Rudina Bleta

Effect of PEG on rheology and stability of nanocrystalline titania hydrosols.

Very stable titania hydrosols were prepared by fast hydrolysis of titanium isopropoxide in a large excess of water. XRD patterns show that these sols contain nanocrystals (5-6 nm) of anatase (70%) and brookite (30%). TEM images indicate that these primary particles form aggregates whose mean hydrodynamic diameter, determined by photon correlation spectroscopy, is in the range of 80-90 nm. The flow curves of these colloids, recorded for several volume fractions of nanoparticles, can be perfectly fitted, in the range 0-100 s(-1), with a power-law model. In this range the behavior is Newtonian but for larger shear rates a shear thinning is observed. The viscosity dependence on particle concentration can be predicted by a Batchelor-type model were the volume fraction of particles is replaced by an effective volume fraction of aggregates, taking into account their fractal dimension. Addition of polyethylene glycol (PEG 2000) induced a marked decrease (more than 50%) of the sol viscosity down to a minimum. This is explained by assuming that PEG adsorbs on the surface of TiO(2) particles producing stabilization by steric effects and leading to formation of more compact aggregates. Without PEG the sol viscosity strongly decreases on aging. This effect is not caused by the growth of primary particles. It is rather interpreted as a progressive reorganization of the aggregates toward a more compact packing.

An efficient route to aqueous phase synthesis of nanocrystalline γ-Al2O3 with high porosity: from stable boehmite colloids to large pore mesoporous alumina.

In this paper we emphasise the important role of Pluronic F127 on the porosity of mesoporous alumina prepared from boehmite colloids. By focusing on the F127/boehmite interactions we show how the concepts of interface science may help to predict and improve the textural characteristics of mesoporous alumina. By varying the synthetic parameters, in particular the copolymer content, we show that the porosity of γ-Al(2)O(3) can be enhanced by 400% and the average pore diameter can be expanded from 5 to 14 nm. These results are discussed in terms of interactions between the Pluronic F127 and boehmite colloids, and are correlated to the critical micelle concentration (CMC) of the copolymer. The textural characteristics of the mesoporous alumina can be further improved either by introducing hydrocarbons in the preformed boehmite/copolymer sols or by concentrating the sols. In comparison with as-synthesised alumina, those prepared with F127 showed improved thermal stability. Furthermore, boehmite/copolymer sols were stable for all surfactant concentrations investigated and can give high quality coatings suitable for catalytic applications.

Photocatalysis of Volatile Organic Compounds in water: Towards a deeper understanding of the role of cyclodextrins in the photodegradation of toluene over titanium dioxide

HYPOTHESIS Cyclodextrin-assisted photodegradation of toluene was investigated in water in the presence of a photo-irradiated commercial titanium dioxide photocatalyst. It was expected that cyclodextrins could form water-soluble supramolecular host/guest complexes with the toluene and thus promote the approach of the pollutant on the TiO2 surface and enhance the phototocatalytic oxidation efficiency. EXPERIMENTS Photodegradation kinetics of toluene were investigated under UV-C and near-visible light radiation in aqueous suspensions of TiO2. Impact of cyclodextrin (CD) on the photocatalytic efficiency of TiO2 was evaluated with different cyclodextrins: α-CD, β-CD, γ-CD and RAME-β-CD. Host-guest association constants were determined by static headspace gas chromatography and affinity of cyclodextrins for the TiO2 surface by isothermal adsorption studies. Issue of the cyclodextrin stability during the degradation process was examined using Total Organic Carbon, NMR and MALDI-TOF analyses. FINDINGS Toluene could be fully mineralized by TiO2 in water within hours, even if the presence of cyclodextrin caused a delay in the photodegradation process. The chemical nature of cyclodextrins was found to exert a significant influence on the extent of inhibitory effect, which was discussed in terms of balance between solubilization efficiency, substrate protection and coverage of active sites of TiO2 by competitive adsorption. The cyclodextrin degradation was also studied and discussed.

