Determination of the Semiconductors Band Gap by UV-Vis Diffuse Reflectance Spectroscopy

For the application of semiconductors, an important factor is the band gap, i.e., the minimum energy required for the transfer of electrons from the valence to the conduction band. One of the possible methods for band gap determination is diffuse reflectance spectroscopy and Tauc plot. In this paper, an overview of the terms and equations related to the said method is given, as well as its utilization in the determination of band gaps of commercial samples of various metal oxides. Thus, the procedure is demonstrated and evaluated through the determination of indirect and direct band gap values of anatase (TiO2), rutile (TiO2), zincite (ZnO), and hematite (Fe2O3). All samples were beforehand analysed and identified by X-ray powder diffraction on Shimadzu XRD 6000 diffractometer with CuKa radiation working in a step scan mode with steps of 0.02 degrees and counting time of 0.6 s. It was determined that all samples are well crystallized with relatively large crystallite sizes. UV-Vis spectra of the samples, as well as barite, which was used as a reference, were obtained on the UV-Vis spectrometer with an integrating sphere in total reflectance mode. The UV-Vis DRS spectra were transformed to Kubelka-Munk function, after which Tauc plot was used for the determination of the indirect and direct band gap values of all samples. The obtained values for anatase were 3.20 eV for indirect transition and 3.41 eV for direct transition, and for rutile 3.00 eV for indirect transition and 3.11 eV for direct transition. The zincite sample showed an indirect band gap of 3.19 eV and direct band gap of 3.25 eV, while the obtained indirect band gap value for hematite was 1.96 eV and direct band gap value 2.15 eV. As may be seen, the method is not particularly useful when distinguishing direct from indirect semiconductors, since, for all samples, the curves in Tauc plot for both indirect and direct electron transitions possess a linear dependence region from which the band gap value is estimated. However, the obtained band gap values for all the studied semiconductors are in relatively good concordance with literature references. The method is perhaps most useful in monitoring the variation of band gap depending on the dopant content. Namely, the studied metal oxides are used in photocatalysis where the addition of dopants is expected to reduce the band gap to visible light area, and thus improve the photocatalytic activity of the semiconductor. It can be concluded that the Tauc method is not perfect in terms of accuracy and differentiation between indirect and direct electron transitions in semiconductors. Nevertheless, it is a very practical way of band gap assessment for semiconducting materials, because it requires no excessively expensive instrumentation, and the processing of experimental data is rather simple.