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While numerous works deal with the properties and applications of porous silicon (PS), some of the related topics are not complete or could be investigated from different aspects. The main objective of this paper is to provide novel information associated with the optical properties of nano-and meso-PS by studying the variation of the effective refractive index (n eff ) and the relative dielectric constant (ε r,eff ) as a function of porosity. For this purpose various PS samples were prepared by electrochemical etching of p and p + -type silicon wafers in order to form silicon supported nano-PS and free-standing meso-PS layers, respectively. The experimental effective optical parameters (n eff , ε r,eff ) of the meso-PS films, were determined from the transmission spectra and the Bragg law. While, in the case of nano-PS layers, we applied the Goodman method, and deduced the values of n eff and ε r,eff from the Bruggeman's effective medium approximation (EMA). In the EMA calculation, the PS structure was considered as being a physical combination of three distinct phases formed by silicon, silicon dioxide and voids with a convenient volume fraction. A good agreement between theory and experiment was found in the case of silicon-supported nano-PS for all porosities. However, for free-standing meso-PS, the theory does not well fit the experimental results for porosities lower than 50% and higher than 70%.

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This study investigates the effect of the diameter of TiO nanotubes and silver decorated nanotubes on optical properties and photocatalytic inactivation of Escherichia coli under visible light. The TiO nanotubes (TiO-NTs) were prepared using the electrochemical method varying the anodization potential starting from 20 V until 70 V. The Ag nanoparticles were carried out using the photoreduction process under the same experimental conditions. The diameter size was determined using the scanning electronic microscopy (SEM). TiO-NTs diameter reached ∼100 nm at 70 V. Transmission electronic microscopy (TEM) imaging confirmed the TiO-NTs surface decoration by silver nanoparticles. The Ag-NPs average size was found to be equal to 8 nm. The X-Ray diffraction (XRD) analysis confirm that all TiO-NTs crystallize in the anatase phases regardless the used anodization potential. The decrease of the photoluminescence (PL) intensity of Ag NPs decorated TiO-NTs indicates the decrease of the specific area when the nanotubes diameter increases. The UV-vis absorbance show that the absorption edges was bleu shifted with the increasing of nanotubes diameter, which can be explained by the increase of the crystallites average size. The bacterial adhesion and inactivation tests were carried in the dark and under light. Bacteria were seen to adhere on TiO-NTs in the dark; however, under light the bacteria were killed before they establish a strong contact with the TiO-NTs and Ag/TiO-NTs surfaces. Bacterial inactivation kinetics were faster when the anodizing potential of the NTs-preparation increases. A total bacterial inactivation was obtained on ∼100 nm nanotubes diameter within 90 min. This result was attributed to the enhancement of the TNTs crystallinity leading to reduced surface defects. Redox catalysis was seen to occur under light on the TiO-NTs and Ag/TiO-NTs. the photo-induced antibacterial activity on the AgO/AgO decorated TiO-NTs was attributed to the interfacial charge transfer mechanism (IFCT).