Particles Morphology and Surface Properties As Investigated by HRTEM, FTIR, and Periodic DFT Calculations: From Pyrogenic TiO2 (P25) to Nanoanatase

By combining electron microscopy, FTIR spectroscopy of different CO isotopic mixtures, and DFT calculations, a complete assignment of the IR spectrum of CO adsorbed on P25 (a mixture of 85% anatase and 15% rutile) at various dehydration states, on its pure rutile component, and on nanoanatase was obtained. It is shown that the measurements at 60 K provide IR spectra of unprecedented quality and that the spectroscopic method is extremely powerful not only to study the surface Lewis and Bronsted acidity but also to investigate the particles morphology. Indeed, as CO adsorbed on different faces is characterized by different stretching frequencies, the IR spectrum contains information on the exposed faces and hence on particles morphology. This information, combined with the study of the IR spectrum of CO adsorbed on defects, of dipole-dipole interactions between parallel oscillators adsorbed on extended faces and with HRTEM results allowed us to fully explore the relations between particles morphology and surface properties. By comparing the spectra of CO obtained on P25, on the pure rutile fraction of P25, and on nanoanatase, the high crystalline character of P25 is inferred, which is likely the key of its outstanding photocatalytic activity. It is also found that the IR spectrum of CO on P25 is the sum of the rutile and anatase contributions and that no additional surface features ascribable to anatase-rutile junctions are noticeable, thus contributing to the strong debate present in the literature.