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Fabrication of transition metal sulfides nanocrystallites via an ethylenediamine-assisted route
Materials Chemistry and Physics 90 (2005) 73–77
Fabrication of transition metal sulfides nanocrystallites via an ethylenediamine-assisted route Qingsheng Wang, Zhude Xu∗ , Haoyong Yin, Qiulin Nie Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Received 28 June 2004; received in revised form 23 September 2004; accepted 4 October 2004
Abstract Many binary transition metal sulfides nanocrystallites MS (M = Zn, Cd, Co, Ni) with different morphologies have been successfully prepared in an ethylenediamine solution of metal salts and (NH4 )2 S (or thiourea) using a facile hydrothermal method. The products are characterized by powder X-ray electron diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and the selected area electron diffraction (SAED), respectively. UV–visible optical absorption spectra clearly indicate the presence of quantum size effects in some transition metal sulfides nanocrystallites and the possible mechanism of ethylenediamine-assisted is also briefly discussed. This method may also be extended to controllable synthesis of other metal chalcogenides with different morphologies, such as nanowires, nanorods, and nanodisks, etc. © 2004 Elsevier B.V. All rights reserved. Keywords: Chalcogenides; Nanostructures; Chemical synthesis; Electron microscopy; Microstructure
1. Introduction In recent year nanoscale materials, such as nanoparticles, nanorods and nanowires have been investigated intensively due to their unique shapes, size-dependent properties and the intriguing prospects for development for novel electro-optical applications and catalysis [1–4]. Transition metal sulfides, in particular, exhibit variable and novel optical and electrical properties, and some of them are used for the fabrication of devices [5]. They are also envisioned for a number of exciting applications including catalysts, heavy metal sponges–absorbents, chemical sensors, luminescent devices, and even superconductors [6–8]. Because of the wide applications mentioned above, highly crystalline particles with almost mono-disperse size distribution and regular morphology are required. Recent breakthroughs in the field of metal chalcogenides with controlled architecture and the preparation of metal sulfide inorganic fullerenes and nanotubes of∗
Corresponding author. Tel.: +86 571 87952477. E-mail addresses: [email protected], [email protected] (Z. Xu). 0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.10.003
fer an excellent insight into this new frontier [9]. It is well known that transition metal sulfide, such as NiS, ZnS and CdS, etc., have been prepared by various methods in the past few decades [10–12]. For example, microwave, solventhermal and electrochemical synthesis have yielded many metastable materials that cannot be produced by direct methods of solid-state synthesis [13–15]. Obtaining new materials via a relative simple route and developing the morphology-controlled synthesis methodologies are a goal and great interest in materials chemistry [16]. Hydrothermal method, which is known as a solutionbased chemical method, might provide an effective and convenient route to generate transition metal sulfides. It is reported that Qian et al. have prepared them using ethylenediamine as the solvent by solventhermal method [17,18]. Compared with other methods mentioned above, hydrothermal method has advantages of convenience and efficiency, without any inert atmosphere protections or expensive equipments. It has been developed and widely used for applications in diluted magnetic semiconductors (DMS), one-dimensional (1D) nanostructured materials, zeolites and ceramic materials, etc. [19–21]. The nanocrystallites as-prepared are high
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crystal powders with narrow grain size distribution and high purity without treatment at high temperatures. Furthermore, the particle properties such as morphology and size can be controlled via the hydrothermal process by adjusting the reaction temperature, time and additives [22]. Consequently, it is great potential to extend this method to the preparation of a wide range of transition metal sulfides nanocrystallites for it is one of the most promising methods of controlling particle size, size distribution and morphology. In this work, we report a straightforward synthetic route to transition metal sulfides nanocrystallites MS (M = Zn, Cd, Co, Ni) through metal salts and sulfur sources under the hydrothermal treatment in the ethylenediamine solution for 48 h. We consider the hydrothermal synthesis of transition metal sulfides powders has three obvious advantages: (1) the reaction is carried out under moderate conditions; (2) powders with nanometer-size and different morphologies can be obtained by adjusting the reaction conditions or reagents; and (3) the as-prepared powders might have different properties from those obtained through other routes.
