Advanced X-ray diffraction for structure-property analysis of photocatalytic TiO2 nanoparticles.

Establishing reliable structure-property relationships in nanostructured TiO2 remains a relevant challenge. In this work, using quantitative X-ray diffraction analysis, including Rietveld refinement and anisotropic line-broadening modeling, empirical correlations between crystallographic parameters and photocatalytic activity have been revealed for a series of TiO2 samples synthesized via non-isothermal decomposition of TiO(acac)2 at controlled final synthesis temperatures. It was found that at a critical TiO2 nanoparticle size of about 20 nm, pronounced non-linear changes occur in the lattice parameters, oxygen site occupancy, and TiO6 octahedral distortion; at the same time, the oxygen atomic coordinate Z(O) passes through a maximum, which may indicate enhanced lattice polarization and the formation of an internal electric field that promotes charge carrier separation. In addition, the coexistence of anatase and rutile phases creates a heterojunction that further improves charge separation efficiency. The highest photocatalytic activity was recorded for the sample obtained at 625 °C. The obtained results show that quantitative XRD analysis can serve as a useful tool for identifying structural factors affecting activity in the investigated systems. However, in order to claim universal predictive capability of the method, the conclusions regarding the relationship between the structural and microstructural parameters of TiO2 and photocatalytic activity require additional verification on samples prepared through different synthesis routes, as well as on other classes of semiconductor photocatalysts.