Tris(<i>tert-</i>butoxy)siloxy Complexes as Single-Source Precursors to Homogeneous Zirconia- and Hafnia-Silica Materials. An Alternative to the Sol-Gel Method

The tris(tert-butoxy)siloxy complexes M[OSi(OtBu)3]4 (1, M = Zr and 2, M = Hf) are efficient single-source precursors to homogeneous zirconia-silica and hafnia-silica materials. The ceramic conversions occur pyrolytically at low temperatures, give the theoretical yield for MO2·4SiO2 compositions, and occur without the incorporation of carbon. Compounds 1 and 2 are isomorphous, and crystallize as a mixture of two isomers. One isomer contains a 4-coordinate metal center, while the other is 5-coordinate and features a chelating η2-OSi(OtBu)3 ligand. The solid-state transformation of 1 to ZrO2·4SiO2 produces an open, fibrous material that is somewhat ordered with pores which are ∼20 nm in diameter. The solution-phase thermolysis of 1 produces a xerogel composed of small primary particles (≤∼5 nm). This xerogel possesses a high surface area (700 m2 g-1 for samples heated to 200 °C). Solutions of the precursors have been used to cast crack-free thin films of MO2·4SiO2. The MO2·4SiO2 materials are amorphous as initially formed, and subsequent crystallizations of tetragonal ZrO2, monoclinic ZrO2, and cristobalite occur at relatively high temperatures. The enhanced stabilization of the amorphous and tetragonal phases of zirconia, relative to those derived from many sol-gel systems, implies that these single-source precursors initially produce homogeneous materials and that subsequent crystallizations are to a large degree diffusion-controlled. Homogeneity in the initially formed ZrO2·4SiO2 material is also suggested by solid-state 29Si NMR spectra. The molecular route to oxide materials described here represents an alternative to the sol-gel method and may offer certain advantages.