Browsing by Subject "Photocatalysis"

Now showing 1 - 4 of 4
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    Decontamination of Particulate Foods Using Intense Pulsed Light and Other Non-Thermal Technologies
    (2020-01) Chen, Dongjie
    Low-moisture particulate foods comprise a wide range of food products such as milk powder, protein powder, egg powder, whey powder, spice, flour, grain, and seeds. Various pathogens or toxins such as C. sakazakii, Salmonella spp., Bacillus cereus spores, and deoxynivalenol (DON) were infected in particulate food matrices. These contaminants are physiologically dormant and metabolically quiescent in low moisture particulate foods and are therefore resistant to conventional thermal process. Conventional heating processes used to eliminate foodborne pathogens may cause some degrees of undesirable flavor and quality changes on particulate food products that are unacceptable for uses by consumers and food industries. This dissertation research focuses on developing non-thermal microbiocidal technologies for dry particulate foods. Lack of knowledge related to continuous nonthermal techniques on low-moisture particulate foods prevents the technology from applications in food industry. In this study, an intense pulsed light (IPL) treatment system was developed, processing parameters such as relative humidity, temperature, water activity, pulsed duration, voltage, pulsed frequency, and residence time, etc., were evaluated on different low-moisture particulate foods. After several generation-improvement of the IPL system and processes, the results showed a maximum of 4 log10 CFU/g reduction of microbe could be obtained after 60s IPL treatment on the conditions of 1 Hz and 3000 voltage. Furthermore, additional one log10 CFU/g microbial reduction could be achieved when combining IPL with TiO2 based catalysts. Food products such wheat, wheat kernels, and NFDM could be potentially subjected to IPL with minimized quality loss. For milk powder process, the IPL step can be fitted after spray drying. On the other hand, cold atmospheric plasma was able to inactivate be used to inactivate ~3 log10CFU/g of C. sakazakii in non-fat dry milk after only 120 s. However, the throughput of the system was limited and thereby, difficult to scale up. With respect to plasma activated water, the system was effective in degrading DON (34.6 %) in germinating barley samples while maintaining sample quality after 5 min. For microwave or catalytic microwave treatments, the results indicated microwave treatment below 60 °C was feasible to inactivate pathogens in wheat kernels up to 5 log10CFU/g at the water activity level of ~0.8. Therefore, the process can be fitted in a step after tempering. In summary, several nonthermal technologies specifically used for particulate food pasteurization were developed in the current research, optimized conditions for disinfection and particulate sample preservation were comprehensively investigated. The findings from this research has filled the key knowledge gaps of preventing the technology from commercialization.
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    Driven by Light: An Ultrafast Look into the Bright Future of Photosensitizers
    (2024-04) Schaffner, Jacob
    This thesis investigates various strong light-absorbing molecules that have potential applications in furthering our progress into replacing fossil fuels with clean energy resources and remediating harmful chemicals in the environment. The research presented in this thesis employs a range of spectroscopic techniques, complemented with computational predictions, to characterize the light absorption and excited state dynamics of newly developed chromophores that have shown promise in these various applications. Chapter 3 investigates a BODIPY-fullerene dyad designed to be used in organic photovoltaics as a triplet sensitizer to form longer-lived excitons. This triplet sensitization occurs via a ping-pong energy transfer mechanism between the BODIPY and fullerene, resulting in a long-lived BODIPY triplet (>1 µs). Chapters 4 and 5 investigate the MB-DIPY chromophore that could potentially displace fullerene as a strong and more versatile electron acceptor in organic photovoltaics. In Chapter 4, the redox potentials and photophysics of four MB-DIPY analogs are explored. The MB-DIPYs had comparable reduction potentials to fullerene and demonstrated efficient intersystem crossing to form long-lived triplet states (>10 µs). In Chapter 5, the MB-DIPY is functionalized with ferrocene, a strong electron donor, and demonstrated sub-ps charge-transfer from the ferrocene to the MB-DIPY followedby charge recombination in 12 ps. Chapters 6 and 7 investigate the Rh-Ga and Co-Ga heterobimetallic photocatalysts that can access challenging bonds via a photoredox mechanism. The excited state nature of these photocatalysts is first explored in Chapter 6. The results were consistent with the naked anionic catalyst being the active participant in the photocatalytic cycle. Chapter 7 investigates the reactivity of the photocatalysts with a chloroadamantane substrate. The results suggested that the substrate binds to the anionic rhodium photocatalyst and that the photocatalytic reactivity is not diffusion-limited. In contrast, the anionic cobalt catalyst was converted into the chlorinated precatalyst upon the addition of the substrate, demonstrating that the chemical reactivity of the rhodium and cobalt photocatalysts differ with this substrate.