Evidence for the existence of crosslinked crystalline domains within cyclodextrin-based supramolecular hydrogels through sol–gel replication

The supramolecular hydrogel network formed from the association of poly(ethylene glycol) and α-cyclodextrin-based polypseudorotaxanes has been successfully replicated into a silica scaffold with a three-dimensional hierarchical pore structure by using a sol–gel process. The examination of the replicas indicates that the columnar polypseudorotaxane-based nanocrystallites act as crosslinkers maintaining the supramolecular hydrogel in a water-swollen state. Depending on the poly(ethylene glycol)/α-cyclodextrin formulation and the pH value, the resulting silica materials may exhibit a pore structure comprised of large mesopores (4–10 nm) interconnected by smaller ones (2–2.5 nm) in a framework where the pore walls are made-up of a microporous corona (<1.8 nm).

Understanding the role of cyclodextrins in the self-assembly, crystallinity, and porosity of titania nanostructures.

A series of mesoporous titania photocatalysts with tailorable structural and textural characteristics was prepared in aqueous phase via a colloidal self-assembly approach using various cyclodextrins (CDs) as structure-directing agents. The photocatalysts and the structure-directing agents were characterized at different stages of the synthesis by combining X-ray diffraction, N2-adsorption, field emission scanning electron microscopy, transmission electron microscopy, UV-visible spectroscopy, dynamic light scattering, and surface tension measurements. The results demonstrate that the cyclic macromolecules efficiently direct the self-assembly of titania colloids, resulting in a fine-tuning of the crystal phase composition, crystallite size, surface area, particle morphology, pore volume, and pore size. Depending on the chemical nature of the substituents in the cyclodextrin ring, synergistic or competitive effects arising from the adsorption capacity of these cyclic oligosaccharides onto titania surface, surface-active properties, and ability to aggregate in water by intermolecular interactions were found to substantially impact the characteristics of the final material. We propose that, in contrast to the native cyclodextrins, which tend to favor the local agglomeration of titania nanoparticles due to the strong intermolecular interactions, the substitution of hydroxyl groups by a relatively large number of methoxyl or 2-hydropropoxyl ones in the β-CD derivatives allows for creating smoother interfaces, thus facilitating the self-assembly of the colloids in a more homogeneous network. The photocatalytic activity of those titania materials was evaluated in the photodegradation of a toxic herbicide, phenoxyacetic acid, and was correlated to the structural and textural characteristics of the photocatalysts.

Block copolymer–cyclodextrin supramolecular assemblies as soft templates for the synthesis of titania materials with controlled crystallinity, porosity and photocatalytic activity

Nanostructured titania materials with tunable porosity and crystalline framework were prepared in an aqueous phase by using a template-directed colloidal self-assembly strategy. The approach employs the supramolecular assemblies formed between the randomly methylated β-cyclodextrin and the block copolymer P123 as soft templates and the sol–gel synthesised TiO2 nanocrystals as building blocks. By combining X-ray diffraction, N2-adsorption, field emission scanning electron microscopy and UV-visible spectroscopy, we show that considerable control over crystallite size, polymorph content and particle morphology could be achieved by using a delicate balance between the composition and structure of the supramolecular template as well as the reaction conditions. The photocatalytic activity of these mesoporous TiO2 materials was evaluated in the photodegradation of a toxic herbicide, the phenoxyacetic acid (PAA), and was correlated to the structural and textural characteristics of the photocatalyst.

Cyclodextrins and Nanostructured Porous Inorganic Materials

The discovery of ordered mesoporous materials in 1992 by Mobil Oil Corporation (Mobils) scientists has opened great opportunities for new applications in many emerging fields such as heterogeneous catalysis, biocatalysis, energy conversion, biosensors, photocatalytic devices and environmental technologies. Porous materials are grouped by the International Union of Pure and Applied Chemistry (IUPAC) into three classes according to their pore diameter: microporous ( 50 nm). One of the most versatile methods for the preparation of these materials is the soft-template approach which combines the sol-gel process with the molecular self-assembly. Materials with monodisperse particle sizes, well-defined architectures and tunable porosity can be built by this approach using micelles formed by ionic or non-ionic surfactants, as well as amphiphilic polymers, as templates. On the other hand, the supramolecular assemblies formed between cyclodextrins and block copolymers have been less investigated in the literature for the preparation of inorganic materials or nanocomposites, despite their large chemical and structural diversity.