abstract
Amongst nanomaterials, metal oxides play an increasingly dominant role, with titanium dioxide (titania, TiO2) being widely used for various applications, such as light-to-energy conversion and storage, and photocatalysis. In this work, TiO2 has been synthesised via an aqueous sol-gel method, using three different mineral acids (HNO3, HCl and HBr) to peptise the sol, and hence provide counter-ions. Dried sols were thermally treated at three different temperatures (450, 600 and 800 degrees C),using three different dwell times (2, 4, and 8 h). Advanced X-ray methods were used to monitor the effect that the counter-ions had on the anatase-to-rutile phase transformation (ART). Quantitative phase analysis (QPA) using the Rietveld method was applied to assess the true amount of crystalline phases in the systems, and the amount of amorphous phase. Furthermore, the average crystalline domain diameter was also investigated, using whole powder pattern modelling (WPPM). With the advanced XRPD data (actual crystalline phase weight fraction in the samples and their average domain diameter and size distribution), it was possible to carry out a semi-quantitative study of the ART transformation kinetics. At a low temperature of 75 degrees C, the Cl-counter-ion was the most favourable to obtain anatase as the major crystalline phase, delaying the onset of the ART. Conversely, the Br-ions, maintained more anatase at 450 degrees C, with a lower ART rate. In general, halides were more effective in delaying the ART than NO3- counterions. Moreover, we observed an inverse linear relationship between the lattice volume expansion of rutile and the increase of its crystalline domain size at 450 and 600 degrees C isotherms. As the domain sizes increased with temperature, this effect reversed and became a direct linear dependence at the 800 degrees C isotherm, suggesting a critical size limit < 90 nm for this effect.
keywords
TITANIUM-DIOXIDE NANOPARTICLES; LATTICE EXPANSION; AMORPHOUS TITANIA; PARTICLE GROWTH; SIZE; STABILITY; POWDER; DIFFRACTION; MECHANISM; BROOKITE
subject category
Chemistry; Crystallography
authors
Tobaldi, DM; Pullar, RC; Gualtieri, AF; Jorge, AB; Binions, R; McMillan, PF; Seabra, MP; Labrincha, JA
our authors
David Maria Tobaldi
Researcher
João António Labrincha Baptista
Full professor
Maria Paula Seabra
Assistant Professor
Rob Pullar
Collaboratoracknowledgements
D.M. Tobaldi is thankful to the ECO-SEE project (funding from the European Union's Seventh Framework Programme for Research, Technological Development and Demonstration under grant agreement no. 609234. Note: The views expressed are purely those of the authors and may not in any circumstances be regarded as stating an official position of the European Commission). Authors acknowledge PEst C/CTM/LA0011/2013. R.C. Pullar acknowledges the support of the FCT grant SFRH/BPD/97115/2013. We would also like to be grateful to an anonymous reviewer, who gave us the stimulus to improve the manuscript.