Preparation of NiO nanoflakes under different calcination temperatures and their supercapacitive and optical properties

Highlights: • The NiO nanocrystals were prepared by calcinating Ni(OH)₂ nanoflakes synthesized via an ion diffusion controlled by ion exchange membrane. • The NiO sample calcinated at 400 °C exhibits the highest specific capacitance of 381 F g⁻¹ and specific surface area of 188.4 m² g⁻¹. • The NiO samples with the lower binding energy are harder to capture OH- than Ni(OH)2, which is disadvantageous to charge storage. • The UV–Vis absorption peak of NiO samples have a red shift with increasing the calcination temperature due to the increase in crystallinity. - Abstract: The NiO nanocrystals were successfully prepared by calcinating Ni(OH)₂ precursor synthesized via a facile ion diffusion controlled by ion exchange membrane without adding any solvent or template. X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) isotherm, X-ray photoelectron spectroscopy (XPS) and Ultraviolet–visible (UV–vis) analysis were used to investigate the crystallinity, morphology, surface and porosity characteristics, chemical composition and optical properties in more detail. The pseudocapacitive behavior of the NiO samples was investigated by cyclic voltammograms (CV) and galvanostatic charge-discharge tests in 2 M KOH. The results analysis reveals that both specific capacitance and surface area decrease with the increase of calcination temperatures. Among the NiO samples, the NiO-400 nanoflakes calcinated at 400 °C possess the highest specific capacitance of 381 F g⁻¹ at a current density of 2 A g⁻¹, but much lower than the Ni(OH)₂ sample. In addition, the UV–vis analysis shows that there is a red shift of absorption peak for the three NiO samples with the increasing temperature and the NiO-400 has a broad band gap of 3.3 eV, which renders the material highly interesting for application in photocatalyst.