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Hydrothermal synthesis and characterization of sea urchin-like nickel and cobalt selenides nanocrystals
Materials Science and Engineering B 140 (2007) 38–43
Hydrothermal synthesis and characterization of sea urchin-like nickel and cobalt selenides nanocrystals Xiaohe Liu a,b,∗ , Ning Zhang a , Ran Yi a , Guanzhou Qiu a , Aiguo Yan a , Hongyi Wu a , Dapeng Meng a , Motang Tang b b
a Department of Inorganic Materials, Central South University, Changsha, Hunan 410083, PR China School of Metallurgical Science & Engineering, Central South University, Changsha, Hunan 410083, PR China
Received 23 October 2006; received in revised form 6 March 2007; accepted 7 March 2007
Abstract Sea urchin-like nanorod-based nickel and cobalt selenides nanocrystals have been selective synthesized via a hydrothermal reduction route in which hydrated nickel chloride and hydrated cobalt chloride were employed to supply Ni and Co source and aqueous hydrazine (N2 H4 ·H2 O) was used as reducing agent. The composition, morphology, and structure of final products could be easily controlled by adjusting the molar ratios of reactants and process parameters such as hydrothermal time. The morphology and phase structure of the final products have been investigated by X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. The probable formation mechanism of the sea urchin-like nanorod-based nickel and cobalt selenides nanocrystals was discussed on the basis of the experimental results. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; Sea urchin; Nickel selenide; Cobalt selenide; Nanocrystal
1. Introduction Semiconducting selenides have attracted enormous attention due to their distinctive electrical and magnetic properties and wide variety of potential applications in, e.g., optical recording materials, solar cell, superionic materials, sensor, laser materials, optical filters, and conductivity fields, etc. [1–6]. Recently, selenides nanomaterials have become the focus owing to their importance in basic scientific research and potential technology applications. The properties of nanomaterials sensitively depend on their compositions and morphologies. Thus it is significant challenges to fabricate selenides nanomaterials with different compositions and novel morphologies. Recent a variety of novel shapes such as quantum dots [7], nanorods [8], nanoribbons [9], nanowires [10], nanotubes [11], and hollow spheres [12] have been synthesized through varied synthetic reactions at room or slightly elevated temperatures. However, to the best of our knowledge, the productions of sea urchin-like selenides nano-
∗ Corresponding author at: Department of Inorganic Materials, Central South University, Changsha, Hunan 410083, PR China. Tel.: +86 731 8879815; fax: +86 731 8830543. E-mail address: [email protected] (X. Liu).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.03.007
materials have still not been realized until now. In recent years, we have been involved for a long time in the synthesis of sea urchin-like nanomaterials, and sea urchin-like metallic nickel and sulfides with nanorod-based architecture have been selective synthesized through a solventothermal or hydrothermal process [13–15]. Traditionally, selenides have been synthesized by solid-state reactions, solid-state metathesis, solution growth technique, and self-propagating high-temperature synthesis [16–20]. Among these approaches, many are based on high-temperature processes that generally required intricate manipulation and the use of high temperature. Gas phase reactions were used frequently for the preparation of selenides based on the reaction of the toxic H2 Se and metal or its compounds; however, the process is often involved relatively dangerous and highly toxic [21]. Over the past several years, organometallic precursors have also been reported to prepare selenides [22], which had to be performed at relatively high temperature with standard airless environments that generally required intricate processing. In recent years, solvothermal and hydrothermal synthesis methods were emerging as the effective synthetic technique for selenides. We recently demonstrated that hollow submicrometer spheres of Ni0.85 Se and NiSe2 could be successfully synthesized through hydrothermal method in the presence of surfactant [12]. In this paper, we
X. Liu et al. / Materials Science and Engineering B 140 (2007) 38–43
