Abstract Here, we demonstrate a pyrolysis route for the synthesis of flower buds like magnesium oxide nanoparticles using a magnesium carbonate precursor without additional chemicals. The effect of heating at different time intervals upon the structure and morphology of the acquired nanostructures were investigated via X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and Fourier transformation infrared spectroscopy. Nitrogen adsorption was employed to study its porosity. The obtained data confirmed the formation of target nanoparticles that exhibited increasing sizes as pyrolysis time was lengthened. As a consequence a high surface area up to 27 m2 g−1 was recorded for the sample heated for 1 h duration. Furthermore, Indigo Carmine dye adsorption was carried out using the largest surface area species which showed an adsorption capacity of 158 mg g−1. The adsorption was found to comply with the Langmuir isotherm and it follows the pseudo-second-order kinetics. The diffusion process showed intra-particle along with film diffusion mode.