Layer-by-layer assembly of sol–gel oxide “glued” montmorillonite-zirconia multilayers

Abstract

This paper reports the layer-by-layer synthesis and growth kinetics of a new class of nanostructured multilayers consisting of montmorillonite (MMT) nanoclays “glued” by sol–gel oxides, such as zirconia (ZrO₂) and tin oxide (SnO₂), {MMT_x-(sol–gel oxide)}_n. The multilayers possess an ordered layer structure with tunable nanoscale periods of thickness. Systematic investigation of growth kinetics revealed unique underlying film growth mechanisms. The growth of the MMT and sol–gel ZrO₂ layers is strongly coupled. For fresh aqueous ZrO₂ precursors, the growth rates of sol–gel ZrO₂ layers on MMT surfaces as functions of time and precursor concentration do not follow the standard mass transfer or interfacial reaction controlled kinetic models. Furthermore, the growth of the sol–gel oxide layers on MMT surfaces is self-limited to a maximum thickness of ∼50–60 nm. These observations suggest a surface-mediated growth of sol–gel oxide layers on MMT surfaces, and such growth is likely influenced or controlled by electrostatic interactions. For the aged precursors, the growth mechanism differs; the growth of sol–gel oxide layers is controlled by hydrodynamics and follows the Landau–Levich model. These new findings on detailed growth kinetics, which have been difficult to observe and quantify via the synthesis of more prevailing polyelectrolyte-based multilayers, significantly advance the general understanding of the layer-by-layer electrostatic assembly. Overall, layer-by-layer assembly of multilayers using sol–gel oxides, instead of polyelectrolytes, as both adhesive and functional components in the structure, is a new concept of nanoscale fabrication, which can lead to the development of a broad range of inorganic nanostructured films. The mechanical properties and potential applications of this new class of multilayers are briefly discussed.