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demonstrated that sea urchin-like nanorod-based nanocrystals of hexagonal Ni0.85 Se, cubic NiSe2 , and hexagonal Co0.85 Se can be selectively synthesized via a hydrothermal reduction route under mild conditions, and the composition of nickel selenides can be easily controlled by adjusting the molar ratios of reactants. 2. Experimental section All chemical solvent and reagents used in this work, such as hydrated nickel chloride (NiCl2 ·6H2 O), hydrated cobalt chloride (CoCl2 ·6H2 O), selenium powder (99.9%), anhydrous alcohol, and aqueous hydrazine (N2 H4 ·H2 O, 50 wt.% content) were analytical grade, and which were used without further purification. 2.1. Preparation of sea urchin-like Ni0.85 Se and NiSe2 nanocrystals NiCl2 ·6H2 O (0.1 mmol) was respectively put into two Teflonlined stainless steel autoclaves of 50 ml capacity and dissolved in 20 ml deionized water. Subsequently, Se (0.12 or 0.2 mmol) was added into an autoclave under vigorous stirring, which was filled with 10 ml N2 H4 ·H2 O (50 wt.% content) and distilled water up to 75% of the total volume. The solution was stirred vigorously for 10 min and sealed and maintained respectively at 100 ◦ C for 12–72 h. The system was allowed to cool to room temperature naturally. The product was collected by filtration and washed with absolute ethanol and distilled water in sequence for several times. The final product was dried in a vacuum box at 60 ◦ C for 4 h. 2.2. Preparation of sea urchin-like Co0.85 Se nanocrystals CoCl2 ·6H2 O (0.1 mmol) and Se (0.12 mmol) were added to a Teflon-lined stainless steel autoclave of 50 ml capacity and dissolved in 20 ml deionized water under stirring. The autoclave was filled with 10 ml N2 H4 ·H2 O (50 wt.% content) and distilled water up to 75% of the total volume, sealed and maintained at 100 ◦ C for 72 h, and then the system was allowed to cooled to room temperature naturally. The washing and collecting of the procedures were the same to those for Ni0.85 Se. 2.3. Characterization The obtained samples were characterized on a Rigaku Dmax2000 X-ray powder diffractometer (XRD) with Cu K␣ radiation ˚ The operation voltage and current were kept at (λ = 1.5418 A). 40 kV and 40 mA, respectively. The size and morphology of the as-synthesized products were determined at 20 kV by a XL30 S-FEG scanning electron microscope (SEM) and at 160 kV by a JEM-200CX transmission electron microscope (TEM). 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of nickel selenides nanocrystals that were obtained respectively at 100 ◦ C
Fig. 1. The evolution of the XRD patterns of the nickel selenides nanocrystals obtained at 100 ◦ C for different reaction time: (A) 12 h; (B) 24 h; (C) 48 h; (D) 72 h.
for 12, 24, 48, and 72 h using 0.12 mmol Se. All the peaks of XRD patterns could be well indexed to hexagonal phase (space group P63 /mmc (1 9 4)) Ni0.85 Se with lattice parame˚ c = 5.288 A ˚ (JCPDS file card no. 18-0888). No ters a = 3.624 A, characteristic peaks of other molar ratio nickel selenides and impurities can be detected, which presents that the products are pure hexagonal phase Ni0.85 Se. With the elevation of time, the diffraction reflections become higher and far narrower, implying that the crystallinity of the products is continuously improved and the size of the products is increased. The morphologies and structures of the as-prepared samples were further examined by transmission electron microscopy (TEM) and scanning electron microscope (SEM). Typical TEM image of Ni0.85 Se obtained at 100 ◦ C for 12 h is shown in Fig. 2A and presents Ni0.85 Se nanocrystals with the size in the range of 200–800 nm. It is carefully observed that there are many similarly subuliform nanostructures on the surface of sphere-like nanocrystals. These subuliform nanostructures radially grow and form similar sea urchin-like morphology. We think the formation of these subuliform nanostructures results from Ni0.85 Se nucleation and growth on the surface of nanocrystals under the function of stochastic diffusive force, and these new nanostructures may grow gradually up and form nanorods. Fig. 2B is a typical SEM image of the as-prepared Ni0.85 Se obtained at 100 ◦ C for 12 h. The surface of nanocrystals is relatively rough and there are lots of spots and subuliform nanostructure on the surface of nanocrystals, and these new nanostructures may grow gradually up and form nanorods. When the reaction time is elongated to 24 h, the TEM image of the product is shown in Fig. 2C. It is more obvious that plenty of nanorods radially grow and form similar sea urchin-like morphology. Fig. 2D shows a Low-magnification SEM image of the as-prepared product at 100 ◦ C for 24 h. The inset of (D) is a higher-magnification SEM image obtained from a selected area of Fig. 2D, from which it is clearly seen that there are a great number of nanorods with the
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X. Liu et al. / Materials Science and Engineering B 140 (2007) 38–43
Fig. 2. TEM and SEM images of sea urchin-like Ni0.85 Se nanocrystals obtained at 100 ◦ C for different reaction time: (A and B) 12 h; (C and D) 24 h.
