Fabrication of Nickel Hydroxide Microtubes with Micro- and Nano-Scale Composite Structure and Improving Electrochemical Performance Feifei Tao,† Mingyun Guan,† Yiming Zhou,‡ Li Zhang,‡ Zheng Xu,*,† and Jun Chen§
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2157–2162
State Key Laboratory of Coordination Chemistry, Laboratory of Solid State Microstructures, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China, Department of Chemistry, Nanjing Normal UniVersity, Nanjing 210097, P. R. China, and Institute of New Energy Material Chemistry, Nankai UniVersity, Tianjin 300071, P. R. China ReceiVed December 10, 2007; ReVised Manuscript ReceiVed March 10, 2008
ABSTRACT: The β-Ni(OH)2 microtubes with hierarchical structure wall composed of interweaved nanolamella were successfully fabricated by the template-free wet-chemical approach. To the best of our knowledge, it is the first report on Ni(OH)2 microtubes with the micro- and nanoscale composite structure. This method is very facile and effective to prepare the tubular materials with high yield and uniform tube diameter. The intermediate product and final composite structures were well characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform IR (FTIR), and thermogravimetric analysis (TGA), and the formation mechanism was deeply studied. In comparison with β-Ni(OH)2 microrod and particle electrodes, β-Ni(OH)2 microtube electrode has the highest discharge capacity in alkaline battery, indicating the potential applications of the microtubes with the hierarchical structures in alkaline Ni-based batteries, supercapacitor, and catalytic areas. 1. Introduction Recently, the electrode materials with micro- and nanostructure have attracted extensive attention due to their potential in improving the performance of batteries.1–9 It has been documented that the overall capability of a battery depends on not only the structure but also the crystallite size and shape of the active material.10,11 As is known, the smaller crystalline size shows a higher proton diffusion coefficient,12 which is of benefit for improving the electrochemical performance. But the resistance between nanoparticles and between the nanoparticles and conductor will remarkably increase, making the electrode have an even worse performance. Therefore, the electrochemical performance of electrodes could be significantly improved only if nanoparticles as adjuvant were added to a micrometer-sized one in an appropriate proportion.13–15 It might be an better way to solve this issue that the electrochemically active particles on the microscale are composed of the hierarchical nanostructures. Ni(OH)2 is an important electrode material. The syntheses of Ni(OH)2 nanocrystals, including nanoparticles,16 singlecrystalline nanorods,17 nanosheets and nanoribbons,18,19 hollow microspheres,20,21 and carnation-like structure,22 have been reported. But there are few of the works on the fabrication of Ni(OH)2 nanotubes,1,23 especially self-assembled microtubes by nanolamella. Recently, Chen et al. 1 reported Ni(OH)2 tubes composed of nanoparticles prepared by a chemical deposition method within anodic alumina membrane, but it is quite difficult to scale up. Therefore, how to synthesize tubular Ni(OH)2 materials without template in large scale is still a big challenge to chemists, let alone the tubes with micro- and nanoscale composite structure, which could have better electrochemical performance. In this paper, we report a facile template-free wet-chemical method for the large-scale synthesis of β-Ni(OH)2 microtubes * To whom correspondence should be addressed. Fax: 86-25-83314502. E-mail:
[email protected]. † Nanjing University. ‡ Nanjing Normal University. § Nankai University.
