Examining the anisotropic behavior of the excitons in anatase TiO2 by angle-resolved electron energy-loss spectroscopy

The behavior of excitons excited in anatase crystalline materials affects the photocatalytic performance of the materials. Thus, investigating the properties of such excitons is essential for understanding the origin of their influence on photocatalytic performance. Here, angle-resolved electron energy-loss spectroscopy was applied to evaluate the size of the spatial spread of excitons in anatase TiO2. Three kinds of excitons (I-III), which were reported by polarized light absorption experiments, were experimentally identified in the electron energy-loss spectroscopy (EELS) spectra. Exciton size was evaluated from absorption intensity, which was derived by the Kramers-Kronig analysis of the EELS spectra, depending on the momentum transfer (q). The sizes of excitons I, II, and III were evaluated to be 8, 5, and 6 nm, respectively. The larger size of exciton I than that of III was with the same tendency as in the theoretical results. Exciton II, which was evaluated as a delocalized one, was evaluated to have a finite size in this experiment. The largest size of exciton I, approximately 8 nm, is the same order of the exciton diffusion lengths of the material. Therefore, exciton I should significantly influence the photocatalytic activity of anatase.

Automated summary

Investigating the behavior of excitons in anatase crystalline materials and their influence on photocatalytic performance is crucial in understanding their properties. Angle-resolved electron energy-loss spectroscopy was used to evaluate the spatial spread of excitons in anatase TiO2. Three types of excitons (I-III), identified through polarized light absorption experiments, were also experimentally detected in the electron energy-loss spectroscopy spectra. The size of each exciton was evaluated based on the absorption intensity derived from the Kramers-Kronig analysis of the spectra. Excitons I, II, and III were found to have sizes of 8, 5, and 6 nm, respectively. Exciton I, being larger than III, exhibits a similar trend as the theoretical results. Exciton II, initially believed to be delocalized, was found to have a finite size in this experiment. The maximum size of exciton I, around 8 nm, is comparable to the exciton diffusion lengths of the material, indicating its significant influence on the photocatalytic activity of anatase.