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

• Published in: Applied materials today 2024-04, Vol.37, p.102094, Article 102094
• Main Authors: Andrade, Pedro H.M., Volkringer, Christophe, Loiseau, Thierry, Tejeda, Antonio, Hureau, Matthieu, Moissette, Alain
• Format: Article
• Language: English
• Subjects: Band gap; Boltzmann regression; Chemical Sciences; Inorganic chemistry; Kramers-Kronig; MOF; or physical chemistry; Theoretical and; UV–vis
• ISSN: 2352-9407; 2352-9415
• DOI: 10.1016/j.apmt.2024.102094

•Challenges in determining whether a material has a direct or an indirect band gap were discussed.•The study proposes methodologies for accurate band gap determination in MOF materials.•The paper underscores the significance of pre-data treatment and baseline correction, particularly when a pre-absorption edge is present.•A robust methodology combining Boltzmann regression and Kramers-Kronig transformation was proposed for determining whether a material has a direct or indirect gap.•The approach presented here offers a comparative method without relying on computer simulations, enhancing the reliability of band gap assessments. 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 s