Facile synthesis of ZnS hollow submicrospheres with open holes in solution containing ethylenediamine and CS₂

Li Lingang ^1,2, Tang Yu¹, Yang Jun¹, Zhang Yuanming¹, Du Biying¹
(¹ Department of Chemistry, Jinan University , Guangzhou 510632, China; ² Department of Chemistry and Life Science, West Anhui University, Lu^,an 237012, China)

Abstract  The novel ZnS hollow submicrospheres with uniform open holes and multiple open holes were prepared by reacting ZnSO₄ with sulfide source from CS₂ and ethylenediamine at 50^oC, then placing the mixture at room temperature. The ZnS hollow submicrospheres were characterized by XRD, SEM, TEM and UV-vis absorption. As the placing time of the reaction mixture increased from 2 days to 5 days, the size of ZnS hollow submicrospheres with holes decreased from about 700-750 nm to 150-200 nm. A rational mechanism is proposed that the spherical shape is formed by ZnS nanocrystals aggregating around the product of ethylenediamine and CS₂, open holes are caused by evaporation of CS₂ in the drying process, and hollow submicrospheres with smaller size are obtained by nanocrystals reaggregating with the aid of product of ethylenediamine and CS₂.
Keywords: zinc sulfide; submicrosphere; ethylenediamine; open hole

1. INTRODUCTION
Transition metal sulfides have attracted much attention because of their variable and special optical and electrical properties, and some of them are used for the fabrication of various devices^[1]. Of them ZnS is well known for its wide applications in photocatalysis ^[2], photoluminescence and electroluminescence ^[3-5]. Highly crystalline particles with almost mono-disperse size distribution and regular morphology are required for these applications. Recently, many efforts have been devoted to the synthesis of hollow spheres of ZnS for the applications of special shape of hollow in carriers for the controlled release of medicines, highly effective catalysts, fuel cell and confined reactors, etc^[6]. For example, ZnS hollow spheres have been synthesized by emulsifier-free emulsion route with the aid of ultrasonic [6], templating the pluronic amphiphilic triblock copolymer P123^[7], using monodisperse PSMA latex spheres as the templates ^[8]. Despite these success examples, shape control is still hard to achieve and turns out to be a great challenge for the future.
    As template and sulfur source, ethylenediamine was employed in the preparation of hollow spheres of CdS^[9]. But no such report about ZnS has been found. The special role of ethylenediamine aroused our much interesting. In order to further investigate the action of ethylenediamine and its product with CS₂, reaction of ZnSO₄ with sulfur source from CS₂ and ethylenediamine was carried out at 50^oC. The effects of placing time on size of ZnS and mechanism of crystal growth of ZnS were discussed also.

2. EXPERIMENTAL
2.1. Synthesis 
All chemicals were of analytical purity and used without further purifications. All of the solutions were prepared with twice distilled water. In a typical synthesis, 0.37 mL of CS₂ was added into 40 mL of 0.15 mol/L ethylenediamine aqueous solution in a flask. Then 20 mL of 0.1 mol/L ZnSO₄ aqueous solution was added. The mixture was heated to 50^oC and kept at this temperature for 20 min. Then, the mixture was cooled and maintained under a static condition for a definite time at room temperature. The white precipitate was centrifuged and washed with water and ethanol for three times, respectively. The final powder product was dried at 60^oC in a vacuum oven for 6 h.
2.2. Characterization            
The products were subject to the characterization of X-ray diffraction (XRD, MSAL XD-2 powder X-ray diffractometer using Cu Ka radiation of wavelength 1.5406Å at 40 kV and 20 mA), scanning electron microscopy (SEM, JEOL JSM-T300), transmission electron microscopy (TEM, Philips, Tecnai-10, 100 kV), UV-visible adsorption spectroscopy (UV-vis, type TU-191, Beijing).

