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

Nanophase TiO2 has been synthesized in an organic medium by two different r outes, yielding two different structural phases viz. rutile and anatase. Th ese differences are reflected in the different characterization techniques used to study the nanoparticles. UV-vis spectroscopy shows absorption-peak related particle sizes of similar to 2.5 nm and agrees with transmission el ectron microscopy (TEM) estimates of 2.5-5.0 nm. Grazing incidence X-ray di ffraction shows rutile and anatase phase with an overlay of Ti2O3 for the d ifferent routes considered. Differences in the nature of the transition fro m absorption plots not withstanding, a direct transition is confirmed. Phot o-luminescence (PL) spectra for the two particulate structures shows promin ent 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 si gnal. In addition a Ti3+ (PL) state is observed at 617.5 nn 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) grou ps, not otherwise seen in bulk titania. A surface consisting of Ti-6(3+)-OH for coordinative saturation (octahedral site), along with Ti-4(4+)-O (tetr ahedral) is thus necessary. Grazing incidence X-ray diffraction studies sho ws the presence of the rutile phase of TiO2 and also a sub-oxide phase of T i (Ti2O3) X-ray photoemission spectre (XPS) of thin films of TiO2 confirms the oxide phase and also the presence of sub-valence states. The XPS and FT IR. spectra confirm the presence of adsorbed sites for coordinative saturat ion of sub-valence states (Ti2+, Ti3+), through hydroxyl incorporation. The se 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 ob served in the data vis-a-vis literature reports. (C) 2000 Elsevier Science S.A. All rights reserved.