Johan E. ten Elshof

Microporous Niobia-Silica Membrane with Very Low CO2 Permeability

A sol-gel-derived microporous ceramic membrane with an exceptionally low permeability for CO(2) from gaseous streams was developed and characterized. The sols were prepared from a mixture of niobium and silicon alkoxide precursors by acid-catalyzed synthesis. Microporous films were formed by coating asymmetric gamma-alumina disks with the polymeric sol (Si/Nb=3:1), followed by calcination at 500 degrees C. The membrane consists of a 150-nm-thick layer with a Si/Nb atomic ratio of about 1.5. The single-gas permeance of small gas molecules such as H(2), CH(4), N(2), and SF(6) decreases steadily with kinetic diameter. Hydrogen, helium, and carbon dioxide follow an activated transport mechanism through the membrane. The permeance of CO(2) in this membrane is much lower than that in pure silica, and its behavior deviates strongly from the general trend observed with the other gases. This is attributed to a relatively strong interaction between CO(2) and adsorption sites in the niobia-silica membrane.

Stable Hybrid Silica Nanosieve Membranes for the Dehydration of Lower Alcohols

A thirst for water: Organic-inorganic hybrid silica nanosieve membranes with narrow pore size distributions were developed for the separation of binary (bio)alcohol/water mixtures, for example, to remove water from wet biofuels during production. These membranes dehydrate lower alcohols and show a stable performance in the presence of significant amounts of acetic acid.

Cat‐in‐a‐Cup: Facile Separation of Large Homogeneous Catalysts

A cat with nine lives: Catalyst separation and recycling is the bane of homogeneous catalysis. This problem is addressed by a two-layered ceramic membrane cylinder that allows the diffusion of reactants and products in and out, but keeps the catalyst trapped. The concept is demonstrated for the enantioselective transfer hydrogenation of acetophenone to (S)-phenylethyl alcohol with large molecular catalysts anchored on Frechet-type dendrimers.

Microporous structure and enhanced hydrophobicity in methylated SiO2 for molecular separation

Methylated microporous silica with high thermal stability and tuneable hydrophobicity was obtained by acid-catalysed sol–gel hydrolysis and condensation of mixtures of tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES). The gels exhibited a trend towards smaller ultramicropores with increasing methyl content, while in addition some supermicropores were formed with sizes of around 2 nm. For low MTES concentration, dilution prior to gelation and ageing resulted in materials with clearly smaller ultramicropores, whereas only a minor effect of dilution on structure was found at high MTES concentration. The small ultramicropore size in ‘diluted’ materials can be associated with a higher extent of condensation of mainly TEOS monomers. Stable structures formed from MTES in an early stage of synthesis may explain the particular micropore structure of MTES-rich gels. With increasing methyl content and with dilution of the sol, the affinity of the surface to water was strongly decreased. The applicability of microporous silica in wet atmospheres may thus be improved by methylation, and their pore structure modified by adaptation of the recipe, which would be highly relevant for industrial gas and liquid separation by inorganic membranes.

Improved Langmuir−Blodgett titanate films via in situ exfoliation study and optimization of deposition parameters

The exfoliation and deposition of large (10-100 μm) Ti0.87O2 and small (0.1-1 μm) Ti0.91O2 nanosheets from lepidocrocite-type protonated titanates was investigated for getting high quality films. Exfoliation was carried out with different tetra-alkyl ammonium ions (TAA(+)) and varying TAA(+)/H(+) ratios, and the colloidal solutions were characterized by small-angle X-ray scattering (SAXS) and ultraviolet-visible (UV-vis) spectroscopy. Using Langmuir-Blodgett deposition the titanate nanosheets were directly transferred onto a Si substrate. The resulting films were characterized by atomic force microscopy (AFM).The results indicate that the H1.07Ti1.73O4 titanate exfoliated at very low ratios of TAA(+)/H(+); no lower threshold for exfoliation was observed for the TAA(+) concentration. Nanosheets exfoliated at very low ratios of TAA(+)/H(+) typically showed a small size and porous surface. Subsequent exfoliation of the remaining layered titanate particles yielded much higher quality nanosheets. The optimized deposition parameters for Langmuir-Blodgett films suggest that the surface pressure is a key parameter to control the coverage of the film. The bulk concentration of nanosheets was found to be a less important deposition parameter in the LB deposition process. It only influenced whether the desired surface pressure could be reached at a given maximum degree of compression.

