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Dye-Sensitized Metal-Organic Frameworks for Photocatalytic Water Splitting

Final Report Summary - PHOTOCATMOF (Dye-Sensitized Metal-Organic Frameworks for Photocatalytic Water Splitting)

Metal-organic frameworks (MOFs) 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. 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. 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 energy-driven 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, 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)) containing 2-amino-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 NH2-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.
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.

A Ti-based cluster [Ti8O8(OOCC6H5)16] 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. Co-catalysts 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. MOG is a kind of amorphous, cross-linking structure of agglomerated nanoparticles which are comprised of polymers of metal-organic coordination units. 1H and 13C NMR and element analyses suggested the MOG are comprised 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. 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 and even under illumination of visible light (>420nm), the observed transient current density is up to 5 µA/cm2.

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. 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, Nb-based 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; alpha = gamma = 90°, beta = 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.

Zr-TCPP MOF (denoted as PCN222) is built from Zr-oxo SBUs and the TCPP linker. This MOF has 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.