Coordination‐Induced Band Gap Reduction in a Metal–Organic Framework

Abstract

Herein, we report on the synthesis of a microporous, three‐dimensional phosphonate metal–organic framework (MOF) with the composition Cu3(H5‐MTPPA)2 ⋅ 2 NMP (H8‐MTPPA=methane tetra‐p‐phenylphosphonic acid and NMP=N‐methyl‐2‐pyrrolidone). This MOF, termed TUB1, has a unique one‐dimensional inorganic building unit composed of square planar and distorted trigonal bipyramidal copper atoms. It possesses a (calculated) BET surface area of 766.2 m2/g after removal of the solvents from the voids. The Tauc plot for TUB1 yields indirect and direct band gaps of 2.4 eV and 2.7 eV, respectively. DFT calculations reveal the existence of two spin‐dependent gaps of 2.60 eV and 0.48 eV for the alpha and beta spins, respectively, with the lowest unoccupied crystal orbital for both gaps predominantly residing on the square planar copper atoms. The projected density of states suggests that the presence of the square planar copper atoms reduces the overall band gap of TUB1, as the beta‐gap for the trigonal bipyramidal copper atoms is 3.72 eV.

Exceptionally Stable And Super‐Efficient Electrocatalysts Derived From Semiconducting Metal Phosphonate Frameworks

“…Hypothetically, the use of narrow band gap or semiconducting MOFs might help improve OER kinetics, and currently there is a limited number of narrow band gap MOFs in the literature [40] . Our research group has recently reported on the electrical conductivity of microporous phosphonate MOFs and layered metal phosphonates [17,20,41–44] . The reported semiconducting phosphonate MOFs by our group have narrow band gaps between 1.4 and 2.5 eV.…”

“…[40] Our research group has recently reported on the electrical conductivity of microporous phosphonate MOFs and layered metal phosphonates. [17,20,[41][42][43][44] The reported semiconducting phosphonate MOFs by our group have narrow band gaps between 1.4 and 2.5 eV. We have previously shown that changing the identity and coordination environment of the metal ions in isostructural MOFs and hydrogen-bonded organic framework (HOFs) are important tools for fine band-gap tuning of the framework.…”

“…We have previously shown that changing the identity and coordination environment of the metal ions in isostructural MOFs and hydrogen-bonded organic framework (HOFs) are important tools for fine band-gap tuning of the framework. [43,45] The band gap of a compound might be an indicator for the ease of an electron transfer step in electrocatalytic reactions. [46,47] For example, the Co 3 + /Co 2 + couple is currently intensely investigated for photocatalytic water splitting because the respective reduction potential at pH = 7 is E 0 ~+ 1.41 V > + 1.23 V for the OER of pure water.…”

“…The reported semiconducting phosphonate MOFs by our group have narrow band gaps between 1.4 and 2.5 eV. We have previously shown that changing the identity and coordination environment of the metal ions in isostructural MOFs and hydrogen‐bonded organic framework (HOFs) are important tools for fine band‐gap tuning of the framework [43,45] . The band gap of a compound might be an indicator for the ease of an electron transfer step in electrocatalytic reactions [46,47] .…”

A Systematic Look at Structural Diversity of Metal Phosphonates

“…Phosphonate-MOFs (MOFs = metal-organic frameworks) have recently been a very active research area, especially with respect to the exceptional thermal and chemical stabilities of phosphonate-MOFs. [1][2][3][4][5][6] The recent developments in phosphonate-MOFs have paved a way to several breakthroughs and generated novel stable microporous materials exhibiting magnetic, semiconducting, and proton conducting properties, and they found further applications in electrocatalysis and photocatalysis. [3,[7][8][9][10][11] The activity in phosphonate MOF research have been documented by many review articles that have been published during the last five years.…”