Au nanoparticles in alumina sols and coatings

The spontaneous reduction of HAuCl4 in the partially acetylacetanato (acac) complexed aluminium tri-sec-butoxide [Al(O-s-Bu)(3-0.66)(acac)(0.66)] solution results in the formation of stable and well dispersed Au nanoparticles (AuNPs). Generation of such AuNPs (Au-0) occurred through unstable Au-r intermediates, and their sizes are controlled by the concentration of Au. The use of acac with aluminium tri-sec-butoxide (which remains bonded with Al in a chelated fashion) also stabilizes the alumina sols after hydrolysis. The resulting alumina sols containing AuNPs are stable for months and can be used to prepare Au nanoparticle doped transparent alumina coatings on glass substrates. The acac chelates attached to Al are most probably acting as a capping (protecting) agent of AuNPs in the alumina sols. The action of acid (HNO3) causes both opening of Al-acac chelates and destabilization of AuNPs resulting in aggregation through fusion. The aggregation of AuNPs in the sol could be controlled by controlling the amount of HNO3. Such aggregation of nanoparticles causes shifting of plasmon absorption positions towards the longer wavelength region in a systematic manner due to plasmon coupling, which is linked with the effective length of the aggregated nanoparticles. Au nanoaggregates incorporated alumina coatings can also be prepared on glass substrates using these sols, which also show the modified plasmonic absorptions in the NIR regions.