Anatase-TiO₂ Nanomaterials:  Morphological/Size Dependence of the Crystallization and Phase Behavior Phenomena

Abstract

Nanoparticulated TiO2 materials with anatase structure were synthesized by using a microemulsion method. Three different syntheses with varying surfactant-to-water molar ratio (ω) were used to obtain amorphous solid precipitates at room temperature. The structural characteristics of these solid precursors were studied by using X-ray absorption structure (X-ray absorption near-edge structure and extended X-ray absorption fine structure) and Raman spectroscopies, which showed that all lack 3D (tridimensional) order but contain a different degree of 2D-confined connectivity. While heating such solid precursors under dry air, marked differences appeared in the phase behavior; the onset temperature for anatase crystallization increases ca. 150 °C while the ω parameter decreases and only one of the samples shows the anatase-to-rutile transformation below 900 °C. In all cases, the crystallization of the anatase structure does not follow a traditional nucleation and growth mechanism and its analysis using the Avrami formalism gives conclusive evidence of a surface nucleation-dominated process. This appears as a distinctive feature of anatase-TiO2 nanomaterials, far from the corresponding behavior of microsized or bulk materials. After nucleation, the grain growth of anatase nanoparticles was found to follow the kinetic equation D 2 − D 0 2 = k 0 exp(−E a/RT), where the activation energy is a function of several structural properties of the solid materials mainly related to the hydration characteristics of the surface layer. A combined in situ X-ray diffraction/Raman/infrared study aimed to unveil the physical basis of the phase behavior and to interpret key variables allowing control of the crystallization mechanism and morphological properties, particularly primary particle size, in the nanometer regime.

Carbon-incorporated TiO2 photoelectrodes prepared via rapid-anodic oxidation for efficient visible-light hydrogen generation

“…The Raman spectrum of the sample prepared at 10 V shows a mixture of anatase and rutile phases with poor crystallinity (inset I in Fig. 3A), which is generally observed for native TieO materials [27]. The observed Raman modes are attributed to the TieO bending vibration (215 and 425 cm À1 ), TieO stretching vibration (610 and 635 cm À1 ), and TieOeTi stretching vibration (850 cm À1 e870 cm À1 ) [25e27].…”

Acetaldehyde degradation under UV and visible irradiation using CeO2–TiO2 composite systems: Evaluation of the photocatalytic efficiencies

“…For nanocomposite samples, after 5 min, titanium tetraisopropoxide was introduced into the previously resulting microemulsion drop by drop from a mixture with isopropanol (2:3). Water/TiO 2 and water/surfactant molar ratios were 110 and 18 for all samples, respectively [33,34]. The resulting mixture was stirred for 24 h, centrifuged, and the separated solid precursors rinsed with methanol and dried at 110 o C for 12h.…”

Photocatalytic Decolorization Study of Methyl Orange by TiO₂–Chitosan Nanocomposites

“…The filtrate was taken out and measured by UV-Vis at 464 nm. This experiment was then repeated for different concentrations of MO (10,20,30,40 and 50 mg/L) and pH (2,3,4,5,6,7,8). As a reference, TiO 2 powder (bulk) was also used for the photocatalytic process using the same conditions.…”

“…It is concomitant with enhancement of chemical activity of TiO 2 . The chemical activity is connected with several structural and electronic size-related effects and also the photochemical and photophysical activities by reduction of light scattering [4]. In addition, immobilization of TiO 2 in a host material may leads to easier handling and separating of TiO 2 from the water phase easier.…”