Synthesis and characterization of nanocrystalline zinc sulfide via zinc thiobenzoate-lutidine single-source precursor

Article abstract of DOI:10.1016/j.ica.2011.02.093

ZnS nanocrystals were prepared both in the form of mesoporous powder and thin films by one step thermal decomposition technique from a single-source procure (SSP) [Zn(SOCPh)₂Lut₂·H₂O]. The final product was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), N₂ adsorption-desorption isotherm, UV-Vis absorption spectroscopy and photoluminescence (PL) study. Structural analyses of the prepared ZnS revealed the formation of cubic crystallites with diameters around 5 and 10 nm for the thin films and powder materials, respectively. On the other hand, the powder form showed mesoporous nature (type IV isotherm) with an average pore diameter of 37.9 and BET specific surface area of 51.73 m²/g.

Full text of DOI:10.1016/j.ica.2011.02.093

Inorganica Chimica Acta 371 (2011) 20–26
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and characterization of nanocrystalline zinc sulfide via zinc
thiobenzoate-lutidine single-source precursor
a,
b,
Swarup Kumar Maji a, Nillohit Mukherjee a, Anup Mondal a, , Bibhutosh Adhikary , Basudeb Karmakar
⇑
⇑
⇑
,
Supriya Dutta c
a Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, West Bengal, India
b Glass Division, Central Glass and Ceramic Research Institute, Kolkata 700 032, West Bengal, India
c Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
ZnS nanocrystals were prepared both in the form of mesoporous powder and thin films by one step ther-
mal decomposition technique from a single-source procure (SSP) [Zn(SOCPh)2Lut2ꢀH2O]. The final product
was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), N2
adsorption–desorption isotherm, UV–Vis absorption spectroscopy and photoluminescence (PL) study.
Structural analyses of the prepared ZnS revealed the formation of cubic crystallites with diameters
around 5 and 10 nm for the thin films and powder materials, respectively. On the other hand, the powder
form showed mesoporous nature (type IV isotherm) with an average pore diameter of 37.9 Å and BET
specific surface area of 51.73 m2/g.
Received 3 November 2009
Received in revised form 12 January 2011
Accepted 27 February 2011
Available online 4 March 2011
Keywords:
Crystal growth
Chemical synthesis
Nanomaterials
Ó 2011 Elsevier B.V. All rights reserved.
Mesoporous structures
Optical properties
1. Introduction
tal contaminant elimination, CO2 reduction, and H2 evolution [15–
19]. Many synthetic methods have been employed to prepare ZnS
During the past decade, ‘‘small-particle’’ research has become
quite popular in various fields of chemistry and physics. The
‘‘small-particles’’, now being called nanostructured materials, are
interesting both for scientific reasons and practical applications
[1,2]. Nanostructured semiconductor materials of various shapes
and sizes (triangles, rods, cubes, arrows and tetrapods) [3,4] have
been successfully synthesized and extensively studied due to their
unique properties that are different from those observed in their
spherical counterparts [5–7]. Moreover, nanoparticles having mes-
oporous nanostructures have higher surface to volume ratio, high-
er chemical activity and better stability compared with the same
size nanoparticles.
ZnS nanoparticles, an important II–VI compound semiconductor
with a large band gap, have been extensively investigated as it has
numerous applications to its credit. It is used as a key material for
light-emitting diodes, cathode-ray tubes, thin film electrolumines-
cence, reflector, dielectric filter, chemical/biological sensors, pho-
tocatalyst and window layers in photovoltaic cells [8–14]. In
recent years, ZnS nanocrystals (NCs) have been extensively studied
and are reported to have potential as photocatalyst in environmen-
nanocrystals like sol–gel process [20], sonochemical preparation
[21], microwave heating [22], hydrothermal or solvothermal route
[23,24], template method [25], reverse micelle [26], chemical va-
por deposition [27,28], chemical bath deposition [29,30], thermol-
ysis of single-source precursor (SSP) [31–34] etc.
The use of SSP can potentially provide several advantages over
the other routes. For example, the existence of preformed bonds
can lead to a material with fewer defects and/or better stoichiom-
etry. It is possible to carry out the deposition with relatively simple
installations. Another advantage is the air stability of the SSP,
which renders easier handling and characterization. For this rea-
son, there is an increasing interest for the synthesis of ZnS nano-
particles through the SSP route. Mu et al. deposited cubic ZnS
nanocrystals through a self-assembled thin film precursor route
at relatively low temperature [32]. It has been found by Lan et al.
