaprasadu, M. A. Malik, Mater. Sci.
Semicond. Process. 36 (2015) 20.
[8] W. Scanlon, J. Phys. Chem. Solids 8 (1959) 423.
[9] F. W. Wise, Acc. Chem. Res. 33 (2000) 773.
[10] J. Tang, E. H. Sargent, Adv. Mater. 23 (2011) 12.
[11] L-D. Zhao, S-H Lo, J. He, H. Li, K. Biswas, J. Androulakis, C-I. Wu, T. P. Hogan, D-Y.
Chung, V. P. Dravid and M. G. Kanatzidis, J. Am. Chem. Soc. 133 (2011) 20476.
[12] S. Johnsen, J. He, J. Androulakis, V. P. Dravid, I. Todorov, D. Y. Chung and M. G.
Kanatzidis, J. Am. Chem. Soc. 133 (2011) 3460.
[13] H. Karami, M. Ghasemi, S. Matini, Int. J. Electrochem. Sci. 8 (2013) 11661.
[14] T. Fu, Sens. Actuators, B: Chem. 140 (2009) 116.
[15] W. Wu, Y. He, Y. Wu, T. Wu, J. Alloys Compd. 509 (2011) 9356.
[16] Z. Tshemese, M. D. Khan, S. Mlowe, N. Revaprasadu, Mater. Sci. Eng. B 227 (2018)
116.
[17] L. D. Nyamen, V. R. Pullabhotla, A. A. Nejo, P. T. Ndifon, J. H. Warner, N. Revaprasadu,
Dalton Trans. 41 (2012) 8297.
[18] P. S. Nair, T. Radhakrishnan, N. Revaprasadu, G. Kolawole, A. Luyt, V. Djoković, Appl.
Phys. A 81 (2005) 835.
[19] S. C. Masikane, S. Mlowe, C. Gervas, N. Revaprasadu, A. S. Pawar, S. S. Garje, J. Mater.
Sci.: Mater. Electron. 29 (2018) 1479.
[20] K. P. Mubiayi, N. Revaprasadu, S. S. Garje, M. J. Moloto, J. Saudi Chem. Soc. 21 (2017)
593.
[21] G. B. Shombe, E. B. Mubofu, S. Mlowe, N. Revaprasadu, Mater. Lett. 185 (2016) 17.
[22] S. Salim, O. Hamid, Renew. Energ. 24 (2001) 575.
[23] T. Kanniainen, S. Lindroos, J. Ihanus, M. Leskelä, J. Mater. Chem. 6 (1996) 983.
[24] N. Fainer, M. Kosinova, Y. M. Rumyantsev, E. Salman, F. Kuznetsov, Thin Solid Films
280 (1996) 16.
[25] M. D. Khan, S. Hameed, N. Haider, A. Afzal, M. C. Sportelli, N. Cioffi, M. A. MaliK, J.
Akhtar, Mater. Sci. Semicond. Process. 46 (2016) 39.
[26] H. Arslan, N. Külcü, U. Flörke, Transition Met. Chem. 28 (2003) 816.
[27] M. Afzaal, K. Ellwood, N. L. Pickett, P. O'Brien, J. Raftery , J. Waters, J. Mater. Chem.
14 (2004) 1310.
15

