Self-Assembled Nickel Hydroxide Three-Dimensional Nanostructures

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CRYSTAL GROWTH & DESIGN

Self-Assembled Nickel Hydroxide Three-Dimensional Nanostructures: A Nanomaterial for Alkaline Rechargeable Batteries

2007 VOL. 7, NO. 1 170-174

Minhua Cao,*,† Xiaoyan He,† Jun Chen,§ and Changwen Hu*,†,‡ Institute of Polyoxometalate Chemistry, Northeast Normal UniVersity, Changchun 130024, P. R. China, Department of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China, and Institute of New Energy Material Chemistry, Nankai UniVersity, Tianjin 300071, P. R. China ReceiVed August 3, 2006; ReVised Manuscript ReceiVed October 30, 2006

ABSTRACT: R- and β-Ni(OH)2 with three-dimensional (3D) nanostructures were synthesized in a water-in-oil reverse micelle/ microemulsion system. The form and shape of Ni(OH)2 can be readily tuned by adjusting experimental parameters of the reverse micelle/microemulsion system. R-Ni(OH)2 with dandelion-like nanostructures was obtained in the reverse microemulsion of cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-pentanol with a molar ratio of H2O to surfactant of 40; β-Ni(OH)2 phase with flower-like nanostructures was formed in the reverse micelles of the same composition with a molar ratio of H2O to surfactant of 10. Electrochemical measurements of the as-synthesized R-Ni(OH)2 phase showed that the 3D R-Ni(OH)2 nanostructures exhibited superior cycling reversibility and improved capacity compared with commercial Ni(OH)2. Introduction Architecturing of highly ordered micro- and nanostructures has attracted considerable interest in areas of materials science.1-7 The most promising and now extensively used method for achieving novel structures is self-assembly processes. Recently, much effort has been focused on the self-assembly of lowdimensional building blocks into two- and three-dimensional (2D and 3D) hierarchical structures.1-7 To date, many selfassembly processes driven by chemical or physical principles have been developed for organizing curved structures, such as “dandelion” formations via a modified Kirkendall effect,8 triangular and Fibonacci number patterns driven by stress on core/shell microstructures,9 and colloidosomes formation using emulsion droplets as templates.10 However, although major advances have been made in “building” the curved structures, developing facile and simple methods is still highly desired for fully understanding and exploiting the self-assembly process. Recently, nanostructured electrode materials have received extensive interest due to their potential in improving the performance of batteries.11,12 It has been documented that the overall activity of a battery depends on not only the microstructure but also the crystallite size and shape of the active material.13 Hence, control over the dimension of the active battery material is very important for manipulating its electrochemical behavior. Nickel hydroxide has been used as the active material of the positive electrode in many alkaline rechargeable batteries, such as Ni/Cd, Ni/H2, Ni/MH, Ni/Fe, and Ni/Zn.14 It has been shown that nickel hydroxide exists in two common forms, R and β, and R-Ni(OH)2 generally exhibits superior electrochemical properties compared with the β-form.15 As inspired by both the potential applications of nickel hydroxide and the novel properties of nanoscale materials, considerable effort recently has been focused on the synthesis of nickel hydroxide nanostructures with different morphologies, such as * To whom correspondence should [email protected]; [email protected]. † Northeast Normal University. ‡ Beijing Institute of Technology. § Nankai University.