with the hierarchical structure wall. The microtube wall is composed of interweaved nanolamella, which usually appears in hollow spherical structures24–26 and few in tubular ones.27,28 To the best of our knowledge, it is the first report on Ni(OH)2 microtubes with micro- and nanoscale composite structure. This method is very facile and effective for preparing the tubular materials with high yield and uniform tube diameter. On the basis of the results of the morphology evolution with time, the formation mechanism of microtubes is proposed. In our reaction system, nickel chloride is used as nickel source and ethylene glycol (EG) as the solvent. A green tubular intermediate complex (Ni-EG coordination polymer) was obtained first by the wet-chemical method. After treated by KOH solution, β-Ni(OH)2 microtubes were formed (see Experimental Section for the details). The electrode of Ni(OH)2 microtubes shows the better electrochemical performance than those of microrods and spherical particles in alkaline batteries. 2. Experimental Section NiCl2 · 6H2O (0.4754 g, 2 mmol) was dissolved in ethylene glycol (EG) (16 mL) to form a clear light green solution, and NaAc (1.44 g) and polyethylene glycol 200 (PEG200) (0.4 g) were then sequentially added into the above solution and stirred to get a clear light green solution. The solution was transferred to a Teflon-lined stainless-steel autoclave (22 mL capacity) and heated to 190 °C for 8 h, and then allowed to naturally cool to room temperature. The green products were collected, rinsed by ethanol five times, and then dried at 60 °C for 3 h, which was the intermediate complex of β-Ni(OH)2 microtubes. After the green product was immersed in a 6 M KOH solution for 10 h, β-Ni(OH)2, microtubes were obtained. The microrods could be fabricated in the reaction temperature range of 80–140 °C with other reaction conditions constant. The lower concentration of NaAc than 0.61 mol L-1 resulted in the formation of the spherical particles with other reaction conditions constant. The as-prepared samples were characterized by transmission electron microscopy (TEM, JEM-200CX TEM, 200 kV), scanning electron microscopy (SEM, JEOL JSM-5610 LV SEM, 20 kV), energydispersive spectrum (EDS, VAWTAGE from Thermo NORAN), X-ray diffraction (XRD, Shimadzu X-6000 X-ray diffractometer, Cu KR radiation), Fourier transform IR (FTIR, VECTOR 22 from BRUKER), and thermogravimetric analysis (TGA, LABSYS from SETERAM).
10.1021/cg7012123 CCC: $40.75 2008 American Chemical Society Published on Web 06/07/2008
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Figure 1. (a-c) SEM and (d, e) TEM images and (f) XRD pattern of Ni(OH)2 microtubes with the hierarchical structure wall composed of interweaved nanolamella. (b) and (c) are the magnification images of the top and the side views of the microtube in (a), respectively. (e) is the magnification image of the side view of (d). Nickel hydroxide electrodes were prepared by coating an active paste into a nickel foam substrate. A paste contained 80 wt % nickel hydroxide microtubes or microrods or spherical particles, 10 wt % carbon black, and 10 wt % polytetrafluoroethylene (PTFE). First, the coated electrode was dried in vacuum at 60 °C for 12 h. Second, the electrode was hot-pressed under a pressure of 20 MPa at 90 °C for 1 min, and dried again in a vacuum at 60 °C for 3 h. Before the electrochemical measurement, the nickel hydroxide electrode must be activated by immersing the electrode into 6 M KOH solution for 12 h. The electrochemical performance was measured on the DC-5 potentionstat analyzer from Shanghai Zhengfang corp. in an electrochemical cell, which contained the nickel hydroxide working electrode, a treated nickel foam opposite electrode, a Hg/HgO reference electrode, and a 6 M KOH solution as the electrolyte. The discharge capacity of the nickel hydroxide in the positive electrode was measured on the basis of the amount of active material (Ni(OH)2) excluding the weight of carbon black and PTFE in the electrode. The discharge capacity of each electrode was expressed in mA h per gram of active material.
3. Results and Discussion 3.1. Structure and Morphology. Figure 1 shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Ni(OH)2 microtubes. The length, wall thickness, and outer diameter of Ni(OH)2 microtubes are 10-40, 0.5-1.5, and 2-4 µm, respectively, and the aspect ratio 5-20. Both the small aspect ratio and open ends of microtubes as clearly seen from Figure 1 are an advantage for chemical species entering in and going out from the microtube channel and the space between nanolamella, and the hierarchical structural wall creates a large specific surface area (Figure 1c). These factors make it a good candidate for the electrode materials and catalysts. Figure 1a shows the SEM image of Ni(OH)2 microtubes in large area and there hardly has any impurity in the as-synthesized products indicating a high yield and purity. Images b and c in Figure 1 further display the hierarchical structural feature of the microtubes. TEM images d and e in Figure 1 further prove such structural features of the microtubes. The morphology of the green intermediate complex before the treatment of KOH solution is shown in images g and h in Figure 3 (the composition of the complex is shown later), which is almost same as the Ni(OH)2 microtubes in Figure 1. It
