We describe a simple, inexpensive coating method to produce thin silica and titania films with surfactant templated, orthogonally tilted cylindrical nanopore arrays. These films can be deposited onto any substrate because orientation of the 2D hexagonally close packed (HCP) mesophases out of the plane of the film is directed by a chemically neutral sacrificial copolymer layer. Orientation of the HCP mesophases through the entire thickness of films cured in open air is achieved by limiting the coating thickness. This generalizes the coating method by making it possible to deposit oriented films on substrates of any curvature and size. We find a critical thickness between 70 and 100 nm, below which the triblock copolymer surfactant-templated HCP phase aligns completely out of the plane of glass and silicon wafer substrates. Above this thickness, the effect of the chemically neutral bottom layer does not propagate across the entire film, and alignment of the HCP mesophases parallel to the (nonpolar) air interface produces a mixed orientation.

Hollow spherical silica particles with hexagonally ordered mesoporous shells are synthesized with the dual use of cetyltrimethylammonium bromide (CTAB) and unmodified polystyrene latex microspheres as templates in concentrated aqueous ammonia. In most of the hollow mesoporous particles, cylindrical pores run parallel to the hollow core due to interactions of CTAB/silica aggregates with the latices. Effects on the product structure of the CTAB:latex ratio, the amount of aqueous ammonia, and the latex size are studied. Hollow particles with hexagonally patterned mesoporous shells are obtained at moderate CTAB:latex ratios. Too little CTAB causes silica shell growth without surfactant templating, and too much induces nucleation of new mesoporous silica particles without latex cores. The concentration of ammonia must be large to induce co-assembly of CTAB, silica, and latex into dispersed particles. The results are consistent with the formation of particles by addition of CTAB/silica aggregates to the surface of latex microspheres. When the size and number density of the latex microspheres are changed, the size of the hollow core and the shell thickness can be controlled. However, if the microspheres are too small (50 nm in this case), agglomerated particles with many hollow voids are obtained, most likely due to colloidal instability.

29Si NMR is used to quantify the polymerization kinetics of silicon alkoxide intermediates in acidic sol−gel processes well before gelation. Reaction conditions are chosen (a) to minimize interference by hydrolysis kinetics and differing degrees of hydrolysis and (b) to investigate the possibility of single-step copolymerization. At the composition chosen hydrolysis reactions rapidly reach pseudoequilibrium, so condensation rate coefficients can be easily determined. We compare condensation rate coefficients of hydrolyzed intermediates from tetraethoxysilane {Si(OEt)4}, ethyltriethoxysilane {EtSi(OEt)3}, and diethyldiethoxysilane {Et2Si(OEt)2} at a constant initial water and catalyst concentration. A surprisingly strong negative first-shell substitution effect is caused by connectivity for each system and is discussed. This substitution effect depends on the degree of condensation of both reacting groups but more so on the less connected one. We also comment on a decrease in condensation reactivity and an increase in hydrolysis reactivity and favorability caused by the nonreactive ethyl groups.

ence electrodes. Films were inserted into a home-built sample holder, with an air-filled gas-phase volume of 0.326 cm 3 encapsulated between the electrode and the polymer film/glass substrate using a 20 mm diameter Viton O'-ring spacer. Six UVA 365 ± 25 nm fluorescent tubes (8W BLB, I = 5.96 mW cm ±2 ) arranged in a semicircle and placed 13 cm above the cell, facing the cell, were used as the irradiation source, resulting in a sample irradiance of 4.8 mW cm ±2 , similar to the intensity of UVA solar irradiance. A fan was used to prevent the cell from heating up due to the heat from the irradiation source. SEM images were obtained with a Philips XL-30 field-emission scanning electron microscope. Samples were coated with Au/Pd prior to observation. For TAS measurements, a PTI GL-3300 N 2 -laser (337 nm, 30 lJ cm ±2 , repetition rate of 0.8 Hz) was used as the excitation source [5]. X-ray diffraction analysis of the Degussa P25 and P90 materials showed that both materials consist of TiO 2 anatase and rutile particles, the anatase contents being 81 % and 90 % for P25 and P90, respectively. The crystal domain size of the anatase particles was estimated as 20 nm for TiO 2 P25 and 13 nm for TiO 2 P90.

