General self-template synthesis of transition-metal oxide microspheres and their excellent charge storage properties
Graphical abstract
Introduction
Many efforts have been devoted to studying the energy storage devices due to the increasing future energy use and consequent environmental impacts. Supercapacitors, one of the energy storage devices, have attracted worldwide attention because of their unique characteristics of high power density, fast charge/discharge process, long cycle life and excellent reversibility [1,2], as well as their potential applications in many fields such as mobile electronics, power resources, transportation and aerospace systems [3,4]. To achieve the high-performance charge storage properties, considerable interests have been taken in electrode materials for electrochemical capacitors using transition metal oxides including NiO, Co3O4 and NiCo2O4 [[5], [6], [7]]. These metal oxides can be considered as the most promising electrode materials for charge storage and power delivery owing to their low cost, environmental friendly and high theoretical capacitance [1,8]. However, the preparation of high specific and good stability electrode materials is still a challenge. To solve these problems, many research have been committed to design the nanostructure and morphology of the materials, such as nanoparticles (Co/CNTs) [9], nanotubes (NiCo2O4) [10], nanofibers (carbon-doped Co3O4) [11] and nanosheets (CoS1.097/N-doped C, CoOx) [12,13].
Nanosheets materials are considered as the new popular materials with admirable electronic properties for their high specific surface area and energy storage [14], which plays great important on electrochemical performance by facilitating the high transport rates for both electrons and electrolyte ions. For example, Du et al. suggested that the NiO nanosheet in microspheres show high specific capacitance (762 F g−1 at 1 A g−1) due to their high surface area and porosity [15]. Liu et al. have synthesized NiCo2O4@NiCo2O4 core/shell nanoflake arrays, which exhibit high areal specific capacitances of 1.55 F cm−2 (787 F g−1) at 2 mA cm−2 (1 A g−1), mainly attributed to their unique core/shell and nanosheet structure [16]. The porosities can also shorten ion/electron diffusion distance and accelerate the kinetic process [[17], [18], [19]]. Liao et al. have prepared hierarchical Co3O4 microspheres, which display a specific capacitance of 483.8 F g−1 at 1 A g−1 and good cycling stability (10.5% lose after 2000 cycles) because of their large surface area and unique hierarchical porous structures [20]. Therefore, it is necessary to develop ultrathin high specific and good stability ultrathin metal oxide electrode materials for the purpose of satisfying the advanced energy storage devices.
In the past few years, extensive research has focused on hollow and porous nanostructures for charge storage, such as porous NiO nanoflake arrays [21], starfish-shaped porous Co3O4/ZnFe2O4 hollow nanocomposite [22] and hollow NiCo2O4 submicrospheres [23]. Unfortunately, a large number of the preparation processes are complex and poisonous. Most of the research only consider the method for individual metal oxide which is not suitable for others. Therefore, universality of preparation becomes increasingly vital to the basic research and application of materials. For example, the research of combination of cellulose nanocrystal aerogels for supercapacitor with enhanced performance is made by Yang et al. [24]. Moreover, Wang et al. demonstrates general synthesis of transition metal oxide (NiO, Co3O4, NiCo2O4) nanotubes with enhanced oxygen evaluation reaction (OER) performances [25]. Thus, it is necessary yet very challenging to seek efficient, clean and sustainable preparation methods for fabricating a variety of materials with enhanced energy storage properties using a universal method.
Herein, we report a universal and general self-template method to synthesize transition metal oxides (NiO, Co3O4 and NiCo2O4) microspheres organized from a large number of ultrathin nanosheets via a facile and universal hydrothermal method. This kind of microspheres exhibit excellent charge storage performances. The universality of the approach is a great promotion to the development of advanced electrode materials for high-power battery-type devices [26,27].
