Chemical Substitution - Alignment of the Surface Potentials for Efficient Charge Transport in Nanocrystalline TiO2 Photocatalysts

Anatase TiO2 is among the most studied photocatalytic materials for solar energy conversion and environmental cleanup. However, its poor visible light absorption and high facet-dependent performance limits its utilization. In this study chemical substitution (doping) of TiO2 nanoparticles with metal ions (Sb, Cr, or Sb/Nb and Cr/Nb) is presented as an alternative strategy to address both issues simultaneously. Highly crystalline doped and codoped TiO2 nanoparticles were successfully synthesized by a microwave-assisted nonaqueous sol gel synthesis. The structural and compositional analysis done by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) showed that depending on the doping applied, variations in particles size and morphology were observed. Doped and codoped samples showed improved absorption in the visible range and in comparison to the undoped TiO2 displayed improved photocatalytic (PC) activity. The variations of the PC activity, observed among different samples, are attributed to the effect of doping on (i) particles size/morphology, (ii) optical activity, and (iii) on the surface potential differences for the various crystal facets. We found that Sb-doping in TiO2 diminishes the surface potential difference for {101} reductive and {001} oxidative sites, which makes all crystal surfaces equally attractive to both electrons and holes. Accordingly, in Sb-doped TiO2 nanoparticles the photocatalytic activity is independent of the exposed crystal facets and thus on the particle morphology. This observation also explains the superior PC performance of this material.