Surface functionalization of nanodiamond with phenylphosphonate.

HYPOTHESIS: This work explores arylation of oxidized nanodiamond (OND) and carboxylated nanodiamond (CND) using phenylphosphonate (PPA) to produce PPA-grafted nanodiamond (PPA@ND). The specific hypothesis is that PPA can be readily grafted onto the surfaces of OND and CND particles via the nanodiamonds' reactive oxygen-rich surface functional groups. EXPERIMENTS: This work explores the effect of varying reaction conditions (reactant ratio, heat, sonication) on PPA graft density and grafting mode. The products were characterized by a variety of techniques including: (31)P MAS NMR and FTIR to confirm PPA grafting, XPS to estimate PPA grafted amounts, and TGA to assess thermal stability. The utility of PPA functionalization for improving ND dispersion in organic solvents was evaluated using dynamic light scattering. FINDINGS: The results confirm PPA grafting on both OND and CND, quantify the extent of PPA grafting, and suggest primarily bidentate grafting. For PPA grafting on OND, the grafted amount does not depend on PPA:ND ratio, application of heat, or use of sonication. The grafted amount of PPA on CND was significantly lower than that on OND. TGA data indicates that grafted PPA improves the thermal stability of both OND and CND. The starting OND disperses in water as aggregates with approximately 100 nm particle size. However, PPA-grafted OND does not disperse at all in water: thus grafted PPA changes the ND surface from hydrophilic to hydrophobic. PPA@OND prepared without sonication does not completely disperse in either DMF or toluene. Only PPA@OND prepared with sonication disperses well in these solvents, suggesting that sonication promotes more complete exposure of the ND surface to PPA grafting.

Terthiophene-containing copolymers and homopolymer blends as high-performance dielectric materials.

This work explores the dielectric and polarization properties of block copolymers and homopolymer blends containing a terthiophene-rich, electronically polarized block (PTTEMA) and an insulating polystyrene block (PS). PTTEMA-b-PS block copolymers were synthesized by reverse addition-fragmentation chain transfer (RAFT) polymerization, and PTTEMA/PS homopolymer blends with the same PTTEMA weight percentages were produced by solution blending. DSC and XRD characterization show that crystallinity increases with PTTEMA content, indicating the presence of terthiophene-rich crystalline domains. Under an applied electric field, these domains are electronically polarized, but the insulating PS block inhibits current leakage, resulting in enhanced dielectric properties. Impedance measurements show that relative permittivity increases with PTTEMA content. The permittivity values are higher in PTTEMA-b-PS copolymers with moderate PTTEMA content due to the ability of the PS block to inhibit PTTEMA association, resulting in a higher density of isolated, terthiophene-rich polarizable domains. Freestanding PTTEMA/PS blend films containing up to 40 wt % PTTEMA have almost 40% greater recoverable energy density compared to pure PS films polarized to the same electric field strength.

Covalent grafting of phenylphosphonate on calcium niobate platelets.

HYPOTHESIS: This work explores covalent grafting of phenylphosphonate (PPA) onto exfoliated, protonated calcium niobate (HCN), a Dion-Jacobson layered perovskite. The specific hypothesis is that PPA can be readily grafted onto the face surfaces of exfoliated HCN, which has reactive apical oxygen atoms. EXPERIMENTS: Previous research has established the conditions required for full exfoliation of HCN in aqueous solutions of tetrabutylammonium hydroxide (TBAOH), denoted as TBACN. This work first explores the effect of reflux conditions on the dispersion state of TBACN suspensions, and then investigates PPA grafting onto both non-exfoliated HCN and exfoliated TBACN dispersed in deionized (DI) water, TBA solution, and various alcohols. The products are characterized by a variety of techniques including light scattering to assess the TBACN dispersion state, (31)P MAS NMR to confirm PPA grafting, and XPS to estimate PPA grafted amounts. FINDINGS: The results confirm the grafting of PPA on HCN and TBACN, quantify the extent of PPA grafting, and identify various grafting modes (mono-, bi-, and tridentate). All of these aspects are found to be dependent on the layered materials' exfoliation state, suspension processing conditions, and solvent composition. The results are rationalized in terms of a plausible mechanism of the grafting process.

Tuning the underwater oleophobicity of graphene oxide coatings via UV irradiation.

Ultraviolet (UV) irradiation was utilized to gradually modify the chemistry and structure of graphene oxide (GO) flakes, as confirmed by XPS and AFM. Ultrathin GO coatings/membranes, made of UV-irradiated flakes, showed tunable underwater oleophobicity. UV-treated, superoleophobic GO membranes exhibited excellent antifouling capability for oil/water separation.

Montmorillonite-levan nanocomposites with improved thermal and mechanical properties.

