title: Osmotically driven crystal morphogenesis: a general approach to the fabrication of micrometer-scale tubular architectures based on polyoxometalates.
abstract: The process of osmotically driven crystal morphogenesis of polyoxometalate (POM)-based crystals is investigated, whereby the transformation results in the growth of micrometer-scale tubes 10-100 μm in diameter and many thousands of micrometers long. This process initiates when the crystals are immersed in aqueous solutions containing large cations and is governed by the solubility of the parent POM crystal. Evidence is presented that indicates the process is general to all types of POMs, with solubility of the parent crystal being the deciding parameter. A modular approach is adopted since different POM precursor crystals can form tubular architectures with a range of large cationic species, producing an ion-exchanged material that combines the large added cations and the large POM-based anions. It is also shown that the process of morphogenesis is electrostatically driven by the aggregation of anionic metal oxides with the dissolved cations. This leads to the formation of a semi-permeable membrane around the crystal. The osmotically driven ingress of water leads to an increase in pressure, and ultimately rupture of the membrane occurs, allowing a saturated solution of the POM to escape and leading to the formation of a "self-growing" microtube in the presence of the cation. It is demonstrated that the growth process is sustained by the osmotic pressure within the membrane surrounding the parent crystal, as tube growth ceases whenever this pressure is relieved. Not only is the potential of the modular approach revealed by the fact that the microtubes retain the properties of their component parts, but it is also possible to control the direction of growth and tube diameter. In addition, the solubility limits of tube growth are explored and translated into a predictive methodology for the fabrication of tubular architectures with predefined physical properties, opening the way for real applications.
authors: School of Chemistry, WestCHEM, The University of Glasgow, University Avenue, Glasgow G12 8QQ, UK.
authors: Geoffrey J T
authors: GJ
authors: Cooper
authors: Antoine G
authors: AG
authors: Boulay
authors: Philip J
authors: PJ
authors: Kitson
authors: Chris
authors: C
authors: Ritchie
authors: Craig J
authors: CJ
authors: Richmond
authors: Johannes
authors: J
authors: Thiel
authors: David
authors: D
authors: Gabb
authors: Roslyn
authors: R
authors: Eadie
authors: De-Liang
authors: DL
authors: Long
authors: Leroy
authors: L
authors: Cronin