Ana Borras

Reversible Superhydrophobic to Superhydrophilic Conversion of Ag@TiO2 Composite Nanofiber Surfaces

A new type of superhydrophobic material consisting of a surface with supported Ag@TiO(2) core-shell nanofibers has been prepared at low temperature by plasma-enhanced chemical vapor deposition (PECVD). The fibers are formed by an inner nanocrystalline silver thread which is covered by a TiO(2) overlayer. Water contact angles depend on the width of the fibers and on their surface concentration, reaching a maximum wetting angle close to 180 degrees for a surface concentration of approximately 15 fibers microm(-2) and a thickness of 200 nm. When irradiated with UV light, these surfaces become superhydrophilic (i.e., 0 degrees contact angle). The decrease rate of the contact angle depends on both the crystalline state of the titania and on the size of the individual TiO(2) domains covering the fibers. To the best of our knowledge, this is one of the few examples existing in the literature where a superhydrophobic surface transforms reversibly into a superhydrophilic one as an effect of light irradiation.

Type of Plasmas and Microstructures of TiO2 Thin Films Prepared by Plasma Enhanced Chemical Vapor Deposition

TiO 2 thin films have been prepared at temperatures between 298 and 523 K by plasma enhanced chemical vapor deposition at working pressures of 4 × 10 -3 and 4 × 10 -4 Torr, in this latter case by using electron cyclotron resonance conditions. Ti isopropoxide has been used as the precursor and oxygen or mixtures of oxygen + argon as the plasma gas. The refraction indexes of the films (from 1.95 to 2.4) have been correlated with their microstructure and surface roughness as observed, respectively, by scanning electron microscopy and atomic force microscopy. Optical emission spectroscopy analysis of plasma indicates a higher fragmentation of the precursor in a plasma of pure O 2 than in another of Ar + O 2 . Heating at T > 503 K induces the crystallization of the films into the anatase structure. The differences in the structure, microstructure, and optical properties are accounted for by assuming that the type of interactions of the plasma species with the growing film change with the deposition conditions.

Correlation lengths, porosity and water adsorption in TiO 2 thin films prepared by glancing angle deposition

This paper reports a thorough microstructural characterization of glancing angle deposited (GLAD) TiO(2) thin films. Atomic force microscopy (afm), grazing-incidence small-angle x-ray scattering (GISAXS) and water adsorption isotherms have been used to determine the evolution of porosity and the existence of some correlation distances between the nanocolumns constituting the basic elements of the films nanostructure. It is found that the deposition angle and, to a lesser extent, the film thickness are the most important parameters controlling properties of the thin film. The importance of porosity and some critical dimensions encountered in the investigated GLAD thin films is highlighted in relation to the analysis of their optical properties when utilized as antireflective coatings or as hosts and templates for the development of new composite materials.

Superhydrophobic supported Ag-NPs@ZnO-nanorods with photoactivity in the visible range

In this article we present a new type of 1D nanostructures consisting of supported hollow ZnO nanorods (NRs) decorated with Ag nanoparticles (NPs). The 3D reconstruction by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) electron tomography reveals that the Ag NPs are distributed along the hollow interior of the ZnO NRs. Supported and vertically aligned Ag-NPs@ZnO-NRs grow at low temperature (135 °C) by plasma enhanced chemical vapour deposition on heterostructured substrates fabricated by sputtered deposition of silver on flat surfaces of Si wafers, quartz slides or ITO. The growth mechanisms of these structures and their wetting behavior before and after visible light irradiation are critically discussed. The as prepared surfaces are superhydrophobic with water contact angles higher than 150°. These surfaces turn into superhydrophilic with water contact angles lower than 10° after prolonged irradiation under both visible and UV light. The evolution rate of the wetting angle and its dependence on the light characteristics are related to the nanostructure and the presence of silver embedded within the ZnO NRs.

Preillumination of TiO2 and Ta2O5 photoactive thin films as a tool to tailor the synthesis of composite materials.

Illumination of TiO 2 thin films with UV light is known to induce the transformation of the surface of this material from partially hydrophobic into fully hydrophilic. The present work shows that this transformation is accompanied by other effects that may be used to control the synthesis of composite materials. For this purpose, TiO 2 and Ta 2O 5 transparent thin films with a columnar structure and open pores were prepared by electron evaporation at glancing angles. Transparent TiO 2 thin films with micropores (i.e., pores smaller than 2 nm) prepared by plasma enhanced chemical vapor deposition (PECVD) were also used. All these films became hydrophilic upon UV illumination. Rhodamine 6G and Rhodamine 800 dyes were irreversibly adsorbed within the columns of the TiO 2 and Ta 2O 5 thin films by immersion into a water solution of these molecules. Isolated and aggregated molecules of these two dyes were detected by visible absorption spectroscopy. The infiltration adsorption efficiency was directly correlated with the acidity of the medium, increasing at basic pHs as expected from simple considerations based on the concepts of the point of zero charge (PZC) in colloidal oxides. The infiltration experiments were repeated with columnar TiO 2 and Ta 2O 5 thin films that were subjected to preillumination with UV light. It was found that this treatment produced a modification in the type (isolated or aggregated) and amount of dye molecules incorporated into the pores. Moreover, the selective adsorption of a given dye in preilluminated areas of the films permitted the lithographic coloring of the films. Preillumination also controls the UV induced deposition of silver on the surface of the microporous TiO 2 thin films. It was found that the size distribution of the formed silver nanoparticles was dependent on the preillumination treatment and that a well-resolved surface plasmon resonance at around 500 nm was only monitored in the preilluminated films. A model is proposed to account for the effects induced by UV preillumination on the TiO 2 and Ta 2O 5 oxide surfaces. The possibilities of this type of light treatment for the tailored synthesis of nanocomposite thin films (i.e., dye-oxide, metal nanoparticles-oxide) are highlighted.

