Film Growth Rates and Activation Energies for Core-Shell Nanoparticles Derived from a CVD Based Aerosol Process.

Frederik Weis,Gerhard Kasper,Martin Seipenbusch

doi:10.3390/ma8030966

Frederik Weis, Gerhard Kasper + Show 1 more

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https://doi.org/10.3390/ma8030966

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Abstract

Silica core-shell nanoparticles of about 60–120 nm with a closed outer layer of bismuth or molybdenum oxide of 1–10 nm were synthesized by an integrated chemical vapor synthesis/chemical vapor deposition process at atmospheric pressure. Film growth rates and activation energies were derived from transmission electron microscopy (TEM) images for a deposition process based on molybdenum hexacarbonyl and triphenyl bismuth as respective coating precursors. Respective activation energies of 123 ± 10 and 155 ± 10 kJ/mol are in good agreement with the literature and support a deposition mechanism based on surface-induced removal of the precursor ligands. Clean substrate surfaces are thus prerequisite for conformal coatings. Integrated aerosol processes are solvent-free and intrinsically clean. In contrast, commercial silica substrate particles were found to suffer from organic residues which hinder shell formation, and require an additional calcination step to clean the surface prior to coating. Dual layer core-shell structures with molybdenum oxide on bismuth oxide were synthesized with two coating reactors in series and showed similar film growth rates.

Highlights

Nanoparticles with core-shell structures have wide applications ranging from catalysis to medical applications and optoelectronic devices
Growth rates for molybdenum oxide films on silica particles were derived from the analysis of a series of transmission electron microscopy (TEM) images at a residence time of 50 s in the coating reactor
No film growth was detected below 110 °C; above 140 °C no further increase of the film growth rate was observed due to the onset of competing homogeneous gas phase decomposition of the precursor resulting in the formation of fractal agglomerates of molybdenum oxide

Summary

Introduction

Nanoparticles with core-shell structures have wide applications ranging from catalysis to medical applications and optoelectronic devices. As an alternative to conventional multistep liquid phase synthesis routes, several gas phase processes have been developed and proven to offer continuous, solvent-free and scalable synthesis methods [1,2,3] Such an integrated chemical vapor synthesis (CVS)/chemical vapor deposition (CVD) process was used to produce well defined core-shell structures with a silica core and an oxide outer layer. A key feature of this approach is that spherical core particles are prepared continuously by decomposition of tetra-ethyl-ortho-silicate (TEOS), and transferred immediately to a CVD coating step with molybdenum and/or bismuth oxide. These coating materials were selected, since they have potential as supported thin film catalysts for the selective oxidation of hydrocarbons [4,5,6,7].

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