Limin Huang

Synthesis, morphology control, and properties of porous metal–organic coordination polymers

Abstract A “direct mixing” synthesis strategy has been demonstrated for the first time that allows fast (e.g., 0.5 h) synthesis of bulk quantity of thermally stable and highly porous metal–organic coordination polymers (MOCP) nanocrystals (30–150 nm diameter) at room temperature with high yield (∼90%). The MOCP materials constructed from Zn ( NO 3 ) 2 · 6 H 2 O and 1,4-benzenedicarboxylic acid (H 2 BDC) were characterized with scanning electron microscope, powder X-ray diffraction, thermal gravimetric analysis, FT-IR, and volumetric Ar adsorption/desorption. The direct mixing method also produced a highly porous nanometer-sized MOCP material, which is likely to be a new phase that has not been discovered by the more commonly used “diffusion” approach. “Soft” and “hard” template approaches were used to successfully manipulate the morphology of MOCP materials at nanometer scale. In addition, water molecules were shown to play an important role in the synthesis and eventual composition of MOCP materials. Exposure of MOCP materials to water resulted in dramatic drop in surface area and porosity because of possible hydrolysis of the framework. An acid hydrolysis process of MOCP materials was also revealed in which the crystals could be hydrolyzed back into metal salts and organic acid under acid treatment.

Nafion-bifunctional silica composite proton conductive membranes

Sol–gel derived sulfonated phenethylsilica with hydrophilic –Si-OH and proton conductive –SO3H functional groups was used as a bifunctional additive to improve the proton conductivity and water uptake characteristics of Nafion. Nafion-bifunctional silica (NBS) composite membranes were prepared by casting Nafion–ethanol solutions mixed with sulfonated phenethylsilica sol. The ion exchange capability of NBS composite membranes increases linearly with the amount of bifunctional silica incorporated, and is about 1.9 × 10−3 mol SO3H g−1 for NBS with 7.5 wt% silica. Liquid water uptake measurements showed that NBS composite membranes have higher water uptake (g H2O g−1 composite membrane) than bare recast Nafion while the degree of hydration (i.e., nH2O–SO3H) remains fairly constant. The NBS composite membranes showed improved proton conductivity when compared with bare recast Nafion and Nafion 117 membranes at 80 °C and over a range of relative humidity.

Barium titanate nanocrystals and nanocrystal thin films: Synthesis, ferroelectricity, and dielectric properties

Advanced applications for high k dielectric and ferroelectric materials in the electronics industry continues to demand an understanding of the underlying physics in decreasing dimensions into the nanoscale. We report the synthesis, processing, and electrical characterization of thin (<100nm thick) nanostructured thin films of barium titanate (BaTiO3) built from uniform nanoparticles (<20nm in diameter). We introduce a form of processing as a step toward the ability to prepare textured films based on assembly of nanoparticles. Essential to this approach is an understanding of the nanoparticle as a building block, combined with an ability to integrate them into thin films that have uniform and characteristic electrical properties. Our method offers a versatile means of preparing BaTiO3 nanocrystals, which can be used as a basis for micropatterned or continuous BaTiO3 nanocrystal thin films. We observe the BaTiO3 nanocrystals crystallize with evidence of tetragonality. We investigated the preparation of wel...

Characterization of Beta/MCM-41 composite molecular sieve compared with the mechanical mixture

Abstract A Beta/MCM-41 composite was prepared for the first time through a two-step crystallization process combining a low crystallized zeolite Beta synthesis gel with a surfactant cetyltrimethylammonium bromide (CTAB) solution. The composite has been investigated and compared with a mechanical mixture of Beta and MCM-41 for their structure, acidity and catalytic activity by means of XRD, SEM, 129 Xe NMR, DTG, N 2 adsorption, IR spectra of pyridine and catalytic cracking reaction. The XRD result shows that the Beta/MCM-41 composite possesses a well-ordered mesoporous MCM-41 phase and a zeolite Beta phase. The result of N 2 adsorption indicates that the composite contains a bimodal mesopore system and the microporous structure of zeolite Beta. Moreover, the results of 129 Xe NMR and DTG suggest that the mesopore wall of the composite is constructed by a lot of secondary building units characteristic of zeolite Beta, which is quite different from the amorphous character of MCM-41 in the mechanical mixture. Although the total number of acid sites of the composite is comparable to that of the mechanical mixture, the composite possesses more strong Bronsted acid sites due to the initially crystallized mesostructure and shows higher catalytic activity for n -heptane cracking compared with the mechanical mixture.

