Metal Oxides

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When illuminated by UV light, a titanium dioxide electrode has been shown to catalyze water oxidation.1  Dye-sensitized solar cell (DSSC) research has led to progress in shifting the absorption of TiO2 into the visible range using dyes attached to the surface of TiO2 nanoparticles.2  However, successful water oxidation using similar methods presents additional requirements which are not relevant to solar cells.  In addition to ultrafast photo-oxidation and electron injection into the conduction band of TiO2, the dyes must be stable in water, resistant to degradative oxidation, and thermodynamically poised to catalyze the oxidation of water. 

Terahertz spectroscopy is an ideal tool for studying the carrier dynamics of metal oxide semiconductors because THz light is strongly absorbed by mobile electrons, such as those in the conduction band, while bound electrons such as those in the valence band and the sensitizer are transparent to THz. Thus, monitoring the absorption of the THz radiation allows for probing of mobile electrons in metal oxides.

Dye-sensitized Metal Oxides

Efficient electron transfer from excited dye molecules to the metal oxide conduction band can sometimes be the limiting factor in device efficiency.  Understanding the dynamics and mechanism of electron transport is crucial for device development and optimization.  Time-resolved THz spectroscopy (TRTS) has been used to evaluate the relative electron injection efficiencies of different dye molecules3, 4 and metal oxides for use in systems for photochemical water oxidation.5

TiO2 Nanotubes

TiO2 nanotubes have unique electron transport properties that make them a promising material for next generation solar cells and photoelectochemical cells.6 Both of these applications require that electrons be transported through a network of TiO2 nanoparticles.   Several authors have suggested that nanotubes might be superior to standard nanoparticle networks because electrons could travel down a continuous nanotube and not have to hop between as many particles.7, 8 Evaluation of electron mobility through nanotubes in solar cells has, surpisingly, found nanotubes to be no better than nanoparticle films.9

TRTS measurements of the photoconductivity in nanotube films confirm the comparably low electron mobility of nanoparticle and nanotube films but reveal different mechanisms for the same observed effect.10 In nanoparticles films, conductivity is inhibited by significant backscattering and/or disorder-induced localization. In nanotube films, the photoconductivity is limited by the formation of exiton-like trap states.

References

  1. Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238 (5358), 37-38.
  2. O’Regan, B.; Gratzel, M., A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidal Tio2 Films. Nature 1991, 353 (6346), 737-740
  3. McNamara, W. R.; Snoeberger, R. C.; Li, G.; Schleicher, J. M.; Cady, C. W.; Poyatos, M.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W.; Batista, V. S., Acetylacetonate Anchors for Robust Functionalization of TiO2 Nanoparticles with Mn(II)-Terpyridine Complexes. Journal of the American Chemical Society 2008, 130 (43), 14329-14338
  4. McNamara, W. R.; Snoeberger, R. C.; Li, G. H.; Richter, C.; Allen, L. J.; Milot, R. L.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W.; Batista, V. S., Hydroxamate anchors for water-stable attachment to TiO2 nanoparticles. Energy & Environmental Science 2009, 2 (11), 1173-1175.
  5. Li, G. H.; Richter, C. P.; Milot, R. L.; Cai, L.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W.; Batista, V. S., Synergistic effect between anatase and rutile TiO2 nanoparticles in dye-sensitized solar cells. Dalton Transactions 2009,  (45), 10078-10085.
  6. Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A., A review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applications. Solar Energy Materials and Solar Cells 2006, 90 (14), 2011-2075.
  7. Allam, N. K.; Grimes, C. A., Formation of vertically oriented TiO2 nanotube arrays using a fluoride free HCl aqueous electrolyte. Journal of Physical Chemistry C 2007, 111 (35), 13028-13032
  8. Kuang, D.; Brillet, J.; Chen, P.; Takata, M.; Uchida, S.; Miura, H.; Sumioka, K.; Zakeeruddin, S. M.; Gratzel, M., Application of highly ordered TiO2 nanotube arrays in flexible dye-sensitized solar cells. ACS Nano 2008, 2 (6), 1113-1116.
  9. Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J., Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays. Nano Letters 2007, 7 (1), 69-74.
  10. Richter, C.; Schmuttenmaer, C. A., Exciton-like trap states limit electron mobility in TiO2 nanotubes. Nat. Nanotechnol. 2010, 5 (11), 769-772.