Electron Dynamics in Ba5Ta4O15 Electrospun Nanofibers Revealed by Transient Absorption Spectroscopy

A nanosecond to millisecond transient absorption (TA) spectroscopy study on electrospun Ba₅Ta₄O₁₅ nanofibers was performed to investigate their charge carrier dynamics and to correlate these to their photocatalytic activity. The shape of the TA spectra of the nanofiber samples differs from the reported powdered transient absorption spectra of Ba₅Ta₄O₁₅, which can be attributed to the morphology differences of the samples, i.e., powdered vs fibrous morphology, which could be revealed by measurements of nanofibers with different nanofiber diameters of 93, 114, and 205 nm. By performing measurements in argon and argon-methanol on undecorated nanofibers and Rh-CrO_x-decorated samples, the absorption signals at 500 nm could be assigned to deep trapped electrons, signals at 675 nm to shallow trapped electrons, and signals at 825 nm could be assigned to free electrons, respectively. TA measurements and decay curve investigations, together with Rietveld refinements of the X-ray diffraction data, further reveal that the photocatalytic activity of the nanofibers depends on the crystallite sizes of the nanofibers and their preferential orientation, and on the nanofiber diameter and surface area. The impact of the nanofiber diameter and surface area on the photocatalytic activity is more pronounced for the cocatalyst-decorated sample, while the impact of the crystallite size is more pronounced for the undecorated samples. Moreover, the charge carrier recombination can be more effectively reduced by the decoration with cocatalyst than by the addition of methanol as a hole scavenger.