Growth mechanism of penniform BaWO4 nanostructures in catanionic reverse micelles involving polymers

The formation of penniform BaWO4 nanostructures made of nanowires or nanobelts under the direction of a block copolymer in catanionic reverse micelles has been studied in detail. On the basis of the experimental results obtained from the BaWO4 crystallization in aqueous polymer solutions and careful transmission electron microscopy (TEM) observations of BaWO4 nanostructures formed in reverse micelles containing polymers, a detailed two-stage growth mechanism has been proposed for the formation of the penniform nanostructures in reverse micelles, which involves the polymer-controlled shaft formation (Stage 1) and the mixed surfactants-controlled barb growth (Stage 2). During Stage 1, poly(ethylene glycol)-block-poly(methacrylic acid) (PEG-b-PMAA) induced the formation of c-axis-oriented shuttle-like nanocrystals and the subsequent oriented attachment of these shuttle-like nanocrystals resulted in the formation of [100]-oriented shafts with many parallel [001]-oriented pricks. During Stage 2, [001]-oriented nanowires or nanobelts grew gradually from the pricks into barbs, leading to the formation of well-defined penniform BaWO4 nanostructures with the barb morphology essentially determined by the mixing ratio r of the anionic to cationic surfactants (i.e., nanowires were formed at r=1 while nanobelts were formed at r deviating from 1). The current understanding of the growth mechanism of penniform BaWO4 nanostructures in catanionic reverse micelles involving polymers may be potentially applied for designing a new synthesis system for the controlled synthesis of other hierarchical 1D nanostructures with desired architectures.