Fabrication of nanoporous titania on glass and transparent conducting oxide substrates by anodization of titanium films
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Nanoporous titania (TiO2) or titania nanotubes could provide a continuous nanostructured electron-conducting anode for organic photovoltaics. In this work, nanoporous titania was formed by anodizing thin films of titanium on both glass and transparent conducting oxide (TCO) substrates. Titanium thin films (500–700 nm) were deposited by radio frequency (RF) magnetron sputtering. Films were anodized in acidic electrolytes containing small amounts of hydrofluoric acid (HF) at constant voltages ranging from 7 to 15 V. Scanning electron microscope (SEM) analysis revealed a nanoporous structure. Nanoporous titania structures were grown on glass in an electrolyte containing sulfuric acid, trisodium citrate, and potassium fluoride, with pore diameters around 50 nm. Analyzing the films at different anodization times, the stages of nanopore formation were elucidated. Additionally, nanoporous titania was formed on a TCO substrate by anodizing in an electrolyte containing acetic acid and hydrofluoric acid. While not completely transparent, the nanoporous titania is promising for use in organic photovoltaics.
I. INTRODUCTION
Nanostructured materials offer new possibilities for the electronics world because structures on the length scale of a few atoms interact with light and electrons differently than in bulk materials. Self-assembled nanostructures are of considerable interest because of their potential scalability and simple processing. Fabrication of ordered alumina (Al2O3) nanotubes by anodization has long been known,1 and more recently titanium has been shown to create a titania (TiO2) nanotube structure when anodized in an electrolyte containing fluoride ions as described below. Titania is of interest due to its semiconducting nature and smaller band gap than alumina and the diversity of properties for its various crystal types, especially anatase and rutile.2 Studies to date have looked at the anodization primarily of Ti foils to produce titania nanotubes from a bulk titanium foil sheet, but anodizing titanium thin films especially on transparent conducting oxide (TCO) materials may provide nanostructured films of titania for new applications suitable a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr_policy. DOI: 10.1557/JMR.2007.0078
for applications such as organic photovoltaics and organic light-emitting diodes. Titania-based nanoparticle anodes have been very successful in the Gratzel dye-sensitized solar cell3 and thus may be suited for polymer solar cells as well. A better TiO2 contact with a continuous path for electron conduction could be provided by an array of titania nanotubes or nanopores of the correct diameter and spacing. Titania nanotube arrays have been the focus of recent research and have been investigated for applications including hydrogen sensors4 and water photoelec
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