[28] M. Khan, M. Malik, J. Akhtar, S. Mlowe, N. Revaprasadu, Thin Solid Films 638 (2017)
338.
[29] R. A. Hussain, A. Badshah, N. Haider, M. D. Khan, B. Lal, J. Chem Sci. 127 (2015) 499.
[30] R. A. Hussain, A. Badshah, F. Yasmin, M. D. Khan, M. N. Tahir, Aust. J. Chem. 68
(2015) 298.
[31] A. A. Memon, M. Dilshad, N. Revaprasadu, M. A. malik, J. Raftery, J. Akhtar, Turk. J.
Chem. 39 (2015), 169.
[32] D. Barreca, A. Gasparotto, C. Maragno, R. Seraglia, E. Tondello, A. Venzo, V. Krishnan
and H. Bertagnolli, Appl. Organomet. Chem. 19 (2005) 59
[33] S-M. Lee, Y-W. Jun, S-N. Cho, J. Cheon, J. Am. Chem. Soc. 124 (2002) 11244.
[34] T. Trindade, P. O’Brien, X-M. Zhang, M. Motevalli, J. Mater. Chem. 7 (1997) 1011.
[35] T. Mandal, G. Piburn, V. Stavila, I. Rusakova, T. Ould-Ely, A. C. Colson and K. H.
Whitmire, Chem. Mater. 23 (2011) 4158.
[36] N. Pradhan, B. Katz, S. Efrima, J. Phys. Chem. B 107 (2003) 13843.
[37] M. D. Khan, S. Hameed, N. Haider, A. Afzal, M. C. Sportelli, N. Cioffi, M. A. MaliK, J.
Akhtar, Mater. Sci. Semicond. Process. 46 (2016) 39
[38] M. Moloto, N. Revaprasadu, G. Kolawole, P. O'Brien, M. Malik, S. Afr. J. Sci. 101
(2005) 463.
[39] T. Duan, W. Lou, X. Wang, Q. Xue, Colloids Surf. A: Physicochem. Eng. Asp. 310
(2007) 86.
[40] Bruker APEX2, SAINT and SADABS, Bruker AXS Inc., Madison, Wisconsin, USA
(2012). [41] G. M. Sheldrick, Acta Crystallogr. C 71 (2015) 3.
[42] L. J. Farrugia, J. Appl. Crystallogr. 45 (2012) 849.
[43] I. B. Douglass, F. Dains, J. Am. Chem. Soc. 56 (1934) 719.
[44] L. Beyer, M. Pulst, H. Paul, E. Hoyer, J. Prakt. Chem. 317 (1975) 265.
[45] M. Mikami, I. Nakagawa, T. Shimanouchi, Spectrochim Acta Part A: Mol. Spectrosc.
23 (1967) 1037.
[46] M. Merdivan, F. Karipcin, N. Kulcu, R. Aygun, J. Therm. Anal. Calorim. 58 (1999) 551.
[47] O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, A. K. Howard, H. Puschmann, J. Appl.
Crystallogr. 42 (2009) 339.
[48] S. Sadovnikov, A. Gusev, J. Alloys Compd. 573 (2013) 65.
[49] D. Saikia, P. Phukan, Thin Solid Films 562 (2014) 239.
[50] N. Bahlawane, E. F. Rivera, K. Kohse-Höinghaus, A. Brechlin, U. Kleineberg, Appl.
Catal. B: Environ. 53 (2004) 245.
[51] M-L. Chan and M. Rossi, Inorg. Chem. 36 (1997) 3609.
[52] L. S. Ramsdell, U-M, (1925).
[53] R. G. Wyckoff, Cryst. Struct. Intersci. Publ. 1 (1963) 239.
[54] Noda, K. Masumoto, S. Ohba, Y. Saito, K. Toriumi, Y. Iwata and I. Shibuya, Acta
Crystallogr. Sect. C: Cryst. Struct. Commun. 43 (1987) 1443.
[55] A. Rakhshani, J. Phys.: Condens. Matter 12 (2000) 4391.
[56] J. N. Zemel, J. D. Jensen, R. B. Schoolar, Phys. Rev. 140 (1965) A330.
[57] M. Abbas, A. A-M. Shehab, N. Hassan, A. Al-Samuraee, Thin Solid Films 519 (2011)
4917.
16

[58]Thangaraju B, Kaliannan P. Spray pyrolytically deposited PbS thin films.
Semiconductor Sci. Tech. 15 (2000) 849.
[59] C. Rajashree, A. Balu, V. Nagarethinam, Int J. Chem. Tech. Res. 6 (2014) 347.
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Table of Content
We report the synthesis of lead N’-benzoylthioureato complexes of N-morpholine and N-
pyrrolidine and their use as single source precursors for the synthesis of lead sulfide thin
films.
AACVD
350 – 450 °C
18

Research Highlights
 PbS thin films have been synthesized using lead N’-benzoylthioureato complexes
 The X-ray single crystal structure of bis(N-morpholine-N’-benzoylthioureato)lead(II)
complex (1)
 The films showed blue shift in their absorption band edge as compared to bulk PbS
 The particle sizes were tunable with the deposition temperature.
19