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Synthesis and characterization of ZnS nanoclusters via hydrothermal processing from [bis(salicylidene)zinc(II)]
Journal of Alloys and Compounds 470 (2009) 502–506
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of ZnS nanoclusters via hydrothermal processing from [bis(salicylidene)zinc(II)] Masoud Salavati-Niasari a,b,∗ , Fatemeh Davar b , Mehdi Mazaheri c a b c
Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317–51167, I. R. Iran Department of Chemistry, Faculty of Science, University of Kashan, Kashan, P.O. Box 87317–51167, I. R. Iran Materials and Energy Research Center, P.O. Box 14155–4777, Tehran, I. R. Iran
a r t i c l e
i n f o
Article history: Received 2 February 2008 Received in revised form 27 February 2008 Accepted 1 March 2008 Available online 22 April 2008 Keywords: Chemical synthesis ZnS Nanostructured materials Inorganic materials
a b s t r a c t A thioglycolic acid (TGA)-assisted hydrothermal process has been developed to synthesize zinc sulfide (ZnS) nanoclusters via the reaction between a new precursor, bis(salicylaldiminato)zinc(II); [Zn(sal)2 ]; and thioacetamide (CH3 CSNH2 ). X-ray diffraction (XRD), transmission electron microscopy (TEM), Xray photoelectron spectroscopy (XPS), UV–vis spectroscopy and Fourier transform infrared (FT-IR) were employed to characterize the obtained product. Furthermore, the possible mechanism and the critical factors for the TGA-assisted hydrothermal synthesis of the ZnS nanoclusters have been preliminarily presented. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured materials have attracted a great deal of attention in the last few years due to their unique properties that are different from the bulk materials [1–3]. Zinc sulfide (ZnS) as an important wide-bandgap (3.6 eV) semiconductor has been used as a key material for the ultraviolet lightemitting diodes and injection lasers [4], flat panel displays [5], electroluminescent devices and infrared windows [6,7]. In recent years, some characteristics of ZnS nanocrystals different from bulk crystal [8] have enlarged the range of application. Thus, the study of ZnS nanostructure is of considerable importance and great efforts have been focused on the synthesis and physical properties [9–12]. ZnS is observed in nature in two polymorphs, zinc blende and wurzite, with cubic and hexagonal lattice structures, respectively, with atoms in the bulk of both polymorphs being four-coordinated, having tetrahedral coordination. Although the zinc blende phase is more stable than wurzite, it is common to find both in the same sample, and it is not clear what factors control which phase predominates. Many efforts have been made in the fabrication of onedimensional (tubes [13], wires [14], rods [15], belts and ribbons [16]), two-dimensional (sheets [17], platelets [18], diskettes [19]) and three-dimensional (spheres [20]) ZnS nanostructures by liquid crystal templating, irradiation, solvothermal, physical evaporation,
∗ Corresponding author. Tel.: +9836215555333; fax: +983615552930. E-mail address: [email protected] (M. Salavati-Niasari). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.048
and surfactant-mediated microemulsion approaches. The existing methods for size and shape control of ZnS nanostructure generally use capping agents, such as surfactants, ligands, polymers, or dendrimers, to confine the growth in the nanometer regime. Among various methods, the hydrothermal method provides a more promising way for the synthesis of crystals due to its low cost, high efficiency and potential for large scale production. Recently, Yang’s groups have developed a mild hydrothermal route to synthesize metal sulfides with the use of thioglycolic acids as nontoxic template [21]. Yan and Xue successfully synthesized ZnO/ZnS nanocable and ZnS nanotube arrays via thioglycolic acid and Na2 S [22]. And so, Qian’s groups have synthesized new ZnS/organic composite nanoribbons by the use of thioglycolic acid [23]. The nanoclusters of metals, metal oxides and semiconductors are interesting because of their unique mechanical, electronic, optical, magnetic, and chemical properties [24–27]. This is largely due to the phenomenon of quantum confinement, whereby in sufficiently small particles, the optical and electronic properties depend strongly on the particle size. Quantum confinement effects may be very large, with one of the results being that the nanocluster band gap may be more than an electronvolt larger than in the bulk semiconductor. In these cases, the spatial extent of photogenerated excitons is comparable to the size of the nanocluster. Semiconductor nanoclusters are thus often referred to as “quantum dots”. The main issue in the preparation of semiconductor nanoclusters is a careful control of semiconductor size and, even more important, of their size distribution. Moreover, their subsequent
