Complex Metal Oxides for Photocatalytic Applications.

Abstract

Synthesizing narrow band gap semiconductors that absorb longer wavelengths of the solar spectrum (λ ≤ 550 nm) is an active research area focusing on using sunlight and a photocatalyst for water splitting to evolve hydrogen gas as a renewable form of energy. Harnessing the energy of sunlight through electron transfer is useful for environmental remediation since industrial effluents often contain a significant concentration of organic dyes that require degradation. In this thesis, a variety of compounds are synthesized via a facile sol-gel approach in order to produce visible light harvesting compounds. The major contribution of the work outlined in this thesis details the composition, structure, absorption, and mechanism of dye degradation for a co-incorporated system, TiO2:(Nb,N). A series of co-incorporated compounds, TiO2:(Nb,N), possessing the anatase structure have been prepared by a sol-gel technique. This technique allows us to synthesize mono-alloyed compositions, Ti1–(5x/4)NbxO2, which are converted to co-incorporated forms by annealing under flowing ammonia to afford Ti1–(5x/4)NbxO2–y–δNy, referred to as NbN-x. We prepare the composition-dependent series and fully characterize the surface and bulk properties. High mole-percent niobium compositions exhibit the fastest rate constants for methylene blue dye degradation, 0.779 h–1 for NbN-25. This composition possesses maximum nitrogen incorporation, ~ 2 mole percent, with minimal Ti3+. NbN-25 is active towards visible light (λ ≤ 515 nm) water oxidation when loaded with 1 wt. % RuO2 in the presence of an electron acceptor. Visible light absorbing metal oxide titanates have been prepared and structurally characterized. Modified sol-gel processes were used to alloy various metal oxide phases— titania, perovskite, and Ruddlesden-Popper. Thin films of TiO2:Mn and SrTiO3:Mn were prepared by spin casting precursor sols and annealing in air, affording phase pure compositions determined by X-ray diffraction and ICP analysis. Both phases absorb light into the visible part of the solar spectrum (λ ≤ 500 nm); however, the presence of Mn3+ results in capacitive behavior that limits the use of the photoanodes towards water oxidation. Ruddlesden-Popper powders, K2La2Ti3-xFexO3, absorb visible light to ~ 515 nm, and electrodes were prepared to examine the photoelectrochemistry.