An investigation of the kinetics of tethering of amine functional-ended polystyrene from good solvent to the surface of a solid substrate showed three distinct regimes of kinetics rather than the two predicted by theory. The first regime was fast and appeared to be controlled by diffusion through the solvent, as predicted by theory. The second regime was slow and appeared to be linear in the natural logarithm of time, as predicted by theory. The third regime, not predicted by theory, was one of accelerated tethering (termed by us "layer-assisted tethering"). The third regime, observed for different solvents, different molecular weights, and different temperatures, ended when saturation was reached. We suggest herein an explanation for the observed acceleration.

Vertically aligned amorphous titania (TiO2) nanotubes are produced by anodizing Ti foils at various applied potentials in a neutral electrolyte solution containing fluoride ions. Pore size and wall thickness are tuned in the range from 30 to 70 nm and 17 to 35 nm, respectively, by adjusting the applied potential, in addition to tuning the tube length from 355 to 550 nm. Utilizing all of these films as negative electrode materials in lithium-ion batteries delivers stable capacities of 130–230 mAh g–1 and 520–880 mAh cm–3 up to 200 cycles. Microstructural analysis shows that there is no structural change or mechanical degradation in the active material, and the amorphous active material maintains good contact with the substrate/current collector. A continuum elasticity model for the tubular geometry is presented to understand the diffusion-induced stresses, fracture tendency, and stability in TiO2 nanotubes. Modeling results indicate that the fracture tendencies of nanotubes with the dimensions in this work are very small; stable reversible capacity retention results from the high ratio of inner to outer diameter of the tubes. In other words, tubes with thinner walls more easily accommodate expansion or contraction during the lithiation/delithiation process. A guideline for designing lithium-ion battery nanotube electrodes is given such that under specific conditions the fracture tendency is small and volumetric charge density is high.

Awl t~tuQ STI AbstractRecent investigations have implicated cage-liie precursors in the unusually high gelation conversion (W 82Yo) of acid-catalyzed tetraethoxysilane. However, the statistical models used so far cannot capture kinetic or composition-dependent features of alkoxysilane polycondensation.Here we take a first step towards unified modeling of the kinetics and structure of silica gelation.Dynamic Monte Carlo simulations [J. Somv&rsky and K. DuSek, Polym. BuJ1. 1994 33:369] are developed which permit competition between extensive cyclization and growth. The model includes well-established kinetic trends (hydrolysis pre-equilibrium and first shell substitution effects). As a first approximation, unimolecular-like terms for cyclization reactivity follow the experimental pattern of bimolecular rate coefficients. The present simulations allow unlimited formation of 3-site rings, giving rise to many structures which are not those of real silicates (where 4-site rings dominate). However, the level of cyclization (both cycles per molecule and per site) is consistent with that of real silicates, and is enough to delay gelation to 82% conversion or higher. These simulations also display a broader range of gelation behavior than prior kinetic models. At high to moderate monomer concentrations, competition between cyclization and growth causes the expected delay of gelation. Upon further dilution, we discover a third regime, absent from prior kinetic gelation models but important for siloxanes: formation of a distribution of polycyclic precursors which still rettin enough functionalisty to gel.

When cationic surfactants are added to the Stöber process, spherical particles with radially oriented mesopores can be prepared by precipitation of silica from a solution of ethanol, water, and ammonia. Van Tendeloo and co-workers proposed that these particles form by epitaxial growth of cylindrical assemblies from the facets of Ia3d cubic (MCM-48) seeds. [J. Phys. Condens. Matter 2003, 15, S3037.] Here, we reexamine this hypothesis by detailed characterization of intermediate and final mesoporous silica particles formed from ethanol/water/ammonia solutions. We find that the presence of a cubic core is not required to explain the synthesis of spherical particles with radially oriented pores. Instead, we hypothesize that the radial orientation originates at the particle surface because of the preferred alignment of CTAB micelles normal to that interface. Consistent with previous studies of the Stöber process, we initially observe small, irregular silica/surfactant clusters. After an induction time, these clusters suddenly form spherical particles larger than 100 nm in diameter because of aggregation or collapse of weakly bound clusters. No ordered micellar structure forms initially, but shortly after the appearance of large spherical particles, cylindrical surfactant micelles appear and align perpendicular to the particle/solution interface. The micelles appear to maintain their alignment normal to the interface even during particle coalescence, supporting the idea that radial orientation originates at the surface rather than the interior of the particles. A study of particles formed with varying amounts of ethanol suggests that ethanol acts as a cosolvent and as a low-dielectric constant solvent to induce cooperative effects on micelle organization and particle morphology, leading to particles with radially oriented pores at a large ethanol concentration.