Silica-titania composite aerogel photocatalysts by chemical liquid deposition of titania onto nanoporous silica scaffolds

Silica-titania composite aerogels were synthesized by chemical liquid deposition of titania onto nanoporous silica scaffolds. This novel deposition process was based on chemisorption of partially hydrolyzed titanium alkoxides from solution onto silica nanoparticle surfaces and subsequent hydrolysis and condensation to afford titania nanoparticles on the silica surface. The titania is homogeneously distributed in the silica-titania composite aerogels, and the titania content can be effectively controlled by regulating the deposition cycles. The resultant composite aerogel with 15 deposition cycles possessed a high specific surface area (SSA) of 425 m(2)/g, a small particle size of 5-14 nm, and a large pore volume and pore size of 2.41 cm(3)/g and 18.1 nm, respectively, after heat treatment at 600 °C and showed high photocatalytic activity in the photodegradation of methylene blue under UV-light irradiation. Its photocatalytic activity highly depends on the deposition cycles and heat treatment. The combination of small particle size, high SSA, and enhanced crystallinity after heat treatment at 600 °C contributes to the excellent photocatalytic property of the silica-titania composite aerogel. The higher SSAs compared to those of the reported titania aerogels (<200 m(2)/g at 600 °C) at high temperatures combined with the simple method makes the silica-titania aerogels promising candidates as photocatalysts.

An engineered anisotropic nanofilm with unidirectional wetting properties

Anisotropic textured surfaces allow water striders to walk on water, butterflies to shed water from their wings and plants to trap insects and pollen. Capturing these natural features in biomimetic surfaces is an active area of research. Here, we report an engineered nanofilm, composed of an array of poly(p-xylylene) nanorods, which demonstrates anisotropic wetting behaviour by means of a pin-release droplet ratchet mechanism. Droplet retention forces in the pin and release directions differ by up to 80 μN, which is over ten times greater than the values reported for other engineered anisotropic surfaces. The nanofilm provides a microscale smooth surface on which to transport microlitre droplets, and is also relatively easy to synthesize by a bottom-up vapour-phase technique. An accompanying comprehensive model successfully describes the film's anisotropic wetting behaviour as a function of measurable film morphology parameters.

Single and binary self-assembled monolayers of phenyl- and pentafluorophenyl-based silane species, and their phase separation with octadecyltrichlorosilane

In this paper, we first present the study of the formation of phenyltrichlorosilane film and self-assembled monolayers of phenylalkyltrichlorosilane (PATCl), pentafluoro-phenylalkyltrichlorosilane (PFATCl), and a mixture of the two, on silicon covered by its native oxide. These monolayers are shown to grow in two steps with characteristic time constants. The first step is characterized by a similar time constant of growth for all the studied trichlorosilane molecules and attributed to chemisorption. The second step corresponds to the arrangement between molecules, accelerated by the presence of the short alkyl chain (3-4 carbon atoms), and by mixing phenyl and pentafluoro-phenyl terminal moieties, which is accounted for by hydrogen bonding CH···FC and/or attractive quadrupolar interactions within a face-to-face phenyl/pentafluoro-phenyl alternating stack arrangement. Such results should allow improvement of intermolecular stacking within conjugated molecular domains, which is particularly important for molecular electronic devices. In the second part, we studied how PATCl, PFATCl, and their mixture phase separate with octadecyltrichlorosilane (OTS) molecules in various ratios. The way to improve phase separation was studied modifying aromatic ring to ring as well as aromatic-aliphatic interactions. OTS island size and coverage are shown to be smaller with the aromatic phase that involves stronger ring to ring interactions, i.e., attractive interactions between the phenyl species by mixing phenyl and pentafluoro-phenyl rings. The best phase separation is obtained with PFATCl as the aromatic molecule. If nanoislands of aromatic molecules could not be observed in these experiments, we show that they are attainable by mixing OTS and aromatic small organotriethoxysilanes whose grafting kinetics is slower. These results pave the way to the control improvement of the composition and nanostructuration of SAMs, essential for their further use within molecular devices.

Noncovalent mechanism for the conformal metallization of nanostructured parylene films

We describe a rapid, reliable method of preparing nanoporous Ni or Co films using nanostructured poly(chloro-p-xylylene) (nanoPPX) films as templates. The nanoPPX films are vapor deposited onto Si substrates using oblique angle polymerization (OAP), resulting in the formation of an obliquely aligned PPX nanorod array on the substrate. The nanoPPX films are then subjected to noncovalent functionalization using an aromatic ligand (i.e., pyridine) by means of treatment with either an aqueous solution of the ligand or ligand vapor. The results of quartz crystal microbalance and X-ray diffraction studies support a model in which pyridine adsorption is facilitated by the formation of pi-pi interactions with aromatic moieties in the amorphous surface regions of nanoPPX. The physisorbed pyridine in the nanoPPX film can subsequently bind a catalytic Pd(II)-based colloidal seed layer. Continuous, conformal Ni or Co films, characterized by FIB/SEM and AFM, are grown on the Pd(II)-laden nanoPPX films using electroless metallization. Analogous metallization of a conventionally deposited planar PPX film results in noncontinuous or patchy metal deposits. Such behavior is attributed to the sluggish adsorption of pyridine in the planar PPX film, resulting in an approximately 22-fold decrease in the quantity of pyridine adsorbed compared to that in a nanoPPX film. Consequently, the level of Pd(II) bound by pyridine on a planar PPX film is insufficient to catalyze continuous metallization. Results of a statistical two-level factorial design indicate that the morphology of the metal layer formed on a nanoPPX film is profoundly influenced by the ligand adsorption condition (i.e., aqueous ligand vs ligand vapor treatment) and is correlated to the catalytic activity of Co films for the production of hydrogen from sodium borohydride decomposition.