same size. The diameter and the length are about 20 and 150 nm, respectively. These nanorods may form due to the further growth of subuliform nanostructures by the function of the directive force. Further elongating the reaction time to 48 h, the product is sea urchin-like Ni0.85 Se nanocrystals with nanorod-based architecture growing radially from one center into sea urchin-like morphology (in Fig. 3A and B). The inset in Fig. 3B is a highmagnification SEM image. Herein, many nanorods with the length of 100–150 nm can be observed clearly. Fig. 3C shows a typical TEM image of as-synthesized Ni0.85 Se nanocrystals obtained at 100 ◦ C for 72 h. In contrast to the product processed 100 ◦ C for 48 h, the sea urchin-like Ni0.85 Se nanocrystals obtained at 100 ◦ C for 72 h are mostly nanorod form with high ratio of length to diameter, as can be observed more clearly from Fig. 3C. The SEM image (in Fig. 3D) presents that Ni0.85 Se nanorods grow radially from one center of an sea urchin-like nanocrystals with mean diameters of about 30 nm and lengths of up to ∼200 nm, which is consistent with the TEM result. The morphology and size of final products mainly depend on reaction time, and prolonged reaction time is advantageous to the formation of sea urchin-like nanocrystals. We think that is because the nanorods gradually grow under the function of directive force. Fig. 4 shows the XRD pattern of the product prepared at 100 ◦ C for 72 h using 0.2 mmol Se. The reflections can be read˚ ily identified as cubic NiSe2 with lattice constant a = 5.991 A (JCPDS file card no. 41-1495). The successful preparation
of NiSe2 indicates that the composition of nickel selenides can be easily controlled by adjusting the molar ratio of reactants. The XRD pattern of the product synthesized at 100 ◦ C for 72 h using 0.12 mmol Se and 0.1 mmol CoCl2 ·6H2 O is shown in Fig. 5. The d value corresponding to the peaks of the pattern ˚ are respectively 2.6759, 2.0145, 1.7873, 1.5345, and 1.4848 A, which are in consistent with the d value corresponding to the diffraction peaks of Ni0.85 Se (space group, P63 /mmc (1 9 4)) in Fig. 1 ((1 0 1), (1 0 2), (1 1 0), (1 0 3), and (2 0 1)). Therefore, this structure is analogous to that of hexagonal Ni0.85 Se with NiAs structure (JCPDS 18-888), and the as-formed product is termed Co0.85 Se [23]. No other impurities, such as metallic cobalt or selenium, can be detected in the XRD pattern, which shows the product is pure Co0.85 Se. Fig. 6A shows a TEM image of NiSe2 obtained at 100 ◦ C for 72 h. Fig. 6B and 6C show the typical SEM images of assynthesized NiSe2 and indicate that large quantity and good uniformity sea urchin-like NiSe2 nanocrystals were achieved using this approach. Fig. 6C is a higher magnification SEM image obtained from a selected area of Fig. 6B. It is more obvious that nanorods radially form sea urchin-like nanocrystals. The diameter of nanorods is in the range of 20–100 nm and the length is up to micrometers. Fig. 6D shows the typical TEM image of a sea urchin-like Co0.85 Se nanocrystal obtained at 100 ◦ C for 72 h. It is clearly seen that the surface of the nanocrystals is relatively rough and there are lots of spots and rod-like nanostructures on the surface of Co0.85 Se nanocrystals.
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Fig. 3. TEM and SEM images of sea urchin-like Ni0.85 Se nanocrystals obtained at 100 ◦ C for different reaction time: (A and B) 48 h; (C and D) 72 h.
The chemical reactions we may employ can be expressed as the following equation: 2MCl2 + N2 H4 + 4OH− → 2M ↓ + N2 ↑ + 4H2 O + 4Cl− (1) N2 H4
,100 ◦ C
xM + ySe −−−−−−→ Mx Sey (x = 0.85, y = 1
or x = 1, y = 2)
(2)
Fig. 4. The XRD patterns of the NiSe2 nanocrystals obtained at 100 ◦ C for 72 h.
First, N2 H4 can reduce M2+ (Ni2+ , Co2+ ) into elemental metal (Ni, Co). Then Ni or Co reacts with Se to form nickel and cobalt selenides monomers. The nickel and cobalt selenides monomers eventually form sea urchin-like nanocrystals due to diffusion-limited aggregation (DLA) mechanism [24]. The sea urchin-like nanorod-based nickel and cobalt selenides nanocrystals obtained is consequence of the interaction between the stochastic diffusive force and directive
Fig. 5. The XRD patterns of the Co0.85 Se nanocrystals obtained at 100 ◦ C for 72 h.
