Showing papers in "Chemistry of Materials in 1994"

Organization of Organic Molecules with Inorganic Molecular Species into Nanocomposite Biphase Arrays

Abstract: The organization of cationic or anionic organic and inorganic molecular species to produce three-dimensional periodic biphase arrays is described. The approach uses cooperative nucleation of molecular inorganic solution species with surfactant molecules and their assembly a t low temperatures into liquid-crystal-like arrays. The organic/inorganic interface chemistry makes use of four synthesis routes with @+I-), @-I+), (S+X-I+), and (S-M+I-) direct and mediated combinations of surfactant (cationic S+, anionic S-) and soluble inorganic (cationic I+, anionic I-) molecular species. The concepts can be widely applied to generate inorganic oxide, phosphate or sulfide framework compositions. Distinct lamellar, cubic silica mesophases were synthesized in a concentrated acidic medium (S+X-I+), with the hexagonal and the cubic phases showing good thermal stability. For the hexagonal mesostructured silica materials high BET surface areas (>lo00 m2/g) are found. Hexagonal tungsten(V1) oxide materials were prepared in the presence of quaternary ammonium surfactants in the pH range 4-8. Cubic (Iu3d) and hexagonal antimony(V) oxides were obtained by acidifying (pH = 6-7) homogeneous solutions of soluble Sb(V) anions and quaternary ammonium surfactants a t room temperature @+I-). Using anionic surfactants, hexagonal and lamellar lead oxide mesostructures were found (S-I+). Crystalline zinc phosphate lamellar phases were obtained a t low synthesis temperatures (4 \"C) and lamellar sulfide phases could be also readily generated a t room temperature. The synthesis procedure presented is relevant to the coorganization of organic and inorganic phases in biomineralization processes, and some of the biomimetic implications are discussed.

Effect of surfactant/silica molar ratios on the formation of mesoporous molecular sieves : inorganic mimicry of surfactant liquid-crystal phases and mechanistic implications

Abstract: The influence of surfactant/silica molar ratio (Sur/Si) in the synthesis of mesoporous molecular sieve materials (M41S) was studied in a simple ternary synthesis system containing tetraethylorthosilicate (TEOS), water, and the cetyltrimethylammonium (CTMA) cation at 100{degrees}C. The resulting silicate materials were characterized by X-ray diffraction, {sup 29}Si NMR, and FTIR. As the Sur/Si molar ratio increased from 0.5 to 2, the siliceous products obtained could be classified into four separate groups: MCM-41 (hexagonal), MCM-48 (cubic), thermally unstable M41S, and a molecular species, the cubic octamer [(CTMA)SiO{sub 2.5}]{sub 8}. One of the thermally unstable structures has been identified as a lamellar phase. These results are consistent with known micellar phase transformations that occur at various surfactant concentrations and reinforce the concept that liquid-crystal structures serve as templating agents for the formation of M41S type materials. 48 refs., 13 figs., 5 tabs.

Organic Salts with Large Second-Order Optical Nonlinearities

Abstract: This paper presents a review of recent work on the development of organic salts for second-order nonlinear optical applications. In particular, salts in which the cation has been designed to have a large molecular hyperpolarizability and in which variation of the counterion facilitates preparation of crystals with the required noncentrosymmetric packing are discussed. In many cases, this approach has led to materials with large powder second harmonic generation (SHG) efficiencies. One salt, N,N-dimethylamino-N'-methylstilbazolium p-toluenesulfonate, DAST, exhibited an SHG efficiency >1000 times that of a urea powder reference. A common layered-polar-sheet crystal packing motif was observed for DAST and several salt crystals that have been examined crystallographically. A possible explanation for the high incidence of noncentrosymmetric packing in these structures is discussed. Finally, the growth and properties of DAST single crystals are reviewed.

Synthesis and characterization of octahedral molecular sieves (OMS-2) having the hollandite structure

