Fabrication of coaxial nanowire heterostructures: SiO x nanowires with conformal TiO2 coatings

Abstract

Silica nanowires, grown via the active oxidation of a silicon substrate, have been coated with TiO 2 using two coating methods: solution-based deposition of Ti-alkoxides and atomic layer deposition. Analysis of as-deposited and annealed films shows that it is possible to produce stable conformal coatings of either the anatase or rutile phases of TiO 2 on nanowires with diameters greater than 100 nm when annealed between 500-600°C and 800-900°C, respectively, with annealing at higher temperatures (1050°C) producing coatings with a highly facetted rutile morphology. The efficacy of the process is shown to depend on nanowire diameter, with nanowires having diameters less than about 100 nm fusing together during solution-based coating and decomposing during TiO 2 atomic layer deposition. The use of a suitable buffer layer is shown to be an effective means of minimizing nanowire decomposition. Finally, annealing coated nanowires under active oxidation conditions (1100°C) is shown to be an effective technique for depositing additional conformal SiO x coatings, thereby providing a means of fabricating multi-layered coaxial nanostructures.

Influence of substrate temperature on the morphology of silicon oxide nanowires synthesized using a tin catalyst

“…All the above factors make these nanostructures suitable for numerous applications. They can be used in optoelectronic devices [5,6] and gas and biological sensors [4]. Silicon oxide nanowires can be coated with various materials, including metals and semiconductors.…”

“…Functionalized silicon oxide nanowires are used to control surface wettability [10], which is important for biological applications and for the enhancement of boiling heat transfer on nanostructured surfaces [10,11]. Usually, silicon oxide nanowires are synthesized by the vapor-liquid-solid mechanism on catalyst particles using the thermal evaporation method [2,4,5]. However, this method is characterized by high temperatures, which is not always suitable for device applications.…”

The Mechanical and Photochemical Properties of Titania Coated Silica Nanowires

“…Further synthesis details and associated TiO 2 phase characterization have already been previously recently reported. 24 Mechanical characterization.-The mechanical properties of the dense NW surfaces were investigated by assuming the NW layer to be an effective thin film layer, measured via cross-section Scanning Electron Microscope (SEM) micrographs obtained from a Zeiss UltraPlus FESEM, to be in the order of tens of microns in thickness.…”

“…Using Ti-alkoxide precursors, both anatase and rutile TiO 2 coatings up to 50 nm in thickness have been successfully deposited on SiO x NWs by both Atomic Layer Deposition (ALD) and wet chemistry coating techniques. 24 Not only do the alkoxides adhere well to nanostructured silica, but it is believed that strong Si-O-Ti chemical bonding at the interface is likely to retard the transformation to the rutile phase. 25 The photoactivity of nanostructured anatase TiO 2 surfaces is more active than that of rutile surfaces due to a combination of effects, including volume fraction (and therefore effective surface area), bandgap and number of hydroxyl groups, all of which are higher for anatase surfaces.…”