Selective synthesis and characterization of sea urchin-like metallic nickel nanocrystals

Materials Science and Engineering B 132 (2006) 272–277

Selective synthesis and characterization of sea urchin-like metallic nickel nanocrystals Xiaohe Liu ∗ , Xudong Liang, Ning Zhang, Guanzhou Qiu, Ran Yi Department of Inorganic Materials, Central South University, Changsha, Hunan 410083, PR China Received 29 December 2005; received in revised form 18 April 2006; accepted 22 April 2006

Abstract Sea urchin-like nanobelt-based and nanorod-based metallic nickel nanocrystals have been selective synthesized via a hydrothermal reduction route in which sodium hydroxide was used as alkaline reagent and aqueous hydrazine (N2 H4 ·H2 O) was used as reducing agent. The morphology and structure of final products could be easily controlled by adjust process parameters such as hydrothermal time, reaction temperature and alkaline concentration. Surfactant cetyltrimethylammonium bromide (CTAB) was also important parameter influencing the morphology of the products. The morphology and phase structure of the final products have been investigated by X-ray diffraction, transmission electron microscopy and selected area electron diffraction. The probable formation mechanism of the sea urchin-like metallic nickel nanocrystals was discussed on the basis of the experimental results. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; Sea urchin; Nickel; Nanocrystal

1. Introduction During the past decades, nanomaterials have been of great research interest due to their unique optical, electrical and magnetic properties and their wide variety of potential applications in, e.g., catalysis [1], solar cells [2], light-emitting diodes [3], biological labeling [4], and electronic fields [5], etc. It is well known that morphologies and structures of nanomaterials have great influence on their properties, and considerable efforts have been placed on the tailoring the morphology and the structure of nanomaterials with unusual and novel architecture to fine-tune their properties. Up to now, many nanomaterials with complex functional architecture have been successfully synthesized through various methods. Zhu et al. [6] have stated they prepared novel urchin-like architecture and snowflake-like pattern CuS via heating different solutions. Lu et al. [7] have synthesized highly ordered snowflakelike structures of bismuth sulfide via a simple bionolecule-assisted approach. The fabrication of curved structures composed of nanoplatelets in a one-pot process and synthesis of ZnO “dandelions” via modified kirkendall process were also reported [8,9]. In recent years, we have been involved

∗

Corresponding author. E-mail address: [email protected] (X. Liu).

0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.04.024

for a long time in the synthesis of ordered superstructures or complex functional architectures. Sea urchin-like hexagonal ␣NiS, rhombohedral ␤-NiS, cubic phase Co9 S8 , and hexagonal CdS nanocrystals with nanorod-based architecture have been selective synthesized [10,11]. Nickel has attracted much attention especially due to its electrochemical, magnetic, mechanical properties and its applications in magnetic storage, fuel cell, and catalytic fields [12–15]. Nickel nanostructures may provide some immediate advantage compared with those in traditional bulk materials. To date, many kinds of novel nanostructures such as wire- [16], belt- [17], rod- [18], tube- [19], sphere-like [20] nickel nanocrystals have been synthesized via various approaches including hydrothermal reduction [21], electrodeposition [22], metal-organic chemical vapor deposition (CVD) of metal [23] into the nanopores of template materials and so on. However, there have been few reports on the synthesis of sea urchin-like nickel nanocrystals. In this paper, we demonstrate that sea urchin-like nanobelt-based and nanorod-based nickel nanocrystals can be selective prepared via a hydrothermal reduction route. The morphology and structure of final products can be easily controlled by adjust process parameters such as hydrothermal time, reaction temperature and alkaline concentration. It is worthy to note that the sea urchinlike nickel nanocrystals may exhibit some novel properties, such as good conductivity and a larger specific surface area, which