M. Salavati-Niasari et al. / Journal of Alloys and Compounds 470 (2009) 502–506
manipulation requires an efficient procedure of recovery and stabilization. For example, such nanoclusters have been prepared in colloids, polymer matrixes, self-assembled monolayers, and zeolites. Other methods have been used to prepare the nanoclusters as microemulsion or thermal decomposed methods [28,29]. Submicrometer-sized ZnS spheres have been synthesized by using silica [30] and polystyrene [31] spheres as sacrificial templates, respectively. Furthermore, Qi and co-workers [20] synthesized ZnS nanospheres in aqueous solutions of an amphiphilic triblock copolymer, in which not micelle but the copolymer aggregates were the template. Zhang’s group [13] fabricated ZnS spheres by templating with in situ generated bubbles under hydrothermal condition. Chu and co-workers [32] presented ZnS nanostructures by surfactant polyethylene glycol (PEG)-assisted method. Pietro et al. have been investigating the chemical activation and reactivity of the surface of cadmium sulfide nanoclusters prepared by the well known kinetic trapping method. This method provides nanoclusters of surprisingly good monodispersity, which are conveniently enveloped in a shell of organic aromatic “capping” molecules [33]. Our strategy has been to use a thioglycolic acid as capping agent for hydrothermal synthesize of ZnS nanoclusters. It still remains a great challenge to develop facile and environmentally benign methods for creating ZnS clusters nanospheres from a simple method. In this paper, we report a novel thioglycolic acid (TGA)-assisted hydrothermal method to fabricate well-crystallized ZnS nanoclusters. The trick in hydrothermal synthesis of ZnS nanoclusters presented here is the application of thioglycolic acid, which was previously used as the stability agent to prevent the chalcogenide nanocrystals from aggregating [34]. TGA makes important roles in crystal growth and self-assembly of ZnS crystal to clusters during the hydrothermal process. 2. Experiment 2.1. Materials All the chemicals reagents used in our experiments such as thioacetamide (TAA), thioglycolic acid (TGA) were of analytical grade and were used as received without further purification. The precursor complexes [bis(salicylaldiminato)zinc(II)]; [Zn(sal)2 ]; were prepared according to the procedure described previously [35]. The water used in this work was distilled and de-ionized. 2.2. Characterization XRD patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu K␣ radiation. Elemental analyses were obtained from Carlo ERBA Model EA 1108 analyzer. The compositional analysis was done by energy dispersive X-ray (EDX, Kevex, Delta ClassI). Transmission electron microscopy (TEM)
503
Fig. 1. XRD pattern of the as-prepared ZnS nanoclusters.
images were obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV. The surface of the zinc manganese compound was characterized by X-ray Photoelectron Spectroscopy (XPS) ESCA-3000 electron spectrometer with non-monochromatic Mg K␣ radiation was used to excite the photoelectrons. Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets. The electronic spectra of the complexes were taken on a Shimadzu UV–visible scanning spectrometer (Model 2101 PC). 2.3. Experimental Fabrication of ZnS nanoclusters was achieved by the reaction between a new precursor [Zn(sal)2 ], TGA and TAA. 10 ml of thioglycolic acid (TGA) were added into 0.6 g [Zn(sal)2 ] under stirring and heating. After 40 min 6 ml of 0.5 mol l−1 thioacetamide (TAA) were added to the solution under stirred vigorously. The final solution was put into a Teflon-lined stainless steel autoclave under stirring. Then, the autoclave was maintained under a static condition at 105 ◦ C for 7 h and then gradually cooled down to room temperature. The mixture turned white due to the formation of ZnS precipitates. The product was filtered out, washed with ethanol and deionized water for several times, and then dried at 80 ◦ C.
