Effect of Silica Additive on the Anatase‐to‐Rutile Phase Transition

Abstract

The effect of SiO 2 addition on the anatase-to-rutile phase transition was investigated by DTA, XRD, FTIR, and XPS. TiO 2 xerogels containing SiO 2 up to 20 mol% were prepared by mixing and hydrolyzing titanium tetraisopropoxide (TTIP) and tetraethylorthosilicate (TEOS) with HNO 3 as a catalyst. With increased amounts of SiO 2 in the xerogels, the following results were obtained: (1) the crystallization temperature of anatase increased from 415°C in pure TiO 2 to 609°C in 20-mol%-SiO 2 -containing xerogel in the DTA curves; (2) the formation temperature of rutile, according to quantitative XRD analysis, increased with increased SiO 2 content up to 5 mol% SiO 2 but became constant at higher SiO 2 contents; (3) the crystallinity of anatase became lower; and (4) the lattice parameter a of the anatase decreased slightly, but the parameter c decreased greatly up to 20 mol% SiO 2 . Although the added silicon atoms were considered from these results to be incorporated into the amorphous TiO 2 and anatase structures, the 29 Si MAS NMR spectra of the xerogels containing 10 mol% SiO 2 showed only tetrahedral silicon, with no indication of silicon in octahedral coordination. When calcined at higher temperatures, the xerogel showed polymerization of the SiO 4 tetrahedra in the NMR spectra and the Si-O-Si vibration in the FTIR spectra. The chemical composition of the xerogel surfaces, measured using XPS, showed increased SiO 2 content with increased calcining temperature, indicating the expulsion of silicon from inside the particles to form an amorphous SiO 2 surface layer. The formation of this amorphous SiO 2 surface layer was considered to be important in retarding the anataseto-rutile phase transition by suppressing diffusion between anatase particles in direct contact and limiting their ability to act as surface nucleation sites for rutile. These effects of silicon additions were similar to those observed in the ␥-Al 2 O 3 -to-␣-Al 2 O 3 transition.

Review of the anatase to rutile phase transformation

“…Conversely, Yang and Ferreira [175] have suggested that the observed contraction in lattice parameters upon SiO 2 and/or Al 2 O 3 doping is evidence of solid solubility. Another study of the effects of SiO 2 doping on the lattice parameter of anatase [82] also suggested that Si 4? enters substitutionally, thereby decreasing the lattice parameter of anatase (and forming interstitial Ti 4?…”

“…It has been suggested that cations of small radii and low valence accelerate the transition to rutile owing to the increase in oxygen vacancies that result from the assumed substitution of Ti 4? ions with cations of lower valences [43,109,157,164,165]. Charge neutrality thus requires an increase in the level of oxygen vacancies and/or the formation of Ti interstitials of lower valence [82,172]. Conversely, when cations of valence higher than 4 are assumed to substitute for Ti ions on the anatase lattice, this gives rise to the annihilation of existing oxygen vacancies and the formation of Ti interstitials of the same or lower valence.…”

“…Phase formation during synthesis of TiO 2 In the synthesis of TiO 2 films by various methods, the initial crystalline TiO 2 phase formed is generally anatase [82,83]. From a structural perspective, this could be due to the greater ease of the short-range ordered TiO 6 octahedra in arranging into long-range ordered anatase structure owing to the less-constrained molecular construction of anatase relative to rutile [84].…”

Influence of pH on the Formulation of TiO<sub>2</sub> Nanocrystalline Powders

“…From the mechanism of A to R transformation, it seems that phase transformation is a nucleation and growth process [22,26]. Also it is stated that in the synthesis of TiO 2 by various methods, the initial crystalline TiO 2 phase formed is generally anatase [27,28]. From a structural perspective, this could be due to the greater ease of the short-range ordered TiO 6 octahedra in arranging into long-range ordered anatase structure owing to the less-constrained molecular construction of anatase relative to rutile [29].…”

Sand Supported Mixed‐Phase TiO2 Photocatalysts for Water Decontamination Applications

“…These elements are reported to inhibit the formation of rutile through the restriction of the atomic rearrangement involved in the phase transformation. [23,[34][35][36][37] The use of lower firing temperatures for similar coatings may result in similar mixed phase coatings if anatase-to rutile transformation promoting dopants, such as V, Co and Ni , are added [38] . However the use of lower firing temperatures may impede the formation of good coating adhesion.…”