Multipodal and Multilayer TiO 2 Nanotube Arrays: Hierarchical Structures for Energy Harvesting and Sensing
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Multipodal and Multilayer TiO2 Nanotube Arrays: Hierarchical Structures for Energy Harvesting and Sensing Arash Mohammadpour1, Samira Farsinezhad1, Ling-Hsuan Hsieh1and Karthik Shankar1,2 1
Department of Electrical and Computer Engineering, University of Alberta, 9107 – 116 St, Edmonton, AB T6G 2V4, Canada.
2 National Institute for Nanotechnology, National Research Council, 11421 Saskatchewan Drive, Edmonton, AB, T6G 2M9, Canada
Abstract Our ability to fabricate multipodal and multilayer TiO2 nanotube arrays enables us to increase performance and functionality in light harvesting devices such as excitonic solar cells and photocatalysts. Using a combination of simulations and experiments, we show that multilayer nanotube arrays enable photon management in the active toward enhancing the absorption and utilization of incident light. We show that the simultaneous utilization of TiO2 nanotubes with large (~450 nm) and small (~80 nm) diameters in stacked multilayer films increased light absorption and photocurrent in solar cells. Such enhanced light absorption is particularly desirable in the near-infrared region of the solar spectrum in which most excitonic solar cells suffer from poor quantum efficiencies and for blue photons at the TiO2 band-edge where significant room exists for improvement of photocatalytic quantum yields. Under AM 1.5 one sun illumination, multilayer nanotube arrays afforded us an approximately 20% improvement in photocurrent over single layer nanotube array films of the same thickness for N-719 sensitized liquid junction solar cells. Also, the possibility of multipodal TiO2 nanotube growth with different electrolyte recipes is presented.
INTRODUCTION Anodically grown TiO2 nanotube arrays are the focus of intense research due to their excellent performance in a variety of applications including but not limited to excitonic solar cells1-3, water photoelectrolysis4, 5, photocatalysis6, 7, hydrogen sensors8, 9, glucose sensors10, 11, stem-cell differentiation12, 13, biomarker assays14 and drug delivery15, 16. However, several applications in energy harvesting and sensing require nanostructures which possess more morphological complexity. Multipodal nanotubes are branched nanotubes possessing two or more pods, formed by the process of nanotube combination17. Multipodal nanotube structures also offer the promise of sorting and handling quantum dots, in building three-terminal nanodevices, and in multiplexed sensing applications by different surface functionalization of the individual legs. Multilayer nanotubes are a closely related architecture where the potential-step induced electrochemical branching of nanotubes is used to create connected bilayer films with nanotubes of two different pore diameters. These structures are so beneficial for applications involving light harvesting such as excitonic solar cells and photocatalytic processes. Small diameter nanotubes provide a larger surface area for the attachment of sensitizing dyes/ quantum dots while on the other hand, large
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