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nanotubes, nanorods, nanosheets, nanoribbons, and mesoporous films.11a,16-21 However, among the methods previously reported on the synthesis of nickel hydroxide, to the best of our knowledge, most were based upon the homogeneous precipitation by employing urea, and no work has been reported on the synthesis of layered nickel hydroxide in a water-in-oil system. In this paper, R- and β-Ni(OH)2 with 3D structure were successfully synthesized by a reverse micelle/microemulsion method.22 The form and shape of Ni(OH)2 can be readily tuned by adjusting experimental parameters of the reverse micelle/ microemulsion system. The electrochemical performance of the as-synthesized R-Ni(OH)2 nanostructures was also studied because of its well-known electrochemical properties. It was found that these 3D R-Ni(OH)2 nanostructures exhibited a superior cycling reversibility and improved capacity compared with commercial Ni(OH)2. In addition, the nickel hydroxide product can be calcined into NiO without changing its shape. Experimental Section Preparation of Nickel Hydroxide 3D Nanostructures. All of the reagents used were of analytical purity and used without further purification. Cetyltrimethylammonium bromide (CTAB) and urea were purchased from Beijing Beihua Fine Chemicals Limited Corporation of China. Cyclohexane and n-pentanol were purchased from Tianjin Chemical Reagent Factory of China. Ni(NO3)2‚2H2O was purchased from Shenyang Chemical Reagents Factory of China. In a typical synthesis, two separate solutions with the same volume and composition were prepared by dissolving 1 g of CTAB and 1.5 mL of n-pentanol into 25 mL of cyclohexane. A total of 2 mL of 0.5 M Ni(NO3)2‚2H2O aqueous solution and 2 mL of 0.5 M urea aqueous solution were added dropwise to the above solutions, respectively, under vigorous stirring. After vigorous stirring of the sample, two optically transparent microemulsion solutions were formed. Then, the two solutions were mixed and stirred for another 30 min. Afterward, the newly formed solution was transferred into an 80 mL stainless Teflonlined autoclave and heated at 140 °C for 12 h. The resulting precipitate was collected and washed several times with absolute ethanol and distilled water. Finally, R-Ni(OH)2 nanostructures were obtained by centrifugation and drying in air atmosphere at room temperature. For the preparation of β-Ni(OH)2 nanostructures, 0.5 mL of 0.5 M of Ni(NO3)2‚2H2O aqueous solution and 0.5 mL of 0.5 M urea aqueous solution should be used.

10.1021/cg060524w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006

Nickel Hydroxide 3D Nanostructures Characterization. X-ray diffraction (XRD) patterns were obtained with a Rigaku X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å). Field emission scanning electron microscopy (FE-SEM) images were obtained on a field emission microscope (JEOL, 7500B) operated at an acceleration voltage of 200 kV. Transmission electron microscopy (TEM) images were obtained on a Hitachi H-800 transmission electron microscope. The elemental analysis was conducted on a PerkinElemental 2400 elemental analyzer. Inductively coupled plasma (ICP) analysis was performed on a Perkin-Elmer Optima 3300Dv spectrometer. Thermogravimetric analysis (TGA) was carried out on a PerkinElmer TGA 7 unit in the air with a heating rate of 10 °C/min. The Fourier transform infrared (FTIR) spectrum was obtained within the 4000-400 cm-1 wavenumber range using a Perkin-Elmer model 580B IR spectrophotometer with the KBr pellet technique. Electrochemical Measurements. Electrochemical measurements were carried out as described elsewhere.11a Nickel hydroxide electrodes were prepared by inserting an active paste into a nickel foam substrate. A paste containing 90 wt % nickel hydroxide nanostructures or spherical powder (Tanaka Chemical, Japan), 5 wt % CoOOH, and 5 wt % polytetrafluoroethylene (PTFE) was used. The electrode was dried at 80 °C for 1 h and cut into a disk (1.2 × 1.2 cm), which was pressed at a pressure of 100 kg cm-2 to a thickness of 0.4 mm. Then, the electrode was spot-welded to a nickel sheet for electrical connection. Electrochemical performance was measured with a Solartron SI 1260 Potentionstat analyzer with 1287 Interface and an Arbin charge/ discharge unit at controlled temperatures in an electrochemical cell, which contained the nickel hydroxide working electrode, a metal hydride electrode, a Hg/HgO reference electrode, and 6 M KOH solution as the electrolyte. The discharge capacity of the nickel hydroxide in the positive electrode was based on the amount of active material (Ni(OH)2) excluding the weight of CoOOH and PTFE in the electrode. The discharge capacity of each electrode was expressed in mA h per gram of active material.

