PUBLISHABLE SUMMARY
Metal-organic frameworks (MOFs) 1 have attracted intensive research interest owing to their
topological diversity, extraordinarily high porosity, tunable pore sizes, tailorable functionalization and
flexibility, and subsequent applications in gas storage and separation, heterogeneous catalysis and
optical, electronic, magnetic properties etc. 2 In general, MOFs are comprised of two main
components: organic linkers and secondary building units (SBUs). The linkers act as ‘struts’ that
bridge SBUs, which in turn act as ‘joints’ in the resulting MOF architecture. Very recently, the concept
of semiconductive MOFs has been proposed and confirmed.3 Photo-induced charge separation, the
hallmark of a semiconductor, has already been experimentally evidenced by laser flash photolysis.
The project is targeted at the exploration of MOFs as photocatalysts for H2 generation. Solar energydriven renewable and clean hydrogen energy could transform the supply of carbon free fuel and make an
enormous impact on the viability of hydrogen as an energy carrier. In comparison with the conventional
inorganic semiconductor photocatalysts, porous, tunable and modifiable MOF materials could be
developed as a new generation of photocatalyst for H2 generation.
In term of photocatalysis, MOFs can be considered as a matrix of semiconductor quantum dots
(secondary building units, SBUs) linked by organic sensitizers (organic linkers). There are three
possible active centers responsible for photocatalytic activities of MOF materials: i) SBUs; ii) organic
motifs (organic linkers); iii) charge transfer from organic linkers to SBUs. The band gap of most
transition metal-based SBUs is too large to efficiently utilise visible light and the conduction band
energy level is too high to be sensitized with organic linkers. In the existing literature, two
photocatalytic mechanisms have been proposed for H2 generation. In the case of Al-PMOF, 4 the
organic linker TCPP (TCPP = meso-Tetra(4-carboxyphenyl)porphyrin) produces H2 upon visible light
illumination, where the electron from TCPP cannot migrate to the SBUs comprised of Al-oxo chains
owing to their higher energy levels. In contrast, a Ti-based MOF ((NH2-MIL-125(Ti))5 containing 2amino-benzenedicarboxylates (NH2-BDC) and Ti-oxo SBUs demonstrated electron transfer from
organic linkers to the SBUs owing to the matched LUMO orbital energy level between NH 2-BDC and
the conduction band energy level of the Ti-oxo SBUs. For H2 generation, in most case co-catalysts are
needed to help photo-generated charge separation and catalyse the reaction of H2 formation. It is
found in this project that a Ti-based cluster having similar structure with SBU in NH2-MIL-125(Ti)
can produce H2 without any co-catalysts. It is noteworthy that the band gaps and energy levels of the
SBUs deviate from their corresponding oxides; however, metal-oxo SBUs demonstrate comparable
behaviours with their corresponding oxides.6
1 H. Li, M, Eddaoudi, M. O'Keeffe, O. M. Yaghi, Nature, 1999, 402, 276.
2 For a selection of current reviews, see the themed issue on “Metal Organic Frameworks”: Long J, Yaghi O, Eds. Chem.
Soc. Rev., 2009, 38, 1203–1508 and “2012 Metal Organic Frameworks”: H-C. Zhou, J. R. Long, O. M. Yaghi, Chem. Rev.,
2012, 112, 673.
3 a) F. X. L. Xamena, A. Corma, H. Garcia, J. Phys. Chem. C, 2007, 111, 80. b) C. G. Silva, A. Corma, H. García, J. Mater.
Chem., 2010, 20, 3141.
4 A. Fateeva, P. A. , Chater, C. P. Ireland, A. A. Tahir, Y. Z. Khimyak, P. V. Wiper, J. R. Darwent, M. J. Rosseinsky, Angew.
Chem., Int. Ed. 2012, 51, 7440.
5 a) Y. Fu, D. Sun, Y. Chen, R. Huang, Z. Ding, X. Fu, Z. Li, Angew. Chem., Int. Ed. 2012, 51, 3364. b) Y. Horiuchi, T.
Toyao, M. Saito, K. Mochizuki, M. Iwata, H. Higashimura, M. Anpo, M. Matsuoka, J. Phys. Chem. C 2012, 116, 20848.