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Fig. 6. TEM and SEM images of sea urchin-like NiSe2 (A–C) and Co0.85 Se (D) nanocrystals obtained at 100 ◦ C for 72 h.
force. First, with the elevation of reaction time, the monomers nucleate and grow into sphere-like selenides nanocrystals due to monomer concentration continuously increases. Secondly continuously increased monomers may nucleate and form spots and similarly subuliform nanostructures on the surface of sphere-like selenides nanocrystals under the function of stochastic diffusive force. Then radially growing nanorods formed due to the further growth of the spots and subuliform nanostructures by the function of the directive force. Finally sea urchin-like nickel and cobalt selenides nanocrystals with nanorod-based architecture were achieved for longer reaction time. 4. Conclusion In summary, we have selectively synthesized the sea urchinlike nanorod-based nickel and cobalt selenides nanocrystals via a hydrothermal reduction route under mild conditions, and the composition of selenides could be easily controlled by adjusting the molar ratios of reactants. The effects of reaction time were investigated, and prolonged reaction time is advantageous to the formation of sea urchin-like nickel and cobalt selenides nanocrystals. It is worthy to note that the research for sea urchinlike nanorod-based nickel and cobalt selenides nanocrystals favor not only the research for crystal growth, but also the functional applications in various fields. Moreover, this synthetic strategy presented here may have a promising prospect in the future application and provide an effective route for the
synthesis of other sea urchin-like selenides nanocrystals with nanorod-based architecture. Acknowledgments Financial support of this work by National Natural Science Foundation of China (Grant no. 50504017) and Hunan Provincial Natural Science Foundation of China (Grant no. 05JJ30104) is gratefully acknowledged. References [1] F. Mongellaz, A. Fillot, R. Griot, J. De Lallee, Proc. SPIE-Int. Soc. Opt. Eng. 156 (1994) 2227. [2] S.T. Lakshmikvmar, Sol. Energy Mater. Sol. Cells 32 (1994) 7. [3] A.A. Korzhuev, Fiz. Khim. Obrab. Mater. 3 (1991) 131. [4] A. Hagfeldt, M. Gratzel, Chem. Rev. 95 (1995) 49. [5] O. Tatsuya, O. Satoru, J. Non-Cryst. Solids 250–252 (1999) 344. [6] W.Z. Wang, Y. Geng, P. Yan, F.Y. Liu, Y. Xie, Y.T. Qian, J. Am. Chem. Soc. 121 (1999) 4602. [7] S.A. Empedocies, M.G. Bawendi, Science 278 (1997) 2114. [8] W.Z. Wang, Y. Geng, P. Yan, F.Y. Liu, Y. Xie, Y.T. Qian, Inorg. Chem. Commun. 2 (1999) 83. [9] Y. Jiang, X.M. Meng, W.C. Yiu, J. Liu, J.X. Ding, C.S. Lee, S.T. Lee, J. Phys. Chem. B 108 (2004) 2784. [10] Q. Li, M.A. Brown, J.C. Hemminger, R.M. Penner, Chem. Mater. 18 (2006) 3432. [11] H.M. Cui, H. Liu, X. Li, J.Y. Wang, F. Han, X.D. Zhang, R.I. Boughton, J. Solid State Chem. 177 (2004) 4001. [12] X.H. Liu, G.Z. Qiu, J.W. Wang, Y.D. Li, Chem. Lett. 33 (2004) 852.
X. Liu et al. / Materials Science and Engineering B 140 (2007) 38–43 [13] X.H. Liu, X.D. Liang, N. Zhang, G.Z. Qiu, R. Yi, Mater. Sci. Eng. B 132 (2006) 272. [14] X.H. Liu, Mater. Chem. Phys. 91 (2005) 212. [15] X.H. Liu, Mater. Sci. Eng. B 119 (2005) 19. [16] T. Ohtani, M. Motoki, Mater. Res. Bull. 30 (1995) 1495. [17] H.C. Yi, J.J. Moore, J. Mater. Sci. 25 (1990) 1159. [18] P. Pramanik, S. Bhattacharya, P.K. Basu, Thin Solid Films 149 (1987) L81. [19] P. Pramanik, S. Biswas, J. Solid State Chem. 65 (1986) 145. [20] I.P. Parkin, Chem. Soc. Rev. 25 (1996) 199.
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[21] H.C. Metcalf, J.E. Williams, J.F. Caskta, Modern Chemistry, Holt, Reinhart, and Winston, New York, 1982, p. 54. [22] V. Ptatschek, B. Schreder, K. Herz, U. Hilbert, W. Ossau, G. Schottner, O. Rahauser, T. Bischof, G. Lermann, A. Materny, W. Kiefer, G. Bacher, A. Forchel, D. Su, M. Giersig, G. Muller, L. Spanhel, J. Phys. Chem. B 101 (1997) 8898. [23] J.H. Zhan, X.G. Yang, S.D. Li, Y. Xie, W.C. Yu, Y.T. Qian, J. Solid State Chem. 152 (2000) 537. [24] T.C. Halsey, B. Duplantier, K. Honda, Phys. Rev. Lett. 78 (1997) 1719.
