Titania Coated Silica Nanowires
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1206-M16-05
Titania Coated Silica Nanowires Avi Shalav, Dinesh K.Venkatachalam, Fabian Reichardt, Frederic Fischer and Robert G. Elliman Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian National University, Canberra, ACT0200, Australia
ABSTRACT The morphology, phase and stability of titania-coated SiOx NWs are investigated for two coating methods: solution-based deposition of Ti-alkoxides and Atomic Layer Deposition (ALD). Analysis of as-deposited and annealed films shows that it is possible to produce stable coatings of the anatase phase of TiO2. The limitations of these two approaches are also discussed. INTRODUCTION Titanium dioxide (Titania), TiO2 is a non-toxic semiconductor that can be directly excited by ultra-violet radiation, with important electronic device applications including photocatalysis, chemical sensing and solar cells. Nanostructured TiO2 materials have particular importance for these applications since high surface to volume ratios can be obtained resulting in enhanced conversion efficiencies. Unfortunately, nanowires (NWs) of the more reactive anatase phase of TiO2, best suited for these applications, is particularly difficult to grow using common high temperature vapor-liquid-solid (VLS) growth techniques since formation of the more stable and less reactive rutile phase dominates. Rutile films have been observed on Si substrates at deposition temperatures above 700oC, but more typically between 900-1100oC. Anatase is a metastable phase and its conversion to rutile is irreversible. Unlike TiO2 NWs, Silica NWs can be grown under very simple conditions. At high temperatures and low oxygen partial pressures, silicon undergoes active oxidation to produce volatile SiO vapor. With the addition of a metal catalyst, profuse sub-stoichiometric silica (SiOx, where x is close to 2), NWs can be readily grown on top of a Si substrate via the VLS growth mechanism, where the substrate itself provides the necessary vapor-phase precursors [1, 2]. These silica NWs have the potential to provide an ideal large-area ‘backbone’ or supporting substrate for enhanced photon energy conversion devices. For example, in a recent study, the presence of an SiOx bottom layer has been shown to improve the photocatalytic activity of sputtered TiO2, most likely by minimizing recombination by introducing electron traps [3]. A detailed explanation for the growth mechanism and resulting morphology of the SiOx nanowires via active oxidation of a Si substrate is still lacking in the literature. Pioneering experiments in the 1950’s suggest that a fibrous microcrystalline chain phase of silica is formed (called silica ‘W’ by Sosman [4]), which rapidly becomes amorphous in the presence of water vapor [4, 5]. The surface properties of the silica are particularly important for the adhesion of dispersed titanium alkoxide coatings since surface non-isolated hydroxyl groups are required and
vary depending on the silica substrate [6]. High silica surface areas (for example fumed silica
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