[33] that when the precursor was heated in NaCl flux, cubic ZnS
nanocrystals were transformed to hexagonal morphology.
Hampden-Smith and co-workers and Vittal et al. prepared
different types of ZnS nanocrystals from SSP using thiocarboxylate
complexes of zinc [34–37]. Researchers also used dimethylthio-
phosphinates [38], [Zn(S2PiBu2)2]2 [39] and dialkyldithiocarbamato
metal complexes with general formula [Zn(S2CNR2)2] (symmetri-
i
cal) or [Zn(S2CNR0R2)2] (unsymmetrical) (R = Me, Et or Pr; R0 =
⇑
Corresponding authors. Tel.: +91 3326684561; fax: +91 3326682916.
E-mail address: bibhutoshadhikary@yahoo.in (B. Adhikary).
alkyl) [40–42] for the preparation of ZnS NCs. As a continuation
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.02.093

S.K. Maji et al. / Inorganica Chimica Acta 371 (2011) 20–26
21
of our studies in related system [43], this work describes the prep-
2.5. Physical measurements
aration and characterization of ZnS nanoparticles both in the form
of thin film and mesoporous powder using Zn(SOCPh)2Lut2ꢀH2O as
the SSP.
Metal thiobenzoate complex we have prepared was typically
characterized by carbon, hydrogen, and nitrogen (CHN) elemental
analyses (Perkin-Elmer 2400II), Fourier transform infra red (FTIR)
spectroscopy (JASCO FTIR-460 Plus), UV–Vis transmission
spectroscopy (JASCO V-530), single-crystal X-ray diffraction for
structural analysis (Bruker-AXS SMART APEX II) and thermogravi-
metric analysis (TGA, Perkin-Elmer USA Diamond-200). The TGA
experiment was carried out by heating the sample from 25 to
600 °C at a rate of 10 °C/min under dinitrogen purge. The crystal-
line structure of the metal sulfide species, obtained after annealing
(in Argon atmosphere at 600 °C for 15 min), were characterized by
X-ray diffraction (XRD) technique (X’pert Pro MPD diffractometer,
PANalytical, Almelo, The Netherlands); operating at 40 kV and
2. Experimental
2.1. Commercial chemicals
All the materials [ZnCO3, Zn(OH)2ꢀ2H2O] (Qualigens), 3,5-
dimethylpyridine/lutidine (C7H9N, Lut) (Aldrich, 98+%), thiobenzo-
ic acid (C7H6OS, PhCOSH) (Aldrich, 90%) and toluene (Rankem,
99.0%) were purchased commercially and used as received.
30 mA using Ni-filtered Cu K
a X-radiation (k = 1.540598 Å) with
2.2. Synthesis of Zn(SOCPh)2Lut2ꢀH2O complex
X’celerator step size 2h = 0.05°, step time 0.5 s, from 10° to 80°.
The UV–Vis absorption and transmittance spectra of the ZnS thin
film were measured using a JASCO V-530 spectrophotometer and
the photoluminescence (PL) measurements were carried out using
a PerkinElmer LS-55 fluoremeter, with an excitation wavelength of
325 nm. The surface morphology of the deposited films and the
powder material was obtained by field emission scanning electron
microscopy (FESEM) (Gemini Zeiss Supra™ 35VP model; Carl Zeiss
Microimaging GmbH, Berlin, Germany) using an accelerating volt-
age of 4.9 kV. N2 adsorption–desorption isotherms were collected
on a Quantachrome Instruments adsorption analyzer at 77 K. The
degassing was done under vacuum and He was used to maintain
the inert atmosphere at 300 °C.
