Bismuth pyrochlores with varying Fe/Co ratio for efficient Multi-functional Catalysis: Structure evolution versus Photo- and Electro-catalytic activities

Bismuth pyrochlores that the crystal structure tolerates lower site misplacements and higher vacancy concentrations exhibit unique stereo-chemical activity, therefore, the manipulation of the property by structural engineering has driven significant research activity, with the goal of realizing their transformative functional potential. Herein, an excellent platform to investigate the role of the strucutre in tuning catalytic activity is provided by varying Fe/Co ratio in a bismuth niobate pyrochlore oxide. Continuous Co-level from 0 to 100% was ingeniously designed on the basis of special phase diagrams and pryocholore-type pure samples were successfully fabricated by a modified sol-gel auto-combustion method. The effect of structural modulation on dielectric property and multi-functional catalytic activities were studied. Significantly, two opposing strucutral shifts were observed with increasing the Co-level, ascribled to the competition between displacive disorders on the Bi sites and strucutral distortions in the octahedral network. Tunable dielectric relaxation was exhibited by electrical modulus spectroscopy and one additional hidden relaxation process implies an outstanding thermal motion of electrons from variable oxidation states of Fe3+/ Co2+ ions when ionized oxygen vacancy becomes weak. For photocatalysis, the low-level Co showed a best visible-light-responsive photocatalytic activity by means of the RhB-dye decomposition, while the high-level Co exhibited a higher photoinduced current density. For electrocatalysis, the sample that maintains its structure with the coexistence of massive oxygen vacancies and low-spinstate Co2+ present a superior oxygen evolution reaction (OER) activity, with a lower OER overpotential (-300 mV at 10 mA/cm2) and a smaller Tafel slope (-70 mV/dec). Moreover, the underlying mechanism of the improved performances was analyzed in detail and the new insights were provied for constructing single-phase multi-functioning catalysts.