3. Results and discussion Fig. 1 is the X-ray powder diffraction pattern of the ZnS nanoclusters showing reflections from (1 1 1), (2 2 0) and (3 1 1) planes, which indicates formation of zinc blende (cubic, -ZnS) structure (JCPDS No. 05-0566). No diffraction peaks from other crystalline forms were detected, which indicated a high purity and well crystallinity of these ZnS nanoclusters. The size of the nanocrystals estimated from the full width at half maxima (FWHM) of the (1 1 1) diffraction peak on the basis of the Scherrer formula:
Fig. 2. EDX analysis of the as-synthesized ZnS.
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Fig. 3. TEM images of (a) ZnS nanoclusters (the inset shows the SAED pattern) and (b) broken part of nanoclusters.
D = 0.89/ˇ cos , is about 4 nm for the nanoparticles synthesized, which is much smaller than the diameters of the sphere. This indicates that the clusters were formed from assembly of nanocrystal masses. The EDX spectrum of the product further confirms the formation of ZnS. Fig. 2 depicts the EDX spectrum of the product. Only Zn and S elements were found. According to the calculation of the peak areas the molar ratio of the Zn/S is 51.4/48.9, which is very close to the stoichiometric ratio of ZnS. Also, no other impurity peak is found in XRD and EDX spectra, indicating that the product is pure. Typical TEM images and selected area electron diffraction (SAED) pattern of the product are shown in Fig. 3. Fig. 3a and b clearly reveals the sphere morphology of ZnS crystals with the diameter ranging from 50 to about 150 nm. Some loose spheres formed by aggregation of small particles can be observed in TEM images. We may consider that when the reaction was carried out at 105 ◦ C, most organic molecules were decomposed. Since only a few thiocarbamide or TGA molecules adsorbed on ZnS nanoparticles, nanoclusters, loose solid spheres, were finally produced. The selected area electron diffraction (SAED) pattern (Fig. 3a inset) shows three sharp rings from (1 1 1), (2 2 0) and (3 1 1) planes of cubic zinc blende ZnS, which is in well agreement with the XRD study. The magnified TEM image of nanoclusters (Fig. 3b) interestingly reveals that the nanospheres are not composed of densely packed, so the loosely packed clusters are the evidence of organic protective agent. These nanoclusters are formed of small nanoparticles of 4–5 nm sizes. The XPS was employed to investigate the composition and purity of as-synthesized ZnS nanocrystal. The XPS sample is shown in Fig. 4. The XPS data of the sample indicated the presence of Zn and S. No impurities were found on the surface of the sample, indicating that the as-synthesized ZnS nanoclusters are relatively pure. The binding energies of Zn 2p1/2 and Zn 2p3/2 for the product were observed at around 1045.0 and 1022.1 eV (Fig. 4a), respectively, while the peak located at around 162.5 eV (Fig. 4b) corresponded to the binding energy of S 2p3/2 . All of the observed binding energy values for Zn 2p and S 2p agreed with the literature data [36]. Fig. 5 is the UV–vis spectrum of the as-synthesized ZnS nanoclusters dispersed in ethanol. An absorption peak centered at
323 nm (3.83 eV), which is considerably blue-shifted relative to the absorption onset of bulk ZnS (ca. 340 nm) because of the quantum size effects, was found. According to Ref. [20], the diameter of particles was about 4.5 nm when the absorption peak was located
Fig. 4. XPS spectra of the as-synthesized ZnS.