Abstract: Hollandite and cryptomelane materials have been prepared using two different methods. Octahedral molecular siev (OMS) having the 2x2 hollandite structure with a one-dimensional pore diameter of 4.6 {Angstrom} Synthetic cryptomelane or OMS-2 can be formed by refluxing or autoclaving an acidic solution of KMnO{sub 4} and Mn{sup 2+}. Temperature, pH, and countercation are important synthetic parameters. The hollandite formed shows thermal stability up to 600 {degrees}C. OMS-2 formed by oxidation of Mn{sup 2+} under basic conditions and calcined at higher temperature (200-800 {degrees}C) is thermally stable up to 800 {degrees}C. X-ray powder diffraction and electron diffraction patterns have been simulated that show good agreement with experimental data supporting a tetragonal crystal system in the I4/m space group. Hollandites were also prepared in the presence of other transition-metal ions such as Cu{sup 2+} and Fe{sup 3+}. Electron paramagnetic resonance (EPR) data show that OMS-2 materials synthesized in the presence of Cu{sup 2+} and Fe{sup 3+} contain nonexchangeable Mn{sup 2+}. EPR data for Cu-OMS-2 showed a characteristic six-line pattern with a g value of 2.0 and A value of 85 G indicative of octahedral Mn{sup 2+} coordination. The Mn{sup 2+} EPR peaks in Fe-OMS-2 showed similar g and A values. EPR spectra,more » ion-exchange data, X-ray diffraction patterns, and the theoretical simulations of diffraction data suggest that Cu{sup 2+} and Fe{sup 3+} are located in the tunnels of OMS-2. 29 refs., 8 figs., 3 tabs.« less

Photoelectrochemical Studies on Nanocrystalline Hematite Films

Abstract: Nanocrystalline hematite films supported on conducting glass have been prepared from colloidal Fe{sub 2}O{sub 3} particles, giving a roughness factor of about 30. The current-potential characteristic showed an abnormal drop in photocurrent under reverse polarization. Photocurrent yields are low over the entire potential range. In the presence of iodide the abnormal drop was removed and the photocurrent yield increased. Action spectra were determined by illumination from the electrolyte side and the conducting glass support, respectively. The incident photon-to-current conversion efficiencies (IPCE) is a factor of 100 higher in the former compared to the latter case if water is used as a hole scavenger. In the presence of iodide this ratio is diminished 4-fold due to an increase in the IPCE value measured by illumination from the electrolyte side. The poor photocurrent yields observed are due to charge carrier recombination. Recombination at surface sites can be partially intercepted by adsorbed iodide. 9 refs., 9 figs.

Polymorphism and Charge Transport in Vacuum-Evaporated Sexithiophene Films

Abstract: The influence of deposition temperature on structure, orientation, and morphology of vacuum-evaporated sexithiophene films has been studied by using X-ray diffraction, UVvisible spectroscopy and scanning electron microscopy. We correlate this with the mobility of these films as measured in field-effect transistors. The X-ray study based on meridional 001 reflections shows evidence for various crystalline phases depending on the substrate temperature during vapor deposition. A high degree of orientation can be achieved even in several-micrometer-thick films deposited above 190 "C. It is shown that the field-effect mobility is substantially enhanced for deposition temperatures close to the melting point (290 "C), which is associated with a suitable orientation (002 being the single contact plane) and eventually a favorable crystalline structure and coalescent lamellae morphology.

Surface Acidity and Photocatalytic Activity of TiO2, WO3/TiO2, and MoO3/TiO2 Photocatalysts

Abstract: TiO[sub 2] powder samples were calcined at various temperatures and their photocatalytic activity toward the degradation of 1,4-dichlorobenzene was measured. The surface acidity of the sample was also determined by the n-butylamine titration method. A linear relationship between photocatalytic activity and surface acidity was observed for the TiO[sub 2] samples. WO[sub 3]/TiO[sub 2] mixed oxide powders and MoO[sub 3]/TiO[sub 2] greatly enhanced its photocatalytic properties. The addition of WO[sub 3] or MoO[sub 3] to TiO[sub 2] also increased the catalysts' surface acidity, as determined by the n-butylamine titration method. A correlation between the maximum photocatalytic activity and the maximum surface acidity was observed. Whereas acidity may not be the only factor which influences activity, it is certainly an important one. 55 refs., 4 figs., 3 tabs.

Novel synthetic routes to carbon-nitrogen thin films

Abstract: New unimolecular carbon-nitride precursors such as C{sub 3}N{sub 3}F{sub 2}N(SiMe{sub 3}){sub 2} and C{sub 3}N{sub 3}Cl{sub 2}N(SiMe{sub 3}){sub s} were synthesized and used to deposit thin films of composition C{sub 3}N{sub 4}-C{sub 3.2}N{sub 4}, the highest nitrogen content observed in C-N solids. The films were formed by the thermal decomposition of the precursors via elimination of SiMe{sub 3}F and SiMe{sub 3}Cl at 400-500 {degrees}C. Film thicknesses between 1200 and 4000 {Angstrom} were deposited on (100) Si, graphite, beryllium, and SiO{sub 2}, and were extensively characterized for composition and chemical purity using RBS, energy-dispersive X-ray analysis, and SIMS. The material was amorphous as indicated by X-ray diffraction. IR, EELS, and {sub 13}C NMR reveal substantial sp{sup 2} hybridization in both the carbon and the nitrogen. This material should be an excellent precursor for the high-pressure synthesis of C{sub 3}N{sub 4}, the highly sought structural and compositional analog of Si{sub 3}N{sub 4}. 12 refs., 4 figs.

