Titanium Dioxide Nanostructures for Photovoltaics and Photocatalysis

The dye-sensitized solar cell (DSSC) is a promising low cost photovoltaic device for the generation of carbon-free energy. DSSC consists of two conducting glass electrodes in a sandwich configuration, with a redox electrolyte filling the free space in between. During illumination of the cell, the dye molecules inject electrons into the semiconductor film and the injected electrons diffuse through the semiconductor nanoparticle network through hopping from particle to particle until being collected at the electron collecting photoanode. Meanwhile, the charged dye is regenerated by an electrochemical reaction with a redox couple in the electrolyte. The oxidized ionic species diffuse towards the counter photocathode and are reduced by electrons that have traveled from the photocathode through the load to complete the circuit. To date, DSSCs with light-to-electric conversion efficiencies of ∼7 to 11% have been demonstrated with ∼10 mum thick electrodes made of 10-30 nm diameter TiO2 nanoparticles sensitized with ruthenium-based dyes, but further device improvement is limited due to the competition between electron transport and recombination. Wide bandgap semiconductor nanowire electrodes have the potential to increase the DSSC performance by increasing the electron transport rate while keeping the electron recombination rate unaltered. Towards this end, the synthesis of single-crystalline TiO2 nanowires on substrates was studied.;Mesoporous anatase TiO2 microspheres composed of abutted TiO 2 nanoparticles were synthesized through a two-step hydrothermal method. Photoanodes assembled from alternating layers of these mesoporous TiO 2 microspheres and TiO2 nanoparticles increase the overall power conversion efficiencies of DSSCs by as much as 26%. This increase is due to enhanced light scattering by porous TiO2 microspheres and is achieved without sacrificing the specific surface area.;Single-crystalline TiO2 nanowire arrays were grown on flexible titanium foil using a three-step solution synthesis. The synthesis method relies on the ability to grow single crystal sodium titanate (Na2Ti 2O5˙H2O) nanowires on titanium foil through a novel alkali hydrothermal growth process. Following growth, the Na 2Ti2O5˙H2O nanowires are converted to protonated bititanate (H2Ti2O5˙H 2O) nanowires through an ion-exchange reaction without changing their morphology or crystal structure. Finally, the protonated bititanate nanowires are converted to single crystalline anatase TiO2 nanowires through a topotactic transformation by calcination. These three sequential steps yield a carpet of 2-- 50 mum long single crystalline nanowires oriented in the [100] direction and primarily normal to the titanium foil. DSSC assembled from 12 mum thick TiO2 nanowire film gives a light-to-electric conversion efficiency of ∼ 1.4%. Further improvements in the cell efficiency should be possible with longer nanowires.;Single-crystalline rutile TiO2 nanorods were grown on transparent conductive fluorine-doped tin oxide (FTO) substrates using a facile, hydrothermal method. The diameter, length, and density of the nanorods could be varied by changing the growth parameters, such as growth time, growth temperature, initial reactant concentration, acidity, and additives. The epitaxial relation between the FTO substrate and rutile TiO2 with a small lattice mismatch plays a key role in driving the nucleation and growth of the rutile TiO2 nanorods on FTO. With TiCl4-treatment, a light-to-electricity conversion efficiency of 3% could be achieved by using 4 mum-long TiO 2 nanorod films as the photoanode in a DSSC.;Single crystal anatase TiO2 nanorods/nanoflakes were grown on FTO substrates though a TiCl4 evaporation-condensation-hydrolyzation process, following by a subsequent thermal treatment. DSSCs assembled from 1 im long TiO2 nanorod and nanoflake films give a light to electricity conversion efficiency of ∼ 2.1%.