Synthesis and Characterization of Sn2+- based and Bi3+- based metal oxides for photocatalytic applications

  • Dalal Noureldine

Student thesis: Doctoral Thesis

Abstract

The main challenge of water splitting technology is to develop stable, visible responsive photocatalysts that satisfy the thermodynamic requirements to achieve water redox reactions. This study investigates development of the semiconductors containing metals with s2d10 electronic configuration such as Sn2+ or Bi3+ which shifts the valence band position negatively. Efficient water splitting can, however, be only achieved by understanding the fundamental semiconductor properties of underlying processes. This work elucidates the semiconductor properties through two approaches: the first is to synthesize the materials of various stoichiometry in various forms (powders, thin film etc.) and the second is to perform a combined experimental-theoretical studies to determine the optoelectronic properties of the synthesized materials. The study includes the synthesis and characterization of a series of Bi3+ based semiconductors (Bi2Ti2O7, Bi12TiO20, and Bi4Ti3O12) to resolve inconsistencies in their optoelectronic properties. The crystal parameters and stoichiometry were confirmed by the Rietveld refinement and XRD measurements. These compounds showed a UV responsive absorption, high dielectric constants, and low electron and hole effective masses in one crystallographic reflecting their good charge separation and carrier diffusion properties. The approach showed to be accurate in determining the optoelectronic properties due to good agreement between experimental and theoretical values. The second study investigated the synthesis of SnNb2O6 and using flux assisted method which afforded control over the surface. Increasing the flux to reactant molar ratio resulted in a 2D platelets with anisotropic growth along bc plane as confirmed by XRD and SEM. The photocatalytic activity increased while increasing the flux to reactant ratio exceeding solid state synthesis. This method minimized the oxidation of the surface and formation of grain boundaries and enabled the synthesis of the compound at lower temperature. Next, the optoelectronic properties of α-SnWO4 structure were studied though a combination of experimental and theoretical approach. α-SnWO4 thin films were deposited by RF-sputtering. An interesting low band gap of ~ 1.95 eV was experimentally for direct band gap and 1.7 eV for indirect band gap, high dielectric constants and low electron effective masses in one crystallographic direction were obtained, exhibiting good charge separation and charge carrier transport of the charge carriers. The PEC performance was limited by the oxidation of Sn2+ in the material under applied potential. Finally, pyrochlore SnSb2O6 structure was synthesized utilizing soft hydrothermal method. The crystal structure was studied by Rietveld refinement and the position of Sn2+ was specified. The material showed interesting absorption edge around 700 which is promising for overall water splitting application.
Date of AwardJul 2016
Original languageEnglish (US)
Awarding Institution
  • Physical Science and Engineering
SupervisorKazuhiro Takanabe (Supervisor)

Keywords

  • Photocatalysis
  • Solar Energy
  • Synthesis
  • Tin-based
  • Bismuth-based
  • ternary metal oxide

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