The promise of semiconductor photocatalysis as a sustainable environmental remediation method has yet to be fully realized, due in large part to the mismatch between the absorption range of effective photocatalysts and the wavelengths present in sunlight and indoor light sources. When these photocatalysts absorb light, they generate electron-hole pairs that produce reactive oxygen species capable of degrading a wide variety of pollutants in air and water. Unfortunately, the photocatalysts best suited to this purpose are wide-bandgap semiconductors whose high bandgap energies (Eg) provide good photocatalytic activity but dismal visible-light absorption. For instance, TiO2 (Eg = 3.2 eV) can only absorb UV photons, and even effective visible-light photocatalysts (e.g., WO3, Eg ~ 2.7-2.8 eV) absorb only up to the blue range of the visible spectrum, wasting the vast majority of the energy present in sunlight and indoor light.
Upconversion (UC) is able to correct this mismatch by converting pairs of sub-bandgap photons (i.e., E < Eg) into higher-energy photons capable of photocatalyst sensitization, extending the effective absorption range of the photocatalyst without compromising the redox potential of its charge carriers. UC occurs through a number of different mechanisms, but only triplet-triplet annihilation (TTA)-UC can efficiently harness the low-intensity noncoherent light emitted by common light sources. The TTA-UC mechanism, which consists of a series of short-range energy transfers between the oxygen-sensitive triplet excited states of sensitizer chromophores that harvest low-energy photons and acceptor chromophores that emit anti-Stokes fluorescence, was originally confined to deoxygenated solution-based systems. Recent advances, however, have made it possible to translate TTA-UC to practical solid host materials. This dissertation research advances TTA-UC toward real-world applications such as environmental remediation by developing effective new UC film architectures and evaluating their device integration potential using proof-of-concept studies.
The ability of TTA-UC to harness sub-bandgap photons for photocatalytic pollutant degradation is demonstrated using photocatalyst devices containing glass-encased polymer films that upconvert sub-bandgap green photons (2.3 eV) to blue photons (2.9 eV) that then sensitize an underlying photocatalyst film (Eg = 2.8 eV). Under low-intensity green LED excitation, these devices successfully mineralize acetaldehyde, a common indoor air pollutant, to carbon dioxide. Incorporating particles with compatible surface plasmon resonance into the TTA-UC films enhances their light absorption, thereby improving the devices' photocatalytic performance. Despite the promise of this initial result, the films' narrow light absorption range (~20 nm) drastically limits their potential in real-world applications.
TTA-UC films with broadened light absorption are developed by incorporating both red-to-blue and green-to-blue sensitizers. A detailed characterization of the UC performance of single- and dual-sensitizer films using red and green laser excitation indicates that only dual-sensitizer films containing impractically low sensitizer concentrations can outperform both of their single-sensitizer analogues. Overlaying the single-sensitizer films within a dual-layer film architecture proves more successful, producing systems containing sensitizer concentrations that maximize light harvesting while minimizing the fluorescence quenching found in the dual-sensitizer films.
Under white LED excitation, these dual-layer red/green-to-blue UC film systems demonstrate superior TTA-UC performance, and incorporating these film systems into photocatalyst devices increases acetaldehyde degradation accordingly. Since spectral overlap between the individual UC films' absorption and emission profiles introduces strong inner-filter effects, the performance of the dual-layer film systems and devices is highly dependent on the order of their films. Probing the effect of employing different light sources, excitation scenarios, and chromophore systems provides a preliminary framework for the development of more complex multilayer TTA-UC film systems.
To maximize TTA-UC films' potential applications, flexible and micropatternable films are also developed. Solution-processing techniques are employed to encase spin- coated films of a new host polymer between layers of flexible oxygen-barrier polymers, producing efficient and photostable UC films without any need for protective layers of glass. This scalable fabrication procedure could prove compatible with thin-film photovoltaics. Finally, incorporating soft lithography methods into this procedure produces UC films patterned with microscale designs, introducing unconventional new applications in fields such as anti-counterfeiting.
|Commitee:||Elimelech, Menachem, Hu, Shu|
|Department:||Chemical and Environmental Engineering|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 81/3(E), Dissertation Abstracts International|
|Subjects:||Chemistry, Chemical engineering, Energy|
|Keywords:||Photocatalysis, Triplet-triplet annihilation, Upconversion|
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