Michael E. Hagerman

Directed Self-Assembly in Laponite/CdSe/Polyaniline Nanocomposites

Laponite films provide versatile inorganic scaffolds with materials architectures that direct the self-assembly of CdSe quantum dots (QDs or EviTags) and catalytic surfaces that promote the in situ polymerization of polyaniline (PANI) to yield novel nanocomposites for light emitting diodes (LEDs) and solar cell applications. Water-soluble CdSe EviTags with varying, overlapping emission wavelengths in the visible spectrum were incorporated using soft chemistry routes within Na-Laponite host film platforms to achieve broadband emission in the visible spectrum. QD concentrations, composition and synthesis approach were varied to optimize photophysical properties of the films and to mediate self-assembly, optical cascading and energy transfer. In addition, aniline tetramers coupled to CdSe (QD-AT) surfaces using a dithioate linker were embedded within Cu-Laponite nanoscaffolds and electronically coupled to PANI via vapor phase exposure. Nanotethering and specific host-guest and guest-guest interactions that mediate nanocomposite photophysical behavior were probed using electronic absorption and fluorescence spectroscopies, optical microscopy, AFM, SEM, powder XRD, NMR and ATR-FTIR. Morphology studies indicated that Lap/QD-AT films synthesized using mixed solvent, layer by layer (LbL) methods exhibited anisotropic supramolecular structures with unique mesoscopic ordering that affords bifunctional networks to optimize charge transport.

Water-Processable Laponite/Polyaniline/Graphene Oxide Nanocomposites for Energy Applications

Graphene-polyaniline (GP) nanocomposites have demonstrated remarkable ability as supercapacitive materials and are typically synthesized via chemical reduction of graphene oxide/polyaniline (GOP) precursors. We report the formation of novel nanomaterials combining GOP nanocomposites with Laponite nanodisks. Host-guest interactions within GOP systems were studied with and without Laponite nanoparticle templating agents. Incorporating Laponite clay into the composite synthesis enhances aqueous dispersibility as well as facilitates the casting of homogeneous films. Structural and morphological characterization confirmed porous heterointerfaces and control of polymer and nanoclay loading. These results may enable the development of flexible supercapacitive and solar nanocomposites with improved device utility, water dispersibility, and film processability. We demonstrate that these films can be easily cast and that the composites maintain their electrical transport properties.

Inorganic/organic host–guest materials: Surface and interclay reactions of styrene with copper(II)-exchanged hectorite

Many important layered silicate–polymer nanocomposite materials may be synthesized using an in-situ polymerization process. Using this technique, organic monomers are intercalated into the interlayer regions of the hosts, where subsequent polymerization may then occur. In this paper, we report on the in-situ polymerization of styrene in Cu(II)-exchanged hectorite thin films. Scanning force microscopy (SFM) images of the polymer surface reveal that the surface polystyrene is generally aggregated into groups of elongated strands. SFM imaging of the interclay regions, in conjunction with X-ray diffraction (XRD) and electron spin resonance (ESR) data, indicates that approximately 20–30% of these regions contain polystyrene, with minimal reduction in the majority of Cu2+ sites observed. XRD data shows little or no intercalation of the monomer into the true intergallery regions. Instead, the polymer likely forms in intercrystallite or planar defect regions. In addition, two distinct phases of polymeric material are found within these defect regions, a highly polymerized polystyrene in addition to a polystyrene form exhibiting greater material stiffness.

Site-Specific Prebiotic Oligomerization Reactions of Glycine on the Surface of Hectorite

Abstract. Condensation reactions of the amino acid glycine on the surface of Cu(II)-exchanged hectorite are investigated using the technique of scanning force microscopy. Prebiotic conditions are simulated using alternate wetting and heating cycles. Concentration, immobilization, and subsequent polymerization resulting in glycine oligomers are seen to occur primarily at step edges or faults in the topmost layer. Condensation reactions also occur within tiny micropores or defects in the topmost layer. These reactions are facilitated by the availability of intergallery metal cations at the step edges or pores in the surface region.

Nanometer-scale structure of hectorite–aniline intercalates

The inclusion of the organic guest aniline into Cu(II)-exchanged hectorite thin films has been investigated. The subsequent polymerization of aniline on the clay surface and in the intergallery regions are studied using scanning force microscopy (SFM), electron paramagnetic resonance (EPR) spectroscopy, powder x-ray diffraction (XRD), and impedance spectroscopy. EPR and XRD data show that in addition to strong polymerization of aniline on the clay surface, polymerization also occurs in the intergallery regions of the clay. Using standard lift-off techniques or razor cleaving, the exposed intergallery polyaniline is successfully imaged for the first time using noncontract SFM in phase-contrast mode. The nearly two-dimensional polymer sheets formed in these regions exhibit none of the nanometer-scale grain or bundle structure commonly associated with polyaniline synthesized using techniques such as electrochemical or vacuum deposition. The electrical response of the resulting clay-conducting polymer composi...

Scanning force microscopy and polymerization studies on cast thin films of hectorite and montmorillonite

Thin films of the smectite clays Na+ exchanged montmorillonite, Ca2+ exchanged montmorillonite, Na2+ and Cu2+ exchanged hectorite, Cu2+ exchanged sintered hectorite, and Cu2+ and Na2+ exchanged hectorite exposed to benzene, aniline, and thiophene were studied using the techniques of scanning force microscopy (SFM), electron spin resonance (ESR), and x‐ray diffraction. The microstructure of the two montmorillonite clays was markedly different, with the Ca‐montmorillonite exhibiting a sturdy, close packed array of crystallites of average dimension 0.4 μm. The Na‐montmorillonite clays consisted of smaller (0.1 μm), more poorly defined crystallites with an overall cauliflowerlike appearance. The pure Na‐hectorite and Cu‐hectorite clays exhibited large flat regions composed on interlocking platelets. Heat sintering as well as exposure to benzene resulted in measurable shifts in the platelets perpendicular to the surface for the Cu‐hectorite. It is proposed that in the sintered case these shifts are due to inte...