6 a) S. Bordiga, C. Lambertia, G. Ricchiardia, L. Reglia, F. Boninoa, A. Damina, K.-P. Lillerudb, M. Bjorgenb, A. Zecchina,
Chem. Commun. 2004, 2300. b) M. Alvaro, E. Carbonell, B. Ferrer, F.X. Llabrés i Xamena, H. Garcia, Chem. Eur. J. 2007,
13, 5106.
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Based on the understanding described above, the project took two coupled approaches: on one side,
some reported MOFs were selected for photocatalysis evaluation and on the other hand, some new
MOFs were designed for photocatalytic H2 generation. Most of the organic linkers in MOFs are not
capable of absorbing visible light which prevents MOF materials from efficiently utilising solar
energy. TCPP (TCPP = meso-tetra(4-carboxyphenyl)porphyrin), a porphyrin derivative with four
carboxylate groups and visible light absorption up to 700 nm, is an excellent candidate for building
photocatalytic MOFs. Since titanium oxide is a good semiconductor with an appropriate conduction
band energy level (ca. -0.1 eV vs NHE) for H2 generation, the research focused on Ti-based MOFs
syntheses. Elements adjacent to Ti, like Zr, V, Nb, etc. were also under investigation due to their
similar properties.
Table 1 H2 generation performance of
[Ti8O8(OOCC6H5)16 in comparison with TiO2
(P25)
and
NH2-MIL-125(Ti).
a
In a typical experiment, 35 mg photocatalyst was
placed in 35 ml distilled H2O or H2O/MeOH (v/v = 1:1)
in a 44 ml Pyrex vial. The vial was exposed to Xe-lamp
(300 W) irradiation with a filter cutting off the light >
420 nm.
Figure
1
Structural
representation
[Ti8O8(OOCC6H5)16 along b axis.
A Ti-based cluster [Ti8O8(OOCC6H5)16]7 (Figure 1) was selected as starting point for H2 generation
study, because this Ti-oxo cluster is structurally similar to SBUs in MOF of NH2-MIL-125(Ti) and the
cluster is water-stable. The band gap of the cluster was determined as 3.25 eV by UV-Vis diffuse
reflectance, which is very close to that of TiO2 (3.2 eV). Under ultraviolet irradiation, both TiO2 (P25)
and NH2-MIL-125(Ti) are not able to produce any H2 from distilled H2O or the mixture of
H2O/MeOH. In contrast, [Ti8O8(OOCC6H5)16] produce a remarkable amount of H2 (17 µmol/g/h) from
the mixture of H2O/MeOH without any precious metal co-catalyst, which is a noteable difference
from oxides such as TiO2 which often require co-catalysts that are critical elements (Table 1). Cocatalysts are usually noble metal nanoparticles, which are costly and of low abundance in earth.
Therefore, developing co-catalyst free photocatalysts like [Ti8O8(OOCC6H5)16] for H2 generation is
important. The powder X-ray diffraction implied the cluster lost its crystallinity. However, 1H-NMR
of the cluster before and after photocatalysis remains unchanged, which reveals the cluster molecule
remains intact during the photocatalytic process.
[Ti8O8(OOCC6H5)16 does not adsorb visible light because it has no chromophore and its band gap is
large. In order to make Ti-based materials responsive to visible light, the syntheses of materials
containing Ti-N bonds through the use of coordinating N-contained ligand was investigated. Heating
Ti(OBu)4 and 3-amino-1H-pyrazole-4-carboxylic acid in DMF solution with triethanolamine,
produced a metal-organic gel (MOG) instead of a MOF (Figure 2a). MOG is a kind of amorphous,
cross-linking structure of agglomerated nanoparticles which are comprised of polymers of metalorganic coordination units. 1H and 13C NMR and element analyses suggested the MOG are comprised
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T. Sébastien, C. Guillaume, L. Capucine, S. M. Popall, C. Sanchez, L. Rozes, Eur. J. Inorg. Chem. 2010, 5650–5659
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of Ti and organic linker. The supercritical CO2 activated aerogel displays a BET surface area of ca.
