From molecules to metastable solids: solid-state and chemical vapour syntheses (CVS) of nanocrystalline ZnO and Zn

The synthesis and morphology of nano-sized ZnO and unusual belt-like Zn particles prepared via thermolysis of the molecular single-source precursors [MeZnOSiMe3](4) (volatile) and [Zn(OSiMe3)(2)](n) (non-volatile) are discussed. These zinc siloxides are simply accessible in high yield through Bronsted acid-base reactions between Me3SiOH and Me2Zn in the molar ratio of 1:1 and 2:1, respectively. These precursors show rather unusual ambivalent decomposition behaviour, leading to ZnO and Zn nanoparticles at the same time. Thus, solid-state thermolysis of [MeZnOSiMe3](4) furnishes, depending on the reaction conditions, nanocrystalline ZnO and Me4Si as kinetic products at 160 degreesC, while elemental Zn and Me3SiOMe are exclusively formed under thermodynamic reaction control at 300 degreesC. Remarkably, even mixtures of ZnO and Zn are easily accessible if the decomposition of [MeZnOSiMe3](4) is achieved at 200 degreesC. The ZnO particles (wurtzite structure) have a particle size of 10-12 nm and a very large surface area of ca 110 m(2) g(-1). Unexpectedly, thermolysis of the non-volatile [Zn(OSiMe3)(2)](n) polymer gives products similar to those obtained in the case of the decomposition of the heterocubane [MeZnOSiMe3](4), that is ZnO and/or elemental Zn, depending on the reaction conditions. It turned out that decomposition of the non-volatile polymer [Zn(OSiMe3)(2)](n) proceeds via a Me-Si/Zn-O metathesis reaction, leading to polysiloxanes and the heterocubane [MeZnOSiMe3](4) as primary products. In contrast, the gas-phase decomposition of [MeZnOSiMe3](4) under chemical-vapour-synthesis (CVS) conditions (high dilution) takes place only at higher temperatures (> 500 degreesC), leading to elemental Zn particles covered by oligodimethylsiloxanes. Spontaneous separation of the Zn particles and their polysiloxane shells is observed at 750 degreesC, affording crystalline Zn with a rod- and belt-like morphology and a typical length of a few micrometers. The latter nano-whiskers were characterized by Electron Dispersion X-Ray Analysis (EDX) and High-Resolution Transmission Electron Microscopy (HR-TEM). Finally, a mechanism for the stepwise CVS decomposition of [MeZnOSiMe3](4) is proposed.