Mixed metal-metal organic frameworks (MM-MOFs) and their use as efficient photocatalysts for hydrogen evolution from water splitting reactions

• Applications of Mixed-Metal Metal Organic Frameworks (MM-MOFs) for photocatalytic hydrogen evolution from water splitting reaction are highlighted. • Induction of heterogeneity in the homometallic MOFs significantly improves the photocatalytic activity and hydrogen evolution rates. • Synergistic interactions between the metal ions reduce the optical band gap for the water splitting reaction. • Presence of different metals in the frameworks reduces the recombination of photo-generated electrons and holes. • Hydrogen evolution rates with reference to the synthetic methods, co-catalyst, photosensitizer, sacrificial electron donor, solvent, light source and pH are summarized. Sustainable energy is the need of the hour to cater the global energy demands with zero carbon emissions. Solar energy is the ultimate, clean and renewable form of energy that can be harvested to split water molecules into molecular hydrogen, a clean fuel with energy density 120 MJ Kg −1 . Homometallic metal organic frameworks (MOFs), an emerging class of porous crystalline materials with tunable porosity and structural controllability, chemical and thermal stabilities, and recyclability, are proclaimed as potent semiconducting photocatalysts for water splitting reactions. Despite these peculiar advantages, there are still many open challenges to tailor MOFs as well-versed photocatalysts because of their certain inadequacies such as UV activity and limited catalytic efficiency. Introduction of structural heterogeneity into the MOF systems via the incorporation of other metal ions or the different organic linkers is envisioned as an unambiguous strategy to ameliorate the performance of the MOFs. Subsequently, the heterogeneity in the mixed metal-MOFs (MM-MOFs) infers the synergistic interactions between the metal orbitals within the framework to bring accentuated enhancement in the catalytic properties, and encapsulation of photoactive antennae leading to the improved photosensitizing properties in the visible region. This review article summarizes the various synthetic approaches to MM-MOFs with their applications in photocatalytic hydrogen production by water splitting reactions.