CRYSTAL GROWTH & DESIGN
Hierarchical β-Ni(OH)2 and NiO Carnations Assembled from Nanosheet Building Blocks Li-Xia Yang, Ying-Jie Zhu,* Hua Tong, Zhen-Hua Liang, and Wei-Wei Wang State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China, and Graduate School of Chinese Academy of Sciences, P. R. China
2007 VOL. 7, NO. 12 2716–2719
ReceiVed August 5, 2006; ReVised Manuscript ReceiVed June 20, 2007
ABSTRACT: β-Ni(OH)2 carnationlike structures assembled from nanosheet building blocks were successfully synthesized via a hydrothermal method through the hydrolysis of nickel acetate in mixed solvents of water and glycerol. NiO carnations assembled from nanosheets with nanopores were obtained by thermal decomposition of the as-synthesized carnationlike β-Ni(OH)2 at 400 °C for 2 h in air. The as-obtained β-Ni(OH)2 carnations showed a different cyclic voltammogram (CV) curve from that of β-Ni(OH)2 nanosheets. Introduction The development of nanoscience and nanotechnology has extended from the stage of individual nanocomponent to that of nanostructured systems assembled from nanosize building blocks. Two approaches to the fabrication of nanostructured systems have been established, namely, bottom-up and top-down strategies. The assembly of nanosize building blocks appears as a bottom-up alternative for the fabrication of small devices in microelectronics and for the magnetic-storage industry. For this reason, strategies have been developed for the long-range organization and assembly of nanostructured phases. Conventional self-assembly involving preprepared components can be controlled by proper design of the component, thus self-assembly always combines chemical and microfabrication methods to produce externally patterned materials, associating spontaneous processes like solvent evaporation, or molecular cross-linking to control the deposition of nanostructure-based superlattices.1–6 Recently, Cölfen et al.7–9 expatiated a concept of mesocrystal, a high-order organization by mesoscale self-assembly and transformation of hybrid nanostructures, which took place through spontaneous chemical assembly and transformation of building blocks across multiple length scales. This process proceeded through a modular nanobuilding block route aggregation-mediated growth mechanism, a challenge to the normal atom-by-atom addition to an existing nucleus.7–9 Many examples of aggregation-based crystallization have been reported, such as monodisperse ZnS spheres constructured by fibrillous structures,10 monodisperse spherical polycrystalline CdS particles,11 the three-dimensionally oriented aggregation of CuO nanoparticles into monocrystalline architectures,12 CaCO3 aggregates with characteristic pseudo-octahedral morphology,13 and nanostructured CoC2O4 particles.14 Recently, we have synthesized flowerlike β-Co(OH)2 consisting of nanosheet networks by a hydrothermal route.15 Nickel hydroxide (Ni(OH)2) is an active material in the positive electrode of alkaline rechargeable batteries, the performance of such batteries depends on the size, morphology, and phase of Ni(OH)2.16–18 The preparation of single-crystalline β-Ni(OH)2 nanosheets has been demonstrated taking advantage of its intrinsic lamellar structure.19–21 One-dimensional β-Ni(OH)2 nanorods and nanotubes have also been reported * To whom correspondence should be addressed. E-mail: y.j.zhu@ mail.sic.ac.cn.