demonstrates that the treatment by alkaline solution has no any effect on the morphology of hollow structures and surface characters, except that the complex was completely converted into β-Ni(OH)2, which was proved by X-ray diffraction (XRD) pattern in Figure 1f. All recorded peaks can be indexed to hexagonal-phased β-Ni(OH)2 in good agreement with the data of JCPDS file No. 14-0117. No peaks from other phases are found, suggesting the intermediate complex completely converted to β-Ni(OH)2. 3.2. Growth Mechanism. To get an insight into the formation mechanism of Ni(OH)2 microtubes with the hierarchical structure wall, we only need to investigate that of the intermediate complex because the morphology has no change after treating with KOH solution. We found that the reaction temperature played a crucial role in the formation of the products. In the reaction temperature range of 80-140 °C (Figure 2a-d), only the microrods with smooth surface can be formed. The diameter of rods becomes thick with the increasing reaction temperatures. When the reaction temperature is higher than 140 °C, the microrods gradually dissolves, and their surfaces become rough, and furthermore, the small particles attach on the surface of rods (Figure 2e). As the dissolution process proceeding, at the higher temperature of 190 °C, the central part of microrods was emptied gradually and finally, the uniform microtubes were formed (images g and h in Figure 3). It is suggested that the dissolution-recrystallization process can proceed only at the temperature higher than 140 °C. Adjusting reaction temperatures can selectively control the formation of microtubes and microrods. Furthermore, time-dependent experiments were carried out at 190 °C and the intermediate complexes were inspected by SEM. The images were shown in Figure 3, from which the evolution process can be clearly seen. At the beginning of the reaction, the microrods were first formed after reacted for 15 min (images a and b in Figure 3). The surface of microrods with the diameter of 1-3 µm was smooth and nothing on it (Figure 3b). The SEM images of the product reacted for 0.5 h indicated that many spherical particles composed of nanolamella attached on the surface of microrods and that the ends of the microrods began to split into several even, thin nanowires (indicated by arrow in Figure 3d). This phenomenon was more
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Figure 2. SEM images of the precursors obtained at various reaction temperatures: (a) 80, (b) 100, (c) 120, (d) 140, and (e) 160 °C. The insets in a-e are the magnification images of corresponding photos.
Figure 3. SEM images of the precursor samples reacted at 190 °C for different reaction times: (a, b) 15 min, (c, d) 0.5 h, (e, f) 1 h, and (g, h) 8 h.
obvious after being reacted for 1 h (images e and f in Figure 3). The sphere-like particles composed of the nanolamella extended gradually until connecting each other to form the hierarchical structure wall. On the other hand, The nanowires became thinner and thinner, and finally all disappeared to form hollow structure (shown in the insets of Figure 3f at right top and left bottom). After the reaction for 8 h, the uniform microtubes with the hierarchical surface and hollow inner
structure were finally formed. XRD data in Figure 4 prove that the samples at the different reaction stages from microrods to microtubes had the same crystalline phase structures. The above experimental results clearly indicate that the formation of microtubes is a dissolution-recrystallization process from inner to outer of microrods at the cost of the microrods. In the ripening process, the microrods, which are thermodynamically less stable at temperatures above 140 °C, are converted to thermodynami-
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Figure 4. XRD patterns of the as-prepared intermediate complexes at the reaction time of (a) 15 min and (b) 8 h.
Figure 5. FTIR spectrum from KBr pellets containing the as-prepared intermediate complex at the reaction time of 15 min.
cally more stable spherelike structures composed of interweaved nanolamella. The latter prefers to nucleate and grow on the surface of microrods, which prevents further dissolution of the surface layer of microrods and promotes the transportation of the inner core to the outer surface and the formation of the microtubes with the hierarchical structure wall composed of nanolamella (shown in images g and h in Figure 3). The rodlike green intermediate complex at the reaction time of 15 min is composed of Ni-EG coordination polymer. As is known to all, ethylene glycol (EG) is a bidentate ligand and can react with metal ions to form the metal-glycolate coordination complex with the chainlike structure.29,30 They further coagulate each other to form a rodlike complex by Van der Waals interactions. The evidence comes from the XRD, Fourier transform IR (FTIR), and thermogravimetric analysis (TGA) experiments. A strong diffraction peak at around 10° in the XRD pattern (see Figure 4a) is the characteristic one of the coordination polymer composed of metal ions and EG.29,30 It is further proved by the FTIR spectrum of the rodlike products (see Figure 5), which shows the existence of the C-OH vibration band at 1059 cm-1 and the CH2 bands at 2868 and 2943 cm-1. The band at 1600-1650 cm-1 belongs to Ac-, which is wrapped in the hierarchical structure wall and can not be washed away. Energy-dispersive spectrometry (EDS) analysis shows that the Na element is present in the sample (the spectrum is not shown), which is accordance with FTIR spectrum. The TGA data (see Figure 6) show a total of about 58 wt % weight loss up to 600
Figure 6. Typical TGA curve for the as-prepared intermediate complex at the reaction time of 15 min.