This work reports on the structure and properties of novel nanocomposites composed of exfoliated montmorillonite clay blended with levan, a polysaccharide produced by Bacillus sp. Dry levan is very brittle, making it difficult to obtain stand-alone films. MMT-levan composites were prepared by solution blending in water, coating on plastic surfaces, partial drying at 50°C, and conditioning in air at 50-60% relative humidity. This process results in freestanding, transparent, and flexible films of pure levan and MMT-levan composites plasticized by 10-15 wt% water. XRD patterns from levan-MMT composites indicate an MMT interlayer spacing 0.62 nm greater than that of the starting MMT, suggesting re-stacking of MMT platelets coated by adsorbed, uncoiled levan molecules. FTIR results suggest that levan adheres to MMT via water-mediated hydrogen bonding between the levan's hydroxyl groups and MMT surface oxygens. MMT-levan composites have improved thermal stability and a well-defined glass transition temperature that increases with MMT loading. The tensile moduli of levan-MMT composites increase by as much as 480% relative to pure levan. The XRD and mechanical property results suggest that MMT reinforces levan through a filler network structure composed of MMT platelets bridged by adsorbed levan molecules, enhanced when the MMT loading becomes high enough (5-10wt% MMT) to induce an isotropic-nematic transition in MMT platelet orientation.

Ultrathin, molecular-sieving graphene oxide membranes for selective hydrogen separation.

Ultrathin, molecular-sieving membranes have great potential to realize high-flux, high-selectivity mixture separation at low energy cost. Current microporous membranes [pore size < 1 nanometer (nm)], however, are usually relatively thick. With the use of current membrane materials and techniques, it is difficult to prepare microporous membranes thinner than 20 nm without introducing extra defects. Here, we report ultrathin graphene oxide (GO) membranes, with thickness approaching 1.8 nm, prepared by a facile filtration process. These membranes showed mixture separation selectivities as high as 3400 and 900 for H2/CO2 and H2/N2 mixtures, respectively, through selective structural defects on GO.

Quantitative analysis of exfoliation and aspect ratio of calcium niobate platelets.

This work employs tapping-mode AFM and dynamic light scattering (DLS) to investigate the effects of suspension composition (alkylammonium cation type, concentration) on the degree of exfoliation of a layered perovskite, calcium niobate (HCa(2)Nb(3)O(10), denoted as CNP). It is well known that in aqueous suspensions, tetrabutylammonium (TBA) cations intercalate CNP's layered structure, producing exfoliated platelets. This work shows that tetramethylammonium and tetraethylammonium cations do not produce significant levels of CNP exfoliation. However, TBA leads to complete exfoliation of CNP (>95%) in suspensions prepared with TBA:CNP ratios greater than 1:1. TGA and XRD data indicate that TBA uptake is limited by steric packing constraints. After depositing TBA-CNP platelets on mica surfaces for AFM imaging, quantitative image analysis provides the degree of exfoliation and distributions of platelet thickness, lateral size, and aspect ratio. The aspect ratio of TBA-CNP platelets has a lognormal distribution. Sedimentation of TBA-CNP suspensions under unit gravity removes unexfoliated particles from suspension, and centrifugation leads to differential sedimentation. The lateral size and aspect ratio of residual suspended TBA-CNP platelets decrease with increasing centrifugation speed. For all fractions of TBA-CNP platelets, the number-average lateral size from AFM agrees remarkably well with the effective spherical particle diameter measured by DLS.

Oligoaniline-containing supramolecular block copolymer nanodielectric materials.

We report a new generation of nanodielectric energy storage materials based on supramolecular block copolymers. In our approach, highly polarizable, conducting nanodomains are embedded within an insulating matrix through block copolymer microphase separation. An applied electric field leads to electronic polarization of the conducting domains. The high interfacial area of microphase-separated domains amplifies the polarization, leading to high dielectric permittivity. Specifically, reversible addition fragmentation transfer (RAFT) polymerization was used to prepare block copolymers with poly(methyl acrylate) (PMA) as the insulating segment and a strongly acidic dopant moiety, poly-(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA), as the basis for the conducting segment. The PAMPSA block was complexed with an oligoaniline trimer to form a dopant-conjugated moiety complex that is electronically conductive after oxidation. For the undoped neat block copolymers, the increase of the PMA block length leads to a transition in dielectric properties from ionic conductor to dielectric capacitor with polarization resulting from migration of protons within the isolated PAMPSA domains. The oligoaniline-doped copolymers show remarkably different dielectric properties. At frequencies above 200 kHz, they exhibit characteristics of dielectric capacitors with much higher permittivity and lower dielectric loss than the corresponding undoped copolymers.

