Hee K. Chae

Reticular synthesis and the design of new materials

The long-standing challenge of designing and constructing new crystalline solid-state materials from molecular building blocks is just beginning to be addressed with success. A conceptual approach that requires the use of secondary building units to direct the assembly of ordered frameworks epitomizes this process: we call this approach reticular synthesis. This chemistry has yielded materials designed to have predetermined structures, compositions and properties. In particular, highly porous frameworks held together by strong metal–oxygen–carbon bonds and with exceptionally large surface area and capacity for gas storage have been prepared and their pore metrics systematically varied and functionalized.

Exceptional chemical and thermal stability of zeolitic imidazolate frameworks

Twelve zeolitic imidazolate frameworks (ZIFs; termed ZIF-1 to -12) have been synthesized as crystals by copolymerization of either Zn(II) (ZIF-1 to -4, -6 to -8, and -10 to -11) or Co(II) (ZIF-9 and -12) with imidazolate-type links. The ZIF crystal structures are based on the nets of seven distinct aluminosilicate zeolites: tetrahedral Si(Al) and the bridging O are replaced with transition metal ion and imidazolate link, respectively. In addition, one example of mixed-coordination imidazolate of Zn(II) and In(III) (ZIF-5) based on the garnet net is reported. Study of the gas adsorption and thermal and chemical stability of two prototypical members, ZIF-8 and -11, demonstrated their permanent porosity (Langmuir surface area = 1,810 m2/g), high thermal stability (up to 550°C), and remarkable chemical resistance to boiling alkaline water and organic solvents.

A route to high surface area, porosity and inclusion of large molecules in crystals

One of the outstanding challenges in the field of porous materials is the design and synthesis of chemical structures with exceptionally high surface areas. Such materials are of critical importance to many applications involving catalysis, separation and gas storage. The claim for the highest surface area of a disordered structure is for carbon, at 2,030 m2 g-1 (ref. 2). Until recently, the largest surface area of an ordered structure was that of zeolite Y, recorded at 904 m2 g-1 (ref. 3). But with the introduction of metal-organic framework materials, this has been exceeded, with values up to 3,000 m2 g-1 (refs 4–7). Despite this, no method of determining the upper limit in surface area for a material has yet been found. Here we present a general strategy that has allowed us to realize a structure having by far the highest surface area reported to date. We report the design, synthesis and properties of crystalline Zn4O(1,3,5-benzenetribenzoate)2, a new metal-organic framework with a surface area estimated at 4,500 m2 g-1. This framework, which we name MOF-177, combines this exceptional level of surface area with an ordered structure that has extra-large pores capable of binding polycyclic organic guest molecules—attributes not previously combined in one material.

Preparation of SrBi2Ta2O9 thin films with a single alkoxide sol–gel precursor

For the first time, a single alkoxide sol–gel precursor solution for the ferroelectric strontium bismuth tantalate (SrBi 2 Ta 2 O 9 , SBT) was synthesized and utilized for the fabrication of its thin films. The precursor was prepared from a 2-methoxyethanol solution of Sr(OCH 2 CH 2 OCH 3 ) 2 , Bi(OCH 2 CH 2 OCH 3 ) 3 , and Ta(OCH 2 CH 2 OCH 3 ) 5 . 1 H and 13 C NMR spectra of the precursor in benzene show only one set of alkoxy groups, indicating the same chemical environment in solution. This observation suggests that it is a single sol–gel precursor, which is ideal for the sol–gel processing of SBT thin films. The SBT films derived from this precursor present outstanding ferroelectric properties and surface morphology.

