Synthesis and application of TiO₂ nanotube-based functional materials


Student thesis: Doctoral Thesis

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  • Fengxia LIANG


Awarding Institution
Award date3 Oct 2012


In this thesis, TiO2 nanostructures conveniently fabricated by the anodization method were investigated. Anodic TiO2 nanostructures hold great promise in various application fields, such as dye-sensitized solar cells, electrochromic devices, hydrogen sensors, water splitting, organic pollutant degradation, lithium ion batteries and surface wettability control, due to their large surface area, easy fabrication, rich surface chemistry, chemical stability and photocatalytic properties. In this dissertation, we studied the fabrication of anodic TiO2 nanostructures and nanocomposites and devices based on them, and further investigated the fabricated nanomaterials and devices for different applications, such as nanocrystal fabrication, sensing, photocatalysis, and photo-detecting. The first chapter provides an overview of the fabrication methods and applications of TiO2 nanostructures, with an emphasis on anodic TiO2 nanostructures. The second chapter reports a convenient bottom-up method to fabricate robust and inert TiO2-based nanowells for growing nanocrystals. These TiO2-based nanowells were fabricated in three steps: fabricating a thin film of self-organized TiO2 nanotubes by the convenient electrochemical anodization method (Step 1), striping the fabricated nanotube thin film to expose the ordered array of nanowells underneath (Step 2), and surface modifying the exposed nanowells to adjust their surface hydrophobicity and hydrophilicity (Step 3). These fabricated nanowells were then used to template the growth of an ordered array of nanocrystals by trapping a tiny droplet of precursor solution inside each nanowell. The diameter and surface hydrophobicity and hydrophilicity of the TiO2-based nanowells were easily tailored by controlling the anodization parameters used in Step 1 and surface modifying parameters used in Step 3, respectively, thus enabling these TiO2-based nanowells a promising template substrate for growing a wide range of nanocrystals with adjustable sizes from aqueous and organic solvent systems. The third chapter investigates the electrical property of individual TiO2 nanotubes enabled by the construction of field effect transistors based on individual TiO2 nanotubes. It is found that individual TiO2 nanotubes exhibit typical n-type electrical conduction characteristics, with electron mobility of 6.9×10-3 cm2/V-1s-1 at Vds= 1 V, and electron concentration of 2.8×1017 cm-3. Moreover, the on-off ratio of the TiO2 nanotube-based field effect transistors is as high as 103. Humidity sensing test shows the sensitive response of the TiO2 nanotubes-based field effect transistors to water vapor. The fourth chapter report on Ni-modified TiO2 nanotubes with improved photocatalytic properties. Using as-anodized TiO2 nanotubes as templates, Ni was electrodeposited from the bottoms of the inter-tubular voids with the tube inside kept empty and tube openings kept unclogged, forming a porous co-axial Ni/TiO2 nanocomposites. Further photo-degradation tests using methyl red revealed that the fabricated Ni/TiO2 nanocomposites possess higher photocatalytic efficiency than the counterparts of pristine TiO2 nanotubes. The observed improved photocatalytic efficiency is ascribed to the schottky barriers formed between Ni and TiO2. The fifth chapter studies the utilization of porous TiO2 (p- TiO2) thin film with optical fringes as chemical sensors for detecting organic vapors. Thin-film optical interference (Fabry-Perot) fringes in the reflectance spectrum are monitored using Reflectometric Interference Fourier Transform Spectroscopy (RIFTS). Three analytes are employed to probe the sensitivity of the porous TiO2-based sensors as a function of analyte vapor pressure: dodecane, isopropyl alcohol (IPA), and pentane. Measured lower limits of detection (3, 30 and 11,000 ppmv for dodecane, IPA, and pentane, respectively) track the saturation vapor pressures (Psat) of the analytes (0.134, 45, and 513 Torr at 25°C for dodecane, IPA, and pentane, respectively); the analyte with the lowest value of Psat shows the lowest LLOD. Recovery of the sensor after a saturation dose of analyte is also dependent on Psat: the sensor displays good recovery from pentane and IPA, and sluggish and incomplete recovery from dodecane. However, irradiation of the porous TiO2 sensor with UV light in the presence of air accelerates recovery, and this process is attributed to photo-catalyzed oxidation of the analyte at the TiO2 surface. The last chapter report on the fabrication of TiO2 nanotubes arrays (TiO2NA)/monolayer graphene schottky junction for UV light detection. The TiO2NA was synthesized by an anodization method and the monolayer graphene was prepared by a simple chemical vapor deposition process. It is observed that as-obtained TiO2NA/monolayer graphene schottky junction shows a prominent rectifying characteristic. Photoconductive analysis shows that this schottky junction photodetector is sensitive to UV light illumination with good stability and reproducibility. Further spectral response reveals a peak photo-sensitivity at ~365 nm. The relationship between the photocurrent and light intensity can be fitted by using a simple power law. Finally, the working mechanism of the TiO2NA/monolayer graphene schottky junction photodetector was elucidated.

    Research areas

  • Nanostructured materials, Titanium dioxide, Nanotubes