Zinc carbonate basic [ZnCO3, Zn(OH)2ꢀ2H2O] (1.0 g, 3.8 mmol),
3,5-lutidine (0.814 g, 7.6 mmol), and 20 ml of toluene were com-
bined in
a round-bottomed flask. Thiobenzoic acid (1.05 g,
7.6 mmol) was dropped into the mixture while stirring rapidly,
and the stirring was continued for 2 h at room temperature. As
the reaction proceeded, CO2 bubble formation was observed, and
the resulting pale yellow solution became canary yellow. The insol-
uble part was removed by filtration and the solvent was removed
under reduced pressure. The resulting light yellow material was
re-dissolved in toluene and filtered to remove any insoluble mate-
rials and then kept for slow evaporation. A crystalline solid was
collected by filtration. Yield: ꢁ1. 55 gm (70.5% yield based on zinc
carbonate).
2.6. Single crystal X-ray diffraction data
2.3. Experimental data
Crystallographic data for the Zn(SOCPh)2Lut2ꢀH2O complex is
summarized in Table 1. Crystals suitable for structure determina-
tions of the molecule were obtained from crystal tube by layering
hexane over the compound solution in toluene. The crystals were
mounted on glass fibers using perfluoropolyether oil. Intensity data
were collected on a Bruker-AXS SMART APEX II diffractometer at
Anal. Calc. for C28H30N2O3S2Zn: C, 58.79; H, 5.29; N, 4.90. Found:
C, 58.70; H, 5.20; N, 4.89%. FT-IR (cmꢂ1): 3452 (br), 3055 (w), 2921
(w), 1602 (s), 1569 (s), 1444 (m), 1382 (w), 1305 (w),1199 (s), 1170
(s), 1151 (m), 1041 (w), 919 (s), 867 (w), 769 (m), 692 (s), 651 (m),
541 (w), UV–Vis [Toluene, kmax/nm (e
/Mꢂ1 cmꢂ1)]: 288 (12,090),
100(2)
K using graphite monochromated Mo Ka radiation
300 (10,400). Thermogravimetric analysis: the sample of Zn(SOC-
(k = 0.710 73 Å). The data were processed with SAINT, and absorp-
tion corrections were made with SADABS [44]. The structures were
solved by direct and Fourier methods and refined by full-matrix
least-squares methods based on F2 using SHELX-97 [45]. For the
structure solutions and refinements, the SHELX-TL software package
[46] was used. The non-hydrogen atoms were refined anisotropi-
cally, while the hydrogen atoms were placed at geometrically cal-
culated positions with fixed thermal parameters.
Ph)2Lut2ꢀH2O complex decomposed between 100 and 550 °C with
16.8 wt.%
remaining,
(MW(ZnS)/MW(Zn(SOCPh)2Lut2ꢀH2O) ꢃ
100 = 17.03%. The inorganic residue in the TGA pan was identified
as sphalerite ZnS by powder X-ray diffraction. Crystal data: tri-
ꢀ
clinic, space group P1; a = 10.273(5) Å,
b
= 11.733(5) Å, c =
= 92.566(5)°; V =
12.130(5) Å;
1341.6(10) Å3;
[I > 2 (I)] for 329 parameters and 5574 data.
a
= 11.037(5)°, b = 98.584(5)°,
c
q
= 1.411 g/cm3; R1a = 0.0327; wR2b = 0.0915
r
3. Results and discussion
2.4. Preparation of ZnS nanoparticles from metal precursor
3.1. Synthesis
To prepare mesoporous ZnS, the metal precursor complex
Zn(SOCPh)2Lut2ꢀH2O (880 mg, 1.59 mmol) was taken in a molybde-
num boat and heated at 600 °C for 15 min in an argon atmosphere,
using a quartz tube furnace. After annealing, the precursor yielded
about 131 mg of the metal sulfide.
In order to prepare the thin films of ZnS, a saturated solution of
the metal precursor complex in toluene (99%) was made and about
five drops of the solution were taken over a properly cleaned trans-
parent conducting oxide (TCO) coated glass substrate of area
2.5 cm ꢃ 1 cm. This was then dried in air for about 15–20 min to
obtain a uniform coating of the precursor on the TCO substrate.
On annealing in argon atmosphere at 600 °C for 15 min, this coat-
ing gets converted into a thin film of ZnS with an average thickness
The complex Zn(SOCPh)2Lut2ꢀH2O was prepared by reacting
zinc carbonate basic with, 3,5-lutidine and thiobenzoic acid in a ra-
tio 1:2:2, where toluene was used as the solvent. The unreacted
Zn(OH)2 and any insoluble materials were removed by filtration.
