TiO2 nanosheets on MOF cubes

16 Nov 2015 NUS professors demonstrated that a heterojunction material consisting of MIL-125 (Ti)-NH₂ cubes and TiO₂ nanosheets shows high photocatalytic activity.

A team led by Prof Stephan JAENICKE from the Department of Chemistry in NUS demonstrated that a heterojunction material made from the metal-organic framework MIL-125(Ti)-NH₂ and TiO₂ nanosheets shows high photocatalytic activity. The efficiency of a heterogeneous photocatalyst depends on two factors: the primary quantum yield of electron-hole pair generation, which is related to the absorption spectrum of the material, and the rate of charge separation and suppression of subsequent recombination. The latter determines how many electrons and holes can reach the surface of the particle where they can initiate chemical reactions such as the oxidation of environmental pollutants.

Metal-organic frameworks are a promising new class of porous solid materials with potential applications in catalysis and environmental remediation. They have frameworks structures formed by inorganic cluster cations that are connected in three dimensions by organic linkers. Photon absorption can take place in the organic linker, or in the inorganic oxo-complexes of the nodes. This gives considerable freedom for the rational design of novel photocatalysts.

The Ti-based metal organic framework Ti₈O₈(OH)₄(bdc-NH₂)₆(NH₂-MIL-125(Ti)) with an amino-group bearing linker is a visible-light-activated photocatalyst, which has been used to reduce CO₂ and to produce hydrogen. Its photo-activity is based on ligand-to-metal charge transfer. It has now been found that the separation of the photogenerated charges by vectorial electron transfer from NH₂-MIL-125(Ti) over a heterojunction into TiO₂ nanoparticles greatly improves the quantum yield (see Figure). Mr Jiulong SUN devised a facile one-pot solvothermal preparation of such heterojunctions. Notably, the optimised material exhibited enhanced photoactivity under visible-light, which offers prospects for solar photo-remediation of wastewater and water-splitting for hydrogen generation.

This work won a best poster award at the Tokyo Conference on Advanced Catalytic Science and Technology (TOCAT-7 KYOTO 2014).

Electron micrograph (a) shows smooth particles of the MOF with the characteristic cubic crystal habit, but with more Ti-precurser in the synthesis gel (b-d), TiO₂ nanosheet structures form at the surface, and the crystallites of the ordered MOF structure become smaller. The thickness of the TiO₂ nanosheets is around 8.2 nm as deduced from the micrographs. [Image credit: Jaenicke Stephan]

Reference

Jaenicke S, Sun J, Han A, Chuah GK. ”MIL-125(Ti)-NH₂–TiO₂ heterojunctions as new approach to visible light photocatalysts.” TOCAT-7 (Kyoto, Japan 2014) poster.