EPR Study of the Surface Characteristics of Nanostructured TiO₂ under UV Irradiation

Abstract

Nanostructured TiO2 samples with primary particle size in the 4−20 nm range were prepared by either hydrothermal (H) or thermal (T) treatment of an amorphous precursor, and their behavior under UV illumination at 77 K was studied by means of EPR spectroscopy. The samples of the H series present the smallest crystallite size and after irradiation in a vacuum show some Ti3+ centers. In contrast, under these conditions only weak signals associated with oxygenated radicals are observed for the T samples. However, when oxygen is preadsorbed, several oxygenated complexes (O-, O2 -, O2H•, and O3 -) are photogenerated in proportions that depend on the characteristics of the material. Subsurface O- species are exclusively detected in the case of the samples of the H series, whereas ozonide radicals and surface O- are stabilized on materials with larger crystalline domains. These oxygenated complexes are thermally unstable, and they disappeared after warming to room temperature in the case of the T samples, but they are transformed to O2 - on the surface of the hydrothermally treated TiO2. Since adsorbed water and different types of free hydroxyls are present on these materials, as revealed by FTIR, a number of surface reactions have to be considered in order to account for the formation and stability of such photogenerated species.

Interesting magnetic behavior from reduced titanium dioxide nanobelts

“…14 The broad EPR signals on the left side of the sharp peak are most likely due to those of surface oxygenated active species. 11,15 This assignment is consistent with our experimental observations where the intensities of these signals are always higher in all air annealed samples, but relatively lower in all samples that were prepared under vacuum annealing.…”

Boosting TiO2-anatase antimicrobial activity: Polymer-oxide thin films

“…6a) show weak signals exclusively attributable to TiO 2 species; an apparently axial shape displaying g ? % 1.99 and unresolved parallel component, ascribable to Ti 3+ species, and another orthorhombic with g 1 = 2.016-7, g 2 = 2.011-2; g 3 % 2.002-4, ascribable to trapped hole oxygen centers (O À species) [32]. It is noticeable the lack of electron capture centers characteristic of the polymer component [33].…”

Photoacoustic spectroscopic analysis of photoinduced change in absorption of titanium(IV) oxide photocatalyst powders: A novel feasible technique for measurement of defect density

“…H 2 O 2 can be produced by reaction of photoexcited electrons with molecular oxygen via superoxide anion (O 2 -) [23,24], and EPR studies have suggested the liberation of peroxide species in the presence of oxygen [6], though we have no information on the origin. Anatase TiO 2 samples of large specific surface area have been shown to have large amounts of hydroxyl groups and physisorbed water through studies of infrared spectroscopy [6,25], thermogravimetry [5,26,27] and 1 H NMR [28], and these species on Type 2 samples might participate in the hole trapping and/or peroxy species productions.…”

“…We have proposed a linearity of the Ti 3+ density with the defect density but with no certified claim that these two species are identical [3]. Electron paramagnetic resonance (EPR) has also been used to measure such species [4][5][6][7][8], though active oxygen species liberated simultaneously may interfere. Spectroscopy is an alternative way for investigation on the dynamics of electrons and positive holes, and ultrafast spectroscopy under various conditions has been reported [9][10][11][12][13].…”