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Reversibly UV-light-induced hydrophobic/oleofilic to amphiphilic surface transition in thin films of organic-capped TiO2 nanorods

Gianvito Caputo 1Athanassia Athanassiou 1Concetta Nobile 1Tobias Kipp 2Roman Krahne 1Elvio Carlino 3Liberato Manna 1Roberto Cingolani 1Davide P. Cozzoli 1

1. National Nanotechnology Laboratory of CNR-INFM (NNL), via per Arnesano Km 5, Lecce 73100, Italy
2. University of Hamburg, Institute of Applied Physics, Jungiusstrasse 11, Hamburg 20355, Germany
3. TASC-INFM-CNR National Laboratory, (TASC), Area Science Park, Basovizza, Bld MM, SS 14, Km 163.5, Trieste 34012, Italy

Abstract

Nanocrystalline TiO2 is among the most studied semiconductor oxides, owing to its low-cost widespread applications in environmental clean-up, in sensing, and in the photodynamic deactivation of bacteria and malignant cells. More recently, it has been discovered that UV-irradiation of TiO2 surfaces rapidly converts them from an initially hydrophobic state to a highly amphiphilic one, followed by a slow recovery of the starting properties under dark storage. These findings have stimulated the fabrication and the study of inorganic coatings which exhibit simultaneous self-cleaning and antifogging behaviour. However, the understanding the mechanism leading to light-induced wettability modification on TiO2 surfaces remains elusive yet and the ability to control them has to be developed further. In this contribution we will present a novel approach to engineer TiO2 surfaces with UV-switchable wettability, which relies on the use of organic-capped TiO2 nanorods (NRs) for the fabrication of thin films made of close-packed laterally aligned arrays of TiO2 crystalline domains exposing well-defined light-active crystal facets. As opposed to conventional polycrystalline or single-crystal TiO2 surfaces, such NR-based films exhibit a reversible surface transition from a highly hydrophobic/oleofilic state (water and octadecene contact angles of 110° and 8°, respectively) to a highly amphiphilic one (water and octadecene contact angles of 20° and 3°, respectively) under remarkably milder UV-irradiation conditions (energy density as low as 1-2 mJ/cm2). By using a combination of techniques (XRD, HRTEM, SEM, FTIR and Raman spectroscopy, and contact angle measurements), we will demonstrate that the observed light-driven wettability changes are accomplished by progressive hydroxylation of the TiO2 surface irrespective of the presence of surfactant molecules, which simultaneously undergo conformational changes without suffering for significant photocatalytic degradation.

 

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Related papers

Presentation: Poster at E-MRS Fall Meeting 2007, Symposium B, by Gianvito Caputo
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-16 14:27
Revised:   2009-06-07 00:44