Hydrothermal synthesis and characterization of sea urchin-like nickel and cobalt selenides nanocrystals Xiaohe Liu a,b,∗ , Ning Zhang a , Ran Yi a , Guanzhou Qiu a , Aiguo Yan a , Hongyi Wu a , Dapeng Meng a , Motang Tang b b
a Department of Inorganic Materials, Central South University, Changsha, Hunan 410083, PR China School of Metallurgical Science & Engineering, Central South University, Changsha, Hunan 410083, PR China
Received 23 October 2006; received in revised form 6 March 2007; accepted 7 March 2007
Abstract Sea urchin-like nanorod-based nickel and cobalt selenides nanocrystals have been selective synthesized via a hydrothermal reduction route in which hydrated nickel chloride and hydrated cobalt chloride were employed to supply Ni and Co source and aqueous hydrazine (N2 H4 ·H2 O) was used as reducing agent. The composition, morphology, and structure of final products could be easily controlled by adjusting the molar ratios of reactants and process parameters such as hydrothermal time. The morphology and phase structure of the final products have been investigated by X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. The probable formation mechanism of the sea urchin-like nanorod-based nickel and cobalt selenides nanocrystals was discussed on the basis of the experimental results. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; Sea urchin; Nickel selenide; Cobalt selenide; Nanocrystal
1. Introduction Semiconducting selenides have attracted enormous attention due to their distinctive electrical and magnetic properties and wide variety of potential applications in, e.g., optical recording materials, solar cell, superionic materials, sensor, laser materials, optical filters, and conductivity fields, etc. [1–6]. Recently, selenides nanomaterials have become the focus owing to their importance in basic scientific research and potential technology applications. The properties of nanomaterials sensitively depend on their compositions and morphologies. Thus it is significant challenges to fabricate selenides nanomaterials with different compositions and novel morphologies. Recent a variety of novel shapes such as quantum dots [7], nanorods [8], nanoribbons [9], nanowires [10], nanotubes [11], and hollow spheres [12] have been synthesized through varied synthetic reactions at room or slightly elevated temperatures. However, to the best of our knowledge, the productions of sea urchin-like selenides nano-
∗ Corresponding author at: Department of Inorganic Materials, Central South University, Changsha, Hunan 410083, PR China. Tel.: +86 731 8879815; fax: +86 731 8830543. E-mail address: [email protected] (X. Liu).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.03.007
materials have still not been realized until now. In recent years, we have been involved for a long time in the synthesis of sea urchin-like nanomaterials, and sea urchin-like metallic nickel and sulfides with nanorod-based architecture have been selective synthesized through a solventothermal or hydrothermal process [13–15]. Traditionally, selenides have been synthesized by solid-state reactions, solid-state metathesis, solution growth technique, and self-propagating high-temperature synthesis [16–20]. Among these approaches, many are based on high-temperature processes that generally required intricate manipulation and the use of high temperature. Gas phase reactions were used frequently for the preparation of selenides based on the reaction of the toxic H2 Se and metal or its compounds; however, the process is often involved relatively dangerous and highly toxic [21]. Over the past several years, organometallic precursors have also been reported to prepare selenides [22], which had to be performed at relatively high temperature with standard airless environments that generally required intricate processing. In recent years, solvothermal and hydrothermal synthesis methods were emerging as the effective synthetic technique for selenides. We recently demonstrated that hollow submicrometer spheres of Ni0.85 Se and NiSe2 could be successfully synthesized through hydrothermal method in the presence of surfactant [12]. In this paper, we
X. Liu et al. / Materials Science and Engineering B 140 (2007) 38–43
39
