Hierarchical Ti-Based MOF with Embedded RuO2 Nanoparticles: a Highly Efficient Photoelectrode for Visible Light Water Oxidation

Abstract

The Ti-based metal–organic framework (MOF) MIL-125-NH2 is one of the promising materials for solar water splitting because it contains a sensitizer and a catalytic center in a single structure. MIL-125-NH2 as many other MOFs has a microporous structure with pore diameters less than 2 nm. Compared with common MOFs, hierarchical mesoporous materials exhibit very large specific surface areas that facilitate diffusion of active species, accelerate subsequent surface reactions, and increase the bubble release rate by providing larger free spaces. Thus, the development of a facile method to create hierarchical porous MOFs with larger pore sizes remains a chemical challenge. Furthermore, MOF-type semiconducting materials usually have low activities in oxygen evolution reaction, and the presence of a suitable cocatalyst is needed to reduce the large O2 overpotential. This study attempted to generate a hierarchical MIL-125-NH2 MOF material with embedded RuO2 nanoparticles as a highly efficient cocatalyst in a simple one-step process for use in efficient solar water oxidation. Different amounts of RuCl3·H2O precursor salt were used simultaneously for creating hierarchical porosity in MIL-125-NH2 and for producing the assumed RuO2 cocatalyst. For comparison, a hydrochloric acid treatment was applied to generate hierarchical porosity in the MOF in the absence of ruthenium. The samples were characterized using high-resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller adsorption, powder X-ray diffraction, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy. HRTEM gave evidence that in the ruthenium oxide-containing MIL-125-NH2 samples, tetragonal RuO2 nanoparticles are present. The materials were applied as photoelectrodes, and photoelectrochemical (PEC) water oxidation performance under visible light illumination was studied. The PEC water oxidation performance of the MIL-125-NH2 layer could be strikingly improved with a photocurrent density of about 10 times more than that of the pure MOF at 1.23 V versus reversible hydrogen electrode in artificial seawater, as a result of the hierarchical MOF structure and the presence of RuO2 as a cocatalyst. Furthermore, density functional theory calculations were performed to shed light on the electronic properties and the role of the RuO2 in the assumed hole transport.