Robert L. Bedard

Synthesis and structure of microporous layered tin(IV) sulfide materials

Synthetic methods have been developed which yield large single crystals and highly crystalline phase-pure microporous layered SnS-n materials. This allows study of the structure–property–function relations of these materials. The tin sulfide layer of the SnS-1 structure type contains hexagonally shaped 24-atom rings which are constituted by six Sn3S4 broken-cube cluster building units, linked together by double bridge Sn(µ-S)2Sn sulfur bonds. The SnS-3 structure type contains elliptically shaped 32-atom rings which are also constructed from six Sn3S4 broken-cube clusters. However, they are linked by double bridge Sn(µ-S)2Sn sulfur bonds as well as tetrahedral edge-bridging Sn(µ-S2SnS2)Sn spacer units. The SnS-1 structure type [A2Sn3S7 ] was obtained in the presence of A+=Et4N+ , DABCOH+ (protonated 1,8-diazabicyclooctane), and a mixed template system of NH4+ /Et4N+ , while the SnS-3 structure type [A2Sn4S9 ] emerged in the presence of A+=Prn4N+ and Bun4N+ . Various SnS-1 and SnS-3 structures are examined and compared in relation to the size/shape of constituent template cations. A particular kind of structure-directing function was observed, that is, larger template molecules create larger void spaces within and between the tin sulfide sheets. Unique framework flexibility was discovered for both structure types. In order to accommodate the size/shape changes of templates, the flexible porous tin(iv) sulfide layers are able to undergo a certain degree of elastic deformation to alter the architecture of void spaces within and between the layers, rather than forming a completely new porous structure type. This is believed to be responsible for the relatively small number of structure types so far discovered for tin(iv) sulfide-based microporous layered materials compared to the myriad of three-dimensional open-framework structure types found for the zeolites and aluminophosphates. The observed differences among the various SnS-1 or SnS-3 structures is significant and has resulted in distinct adsorption behavior towards guest molecules. The TPA-SnS-3 framework is also found to be pressure sensitive. This all bodes well for envisaged chemical sensor applications for this class of porous materials.

Intermediates in the formation of microporous layered tin(IV) sulfide materials

The formation pathway of microporous layered tin(iv) sulfides A2Sn3S7 and A2Sn4S9 (A=cation), respectively denoted SnS-1 and SnS-3, synthesized from elemental Sn and S sources in the presence of templates and mineralizers has been studied. Three types of reaction intermediates, dimeric [Sn2S6 ]4– , polysulfides [Sn(S4)3 ]2– /[Sn(S4)2(S6)]2– and thiosulfate [S2O3 ]2– , have been isolated and characterized by single crystal X-ray diffraction structure analysis. UV–VIS and 119Sn NMR studies of mother-liquors of representative SnS-n systems indicate that the dimeric [Sn2S6 ]4– anion is the predominant solution tin-containing species and a likely basic building unit for the SnS-n frameworks. The role of template cations has also been examined. The pH dependent condensation–polymerization of the dimeric [Sn2S6 ]4– precursor in the presence of template cations is believed to be responsible for the formation of the SnS-n structures. The dissolution and redox reactions of the elemental Sn metal and S8 powders are found to be extremely sensitive to pH, mineralizers and temperature. Reagents, such as sulfide, fluoride, hydroxide and amines that can react with tin, sulfur and various tin sulfide and polysulfide intermediates are good mineralizers. To produce large single crystals of the SnS-n materials, the concentration and strength of mineralizers need to be kept low, however, to prepare phase pure materials, the presence of excess mineralizer is required. In general, amine templates are strong mineralizers for tin metal particles, presumably through coordination to various tin polysulfide and sulfide intermediates. As a result, amine templates/mineralizers provide cleaner and smaller product crystals under reaction conditions that are similar to those used with tetraalkylammonium hydroxide templates. Surprisingly, it was found that (DABCOH)2Sn3S7 , denoted DABCOH-SnS-1, can be formed using ‘very soft chemistry’ at room temperature and atmospheric pressure conditions, from an aqueous solution of [Sn2S6 ]4– simply by the precise control of pH. The hydrothermal reaction conditions that are generally employed in the synthesis of SnS-n materials only serve to digest the starting material.