2. Experimental section 2.1. Chemicals (NH4 )2 S; absolute ethanol; ethylenediamine, ≥99.0%; thiourea, ≥99.0%; ZnCl2 , ≥98.0%; CdCl2 ·2.5H2 O, ≥99.0%; CoCl2 ·6H2 O, ≥99.0%; NiCl2 ·6H2 O, ≥98.0%. All the chemicals used in the present work were analytical grade and were used as received without any further purification. Distilled water was used throughout the experiments.
(50 ml). The autoclaves were sealed carefully and put into an oven maintained at 180 ◦ C for 48 h, then cooled down to room temperature gradually. The precipitates were separated from the aqueous solution and washed with distilled water and absolute ethanol repeatedly. The final products were dried in a vacuum at 60 ◦ C for 4 h and were collected for the further characterization. 2.3. Instruments and characterization All the measurements in this work were conducted at room temperature. TEM images were taken on a JEM-200CX transmission electron microscope with an accelerating voltage of 160 kV and the crystallinity was confirmed by the selected area electron diffraction pattern (SAED). Samples for TEM were prepared by putting a few drops of an absolute ethanol solution of ultrasonically dispersed crystallite samples onto a copper grid and then allowing the solvent to evaporate at room temperature. The morphology of CdS nanorods was also characterized by a scanning electron microscopy (SEM Hitachi S-4700) in which the accelerating voltage and take off angle were 15 kev and 32.42◦ , respectively. Powder XRD patterns were collected on an X’pert MPD Philips X-ray diffractometer with Cu K␣ radiation with the scanning step of 0.2◦ s−1 . The operation voltage and current were 40 kV and 40 mA, respectively. UV–visible absorption spectra were recorded on a Specord 200 UV-visual spectrophotometer by scanning the absolute ethanol solution in a 1 cm quartz cell. The scanning range was from 200 to 600 nm with absolute ethanol as the reference.
3. Results and discussion 2.2. Synthesis The major steps involved in this hydrothermal process were carried out as follows: appropriate amount of metal salts and thiourea or (NH4 )2 S were prepared to obtain metal sulfides, respectively (Table 1). The mainly synthetic process of ZnS was presented as follows: 0.005 mol of ZnCl2 was dissolved in 30 ml distilled water and 0.006 mol of (NH4 )2 S (molar ratio, 1:1.2) was injected dropwise into this solution with a syringe. Then the solution was placed into a 50 ml Teflon-lined stainless steel autoclave and was filled with about 18 ml ethylenediamine up to 95% of the capacity
Table 1 The results of the products and morphologies of various transition metal sulfides Metal salt
Sulfur source Product Morphology JCPDS reference
ZnCl2 CdCl2 ·2.5H2 O NiCl2 ·2.5H2 O CdCl2 ·2.5H2 O CoCl2 ·6H2 O
(NH4 )2 S (NH4 )2 S (NH4 )2 S Thiourea Thiourea
ZnS CdS NiS CdS CoS
Particle Rod-like Particle Rod-like Particle
5-0566 41-1049 75-0613, 12-0041 41-1049 75-0605
Fig. 1 shows TEM images of as-prepared transition metal sulfides. The products of ZnS, CoS, NiS are all found to be spherical and their average particles sizes are about 40, 25 and 160 nm, respectively. However, some samples such as CoS exhibit that they can easily aggregate due to their small dimensions and high surface energy. Meanwhile, from TEM images, we can get that CdS prepared with (NH4 )2 S as sulfur source is rod-like with a diameter of around 20 nm and a length of up to 200 nm, which can be clearly proved by typical SEM image showed in Fig. 2. A selected area electron diffraction pattern (shown in Fig. 1C) taken from a selected area in Fig. 1 B confirms single crystalline of these materials, which coincides with the result from its XRD pattern (shown in Fig. 3). The hexagonal lattice has cell constants: a = 0.41 nm and c = 0.67 nm. We notice that the rod-shaped crystal is preferentially grown along the c-axis, which is also close to the c-value estimated from the XRD pattern. In this paper, we also select the thiourea as sulfur source to prepare CdS nanorods through hydrothermal process. Although the average diameter of the rods increases to about 140 nm, that their morphologies remain rod-like indicates the morphologies of the CdS products are not influenced by the sulfur source.