Self-organizing high-density single-walled carbon nanotube arrays from surfactant suspensions

Very thin films of oriented and densely packed single-walled carbon nanotubes (SWNTs) can be self-assembled on substrates from surfactant sodium dodecyl sulfate (SDS-) coated SWNT suspensions at ambient conditions. The evaporation of water causes a concentration of the SDS-coated nanotubes above critical micelle concentrations for SDS, and it is believed that self-organization of the SDS molecules serves as a driving force for the oriented and dense assembly of the nanotubes. The high degree of alignment in the SWNT thin films was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and polarized Raman spectroscopy.

Synthesis and characterization of composite molecular sieves comprising zeolite Beta with MCM-41 structures

Zeolite Beta/MCM-41 composites have been prepared with various crystallinities of zeolite Beta through a two-step crystallization process involving the combination of low crystallinity zeolite Beta synthesis gel with cetyltrimethylammonium bromide surfactant solution. The composites are characterized by XRD, SEM, 29Si and 27Al MAS NMR, N2 adsorption, NH3-TPD and catalytic cracking. The experimental results show that the relative crystallinity of Beta in the composites greatly affects the morphology and surface acidity of the materials, while exhibiting similar 29Si, 27Al MAS NMR spectra and N2 adsorption–desorption isotherms. The pore structural data indicate that the composites contain bimodal mesopore systems and microporous structures of zeolite Beta. Although the number of weak acid sites on the composites with various crystallinities of Beta is comparable, composites with higher Beta crystallinity possess more medium acid sites and show higher catalytic activity for n-heptane cracking.

Determination of the Young's Modulus of Structurally Defined Carbon Nanotubes

We have combined optical characterization with a magnetic actuation technique to measure the stiffness of single-walled carbon nanotubes of defined crystal structure. The measured stiffnesses correspond to an average Youngs modulus of E = 0.97 +/- 0.16 TPa. For the structures investigated, no dependence on the nanotube chiral index was observed within the indicated experimental accuracy.

Precise positioning of single-walled carbon nanotubes by ac dielectrophoresis

The precise placement of single-walled carbon nanotubes (SWCNTs) in device architectures by ac dielectrophoresis involves the optimization of the electrode geometry, applied voltage and frequency, load resistance, and type of nanotube sample used. The authors have developed a toolkit to controllably integrate SWCNTs in device structures by the use of floating potential metal posts and appropriate electrode geometries, as designed using electric field simulations, and used it to fabricate structures such as crossed nanotube junctions.

Orientated assembly of single-walled carbon nanotubes and applications

Single-walled carbon nanotubes (SWNTs) have unique structural, mechanical, thermal and electrical properties, which make them attractive and important building blocks for nanotechnology. Orientated assembly of SWNTs presents a crucial prerequisite for both fundamental research at the individual SWNT level and nanotube-based device fabrication and applications. In this feature article, we review recent progress in the field with a focus on the orientated assembly of SWNTs (horizontally aligned and vertically aligned) from solution deposition and direct chemical vapor deposition, and on possible growth mechanisms. We also discuss our own research efforts in orientation control by chemical vapor deposition, and how this enables the fundamental characterization of individual SWNTs, and contributes to nanotube-based device fabrication and applications.

Synthesis and Corrosion Resistance of High-Silica Zeolite MTW, BEA, and MFI Coatings on Steel and Aluminum

High-silica zeolite ZSM-12Na n (Al n Si 28-n O 56 )∼4H 2 O with n = 2 (MTW), Beta Na n (Al n Si 64-n O 128 ) with n = 7 (BEA), and ZSM-5 Na n (SiO 96-n O 192 )∼16H 2 O with n < 27 (MFI) coatings were synthesized by hydrothermal crystallization without seeding using a new two-silica method on Al-alloy (Al-2024-T3 and Al-6061-T4) and stainless steel (SS-304). The pure-silica MTW and BEA coatings synthesized in this study have not been previously reported. All coatings are continuous and have excellent adhesion The as-synthesized zeolite coatings were tested by a dc polarization technique and found to he highly corrosion resistant in severely corrosive acidic (0.5 M H 2 SO 4 aqueous solution) and alkaline (0.1 M NaOH aqueous solution) media. In particular, a pure-silica MFI coating of only a few hundreds of nanometers thickness was formed within a relatively short crystallization time (e.g., 15 min), and was shown to provide sufficient protection against corrosion.