The formation of poly(methyl-methacrylate) on transition metal-exchanged hectorite

Abstract Hectorite clay films exchanged with Cu 2+ or Fe 3+ react with methyl-methacrylate monomer in a solventless process to form poly(methyl-methacrylate) (PMMA). Scanning force microscopy (SFM), electron spin resonance (ESR), and X-ray diffraction (XRD) are useful in studying the resulting clay/polymer composite. SFM shows that PMMA forms on the surface of both the Cu 2+ - and Fe 3+ -exchanged films. In addition, XRD shows PMMA formation in the interlayer region of the Fe 3+ -exchanged hectorite but not the interlayer region of Cu 2+ -exchanged hectorite. SFM shows that the morphology of the polymer formed on the film surface depends on: (1) the type of transition metal exchanged into the interlayer region. (2) The mode of delivery (vapor vs. liquid) of methyl-methacrylate monomer to the surface of the clay. PMMA does not form to a significant extent on or within Ca 2+ -exchanged hectorite, and ESR shows no evidence of free radical formation or reduction of the transition metals during any of the polymerization reactions. The results suggest the possibility of an unusual cationic polymerization of methyl-methacrylate to form PMMA with organic cations being stabilized by interaction with the silicate surface. The experimental results have implications for “solventless synthesis” of PMMA and other polymers.

POLYMERIZATION OF BENZENE AND ANILINE ON Cu(II)-EXCHANGED HECTORITE CLAY FILMS: A SCANNING FORCE MICROSCOPE STUDY

The technique of scanning force microscopy (SFM) was used to study the nanometer-scale structure of Cu(II)-exchanged hectorite thin films. Supporting data were also obtained from Electron Spin Resonance (ESR) and X-ray diffraction (XRD) techniques. The surfaces studied included pure Cu(II)-exchanged hectorite, Cu(II)-exchanged hectorite exposed to benzene and Cu(II)-exchanged hectorite exposed to aniline. SFM images of the unexposed Cu(II)-exchanged hectorite surface revealed a smooth surface composed of interlocking platelets. The lateral dimension of these platelets ranged from a few nm to about 1 μm. After exposure to refluxing benzene, the SFM showed that the platelets underwent vertical shifts in position. This is believed to have occurred from intercalated benzene that polymerized in the interlayer region. No SFM evidence was obtained for benzene polymerization on the surface of the hectorite. Hectorite films exposed to aniline at room temperature revealed a post-polymerization structure on the hectorite surface consisting of small polymer bundles. The diameter of these bundles was measured to be 300–3000 Å, similar to the structure seen on electropolymerized polyaniline films. Aniline polymerized on the surface of hectorite films at 180 °C revealed a structure similar to undoped n-methyl-pyrrolidinone (NMP) cast polyaniline films. In this case, the polymer bundles are only 300 Å in dimension on average. XRD and ESR data also indicated interlayer aniline polymerization in Cu(II) exchanged hectorite. Mechanistic considerations affecting these polymerization reactions are presented.

Surface and intergallery catalytic properties of Cu(II)-exchanged hectorite: A scanning force microscope study

Many layered silicate clay minerals such as hectorite readily adsorb or intercalate organic species. Surface or intergallery structural effects, electric fields, or charged particles may then facilitate polymerization or other reactions, resulting in novel inorganic/organic host–guest composite systems. We have studied the interaction of Cu(II)-exchanged hectorite with the polar monomer aniline, nonpolar styrene, and the amino acid glycine. Hectorite is known to readily intercalate aniline into its intergallery regions, where spontaneous polymerization occurs, resulting in nearly two-dimensional sheets of the conducting polymer polyaniline. In addition, aniline vapor is spontaneously polymerized on the surface of Cu(II)-exchanged hectorite, due to the presence of micro-pore sites on the silicate surface where oxidizing Cu2+ cations are available. For the nonpolar styrene molecule, little or no intercalation occurs. Surface polymerization of styrene is observed for long, intense exposures to the monomer in...

Hybrid Inorganic/Organic Self-Assembled Clays Nanocomposites for Roll to Roll Fabrication in Photovoltaics

We have been developing a new approach to layered hybrid (inorganic/organic) photovoltaic materials for fabrication by Roll-to-Roll (R2R) manufacturing. In this report, we combine the low cost and processability of organic electrically conducting polymers with the efficiency of dye sensitized titanium dioxide, semi-conductor quantum dots (CdSe) self-assembled on layered clay materials (Laponite) onto indium tin oxide coated flexible substrate polyethylene terephthalate (PET) substrates. We have shown electron transfer, guest-guest and host-guest interactions, charge separation, spectral line broadening, and quenching of fluorescence signals which indicate electronic coupling of the dye [Ru(bpy) 3 ] 2+ on a CdSe nanocrystal and titanium dioxide nanoparticles. Scanning electron microscopy and atomic force microscopy demonstrate successful nanoparticle formation and thin film self-assembly, as well as surface morphology and polymer thickness.