Aqueous Assembly of Oxide and Fluoride Nanoparticles into 3D Microassemblies.

We demonstrate rapid [∼mm3/(h·L)] organic ligand-free self-assembly of three-dimensional, >50 µm single-domain microassemblies containing up to 107 individual aligned nanoparticles through a scalable aqueous process. Organization and alignment of aqueous solution-dispersed nanoparticles are induced by decreasing their pH-dependent surface charge without organic ligands, which could be temperature-sensitive or infrared light absorbing. This process is exhibited by transforming both dispersed iron oxide hydroxide nanorods and lithium yttrium fluoride nanoparticles into high packing density microassemblies. The approach is generalizable to nanomaterials with pH-dependent surface charge (e.g., oxides, fluorides, and sulfides) for applications requiring long-range alignment of nanostructures as well as high packing density.

Stable cycling and excess capacity of a nanostructured Sn electrode based on Sn(CH3COO)2 confined within a nanoporous carbon scaffold.

A high capacity, electrochemically stable, nanostructured Sn electrode for Li ion battery anodes is described. This electrode utilizes a rigid, electrically conductive, nanoporous carbon aerogel scaffold by incorporating tin acetate, Sn(CH3COO)2, into the scaffold pore volume through melt infusion. Incorporation of the Sn(CH3COO)2 by melt infusion ensures a chemically stable contact with the scaffold. The mechanical rigidity of the pore volume confines the Sn to nanometer dimensions without sintering, leading to stable cycling. Separation of the synthesis of the scaffold from the loading with Sn(CH3COO)2 permits optimized division of the scaffold pore volume for expansion and electrolyte access during reaction with Li. Using this design, an electrode based on an aerogel with a 5 nm mode pore size was cycled over 300 times without degradation. In addition, after subtracting the contribution from the carbon scaffold, the capacity exceeded the theoretical capacity for Sn, due to an oxidation reaction occurring at 1.2 V. This excess capacity may be related to the solid-solid or solid-electrolyte interfaces in the electrode, possibly representing a new reversible Li ion reaction.

Reversible ligand exchange in a metal-organic framework (MOF): toward MOF-based dynamic combinatorial chemical systems.

Reversible benzene dicarboxylate/2-bromobenzene dicarboxylate ligand exchange has been studied in the cubic metal-organic framework MOF-5. Significant exchange (up to ∼50%), with continuous compositional variation, was observed using ex-situ (1)H NMR following treatment over ∼6 h at ∼85 °C in 10-40 mM ligand solutions. Exchange occurred without significant structural degradation as characterized by X-ray diffraction, nitrogen adsorption, and scanning electron microscopy. Solid-state (13)C NMR was used to show that exchanged ligands were incorporated into the framework lattice and not simply adsorbed within the pores. Exchange was found to be sensitive to the small free energy changes caused by the ligand concentration in the exchanging solution indicating that exchange is energetically nearly degenerate. This demonstration of reversible, nearly isoenergetic exchange indicates that mixed ligand MOFs could be developed as dynamic combinatorial chemical systems.

Dual Component Passive Icephobic Coatings with Micron-Scale Phase-Separated 3D Structures.

A passive icephobic coating (τice < 20 kPa) is an enabling technology to many industries, including aerospace and energy and power generation, with recent efforts in materials research identifying strategies to achieve this low adhesion threshold. To better meet this need, we have combined low surface energy perfluoropolyether (PFPE) and hydrophilic poly(ethylene glycol) (PEG) species in a segmented polyurethane thermoplastic elastomer. Coating microstructure presents a segregated 3D morphology at the micron-scale (1-100 µm) with discrete PFPE and continuous PEG phases self-similar through the thickness. Spray application produces a solid, mechanically tough film free of additive fluids or sacrificial elements, demonstrating exceptional ice adhesion reduction up to 1000× lower versus aluminum (τice < 1 kPa), as measured under environmentally realistic accretion and centrifugal test shedding conditions. Finally, the modular nature of the synthetic system allows PEG and PFPE to be exchanged for poly(tetramethylene oxide) to investigate performance drivers.

Hierarchical carbon foams with independently tunable mesopore and macropore size distributions.

Hierarchical carbon foams with independently tunable mesopore and macropore size distributions were formed in a high internal phase emulsion (HIPE) template. The HIPE consists of an internal oil phase that controls the macropore dimensions and an aqueous resorcinol-formaldehyde precursor solution external phase that directs the mesopore size distribution. Once the emulsion is formed, the precursor solution is cured, fluid elements are extracted from the monolith via solvent exchange, and then the sample is pyrolyzed to create a hierarchical open-cell foam consisting of macropores with mesoporous carbon xerogel walls. Both mesopore and macropore size distributions may be independently tuned by changing the synthesis parameters. These samples have a peak in the mesopore size distribution that may be tuned to between 5 and 8 nm and macropore average diameters that may be tuned to between 0.7 and 2.1 microm. Furthermore, the 0.7 and 2.1 microm average diameter macropores have 0.18 and 0.53 microm diameter macropore windows between adjacent pores, respectively. Pore volumes up to 5.26 cm(3)/g and electrical conductivities as high as 0.34 S/cm are observed after 1200 degrees C carbonization of the framework. These foams may have potential applications as 3-D current collectors in batteries and as fuel cell catalyst supports.

