Limited Crystallite Growth upon Isothermal Annealing of Nanocrystalline Anatase

Abstract

The crystalline growth kinetics during isothermal sintering of two nanotitania powders synthesized by similar routes, but with and without the presence of chlorine in the synthesis batch) were studied by X-ray powder diffraction and modelled by several grain growth models. Both nanopowders contained anatase as the initial titania phase with similar crystallite dimension. Crystal growth curves at three isotherms per each samples were sampled over a period of time up to 40 h. Temperature steps within different ranges (375, 425, and 475 °C for Cl-free sample; 500, 550, and 600 °C for Cl-containing sample) were chosen. The XRD analysis of samples heated at 900°C revealed that, while the entire Cl-free sample was converted to rutile, only 10% of the Cl-containing sample had transformed to rutile with a very limited crystal growth. Direct comparison of the crystal growth curves show different behaviour which was modelled by different grain growth kinetic equations which include a growth limiting factor. Although the generalized parabolic grain growth model with time-exponent (so called "Höfler-Averback equation") provided good fits for both the Cl-free and Cl-containing nanopowders at all temperature, only the isothermal curve at 500°C of the Cl-containing sample was satisfactorily fitted with a modified KJMA equation. The activation energy of the grain growth are very similar and in line with the previously reported values. We conclude that the crystal size locking phenomenon observed for Cl-bearing anatase can be ascribed to the effects of chlorine ions adsorbed on grain surface as previously suggested. The blocked crystal growth of nanoanatase obtained by the reflux synthesis of organic solvents in the presence of hydrochloric acid as catalyst makes this material very appealing for devices that require a given nanosize and for anatase phase in spite of the high temperature processing.

Selective suppression of {112} anatase facets by fluorination for enhanced TiO2 particle size and phase stability at elevated temperatures

“…UV-Vis measurements showed a significant redshift for the fluorinated samples, when compared with TiO2, at the temperature where the phase transition took place (Figure S5). [44,45,46,47] As discussed earlier, the anatase-to-rutile phase transition temperature can be affected or controlled by how efficiently {112} twin boundaries interact with each other. [25,26] Therefore, we evaluated the effect of fluorine on the {112} facets growth by comparing the equivalent diffraction peaks related to the {112} plane at 38.60° and to {103} plane at 36.95° (PDF number # 21-1272) (see Tables S2 and S3).…”

Investigation on Sensing Performance of Highly Doped Sb/SnO2

“…It is well worth noting that increasing the antimony doping level in SnO 2 usually leads to a decreased particle diameter, which suggests that antimony doping inhibits the coalescence of the SnO 2 -based particles during the powder calcination. Growth of the nanoparticles during high temperature thermal treatment can be explained by referring to the surface energy [ 56 , 57 , 58 ]. Indeed, smaller particle sizes result in high surface energy, which can be conveniently minimized, during the heat treatment by grain coalescence, which naturally results in a reduction of the surface area with a concomitant increase in particle size.…”

“…Indeed, smaller particle sizes result in high surface energy, which can be conveniently minimized, during the heat treatment by grain coalescence, which naturally results in a reduction of the surface area with a concomitant increase in particle size. Thus, the presence of antimony doping leads to a decline of the surface energy of SnO 2 compared to pure SnO 2 , which results in small particle size formation during the nanoparticle synthesis and also hinders nanoparticle coalescence during the heat treatment [ 56 , 58 ].…”

A Semiconducting Material Exhibiting Visible-Light Promoted Photochromism, Photoluminescent and Photocatalytic Activity

“…Such a delay in the ART, when introducing Cu and Nd in the titania system, might be explained by a grain-boundary pinning (Zener drag). 63,64 Unit cell parameters and unit cell volumes are displayed in Table 2. A steady expansion of anatase unit cell parameters (and volume, accordingly), it is seen in the specimens with the addition of copper and neodymium.…”