The Zn(SOCPh)2Lut2ꢀH2O complex undergoes thiobenzilic anhy-
dride elimination to form ZnS nanoparticles. The reaction scheme
that may be involved in this process can be described as:
ZnCO3; ZnðOHÞ2 ꢀ 2H2O þ 3; 5-Lut þ PhCOSH ! ZnðSOCPhÞ2Lut2
!
! H2OZnðSOCPhÞ2Lut2 ꢀ H2O
D
ZnS
of about 0.5
Fig. 5b).
lm (as obtained from cross-sectional SEM, inset of
Similar precursor was also synthesized by us using 2,6-
dimethylpyridine instead of 3,5-dimethylpyridine, which give very

22
S.K. Maji et al. / Inorganica Chimica Acta 371 (2011) 20–26
Table 1
Summary of crystallographic data.
Empirical formula
C28H30N2O3S2Zn
Formula weight
T (K)
572.03
100(2)
ꢀ
Crystal system, space group
triclinic, P1
10.273(5)
11.733(5)
a (Å)
b (Å)
c (Å)
12.130(5)
a
b (°)
(°)
111.037(5)
98.584(5)
c
(°)
92.566(5)
1341.6(10)
2, 1.416
V (Å3)
Z value, Dcalc (Mg mꢂ3
l
)
(mmꢂ1
)
1.103
F (0 0 0)
Crystal size (mm3)
Number of data/restraints/parameters
Number of reflections [I > 2 (I)]
Goodness-of-fit (GOF) on F2
596
0.16 ꢃ 0.20 ꢃ 0.32
5970, 0, 329
5574
r
1.045
Final R indices [I > 2
R indices (all data)
r(I)]
R1a = 0.0327, wR2b = 0.0915
R1a = 0.0350, wR2b = 0.0939
Fig. 2. An ellipsoid diagram of the unit cell, in which two Zn(SOCPh)2Lut2ꢀH2O
molecules are bonded through hydrogen bond.
P
P
a
R1(F) = |Fo| ꢂ |Fc||/ |Fo|.
P
wR2(F2) = [w(F2o ꢂ F2c )2/ w(F2o)2]1/2
.
b
tal system with P-1 space group. In a unit cell two water molecules
are present as water of crystallization as shown in Fig. 2. The two
complex units [Zn(SOCPh)2Lut2] are attached through H-bonds be-
tween one of the carbonyl O atom of thiobenzoate group with two
water molecules. The O–H bond distances are different, i.e., 3.024
and 2.688 Å, respectively. Two nitrogen atoms from lutidine and
two sulfur atoms from thiobenzoate ligands are bonded to the cen-
tral metal ion to provide a distorted tetrahedron ZnN2S2 coordina-
tion environment around the central metal ion. The relevant bond
lengths (Å) and angles (°) are shown in Table 2. The geometry
around the metal is somewhat similar to the monomers, Zn(SOC-
CH3)2Lut2 and Zn(SOC(CH3)3)2Lut2 reported by Hampden-Smith
and co-workers [34]. Two Zn–N bond lengths are 2.070 and
2.042 Å, where as two Zn–S bond lengths are 2.329 and 2.297 Å,
respectively. The difference in the two Zn–S (0.032 Å) and two
Zn–N (0.028 Å) bond lengths are more than that obtained by
Hampden-Smith and co-workers, who have used thioacetic acid
(Zn–S, 0.003 Å, and Zn–N, 0.004 Å), and thiopivalic acid (Zn–S,
low yield (2–5% based on zinc carbonate) and hence the results are
not reported.
The infrared spectra for the crystalline Zn(SOCPh)2Lut2ꢀH2O
sample showed the presence of band at 1602 cmꢂ1 which was as-
signed to uncoordinated
(C@N) band was found at 1569 cmꢂ1 and the
served at 919 cmꢂ1, consistent with coordinated
and N elemental analyses agreed with the calculated values for
the proposed empirical formulae within the experimental error.
m
(C@O) in thiobenzoate carbonyls, the
(C–S) band was ob-
(C–S). All C, H
m
m
m
3.2. Solid-state structures of the precursor [Zn(SOCPh)2Lut2ꢀH2O]
The molecular structures of the Zn(SOCPh)2Lut2ꢀH2O molecule
determined by single-crystal X-ray diffraction technique are
shown in Figs. 1 and 2. The molecule crystallizes in a triclinic crys-
Fig. 1. An ORTEP diagram of Zn(SOCPh)2Lut2ꢀH2O complex with 50% probability thermal ellipsoid and the numbering scheme. The hydrogen atoms are omitted for clarity.

