Kinetics of the Anatase‐Rutile Transformation

Abstract

Transformation of three commercial anatase powders resulted in sigmoid curves of transformation vs. time, which were interpreted in terms of a nucleation‐growth process. Analysis of these curves by several rate laws led to activation energies for nucleation of 109, 136, and 148 kcal/mole and for growth of 100, 115, and 190 kcal/mole, respectively. The rate of transformation and activation energy are governed by the nature and amount of impurities which determine the defect structure of the TiO2, i.e. the concentration of oxygen vacancies or interstitials. It is suggested that, in general, oxygen vacancies accelerate and the interstitials inhibit the transformation. CuO additions and hydrogen atmospheres accelerated the transformation by the introduction of oxygen vacancies and/or by reduction to a second TinO2n−1 Magnéli phase which could act as a nucleating agent. The transformation is retarded in vacuum; this retardation is attributed to the introduction of titanium interstitials.

Dispersion of mixtures of submicrometer and nanometre sized titanias to obtain porous bodies

“…The first one, occurring at about 1060 ºC was identified as the transformation of anatase to rutile, which involves a sudden shrinkage of about 8.5 vol.% (calculated using the theoretical densities of anatase−TiO 2 , 3.89 g/cm 3 , and rutile-TiO 2 , 4.25 g/cm 3 ). This transformation can occur between 400 and 1200 ºC depending on several parameters, such as grain size, impurities and atmosphere [38,39]. The second slope change (∼1220 ºC) corresponded to the maximum shrinkage rate and it was wider than the previous one indicating that it is a progressive process.…”

Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies

“…Briefly, as oxygen becomes depleted in the anatase phase in an atmosphere with a very low partial pressure of oxygen, the rate of anatase to rutile transformation can be greatly increased, resulting in an anatase to rutile phase transformation, a process that normally requires temperatures in excess of 700 C. Oxygen depletion results in the spatial disturbance of the oxygen framework, which, in turn, results in the breakage of a number of Ti-O bonds and the rearrangement of the anatase pseudo-closepacked planes of oxygen {112} to the rutile close-packed planes {100}. 35 The main change induced by oxygen depletion is the strong enhancement, change of position, and broadening of the main rutile peak. Further oxygen depletion of the structure, for the samples heat-treated at 630 C for two hours in air and argon (III) or only argon atmosphere (IV), makes the rutile band broader and causes a shift to 438 cm À1 , as seen in Fig.…”

“…19 Furthermore, the kinetics of the phase transformation from anatase to rutile (A / R) is well studied by different researchers. [35][36][37] It is well understood that in the A / R transformation, the Ti-O bonds of the anatase phase structure break and are rearranged to form rutile. Oxygen vacancies overcome the activation energy of this transformation and accelerate it.…”

Control of nanoparticle size and agglomeration through electric-field-enhanced flame synthesis

“…9 Specific surface area of titania powder synthesized as a function of applied electric field. d BET represents the equivalent particle diameter based on BET surface area measurement rearrangement (where oxygens are in a distorted face-centered cubic arrangement for anatase, while for rutile, they are in a distorted hexagonal arrangement (Shannon and Pask 1964)). However, in our setup, particle residence time is on the order of 12-30 ms.…”