Fabrication and hydrogen sorption behaviour of nanoparticulate MgH2 incorporated in a porous carbon host.

Nanoparticles of MgH2 incorporated in a mesoporous carbon aerogel demonstrated accelerated hydrogen exchange kinetics but no thermodynamic change in the equilibrium hydrogen pressure. Aerogels contained pores from <2 to approximately 30 nm in diameter with a peak at 13 nm in the pore size distribution. Nanoscale MgH2 was fabricated by depositing wetting layers of nickel or copper on the aerogel surface, melting Mg into the aerogel, and hydrogenating the Mg to MgH2. Aerogels with metal wetting layers incorporated 9-16 wt% MgH2, while a metal free aerogel incorporated only 3.6 wt% MgH2. The improved hydrogen sorption kinetics are due to both the aerogel limiting the maximum MgH(2) particle diameter and a catalytic effect from the Ni and Cu wetting layers. At 250 degrees C, MgH2 filled Ni decorated and Cu decorated carbon aerogels released H(2) at 25 wt% h(-1) and 5.5 wt% h(-1), respectively, while a MgH(2) filled aerogel without catalyst desorbed only 2.2 wt% h(-1) (all wt% h(-1) values are with respect to MgH2 mass). At the same temperature, MgH2 ball milled with synthetic graphite desorbed only 0.12 wt% h(-1), which demonstrated the advantage of incorporating nanoparticles in a porous host.

The kinetic enhancement of hydrogen cycling in NaAlH(4) by melt infusion into nanoporous carbon aerogel.

Enhanced kinetic performance and reversibility have been achieved with uncatalyzed NaAlH4 by incorporation into nanoporous carbon aerogel. Aerogel with a pore size distribution peaked at 13 nm and a pore volume of 0.8 cm(3) g(-1) was filled with NaAlH4 to 94% capacity by melt infusion at 189 degrees C under 183 bar H(2) gas overpressure. Dehydrogenation to NaH + Al with reasonable kinetics was accomplished at 150 degrees C, well below the NaAlH4 melting temperature (183 degrees C), compared to hydrogen release above 230 degrees C for bulk uncatalyzed NaAlH4. Uncatalyzed bulk samples did not rehydrogenate under laboratory conditions, whereas NaAlH4 in a carbon aerogel host was readily rehydrogenated at approximately 160 degrees C and 100 bar H(2) to approximately 85% of its initial capacity. Ball-milled NaAlH4 catalyzed with 4 mol% TiCl3 showed somewhat better kinetics compared to the infused aerogel; nevertheless, the large kinetic enhancement obtained by incorporation into carbon aerogel, even in the absence of a catalyst, demonstrates the substantial benefit of confining the NaAlH4 to nanoscale dimensions.

The synthesis and hydrogen storage properties of a MgH(2) incorporated carbon aerogel scaffold.

A new approach to the incorporation of MgH2 in the nanometer-sized pores of a carbon aerogel scaffold was developed, by infiltrating the aerogel with a solution of dibutylmagnesium (MgBu2) precursor, and then hydrogenating the incorporated MgBu2 to MgH2. The resulting impregnated material showed broad x-ray diffraction peaks of MgH2. The incorporated MgH2 was not visible using a transmission electron microscope, which indicated that the incorporated hydride was nanosized and confined in the nanoporous structure of the aerogel. The loading of MgH2 was determined as 15-17 wt%, of which 75% is reversible over ten cycles. Incorporated MgH2 had >5 times faster dehydrogenation kinetics than ball-milled activated MgH2, which may be attributed to the particle size of the former being smaller than that of the latter. Cycling tests of the incorporated MgH(2) showed that the dehydrogenation kinetics are unchanged over four cycles. Our results demonstrate that confinement of metal hydride materials in a nanoporous scaffold is an efficient way to avoid aggregation and improve cycling kinetics for hydrogen storage materials.

Size effects on the hydrogen storage properties of nanoscaffolded Li3BN2H8.

The use of Li3BN2H8 complex hydride as a practical hydrogen storage material is limited by its high desorption temperature and poor reversibility. While certain catalysts have been shown to decrease the dehydrogenation temperature, no significant improvement in reversibility has been reported thus far. In this study, we demonstrated that tuning the particle size to the nanometer scale by infiltration into nanoporous carbon scaffolds leads to dramatic improvements in the reversibility of Li3BN2H8. Possible changes in the dehydrogenation path were also observed in the nanoscaffolded hydride.

