Systematic variation of the optical bandgap in titanium based isoreticular metal-organic frameworks for photocatalytic reduction of CO2 under blue light

Article abstract of DOI:10.1039/c7ta00437k

A series of metal-organic frameworks isoreticular to MIL-125-NH₂ were prepared, where the 2-amino-terephthalate organic links feature N-alkyl groups of increasing chain length (from methyl to heptyl) and varying connectivity (primary and secondary). The prepared materials display reduced optical bandgaps correlated with the inductive donor ability of the alkyl substituent as well as high photocatalytic activity towards the reduction of carbon dioxide under blue illumination operating over 120 h. Secondary N-alkyl substitution (isopropyl, cyclopentyl and cyclohexyl) exhibits larger apparent quantum yields than the primary N-alkyl analogs directly related to their longer lived excited-state lifetime. In particular, MIL-125-NHCyp (Cyp = cyclopentyl) exhibits a small bandgap (E_g = 2.30 eV), a long-lived excited-state (τ = 68.8 ns) and a larger apparent quantum yield (Φ_app = 1.80%) compared to the parent MIL-125-NH₂ (E_g = 2.56 eV, Φ_app = 0.31%, τ = 12.8 ns), making it a promising candidate for the next generation of photocatalysts for solar fuel production based on earth-abundant elements.

Full text of DOI:10.1039/c7ta00437k

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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DOI: 10.1039/C7TA00437K
Systematic Variation of the Optical Bandgap in Titanium Based
Isoreticular Metal-Organic Frameworks for Photocatalytic Reduction
of CO2 under Blue Light
Matthew W. Logan,1 Suliman Ayad,2 Jeremy D. Adamson,1 Tristan Dilbeck,2 Kenneth
Hanson,2 and Fernando J. UribeꢀRomo.1*
1
Department of Chemistry, University of Central Florida, 4111 Libra Dr. Rm. 251 PSB,
Orlando, FL 32816ꢀ2366, USA.
2
Department of Chemistry, Florida State University, 95 Chieftan Way Rm. 118 DLC,
Tallahassee, FL 32306ꢀ4390, USA.
* To whom correspondence should be addressed: fernando@ucf.edu
Abstract
A series of metalꢀorganic frameworks isoreticular to MILꢀ125ꢀNH2 were prepared, where
the 2ꢀaminoꢀterephthalate organic links feature Nꢀalkyl groups of increasing chain length
(from methyl to heptyl) and varying connectivity (primary and secondary). The prepared
materials display reduced optical bandgaps correlated to the inductive donor ability of the
alkyl substituent as well as high photocatalytic activity towards the reduction of carbon
dioxide under blue illumination operating over 120 h. Secondary Nꢀalkyl substitution
(isopropyl, cyclopentyl and cyclohexyl) exhibit larger apparent quantum yields than the
primary Nꢀalkyl analogs directly related to their longer lived excitedꢀstate lifetime. In
particular, MILꢀ125ꢀNHCyp (Cyp = cyclopentyl) exhibits a small bandgap (Eg = 2.30
eV), a longꢀlived excitedꢀstate (τs = 68.8 ns) and the larger apparent quantum yield (Φapp
= 1.80%) compared to the parent MILꢀ125ꢀNH2 (Eg = 2.56 eV, Φapp = 0.31%, τs = 12.8
ns), making it a promising candidate for the next generation of photocatalysts for solar
fuel production based on earthꢀabundant elements.

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Introduction
The accumulation of carbon dioxide in the atmosphere as a result of human
activity is one of the largest contributing factors in the gradual warming of the Earth.1
Our planet has a natural system of negative feedback processes to balance these
accumulations, primarily by carbon fixation through photosynthesis. The amount of
carbon dioxide already in the atmosphere is far too large for natural photosynthesis to
reduce its accumulation in the short term. Therefore, it is urgent that technologies be
developed that are capable of fixating large quantities of CO2 into organic matter, which
are both energetically efficient and capable of creating a usable feedstock of reduced
carbon (soꢀcalled solar fuel).2 These new technologies require the use of a naturally
abundant and sustainable source of energy. Early transition metal oxides, in particular
titanium oxides, are capable of reducing CO2 when excited by UV light.3 Using sunlight
to reduce CO2 directly would be ideal; however, the majority of the suns rays are in the
visibleꢀIR range,4 which is too low energy to induce photoreduction with these materials,
limiting their applicability on environmentally relevant scales. Thus, materials that are
able to reduce CO2 with the use of visible light are an imminent and currently unfulfilled
need.
Metalꢀorganic frameworks (MOFs)5 are a class of porous materials composed of
metalꢀoxide clusters connected through organic links forming highly crystalline open
structures that offer a promising alternative to metalꢀoxide semiconductor photocatalysis.
Research in the design and synthesis of new MOFs (reticular chemistry6) has allowed for
the creation of functional materials with unprecedented high porosity (surface areas up to
10 000 m2 gꢀ1)7 and exceptional chemical and thermal stability, with applications in gas
storage,8 gas separation,9 heterogeneous catalysis, 10 payload release,11 and sensing,12
among others. MOFs are materials with extraordinary chemical and functional versatility,
because both the organic and inorganic components can be systematically varied towards
targeted applications in a systematic manner.13
Titanium based MOFs in particular, combine the photocatalytic activity of
titanium oxide clusters with the light absorption properties of organic linkers, producing
materials that can be photocatalytically active under UVꢀvisible light.14 These catalytic
reactions are facilitated through either oxidative,14a, 15 or reductive 16 photoinduced

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electron transfer within the TiꢀMOF under irradiation. MILꢀ125ꢀNH2 (MIL = Materials
Institut Lavoisier) is a titaniumꢀbased MOF constructed from 2ꢀaminoterephthalate and a
Ti8O12 ringꢀshaped cluster (Figure 1) that has shown activity towards photocatalytic
reduction of carbon dioxide.17 The biggest challenge in increasing and optimizing this
Figure 1. Top: Crystal structure of the parent MOF MILꢀ125 utilized in this study.
Bottom: Molecular structure of the organic link in MILꢀ125ꢀNH2 and the Nꢀsubstituted
isoreticular MOFs prepared in this work. Gray spheres = carbon, white = hydrogen, red =
oxygen, blue polyhedral = titanium, the yellow sphere represents the size of the pore in
the MOF.