Extraction of band gap energies and composition of mixed-phase polycrystalline semiconductors; a possible alternative...

The optical band gap energy,E_g, of semiconductors is routinely determined using UV-Vis absorption measurements followed by Tauc-plot analysis. This method requires knowledge of the Tauc exponentn, which corresponds to the nature of the electronic transition. While Tauc analysis is effective for single-phase semiconductors, it may not be applicable to mixed-phase materials that exhibit two different types of transitions. In this work, the absorbance of mixed-phase TiO₂(anatase and rutile) with varying compositions is examined. Anatase has an indirect band gap (n= 2), whereas rutile has a direct band gap (n= ½). It is shown that the first derivative of the absorbance with respect to the wavelength, dA/dλ, yields phase-composition information largely similar to that obtained from XRD analysis. Furthermore, the positive peak of the second derivative, d²A/dλ², provides a band-gap value that does not shift (within <0.05 eV) with changes in TiO₂phase composition. The method was also tested on two additional semiconductors (ZnO and g-C₃N₄), yielding band-gap energies comparable to those obtained using established techniques. These results demonstrate that the band gap energy can be determined without prior knowledge of the transition type by using the second derivative of the absorbance.