3. RESULTS AND DISCUSSION
3.1 Effect of placing time on the size of ZnS hollow submicrospheres with open holes
Fig. 1 presents the SEM images of ZnS hollow submicrospheres prepared at different placing time. In Fig. 1a and 1b, obvious holes appear as round dark centers covered by pale outer shells. In Fig. 1c, the holes of the particles are too small to be seen. But the holes can be observed from the corresponding TEM image in Fig. 2 which shows strong contrast between the dark center holes and the pale outer shells. Such appearance is also consistent with that in SEM images, but different with the ZnS hollows without open holes appearing as pale center covered by dark outer shells in the reported TEM images^[6,7]. To the best of our knowledge, the present hollow shape of ZnS with open holes is different from that of reported hollow ZnS^[6,7]. As shown in Fig. 1 and 2, the ZnS spheres and holes are obviously uniform. The ZnS particles with size about 700-750 nm (Fig. 1a), 280-330 nm (Fig. 1b) and 150-200 nm (Fig. 1c) are corresponding to setting of the reaction mixture for 2 days, 3 days and 5 days, respectively. The result indicates that ZnS particles become smaller and smaller with increasing setting time. As shown in Fig. 2a, the hollow submicrospheres obtained after setting the reaction mixture for 5 days display the diameter of about 150-200 nm (average about 180 nm) and the shell thickness of about 20 nm. Three rings appearing in related electron diffraction (ED) pattern (Fig. 2a) correspond to the (111), (220), and (311) planes of zinc blende, the cubic ZnS phase, respectively. The enlarged TEM image in Fig. 2b shows that the diameter of the hole is about 80 nm, and the shells of ZnS particles are coarse, which suggest the products are consisted of primary ZnS nanoparticles ^[7].



Fig. 1. SEM images of ZnS hollow submicroscpheres by placing the reaction mixture for a) 2 days, b) 3 days, c) 5 days

Fig. 2. TEM images of ZnS hollow submicroscpheres by placing the reaction mixture for 5 days

Fig. 3. X-ray diffraction pattern of ZnS hollow submicrosphere

Fig. 4. UV-vis absorption spectrum of ZnS hollow submicroscphere

    Different size ZnS particles obtained at different setting time exhibit similar X-ray diffraction and UV-vis absorption. This result confirms they are consisted of nanoparticles again. Fig. 3 shows the XRD pattern of ZnS hollow submicrospheres. The three broad characteristic diffraction peaks appear at about 28.5°, 47.5°and 56.3°, corresponding to the (111), (220), and (311) planes of the zinc blende, the cubic ZnS phase. The average crystallite size is estimated to be about 4 nm according to the line width analysis of the (111) diffraction peak based on the Scherrer formula, suggesting that the submicrospheres with open holes could be consisted of primary nanoparticles, which is in good agreement with the enlarged TEM.
    Fig. 4 presents the UV-vis absorption spectrum of the ZnS hollow submicrospheres, which were obtained after the dried product was redispersed in ethanol. It shows an absorption peak at 294 nm, which is obviously blue-shifted from 340 nm for bulk zinc blende ZnS due to quantum size effects.
    It has been documented that the optical absorption of the ZnS nanocrystals with a size of 3.5 nm exhibits an excitonic peak at 288 nm ^[10], the size of crystallite constituting ZnS hollow submicrspheres could be estimated to be 3.8 nm according to the excitonic peak at 294nm, which is in good agreement with the result obtained from XRD.
3.2 Possible mechanism                                  
CS₂ and ethylenediamine were employed as the sulfur source for ZnS. The synthetic design was motivated by the known reactions between CS₂ and ethylenediamine^[11], as described by the following equations:
H₂N-CH₂-CH₂-NH₂+CS₂→H₂N-CH₂-CH₂-NH-CS-SH (1)
n(H₂N-CH₂-CH₂-NH-CS-SH) →(-HN-CH₂-CH₂-NH-CS-)_n + H₂S (2)
    It is well known that ethylenediamine is a bidentate ligand and can react with metal ions to form relatively stable complexes, which may serve as molecular templates in control of the crystal growth^[12]. Thus, Zn²⁺ could be chelated by ethylenediamine in aqueous solution, and carried from water to CS₂-water interface^[9,13]. Then ZnS nanocrystals would be formed by reacting Zn²⁺ with H₂S produced immediately after the contact of ethylenediamine with CS₂. The amino groups in the product in reaction (2) can still act as ligands with ZnS, and would attract ZnS to precipitate around the organic phase. In aqueous solution, the hydrophobility of organic phase drive the organic molecule to form microscale spherical oil droplet, which resulting in the spherical ZnS particles precipitated around the organic core containing the product in reaction (2) and CS₂. The interesting uniform open holes in present ZnS submicrospheress are probably caused by the evaporation of CS₂ in the drying process, which is proposed to result in the close hollow CdS spheres^[9]. This is supported by the ZnS submicrospheres with novel multiple holes obtained by employing higher ratio CS₂ to ethylenediamine as shown in Fig. 5a and b. The more amount of CS₂ in the core cause more holes in ZnS submicrospheres.
  

