(641b) Synthesis Of TiO2 Nanowires For Dye-Sensitized Solar Cells

Nanostructured mesoporous TiO₂ films are used as photoanodes in novel solar-to-electric energy conversion devices such as dye-sensitized solar cells (DSSCs). In a DSSC, a mesoporous TiO₂ film, made up of 5-10 nm diameter nanoparticles is deposited on a transparent conducting oxide (TCO) and a monolayer of a dye is adsorbed onto the nanoparticle surfaces to form a photosensitized anode. The porous space between the particles is filled with an electrolyte containing I₃^-/I^- redox couple and the cell is completed by sandwiching this nanostructured TiO₂-dye-electrolyte interface between the TCO and a photocathode. The incident photons excite electrons from the HOMO to LUMO levels in the dye and these photoexcited electrons are injected into the TiO₂ nanoparticles. The charged dye is reduced through an electrochemical reaction with I^- in the electrolyte. The electrons hop from particle to particle, flow through the external load and reduce I₃^- at the cathode to complete the circuit. Electron transport through the nanoparticle film is via random walk with multiple trapping and detrapping events. This hopping mechanism increases transport time across the mesoporous film and puts stringent limits on the electron recombination kinetics at the TiO₂-electrolyte interface; essentially, only electrolytes or hole conducting media with extraordinarily slow electron recombination at TiO₂ surface can be used to achieve high electron collection efficiencies. Thus, high efficiency DSSCs have been limited to liquid electrolytes containing I₃^-/I^-, the only known hole conducting media with electron recombination much slower than electron transport. This inflexibility with the hole conducting medium has limited the DSSC open circuit voltages, which is determined by the difference between the I₃^-/I^- redox level and conduction band edge of the TiO₂. Slow electron transport also limits the film thicknesses to less than the electron diffusion length and prevents further increase of the film's optical density in the infrared region of the spectrum where dye absorption cross sections are smaller.

Recently, there has been increased interest in replacing the nanoparticles with one dimensional nanostructures such as nanowires, nanotubes or chains of nanoparticles. This interest is based on the conjecture that such one dimensional nanostructures would provide direct transport from the point of injection to the anode without particle-to-particle hopping. This may result in faster electron transport and/or slower recombination allowing thicker photoanodes to be assembled with hole conducting media other than the ubiquitous I₃^-/I^- electrolyte.

In this presentation, we describe a method for growing TiO₂ nanowire films on titanium foil for use in DSSCs and explain the growth mechanism in detail. The synthesis method takes advantage of the ability to grow sodium titanate Na₂Ti₂O₅?H₂O nanotubes through hydrothermal treatment of titanium metal in basic solutions. The Na₂Ti₂O₅?H₂O nanotubes are transformed to hydrodogen titanate nanotubes by ion exchange and to anatase nanowires through annealing. Specifically, the titanium foil was placed in a pressure vessel with 10 M NaOH and 35 wt% H₂O₂ and heated at 220°C. A 10 micron thick film of Na₂Ti₂O₅?H₂O nanotubes formed on the surface of the titanium foil after four hours. The sodium titanate nanotubes are transformed, without loss of microstructure, to hydrogen titanate (H₂Ti₂O₅?H₂O ) by exchanging the Na⁺ with H⁺ in 0.6 M HCl solution. Finally, the hydrogen titanate nanotubes are converted to anatase TiO₂ nanowires by heating them to 500°C for 1 hour.

The nanotubes and nanowires were examined at different stages of the synthesis using x-ray microdiffraction and scanning and transmission electron microscopies. The body-centered orthorhombic Na₂Ti₂O₅?H₂O crystal structure is made up of 2-dimensional sheets of TiO₆ octahedra combined via edge-sharing with Na⁺, H⁺ and H₂O in between the layers. The layers scroll to form the Na₂Ti₂O₅?H₂O nanotubes. After the ion-exchange, the TiO₆ octahedra layers remain unchanged but the Na⁺ ions are replaced with H⁺ ions. During the subsequent annealing step, the water between the TiO₂ octahedra layers leaves and the layers rearrange from edge-sharing octahedra to corner-sharing octahedra and come in contact with each other, forming anatase nanowires. HRTEM shows that the nanowires formed using this method are polycrystalline.

Solar cells were assembled by sandwiching the TiO₂ nanowires between the Ti foil substrate and a fluorine doped tin oxide (FTO) glass coated with a thin platinum layer. A ruthenium based organic dye cis-Di-(thiocyanato)bis(2,2'-bipryidyl)-4-4'-dicarboxylate) ruthenium-(II) (N179) was absorbed onto the surface of the TiO₂ nanowires. A liquid electrolyte containing the redox pair I^-/I₃^- was injected in the space between the nanowires and the platinized FTO cathode. The TiO₂ nanowire-based dye-sensitized solar cell exhibited short circuit currents of 4 mA/cm₂, open-circuit voltages of 0.55 V with 60% fill factor and 1.3% overall efficiency under AM1.5 illumination.