Effect of Architecture and Doping on the Photoelectrochemical Properties of Titania Nanotubes

Open Access
Author:
Shankar, Karthik
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 19, 2007
Committee Members:
  • Craig A Grimes, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Jerzy Ruzyllo, Committee Member
  • Srinivas A Tadigadapa, Committee Member
Keywords:
  • Photovoltaic
  • Nanotubes
  • Anodization
  • Electrochemical
  • formamide
  • protophilic solvents
  • solar cells
  • dye sensitized
  • photocatalyst
  • quantum efficiency
  • Incident Photon Conversion Efficiency
  • Flame Annealing
  • Anionic Doping
  • Electron Transport in TiO2
  • Open circuit voltage decay technique
  • high surface area nanostructures
  • ordered heterojunction
Abstract:
N-type nanocrystalline titania has attracted significant attention in the scientific community for its unique properties such as size quantization effects, large specific surface area and the possibility of large-scale use in high-efficiency semiconducting photo-electrochemical cells. While the nanocrystalline titania typically used in photoelectrochemical cells is prepared from a colloidal sol of nanoparticles, the titania nanotubes in the present study are robust immobilized structures grown anodically upright to form a compact self-organized non-particulate film. This dissertation investigates the effect of the formation parameters such as the anodization potential, the concentration of water and fluoride species in the anodization electrolyte, the nature of the cation, the anodization temperature and the nature of the solvent used on the architecture of TiO2 nanotube arrays fabricated by anodization of a starting Ti foil in a fluoride ion containing electrolyte. By varying the nature of the anodization electrolyte, an unprecedented degree of control over the architecture of TiO2 nanotube arrays has been achieved. Nanotube arrays ranging from 0.1 ìm to 100 ìm in length, wall-thickness from 6 nm to 34 nm and pore diameters ranging from 12 nm to 250 nm were fabricated. While the nanotube length in all-aqueous electrolytes is limited to less than 10 ìm, nanotube lengths as great as 360 ìm were obtained by employing organic solvents in conjunction with water. The fast formation of very long high-aspect ratio TiO2 nanotubes in electrolytes containing formamide and ethylene glycol are considered in terms of a growth model. It is suggested that faster high field ionic conduction through a thinner barrier layer is responsible for the higher growth rates observed in electrolytes containing formamide and ethylene glycol. Also examined are the photoelectrochemical properties that result as a consequence of the nanotubular architecture of TiO2. The magnitude of the anodic photocurrents obtained from the nanotube photoelectrodes under band-gap illumination are significantly higher than that reported for any other form of nanocrystalline titania. Open circuit voltage decay experiments revealed that the electron recombination lifetimes in titania nanotube array photo-electrodes are superior to those in nanoparticulate electrodes of similar thickness and as a consequence, the nanotubular electrodes are expected to have superior charge collection efficiencies. Under band-gap illumination, 30 ìm long TiO2 nanotube array based photoanodes performed photo-assisted water splitting at an efficiency as high as 16.25 %. Anionic doping of TiO2 nanotube arrays has been investigated as a technique to extend the photoresponse of titania into the visible region with emphasis on the carbon and nitrogen as dopants. An electrochemical doping technique was successful in introducing a nitrogen doping level as high as 12 % with nitrogen occupying the photo-electrochemically useful substitutional sites in the TiO2 lattice instead of the more typical interstitial sites. The nanotube arrays have an extremely large internal surface area compared to a planar surface. TiO2 nanotube arrays 20 ìm in length and with a pore diameter of 90 nm are found to have an internal surface area 3000 times that of a planar unstructured film. Dye sensitized solar cells using different architectures and dyes have been constructed and photoconversion efficiencies as high as 6.9 % have been demonstrated.