Microstructure and charge trapping assessment in highly reactive mixed phase TiO2 photocatalysts

Abstract

The structural-microstructural characteristics and interfacial charge transfer are key issues to the development of efficient mixed phase TiO2 photocatalysts. In this work, the interplay of lattice deformation and microstrains as well as the identification of charge trapping sites and electron transfer were investigated for a series of nanostructured titania photocatalysts by X-ray powder diffraction analysis, Raman and electron paramagnetic resonance (EPR) spectroscopy. These mixed phase nanomaterials were selected as model sol-gel TiO2 systems based on their exceptional photocatalytic performance over a wide range of hazardous water pollutants (including degradation/mineralization of phenol, 2,4-dichlorophenoxyacetic acid and imazalil) under UV light. Lattice contraction with respect to the bulk anatase together with anisotropic microstrains was identified for the smallest (11 nm) anatase nanoparticles. Both effects gradually relaxed with the increase of calcination temperature and the concomitant particle growth, with microstrains scaling linearly with the relative change of the c-axis lattice constant and the broadening of the main anatase Raman mode. The growth of anatase nanoparticles at 1023 K with minimal lattice deformation and microstrains resulted in the optimal photocatalytic efficiency, outperforming the benchmark Aeroxide® P25 catalyst. This mixed phase catalyst comprised also larger, though more strained, rutile nanocrystals than P25, and presented an additional deeper electron trapping lattice site according to light-induced EPR measurements. More importantly, electron transfer from rutile to anatase lattice traps was identified by EPR under visible light in the mixed phase photocatalyst. The improved crystal quality of the anatase nanocrystals combined with the enhanced charge separation in anatase/rutile interfaces is concluded crucial to the design of competent solar photocatalytic nanomaterials.