Q. Wang et al. / Materials Chemistry and Physics 90 (2005) 73–77
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Fig. 1. TEM images of the metal sulfides of (A) ZnS, (B) CdS prepared with (NH4 )2 S, (C) SAED pattern carried out on the samples shown in (B), (D) CdS prepared with thiourea, (E) CoS, and (F) NiS.
Fig. 2. Typical SEM images of CdS nanorods prepared with (NH4 )2 S.
Figs. 3 and 4 are the XRD patterns of ZnS, CdS, CoS, NiS, which show that the phases of all the products are identical with the literature data (shown in Table 1). We can also get the result that most of these products prepared using hydrothermal method have high purity. However, from the XRD pattern of NiS (shown in Fig. 4), we find that in addition to the peaks belonging to NiS (JCPDS 75-0613), there are obviously another set of peaks, which can be indexed to millerite-NiS (JCPDS 12-0043). The shape of the strong diffraction peaks indicates that the samples are fairly well crystallized and there are two different crystals in our final products of NiS powder. UV–visible absorption spectra of the metal sulfide, which were ultrasonically dispersed in absolute ethanol, are shown in Fig. 5. Although NiS has no obvious shoulder position between 200 and 600 nm in our experiment, ZnS, CdS, CoS have a shoulder position (relate to the absorption edges) located at 326, 484, 288 nm, respectively. As we know, the UV–visible absorption reflects corresponding grain size due to the quantum confine effect. In Fig. 5, there are blue shifted
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Fig. 5. UV–visual optical absorption spectra of the transition metal sulfides.
Fig. 3. Power XRD patterns of the transition metal sulfides of ZnS and CdS.
from the absorption edges of bulk materials and this clearly indicates the presence of the quantum confine effects which can be described with the Brus formula [23]. Through the formula we can estimate the grain size of as-prepared products and confirm the results of their relative particle sizes, which are obtained from the TEM images above (shown in Fig. 1). In this ethylenediamine-assisted route, the addition of ethylenediamine is the key factor to the purity and morphology of the products. The presence of ethylenediamine results in the formation of phase pure mental sulfides, which can be confirmed by XRD characterizations mentioned above. Without the addition of ethylenediamine or when the concentration of it is lower than 10 wt.% (about less than 5 ml of volume) the final products are all spherical particles in un-
ordered shapes even other experimental conditions are kept all the same. The result fits well with the reference reported before [24]. It is well known that ethylenediamine is a bidentate ligand and it can react with metal ions to form relatively stable complexes, which may serve as molecular templates in control of the crystal growth [25]. Deng et al. have declared that the size of the intermediate complex may affect the product morphology and they also estimated the approximate size of precursor [Zn(en)n ]S and [Zn(en)n ]Se in a similar chemical process [26]. When we substitute ethylenediamine with NaOH, ammonia or pyridine, all the products have the same spherical morphology in unordered shapes. Based on our experiments, we may acquire that the template ethylenediamine really plays an important role in the formation of CdS nanorods. However, in the present work, although the reaction conditions are kept the same, the morphologies of some metal sulfides crystallites are particles, while others are rod-like. It is quite possible that the origin of the morphology of metal sulfides crystallites might relate to many crucial factors, including the stability and geometric structure of the intermediate complex. First, in our preparations the stability of the intermediate complex of various metal sulfides with coordination agent ethylenediamine is quite different from each other (shown in Table 2), which might influence the morphologies of metal sulfide nanocrystallites dramatically [27]. Li et al. have proposed a solvent coordinating molecular template Table 2 The cumulative stability constants for ethylenediamine when coordinated with various metals and morphologies of the products Metal
Fig. 4. Power XRD patterns of the transition metal sulfides of CoS and NiS.