Selective Dichroic Patterning by Nanosecond Laser Treatment of Ag Nanostripes

Authors thank the Projects FUNCOAT CONSOLIDER-INGENIO CDS2008 – 0023, MAT2007 – 65764, MAT2010-18447, MAT2010-21228, CEN2007 – 2014, P09-TEP-5283, Domingo Martinez Foundation, and Juan de la Cierva Grant No JCI-2009 – 05098 for J.T.

Transparent nanometric organic luminescent films as UV-active components in photonic structures.

A new kind of visible-blind organic thin-film material, consisting of a polymeric matrix with a high concentration of embedded 3-hydroxyflavone (3HF) dye molecules, that absorbs UV light and emits ...

Wetting Properties of Polycrystalline TiO2 Surfaces: A Scaling Approach to the Roughness Factors

This work presents a thorough study on the wettability of polycrystalline anatase TiO(2) thin films prepared at 250 °C in a microwave plasma enhanced chemical vapor deposition (MW-PECVD) reactor with Ar/O(2) plasmas. Anatase polycrystalline thin films with different microstructures, textures, and surface roughness were obtained as a function of their thickness. The water contact angle of the samples was analyzed within the assumptions of the Wenzel, Cassie, and Miwa models to ascertain the effect of roughness and other surface heterogeneities on their characteristic parameters. The roughness factors defined in the different models were calculated from the atomic force microscopy (AFM) images of the films for two different observation scales within the premises of the dynamic scaling theories. The obtained results indicate that the wetting angle of an equivalent flat anatase surface with a value of 82° can only be properly estimated for observation scales of 5 × 5 μm(2) and using the Miwa model. The analysis of the UV induced hydrophilization of the surface state of the anatase films and the posterior recovery of the partially hydrophobic character of these surfaces in the absence of UV photons suggest a clear dependence of the light induced wettability on their texture and size of crystalline domains.

One-Step Dry Method for the Synthesis of Supported Single-Crystalline Organic Nanowires Formed by π-Conjugated Molecules

We present for the first time a general vacuum process for the growth of supported organic nanowires formed by pi-conjugated molecules, including metalloporphyrins, metallophthalocyanines, and perylenes. This methodology consists on a one-step physical vapor deposition of the pi-conjugated molecules. The synthesis is carried out at controlled temperature on substrates with tailor morphology which allows the growth of organic nanowires in the form of squared nanofibers and nanobelts. The study of the nanowires by electron diffraction and HRTEM combining with the results of a theoretical analysis of the possible arrangement of the pi-conjugated molecules along the nanowires reveals that the nanowires show a columnar structure along the fiber axis consisting of pi-stacked molecules having a herringbone-like arrangement. The formation of these nanowires on different substrates demonstrates that the growth mechanism is independent of the substrate chemical composition. An in-depth phenomenological study of the formation of the nanowires drives us to propose a growth mechanism based on a crystallization process. Furthermore, the growth method allows the fabrication of two particular 1D heterostructures: binary and open core@shell organic nanofibers.

Laser treatment of Ag@ZnO nanorods as long-life-span SERS surfaces.

UV nanosecond laser pulses have been used to produce a unique surface nanostructuration of Ag@ZnO supported nanorods (NRs). The NRs were fabricated by plasma enhanced chemical vapor deposition (PECVD) at low temperature applying a silver layer as promoter. The irradiation of these structures with single nanosecond pulses of an ArF laser produces the melting and reshaping of the end of the NRs that aggregate in the form of bundles terminated by melted ZnO spherical particles. Well-defined silver nanoparticles (NPs), formed by phase separation at the surface of these melted ZnO particles, give rise to a broad plasmonic response consistent with their anisotropic shape. Surface enhanced Raman scattering (SERS) in the as-prepared Ag@ZnO NRs arrays was proved by using a Rhodamine 6G (Rh6G) chromophore as standard analyte. The surface modifications induced by laser treatment improve the stability of this system as SERS substrate while preserving its activity.