Fig. 5 SEM images of ZnS hollow submicroscpheres with different molar ratio of CS₂ : ethylenediamine by setting the reaction mixture for 2 days: (a) 3:1, (b) 5:1, (c) 1:3, c), inside corresponding TEM images,Scale bars: 500nm

    Ethylenediamine has been employed to form complex precursor of ZnS and probably acts as connecting molecular bridges between neighboring ZnS chains or layers^[14]. In the synthesis of binary transition metal sulfides nanocrystallites MS (M = Zn, Cd, Co, Ni) with hydrothermal method, ethylenediamine is the key factor to the purity and morphology of the products^[15]. The controlled fabrication of wurtzite ZnS nanorods is due to a mediated generation of the lamellar phase, ZnS·0.5ethylenediamine, a covalent organic-inorganic network based on ZnS slabs, and to its subsequent thermolysis in aqueous solution ^[16]. Thus, ethylenediamine can act not only with Zn²⁺ ions but also with ZnS molecules. For this reason, the product with similar structure of ethylenediamine unit in reaction (2) is suggested to template the ZnS nanocrystals reaggregate from the larger one during setting. Thus, the same formation process as initial hollow ZnS submicrospheres with open holes is repeated to give the corresponding smaller ZnS particles. The template effect of ethylenediamine and its product can be supported by disappearance of hole after adding surfactants such as SDS, SDBS. Due to the well known strong action of template, surfactants would induce the formation of ZnS particles instead of ethylenediamine and its product. Thus the forming process proposed above is replaced by the role of surfactants also, which causes the disappearance of special morphology correspondingly. Further support comes from the smaller ZnS particles obtained not by long setting time, but by employing more amount of ethylenediamine and setting for only 2 days as shown in Fig. 5c. The SEM and related inside TEM show no hole in the ZnS particles. In the case of excess ethylenediamine, almost all CS₂ have been consumed, and the left ethylenediamine would induce the formation of ZnS nanocrystals instead of its product with CS₂ for its lower molecular weight and stronger interaction with Zn²⁺. Furthermore, the good solubility in water of ethylenediamine would not put the molecules exist as sphere. Upon these reasons, small size ZnS particles without uniform spherical shape and hole are resulted in.

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在乙二胺和CS2溶液中简便合成带有开口孔的中空ZnS亚微球
李林刚^1,2，唐渝¹，杨骏¹，张渊明¹，杜碧莹¹
(¹暨南大学化学系，广东 广州510632；²皖西学院化学与生命科学系，安徽 六安 237012）
摘要  在50℃通过ZnSO₄与CS₂与乙二胺（en）产生的硫源反应，然后于室温下放置合成了具有均一开口孔和多孔新形貌的中空ZnS亚微球。对合成的ZnS亚微球进行了XRD, SEM, TEM 以及 UV-vis等测试手段的表征，当放置时间由2天增加到4天，带孔的中空ZnS亚微球的大小由700-750 nm 减小到 150-200 nm。提出了合理的生长机理，认为由ZnS纳米晶在en与CS₂形成的带有氨基的产物周围聚集形成了球形，CS₂在干燥过程中蒸发形成了开口孔，在带氨基产物的协助下纳米晶再聚集形成了尺寸变小的中空亚微球。
关键词 硫化锌；亚微球；乙二胺；开口孔

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