Patterning Lead Zirconate Titanate Nanostructures at Sub-200-nm Resolution by Soft Confocal Imprint Lithography and Nanotransfer Molding

Patterned sol-gel-derived lead zirconate titanate (PZT) thin films with lateral resolutions down to 100 nm on silicon are reported. Both an imprint and a transfer-molding method were employed. The formed patterns after annealing were characterized with scanning electron microscopy, atomic force microscopy, and X-ray diffraction. Despite the small dimensions and flexibility of the poly(dimethylsiloxane) (PDMS) stamps used for patterning, the quality of replication was found to be good. The influence of the surface energies of the substrate, PDMS mold, and precursor solution on the quality of pattern replication is discussed. The colloidal structure of the PZT sol-gels from which the patterns were made was studied with small-angle X-ray scattering. The sols were found to be chemically homogeneous down to a length scale of approximately 2 nm and higher, which is sufficient for pattern replication on approximately 100 nm scale.

Fabrication of La0.3Sr0.7CoO3 – thin layers on porous supports by a polymeric sol–gel process

A polymeric sol–gel process was developed to fabricate porous thin layers of the perovskite-type La0.3Sr0.7CoO3 – for membrane applications. A spin-coating technique was used for deposition of the layer on porous -and -Al2O3 supports. Both supported and non-supported membranes were characterized by means of thermal analysis, X-ray powder diffraction, particle size analysis, scanning electron microscopy and permporometry. The main phase formed upon heating in air above 400 °C is cubic perovskite, although traces of SrCO3 and of an unknown phase can be observed up to 800 °C. Above this temperature, strong chemical interaction occurs between the deposited perovskite layer and the support material. The effects of the sintering temperature and the type of the La-precursor on the pore-size distribution were investigated.

Hydrogen generation from photocatalytic silver|zinc oxide nanowires: towards multifunctional multisegmented nanowire devices.

Photoactive nanowires: A novel photo-electrochemical nanowire diode that catalyzes the conversion of methanol and water to hydrogen under UV light is demonstrated. The wire consists of a metal and a metal oxide segment that are connected via a Schottky barrier. Other functions, such as remote- controlled autonomous movement, can be easily added to these wires in a modular fashion

Thin Films of Conductive ZnO Patterned by Micromolding Resulting in Nearly Isolated Features

Patterned and continuous thin films of conductive Al-doped zinc oxide (ZnO:Al) were prepared on different substrates from a polymeric precursor solution. Their electric conductivity and light transmittance (for visible and UV light) was measured at room temperature. By means of a simple device, conductive ZnO:Al films with high fidelity patterns with features of 2-20 microm width could be obtained by simply micromolding the liquid precursor film prior to heat treatment. The individual features were interconnected by a very thin residual ZnO layer.

Nanopatterning from the gas phase: high resolution soft lithographic patterning of organosilane thin films

A general methodology for nanopatterning organosilane thin films directly from vapor phase precursors is presented. Aminosilane line patterns with a width of approximately 200 nm in an area of 1 cm(2) were fabricated on silicon substrates by diffusion of aminosilane vapor through the open channels of PDMS stamps bonded to a substrate. The patterned thin films were characterized by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Patterns were initially formed at the three-phase boundary lines between substrate, PDMS mold, and vapor by exploiting the fact that vapors condense preferentially in geometrically restricted areas with a concave shape compared to flat surfaces. The lateral resolution of the formed patterns is about 1 order of magnitude smaller than the feature sizes of the PDMS stamp. Prolonged exposure to the precursor vapor resulted in micrometer-sized patterns with similar features and dimensions as the stamp. This methodology provides an easy and low cost parallel fabrication route of functional organosilane nanoscale patterns of arbitrary shape and composition from micrometer-size patterned stamps.