Spontaneous generation and shape conversion of silver nanoparticles in alumina sol, and shaped silver nanoparticle incorporated alumina films

Stable and well-dispersed Ag nanoparticles were generated by spontaneous reduction of AgNO3 in the partially acetylacetonato complexed aluminium tri-sec-butoxide solution without using any additional reducing agent. The addition of higher concentration of AgNO3 caused agglomeration of Ag nanoparticles resulting in precipitation. Use of polyvinylpyrrolidone as capping agent not only prevents the agglomeration but also causes a controlled growth of Ag nanoparticles in different shapes in alumina sol. The addition of varying concentrations of polyvinylpyrrolidone caused transformation of yellow alumina sol to green and blue coloured sol. These changes were due to the conversion of spherical (yellow) to a mixture of hexagonal and triangular (green), and finally truncated/filleted triangular (blue) Ag nanoparticles. These yellow, green and blue coloured alumina sols were used to prepare different shaped Ag nanoparticle doped optically transparent amorphous alumina films on glass substrates. The shape conversions of the Ag nanoparticles were studied systematically by UV/visible spectrometry, transmission electron microscopy and grazing incidence X-ray diffraction, and a mechanism of growth of the Ag nanoparticles in alumina sol was established.