Ag Cd Zn Co Ni
Cumulative stability constant
Morphology
log k1
log k2
log k3
4.7 5.47 5.77 5.81 7.52
7.70 10.09 10.63 10.84 13.84
12.09 14.11 13.04 18.33
Rod-like Rod-like Particle Particle Particle
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mechanism to explain the growth process of nanocrystallites [28]. But in our experiment, when a moderate stability intermediate complex forms (for instance, ethylenediamine coordinated to Ag+ and Cd2+ ) it favors the formation of the onedimensional nanostructures (shown in Table 2). According to the cumulative stability constants for ethylenediamine coordinated with different metal ions such as Zn2+ , Co2+ and Ni2+ [27], the much higher stability of the intermediate complex may inversely make it difficult for crystallites to grow epitaxially and as a result spherically nanocrystallites are dominant. Second, based on the modern coordination chemical theory, it is much easy for the anion such as S2− to attack the complex, which has the structure of tetrahedron, square, or octahedron [29]. In our experiments, if the complex formed by ethylenediamine with metallic cation (for instance Cd2+ ) can provide square planar structure, there may be faster lined up along the vertical direction and consequently the products display a rod-like morphology; otherwise, if the geometric structure of the intermediate complex is either octahedral or tetrahedral, there is no preferential direction for S2− to approach the complex. Thus, S2− may combine with metallic cation along every direction equally, which will engender a spherical morphology. The more detailed studies about the effect of the coordination agent ethylenediamine and the mechanism of the reaction are in progress. Although an extended study of this simple method is needed, we believe that many other binary metal sulfides not listed in Table 1 and some ternary sulfides can also be prepared by using this method. Substituted sulfur sources (for instance (NH4 )2 S and thiourea in our experiments) with appropriate selenium and tellurium sources, this process in principle can be used to synthesize metal selenides and tellurides. Moreover, we also found that the size and the shape of ZnS, CdS, CoS and NiS are different even in the same synthetic condition. The coordination agent ethylenediamine as the template plays a key role in this hydrothermal synthesis, which shows that this progress can influence selective nucleation and growth rates of different compounds. Given the generality of this method, coating and modification of the surfaces may also be fulfilled and the more detailed researches about hydrothermal synthesis are in progress.
4. Conclusion In summary, various transition metal sulfides nanocrystallites MS (M = Zn, Cd, Co, Ni) were hydrothermal synthesized by the reaction of metal salts and (NH4 )2 S (or thiourea) with ethylenediamine as the solvent at 180 ◦ C. The experiments show that ethylenediamine is the key factor to the morphology of the final products. We can foresee the up-scaling of the process to form large quantities of this kind of nanoscale materials. Based on the simple route, synthesis of metal sulfides
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nanocrystallites materials with exactly controlled morphologies is in progress. Acknowledgements This work was support by the National Natural Science Foundation of China (No. 20171039). We thank Prof. Rizhi Wang at the University of British Columbia and Ms. Jiejia Wang at the University of Nottingham for the assistance with our manuscript. References [1] L. Brus, Curr. Opin. Coll. Interf. Sci. 2 (1996) 197. [2] A.K. Aboul-Gheit, S.A. Ghoneim, A.A. Al-Owais, Appl. Catal. A 170 (1998) 277. [3] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [4] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353. [5] R.W.J. Scott, M.J. MacLachlan, G.A. Ozin, Curr. Opin. Solid State Mater. Sci. 4 (1999) 113. [6] L.N. Lewis, Chem. Rev. 93 (1993) 2693. [7] G.A. Ozin, Adv. Mater. 4 (1992) 612. [8] G.A. Ozin, Supramol. Chem. 6 (1995) 125. [9] R. Tenne, M. Homyonfer, Y. Feldman, Chem. Mater. 