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    Titanium dioxide nanostructures for photovoltaics and photocatalysis.
    (2011-08) Liu, Bin
    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%.
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    UV-Vis and MCD Spectroscopy and TDDFT investigations into N-Confused porphyrins and properties of mono-functionalized tetraferrocenyl porphyrins in solution and on a gold surface
    (2014-08) Erickson, Nathan Robert
    N-confused porphyrin (NCP) and its externally methylated variant (MeNCP) were investigated using UV-visible and magnetic circular dichrosim (MCD) spectroscopies. In addition to evaluating the spectroscopy of the neutral compounds, the acid/base chemistry of these macrocycles was examined by the same methods. NCP exhibits two tautomeric states depending on the polarity of the solvent, and their protonation/deprotonation chemistries also differ depending on solvent polarity. DFT and TDDFT calculations were employed to evaluate the observed spectroscopic changes. Using both experimental and calculated results, we were able to determine the sites of protonation/deprotonation for both tautomeric forms of NCP. Inspection of the MCD Faraday B terms for all of the macrocycles presented in this report showed that the ΔHOMO > ΔLUMO condition is maintained in all cases and these observations were in good agreement with the DFT calculations. Various methylated N-confused tetraphenylporphyrins (MeNCTPP) were investigated using UV-Vis and magnetic circular dicrhoism (MCD) spectroscopy in polar and apolar solvents as well as the protonation and deprotonation of the compounds. The MeNCTPPs were also investigated using time-dependent density functional theory (TDDFT) methods in the gas phase. Experimentally, the MCD spectra showed that all the molecules investigated had transitions with the ΔHOMO > ΔLUMO. TDDFT results confirmed these energy gaps. TDDFT calculations also showed that nearly all low energy transitions were from the HOMO to the LUMO and LUMO+1, except in the case of 2c, 5a, and 5c. Otherwise, results show that the addition of a methyl group to the N-confused tautomer of tetraphenylporphyrin effectively regulates the proton tautomerization of the N-confused tautomer. MeNCTPPs are unaffected by the polarizability of the solvent. Two unsymmetric meso-tetraferrocenyl-containing porphyrins of general formula H2Fc3Fc(COR)P [Fc = ferrocenyl, R = -CH3 or -(CH2)5Br, P = porphyrin(2-)] have been prepared and characterized by variety of spectroscopic methods, while their redox properties were investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) approaches. Spectroscopic signature of the mixed-valence [H2Fc3Fc(COR)P ]n+ (n = 1, 3) were investigated using spectroelectrochemical as well as chemical oxidation methods and corroborated with DFT and TDDFT calculations. Inter-valence charge-transfer (IVCT) transitions in [H2Fc3Fc(COR)P ]+ were analyzed using band deconvolution analysis in the borders of Hush model. The resulting data from the mixed-valence [H2Fc3Fc(COR)P ]+ derivatives matched very closely to the previously reported MTFcP and metal-free poly(ferrocenyl)porphyrins complexes and were assigned as Robin and Day Class II mixed-valence compounds. Following previous results indicating the ability of gold supported TFcP monolayers in photo-catalytic reduction of dioxygen, self-assembled monolayers (SAMs) of a thioacetyl derivative (H2Fc3Fc(CO(CH2)5SCOCH3)P) were also prepared and characterized using UV-vis spectroscopy and CV methods. Photoelectrochemical properties of SAMs in different electrolyte systems were investigated by electrochemical techniques and photocurrent generation experiments, showing that the choice of electrolyte is critical for efficiency of redox-active SAMs.