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Summary

The goal of this project is the production of high performance photoanodes for photoelectrochemical (PEC) solar cell devices based on titania nanotube surfaces modified with low band gap metal oxide semiconductors for light absorption in the visible range and with suitable catalysts to accelerate the anodic reaction.

In this second year of the project we have improved on our method of preparation of titania nanotubes, and we have successfully demonstrated PEC response in the visible using electrodeposited Cu2O or Fe2O3. In addition, we have proposed an explanation for the observed evolution with time of the PEC properties of Cu2O.

1. Production and Characterization of TiO2 nanotubes

TiO₂ nanotubes are formed by the anodization of Ti foil in an aqueous solution containing 0.15-0.25M HF, 1M H₂SO₄, 0.02M CH₃COOH. The nanotubes are formed as an organized array of cylinders with diameter of 100nm and length of 500 nm. As-synthesized TiO2 nanotubes are amorphous, but they can be transformed to the high mobility anatase phase by thermal annealing at 500°C. We have recently observed that an intermediate HF concentration is necessary to avoid oxide debris observed on the surface at low HF, or extensive cracking observed at high HF.

2. Photoelectrochemical Stability of Cu2O films

Preliminary experiments of electrodeposition of Cu2O into TiO2 nanotubes evidenced the formation of Cu2O clusters on the TiO2 surface, whose morphology evolved during polarization in solution and illumination; this observation prompted an investigation of this phenomenon. p-type Cu₂O was electrodeposited from 0.3M CuSO₄ + 3M lactic acid, adjusted to pH of 12 by 5M NaOH. By running repeated cyclic voltammograms, we observed a transformation from densely packed triangular grains to a network of leaves and flowers. The driving force for this transformation was attributed to surface energy differences among the various crystal facets. We have proposed a mechanism whereby Cu₂O rearranges to expose the more stable {111} facets to the electrolyte, resulting in the leaf and flower structures. In characterizing the photoelectrochemical performance, we have observed an initial increase in the photocurrent which we attribute to the increase in surface area. As the {111} facets become dominant, the differences in Fermi levels among the {111}, {110}, and {100} facets that aid in the separation of electron hole pairs are no longer present and photocurrent is observed to decay back to the initial value.

3. Electrodeposition of Cu2O in and on TiO2 nanotubes

In order to optimize the deposition process and favor growth of Cu2O from the bottom of the nanotubes, we have used an electrochemical self-doping process whereby polarization in ammonium sulfate solution cause reduction of Ti4+ to Ti3+ and local self-doping, increasing conductivity at the pore bottom. Cu₂O deposits as hemispherical particles with the self-doping and as sparsely scattered faceted nanocrystals without this pretreatment (Figure 4). Measurements in monochromated light showed no photocurrent response with the pre-treated samples, while photocurrents were observed in the sample without the pretreatment. Moreover, the pre-treated samples on TiO2 showed the same morphological instability as on gold, while the Cu₂O nanocrystals on the non-pretreated samples appear to be stable. This is likely due to band-structure changes during the self-doping process which makes the contact between Cu₂O and TiO₂ behave ohmically rather than as a rectifying junction. Highest efficiencies are observed in the UV and near 700nm. The observed values are similar to or higher than those reported in the literature. The results of this work were presented as a poster at the 2010 Gordon Conference on Electrodeposition.

4. Future Work

Future work on the Cu₂O-TiO₂ nanotube system will involve optimizing the deposition conditions so that the coverage of TiO2 may become more uniform and the nanotubes can be completely filled. UV illumination prior to deposition has been shown to improve the wetting of the TiO₂ nanotube walls and achieve better filling behavior.  Pulse plating may also be used to obtain a more uniform deposit. We have also recently completed a series of preliminary experiments where Fe2O3 was successfully plated on and into the TiO2 nanotubes. The PEC performance of Fe₂O₃ has been evaluated and response in the visible has been observed. A better understanding of the potential of metal oxide/TiO2 combinations in providing high PEC efficiencies will require the comparison of various narrow bandgap semiconductors: p-Cu₂O, n-Cu₂O, and Fe₂O₃, which we are in the process of completing.

5. Future Funding

A grant request was sent to the National Science Foundation, Division of Chemistry, in September 2009. The proposal was rejected in March 2010 despite very good reviews. Proposal submissions are currently being prepared to NSF and DoE for the fall 2010.