demonstrated that sea urchin-like nanorod-based nanocrystals of hexagonal Ni0.85 Se, cubic NiSe2 , and hexagonal Co0.85 Se can be selectively synthesized via a hydrothermal reduction route under mild conditions, and the composition of nickel selenides can be easily controlled by adjusting the molar ratios of reactants. 2. Experimental section All chemical solvent and reagents used in this work, such as hydrated nickel chloride (NiCl2 ·6H2 O), hydrated cobalt chloride (CoCl2 ·6H2 O), selenium powder (99.9%), anhydrous alcohol, and aqueous hydrazine (N2 H4 ·H2 O, 50 wt.% content) were analytical grade, and which were used without further purification. 2.1. Preparation of sea urchin-like Ni0.85 Se and NiSe2 nanocrystals NiCl2 ·6H2 O (0.1 mmol) was respectively put into two Teflonlined stainless steel autoclaves of 50 ml capacity and dissolved in 20 ml deionized water. Subsequently, Se (0.12 or 0.2 mmol) was added into an autoclave under vigorous stirring, which was filled with 10 ml N2 H4 ·H2 O (50 wt.% content) and distilled water up to 75% of the total volume. The solution was stirred vigorously for 10 min and sealed and maintained respectively at 100 ◦ C for 12–72 h. The system was allowed to cool to room temperature naturally. The product was collected by filtration and washed with absolute ethanol and distilled water in sequence for several times. The final product was dried in a vacuum box at 60 ◦ C for 4 h. 2.2. Preparation of sea urchin-like Co0.85 Se nanocrystals CoCl2 ·6H2 O (0.1 mmol) and Se (0.12 mmol) were added to a Teflon-lined stainless steel autoclave of 50 ml capacity and dissolved in 20 ml deionized water under stirring. The autoclave was filled with 10 ml N2 H4 ·H2 O (50 wt.% content) and distilled water up to 75% of the total volume, sealed and maintained at 100 ◦ C for 72 h, and then the system was allowed to cooled to room temperature naturally. The washing and collecting of the procedures were the same to those for Ni0.85 Se. 2.3. Characterization The obtained samples were characterized on a Rigaku Dmax2000 X-ray powder diffractometer (XRD) with Cu K␣ radiation ˚ The operation voltage and current were kept at (λ = 1.5418 A). 40 kV and 40 mA, respectively. The size and morphology of the as-synthesized products were determined at 20 kV by a XL30 S-FEG scanning electron microscope (SEM) and at 160 kV by a JEM-200CX transmission electron microscope (TEM). 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of nickel selenides nanocrystals that were obtained respectively at 100 ◦ C
Fig. 1. The evolution of the XRD patterns of the nickel selenides nanocrystals obtained at 100 ◦ C for different reaction time: (A) 12 h; (B) 24 h; (C) 48 h; (D) 72 h.
for 12, 24, 48, and 72 h using 0.12 mmol Se. All the peaks of XRD patterns could be well indexed to hexagonal phase (space group P63 /mmc (1 9 4)) Ni0.85 Se with lattice parame˚ c = 5.288 A ˚ (JCPDS file card no. 18-0888). No ters a = 3.624 A, characteristic peaks of other molar ratio nickel selenides and impurities can be detected, which presents that the products are pure hexagonal phase Ni0.85 Se. With the elevation of time, the diffraction reflections become higher and far narrower, implying that the crystallinity of the products is continuously improved and the size of the products is increased. The morphologies and structures of the as-prepared samples were further examined by transmission electron microscopy (TEM) and scanning electron microscope (SEM). Typical TEM image of Ni0.85 Se obtained at 100 ◦ C for 12 h is shown in Fig. 2A and presents Ni0.85 Se nanocrystals with the size in the range of 200–800 nm. It is carefully observed that there are many similarly subuliform nanostructures on the surface of sphere-like nanocrystals. These subuliform nanostructures radially grow and form similar sea urchin-like morphology. We think the formation of these subuliform nanostructures results from Ni0.85 Se nucleation and growth on the surface of nanocrystals under the function of stochastic diffusive force, and these new nanostructures may grow gradually up and form nanorods. Fig. 2B is a typical SEM image of the as-prepared Ni0.85 Se obtained at 100 ◦ C for 12 h. The surface of nanocrystals is relatively rough and there are lots of spots and subuliform nanostructure on the surface of nanocrystals, and these new nanostructures may grow gradually up and form nanorods. When the reaction time is elongated to 24 h, the TEM image of the product is shown in Fig. 2C. It is more obvious that plenty of nanorods radially grow and form similar sea urchin-like morphology. Fig. 2D shows a Low-magnification SEM image of the as-prepared product at 100 ◦ C for 24 h. The inset of (D) is a higher-magnification SEM image obtained from a selected area of Fig. 2D, from which it is clearly seen that there are a great number of nanorods with the
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X. Liu et al. / Materials Science and Engineering B 140 (2007) 38–43
Fig. 2. TEM and SEM images of sea urchin-like Ni0.85 Se nanocrystals obtained at 100 ◦ C for different reaction time: (A and B) 12 h; (C and D) 24 h.
