  Bismuth vanadate (BiVO4) has emerged as a highly prospective material for
photoanodes in photoelectrochemical (PEC) water oxidation. However, current
limitations with this material lie in the difficulties in producing stable and
continuous BiVO4 layers with efficient carrier transfer kinetics, thereby
impeding its widespread application in water splitting processes. This study
introduces a new fabrication approach that yields continuous, monoclinic
nanostructured BiVO4 films, paving the way for their use as photoanodes in
efficient PEC water oxidation for hydrogen production under solar light
conditions. The fabrication involves the intercalation of vanadium (V) ions
into Bi2O3 films at 450oC. Upon interaction with V ions, the film undergoes a
transformation from tetragonal Bi2O3 to monoclinic scheelite BiVO4. This
synthesis method enables the fabrication of single monoclinic phase BiVO4 films
with thicknesses up to 270 nm. The engineered monoclinic BiVO4 film, devoid of
any pinholes that could cause carrier loss, exhibits a robust photocurrent of
1.0 mA/cm2 at 1.23 VRHE in a neutral electrolyte, without requiring additional
modifications or doping. Moreover, we demonstrate that the incorporation of a
cobalt phosphate (Co-Pi) co-catalyst into the BiVO4 photoanode significantly
enhances the lifetime of photogenerated holes by a factor of nine, resulting in
a further elevation of the photocurrent to 2.9 mA/cm2. This remarkable PEC
enhancement can be attributed to the surface state passivation by the Co-Pi
co-catalyst. Our fabrication approach opens up a new facile route for producing
large-scale, highly efficient BiVO4 photoanodes for PEC water splitting
technology.