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505
Scheme 1. Schematic diagram illustrating the formation of ZnS nanoclusters.
at 325 nm. This implied that the size of the as-synthesized ZnS nanoparticles should be below 4.5 nm, which is in good agreement with the result of XRD analysis. To investigate whether the surface of the nanoparticles was capped with organic surface the Fourier transformed infrared (FTIR) of the as-synthesized samples were performed. In typically Fig. 6 shows the FT-IR spectra of ZnS nanoclusters. The spectra of ZnS nanoclusters show two weak stretch vibrations between 1500 and 2500 cm−1 attributing to the C–H and C O stretching models of the thioglycolic acid, indicating TGA molecules are absorbed on the surface ZnS nanoparticles. There was no evidence of free precursor [Zn(sal)2 ], in the sample, because stretch vibration of C–H (C–H ) and the stretching vibrations of C O and C C benzene ring disappeared. So the thioglycolic acid serves as the capping ligand that controls growth of ZnS nanoclusters. As shown in EDX and XPS analysis, there are no ZnO products in the as-synthesized samples and only ZnS nanoclusters were obtained without any oxidation. The nanocrystal possess good transmittance at 400–4000 cm−1 , it indicating that nano-ZnS is a better infrared-transmittance material. Previously, thioglycolic acid (TGA) was widely used as the stability agent preventing the nanocrystals from aggregation [34]. In our synthesis route, TGA is critical for the formation of the clusters structure. But, in our case extensive study is necessary to understand the concept of ‘magic number’ on self-assembled ZnS nanoclusters. The possible mechanism can be illustrated in
Fig. 6. FT-IR spectra of as-synthesized ZnS nanoclusters.
Scheme 1. Initially, [Zn(sal)2 ] heated and acted as a Zn+2 source. Zn+2 in the reaction medium formed complex coordinated bonds with TGA. Since the S atom in the thioacetamide molecules, generated via thermal decomposition of thioacetamide, has a strong affinity toward the Zn ion, the excess TGA can be adsorbed on the surface of ZnZ nanoparticles. When the small ZnS nanoparticles aggregated, the bulk volume of TGA adsorbed on ZnS forced the aggregates to take some extent of curvature. On the other hand TGA presented a key role to assembly of nanoparticles to nanoclusters. As the ZnS nanoparticles aggregated, spherical structures progressively developed and finally formed clusters. TGA and thioacetamide make critical roles in hydrothermal formation of the ZnS nanospheres and the self-assembly [37]. Exact mechanism of the formation of the ZnS nanoclusters under hydrothermal condition is still under investigation. 4. Conclusions
Fig. 5. UV–vis absorption spectrum of the ZnS nanoclusters dispersed in ethanol.
ZnS nanoclusters have been successfully synthesized through a TGA-assisted hydrothermal processing using a new precursor [bis(salicylaldiminato)zinc(II)]; [Zn(sal)2 ]; as the source of Zn2+ and thioacetamide (CH3 CSNH2 ) as sulfiding reagent. It is reasonable to believe that the TGA-assisted hydrothermal process offers great opportunity for scale-up preparation of other morphology chalcogenides. The possible growth mechanism of formation of the ZnS
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nanoclusters was discussed and the effects of TGA and thioacetamide were proposed. Acknowledgment Authors are grateful to council of University of Kashan for providing financial support to undertake this work. References [1] C.C. Chen, A.B. Herhold, C.S. Johnson, A.P. Alivisatos, Science 276 (1997) 398. [2] N. Herron, Y. Wang, H. Eckert, J. Am. Chem. Soc. 112 (1990) 1322. [3] L. Motte, F. Billoudet, E. Laxaze, J. Douin, M.P. Pileni, J. Phys. Chem. B 101 (1997) 138. [4] T. Yamamoto, S. Kishimoto, S. Iida, Physics B 308 (2001) 916. [5] M. Bredol, J. Merikhi, J. Mater. Sci. 33 (1998) 471. [6] R. Vacassy, S.M. Scholz, J. Dutta, H. Hofmann, C.J.G. Plummer, G. Carrot, J. Hilborn, M. Akine, Mater. Res. Soc. Symp. Proc. 501 (1998) 369. [7] P. Calandra, M. Goffredi, V.T. Liveri, Colloids Surf. A 160 (1999) 9. [8] N.A. Dhas, A. Zaban, A. Gedankan, Chem. Mater. 