Band gap analysis in MOF materials: Distinguishing direct and indirect transitions using UV–vis spectroscopy

Metal-Organic Frameworks (MOFs) have gained considerable attention due to their potential applications in gas storage, separation, and catalysis. These porous materials exhibit properties of interest for semiconductor physics and homogeneous photocatalysis, in which concepts from coordination chemistry and semiconductor physics are often mixed. In the photocatalysis field, the optical band gap of the semiconductors is a crucial parameter that determine their functionality. Despite all the interest of MOFs, there is still a considerable lack of information about their band gap evaluation (especially if the gap is direct or indirect) using UV–Vis spectroscopy, and there is a considerable scattering in these values. The Tauc plot method is frequently used to access band gaps, even though it is not always accurate, especially for distinguishing direct and indirect band gaps. A more complete and precise analysis can be reached by using additional experimental techniques (XPS, UPS, and IPES spectroscopies), that are not always of easy access. This work examines several approaches for determining the band gap of MOF materials and proposes methodologies for a correct data interpretation, which leads to a better agreement between experimental and theoretical gaps. Several methods were analyzed to access the band gap of different MOF materials – UiO-66 (Zr), UiO-66 (Hf), UiO-66 (Zr/Ti), UiO-66 (Hf/Ti), UiO-67 (Zr) NH2, UiO-67 (Zr/Hf) NH2, UiO-67 (Hf) NH2, MIL-125 (Ti), and MIL-125 (Ti) NH2 – purely from diffuse reflectance UV–vis (DR-UV–vis) data. The Kubelka-Munk and log (1/R) approaches were considered for transforming the DR-UV–vis spectra and the results demonstrate that the former method is more suitable, as it provides spectra with sharper absorption edges, which facilitates the interpretation and characterization of the optical band gaps. This study also highlights the importance of pre-data treatment and baseline correction in cases where a pre-absorption edge is present. Finally, by applying the Kramers-Kronig transformation to the reflectance spectra, and the Boltzmann regression to the Kubelka-Munk data, a solid base was created for determining if a material has a direct or an indirect gap. In addition, for some materials, the need for acquiring both the indirect and direct band gap values was discussed, as in some of these hybrid materials, both of these transitions can occur simultaneously. This paper guides the research community towards a most suitable methodology for assessing optical band gaps in hybrid materials, as it assists researchers in selecting the best methodology for their needs while avoiding typical mistakes in data interpretation.