The use of mesoporous silica as a template for the nanocasting of metal oxide arrays
The mesoporous silica SBA-15 was used as a template for the formation of metal oxide nanowire arrays using the nanocasting approach. This approach utilizes solutions that fill the mesopores of the silica template with a metal salt precursor, which is converted to the oxide by heating to high temperature. The template can be removed by dissolving with an acidic or basic etch solution. The effects on the quality of product produced were assessed in regards to precursor type (metal nitrate vs. metal halide), precursor solvent, method of infiltration, and atmosphere (air, methanol/nitrogen, hydrogen/nitrogen) under which the precursor was converted. The nanowires were characterized by x-ray diffraction (XRD) and transmission electron microscopy (TEM), which confirmed the composition and the shape and length, respectively. Cerium, cobalt, chromium, iron, and nickel nitrate precursors were successfully decomposed to their most stable oxide under air as determined by XRD. Cerium nitrate and cobalt nitrate precursors were decomposed to their oxides under methanol, and cerium nitrate decomposed to the oxide under hydrogen. Short wires or particles were formed in the remaining samples. None of the ethanolic metal halide precursors formed nanowires or nanoparticles visible by TEM. XRD showed diffraction patterns from the decomposition of molybdenum chloride under air and methanol, and from the decomposition of vanadium chloride under hydrogen. As distinct differences were seen between the use of aqueous metal nitrates and ethanolic metal halides, two parallel experiments using cobalt precursors were performed to determine if the differences were due to different solvents or different precursors. The aqueous cobalt chloride precursor formed cobalt oxide nanowires that were less well formed than those observed from aqueous cobalt nitrate precursors. It was also found that the template periodicity was disrupted in the cobalt chloride sample. Cobalt nitrate in ethanol and in acetone gave superior oxide nanowires to those found using aqueous cobalt nitrate, the best structures resulting when the precursor solution was introduced under vacuum. From this, it was concluded that the poor results from aqueous metal halide precursors was primarily the result of differences in the chemistry when converting from the metal halides to the oxides.