recently.17,18,22 However, only a few reports involved the formation of β-Ni(OH)2 hierarchical structures, such as hollow spheres23,24 and stacks of pancakes.25 The search for simple, high-yield, and environmentally benign methods to synthesize hierarchical structures is still an on-going process. Herein, we report a simple template- and surfactant-free hydrothermal approach for the synthesis of carnationlike hierarchical β-Ni(OH)2 structures assembled from nanosheet building blocks. Furthermore, cubic nickel oxide (NiO) carnations with nanopores have also been successfully synthesized by the thermal decomposition of the as-synthesized carnationlike β-Ni(OH)2 in air. Experimental Section All of the reagents were of analytical grade and purchased and used without further purification. In a typical experiment for the synthesis of carnationlike β-Ni(OH)2, 0.208 g of Ni(CH3COO)2 · 4H2O was dissolved in the mixed solvents of 23 mL of deionized water and 1 mL of glycerol under magnetic stirring to form a homogeneous solution at room temperature. The solution was then transferred into a Teflonlined stainless steel autoclave (40 mL), sealed, and heated at 200 °C for 3.5 h. The greenish product was collected by centrifugation, washed with deionized water and ethanol, and dried at 60 °C in air. The asobtained product was characterized by X-ray powder diffraction (XRD) collected on a Rigaku D/max 2550 V X-ray diffractometer with highintensity Cu KR radiation (λ ) 1.54178 Å) and a graphite monochromator, field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F), transmission electron microscopy (TEM), and selectedarea electron diffraction (SAED) on a JEOL JEM-2100F field-emission transmission electron microscope. The cyclic voltammetric studies were performed in a three-compartment electrolysis cell at 25 °C using platinum powder microelectrode with a diameter of 100 µm as the working electrode.
Results and Discussion The phase of the product was investigated by XRD. The product prepared by the above method consisted of a single phase of β-Ni(OH)2 with a hexagonal structure (Figure 1a, JCPDS 74-2075). Cubic NiO formed by the thermal transformation of β-Ni(OH)2 carnations as the precursor at 400 °C for 2 h in air (Figure 1b, JCPDS 78-0643). No peaks from β-Ni(OH)2 were observed, indicating that β-Ni(OH)2 was completely transformed to NiO after heated at 400 °C for 2 h. The morphology of the as-synthesized β-Ni(OH)2 was investigated by SEM. The SEM micrographs (Figure 2) show
10.1021/cg060530s CCC: $37.00 2007 American Chemical Society Published on Web 10/18/2007
β-Ni(OH)2 and NiO Carnations
Crystal Growth & Design, Vol. 7, No. 12, 2007 2717
Figure 1. XRD patterns of (a) β-Ni(OH)2 and (b) NiO. Figure 3. (a) TEM micrograph of a single β-Ni(OH)2 carnation, the inset of (a) is the corresponding SAED pattern; (b) enlarged image of the portion labeled by a square in (a); (c) HRTEM image.
Figure 2. SEM micrographs of β-Ni(OH)2 carnations showing multilayered highly ordered texture and a hole at the center.
that the β-Ni(OH)2 product consisted of carnationlike hierarchical structures assembled from nanosheet building blocks. A large number of the β-Ni(OH)2 flower-mimetic structures could be produced by this simple one-step method. The carnations had sizes in the range of 800 nm to 3 µm. In a few cases, the crescent-shaped β-Ni(OH)2 was observed as a minor product. A high-magnification SEM micrograph (Figure 2b) shows that the carnation comprised densely packed uniform nanosheets with a thickness of about 30 nm. The β-Ni(OH)2 carnation had a multilayered highly ordered texture and a hole at the center. They could not be broken up into individual nanosheets when submitted to ultrasonication. β-Ni(OH)2 carnations were much more stable than the reported doughnut-shaped micrometer-sized aragonite particles produced by the surfactant-mediated transformation and crystallization of the amorphous calcium carbonate precursor nanoparticles.26 It was reported that van der Waals’ interactions were too weak to stabilize the superstructures,27 and it is believed that the strong chemical bonding is the integrating force between the contacting surfaces formed during growth.28