High-dielectric polymer composite materials from a series of mixed-metal phenylphosphonates, ATi(C6H5PO3)3 for dielectric energy storage.

We report the preparation of new polymer composite dielectric materials for energy storage applications. New layered 1:1 mixed A+2/Ti4+ metal phenylphosphonates, ATi(O3PC6H5)3, A=Mg, Ca, Sr, Ba, and Pb, have been prepared via a melt route, in which mixed metal oxides, ATiO3, were reacted with molten phenyl phosphonic acid. The mixed-metal phosphonates were combined with polystyrene (PS) via a solution route and cast as thin films for dielectric permittivity measurements. The ATi(O3PC6H5)3-PS composites exhibit a substantial enhancement in the dielectric permittivity as a function of weight loading relative to the parent ATiO3-PS composites. For both ATiO3-PS and ATi(O3PC6H5)3-PS, the composites' dielectric permittivity increases with A cation polarizability. Unusually large increases for 40 wt% ATi(O3PC6H5)3-PS composites (A=Sr, Ba, and Pb) indicate permittivity enhancement that goes beyond the effect of varying filler composition.

Pt/glassy carbon model catalysts prepared from PS-b-P2VP micellar templates.

Poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer was used as a micellar template to fabricate arrays of Pt nanoparticles on mica and glassy carbon (GC) supports. Polymer micellar deposition yields Pt nanoparticles with tunable particle size and surface number density on both mica and GC. After deposition of precursor-loaded micelles onto GC, oxygen plasma etching removes the polymer shell, followed by thermal treatment with H2 gas to reduce the Pt. Etching conditions were optimized to maximize removal of the polymer while minimizing damage to the GC. Arrays of Pt nanoparticles with controlled size and surface number density can be prepared on mica (for particle size characterization) and GC to make Pt/GC model catalysts. These model catalysts were characterized by tapping mode atomic force microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry to measure activity for oxidation of carbon monoxide or methanol. Cyclic voltammetry results demonstrate the existence of a correlation between Pt particle size and electrocatalytic properties including onset potential, tolerance of carbonaceous adsorbates, and intrinsic activity (based on active Pt area from CO stripping voltammetry). Results obtained with Pt/GC model catalysts duplicate prior results obtained with Pt/porous carbon catalysts therefore validating the synthesis approach and offering a new, tunable platform to study catalyst structure and other effects such as aging on proton exchange membrane fuel cell (PEMFC) reactions.

EXAFS characterization of dendrimer-Pt nanocomposites used for the preparation of Pt/gamma-Al2O3 catalysts.

Pt/gamma-Al2O3 catalysts were prepared using hydroxyl-terminated generation four (G4OH) PAMAM dendrimers as the templating agents and the various steps of the preparation process were monitored by extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS results indicate that, upon hydrolysis, chlorine ligands in the H(2)PtCl(6) and K(2)PtCl(4) precursors were partially replaced by aquo ligands to form [PtCl3(H2O)3]+ and [PtCl2(H2O)2] species, respectively. The results further suggest that, after interaction of such species with the dendrimer molecules, chlorine ligands from the first coordination shell of Pt were replaced by nitrogen atoms from the dendrimer interior, indicating that complexation took place. This process was accompanied by a substantial transfer of electron density from the dendrimer to platinum, indicating that the dendrimer plays the role of a ligand. Following treatment of the H(2)PtCl(6)/G4OH and K(2)PtCl(4)/G4OH complexes with NaBH4, no substantial changes were observed in the electronic or coordination environment of platinum, indicating that metal nanoparticles were not formed during this step under our experimental conditions. However, when the reduction treatment was performed with H2, the formation of extremely small platinum clusters, incorporating no more than four Pt atoms was observed. The nuclearity of these clusters depends on the length of the hydrogen treatment. These Pt species remained strongly bonded to the dendrimer. Formation of larger platinum nanoparticles, with an average diameter of approximately 10 A, was finally observed after the deposition and drying of the H(2)PtCl(6)/G4OH nanocomposites on a gamma-Al(2)O(3) surface, suggesting that the formation of such nanoparticles may be related to the collapse of the dendrimer structure. The platinum nanoparticles formed appear to have high mobility because subsequent thermal treatment in O2/H2, used to remove the dendrimer component, led to further sintering.

AFM characterization of dendrimer-stabilized platinum nanoparticles.

