Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016)
    Nanomaterials Tuesday Sessions
       Session NM-TuE

Paper NM-TuE3
Low-temperature Localized Growth of High Aspect-Ratio Multi-Wall Titanium Dioxide Nanotubes for Orthogonal Frequency Coded SAW Gas Sensors

Tuesday, December 13, 2016, 6:20 pm, Room Hau

Session: Nanofabrication and Nanodevices II
Presenter: William Clavijo, Virginia Commonwealth University, USA
Authors: W. Clavijo, Virginia Commonwealth University, USA
C. Castano, Virginia Commonwealth University, USA
W. Wilson, NASA Langley Research Center, USA
G. Atkinson, Virginia Commonwealth University, USA
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We expand the potential application and sensitivity of solid acoustic wave (SAW) gas sensors by incorporating ultra-high surface area, multi-wall TiO2 nanotubes into orthogonal frequency coded (OFC) SAW gas sensors. This process relies on synthetization of multi-wall tube-in-tube polycrystalline TiO2 nanotubes utilizing nanoporous anodic aluminum oxide (AAO) templates in a thin (2.5µm) aluminum film deposited on a lithium niobate substrate. We have demonstrated this method by integrating multi-wall nanotubes onto the delay line of an OFC SAW device to form an integrated sensor structure. The multi-wall TiO2 nanotube growth uses a combination of multi-stage aluminum anodization, alumina barrier layer removal, TiO2 and Al2O3 atomic layer deposition (ALD), and wet release etching. This growth process selectively forms TiO2 nanotubes on the delay line while the aluminum film remains intact for interdigital transducers (IDT) and reflector banks. The self-assembled high density AAO template was selectively formed in an ultra-smooth (Ra=1.5nm) 2.5 µm thick aluminum layer deposited through e-beam evaporation. The resulting AAO template consists of nanopores of 100 nm in diameter and 1.5 µm in height with an aerial density of 1.3 x 1010 nanopores/cm2. This AAO template was then filled with successive ALD nanotubes by alternating Al2O3 sacrificial spacers and TiO2 at 200 ˚C. The alumina template and Al2O3 sacrificial spacers were then removed, leaving free standing multi-wall coaxial TiO2 nanotubes of 1.5 µm in height and 100 nm in diameter, offering an increase in 112X the surface area over a standard flat TiO2 film for sensing applications. The TiO2 nanostructures were characterized by SEM, and TEM to examine internal structure, composition, and verify crystal structure. In addition, the OFC SAW gas sensor with a center frequency of 229 MHz and 3 reflectors on each side of the IDT was tested using 200 ppm NH3 to demonstrate functionality and measure sensitivity. Mass loading induced by the NH3 gas absorbed onto the multi-wall TiO2 nanotube resulted in amplitude shift of 0.027 dB upon exposure to 200 ppm NH3 at room temperature during interrogation of the 3rd reflector in sensitive channel. The sensor shows a promising room temperature operation with reproducible performance. Furthermore, OFC SAW gas sensors may be coded and implemented both passively and wirelessly in addition of taking advantages of the benefits of SAW technology.