TEM was employed for the crystal structure investigation of β-Ni(OH)2 carnation-like structures. Figure 3a shows a typical TEM micrograph of a single carnation, which reveals that the thickness at the center of the carnation was much thinner than that at other sites, consistent with the morphology observed in SEM (Figure 2b). An enlarged micrograph of the portion indicated by a square in Figure 3a is shown in Figure 3b, indicating a configuration of overlapped nanosheets. The SAED pattern of this single carnation (inset of Figure 3a) exhibits a high degree of crystallization and orientation and a hexagonal symmetry, indicating that the surface of the nanosheet was the (00–1) plane of β-Ni(OH)2, and the thickness direction of β-Ni(OH)2 was along the [00–1] direction (c-axis of hexagonal β-Ni(OH)2). High-resolution TEM (HRTEM) was performed on the thin edges of a single carnation; the single-crystalline structure of a single nanosheet was observed with the d spacing of 0.236 nm, corresponding to the d spacing between (011) planes of a hexagonal β-Ni(OH)2. Both the volume ratio of glycerol to water (keeping the total volume constant at 24 mL) and the Ni(CH3COO)2 concentration were crucial to the formation of β-Ni(OH)2 carnations. The sole use of water as the solvent yielded only irregular nanosheets through the hydrolysis of Ni(CH3COO)2 (see the Supporting Information, Figure S1). Ni(OH)2 carnations were not observed in the absence of glycerol. The volume ratio of glycerol to H2O was an important factor for the formation of β-Ni(OH)2 carnations. No product was obtained if the glycerol/H2O volume ratio was higher than 1:5 via hydrothermal treatment at 200 °C, even for a prolonged time (10.5 h). Mixed morphologies such as nanosheets, crescents, and carnations were obtained at a glycerol/water volume ratio of 1:11 (see the Supporting Information, Figure S2a). As discussed above, β-Ni(OH)2 carnations were obtained at the glycerol/H2O volume ratio of 1:23. On the other hand, the concentration of nickel acetate is another important factor for the formation of β-Ni(OH)2 carnations (1:23 glycerol:H2O). When the concentration of nickel acetate was significantly increased, individual nanosheets formed as a major morphology and carnations formed as a minor morphology (see the Supporting Information, Figure S2b). Thus, the concentration of nickel acetate and the glycerol:H2O volume ratio are the two dominant parameters in the formation of β-Ni(OH)2 carnations, and these two parameters should be in a good coordination in order to get monodisperse carnations. By changing experimental parameters, the size of the β-Ni(OH)2 carnations was tunable. With the hydrothermal temperature
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Figure 5. Cyclic voltammograms of β-Ni(OH)2 samples. (a) β-Ni(OH)2 nanosheets, and (b) carnationlike β-Ni(OH)2. Figure 4. (a) SEM micrograph, (b) TEM micrograph, and (c) HRTEM image of NiO carnations.
varying between 120 and 200 °C, carnations with similar sizes could be obtained if the hydrothermal time was sufficiently long. From the thermodynamics point of view, the surface energy of an individual nanosheet is quite high with two main exposed planes, and thus they tend to aggregate perpendicularly to the surface planes to decrease the surface energy by reducing exposed areas. In this process, glycerol plays a key role. Glycerol can form the complex with Ni2+ in the basic condition, which decreases the free Ni2+ ion concentration and reduces the rate of crystal growth. In addition, glycerol changes the characteristic of the solution to be stable and viscous, which is favorable for the aggregation growth of the nanosheets. On the basis of the information we have gathered, we suggest a possible aggregation-mediated growth mechanism of β-Ni(OH)2 carnations that is similar to the mesocrystal growth mechanism proposed by Cölfen et al.7–9 There is a hydrolysis equilibrium of acetate anions in water, which provides OH- ions for the formation of Ni(OH)2 nanosheets. TEM studies indicated that the intermediate nanosheets and their aggregates formed at the early stage (200 °C for 1 h) (see the Supporting Information, Figure S3). The thin nanosheets seemed to have a trend of curling because of the unstable large surface area (see the Supporting Information, Figure S3d). A similar process was reported for the formation of zeolite.29 Because the obtained carnations showed the oriented electron diffraction pattern, we suggest that the oriented attachment mechanism takes effect in this process.30–32 The surface energy is substantially reduced when the neighboring nanosheets are self-oriented and assembled. As a result, carnationlike hierarchical β-Ni(OH)2 structures assembled from nanosheet building blocks are constructed by this mesoscale assembly. It is interesting that the carnationlike β-Ni(OH)2 morphology was successfully sustained during the thermal transformation to NiO and that NiO carnations assembled from nanosheets were obtained (see the Supporting Information, Figure S4). A single NiO carnation is shown in Figure 4a. Nanopores with lateral dimensions in the range of 4–20 nm were observed on the nanosheets of NiO carnations (images b and c in Figure 4). The HRTEM image recorded at the edge of a single carnation indicates the lattice fringes of 0.240 nm, which agrees well with the d spacing of the (111) plane of a cubic NiO. Figure 5 shows the typical cyclic voltammograms (CVs) of the β-Ni(OH)2 nanosheets (see the Supporting Information, Figure S1) and carnationlike morphology (Figure 2) recorded at a scan rate of 10 mV/s in 6 M KOH solution vs the Hg/HgO reference electrode. The important feature on the CVs of Figure
