Athanassios G Kontos

The optical and structural properties of cadmium and lead sulfide nanocrystals deposited on mesoporous TiO2 substrates via the successive ionic layer adsorption and reaction method were comparatively investigated by reflectance, transmittance, micro-Raman and photoluminescence measurements. Enhanced interfacial electron transfer is evidenced upon direct growth of both CdS and PbS on TiO2 through the marked quenching of their excitonic emission. The optical absorbance of CdS/TiO2 can be tuned over a narrow spectral range. On the other side PbS/TiO2 exhibits a remarkable band gap tunability extending from the visible to the near infrared range, due to the distinct quantum size effects of PbS quantum dots. However, PbS/TiO2 suffers from severe degradation upon air exposure. Degradation effects are much less pronounced for CdS/TiO2 that is appreciably more stable, though it degrades readily upon visible light illumination.

Nitrogen and fluorine co-doped TiO₂ films have been prepared by dip coating of a modified titania sol–gel based on a nitrogen precursor and a nonionic fluorosurfactant as pore template and fluorine source. The modified NF–TiO₂ films absorb in the visible spectral range, between 400–510 nm and undergo reversible hydrophilic conversion under visible light to a final contact angle of 8°, in contrast to the UV limited optical response of their undoped anatase TiO₂ analogues. The phenomenon takes place at a rate slower than the corresponding one observed for the UV stimulated superhydrophilic effect. The wetting response of the N–F doped TiO₂ films correlates well with the variation of their optical properties and surface morphological characteristics and most importantly with their photocatalytic activity, rendering these materials very promising for self-cleaning applications under visible light.

Self-organized TiO(2) nanotubes with packed, vertically aligned morphology and different lateral characteristics were grown on Ti metal substrates by controlled electrochemical anodization in phosphate/HF and ethylene glycol/HF electrolytes. The wetting, photo-induced superhydrophilicity, and photocatalytic activity of the nanotubular materials were investigated under ultraviolet irradiation. The photoactivity of the TiO(2) nanotube arrays was analysed in terms of their morphological characteristics that were determined by means of scanning electron microscopy and atomic force microscopy in conjunction with geometrical modelling. The wetting and the UV-induced superhydrophilicity could be accordingly modelled by the Cassie-Baxter mode arising from the large scale roughness of the nanotubular arrays in combination with the Wenzel mode due to the small scale roughness induced by ridges at the outer tube surface. The photocatalytic activity of the TiO(2) nanotube arrays was further found to correlate quantitatively with the variation of the geometric roughness factor, verifying the strong impact of morphology on the photo-induced properties of the vertically oriented TiO(2) tubular architecture.