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

Abstract

The synthesis and morphology of nano-sized ZnO and unusual belt-like Zn particles prepared via thermolysis of the molecular single-source precursors [MeZnOSiMe 3] 4 (volatile) and [Zn(OSiMe 3) 2] n (non-volatile) are discussed. These zinc siloxides are simply accessible in high yield through Brønsted acid–base reactions between Me 3SiOH and Me 2Zn 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 [MeZnOSiMe 3] 4 furnishes, depending on the reaction conditions, nanocrystalline ZnO and Me 4Si as kinetic products at 160 °C, while elemental Zn and Me 3SiOMe are exclusively formed under thermodynamic reaction control at 300 °C. Remarkably, even mixtures of ZnO and Zn are easily accessible if the decomposition of [MeZnOSiMe 3] 4 is achieved at 200 °C. 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(OSiMe 3) 2] n polymer gives products similar to those obtained in the case of the decomposition of the heterocubane [MeZnOSiMe 3] 4, that is ZnO and/or elemental Zn, depending on the reaction conditions. It turned out that decomposition of the non-volatile polymer [Zn(OSiMe 3) 2] n proceeds via a Me–Si/Zn–O metathesis reaction, leading to polysiloxanes and the heterocubane [MeZnOSiMe 3] 4 as primary products. In contrast, the gas-phase decomposition of [MeZnOSiMe 3] 4 under chemical-vapour-synthesis (CVS) conditions (high dilution) takes place only at higher temperatures (> 500 °C), leading to elemental Zn particles covered by oligodimethylsiloxanes. Spontaneous separation of the Zn particles and their polysiloxane shells is observed at 750 °C, 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 (HRTEM). Finally, a mechanism for the stepwise CVS decomposition of [MeZnOSiMe 3] 4 is proposed. To cite this article: M. Driess et al., C. R. Chimie 6 (2003).