Self-Assembled 3D Flower-Like Hierarchical β-Ni(OH)2Hollow Architectures and their In Situ Thermal Conversion to NiO
Three-dimensional (3D) flower-like hierarchicalβ-Ni(OH)2hollow architectures were synthesized by a facile hydrothermal route. The as-obtained products were well characterized by XRD, SEM, TEM (HRTEM), SAED, and DSC-TGA. It was shown that the 3D flower-like hierarchicalβ-Ni(OH)2hollow architectures with a diameter of several micrometers are assembled from nanosheets with a thickness of 10–20 nm and a width of 0.5–2.5 μm. A rational mechanism of formation was proposed on the basis of a range of contrasting experiments. 3D flower-like hierarchical NiO hollow architectures with porous structure were obtained after thermal decomposition at appropriate temperatures. UV–Vis spectra reveal that the band gap of the as-synthesized NiO samples was about 3.57 eV, exhibiting obviously red shift compared with the bulk counterpart.
KeywordsNi(OH)2 NiO Hollow architecture Hydrothermal synthesis
Ordered self-assembly of nanoscale building blocks, such as nanoparticles, nanorods, nanoribbons, and so forth, into complex architectures has recently become a hot topic in material research fields. Remarkable progress has been made in the self-assembly of highly organized building blocks of metals [1, 2, 3, 4], semiconductors [5, 6, 7, 8], copolymers , and organic–inorganic hybrid materials  based on different driving mechanisms, such as Ostwald ripening , Kirkendall effect , and self-assembly of nanoscale blocks through hydrophobic interactions . However, controlled organization into curved hollow structures from the primary building units, for example sheets, remains a challenge for materials self-assembly . The ability to assemble primary units into hollow structures is in great demand not only because of their role in better understanding the concept of self-assembly with artificial building blocks but also due to its great potential for technological applications .
Nickel hydroxide (Ni(OH)2), as one of the most important transition metal hydroxides, has received increasing attention due to its extensive applications, especially as a positive electrode active material, in alkaline rechargeable Ni-based batteries . It has been reported that the capacity of the positive electrode could be significantly increased when nanophase Ni(OH)2 was added to micrometer-size spherical Ni(OH)2 [17, 18]. Further efforts have focused on searching for new synthetic methods of Ni(OH)2 nanocrystals with high quality and various exciting morphologies. 1D, 2D, and 3D nanostructures of Ni(OH)2, including nanorods , nanoribbons , nanotubes , nanosheets , and superstructures patterns [23, 24, 25, 26, 27, 28], have been fabricated successfully by a variety of methods. Nickel oxide (NiO) is a very prosperous inorganic material which was widely applied in the fields of smart window, electrochemical supercapacitor, battery cathodes, catalyst, etc. [29, 30, 31, 32]. NiO can be conveniently prepared by thermal decomposition of its precursors . By contrast, there are only limited reports concerning the synthesis of Ni(OH)2 and NiO hollow architectures and their interesting properties. For example, Wang’s group synthesized hollow architectures of Ni(OH)2 with unusual form and hierarchical structures by using styrene-acrylic acid copolymer (PSA) latex particles as the templates . Hierarchically porous β-Ni(OH)2 microspheres constructed with nanoflakes were recently prepared with the help of hexamethylenetetramine (HMTA) as the basic source, exhibiting small blue shift compared with the bulk counterpart . Duan et al. reported the fabrication of hierarchical Ni(OH)2 monolayer hollow-sphere arrays with a fine structure of nanoflakelets by an electrochemical strategy based on a polystyrene (PS) sphere colloidal monolayer. Such hierarchically structured hollow-sphere arrays have demonstrated a tunable optical transmission stop band in the visible-near-IR (Vis–NIR) region from 455 to 1855 nm, depending on the hollow-sphere size and the fine structure . However, hollow structures prepared from hard templating routes (e.g. PS latex particles) usually suffer from disadvantages related to high cost and tedious synthetic procedures, which may prevent them from being used in large-scale applications . Thus, it still remains a challenge to develop simple approaches to synthesize hierarchical Ni(OH)2 and NiO hollow architectures.
Herein we describe a facile hydrothermal route to synthesize highly ordered 3D flower-like hierarchicalβ-Ni(OH)2hollow architectures with a high yield. The formation mechanism of the 3D flower-like hierarchicalβ-Ni(OH)2hollow architectures was proposed. The morphology-retained NiO hollow architectures with porous structure were readily obtained by thermal decomposition of the as-obtainedβ-Ni(OH)2products. Finally, the optical property of NiO sample was investigated with the help of UV–Vis spectrum.
Synthesis of 3D Flower-Like Hierarchicalβ-Ni(OH)2and NiO Hollow Architectures
In a typical synthesis, 1 mmol of NiCl2·6H2O was dissolved in 5 mL of deionized (DI) water, followed by an addition of 15 mL of ethanol and 5 mL of CO(NH2)2solution (2 mol L−1) under vigorous stirring. Then, 2 mL of NH3·H2O (35% by v/v) was added dropwise into the above solution to form a clear blue solution. The final solution was transferred to a 50 mL Teflon-lined autoclave. The autoclave was sealed and heated in an oven at 120 °C for 12 h and then allowed to cool to room temperature. The resulting pale green slurry was rinsed with DI water several times to remove soluble impurities. The product was dried in an oven at 50 °C for 8 h to get the sample ofβ-Ni(OH)2. To obtain NiO the as-prepared sample ofβ-Ni(OH)2was calcined in air for 4 h.
