Preparation and thermal stability of gold nanoparticles in silk-templated porous filaments of titania and zirconia

Porous metal-oxide filaments were readily fabricated using hydrophobic silk as template via the surface sol-gel process, followed by removal of the organic components. Such biotemplated porous matrixes were employed as nanoreactors for in situ synthesis of gold nanoparticles by NaBH₄ reduction of the incorporated gold ion. The as-synthesized nanoparticle (ca. 4 nm) was apparently stabilized by the porous morphology of the silk-templated matrix, as well as by the coordinative interaction between the surface Au atom of the nanoparticle with the surrounding oxygen linkage of the matrix. The nanoparticle showed only a small increase (to ca. 6 nm) in particle size after annealing at 500 °C for 5 h, but it became much larger (∼40 nm) at 800 °C. Under the otherwise identical conditions, amorphous zirconia matrix gave rise to a much smaller size increase (ca. 10 nm at 800 °C), indicating more effective suppression of fusion of molten gold nanoparticles. Apparently, better physical isolation of the nanoparticle was attained for amorphous zirconia than nanocrystalline titania. General features of the formation of metal nanoparticles in solid matrixes were discussed, in terms of morphology and functionality of matrixes.