S.K. Maji et al. / Inorganica Chimica Acta 371 (2011) 20–26
23
Table 2
dine groups (calc. wt.% = 18.74). In the final step, weight loss is ob-
served in the temperature range between 330 and 550 °C, which
matched well with the elimination of two water molecules by
the formation of 2ZnS, with calculated and measured weight losses
of 3.15% and 3.4%, respectively. The possible chemical transforma-
tion schemes for the thermal decomposition of Zn(SOC-
Ph)2Lut2ꢀH2O are presented below:
Relevant bond lengths (Å) and angles (°) of Zn(SOCPh)2Lut2ꢀH2O complex.
Bond lengths
Bond angles
Zn–S(1)
Zn–S(2)
Zn–N(1)
Zn–N(2)
S(1)–C(21)
S(2)–C(28)
O(1)–C(21)
O(2)–C(28)
2.329(12)
2.297(12)
2.070(18)
2.042(2)
1.761(2)
1.7401(19)
1.220(2)
1.233(2)
S(1)–Zn–S(2)
S(1)–Zn–N(1)
S(2)–Zn–N(1)
S(1)–Zn–N(2)
S(2)–Zn–N(2)
N(1)–Zn–N(2)
113.74(2)
102.11(5)
110.67(5)
110.87(5)
112.19(5)
106.58(6)
2½ZnðSOCPhÞ Lut2 ꢀ H2Oꢄ ! 2½ZnðSOCPhÞ2Lut1:5 ꢀ H2Oꢄ þ Lut
ð1Þ
2½ZnðSOCPhÞ2Lut1:5 ꢀ H2Oꢄ ! 2½ZnSLut ꢀ H2Oꢄ þ 2½SðOCPhÞ ꢄ þ Lutð2Þ
2
2
2½ZnSLut ꢀ H2Oꢄ ! 2½ZnS ꢀ H2Oꢄ þ 2Lut
2½ZnS ꢀ H2Oꢄ ! 2ZnS þ 2H2O
ð3Þ
ð4Þ
0.019 Å, and Zn–N, 0.012 Å) as complexing agents. This is probably
due to the presence of two phenyl rings which influence the geo-
metrical distortion in Zn(SOCPh)2Lut2ꢀH2O compared to thioace-
tate and thiopivalate complexes. The steric hindrance of bulky
thiocarboxylates, thiobenzoate and thiopivalte is also reflected in
the bond angles of their complexes. The difference between maxi-
mum and minimum bond angles of thiobenzoate, thiopivalate and
thioacetate are 11.63°, 19.0° and 6.1°, respectively. The minimum
difference of bond angles (6.1°) of thioacetate complex indicates
more regular geometry due to presence of methyl group.
The formation of pure crystalline ZnS (as observed from the
XRD analyses) in the solid-state thermal decomposition products
of the Zn(SOCPh)2Lut2ꢀH2O, suggests that this is the logical precur-
sor to the metal sulfide.
3.4. X-ray diffraction analysis
Fig. 4a and b represents the X-ray diffraction (XRD) pattern of
the powder material and thin film, obtained after annealing the
precursor complex, respectively. The patterns indicate the forma-
tion of sphalerite or cubic ZnS (JCPDS ID: 05-0566) with three ma-
jor diffractions from the (1 1 1), (2 2 0) and (3 1 1) planes for both
the powder and thin film. Some small diffractions from (2 0 0),
(4 0 0) and (3 3 1) planes were also obtained, but the (4 0 0) peak
was absent for the thin film. The high intensity of the peaks indi-
cates high crystallinity of the formed material; however, the rela-
tive intensity of the diffraction peaks for thin film was lesser,
3.3. TGA–DTG studies of Zn(SOCPh)2Lut2ꢀH2O complex
The curves obtained from the thermogravimetric analysis (TGA)
and differential thermogravimetric (DTG) analysis for Zn(SOC-
Ph)2Lut2ꢀH2O complex are shown in Fig. 3. The compound was
found to decompose between 100 and 550 °C following several
steps, resulting in the formation of metal sulfide (calc.
