Electronic Band Engineering in TiO2 Particulate Layers for Photocatalysis

Electronic Band Engineering in TiO₂ Particulate Layers for Photocatalysis

Qilong Huang and Edmund G. Seebauer

University of Illinois at Urbana-Champaign, Urbana, IL 61801

Many large-scale photocatalysis applications employ TiO₂ deposited as a thin film by liquid-based methods from a particulate-based starting material. With solar illumination, photogenerated charge carriers form throughout the film and diffuse to nearby interior surfaces within the pore structure.  For very thin films with wide pores, catalyzing gas-phase reactions at modest light intensity, reactants have little trouble diffusing to meet the charge carriers, and the photocatalyst effectiveness factor is near unity.  For thicker films with narrow pores, catalyzing liquid-phase reactions of low ionic strength at high light intensity, catalyst effectiveness would improve if electric fields within the structure could be adjusted to pull minority photocarriers toward the nominal free surface, thereby circumventing the fluid transport problem.  Such a strategy requires electronic band engineering of a sort not normally practiced in particulate-based films.  The present work demonstrates experimentally how such band engineering may be implemented in the case of methylene blue photo-oxidation.  Variations in film thickness, pore structure, light intensity, penetration depth and fluid viscosity highlight the interplay of reactant transport within the fluid and photocarrier transport within the solid.  The concept of changing average electric field normal to the nominal surface via variations in the surface potential and majority carrier concentration are illustrated.