10 (1998) 3225. [10] N. Chen, W.Q. Zhang, W.C. Yu, Y.T. Qian, Mater. Lett. 55 (2002) 230. [11] Z.P. Qiao, Y. Xie, Y.T. Qian, Y.J. Zhu, Mater. Chem. Phys. 62 (2000) 88. [12] J. Yang, J.H. Zeng, S.H. Yu, L. Yang, G.E. Zhou, Y.T. Qian, Chem. Mater. 12 (2000) 3259. [13] X.C. Xu, W.S. Yang, J. Liu, L.W. Lin, Adv. Mater. 3 (12) (2000) 195. [14] O. Palchik, I. Felner, G. Kataby, A. Gedanken, J. Mater. Res. 15 (10) (2000) 2176. [15] D. Routkevitch, T. Bigioni, M. Moskovits, J.M. Xu, J. Phys. Chem. B 100 (1996) 14037. [16] X.F. Duan, C.M. Lieber, Adv. Mater. 12 (2000) 298. [17] B. Li, Y. Xie, J.X. Huang, H.L. Su, Y.T. Qian, Nanostruct. Mater. 11 (1999) 1067. [18] M.S. Mo, M.W. Shao, H.M. Hu, L. Yang, W.C. Yu, Y.T. Qian, J. Cryst. Growth 244 (2000) 364. [19] Q.S. Wang, Z.D. Xu, Q.L. Nie, L.H. Yue, W.X. Chen, Y.F. Zheng, Solid State Comm. 130 (2004) 607. [20] H.M. Guan, Y.J. Zhang, J. Solid State Chem. 177 (2004) 781. [21] D. V¨oltzke, S. Gablenz, H.P. Abicht, R. Schneider, E. Pippel, J. Woltersdorf, Mater. Chem. Phys. 61 (1999) 110. [22] E. Matijevic, J. Eur. Ceram. Soc. 18 (9) (1998) 1357. [23] L. Brus, J. Phys. Chem. 90 (1986) 2555. [24] X.J. Chen, H.F. Xu, N.S. Xu, F.H. Zhao, W.J. Lin, G. Lin, Y.L. Fu, Z.L. Huang, H.Z. Wang, M.M. Wu, Inorg. Chem. 42 (9) (2003) 3100. [25] B. Li, Y. Xie, J.X. Huang, Y.T. Qian, Adv. Mater. 11 (1999) 1456. [26] Z.X. Deng, C. Wang, X.M. Sun, Y.D. Li, Inorg. Chem. 41 (4) (2002) 869. [27] J.A. Dean, Lang’s Handbook of Chemistry, 13th ed., Mcgraw-Hill, New York, 1985, pp. 5–82. [28] Y.D. Li, H.W. Liao, Y. Ding, Y. Fan, Y. Zhang, Y.T. Qian, Inorg. Chem. 38 (1999) 1382. [29] Q.L. Nie, Q.L. Yuan, W.X. Chen, Z.D. Xu, J. Cryst. Growth 265 (2004) 420.
Fabrication of transition metal sulfides nanocrystallites via an ethylenediamine-assisted route Qingsheng Wang, Zhude Xu∗ , Haoyong Yin, Qiulin Nie Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Received 28 June 2004; received in revised form 23 September 2004; accepted 4 October 2004
Abstract Many binary transition metal sulfides nanocrystallites MS (M = Zn, Cd, Co, Ni) with different morphologies have been successfully prepared in an ethylenediamine solution of metal salts and (NH4 )2 S (or thiourea) using a facile hydrothermal method. The products are characterized by powder X-ray electron diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and the selected area electron diffraction (SAED), respectively. UV–visible optical absorption spectra clearly indicate the presence of quantum size effects in some transition metal sulfides nanocrystallites and the possible mechanism of ethylenediamine-assisted is also briefly discussed. This method may also be extended to controllable synthesis of other metal chalcogenides with different morphologies, such as nanowires, nanorods, and nanodisks, etc. © 2004 Elsevier B.V. All rights reserved. Keywords: Chalcogenides; Nanostructures; Chemical synthesis; Electron microscopy; Microstructure
1. Introduction In recent year nanoscale materials, such as nanoparticles, nanorods and nanowires have been investigated intensively due to their unique shapes, size-dependent properties and the intriguing prospects for development for novel electro-optical applications and catalysis [1–4]. Transition metal sulfides, in particular, exhibit variable and novel optical and electrical properties, and some of them are used for the fabrication of devices [5]. They are also envisioned for a number of exciting applications including catalysts, heavy metal sponges–absorbents, chemical sensors, luminescent devices, and even superconductors [6–8]. Because of the wide applications mentioned above, highly crystalline particles with almost mono-disperse size distribution and regular morphology are required. Recent breakthroughs in the field of metal chalcogenides with controlled architecture and the preparation of metal sulfide inorganic fullerenes and nanotubes of∗
Corresponding author. Tel.: +86 571 87952477. E-mail addresses: [email protected], [email protected] (Z. Xu). 0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.10.003