Crystal Growth & Design, Vol. 7, No. 1, 2007 171

Figure 1. SEM images of a R-Ni(OH)2 sample obtained with the concentration of Ni(NO3)2‚2H2O aqueous solution at 0.5 M and the molar ratio of H2O to CTAB at 40: (a) overview of sample; (b) a single dandelion-like R-Ni(OH)2 microsphere; (c) detailed sphere structure assembled by nanowires; and (d) R-Ni(OH)2 nanospheres.

Results and Discussion The nickel hydroxide 3D nanostructures were fabricated by the reverse micelle/ microemulsion system, CTAB/water/ cyclohexane/n-pentanol. Using this reverse micelle/microemulsion system, nanocrystals, nanorods, and nanowires have been successfully prepared by our group.22,23 Here we describe in detail that this system can be extended to synthesize 3D nanostructures of layered nickel hydroxide. When an Ni(NO3)2‚ 2H2O aqueous solution (0.5 M) was used and the molar ratio of H2O to CTAB (defined as w) was kept at 40, R-Ni(OH)2 nanostructures were formed. The product is mainly composed of dandelion-like microspheres with diameters of 3-6 µm as shown in the low-magnification scanning electron microscopy (SEM) images (Figure 1a and Figure S1a of Supporting Information). The high-magnification SEM image in Figure 1b clearly demonstrates the shape of a single dandelion-like microsphere. As shown in the higher-magnification SEM image in Figure 1c, the 3D dandelion-like microsphere consists of nanowires with widths ranging from 20 to 30 nm. The SEM images also reveal that some dandelion-like microspheres selforganized into a rope- or dendritic-like structure (Figure S1b,c of Supporting Information), and some randomly aggregated together (Figure S1d of Supporting Information). In addition, a small number of nanospheres with diameters of 400-500 nm can also be observed in Figure 1a. The high-magnification SEM image in Figure 1d clearly reveals the surface structure of these nanospheres. In fact, these nanospheres consisted of many intercrossed nanoplatelets with thicknesses of about 20-30 nm. The phase of the as-synthesized sample was determined by X-ray powder diffraction (XRD). As shown in Figure 2a, the peaks at d ) 7.31, 3.64, 2.69, and 1.55 Å could be assigned to (003), (006), (101), and (110) planes, indicating the formation of a R-Ni(OH)2 layered structure.21 To reveal the stoichiometry of the synthesized R-Ni(OH)2 nanostructures, elemental analysis

Figure 2. X-ray diffraction patterns of (a) R-Ni(OH)2, (b) a mixture of R-Ni(OH)2 and β-Ni(OH)2, (c) β-Ni(OH)2 (0: R-Ni(OH)2; 1: β-Ni(OH)2), and (d) NiO.

and thermal gravimetric measurements were carried out. Elemental analysis gives 63.20% of Ni, 3.52% C, 2.02% H, and 2.45% N. The formula of the synthesized nickel hydroxide was determined to be [Ni(OH)1.38](CO3)0.21(OCN)0.12(NO3)0.08(H2O)0.44 based on elemental and thermal gravimetric analyses (TGA, see Figure S2 of Supporting Information). Furthermore, the Fourier transform infrared spectroscopy (FTIR, see Figure S3 of Supporting Information) also further proved this composition. Transmission electron microscopy (TEM) images (Figure 3a) obtained from the same R-Ni(OH)2 sample further confirmed that the dandelion-like mirosphere was composed of nanowires. The high-magnification TEM image in Figure 3b clearly showed that the nanowires were about 30-50 nm in diameter. The highresolution TEM (HRTEM) and electron diffraction pattern shown in Figure 3c indicated that the nanowire was polycrystalline, which was composed of nanoclusters. When the R-Ni(OH)2 sample was calcined in air by applying a heating rate of

172 Crystal Growth & Design, Vol. 7, No. 1, 2007

Figure 3. (a) Low-magnification TEM image of R-Ni(OH)2, (b) highmagnification TEM image of R-Ni(OH)2, (c) high-resolution TEM image of nanowires (the inset is the electron diffraction image), and (d) TEM image of NiO.