An organometallic mercaptopyridine complex with unusual bond shift fluxionality: metal-mediated tautomerism of (pentamethylcyclopentadienyl)bis(pyridine-2-thiolato)rhodium(III)

Abstract Studies have been carried out on unique molecular non-rigidity of Cp*RhIII(PyS)2 (PyS=pyridine-2-thiolato). One PyS ligand bonds to the rhodium ion in an S-monodentate mode (RhS(2)=2.3799(9) A) while the other ligand chelates to the metal in an N,S-bidentate mode (RhS(1)=2.4380(9); RhN(1)=2.089(2) A). Both PyS ligands possess a significant contribution from the thiol tautomer in the solid state. Even though the conformation and configuration of the molecule are still retained in solution, an unusual metal-mediated tautomeric non-rigidity for the PyS region is observed in the solution around room temperature.

The Sol-Gel Chemistry of Ferroelectric SrBi2Ta2O9 Thin Layers

A characterization of sol-gel precursors of SrBi2Ta2O9 (SBT) has been carried out. Each molecular precursor, [SrBi2+xTa2(OCH2CH2OCH3)18]n (1) (x = 0.0, 0.2, and 0.4) and [SrTa2(OEt)(OiPr)11(iPrOH)2] (2) was prepared from the mixtures of Bi(OR)3, Sr(OR)2, and Ta(OR)5 (R = Et, iPr, and CH2CH2OCH3), respectively. 1H and 13C NMR spectroscopy and powder X-ray diffractometry have been used to characterize precursor molecules and their oxides, respectively. Especially, X-ray single crystal studies of complex 2 show that the molecule is made up of three octahedra; the central [SrO6] octahedron is sharing two neighboring edges with the peripheral [TaO6] ones. The SBT film derived from the precursor 1 presents outstanding ferroelectric properties (Pr = 9 μC/cm2; Ec = 0.8 V).

Pressure Effect on the Formation of Ferroelectric SrBi_2Ta_2O_9 Thin Films

It has been found that the formation of a ferroelectric phase for the SrBi2Ta2O9 (SBT) thin films derived from sol–gel solution is dependent on the oxygen pressure during the heat-treatment process. Under an elevated oxygen pressure as high as 30 atm, the heat-treatment temperature inducing the ferroelectric SBT phase can be lowered to 650°C. Those films processed at 650°C present satisfactory ferroelectric properties, that is, the remanent polarization (Pr) is 5.0 µC/cm2, and the coercive field (Ec) is 43 kV/cm with 5 V application.

Sol–gel precursor effect on the formation of ferroelectric strontium bismuth tantalate thin films

Abstract Two chemical solutions have been prepared for the derivation of strontium bismuth tantalate [SrBi2Ta2O9 (SBT)] thin films. One is a single alkoxide sol–gel precursor, chemically linked at a molecular level, prepared from 2-methoxyethanol solutions of strontium 2-methoxyethoxide [Sr(OCH2CH2OCH3)2], bismuth 2-methoxyethoxide [Bi(OCH2CH2OCH3)3], and tantalum 2-methoxyethoxide [Ta(OCH2CH2OCH3)5]. The other is a simple mixture solution prepared from strontium bis(2,2,6,6- tetramethyl-3,5-heptanedionate [Sr(TMHD)2], triphenyl bismuth [Bi(C6H5)3], and Ta(OCH2CH2OCH3)5. The chemical structures of the precursors were analyzed by 1H and 13C nuclear magnetic resonance (NMR), and their thermal decomposition behaviors were investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). We have discussed the role of precursors in the formation of a crystallographic phase, grain structure, and ferroelectric properties for the derived SBT films. It has been found that the process temperature and the amount of excess Bi, needed to form the ferroelectric phase, are strongly dependent on the sol–gel solution. By heat-treatment at 700°C for 1 h, utilizing our single alkoxide sol–gel solution, we have obtained SBT films demonstrating a saturated hysteresis loop at 3 V.

Preparation of Impurity-Doped TiO2 Nanoparticles and Their Applications for Photocatalysis and Biopanning

A series of surface-modified TiO2 nanoparticles were prepared and their photocatalytic effects were evaluated by degradation of Acid red dye in aqueous solution under UV light irradiation. Phage display peptide libraries were used for peptide sequence to bind the doped TiO2 particles selectively.