X-ray absorption spectroscopy studies of phase transformations and amorphicity in nanotitania powder and silica–titania core–shell photocatalysts

Abstract

The local environment of titanium in nanocrystalline sol‐gel synthesized titania, cobaltiferous titania and silica–titania core–shell photocatalysts was investigated using X‐ray absorption spectroscopy (XAS). Anatase reconstructively transforms to rutile via a persistent amorphous phase that is retained, in part, up to 1273 K. In nanotitania, temperature‐dependent trends in Ti order correlation observed by XAS parallel the development of amorphous content extracted from powder X‐ray diffraction patterns, such that amorphicity shows a transient maximum at ∼873 K with the onset of rutile crystallization. Cobaltiferous and core–shell materials behaved similarly, but with anatase retained to 973 and 1273 K, respectively. In the former, cobalt redox reactions may stabilize anatase to higher temperatures by ready charge‐balancing during the loss of hydroxyl and the formation of oxygen vacancies. In the core–shell architecture, higher Ti coordination and interatomic distance variance in the first‐ and second‐nearest‐neighbour shells are maintained to 1273 K by interaction of a substantially aperiodic TiO6 network with the glassy silica substrate, which inhibits crystallization of rutile from the amorphous intermediate. Comparisons are also drawn with the commercial P25 catalyst. The overall transformation mechanism can be summarized as gel → non‐stoichiometric anatase → amorphous titania → rutile. Smaller anatase crystals and a higher average Ti—Ti coordination environment in the core–shell structure may enhance photocatalytic activity directly, by creating larger specific surface areas and hosting reactive defects, or indirectly, by inhibiting exciton annihilation in aperiodic titania and delaying the crystallization of less photoactive rutile.

Influence of the OH groups on the photocatalytic activity and photoinduced hydrophilicity of microwave assisted sol–gel TiO2 film

“…XRD investigation of the prepared films did however not show any evidence of formation of rutile. In recent work reported by Lim et al [45] the amount of X-ray amorphous material was found to increase at temperatures from 500 8C to 700 8C due to the way of conversion of anatase to rutile in nanotitania. However, in the present work it was found that the amount of crystalline material (Anatase) did not change due to heat treatment at temperatures up to 550 8C.…”

Nonstoichiometry, amorphicity and microstructural evolution during phase transformations of photocatalytic titania powders

“…This assumption was considered reliable because in powder XRD the broad X-ray scattering feature, characteristic of amorphicity, was generally absent after low-temperature treatments. However, quantitative XRD and X-ray absorption spectroscopy (XAS) found that aperiodic titania (several tens of percent) coexists with the dimorphs, even after the complete removal of anatase (Grey & Wilson, 2007;Lim et al, 2008), as recognized in early electron microscope studies (Bickey et al, 1991). It is now apparent that a significant non-Bragg diffracting portion is retained without the telltale amorphous hump, and that a new aperiodic phase, distinct from the amorphous precursor, appears in the crystallization sequence gel !…”

“…rutile, with the temperatures of the phase transformations dependent on chemistry and form. Moreover, the photocatalytic degradation of methylene blue may correlate with the maximum amorphous content (Lim et al, 2008). This might be due to the relatively high surface areas afforded by the intervening aperiodic material, or the modification of the space-charge potential at the disordered interfaces and the creation of localized electronic states.…”

“…While the progression from gel to rutile is certain, the absolute quantity of aperiodic titania determined by Rietveld analysis of powder X-ray diffraction data (Cu K) remained doubtful as attenuation of Bragg diffraction by microabsorption proved difficult to eliminate, leading to an overestimation of amorphous content (Lim et al, 2008). In this study, we extract the non-Bragg diffracting content in sol-gel-derived titania powders as a function of calcination temperature using neutron diffraction, where absorption is expected to be negligible, and multiple-wavelength X-ray diffraction, to monitor the energy dependence of microabsorption and evaluate the evolution of microstructure.…”

Characterization of nickel-titanium based thin films crystallized by fast and ultra-fast annealing methods

“…X-ray absorption spectroscopy (XAS) is a powerful technique to examine near-neighbor atomic pairs and assess order-disorder phenomena [150,151]. However, there is no information on the evolution of co-ordination shells of TiNi-based thin films during the aperiodic-crystalline transition.…”