Hydrothermally Synthesized BiVO $_4$ : The Role of KCl as Additive for Improved Photoelectrochemical and Photocatalytic Oxygen Evolution Activity

With sunlight as the most abundant energy source on earth, solar water splitting has the potential to produce renewable hydrogen at a commercially competitive cost. Monoclinic BiVO $_{4}$ is a promising n‐type semiconductor for photocatalytic and photoelectrochemical (PEC) water oxidation. One of the simplest and most energy‐efficient approaches for producing BiVO $_{4}$ is hydrothermal synthesis. This method is carried out at moderate temperatures, while particle size, shape, and crystallinity are controlled by a wide range of synthesis parameters, which can be further expanded by using additives. In this work, these parameters systematically vary to study their influence on the hydrothermal synthesis of BiVO $_{4}$ , with a focus on KCl as an additive are systematically vary. By X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy is shown that KCl acts as structure‐directing agent, leading to significant changes in morphology and crystallinity. Since the color and the optical spectra of BiVO $_{4}$ powders indicate a redshift with increasing KCl concentration, an additional anionic substitution by Cl $^{−}$ takes place is proposed, a hypothesis supported by X‐ray photoelectron spectroscopy measurements and density functional theory calculations. The highest photocatalytic performance (1328 µmol g $^{−1 }$ h $^{−1}$ ) is reached for 25 mmol L $^{−1}$ KCl, while particle‐based photoelectrodes decorated with CoPi cocatalysts showed an improved photocurrent density (393 µA cm $^{−2}$ ) at 1.23 V vs. reversible hydrogen electrode (RHE).