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can lead to new and important applications in hydrogenation catalysts [24], electrode materials [25] and magnetic device [26]. 2. Experimental All chemical solvent and reagents used in this work, such as hydrated nickel nitrate (Ni(NO3 )2 ·6H2 O), cetyltrimethylammonium bromide (CTAB), sodium hydroxide (NaOH), anhydrous alcohol, and aqueous hydrazine (N2 H4 ·H2 O, 50%) were analytical grade, and which were used without further purification. In a typical procedure, Ni(NO3 )2 ·6H2 O (0.546 g, 2 mmol) was put into a Teflon-lined stainless steel autoclave of 50 mL capacity and dissolved in 15 mL deionized water. Ten milliliters 1–3 mol/L NaOH solution and CTAB (2 mmol) were added into the autoclave under vigorous stirring. Then, the autoclave was filled with 10 mL N2 H4 ·H2 O. The solution was stirred vigorously for 10 min and sealed and maintained respectively at 120–180 ◦ C for 24–48 h. Subsequently, the system was allowed to cool to room temperature naturally. The resulting black precipitate 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 50 ◦ C for 4 h. 3. Characterization X-ray powder diffraction patterns were obtained on a Japan Rigaku D/max ␥. A rotiating anode X-ray diffractometer with ˚ radiation. The graphite-monochromatized Cu K␣ (λ = 1.5418 A) operation voltage and current were kept at 40 kV and 40 mA, respectively. XRD patterns were recorded from 10◦ to 70◦ (2θ) with a scanning step of 0.02◦ . TEM patterns were recorded on a Hitachi Model H-800 transmission electron microscope at an accelerating voltage of 200 kV. The samples were dispersed in absolute ethanol in an ultrasonic bath. Then the suspensions were dropped onto Cu grids coated with amorphous carbon films. Selected area electron diffraction (SAED) was further performed to identify the crystallinity. Scanning electron microscopy (SEM) was taken on a XL30 S-FEG scanning electron microscope operated at 10 kV.

Fig. 1. XRD pattern of metallic nickel obtained using 1 mol/L NaOH solution as alkaline agent at different temperatures for 24 h: (A) 120 ◦ C; (B) 140 ◦ C; (C) 180 ◦ C.

4. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns (2θ scan) from products obtained respectively at 120, 140, and 180 ◦ C for 24 h using 1 mol/L NaOH solution as alkaline agent. All the reflections of XRD pattern can be finely indexed to a facecentered cubic phase (space group Fm3m(225)) Ni with lattice ˚ (JCPDS file card no. 04-0850). With the parameters a = 3.5238 A elevation of temperature, the diffraction peaks become higher and far narrower, implying that the crystallinity of the products is continuously improved or the size of the products is increased. The morphologies and structures of the as-prepared samples were further examined by transmission electron microscopy (TEM). Fig. 2A shows a typical TEM image of nickel obtained at 120 ◦ C for 24 h using 1 mol/L NaOH solution as alkaline agent. It is the mixture of nanobelt and nanosheet, and implying the poor crystallized products. When the temperature was lifted to 140 ◦ C, the product is sea urchin-like metallic nickel nanocrystals with the size in the range of 500–1500 nm, which consist of a large quantities of nanobelts and nanosheets with the length of

Fig. 2. TEM images of metallic nickel nanocrystals obtained using 1 mol/L NaOH solution as alkaline agent at different temperatures for 24 h: (A) 120 ◦ C; (B) 140 ◦ C; (C) 180 ◦ C.

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Fig. 3. (A) Low- and (B) high-magnification TEM images of sea urchin-like nickel nanocrystals prepared at 140 ◦ C for 48 h, the inset of (B) shows an individual tubular nanostructure. Image (C) shows the SAED pattern of nanorod (B); (D and E) TEM and (inset of E) SEM images of sea urchin-like nickel nanocrystals prepared at 180 ◦ C for 48 h. (F) The SAED pattern of an individual nanorod.

about several hundreds nanometers growing radially from one center into sea urchin-like nanocrystals (in Fig. 2B). When the samples maintained at 180 ◦ C, sea urchin-like metallic nickel nanocrystals with the size of 1000–2000 nm are mostly comprise of radially growing nanobelts with the length of 500–1000 nm, as can be carefully observed in Fig. 2C. Compared to the products obtained at 140 ◦ C, both the average size and crystallinity increase for the products obtained at 180 ◦ C, which agrees well with the XRD results. Sheet-like leaves of trees or grasses have trend of curling during its drying process. We get enlightenment from this phenomenon: Do nanosheet or broad nanobelt curl into nanorod or nanotube at higher temperature or for longer reaction time? We tried to elongate reaction time in order to test whether the presume is true or not. All the reflections of XRD pattern of the products produced through the same method at 140–180 ◦ C for 48 h can be indexed to face-centered cubic phase Ni with lat˚ Fig. 3A and B shows typical TEM tice parameters a = 3.5238 A. images of products prepared at 140 ◦ C for 48 h. Fig. 3B is a higher magnification TEM image. It is obvious that a large quantity of nanorods and some nanobelts radially form sea urchin-like metallic nickel nanocrystals with the size of 1500–2500 nm. Compared to the product obtained for 24 h, the size of the sea urchin-like nanocrystals increases and the diameter and length of nanorods is about 50–80 nm and several micrometers, respec-