Scheme 1. Presumed Molecule Structural Scheme of the Intermediate Complex Prepared at a Reaction Time of 15 min
°C under air, which is close to the calculated value of 58.7 wt % by losing two coordinate ethylene glycol molecules and forming NiO. A presumed molecule structure is shown in Scheme 1. The XRD pattern of the tubular intermediate product prepared at the reaction time of 8 h is similar with that of the microrods, especially the characteristic peak at ∼10° (see Figure 4), which proves that both of rodlike and tubular products are Ni-EG coordination polymer with the same phase structure. In addition, polyethylene glycol 200 (PEG200) and sodium acetate (CH3COONa, NaAc) also play an important role in the formation of tubular structure. Without PEG200, the outer surface and interior of the microrods simultaneously began to dissolve and to form the stripelike structure and the final products were particles and thinner microrods (see Figure 7). It is obvious that PEG200 protects the surface of the microrods
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Figure 7. SEM images of the intermediate complexes obtained without PEG200 at 190 °C for (a, b) 0.5 h and (c) 8 h.
electrode. Therefore, the discharge capacity difference between Ni(OH)2 microtubes with micro- and nanoscale composite structure and the Ni(OH)2 spherical particles reaches 74.9 mA h g-1, which is larger than that of 50 mA h g-1 in previous work.1 Further investigations of the discharge capacity are in progress. 4. Conclusions
Figure 8. Discharge curves of sixth cycle as a function of capacity for β-Ni(OH)2 microtubes (solid line), β-Ni(OH)2 microrods (dashed line), and β-Ni(OH)2 spherical particles (dotted line) electrodes at the current density of 80 mA g-1 and 20 °C.
from dissolution and promotes transportation from inner part to exterior of the microrods. Also, the experimental results show that the formation of microtubes is very sensitive to the concentration of NaAc. There has an optimum concentration range of 0.76-1.52 mol L-1. The lower concentration (less than 0.61 mol L-1) leads to the formation of particles, while the higher one (greater than 1.52 mol L-1) results in the coexistence of particles and microrods. The exact role of NaAc in the formation of microtubes is not clear at this moment and a further study is needed. 3.3. Electrochemical Property of the Products. To investigate the effect of the different morphology of the synthesized β-Ni(OH)2 structures on the electrochemical properties, their discharge capacities were measured (shown in Figure 8). The primary results show that the discharge curve of the β-Ni(OH)2 microtube electrode displays a longer plateau than those of the Ni(OH)2 microrod and spherical particle electrodes, indicating that β-Ni(OH)2 tubular structure electrode has a high-output behavior. The highest discharge capacity for the β-Ni(OH)2 microtube electrode is 232.4 mA h g-1 at 80 mA g-1 and 20 °C, whereas it is 135.0 and 157.5 mA h g-1 for β-Ni(OH)2 microrods and spherical particles, respectively. It is obvious that the significantly improved discharge capacity of β-Ni(OH)2 microtube electrode, compared with microrod and spherical particle electrodes, may be attributed to its unique morphology. The hierarchical structure wall of β-Ni(OH)2 microtubes selfassembled by nanolamella is favorable for increasing the interface area between electrode and electrolyte, which can result in a higher diffusion rate and faster electrode kinetics, and can increase the usage factor of the active component of the
In conclusion, the β-Ni(OH)2 microtubes with micro- and nanoscale composite structure have been successfully fabricated using a facile template-free wet-chemical method combined with alkaline solution treatment. It is the first time that the effective fabrication of Ni(OH)2 microtubes with the hierarchical structural wall is reported. The microrods and microtubes composed of the Ni-EG complex can be fabricated selectively by adjusting the reaction temperature. The β-Ni(OH)2 microtubes can be obtained by immersing the tubular precursors composed of NiEG complex into the alkaline solution for 10 h. The influence of reaction parameters on the formation of microtubes was discussed and the possible formation mechanism of microtubes with micro- and nanoscale composite structure was suggested. β-Ni(OH)2 microtubes with novel structural features show a higher discharge capacity, which makes them a potential candidate for the electrode materials in alkaline rechargeable Ni-based batteries. Acknowledgment. Financial support from the major research programme of nanoscience and technology (90606005) and the National Natural Science Foundation of China (NSFC) under major project (20490210) and surface project (20571040 and 20371026) is greatly appreciated.
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