Non-Hydrolytic Synthesis of Visible Light Active Stable Metal Oxides and Metal Oxide Coatings for Fuel Forming Reactions

Abstract

This thesis addresses current challenges in the synthesis and photochemistry of metal oxide and metal chalcogenide materials, while providing potentially useful avenues for expanding their application. While water splitting is a means to achieve solar fuel production in the formation of hydrogen gas (H2), the slow kinetics of water oxidation limit large scale utility. As a replacement, the transformation of organic substrates into value-added chemicals has emerged as a scalable alternative. Materials commonly employed for this strategy are metal oxide and metal chalcogenide semiconductors that through excitation from solar light can generate charge carriers capable of facilitating redox chemistry. However, they are limited by energy barriers due to their large band gap, poor charge carrier mobility and photocorrosion during the catalytic transformation. While conventional synthesis eventually leads to pure materials, the attractive aspect of tuning the particle’s size and composition comes with great difficulty. This work has focused on investigating synthetic routes for preparing phase pure, metal oxide semiconductor nanomaterials using solvent-assisted low temperature methods through controlled microwave heating. With the aid of direct heating through microwave irradiation in non-aqueous media, nanocrystalline tungsten (VI) oxide is achievable in 30 minutes at 200°C, faster and at lower temperatures than conventional methods. Forming in a platelet morphology, these particles are 20 nm with a surface area of 37 m2g–1 WO3. These nanoplatelets are active for the photocatalytic oxidation of the benzylamine (rate constant, k of 0.021 h–1 WO3), benzyl alcohol (k of 2.6x10–3 h–1), and 5-(hydroxymethyl)-2- furfural (k of 0.01 h–1) using 10 mg of WO3 with 2 mL of 0.250 M substrate in acetonitrile and a 150 mW•cm–2 460 nm blue LED source. These rate constants are larger than those observed for commercially prepared, micron-sized WO3. XPS analysis shows that during catalysis, the concentration of W5+ on the surface increases, but the nanoplatelets are stable under these reaction conditions. The morphology and size of the particles are retained through the reactions. Moreover, the nanoplatelets are recyclable—showing no loss in activity for four reaction cycles. Expanding the synthetic scope, through microwave heating in ethanol and with subsequent annealing, crystalline MgFe2O4 nanoparticles are produced rapidly and in high yields >99%. Under varied annealing temperatures, the degree of Mg and Fe site inversion changes the optical, electronic, and composition of the nanoparticles. A small particle size of ∼10 nm is achievable with the aid of an ammonium salt mineralizer that caps the particles during nucleation and growth. Particles with the lowest inversion parameter and limited sintering upon annealing (at 600 °C) exhibit the greatest production of hydroxyl radicals under visible light illumination. As such, these particles also facilitate the degradation of methylene blue dye (rate constant k of 0.061 h–1) faster than those particles annealed at higher temperature with 20 mg of catalyst. This synthetic protocol is successively expanded to other ternary metal ferrites with similar crystalline structures. Finally, from the knowledge gained in this work, the synthesis and photostability of metal chalcogenide materials can be explored for routes to support large scale use. Known approaches to surface modification of nanoparticles are adapted to design complex core@shell structures of well-studied photocatalytic semiconductors. Insights into how the synthetic design can be improved are provided based on the observations of incomplete shell coverage of cadmium selenide (CdSe).