Size-controlled nickel oxide nanoparticle synthesis using mesoporous silicon thin films

A process for chemically synthesizing size-controllable nickel oxide (NiO) nanoparticles (NPs) within the interior of mesoporous silicon (PSi) thin films is presented. The method is demonstrated to provide control of the average NP size over an order of magnitude, from 9 nm to 128 nm diameter, by fabricating PSi films with mean pore diameters ranging from 32 to 140 nm and annealing at temperatures between 300 and 1100 °C. NiO NPs are readily detached from the PSi films through electrolytic dissolution of the PSi host matrix. Nanocomposite films and NPs are characterized through x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy-dispersive x-ray spectroscopy. Optical absorbance measurements of free NiO NPs in aqueous suspension indicate that the optical bandgap is tuned from 3.65 to 3.9 eV, as expected from the effects of quantum confinement. This synthesis process is amenable to the batch fabrication of a wide variety of metal oxide NPs at temperatures up to 1000 °C with sizes below 100 nm. The method is advantageous over conventional chemical synthesis techniques as it facilitates control of the resulting NP size across a wide range and also permits high-temperature annealing while precluding extended crystallite formation. Furthermore, the use of a PSi template enables direct integration of nanoparticulate metal oxide into Si-based, on-chip applications. NiO was selected here as the model system to demonstrate this technique due to its numerous applications including energy storage and memristor technologies.