400 m2/g. The MOG shows visible light absorbance up to 900 nm. However, the MOG is not able to
produce H2 under visible light in a H2O/MeOH solution with or without Pt nanoparticles as cocatalyst,
whereas a H2 generation rate of 380 µmol/g/h was reached under UV irradiation (Figure 2b). This
means the energy of visible-light generated electron is not high enough to drive the reaction of H2
formation. Nevertheless, this photo-generated electron both from UV and visible irradiation can
produce photocurrent. The MOG thin film was prepared by spray method on platinized fluorine doped
tin oxide (FTO) glass substrates for photovoltaic property study, revealing a transient current of 50
µA/cm2 at a power of 100 mW/m2 under AM 1.5 (air mass) radiation (Figure 2c) and even under
illumination of visible light (>420nm), the observed transient current density is up to 5 µA/cm2.
(Figure 2c)
Figure 2 (a) images of Ti-based sol-gel, (b) H2 generation performance of MOG sample and (c)
transient current density under visible (> 420nm, red curve) and AM1.5 (black curve) irradiation.
In order to enable Ti-based materials active in visible light irradiation, the visible light absorbing
TCPP was selected as the organic ligands for Ti-based MOF synthesis. Microwave-assisted
hydrothermal reaction of Ti(isoproxide)4 and TCPP produced a crystalline Ti-TCPP material with a
BET surface area ca. 1400 m2/g, which is isostructural to the reported Al-TCPP MOF.4 Nevertheless,
this material is not stable in water. So it is not applicable for H2 generation from aqueous solution. We
have recently discovered new materials with similar structures that are stable over a wide pH range
(3-11) and at up to 100 oC in water.
Niobium is adjacent to Titanium in the
periodic table of the elements. However, Nbbased MOFs are still unexplored with no
structures reported in the Cambridge Structural
Database. This project has produced the first
Nb-based framework (Nb-TCPP), synthesised
by solvothermal reaction. The compound
crystallized in the monoclinic system (Space
group: C2/c, a = 7.5012, b = 24.212, c=
30.3700; α = γ = 90°, β = 94.531°), where each
Nb atom adapts eight-coordination geometry
by coordinating with four carboxylate group
from different TCPP ligands. The framework
is a 3-fold interpenetrating structure as
displayed in Figure 3.
Figure 3 (a) single network of NbTCPP with
the niobium coordination geometry (inserted)
and (b) 3-fold interpenetrating feature of the
NbTCPP compound.
Zr-TCPP MOF (denoted as PCN222) is built from Zr-oxo SBUs and the TCPP linker.8 This MOF has
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a mesoporous channel with a diameter of 3.7 nm and superior water stability. Therefore, PCN222 is
an excellent candidate for photocatalytic H2 generation. Photocatalytic H2 generation experiments
under visible light illumination (>420 nm) gave rise to a H2 generation rate of 30 µmol/h/g. Similar to
Al-TCPP, the excited electron from TCPP is not able to transfer to the Zr-oxo SBUs because of their
high conduction band energy level, which is not favorable for efficient charge separation. Preventing
electron-hole recombination and improving the charge separation are critical to enhance
photocatalytic performance, and this was achieved by chemical manipulation to generate a much
higher H2 generation rate of 80 µmol/h/g.
In summary, a series of compounds have been prepared and characterized and photocatalytic H2
generation properties have been studied. The feasibility of metal-organic framework materials as
photocatalysts for H2 generation has been generated, and the electron-hole dynamics within the MOF
materials controlled to further increase the H2 generation rate. However, in order to compete with
traditional inorganic photocatalysts, the stability and efficiency of MOF-based photocatalysts need to
be further improved through further synthesis and characterisation efforts, in order to realise the
promise of this class of materials.
CONTACT DETAILS:
Dr. Bo Liu
Tel: +44 (0) 151 794 3542
Research Fellow
bo.liu@liverpool.ac.uk
Department of Chemistry
University of Liverpool
Crown Street
L69 7ZD, Liverpool
8 D. Feng, Z. Gu, J. Li, H. Jiang, Z. Wei, H-C. Zhou, Angew. Chem. Int. Ed. 2012, 51, 10307.
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