tively. With careful observation, the sea urchin-like morphology can be found to be composed of many nanorods or nanotubes due to nanobelts or nanosheets curled. The SEM image (inset of Fig. 3B) shows the as-prepared sea urchin-like nanocrystals with tubular architecture. Fig. 3C shows an electron diffraction (SAED) pattern collected from the selected area of nickel nanorods of Fig. 3B, implying that these radial-aligned growing nanorods are single crystal. When the temperature was elevated to 180 ◦ C, the diameter of these sea urchin-like products can be up to several micrometers (Fig. 3D and E). The inset SEM image in Fig. 3E shows as-prepared sea urchin-like metallic nickel with roll-like nanostructures. Fig. 3F is the SAED pattern collected from Fig. 3E, indicating nanorod with radial-aligned mode are well crystallized single crystal. Roll-like nanostructures of Fig. 3E imply it is feasible that nanosheet or broad nanobelt structures roll and form nanorod or nanotube structures through long reaction time. In our experiments, the influences of alkaline concentration on the shapes of products were also explored. When we added 10 mL 2 mol/L NaOH solution to the reaction system, the product obtained at 120–180 ◦ C for 24 h is still fcc phase metallic nickel, and Ni(OH)2 or impurities cannot be observed. If the NaOH concentration is further lifted, the result is different. Fig. 4 shows the XRD pattern of products obtained at 120–180 ◦ C for 24 h using 10 mL 3 mol/L NaOH solution as alkaline reagent

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Fig. 4. The XRD pattern of product obtained by elevating NaOH concentration at different temperatures for 24 h (NaOH (10 mL, 3 M)).

in the experiments. All the peaks in Fig. 4A can be indexed to ¯ a hexagonal phase (space group P 3m1(164)) of Ni(OH)2 with ˚ c = 4.605 A ˚ (JCPDS file card no. lattice parameters a = 3.126 A, 14-0117). The peaks of hexagonal phase Ni(OH)2 are weak in Fig. 4B. However, when the temperature was lifted to 180 ◦ C, all the peaks can be finely indexed to fcc metallic nickel and no peaks of Ni(OH)2 appear in Fig. 4C, which indicates that the ˚ products are pure phase with lattice parameters of a = 3.5238 A (JCPDS file card no. 04-0850). Surfactant CTAB is of great importance in the formation of sea urchin-like nanobelt-based metallic nickel nanocrystals. We find out that a small quantity of CTAB can benefit to the formation of sea urchin-like nanobelt-based metallic naocrystals through a series of experiments by changing the concentration of CTAB. Fig. 5 shows XRD pattern of the products obtained with the absence of surfactant CTAB at 120–180 ◦ C for 24 h using 1 mol/L NaOH solution as alkaline agent. All the reflections can be finely indexed to face-centered cubic phase (space

275

˚ group Fm3m(225)) Nickel with lattice parameters a = 3.5238 A (JCPDS file card no. 04-0850). TEM was also employed to further investigate the morphologies and structures of the as-prepared products obtained without using surfactant CTAB. Fig. 6A is a TEM image of the products obtained at 120 ◦ C that clearly indicates a large quantity of nanocrystals with diameters of 50–150 nm. The inset in Fig. 6A is a higher magnification TEM image, from which it is clearly seen that nanocrystals are similarly sphere and there is no new nanostructure on the surface of nanocrystals. The TEM image of the product obtained at 140 ◦ C is shown in Fig. 6B and presents Ni nanocrystal with the size ranging from 200 to 800 nm. It is carefully observed that there are plenty of nanorods on the surface of every nanocrystal. These rodlike structures radially grow from the center and form similar sea urchin-like morphology. The inset of Fig. 6B is a typical SEM image of the as-prepared product obtained at 140 ◦ C. It is clearly seen that the surface of nanocrystals is relatively rough and there are lots of spots and rodlike nanostructure on the surface of nanocrystals, and these new nanostructures may grow gradually up and form nanorods. Fig. 6C is a higher magnification TEM image. It is more obvious that nanorods grow from spherelike nanocrystals. The length and the diameter of nanorods are 50–150 and 10–30 nm, respectively. When the temperature was lifted to 180 ◦ C, the TEM image of products is shown in Fig. 6D and E. Fig. 6D is a lower magnification TEM image. In contrast to the samples obtained at low temperature, the size of sea urchin-like nanorod-based metallic nickel increases and is up to about several micrometers. Meanwhile both the length and the diameter of nanorods also increase and are 400–800 and 100–200 nm, respectively, because nanorods grow faster at high temperature. The inset SEM image in Fig. 6E obviously shows that many nanorods radially form sea urchin-like nanocrystals. Fig. 6F is a higher magnification TEM image obtained from a selected area of Fig. 6E. The inset in Fig. 6F shows the selected area electron diffraction (SAED) pattern of single nanorod, which demonstrates these nanorods are single crystal structure. In contrast above products to the products produced with the presence of CTAB, it is easily found that CTAB plays critical role during the formation of nanobelt or nanosheet structure in the sea urchin-like metallic nickel nanocrystal. In above synthetic systems, reaction velocity can be reduced through precipitation show-release method, which can regulate the kinetics of nucleation and growth of products, finally the morphology, structure of final products can be efficiently controlled. The chemical reactions we employed can be expressed as the following equation: Ni(NO3 )2 + 2NaOH → Ni(OH)2 ↓ +2Na+ + 2NO3 − Ni(OH)2 ↓

