Of late, ZnO nanoparticles have been regarded as favourable material for designing highly sensitive and selective gas sensors owing to their striking and multifunctional features. A facile sol–gel process chemical technique was used in this study for the preparation of undoped ZnO nanoparticles. By deploying x-ray diffraction (XRD), transmission electron microscopy (TEM), UV–visible near infrared and photoluminescence spectroscopy (PL), we examined the optical attributes and structural characterisation of the undoped ZnO nanoparticles through a change in the concentration of the KOH solution. The typical peaks inspected in the XRD spectrum corroborate the wurtzite hexagonal arrangement in the undoped ZnO nanoparticles. Moreover, ZnO nanoparticles possess a non-agglomerated crystalline with spherical morphology, as acquired by TEM. In addition, the vibrational modes in FTIR spectra vary with the change of concentration of KOH solution in ZnO NPs. The Raman spectroscopic techniques were used to characterise the ZnO nanoparticles, which revealed their nanocrystalline nature.