Porous nickel oxide nanoparticles with a hierarchical structure and high specific surface area were obtained by green synthesis followed by thermal annealing. The influence of the choice of precursor plant extract (Fumaria officinalis L. and Origanum vulgare L.) and the extractants in aqueous solutions on the parameters of the synthesized particles was studied. Characterization of the NiO morphology and composition, as well as the specific surface area, was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and the BET method of nitrogen thermal desorption. Resulting particles have a spherical shape and a size from 30 to 50 nm. According to the data obtained, it can be seen that when the precursor is changed from Fumaria officinalis L. to Origanum vulgare L., the size of the synthesized particles increases, while the structure becomes more friable. It has been revealed that certain parameters and the nature of the assembly of porous particles lead to an increase in the surface area: the highest value of the SSA of 130.0 m²/g is observed in NiO nanoparticles obtained using Fumaria officinalis L. extract based on isopropyl alcohol. Also, a relatively high SSA value of 73.5 m²/g is observed in nanoparticles obtained using the same extractant for Origanum vulgare L. extract, while the use of an ethyl alcohol-based extractant for Fumaria officinalis L. resulted in the lowest value of 40.2 m²/g. The developed semiconductor particles are promising for use in catalysis, sensors, and as part of supercapacitor electrodes and functional layers in device structures for solar cells.

A technological approach to the formation of a 3D nanocomposition material based on hierarchical porous nickel oxide nanoparticles incorporated into porous silicon with a dendritic porous structure is proposed. Porous silicon was used as a 3D porous template, in the presence of which porous hierarchical nickel oxide nanoparticles were synthesized using a “green” synthesis method followed by annealing in an oxygen-containing atmosphere. The resulting materials were characterized using scanning electron microscopy, transmission electron microscopy, X-ray spectral microanalysis, X-ray diffraction, and the BET method. The potential of a developed composition based on porous hierarchical nickel and silicon oxide nanoparticles to enhance the sensitivity of adsorption gas sensors was assessed using impedance spectroscopy in the presence of a probe gas (isopropanol). Gas sensitivity measurements were conducted at room and elevated temperatures in the frequency range from 100 Hz to 500 kHz. Differences in the dependences of the real part of impedance on the imaginary part were revealed for the porNiO-porSi composition in Nyquist coordinates. The results are discussed in terms of percolation theory and fractal organization.