Visualizing the Nano Cocatalyst Aligned Electric Fields on Single Photocatalyst Particles

The cocatalysts or dual cocatalysts of photocatalysts are indispensable for high efficiency in artificial photosynthesis for solar fuel production. However, the reaction activity increased by cocatalysts cannot be directly ascribed to the accelerated catalytic kinetics, since photogenerated charges are involved in the elementary steps of photocatalytic reactions. To date, diverging views about cocatalysts show that their exact role for photocatalysis is not well understood yet. Herein, we image directly the local separation of photogenerated charge carriers across single crystals of the BiVO₄ photocatalyst which loaded locally with nanoparticles of a MnO_x single cocatalyst or with nanoparticles of a spatially separated MnO_x and Pt dual cocatalyst. The deposition of the single cocatalyst resulted not only in a strong increase of the interfacial charge transfer but also, surprisingly, in a change of the direction of built-in electric fields beneath the uncovered surface of the photocatalyst. The additive electric fields caused a strong increase of local surface photovoltage signals (up to 80 times) and correlated with the increase of the photocatalytic performance. The local electric fields were further increased (up to 2.5 kV·cm^-1) by a synergetic effect of the spatially separated dual cocatalysts. The results reveal that cocatalyst has a conclusive effect on charge separation in photocatalyst particle by aligning the vectors of built-in electric fields in the photocatalyst particle. This effect is beyond its catalytic function in thermal catalysis.