Ultrasmall NiO nanoclusters modified with conical Ni(ii)-SR staples for high performance supercapacitor applicationsElectronic supplementary information (ESI) available: Fig. S1-S7. See DOI: 10.1039/c6nj03678c

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 NaBH 4 , 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 NaBH 4 i.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. We report MPS stabilized ultrasmall NiO NCs for high performance supercapacitors.