Investigating the structural, electrochemical, and optical properties of p-type spherical nickel oxide (NiO) nanoparticles

A facile and efficient co-precipitation method has been used to synthesize pure nickel oxide (NiO) nanoparticles. The synthesis of NiO has been conducted at two different pH values (11.00, 12.33) and in alkaline media of LiOH and NaOH, and the variations in structural, electrochemical, and optical properties have been analyzed. The particle sizes of all of the synthesized samples were in the range 8.000-20.00 nm, and their energy band gaps were in the range 3.500-4.000 eV. Williamson-Hall plots indicate that strain was introduced in the samples during the synthesis process. NiO nanoparticles synthesized in the LiOH medium at pH 12.33 showed the smallest crystallite size of 8.830 nm, with a lower strain value of 9.620E-6. The charge-transport and charge-transfer properties of the NiO samples have been investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. CV study revealed that the redox process is diffusion-controlled, with a diffusion constant of the order of 10(-)(10) cm(2) s(-1). Nyquist plots obtained by the EIS technique indicate how charge-transfer resistance and Warburg diffusion vary with pH. The NiO nanoparticles synthesized in LiOH medium at pH 12.33 showed the lowest charge-transfer resistance, indicating high electrical conductivity. In the present study, NiO nanoparticles with small crystallite size and lower strain value has been obtained, which form a better thin film suitable for use as a hole-transporting layer in solar cells. Lower charge-transfer resistance facilitates better charge transport in the obtained NiO nanomaterial, facilitating use in electrochromic devices.