Synthesis and characterization of ZnS nanopowder prepared
by microwave-assisted heating
Wei Huang1, Min-Hung Lee1, San Chan2, Yueh-Chien Lee2,*, Ming-Kwen Tsai2, Sheng-Yao Hu3,
Jyh-Wei Lee4
1
Institute of Electro-Optical Science and Technology, National Taiwan Normal University, Taipei,
Taiwan
2
Department of Electronic Engineering, Tungnan University, New Taipei City, Taiwan
3
Department of Electrical Engineering, Tungfang Design University, Kaohsiung, Taiwan
4
Department of Materials Engineering, Ming Chi University of Technology, New Taipei City, Taiwan
Abstract
In this study, we present the zinc sulfide (ZnS) nanopowder (NP) synthesized via
microwave-assisted heating of a mixed zinc sulfate (ZnSO4) and thioacetamide
(CH3CSNH2: TAA) precursors in deionized water (DI water). The variations in the
crystalline performance and morphology of ZnS synthesized with different molar ratio
of ZnSO4 to TAA are investigated by X-ray diffraction (XRD), Fourier transform
infrared spectroscopy (FTIR) spectra, and scanning electron microscopy (SEM)
images. With increasing the molar ratio of ZnSO4 to TAA, the XRD results indicate
the synthesized ZnS NP tending towards crystallization, while the FTIR spectra show
the increase in the amount of hydroxyl group and C=O vibration modes. SEM images
confirm that the higher concentration in TAA is responsible for increasing the
diameter of spherical particles consisted of chains agglomeration.
Keywords: Zinc Sulfide; Microwave-assisted synthesis; X-ray diffraction; Scanning
electron microscopy
*Corresponding author: Yueh-Chien Lee, Tel: +886-2-86625911#265
Fax: +886-2-26629592; E-mail address: jacklee@mail.tnu.edu.tw
1
1. Introduction
Recently, nanocrystalline II-VI semiconductor materials have been studied
extensively because they have the unique electrical and optical properties as compared
to those of the bulk material [1-3]. Among the II-VI compounds, ZnS has attached
considerable attention due to its applications in flat panel displays,
electroluminescence devices, photonic crystal devices, sensor, laser and photocatalysis
[1-3]. Up to now, several methods for synthesizing ZnS nanostructure have been
reported such as the sol-gel technique [4], solvothermal method [5], thermal
evaporation [6], and microwave irradiation [7-8], etc.
Among different synthesis methods, the microwave-assisted synthesis has
advantages over other approaches such as lost cost, rapid heating, thermal uniformity
and energy efficiency [7-8]. In the microwave-assisted synthesis, the precursor
solution is irradiated with a microwave source and the efficient energy transfer
through either resonance or relaxation can result in a rather rapid heating process.
Furthermore, microwave heating process can result in homogeneous heating of the
precursor solution in a rather short duration to achieve a uniform distribution of
particle size. Consequently, the microwave hydrothermal process is kinetically more
efficient than the conventional hydrothermal process for preparing various
nanostructures [7-8].
In this work, we synthesized ZnS NP by the controlled microwave-assisted
heating of ZnSO4 and TAA solution with DI water as the solvent. The structural
properties and morphology of the ZnS synthesized with different molar ratio of ZnSO4
to TAA are characterized by XRD, FTIR, and SEM measurements, respectively.
2. Experimental
The ZnS NP were prepared by microwave-assisted heating process. The
precursor solution was prepared by stirring a mixture with various molar ratio (1:1,
1:2, 1:3, 1:4, and 1:5) of ZnSO4 to CH3CSNH2 in DI water at room temperature for 30
minutes to obtain a well-dissolved solution. The solution was then placed into a
microwave oven and heated from room temperature up to 95 °C with magnetic
stirring for one hour. The reactions occurring during microwave irradiation, leading to
form ZnS nanoparticles, can be described as [9]:
CH 3CSNH 2 +H 2O  CH 3CONH 2 +H 2S
(1)
S2- +Zn 2+  ZnS
(2)
Equation (1) describes the irreversible reaction releasing H2S in a homogeneous
way in the solution. Then Sulfide (S2−) anions coming from H2S react with Zn2+
cations to yield ZnS [9]. Afterwards, the semi-clear solution was cooled down to the
2
room temperature and the ZnS NP were rinsed with an ethanol solution and dried at
95 °C in the microwave oven for 5 minutes.