11 (1999) 806. [9] Z.W. Wang, L.L. Daemen, Y.S. Zhao, C.S. Zha, R.T. Downs, X.D. Wang, Z.L. Wang, R.J. Hemley, Nat. Mater. 4 (2005) 922. [10] Y.F. Hao, G.W. Meng, Z.L. Wang, C.H. Ye, L.D. Zhang, Nano Lett. 6 (2006) 1650. [11] Y. Jiang, X.M. Meng, J. Liu, Z.Y. Xie, C.S. Lee, S.T. Lee, Adv. Mater. 15 (2003) 323. [12] C. Ma, D. Moore, J. Li, Z.L. Wang, Adv. Mater. 15 (2003) 228. [13] H. Zhang, S.Y. Zhang, S. Pan, G.P. Li, J.H. Hou, Nanotechnology 15 (2004) 945. [14] S. Kar, S. Biswas, S. Chaudhuri, Nanotechnology 16 (2005) 737.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of ZnS nanoclusters via hydrothermal processing from [bis(salicylidene)zinc(II)] Masoud Salavati-Niasari a,b,∗ , Fatemeh Davar b , Mehdi Mazaheri c a b c
Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317–51167, I. R. Iran Department of Chemistry, Faculty of Science, University of Kashan, Kashan, P.O. Box 87317–51167, I. R. Iran Materials and Energy Research Center, P.O. Box 14155–4777, Tehran, I. R. Iran
a r t i c l e
i n f o
Article history: Received 2 February 2008 Received in revised form 27 February 2008 Accepted 1 March 2008 Available online 22 April 2008 Keywords: Chemical synthesis ZnS Nanostructured materials Inorganic materials
a b s t r a c t A thioglycolic acid (TGA)-assisted hydrothermal process has been developed to synthesize zinc sulfide (ZnS) nanoclusters via the reaction between a new precursor, bis(salicylaldiminato)zinc(II); [Zn(sal)2 ]; and thioacetamide (CH3 CSNH2 ). X-ray diffraction (XRD), transmission electron microscopy (TEM), Xray photoelectron spectroscopy (XPS), UV–vis spectroscopy and Fourier transform infrared (FT-IR) were employed to characterize the obtained product. Furthermore, the possible mechanism and the critical factors for the TGA-assisted hydrothermal synthesis of the ZnS nanoclusters have been preliminarily presented. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured materials have attracted a great deal of attention in the last few years due to their unique properties that are different from the bulk materials [1–3]. Zinc sulfide (ZnS) as an important wide-bandgap (3.6 eV) semiconductor has been used as a key material for the ultraviolet lightemitting diodes and injection lasers [4], flat panel displays [5], electroluminescent devices and infrared windows [6,7]. In recent years, some characteristics of ZnS nanocrystals different from bulk crystal [8] have enlarged the range of application. Thus, the study of ZnS nanostructure is of considerable importance and great efforts have been focused on the synthesis and physical properties [9–12]. ZnS is observed in nature in two polymorphs, zinc blende and wurzite, with cubic and hexagonal lattice structures, respectively, with atoms in the bulk of both polymorphs being four-coordinated, having tetrahedral coordination. Although the zinc blende phase is more stable than wurzite, it is common to find both in the same sample, and it is not clear what factors control which phase predominates. Many efforts have been made in the fabrication of onedimensional (tubes [13], wires [14], rods [15], belts and ribbons [16]), two-dimensional (sheets [17], platelets [18], diskettes [19]) and three-dimensional (spheres [20]) ZnS nanostructures by liquid crystal templating, irradiation, solvothermal, physical evaporation,
∗ Corresponding author. Tel.: +9836215555333; fax: +983615552930. E-mail address: [email protected] (M. Salavati-Niasari). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.048
