Photocatalysis with TiO2 Nanotubes: “Colorful” Reactivity and Designing Site-Specific Photocatalytic Centers into TiO2 Nanotubes (original) (raw)
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Anodic TiO2 nanotubes: Influence of top morphology on their photocatalytic performance
Electrochemistry Communications, 2012
The present work investigates the photocatalytic performance of TiO 2 nanotubes with different tube top morphologies. Such morphological variations are obtained when growing self-organized nanotube arrays under different (but widely used) anodization conditions. We compare the most typical appearances of tube tops that are: i) "grassy" tubes, ii) tubes with initiation layers, iii) doubled anodized and iv) "open" tubes. The results show that tubes with an open, defined mouth have a significantly enhanced photocatalytic performance in open circuit, dye degradation experiments as well as when used as a photoanodes for photoelectrochemical water splitting.
Some critical factors for photocatalysis on self-organized TiO2 nanotubes
Journal of Solid State Electrochemistry, 2012
In the present work, different intrinsic and extrinsic parameters are investigated that affect the photocatalytic activity of self-organized TiO 2 nanotube layers. Particularly, the influence of annealing temperature and annealing atmosphere, the influence of different gas purging in the electrolyte, and the effect of applied voltage on the photocatalytic degradation rates of acid orange (AO7) are discussed. We find that the effect of the reducing gas atmosphere dominates over the anatase/rutile ratio in activating the nanotube layers. Moreover, we show that the effect of different gas purging (Ar and O 2) of the electrolyte affects the reaction rate twofold: (1) by providing electron acceptor states and also by (2) a different change in the red-ox potential, i.e., the band bending in TiO 2. By an external anodic voltage, the reaction rates can be increased drastically due to increased band bending. Nevertheless, the magnitude of the effect is also affected by the presence or absence of O 2 in the electrolyte.
Small, 2012
Photocatalytic approaches, that is the reaction of lightproduced charge carriers at a semiconductor surface with their environment, currently attract an extremely wide scientifi c interest. This is to a large extent due to the high expectations: i) to convert sunlight directly into an energy carrier (H 2 ), ii) to stimulate chemical synthetic reactions, or iii) to degrade unwanted environmental pollutants. Since the early reports in 1972, TiO 2 has been the most investigated photocatalytic material by far; this originates from its outstanding electronic properties that allow for a wide range of applications. Not only the material, but also its structure and morphology, can have a considerable infl uence on the photocatalytic performance of TiO 2 . In recent years, particularly 1D (or pseudo 1D) structures such as nanowires and nanotubes have received great attention. The present Review focuses on TiO 2 nanotube arrays (and similar structures) that grow by self-organizing electrochemistry (highly aligned) from a Ti metal substrate. Herein, the growth, properties, and applications of these tubes are discussed, as well as ways and means to modify critical tube properties. Common strategies are addressed to improve the performance of photocatalysts such as doping or band-gap engineering, co-catalyst decoration, junction formation, or applying external bias. Finally, some unique applications of the ordered tube structures in various photocatalytic approaches are outlined.
Titanium Dioxide - Material for a Sustainable Environment
This book chapter reports some spectacular and interesting 1D nanostructures of TiO 2 , which are grown by the anodic oxidation. Under suitable conditions, conventional onestep anodic oxidation is available to grow TiO 2 nanotube arrays (TNAs) and TiO 2 nanowires/nanotubes; meanwhile, two-step anodic oxidation allows fabricating some novel TNAs with spectacular morphologies such as highly ordered TNAs, bamboo-type TNAs, and lotus root-shaped TNAs. The formation mechanisms of these nanostructures during the anodic oxidation processes are elusive via studying effects of several key parameters such as oxidizing voltage, processing time, and electrolytes. In addition, the photocatalytic activity of the TNA-based nanomaterials is characterized by the degradation of pharmaceutical model, methylene blue, or the photoelectrochemical effect.
Applied Surface Science, 2014
We report the significance of crystallinity on photoelectrochemical and the photocatalytic degradation of methyl orange of titanium dioxide (TiO 2) nanotube arrays. The TiO 2 nanotube arrays are fabricated by electrochemical anodization of titanium substrates in fluoride based aqueous electrolyte for various anodization time. The degree of crystallinity and phase purity (anatase) is confirmed from X-ray diffraction and Raman spectra. High resolution scanning electron microscope is used to analyze the surface morphology of forming nanotubes. The UV-visible absorption spectrum shows the enhanced absorption in the visible region which is further confirmed using photoluminescence spectra. The photoelectrochemical properties of the prepared samples are studied from linear sweep photovoltammetry measurements and a maximum photocurrent density of 1.32 mA/cm 2 is observed. The enhanced photoelectrochemical activity is attributed to the higher crystallinity which increases the charge carrier separation and extends its light absorption from ultraviolet to visible region owing to lower band gap of 2.751(7) eV.
Electrochemically assisted photocatalysis on self-organized TiO2 nanotubes
Electrochemistry Communications, 2007
In this work we investigate influence of an externally applied bias on the photocatalytic performance of self-organized TiO 2 nanotube layers. These layers were grown by anodization of titanium in fluoride containing electrolytes and have different geometric dimensions. Since the layers are grown directly on the Ti substrate, a very good electrical backside contact is directly provided. Therefore, we use the nanotube layers/Ti structures as photo-anodes for the UV light induced photocatalytic decomposition of acid orange 7. For comparison, we use TiO 2 nanopowder (Degussa P25) compacted also on a Ti sheet. The present results demonstrate that the photocatalytic activity of self-organized TiO 2 nanotube layers can significantly be increased by electrochemical bias.
Fabrication of carbon-modified TiO2 nanotube arrays and their photocatalytic activity
Materials Letters, 2008
TiO 2 nanotube (TN) arrays were fabricated by an anodic oxidation process. Through a heat treatment of the as-fabricated TN arrays under a continuous Ar and acetylene flux, carbon-modified TN (C-TN) arrays were obtained. The as-fabricated catalysts were characterized by FE-SEM, HRTEM, XPS, Raman and UV-Vis spectra. Moreover, photocatalytic activity of the C-TN arrays was evaluated through the photodegradation of aqueous methyl blue. The experiments demonstrated that the C-TN arrays display an excellent photocatalytic activity. Under sunlight irradiation, the C-TN arrays are able to almost completely decompose the methylene blue pollutant of 1 × 10 − 5 M within 300 min.
A systematic study of titanium dioxide (TiO 2 ) nanotube arrays grown by electrochemical anodisation in an ethylene glycol electrolyte containing 0·5 wt-% ammonium fluoride has been carried out, with a range of anodisation voltage of 15-60 V for 1 hour. Among all of the applied anodisation voltages, 60 V resulted in the highest aspect ratio TiO 2 nanotube arrays with the tube length of approximately 2 μm and pore size of 105 nm. The diameter and length of nanotubes were found to be increased with anodisation voltage because of the high electric field dissolution at the barrier layer of nanotubes. Besides, the anatase phase of TiO 2 could be detected from the X-ray diffraction patterns after subjecting the annealing process at 400°C in argon atmosphere for 4 hours. Based on the photocatalytic studies, it was observed that TiO 2 nanotube arrays with the highest aspect ratio (length/pore's size) exhibited preferably high photocatalytic activity among the samples owing to the larger active surface area to generate more photo-induced electron-hole pairs. This condition will enhance the photocatalytic degradation efficiency of methyl orange.