Fabrication of hierarchical noble metal nanostructures is of interest due to the unique structural arrangement of nanoscale building blocks: nanocrystals or nanoparticles, composition and size are important in taking the physical and chemical usefulness of metal nanomaterials to a macroscopic scale. Various methods have been employed in achieving nanostructures of unique morphological arrangement. Amongst which are chemical and electrochemical reduction, or electrodeposition. In this research, platinum hierarchical nanostructures (PtHNs) were prepared using bicontinuous microemulsion (BC-ME) as soft templates. The PtHNs were synthesized using electrodeposition method as well as chemical reduction method. PtHNs was electrodeposited on Au macroelectrode from the bicontinuous microemulsion as well as from aqueous solution of chloroplatinic acid. Linear sweep voltammetry and chronopotentiometry was employed to achieve the deposition of Pt⁰ from the aqueous and bicontinuous platinum microemulsion electrolytic media. In the chemical method, sodium borohydride was used to reduce the hexachloroplatinic ions present in the bicontinuous microemulsion to platinum nanomaterial. In using bicontinuous microemulsions as a soft templates, the channels serve as continuous vessels for the converging growth of structures, which are dimensionally restricted by the diameter of the channels.

PtHNs were characterized by SEM, EDX, TEM, XRD and cyclic voltammetry. The SEM images obtained, showed that the structure was unique, hierarchical with a regular morphology. From the SEM images, it was observed that the chemically synthesized PtHNs, were made up of interconnected spheres and self-assembled nanoneedles forming a nanocoral shape. The PtHNs XRD patterns confirm the polycrystalline nature of the platinum nanomaterials.

Cyclic voltammetry (CV) of PtHNs synthesized through chemical reduction and electrodeposition method was conducted in degassed 0.5 M H₂SO₄. CV measurements (current voltage curve) essentially provide information about the electrochemical reactions that occurs on a working electrode surface. Electrochemical active surface area (ECSA) values of 35.1 m²/g and 20.9 m²/g were recorded for PtHNs synthesized by chemical reduction of 2% and 4% water soluble Pt precursor respectively. The highest ECSA value of 35.1 m²/g recorded from the PtHNs is higher than Pt dendritic tubes (23.3 m²/g) reported by Zhang et al. [1]. This clearly shows that the catalytic ability of the nanostructures synthesized in this work are highly desirable. The electrochemical surface area (ECSA) of 14.6 m²/g was recorded for the PtHNs electrodeposited on Au macroelectrode from aqueous Pt electrolyte while 12.2 m²/g was obtained when bicontinuous microemulsion of Pt was used as the platinum electrodeposition media. The PtHNs synthesized by the chemical method exhibited larger surface area than the platinum nanomaterial synthesized by electrodeposition. Broadly, this report presents a novel study of different synthetic methods for obtaining platinum nanostructures demonstrating the “template effect” of bicontinuous microemulsions towards the synthesis of metallic superstructures.

Reference

[1] Zhang, G., Sun, S., Cai, M., Zhang, Y., Li, R., and Sun, X., 2013, Porous dendritic platinum nanotubes with extremely high activity and stability for oxygen reduction reaction: Sci. Rep., 3, 1526.