The synthesized ZnS NP were characterized by XRD, FTIR, and SEM
measurements. The XRD spectrometer (Shimadzu XRD-6000) with a CuKα line of
1.5405 Å was used to study the crystal phases in the synthesized samples. The
variations in the vibration modes of the ZnS NP synthesized as function of molar ratio
were measured by FTIR spectroscopy. The SEM images were taken on a JEOLJSM7001F to observe the morphology.
3. Results and discussion
Figure 1 shows the XRD patterns of the synthesized-ZnS samples. All samples
can exhibit three significant diffraction peaks, which are corresponded to the lattice
planes of (111), (220), and (311) by the JCPDS card No. 05-0566. The results indicate
all synthesized samples having the cubic zinc blended structure [7,10]. Additionally,
three weak peaks at about 33.1, 69.5, and 76.8, assigned to the lattice planes of (200),
(400), and (311), can be observed in the samples synthesized with higher molar ratio.
Furthermore, it is noted that the full width at half maximum (FWHM) of the
pronounced peaks are narrowed with increasing the molar ratio of precursor, which
implies the increases in the grain size. The average particle size in the calcined ZnS
NP can be estimated by the Sherrer’s relation [8]:
D
0.9
B cos 
(3)
where D,  ,  , and B are respectively, the crystal size, X-ray wavelength, Bragg
diffraction angle, and FWHM in radians. The average particle size of the ZnS NP
synthesized with controlled molar ratio of 1:1, 1:2, 1:3, 1:4, and 1:5 are estimated to
be about 5.79, 6.02, 6.62, 7.94 and 8.34 nm, respectively.
3
Figure 1 XRD patterns of ZnS NP synthesized with different molar ratio of precursor:
(a) 1:1, (b) 1:2, (c) 1:3, (d) 1:4, and (e) 1:5.
Figure 2 displays the FTIR results, which are measured to investigate the
variations of vibration modes in the ZnS NP synthesized with different molar ratio. As
shown in Fig. 2(a), the FTIR spectrum of the ZnS NP synthesized with 1:1 molar ratio
exhibits several typical vibration peaks [10-11]. It can be observed that the
characteristic Zn-S vibration peaks are at 620 and 1120cm-1 and the intensity of that
are enhanced as a function of molar ratio of precursor. The bands at 1500−1650 cm-1
are attributed to the C=O stretching modes arising from the absorption of atmospheric
CO2 on the surface of nanoparticles. A broad absorption peak in the range of
3000−3600 cm-1 is assigned to the OH stretching mode of hydroxyl group, which
indicates the existence of water absorbed in the surface of nanocrystals. However,
with increasing the molar ratio of precursor, the amount of the OH and C=O
stretching modes would be much more than that of the Zn-S vibration, resulting in a
much broader absorption range in the FTIR result. The phenomenon could be due to
the increased amount of the hydroxyl group and C=O produced form the TAA
decomposition.
4
Figure 2 FTIR spectra of ZnS NP synthesized with different molar ratio of precursor:
(a) 1:1, (b) 1:2, (c) 1:3, (d) 1:4, and (e) 1:5.
The morphology of the ZnS NP synthesized with different molar ratio of
precursor is revealed by SEM images as presented in Fig. 3 and the showed
magnification of the SEM images is fixed to be 20,000 to compare. As shown in Fig.
3(a)−(e), it can be observed clearly a trend in the increase in average size of the ZnS
NP with increasing the molar ratio, which is consistent with the estimation from XRD
results. However, the estimated particle size in SEM images is much larger than the
crystal size calculated by the Scherrer’s equation. It has been known that the
discrepancy may be understood by noting that the SEM images give the size of the
ZnS NP which may be as a result of the agglomeration of many nano-particles.
