Effect of unsaturated coordination on photoelectrochemical properties of Ni-MOF/TiO2 photoanode for water splitting

Abstract

Hydrogen energy has attracted a lot of attention recently, which is expected to replace fossil fuels because it is clean and pollution-free. Photoelectrochemical (PEC) water splitting is an excellent strategy for hydrogen production at a low cost using solar energy. However, pristine TiO2 only shows the expected low photocurrent (0.42 mA/cm2 at 1.23 V vs. RHE), indicating charge separation and transfer efficiencies are not ideal. The robust water splitting reaction has been achieved by metal-organic framework (MOF), duo to the abundant unsaturated metal sites at the edge face of MOF. However, the controllable design of surface coordination environment to directly understand the interaction between unsaturated metal sites and water splitting reaction is still challenging. Here, we propose a secondary coordination strategy for selectively quenching the unsaturated metal sites of MOF with guest small ligands, in order to establish a contrast system based on the MOF with unsaturated metal sites. TiO2 decorated with Ni-MOF was applied as a genuine mode, of which the PEC water splitting revealed the practical function of unsaturated Ni2+ sites. Ligands including PTA, Ac−, PO43−, C2O42− and EDTA2− manifested an obvious secondary coordination toward Ni2+ sites, and the photocurrent densities regularly decayed, indicating that the reduction of unsaturated Ni2+ sites depress the water splitting reaction. Meanwhile, the difference of the size and coordination ability also influenced on the photocurrent density, attributed to their diversity of secondary coordination mode of unsaturated Ni2+ sites and ligands. The samples treated by secondary coordination were systematically evaluated through Mott-Schottky plots and photoelectrochemical impedance spectrum (PEIS), compared with TiO2-MOF. Therefore, this work provides a new insight into the unsaturated metal sites for water splitting, which theoretically supports the rational fabrication of efficient photoelectrode in PEC system.