4 °C/min and holding the calcinations temperature of 350 °C for 5 h, pure NiO was obtained, as confirmed by XRD (Figure 2d) and TGA (Figure S2 of Supporting Information). All reflections can be assigned to cubic NiO (JCPDS [78-0429]). The TEM image of NiO sample is shown in Figure 3d. It can be seen that the NiO sample still preserved the shape of R-Ni(OH)2 precursor. When the w value was decreased to 20 but keeping other reaction conditions constant, a mixed phase of R-Ni(OH)2 and β-Ni(OH)2 was obtained, as proven by the XRD pattern (Figure 2b). If the w value was decreased to 10, β-Ni(OH)2 was formed. Figure 2c displays the XRD pattern of the synthesized sample. Only a very weak peak of R-Ni(OH)2 has been observed in Figure 2c, and the main peaks can be indexed to be the hexagonal β-Ni(OH)2 phase (JCPDS [73-1520]), suggesting the conversion between R- and β-form under current experimental conditions. Figure 4 shows SEM images of the β-Ni(OH)2 sample at different magnifications. Figure 4a is an overall view of the product. It can be seen that the product consisted of microflowers and microspheres. The diameter of the microflower is approximately 5 µm. The microsphere in shape is same as that of R-Ni(OH)2, but its size is larger than that of R-Ni(OH)2 nanospheres, about 1 µm in diameter. To reveal the actual structure of the microflowers and microspheres, high magnification SEM images were recorded. Figure 4d clearly shows the surface structure of a microsphere, from which it can be seen that lots of nanoplatelets intercrossed with each other formed microspheres by self-assembly, same as that of R-Ni(OH)2. The edge thickness of these nanoplatelets is about 30 nm. More detailed morphology structure of the microflowers is shown in Figure 4b,c. Figure 4, panels b and c, are over and side views of a single microflower, respectively, clearly revealing that the microflower is composed of a large number of nanopetals in a highly close-packed assembly. The thickness of these nanopetals is about 100 nm. The reverse micelle/microemulsion system has been extensively used for preparing well-defined inorganic micro- or nanostructures. A reverse micelle or microemulsion is a transparent and isotropic liquid medium with nanosized water pools dispersed in a continuous oil phase and stabilized by surfactants. Studies have shown that this system exhibits distinct

Cao et al.

Figure 4. SEM images of β-Ni(OH)2 synthesized with the concentration of Ni(NO3)2‚2H2O aqueous solution of 0.5 M and the molar ratio of H2O to CTAB of 10 at 140 °C. (a) Low-magnification, (b) individual flower-like structure, (c) side face of the individual flower-like structure, and (d) detailed microsphere structures of β-Ni(OH)2.