same size. The diameter and the length are about 20 and 150 nm, respectively. These nanorods may form due to the further growth of subuliform nanostructures by the function of the directive force. Further elongating the reaction time to 48 h, the product is sea urchin-like Ni0.85 Se nanocrystals with nanorod-based architecture growing radially from one center into sea urchin-like morphology (in Fig. 3A and B). The inset in Fig. 3B is a highmagnification SEM image. Herein, many nanorods with the length of 100–150 nm can be observed clearly. Fig. 3C shows a typical TEM image of as-synthesized Ni0.85 Se nanocrystals obtained at 100 ◦ C for 72 h. In contrast to the product processed 100 ◦ C for 48 h, the sea urchin-like Ni0.85 Se nanocrystals obtained at 100 ◦ C for 72 h are mostly nanorod form with high ratio of length to diameter, as can be observed more clearly from Fig. 3C. The SEM image (in Fig. 3D) presents that Ni0.85 Se nanorods grow radially from one center of an sea urchin-like nanocrystals with mean diameters of about 30 nm and lengths of up to ∼200 nm, which is consistent with the TEM result. The morphology and size of final products mainly depend on reaction time, and prolonged reaction time is advantageous to the formation of sea urchin-like nanocrystals. We think that is because the nanorods gradually grow under the function of directive force. Fig. 4 shows the XRD pattern of the product prepared at 100 ◦ C for 72 h using 0.2 mmol Se. The reflections can be read˚ ily identified as cubic NiSe2 with lattice constant a = 5.991 A (JCPDS file card no. 41-1495). The successful preparation
of NiSe2 indicates that the composition of nickel selenides can be easily controlled by adjusting the molar ratio of reactants. The XRD pattern of the product synthesized at 100 ◦ C for 72 h using 0.12 mmol Se and 0.1 mmol CoCl2 ·6H2 O is shown in Fig. 5. The d value corresponding to the peaks of the pattern ˚ are respectively 2.6759, 2.0145, 1.7873, 1.5345, and 1.4848 A, which are in consistent with the d value corresponding to the diffraction peaks of Ni0.85 Se (space group, P63 /mmc (1 9 4)) in Fig. 1 ((1 0 1), (1 0 2), (1 1 0), (1 0 3), and (2 0 1)). Therefore, this structure is analogous to that of hexagonal Ni0.85 Se with NiAs structure (JCPDS 18-888), and the as-formed product is termed Co0.85 Se [23]. No other impurities, such as metallic cobalt or selenium, can be detected in the XRD pattern, which shows the product is pure Co0.85 Se. Fig. 6A shows a TEM image of NiSe2 obtained at 100 ◦ C for 72 h. Fig. 6B and 6C show the typical SEM images of assynthesized NiSe2 and indicate that large quantity and good uniformity sea urchin-like NiSe2 nanocrystals were achieved using this approach. Fig. 6C is a higher magnification SEM image obtained from a selected area of Fig. 6B. It is more obvious that nanorods radially form sea urchin-like nanocrystals. The diameter of nanorods is in the range of 20–100 nm and the length is up to micrometers. Fig. 6D shows the typical TEM image of a sea urchin-like Co0.85 Se nanocrystal obtained at 100 ◦ C for 72 h. It is clearly seen that the surface of the nanocrystals is relatively rough and there are lots of spots and rod-like nanostructures on the surface of Co0.85 Se nanocrystals.
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Fig. 3. TEM and SEM images of sea urchin-like Ni0.85 Se nanocrystals obtained at 100 ◦ C for different reaction time: (A and B) 48 h; (C and D) 72 h.
The chemical reactions we may employ can be expressed as the following equation: 2MCl2 + N2 H4 + 4OH− → 2M ↓ + N2 ↑ + 4H2 O + 4Cl− (1) N2 H4
,100 ◦ C
xM + ySe −−−−−−→ Mx Sey (x = 0.85, y = 1
or x = 1, y = 2)
(2)
Fig. 4. The XRD patterns of the NiSe2 nanocrystals obtained at 100 ◦ C for 72 h.
First, N2 H4 can reduce M2+ (Ni2+ , Co2+ ) into elemental metal (Ni, Co). Then Ni or Co reacts with Se to form nickel and cobalt selenides monomers. The nickel and cobalt selenides monomers eventually form sea urchin-like nanocrystals due to diffusion-limited aggregation (DLA) mechanism [24]. The sea urchin-like nanorod-based nickel and cobalt selenides nanocrystals obtained is consequence of the interaction between the stochastic diffusive force and directive
Fig. 5. The XRD patterns of the Co0.85 Se nanocrystals obtained at 100 ◦ C for 72 h.
