Defect engineering in MIL-125-(Ti)-NH2 for enhanced photocatalytic H2 generation

Abstract

Pre-designing starting materials is a sensible approach to tailor the synthetic, optoelectronic, and physicochemical properties of a photocatalyst towards higher activity without the need for additional active species. MIL-125-(Ti)-NH₂, a metal–organic framework (MOF) photocatalytically active for H₂ evolution, was first successfully synthesised at a relatively low temperature of 70 °C upon employing pre-designed titanium-oxo-carboxylate clusters. While rearrangement of the original cluster enabled successful MIL-125-(Ti)-NH₂ formation, its ligand stoichiometry favoured MOFs with abundant “defects” at the Ti centres which in turn acted as accessible active sites for H₂ generation. The catalytic sites and their local geometry were studied by pyridine-adsorbed Fourier transform infrared spectroscopy, X-ray absorption near-edge structure, and extended X-ray absorption fine structure. Interestingly, the frameworks prepared using pre-designed titanium-oxo clusters can alter electronic optical properties and energy levels. In the presence of triethanolamine as an electron donor and under visible light irradiation, this led to a ∼3.5 times higher H₂ evolution rate in the titanium-oxo cluster MOF compared to MIL-125-(Ti)-NH₂ obtained by typical hydrothermal synthesis. The obtained catalyst also exhibits a good-reusable performance for at least three consecutive runs without any loss in its reactivity. Pre-designed clusters can be simply utilised to generate accessible active sites and manipulate electrical properties for enhancing catalytic performance.