Scientists Jian Zhu, Fengtao Fan and Can Li et al. at Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), imaged directly the local separation of photogenerated charge carriers across cocatalyst loaded photocatalyst particles, by home-made surface photovoltage microscopy. They found 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. This work has been published online in Nano Letters （Jian Zhu，Fengtao Fan, Can Li, et al., Nano Letters, DOI: 10.1021/acs.nanolett.7b02799）.
It is well known that the use of nano cocatalysts or dual cocatalysts in semiconductor based photocatalysts can significantly improves solar fuel production in artificial photosynthesis, but the exact roles of the cocatalysts in photocatalysis is not understood yet. A big difference between photocatalysis and thermal catalysis is that the photogenerated charges are involved directly in the elementary steps of photocatalytic reactions. So imaging the photogenerated charges would provide unprecedented insight into the nature of the photocatalysis, but requires high sensitivity and high spatial resolution.
Fengtao Fan and Can Li et al. delivered the first report to image the photogenerated charge redistribution on a single particle photocatalyst deposited with nanosize cocatalyst, by using surface photovoltage microscopy. It is found that cocatalysts with nano size shows big impact on the charge separation of single particle photocatalyst. The deposition of cocatalysts on to photocatalyst not only strongly improve the interfacial charge transfer but also surprisingly change the direction of the built-in electric fields beneath the uncovered surface of photocatalyst, resulting in additive electric fields up to 80 times enhancement in localized photovoltage and correspondingly improved photocatalytic performances. A synergetic effect of spatially separated dual cocatalysts(e.g., MnOx and Pt) on the photocatalyst particle is also found to further increase the electric field strength up to 2.5 kV/cm. This understanding may become a new principle of photocatalyst design and fabrication by rationally aligning the distribution of built-in electric fields into additive one.
(By Jian Zhu and Fengtao Fan)