Diffusion-controlled self-assembly and dendrite formation in silver-seeded anatase titania nanospheres

We study diffusion-controlled, solvent-mediated self-assembly and dendritic structure formation as anatase TiO2 is seeded with silver oxides and silver that are formed using silver nitrate (zinc-catalyzed). Anatase TiO2 nanocolloid solutions are seeded with silver nitrate over a concentration range extending to saturation as the zinc-catalyzed transformation of the nitrate is studied in a number of solvents used in the formation and study of porous silicon interfaces. In acetone alone and, to a lesser extent, butanone, the silver/silver-oxide-seeded titania nanoparticles assemble into needlelike arrays of significant extent. This assembly is greatly accentuated in benzene/acetone mixtures and, to a much lesser extent, in diethyl ether/acetone mixtures, due primarily to the significant vapor pressure of the ether. In contrast to these aprotic solvents, self-assembly cannot be induced in the protic solvents water, methanol, and ethanol or in the aprotic solvents acetonitrile, methylene chloride, and dimethylformamide, even in mixtures with acetone. These results are explained and attributed to several controlling factors: the titania crystal structure and the nature of its subsequent silver seeding; the nature of diffusion through the considered solvents; the formation of silver acetate, propionate, and benzoate; the formation of strong cyano and chloride bonds. At saturated silver nitrate concentrations, distinctly different solvent-mediated dendrite growth in water and acetone solvents is explained from the perspective of a diffusion-limited aggregation (DLA) process.