fer an excellent insight into this new frontier [9]. It is well known that transition metal sulfide, such as NiS, ZnS and CdS, etc., have been prepared by various methods in the past few decades [10–12]. For example, microwave, solventhermal and electrochemical synthesis have yielded many metastable materials that cannot be produced by direct methods of solid-state synthesis [13–15]. Obtaining new materials via a relative simple route and developing the morphology-controlled synthesis methodologies are a goal and great interest in materials chemistry [16]. Hydrothermal method, which is known as a solutionbased chemical method, might provide an effective and convenient route to generate transition metal sulfides. It is reported that Qian et al. have prepared them using ethylenediamine as the solvent by solventhermal method [17,18]. Compared with other methods mentioned above, hydrothermal method has advantages of convenience and efficiency, without any inert atmosphere protections or expensive equipments. It has been developed and widely used for applications in diluted magnetic semiconductors (DMS), one-dimensional (1D) nanostructured materials, zeolites and ceramic materials, etc. [19–21]. The nanocrystallites as-prepared are high
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crystal powders with narrow grain size distribution and high purity without treatment at high temperatures. Furthermore, the particle properties such as morphology and size can be controlled via the hydrothermal process by adjusting the reaction temperature, time and additives [22]. Consequently, it is great potential to extend this method to the preparation of a wide range of transition metal sulfides nanocrystallites for it is one of the most promising methods of controlling particle size, size distribution and morphology. In this work, we report a straightforward synthetic route to transition metal sulfides nanocrystallites MS (M = Zn, Cd, Co, Ni) through metal salts and sulfur sources under the hydrothermal treatment in the ethylenediamine solution for 48 h. We consider the hydrothermal synthesis of transition metal sulfides powders has three obvious advantages: (1) the reaction is carried out under moderate conditions; (2) powders with nanometer-size and different morphologies can be obtained by adjusting the reaction conditions or reagents; and (3) the as-prepared powders might have different properties from those obtained through other routes.
2. Experimental section 2.1. Chemicals (NH4 )2 S; absolute ethanol; ethylenediamine, ≥99.0%; thiourea, ≥99.0%; ZnCl2 , ≥98.0%; CdCl2 ·2.5H2 O, ≥99.0%; CoCl2 ·6H2 O, ≥99.0%; NiCl2 ·6H2 O, ≥98.0%. All the chemicals used in the present work were analytical grade and were used as received without any further purification. Distilled water was used throughout the experiments.
(50 ml). The autoclaves were sealed carefully and put into an oven maintained at 180 ◦ C for 48 h, then cooled down to room temperature gradually. The precipitates were separated from the aqueous solution and washed with distilled water and absolute ethanol repeatedly. The final products were dried in a vacuum at 60 ◦ C for 4 h and were collected for the further characterization. 2.3. Instruments and characterization All the measurements in this work were conducted at room temperature. TEM images were taken on a JEM-200CX transmission electron microscope with an accelerating voltage of 160 kV and the crystallinity was confirmed by the selected area electron diffraction pattern (SAED). Samples for TEM were prepared by putting a few drops of an absolute ethanol solution of ultrasonically dispersed crystallite samples onto a copper grid and then allowing the solvent to evaporate at room temperature. The morphology of CdS nanorods was also characterized by a scanning electron microscopy (SEM Hitachi S-4700) in which the accelerating voltage and take off angle were 15 kev and 32.42◦ , respectively. Powder XRD patterns were collected on an X’pert MPD Philips X-ray diffractometer with Cu K␣ radiation with the scanning step of 0.2◦ s−1 . The operation voltage and current were 40 kV and 40 mA, respectively. UV–visible absorption spectra were recorded on a Specord 200 UV-visual spectrophotometer by scanning the absolute ethanol solution in a 1 cm quartz cell. The scanning range was from 200 to 600 nm with absolute ethanol as the reference.
