Activation energies for phase transformations in electrospun titania nanofibers: comparing the influence of argon and air atmospheres

This paper reports on titania absolute phase level (amorphous, anatase, and rutile forms) changes in electrospun amorphous titania nanofibers from 25 to 900 degrees C in air and argon atmospheres. A novel method was developed to extract absolute levels of amorphous titania and crystalline anatase and rutile from the synchrotron radiation diffraction (SRD) data. This is a sequel to a relative phase concentrations study that has been reported previously by Albetran et al. (Appl Phys A 116: 161 [2014]). Determination of absolute phase levels facilitated estimation of the activation energies for the amorphous-to-anatase transformation of 45(9) kJ/mol in argon and 69(17) in air, and for the anatase-to-rutile transformation energies of 97(7) kJ/mol for argon and 129(5) for air. An activation energy estimate for amorphous-to-crystalline titania in argon of 142(21) kJ/mol, achieved using differential scanning calorimetry (DSC), is consistent with the SRD results. The differences in phase transition and activation energies when the titania nanofibers are heated in argon is attributed to the presence of substantial oxygen vacancies in anatase. Estimates of anatase and rutile oxygen site occupancies from the SRD data show that anatase has discernible oxygen vacancies in argon, which correspond to stoichiometric TiO2-x with x < 0.4 that the anatase stoichiometry in air is TiO2. Rutile does not have significant oxygen vacancies in either argon or air.