and surfactant-mediated microemulsion approaches. The existing methods for size and shape control of ZnS nanostructure generally use capping agents, such as surfactants, ligands, polymers, or dendrimers, to confine the growth in the nanometer regime. Among various methods, the hydrothermal method provides a more promising way for the synthesis of crystals due to its low cost, high efficiency and potential for large scale production. Recently, Yang’s groups have developed a mild hydrothermal route to synthesize metal sulfides with the use of thioglycolic acids as nontoxic template [21]. Yan and Xue successfully synthesized ZnO/ZnS nanocable and ZnS nanotube arrays via thioglycolic acid and Na2 S [22]. And so, Qian’s groups have synthesized new ZnS/organic composite nanoribbons by the use of thioglycolic acid [23]. The nanoclusters of metals, metal oxides and semiconductors are interesting because of their unique mechanical, electronic, optical, magnetic, and chemical properties [24–27]. This is largely due to the phenomenon of quantum confinement, whereby in sufficiently small particles, the optical and electronic properties depend strongly on the particle size. Quantum confinement effects may be very large, with one of the results being that the nanocluster band gap may be more than an electronvolt larger than in the bulk semiconductor. In these cases, the spatial extent of photogenerated excitons is comparable to the size of the nanocluster. Semiconductor nanoclusters are thus often referred to as “quantum dots”. The main issue in the preparation of semiconductor nanoclusters is a careful control of semiconductor size and, even more important, of their size distribution. Moreover, their subsequent
M. Salavati-Niasari et al. / Journal of Alloys and Compounds 470 (2009) 502–506
manipulation requires an efficient procedure of recovery and stabilization. For example, such nanoclusters have been prepared in colloids, polymer matrixes, self-assembled monolayers, and zeolites. Other methods have been used to prepare the nanoclusters as microemulsion or thermal decomposed methods [28,29]. Submicrometer-sized ZnS spheres have been synthesized by using silica [30] and polystyrene [31] spheres as sacrificial templates, respectively. Furthermore, Qi and co-workers [20] synthesized ZnS nanospheres in aqueous solutions of an amphiphilic triblock copolymer, in which not micelle but the copolymer aggregates were the template. Zhang’s group [13] fabricated ZnS spheres by templating with in situ generated bubbles under hydrothermal condition. Chu and co-workers [32] presented ZnS nanostructures by surfactant polyethylene glycol (PEG)-assisted method. Pietro et al. have been investigating the chemical activation and reactivity of the surface of cadmium sulfide nanoclusters prepared by the well known kinetic trapping method. This method provides nanoclusters of surprisingly good monodispersity, which are conveniently enveloped in a shell of organic aromatic “capping” molecules [33]. Our strategy has been to use a thioglycolic acid as capping agent for hydrothermal synthesize of ZnS nanoclusters. It still remains a great challenge to develop facile and environmentally benign methods for creating ZnS clusters nanospheres from a simple method. In this paper, we report a novel thioglycolic acid (TGA)-assisted hydrothermal method to fabricate well-crystallized ZnS nanoclusters. The trick in hydrothermal synthesis of ZnS nanoclusters presented here is the application of thioglycolic acid, which was previously used as the stability agent to prevent the chalcogenide nanocrystals from aggregating [34]. TGA makes important roles in crystal growth and self-assembly of ZnS crystal to clusters during the hydrothermal process. 2. Experiment 2.1. Materials All the chemicals reagents used in our experiments such as thioacetamide (TAA), thioglycolic acid (TGA) were of analytical grade and were used as received without further purification. The precursor complexes [bis(salicylaldiminato)zinc(II)]; [Zn(sal)2 ]; were prepared according to the procedure described previously [35]. The water used in this work was distilled and de-ionized. 2.2. Characterization XRD patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu K␣ radiation. Elemental analyses were obtained from Carlo ERBA Model EA 1108 analyzer. The compositional analysis was done by energy dispersive X-ray (EDX, Kevex, Delta ClassI). Transmission electron microscopy (TEM)
503
Fig. 1. XRD pattern of the as-prepared ZnS nanoclusters.
