Titanium dioxide nanostructures for photovoltaics and photocatalysis.
2011-08
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Titanium dioxide nanostructures for photovoltaics and photocatalysis.
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2011-08
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Abstract
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 mm 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 TiO2 nanoparticles
were synthesized through a two-step hydrothermal method. Photoanodes assembled from
alternating layers of these mesoporous TiO2 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 (Na2Ti2O5·H2O) nanowires on titanium foil through a novel alkali hydrothermal growth process. Following growth, the Na2Ti2O5·H2O nanowires are
converted to protonated bititanate (H2Ti2O5·H2O) 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 μm long single crystalline nanowires oriented in the [100] direction and primarily
normal to the titanium foil. DSSC assembled from 12 μm 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 μm-long TiO2 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 μm long TiO2 nanorod and
nanoflake films give a light to electricity conversion efficiency of ~ 2.1%.
Description
University of Minnesota Ph.D. dissertation. August 2011. Major: Chemical Engineering. Advisor: Eray S. Aydil. 1 computer file (PDF); xv, 124 pages.
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Liu, Bin. (2011). Titanium dioxide nanostructures for photovoltaics and photocatalysis.. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/115917.
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