Visible Light Sensitization by cis-Bis(thiocyanato)bis(2,2"-bipyridyl-4,4"-dicarboxylato)ruthenium(II) of a Transparent Nanocrystalline ZnO Film Prepared by Sol-Gel Techniques

Abstract: Reference LPI-ARTICLE-1994-002doi:10.1021/cm00041a020View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Hydrothermal Synthesis and Dielectric Properties of Tetragonal BaTiO3

Abstract: Hydrothermal synthesis of barium titanate (BaTiO{sub 3}) at 240{degrees}C using different barium salts as starting reactants is examined. It is found that halide salts (I{sup {minus}}, Br{sup {minus}}, Cl{sup {minus}}) result in the highest tetragonal content, followed by acetate, nitrate, and hydroxide salts. The tetragonal content was quantified from differential scanning calorimetry by following the endothermic Curie transition around 130{degrees}C and reached as high as 37-39% for the materials made with the barium halides. The particle sizes ranged from 0.15 to 0.37 {mu}m, as estimated by scanning electron microscopy. Sintering and dielectric properties of the barium titanates synthesized from different barium sources were examined. BaTiO{sub 3} made from the chloride salt sintered with high densities at 1250{degrees}C with grain sizes of 1-5 {mu}m and a room-temperature dielectric constant (1 kHz) of 2960. 35 refs., 10 figs., 1 tab.

Molecular Construction of Oriented Inorganic Materials: Controlled Nucleation of Calcite and Aragonite under Compressed Langmuir Monolayers

Abstract: The oriented nucleation of the calcium carbonate polymorphs calcite and aragonite under compressed langmuir monolayers of anionic surfactants was studied by optical and analytical electron microscopy. Whereas n-octadecanoic acid (CH 3 (CH 2 ) 16 COOH) monolayers induced the nucleation of the {1-1.0} face of calcite, n-eicosyl sulfate (CH 3 (CH 2 ) 19 OSO 3 H), and n-eicosyl phosphonate (CH 3 (CH 2 ) 19 PO 3 H 2 ) promoted nucleation of the calcite (00.1) face. The precipitation of the CaCO 3 polymorph araognite is favored by the presence of Mg 2+ in the crystallization medium. Aragonite nucleation under compressed monolayers of these surfactants showed a similar dependence on headgroup stereochemistry. The (100) face was nucleated under eicosanoic acid monolayers, whereas the (001) face was observed in the presence of eicosyl sulfate or eicosyl phosphonate. The nucleation selectivity observed in these experiments can be explained by an interfacial mechanism that involves stereochemical complementarity between the oxygen atoms of the anionic headgroup and those of carbonate ions located in lattice positions in the crystal faces of nuclei forming at the monolayer/solution interface.

Solid-State and Surface Spectroscopic Characterization of BaTiO3 Fine Powders

Abstract: The bulk and surface structure of two BaTiO 3 powders has been investigated by FT-IR and FT-Raman spectroscopies. A high-area sample (39 m 2 /g), produced by a hydrothermal method, cubic to XRD, appears to have lower symmetry to Raman spectroscopy, with unit-cell distortions similar to those of tetragonal ferroelectric BaTiO 3 . It contains BaCO 3 impurities, leading to a bulk Ba/Ti ratio lower than 1, as also evidenced by Raman spectra. Bulk hydroxy groups are evident, possibly needed to balance such a charge defect. Calcination at 1223 K causes the disappearance of these impurities and converts this material in the usual tetragonal phase. The surface properties of both powders, investigated through water and pyridine adsorption, are similar and appear to be dominated by Ba 2+ ions, so leading to a solid characterized by extremely weak Lewis acidity. An interpretaion for the solid-state behavior of the starting powder is proposed

Synthesis, Crystal Structures, and Ion-Exchange Properties of a Novel Porous Titanosilicate