This work describes the use of atomic force microscopy (AFM) to measure the size of dendrimer-stabilized Pt nanoparticles (Pt DNs) deposited from aqueous solutions onto mica surfaces. Despite considerable previous work in this area, we do not fully understand the mechanisms by which PAMAM dendrimers template the formation of Pt DNs. In particular, Pt DN sizes measured by high-resolution transmission electron microscopy (HRTEM) are reported to be larger than expected if one assumes that each PAMAM molecule templates one spherical Pt nanoparticle. AFM provides a vertical height measurement that complements the lateral dimension measurement from HRTEM. We show that AFM height measurements can distinguish between "empty" PAMAM and Pt DNs. If the complexation of Pt precursor with PAMAM is prematurely terminated, AFM images and feature height distributions show evidence of arrested precipitation of Pt colloids. In contrast, sufficient Pt-PAMAM complexation time leads to AFM images and height distributions that have relatively narrow, normal distributions with mean values that increase with the nominal Pt:PAMAM ratio. The surface density of features in AFM images suggest that these Pt DNs reside on the mica surface as two-dimensional surface aggregates. These observations are consistent with an intradendrimer templating mechanism for Pt DNs. However, we cannot determine if the mechanism obeys a fixed loading law because we do not have definitive information about Pt DN shape. A second peak in the Pt DN height distribution appears when the Pt loading exceeds about 66% of PAMAM's theoretical capacity for Pt. Excluding these secondary particles, the dependence of mean feature height on the Pt:PAMAM ratio follows a power-law relationship. Also considering the magnitudes of the measured mean height values, the data suggest that Pt DNs exist as ramified, noncompact aggregates of Pt atoms interspersed within the PAMAM framework.

Dendrimer-mediated synthesis of platinum nanoparticles: new insights from dialysis and atomic force microscopy measurements.

In this work, we use AFM measurements in conjunction with dialysis experiments to study the synthesis mechanism and physical state of dendrimer-stabilized platinum nanoparticles. For characterizing particle size distributions by high resolution transmission electron microscopy and AFM, sample preparation by drop evaporation presumably minimizes the risk of sample bias that might be found in spin coating or dip-and-rinse methods. However, residual synthesis by-products (mainly salts) must be removed from solutions of dendrimer-stabilized metal nanoparticles prior to AFM imaging. Purification by dialysis is effective for this purpose. We discovered, by UV-visible spectrophotometry and atomic absorption (AA) spectroscopy, that dialysis using 'regular' cellulose dialysis tubing (12 000 Da cut-off) used in all previous work leads to substantial losses of poly(amidoamine) (PAMAM) dendrimer (G4OH), PAMAM-Pt(+2) complex, and PAMAM-stabilized Pt nanoparticles. Use of benzoylated dialysis tubing (1200 Da cut-off) shows no losses of G4OH or G4OH-Pt mixtures. We use AFM to see whether selective filtration during dialysis introduces sampling bias in the measurement of particle size distributions. We compare results (UV-visible spectra, AA results, and AFM-based particle size distributions) for a sample of G4OH-Pt(40) divided into two parts, one part dialysed with regular dialysis tubing and the other with benzoylated tubing. Exhaustive dialysis using benzoylated tubing may lead to the loss of colloidal Pt nanoparticles stabilized by adsorbed dendrimer, but not Pt nanoparticles encapsulated by the dendrimer. The comparisons also lead to new insights concerning the underlying synthesis mechanisms for PAMAM-stabilized Pt nanoparticles.

Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments.

Arc-synthesized single-walled carbon nanotubes have been purified through preparative electrophoresis in agarose gel and glass bead matrixes. Two major impurities were isolated: fluorescent carbon and short tubular carbon. Analysis of these two classes of impurities was done. The methods described may be readily extended to the separation of other water-soluble nanoparticles. The separated fluorescent carbon and short tubule carbon species promise to be interesting nanomaterials in their own right.

Platinum ion uptake by dendrimers: an NMR and AFM study.

The reaction of generation 2 and generation 4 poly(amidoamine) (PAMAM) dendrimers with K2PtCl2 was studied by several NMR methods. The time dependency of the Pt(II) complexation was followed with 195Pt NMR for both dendrimers and the equilibrium product was further characterized with (1)H NMR, and indirectly detected 13C NMR, in the case of the generation 2 dendrimer. After 2 days, a black precipitate of Pt(0) was observed, half the original 195Pt signal was lost, and approximately 20% of the initial Pt(II) was coordinated to the tertiary and secondary nitrogens of the generation 2 dendrimer. The uptake of Pt(II) by the generation 4 dendrimer was much slower, consistent with the steric crowding of the surface groups on the generation 4 dendrimer compared to the more open generation 2. After 10 days, 80% of the Pt(II) was deep within the generation 4 dendrimer; the remaining 20% was unreacted or bound near the surface nitrogens of a single dendrimer. The location and time course of the platinum ion uptake by the dendrimers provides valuable insight into the formation of Pt(0) nanoparticles made in the presence of dendrimers as stabilizers, visualized by atomic force microscopy.