Photocatalysts on Microresonators

Abstract

In this thesis, various types of photocatalysts based on TiO2 and ZnO are fabricated, and the photocatalyst nanomaterials have been studied using microresonators. Fundamental study on the kinetics and mechanisms of photocatalytic reactions is investigated, and a lot of application areas related with energy, environmental, and bioengineering are also reported. The photocatalytic activity is measured using microresonators

to our knowledge, this is the first study to use this approach to determine photocatalytic activity. Furthermore, combination of photocatalytic nanomaterials and microresonators will bring breakthrough technologies for future-based engineering applications. The thesis consists of 6 chapters. Chapter 1 describes the general background of photocatalysis and microresonators. Chapter 2 through Chapter 6 are research works for fabrication of (TiO2 or ZnO)-based semiconductor photocatalysts and their applications.Chapter 2 is investigation of ZnO nanorod-coated quartz crystals as self-cleaning thiol sensors for natural gas fuel cells. ZnO nanorods were directly grown on quartz crystals in zinc nitrate hexahydrate solution. When exposed to hexanethiol vapor, the hexanethiol molecules adsorbed onto the nanorod-coated surface. Subsequent exposure of the adsorbed hexanethiol molecules to UV irradiation led to the photodegradation of the molecules. The adsorption and photocatalytic degradation processes were monitored by measuring changes in the resonant frequency of the crystal using a quartz crystal microbalance (QCM). UV irradiation of the nanorod-coated quartz crystals on which hexanethiol was adsorbed caused the frequency to return to the value for the bare ZnO surface, indicating that the hexanethiol molecules adsorbed on the ZnO surfaces were completely removed. The results highlight the potential of ZnO nanorod-grown QCMs as self-cleaning sensors capable of long-term operation under harsh conditions. This chapter was published in Sensors and Actuators B in 2009.Chapter 3 describes the examination of the photocatalytic activity of various TiO2 films using silicon resonators. Ordered mesoporous TiO2 was synthesized using the combined assembly of soft and hard chemistries method, and deposited as a film coating on a microcantilever array along with two other types of TiO2 film: one from nanoparticles and one prepared via a sol-gel reaction. After loading methylene blue molecules on the TiO2 films, the films were exposed to ultraviolet radiation. The photocatalytic decomposition of methylene blue was monitored by measuring changes in the resonance frequency of each cantilever. The mesoporous TiO2 film showed higher photocatalytic activity than conventional TiO2 films fabricated from nanoparticles or via a sol-gel reaction. This chapter was published in Analytical Chemistry in 2010.Chapter 4 explores the ZnO/CdS core/shell nanowires synthesized by a spin-coating-based successive ion layer adsorption and reaction method. We developed a successive ion layer adsorption and reaction method based on spin-coating (spin-SILAR) and applied the method to the fabrication of highly uniform ZnO/CdS core/shell nanowire arrays. Because the adsorption, reaction, and rinsing steps occur simultaneously during spin-coating, the spin-SILAR method does not require rinsing steps between the alternating ion adsorption steps, making the growth process simpler and faster than conventional SILAR methods based on dip-coating (dip-SILAR). The ZnO/CdS core/shell nanowire arrays prepared by spin-SILAR had a denser and more uniform structure than those prepared by dip-SILAR, resulting in the higher power efficiency for use in photoelectrochemical cells. This chapter was published in Nanotechnology in 2010.Chapter 5 reports a facile route to the synthesis of hybrid nanoclusters consisting of Fe3O4/SiO2/TiO2 core/shell structured nanoparticles and TiO2 nanocrystals. The hybrid nanoclusters were synthesized by a solvothermal reaction and produced well-defined anatase crystalline phase TiO2 without a calcination process. The hybrid nanoclusters were dispersed in a methylene blue solution, and the photocatalytic activity was measured by UV-Vis spectroscopy. Enhanced photocatalytic activity was achieved due to the high crystallinity and large surface area of the nanoclusters. Further, the nanoclusters could be recovered from the suspension simply by applying an external magnetic field. The recovered nanoclusters were found to maintain their initial photocatalytic activity after at least ten cycles of use.Chapter 6 describes the development of a novel gravimetric immunosensor for sensitive detection of protein biomarkers. The sensor consisted of an array of microcantilevers, and a different antibody was immobilized on each microcantilever to detect a specific protein biomarker using sandwich immunoassay. Multiple analytes could be determined simultaneously by performing a single assay. This provides significant advantages in terms of cost, test throughput, and convenience. Moreover, enhanced sensitivity was achieved by introducing the multifunctional magnetic-photocatalytic nanoparticles: (1) Antigen-conjugated hybrid nanoparticles are easily separated from serum samples and preconcentrated for sensitive detection. (2) After the antigen-conjugated nanoparticles bind to detection antibody on the microcantilever, photocatalytic reduction of silver ions results in the formation of metallic silver onto the nanoparticles under UV irradiation and induces enhanced mass changes. The resonance frequency change by photocatalytic silver reduction is larger than the change by antigen binding alone. The results further highlight the potential of the magnetic-photocatalytic nanoparticles for the detection of low concentration of multiple protein biomarkers using microresonator arrays.