Effect of a Cocatalyst on a Photoanode in Water Splitting: A Study of Scanning Electrochemical Microscopy.

With a proper band gap of ∼2.4 eV for solar light absorption and suitable valence band edge position for oxygen evolution, scheelite-monoclinic bismuth vanadate (BiVO4) has become one of the most attractive photocatalysts for efficient visible-light-driven photoelectrochemical (PEC) water splitting. Several studies have indicated that surface modification of BiVO4 with a cocatalyst such as NiFe layered double hydroxide (LDH) can significantly increase the PEC water splitting performance of the catalyst. Herein, we experimentally investigated the charge transfer dynamics and charge carrier recombination processes by scanning electrochemical microscopy (SECM) with the feedback mode on the surface of BiVO4 and BiVO4/NiFe-LDH as model samples. The ratio of rate constants for photogenerated hole (kh+0) to electron (ke-0) via the photocatalyst of BiVO4/NiFe-LDH reacting with the redox couple is found to be five times larger than that of BiVO4 under illumination. In this case, the ratio of the rate constants kh+0/ke-0 stands for the interfacial charge recombination process. This implies the cocatalyst NiFe-LDH suppresses the electron back transfer greatly and finally reduces the surface recombination. Control experiments with cocatalysts CoPi and RuOx onto BiVO4 further verify this conclusion. Therefore, the SECM characterization allows us to make an overall analysis on the function of cocatalysts in the PEC water splitting system.