Therefore, the size of the ZnS NP as shown by the SEM image is larger than the
average particle size calculated from the XRD spectra. It is also observed that
needle-like structures would be covered on the surface of spherical particle and can be
presented clearly with increasing the molar ratio of precursor. M. K. Mekki Berrada et
al. have attributed the needle-like structures to the agglomerated chains of about
twenty crystallites [9]. These chains then agglomerate and form spherical porous
particles of 1−5 μm and the diameter of spherical particle would be increased with
increasing the concentration of TAA, which is in agreement with the presented SEM
images.
5
Figure 3 SEM images of ZnS NP synthesized with different molar ratio of precursor:
(a) 1:1, (b) 1:2, (c) 1:3, (d) 1:4, and (e) 1:5.
4. Conclusion
In summary, we have successfully synthesized ZnS NP by microwave-assisted
process using ZnSO4 and TAA solution with DI water as the solvent. The XRD result
presents an increase in the crystalline performance and grain size as a function of
molar ratio of ZnSO4 to TAA. However, the absorption peaks of hydroxyl group and
C=O vibration modes would broaden FTIR spectrum with increasing the molar ratio
of precursor, which is related to the higher concentration in TTA decomposition. SEM
images shows that the diameter of ZnS spherical particles consisted of chains
agglomeration becomes larger with increasing the molar ratio of precursor, which is
consistent with the results by XRD.
Acknowledgement
The author Y.C. Lee would like to acknowledge the support of the National Science
Council Project No. NSC 99-2112-M-236-001-MY3
6
References:
[1] M. Lin, T. Sudhiranjan, C. Boothroyd, K. P. Loh, Influence of Au catalyst on the
growth of ZnS nanowires, Chemical Physics Letters, Vol. 400, 2004, pp.
175-178.
[2] H. Zhang, L. Oi, Low-temperature, template-free synthesis of wurtzite ZnS
nanostructures with hierarchical architectures, Nanotechnology, Vol. 17, 2006, pp.
3984-3988.
[3] X. Fang, L. Wu, L. Hu, ZnS nanostructure arrays: a developing material star,
Advanced Materials, Vol. 23, 2011, pp. 585-598.
[4] N. I. Kovtyukhova, E. V. Buzaneva, C. C. Waraksa, and T. E. Mallouk, Ultrathin
Nanoparticle ZnS and ZnS:Mn Films: Surface Sol-Gel Synthesis, Morphology,
Photophysical Properties, Materials Science and Engineering: B, Vol. 69-70,
2000, pp. 411-417.
[5] Q. Zhao, L. Hou, R. Huang, Synthesis of ZnS nanorods by a surfactant-assisted
soft chemistry method, Inorganic Chemistry Communications, Vol. 6, 2011, pp.
971-973.
[6] Y. Wang, L. Zhang, C. Liang, G. Wang, X. Peng, Catalytic growth and
photoluminescence properties of semiconductor single-crystal ZnS nanowires,
Chemical Physics Letters, Vol. 357, 2002, pp. 314-318.
[7] Y. Zhao, J. M. Hong, J. J. Zhu, Microwave-assisted self-assembled ZnS
nanoballs, Journal of Crystal Growth, Vol. 270, 2004, pp. 438-445.
[8] Y. C. Lee, C. S. Yang, H. J. Huang, S. Y. Hu, J. W. Lee, C. F. Cheng, C. C.
Huang, M. K. Tsai, H. C. Kuang, Structural and optical properties of ZnO
nanopowder prepared by microwave-assisted synthesis, Journal of Luminescence,
Vol. 130, 2010, pp. 1756-1759.
[9] M. K. Mekki Berrada, F. Gruy, M. Cournil, Synthesis of zinc sulfide multi-scale
agglomerates by homogeneous precipitation–parametric study and mechanism,
Journal of Crystal Growth, Vol. 311, 2009, pp. 2459-2465.
[10] M. Kuppayee, G.K. Vanathi Nachiyar, V. Ramasamy, Synthesis and
characterization of Cu2+ doped ZnS nanoparticles using TOPO and SHMP as
capping agents, Applied Surface Science, Vol. 257, 2011, pp. 6779-6786.
[11] S. Ummartyotin, N. Bunnak, J. Juntaro, M. Sain, H. Manuspiya, Synthesis and
luminescence properties of ZnS and metal (Mn, Cu)-doped-ZnS ceramic powder,
Solid State Sciences, Vol. 14, 2012, pp. 299-304.
7