Krengvirat, Warapong
(2013)
Development Of Carbon And Potassium-Incorporated Titanium Dioxide Nanotube Arrays For Solar Energy Harvesting Applications.
PhD thesis, Universiti Sains Malaysia.
Abstract
TiO2 nanotube arrays have attracted great interest as the most promising candidate for solar energy harvesting applications. However, poor visible-light absorption and high recombination of charge carriers still remain as challenging issues for their practical applications. Hence, the objective of this work was to develop carbon and potassium-incorporated TiO2 nanotube arrays for solar energy harvesting applications, including photodecolorization, photoelectrochemical cell (PEC) and dye-sensitized solar cells (DSSC). Visible-light responsive TiO2 nanotube arrays were rapidly grown with a rate of ~289 nm min-1 by anodic oxidation in ethylene glycol (EG) containing 0.5 wt% ammonium fluoride (NH4F) and 1 wt% H2O. The presence of adsorbed-carbonate species and interstitial carbon in TiO2 nanotubes originated from the pyrogenation of EG resulted in the generation of localized state, and thus enabled the visible-light absorption. Anatase TiO2 nanotube arrays with high surface area (110.9 m2 g-1) were obtained by facile immersion of as-anodized nanotube arrays in hot water at ~90C. Such hot water-treated nanotube arrays exhibited efficient visible-light photodegradation of methylene blue with the decomposition rate of ~11 % h-1. This value is relatively higher than heat-treated arrays (~9 % h-1) and P25 powder (~2 % h-1). However, heat treatment at 400C for 4 h was found as essential approach to obtain better crystallinity for high PEC and DSSC properties. Heat-treated TiO2 nanotube arrays with average nanotube lengths
xxxiv
of 18 m, thick walls (13 nm) and large pore sizes (115 nm), with high aspect ratio (~123.6) exhibited remarkable ability to generate H2 at a rate of ~508.3 L min-1 cm-2 and photoconversion efficiency () of ~2.3%. The growth of TiO2 nanotube arrays and their electrochemical properties were further improved by simple addition of potassium hydroxide (KOH) into fluorinated-EG. The incorporation of 1 wt% of 1.0 M KOH facilitated an equilibrium growth of nanotube arrays with a rate of ~353 nm min-1. The adsorbed-potassium species further extended the light visible-light absorption to 780 nm. Furthermore, the electron donation nature of adsorbed-potassium promoted larger number of charge carriers (9.7 × 1021 cm-3). Carbon and potassium-incorporated TiO2 nanotube arrays with aspect ratio of 140.5 exhibited superior photoelectrochemical H2 generation with an evolution rate of ~658.3 L min-1 cm-2 and of ~2.5%, which is 30 % higher than that of without potassium. Carbon and potassium-incorporated TiO2 nanotube arrays were assembled to back-side illumination DSSCs using N719 dye and iodide/triodide redox electrolyte. Well-aligned nanotubes with average nanotube lengths of 18 m, thick walls (13 nm), and large pore sizes (130 nm) allowed a greater penetration of excited h and ease charge carrier diffusion. Furthermore, high geometric surface area up to 755 could harvest higher dye adsorption. A maximum of 2.78% was achieved from a 17.8 m length TiO2 nanotube arrays, with open circuit potential of 0.67V, current density of 8.95 mA cm-2, and filled factor of 46.39%.
Actions (login required)
 |
View Item |
![[img]](http://eprints.usm.my/style/images/fileicons/application_pdf.png)