Aqueous room temperature synthesis of cobalt and zinc sodalite zeolitic imidizolate frameworks.

Sodalite zeolitic imidazolate frameworks containing Co (ZIF-67) and Zn (ZIF-8) were synthesized at room temperature under aqueous conditions in 10 min. A trialkylamine deprotonated the 2-methylimidazole ligand and nucleated the frameworks. Furthermore, the ligand acted as a structure directing agent in place of an organic solvent.

Examining the role of surfactant packing in phase transformations of periodic templated silica/surfactant composites.

In this work, we examine the role of curvature and surfactant packing in controlling the structure of periodic silica/surfactant composites by driving such materials through a transformation from a hexagonal to a lamellar phase. We focus on how the interplay of desired packing and volume constraints dictates the resulting structures. In general, surfactants expand in a complex way upon heating, and this can cause a change in the optimal packing geometry. However, the presence of a rigid silica framework may prevent surfactants from reaching this preferred volume and/or curvature. Real-time in situ X-ray diffraction is used to monitor the structural evolution of these materials heated under hydrothermal treatments. Because the thermal-driven disorder of the surfactant tails drives the phase transition, we examine four types of composites with varying tail density. Ordinarily, composites consist of surfactants with one 20-carbon tail and one positively charged ammonium headgroup. Tail density is varied by replacing a small amount (0-16%) of these single-tail, single-head surfactants with single-tail, double-head 'gemini' surfactants. A greater head--tail ratio indeed produces different results, causing the phase transition to occur at higher temperatures. Using simple geometric models to gain better understanding of our experimental results, we find that, while both unfavorable curvature and limited volume may exist for the surfactants in these composites, the constrained curvature appears to be the dominant effect in driving structural rearrangement.

Probing the effects of interfacial chemistry on the kinetics of phase transitions in amorphous and tetragonal zirconia nanocrystals.

In this work, we examine the phase stability of both uncoated and alumina-coated zirconia nanoparticles using in-situ X-ray diffraction. By tracking structural changes in these particles, we seek to understand how changing interfacial bonding affects the kinetics of amorphous zirconia crystallization and the kinetics of grain growth in both initially amorphous and initially crystalline zirconia nanocrystals. Activation energies associated with crystallization are calculated using nonisothermal kinetic methods. The crystallization of the uncoated amorphous zirconia colloids has an activation energy of 117 +/- 13 kJ/mol, while that for the alumina-coated amorphous colloids is 185 +/- 28 kJ/mol. This increase in activation energy is attributed to inhibition of atomic rearrangement imparted by the alumina coating. The kinetics of grain growth are also studied with nonisothermal kinetic methods. The alumina coating again dramatically affects the activation energies. For colloids that were coated with alumina when they were in an amorphous structure, the coating imparts a 5x increase in the activation energy for grain growth (33 +/- 8 versus 150 +/- 30 kJ/mol). This increase shows that the alumina coating inhibits zirconia cores from coarsening. When the colloids are synthesized in the tetragonal phase and then coated with alumina, the effect of surface coating on coarsening kinetics is even more dramatic. In this case, a 10x increase in activation energies, from 28 +/- 3 kJ/mol for the uncoated particles to 300 +/- 25 kJ/mol for the alumina-coated crystallites, is found. The results show that one can alter phase stability in colloidal systems by using surface coatings and interfacial energy to dramatically change the kinetic barriers to structural rearrangement.

Chemical control of phase transformation kinetics in periodic silica/surfactant composites.

Control of phase stability is investigated through control of silica chemistry in ordered silica/surfactant composites under hydrothermal conditions. The composites were hydrothermally treated in pH 9 through pH 11 buffers while using in situ real time X-ray diffraction to follow a p6mm hexagonal-to-lamellar structural transition. The data were analyzed using both isothermal and nonisothermal (temperature-ramped) kinetics to determine activation energies. It was found that the most mildly basic conditions utilized (pH 9), which favor silica condensation, best inhibit the phase transition and thus produce the most kinetically stable composites. High-pH treatment, conversely, allows for the most facile rearrangements. Condensation occurring during composite synthesis rather than during hydrothermal treatment has a much smaller effect on phase stability, probably because much of the condensation that occurs during synthesis is random and not optimally coupled to the nanoscale architecture. Materials that start out poorly condensed, by contrast, can be extensively hydrothermally modified so that the final material has an inorganic framework with a highly uniform silica density; this provides the maximum resistance to transformation and the highest kinetic stability. In all cases, very good agreement is found between the results of isothermal and nonisothermal kinetic methods. The trends across pHs indicate that both isothermal and nonisothermal measurements are accurate and that differences between them are meaningful and represent physical differences in the transforming materials resulting from the different heating processes.