To date, researchers have paid attention to improving the performance of materials for practical applications. In essence, the functionality of materials depends on their compositions, crystallographic structures, morphologies, as well as scales and dimensions. Since the physical and/or chemical properties of materials with the same composition but different structures or morphologies can vary substantially [1], the design and characterization of functional materials with specified nanoscale structures is crucial. For instance, the synthesis of transition metal oxides with a controlled structure and morphology allows the generation of active sites; and hence, it could lead to the improvement of their performance. Co3O4 is a versatile metal oxide that is used in many applications, including gas sensors [2], biosensors [3], lithium ion batteries [4] and supercapacitors [5]. Nanoscale Co3O4 with different morphologies such as nanowires, nanocones, nanospheres, nanocubes, nanorods, and mesoporous structures were reported using various technologies, such as electrodeposition, hydrothermal and subsequent calcination [6].
The self-assembly process is widely used as a bottom-up approach for micro-/nano patterning and manufacturing because the driving force of forming molecules and colloids, materials and polymers into an organized structure or pattern can be expanded beyond these, such as conventional ionic, covalent, metallic, hydrogen and coordination bonds, to include weaker interactions, e.g. van der Waals and Casimir, π–π and hydrophobic, colloidal and capillary, convective and shear, magnetic, electrical and optical forces [7]. Nowadays, the efficiency of the self-assembly process has been demonstrated for the synthesis of nanomaterials with high yield and low cost. Furthermore, template-assisted self-assembly has been shown to be a promising method for the preparation of mesoporous materials [8], [9], [10] and [11]. For example, Balaya et al. prepared mesoporous titanium dioxide with nanograins of dimensions in the range of 16–20 nm through the soft-templating approach, using various cationic surfactants such as octyl-, dodecyl-, cetyl trimethylammonium bromide with different surfactant compositions and titania precursor concentrations [10]. Moreover, Akagi et al. reported that helical carbon and graphite films obtained from helical poly(3,4-ethylenedioxythiophene) films can be synthesized through electrochemical polymerization in a chiral nematic liquid-crystal field [11]. By tuning the sources and the concentration of soft templates, usually surfactants, the nanostructure of specified materials can be obtained in a controlled fashion. Meanwhile, well-aligned nanoarrays with active components directly grown on the current collectors have been considered as a new generation of highly effective electrodes for supercapacitors or lithium ion batteries due to their intrinsic advantages, such as binder-free, fast ion transportation and accommodation of swelling and expansion [12]. Specifically, the open space within nanoarrays facilitates the electrolyte penetration and diffusion of ionic species, resulting in high utilization efficiency of the active materials. The direct contact of nanowire arrays with the underlying conductive current collectors and their strong binding ensure good electrical conductivity between them, as well as high structural stability of the obtained electrodes [6]. Thus, these electrodes show great potential for direct use in electrochemical energy storage cells, and are also beneficial for biosensor applications [3].
This issue's cover of Materials Today, show Co3O4 microscale flowers we have synthesizes, comprised of curved nanosheets on three-dimensional (3D) nickel foam, using a combination of hydrothermal and sodium dodecyl sulfate (SDS) soft-template-based self-assembly methods. The nanostructure of Co3O4 synthesized by hydrothermal methods can be tuned by using different surfactants, for instance, flower-like and nanowire-like microstructures are obtained using SDS and hexadecyl trimethyl ammonium bromide (CTAB), respectively [5]. The formation mechanism is proposed that the precursor of Co3O4 will selectively nucleate on the long chains or hydrophilic terminals of surfactant and then form into various nanostructures. Specifically, since CTAB is a cationic surfactant, the hydrophilic terminal of CTAB firstly adsorbs anions, like OH− or NO3− in the solution. Subsequently, the distance of hydrophilic terminals of CTAB molecules is enlarged and the hydrophobic terminals remain close to each other, which results in the side-by-side growth of nanowires. In addition, since SDS is an anionic surfactant, whose hydrophilic terminal will adsorb the Co2+ ion directly, it resulted in the nucleation and growth of Co2+ around the hydrophilic terminals of SDS to form flower-like microstructures.
Acknowledgement
We would like to extend our thanks to the Shun Hing Institute of Advanced Engineering (SHIAE), which partially supported this work with the Project No. 8115045, and the Early Career Scheme (ECS) grant from the Research Grants Council of Hong Kong SAR, with the Project No. 439113. The authors also thank the Li Ka Shing Institute of Health Sciences, and Micro and Nano Fabrication Laboratory in The Chinese University of Hong Kong (CUHK) for their technical assistance.
Further reading
1. T.Y. Ma, et al., J. Am. Chem. Soc., 136 (2014), pp. 13925–13931
2. W.Y. Li, L.N. Xu, J. Chen, Adv. Funct. Mater., 15 (2005), pp. 851–857
3. X.Y. Lang, et al., Nat. Commun., 4 (2013), p. 2169
4. Y.G. Li, B. Tan, Y.Y. Wu, Nano Lett., 8 (2008), pp. 265–270
5. L.J. Han, P.Y. Tang, L. Zhang, Nano Energy, 7 (2014), pp. 42–51
6. K.K. Lee, W.S. Chin, C.H. Sow, J. Mater. Chem. A, 2 (2014), pp. 17212–17248
7. G.A. Ozin, et al., Mater. Today, 12 (2009), pp. 5–23
8. G.M. Whitesides, B. Grzybowski, Science, 295 (2002), pp. 2418–2421
9. Y.J. Song, et al., Nano Lett., 7 (2007), pp. 3650–3655
10. S.R. Gajjela, et al., Energy Environ. Sci., 3 (2010), pp. 838–845
11. S. Matsushita, et al., Angew. Chem. Int. Ed., 53 (2014), pp. 1659–1663
12. Y. Jiang, et al., Nano Lett., 14 (2014), pp. 365–372