images were obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV. The surface of the zinc manganese compound was characterized by X-ray Photoelectron Spectroscopy (XPS) ESCA-3000 electron spectrometer with non-monochromatic Mg K␣ radiation was used to excite the photoelectrons. Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets. The electronic spectra of the complexes were taken on a Shimadzu UV–visible scanning spectrometer (Model 2101 PC). 2.3. Experimental Fabrication of ZnS nanoclusters was achieved by the reaction between a new precursor [Zn(sal)2 ], TGA and TAA. 10 ml of thioglycolic acid (TGA) were added into 0.6 g [Zn(sal)2 ] under stirring and heating. After 40 min 6 ml of 0.5 mol l−1 thioacetamide (TAA) were added to the solution under stirred vigorously. The final solution was put into a Teflon-lined stainless steel autoclave under stirring. Then, the autoclave was maintained under a static condition at 105 ◦ C for 7 h and then gradually cooled down to room temperature. The mixture turned white due to the formation of ZnS precipitates. The product was filtered out, washed with ethanol and deionized water for several times, and then dried at 80 ◦ C.
3. Results and discussion Fig. 1 is the X-ray powder diffraction pattern of the ZnS nanoclusters showing reflections from (1 1 1), (2 2 0) and (3 1 1) planes, which indicates formation of zinc blende (cubic, -ZnS) structure (JCPDS No. 05-0566). No diffraction peaks from other crystalline forms were detected, which indicated a high purity and well crystallinity of these ZnS nanoclusters. The size of the nanocrystals estimated from the full width at half maxima (FWHM) of the (1 1 1) diffraction peak on the basis of the Scherrer formula:
Fig. 2. EDX analysis of the as-synthesized ZnS.
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Fig. 3. TEM images of (a) ZnS nanoclusters (the inset shows the SAED pattern) and (b) broken part of nanoclusters.
D = 0.89/ˇ cos , is about 4 nm for the nanoparticles synthesized, which is much smaller than the diameters of the sphere. This indicates that the clusters were formed from assembly of nanocrystal masses. The EDX spectrum of the product further confirms the formation of ZnS. Fig. 2 depicts the EDX spectrum of the product. Only Zn and S elements were found. According to the calculation of the peak areas the molar ratio of the Zn/S is 51.4/48.9, which is very close to the stoichiometric ratio of ZnS. Also, no other impurity peak is found in XRD and EDX spectra, indicating that the product is pure. Typical TEM images and selected area electron diffraction (SAED) pattern of the product are shown in Fig. 3. Fig. 3a and b clearly reveals the sphere morphology of ZnS crystals with the diameter ranging from 50 to about 150 nm. Some loose spheres formed by aggregation of small particles can be observed in TEM images. We may consider that when the reaction was carried out at 105 ◦ C, most organic molecules were decomposed. Since only a few thiocarbamide or TGA molecules adsorbed on ZnS nanoparticles, nanoclusters, loose solid spheres, were finally produced. The selected area electron diffraction (SAED) pattern (Fig. 3a inset) shows three sharp rings from (1 1 1), (2 2 0) and (3 1 1) planes of cubic zinc blende ZnS, which is in well agreement with the XRD study. The magnified TEM image of nanoclusters (Fig. 3b) interestingly reveals that the nanospheres are not composed of densely packed, so the loosely packed clusters are the evidence of organic protective agent. These nanoclusters are formed of small nanoparticles of 4–5 nm sizes. The XPS was employed to investigate the composition and purity of as-synthesized ZnS nanocrystal. The XPS sample is shown in Fig. 4. The XPS data of the sample indicated the presence of Zn and S. No impurities were found on the surface of the sample, indicating that the as-synthesized ZnS nanoclusters are relatively pure. The binding energies of Zn 2p1/2 and Zn 2p3/2 for the product were observed at around 1045.0 and 1022.1 eV (Fig. 4a), respectively, while the peak located at around 162.5 eV (Fig. 4b) corresponded to the binding energy of S 2p3/2 . All of the observed binding energy values for Zn 2p and S 2p agreed with the literature data [36]. Fig. 5 is the UV–vis spectrum of the as-synthesized ZnS nanoclusters dispersed in ethanol. An absorption peak centered at
323 nm (3.83 eV), which is considerably blue-shifted relative to the absorption onset of bulk ZnS (ca. 340 nm) because of the quantum size effects, was found. According to Ref. [20], the diameter of particles was about 4.5 nm when the absorption peak was located
Fig. 4. XPS spectra of the as-synthesized ZnS.