3. Results and discussion 2.2. Synthesis The major steps involved in this hydrothermal process were carried out as follows: appropriate amount of metal salts and thiourea or (NH4 )2 S were prepared to obtain metal sulfides, respectively (Table 1). The mainly synthetic process of ZnS was presented as follows: 0.005 mol of ZnCl2 was dissolved in 30 ml distilled water and 0.006 mol of (NH4 )2 S (molar ratio, 1:1.2) was injected dropwise into this solution with a syringe. Then the solution was placed into a 50 ml Teflon-lined stainless steel autoclave and was filled with about 18 ml ethylenediamine up to 95% of the capacity
Table 1 The results of the products and morphologies of various transition metal sulfides Metal salt
Sulfur source Product Morphology JCPDS reference
ZnCl2 CdCl2 ·2.5H2 O NiCl2 ·2.5H2 O CdCl2 ·2.5H2 O CoCl2 ·6H2 O
(NH4 )2 S (NH4 )2 S (NH4 )2 S Thiourea Thiourea
ZnS CdS NiS CdS CoS
Particle Rod-like Particle Rod-like Particle
5-0566 41-1049 75-0613, 12-0041 41-1049 75-0605
Fig. 1 shows TEM images of as-prepared transition metal sulfides. The products of ZnS, CoS, NiS are all found to be spherical and their average particles sizes are about 40, 25 and 160 nm, respectively. However, some samples such as CoS exhibit that they can easily aggregate due to their small dimensions and high surface energy. Meanwhile, from TEM images, we can get that CdS prepared with (NH4 )2 S as sulfur source is rod-like with a diameter of around 20 nm and a length of up to 200 nm, which can be clearly proved by typical SEM image showed in Fig. 2. A selected area electron diffraction pattern (shown in Fig. 1C) taken from a selected area in Fig. 1 B confirms single crystalline of these materials, which coincides with the result from its XRD pattern (shown in Fig. 3). The hexagonal lattice has cell constants: a = 0.41 nm and c = 0.67 nm. We notice that the rod-shaped crystal is preferentially grown along the c-axis, which is also close to the c-value estimated from the XRD pattern. In this paper, we also select the thiourea as sulfur source to prepare CdS nanorods through hydrothermal process. Although the average diameter of the rods increases to about 140 nm, that their morphologies remain rod-like indicates the morphologies of the CdS products are not influenced by the sulfur source.
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Fig. 1. TEM images of the metal sulfides of (A) ZnS, (B) CdS prepared with (NH4 )2 S, (C) SAED pattern carried out on the samples shown in (B), (D) CdS prepared with thiourea, (E) CoS, and (F) NiS.
Fig. 2. Typical SEM images of CdS nanorods prepared with (NH4 )2 S.
Figs. 3 and 4 are the XRD patterns of ZnS, CdS, CoS, NiS, which show that the phases of all the products are identical with the literature data (shown in Table 1). We can also get the result that most of these products prepared using hydrothermal method have high purity. However, from the XRD pattern of NiS (shown in Fig. 4), we find that in addition to the peaks belonging to NiS (JCPDS 75-0613), there are obviously another set of peaks, which can be indexed to millerite-NiS (JCPDS 12-0043). The shape of the strong diffraction peaks indicates that the samples are fairly well crystallized and there are two different crystals in our final products of NiS powder. UV–visible absorption spectra of the metal sulfide, which were ultrasonically dispersed in absolute ethanol, are shown in Fig. 5. Although NiS has no obvious shoulder position between 200 and 600 nm in our experiment, ZnS, CdS, CoS have a shoulder position (relate to the absorption edges) located at 326, 484, 288 nm, respectively. As we know, the UV–visible absorption reflects corresponding grain size due to the quantum confine effect. In Fig. 5, there are blue shifted
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Fig. 5. UV–visual optical absorption spectra of the transition metal sulfides.
Fig. 3. Power XRD patterns of the transition metal sulfides of ZnS and CdS.
