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TiO2 anatase-based membranes with hierarchical porosity and photocatalytic properties

https://doi.org/10.1016/j.jcis.2006.09.064Get rights and content

Abstract

Taking benefit of previously obtained results, stable complex organic–inorganic hybrid suspensions are successfully prepared by mixing a polystyrene latex aqueous suspension, a titania hydrosol and a nonionic triblock copolymer. These suspensions can be then deposited as thin films on dense or porous substrates. Solvent evaporation induces the formation of spherical micelles by self-assembly of the amphiphilic molecules during the drying of the films. Two types of isolated spherical macropores (few ten nanometers) and mesopores (4–5 nm) are generated inside the layers by the thermal removal of the polystyrene particles and of the micelles, respectively. The remaining inorganic network exhibits an additional interconnected microporosity with a mean pore size of 1.5 nm, resulting from the aggregation of the anatase nanoparticles. A complete removal of the templating units at low temperature is possible using the photocatalytic properties of the anatase network. Such layers exhibit attractive properties for the design of ceramic membranes. They can be advantageously used in order to increase the permeability of the separative layer and to reduce the number of intermediate layers of these asymmetric structures.

Graphical abstract

Hierarchical anatase-based membrane consisting in isolated spherical macropores (a), isolated mesopores (b), and nanocrystalline anatase walls with interconnected micropores (c).

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Introduction

Intensive research is currently devoted to materials with a hierarchical structure [1], [2], [3], [4], [5], [6], [7]. This approach is here applied to the design of the porosity for the top-layer of an asymmetric ceramic membrane. In order to increase the permeability of such separative layer, it can be advantageous to generate extraporosity. The main condition which has to be respected is that the additional porosity must not be directly interconnected in order to preserve the cut-off fixed by the porosity of the continuous phase. In the case of sol–gel derived porous membranes, removable templating particles and amphiphilic molecules giving rise to individual micelles by self-assembly can be added into the starting sol. Moreover, the presence of these removable additives inside the starting suspensions modifies their rheology and decreases their ability to infiltrate the porous substrates. It can be used to decrease the number of intermediate layers of the asymmetric membranes. In the present study, TiO2 anatase-based layers were prepared with three levels of porosity: macropores, mesopores, and micropores.

Section snippets

Materials and methods

The combination of two types of porogens and of nanoparticles offers an efficient way for the structure of ordered microporous–mesoporous–macroporous architectures. The hierarchical thin layers and membranes were prepared from an anatase hydrosol. The synthesis conditions of this titania sol have been previously detailed [8], [9], [10]. The templating particles used to generate isolated macropores were polystyrene (PS) latex spheres, all having the same diameter (∼130 nm). The latex particles

Results and discussion

TEM images of layers TLP and TLF are given in Figs. 1a and 1b. It is possible to observe the macropores and the mesopores in the area between them. The mesopores measured for TLP layer are larger than the TLF one. These pictures suggested spherical mesopores for TLF as previously reported for F67 layers. But for TLP it is not possible to evidence the hexagonal close-packing of cylindrical mesopores as expected with the initial volume fraction of P123 triblock copolymer. However the alignment of

Conclusion

This preliminary study demonstrates the feasibility of TiO2 anatase based membranes exhibiting a hierarchical porosity. Additional experiments are now in progress to experimentally demonstrate the potential interest of such membranes in term of permanence and cut-off and also of simplification of the preparation process.

Acknowledgements

The authors thank L. Datas (CIRIMAT, Toulouse, France), A. El-Mansouri, and D. Cot (I.E.M., Montpellier, France) for their help with the TEM, nitrogen adsorption–desorption, and SEM characterizations, respectively.

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