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 (S_c) 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 C_sp 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 C_sp for NiO@Ni(II)-SR NCs.