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X. Liu et al. / Materials Science and Engineering B 140 (2007) 38–43
Fig. 6. TEM and SEM images of sea urchin-like NiSe2 (A–C) and Co0.85 Se (D) nanocrystals obtained at 100 ◦ C for 72 h.
force. First, with the elevation of reaction time, the monomers nucleate and grow into sphere-like selenides nanocrystals due to monomer concentration continuously increases. Secondly continuously increased monomers may nucleate and form spots and similarly subuliform nanostructures on the surface of sphere-like selenides nanocrystals under the function of stochastic diffusive force. Then radially growing nanorods formed due to the further growth of the spots and subuliform nanostructures by the function of the directive force. Finally sea urchin-like nickel and cobalt selenides nanocrystals with nanorod-based architecture were achieved for longer reaction time. 4. Conclusion In summary, we have selectively synthesized the sea urchinlike nanorod-based nickel and cobalt selenides nanocrystals via a hydrothermal reduction route under mild conditions, and the composition of selenides could be easily controlled by adjusting the molar ratios of reactants. The effects of reaction time were investigated, and prolonged reaction time is advantageous to the formation of sea urchin-like nickel and cobalt selenides nanocrystals. It is worthy to note that the research for sea urchinlike nanorod-based nickel and cobalt selenides nanocrystals favor not only the research for crystal growth, but also the functional applications in various fields. Moreover, this synthetic strategy presented here may have a promising prospect in the future application and provide an effective route for the
synthesis of other sea urchin-like selenides nanocrystals with nanorod-based architecture. Acknowledgments Financial support of this work by National Natural Science Foundation of China (Grant no. 50504017) and Hunan Provincial Natural Science Foundation of China (Grant no. 05JJ30104) is gratefully acknowledged. References [1] F. Mongellaz, A. Fillot, R. Griot, J. De Lallee, Proc. SPIE-Int. Soc. Opt. Eng. 156 (1994) 2227. [2] S.T. Lakshmikvmar, Sol. Energy Mater. Sol. Cells 32 (1994) 7. [3] A.A. Korzhuev, Fiz. Khim. Obrab. Mater. 3 (1991) 131. [4] A. Hagfeldt, M. Gratzel, Chem. Rev. 95 (1995) 49. [5] O. Tatsuya, O. Satoru, J. Non-Cryst. Solids 250–252 (1999) 344. [6] W.Z. Wang, Y. Geng, P. Yan, F.Y. Liu, Y. Xie, Y.T. Qian, J. Am. Chem. Soc. 121 (1999) 4602. [7] S.A. Empedocies, M.G. Bawendi, Science 278 (1997) 2114. [8] W.Z. Wang, Y. Geng, P. Yan, F.Y. Liu, Y. Xie, Y.T. Qian, Inorg. Chem. Commun. 2 (1999) 83. [9] Y. Jiang, X.M. Meng, W.C. Yiu, J. Liu, J.X. Ding, C.S. Lee, S.T. Lee, J. Phys. Chem. B 108 (2004) 2784. [10] Q. Li, M.A. Brown, J.C. Hemminger, R.M. Penner, Chem. Mater. 18 (2006) 3432. [11] H.M. Cui, H. Liu, X. Li, J.Y. Wang, F. Han, X.D. Zhang, R.I. Boughton, J. Solid State Chem. 177 (2004) 4001. [12] X.H. Liu, G.Z. Qiu, J.W. Wang, Y.D. Li, Chem. Lett. 33 (2004) 852.
X. Liu et al. / Materials Science and Engineering B 140 (2007) 38–43 [13] X.H. Liu, X.D. Liang, N. Zhang, G.Z. Qiu, R. Yi, Mater. Sci. Eng. B 132 (2006) 272. [14] X.H. Liu, Mater. Chem. Phys. 91 (2005) 212. [15] X.H. Liu, Mater. Sci. Eng. B 119 (2005) 19. [16] T. Ohtani, M. Motoki, Mater. Res. Bull. 30 (1995) 1495. [17] H.C. Yi, J.J. Moore, J. Mater. Sci. 25 (1990) 1159. [18] P. Pramanik, S. Bhattacharya, P.K. Basu, Thin Solid Films 149 (1987) L81. [19] P. Pramanik, S. Biswas, J. Solid State Chem. 65 (1986) 145. [20] I.P. Parkin, Chem. Soc. Rev. 25 (1996) 199.
43
[21] H.C. Metcalf, J.E. Williams, J.F. Caskta, Modern Chemistry, Holt, Reinhart, and Winston, New York, 1982, p. 54. [22] V. Ptatschek, B. Schreder, K. Herz, U. Hilbert, W. Ossau, G. Schottner, O. Rahauser, T. Bischof, G. Lermann, A. Materny, W. Kiefer, G. Bacher, A. Forchel, D. Su, M. Giersig, G. Muller, L. Spanhel, J. Phys. Chem. B 101 (1997) 8898. [23] J.H. Zhan, X.G. Yang, S.D. Li, Y. Xie, W.C. Yu, Y.T. Qian, J. Solid State Chem. 152 (2000) 537. [24] T.C. Halsey, B. Duplantier, K. Honda, Phys. Rev. Lett. 78 (1997) 1719.