Atomically-precise Janus polyoxometalate catalyst with tunable water splitting activity

The development of bifunctional catalysts for solar-driven hydrogen and oxygen evolution has been regarded as a challenging but interesting research topic. As promising multi-electron-transfer catalysts, reported polyoxometalate (POM)-based catalysts often contain only a single type of transition metal substitution for driving either hydrogen production or oxygen evolution. Herein, a viable two-step parental substitution approach has been developed to synthesize two structurally-new mixed-transition-metal-substituted polyoxometalates (mixed TMSPs), K6Na4[Mn2Ni2(H2O)2(PW9O34)2]·21H2O ({Mn2Ni2}) and K10[Mn2Co2-(H2O)2(PW9O34)2]·35H2O ({Mn2Co2}), using Na12[Mn2Na2(PW9O34)2]·36H2O ({Mn2Na2}) as the precursor. Characterization results confirmed the nearly quantitative substitution of Na+ with Ni2+ and Co2+ ions. X-ray absorption fine structure (XAFS) spectroscopy revealed that the Mn atoms are preferentially located in the internal positions of the central belt while Ni and Co atoms preferentially reside in the external, solvent-accessible positions. Benefiting from the second substitution of catalytically active transition metals, the resulting {Mn2Ni2} and {Mn2Co2} can be utilized as Janus catalysts towards H2 evolution and O2 evolution under visible light irradiation with greatly-enhanced activity compared to that of parental {Mn2Na2}. The introduction of mixed transition metals into POM structures not only enriches the POMs family, but also provides an effective strategy to control electronic structures and catalytic properties of POM-based catalysts at the atomic level.