from the absorption edges of bulk materials and this clearly indicates the presence of the quantum confine effects which can be described with the Brus formula [23]. Through the formula we can estimate the grain size of as-prepared products and confirm the results of their relative particle sizes, which are obtained from the TEM images above (shown in Fig. 1). In this ethylenediamine-assisted route, the addition of ethylenediamine is the key factor to the purity and morphology of the products. The presence of ethylenediamine results in the formation of phase pure mental sulfides, which can be confirmed by XRD characterizations mentioned above. Without the addition of ethylenediamine or when the concentration of it is lower than 10 wt.% (about less than 5 ml of volume) the final products are all spherical particles in un-
ordered shapes even other experimental conditions are kept all the same. The result fits well with the reference reported before [24]. It is well known that ethylenediamine is a bidentate ligand and it can react with metal ions to form relatively stable complexes, which may serve as molecular templates in control of the crystal growth [25]. Deng et al. have declared that the size of the intermediate complex may affect the product morphology and they also estimated the approximate size of precursor [Zn(en)n ]S and [Zn(en)n ]Se in a similar chemical process [26]. When we substitute ethylenediamine with NaOH, ammonia or pyridine, all the products have the same spherical morphology in unordered shapes. Based on our experiments, we may acquire that the template ethylenediamine really plays an important role in the formation of CdS nanorods. However, in the present work, although the reaction conditions are kept the same, the morphologies of some metal sulfides crystallites are particles, while others are rod-like. It is quite possible that the origin of the morphology of metal sulfides crystallites might relate to many crucial factors, including the stability and geometric structure of the intermediate complex. First, in our preparations the stability of the intermediate complex of various metal sulfides with coordination agent ethylenediamine is quite different from each other (shown in Table 2), which might influence the morphologies of metal sulfide nanocrystallites dramatically [27]. Li et al. have proposed a solvent coordinating molecular template Table 2 The cumulative stability constants for ethylenediamine when coordinated with various metals and morphologies of the products Metal
Fig. 4. Power XRD patterns of the transition metal sulfides of CoS and NiS.
Ag Cd Zn Co Ni
Cumulative stability constant
Morphology
log k1
log k2
log k3
4.7 5.47 5.77 5.81 7.52
7.70 10.09 10.63 10.84 13.84
12.09 14.11 13.04 18.33
Rod-like Rod-like Particle Particle Particle
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mechanism to explain the growth process of nanocrystallites [28]. But in our experiment, when a moderate stability intermediate complex forms (for instance, ethylenediamine coordinated to Ag+ and Cd2+ ) it favors the formation of the onedimensional nanostructures (shown in Table 2). According to the cumulative stability constants for ethylenediamine coordinated with different metal ions such as Zn2+ , Co2+ and Ni2+ [27], the much higher stability of the intermediate complex may inversely make it difficult for crystallites to grow epitaxially and as a result spherically nanocrystallites are dominant. Second, based on the modern coordination chemical theory, it is much easy for the anion such as S2− to attack the complex, which has the structure of tetrahedron, square, or octahedron [29]. In our experiments, if the complex formed by ethylenediamine with metallic cation (for instance Cd2+ ) can provide square planar structure, there may be faster lined up along the vertical direction and consequently the products display a rod-like morphology; otherwise, if the geometric structure of the intermediate complex is either octahedral or tetrahedral, there is no preferential direction for S2− to approach the complex. Thus, S2− may combine with metallic cation along every direction equally, which will engender a spherical morphology. The more detailed studies about the effect of the coordination agent ethylenediamine and the mechanism of the reaction are in progress. Although an extended study of this simple method is needed, we believe that many other binary metal sulfides not listed in Table 1 and some ternary sulfides can also be prepared by using this method. Substituted sulfur sources (for instance (NH4 )2 S and thiourea in our experiments) with appropriate selenium and tellurium sources, this process in principle can be used to synthesize metal selenides and tellurides. Moreover, we also found that the size and the shape of ZnS, CdS, CoS and NiS are different even in the same synthetic condition. The coordination agent ethylenediamine as the template plays a key role in this hydrothermal synthesis, which shows that this progress can influence selective nucleation and growth rates of different compounds. Given the generality of this method, coating and modification of the surfaces may also be fulfilled and the more detailed researches about hydrothermal synthesis are in progress.
4. Conclusion In summary, various transition metal sulfides nanocrystallites MS (M = Zn, Cd, Co, Ni) were hydrothermal synthesized by the reaction of metal salts and (NH4 )2 S (or thiourea) with ethylenediamine as the solvent at 180 ◦ C. The experiments show that ethylenediamine is the key factor to the morphology of the final products. We can foresee the up-scaling of the process to form large quantities of this kind of nanoscale materials. Based on the simple route, synthesis of metal sulfides
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