GRADUATE SCHOOL OF SCIENCES & ENGINEERING
CHEMICAL AND BIOLOGICAL ENGINEERING
PhD THESIS DEFENSE BY YAPRAK ÖZBAKIR
Title: DEVELOPMENT OF AEROGEL BASED OPTOFLUIDIC MICROREACTORS
Speaker: Yaprak Özbakır
Time: September 14, 2018, 10:00
Place: ENG B29
Rumeli Feneri Yolu
Thesis Committee Members:
Prof. Dr. Can Erkey (Advisor, Koç University)
Prof. Dr. Alper Kiraz (Co-advisor, Koç University)
Prof. Dr. Yaman Arkun (Koç University)
Prof. Dr. Ahmet Kerim Avcı (Boğaziçi University)
Asst. Prof. Zeynep Ülker Demir (Altınbaş University)
Asst. Prof. Erdal Uzunlar (İzmir İnstitute of Technology)
Efficient light distribution in reaction medium in optofluidic microphotoreactors is highly desirable to fully harness their potential in light-driven chemical processes. The interaction of light with reactants and photocatalysts can be dramatically improved by enabling light propagation directly within the reactors using light guiding. In this study, liquid core optofluidic waveguides are utilized in microphotoreactors for guiding of light. In this approach, a suitable material confines the core liquid within internal channels and, simultaneously, behaves like waveguide cladding. For the propagation of non-lossy optical modes guided in the liquid by total internal reflection (TIR) of light from the channel walls, the cladding material should have a low absorption coefficient at working light wavelength and a lower refractive index than that of the core liquid (ncore>ncladding). In applications with aqueous solutions, a material with a refractive index lower than the refractive index of water, 1.33, is required and only a narrow choice of solid host materials are available. An aerogels is a highly porous nanostructured material consisting of an interconnected open network of loosely packed, bonded particles separated by air gaps, and they feature extremely low refractive index of ~1 which makes them remarkable as solid cladding of liquid-core optofluidic waveguides without any additional coating.
A new, straightforward technique that uses direct manual drilling to manufacture TIR-based liquid-core optofluidic waveguides was first developed for aerogel monoliths. Fabrication of channels in aerogel blocks by manual drilling preserving nanoporous and monolithic structure of aerogels was demonstrated for the first time. Silica aerogels with densities ranging from 0.15 g/cm3 to 0.39 g/cm3 were produced by aging of alcogels in tetraethylorthosilicate solution for various time periods, followed by supercritical extraction of the solvent from the alcogel network. Subsequently, the resulting hydrophilic aerogel samples were made hydrophobic by hexamethyldisilazane (HMDS) vapor treatment. This method of fabrication is capable of producing channels with lengths significantly larger than those reported previously in the literature and it also provides relative flexibility in the channel shaping. Long channels (up to ~7.5 cm) with varying geometries such as straight and inclined L-shaped channels could be fabricated. Multimode optofluidic waveguides prepared by filling the drilled channels in aerogel monoliths with water yielded high numerical aperture values (~ 0.8). Efficient guiding of light by TIR in the water-filled channels in aerogels was visually revealed and characterized by monitoring the channel output. The resulting aerogel-based optofluidic waveguides exhibited efficient light guiding features, as verified by direct imaging of guided light modes and measuring the transmittance of the waveguides. The synthesized samples retained their low refractive index (below ~1.09) and, hence, they could serve as suitable optical cladding materials for aqueous waveguide cores (refractive index ncore = 1.33). Hydrophobic silica aerogel samples produced by the above technique also had low absorption coefficients in the visible part of the spectrum.
Next, a new type of aerogel-based microphotoreactor with integrated optofluidic waveguide was developed. This optofluidic microphotoreactor consists of a single liquid-filled channel fabricated inside a monolithic silica aerogel within which photochemical reactions are carried out. Due to the contrast of refractive indices between the aerogel and the liquid, optofluidic waveguides based on TIR are naturally formed to deliver the light to the liquid reaction medium inside the channel. This configuration is demonstrated to provide an excellent overlap between guided light and fluids in the channel for efficient photochemical activation. The microphotoreactor was shown to be well suited for photochemical degradation of a model organic compound – methylene blue (MB) dye – and the efficiency of the dye photoconversion as a function of the incident light power was characterized.
Finally, the fields of optofluidics and highly porous, membrane-like properties of aerogels combined with a fully tunable three-dimensional structure were both exploited to construct photocatalytic microphotoreactors for controlled distribution of light through the reaction medium maintaining good interaction of light, fluid and the solid photocatalyst particles. Anatase TiO2 nanoparticles were used as photocatalysts and they were successfully introduced into the mesoporous network of silica aerogels during sol-gel step of aerogel synthesis obtaining monolithic composite aerogels with varying titania content from 1 wt % to 50 wt % for the first time. The presence of TiO2 and its desired crystalline structure in aerogel matrix was confirmed by XRD patterns and FE-SEM images. The silica-titania composite aerogels retained their interconnected mesoporous network with high porosity and pore volume as well as high surface area. Surface modification by HMDS was devised to alter the wetting conditions of the reactor walls for construction of liquid-core optofluidic waveguides in the channel. Cylindrical straight channels were then fabricated in the synthesized monolithic composites. Light was confined in the liquid in the channel and was guided in a controlled manner by total internal reflection (TIR) from the channel walls. Low and favorable propagation losses ranged from 2.6 dB/cm to 3.9 dB/cm with increasing amount of the TiO2 in the structure from 1 wt % to 50 % wt. The band gap of the SiO2– TiO2 composites was estimated from Tauc plot calculated by Kubelka-Munk function from diffuse reflectance spectra of samples obtained using UV-visible reflectance spectroscopy. Using this technique, the anatase TiO2 band gap was observed to be near expected value of ≈ 3.2 eV. The photocatalytic degradation of phenol over the immobilized photocatalysts in the channel walls by the light delivered through the constructed waveguide was used as model reaction to test the reactor and demonstrating very promising performance. The effect of incident light power, flow rate of the reactant and mass fraction of the photocatalyst in aerogel composites and additional oxygen supply on the performance of the reactor were investigated. Along with experimental studies, a simple model for immobilized photocatalytic microphotoreactors with integrated waveguide following a first order reaction rate with light dependency was developed and compared with experimental data for various conditions investigated. The reaction rate parameters were regressed from an exponential model. The model results and experimental data were found to be in good agreement. Lastly, carbon nanotubes (CNTs) were incorporated into the silica-titania matrix during sol-gel method resulting in monolithic composites and to their function on the photocatalytic degradation was investigated. As compared to the silica-titania composite aerogels, simply mixing of CNTs with TiO2 presumably exhibited lower photocatalytic efficiency.