The University of Massachusetts Amherst
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Seminar: Eric I. Altman, Yale University, “Two Dimensional Tetrahedral Oxides: From Model Catalysts to Atomically Thin Membranes”


Tuesday, May 2, 2017 - 11:30am


LGRT 201



Recent years have seen an explosion of interest in materials constructed of two-dimensional (2D) layers that interact with each other and their surroundings solely through van der Waals (VDW) interactions.  Our focus has been on 2D VDW tetrahedral oxides with surfaces that resemble the catalytically relevant internal surfaces of zeolites while intrinsically featuring molecule-sized openings with the potential to serve as “ultimate” membranes.  The parent member of this new family of materials is SiO2 which forms 2D VDW bilayers constructed of mirror image planes of rings of corner-sharing SiO4 tetrahedra arranged in crystalline or amorphous structures. Aluminum can replace Si in the structure generating catalytic sites, thus allowing us to use surface science tools including scanning probe microscopy to characterize zeolite catalysis down to the atomic level.  It will be shown that when protonated the 2D material can perform the same acid-catalyzed reactions as 3D zeolites.  Although the crystalline form of 2D VDW SiO2 resembles graphene constructed from SiO4 tetrahedra, the flexibility of the linkages between the tetrahedra opens the door to forming a wide range of 2D structural polymorphs as evidenced by the observation of an amorphous phase and nearly periodic defects induced in the structure by uniaxial strain imparted by a Pd(100) substrate.  We have used theory to guide a search for methods to realize this polymorphism in a controlled way.  The results highlight the potential role of lattice strain and charge-balancing cations in 2D aluminosilicates as viable routes to directing the structure of the materials with the goals of replicating the structural diversity seen in three dimensions and designing efficient materials for size-based separations.  Experimentally, epitaxial Ni-Pd alloys with continuously tunable lattice constants have been developed for exploring the role of lattice strain.  Results obtained thus far indicate that despite the VDW nature of the interactions of the 2D materials, the chemical properties of the surface limits how much lattice strain can be imparted; we are investigating putting a reactive Ru layer on top of the alloy surface to enhance the surface interaction and increase the imparted strain.  Finally, we have used theory to search for other oxides that may adopt similar 2D structures.  The most promising materials identified were AlPO4 and GaPO4.  The results reveal general guidelines based on ionic size and reducibility that favor 2D tetrahedral oxide growth.

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