advantages over other methods on the synthesis of nanomaterials. This is because the microreactors formed in this system provide a unique space for the proceeding reaction. It has been demonstrated that the behavior of a reverse micelle/microemulsion is mainly determined by the w value, i.e., the size of the water pools. It has been generally defined that the aggregates with w values less than 15 are taken as reverse micelles, whereas systems with the value above are named as reverse microemulsions.24 On the basis of this definition, it can be inferred that R-Ni(OH)2 nanostructures (obtained at w ) 40) in the present study were formed in reverse microemulsions, and β-Ni(OH)2 with flower-like nanostructure (obtained at w ) 10) were formed in reverse micelles of the same composition. The formation mechanism of Ni(OH)2 with different forms and shapes is similar to that of layered double hydroxides obtained in a reverse microemulsion reported by Hu et al.25 In their experiment, a triblock copolymer-modified reverse microemulsion system was used, in which the addition of the copolymer in the reverse microemulsion system can significantly change the interior environment and the structure of the micellar aggregates. In our case, the solvothermal treatment of the reverse micelle/microemulsion reaction may serve as the same function as the copolymer mentioned above, i.e., modifying the microstructure of the micellar aggregates. To further investigate the growth mechanism, we studied the effect of reaction time and temperature on the morphology of R-Ni(OH)2 product. Figure 5 shows the TEM images of R-Ni(OH)2 nanostructures synthesized at 140 °C but with reaction times of 3, 6, and 9 h, respectively. It can be clearly seen that when the shorter reaction time (3 h) was used, the flower-like nanostructures were obtained (Figure 5a). If the reaction time was increased to 6 h, the petals of the flower-like nanostructures have partly changed to nanowires (Figure 5b). When the reaction time was prolonged to 9 h, as shown in Figure 5c, the flowerlike nanostructures have completely changed to dandelion-like microspheres. If the same reaction was carried out at 80 °C, only flower-like nanostructures, not the nanowire nanostructures, were obtained (Figure 5d). Therefore, it can be concluded that for the formation of the R-Ni(OH)2 nanostructures, the higher temperature under the solvothermal conditions in connection

Nickel Hydroxide 3D Nanostructures

Crystal Growth & Design, Vol. 7, No. 1, 2007 173

Figure 6. Discharge curves as a function of capacity for R-Ni(OH)2 microstructures (solids) and spherical-powder (dots) electrodes at 20 °C. Figure 5. TEM images of R-Ni(OH)2 synthesized with the concentration of Ni(NO3)2‚2H2O aqueous solution of 0.5 M and the molar ratio of H2O to CTAB of 40 at 140 °C with different reaction times: (a) 3 h; (b) 6 h; and (c) 9 h. (d) TEM image of R-Ni(OH)2 synthesized with the concentration of Ni(NO3)2‚2H2O aqueous solution of 0.5 M and the molar ratio of H2O to CTAB of 40 at 80 °C.

with the larger water pools may be the main factors, which result in the formation of dandelion-like R-Ni(OH)2 nanostructures. The solvothermal conditions may change the interior environment of the reverse microemulsions. During the reaction process, the fused water pools may gradually split into two types of water pools under solvothermal conditions: the smaller spheroidal ones and the larger nearly spherical hyper-branched reservoirs. Thus, the reaction within the spheroidal water pools resulted in the formation of randomly distributed R-Ni(OH)2 nanoplatelets, which then assembled into nanospheres, in agreement with the shape of the water pools, whereas that in the hyperbranched reservoirs was responsible for the growth of the dandelion-like microspheres. When the lower w value was used, the reaction conditions might have little effect on the shape of water pools due to their small size. So the reverse micelles may produce the smaller spheroidal and the ellipsoidal water pools. Similar to the above case, the smaller spheroidal water pools lead to the formation of microspheres, and the ellipsoidal ones result in the flower nanostructures. As for the different phases produced in the two cases, although the reason is not clear, we speculate it may be closely related to the different microstructures of the microemulsion system, resulting from the w value change. To evaluate the electrochemical properties of the synthesized R-Ni(OH)2 nanostructures, we chose commercial spherical Ni(OH)2 with a diameter of 500 nm as a reference, and batteries were assembled as follows: the as-synthesized R-Ni(OH)2 nanostructures or commercial spherical Ni(OH)2 was used as the working electrode, the opposite electrode was MH, and the reference electrode was Hg/HgO. To improve the conductivity of the working electrode, β-CoOOH nanospheres as additives (see Figure S4 of Supporting Information), same in shape as the synthesized Ni(OH)2 spheres, have been prepared by our group by treating the [Co(NH3)5H2O]Cl3 aqueous solution under ultrasonic conditions published elsewhere.26 The conductivity of β-CoOOH nanospheres has been tested to be 0.08 S/m. Figure 6 shows a comparison of the discharge curves of the as-prepared R-Ni(OH)2 (solids) and the spherical-powder (dots)