The phase purity of the products was examined by X-ray powder diffraction (XRD) using a Rigaku D/max 2500 diffractometer at a voltage of 40 kV and a current of 200 mA with Cu-Kα radiation (λ = 1.5406 Å), employing a scanning rate 0.02°/s in the 2θ ranging from 30 to 80°. Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) analysis were obtained using a HITACHI S-4300 microscope (Japan). Transmission electron microscope (TEM) images and the corresponding selected area electron diffraction (SAED) pattern were taken on a Hitachi-600 transmission electron microscope at an accelerating voltage of 200 kV. High-resolution transmission electron microscope (HRTEM) images were carried out for the as-prepared sample using JEOL JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. The size distribution of the sample was measured using a scale on the magnified SEM micrographs. Thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses were carried out on a NETZSCH STA-409 PC thermal analyzer with a heating rate of 10 °C min−1in flowing oxygen atmosphere. Room-temperature UV–Vis absorption spectrum was recorded on a Shimadzu UV-1601 PC UV–Vis recording spectrophotometer.
Results and Discussion
The morphologies and structures of as-synthesized samples were further characterized by TEM. As shown in Fig. 2e, TEM observations demonstrate that the products are flower-like structures similar to the SEM observation. The remarkable feature of the hollow architectures is the obvious contrast between the dark edge and the pale center, as reported for other hollow particles with a central cavity. To further obtain structural information for the well-aligned sheets, high-resolution TEM (HRTEM) images and the corresponding selected area electron diffraction (SAED) patterns were also recorded on single sheet. In a HRTEM image (Fig. 2f) taken from the edge of a sheet, the lattice fringes are clearly visible with a spacing of 0.27 nm, which is in good agreement with the spacing of the (01-10) planes ofβ-Ni(OH)2(JCPDS No: 14-0117). The corresponding SAED pattern is shown in the inset of Fig. 2f. The SAED and HRTEM analyses reveal that the building units are single-crystal.
In addition, the roles of urea and ammonia were found to be very important for the formation feature of 3D flower-like hollow architectures. In a control experiment, when no urea was added under the same reaction conditions, the products take on a flake-like shape (Fig. 3d) rather than 3D flower-like hierarchical hollow architectures, while the ammonia was absent, only honeycomb-structured micro-architectures can be obtained, as shown in Fig. 3e and f.
Most probably, the bubbles of CO2 gas produced in the reaction with the participation of CO(NH2)2 must have played a key role, since no other templates/surfactants/emulsions were used in this work. A possible formation process involving the assembly-then-assemble mechanism can be schematically illustrated in Fig. 3g. In the beginning, Ni2+ in solution reacts first with NH3 to form a relatively stable complex, [Ni(NH3)62+, because of its strong affinity to Ni2+ at room temperature. Afterwards, the complex was decomposed and released NH3 to provide OH− ions for the formation of Ni(OH)2 by a hydrothermal treatment. At the same time, with the participation of CO(NH2)2, many micrometer/sub-micrometer CO2 bubbles are produced in the system at 120 °C (step a). The freshly crystalline nanoparticles are unstable because of their high surface energy and tend to aggregate and form higher nanoparticles, driven by the minimization of interfacial energy. In our synthesis, the formation of [Ni(NH3)62+ complex would sharply decreased the free Ni2+ concentration in the solution, which resulted in a relatively low reaction rate of Ni2+ ions with OH− ions. A slow reaction rate caused the separation of nucleation and growth steps, which is crucial for high-quality crystal synthesis. As a result, the sheet-like high crystalline Ni(OH)2 was firstly formed (step b), which may be related to the nature of the initial crystal structures . Then the self-assembly and Ostwald ripening process occurs around the gas/liquid interface of CO2 and water, and finally 3D flower-like hierarchical hollow architectures (step c). Here, CO2 bubbles decomposed from CO(NH2)2 can act as soft templates to induce the self-assembly of nanosheets on their surfaces. A similar gaseous bubble has also been used as a template for TiO2 and VOOH hollow nanostructures [35, 36], which is different from the assembly-then-inside-out evacuation mechanism in the formation of Fe3O4 hollow spheres . Our time-dependent experiments support the above aggregation-then-assembly mechanism; it is found that the assembly process occurs after the formation of the nanosheets.
The 3D flower-like hierarchicalβ-Ni(OH)2hollow architectures have been synthesized by a facile hydrothermal route in the presence of urea and ammonia. The 3D flower-like hollow architectures with the size of several micrometers are composed of nanosheets of 10–20 nm in thickness. The results indicated that the reaction time, urea and ammonia play important roles in the formation of 3D flower-like hierarchicalβ-Ni(OH)2hollow architectures. By calcining the as-prepared flower-likeβ-Ni(OH)2hollow architectures, hierarchical NiO crystallites with porous single-crystalline nanosheets were obtained, well inheriting the shapes of theβ-Ni(OH)2samples. The optical absorption band gap of the as-obtained NiO samples is determined to be 3.57 eV. Due to the unique architectures, the as-obtained products may have potential applications in water treatment, electrode, sensors, catalysts, biomarkers, microelectronics, energy storage, and other related micro/nanoscale devices due to their unique architectures.
This work was financially supported by the National Natural Science Foundation of China (Nos.: 50573090 and 10672161) and Beijing Municipal Natural Science Foundation (No. 2082023).
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