Effect of calcination on band gaps for electrospun titania nanofibers heated in air-argon mixtures

Abstract

The relationship between the band gap in electrospun titania nanofibers at ambient temperature and the nature of the air-argon atmosphere in which the material has been heated non-isothermally to 900. °C was investigated by ultraviolet-visible absorption spectrometry at room temperature. The results for heating in different selected air-argon mixtures show that the UV-region band gap found in unheated as-spun amorphous nanofibers, 3.33. eV, may be shifted well into the visible region by calcining in the different air-argon mixtures. The band gap value found for heating in air, 3.09. eV, reduces systematically when the material is heated in an air-argon mixture, with the gap in pure argon being 2.18. eV. The progressive lowering of the band gap is attributed to the development of crystallinity in the fibers as the material is calcined and the associated development of oxygen vacancies when heated in argon, and therefore to the formation of defect states below the conduction band.