Sol−Gel Pure and Mixed-Phase Titanium Dioxide for Photocatalytic Purposes: Relations between Phase Composition, Catalytic Activity, and Charge-Trapped Sites

Abstract

The sol-gel synthesis of TiO 2 from TiCl 4 assisted by the triblock copolymer EO 20 -PO 70 -EO 20 (EO ) -CH 2 CH 2 O-, PO ) -CH 2 (CH 3 )CHO-) as templating agent was carried out by systematically changing H 2 O:Ti (r w ) and HCl:Ti (r a ) molar ratios. Mesoporous and nanocrystalline TiO 2 samples with well-defined and controlled phase composition (anatase, rutile, and mixed phase) were obtained after calcination at 400 °C and characterized for the morphology, particle size, and shape using TEM, HRTEM, XRD, and surface area measurements. The role of r w and r a and influence of the copolymer in determining the phase composition was demonstrated. Rutile becomes the main phase by increasing r w . In fact, the decrease of Ti concentration slows down the condensation rate, favoring formation of rutile seeds in the gel. The photocatalytic activity of TiO 2 in the UV photomineralization of phenol depends on the phase composition and oxidizing agent, H 2 O 2 or O 2 . When the oxidation is performed by H 2 O 2 , rutile, formed by large crystalline rods with high aspect ratios (size 15-20 × 100-120 nm), shows higher catalytic activity with respect to the small, almost cubic, anatase particles (5-15 nm). If O 2 is used, the catalytic activity generally decreases and the behavior of polymorphous species is reverse. EPR investigation of the paramagnetic charge carriers, formed under UV irradiation at 10 K, showed the resonance lines of holes trapped at Olattice sites and electrons trapped at Ti 3+ and O 2 sites. The rutile crystalline rods present the largest quantity of Oand Ti 3+ centers. The overall results suggest correlation between TiO 2 particle size and shape and the photocatalytic activity and indicate that electron-hole recombination is the most probable rate-controlling process.

Photogenerated Defects in Shape-Controlled TiO2 Anatase Nanocrystals: A Probe To Evaluate the Role of Crystal Facets in Photocatalytic Processes

“…This indicates an enhanced reactivity of these traps, mainly located at the surface in SP, R, and RE nanoparticles, which in principle are able to give, by electron transfer, Ti 4+ ÀO 2 À species that are active in the oxidative processes. 55,58,59 Finally, it can be observed that the amount of superoxide species is very high for SP nanoparticles, while for faceted nanocrystals it is generally low ( Figure 11B) and increases as the SSA of exposed {101} faces increases, becoming the highest for NB nanoparticles (inset in Figure 11A). This, on one hand, can explain the relatively good photoefficacy of the SP nanocrystals and, on the other, suggests that O 2 À species locate mainly on {101} faces, confirming an indirect involvement of these surfaces in the photooxidative processes.…”

“…In all the performed experiments, during the initial stage of recirculation in the dark (30 min), phenol adsorption on the nanoparticle surfaces causes depletion of the loaded PhOH by about 5.0 ( 2%. The data of TOC disappearance and the fitted curves 55 of PhOH mineralization for shape-controlled TiO 2 nanocrystals are shown in Figure 7. The Blank test is also reported for comparison.…”

Influence of physicochemical–electronic properties of transition metal ion doped polycrystalline titania on the photocatalytic degradation of Indigo Carmine and 4-nitrophenol under UV/solar light

“…The bicrystalline framework with high intimate contact in Mn 2+ -TiO 2 sample along with unique half filled electronic configuration of Mn 2+ and high standard reduction potential of Mn 2+ /Mn 3+ pairs accelerates the interfacial charge transfer process leading to the enhanced generation of hydroxyl radicals under UV/solar light, which obviously results in higher reaction rates. In the mixed-phase of anatase and rutile, photogenerated hole and electron preferentially gets trapped on O − and Ti 3+ centers of the rutile phase, even when anatase is the main component [117]. Under UV excitation, anatase in the mixed phase gets activated as it is a good absorber of UV light photons (Fig.…”

Lattice distortion mechanism study of TiO2 nanoparticles during photocatalysis degradation and reactivation

“…The photo-induced electrons and holes can freely move, recombine or migrate to the surface and participate in the photocatalytic reactions as triggers. [4][5][6] Scotti et al, 7 Lv et al 8 and Jiang et al 9 proposed that the key role in controlling TiO 2 photocatalytic reactivity was to adjust the rate of the electron and hole recombination. Thompson et al 10 figured that some lattice defects could turn into the surface light activation points.…”