Growth of cookie-like Au/NiO nanoparticles in SiO2 sol-gel films and their optical gas sensing properties

The favorable lattice matching between Au and NiO crystals made possible the growth of unique cookie-like nanoparticles (25 nm mean diameter) inside a porous SiO2 film by simply tailoring the film synthesis parameters. The unusual aggregates result from the coupling of well distinguishable Au and NiO hemispheres, which respectively face each other through the (100) and (200) lattice planes. High resolution transmission electron microscopy analysis revealed that the 2-fold nanostructures show a sharp flat interface with epitaxial coherence between the Au and NiO phases. The surface plasmon resonance (SPR) bands observed in optical absorption spectra provides evidence for the effect of the atypical dielectric nature of the media surrounding the Au aggregates. The films show noticeable and reversible change in the optical transmittance when exposed to CO and H2, with different features according to the detected species. The presence of CO does not modify the maximum SPR band wavelength, while H2 induces a clear shift of the overall plasmonic resonance frequencies.