(600b) 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.