Figure 7. Cycle life of the electrodes made by the as-prepared R-Ni(OH)2 microstructures (solids) and spherical-powder (dots) at 20 °C.

electrodes. It is clear that the discharge curve of the R-Ni(OH)2 nanostructure electrode displays a higher discharge voltage and a longer plateau than that of the spherical-powder electrode, indicating that the R-Ni(OH)2 nanostructure electrode has a highoutput behavior. The highest discharge capacity for the 3D nanostructure electrode, achieved at 50 mA g-1 and 20 °C, is 268.57 mA h g-1, whereas it is 251.25 mA h g-1 for the spherical-powder electrode. Yang et al.18a recently reported the synthesis of R-Ni(OH)2, and its highest discharge capacity was as low as 78 mA h g-1. With respect to the low capacity, it is generally accepted that it may be caused by the structure transformation from R-Ni(OH)2 to the β-form and the capacity fading in a strong alkaline medium. In contrast, the higher discharge capacity of 268.57 mA h g-1, in our case, implies that the R-Ni(OH)2 nanostructures synthesized by our method have an excellent stability in a strong alkaline medium, which is further confirmed by the following cycling life study. Figure 7 shows the curves of discharge capacity versus cycle number for the electrodes made from R-Ni(OH)2 nanostructures and commercial spherical Ni(OH)2. It can be seen that the discharge capacity of the R-Ni(OH)2 nanostructure electrodes gradually decreased with increasing cycles, from the initial 268.57 to 255.46 mA h g-1 after 200 cycles. The rate of the capacity decay is almost constant, and the capacity decay was

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only about 5% of its initial capacity. While in case of commercial spherical Ni(OH)2, the rate of the capacity decay from the first to the 100 cycle, same as that for the R-Ni(OH)2 nanostructure electrodes, is much higher than that from the 100 cycle to the 200 cycle. It is clear that the present cycling decay of 3D R-Ni(OH)2 microstructures is markedly better than that of commercial spherical Ni(OH)2. Recently, studies on the correlations between morphology, structure of nickel hydroxide, and its electrochemical properties in the alkaline rechargeable battery have been reported.11a,18a In our case, it can be seen that the significantly improved cycling stability of the 3D R-Ni(OH)2 nanostructure electrode may be related to its unique morphology. The dandelion-like R-Ni(OH)2 nanostructures with interconnected nanowires are favorable in increasing the interface area between electrode and electrolyte and can therefore result in a higher diffusion rate and significantly faster electronic kinetics. Thus, the solid-state diffusion and intercalation/deintercalation processes are remarkably improved, leading to a better cycle stability. Conclusions In summary, Ni(OH)2 nanostructures have been synthesized in a water-in-oil system. Higher w value (reverse microemulsions) leads to R-Ni(OH)2 nanowire nanostructures, whereas lower (reverse micelles) generally favors the formation of the β-Ni(OH)2 phase with flower-like nanostructures. The R-Ni(OH)2 nanostructures are found to exhibit a superior cycling reversibility and improved capacity when they were used as positive electrode materials of alkaline rechargeable batteries. Acknowledgment. This work was supported by the Natural Science Foundation Council of China (NSFC) (Nos. 20271007, 20331010, 90406002, and 20401005), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (No. 20030007014), Jilin Distinguished Young Scholars Program, the Huo Yingdong Foundation, and analysis and test fund of Northeast Normal University. Supporting Information Available: SEM images of R-Ni(OH)2 sample obtained with Ni(NO3)2‚2H2O (Figure S1); TGA curve of the R-Ni(OH)2 sample (Figure S2); FTIR spectrum of the R-Ni(OH)2 sample (Figure S3); SEM image of CoOOH nanospheres (Figure S4). This material is available free of charge via the Internet at http:// pubs.acs.org.

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