wt.% = 17.03, Found wt.% = 16.8). This fact was also supported by
the powder X-ray diffraction pattern of the residue obtained from
the TGA pan, where the peaks correspond to only ZnS were ob-
tained and no peaks for the metal oxide (ZnO) were present. The
curve showed four pronounced weight loss steps, which was also
supported by the DTG maxima. The first weight loss (9.27%) be-
tween 70 and 160 °C corresponds to the loss of one lutidine group
(calc. wt.% = 9.37). The major weight loss in the second step is ob-
served in the temperature range of 160–250 °C, which matched
with the elimination of two S(OCPh)2 groups and one lutidine
group, having calculated and measured weight losses of 51.7%
and 52.43 %, respectively. The decomposition in the temperature
range between 250 and 330 °C in the third step has been found
to be 18.1%, which is very close to the loss of two remaining luti-
Fig. 4. X-ray diffraction pattern of the sphalerite ZnS (a) powder material and (b)
thin film (the‘ꢅ’marked peaks are for the SnO2 of the TCO substrate).
Fig. 3. TGA–DTG curve of the Zn(SOCPh)2Lut2ꢀH2O complex.

24
S.K. Maji et al. / Inorganica Chimica Acta 371 (2011) 20–26
since the thickness of the deposited film was very low (0.5
l
m). On
represents the cross sectional view of the thin film deposited on
TCO taking five drops of the saturated precursor solution. From this
cross sectional image, the thickness of the film was determined,
the other hand, the broadness of the diffraction peaks for both the
systems indicates the formation of particles with very small
dimension. Since, the produced ZnS particles were of regular
shape/size, it is possible to calculate the crystallite size using the
Debye–Scherrer equation (D = 0.9k/(b cos h)), where, D is the crys-
which was found to be about 0.5 lm.
3.6. N2-sorption studies
tallite size (diameter),
k is the wave length of X-ray, i.e.,
1.540598 Å, b is the value of full width at half maximum of the
most intense peak (after correcting the instrumental broadening),
which is expressed in radians and h is the Bragg’s angle. The calcu-
lated value of crystallite size was found to be around 5 and 10 nm
for the thin film and powder systems, respectively.
Typical nitrogen adsorption–desorption isotherms at 77 K for
the powder ZnS material and the corresponding pore size distribu-
tion are shown in Fig. 6. The resulting isotherm could be identified
as a Type IV isotherm, characteristic of the mesoporous materials
(Fig 6a). The small hysteresis loop in the isotherm was generated
due to the capillary condensation of the absorbate in the mesop-
ores of the solid. This loop also indicates that the pores are acces-
sible by the gas molecules without much hindrance. Similar
isotherm for mesoporous ZnS nanoparticles was also observed by
Rana et al. [47]. As calculated using the Barrett–Joyner–Halenda
(BJH) method from the adsorption isotherm, the average pore
diameter was found to be around 37.9 Å (Fig. 6b), whereas, the spe-
cific surface area for the sample was found to be 51.73 m2/g from
the Brunauer–Emmett–Teller (BET) analysis.
3.5. FESEM image analyses
Fig. 5a and b shows the surface morphology obtained by FESEM
for the powder and the thin film of ZnS, respectively. The powder
material shows porous structure formed by the network of the
interlinked or agglomerated ZnS crystals. Whereas, highly compact
surface morphology composed of uniformly distributed spherical
grains, is evident from the FESEM image of the ZnS thin film. The
compactness of the film surface indicates that they could be used
successfully in various optical and electrical devices. The average
grain sizes, as obtained from the FESEM images were 10 and
20 nm for the thin film and the powder, respectively. It can be re-
called that the crystallite sizes as derived from the XRD data anal-
ysis were 5 and 10 nm, respectively for the thin film and powder. It
is evident that, for both the systems, the cubic crystallites coa-
lesced together to form spherical grains with comparatively larger
sizes, in order to lower the Gibb’s free energy. The inset of Fig. 5b
3.7. Optical analyses
3.7.1. UV–Vis studies
Optical study (i.e., UV–Vis absorption spectroscopy and PL) is
one of the most important methods to reveal the energy structures
Fig. 5. FESEM images of (a) powder ZnS nanoparticles and (b) thin film (inset:
Fig. 6. (a) N2 adsorption–desorption isotherms for powder ZnS and (b) the
cross-sectional FESEM image).
corresponding pore size distribution curve.

