Band Gap Modulations in UiO Metal–Organic Frameworks

Abstract

We address the metal–organic frameworks UiO-6x (x = 6, 7, 8), their band gaps, and the changes in the band gaps upon perturbations in the metal–organic framework structures. Computational studies were performed with complementary experimental band gap measurements. Band gap modulations upon hydrogen substitutions by NH2 and NO2 on the organic linker, hydroxylation and dehydroxylation of the metal center, different linker lengths (x = 6, 7, 8), and Ti and Hf substitutions for Zr were analyzed in detail. The origin of the band gap changes was thoroughly investigated, and this work confirmed a reduction in the band gap upon NH2 and NO2 substitutions. Furthermore, this work explicitly illustrated that changes in the band gap were also observed by changing the coordination around the Zr atom, whereas isovalent substitutions on the metal center did not yield significant perturbations of the band gap.

Vibrational fingerprint of the absorption properties of UiO-type MOF materials

“…Recent studies on the periodic UiO materials confirmed the experimental band gap modulations via calculations based on density functional theory (DFT) and attributed it to an extra electronic band appearing in the materials band gap [24,25]. Some of the presenting authors also investigated the optical characteristics of mono-and disubstituted UiO-66-X frameworks (with X=OH, NH 2 and SH) using ab initio methods in direct comparison with experimental UV/Vis measurements [23].…”

Electronic effects of ligand substitution on metal–organic framework photocatalysts: the case study of UiO-66

“…In independent studies, Li et al 31 and Matsuoka et al 32 reported the successful tuning of the light absorption of a Ti-based MOF by the introduction of NH 2 groups to the MIL-125(Ti), and its applications in the photocatalytic CO 2 reduction and hydrogen production. [35][36][37][38] The photocatalytic activity of functional MOFs except the NH 2 substituent has not been experimentally uncovered. 33,34 Despite the progress, work in this area is still in its early stages, particularly, we note that, engineering the optical response of the MOF-based photocatalysts by the functionalization of a ligand almost exclusively focuses on the NH 2 substituent.…”

Computational characterization and prediction of metal–organic framework properties

“…A lot of the early work on the modeling of electronic properties of MOFs focused on the characterization of their band gaps, [129] and the possibilities for tuning these by metal exchange [127,181] or ligand functionalization [182] or substitution. [183] Probably one of the most comprehensive examples of this kind of studies is that of Hendon et al, [184] who explored through a combination of synthetic and computational work the influence of linker functionalization on the band gap of MIL-125, a photochromic MOF based on TiO 2 and 1,4-benzenedicarboxylate.…”