Section snippets
Synthesis of NiO microspheres
Firstly, 0.3993 g L-aspartic was added into 15 mL deionized water. Next, 3 mL NaOH (2 M) aqueous solution was slowly added to the previous solution under stirring. Finally, 18 mL glycol (EG) and 0.872 g nickel nitrate hexahydrate (Ni(NO3)2·6H2O) were mixed to the solution with vigorous stirring until the solution turn blue. Subsequently, the transparent blue solution was transferred to 50 mL Teflon-lined autoclave and maintained at 180 °C for 16 h. The obtained precipitates were collected and
Synthetic process
The formation process of NiO, Co3O4 and NiCo2O4 microspheres is more similar to a self-template mechanism [25,28,29], as illustrated in Fig. 1. Take NiO as an example, Ni2+ ions will initially connect with the L-aspartic molecules (Ni(II)-Asp) under the coordination characteristics of transition metal ions [30] to form Ni(II)-Asp complexes which will be fabricated as precursors aggregating to form a core center [25,31,32]. These nucleation centres that allow the subsequent adsorption of Ni(II)
Conclusions
In summary, a universal and general self-template method has been developed to fabricate flower-like transition metal oxides (NiO, Co3O4, NiCo2O4) microspheres. Those transition metal oxides exhibit excellent charge storage properties, which can be ascribed to the large number of ultrathin nanosheets around the surface of microspheres. This unique structures can provide high surface area and abundant mesoporous. This work provides a universal and general method for fabricating metal oxides
Acknowledgements
This work was supported by the Excellence Foundation of BUAA for PhD Students under Grant no. 2017008 and the National Natural Science Foundation of China under Grant nos. 51671010 and 51101007.
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2022, Nano Materials ScienceCitation Excerpt :After calcination of Co-phyllosilicate in the air, the wrapped Co2+ will be converted into cobalt oxide. The obtained cobalt oxide has a phase consistent with the Co3O4 phase (JCPDS No. 42–1467) [31]. Additionally, after phosphorization, the wrapped Co2+ will be converted into cobalt phosphate.
3-D hierarchical urchin-like Fe<inf>3</inf>O<inf>4</inf>/CNTs architectures enable efficient electromagnetic microwave absorption
2022, Materials Science and Engineering: BCitation Excerpt :The steric hindrance and magnetic interaction promoted the radial orientation of the nanofibers, and finally formed an urchin-like morphology [30]. In this work, urchin-like Fe3O4 was synthesized by a simple hydrothermal method, which is different from other work using complex surfactant or structure-directing agents [31–33]. Compared with spherical, cubic, rod-shaped, or other morphologies, the urchin-like structure has a larger specific surface area and richer interfaces, which is very conducive to the interface polarization of nanomaterials and further promotes the EMW absorption properties.
Construction of NiCo<inf>2</inf>O<inf>4</inf> nanosheets-covered Ti<inf>3</inf>C<inf>2</inf>T<inf>x</inf> MXene heterostructure for remarkable electromagnetic microwave absorption
2022, CarbonCitation Excerpt :Fig. S9 further confirms that the synthesized NiCo2O4 is a flower-like structure that is difficult to fully expose the surface. High-resolution TEM (HRTEM) image shows the lattice fringers of 0.25 nm, 0.28–0.30 nm, 0.47 nm, corresponding to the (311), (220), and (111) planes of NiCo2O4 (Fig. 4d), respectively [33,34]. Importantly, a lattice finger of 0.50 nm is observed, corresponding to the (0004) plane of Mxene [35], further suggesting the successful formation of NiCo2O4 and MXene in the heterostructure.
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2021, Journal of MateriomicsCitation Excerpt :Metal oxides are widely used in the field of energy storage and conversion due to their high natural abundance, low-cost, simple preparation, ecofriendliness, and multiple valences [65–69], such as supercapacitors [70–78], lithium batteries [79–84], fuel cells [85–89] and electrocatalysis [90–94]. Among them, transition metal oxides have attracted much attention because of their low-cost and excellent ecofriendliness potential [95–98]. It is widely accepted that the charge storage mechanisms of the electrochemical behavior of metal oxide electrodes (e.g. MnO2, NiO, Co3O4) in aqueous electrolytes are based on the valence changes of metallic species through Faradaic proton/alkali metal intercalation/deintercalation [99], surface adsorption/desorption [100], or surface redox reactions with anions [101].
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These authors contributed equally to this work.