Au and NiO nanoparticles dispersed inside porous SiO 2 sol-gel film: correlation between localized surface plasmon reson
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Au and NiO nanoparticles dispersed inside porous SiO2 sol-gel film: correlation between localized surface plasmon resonance and structure upon thermal annealing Enrico Della Gaspera1, Giovanni Mattei2 and Alessandro Martucci*1 1 Università di Padova, Dipartimento di Ingegneria Industriale, Padova, Italy 2 Università di Padova, Dipartimento di Fisica e Astronomia, Padova, Italy ABSTRACT 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 after annealing at 700 °C. 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. The thermal evolution of the Au and NiO nanoparticles structure has been studied by high resolution transmission electron microscopy and UV-visible absorption spectroscopy and correlated with the evolution of the Au surface plasmon resonance peak. INTRODUCTION Films containing NiO, a p-type semiconductor with a wide band gap of 4.2 eV, have been recently proposed as sensitive materials for chemoresistive1 and optical gas sensors2. The working mechanism of NiO-based platforms consists in a change of the electrical resistance or optical transmittance of the material as a consequence of physisorption/chemisorption and reactions of the analyte with its surface. The interaction extent with the target gas can be maximized either by increasing the sensor’s surface area3, or by doping the oxide with noble metal nanoparticles (NPs)4. In previous works we demonstrated that SiO2 porous films containing NiO NPs have remarkable gas sensing properties2,5; moreover the optical gas sensing effect can be enhanced by the introduction of Au NPs together with the NiO nanocrystals6,7. In fact, the surface plasmon resonance (SPR) of Au NPs depends strongly upon the surrounding medium properties, particularly refractive index8. Even a small change of the matrix properties leads to a large variation of the plasmonic frequencies, allowing for detection of the target gas at low concentrations. Moreover, thanks to the Au SPR peak in the visible region, it is possible to tune the response of the sensors, by selecting an appropriate wavelength of analysis9,10. In fact, the variation of the dielectric constant around the SPR bands will differ for different gas species; this leads to a diverse variation of the optical properties at different wavelengths. In this way it is possible to enhance the selectivity towards interfering gases, a feature that is not achievable with conventional electrical sensors We already have shown that by tailoring the annealing conditions it is possible to change the NP morphology inside the porous SiO2 matrix11. At 500 °C the NiO and Au NPs are separated while at 700 °C the clusters show a twofold structure formed by two hemispheres of NiO and Au arranged in a cookie-like structure, with the Au (111) planes parallel to NiO (200) ones. The objective of this work is to study
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