Nanostructures fabrication porous Anodic Alumina and ZnO nanorods

• Autores: Ignacio Mínguez Bacho
• Directores de la Tesis: Manuel Hernández Vélez (dir. tes.)
• Lectura: En la Universidad Autónoma de Madrid ( España ) en 2012
• Idioma: español
• Tribunal Calificador de la Tesis: Aurelio Climent Font (presid.), José Luis Pau Vizcaino (secret.), Rastislav Varga (voc.), Cristina Gómez-Aleixandre Fernández (voc.), Agustina Asenjo Barahona (voc.)
• Materias:
  • Física
• Enlaces
  • Tesis en acceso abierto en: Biblos-e Archivo
• Resumen

  • Through the following report it is going to be presented and discussed the results obtained from the investigation carried out for during the development of the doctoral thesis.

    This work is based fundamentally on the development and fabrication of Nanoporous Anodic Alumina Films, (NAAF), and Zinc Oxide nanostructures. These materials have been widely and systematically charecterised based on the use of different techniquesin order to conveniently study their composition,structure and properties.

    Firstly, it is explained the fabrication process of NAAF grown with the objective of reducing geometric parameters (i.e. pore diameter and interpore distance) of such. With this purpose, double anodization process have been carried out, using aqueous electrolytes based on sulphuric acid with three distinct concentrations, being 3, 10 and 20wt%. In addition, the voltage applied during the process of anodization varied between 28 and 5 voltsfor each one of the used electrolytes. The NAAFs obtained have certain geometric characteristics that vary based on the parameters used to fabricate them. These variations in their geometry can be small between each other, for which a methodology for their study was carried out which allows for differentiation. This method is based on the application of the self¿correlation function (SCF) on surface micrographs of NAAF obtained by high resolution scanning electron microscope (HR¿SEM). The process of transforming micrographs in self¿correlation images, allows us to obtain precise measurements of the geometric parameters of each one of the NAAFs, which in turn givesisthe possibility to carry out a qualitative and quantitative study of the hexagonal ordering degree which was generated among the nanopores.

    On the other hand, on the same samples, a compositional and structural study has been carried out using the combination of techniques as Rutherford and Backscattering Spectronemetry (RBS), thermo¿gravimetry (TG), Infrared spectrometry IR) and the diffraction of X¿rays and the complementation among them.

    Anotherstudy carried out wastheir growth¿rate, volume expansion and the efficiency Finally, the above lead us to the fabrication and morphology and magnetic characterization of Co nanowires by using theseNAAFs astemplates.

    Other subject of interest in the scientific community discussed in this thesis has been the growth of anodic alumina through a processso called hard anodization (HA). In this case, it wasstudied in two ways, both potentiostatic processes. In one ofthe experimental procedures the voltage rate applied increased between 25 and 60V, stopping the process at various distinct moments. After the analysis of the morphology characterization, it has been possible to observe a high degree of ordering achieved within short times and a dependence on the time with the centre to centre distance ofthe anodic alumina cells.

    In the second study, aluminumanodization was carried out by pulsed anodization (PA).

    This method is based on alternating periodic HA and mild anodization (MA) (i.e., the more typical). The alternation of these two modes of anodization provokes a variation in diameter and structure on the lengths ofthe nano¿channels of anodic alumina.During the length of both types of segments can be controlled by the duration of these pulses. Also, we were able to control the shape of the HA pulses, between squares, exponentials and triangles signals. The structures obtained were used as a template for the fabrication of modulated Co nanowires with high aspectratio.

    Finally, some studies have been presented investigating the obtention of ZnO nanowires within NAAF templates; however the results were not the expected. One simple and low¿cost alternative for obtaining nanostructures of ZnO has been the hydrothermal method. This chemical method is based on the chemical reaction that is produced based on the aqueous solution through which length and diameter of the remaining nanorods can be controlled by varying the concentration of ZnO precursor and growth time. The lengths obtained vary approximately between 1 and 4µm and its diameters between 40 and 600nm, being greater due to the distribution in size. These ZnO nanorods have been grown in a substrate of Fluor doped Tin oxide, which is a transparent conductor oxide.

    Thistransparentsubstrate, along with the photo¿catalytic properties of ZnO improved due to the deposit of CdS, allows for the development of a device capable of producing Hydrogen starting from solar light by dissociation of water molecules thanks to the photo¿ current generated by carriers ofthe semi¿conductorsin question.

    of anodisation depending on their experimental parameters. Starting from the reflectance spectra, we were also able to calculate the refraction index based on a mathematic algorithm based on Snell¿s law and Bragg¿s equation. Also, the samples thickness was estimated with great precision through a non¿destructive method based on Maxwell¿Garnett¿s effective model