Carol L. Bowes

Effect of microgravity on the crystallization of a self-assembling layered material

In microgravity, crystals of semiconductors and proteins can be grown with improved crystallinity, offering the prospect of improved structural analyses (for proteins) and better electronic properties (for semiconductors). Here we study the effect of a microgravity environment on the crystallization of a class of materials—layered microporous tin(IV) sulphides—whose crystal structure is determined by weak interlayer interactions (electrostatic, hydrogen-bonding and van der Waals) as well as strong intralayer covalent bonds. We find that the crystals grown in microgravity (on board the Space Shuttle Endeavour) show improved crystal habits, smoother faces, greater crystallinity, better optical quality and larger void volumes than the materials grown on Earth. These differences are due at least in part to the profound influence of microgravity on the layer registry over length scales of around a nanometre, which is shown by X-ray and electron diffraction to be better in space than on Earth. Thus we can see a clear distinction between the covalent bonds in these materials, which are not significantly affected by microgravity, and the weaker forces (like those that determine the structure of proteins over length scales of around 0.3–0.4 nm) which are more susceptible to the dynamic disturbances that operate in crystallization on Earth.

Microporous layered tin sulfide, SnS-1: molecular sieve or intercalant?

The results of a detailed study of the structure, thermal/pressure stability, ion-exchange, amine intercalation and adsorption property relations, of microporous layered tin(iv) sulfides A2Sn3S7 , denoted SnS-1, where A=tetramethylammonium (TMA), quinuclidinium (QUIN), or tert-butylammonium (TBA), are reported and comparisons are made to zeolites and layered metal sulfides. The thermal stability of the materials indicates potential for the removal of void filling organic template without affecting the integrity of the lattice. The sensitivity of the materials to applied pressure is found to depend on the nature of the occluded template. Aqueous ion-exchange and gas-phase exchange by amines are observed in SnS-1 templated with the tertiary ammonium cation. This allows for an intercalation-like process similar to the layered chalcogenides. Adsorption isotherms for SnS-1 show it to exclude gaseous species of kinetic diameter >3.4 A, thus acting as a molecular sieve like zeolites and showing potential for catalysis, separation and chemical sensing applications.

Nanomaterials: Tin(IV) Sulfide Endo-and Exosemiconductors

Endosemiconductors, materials produced when atom constituents of bulk semiconductors are reorganized into ordered arrays of single size and shape clusters encapsulated within a nanoporous host, are introduced with the report of a novel tin(IV) sulfide endosemiconductor synthesized by a two-step MOCVD-like self-assembly process. Template-mediated hydrothermal syntheses of phase-pure and large single crystal tin(IV) sulfide exosemiconductors, materials resulting from reorganization of those same atomic constituents into an open-framework crystalline nanoporous structure is also discussed. The properties of these nanomaterials are considered in terms of the degree of coupling between molecule-like constituent clusters, a concept which mediates the nanoworld of matter intermediate between molecular and bulk.

Tin sulfide clusters in zeolite Y, Sn4S6-Y

The synthesis of a tin sulfide cluster array, denoted Sn4S6-Y, using the quantitative sequential surface anchoring and reaction of tetramethyltin with hydrogen sulfide (metal organic chemical vapor deposition, MOCVD, reagents) within the supercages of acid zeolite-Y is detailed. The tethered methyltin species and their transformation to the encapsulated tin sulfide clusters are elucidated through gravimetry, coupled with mid-IR and 119Sn Mossbauer spectroscopy. Transmission electron microscopy (TEM) and Rietveld refinement of synchrotron powder X-ray diffraction (PXRD) data show that the clusters are internally confined and homogeneously dispersed in the supercages of the host zeolite, while optical spectroscopy show a cluster size dependent blue-shift of the absorption edge with respect to the bulk tin sulfide phase. A geometry is proposed for the encapusulated Sn4S6-Y cluster.

Electrical Sieves for Molecule Recognition

Nanoporous materials with crystallographically defined pores represent an attractive class of materials for molecule discriminating chemical sensor applications. Recently, zeolite-modified surface acoustic wave (SAW), zeolite-loaded piezoelectric quartz crystal microgravimetric (QCM), and aluminophosphate molecular sieve coated capacitance devices have shown promise in selectively detecting organic molecules. As zeolites are electrical insulators it would be interesting to find electrically conducting open-framework materials whose charge-transport response is sensitive to the size and shape of adsorbed molecules. Recently, nanoporous tin(IV) sulfides have been synthesised and structurally characterized. Considering the semiconductor character of tin(IV) disulfide, the nanoporous tin(IV) sulfide versions are potentially interesting electronic molecular sieves for chemical sensing. In this paper synthetic, structural, ionexchange, thermochemical and vibrational information for various members of the R2Sn3S7 structure class of nanoporous tin(IV) sulfides, where R represents an alklyammonium template cation, will be briefly highlighted. The adsorption, optical and electrical responses of these materials are found to be sensitive to the identity and concentration of a range of molecular guests. Furthermore, their adsorption processes are fast and reversible. These are attributes of a molecule recognition material for sensing applications.