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Vapor phase synthesized MgO nanocubes as seed structures in solution

Subject Area Solid State and Surface Chemistry, Material Synthesis
Term from 2012 to 2016
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 215441693
 
Final Report Year 2016

Final Report Abstract

This project has aimed at the knowledge-based transformation of vapor phase grown MgO nanoparticle powders into colloidal particle dispersions featuring entirely new types of interfaces. The major motivation for this study has been to link the opportunity to use vapor phase based approaches for surface functionalization and defect engineering to chemical growth processes in colloidal dispersion. The most important reactivity changes we observed depend on aqueous species which are either provided by the gas phase or by organic solvents as dispersion agents. We learned that water, depending on its concentration and state - e.g. as individual molecules producing site selective hydroxylation or as thin films of condensed water on top of the oxide nanostructures - determine the mutual interaction between nanoparticles, their self-assembly, their morphological stability and, last but not least, their dissolution behavior. Thin films of water covering chlorinated metal oxide nanoparticles were found to sustain spontaneous and spatially controllable growth of hydroxide fibers in the ambient. We explored the transformation of surface chlorinated MgO nanocubes into magnesium oxychloride fibers upon contact with water. Specifically we studied the reactivity of MgO nanocubes and larger sized cubes towards water inside the colloidal dispersion or from the gas phase and show how the functionalization process and material dispersion determine the reaction pathway towards the different types of hydroxides. Lessons to be learned from this route to utilize reactive oxide interfaces under ambient conditions can be applied to a variety of microstructural evolution processes that involve high surface area materials and superficial and condensed water acting both as a reactant and as a reaction medium. By means of ZnO nanoparticles which were grown by metal organic chemical vapor synthesis we analyzed the experimental challenges one faces when nanoparticle powders, which have been synthesized in dry and solvent free reaction media, are compared to their colloidal materials counterparts in an aqueous continuous phase. Using vapor phase transfer techniques, we transformed the ZnO nanoparticle powders into aqueous colloidal systems. In the absence of surface active stabilizers, typically employed in colloidal chemistry, these nanoparticles undergo agglomeration. Using defect specific spectroscopic fingerprints such as photoluminescence emission from oxygen interstitials or paramagnetic electron centres we learned to discriminate between artifacts which arise from agglomeration and sedimentation induced local concentration changes and real adsorption induced changes in the electronic structure of the semiconductor in liquid water. Knowledge about the stability of engineered nanomaterials in aqueous systems is critical for predicting their functionality under environmental conditions. For transient electronics, for example, it has become a major challenge to control the factors of materials transformation in different aqueous environments. Our findings do also underline the metastability of nanomaterials under ambient conditions and show the substantial effect ubiquitous water films can have on oxide nanomaterials during storage and characterization.

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