5a (β-Ni(OH)2 nanosheets) is that there were two partially overlapped oxidation current peaks located at the potentials of about 469 and 525 mV and one reduction current peak at the potential of about 352 mV. In contrast, one can see from Figure 5b that only one couple of oxidation/reduction peaks of carnationlike β-Ni(OH)2 on the CVs, located at the potentials of about 496 and 327 mV, respectively, show a curve similar to that of commercial micrometer-sized spherical β-Ni(OH)2 (see the Supporting Information, Figure S5). The first oxidation peak for the nanosheet sample may be explained as follows: β-Ni(OH)2 was overoxidized to form γ-NiOOH by intensive oxygen evolution on the anodic polarization, and γ-NiOOH was reduced to R-Ni(OH)2 by the cathodic polarization, which was unstable and transformed to β-Ni(OH)2 in the strong alkaline medium at the reverse scan.16,33 The formation of γ-NiOOH resulted in the volume expansion of the electrode, which affected the effective contact between particles of active materials and thus increased the resistance of the electrode reaction and led to faster capacity decay.34 However, when these nanosheets were organized to form a carnationlike morphology, the formation of γ-NiOOH could be restrained, preventing the swelling of the electrode, which was favorable for better utilization of the active materials. This phenomenon showed that when the nanosheets were assembled into carnations, the cyclic voltammogram of the carnation electrode no longer presented the pattern of the nanosheet one. In addition, it can be seen that the spherical electrode shows a larger value of EO – ER and a smaller value of EOE – EO, illustrating that the reaction reversibility of carnations is better than that of the spherical particles. This may be related to the enhancement of surface reactivity with the increased surface area, as there were interspaces between the neighboring nanosheets in the carnation. However, the electrochemical properties of these interesting hierarchical β-Ni(OH)2 structures should be further investigated. Conclusion In conclusion, we have successfully synthesized novel flowermimetic hierarchical superstructures of β-Ni(OH)2 assembled from nanosheets through the hydrolysis of nickel acetate in mixed solvents of water and glycerol by a hydrothermal method. The method has advantages such as being simple and templateand surfactant-free. The volume ratio of glycerol to water and the Ni(CH3COO)2 concentration are crucial to the formation of β-Ni(OH)2 carnations. Cubic NiO carnations assembled from nanosheets with nanopores have also been synthesized by the thermal decomposition of the as-synthesized carnationlike β-Ni(OH)2 in air. The carnationlike morphology can be wellsustained during the transformation process from β-Ni(OH)2 to
β-Ni(OH)2 and NiO Carnations
NiO. The understanding of this carnationlike morphology will be helpful in controlling the aggregation-mediated crystallization of ordered superstructures and shed new insight into the morphology–property relationship. Acknowledgment. Financial support from the National Natural Science Foundation of China (50472014) and Chinese Academy of Sciences under the Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) is gratefully acknowledged. We also thank Professor Meiling Ruan for assistance in the TEM experiments, Dr. Xian-Zhu Fu and Professor Dai-Wei Liao from State Key Laboratory of Physical Chemistry on Solid Surfaces, Department of Chemistry, Institute of Physical Chemistry, Xiamen University, P. R. China for their assistance in the cyclic voltammetirc measurement and helpful discussion. Supporting Information Available: TEM and SEM images of different products and cyclic voltammogram of micrometer-sized spherical β-Ni(OH)2 sample (in Word format). This material is available free of charge via the Internet at http://pubs.acs.org.
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