In situ chemical etching of tunable 3D Ni 3 S 2 superstructures for bifunctional electrocatalysts for overall water splitting† Ting Zhu, a Liangliang Zhu, a Jing Wang a and Ghim Wei Ho * abc Three-dimensional (3D) nanomaterials are rendered with large specific surface areas as well as desired physicochemical, electrical and catalytic properties for a large variety of functional applications. In this work, 3D Ni 3 S 2 superstructures (needle array and leaf pattern) were created on nickel foams (NFs) through an in situ chemical etching (ICE) method. This anisotropic growth involves chemical dissolution, in situ nucleation and re-deposition processes, which endows the as-fabricated Ni 3 S 2 with large surface areas and warrants firm adhesion to the NF as well. Importantly, the Ni 3 S 2 @NF can directly serve as effective binder-free electrodes for hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs) from electrocatalytic water splitting. Introduction As compared to one-dimensional (1D) and two-dimensional (2D) nanostructures, 3D semiconducting nanomaterials are usually endowed with large specic surface areas in combina- tion with unique physicochemical, electrical and catalytic properties. 1,2 Moreover, the spatial conguration of the inter- connected networks of building subunits in the 3D structures can further expand specic properties, such as efficient mass transport and charge carrier transfer, 3 making the relevant nano/microfunctional materials popular with diverse applica- tions in energy storage, harvesting and conversion. 4–8 However, fabrication of 3D functional nanostructures, especially those generated on underlying substrates, still remains a signicant challenge because of the high complexity and diversity of the manufacturing process. 9 For instance, some conventional chemical methods that have been applied to achieve 3D nano- structures either involve tedious experimental steps or lead to poor physical contact between the target materials and substrates. 10–13 Chemical etching (chemical milling) is commonly employed in the industrial metal removal to obtain parts that cannot be machined easily through traditional machining methods. Usually, a strong chemical bath with regulated temperature is applied to a metal surface, where metal ions are dissolved from the metal substrate into the chemical solution to achieve desired shapes. 14–16 Taking advantage of this technique, we speculate that fabrication of 3D nano/microstructures could be feasible in a hydro/solvo-thermal solution, because the exfoli- ated metal species may nucleate into crystal grains if a new phase can be formed at the metal–solution interface. 17 Driven by the energy uctuation, further growth of the as-formed grains may take place in the solution, which will grow into secondary architectures of target materials. 18–20 Hence the chemical etching assisted growth involved process in a hydro- thermal synthesis may bring about target materials with unique structural features, high surface areas, and excellent physical contact at a molecular level, thus conferring the materials with unique functionalities for diverse applications, such as electrocatalysis. 21–25 3D nanostructures composed of transitional metal chalco- genides have been recently studied as bifunctional electro- catalysts for hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs) owing to their high electrocatalytic activity, good electric conductivity, low cost and abundance on earth. 26–28 For example, a recent study by Wang et al. presented nickel sulde microsphere lm on Ni foam as a bifunctional electrocatalyst for water splitting, in which superior activity with good durability has been demonstrated. 29 Liu et al. demon- strated NiCo 2 S 4 nanowires on carbon cloth serving as an efficient electrocatalyst for both HER and OER, where low overpotentials are reported to reach a current density of 100 mA cm 2 . 30 Therefore, it is of great importance to explore facile methods for the construction of 3D structures to enhance the bifunctional electrocatalytic properties. In this work, we present a facile design of in situ grown Ni 3 S 2 superstructures on nickel foams (NFs) via a one-step ICE approach. NF here plays dual roles of the a Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583. E-mail: elehgw@nus.edu.sg b Engineering Science Programme, National University of Singapore, 9 Engineering Drive 1, Singapore 117575 c Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602, Singapore † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta05618k Cite this: J. Mater. Chem. A, 2016, 4, 13916 Received 5th July 2016 Accepted 10th August 2016 DOI: 10.1039/c6ta05618k www.rsc.org/MaterialsA 13916 | J. Mater. Chem. A, 2016, 4, 13916–13922 This journal is © The Royal Society of Chemistry 2016 Journal of Materials Chemistry A PAPER Published on 10 August 2016. 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