slow-release

−→

Ni2+ + 2OH−

2Ni2+ + N2 H4 + 4OH− Fig. 5. The XRD patterns of nickel obtained using 1 mol/L NaOH solution as alkaline agent with the absence of surfactant CTAB at different temperatures for 24 h.

H2 O, 120–180◦ C

−→

2Ni ↓ +N2 ↑ +4H2 O

Firstly, Ni(NO3 )2 could react with NaOH forming the Ni(OH)2 precipitations, which slowly release Ni2+ in the solution. The reaction kinetics can be adjusted through the balance

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Fig. 6. (A) TEM images of product obtained with the absence of surfactant CTAB at 120 ◦ C for 24 h. The inset shows higher magnification image of single metallic nickel nanocrystal. (B) Low- and (C) high-magnification TEM images of product obtained without CTAB at 140 ◦ C for 24 h. The inset of (B) shows typical SEM image of the as-prepared product at 140 ◦ C. TEM (D, E and F) images of product obtained without CTAB at 180 ◦ C for 24 h. The inset of (E) shows SEM image of the corresponding product. The inset of (F) shows the SAED pattern of an individual nanorod.

of precipitation dissolution. Then N2 H4 can reduce Ni2+ in nickel monomer. The nickel monomers eventually form sea urchin-like nickel nanocrycrystals through diffusion-limited aggregation (DLA) mechanism [27]. The sea urchin-like nanorod-based metallic nickel nanocrystals obtained without CTAB is consequence of the interaction between the stochastic diffusive force and directive force. Firstly, with the elevation of the temperature, the products nucleate and grow quickly due to monomer concentration continuously increases. Then because of relatively strong stochastic diffusive force, the metallic nickel monomers may form spot structure on the surface of nanocrystals, and directive force makes the spot structure crystallize at certain direction and form the nanorods. Fig. 6 can provide importance evidences for formation process of sea urchin-like nanorod-based nanocrystals. When CTAB is added to the reaction system, the product is sea urchin-like nanobelt-based or nanosheet-based metallic nickel nanocrystals. We think that the layer-structure which may form due to the interaction of surfactant and ion in the formation of nanobelt or nanosheet. In reaction system, CTA-Ni(OH)4 ion pair is obtained through the interaction of CTA+ cationic formed due to CTAB addition and Ni(OH)4 2− anionic formed by Ni(OH)2 in the alkaline condition, and fabricates well order layer-like structure by Van der Waals force interactions [28,29].

Surfactants in layer structure gradually desorbe and nanobelt or nanosheet finally form through further process under hydrothermal condition, then some broad nanobelts or nanosheets roll into nanorods at elevated temperature or for longer reaction time. The formation process of final products is as follow: firstly, the nucleation and growth occur and nanocrystals form when the Ni monomer concentration comes up to a certain degree. Secondly continuous nickel monomers may nucleate and form spot structure on the surface of nanocrystals under the function of stochastic diffusive force. Then radial-aligned growing nanobelt and nanosheet formed by the function of the directive force and CTAB, finally nanobelts or nanosheets curl into nanorods with radial-aligned mode at elevated temperature or for longer reaction time. 5. Conclusion In summary, we have successfully synthesized the sea urchin-like nanobelt-based and nanorod-based nanocrystals via a hydrothermal reduction route in which sodium hydroxide was used as alkaline reagent and N2 H4 ·H2 O was used as reducing agent. The experimental results demonstrated that it is possible to control the morphology and structure of sea urchin-like nickel nanocrystals by adjusting process parameters such as

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hydrothermal time, reaction temperature and alkaline concentration. CTAB was also an important parameter influencing the morphology of the products. It is worthy to note that the research for sea urchin-like nanobelt-based and nanorod-based metallic nickel favor not only the research for crystal growth, but also the functional applications in various fields.

[8] [9] [10] [11] [12]

[13]

Acknowledgments

[14]

Financial support of this work by National Natural Science Foundation of China (grant no. 50504017, 50374076) and Hunan Provincial Natural Science Foundation of China (grant no. 05JJ30104) is gratefully acknowledged.

[15]

Appendix A. Supplementary data

[18]

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.mseb.2006.04.024.

[19] [20] [21] [22]

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