Ultrasmall NiO nanoclusters modified with conical Ni(ii)-SR staples for high performance supercapacitor applications†
Abstract
Herein, we report 3-mercaptopropylsulfonate (MPS) stabilized ultrasmall NiO nanoclusters modified with conical Ni(II)-SR staples (NiO@Ni(II)-SR NCs). The surface tuning of MPS (-SR) staples over NiO NCs by varying metal to ligand (M–L) ratio resulted in the size-dependent supercapacitor behaviors. The formation of NiO@Ni(II)-SR NCs was ligand-specific with trace amounts of NaOH in the absence of NaBH4, wherein similar attempts with glutathione (GS) were unsuccessful. Atomic force microscopic (AFM) studies have confirmed the decreasing trend in the sizes of ultrasmall NiO@Ni(II)-SR NCs from ∼10 nm to ∼2 nm on increasing the M–L ratio and that of NiO nanoparticles synthesized with NaBH4i.e. the sizes of the NiO@Ni nanoparticles were found to be much greater (∼250 nm). High resolution scanning electron microscopy (HRSEM) images suggested the presence of lesser NiO aggregates in NiO@Ni(II)-SR NCs than NiO@Ni. X-ray diffraction studies confirm the amorphous nature of the ultrasmall NiO@Ni(II)SR NCs than NiO@Ni nanoparticles. Raman peaks around 510, 740 and 1090 cm−1 validate the presence of Ni–O vibrations, which get intensified with laser exposure, and peaks at 141, 242, 255, 285, 343 cm−1 suggest Ni–S vibrational modes indicating the formation of Ni(II)-SR staples over the core NiO nanoclusters. The specific capacitance (Sc) for NiO@Ni(II)-SR NCs is ∼449 F g−1, which is higher than that of NiO@Ni at 5 mV s−1 by CV analyses. The charge–discharge studies observed a maximum Csp of ∼512 F g−1 at a current density of 1 A g−1, and the cyclic stability for 1000 cycles retained ∼82% of its initial Csp for NiO@Ni(II)-SR NCs.