Adsorption and sensing properties of microporous layered tin sulfide materials

The adsorption and sensing behavior of three structurally well defined microporous layered tin sulfides R2Sn3S7 , denoted DABCOH-SnS-1, ATEA-SnS-1 and TEA-SnS-1, have been explored. In the case of small non-polar guests a micropore-filling process is observed, whereas for certain larger guests an integration of micropore-filling and intercalation is found to occur. The former is controlled by the size while the latter depends on the adsorption properties of the guest molecules. It seems that guests with a propensity for hydrogen bonding are favored for intercalation. Electrical and optical responses with respect to adsorption of specific guests, such as NH3 , H2S and alcohols, show high sensitivity, reversibility and fast reaction times that are comparable to some commercial semiconductor sensors. This makes microporous layered tin sulfides potentially interesting in environmental, industrial and biomedical monitoring.

Modular assembly and phase study of two- and three-dimensional porous tin(IV) selenides

The work presented focuses attention on a detailed powder and single crystal XRD structure analysis of materials that emerge successively in the hydrothermal synthesis of tetramethylammonium-templated porous tin(IV) selenides. Four phases have been identified in this system and three of them, orthorhombic and monoclinic polymorphs of two-dimensional porous layered TMA 2 Sn 3 Se 7 (TMA=tetramethylammonium) plus a novel tetragonal three-dimensional open-framework TMA 2 Sn 5 Se 10 O have been structurally characterized by single crystal XRD. The order of appearance and interconversion of these materials is shown to follow Oswalds law of successive reactions, which is further supported by Connolly surface calculations. In addition the thermal stability of the orthorhombic TMA 2 Sn 3 Se 7 phase is explored by in situ variable temperature PXRD under nitrogen and in vacuum, from which it is determined that the structural integrity of the framework is retained up to the point of template removal around 250 and 300 °C, respectively. The accumulated knowledge that has emerged from this study and earlier work allows a modular assembly pathway to be proposed that can rationalize the formation of the observed phases.

Spectroscopic properties of microporous layered tin sulfide materials

119 Sn solid state nuclear magnetic resonance, 119Sn Mossbauer, Fourier transform Raman and variable temperature single crystal UV–VIS absorption, emission and excitation spectra of some archetypal microporous layered tin sulfides, denoted (Et4N)2Sn3S7 (TEA-SnS-1), (C6H12N2H)2Sn3S7 (DABCOH-SnS-1), (NH4)0.5(Et4N)1 .5Sn3S7 (ATEA-SnS-1) and (Pr4N)2Sn4S9 (TPA-SnS-3), are reported. The combined spectroscopic results provide a rather comprehensive picture of structure–property relations for this novel class of solids, which have the unusual ability of being able to behave as both molecular sieve and intercalation materials to a range of guests.

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.

Synthesis and Crystal Structure ofδ-GeS2, the First Germanium Sulfide with an Expanded Framework Structure

Polycondensation of molecular adamantanoid [Ge4 S10 ]4- precursors at a remarkably low temperature (50°C) affords the crystalline binary dichalcogenide δ-GeS2 . Its crystal structure contains two interpenetrating cristobalite-like frameworks composed of adamantanoid [Ge4 S6 S4/2 ] building blocks. Rings containing 24 atoms form the largest pores of each network (shown on the right).

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.

Synthese und Kristallstruktur von δ-GeS2, dem ersten Germaniumsulfid mit ausgedehnter Gerüststruktur

Durch Polykondensation bei bemerkenswert niedriger Temperatur (50 °C) werden molekulare, adamantanartige [Ge4S10]4−-Vorstufen in das kristalline, binare Dichalkogenid δ-GeS2 uberfuhrt. Dessen Kristallstruktur enthalt zwei einander durchdringende cristobalitartige Geruste, die aus adamantanartigen [Ge4S6S4/2]-Baueinheiten zusammengesetzt sind. Ringe aus 24 Atomen bilden die grosten Poren jedes Gerusts (siehe rechts).

Cobalt caged for catalysis

Zeolites are materials that have open pores in their structure, large enough to allow some molecules inside, but not others. This, and their acidic properties, make them excellent catalysts — but limited in the reactions that they can catalyse. A new method has been found to make similar structures that contain a high proportion of cobalt, a transition metal, which should allow a wider range of reactions including reduction and oxidation.