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 Brønsted 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
°C, while elemental Zn and Me3SiOMe are exclusively formed under
thermodynamic reaction control at 300 °C. Remarkably, even mixtures of ZnO and
Zn are easily accessible if the decomposition of [MeZnOSiMe3]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 m2 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 °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 [MeZnOSiMe3]4
is proposed.