Abstract: A sodium titanosilicate of ideal composition Na{sub 2}Ti{sub 2}O{sub 3}SiO{sub 4}{center_dot}2H{sub 2}O was synthesized hydrothermally in highly alkaline media. Its crystal structure was solved from X-ray powder data by ab initio methods. The compound is tetragonal, a = 7.8082(2), c = 11.9735(4) {angstrom}, space group P4{sub 2}/mcm, Z = 4. The titanium atoms occur in clusters of four grouped about the 4{sub 2} axis and are octahedrally coordinated by oxygen atoms. The silicate groups serve to link the titanium clusters into groups of four arranged in a square of about 7.8 {angstrom} in length. These squares are linked to similar ones in the c direction by sharing corners to form a framework which encloses a tunnel. Half the Na{sup +} ions are situated in the framework coordinated by silicate oxygen atoms and water molecules. The remaining sodiums are present in the cavity, but evidence indicates that some of them are replaced by protons. The composition is then closer to Na{sub 1.64}H{sub 0.36}Ti{sub 2}O{sub 3}SiO{sub 4}{center_dot}1.8H{sub 2}O. The sodium ions within the tunnels are exchangeable. Exhaustive exchange with CsNO{sub 3} yielded a Cs{sup +} phase of composition Na{sub 1.49}Cs{sub 0.2}H{sub 0.31}Ti{sub 2}O{sub 3}SiO{sub 4}{center_dot}H{sub 2}O whose structure was revealed from applicationmore » of Rietveld methods to X-ray powder data. 19 refs., 7 figs., 5 tabs.« less

Polyaniline Wires in Oxidant-Containing Mesoporous Channel Hosts

Abstract: We report the encapsulation of polyaniline filaments in mesoporous hosts with 3 nm channel diameter. The channel size permits encapsulation of several polyaniline chains, and the resulting filaments show microwave conductivity. Processing of information at the molecular level is an intriguing and important challenge. Efforts to create electronic functions and devices based on molecules instead of bulk semiconductors are inspired by the anticipated enormous increase in computing speed and storage density.'Y2 However, this approach requires that communication with nanometer structures or molecules can be established. A major challenge is to electrically isolate the conducting structures, and to achieve charge transfer with low fields as in metallic wires. We have recently demonstrated the encapsulation of several different conjugated polymers such as polypyrrole in the welldefined channels of zeolite molecular sievese3 The synthesis of polyaniline in sol-gel-derived silica gel,4 and the template synthesis of conducting polymers in the pores (ca. 0.1-1 pm) of insulating host membranes has also been reported.5 Here we demonstrate the stabilization of conducting polyaniline filaments in the 3 nm wide hexagonal channels of transition-metal-containing mesoporous aluminosilicate host6 MCM-41. Polyaniline is an unusual conducting polymer because its conductivity is not only controlled by the degree of chain oxidation but also by the level of protonation of [ (-BNHBNH-),(-BN=Q=N-)~.,l., leading to the salt ((PhNH-)A,),.7s In the conducting salt form (emeraldine salt or PANI) x and y are close to 0.5; B, Q, and Ph are C6H4 rings in the benzenoid, quinoid, and intermediate states, respectively, and HA is a strong acid. A proton-induced spin unpairing process on imine nitrogens of the emeraldine salt form of polyaniline is believed to create a conducting polaron lattice. Clays and zeolites ion-exchanged with Cu2+ or Fe3+ have been shown to oxidatively polymerize various monomers of conjugated polymers such as anilines,g pyrrole,lO and thiophene and its de r iva t i~es~~J l inside their cavities. The in situ oxidative polymerization/intercalation of conducting polymers in layered materials containing strong oxidants has also been demonstrated.12 Here we have carried out in situ oxidative adsorption/ polymerization of aniline vapor in Cu-MCM (or FeMCM), either in the absence or presence of air (details in Table 1). The encapsulated emeraldine salt was formed by first adsorbing aniline vapor into the host;13 a maximum of 0.3 g of aniline adsorbed in 1.0 g of Cu-MCM or FeMCM (Table 1). Under exclusion of air, the saturated pink adduct (AN-CuMCM-0, AN-FeMCM-0) was then immersed in an acidic aqueous solution of peroxydisulfate, and a drastic color change to dark green was observed. After thorough washing with water, the materials were dried under vacuum. A typical polymer loading is 0.16 g/1.00 g of Cu-MCM host. The following will address the nature of the encapsulated material, its location, and its ac conductivity. The Cu or Fe ion content in MCM-41 is low due to the low negative charge on the framework (%/A1 = 18) and incomplete ion exchange (Cu/Al ratio equal to 0.21). Therefore there may not be sufficient oxidant to polymerize large amounts of polyaniline inside the host. By carrying out the reaction in air, oxygen could be an indirect oxidant which continues to oxidize the Cu+ (Fe2+) to Cu2+ (Fe3+) during the reaction. This effect has been observed in other cases.14 However, when the reaction was carried out in air, the resulting product AN-FeMCM-2 is not emeraldine salt ES (nor emeraldine base which shows a strong IR peak at 1594 cm-l), but predominantly the nonconducting leucoemeraldine LEB, as demonstrated with IR and NMR spectroscopy (major IR peaks at 1558 sh, 1497 vs, and 1296 m cm-').15 The 13C MAS NMR spectra of conducting polyaniline are featureless due to the presence of charges, which together with the inhomogeneous character of the materials, results in broadened