Nanocrystalline TiO₂ studied by optical, FTIR and X-ray photoelectron spectroscopy: correlation to presence of surface states

Abstract

Nanophase TiO₂ has been synthesized in an organic medium by two different routes, yielding two different structural phases viz. rutile and anatase. These differences are reflected in the different characterization techniques used to study the nanoparticles. UV-vis spectroscopy shows absorption-peak related particle sizes of ~2.5 nm and agrees with transmission electron microscopy (TEM) estimates of 2.5-5.0 nm. Grazing incidence X-ray diffraction shows rutile and anatase phase with an overlay of Ti₂O₃ for the different routes considered. Differences in the nature of the transition from absorption plots not withstanding, a direct transition is confirmed. Photo-luminescence (PL) spectra for the two particulate structures shows prominent red shifted peaks at 314 nm and 399 nm, respectively (0.81 eV from the excitation), and also reveals vibrational features around the maximum PL signal. In addition a Ti³⁺ (PL) state is observed at 617.5 nm for both cases, a feature governed by the Auger process. Fourier transform infrared (FTIR) studies reveal weak complex vibrations between the Ti-O oxide species and also additional unsaturated sites (Ti³⁺) through incorporation of (OH) groups, not otherwise seen in bulk titania. A surface consisting of ₆Ti³⁺-OH for coordinative saturation (octahedral site), along with ₄Ti⁴⁺-O (tetrahedral) is thus necessary. Grazing incidence X-ray diffraction studies shows the presence of the rutile phase of TiO₂ and also a sub-oxide phase of Ti (Ti₂O₃). X-ray photoemission spectra (XPS) of thin films of TiO₂ confirms the oxide phase and also the presence of sub-valence states. The XPS and FTIR spectra confirm the presence of adsorbed sites for coordinative saturation of sub-valence states (Ti²⁺, Ti³⁺), through hydroxyl incorporation. These sites are amplified as the particle size is reduced, opening avenues for additional coordination, leading to important applications. In this case, a Ti₂O₃ overlay saturates the surface of titania. Subtle differences are observed in the data vis-a-vis literature reports.