S.K. Maji et al. / Inorganica Chimica Acta 371 (2011) 20–26
25
since their formation energies are normally unequal [49]. It is also
established previously that, Schottky defects are dominant in cubic
ZnS system [50]. Therefore, deep traps in cubic ZnS mainly in-
volved internal Zn2+ and S2ꢂ vacancies. From the PL spectrum, a
strong and broad emission peak centered at 358 nm along with
two weak emissions at ꢁ450 and ꢁ493 nm were observed. The
peak centered at 358 nm is assigned to the band-to-band transition
of ZnS and the emission at 450 nm may be attributed to the pres-
ence of sulfur vacancies in the lattice, which was also reported ear-
lier by Manam et al. [48], Zhang et al. [51] and other researchers
[52,53]. The very weak green emission at 493 nm might be due
to the presence of trapped surface states in the ZnS lattice [54,55].
4. Conclusion
A
new approach has been introduced to synthesize ZnS
nanoparticles both in the form of powder and thin films, from a
single-source precursor complex [Zn(SOCPh)2Lut2ꢀH2O]. Both pow-
der material and thin film were found to have cubic crystallites (5
and 10 nm crystal diameter, respectively) which coalesced to form
spherical grains (10 and 20 nm grain diameter, respectively). The
homogeneity in the surface morphology for the deposited films
was also evident from its FESEM images. For the powdered mate-
rial, BET analysis showed that the specific surface area was
51.73 m2/g, whereas, from the pore size distribution curve, meso-
porous nature of the material was established. The percentage
transmittance (80–90%) of ZnS thin film was high, indicating their
probable potential applications as solar cell window material.
2
Fig. 7. UV-Vis absorption (plot a), transmittance (plot b) specrua and in inset (
vs. h plot for ZnS thin film.
ahm)
m
of semiconducting materials. Fig. 7 is the UV–Vis absorption and
2
transmittance spectra and in inset the Tauc plot [(
ahm) versus
hm
] for the ZnS thin film is given. The UV–Vis absorption spectrum
(Fig. 7a) showed an onset of absorption at ꢁ350 nm and the trans-
mittance spectrum (Fig. 7b) showed 80–90% transmittance in the
wavelength range of 370–550 nm. The band gap energy was calcu-
lated using the Tauc’s relation [(
ahm
)
1/n = A(h
m
ꢂ Eg)] [48], where,
hm
is the incident photon energy, ‘A’ is a constant and ‘n’ is the
exponent the value of which is determined by the type of elec-
tronic transition causing the absorption and can take the values
1/2 or 2 depending upon whether the transition is direct or indi-
rect, respectively. Since, ZnS is well established as a direct band
gap semiconductor [48], we can evaluate the value of Eg, from
Acknowledgements
One of the authors (S.K.M.) is indebted to UGC for his Project
Fellowship (F. No. 33-31/2007 (SR)) and another author (N.M.) is
indebted to CSIR, India, for his Research Associateship (Award
No. 08/003(68)/2010-EMR-I). Thanks are also to the Department
of Science and Technology, Government of India for establishing
the National X-ray Diffractrometer facility at the Department of
Inorganic Chemistry, Indian Association for the Cultivation of Sci-
ence. We also thank Dr. P. Biswas of the Department of Chemistry,
Bengal Engineering and Science University, Shibpur for his help to
sort out a crystallographic problem.
the intercept of the straight line at
sus h . From the Tauc plot (inset of Fig. 7) the value of Eg for the
a = 0 from the plot of (ahm
)2 ver-
m
deposited ZnS films was found to be about 3.58 eV. Bandgap en-
ergy in the similar range was also observed by Rana et al. [47]
for their mesoporous ZnS.
3.7.2. Photoluminescence (PL) studies
ZnS is well known for its photoluminescence properties. The
room temperature PL spectrum of the ZnS thin film on TCO sub-
strate was measured with an excitation wavelength (kexc) of
325 nm, which is shown in Fig. 8. In general, both Schottky and
Frenkel defects exist in all solids, but one type should be dominant
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.02.093.
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