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505
Scheme 1. Schematic diagram illustrating the formation of ZnS nanoclusters.
at 325 nm. This implied that the size of the as-synthesized ZnS nanoparticles should be below 4.5 nm, which is in good agreement with the result of XRD analysis. To investigate whether the surface of the nanoparticles was capped with organic surface the Fourier transformed infrared (FTIR) of the as-synthesized samples were performed. In typically Fig. 6 shows the FT-IR spectra of ZnS nanoclusters. The spectra of ZnS nanoclusters show two weak stretch vibrations between 1500 and 2500 cm−1 attributing to the C–H and C O stretching models of the thioglycolic acid, indicating TGA molecules are absorbed on the surface ZnS nanoparticles. There was no evidence of free precursor [Zn(sal)2 ], in the sample, because stretch vibration of C–H (C–H ) and the stretching vibrations of C O and C C benzene ring disappeared. So the thioglycolic acid serves as the capping ligand that controls growth of ZnS nanoclusters. As shown in EDX and XPS analysis, there are no ZnO products in the as-synthesized samples and only ZnS nanoclusters were obtained without any oxidation. The nanocrystal possess good transmittance at 400–4000 cm−1 , it indicating that nano-ZnS is a better infrared-transmittance material. Previously, thioglycolic acid (TGA) was widely used as the stability agent preventing the nanocrystals from aggregation [34]. In our synthesis route, TGA is critical for the formation of the clusters structure. But, in our case extensive study is necessary to understand the concept of ‘magic number’ on self-assembled ZnS nanoclusters. The possible mechanism can be illustrated in
Fig. 6. FT-IR spectra of as-synthesized ZnS nanoclusters.
Scheme 1. Initially, [Zn(sal)2 ] heated and acted as a Zn+2 source. Zn+2 in the reaction medium formed complex coordinated bonds with TGA. Since the S atom in the thioacetamide molecules, generated via thermal decomposition of thioacetamide, has a strong affinity toward the Zn ion, the excess TGA can be adsorbed on the surface of ZnZ nanoparticles. When the small ZnS nanoparticles aggregated, the bulk volume of TGA adsorbed on ZnS forced the aggregates to take some extent of curvature. On the other hand TGA presented a key role to assembly of nanoparticles to nanoclusters. As the ZnS nanoparticles aggregated, spherical structures progressively developed and finally formed clusters. TGA and thioacetamide make critical roles in hydrothermal formation of the ZnS nanospheres and the self-assembly [37]. Exact mechanism of the formation of the ZnS nanoclusters under hydrothermal condition is still under investigation. 4. Conclusions
Fig. 5. UV–vis absorption spectrum of the ZnS nanoclusters dispersed in ethanol.
ZnS nanoclusters have been successfully synthesized through a TGA-assisted hydrothermal processing using a new precursor [bis(salicylaldiminato)zinc(II)]; [Zn(sal)2 ]; as the source of Zn2+ and thioacetamide (CH3 CSNH2 ) as sulfiding reagent. It is reasonable to believe that the TGA-assisted hydrothermal process offers great opportunity for scale-up preparation of other morphology chalcogenides. The possible growth mechanism of formation of the ZnS
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